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Zinc–carbon battery

Zinc-carbon batteries of different sizes. A zinc-carbon dry cell or battery is packaged in a zinc can that serves as both a container and negative terminal. It was developed from the wet Leclanché cell ( /lɛklɑːnˈʃeɪ/). The positive terminal is a carbon rod surrounded by a mixture of manganese dioxide and carbon powder . The electrolyte used is a paste of zinc chloride and ammonium chloride dissolved in water. Zinc chloride cells are an improved version from the original ammonium chloride variety. Zinccarbon batteries are the least expensive primary batteries and thus a popular choice by manufacturers when devices are sold with (low quality) batteries included. They are commonly labeled as "General Purpose" batteries. They were the first commercial dry battery and made things such as flashlights and other portable devices possible, because the battery can function in any position. They can be used in remote controls, flashlights, clocks, or transistor radios, since the power drain is not too heavy (suited for low drain use). They are progressively replaced, in many usages, by alkaline cells, and rechargeable NiMH batteries (which of course did not exist at Leclanché's time). Zinc-carbon batteries account for only 6% of all primary battery sales in Japan and only 7% of all types of batteries sold in Switzerland. In the UK 20% of all portable batteries sold are zinccarbon and in the EU 30%. [1] [2] [3] [4]


History By 1876, the wet Leclanché cell was made with a compressed block of manganese dioxide. In 1886 Dr. Carl Gassner patented a "dry" version by using a zinc cup as the anode and making the electrolyte with a paste of plaster of Paris (and later, wheat flour) to gel and immobilize the electrolyte. In 1898 Conrad Hubert used consumer batteries manufactured by W. H. Lawrence to power what was the first flashlight, and subsequently the two formed the Ever Ready battery company.[5] In 1900 Gassner demonstrated dry cells for portable lighting at the World's Fair in Paris. Continual improvements were made to the stability and capacity of zinc-carbon cells throughout the 20th Century; by the end of the century the capacity of a zinc-carbon cell had increased fourfold over the 1910 equivalent.[6] Improvements include the use of purer grades of manganese dioxide, better sealing, and purer zinc for the negative electrode.

Old zinc-carbon 3V battery (around 1960), with poor-quality cardboard casing

Chemical reactions In a zinc-carbon dry cell, the outer zinc container is the negative terminal. The zinc is oxidised according to the following half-equation. Zn(s) → Zn2+(aq) + 2 e- [e° = -1.04 volts] A graphite rod surrounded by a powder containing manganese(IV) oxide is the positive terminal. The manganese dioxide is mixed with carbon powder to increase the electrical conductivity. The reaction is as follows: 2MnO2(s) + 2 e- + 2NH4Cl(aq) → Mn2O3(s) + 2NH3(aq) + H2O(aq) + 2 Cl- [e° ≈ +.5 v] and the Cl- combines with the Zn2+. In this half-reaction, the manganese is reduced from an oxidation state of (+4) to (+3). There are other possible side-reactions, but the overall reaction in a zinc-carbon cell can be represented as: Zn(s) + 2MnO2(s) + 2NH4Cl(aq) → Mn2O3(s) + Zn(NH3)2Cl2 (aq) + H2O(l) The battery has an e.m.f. of about 1.5 V. The approximate nature of the e.m.f is related to the complexity of the cathode reaction. The anode (zinc) reaction is comparatively simple with a known potential. Side reactions and depletion of the active chemicals increases the internal resistance of the battery, and this causes the e.m.f. to drop. Although carbon is an important element of the battery's construction, it takes no part in the electrochemical reaction, instead only serving to collect current and reduce the resistance of the manganese dioxide mix. The cell could more properly be called a "zinc-manganese" cell.

A zinc-carbon dry cell is considered as a primary cell because the cell is not intended to be recharged.

Construction The container of the zinc-carbon dry cell is a zinc can. This contains a layer of NH4Cl and ZnCl2 aqueous paste impregnating a paper layer that separates the zinc can from a mixture of powdered carbon (usually graphite powder) & manganese (IV) oxide (MnO2) which is packed around a carbon rod. Carbon is the only practical conductor material because every common metal will quickly corrode away in the positive electrode in salt based electrolyte.

Cross-section of a zinc-carbon battery.

Disassembled zinc chloride cell (similar to zinc carbon cell). 1:entire cell, 2:steel casing, 3:zinc negative electrode, 4:carbon rod, 5:positive electrode (Manganese dioxide mixed with carbon powder and electrolyte), 6:paper separator, 7:polyethylene leak proof isolation, 8:sealing rings, 9-negative terminal, 10-positive terminal (originally connected to carbon rod) Early types, and low-cost cells, use a separator consisting of a layer of starch or flour. A layer of starch-coated paper is used in modern cells, which is thinner and allows more manganese dioxide to be used. Originally cells were sealed with a layer of asphalt to prevent drying out of the electrolyte; more recently a thermoplastic washer sealant is used. The carbon rod is slightly porous, which allows accumulated gas to escape while retaining the water for the electrolyte. The ratio of manganese dioxide and carbon powder in the cathode paste affects the characteristics of the cell; more carbon powder lowers the internal resistance, but more manganese dioxide improves capacity.[6] Flat cells are also made for assembly into batteries with higher voltages, up to about 450 volts. A number of flat cells are stacked up, and the whole assembly is coated in wax to prevent evaporation of water from the electrolyte. Leakage and environmental concerns

Leaked and corroded zinc-carbon AAA cells. Generally, the materials of the cell are inexpensive and cheap to reproduce even though the cell is non-rechargeable. These cells have a low energy density; the voltage falls during use due to a drop in electrolyte concentration around the cathode and the time required for the Mn2O3 to diffuse away from the cathode. The zinc-carbon is small and portable and provides a current large enough for most small devices. These cells have a short shelf life as the zinc is attacked by ammonium chloride. As well, the zinc casing oxidises during discharge and the acidic paste can leak out through cracks and corrode other components. The zinc container becomes thinner as the cell is used, because zinc metal is oxidized to zinc ions. When the zinc case thins enough, zinc chloride begins to leak out of the battery. Good quality seals can of course prevent leakage. The old dry cell is not leak proof and becomes very sticky as the paste leaks through the holes in the zinc case. The service life of the battery is short, with a shelf life of around 1.5 years. The zinc casing in the dry cell gets thinner even when the cell is not being used. It is because the ammonium chloride inside the battery is acidic, reacting with the zinc. An "inside-out" form with a carbon cup and zinc vanes on the interior, while more leak resistant, has not been made since the 1960s.[6] This picture shows the zinc container :
 

of fresh batteries (a), of discharged batteries (b) and (c). The battery shown at (c) has no polyethylene protection film, to keep the zinc oxide inside the casing.

Progressive corrosion of zinc carbon batteries. The environmental impact has been reduced since the addition of mercury was made illegal in most of countries (in the U.S., it was under 1996 federal battery legislation). Conveniently manufactured, and disposed, a single zinc-carbon dry cell has low environmental impact on disposal, compared with some other battery types. The manganese (III) is readily oxidized and so becomes immobilized; minute amounts of zinc, carbon and ammonium salts are also harmless.[citation needed] But tens of thousands tons of zinc carbon batteries are disposed every year around the world, and many of them (the percentages vary considerably from a country to another) are not correctly recycled. The zinc chloride cell The zinc chloride cell is an improvement on the original zinc-carbon cell, using purer chemicals and giving a longer life and steadier voltage output as it is used. These cells are often marketed as Heavy Duty cells, to differentiate them from "General Purpose" zinc-carbon cells. This has been a source of consumer confusion after the introduction of alkaline cells, which last longer than the zinc-chloride "Heavy Duty" cell. Instead of an electrolyte mixture containing much NH4Cl, it is largely only ZnCl2 paste. The cathode reaction is thus a little different: MnO2(s) + H2O(l) + e- → MnO(OH)(s) + OH-(aq) as is the overall reaction: Zn(s) + 2 MnO2(s) + ZnCl2(aq) + 2 H2O(l) → 2 MnO(OH)(s) + 2 Zn(OH)Cl(aq)

Electrodes The words anode and cathode can be very confusing. In electrolytic cells, the anode is referred as the positive terminal since all the anions (negative ions) will migrate to the anode to be selectively discharged while the cathode is the negative terminal because the cations (positive ions) will move to the cathode to be selectively discharged. Meanwhile, for voltaic cells, the anode and cathode are opposite to each other. This means that the anode is the negative terminal, while the cathode is the positive terminal. This is due to the convention which states that all anodes are terminals that undergo oxidation or release of electrons, and all cathodes are terminals which undergo reduction. Storage Manufacturers recommend storage of zinc-carbon batteries at room temperature; storage at higher temperatures reduces the expected service life.[7] While batteries may be frozen without damage, manufacturers recommend that they be returned to normal room temperature before use, and that condensation on the battery jacket must be avoided. By the end of the 20th century, storage life of zinc-carbon cells had improved fourfold over expected life in 1910

Alkaline batteries
are a type of primary battery or rechargeable battery dependent upon the reaction between zinc and manganese dioxide (Zn/MnO2). Compared with zinc-carbon batteries of the Leclanché or zinc chloride types, alkaline batteries have a higher energy density and longer shelf-life, with the same voltage. Button cell silveroxide batteries have higher energy density and capacity but also higher cost than similar-size alkaline cells.

The alkaline battery gets its name because it has an alkaline electrolyte of potassium hydroxide, instead of the acidic ammonium chloride or zinc chloride electrolyte of the zinc-carbon batteries. Other battery systems also use alkaline electrolytes, but they use different active materials for the electrodes. Alkaline batteries account for 80% of manufactured batteries in the US and over 10 billion individual units produced worldwide. In Japan alkaline batteries account for 46% of all primary battery sales. In Switzerland alkaline batteries account for 68%, in the UK 60% and in the EU 47% of all battery sales including secondary types. [1] [2] [3] [4] [5]


History The alkaline dry battery was invented by Canadian engineer Lewis Urry in the 1950s while working for the Eveready Battery company. On October 9, 1957, Lewis Urry, Karl Kordesch, and P.A. Marsal filed US patent (2,960,558) for the alkaline battery. It was granted in 1960 and was assigned to Union Carbide Corporation.[6] Chemistry In an alkaline battery, the anode (negative terminal) is made of zinc powder, which gives more surface area for increased current, and the cathode (positive terminal) is composed of manganese dioxide. Unlike zinc-carbon (Leclanché) batteries, the electrolyte is potassium hydroxide rather than ammonium chloride or zinc chloride.

Section through an alkaline battery. The half-reactions are:[7] Zn (s) + 2OH− (aq) → ZnO (s) + H2O (l) + 2e− 2MnO2 (s) + H2O (l) + 2e− →Mn2O3 (s) + 2OH− (aq) Capacity

Several sizes of button and coin cells. Some are alkaline and others are silver oxide. Two 9v batteries were added as a size comparison. Enlarge to see the size code markings. Capacity of an alkaline battery is greater than an equal size Leclanché or zinc-chloride cell because the manganese dioxide anode material is purer and denser, and space taken up by internal components such as electrodes is less. An alkaline cell can provide between three and five times capacity.[8] The capacity of an alkaline battery is strongly dependent on the load. An AA-sized alkaline battery might have an effective capacity of 3000 mAh at low drain, but at a load of 1 ampere, which is common for digital cameras, the capacity could be as little as 700 mAh.[9] The voltage of the battery declines steadily during use, so the total usable capacity depends on the cut-off voltage of the application. Unlike Leclanche cells the alkaline cell delivers about as much capacity on intermittent or continuous light loads. On a heavy load, capacity is reduced on continuous discharge compared with intermittent discharge, but the reduction is less than for Leclanche cells. Voltage The nominal voltage of a fresh alkaline cell is 1.5 V. Multiple voltages may be achieved with series of cells. The effective zero-load voltage of a non discharged alkaline battery varies from 1.50 to 1.65 V, depending on the chosen manganese dioxide and the contents of zinc oxide in the electrolyte. The average voltage under load depends on discharge and varies from 1.1 to 1.3 V. The fully discharged cell has a remaining voltage in the range of 0.8 to 1.0 V. Current The amount of current an alkaline battery can deliver is roughly proportional to its physical size. This is a result of decreasing internal resistance as the internal surface area of the cell increases. A general rule of thumb is that an AA alkaline battery can deliver 700 mA without any significant heating. Larger cells, such as C and D cells, can deliver more current. Applications requiring high currents of several amperes, such as high powered flashlights and portable stereos, will require D-sized cells to handle the increased load. Construction Alkaline batteries are manufactured in standardized cylindrical forms interchangeable with zinccarbon batteries, and in button forms. Several individual cells may be interconnected to form a true "battery", such as those sold for use with flashlights and the 9 volt transistor-radio battery.[10]

A cylindrical cell is contained in a drawn steel can, which is the cathode connection. The cathode mixture is a compressed paste of manganese dioxide with carbon powder added for increased conductivity. The paste may be pressed into the can or deposited as pre-molded rings. The hollow center of the cathode is lined with a separator, which prevents mixing of the anode and cathode materials and short-circuiting of the cell. The separator is made of a non-woven layer of cellulose or a synthetic polymer. The separator must conduct ions and remain stable in the highly alkaline electrolyte solution. The anode is composed of a dispersion of zinc powder in a gel containing the potassium hydroxide electrolyte. To prevent gassing of the cell at the end of its life, more manganese dioxide is used than required to react with all the zinc. When describing standard AAA, AA, C, sub-C and D size cells, the anode is connected to the flat end while the cathode is connected to the end with the raised button. Recharging of alkaline batteries Some alkaline batteries are designed to be recharged (see rechargeable alkaline battery), but most are not. Attempts to recharge may cause rupture, or the leaking of hazardous liquids which will corrode the equipment. Leaks

Leaked alkaline battery Over time, alkaline batteries are prone to leaking potassium hydroxide, a caustic agent that can cause respiratory, eye and skin irritation [11] This can be avoided by not attempting to recharge disposable alkaline cells, not mixing different battery types in the same device, replacing all of the batteries at the same time, storing in a dry place, and removing batteries for storage of devices. Once a leak has formed due to corrosive penetration of the outer steel shell, potassium hydroxide forms a feathery crystalline structure that grows and spreads out from the battery over time, following up metal electrodes to circuit boards where it commences oxidation of copper tracks and other components, leading to permanent circuitry damage.

The leaking crystalline growths can also emerge from seams around battery covers to form a furry coating outside the device, that then corrodes any objects in contact with the leaking device. Disposal When introduced in the 1960s, alkaline batteries contained a small amount of mercury amalgam to control side reactions at the zinc cathode. Improvements in the purity and consistency of materials have allowed manufacturers to reduce the mercury content in modern cells.[12] Unlike other types of batteries, alkaline batteries are allowed to be disposed of as regular domestic waste in some locations. This, however, may not be environmentally friendly, as some alkaline batteries produced before 1996 contain mercury.[13][14] For example the state of California has deemed all batteries as hazardous waste when discarded, and has banned the disposal of batteries with other domestic waste.[15] In the US, one company shreds and separates the battery case metals, manganese and zinc.[6] Another company mixes batteries in as a feedstock in steel making furnaces, to make low-grade steel such as rebar; the zinc fumes are recovered separately.[7] In Europe battery disposal is controlled by the WEEE regulations, and as such alkaline batteries must not be thrown in with domestic waste. They should be disposed through local recycling stations/waste dumps. In the EU most stores that sell batteries are required by law to accept old batteries for recycling.

Fuel cell
.

Demonstration model of a direct-methanol fuel cell. The actual fuel cell stack is the layered cube shape in the center of the image A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used. Fuel Cells are different from batteries in that they require a constant source of fuel and oxygen to run, but they can produce electricity continually for as long as these inputs are supplied. Welsh Physicist William Grove developed the first crude fuel cells in 1839. The first commercial use of fuel cells was in NASA space programs to generate power for probes, satellites and space capsules. Since then, fuel cells have been used in many other applications. Fuel cells are used for primary and backup power for commercial, industrial and residential buildings and in remote or inaccessible areas. They are used to power fuel cell vehicles, including automobiles, buses, forklifts, airplanes, boats, motorcycles and submarines. There are many types of fuel cells, but they all consist of an anode (negative side), a cathode (positive side) and an electrolyte that allows charges to move between the two sides of the fuel cell. Electrons are drawn from the anode to the cathode though an external circuit, producing direct current electricity. As the main difference among fuel cell types is the electrolyte, fuel cells are classified by the type of electrolyte they use. Fuel cells come in a variety of sizes. Individual fuel cells produce very small amounts of electricity, about 0.7 volts, so cells are "stacked", or placed in series or parallel circuits, to increase the voltage and current output to meet an application‘s power generation requirements.[1] In addition to electricity, fuel cells produce water, heat and, depending on the fuel source, sometimes carbon dioxide, very small amounts of nitrogen dioxide and other emissions. The energy efficiency of a fuel cell is generally between 40-60%, or up to 85% efficient if waste heat is captured for use.


Design Fuel cells come in many varieties; however, they all work in the same general manner. They are made up of three segments which are sandwiched together: the anode, the electrolyte, and the cathode. Two chemical reactions occur at the interfaces of the three different segments. The net result of the two reactions is that fuel is consumed, water or carbon dioxide is created, and an electric current is created, which can be used to power electrical devices, normally referred to as the load. At the anode a catalyst oxidizes the fuel, usually hydrogen, turning the fuel into a positively charged ion and a negatively charged electron. The electrolyte is a substance specifically designed so ions can pass through it, but the electrons cannot. The freed electrons travel through a wire creating the electric current. The ions travel through the electrolyte to the cathode. Once reaching the cathode, the ions are reunited with the electrons and the two react with a third chemical, usually oxygen, to create water or carbon dioxide.

A block diagram of a fuel cell The most important design features in a fuel cell are:
   

The electrolyte substance. The electrolyte substance usually defines the type of fuel cell. The fuel that is used. The most common fuel is hydrogen. The anode catalyst, which breaks down the fuel into electrons and ions. The anode catalyst is usually made up of very fine platinum powder. The cathode catalyst, which turns the ions into the waste chemicals like water or carbon dioxide. The cathode catalyst is often made up of nickel. A typical fuel cell produces a voltage from 0.6 V to 0.7 V at full rated load. Voltage decreases as current increases, due to several factors:

  

Activation loss Ohmic loss (voltage drop due to resistance of the cell components and interconnects) Mass transport loss (depletion of reactants at catalyst sites under high loads, causing rapid loss of voltage).[2] To deliver the desired amount of energy, the fuel cells can be combined in series and parallel circuits, where series yields higher voltage, and parallel allows a higher current to be supplied. Such a design is called a fuel cell stack. The cell surface area can be increased, to allow stronger current from each cell. Proton exchange membrane fuel cells In the archetypical hydrogen–oxygen proton exchange membrane fuel cell[3] (PEMFC) design, a proton-conducting polymer membrane, (the electrolyte), separates the anode and cathode sides.

This was called a "solid polymer electrolyte fuel cell" (SPEFC) in the early 1970s, before the proton exchange mechanism was well-understood. (Notice that "polymer electrolyte membrane" and "proton exchange mechanism" result in the same acronym.) On the anode side, hydrogen diffuses to the anode catalyst where it later dissociates into protons and electrons. These protons often react with oxidants causing them to become what is commonly referred to as multi-facilitated proton membranes. The protons are conducted through the membrane to the cathode, but the electrons are forced to travel in an external circuit (supplying power) because the membrane is electrically insulating. On the cathode catalyst, oxygen molecules react with the electrons (which have traveled through the external circuit) and protons to form water — in this example, the only waste product, either liquid or vapor. In addition to this pure hydrogen type, there are hydrocarbon fuels for fuel cells, including diesel, methanol (see: direct-methanol fuel cells and indirect methanol fuel cells) and chemical hydrides. The waste products with these types of fuel are carbon dioxide and water.

Construction of a high temperature PEMFC: Bipolar plate as electrode with in-milled gas channel structure, fabricated from conductive composites (enhanced with graphite, carbon black, carbon fiber, and/or carbon nanotubes for more conductivity);[4] Porous carbon papers; reactive layer, usually on the polymer membrane applied; polymer membrane.

Condensation of water produced by a PEMFC on the air channel wall. The gold wire around the cell ensures the collection of electric current.[5] The different components of a PEMFC are (i) bipolar plates, (ii) electrodes, (iii) catalyst, (iv) membrane, and (v) the necessary hardwares.[6] The materials used for different parts of the fuel cells differ by type. The bipolar plates may be made of different types of materials, such as, metal, coated metal, graphite, flexible graphite, C–C composite, carbon–polymer composites etc.[7] The membrane electrode assembly (MEA), is referred as the heart of the PEMFC and usually made of a proton exchange membrane sandwiched between two catalyst coated carbon papers. Platinum and/or similar type of noble metals are usually used as the catalyst for PEMFC. The electrolyte could be a polymer membrane. Proton exchange membrane fuel cell design issues




Costs. In 2002, typical fuel cell systems were project to cost US$100 per kilowatt of electric power output, assuming high-volume production of contemporary designs.[8] At lower production volumes that do not incorporate economies of scale or a well-developed supply chain, costs are roughly one order of magnitude higher.[9] In 2009, the Department of Energy reported that 80-kW automotive fuel cell system costs in volume production (projected to 500,000 units per year) are US$61 per kilowatt.[10] The goal is US$35 per kilowatt. Cost reduction over a rampup period of about 20 years is needed in order for PEM fuel cells to compete with current market technologies, including gasoline internal combustion engines.[9] Many companies are working on techniques to reduce cost in a variety of ways including reducing the amount of platinum needed in each individual cell. Ballard Power Systems has experiments with a catalyst enhanced with carbon silk which allows a 30% reduction (1 mg/cm² to 0.7 mg/cm²) in platinum usage without reduction in performance.[11] Monash University, Melbourne uses PEDOT as a cathode.[12] A 2011 published study[13] documented the first metal-free electrocatalyst using relatively inexpensive doped carbon nanotubes that are less than 1% the cost of platinum and are of equal or superior performance. Water and air management[14] (in PEMFCs). In this type of fuel cell, the membrane must be hydrated, requiring water to be evaporated at precisely the same rate that it is produced. If water is evaporated too quickly, the membrane dries, resistance across it increases, and eventually it will crack, creating a gas "short circuit" where hydrogen and oxygen combine directly, generating heat that will damage the fuel cell. If the water is evaporated too slowly, the electrodes will flood, preventing the reactants from reaching the catalyst and stopping the







reaction. Methods to manage water in cells are being developed like electroosmotic pumps focusing on flow control. Just as in a combustion engine, a steady ratio between the reactant and oxygen is necessary to keep the fuel cell operating efficiently. Temperature management. The same temperature must be maintained throughout the cell in order to prevent destruction of the cell through thermal loading. This is particularly challenging as the 2H2 + O2 -> 2H2O reaction is highly exothermic, so a large quantity of heat is generated within the fuel cell. Durability, service life, and special requirements for some type of cells. Stationary fuel cell applications typically require more than 40,000 hours of reliable operation at a temperature of 35 °C to 40 °C (-31 °F to 104 °F), while automotive fuel cells require a 5,000 hour lifespan (the equivalent of 240,000 km (150,000 mi)) under extreme temperatures. Current service life is 7,300 hours under cycling conditions.[15] Automotive engines must also be able to start reliably at -30 °C (-22 °F) and have a high power to volume ratio (typically 2.5 kW per liter). Limited carbon monoxide tolerance of some (non-PEDOT) cathodes. High temperature fuel cells SOFC Main article: Solid oxide fuel cell Solid oxide fuel cells use a solid material, most commonly a ceramic material called yttriastabilized zirconia (YSZ), as the electrolyte. Because SOFCs are made entirely of solid materials, they are not limited to the flat plane configuration of other types of fuel cells and are often designed as rolled tubes. They require high operating temperatures (800°C to 1000°C) and can be run on a variety of fuels including natural gas.[16] SOFCs are unique in that negatively charged oxygen ions travel from the cathode (negative side of the fuel cell) to the anode (positive side of the fuel cell) instead of positively charged hydrogen ions travelling from the anode to the cathode, as is the case in all other types of fuel cells. Oxygen gas is fed through the cathode, where it reacts with electrons to create oxygen ions. The oxygen ions then travel through the electrolyte to react with hydrogen gas at the anode. The reaction at the anode produces electricity and water as by-products. Carbon dioxide may also be a by-product depending on the fuel, but the carbon emissions from an SOFC system are less than those from a fossil fuel combustion plant.[17] The chemical reactions for the SOFC system can be expressed as follows:[18] Anode Reaction: 2H2 + 2O–2 → 2H2O + 4e– Cathode Reaction: O2 + 4e– → 2O–2 Overall Cell Reaction: 2H2 + O2 → 2H2O SOFC systems can run on fuels other than pure hydrogen gas. However, since hydrogen is necessary for the reactions listed above, the fuel selected must contain hydrogen atoms. In order for the fuel cell to operate, the fuel must be converted into pure hydrogen gas. SOFCs are capable of internally reforming light hydrocarbons such as methane (natural gas), propane and

butane. Heavier hydrocarbons including gasoline, diesel, jet fuel and biofuels can serve as fuels in a SOFC system, but an external reformer is required. Challenges exist in SOFC systems due to their high operating temperatures. One such challenge is the potential for carbon dust to build up on the anode, which slows down the internal reforming process. Research to address this ―carbon coking‖ issue at the University of Pennsylvania has shown that the use of copper-based cermet (heat-resistant materials made of ceramic and metal) can reduce coking and the loss of performance.[19] Another disadvantage of SOFC systems is slow start-up time, making SOFCs less useful for mobile applications. Despite these disadvantages, a high operating temperature provides an advantage by removing the need for a precious metal catalyst like platinum, thereby reducing cost. Additionally, waste heat from SOFC systems may be captured and reused, increasing the theoretical overall efficiency to as high as 80%-85%.[16] The high operating temperature is largely due to the physical properties of the YSZ electrolyte. As temperature decreases, so does the ionic conductivity of YSZ. Therefore, to obtain optimum performance of the fuel cell, a high operating temperature is required. According to their website, Ceres Power, a UK SOFC fuel cell manufacturer, has developed a method of reducing the operating temperature of their SOFC system to 500-600 degrees Celsius. They replaced the commonly used YSZ electrolyte with a CGO (cerium gadolinium oxide) electrolyte. The lower operating temperature allows them to use stainless steel instead of ceramic as the cell substrate, which reduces cost and start-up time of the system.[20] MCFC Molten carbonate fuel cells (MCFCs) require a high operating temperature (650°C), similar to (SOFCs). MCFCs use lithium potassium carbonate salt as an electrolyte, and at high temperatures, this salt melts into a molten state that allows for the movement of charge (in this case, negative carbonate ions) within the cell.[21] Like SOFCs, MCFCs are capable of converting fossil fuel to a hydrogen-rich gas in the anode, eliminating the need for to produce hydrogen externally. The reforming process creates CO2 emissions. MCFC-compatible fuels include natural gas, biogas and gas produced from coal. The hydrogen in the gas reacts with carbonate ions from the electrolyte to produce water, carbon dioxide, electrons and small amounts of other chemicals. The electrons travel through an external circuit creating electricity and return to the cathode. There, oxygen from the air and carbon dioxide recycled from the anode react with the electrons to form carbonate ions that replenish the electrolyte, completing the circuit.[21] The chemical reactions for an MCFC system can be expressed as follows:[22] Anode Reaction: CO3-2 + H2 → H2O + CO2 + 2eCathode Reaction: CO2 + ½O2 + 2e- → CO3-2 Overall Cell Reaction: H2 + ½O2 → H2O

As with SOFCs, MCFCs disadvantages include slow start-up times because of their high operating temperature. This makes MCFC systems not suitable for mobile applications, and this technology will most likely be used for stationary fuel cell purposes. The main disadvantage of MCFC technology is the cells' short life span. The high temperature and carbonate electrolyte lead to corrosion of the anode and cathode. These factors accelerate the degradation of MFCF components, decreasing the durability and cell life. Researchers are addressing this problem by exploring corrosion-resistant materials for components as well as fuel cell designs that may increase cell life without decreasing performance.[16] MCFCs hold several advantages over other fuel cell technologies, including their resistance to impurities. They are not prone to ―carbon coking‖, which refers to reduced performance through the build-up of carbon and would otherwise slow the fuel reforming process. Therefore, carbonrich fuels like gases made from coal are compatible with the system. The Department of Energy claims that coal, itself, might even be a fuel option in the future, assuming the system can be made resistant to impurities such as sulfur and particulates that result from converting coal into hydrogen.[16] MCFCs also have relatively high efficiencies. They can reach a fuel-to-electricity efficiency of 50%, considerably higher than the 37-42% efficiency of a phosphoric acid fuel cell plant. Efficiencies can be as high as 65% when the fuel cell is paired with a turbine, and 85% if heat is captured and used in a Combined Heat and Power (CHP) system.[21] FuelCell Energy, a Connecticut-based fuel cell manufacturer, develops and sells MCFC fuel cells. The company says that their MCFC products range from 300kW to 2.8MW systems that achieve 47% electrical efficiency and can utilize CHP technology to obtain higher overall efficiencies. One product, the DFC-ERG, is combined with a gas turbine and, according to the company, it achieves an electrical efficiency of 65%.[23] History Main article: Timeline of hydrogen technologies

Sketch of William Grove's 1839 fuel cell The principle of the fuel cell was discovered by German scientist Christian Friedrich Schönbein in 1838 and published in one of the scientific magazines of the time.[24] Based on this work, the first fuel cell was demonstrated by Welsh scientist and barrister Sir William Robert Grove in the February 1839 edition of the Philosophical Magazine and Journal of Science[25] and later sketched, in 1842, in the same journal.[26] The fuel cell he made used similar materials to today's phosphoric-acid fuel cell. In 1955, W. Thomas Grubb, a chemist working for the General Electric Company (GE), further modified the original fuel cell design by using a sulphonated polystyrene ion-exchange membrane as the electrolyte. Three years later another GE chemist, Leonard Niedrach, devised a way of depositing platinum onto the membrane, which served as catalyst for the necessary hydrogen oxidation and oxygen reduction reactions. This became known as the 'Grubb-Niedrach fuel cell'. GE went on to develop this technology with NASA and McDonnell Aircraft, leading to its use during Project Gemini. This was the first commercial use of a fuel cell. It wasn't until 1959 that British engineer Francis Thomas Bacon successfully developed a 5 kW stationary fuel cell. In 1959, a team led by Harry Ihrig built a 15 kW fuel cell tractor for Allis-Chalmers which was demonstrated across the US at state fairs. This system used potassium hydroxide as the electrolyte and compressed hydrogen and oxygen as the reactants. Later in 1959, Bacon and his colleagues demonstrated a practical five-kilowatt unit capable of powering a welding machine. In the 1960s, Pratt and Whitney licensed Bacon's U.S. patents for use in the U.S. space program to supply electricity and drinking water (hydrogen and oxygen being readily available from the spacecraft tanks). In 1991, the first hydrogen fuel cell automobile was developed by Roger Billings.[27] United Technologies Corporation's UTC Power subsidiary was the first company to manufacture and commercialize a large, stationary fuel cell system for use as a co-generation power plant in hospitals, universities and large office buildings. UTC Power continues to market this fuel cell as the PureCell 200, a 200 kW system (although soon to be replaced by a 400 kW version, expected for sale in late 2009[dated info]).[28] UTC Power continues to be the sole supplier of fuel cells to NASA for use in space vehicles, having supplied fuel cells for the Apollo missions,[29] and the Space Shuttle program, and is developing fuel cells for automobiles, buses, and cell phone towers; the company has demonstrated the first fuel cell capable of starting under freezing conditions with its proton exchange membrane. ] Types of fuel cell

Efficiency of leading fuel cell technologies

Theoretical maximum efficiency Energy is lost when it is converted from one form to another. The energy efficiency of a system or device that converts energy is measured by the ratio of the amount useful energy put out by the system ("output energy") to the total amount of energy that is put in ("input energy") or by useful output energy as a percentage of the total input energy. In the case of fuel cells, useful output energy is measured in electrical energy produced by the system. Input energy is the energy stored in the fuel. According to the U.S. Department of Energy, fuel cells are generally between 40–60% energy efficient.[34] This is higher than some other systems for energy generation. For example, the typical internal combustion engine of a car is about 25% energy efficient.[35] In combined heat and power (CHP) systems, the heat produced by the fuel cell is captured and put to use, increasing the efficiency of the system to up to 85–90%.[16] The theoretical maximum efficiency of any type of power generation system is rarely reached in practice, and it does not consider other steps in power generation, such as production, transportation and storage of fuel and conversion of the electricity into mechanical power. In addition, drawing more power from the fuel cell decreases efficiency. However, this calculation allows the comparison of different types of power generation. The maximum theoretical energy efficiency of a fuel cell is 83%, operating at low power density and using pure hydrogen and oxygen as reactants (assuming no heat recapture)[36] According to the World Energy Council, this compares with a maximum theoretical efficiency of 58% for internal combustion engines.[36] While these efficiencies are not approached in real world applications, high temperature fuel cells (solid oxide fuel cells or molten carbonate fuel cells) can theoretically be combined with gas turbines to allow stationary fuel cells to come closer to the theoretical limit. A gas turbine would capture heat from the fuel cell and turn it into mechanical energy to increase the fuel cell‘s operational efficiency. This solution has been predicted to increase total efficiency to as much as 70%.[37] In practice For a fuel cell operating on air, losses due to the air supply system must also be taken into account. This refers to the pressurization of the air and dehumidifying it. This reduces the efficiency significantly and brings it near to that of a compression ignition engine. Furthermore, fuel cell efficiency decreases as load increases. The tank-to-wheel efficiency of a fuel cell vehicle is greater than 45% at low loads[38] and shows average values of about 36% when a driving cycle like the NEDC (New European Driving Cycle) is used as test procedure.[39] The comparable NEDC value for a Diesel vehicle is 22%. In 2008 Honda released a demonstration fuel cell electric vehicle (the Honda FCX Clarity) with fuel stack claiming a 60% tank-to-wheel efficiency.[40] It is also important to take losses due to fuel production, transportation, and storage into account. Fuel cell vehicles running on compressed hydrogen may have a power-plant-to-wheel efficiency of 22% if the hydrogen is stored as high-pressure gas, and 17% if it is stored as liquid hydrogen.[41] Fuel cells cannot store energy like a battery,[42] but in some applications, such as stand-alone power plants based on discontinuous sources such as solar or wind power, they are

combined with electrolyzers and storage systems to form an energy storage system. Most hydrogen, however, is produced by steam methane reforming, and so most hydrogen production emits carbon dioxide.[43] The overall efficiency (electricity to hydrogen and back to electricity) of such plants (known as round-trip efficiency), using pure hydrogen and pure oxygen can be "from 35 up to 50 percent", depending on gas density and other conditions.[44] While a much cheaper lead-acid battery might return about 90%, the electrolyzer/fuel cell system can store indefinite quantities of hydrogen, and is therefore better suited for long-term storage. Solid-oxide fuel cells produce exothermic heat from the recombination of the oxygen and hydrogen. The ceramic can run as hot as 800 degrees Celsius. This heat can be captured and used to heat water in a micro combined heat and power (m-CHP) application. When the heat is captured, total efficiency can reach 80-90% at the unit, but does not consider production and distribution losses. CHP units are being developed today for the European home market. Professor Jeremy P. Meyers, in the Electrochemical Society journal Interface in 2008, wrote, "While fuel cells are efficient relative to combustion engines, they are not as efficient as batteries, due primarily to the inefficiency of the oxygen reduction reaction (and ... the oxygen evolution reaction, should the hydrogen be formed by electrolysis of water). ... [T]hey make the most sense for operation disconnected from the grid, or when fuel can be provided continuously. For applications that require frequent and relatively rapid start-ups ... where zero emissions are a requirement, as in enclosed spaces such as warehouses, and where hydrogen is considered an acceptable reactant, a [PEM fuel cell] is becoming an increasingly attractive choice [if exchanging batteries is inconvenient]".[9] [edit] Applications

Type 212 submarine with fuel cell propulsion of the German Navy in dry dock Power Stationary fuel cells are used for commercial, industrial and residential primary and backup power generation. Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, large parks, communications centers, rural locations including research stations, and in certain military applications. A fuel cell system running on hydrogen can be compact and lightweight, and have no major moving parts. Because fuel cells have no moving parts and do not involve combustion, in ideal conditions they can achieve up to 99.9999% reliability.[45] This equates to less than one minute of downtime in a six year period.[46]

Since fuel cellelectrolyzer systems do not store fuel in themselves, but rather rely on external storage units, they can be successfully applied in large-scale energy storage, rural areas being one example.[47] There are many different types of stationary fuel cells so efficiencies vary, but most are between 40% and 60% energy efficient.[16] However, when the fuel cell‘s waste heat is used to heat a building in a cogeneration system this efficiency can increase to 85%.[16] This is significantly more efficient than traditional coal power plants, which are only about one third energy efficient.[48] Assuming production at scale, fuel cells could save 20-40% on energy costs when used in cogeneration systems.[49] Fuel cells are also much cleaner than traditional power generation; a fuel cell power plant using natural gas as a hydrogen source would create less than one ounce of pollution (other than CO2) for every 1,000 kW produced, compared to 25 pounds of pollutants generated by conventional combustion systems.[50] Fuel Cells also produce 97% less nitrogen oxide emissions then conventional coal-fired power plants. Coca-Cola, Google, Sysco, FedEx, UPS, Ikea, Staples, Whole Foods, Gills Onions, Nestle Waters, Pepperidge Farm, Sierra Nevada Brewery, Super Store Industries, Brigestone-Firestone, Nissan North America, Kimberly-Clark, Michelin and more have installed fuel cells to help meet their power needs.[51] One such pilot program is operating on Stuart Island in Washington State. There the Stuart Island Energy Initiative[52] has built a complete, closed-loop system: Solar panels power an electrolyzer which makes hydrogen. The hydrogen is stored in a 500 US gallons (1,900 L) at 200 pounds per square inch (1,400 kPa), and runs a ReliOn fuel cell to provide full electric back-up to the off-the-grid residence. Cogeneration Combined heat and power (CHP) fuel cell systems, including Micro combined heat and power (MicroCHP) systems are used to generate both electricity and heat for homes (see home fuel cell), office building and factories. These stationary fuel cells are already in the mass production phase. The system generates constant electric power (selling excess power back to the grid when it is not consumed), and at the same time produces hot air and water from the waste heat. MicroCHP is usually less than 5 kWe for a home fuel cell or small business.[53] Co-generation systems can reach 85% efficiency (40-60% electric + remainder as thermal).[16] Phosphoric-acid fuel cells (PAFC) comprise the largest segment of existing CHP products worldwide and can provide combined efficiencies close to 90%.[54] Molten Carbonate (MCFC) and Solid Oxide Fuel Cells (SOFC) are also used for combined heat and power generation and have electrical energy effciences around 60%.[55] [edit] Fuel Cell Electric Vehicles (FCEVs) Main articles: Fuel Cell Vehicle and List of fuel cell vehicles

Configuration of components in a fuel cell car.

Toyota FCHV PEM FC fuel cell vehicle.

Mercedes-Benz (Daimler AG) Citaro fuel cell bus on Aldwych, London.

Toyota FCHV-BUS at the Expo 2005.

Element One fuel cell vehicle.

The world's first certified Fuel Cell Boat (HYDRA), in Leipzig/Germany Automobiles Although there are currently no Fuel cell vehicles available for commercial sale, over 20 FCEVs prototypes and demonstration cars have been released since 2009.[56] As of June 2011 demonstration FCEVs had driven more than 4,800,000 km (3,000,000 mi), with more than 27,000 refuelings.[57] Automobiles such as the Honda FCX Clarity, Toyota FCHV-adv, and Mercedes-Benz F-Cell are all pre-commercial examples of fuel cell electric vehicles. Several of the car manufacturers have announced plans to introduce a production model of a fuel cell car in 2015. Toyota has stated that it plans to introduce such a vehicle at a price of around US$50,000.[58] In June 2011, Mercedes-Benz announced that they would move the scheduled production date of their fuel cell car from 2015 up to 2014.[59] In July 2011, the Chairman and CEO of General Motors, Daniel Akerson, stated that while the cost of hydrogen fuel cell cars is decreasing: "The car is still too expensive and probably won't be practical until the 2020-plus period, I don't know."[60] Fuel cell electric vehicles have been produced with "a driving range of more than 400 km (250 mi) between refueling".[61] They can be refueled in less than 5 minutes.[62] EERE‘s Fuel Cell Technology Program claims that, as of 2011, fuel cells achieved a 42 to 53% fuel cell electric vehicle efficiency at full power,[63] and a durability of over 120,000 km (75,000 mi) with less than 10% degradation, double that achieved in 2006.[61] In a Well-to-Wheels analysis, the U.S. Department of Energy estimated that fuel cell electric vehicles using hydrogen produced from natural gas would result in emissions of approximately 55% of the CO2 per mile of internal combustion engine vehicles and have approximately 25% less emissions than hybrid vehicles.[64] Other analyses conclude, however, that numerous challenges remain before fuel cell cars can become economically competitive with other technologies. They cite the lack of an extensive hydrogen infrastructure in the U.S. and stating: "the large amount of energy required to isolate hydrogen from natural compounds (water, natural gas, biomass), package the light gas by compression or liquefaction, transfer the energy carrier to the user, plus the energy lost when it is converted to useful electricity with fuel cells, leaves

around 25% for practical use."[38][65] Experts believe that it would take at least 20 years for manufacturers to achieve profitable production.[9] In 2003 US President George Bush proposed the Hydrogen Fuel Initiative (HFI). This aimed at further developing hydrogen fuel cells and infrastructure technologies with the goal of producing commercial fuel cell vehicles. By 2008, the U.S. had contributed 1 billion dollars to this project.[66] The Obama Administration has sought to reduce funding for the development of fuel cell vehicles, concluding that other vehicle technologies will lead to quicker reduction in emissions in a shorter time.[67] Steven Chu, the US Secretary of Energy, asserted that hydrogen vehicles "will not be practical over the next 10 to 20 years".[68] He told MIT's Technology Review that he is skeptical about hydrogen's use in transportation because of four problems: "the way we get hydrogen primarily is from reforming [natural] gas. ... You're giving away some of the energy content of natural gas. ... [For] transportation, we don't have a good storage mechanism yet. ... The fuel cells aren't there yet, and the distribution infrastructure isn't there yet. ... In order to get significant deployment, you need four significant technological breakthroughs.[69] The National Hydrogen Association and the U.S. Fuel Cell Council have criticized this position.[70] Buses In total there are over 100 fuel cell buses deployed around the world today. Most buses are produced by UTC Power, Toyota, Ballard, Hydrogenics, and Proton Motor. UTC Buses have already accumulated over 970,000 km (600,000 mi) of driving.[71] Fuel cell buses have a 30141% higher fuel economy than diesel buses and natural gas buses.[72] Fuel cell buses have been deployed around the world including in Whistler Canada, San Francisco USA, Hamburg Germany, Shanghai China, London England, São Paulo Brazil as well as several others.[73] The Fuel Cell Bus Club is a global cooperative effort in trial fuel cell buses. Notable Projects Include:
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12 Fuel cell buses are being deployed in the Oakland and San Francisco Bay area of California.[73] Daimler AG, with thirty-six experimental buses powered by Ballard Power Systems fuel cells completed a successful three-year trial, in eleven cities, in January 2007.[74][75] A fleet of Thor buses with UTC Power fuel cells was deployed in California, operated by SunLine Transit Agency.[76] The first Brazilian hydrogen fuel cell bus prototype in Brazil was deployed in São Paulo. The bus was manufactured in Caxias do Sul and the hydrogen fuel will be produced in São Bernardo do Campo from water through electrolysis. The program, called "Ônibus Brasileiro a Hidrogênio" (Brazilian Hydrogen Autobus), includes three additional buses.[77][78] ] Forklifts Fuel cell powered forklifts are one of the largest sectors of fuel cell applications in the industry.[79] Most fuel cells used for material handling purposes are powered by PEM fuel cells, although some direct methanol fuel forklifts are coming onto the market. Fuel cell fleets are currently being operated by a large number of companies, including Sysco Foods, FedEx Freight,

GENCO (at Wegmans, Coca-Cola, Kimberly Clark, Sysco Foods, and Whole Foods), and H-E-B Grocers.[80] Fuel cell powered forklifts provide significant benefits over both petroleum and battery powered forklifts as they produce no local emissions, can work for a full 8 hour shift on a single tank of hydrogen, can be refueled in 3 minutes and have a lifetime of 8–10 years. Fuel cell powered forklifts are often used in refrigerated warehouses as their performance is not degraded by lower temperatures. Many companies do not use petroleum powered forklifts, as these vehicles work indoors where emissions must be controlled and instead are turning towards electric forklifts. Fuel cell forklifts offer green house gas, product lifetime, maintenance cost, refueling and labor cost benefits over battery operated fork lifts.[81] Motorcycles and bicycles In 2005 the British firm Intelligent Energy produced the first ever working hydrogen run motorcycle called the ENV (Emission Neutral Vehicle). The motorcycle holds enough fuel to run for four hours, and to travel 160 km (100 mi) in an urban area, at a top speed of 80 km/h (50 mph).[82] In 2004 Honda developed a fuel-cell motorcycle which utilized the Honda FC Stack.[83][84] There are other examples of bikes[85] and bicycles[86] with a hydrogen fuel cell engine. Airplanes Boeing researchers and industry partners throughout Europe conducted experimental flight tests in February 2008 of a manned airplane powered only by a fuel cell and lightweight batteries. The Fuel Cell Demonstrator Airplane, as it was called, used a Proton Exchange Membrane (PEM) fuel cell/lithium-ion battery hybrid system to power an electric motor, which was coupled to a conventional propeller.[87] In 2003, the world's first propeller driven airplane to be powered entirely by a fuel cell was flown. The fuel cell was a unique FlatStackTM stack design which allowed the fuel cell to be integrated with the aerodynamic surfaces of the plane.[88] There have been several fuel cell powered unmanned aerial vehicles (UAV). A Horizen fuel cell UAV set the record distance flow for a small UAV in 2007.[89] The military is especially interested in this application because of the low noise, low thermal signature and ability to attain high altitude. In 2009 the Naval Research Laboratory‘s (NRL‘s) Ion Tiger utilized a hydrogenpowered fuel cell and flew for 23 hours and 17 minutes.[90] Boeing is completing tests on the Phantom Eye, a high-altitude, long endurance (HALE) to be used to conduce research and surveillance flying at 20,000 m (65,000 ft) for up to four days at a time.[91] Fuel cells are also being used to provide auxiliary power power aircraft, replacing fossil fuel generators that were previously used to start the engines and power on board electrical needs.[91] Fuel cells can help airplanes reduce CO2 and other pollutant emissions and noise. Boats The world's first Fuel Cell Boat HYDRA used an AFC system with 6.5 kW net output.

For each liter of fuel consumed, the average outboard motor produces 140 times the hydrocarbonss produced by the average modern car. Fuel cell engines have higher energy efficiencies than combustion engines, and therefore offer better range and significantly reduced emissions.[47] Iceland has committed to converting its vast fishing fleet to use fuel cells to provide auxiliary power by 2015 and, eventually, to provide primary power in its boats. Amsterdam recently introduced its first fuel cell powered boat that ferries people around the city's famous and beautiful canals.[92] Submarines The Type 212 submarines of the German and Italian navies use fuel cells to remain submerged for weeks without the need to surface. The latest in fuel cell submarines is the U212A—an ultra-advanced non-nuclear sub developed by German naval shipyard Howaldtswerke Deutsche Werft, who claim it to be "the peak of German submarine technology."[93] The system consists of nine PEM (polymer electrolyte membrane) fuel cells, providing between 30 kW and 50 kW each. The ship is totally silent giving it a distinct advantage in the detection of other submarines.[94] Fuel cells offer some distinct advantages to submarines, in addition to being completely silent, and can be distributed throughut a ship to improve balance and require far less air to run, allowing ships to be submerged for longer periods of time. Fuel cells offer a good alternative to nuclear fuels. Other applications
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Providing power for base stations or cell sites[95][96] Off-grid power supply Distributed generation Fork Lifts Emergency power systems are a type of fuel cell system, which may include lighting, generators and other apparatus, to provide backup resources in a crisis or when regular systems fail. They find uses in a wide variety of settings from residential homes to hospitals, scientific laboratories, data centers,[97] telecommunication[98] equipment and modern naval ships. An uninterrupted power supply (UPS) provides emergency power and, depending on the topology, provide line regulation as well to connected equipment by supplying power from a separate source when utility power is not available. Unlike a standby generator, it can provide instant protection from a momentary power interruption. Base load power plants Electric and hybrid vehicles. Notebook computers for applications where AC charging may not be available for weeks at a time. Portable charging docks for small electronics (e.g. a belt clip that charges your cell phone or PDA). Smartphones with high power consumption due to large displays and additional features like GPS might be equipped with micro fuel cells. Small heating appliances [99]

Fueling stations Main articles: Hydrogen station and Hydrogen highway

Hydrogen fueling station. There are already over 85 hydrogen refueling stations in the U.S.[100] The National Research Council estimated that creating the infrastructure to supply fuel for 10 million FCVs through 2025 would cost the government US$8 billion over 16 years.[101] The first public hydrogen refueling station was opened in Reykjavík, Iceland in April 2003. This station serves three buses built by DaimlerChrysler that are in service in the public transport net of Reykjavík. The station produces the hydrogen it needs by itself, with an electrolyzing unit (produced by Norsk Hydro), and does not need refilling: all that enters is electricity and water. Royal Dutch Shell is also a partner in the project. The station has no roof, in order to allow any leaked hydrogen to escape to the atmosphere.[citation needed] As part of the California Hydrogen Highway initiative California has the most extensive hydrogen refueling infrastructure in the U.S.A. As of June 2011 California had 22 hydrogen refueling stations in operation.[100] Honda announced plans in March 2011 to open the first station that would generate hydrogen through solar-powered renewable electrolysis.[citation needed] South Carolina also has two hydrogen fueling stations, in Aiken and Columbia, SC. According to the South Carolina Hydrogen & Fuel Cell Alliance, the Columbia station has a current capacity of 120 kg a day, with future plans to develop on-site hydrogen production from electrolysis and reformation. The Aiken station has a current capacity of 80 kg. The University of South Carolina, a founding member of the South Carolina Hydrogen & Fuel Cell Alliance, received 12.5 million dollars from the United States Department of Energy for its Future Fuels Program.[102] Japan also has a hydrogen highway, as part of the Japan hydrogen fuel cell project. Twelve hydrogen fueling stations have been built in 11 cities in Japan. Canada, Sweden and Norway also have hydrogen highways implemented. Markets and economics In 2010, fuel cell industry revenues exceeded a $750 million market value worldwide,[103] although, as of 2010, no public company in the industry had yet become profitable.[104] There

were 140,000 fuel cell stacks shipped globally in 2010, up from 11 thousand shipments in 2007, and in 2010 worldwide fuel cell shipments had an annual growth rate of 115%.[105] Approximately 50% of fuel cell shipments in 2010 were stationary fuel cells, up from about a third in 2009, and the four dominant producers in the Fuel Cell Industry remain the United States, Germany, Japan and South Korea.[106] The Department of Energy Solid State Energy Conversion Alliance found that, as of January 2011, stationary fuel cells generated power at approximately $724 to $775 per kilowatt installed.[107] Bloom Energy, a major fuel cell supplier, says its fuel cells will meet a return on investment in 3–5 years, as its fuel cells generate power at 9-11 cents per kilowatt-hour, including the price of fuel, maintenance, and hardware.[108][109] Low temperature fuel cell stacks proton exchange membrane fuel cell (PEMFC), direct methanol fuel cell (DMFC) and phosphoric acid fuel cell (PAFC) use a platinum catalyst. Impurities create catalyst poisoning (reducing activity and efficiency) in these low-temperature fuel cells, thus high hydrogen purity or higher catalyst densities are required.[110] Although there are sufficient platinum resources for future demand,[111] most predictions of platinum running out and/or platinum prices soaring do not take into account effects of reduction in catalyst loading and recycling. Recent research at Brookhaven National Laboratory could lead to the replacement of platinum by a gold-palladium coating which may be less susceptible to poisoning and thereby improve fuel cell lifetime considerably.[112] Another method would use iron and sulphur instead of platinum. This is possible through an intermediate conversion by bacteria. This would lower the cost of a fuel cell substantially (as the platinum in a regular fuel cell costs around US$1,500, and the same amount of iron costs only around US$1.50). The concept is being developed by a coalition of the John Innes Centre and the University of Milan-Bicocca.[113] PEDOT cathodes are immune to monoxide poisoning.[114] Current targets for a transport PEM fuel cells are 0.2 g/kW Pt – which is a factor of 5 decrease over current loadings – and recent comments from major original equipment manufacturers (OEMs) indicate that this is possible. Recycling of fuel cells components, including platinum, will conserve supplies. High-temperature fuel cells, including molten carbonate fuel cells (MCFC's) and solid oxide fuel cells (SOFC's), do not use platinum as catalysts, but instead use cheaper materials such as nickel and nickel oxide. They also do not experience catalyst poisoning by carbon monoxide, and so they do not require high-purity hydrogen to operate. They can use fuels with an existing and extensive infrastructure, such as natural gas, directly, without having to first reform it externally to hydrogen and CO followed by CO removal.

SOLAR CELLS
A solar cell (also called photovoltaic cell or photoelectric cell) is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect. Assemblies of cells used to make solar modules which are used to capture energy from sunlight, are known as solar panels. The energy generated from these solar modules, referred to as solar power, is an example of solar energy.

Photovoltaics is the field of technology and research related to the practical application of photovoltaic cells in producing electricity from light, though it is often used specifically to refer to the generation of electricity from sunlight. Cells are described as photovoltaic cells when the light source is not necessarily sunlight. These are used for detecting light or other electromagnetic radiation near the visible range, for example infrared detectors, or measurement of light intensity.

History of solar cells Main article: Timeline of solar cells The term "photovoltaic" comes from the Greek φῶς (phōs) meaning "light", and "voltaic", from the name of the Italian physicist Volta, after whom a unit of electro-motive force, the volt, is named. The term "photo-voltaic" has been in use in English since 1849.[1] The photovoltaic effect was first recognized in 1839 by French physicist A. E. Becquerel. However, it was not until 1883 that the first photovoltaic cell was built, by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient. In 1888 Russian physicist Aleksandr Stoletov built the first photoelectric cell (based on the outer photoelectric effect discovered by Heinrich Hertz earlier in 1887). Albert Einstein explained the photoelectric effect in 1905 for which he received the Nobel prize in Physics in 1921.[2] Russell Ohl patented the modern junction semiconductor solar cell in 1946,[3] which was discovered while working on the series of advances that would lead to the transistor. Bell produces the first practical cell The modern photovoltaic cell was developed in 1954 at Bell Laboratories.[4] The highly efficient solar cell was first developed by Daryl Chapin, Calvin Souther Fuller and Gerald Pearson in 1954 using a diffused silicon p-n junction.[5] At first, cells were developed for toys and other minor uses, as the cost of the electricity they produced was very high; in relative terms, a cell that produced 1 watt of electrical power in bright sunlight cost about $250, comparing to $2 to $3 for a coal plant. Solar cells were rescued from obscurity by the suggestion to add them to the Vanguard I satellite. In the original plans, the satellite would be powered only by battery, and last a short time while this ran down. By adding cells to the outside of the fuselage, the mission time could be extended with no major changes to the spacecraft or its power systems. There was some skepticism at first, but in practice the cells proved to be a huge success, and solar cells were quickly designed into many new satellites, notably Bell's own Telstar. Improvements were slow over the next two decades, and the only widespread use was in space applications where their power-to-weight ratio was higher than any competing technology.

However, this success was also the reason for slow progress; space users were willing to pay anything for the best possible cells, there was no reason to invest in lower-cost solutions if this would reduce efficiency. Instead, the price of cells was determined largely by the semiconductor industry; their move to integrated circuits in the 1960s led to the availability of larger boules at lower relative prices. As their price fell, the price of the resulting cells did as well. However these effects were limited, and by 1971 cell costs were estimated to be $100 per watt.[6] Berman's price reductions In the late 1960s, Elliot Berman was investigating a new method for producing the silicon feedstock in a ribbon process. However, he found little interest in the project and was unable to gain the funding needed to develop it. In a chance encounter, he was later introduced to a team at Exxon who were looking for projects 30 years in the future. The group had concluded that electrical power would be much more expensive by 2000, and felt that this increase in price would make new alternative energy sources more attractive, and solar was the most interesting among these. In 1969, Berman joined the Linden, New Jersey Exxon lab, Solar Power Corporation (SPC).[7] His first major effort was to canvas the potential market to see what possible uses for a new product were, and they quickly found that if the dollars per watt was reduced from then-current $100/watt to about $20/watt there was significant demand. Knowing that his ribbon concept would take years to develop, the team started looking for ways to hit the $20 price point using existing materials.[7] The first improvement was the realization that the existing cells were based on standard semiconductor manufacturing process, even though that was not ideal. This started with the boule, cutting it into disks called wafers, polishing the wafers, and then, for cell use, coating them with an anti-reflective layer. Berman noted that the rough-sawn wafers already had a perfectly suitable anti-reflective front surface, and by printing the electrodes directly on this surface, two major steps in the cell processing were eliminated. The team also explored ways to improve the mounting of the cells into arrays, eliminating the expensive materials and hand wiring used in space applications. Their solution was to use a printed circuit board on the back, acrylic plastic on the front, and silicone based glue between the two, potting the cells. But the largest improvement in price point was Berman's realization that existing silicon was effectively "too good" for solar cell use; the minor imperfections that would ruin a boule (or individual wafer) for electronics would have little effect in the solar application.[8] Solar cells could be made using cast-off material from the electronics market. Putting all of these changes into practice, the company started buying up "reject" silicon from existing manufacturers at very low cost. By using the largest wafers available, thereby reducing the amount of wiring for a given panel area, and packaging them into panels using their new methods, by 1973 SPC was producing panels at $10 per watt and selling them at $20 per watt, a fivefold decrease in prices in two years.

Navigation market SPC approached companies making navigational buoys as a natural market for their products, but found a curious situation. The primary company in the business was Automatic Power, a battery manufacturer. Realizing that solar cells might eat into their battery profits, Automatic purchased the rights to earlier solar cell designs and suppressed them.[citation needed] Seeing there was no interest at Automatic, SPC turned to Tideland Signal, another battery company formed by ex-Automatic managers. Tideland introduced a solar-powered buoy and was soon ruining Automatic's business. The timing could not be better; the rapid increase in the number of offshore oil platforms and loading facilities produced an enormous market among the oil companies. As Tideland's fortunes improved, Automatic started looking for their own supply of solar panels. They found Bill Yerks of Solar Power International (SPI) in California, who was looking for a market. SPI was soon bought out by one of its largest customers, the ARCO oil giant, forming ARCO Solar. ARCO Solar's factory in Camarillo, California was the first dedicated to building solar panels, and has been in continual operation from its purchase by ARCO in 1977 to this day. This market, combined with the 1973 oil crisis, led to a curious situation. Oil companies were now cash-flush due to their huge profits during the crisis, but were also acutely aware that their future success would depend on some other form of power. Over the next few years, major oil companies started a number of solar firms, and were for decades the largest producers of solar panels. Exxon, ARCO, Shell, Amoco (later purchased by BP) and Mobil all had major solar divisions during the 1970s and 80s. Technology companies also had some investment, including General Electric, Motorola, IBM, Tyco and RCA.[9] Further improvements In the time since Berman's work, improvements have brought production costs down under $1 a watt, with wholesale costs on the order of $2. "Balance of system" costs are now more than the panels themselves, with large commercial arrays falling to $3.40 a watt,[10] fully commissioned, in 2010. As the semiconductor industry moved to ever-larger boules, older equipment became available at fire-sale prices. Cells have grown in size as older equipment became available on the surplus market; ARCO Solar's original panels used cells with 2 to 4 inch diameter. Panels in the 1990s and early 2000s generally used 5 inch wafers, and since 2008 almost all new panels use 6 inch cells. Another major change was the move to polycrystalline silicon. This material has less efficiency, but is less expensive to produce in bulk. The widespread introduction of flat screen televisions in the late 1990s and early 2000s led to the wide availability of large sheets of highquality glass, used on the front of the panels. Other technologies have tried to enter the market. First Solar was briefly the largest panel manufacturer in 2009, in terms of yearly power produced, using a thin-film cell sandwiched between two layers of glass. Since then Silicon panels reasserted their dominant position both in

terms of lower prices and the rapid rise of Chinese manufacturing resulted in the top producers being Chinese. Applications

Polycrystalline photovoltaic cells laminated to backing material in a module

Polycrystalline photovoltaic cells Main article: photovoltaic system Solar cells are often electrically connected and encapsulated as a module. Photovoltaic modules often have a sheet of glass on the front (sun up) side, allowing light to pass while protecting the semiconductor wafers from abrasion and impact due to wind-driven debris, rain, hail, etc. Solar cells are also usually connected in series in modules, creating an additive voltage. Connecting cells in parallel will yield a higher current. Modules are then interconnected, in series or parallel, or both, to create an array with the desired peak DC voltage and current.

To make practical use of the solar-generated energy, the electricity is most often fed into the electricity grid using inverters (grid-connected photovoltaic systems); in stand-alone systems, batteries are used to store the energy that is not needed immediately. Solar panels can be used to power or recharge portable devices. Theory Main article: Theory of solar cell The solar cell works in three steps: 1. Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon. 2. Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. Due to the special composition of solar cells, the electrons are only allowed to move in a single direction. 3. An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity. Efficiency It has been suggested that Fill factor be merged into this article or section. (Discuss) Proposed since June 2011. Main article: Solar cell efficiency The efficiency of a solar cell may be broken down into reflectance efficiency, thermodynamic efficiency, charge carrier separation efficiency and conductive efficiency. The overall efficiency is the product of each of these individual efficiencies. Due to the difficulty in measuring these parameters directly, other parameters are measured instead: thermodynamic efficiency, quantum efficiency, integrated quantum efficiency, VOC ratio, and fill factor. Reflectance losses are a portion of the quantum efficiency under "external quantum efficiency". Recombination losses make up a portion of the quantum efficiency, VOC ratio, and fill factor. Resistive losses are predominantly categorized under fill factor, but also make up minor portions of the quantum efficiency, VOC ratio. Crystalline silicon devices are now approaching the theoretical limiting efficiency of 29%. Cost The cost of a solar cell is given per unit of peak electrical power. Manufacturing costs necessarily include the cost of energy required for manufacture. Solar-specific feed in tariffs vary worldwide, and even state by state within various countries.[11] Such feed-in tariffs can be highly effective in encouraging the development of solar power projects.

High-efficiency solar cells are of interest to decrease the cost of solar energy. Many of the costs of a solar power plant are proportional to the area of the plant; a higher efficiency cell may reduce area and plant cost, even if the cells themselves are more costly. Efficiencies of bare cells, to be useful in evaluating solar power plant economics, must be evaluated under realistic conditions. The basic parameters that need to be evaluated are the short circuit current, open circuit voltage.[12] The chart at the right illustrates the best laboratory efficiencies obtained for various materials and technologies, generally this is done on very small, i.e. one square cm, cells. Commercial efficiencies are significantly lower.

Grid parity, the point at which photovoltaic electricity is equal to or cheaper than grid power, can be reached using low cost solar cells. It is achieved first in areas with abundant sun and high costs for electricity such as in California and Japan.[13] Grid parity has been reached in Hawaii and other islands that otherwise use diesel fuel to produce electricity. George W. Bush had set 2015 as the date for grid parity in the USA.[14][15] Speaking at a conference in 2007, General Electric's Chief Engineer predicted grid parity without subsidies in sunny parts of the United States by around 2015.[16]

The price of solar panels fell steadily for 40 years, until 2004 when high subsidies in Germany drastically increased demand there and greatly increased the price of purified silicon (which is used in computer chips as well as solar panels). The great recession of 2008, and the onset of Chinese manufacturing, caused prices to resume their decline with vehemence. In the four years after January 2008 spot prices for solar modules in the German market prices dropped from €3 to €1 per Watt peak. During that same times production capacity surged with an annual growth of more than 50% and the Mainland Chinese went from single digit to over 50% market share.[17] Materials

The Shockley-Queisser limit for the theoretical maximum efficiency of a solar cell. Semiconductors with bandgap between 1 and 1.5eV have the greatest potential to form an efficient cell. (The efficiency "limit" shown here can be exceeded by multijunction solar cells.) Different materials display different efficiencies and have different costs. Materials for efficient solar cells must have characteristics matched to the spectrum of available light. Some cells are designed to efficiently convert wavelengths of solar light that reach the Earth surface. However, some solar cells are optimized for light absorption beyond Earth's atmosphere as well. Light absorbing materials can often be used in multiple physical configurations to take advantage of different light absorption and charge separation mechanisms. Materials presently used for photovoltaic solar cells include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium selenide/sulfide.[18] Many currently available solar cells are made from bulk materials that are cut into wafers between 180 to 240 micrometers thick that are then processed like other semiconductors. Other materials are made as thin-films layers, organic dyes, and organic polymers that are deposited on supporting substrates. A third group are made from nanocrystals and used as quantum dots (electron-confined nanoparticles). Silicon remains the only material that is wellresearched in both bulk and thin-film forms.

Crystalline silicon Main articles: Monocrystalline silicon, Polycrystalline silicon, Silicon, and list of silicon producers

Basic structure of a silicon based solar cell and its working mechanism. By far, the most prevalent bulk material for solar cells is crystalline silicon (abbreviated as a group as c-Si), also known as "solar grade silicon". Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon, or wafer. 1. monocrystalline silicon (c-Si): often made using the Czochralski process. Single-crystal wafer cells tend to be expensive, and because they are cut from cylindrical ingots, do not completely cover a square solar cell module without a substantial waste of refined silicon. Hence most c-Si panels have uncovered gaps at the four corners of the cells. 2. Poly- or multicrystalline silicon (poly-Si or mc-Si): made from cast square ingots — large blocks of molten silicon carefully cooled and solidified. Poly-Si cells are less expensive to produce than single crystal silicon cells, but are less efficient. US DOE data shows that there were a higher number of multicrystalline sales than monocrystalline silicon sales. 3. Ribbon silicon[19] is a type of multicrystalline silicon: it is formed by drawing flat thin films from molten silicon and results in a multicrystalline structure. These cells have lower efficiencies than poly-Si, but save on production costs due to a great reduction in silicon waste, as this approach does not require sawing from ingots. Analysts have predicted that prices of polycrystalline silicon will drop as companies build additional polysilicon capacity quicker than the industry‘s projected demand. On the other hand,

the cost of producing upgraded metallurgical-grade silicon, also known as UMG Si, can potentially be one-sixth that of making polysilicon.[20] Manufacturers of wafer-based cells have responded to high silicon prices in 2004-2008 prices with rapid reductions in silicon consumption. According to Jef Poortmans, director of IMEC's organic and solar department, current cells use between eight and nine grams of silicon per watt of power generation, with wafer thicknesses in the neighborhood of 0.200 mm. At 2008 spring's IEEE Photovoltaic Specialists' Conference (PVS'08), John Wohlgemuth, staff scientist at BP Solar, reported that his company has qualified modules based on 0.180 mm thick wafers and is testing processes for 0.16 mm wafers cut with 0.1 mm wire. IMEC's roadmap, presented at the organization's recent annual research review meeting, envisions use of 0.08 mm wafers by 2015.[21] Thin films Main article: Thin film solar cell

Marketshare of the different PV technologies In 2010 the marketshare of thin film declined by 30% as thin film technology was displaced by more efficient crystalline silicon solar panels (the light and dark blue bars). Thin-film technologies reduce the amount of material required in creating the active material of solar cell. Most thin film solar cells are sandwiched between two panes of glass to make a module. Since silicon solar panels only use one pane of glass, thin film panels are approximately twice as heavy as crystalline silicon panels. The majority of film panels have significantly lower conversion efficiencies, lagging silicon by two to three percentage points.[22] Thin-film solar technologies have enjoyed large investment due to the success of First Solar and the, largely unfulfilled, promise of lower cost and flexibility compared to wafer silicon cells, but they have not become mainstream solar products due to their lower efficiency and corresponding larger area consumption per watt production. Cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and amorphous silicon (A-Si) are three thin-film techologies often used as outdoor photovoltaic solar power production. CdTe technology is most cost competitive among them.[23] CdTe technology costs about 30% less than CIGS technology and 40% less than A-Si technology in 2011.

Cadmium telluride solar cell Main article: Cadmium telluride photovoltaics A cadmium telluride solar cell uses a cadmium telluride (CdTe) thin film, a semiconductor layer to absorb and convert sunlight into electricity. Solarbuzz[24] has reported that the lowest quoted thin-film module price stands at US$1.76 per watt-peak, with the lowest crystalline silicon (c-Si) module at $2.48 per watt-peak. The cadmium present in the cells would be toxic if released. However, release is impossible during normal operation of the cells and is unlikely during fires in residential roofs.[25] A square meter of CdTe contains approximately the same amount of Cd as a single C cell Nickel-cadmium battery, in a more stable and less soluble form.[25] Copper indium galium selenide Main article: Copper indium gallium selenide solar cell Copper indium gallium selenide (CIGS) is a direct-bandgap material. It has the highest efficiency (~20%) among thin film materials (see CIGS solar cell). Traditional methods of fabrication involve vacuum processes including co-evaporation and sputtering. Recent developments at IBM and Nanosolar attempt to lower the cost by using non-vacuum solution processes. Gallium arsenide multijunction Main article: Multijunction photovoltaic cell High-efficiency multijunction cells were originally developed for special applications such as satellites and space exploration, but at present, their use in terrestrial concentrators might be the lowest cost alternative in terms of $/kWh and $/W.[26] These multijunction cells consist of multiple thin films produced using metalorganic vapour phase epitaxy. A triple-junction cell, for example, may consist of the semiconductors: GaAs, Ge, and GaInP2.[27] Each type of semiconductor will have a characteristic band gap energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb electromagnetic radiation over a portion of the spectrum. The semiconductors are carefully chosen to absorb nearly all of the solar spectrum, thus generating electricity from as much of the solar energy as possible. GaAs based multijunction devices are the most efficient solar cells to date. In October 2010, triple junction metamorphic cell reached a record high of 42.3%.[28] This technology is currently being utilized in the Mars Exploration Rover missions which have run far past their 90 day design life. Tandem solar cells based on monolithic, series connected, gallium indium phosphide (GaInP), gallium arsenide GaAs, and germanium Ge pn junctions, are seeing demand rapidly rise. Between December 2006 and December 2007, the cost of 4N gallium metal rose from about $350 per kg to $680 per kg. Additionally, germanium metal prices have risen substantially to

$1000–$1200 per kg this year. Those materials include gallium (4N, 6N and 7N Ga), arsenic (4N, 6N and 7N) and germanium, pyrolitic boron nitride (pBN) crucibles for growing crystals, and boron oxide, these products are critical to the entire substrate manufacturing industry. Triple-junction GaAs solar cells were also being used as the power source of the Dutch four-time World Solar Challenge winners Nuna in 2003, 2005 and 2007, and also by the Dutch solar cars Solutra (2005), Twente One (2007) and 21Revolution (2009). The Dutch Radboud University Nijmegen set the record for thin film solar cell efficiency using a single junction GaAs to 25.8% in August 2008 using only 4 µm thick GaAs layer which can be transferred from a wafer base to glass or plastic film.[29] Light-absorbing dyes (DSSC) Main article: Dye-sensitized solar cells Dye-sensitized solar cells (DSSCs) are made of low-cost materials and do not need elaborate equipment to manufacture, so they can be made in a DIY fashion, possibly allowing players to produce more of this type of solar cell than others. In bulk it should be significantly less expensive than older solid-state cell designs. DSSC's can be engineered into flexible sheets, and although its conversion efficiency is less than the best thin film cells, its price/performance ratio should be high enough to allow them to compete with fossil fuel electrical generation. The DSSC has been developed by Prof. Michael Grätzel in 1991 at the Swiss Federal Institute of Technology (EPFL) in Lausanne (CH). Typically a ruthenium metalorganic dye (Ru-centered) is used as a monolayer of light-absorbing material. The dye-sensitized solar cell depends on a mesoporous layer of nanoparticulate titanium dioxide to greatly amplify the surface area (200–300 m2/g TiO2, as compared to approximately 10 m2/g of flat single crystal). The photogenerated electrons from the light absorbing dye are passed on to the n-type TiO2, and the holes are absorbed by an electrolyte on the other side of the dye. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. This type of cell allows a more flexible use of materials, and is typically manufactured by screen printing and/or use of Ultrasonic Nozzles, with the potential for lower processing costs than those used for bulk solar cells. However, the dyes in these cells also suffer from degradation under heat and UV light, and the cell casing is difficult to seal due to the solvents used in assembly. In spite of the above, this is a popular emerging technology with some commercial impact forecast within this decade. The first commercial shipment of DSSC solar modules occurred in July 2009 from G24i Innovations.[30] Organic/polymer solar cells Organic solar cells are a relatively novel technology, yet hold the promise of a substantial price reduction (over thin-film silicon) and a faster return on investment. These cells can be processed from solution, hence the possibility of a simple roll-to-roll printing process, leading to inexpensive, large scale production.

Organic solar cells and polymer solar cells are built from thin films (typically 100 nm) of organic semiconductors including polymers, such as polyphenylene vinylene and small-molecule compounds like copper phthalocyanine (a blue or green organic pigment) and carbon fullerenes and fullerene derivatives such as PCBM. Energy conversion efficiencies achieved to date using conductive polymers are low compared to inorganic materials. However, it has improved quickly in the last few years and the highest NREL (National Renewable Energy Laboratory) certified efficiency has reached 8.3% for the Konarka Power Plastic.[31] In addition, these cells could be beneficial for some applications where mechanical flexibility and disposability are important. These devices differ from inorganic semiconductor solar cells in that they do not rely on the large built-in electric field of a PN junction to separate the electrons and holes created when photons are absorbed. The active region of an organic device consists of two materials, one which acts as an electron donor and the other as an acceptor. When a photon is converted into an electron hole pair, typically in the donor material, the charges tend to remain bound in the form of an exciton, and are separated when the exciton diffuses to the donor-acceptor interface. The short exciton diffusion lengths of most polymer systems tend to limit the efficiency of such devices. Nanostructured interfaces, sometimes in the form of bulk heterojunctions, can improve performance.[32] Silicon thin films Silicon thin-film cells are mainly deposited by chemical vapor deposition (typically plasmaenhanced, PE-CVD) from silane gas and hydrogen gas. Depending on the deposition parameters, this can yield:[33] 1. Amorphous silicon (a-Si or a-Si:H) 2. Protocrystalline silicon or 3. Nanocrystalline silicon (nc-Si or nc-Si:H), also called microcrystalline silicon. It has been found that protocrystalline silicon with a low volume fraction of nanocrystalline silicon is optimal for high open circuit voltage.[34] These types of silicon present dangling and twisted bonds, which results in deep defects (energy levels in the bandgap) as well as deformation of the valence and conduction bands (band tails). The solar cells made from these materials tend to have lower energy conversion efficiency than bulk silicon, but are also less expensive to produce. The quantum efficiency of thin film solar cells is also lower due to reduced number of collected charge carriers per incident photon. An amorphous silicon (a-Si) solar cell is made of amorphous or microcrystalline silicon and its basic electronic structure is the p-i-n junction. a-Si is attractive as a solar cell material because it is abundant and non-toxic (unlike its CdTe counterpart) and requires a low processing temperature, enabling production of devices to occur on flexible and low-cost substrates. As the amorphous structure has a higher absorption rate of light than crystalline cells, the complete light spectrum can be absorbed with a very thin layer of photo-electrically active material. A film only 1 micron thick can absorb 90% of the usable solar energy.[35] This reduced material requirement along with current technologies being capable of large-area deposition of a-Si, the scalability of this type of cell is high. However, because it is amorphous, it has high inherent disorder and

dangling bonds, making it a bad conductor for charge carriers. These dangling bonds act as recombination centers that severely reduce the carrier lifetime and pin the Fermi energy level so that doping the material to n- or p- type is not possible. Amorphous Silicon also suffers from the Staebler-Wronski effect, which results in the efficiency of devices utilizing amorphous silicon dropping as the cell is exposed to light. The production of a-Si thin film solar cells uses glass as a substrate and deposits a very thin layer of silicon by plasma-enhanced chemical vapor deposition (PECVD). A-Si manufacturers are working towards lower costs per watt and higher conversion efficiency with continuous research and development on Multijunction solar cells for solar panels. Anwell Technologies Limited recently announced its target for multi-substrate-multichamber PECVD, to lower the cost to USD0.5 per watt.[36] Amorphous silicon has a higher bandgap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), which means it absorbs the visible part of the solar spectrum more strongly than the infrared portion of the spectrum. As nc-Si has about the same bandgap as c-Si, the nc-Si and a-Si can advantageously be combined in thin layers, creating a layered cell called a tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nc-Si. Recently, solutions to overcome the limitations of thin-film crystalline silicon have been developed. Light trapping schemes where the weakly absorbed long wavelength light is obliquely coupled into the silicon and traverses the film several times can significantly enhance the absorption of sunlight in the thin silicon films.[37] Minimizing the top contact coverage of the cell surface is another method for reducing optical losses; this approach simply aims at reducing the area that is covered over the cell to allow for maximum light input into the cell. Antireflective coatings can also be applied to create destructive interference within the cell. This can by done by modulating the Refractive index of the surface coating; if destructive interference is achieved, there will be no reflective wave and thus all light will be transmitted into the semiconductor cell. Surface texturing is another option, but may be less viable because it also increases the manufacturing price. By applying a texture to the surface of the solar cell, the reflected light can be refracted into striking the surface again, thus reducing the overall light reflected out. Light trapping as another method allows for a decrease in overall thickness of the device; the path length that the light will travel is several times the actual device thickness. This can be achieved by adding a textured backreflector to the device as well as texturing the surface. If both front and rear surfaces of the device meet this criteria, the light will be 'trapped' by not having an immediate pathway out of the device due to internal reflections. Thermal processing techniques can significantly enhance the crystal quality of the silicon and thereby lead to higher efficiencies of the final solar cells.[38] Further advancement into geometric considerations of building devices can exploit the dimensionality of nanomaterials. Creating large, parallel nanowire arrays enables long absoprtion lengths along the length of the wire while still maintaining short minority carrier diffusion lengths along the radial direction. Adding nanoparticles between the nanowires will allow for conduction through the device. Because of the natural geometry of these arrays, a textured surface will naturally form which allows for even more light to be trapped. A further advantage of this geometry is that these types of devices require about 100 times less material than conventional wafer-based devices. Manufacture

Early calculator solar battery Because solar cells are semiconductor devices, they share some of the same processing and manufacturing techniques as other semiconductor devices such as computer and memory chips. However, the stringent requirements for cleanliness and quality control of semiconductor fabrication are more relaxed for solar cells. Most large-scale commercial solar cell factories today make screen printed poly-crystalline or single crystalline silicon solar cells. Poly-crystalline silicon wafers are made by wire-sawing block-cast silicon ingots into very thin (180 to 350 micrometer) slices or wafers. The wafers are usually lightly p-type doped. To make a solar cell from the wafer, a surface diffusion of n-type dopants is performed on the front side of the wafer. This forms a p-n junction a few hundred nanometers below the surface. Antireflection coatings, to increase the amount of light coupled into the solar cell, are typically next applied. Silicon nitride has gradually replaced titanium dioxide as the antireflection coating because of its excellent surface passivation qualities. It prevents carrier recombination at the surface of the solar cell. It is typically applied in a layer several hundred nanometers thick using plasma-enhanced chemical vapor deposition (PECVD). Some solar cells have textured front surfaces that, like antireflection coatings, serve to increase the amount of light coupled into the cell. Such surfaces can usually only be formed on single-crystal silicon, though in recent years methods of forming them on multicrystalline silicon have been developed. The wafer then has a full area metal contact made on the back surface, and a grid-like metal contact made up of fine "fingers" and larger "busbars" are screen-printed onto the front surface using a silver paste. The rear contact is also formed by screen-printing a metal paste, typically

aluminium. Usually this contact covers the entire rear side of the cell, though in some cell designs it is printed in a grid pattern. The paste is then fired at several hundred degrees celsius to form metal electrodes in ohmic contact with the silicon. Some companies use an additional electro-plating step to increase the cell efficiency. After the metal contacts are made, the solar cells are interconnected in series (and/or parallel) by flat wires or metal ribbons, and assembled into modules or "solar panels". Solar panels have a sheet of tempered glass on the front, and a polymer encapsulation on the back. Lifespan Most commercially available solar panels are capable of producing electricity for at least twenty years. The typical warranty given by panel manufacturers is over 90% of rated output for the first 10 years, and over 80% for the second 10 years. Panels are expected to function for a period of 30 – 35 years.[39] China Backed by Chinese government's unprecedented plan to offer subsidies for utility-scale solar power projects that is likely to spark a new round of investment from Chinese solar panel makers. Chinese companies have already played a more important role in solar panels manufacturing in recent years. China produced solar cells/modules with an output of 13 GW in 2010 which represents about half of the global production and makes China the largest producer in the world.[41] Some Chinese companies such as Suntech Power, Yingli, LDK Solar Co, JA Solar and ReneSola have already announced projects in cooperation with regional governments with hundreds of megawatts each after the ‗Golden Sun‘ incentive program was announced by the government.[42] The rapid expansion of silicon and wafer production by GCL, China's largest private power producer, will further fuel China's growth as the world's solar manufacturer. United State Main article: Photovoltaics in the United States This section needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (August 2010)

New manufacturing facilities for solar cells and modules in Massachusetts, Michigan, New York, Ohio, Oregon, and Texas promise to add enough capacity to produce thousands of megawatts of solar devices per year within the next few years from 2008.[43] In late September 2008, Sanyo Electric Company, Ltd. announced its decision to build a manufacturing plant for solar ingots and wafers in Salem, Oregon. The plant began operating in October 2009 and reached its full production capacity of 70 megawatts (MW) of solar wafers per year in April 2010.

In early October 2008, First Solar, Inc. broke ground on an expansion of its Perrysburg, Ohio, facility that will add enough capacity to produce another 57 MW per year of solar modules at the facility, bringing its total capacity to roughly 192 MW per year. The company expects to complete construction early next year and reach full production by mid-2010. In mid-October 2008, SolarWorld AG opened a manufacturing plant in Hillsboro, Oregon, that is currently producing 500 MW of solar cells per year in 2011. Solyndra has a manufacturing facility for its unique tubular CIGS technology in California. In March 2010, SpectraWatt, Inc. began production at its manufacturing plant in Hopewell Junction, NY, which was expected to produce 120 MW of solar cells per year when it reached full production in 2011. However, the closure of this plant was announced in late 2010 due to deteriorating market conditions coupled with demand drops from Europe.[44]

Solar Power (Solar Cells)
Once restricted to exotic applications such as satellites, solar panels have found their way into our everyday lives through pocket. The meat of our "Electric Sandwich" is two slices of impure silicon. (More on that later.) On top of both of those layers is a conductive material that helps in harnessing the electric current made by the silicon. On top of the conductive material is an antireflective coating. This is important because you don't want what all that electricity-making calculators and electronic road signs. But how does something like that just create electricity from thin air? The process is really just a matter of elementary chemistry!

Making an Electric Sandwich: The Components of a Solar Cell
A solar cell or module is constructed of several different layers, all serving a different purpose light to bounce off. And on top of all of this is a sheet of glass to protect the delicate silicon and other layers. Below all of the silicon, conductive material, antireflective coating and glass, is another conductive material layer. By attaching wires to the top and bottom of your cell, you get fullfledged, electricity-producing, solar cell.

Impurities Are Your Friend: How Silicon Produces Electricity
If someone were trying to sell you an impure water, or impure gold you would rightly deny the transaction. But if someone tries to give you impure silicon, take it! That's the key to your solar cell. (Why else would you buy silicon?) Silicon is a semiconductor. You may have heard the term "semiconductor" before, but do you know what that means? It's very simple when you break the word down. "Semi" means "partial" and "conductor" can either be someone that drives a train or something that carries an electric current. And since something as dangerous as a train being "partially driven" doesn't create electricity, we

can assume that we're referring to the latter definition. Silicon is something that partially carries electricity. That means it's a poor conductor of electricity, unlike metals or your finger. Silicon atoms have 14 atoms in three electron levels. The third level of any atom usually holds 8 electrons. But poor deprived silicon, has but four. This means silicon atoms are on a constant search for four more atoms. Luckily, every silicon atom has four electrons in its outer level also! This means that wherever you see silicon, four more atoms are sure to follow, all arranged in a crystalline structure, with full electron levels. This means that all of the electrons are happy and won't budge from their cozy orbital levels. If electricity is directed through pure silicon crystals, it will heat up (as any semiconductor does) and some electrons will break free. These "free carriers" as they're called, will wander about until they find an opening in another crystalline structure. But when we remove some silicon atoms (say one per million) and replace them with phosphorous atoms, we get a whole new situation. Now there's an extra electron that does not participate in the crystalline structure. This left out electron is held in place only by the force of the atom's positively charged nucleus (an electron is negatively charged; and thus attracted.) When electricity is added to this setup, there are much more free carriers. This process of adding impurities is called doping. And a silicon sample doped with phosphorous is called N-type silicon (N for negative.) N-type silicon is the one of the two layers of silicon in a solar cell. The other layer is called P-Type silicon (P for Positive). P-type silicon is doped with boron, an atom with only three valence (outer) electrons. This creates perfect little holes for our N-type silicon electrons to fall into.

A Match Made In a Laboratory: The Relationship between N and P-Type Silicon
When N and P-type silicon come into contact, they create their own electric field, (this isn't where the electricity comes from, we haven't gotten to the sun's role in all of this, be patient.) This electric field is created when the electrons on the N-type silicon close-by fall into the holes in the P-type silicon. The result is a barrier between the positive and negative sides that allows electrons to travel one way. The barrier is called a diode. The diode allows electrons to travel from the P-type silicon to the N-type silicon, but not the other way around. When light gets into the fray, the whole puzzle starts to fit together.

Atomic Bumper Cars: Photons' role in Solar Cells
When light strikes the solar cell's silicon sheets, one photon will bump exactly one electron away to the N-type silicon. A photon will also rearrange the electrons on the N-type silicon and will effectively move a "hole" over to the P-type silicon. The result is a electron pump, that moves electrons from one side to the other. When a wire is attached on either type of silicon, the electrons follow the path of least resistance and travel along the wires. This flow of electrons creates a current, while the cell's electric field creates voltage. The two forces, voltage and current create power. The power that give you the answer to the math question on the SAT, and allows the electronic sides to tell you there's an accident at the next exit.

Even Impurities Aren't Perfect: The Limits of Solar Cells
Even though the sun produces about 1,000 watts of energy per square foot on a sunny day, a solar cell can use a maximum of about 25 percent of that. The problem isn't in the solar cell's

construction, but in the light itself. Light that the sun produces, spans an entire spectrum of frequencies and strengths. Thus, the photons that can bump the electrons are a select few. Most are either too powerful, or too weak. The only exception is if a photon can displace exactly two electrons. But this isn't enough to compensate for the loss. Another problem with solar cells, are the collection difficulties. As you expand the contact points of the wires, you block the sun. The result is a careful balance of electrode surface area and light collection surface area. Some solar cells overcome this problem with transparent conductors, but this is expensive and few employ such a technique.

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