8.1 Insolation and energy 8.2 Tracking the sun 8.3 Shading and dirt 8.4 Temperature 8.5 Module efficiency 8.6 Monitoring 8.7 Performance factors 8.8 Module life
• 9-Power costs:
Secondly solar thermal energy:
10-Low-temperature collectors o 10.1 Heating, cooling, and ventilation o 10.2 Process heat 11- Medium-temperature collectors o 11.1 Solar drying o 11.2 Cooking o 11.3 Distillation 12-High-temperature collectors o 12.1 System designs 12.1.1 Parabolic trough designs 12.1.2 Power tower designs
• • •
12.1.3 Dish designs 12.1.4 Fresnel reflectors 12.1.5 Linear Fresnel reflector technologies 12.1.6 Fresnel lenses 12.1.7 MicroCSP 12.1.8 Enclosed parabolic trough 13-Heat collection and exchange 14-Heat storage o 14.1 Steam accumulator o 14.2 Molten salt storage o 14.3 Graphite heat storage o 14.4 Phase-change materials for storage 15-Use of water 16-Conversion rates from solar energy to electrical energy 17- Levelised cost
17-HYBRID SOLAR PANEL 17.1 Introduction 17.2 Review of models and systems 17.2.1 Liquid PVT collectors 17.2.2 Air PVT Collectors 17.2.3 Ventilated PV with heat recovery 17.2.4 PVT Concentrator 17.3 Market of survey of PVT Collectors 17.3.1 Liquid PVT collectors market 17.3.2 Air PVT collectors market 17.3.3 Market of ventilated PV with heat recovery 17.3.4 PVT concentrators market 17.3.5 Other interesting projects developed with PVT collectors
Nellis Solar Power Plant at Nellis Air Force Base in the USA. These panels track the sun in one axis.
Photovoltaic SUDI shade is an autonomous and mobile station in France that replenishes energy for electric vehicles using solar energy.
Solar panels on the International Space Station
Photovoltaic’s (PV) is a method of generating electrical power by converting solar radiation into direct current
electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material. Materials presently used for photovoltaic’s include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/sulfide. Due to the growing demand for renewable energy sources, the manufacturing of solar cells and photovoltaic arrays has advanced considerably in recent years. By the end of 2011, a total of 67.4 GW had been installed, sufficient to generate 85 TWh/year.  Solar photovoltaic's is now, after hydro and wind power, the third most important renewable energy source in terms of globally installed capacity. More than 100 countries use solar PV. Installations may be ground-mounted (and sometimes integrated with farming and grazing) or built into the roof or walls of a building (either building-integrated photovoltaics or simply rooftop). Driven by advances in technology and increases in manufacturing scale and sophistication, the cost of photovoltaic’s has declined steadily since the first solar cells were manufactured  and the levelised cost of electricity (LCOE) from PV is competitive with conventional electricity sources in an expanding list of geographic regions. Net metering and financial incentives, such as preferential feed-in tariffs for solar-generated electricity; have supported solar PV installations in many countries. With current technology, photovoltaic’s recoup the energy needed to manufacture them in 1 to 4 years.
Solar cells produce electricity directly from sunlight
Average solar irradiance, watts per square meter. Note that this is for a horizontal surface, whereas solar panels are normally mounted at an angle and receive more energy per unit area. The small black dots show the area of solar panels needed to generate all of the world's energy using 8% efficient photovoltaics.
Solar cell productions by region
Photovoltaic’s are best known as a method for generating electric power by using solar cells to convert energy from the sun into a flow of electrons. The photovoltaic effect refers to photons of light exciting electrons into a higher state of energy, allowing them to act as charge carriers for an electric current. The photovoltaic effect was first observed by Alexander-Edmond Becquerel in 1839. The term photovoltaic denotes the unbiased operating mode of a photodiode in which current through the device is entirely due to the transduced light energy. Virtually all photovoltaic devices are some type of photodiode. Solar cells produce direct current electricity from sun light, which can be used to power equipment or to recharge a battery. The first practical application of photovoltaic’s was to power orbiting satellites and other spacecraft, but today the majority of photovoltaic modules are used for grid connected power generation. In this case an inverter is required to convert the DC to AC. There is a smaller market for off-grid power for remote dwellings, boats, recreational vehicles, electric cars, roadside emergency telephones, remote sensing, and cathodic protection of pipelines.
Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material. Materials presently used for photovoltaics include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/sulfide. Due to the growing demand for renewable energy sources, the manufacturing of solar cells and photovoltaic arrays has advanced considerably in recent years. Cells require protection from the environment and are usually packaged tightly behind a glass sheet. When more power is required than a single cell can deliver, cells are electrically connected together to form photovoltaic modules, or solar panels. A single module is enough to power an emergency telephone, but for a house or a power plant the modules must be arranged in multiples as arrays.
Photovoltaic power capacity is measured as maximum power output under standardized test conditions (STC) in "Wp" (Watts’s peak). The actual power output at a particular point in time may be less than or greater than this standardized, or "rated," value, depending on geographical location, time of day, weather conditions, and other factors. Solar photovoltaic array capacity factors are typically under 25%, which is lower than many other industrial sources of electricity. A significant market has emerged in off-grid locations for solar-power-charged storage-battery based solutions. These often provide the only electricity available. The first commercial installation of this kind was in 1966 on Ogami Island in Japan to transition Ogami Lighthouse from gas torch to fully self-sufficient electrical power.
Map of solar electricity potential in Europe.
Photovoltaic panels based on crystalline silicon modules are encountering competition in the market by panels that employ thin-film solar cells (CdTe CIGS, amorphous Si, microcrystalline Si), which had been rapidly evolving and are expected to account for 31% of the global installed power by 2013. However, precipitous drops in prices for polysilicon and their panels in late 2011 have caused some thin-film makers to exit the market and others to experience severely squeezed profits. Other developments include casting wafers instead of sawing, concentrator modules, 'Sliver' cells, and continuous printing processes.
The San Jose-based company Sunpower produces cells that have an energy conversion ratio of 19.5%, well above the market average of 12–18%. The most efficient solar cell so far is a multi-junction concentrator solar cell with an efficiency of 43.5% produced by Solar Junction in April 2011. The highest efficiencies achieved without concentration include Sharp Corporation at 35.8% using a proprietary triple-junction manufacturing technology in 2009, and Boeing Spectrolab (40.7% also using a triple-layer design). A March 2010 experimental demonstration of a design by a Caltech group led by Harry Atwater which has an absorption efficiency of 85% in sunlight and 95% at
certain wavelengths is claimed to have near perfect quantum efficiency. However, absorption efficiency should not be confused with the sunlight-to-electricity conversion efficiency.
For best performance, terrestrial PV systems aim to maximize the time they face the sun. Solar trackers achieve this by moving PV panels to follow the sun. The increase can be by as much as 20% in winter and by as much as 50% in summer. Static mounted systems can be optimized by analysis of the sun path. Panels are often set to latitude tilt, an angle equal to the latitude, but performance can be improved by adjusting the angle for summer or winter. Generally, as with other semiconductor devices, temperatures above room temperature reduce the performance of photovoltaics.
A number of solar panels may also be mounted vertically above each other in a tower, if the zenith distance of the Sun is greater than zero, and the tower can be turned horizontally as a whole and each panels additionally around a horizontal axis. In such a tower the panels can follow the Sun exactly. Such a device may be described as a ladder mounted on a turnable disk. Each step of that ladder is the middle axis of a rectangular solar panel. In case the zenith distance of the Sun reaches zero, the “ladder” may be rotated to the north or the south to avoid a solar panel producing a shadow on a lower solar panel. Instead of an exactly vertical tower one can choose a tower with an axis directed to the polar star, meaning that it is parallel to the rotation axis of the Earth. In this case the angle between the axis and the Sun is always larger than 66 degrees. During a day it is only necessary to turn the panels around this axis to follow the Sun.
Solar photovoltaic is growing rapidly, albeit from a small base, to a total global capacity of 69,684 megawatts (MW) at the end of 2011. The total power output of the world’s PV capacity run over a calendar year is equal to some 80 billion kWh of electricity. This is sufficient to cover the annual power supply needs of over 20 million households in the world, and represents 0.5% of worldwide electricity demand.More than 100 countries use solar PV. World solar PV capacity (grid-connected) was 7.6 GW in 2007, 16 GW in 2008, 23 GW in 2009, and 40 GW in 2010. Installations may be ground-mounted (and sometimes integrated with farming and grazing) or built into the roof or walls of a building (building-integrated photovoltaics). Photovoltaic is now, after hydro and wind power, the third most important renewable energy source in terms of globally installed capacity.
The 2011 European Photovoltaic Industry Association (EPIA) report predicted that, "[In 2011] Europe once again was the global leader in PV market growth, with 75% of all newly connected capacity and about 75% of global installed capacity. But non-European markets are showing signs that they may soon shift this balance in their favour". 2012 could see the installation of 20–30 GW of PV — about the same as in 2011. The industry's capacity continues to expand, to perhaps as much as 38 GW, furthering the PV systems price decline. With proper policy support, balanced market development, and continued industry innovation, photovoltaic (PV) can continue its remarkable growth rate over the short-, medium- and long-term, and even beyond.
The EPIA/Greenpeace Solar Generation Paradigm Shift Scenario (formerly called Advanced Scenario) from 2010 shows that by the year 2030, 1,845 GW of PV systems could be generating approximately 2,646 TWh/year of electricity around the world. Combined with energy use efficiency improvements, this would represent the electricity needs of more than 9% of the world's population. By 2050, over 20% of all electricity could be provided by photovoltaics. However, the EPIA prediction may be pessimistic since official agencies keep underestimating the growth rate of renewables. A report based on the 2012 BP Statistical Review shows an exponential growth in global solar generation from 2001 to end 2011, with an approximate doubling of generation every two years. This raises the possibility that solar power could reach 10% of total global power generation by the end of this decade. To accomplish this gain in primary energy share, solar will need to advance from the 55.7 TWh generated in 2011 to approximately 2200 TWh. At current exponential growth rates, those levels could be achieved as early as 2018 rather than around 2030 as suggested by the EPIA. Solar would provide 100 percent of the current world energy needs by 2027 if the biannual doubling of generation continues.
Photovoltaic power worldwide GWp 2005 2006 2007 2008 2009 2010 2011
Year end capacities
5.4 7.0 9.4 15.7 22.9 39.7 67.4
The output of a photovoltaic array is a product of the area, the efficiency, and the insulation. The capacity factor, or duty cycle, of photovoltaics is relatively low, typically from 0.10 to 0.30, as insolation ranges, by latitude and prevailing
weather, and is location specific from about 2.5 to 7.5 sun hours/day. Panels are rated under standard conditions by their output power. The DC output is a product of the rated output times the number of panels times the insulation times the number of days. The sunlight received by the array is affected by a combination of tilt, tracking and shading. Tracking increases the yield but also the cost, both installation and maintenance. A dual axis tracker can increase the effective insolation by roughly 35–40%, while temperature effects can reduce efficiency by 10%. The AC output is roughly 25% lower due to various losses including the efficiency of the inverter. For example, for a 4 kW array in Paris, where the average insolation is 3.34 sun hours/day, the annual (AC) output would be approximately 3.34x4x365x0.75=3657 kWh, and the monthly output, from the following chart, would range from 67 kWh in December to 498 kWh in July. The weather strongly affects the output. Monthly and annual energy production varies substantially from year to year (by +/-40% monthly and +/-20% annually). Published insolation values are normally 10 year averages, and long term output tends to be accurate within 10 to 12%. There are many live data sites that can be monitored, and compared.
Financial incentives for photovoltaic’s, such as feed-in tariffs, have often been offered to electricity consumers to install and operate solar-electric generating systems. Government has sometimes also offered incentives in order to encourage the PV industry to achieve the economies of scale needed to compete where the cost of PV-generated electricity is above the cost from the existing grid. Such policies are implemented to promote national or territorial energy independence, high tech job creation and reduction of carbon dioxide emissions which cause global warming. Due to economies of scale solar panels get less costly as people use and buy more — as manufacturers increase production to meet demand, the cost and price is expected to drop in the years to come.
Reported timeline of solar cell energy conversion efficiencies (from National Renewable Energy Laboratory (USA)
Solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 43.5% with multiple-junction concentrated photovoltaics. Solar cell energy conversion efficiencies for commercially available photovoltaics are around 14-22%. As of 2011, the price of PV modules per MW has fallen by 60% since the summer of 2008, according to Bloomberg New Energy Finance estimates, putting solar power for the first time on a competitive footing with the retail price of electricity in a number of sunny countries. The levelised cost of electricity (LCOE) from PV is competitive with conventional electricity sources in an expanding list of geographic regions, particularly when the time of generation is included, as electricity is worth more during the day than at night. There has been fierce competition in the supply chain, and further improvements in the levelised cost of energy for solar lie ahead, posing a growing threat to the dominance of fossil fuel generation sources in the next few years. As time progresses, renewable energy technologies generally get cheaper, while fossil fuels generally get more expensive: The less solar power costs, the more favorably it compares to conventional power, and the more attractive it becomes to utilities and energy users around the globe. Utility-scale solar power can now be delivered in California at prices well below $100/MWh ($0.10/kWh) less than most other peak generators, even those running on low-cost natural gas. Lower solar module costs also stimulate demand from consumer markets where the cost of solar compares very favorably to retail electric rates. As of 2011, the cost of PV has fallen well below that of nuclear power and is set to fall further. The average retail price of solar cells as monitored by the Solarbuzz group fell from $3.50/watt to $2.43/watt over the course of 2011. For large-scale installations, prices below $1.00/watt were achieved. A module price of 0.60 Euro/watt (0.78 $/watt) was published for a large scale 5-year deal in April 2012. In some locations, PV has reached grid parity, which is usually defined as PV production costs at or below retail electricity prices (though often still above the power station prices for coal or gas-fired generation without their distribution and other costs). Photovoltaic power is also generated during a time of day that is close to peak demand (precedes it) in electricity systems with high use of air conditioning. More generally, it is now evident that, given a carbon price of $50/ton, which would raise the price of coal-fired power by 5c/kWh, solar PV will be cost-competitive in most locations. The declining price of PV has been reflected in rapidly
growing installations, totaling about 23 GW in 2011. Although some consolidation is likely in 2012, due to support cuts in the large markets of Germany and Italy, strong growth seems likely to continue for the rest of the decade. Already, by one estimate, total investment in renewables for 2011 exceeded investment in carbon-based electricity generation. In the case of self consumption payback time is calculated based on how much electricity is not brought from the grid. For example in Germany with electricity prices of 0.25 euro/KWh and Insolation of 900 KWh/KW one KWp will save 225 euro per year and with installation cost of 1700 euro/KWp means that the system will pay back in less than 7 years. 
80 MW Okhotnykovo Solar Park in Ukraine.
Many solar photovoltaic power stations have been built, mainly in Europe. As of July 2012, the largest photovoltaic (PV) power plants in the world are the Agua Caliente Solar Project (USA, 247 MW), Charanka Solar Park (India, 214 MW), Golmud Solar Park (China, 200 MW), Perovo Solar Park (Ukraine 100 MW), Sarnia Photovoltaic Power Plant (Canada, 97 MW), Brandenburg-Briest Solarpark (Germany 91 MW), Solarpark Finow Tower (Germany 84.7 MW), Montalto di Castro Photovoltaic Power Station (Italy, 84.2 MW), Eggebek Solar Park (Germany 83.6 MW), Senftenberg Solarpark (Germany 82 MW), Finsterwalde Solar Park (Germany, 80.7 MW), Okhotnykovo Solar Park (Ukraine, 80 MW), Lopburi Solar Farm (Thailand 73.16 MW), Rovigo Photovoltaic Power Plant (Italy, 72 MW), and the Lieberose Photovoltaic Park (Germany, 71.8 MW). There are also many large plants under construction. The Desert Sunlight Solar Farm under construction in Riverside County, California and Topaz Solar Farm being built in San Luis Obispo County, California are both 550 MW solar parks that will use thin-film solar photovoltaic modules made by First Solar. The Blythe Solar Power Project is a 500 MW photovoltaic station under construction in Riverside County, California. The California Valley Solar Ranch (CVSR) is a 250 megawatt (MW) solar photovoltaic power plant, which is being built by SunPower in the Carrizo Plain, northeast of California Valley. The 230 MW Antelope Valley Solar Ranch is a First Solar photovoltaic project which is under construction in the Antelope Valley area of the Western Mojave Desert, and due to be completed in 2013. The Mesquite Solar project is a photovoltaic solar power plant being built in Arlington, Maricopa County, Arizona, owned by Sempra Generation. Phase 1 will have a nameplate capacity of 150 megawatts.
Many of these plants are integrated with agriculture and some use innovative tracking systems that follow the sun's daily path across the sky to generate more electricity than conventional fixed-mounted systems. There are no fuel costs or emissions during operation of the power stations.
Photovoltaic wall at MNACTEC Terrassa in Spain
Photovoltaic arrays are often associated with buildings: either integrated into them, mounted on them or mounted nearby on the ground. Arrays are most often retrofitted into existing buildings, usually mounted on top of the existing roof structure or on the existing walls. Alternatively, an array can be located separately from the building but connected by cable to supply power for the building. In 2010, more than four-fifths of the 9,000 MW of solar PV operating in Germany were installed on rooftops. Building-integrated photovoltaic’s (BIPV) are increasingly incorporated into new domestic and industrial buildings as a principal or ancillary source of electrical power. Typically, an array is incorporated into the roof or walls of a building. Roof tiles with integrated PV cells are also common. A 2011 study using thermal imaging has shown that solar panels, provided there is an open gap in which air can circulate between them and the roof, provide a passive cooling effect on buildings during the day and also keep accumulated heat in at night. The power output of photovoltaic systems for installation in buildings is usually described in kilowatt-peak units (kWp).
PV has traditionally been used for electric power in space. PV is rarely used to provide motive power in transport applications, but is being used increasingly to provide auxiliary power in boats and cars. A self-contained solar vehicle would have limited power and low utility, but a solar-charged vehicle would allow use of solar power for transportation. Solar-powered cars have been demonstrated.
Solar parking paystation.
Until a decade or so ago, PV was used frequently to power calculators and novelty devices. Improvements in integrated circuits and low power liquid crystal displays make it possible to power such devices for several years between battery changes, making PV use less common. In contrast, solar powered remote fixed devices have seen increasing use recently in locations where significant connection cost makes grid power prohibitively expensive. Such applications include water pumps, parking meters, emergency telephones, trash compactors, temporary traffic signs, and remote guard posts and signals.
Unlike the past decade, which saw solar solutions purchased mainly by international donors, it is now the locals who are increasingly opening their wallets to make the switch from their traditional energy means. That is because solar products prices in recent years have declined to become cheaper than kerosene and batteries. In Cambodia, for example, villagers can buy a solar lantern at US$25 and use it for years without any extra costs, where their previous spending on kerosene for lighting was about $2.5 per month, or $30 per year. In Kenya a solar kit that provides bright light or powers a radio or cell phone costs under $30 at retail stores. By switching to this kit Kenyans can save $120 per year on kerosene lighting, radio batteries and cell phone recharging fees. Developing countries where many villages are often more than five kilometers away from grid power are increasingly using photovoltaics. In remote locations in India a rural lighting program has been providing solar powered LED lighting to replace kerosene lamps. The solar powered lamps were sold at about the cost of a few months' supply of kerosene. Cuba is working to provide solar power for areas that are off grid. These are areas where the social costs and benefits offer an excellent case for going solar though the lack of profitability could relegate such endeavors to humanitarian goals.
The 104kW solar highway along the interchange of Interstate 5 and Interstate 205 near Tualatin, Oregon in December 2008.
In December 2008, the Oregon Department of Transportation placed in service the nation’s first solar photovoltaic system in a U.S. highway right-of-way. The 104-kilowatt (kW) array produces enough electricity to offset approximately one-third of the electricity needed to light the Interstate highway interchange where it is located. A 45 mi (72 km) section of roadway in Idaho is being used to test the possibility of installing solar panels into the road surface, as roads are generally unobstructed to the sun and represent about the percentage of land area needed to replace other energy sources with solar power.
(4.7)Plug in solar:
In 2012, a UL approved solar panel was introduced which is simply plugged into an electrical outlet. It senses mains voltage and waits 5 minutes before activating the inverter, and shuts down immediately if lines voltage is removed, eliminating any shock hazard from touching the plug prongs. Up to five 240 watt panels can be connected to one outlet.
The 89 PW of sunlight reaching the Earth's surface is plentiful – almost 6,000 times more than the 15 TW equivalent of average power consumed by humans. Additionally, solar electric generation has the highest power density (global mean of 170 W/m2) among renewable energies. Solar power is pollution-free during use. Production end-wastes and emissions are manageable using existing pollution controls. End-of-use recycling technologies are under development  and policies are being produced that encourage recycling from producers. PV installations can operate for many years with little maintenance or intervention after their initial set-up, so after the initial capital cost of building any solar power plant, operating costs are extremely low compared to existing power technologies. Grid-connected solar electricity can be used locally thus reducing transmission/distribution losses (transmission losses in the US were approximately 7.2% in 1995). Compared to fossil and nuclear energy sources, very little research money has been invested in the development of solar cells, so there is considerable room for improvement. Nevertheless, experimental high efficiency solar cells already have efficiencies of over 40% in case of concentrating photovoltaic cells  and efficiencies are rapidly rising while mass-production costs are rapidly falling.
In some states of the United States of America, much of the investment in a home-mounted system may be lost if the home-owner moves and the buyer puts less value on the system than the seller. The city of Berkeley developed an innovative financing method to remove this limitation, by adding a tax assessment that is transferred with the home to pay for the solar panels. Now known as PACE, Property Assessed Clean Energy, 28 U.S. states have duplicated this solution. There is evidence, at least in California, that the presence of a home-mounted solar system can actually increase the value of a home. According to a paper published in April 2011 by the Ernest Orlando Lawrence Berkeley National Laboratory entitled An Analysis of the Effects of Residential Photovoltaic Energy Systems on Home Sales Prices in California,
"The research finds strong evidence that homes with PV systems in California have sold for a premium over comparable homes without PV systems. More specifically, estimates for average PV premiums among a large number of different model specifications coalesced near $17,000 for a relatively new “average-sized” - based on the sample of homes studied - PV system of 3,100 watts (DC). This corresponds to an average home sales price premium of $5.5/watt (DC), with the range of results across various models being $3.9 to $6.4/watt."
7-PV TYPES: There are two type types of PV systems:
1-Flat-Plate Systems 2-Concentrator Systems
1-Flat-Plate Photovoltaic Systems
One typical flat-plate module design uses a substrate of metal, glass, or plastic to provide structural support in the back; an encapsulant material to protect the cells; and a transparent cover of plastic or glass. The most common photovoltaic (PV) array design uses flat-plate PV modules or panels. These panels can be fixed in place or allowed to track the movement of the sun. They respond to sunlight that is direct or diffuse. Even in clear skies, the diffuse component of sunlight accounts for between 10% and 20% of the total solar radiation on a horizontal surface. On partly sunny days, up to 50% of that radiation is diffuse, and on cloudy days, 100% of the radiation is diffuse. The simplest PV array consists of flat-plate PV panels in a fixed position. The advantages of fixed arrays are that they lack moving parts, there is virtually no need for extra equipment, and they are relatively lightweight. These features make them suitable for many locations, including most residential roofs. But because the panels are fixed in place, their orientation to the sun is usually at an angle that is less than optimal. Therefore, fixed arrays collect less energy per unit area of array than tracking arrays. However, this drawback must be balanced against the higher cost of the tracking system.
A-Modules B-Balance of System Components. A-Flat-Plate Photovoltaic Modules:
Flat-plate photovoltaic (PV) modules are made of several components, including the front surface materials, encapsulant, rear surface, and frame.
Front Surface Materials
The front surface of a flat-plate PV module must have a high transmission in the wavelengths that can be used by the solar cells in the module. For example, for silicon solar cells, the top surface must have high transmission of light with wavelengths from 350 to 1200 nm. Also, reflection from the front surface should be minimal. An antireflection coating added to the top surface can greatly reduce the reflection of sunlight, and texturing of the surface can cause light that strikes the surface
to stay within the cells. Unfortunately, these textured modules are not "self-cleaning," and the advantage of reduced reflection is usually outweighed by losses because of dust sticking to the surface. The top surface should also be impervious to water, be able to resist damage from hail, be stable under longterm exposure to ultraviolet radiation, and have low thermal resistivity. If water, as liquid or vapor, is able to get inside a PV module, it corrode the metal contacts and interconnects, which will greatly shorten the life span of the module. A common front surface material is tempered, low-iron glass, which is low-cost, strong, stable, highly transparent, and impervious to water and gases. It also has self-cleaning properties.
An encapsulant helps hold together the top surface, PV cells, and rear surface of the PV module. The encapsulant must be stable at high temperatures and high levels of ultraviolet radiation. It must also be optically transparent and have a low thermal resistance. Ethyl vinyl acetate—or EVA—is the most commonly used encapsulant. Thin sheets of EVA are inserted between the solar cells and the top and rear surfaces. Heating this "sandwich" causes the EVA to polymerize, thus bonding the module into one piece.
The material used as the rear surface of the PV module must have low thermal resistance and prevent the ingress of water and gases. In many modules, the rear surface material is a thin polymer sheet, typically made of Tedlar.
The final structural component of the module is the frame, which is typically made of aluminum.
B-Flat-Plate Photovoltaic Balance of System:
Complete photovoltaic (PV) energy systems are composed of three subsystems.
On the power-generation side, the first subsystem of PV devices (cells, modules, and arrays) converts sunlight to direct-current (DC) electricity. On the power-use side, the second subsystem consists of the load, which is the application of the PV electricity.
This illustration shows the elements needed to get the power created by a PV system to the load (in this example, a house). The stand-alone PV system (a) uses battery storage to provide dependable DC electricity day and night. Even for a home connected to the utility grid (b), PV can produce electricity (converted to AC by a power conditioner) during the day. The extra electricity can then be sold to the utility during the day, and the utility can in turn provide electricity at night or during poor weather.
A typical PV array mounting rack.
Between these two, a third subsystem enables the PV-generated electricity to be properly applied to the load. This subsystem is often called the balance of system, or BOS. The BOS typically consists of structures for mounting the PV arrays or modules and power-conditioning equipment that adjusts and converts the DC electricity to the proper form and magnitude required by an alternating-current (AC) load. The BOS can also include storage devices, such as batteries, so PV-generated electricity can be used during cloudy days or at night.
PV arrays must be mounted on a stable, durable structure that can support the array and withstand wind, rain, hail, and other adverse conditions. Sometimes, this mounting structure is designed to track the sun. However,
stationary structures are usually used with flat-plate systems. These structures tilt the PV array at a fixed angle determined by the latitude of the site, the requirements of the load, and the availability of sunlight. Among the choices for stationary mounting structures, rack mounting may be the most versatile. It can be constructed fairly easily and installed on the ground or on flat or slanted roofs. There are two basic kinds of tracking structures: one-axis and two-axis. One-axis trackers are typically designed to track the sun from east to west. They are used with flat-plate systems and sometimes with concentrator systems. The two-axis type is used primarily with PV concentrator systems. These units track the sun's daily course and its seasonal course between the northern and southern hemispheres. Naturally, the more sophisticated systems are the more expensive ones, and they usually require more maintenance.
Power conditioners process the electricity produced by a PV system so it will meet the specific demands of the load. Although most equipment is standard, it is important to select equipment that matches the characteristics of the load. Power conditioners may:
• • • •
Limit current and voltage to maximize power output Convert DC power to AC power Match the converted AC electricity to a utility's electrical network Have safeguards that protect utility personnel and the electrical network from harm during repairs.
Specific requirements of power conditioners depend on the type of PV system they are used with and the applications of that system. For DC applications, power conditioning is often done with regulators, which control output at some constant level of voltage and current to maximize output. For AC loads, power conditioning must include an inverter that converts the DC power generated by the PV array into AC power. Many simple devices—for example, ones that run on batteries—use DC electricity. However, AC electricity, which is what is generated by utilities, is needed to run most modern appliances and electronic devices.
Electricity is needed at night and on cloudy days, when PV power generation may not be possible. If tapping into the utility grid is not an option, a battery backup system is necessary for energy storage. However, batteries lower the efficiency of a PV system because only about 80% of the energy that goes into them can be reclaimed. They take up considerable floor space, pose a few possible safety problems, and require periodic maintenance. Still, they provide one way to store PV electricity for later use. Like PV cells, batteries are DC devices that are directly compatible only with DC loads. However, batteries can also serve as a power conditioner for these loads by regulating power. This allows the PV array to operate closer to its optimum power output.
An inverter (left) and charge controller (right) are the power conditioning components of a PV system.
Inverter convert the DC electricity generated by the PV array into AC electricity, and charge controllers protect batteries from overcharging and excessive discharge. Most batteries must be protected from overcharge and excessive discharge, which can cause electrolyte loss and even damage or ruin the battery plates. Most charge controllers also have a mechanism that prevents current from flowing from the battery back into the array at night.
2-Concentrator Photovoltaic Systems:
Concentrator photovoltaic (PV) systems use less solar cell material than other PV systems. PV cells are the most expensive components of a PV system, on a per-area basis. A concentrator makes use of relatively inexpensive materials such as plastic lenses and metal housings to capture the solar energy shining on a fairly large area and focus that energy onto a smaller area—the solar cell. One measure of the effectiveness of this approach is the concentration ratio—in other words, how much concentration the cell is receiving. Concentrator PV systems have several advantages over flat-plate systems. First, concentrator systems reduce the size or number of cells needed and allows certain designs to use more expensive semiconductor materials which would otherwise be cost prohibitive. Second, a solar cell's efficiency increases under concentrated light. How much that efficiency increases depends largely on the design of the solar cell and the material used to make it. Third, a concentrator can be made of small individual cells. This is an advantage because it is harder to produce large-area, high-efficiency solar cells than it is to produce small-area cells.
A typical concentrator unit consists of a lens to focus the light, a cell assembly, a housing element, a secondary concentrator to reflect off-center light rays onto the cell, a mechanism to dissipate excess heat produced by concentrated sunlight, and various contacts and adhesives.
However, challenges exist for concentrators. First, the required concentrating optics are significantly more expensive than the simple covers needed for flat-plate solar systems, and most concentrators must track the sun throughout the day and year to be effective. Thus, achieving higher concentration ratios means using not only expensive tracking mechanisms but also more precise controls. Both reflectors and lenses have been used to concentrate light for PV systems.
The most promising lens for PV applications is the Fresnel lens, which uses a miniature sawtooth design to focus incoming light. When the teeth run in straight rows, the lenses act as line-focusing concentrators. When the teeth are arranged in concentric circles, light is focused at a central point. However, no lens can transmit 100% of the incident light. The best that lenses can transmit is 90% to 95%, and in practice, most transmit less. Furthermore, concentrators cannot focus diffuse sunlight, which makes up about 30% of the solar radiation available on a clear day.
High concentration ratios also introduce a heat problem. When solar radiation is concentrated, so is the amount of heat produced. Cell efficiencies decrease as temperatures increase, and higher temperatures also threaten the long-term stability of solar cells. Therefore, the solar cells must be kept cool in a concentrator system, requiring sophisticated heat sync cooling designs.
One of the most important design goals of concentrator systems is to minimize electrical resistance where the electrical contacts on the cell carry off the current generated by the cell. A pattern using wide grid lines, known as fingers, in the contacting grid on top of the cell are ideal for low resistance, but they block too much light from reaching the cell because of their shadow. One solution to the problems of resistance and shadowing is prismatic covers. These special covers act like a prism and direct incoming light to parts of the cell's surface that are between the metal fingers of the electrical contact grid. Another solution is a backcontact cell, which differs from conventional cells in that both the positive and negative electrical contacts are on the back. Placing all the electrical contacts on the back of the cell eliminates power losses from shadowing, but it also requires exceptionally good-quality silicon material.
(8.1)-Insolation and energy
At high noon on a cloudless day at the equator, the power of the sun is about 1 kW/m², on the Earth's surface, to a plane that is perpendicular to the sun's rays. As such, PV arrays can track the sun through each day to greatly enhance energy collection. However, tracking devices add cost, and require maintenance, so it is more common for PV arrays to have fixed mounts that tilt the array and face due South in the Northern Hemisphere (in the Southern Hemisphere, they should point due North). The tilt angle, from horizontal, can be varied for season, but if fixed, should be set to give optimal array output during the peak electrical demand portion of a typical year for a stand alone system. This optimal module tilt angle is not necessarily identical to the tilt angle for maximum annual array output. For the weather and latitudes of the United States and Europe, typical insolation ranges from 4 kWh/m²/day in northern climes to 6.5 kWh/m²/day in the sunniest regions. Typical solar panels have an average efficiency of 15%, with the best commercially available panels at 21%. Thus, a photovoltaic installation in the southern latitudes of Europe or the United States may expect to produce 1 kWh/m²/day. A typical "150 watt" solar panel is about a square meter in size. Such a panel may be expected to produce 0.75 kWh every day, on average, after taking into account the weather and the latitude, for an insolation of 5 sun hours/day. A typical 1 kW photovoltaic installation in Australia or the southern latitudes of Europe or United States may produce 3.5-5 kWh per day, dependent on location, orientation, tilt, insolation and other factors. In the Sahara desert, with less cloud cover and a better solar angle, one could ideally obtain closer to 8.3 kWh/m²/day provided the nearly ever present wind would not blow sand on the units. The area of the Sahara desert is over 9 million km². 90,600 km², or about 1%, could generate as much electricity as all of the world's power plants combined.
(8.2)-Tracking the sun
Trackers and sensors to optimize the performance are often seen as optional, but tracking systems can increase viable output by up to 45%. PV arrays that approach or exceed one megawatt often use solar trackers. Accounting for clouds, and the fact that most of the world is not on the equator, and that the sun sets in the evening, the correct measure of solar power is insolation – the average number of kilowatt-hours per square meter per day. For the weather and latitudes of the United States and Europe, typical insolation ranges from 2.26 kWh/m²/day in northern climes to 5.61 kWh/m²/day in the sunniest regions. For large systems, the energy gained by using tracking systems can outweigh the added complexity (trackers can increase efficiency by 30% or more). For very large systems, the added maintenance of tracking is a substantial detriment. Tracking is not required for flat panel and low concentration concentrated photovoltaic systems. For high concentration concentrated photovoltaic systems, dual axis tracking is a necessity.[93
(8.3)-Shading and dirt
Photovoltaic cell electrical output is extremely sensitive to shading. When even a small portion of a cell, module, or array is shaded, while the remainder is in sunlight, the output falls dramatically due to internal 'short-circuiting' (the electrons reversing course through the shaded portion of the p-n junction).
If the current drawn from the series string of cells is no greater than the current that can be produced by the shaded cell, the current (and so power) developed by the string is limited. If enough voltage is available from the rest of the cells in a string, current will be forced through the cell by breaking down the junction in the shaded portion. This breakdown voltage in common cells is between 10 and 30 volts. Instead of adding to the power produced by the panel, the shaded cell absorbs power, turning it into heat. Since the reverse voltage of a shaded cell is much greater than the forward voltage of an illuminated cell, one shaded cell can absorb the power of many other cells in the string, disproportionately affecting panel output. For example, a shaded cell may drop 8 volts, instead of adding 0.5 volts, at a particular current level, thereby absorbing the power produced by 16 other cells. It is important that a PV installation is not shaded by trees or other obstructions. Most modules have bypass diodes between each cell or string of cells that minimize the effects of shading and only lose the power of the shaded portion of the array. The main job of the bypass diode is to eliminate hot spots that form on cells that can cause further damage to the array, and cause fires. Sunlight can be absorbed by dust, snow, or other impurities at the surface of the module. This can cut down the light that strikes the cells. Maintaining a clean module surface will increase output performance over the life of the module. Google found that cleaning the flat mounted solar panels after 15 months increased their output by almost 100%, but that the 5% tilted arrays were adequately cleaned by rainwater.
Module output and life are also degraded by increased temperature. Allowing ambient air to flow over, and if possible behind, PV modules reduces this problem.
In 2012, solar panels available for consumers can have a yield of up about 17%, while commercially available panels can go as far as 27%.
Photovoltaic systems need to be monitored to detect breakdown and optimize their operation. Several photovoltaic monitoring strategies depending on the output of the installation and its nature. Monitoring can be performed on site or remotely. It can measure production only, retrieve all the data from the inverter or retrieve all of the data from the communicating equipment (probes, meters, etc.). Monitoring tools can be dedicated to supervision only or offer additional functions. Individual inverters may include monitoring using manufacturer specific protocols and software. Energy metering of an inverter may be of limited accuracy and not suitable for revenue metering purposes. A third-party data acquisition system can monitor multiple inverters, using the inverter manufacturer's protocols, and also acquire weather-related information. Independent smart meters may measure the total energy production of a PV array system. Separate measures such as satellite image analaysis or a solar radiation meter (a pyranometer) can be used to estimate total insolation for comparison. Data collected from a monitoring system can be displayed remotely over the World Wide Web. For example, the Open Solar Outdoors Test Field (OSOTF) is a grid-connected photovoltaic test system, which continuously monitors the output of a number of photovoltaic modules and correlates their performance to a long list of highly accurate meteorological readings. The OSOTF is organized under open source principles—
All data and analysis is be made freely available to the entire photovoltaic community and the general public. The Fraunhofer Center for Sustainable Energy Systems maintains two test systems, one in Massachusetts, and the Outdoor Solar Test Field OTF-1 in Albuquerque, New Mexico, which opened in June 2012. A third site, OTF-2, also in Albuquerque, is under construction. Some companies offer analysis software to analyze system performance. Small residential systems may have minimal data analysis requirements other than perhaps total energy production; larger grid-connected power plants can benefit from more detailed investigations of performance.
Uncertainties in revenue over time relate mostly to the evaluation of the solar resource and to the performance of the system itself. In the best of cases, uncertainties are typically 4% for year-to-year climate variability, 5% for solar resource estimation (in a horizontal plane), 3% for estimation of irradiation in the plane of the array, 3% for power rating of modules, 2% for losses due to dirt and soiling, 1.5% for losses due to snow, and 5% for other sources of error. Identifying and reacting to manageable losses is critical for revenue and O&M efficiency. Monitoring of array performance may be part of contractual agreements between the array owner, the builder, and the utility purchasing the energy produced. Access to the Internet has allowed a further improvement in energy monitoring and communication. Dedicated systems are available from a number of vendors. For solar PV system that use microInverters (panel-level DC to AC conversion), module power data is automatically provided. Some systems allow setting performance alerts that trigger phone/email/text warnings when limits are reached. These solutions provide data for the system owner and the installer. Installers are able to remotely monitor multiple installations, and see at-aglance the status of their entire installed base.
Effective module lives are typically 25 years or more. The payback period for an investment in a PV solar installation varies greatly and is typically less useful than a calculation of return on investment. While it is typically calculated to be between 10 and 20 years, the payback period can be far shorter with incentives.
The table below shows the total cost in US cents per kWh of electricity generated by a photovoltaic system. The row headings on the left show the total cost, per peak kilowatt (kWp), of a photovoltaic installation. The column headings across the top refer to the annual energy output in kWh expected from each installed kWp. This varies by geographic region because the average insolation depends on the average
cloudiness and the thickness of atmosphere traversed by the sunlight. It also depends on the path of the sun relative to the panel and the horizon.
Panels are usually mounted at an angle based on latitude, and often they are adjusted seasonally to meet the changing solar declination. Solar tracking can also be utilized to access even more perpendicular sunlight, thereby raising the total energy output. The calculated values in the table reflect the total cost in cents per kWh produced. They assume a 10% total capital cost (for instance 4% interest rate, 1% operating and maintenance cost, and depreciation of the capital outlay over 20 years). Normally, photovoltaic modules have a 25 year warranty.
2400 2200 2000 1800 1600 1400 1200 1000 800 20 years kWh/kWp kWh/kWp kWh/kWp kWh/kWp kWh/kWp kWh/kWp kWh/kWp kWh/kWp kWh/kWp
y y y y y y y y y
200 $/kWp 600 $/kWp 1000
0.8 2.5 4.2 5.8 7.5 9.2 10.8 12.5
0.9 2.7 4.5 6.4 8.2 10.0 11.8 13.6
1.0 3.0 5.0 7.0 9.0 11.0 13.0 15.0
1.1 3.3 5.6 7.8 10.0 12.2 14.4 16.7
1.3 3.8 6.3 8.8 11.3 13.8 16.3 18.8
1.4 4.3 7.1 10.0 12.9 15.7 18.6 21.4
1.7 5.0 8.3 11.7 15.0 18.3 21.7 25.0
2.0 6.0 10.0 14.0 18.0 22.0 26.0 30.0
2.5 7.5 12.5 17.5 22.5 27.5 32.5 37.5
Cost per kilowatt hour (US cents/kWh)
Secondly Solar thermal energy:
Solar thermal system for water heating in Santorini, Greece.
Solar thermal energy (STE) is an innovative technology for harnessing solar energy for thermal energy
(heat). Solar thermal collectors are classified by the United States Energy Information Administration as low-, medium-, or high-temperature collectors. Low-temperature collectors are flat plates generally used to heat swimming pools. Medium-temperature collectors are also usually flat plates but are used for heating water or air for residential and commercial use. High-temperature collectors concentrate sunlight using mirrors or lenses and are generally used for electric power production. STE is different from and much more efficient than  photovoltaics, which converts solar energy directly into electricity. While existing generation facilities provide only 600 megawatts of solar thermal power worldwide in October 2009, [note 1] plants for an additional 400 megawatts are under construction and development is underway for concentrated solar power projects totalling 14,000 megawatts.
Of the 21,000,000 square feet (2,000,000 m2) of solar thermal collectors produced in the United States in 2007, 16,000,000 square feet (1,500,000 m2) were of the low-temperature variety. Low-temperature collectors are generally installed to heat swimming pools, although they can also be used for space heating. Collectors can use air or water as the medium to transfer the heat to their destination.
(10.1)Heating, cooling, and ventilation
MIT's Solar House #1 built in 1939 used seasonal thermal storage for year round heating.
In the United States, heating, ventilation, and air conditioning (HVAC) systems account for over 25 percent (4.75 EJ) of the energy used in commercial buildings and nearly half (10.1 EJ) of the energy used in
residential buildings. Solar heating, cooling, and ventilation technologies can be used to offset a portion of this energy. Thermal mass materials store solar energy during the day and release this energy during cooler periods. Common thermal mass materials include stone, concrete, and water. The proportion and placement of thermal mass should consider several factors such as climate, daylighting, and shading conditions. When properly incorporated, thermal mass can passively maintain comfortable temperatures while reducing energy consumption. A solar chimney (or thermal chimney) is a passive solar ventilation system composed of a hollow thermal mass connecting the interior and exterior of a building. As the chimney warms, the air inside is heated causing an updraft that pulls air through the building. These systems have been in use since Roman times and remain common in the Middle East. Solar space heating with solar air heat collectors is more popular in the USA and Canada than heating with solar liquid collectors since most buildings already have a ventilation system for heating and cooling. The two main types of solar air panels are glazed and unglazed. Glazed Solar Collectors are designed primarily for space heating and they recirculate building air through a solar air panel where the air is heated and then directed back into the building. These solar space heating systems require at least two penetrations into the building and only perform when the air in the solar collector is warmer than the building room temperature. Most glazed collectors are used in the residential sector.
Unglazed, "transpired" air collector
Unglazed Solar Collectors are primarily used to pre-heat make-up ventilation air in commercial, industrial and institutional buildings with a high ventilation load. They turn building walls or sections of walls into low cost, high performance, unglazed solar collectors. Also called, "transpired solar panels", they employ a painted perforated metal solar heat absorber that also serves as the exterior wall surface of the building. Heat conducts from the absorber surface to the thermal boundary layer of air 1 mm thick on the outside of the absorber and to air that passes behind the absorber. The boundary layer of air is drawn into a nearby perforation before the heat can escape by convection to the outside air. The heated air is then drawn from behind the absorber plate into the building's ventilation system. A Trombe wall is a passive solar heating and ventilation system consisting of an air channel sandwiched between a window and a sun-facing thermal mass. During the ventilation cycle, sunlight stores heat in the
thermal mass and warms the air channel causing circulation through vents at the top and bottom of the wall. During the heating cycle the Trombe wall radiates stored heat. Solar roof ponds are unique solar heating and cooling systems developed by Harold Hay in the 1960s. A basic system consists of a roof-mounted water bladder with a movable insulating cover. This system can control heat exchange between interior and exterior environments by covering and uncovering the bladder between night and day. When heating is a concern the bladder is uncovered during the day allowing sunlight to warm the water bladder and store heat for evening use. When cooling is a concern the covered bladder draws heat from the building's interior during the day and is uncovered at night to radiate heat to the cooler atmosphere. The Skytherm house in Atascadero, California uses a prototype roof pond for heating and cooling. Active solar cooling can be achieved via absorption refrigeration cycles, desiccant cycles, and solar mechanical processes. In 1878, Auguste Mouchout pioneered solar cooling by making ice using a solar steam engine attached to a refrigeration device.
Solar Evaporation Ponds in the Atacama Desert.
Solar process heating systems are designed to provide large quantities of hot water or space heating for nonresidential buildings. Evaporation ponds are shallow ponds that concentrate dissolved solids through evaporation. The use of evaporation ponds to obtain salt from sea water is one of the oldest applications of solar energy. Modern uses include concentrating brine solutions used in leach mining and removing dissolved solids from waste streams. Altogether, evaporation ponds represent one of the largest commercial applications of solar energy in use today. Unglazed transpired collectors (UTC) are perforated sun-facing walls used for preheating ventilation air. UTCs can raise the incoming air temperature up to 22 °C and deliver outlet temperatures of 45-60 °C. The short payback period of transpired collectors (3 to 12 years) make them a more cost-effective alternative to glazed collection systems. As of 2009, over 1500 systems with a combined collector area of 300,000 m² had been installed worldwide. Representatives include an 860 m² collector in Costa Rica used for drying coffee beans and a 1300 m² collector in Coimbatore, India used for drying marigolds.
A food processing facility in Modesto, California uses parabolic troughs to produce steam used in the manufacturing process. The 5,000 m² collector area is expected to provide 15 TJ per year.
These collectors could be used to produce approximately 50% and more of the hot water needed for residential and commercial use in the United States. In the United States, a typical system costs $4000–$6000 retail ($1400 to $2200 wholesale for the materials) and 30% of the system qualifies for a federal tax credit + additional state credit exists in about half of the states. Labor for a simple open loop system in southern climates can take 3–5 hours for the installation and 4–6 hours in Northern areas. Northern system require more collector area and more complex plumbing to protect the collector from freezing. With this incentive, the payback time for a typical household is four to nine years, depending on the state. Similar subsidies exist in parts of Europe. A crew of one solar plumber and two assistants with minimal training can install a system per day. Thermosiphon installation have negligible maintenance costs (costs rise if antifreeze and mains power are used for circulation) and in the US reduces a households' operating costs by $6 per person per month. Solar water heating can reduce CO2 emissions of a family of four by 1 ton/year (if replacing natural gas) or 3 ton/year (if replacing electricity). Medium-temperature installations can use any of several designs: common designs are pressurized glycol, drain back, batch systems and newer low pressure freeze tolerant systems using polymer pipes containing water with photovoltaic pumping. European and International standards are being reviewed to accommodate innovations in design and operation of medium temperature collectors. Operational innovations include "permanently wetted collector" operation. This innovation reduces or even eliminates the occurrence of no-flow high temperature stresses called stagnation which would otherwise reduce the life expectancy of collectors.
Solar thermal energy can be useful for drying wood for construction and wood fuels such as wood chips for combustion. Solar is also used for food products such as fruits, grains, and fish. Crop drying by solar means is environmentally friendly as well as cost effective while improving the quality. The less money it takes to make a product, the less it can be sold for, pleasing both the buyers and the sellers. Technologies in solar drying include ultra low cost pumped transpired plate air collectors based on black fabrics. Solar thermal energy is helpful in the process of drying products such as wood chips and other forms of biomass by raising the heat while allowing air to pass through and get rid of the moisture.
The Solar Bowl above the Solar Kitchen in Auroville, India concentrates sunlight on a movable receiver to produce steam for cooking.
Solar cookers use sunlight for cooking, drying and pasteurization. Solar cooking offsets fuel costs, reduces demand for fuel or firewood, and improves air quality by reducing or removing a source of smoke. The simplest type of solar cooker is the box cooker first built by Horace de Saussure in 1767. A basic box cooker consists of an insulated container with a transparent lid. These cookers can be used effectively with partially overcast skies and will typically reach temperatures of 50–100 °C. Concentrating solar cookers use reflectors to concentrate solar energy onto a cooking container. The most common reflector geometries are flat plate, disc and parabolic trough type. These designs cook faster and at higher temperatures (up to 350 °C) but require direct light to function properly. The Solar Kitchen in Auroville, India uses a unique concentrating technology known as the solar bowl. Contrary to conventional tracking reflector/fixed receiver systems, the solar bowl uses a fixed spherical reflector with a receiver which tracks the focus of light as the Sun moves across the sky. The solar bowl's receiver reaches temperature of 150 °C that is used to produce steam that helps cook 2,000 daily meals. Many other solar kitchens in India use another unique concentrating technology known as the Scheffler reflector. This technology was first developed by Wolfgang Scheffler in 1986. A Scheffler reflector is a parabolic dish that uses single axis tracking to follow the Sun's daily course. These reflectors have a flexible reflective surface that is able to change its curvature to adjust to seasonal variations in the incident angle of sunlight. Scheffler reflectors have the advantage of having a fixed focal point which improves the ease of cooking and are able to reach temperatures of 450-650 °C. Built in 1999, the world's largest Scheffler reflector system in Abu Road, Rajasthan India is capable of cooking up to 35,000 meals a day. By early 2008, over 2000 large cookers of the Scheffler design had been built worldwide.
Solar stills can be used to make drinking water in areas where clean water is not common. Solar distillation is necessary in these situations to provide people with purified water. Solar energy heats up the water in the still. The water then evaporates and condenses on the bottom of the covering glass.
12- High-temperature collectors
The solar furnace at Odeillo in the French Pyrenees-Orientales can reach temperatures up to 3,800 degrees Celsius.
Concentrated solar power plant using parabolic trough design.
Where temperatures below about 95 °C are sufficient, as for space heating, flat-plate collectors of the nonconcentrating type are generally used. Because of the relatively high heat losses through the glazing, flat plate collectors will not reach temperatures much above 200 °C even when the heat transfer fluid is stagnant. Such temperatures are too low for efficient conversion to electricity. The efficiency of heat engines increases with the temperature of the heat source. To achieve this in solar thermal energy plants, solar radiation is concentrated by mirrors or lenses to obtain higher temperatures – a technique called Concentrated Solar Power (CSP). The practical effect of high efficiencies is to reduce the plant's collector size and total land use per unit power generated, reducing the environmental impacts of a power plant as well as its expense. As the temperature increases, different forms of conversion become practical. Up to 600 °C, steam turbines, standard technology, have an efficiency up to 41%. Above 600 °C, gas turbines can be more efficient. Higher temperatures are problematic because different materials and techniques are needed. One proposal for very high temperatures is to use liquid fluoride salts operating between 700 °C to 800 °C, using multi-stage turbine systems to achieve 50% or more thermal efficiencies. The higher operating temperatures permit the plant to use higher-temperature dry heat exchangers for its thermal exhaust, reducing the plant's water use – critical in the deserts where large solar plants are practical. High temperatures also make heat storage more efficient, because more watt-hours are stored per unit of fluid. Since the CSP plant generates heat first of all, it can store the heat before conversion to electricity. With current technology, storage of heat is much cheaper and more efficient than storage of electricity. In this way, the CSP plant can produce electricity day and night. If the CSP site has predictable solar radiation, then the
CSP plant becomes a reliable power plant. Reliability can further be improved by installing a back-up combustion system. The back-up system can use most of the CSP plant, which decreases the cost of the backup system. With reliability, unused desert, no pollution, and no fuel costs, the obstacles for large deployment for CSP are cost, aesthetics, land use and similar factors for the necessary connecting high tension lines. Although only a small percentage of the desert is necessary to meet global electricity demand, still a large area must be covered with mirrors or lenses to obtain a significant amount of energy. An important way to decrease cost is the use of a simple design.
(12.1) System designs:
During the day the sun has different positions. For low concentration systems (and low temperatures) tracking can be avoided (or limited to a few positions per year) if nonimaging optics are used. For higher concentrations, however, if the mirrors or lenses do not move, then the focus of the mirrors or lenses changes (but also in these cases nonimaging optics provides the widest acceptance angles for a given concentration). Therefore it seems unavoidable that there needs to be a tracking system that follows the position of the sun (for solar photovoltaic a solar tracker is only optional). The tracking system increases the cost and complexity. With this in mind, different designs can be distinguished in how they concentrate the light and track the position of the sun.
(12.1.1)Parabolic trough designs
Sketch of a parabolic trough design. A change of position of the sun parallel to the receiver does not require adjustment of the mirrors.
Parabolic trough power plants use a curved, mirrored trough which reflects the direct solar radiation onto a glass tube containing a fluid (also called a receiver, absorber or collector) running the length of the trough, positioned at the focal point of the reflectors. The trough is parabolic along one axis and linear in the orthogonal axis. For change of the daily position of the sun perpendicular to the receiver, the trough tilts east to west so that the direct radiation remains focused on the receiver. However, seasonal changes in the in angle of sunlight parallel to the trough does not require adjustment of the mirrors, since the light is simply concentrated elsewhere on the receiver. Thus the trough design does not require tracking on a second axis. The receiver may be enclosed in a glass vacuum chamber. The vacuum significantly reduces convective heat loss.
A fluid (also called heat transfer fluid) passes through the receiver and becomes very hot. Common fluids are synthetic oil, molten salt and pressurized steam. The fluid containing the heat is transported to a heat engine where about a third of the heat is converted to electricity. Andasol 1 in Guadix, Spain uses the Parabolic Trough design which consists of long parallel rows of modular solar collectors. Tracking the sun from East to West by rotation on one axis, the high precision reflector panels concentrate the solar radiation coming directly from the sun onto an absorber pipe located along the focal line of the collector. A heat transfer medium, a synthetic oil like in car engines, is circulated through the absorber pipes at temperatures up to 400 °C and generates live steam to drive the steam turbine generator of a conventional power block. Full-scale parabolic trough systems consist of many such troughs laid out in parallel over a large area of land. Since 1985 a solar thermal system using this principle has been in full operation in California in the United States. It is called the SEGS system. Other CSP designs lack this kind of long experience and therefore it can currently be said that the parabolic trough design is the most thoroughly proven CSP technology. The Solar Energy Generating System (SEGS) is a collection of nine plants with a total capacity of 350 MW. It is currently the largest operational solar system (both thermal and non-thermal). A newer plant is Nevada Solar One plant with a capacity of 64 MW. Under construction are Andasol 1 and Andasol 2 in Spain with each site having a capacity of 50 MW. Note however, that those plants have heat storage which requires a larger field of solar collectors relative to the size of the steam turbine-generator to store heat and send heat to the steam turbine at the same time. Heat storage enables better utilization of the steam turbine. With day and some nighttime operation of the steam-turbine Andasol 1 at 50 MW peak capacity produces more energy than Nevada Solar One at 64 MW peak capacity, due to the former plant's thermal energy storage system and larger solar field. 553 MW new capacity is proposed in Mojave Solar Park, California. Furthermore, 59 MW hybrid plant with heat storage is proposed near Barstow, California. Near Kuraymat in Egypt, some 40 MW steam is used as input for a gas powered plant. Finally, 25 MW steam input for a gas power plant in Hassi R'mel, Algeria. The Government of India and its federation have embarked on a major initiative; The Jawaharlal Nehru National Solar Mission (also known as the National Solar Mission) to address India's energy security challenges. The first phase (up to 2013) will focus on capturing of the low hanging options in solar thermal specifically in Jaisalmer in the Western state of Rajasthan. Shriram EPC, one of the major EPC companies in India and a part of the Shriram Group has embarked on a 50 MW solar thermal plant in Jaisalmer for Corporate Ispat and Alloys Ltd. which is to be commissioned by April 2013. Several other projects too are in the anvil.
(12.1.2)Power tower designs
Solar Two. Flat mirrors focus the light on the top of the tower. The white surfaces below the receiver are used for calibrating the mirror positions.
Power towers (also known as 'central tower' power plants or 'heliostat' power plants) capture and focus the sun's thermal energy with thousands of tracking mirrors (called heliostats) in roughly a two square mile field. A tower resides in the center of the heliostat field. The heliostats focus concentrated sunlight on a receiver which sits on top of the tower. Within the receiver the concentrated sunlight heats molten salt to over 1,000 °F (538 °C). The heated molten salt then flows into a thermal storage tank where it is stored, maintaining 98% thermal efficiency, and eventually pumped to a steam generator. The steam drives a standard turbine to generate electricity. This process, also known as the "Rankine cycle" is similar to a standard coal-fired power plant, except it is fueled by clean and free solar energy. The advantage of this design above the parabolic trough design is the higher temperature. Thermal energy at higher temperatures can be converted to electricity more efficiently and can be more cheaply stored for later use. Furthermore, there is less need to flatten the ground area. In principle a power tower can be built on the side of a hill. Mirrors can be flat and plumbing is concentrated in the tower. The disadvantage is that each mirror must have its own dual-axis control, while in the parabolic trough design single axis tracking can be shared for a large array of mirrors. In June 2008, eSolar, a Pasadena, CA-based company founded by Idealab CEO Bill Gross with funding from Google, announced a Power Purchase Agreement (PPA) with the utility Southern California Edison to produce 245 megawatts of power. Also, in February 2009, eSolar announced it had licensed its technology to two development partners, the Princeton, N.J.-based NRG Energy, Inc., and the India-based ACME Group. In the deal with NRG, the companies announced plans to jointly build 500 megawatts of concentrating solar thermal plants throughout the United States. The target goal for the ACME Group was nearly double; ACME plans to start construction on its first eSolar power plant this year, and will build a total of 1 gigawatt over the next 10 years. eSolar's proprietary sun-tracking software coordinates the movement of 24,000 1 meter-square mirrors per 1 tower using optical sensors to adjust and calibrate the mirrors in real time. This allows for a high density of reflective material which enables the development of modular concentrating solar thermal (CSP) power plants in 46 megawatt (MW) units on approximately π square mile parcels of land, resulting in a land-to-power ratio of 4 acres (16,000 m2) per 1 megawatt.
BrightSource Energy entered into a series of power purchase agreements with Pacific Gas and Electric Company in March 2008 for up to 900 MW of electricity, the largest solar power commitment ever made by a utility. BrightSource is currently developing a number of solar power plants in Southern California, with construction of the first plant planned to start in 2009. In June 2008, BrightSource Energy dedicated its 4-6 MW  Solar Energy Development Center (SEDC) in Israel's Negev Desert. The site, located in the Rotem Industrial Park, features more than 1,600 heliostats that track the sun and reflect light onto a 60 meter-high tower. The concentrated energy is then used to heat a boiler atop the tower to 550 degrees Celsius, generating superheated steam. A working tower power plant is PS10 in Spain with a capacity of 11 MW. The 15 MW Solar Tres plant with heat storage is under construction in Spain. In South Africa, a 100 MW solar power plant is planned with 4000 to 5000 heliostat mirrors, each having an area of 140 m². A 10 MW power plant in Cloncurry, Australia (with purified graphite as heat storage located on the tower directly by the receiver). Morocco is building five solar thermal power plants around Ouasarzate. The sites will produce about 2000 MW by 2012. Over ten thousand hectors of land will be needed to sustain all of the sites. Out of commission are the 10 MW Solar One (later redeveloped and made into Solar Two) and the 2 MW Themis plants. A cost/performance comparison between power tower and parabolic trough concentrators was made by the NREL which estimated that by 2020 electricity could be produced from power towers for 5.47 ¢/kWh and for 6.21 ¢/kWh from parabolic troughs. The capacity factor for power towers was estimated to be 72.9% and 56.2% for parabolic troughs. There is some hope that the development of cheap, durable, mass producible heliostat power plant components could bring this cost down.
A parabolic solar dish concentrating the sun's rays on the heating element of a Stirling engine. The entire unit acts as a solar tracker.
A dish Stirling system uses a large, reflective, parabolic dish (similar in shape to satellite television dish). It focuses all the sunlight that strikes the dish up onto a single point above the dish, where a receiver captures the heat and transforms it into a useful form. Typically the dish is coupled with a Stirling engine in a Dish-Stirling
System, but also sometimes a steam engine is used. These create rotational kinetic energy that can be converted to electricity using an electric generator. The advantage of a dish system is that it can achieve much higher temperatures due to the higher concentration of light (as in tower designs). Higher temperatures lead to better conversion to electricity and the dish system is very efficient on this point. However, there are also some disadvantages. Heat to electricity conversion requires moving parts and that results in maintenance. In general, a centralized approach for this conversion is better than the decentralized concept in the dish design. Second, the (heavy) engine is part of the moving structure, which requires a rigid frame and strong tracking system. Furthermore, parabolic mirrors are used instead of flat mirrors and tracking must be dual-axis. In 2005 Southern California Edison announced an agreement to purchase solar powered Stirling engines from Stirling Energy Systems over a twenty year period and in quantities (20,000 units) sufficient to generate 500 megawatts of electricity. In January 2010, Stirling Energy Systems and Tessera Solar commissioned the first demonstration 1.5-megawatt power plant ("Maricopa Solar") using Stirling technology in Peoria, Arizona. At the beginning of 2011 Stirling Energy's development arm, Tessera Solar, sold off its two large projects, the 709 MW Imperial project and the 850 MW Calico project to AES Solar and K.Road, respectively, and in the fall of 2011 Stirling Energy Systems applied for Chapter 7 bankruptcy due to competition from low cost photovoltaics.
A linear Fresnel reflector power plant uses a series of long, narrow, shallow-curvature (or even flat) mirrors to focus light onto one or more linear receivers positioned above the mirrors. On top of the receiver a small parabolic mirror can be attached for further focusing the light. These systems aim to offer lower overall costs by sharing a receiver between several mirrors (as compared with trough and dish concepts), while still using the simple line-focus geometry with one axis for tracking. This is similar to the trough design (and different from central towers and dishes with dual-axis). The receiver is stationary and so fluid couplings are not required (as in troughs and dishes). The mirrors also do not need to support the receiver, so they are structurally simpler. When suitable aiming strategies are used (mirrors aimed at different receivers at different times of day), this can allow a denser packing of mirrors on available land area. Recent prototypes of these types of systems have been built in Australia (CLFR) and by Solarmundo in Belgium. The Solarmundo research and development project, with its pilot plant at Liège, was closed down after successful proof of concept of the Linear Fresnel technology. Subsequently, Solar Power Group GmbH (SPG),
based in Munich, Germany, was founded by some Solarmundo team members. A Fresnel-based prototype with direct steam generation was built by SPG in conjunction with the German Aerospace Center (DLR). Based on the Australian prototype, a 177 MW plant had been proposed near San Luis Obispo in California and would be built by Ausra. But Ausra sold its planned California solar farm to First Solar. First Solar (a manufacturer of thin-film photovoltaic solar cells) will not build the Carrizo project, and the deal has resulted in the cancellation of Ausra’s contract to provide 177 megawatts to P.G.& E. Small capacity plants are an enormous economical challenge with conventional prabolic trough and drive design – few companies build such small projects. There are plans for SHP Europe, former Ausra subsidiary, to build a 6.5 MW combined cycle plant in Portugal. The German company SK Energy GmbH has plans to build several small 1-3 MW plants in Southern Europe (esp. in Spain) using Fresnel mirror and steam drive technology (Press Release). In May 2008, the German Solar Power Group GmbH and the Spanish Laer S.L. agreed the joint execution of a solar thermal power plant in central Spain. This will be the first commercial solar thermal power plant in Spain based on the Fresnel collector technology of the Solar Power Group. The planned size of the power plant will be 10 MW a solar thermal collector field with a fossil co-firing unit as backup system. The start of constructions is planned for 2009. The project is located in Gotarrendura, a small renewable energy pioneering village, about 100 km northwest of Madrid, Spain. A Multi-Tower Solar Array (MTSA) concept, that uses a point-focus Fresnel reflector idea, has also been developed, but has not yet been prototyped. Since March 2009, the Fresnel solar power plant Puerto Errado 1 (PE 1) of the German company Novatec Solar is in commercial operation in southern Spain . The solar thermal power plant is based on linear Fresnel collector technology and has an electrical capacity of 1.4 MW. Beside a conventional power block, PE 1 comprises a solar boiler with mirror surface of around 18,000m². The steam is generated by concentrating direct solar irradiation onto a linear receiver which is 7.40m above the ground. An absorber tube is positioned in the focal line of the mirror field in which water is evaporated directly into saturated steam at 270 °C and at a pressure of 55 bar by the concentrated solar energy. Since September 2011, due to a new receiver design developed by Novatec Solar, superheated steam with temperatures above 500°C can be generated. The 30 MW solar thermal power plant Puerto Errado 2 (PE2) is a scale up of PE 1 and also based on the Fresnel collector technology developed by the German company Novatec Solar. It comprises a mirror surface of 302,000m² and is in operation since August 2012. The plant is located in the region of Murcia. Five Swiss utilities (EBL, IWB, Ekz, ewz, ewb) are the owners of the world's largest Fresnel CSP power station.
30 MW Fresnel CSP Power Station Puerto Errado 2
(12.1.5)Linear Fresnel reflector technologies
Fresnel solar power plant PE 1 in southern Spain
Rival single axis tracking technologies include the relatively new linear Fresnel reflector (LFR) and compactLFR (CLFR) technologies. The LFR differs from that of the parabolic trough in that the absorber is fixed in space above the mirror field. Also, the reflector is composed of many low row segments, which focus collectively on an elevated long tower receiver running parallel to the reflector rotational axis. This system offers a lower cost solution as the absorber row is shared among several rows of mirrors. However, one fundamental difficulty with the LFR technology is the avoidance of shading of incoming solar radiation and blocking of reflected solar radiation by adjacent reflectors. Blocking and shading can be reduced by using absorber towers elevated higher or by increasing the absorber size, which allows increased spacing between reflectors remote from the absorber. Both these solutions increase costs, as larger ground usage is required. The CLFR offers an alternate solution to the LFR problem. The classic LFR has only one linear absorber on a single linear tower. This prohibits any option of the direction of orientation of a given reflector. Since this technology would be introduced in a large field, one can assume that there will be many linear absorbers in the system. Therefore, if the linear absorbers are close enough, individual reflectors will have the option of directing reflected solar radiation to at least two absorbers. This additional factor gives potential for more densely packed arrays, since patterns of alternative reflector inclination can be set up such that closely packed reflectors can be positioned without shading and blocking. CLFR power plants offer reduced costs in all elements of the solar array. These reduced costs encourage the advancement of this technology. Features that enhance the cost effectiveness of this system compared to that of the parabolic trough technology include minimized structural costs, minimized parasitic pumping losses, and low maintenance. Minimized structural costs are attributed to the use of flat or elastically curved glass reflectors instead of costly sagged glass reflectors are mounted close to the ground. Also, the heat transfer loop is separated from the reflector field, avoiding the cost of flexible high pressure lines required in trough systems. Minimized parasitic pumping losses are due to the use of water for the heat transfer fluid with passive direct boiling. The use of glass-evacuated tubes ensures low radiative losses and is inexpensive.
Studies of existing CLFR plants have been shown to deliver tracked beam to electricity efficiency of 19% on an annual basis as a preheater.
Prototypes of Fresnel lens concentrators have been produced for the collection of thermal energy by International Automated Systems. No full-scale thermal systems using Fresnel lenses are known to be in operation, although products incorporating Fresnel lenses in conjunction with photovoltaic cells are already available. The advantage of this design is that lenses are cheaper than mirrors. Furthermore, if a material is chosen that has some flexibility, then a less rigid frame is required to withstand wind load. A new concept of a lightweight, 'non-disruptive' solar concentrator technology using asymmetric Fresnel lenses that occupies minimal ground surface area and allows for large amounts of concentrated solar energy per concentrator is seen in the 'Desert Blooms'  project, though a prototype has yet to be made.
"MicroCSP" references solar thermal technologies in which concentrating solar power (CSP) collectors are based on the designs used in traditional Concentrating Solar Power systems found in the Mojave Desert but are smaller in collector size, lighter and operate at lower thermal temperatures usually below 315 °C (600 °F). These systems are designed for modular field or rooftop installation where they are easy to protect from high winds, snow and humid deployments. Solar manufacturer Sopogy completed construction on a 1 MW CSP plant at the Natural Energy Laboratory of Hawaii. MicroCSP is used for community-sized power plants (1 MW to 50 MW), for industrial, agricultural and manufacturing 'process heat' applications, and when large amounts of hot water are needed, such as resort swimming pools, water parks, large laundry facilities, sterilization, distillation and other such uses.
(12.1.8)Enclosed parabolic trough
The enclosed parabolic trough solar thermal system encapsulates the components within a greenhouse-like glasshouse. The glasshouse protects the components from the elements that can negatively impact system reliability and efficiency. Lightweight curved solar-reflecting mirrors are suspended from the ceiling of the glasshouse by wires. A single-axis tracking system positions the mirrors to retrieve the optimal amount of sunlight. The mirrors concentrate the sunlight and focus it on a network of stationary steel pipes, also suspended from the glasshouse structure. Water is pumped through the pipes and boiled to generate steam when intense sun radiation is applied. The steam is available for process heat. Sheltering the mirrors from the wind allows them to achieve higher temperature rates and prevents dust from building up on the mirrors as a result from exposure to humidity.
13-Heat collection and exchange:
More energy is contained in higher frequency light based upon the formula of , where h is the Planck constant and is frequency. Metal collectors down convert higher frequency light by producing a series of Compton shifts into an abundance of lower frequency light. Glass or ceramic coatings with high transmission in the visible and UV and effective absorption in the IR (heat blocking) trap metal absorbed low
frequency light from radiation loss. Convection insulation prevents mechanical losses transferred through gas. Once collected as heat, thermos containment efficiency improves significantly with increased size. Unlike Photovoltaic technologies that often degrade under concentrated light, Solar Thermal depends upon light concentration that requires a clear sky to reach suitable temperatures. Heat in a solar thermal system is guided by five basic principles: heat gain; heat transfer; heat storage; heat transport; and heat insulation. Here, heat is the measure of the amount of thermal energy an object contains and is determined by the temperature, mass and specific heat of the object. Solar thermal power plants use heat exchangers that are designed for constant working conditions, to provide heat exchange. Heat gain is the heat accumulated from the sun in the system. Solar thermal heat is trapped using the greenhouse effect; the greenhouse effect in this case is the ability of a reflective surface to transmit short wave radiation and reflect long wave radiation. Heat and infrared radiation (IR) are produced when short wave radiation light hits the absorber plate, which is then trapped inside the collector. Fluid, usually water, in the absorber tubes collect the trapped heat and transfer it to a heat storage vault. Heat is transferred either by conduction or convection. When water is heated, kinetic energy is transferred by conduction to water molecules throughout the medium. These molecules spread their thermal energy by conduction and occupy more space than the cold slow moving molecules above them. The distribution of energy from the rising hot water to the sinking cold water contributes to the convection process. Heat is transferred from the absorber plates of the collector in the fluid by conduction. The collector fluid is circulated through the carrier pipes to the heat transfer vault. Inside the vault, heat is transferred throughout the medium through convection. Heat storage enables solar thermal plants to produce electricity during hours without sunlight. Heat is transferred to a thermal storage medium in an insulated reservoir during hours with sunlight, and is withdrawn for power generation during hours lacking sunlight. Thermal storage mediums will be discussed in a heat storage section. Rate of heat transfer is related to the conductive and convection medium as well as the temperature differences. Bodies with large temperature differences transfer heat faster than bodies with lower temperature differences. Heat transport refers to the activity in which heat from a solar collector is transported to the heat storage vault. Heat insulation is vital in both heat transport tubing as well as the storage vault. It prevents heat loss, which in turn relates to energy loss, or decrease in the efficiency of the system.
Heat storage allows a solar thermal plant to produce electricity at night and on overcast days. This allows the use of solar power for baseload generation as well as peak power generation, with the potential of displacing both coal- and natural gas-fired power plants. Additionally, the utilization of the generator is higher which reduces cost. Heat is transferred to a thermal storage medium in an insulated reservoir during the day, and withdrawn for power generation at night. Thermal storage media include pressurized steam, concrete, a variety of phase change materials, and molten salts such as calcium, sodium and potassium nitrate.
The PS10 solar power tower stores heat in tanks as pressurized steam at 50 bar and 285 °C. The steam condenses and flashes back to steam, when pressure is lowered. Storage is for one hour. It is suggested that longer storage is possible, but that has not been proven yet in an existing power plant.
(14.2)Molten salt storage
A variety of fluids have been tested to transport the sun's heat, including water, air, oil, and sodium, but molten salt was selected[who?] as best. Molten salt is used in solar power tower systems because it is liquid at atmosphere pressure, it provides an efficient, low-cost medium in which to store thermal energy, its operating temperatures are compatible with today's high-pressure and high-temperature steam turbines, and it is non-flammable and nontoxic. In addition, molten salt is used in the chemical and metals industries as a heattransport fluid, so experience with molten-salt systems exists in non-solar settings. There are several molten salts mixtures used in this field. The first commercial mixture was made of 60 percent sodium nitrate and 40 percent potassium nitrate, commonly called saltpeter. New studies show that calcium nitrate could be included in the salts mixture to reduce costs and with technical and economical benefits. The binary potassium and sodium salt melts at 220 °C (430 °F) and is kept liquid at 290 °C (550 °F) in an insulated storage tank. The use of Calcium Nitrate can reduce the melting point at 131°C and today there are several technical calcium nitrate grades stable at more than 500°C. The uniqueness of this solar system is in de-coupling the collection of solar energy from producing power, electricity can be generated in periods of inclement weather or even at night using the stored thermal energy in the hot salt tank. Normally tanks are well insulated and can store thermal energy for up to a week. As an example of their size, tanks that provide enough thermal storage to power a 100-megawatt turbine for four hours would be about 9 m (30 ft) tall and 24 m (80 ft) in diameter. The Andasol power plant in Spain is the first commercial solar thermal power plant to utilize molten salt for heat storage and nighttime generation. It came online March 2009. On July 4, 2011, a company in Spain celebrated an historic moment for the solar industry: Torresol’s 19.9 MW concentrating solar power plant became the first ever to generate uninterrupted electricity for 24 hours straight. It achieved this using a molten salt heat storage design.
(14.3)Graphite heat storage
Direct The proposed power plant in Cloncurry Australia will store heat in purified graphite. The plant has a power tower design. The graphite is located on top of the tower. Heat from the heliostats goes directly to the storage. Heat for energy production is drawn from the graphite. This simplifies the design. Indirect Molten salt coolants are used to transfer heat from the reflectors to heat storage vaults. The heat from the salts are transferred to a secondary heat transfer fluid via a heat exchanger and then to the storage media, or alternatively, the salts can be used to directly heat graphite. Graphite is used as it has relatively low costs and compatibility with liquid fluoride salts. The high mass and volumetric heat capacity of graphite provide an efficient storage medium.
(14.4)Phase-change materials for storage
Phase Change Material (PCMs) offer an alternative solution in energy storage. Using a similar heat transfer infrastructure, PCMs have the potential of providing a more efficient means of storage. PCMs can be either organic or inorganic materials. Advantages of organic PCMs include no corrosives, low or no undercooling, and chemical and thermal stability. Disadvantages include low phase-change enthalpy, low thermal conductivity, and flammability. Inorganics are advantageous with greater phase-change enthalpy, but exhibit disadvantages with undercooling, corrosion, phase separation, and lack of thermal stability. The greater phasechange enthalpy in inorganic PCMs make hydrate salts a strong candidate in the solar energy storage field.
15-Use of water:
A design which requires water for condensation or cooling may conflict with location of solar thermal plants in desert areas with good solar radiation but limited water resources. The conflict is illustrated by plans of Solar Millennium, a German company, to build a plant in the Amargosa Valley of Nevada which would require 20% of the water available in the area. Some other projected plants by the same and other companies in the Mojave Desert of California may also be affected by difficulty in obtaining adequate and appropriate water rights. California water law currently prohibits use of potable water for cooling. Other designs require less water. The proposed Ivanpah Solar Power Facility in south-eastern California will conserve scarce desert water by using air-cooling to convert the steam back into water. Compared to conventional wet-cooling, this results in a 90 percent reduction in water usage at the cost of some loss of efficiency. The water is then returned to the boiler in a closed process which is environmentally friendly.
16-Conversion rates from solar energy to electrical energy:
Of all of these technologies the solar dish/Stirling engine has the highest energy efficiency. A single solar dish-Stirling engine installed at Sandia National Laboratories National Solar Thermal Test Facility (NSTTF) produces as much as 25 kW of electricity, with a conversion efficiency of 31.25%. Solar parabolic trough plants have been built with efficiencies of about 20%. Fresnel reflectors have an efficiency that is slightly lower (but this is compensated by the denser packing). The gross conversion efficiencies (taking into account that the solar dishes or troughs occupy only a fraction of the total area of the power plant) are determined by net generating capacity over the solar energy that falls on the total area of the solar plant. The 500-megawatt (MW) SCE/SES plant would extract about 2.75% of the radiation (1 kW/m²; see Solar power for a discussion) that falls on its 4,500 acres (18.2 km²). For the 50 MW AndaSol Power Plant that is being built in Spain (total area of 1,300×1,500 m = 1.95 km²) gross conversion efficiency comes out at 2.6% Furthermore, efficiency does not directly relate to cost: on calculating total cost, both efficiency and the cost of construction and maintenance should be taken into account.
Since a solar power plant does not use any fuel, the cost consists primarily of capital cost with minor operational and maintenance cost. If the lifetime of the plant and the interest rate is known, the cost per kWh can be calculated. This is called the levelised energy cost.
The first step in the calculation is to determining the investment for the production of 1 kWh in a year. Example, the fact sheet of the Andasol 1 project shows a total investment of 310 million euros for a production of 179 GWh a year. Since 179 GWh is 179 million kWh, the investment per kWh a year production is 310 / 179 = 1.73 euro. Another example is Cloncurry solar power station in Australia. It was planned to produce 30 million kWh a year for an investment of 31 million Australian dollars. So, if this is achieved in reality, the cost would have been 1.03 Australian dollar for the production of 1 kWh in a year. This would have been significantly cheaper than Andasol 1, which can be partially explained by the higher radiation in Cloncurry over Spain. The investment per kWh cost for one year should not be confused with the cost per kWh over the complete lifetime of such a plant. In most cases the capacity is specified for a power plant (for instance Andasol 1 has a capacity of 50 MW). This number is not suitable for comparison, because the capacity factor can differ. If a solar power plant has heat storage, it can also produce output after sunset, but that will not change the capacity factor; it simply displaces the output. The average capacity factor for a solar power plant, which is a function of tracking, shading and location, is about 20%, meaning that a 50 MW capacity power plant will typically provide a yearly output of 50 MW × 24 hrs × 365 days × 20% = 87,600 MWh/year, or 87.6 GWh/yr.
Although the investment for one kWh year production is suitable for comparing the price of different solar power plants, it does not give the price per kWh yet. The way of financing has a great influence on the final price. If the technology is proven, an interest rate of 7% should be possible. However, for a new technology investors want a much higher rate to compensate for the higher risk. This has a significant negative effect on the price per kWh. Independent of the way of financing, there is always a linear relation between the investment per kWh production in a year and the price for 1 kWh (before adding operational and maintenance cost). In other words, if by enhancements of the technology the investments drop by 20%, the price per kWh also drops by 20%.
If a way of financing is assumed whereby the money is borrowed and repaid every year, in such way that the debt and interest decreases, the following formula can be used to calculate the division factor: (1 - (1 + interest / 100) ^ -lifetime) / (interest / 100). For a lifetime of 25 years and an interest rate of 7%, the division factor is 11.65. For example, the investment of Andasol 1 was 1.73 euro per kWh, divided by 11.65 results in a price of 0.15 euro per kWh. If one cent operation and maintenance cost is added, then the levelized cost is 0.16 euro per kWh. Other ways of financing, different way of debt repayment, different lifetime expectation, different interest rate, can lead to a significantly different number.
If the cost per kWh may follow the inflation, the inflation rate can be added to the interest rate. If an investor puts his money in a savings account for 7%, then he is not compensated for inflation. However, if the cost per kWh is raised with inflation, then he is compensated and can add 2% (a normal inflation rate) to his return. The Andasol 1 plant has a guaranteed feed-in tariff of 0.21 euro for 25 years. If this number is fixed, after 25 years with 2% inflation, 0.21 euro will have a value comparable with 0.13 euro now.
Finally, there is some gap between the first investment and the first production of electricity. This increases the investment by the interest realised over the period in which the plant is not active. The modular solar dish (but also solar photovoltaic and wind power) have the advantage that electricity production starts after first construction.
Given the facts that solar thermal power is reliable, can deliver peak load and does not cause pollution, a price of US$0.10 per kWh starts to become competitive, although a price of US$0.06 has been claimed With some operational cost a simple target is 1 dollar (or lower) investment for 1 kWh production in a year
Thirdly hybrid photovoltaic’s solar thermal systems:
17- HYBRID SOLAR PANEL:
As we know all the systems that work with electricity have negative effects when the temperature is increased, for example electric engines, batteries and photovoltaic cells as well. In the previous charts the efficiency of the PV modules given by the manufactures are obtained with standard parameters (for example Temperature=25ºC and Radiation 1000 W/m2).In real life, the temperature of the PV cell is higher, reducing the electrical output by 0.5% for every 1°C above its rated output temperature. High temperature in the PV cells represents an increase in the current but also a significant reduction in the voltage, as a result the power of the panel decrease when the temperature increases as we can see in the Figure 1.19:
Fig. 1.19, Current vs. Voltage at different temperatures
Manufactures of photovoltaic panels recommend installing the panels in good ventilated places to reduce the negative effects of the temperature on the PV cells, but with this solution energy in form of heat that exits in the cells is wasted.
Is there any other alternative to solve this problem? The answer is yes. A solar hybrid photovoltaic thermal panel, PV/T is a combination of photovoltaic components and solar thermal components, which produce both electricity and heat from one integrated component or system. The PV/T cogeneration technology offers a solution that actually makes PV systems financially feasible in standard commercial and industrial applications.
Fig. 1.20, Photovoltaic thermal panel
A PVT-panel (Figure 1.20) has a higher electrical annual output than a conventional PV-module because the thermal flow permits to cool down the PV cells. The electrical yield of the PVT can either be used directly or be supplied to the grid while thermal yield, the user determines how heat is used. The thermal output depends strongly on the thermal system design and the amount of heat that is extracted by the user. On the other hand, the idea of combining photovoltaic and solar thermal energy in a simple panel is an innovative concept and interest to research, the use of these panels could have the next advantages: • Generates both electricity and heat energy.
Improve total operating efficiency due to an appropriate working temperature for the PV panel and for the production of thermal energy. The total area used to extract a given amount of electricity and heat may be smaller than for two separate systems.
The semiconductors that compose the cell will operate in lower temperatures where they are more efficient, for this reason the lifetime of the panels is larger.
When using integrated elements a potential saving in installation costs compared to separate systems can turn out to be an important factor for future development of the market for photovoltaic/thermal solar collectors.
Make a good use of the solar radiation per square meter.
PVT collectors provide architectural uniformity on a roof, in contrast to a combination of separate and PVThermal system. It is important to notice that improvement produced either in PV or Solar Thermal Panel is also an improvement for the PV/T panel, for instance if the price of the PV cells decreases, the same reduction will affect the PV/T panel. •
17.2 Review of models and systems:
PVT devices can be very different in design, ranging from PVT domestic hot water systems to ventilated PV facades and actively cooled PV concentrators. The main difference is the fluid used to heat (air or water) .At first, PVT with liquid as heat transfer fluid seems more promise because water has better heat transporting qualities than air. Moreover pipes and pumps are cheaper than funs and ducts. The collectors can be divided into the following categories :
PVT Liquid Collector.
PVT Air Collector.
Ventilated PV with heat recovery.
In choosing what type of PV/T system is most suitable the project demands need to be considered :
Temperature and characteristics of thermal load.
Thermal load (kW).
Electrical load (kW).
Suitable mounting locations.
17.2.1 Liquid PVT collectors :
In these devices PV modules and thermal units using water (or glycol) are mounted together, the systems convert solar radiation to electricity and hot water. According to temperature levels of the liquid, the collectors can be used in different applications: • Low temperature: swimming pool and heat pump applications.
Medium temperature: domestic hot water.
High temperature: hydrogen production.
Several design concepts have been evaluated in PVT liquid, some of them are shown in the Figure 1.21 :
Fig. 1.21, Various PVT liquid collector concepts
Only model denominated sheet and tube PVT has been produced commercially, others are under development phase. Sheet and tube PVT is similar to conventional flat plate liquid collectors; an absorber, usually cooper plate painted with a highly absorptive paint, with a serpentine tube or a series of parallel risers is applied (Figure 1.22), onto which PV that converts sunlight into electricity has been laminated or glued.
Fig. 1.22, Sheet and tube PVT liquid collector
It is important notice that direct application of water heat may require a good control of the flow, to be able to adapt to variations in irradiance. Lower flow rates can cause the temperature gradients in the PVT to be too high and can be a problem for the PV cell. As conventional solar collector, PVT also can be glazed or unglazed (Figure 1.23). Sometimes PVT collectors may have a glass cover over the absorber to reduce the thermal losses. If such a cover is present, the collector is referred to as glazed, otherwise as unglazed .
From the point of view of electrical efficiency the amount of reflecting layers above the PV panel should be minimized. On the other hand the thermal efficiency will drop if no insulating air layers are present . In the discussion whether the collector should be glazed or not, it is important to find a good balance between the increased thermal yield on one hand, and the reduction in electrical yield and the issues related to possible degradation on the other hand.
Fig. 1.23, Cross section of the PVT experimental mode
17.2.2 Air PVT Collectors:
In this case air is used as heat transfer fluid. The PV cells are either pasted to the interior of the cover plate or to an absorber or the PV cells are acting as an absorber or cover plate itself. The air can be circulated by either natural ventilation or forced ventilation. 
As it happens in Liquid PVT, this type of panel also can either be glazed or unglazed.
17.2.3 Ventilated PV with heat recovery :
In conventional PV facades or PV roofs, an air gap is often present at the rear in order to allow the air to cool the PV by means of natural convection (ventilated PV). If this heat can be recovered from the PV and be used in the building, the PV functions as a PVT collector. 
17.2.4 PVT Concentrator :
It is a concentrating parabolic trough system that combines photovoltaic (PV) cells to produce electricity, with thermal energy absorption to produce hot water at high temperature. In the Figure 1.25 is shown an example of one commercial photovoltaic thermal concentrator, in this link can see an illustrative animation about how works. http://www.arontis.se/video/Solar8_solar_concentrator.swf
Fig. 1.25, Photovoltaic Thermal Concentrator
17.3 Market of survey of PVT Collectors :
The markets for both solar thermal and PV are growing rapidly and have reached a very substantial size. For PV-Thermal a similar growth can be expected. However, the present PVT market is still very small. The cost of the PVT system can be assumed to be similar to the cost of the solar thermal system plus the cost of the PV laminate minus the cost of saved materials through integrated production/ installation and reduced installation costs. From the market point of view, standardization and certification of performance and reliability are essential requirements to achieve a successful market introduction in the building sector. Currently several institute and manufacturers have made an effort to standardize these systems.
17.3.1 Liquid PVT collectors market :
Most PVT liquid collectors are developed based on a commercial solar thermal collector that has been modified to include PV in the surface of the absorber. Examples of product developments and market attempts are: • Millennium Electric is a company from Israel which has as its star product a PVT denominated Multi Solar System (MSS). It consists of unglazed flat plate solar collector with PV cells integrated on top of the panel. Underneath the PV cells the water based collector is placed. Moreover there is under the collector a heat exchanger integrated which heat air. According to data of the company is possible achieve efficiencies around 85 % (15% electric and 70% thermal).More information can find in its web page: http://www.millenniumsolar.com/
Fig. 1.26, Multi Solar System
PVTWINS is a spin-off from Netherlands Energy Center (ECN). PVTWINS offers two types of PVT-products namely PVT-panels and PVT collectors.
Fig. 1.27, PVT Collector designed by PVTWINS Fig.1.28, PVT Panel designed by PVTWINS
PVT-collectors (Figure 1.27) consist of several PVT-laminates in a insulated aluminum box with a covering glass plate. The PVT-collector generates a higher temperature up to 90°C. Due to the higher temperature the annual electrical output is equal to a conventional PV-module. The PVT panel (Figure 1.28) is a PVT-laminate in an aluminum frame and insulation on the back side. A PVT-panel has a higher electrical annual output than a conventional PV-module and generates water up to 50°C .
ICEC AG. is a company from Switzerland, they have developed a PV/T collector HYSOLAR combining thermal and photovoltaic solar system. This hybrid solar system reduces the energy cost thanks to a simplified installation method and better roof space.
Fig. 1.29, Schematic of one installation using HYSOLAR
In Japan a company called Sekisui Chemical Co. developed around 1999 a PV/T for domestic hot water which converts about 10 % of the solar energy into electricity and 30% into hot water. Several installations of this were sited in Japan (Figure 1.30) but actually is not in the market. In Germany two companies SolarWerk and SolarWatt developed two similar systems which consist of a flat plate collector with PV cells integrated on the absorber, but both companies have problems maintaining long time stability of the PV cells as they are integrated on an absorber. This seems to be a common problem for companies interested in commercializing PV/T systems where the PV cells are integrated on the absorber. Others prototypes were developed around year 2000 but anyone was commercial.
Fig. 1.30, Installation made by Sekisui Chemical in Japan
17.3.2 Air PVT collectors market :
In this case not many prototypes have been developed although the level of commercialization is higher than liquid PVT collectors:
Aidt Miljø A/S is a company from Denmark which has developed a solar air collector with integrated PV cells and fan. This product preheats ventilation air, but the main purpose of the product is actually to provide dehumidification of the air in cabins, garages. The PV cells supply a fan in the top of the collector with electricity.
Fig. 1.31, “Solar Venti” collector designed by Aidt Miljø
http://www.solarventi.com/generelt/SunModel_UK/index.htm • Grammer Solar is commercially producing a PVT collector namely Twinsolar, which has been applied in a number of large demonstration projects, Solar farms, swimming pools (Figure 1.32), mountain refuges (Figure 1.33) and cabins.
Fig. 1.32, Twin solar installed in public Swimming pools
Fig. 1.33, Twin solar installed in a mountain refuge
Due to the safe and independent operation of the Twinsolar system, it is also ideal for second homes and houses not normally in use. The annual maintenance cost of the independent system at home is only the cost of changing the filter once or twice a year.
Twinsolar is a PVT collector with PV over the whole absorber. Air is circulating in the panel, entering and exiting in the same (lowest) side. Hot air is used as ventilation air in the building. (Figure 1.34)
Fig. 1.34, Picture Twinsolar system
Conserval Engineering Inc. invented and commercialized the transpired collector branded as SolarWall (Figure 1.35) which creates electricity and hot air. This system was tested at the Canadian National Solar Test Facility. The results documented that adding a solar thermal component to a PV array boosts the total solar efficiency to over 50%, compared with 10 to 15% efficiency for most PV modules alone .
Fig. 1.35, SolarWall
17.3.3 Market of ventilated PV with heat recovery:
The systems belonging to the group of Ventilated PV with Heat Recovery typically have emerged from solutions for specific buildings, where the primary focus has been building integration of PV and where the need for ventilation of the PV-systems in order to maximize the electrical yield has been combined with utilization of this heat for preheating of ventilation air, space heating or similar .
Secco Sistemi is an Italian enterprise which design system on roof or façade with integrated PV with heat recovery, this system is part of a roofing system and cannot be applied as add-on on existing roofs.
Fig. 1.36, Library from Mataro (Spain)
Thermal energy collected from a PV-solar air heating system is being used to provide cooling for the Mataro Library (Figure 1.36), near Barcelona. The system is designed to utilize surplus heat available from the ventilated PV facade and PV shed elements during the summer season to provide building cooling.
17.3.4 PVT concentrators market :
• Heliodynamics has developed a commercially available PVT concentrator, based upon tracking technology. The HD series of solar concentrators use mirror banks which concentrate solar radiation onto a receiver unit to produce heat or a combination of heatand-power. Each mirror bank moves separately to accurately track the sun during the day and can fold over to protect itself automatically at night and in case of inclement weather. The receiver unit is a stationery band of devices set at the focus of the mirrors which converts the solar radiation into useful electrical and/or thermal energy .
Fig. 1.37,Installation of PVT concentrator produced by Heliodynamics on the roof
Arontis AB, a Swedish clean technology company that manufactures a PVT concentrator called Solar8 which converts sunlight into both electricity and thermal
heat. Solar8 consists of a sun-following reflector in the shape of a parabolic trough, water-cooled solar cells, electronic steering and actuator. Life cycle analysis shows that Solar8 has an up to four times better environmental performance than normal solar cell modules . - Glass encased construction that protects reflectors and solar cells. - Reflector plate with high reflectance, low price and diffusive surface. - Steering system with double linear actuator. - Solar cells with 17% efficiency. - Three patent applications and design protection in the EU.
Fig. 1.38, Installation of a concentrator solar collector- Solar 8 in the World Heritage Museum in Skule(Sweden)
In Canada, the company Menova Energy Inc. has developed a commercial PVT concentrator mainly for domestic application. The Power Spar, as it is called, is a high efficiency solar concentrator that can be configured for electricity, heat, cooling and lighting solutions.
Fig. 1.39, Solar thermal concentrator plant with Power Spar
The Power-Spar system consists of a parabolic trough reflector which concentrates the sun's energy onto a modular absorber. The absorber converts the sun's energy to electricity (via high efficiency multi-sun photovoltaic cells), or to heat (via a patented absorption surface) or transports the light to the buildings' interior (via optical cabling). According to data obtained from the enterprise, the Power Spar is capable of capturing up to 80% of the sun’s energy.
17.3.5 Other interesting projects developed with PVT collectors:
• A combined photovoltaic / thermal (PV/T) collector was constructed by pasting singlecrystal silicon cells onto a black plastic solar heat absorber. The absorber plate of modified polyphenylenoxid (PPO) plastics contains internal, wall-to-wall channels filled with ceramic granulates.(Figure 1.40) The heat carrier fluid (water) is pumped up to an internal distribution channel at the top of the collector, and, by force of gravity, flows down through the parallel absorber channels. Water fills the vacant space between the ceramic particles and is brought in contact with the top absorber sheet, enabling good heat transport from absorbing surface to heat carrier fluid. The fluid flow in square wallto-wall channels covers the entire back of the absorber surface, resulting in a uniform temperature distribution across the width of the absorber. 
Fig. 1.40,Flat plate photovoltaic thermal • In the Center Thermal of Lyon (France) a new concept of photovoltaic/ thermal collector has been developed . The main difference is the alternate position of the thermal collector and the PV cells that permits obtain water at higher temperatures than most existing hybrids.
Fig. 1.41, Model PVT designed in Center Thermal of Lyon
Y. Tripanagnostopoulos et al.  in the University of Patras (Greece) is one of the people who more have researched about new design concepts in PVT collector. In the next figure (Figure 1.42) we can see an example that proposes the use of booster diffuse reflectors. These reflectors have been placed stationary from the higher part of the modules of one row to the lower part of the modules of next row. This installation increases solar input on PV modules almost all year resulting to an increase of electrical and thermal output of the PV/T systems. The suggested diffuse reflectors don’t contribute to electrical efficiency drop, as they provide an almost uniform distribution of reflected solar radiation on PV module surface .
Fig. 1.42, PV/T systems with booster diffuse reflectors: (a) horizontal building roof system installation; (b) PV/T experimental system with indication of diffuse reflected solar rays.
Solar decathlon is a competition in which 20 teams of students compete to design, build, and operate the most attractive, effective, and energy-efficient solar-powered
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Fig. 1.43, Solar powered house- project Crowder Collegue
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