Applications of Solar Energy
Content
NEU FACULTY OF ENGINEERING DEPARTMENT OF MECHANICAL ENGINEERING
APPLICATIONS OF SOLAR ENERGY: SOLAR HEATING SYSTEMS
PREPARED FOR: ME416 SOLAR ENERGY
PREPARED BY: OSUDE BENEDICT
May 5, 2011
0
PREFACE
The materials provided in this report cover as much information as possible for the conversion of solar energy to provide heating to domestic homes. their backgrounds.
In the first chapter, I will be making a general introduction to the solar heating systems and In chapter 2, I will be reviewing the types of solar heating systems and will mention a means to differentiate the available types. systems that fall under this group.
Chapter 3 is centered on the passive systems and an elaborate description of the types of of the types that make up this system.
The fifth chapter is all about the various components that make up the solar heating system and I will go into details; classes, functions. The last chapter is focused on the technological improvements that have been made to the solar heating systems and the future of the solar heaters.
In the fourth chapter, I will then be discussing the active heating systems and a description
1
ACKNOWLEDGEMENT
blessings all through the training.
First and foremost a special thanks to almighty God for his abundant
special thanks to you all.
To the authors of the published texts I referred to for information. A
the difficulties while getting accustomed the mill.
To my supervisor and the course lecturer that helped me through all
2
TABLE OF CONTENT
CHAPTER 1
CHAPTER 2
2.1 SOLAR WATER HEATING SYSTEMS………………………………………………………………………….7 CHAPTER 3 3.1 PASSIVE SYSTEMS…………………………………………………………………………………………………..9 CHAPTER 4
1.1 INTRODUCTION………………………………………………………………………………………………………6
TABLE OF FIGURES……………………………………………………………………………………………………..5
3.2 THERMOSIPHON……………………………………………………………………………………………………..9 4.1 ACTIVE SYSTEMS……………………………………………………………………………………………………12 4.2 Forced circulation water heater systems………………………………………………………………….12
4.3 Air-Water heating system………………………………………………………………………………………..14 CHAPTER 5 5.1 COMPONENTS OF A SOLAR HEATING SYSTEM…………………………..……………………………17 5.2 THERMAL STORAGE TANK…………………………………………………………….………………………17
4.4 Pool heating systems……………………………………………………………………………………………….15
5.2.1 Air system thermal storage…………………………………………………………….…………………….18
5.2.2 Liquid system thermal storage…………………………………………………………….………………..20 5.3 SOLAR COLLECTORS……………………………………………………………………………………………….22 5.3.1 Flat-Plate Collectors……………………………………………………………………………………………..23
3
5.3.2 Air Collectors……………………………………………………………………………………………………….27 5.3.3 Evacuated-Tube Collectors……………………………………………………..……………………………28 CHAPTER 6 6.1 TECHNOLOGICAL IMPROVEMENTS AND FUTURE OF SOLAR HEATERS……………………32 CHAPTER 7 7.1 CONCLUSION………………………………………………………………………………………………………….33 REFERENCES……………………………………………………………………………………………………………34
5.3.4 Concentrating Collectors………………………………………………………………………………………30
5.3.1.1 Liquid Collectors……………………………………………………………………………………………….25
4
TABLE OF FIGURES
A natural circulation water heater system………………………………………………………………………9 Forced circulation water heater system………………………………………………………………………..12
Water heater system with antifreeze loop and internal heat exchanger………………………….14 Water heater system with antifreeze loop and external heat exchanger…………………………15
Air system thermal storage……………………………………………………………………………………………19 Liquid system thermal storage………………………………………………………………………………………20 A typical flat-plate collector…………………………………………………………………………………………..23 Unglazed collectors………………………………………………………………………………………………………25
An air collector……………………………………………………………………………………………………………..27 Concentrating Collectors……………………………………………………………………………………………….30 An evacuated-tube collector………………………………………………………………………………………….28
5
CHAPTER 1 INTRODUCTION The origin and continuation of humankind is based on solar energy. The most basic solar energy is in humankinds benefit. and does not harm the environment.
processes supporting life on earth such as photosynthesis and the rain cycle are driven by Despite this, only recently, during the last 40 years, has solar energy been harnessed with
solar energy. From the very beginning of its history humankind realized that a good use of specialized equipment and used as an alternative source of energy, mainly because it is free The cost of fuel oil for residential heating has more than doubled in the past several years. become less available and not to mention the recent outbreak of chaos in the major oil energy has shown to be one of the promising sources of steady clean energy. producing nations. These situations have forced a research into other sources for not just Solar heating systems have been in use for years. Solar heat storage in iron was used in home. In the 1930s solar water heaters were in widespread but were quickly replaced been constructed and operated successfully over the past 40 years. The technology also simple and reliable in operation and relatively easy to install by the home owner. for the solar system components and their economic evaluations. water, by both active and passive processes. 1877 as reported, then it moved on to air blowing over heated metal been used to heat a Further cost increase in the future are likely as the limited supplies are consumed and fuels supplying our energy needs but friendly to the environment and at reasonable costs. Solar
when low cost natural gas became available in the 1940s. Several solar heated homes have involved in solar heating systems is therefore well understood. The components used are In this paper i will be dealing with the fundamental concepts and laws that determine the I will be dealing mostly with the application of solar energy systems in building for hot
operation of solar process components and systems, the formulation of working equations
6
CHAPTER 2 SOLAR WATER HEATING SYSTEMS A solar water heater is basically a combination of solar collector array, an energy transfer system, and a storage tank. The main part of the solar water heater is the solar collector heat transfer fluid (In most cases water, or in some cases air) that passes through the collector. The heat can then be stored or used directly. levels during periods of low energy demand. Because there are portions of the solar energy system that are exposed to weather There are two types of solar water heating systems:
array, which absorbs solar radiation and converts it to heat. The heat is then absorbed by a conditions, they must be protected from freezing and overheating caused by high insolation Direct(open) loop systems, in which potable water is heated directly in the collector Indirect (closed) loop systems, in which potable water is heated indirectly by a heat transfer fluid that is heated in the collector and passes through a heat exchanger to transfer its heat to the domestic or service water.
The solar water heating systems can also differ with respect to the way the heat transfer fluid is transported: Passive systems, in which the heat transfer fluid is circulated by heat driven convection or heat pipes water. They are cheap and have an extremely low overheating and freezing problems. systems: circulate the heat transfer fluid; even though it seems to be slightly more system design and allowing pre-existing storage tanks to be used.
Active systems in which mechanical components in the form of pumps are used to expensive than the passive systems they offer more advantages over the passive o The storage tank can be situated anywhere, allowing increased freedom in
maintenance cost, but on the other hand their efficiency is significantly low with
7
o Drain back tanks can be used. o Superior efficiency.
o The storage tank can always be hidden from view. reducing heat loss.
o The storage tank can be placed in conditioned or semi-conditioned space,
o Increased control over the system.
8
CHAPTER 3 3.1 PASSIVE SYSTEMS The main type of system that belong this category is the thermosiphon; 3.2 THERMOSIPHON
Fig. 3.1 A natural circulation water heater system
9
Thermosiphon systems heat potable heat transfer fluid and use natural convection to
transport it from the collector to either direct use or storage. The thermosiphoning effect by the action of solar radiation absorbed, the water in the collector is heated and thus expands, becoming less dense, and rises through the collector into the top of the storage which it flows down the collector. Circulation is continuous as long as the sun is shining. loss and sloped to prevent formation of air pockets, which would stop circulation. The advantages of thermosiphon systems are that they do not rely on pumps and
occurs because the density of water drops with the increase of the temperature. Therefore tank. There it is replaced by the cooler water that has sunk to the bottom of the tank, from be used to minimize pipe friction. Connecting lines are also well insulated to prevent heat controllers, are more reliable, and have a longer life than forced circulation systems. attractive aesthetically.
Since the driving force is only a small density difference, larger than normal pipe sizes must
Moreover, they do not require an electrical supply to operate and they naturally modulate The two types of thermosiphon systems are pressurized and unpressurized.
the circulation flow rate in phase with the radiation levels, while the main disadvantage of In pressurized thermosiphon units, the make-up water is from city mains or than the working pressure of the collectors and storage tank.
thermosiphon systems is that they are comparatively tall units, which makes them not very pressure units and the collectors and storage tanks must be able to withstand the working pressure. When city water is used directly, pressure-reducing and relief intermittent, a cold water storage tank is installed on top of the solar collector, makes the collector unit taller and less attractive. Another disadvantage of the passages.
In unpressurized systems, usually installed where the city water supply is
valves must be installed to protect the system because the pressure can be greater supplying both the hot water cylinder and the cold water needs of the house. This
Thermosiphon systems can be built with freeze protection devices, ranging from dump
system is related to the quality of the water used. As the system is open, extremely
valves or heaters in the bottom of the collector header for mild freeze areas to inherent
hard or acidic water can cause scale deposits that clog or corrode the absorber fluid
10
collector and the tank.
freeze resistance by using a natural circulation, antifreeze closed loop between the
11
CHAPTER 4 In active systems, a heat transfer fluid is pumped through the collectors. These are usually 4.1 ACTIVE SYSTEMS more expensive and a little less efficient than passive systems, particularly if antifreeze There are three types of systems belong in this category; 4.2 Forced circulation water heater systems: especially where there is no basement, because space is required for the additional equipment, such as the hot water cylinder.
measures are required. Additionally, active systems are more difficult to retrofit in houses,
the water, the collectors can be mounted either above or below the storage tank. Forced
In this system, a pump is used to circulate potable water from storage to the collectors
Fig. 4.1 Forced circulation water heater system
circulation systems often use a single storage tank equipped with an auxiliary water heater,
when there is enough available solar energy to increase its temperature and then return
the heated water to the storage tank until it is needed. Because a pump is used to circulate
12
but two-tank storage systems can also be used. An important feature of this configuration is the spring-loaded check valve, which is used to prevent reverse thermosiphon circulation or connected directly to city water mains. Pressure-reducing valves and pressure relief the collectors. valves are required, however, when the city water pressure is greater than the working energy losses when the pump is not running. Forced circulation systems can be used with water supplied from a cold water storage tank the water is extremely hard or acidic, because scale (calcium) deposits may clog or corrode Forced circulation systems can be used in areas where freezing is infrequent. For extreme weather conditions, freeze protection is usually provided by recirculating warm water Such recirculation freeze protection should be used only for locations where freezing occurs rarely (a few times a year), since stored heat is dumped in the process. A installed at the bottom of the collectors to provide additional protection. pump will not work and the system could freeze. In such a case, a dump valve can be from the storage tank. This loses some heat but protects the system. A special thermostat
pressure of the collectors. Forced water heating systems should not be used in areas where
that operates the pump when temperature drops below a certain value is used in this case. disadvantage of this system occurs in cases when there is power failure, in which case the For freeze protection, a variation of the forced circulation system, called the drain-down system is used. In this case, potable water is also pumped from storage to the collector make-up water supply with the normally closed valve and draining it using the two
normally open valves. It should be noted that the solar collectors and associated piping must be carefully sloped to drain the collector’s exterior piping when circulation stops.
array, where it is heated. Under a freezing condition or in the case of a power failure, the
systems drains automatically by isolating the collector array and exterior piping from the
13
4.3 Air-Water heating system:
Air systems are indirect water heating systems because air, circulated through air
Fig. 4.2 Water heater system with antifreeze loop and internal heat exchanger
collectors and via ductworks, is directed to an air-to-water heat exchanger. In the heat
exchanger, heat is transferred to the potable water, which is also circulated through the heat exchanger and returned to the storage tank. This type of system is used most often
because air systems are generally used for preheating domestic hot water and hence the auxiliary heater is used in only one tank. The advantages of this system are that air does not need to be protected from freezing or boiling, is non-corrosive, does not suffer from
handling equipment (ducts and fans) needs more space than piping and pumps, air leaks is generally higher than that of liquid systems.
are difficult to detect, and parasitic power consumption (electricity used to drive the fans)
heat transfer fluid degradation, and is free. Additionally, the system is more cost effective
because no safety valves or expansion vessels are required. The disadvantages are that air-
14
4.4 Pool heating systems
A radiant heating system is considered "open" any time the same hot water is used for both heating and domestic hot water. This type of system is very efficient because a single heat source (in this case, solar with a fossil fuel back-up) provides for all the home's hot water needs. In other words, the homeowner doesn't need two completely separate systems, the most common use of this category of system is in pool cooling. many times with overlapping mechanical components, performing separate heating tasks; as storage. In most cases, the pool’s filtration pump is used to circulate the water through solar panels or plastic pipes. For daylong operation, no automatic controls are required, because the pool usually operates when the sun is shining. If such controls are employed, they are used to direct the flow of filtered water to the collectors only when solar heat is off; thus the collectors are inherently freeze protected.
Fig. 4.3 Water heater system with antifreeze loop and external heat exchanger
Solar pool heating systems require no separate storage tank, because the pool itself serves
available. This can also be achieved by a simple manually operated valve. Normally, these Additionally, plastic pipes or tube-on-sheet panels can be used. In all cases, however, a large area is required and the roof of a nearby building can be used for this purpose.
kinds of solar systems are designed to drain down into the pool when the pump is switched
15
The pool heating load is the total heat loss less any heat gains from incident radiation. The total heat loss is the sum of losses due to evaporation, radiation, and convection. This calculation requires knowledge of the air temperature, wind speed, and relative humidity which at substantial quantities can lower the pool temperature. The addition of make-up temperature. Pools usually operate in a narrow temperature range of 24–32°C. Since the pool has a large mass, its temperature does not change quickly. The use of pool a cover will be in place. In addition, the cover may not have a perfect fit. Hence, a conservative approach should be taken when allowing for the effect of a cover.
or partial vapor pressure. Other causes of heat losses, which have a much smaller effect, are turbulence caused by swimmers, conduction to the ground (usually neglected), and rainfall, water should be considered if the temperature differs considerably from the pool operating cover reduces heat losses, particularly evaporative losses; however, when designing a solar pool-heating system, it is often not possible to know with certainty the times during which
16
CHAPTER 5 5.1 COMPONENTS OF A SOLAR HEATING SYSTEM There are three main components that make up a solar heating system Storage tank to store the solar heated water or air, potable water. Solar collector, which converts solar radiation to usable heat,
Heat exchanger module which transfers the heat from the solar collector to the
5.2 THERMAL STORAGE TANK Thermal storage is one of the main parts of a solar heating, cooling, and power generating necessary if the solar system must operate continuously. system. Because for approximately half the year any location is in darkness, heat storage is
For some applications, such as swimming pool heating, daytime air heating, and irrigation operating at night and when the sun is hidden behind clouds. Usually the design and selection of the thermal storage equipment is one of the most storage system has an enormous influence on overall system cost, performance, and such as the collector loop and the thermal distribution system. The most important functions of the storage tank are;
pumping, intermittent operation is acceptable, but most other uses of solar energy require
neglected elements of solar energy systems. It should be realized, however, that the energy reliability. Furthermore, the design of the storage system affects the other basic elements, Improvement of the utilization of collected solar energy by providing thermal system response to sudden peak loads or loss of solar input. quickly reaching high temperatures, which lower the collector efficiency. capacitance to alleviate the solar availability and load mismatch and improve the
Improvement of system efficiency by preventing the array heat transfer fluid from
17
Generally, solar energy can be stored in liquids, solids, or phase-change materials (PCM). Water is the most frequently used storage medium for liquid systems, even though the collector loop may use water, oils, water-glycol mixtures, or any other heat transfer easy to transport using conventional pumps and plumbing. For service water heating sometimes the structural mass of the building is used. a high storage capacity, based on both weight and volume. Additionally, as a liquid, it is tank, which is usually circular. Air systems typically store heat in rocks or pebbles, but be appropriate for the intended application. The lower the temperature of the fluid supplied to the collectors, the higher is the efficiency of the collectors.
applications and most building space heating, water is normally contained in some type of
medium as the collector fluid. This is because water is inexpensive and non-toxic and it has
The location of the storage tank should also be given careful consideration. The best outside above the ground or on the roof. arrays to avoid long pipe runs.
An important consideration is that the temperature of the fluid delivered to the load should
location is indoors, where thermal losses are minimal and weather deterioration will not be a factor. If the tank cannot be installed inside the building, then it should be located Such a storage tank should have a good insulation and good outside protection of the insulation. The storage tank should also be located as close as possible to the collector 5.2.1 Air system thermal storage. The most common storage media for air collectors are rocks. Other possible media include PCM, water, and the inherent building mass. Gravel is widely used as a storage medium because it is abundant and relatively inexpensive. be distributed directly to the space. In cases where large interior temperature swings can be tolerated, the inherent structure of the building may be sufficient for thermal storage. Loads requiring no storage are usually the most cost-effective applications of air collectors, and heated air from the collectors can
18
Generally, storage may be eliminated in cases where the array output seldom exceeds the thermal demand. The main requirements for gravel storage are good insulation, low air leakage, and low pressure drop. Many different designs can fulfill these requirements. these materials. Airflow can be vertical or horizontal. The container is usually constructed from concrete, masonry, wood, or a combination of
Fig. 5.1 Air systems thermal storage tank can work as effectively as a horizontal flow bed. In these systems, it is important to the opposite direction. In this way, pebble beds perform as effective counter-flow heat In this arrangement, the solar-heated air enters at the top and exits from the bottom. This
exchangers. The size of rocks used in the pebble bed range from 35 to 100 mm in diameter, depending on airflow, bed geometry, and desired pressure drop. The volume of the rock needed depends on the fraction of collector output that must be stored. For residential systems, storage volume is typically in the range of 0.15–0.3m3 per square meter of change materials and water. PCMs are functionally attractive because of their high a pebble bed. collector area. For large systems, pebble beds can be quite large and their large mass and
heat the bed with the hot air flow in one direction and to retrieve the heat with airflow in
volume may lead to location problems. Other storage options for air systems include phase volumetric heat storage capabilities, since they require only about one tenth the volume of
19
storage tank. This option has two advantages: pebble bed’s volume.
Water can also be used as a storage medium for air collectors through the use of a Water storage is compatible with hydraulic heating systems.
conventional water-to-air heat exchanger to transfer heat from the air to the water in the
It is relatively compact; the required storage water volume is roughly one third the
5.2.2 Liquid system thermal storage.
Fig. 5.2 Liquid system thermal storage There are two types of water storage for liquid systems available; Pressurized and unpressurized, the basic differentiation is the use of an external internal heat exchanger and single or multiple tank configurations. Water may be stored in copper, galvanized mains water supply. Pressurized storage is preferred for small service water heating insulated and large tanks should be provided with Pressurized systems are open to city systems, although in cases like Cyprus, where the water supply is intermittent, it is not
metal or concrete tanks. Whatever storage vessel is selected, however, this should be well suitable. Typical storage size is about 40 to 80 L per square meter of collector area. With
20
pressurized storage, the heat exchanger is always located on the collector side of the tank. Either internal or external heat exchanger configurations can be used. addition to the extra storage volume, increased heat exchanger surface (when a heat exchangers are connected in a reverse return mode to improve flow balance. one large one, if such a large-capacity tank is not available. Additional tanks offer, in
An external heat exchanger provides greater flexibility because the tank and the exchanger can be selected independently of other. The disadvantage of this system is the parasitic pump. energy consumption, in the form of electrical energy that occurs because of the additional For small systems, an internal heat exchanger–tank arrangement is usually used, which has
exchanger is used in each tank) and reduced pressure drop in the collection loop. The heat
Sometimes, because of the required storage volume, more than one tank is used instead of
the advantage of preventing the water side of the heat exchanger from freezing. However, the energy required to maintain the water above freezing is extracted from storage, thus exchanger without causing freezing or extraction of heat from storage. If necessary, this arrangement can also be used with internal heat exchangers to improve performance. cost effective than pressurized. This system, however, can also be employed in small domestic flat-plate collector systems, and in this case, the make-up water is usually supplied from a cold water storage tank located on top of the hot water cylinder. For systems with sizes greater than about 30 m3, unpressurized storage is usually more about 25°C. When the heat transfer fluid is warmed to this level, it can enter the heat
the overall system performance is decreased. With this system, a bypass can be arranged to
divert cold fluid around the heat exchanger until it has been heated to an acceptable level of
Unpressurized storage for water and space heating can be combined with the pressurized city water supply. This implies the use of a heat exchanger on the load side of the tank to cooled water is returned to the bottom of the tank so as not to distract stratification. For returns to the top. Where a heat transfer fluid is circulated in the collector loop, the heat isolate the high-pressure mains’ potable water loop from the low-pressure collector loop.
In an unpressurized system heat is extracted from the top of the solar storage tank and the the same reason, on the load side of the heat exchanger, the water to be heated flows from
the bottom of the backup storage tank, where relatively cold water exists, and heated water
21
exchanger may have a double-wall construction to protect the potable water supply from of the heat exchanger. When small pumps are used, both may be controlled by the same controller without overloading problems. The external heat exchanger provides good parasitic power consumption may be reduced by an internal heat exchanger. system flexibility and freedom in component selection. In some cases, system cost and
contamination. A differential temperature controller controls the two pumps on either side
Stratification is the collection of hot water to the top of the storage tank and cold water to the bottom. This improves the performance of the tank because hotter water is available at higher efficiency.
for use and colder water is supplied to the collectors, which enables the collector to operate 5.3 SOLAR COLLECTORS Solar collectors are the heart of most solar energy systems. The collector absorbs the sun’s light energy and changes it into heat energy. This publication describes the different types of solar collectors used for residences. It also briefly covers the solar heating systems for
which they are best suited indirectly the following;
Solar collectors heat a fluid, either air or liquid. This fluid then is used to heat directly or Water for household use Indoor spaces
Water for swimming pools
There are several types of solar collectors used for residences.
Water or air for commercial use
Air to regenerate desiccant (drying) material in a desiccant cooling system.
22
5.3.1 Flat-Plate Collectors
Fig. 5.3 A typical flat-plate collector space-heating installations. A typical flat plate collector is an insulated metal box with a percentage of the total available solar energy. The glazing allows the light to strike the be transparent or translucent. Translucent (transmitting light only), low-iron glass is a Flat-plate collectors are the most common collector for residential water-heating and
glass or plastic cover called the glazing and a dark-colored absorber plate. The glazing can common glazing material for flat-plate collectors because low-iron glass transmits a high
black because dark colors absorb more solar energy than light colors. Sunlight passes
through the glazing and strikes the absorber plate, which heats up, changing solar radiation
absorber plate but reduces the amount of heat that can escape. The sides and bottom of the collector are usually insulated, further minimizing heat loss. The absorber plate is usually
23
into heat energy. The heat is transferred to the air or liquid passing through the collector. Absorber plates are commonly covered with “selective coatings,” which retain the are often made of metal usually copper or aluminum because they are both good heat conductors. Copper is more expensive, but is a better conductor and is less prone to categories; corrosion than aluminum. Flat-plate collectors fall into two basic glazed or unglazed
absorbed sunlight better and are more durable than ordinary black paint. Absorber plates
24
5.3.1.1 Liquid Collectors
Fig. 5.4 Unglazed collectors In a liquid collector, solar energy heats a liquid as it flows through tubes in or adjacent to the absorber plate. For this type of collector, the flow tubes are attached to the absorber plate so the heat absorbed by the absorber plate is readily conducted to the liquid. pattern. A serpentine pattern eliminates the possibility of header leaks and ensures uniform flow. A serpentine pattern is not appropriate, however, for systems that must
The flow tubes can be routed in parallel, using inlet and outlet headers, or in a serpentine
25
through the collector and then flows to the house to be used for domestic purposes. the temperature drops or use an antifreeze type of heat-transfer fluid. loop” (or “indirect”) systems
drain for freeze protection because the curved flow passages will not drain completely. The simplest liquid systems use potable household water, which is heated as it passes directly This design is known as an “open-loop” (or “direct”) system. In areas where freezing temperatures are common, however, liquid collectors must either drain the water when
In systems with heat-transfer fluids, the transfer fluid absorbs heat from the collector and storage tank inside the house, transfers heat to the water. Such designs are called “closed Glazed liquid collectors are used for heating household water and sometimes for space because swimming pools are generally used only in warm weather. Because these collectors need not withstand high temperatures, they can use less
then passes through a heat exchanger. The heat exchanger, which generally is in the water
heating. Unglazed liquid collectors are commonly used to heat water for swimming pools. expensive materials such as plastic or rubber. They also do not require freeze-proofing
26
5.3.2 Air Collectors
Fig. 5.5 An air collector Air collectors are simple, flat-plate collectors used primarily for space heating. The absorber plates in air collectors can be metal sheets, layers of screen, or nonmetallic
routed between the absorber plate and the back insulation to reduce heat loss through the
The disadvantage of this strategy is that it can also increase the amount of power needed
for fans and, thus, increase the costs of operating the system. In colder climates, the air is
systems, fins or corrugations on the absorber are used to increase air turbulence and improve heat transfer.
materials. The air flows past the absorber by natural convection or when forced by a fan. Because air conducts heat much less readily than liquid does, less heat is transferred between the air and the absorber than in a liquid collector. In some solar air-heating
27
glazing. However, if the air will not be heated more than 17°C above the outdoor efficiency.
with liquid systems. Although leaks are harder to detect and plug in an air system, they are also less troublesome than leaks in a liquid system. operating temperatures are usually lower than those of liquid collectors. 5.3.3 Evacuated-Tube Collectors Air systems can often use less expensive materials, such as plastic glazing, because their
temperature, the air can flow on both sides of the absorber plate without sacrificing
Air systems have the advantage of eliminating the freezing and boiling problems associated
Fig. 5.6 An evacuated-tube collector Evacuated-tube collectors heat water in residential applications that require higher temperatures. In an evacuated-tube collector, sunlight enters through the outer glass tube,
strikes the absorber tube, and changes to heat. The heat is transferred to the liquid flowing
28
collector) covered with a selective coating. Evacuated-tube collectors are modular tubes can be added or removed as hot-water needs change. tubes, forming a vacuum. Conductive and convective heat losses are eliminated because some radiant heat loss (heat energy will move through space from a warmer to a cooler surface, even across a vacuum). However, this loss is small and of little consequence compared with the amount of heat transferred to the liquid in the absorber tube.
through the absorber tube. The collector consists of rows of parallel transparent glass
tubes, each of which contains an absorber tube (in place of the absorber plate in a flat-plate When evacuated tubes are manufactured, air is evacuated from the space between the two
there is no air to conduct heat or to circulate and cause convective losses. There can still be Evacuated-tube collectors are available in a number of designs. Some use a third glass tube
inside the absorber tube or other configurations of heat-transfer fins and fluid tubes. One eliminating the need for a separate solar storage tank. Reflectors placed behind the evacuated tubes can help to focus additional sunlight on the collector. collectors particularly useful in areas with cold, cloudy winters. These collectors are more efficient than flat plate collectors for a couple of reasons. commercially available evacuated-tube collector stores 19 liters of water in each tube,
First, they perform well in both direct and diffuse solar radiation. This characteristic,
collectors achieve both higher temperatures and higher efficiencies than flat-plate collectors, they are also more expensive.
position, the sun is only perpendicular to the collector at noon. While evacuated-tube
absorber for most of the day. For comparison, in a flat-plate collector that is in a fixed
combined with the fact that the vacuum minimizes heat losses to the outdoors, makes these
Second, because of the circular shape of the evacuated tube, sunlight is perpendicular to the
29
5.3.4 Concentrating Collectors
Fig. 5.7 Concentrating Collectors Concentrating collectors use mirrored surfaces to concentrate the sun’s energy on an to achieve high temperatures. absorber called a receiver. Concentrating collectors also achieve high temperatures, but Some designs concentrate solar energy onto a focal point, while others concentrate the along the focal line. A heat-transfer fluid flows through the receiver and absorbs heat. These collectors reach much higher temperatures than flat-plate collectors. However, concentrators can only focus direct solar radiation, with the result being that their
unlike evacuated-tube collectors, they can do so only when direct sunlight is available. The mirrored surface focuses sunlight collected over a large area onto a smaller absorber area sun’s rays along a thin line called the focal line. The receiver is located at the focal point or
30
tracking mechanisms to move the collectors during the day to keep them focused on the sun. Single-axis trackers move east to west; dual-axis trackers move east and west and north and south (to follow the sun throughout the year). In addition to these mechanical trackers, there are passive trackers that use Freon to alternative to mechanical systems. Concentrators are used mostly in commercial supply the movement. While not widely used, they do provide a low-maintenance
performance is poor on hazy or cloudy days. Concentrators are most practical in areas of high insolation (exposure to the sun’s rays), such as those close to the equator.
Concentrators perform best when pointed directly at the sun. To do this, these systems use
applications because they are expensive and because the trackers need frequent than dual-axis trackers.
maintenance. Some residential solar energy systems use parabolic-trough concentrating Most residential systems use single-axis trackers, which are less expensive and simpler
systems. These installations can provide hot water, space heating, and water purification.
31
CHAPTER 6 6.1 TECHNOLOGICAL IMPROVEMENTS AND FUTURE OF SOLAR HEATERS The efficiency of solar heating systems and collectors has improved from the early 1970s and costs have dropped somewhat. The efficiencies can be attributed to the use of low-iron,
tempered glass for glazing (low-iron glass allows the transmission of more solar energy coatings.
capable of heating 0.002 cubic meters per second of outside air. On a sunny winter day, the panel can produce temperatures up to 28°C higher than the outdoor air temperature. Transpired air collectors not only heat air, but also improve indoor air quality by directly preheating fresh outdoor air. These collectors have achieved very high efficiencies more or insulation, they are inexpensive to manufacture. All these factors make transpired air collectors a very cost-effective source of solar heat. There are other prototype cooling systems operating today. Some use heat from solar than 70% in some commercial applications. Plus, because the collectors require no glazing collectors for absorption cooling. Others are being used to renew the desiccant material in
heats the metal, and a fan pulls air through the holes in the metal, which heats the air. For residential installations, these collectors are available in 2.4-meter by 0.8-meter panels
available for homes. Called a transpired collector, it eliminates the cost of the glazing, the metal box, and the insulation. This collector is made of black, perforated metal. The sun
than conventional glass), improved insulation, and the development of durable selective Also, a new solar air collector, formerly used primarily for commercial buildings, is now
desiccant cooling systems. Desiccants, such as silica gel, naturally attract moisture. They are used to reduce humidity and the resulting cooling loads in hot, humid climates. collectors will be integrated into many applications. Solar collectors can be used for nearly any process that requires heat. As environmental
laws become stricter and the price of conventional power increases, it is likely that solar
32
CHAPTER 7 7.1 CONCLUSION Water heating is a practical application of solar energy in many parts of the world. Natural circulation systems are widely used in climates where freezing does not occur. Forced are used in climates where freezing is a problem. circulation systems using drain down or non-freezing fluids in collector exchanger loops must be taken to use auxiliary in such a way it does not drive up collector temperature. Auxiliary energy is used in essentially all systems where high reliability is wanted and care
33
REFERENCES Books, Pamphlets, and Reports S. Sklar and K. Sheinkopf, Bonus Books, Inc., Consumer Guide to Solar Energy, 160 East Illinois Street, The Fuel Savers, Chicago, IL 60611, 1991. Lafayette, CA, 1991. Brick House, Periodicals Home Energy Magazine, Berkeley. 2124 Kittredge Street, No. 95,
B. Anderson, Morning Sun Press, B. Anderson and M. Riorden, The New Solar Home Book, NH. Amherst.
Solar Energy Industries Association, Solar Industry Journal, NW. Washington DC. Solar Today, Unit G-1, 4th Floor 122 C Street, 2400 Central Avenue, Boulder, CO 80301.
34
APPLICATIONS OF SOLAR ENERGY: SOLAR HEATING SYSTEMS
PREPARED FOR: ME416 SOLAR ENERGY
PREPARED BY: OSUDE BENEDICT
May 5, 2011
0
PREFACE
The materials provided in this report cover as much information as possible for the conversion of solar energy to provide heating to domestic homes. their backgrounds.
In the first chapter, I will be making a general introduction to the solar heating systems and In chapter 2, I will be reviewing the types of solar heating systems and will mention a means to differentiate the available types. systems that fall under this group.
Chapter 3 is centered on the passive systems and an elaborate description of the types of of the types that make up this system.
The fifth chapter is all about the various components that make up the solar heating system and I will go into details; classes, functions. The last chapter is focused on the technological improvements that have been made to the solar heating systems and the future of the solar heaters.
In the fourth chapter, I will then be discussing the active heating systems and a description
1
ACKNOWLEDGEMENT
blessings all through the training.
First and foremost a special thanks to almighty God for his abundant
special thanks to you all.
To the authors of the published texts I referred to for information. A
the difficulties while getting accustomed the mill.
To my supervisor and the course lecturer that helped me through all
2
TABLE OF CONTENT
CHAPTER 1
CHAPTER 2
2.1 SOLAR WATER HEATING SYSTEMS………………………………………………………………………….7 CHAPTER 3 3.1 PASSIVE SYSTEMS…………………………………………………………………………………………………..9 CHAPTER 4
1.1 INTRODUCTION………………………………………………………………………………………………………6
TABLE OF FIGURES……………………………………………………………………………………………………..5
3.2 THERMOSIPHON……………………………………………………………………………………………………..9 4.1 ACTIVE SYSTEMS……………………………………………………………………………………………………12 4.2 Forced circulation water heater systems………………………………………………………………….12
4.3 Air-Water heating system………………………………………………………………………………………..14 CHAPTER 5 5.1 COMPONENTS OF A SOLAR HEATING SYSTEM…………………………..……………………………17 5.2 THERMAL STORAGE TANK…………………………………………………………….………………………17
4.4 Pool heating systems……………………………………………………………………………………………….15
5.2.1 Air system thermal storage…………………………………………………………….…………………….18
5.2.2 Liquid system thermal storage…………………………………………………………….………………..20 5.3 SOLAR COLLECTORS……………………………………………………………………………………………….22 5.3.1 Flat-Plate Collectors……………………………………………………………………………………………..23
3
5.3.2 Air Collectors……………………………………………………………………………………………………….27 5.3.3 Evacuated-Tube Collectors……………………………………………………..……………………………28 CHAPTER 6 6.1 TECHNOLOGICAL IMPROVEMENTS AND FUTURE OF SOLAR HEATERS……………………32 CHAPTER 7 7.1 CONCLUSION………………………………………………………………………………………………………….33 REFERENCES……………………………………………………………………………………………………………34
5.3.4 Concentrating Collectors………………………………………………………………………………………30
5.3.1.1 Liquid Collectors……………………………………………………………………………………………….25
4
TABLE OF FIGURES
A natural circulation water heater system………………………………………………………………………9 Forced circulation water heater system………………………………………………………………………..12
Water heater system with antifreeze loop and internal heat exchanger………………………….14 Water heater system with antifreeze loop and external heat exchanger…………………………15
Air system thermal storage……………………………………………………………………………………………19 Liquid system thermal storage………………………………………………………………………………………20 A typical flat-plate collector…………………………………………………………………………………………..23 Unglazed collectors………………………………………………………………………………………………………25
An air collector……………………………………………………………………………………………………………..27 Concentrating Collectors……………………………………………………………………………………………….30 An evacuated-tube collector………………………………………………………………………………………….28
5
CHAPTER 1 INTRODUCTION The origin and continuation of humankind is based on solar energy. The most basic solar energy is in humankinds benefit. and does not harm the environment.
processes supporting life on earth such as photosynthesis and the rain cycle are driven by Despite this, only recently, during the last 40 years, has solar energy been harnessed with
solar energy. From the very beginning of its history humankind realized that a good use of specialized equipment and used as an alternative source of energy, mainly because it is free The cost of fuel oil for residential heating has more than doubled in the past several years. become less available and not to mention the recent outbreak of chaos in the major oil energy has shown to be one of the promising sources of steady clean energy. producing nations. These situations have forced a research into other sources for not just Solar heating systems have been in use for years. Solar heat storage in iron was used in home. In the 1930s solar water heaters were in widespread but were quickly replaced been constructed and operated successfully over the past 40 years. The technology also simple and reliable in operation and relatively easy to install by the home owner. for the solar system components and their economic evaluations. water, by both active and passive processes. 1877 as reported, then it moved on to air blowing over heated metal been used to heat a Further cost increase in the future are likely as the limited supplies are consumed and fuels supplying our energy needs but friendly to the environment and at reasonable costs. Solar
when low cost natural gas became available in the 1940s. Several solar heated homes have involved in solar heating systems is therefore well understood. The components used are In this paper i will be dealing with the fundamental concepts and laws that determine the I will be dealing mostly with the application of solar energy systems in building for hot
operation of solar process components and systems, the formulation of working equations
6
CHAPTER 2 SOLAR WATER HEATING SYSTEMS A solar water heater is basically a combination of solar collector array, an energy transfer system, and a storage tank. The main part of the solar water heater is the solar collector heat transfer fluid (In most cases water, or in some cases air) that passes through the collector. The heat can then be stored or used directly. levels during periods of low energy demand. Because there are portions of the solar energy system that are exposed to weather There are two types of solar water heating systems:
array, which absorbs solar radiation and converts it to heat. The heat is then absorbed by a conditions, they must be protected from freezing and overheating caused by high insolation Direct(open) loop systems, in which potable water is heated directly in the collector Indirect (closed) loop systems, in which potable water is heated indirectly by a heat transfer fluid that is heated in the collector and passes through a heat exchanger to transfer its heat to the domestic or service water.
The solar water heating systems can also differ with respect to the way the heat transfer fluid is transported: Passive systems, in which the heat transfer fluid is circulated by heat driven convection or heat pipes water. They are cheap and have an extremely low overheating and freezing problems. systems: circulate the heat transfer fluid; even though it seems to be slightly more system design and allowing pre-existing storage tanks to be used.
Active systems in which mechanical components in the form of pumps are used to expensive than the passive systems they offer more advantages over the passive o The storage tank can be situated anywhere, allowing increased freedom in
maintenance cost, but on the other hand their efficiency is significantly low with
7
o Drain back tanks can be used. o Superior efficiency.
o The storage tank can always be hidden from view. reducing heat loss.
o The storage tank can be placed in conditioned or semi-conditioned space,
o Increased control over the system.
8
CHAPTER 3 3.1 PASSIVE SYSTEMS The main type of system that belong this category is the thermosiphon; 3.2 THERMOSIPHON
Fig. 3.1 A natural circulation water heater system
9
Thermosiphon systems heat potable heat transfer fluid and use natural convection to
transport it from the collector to either direct use or storage. The thermosiphoning effect by the action of solar radiation absorbed, the water in the collector is heated and thus expands, becoming less dense, and rises through the collector into the top of the storage which it flows down the collector. Circulation is continuous as long as the sun is shining. loss and sloped to prevent formation of air pockets, which would stop circulation. The advantages of thermosiphon systems are that they do not rely on pumps and
occurs because the density of water drops with the increase of the temperature. Therefore tank. There it is replaced by the cooler water that has sunk to the bottom of the tank, from be used to minimize pipe friction. Connecting lines are also well insulated to prevent heat controllers, are more reliable, and have a longer life than forced circulation systems. attractive aesthetically.
Since the driving force is only a small density difference, larger than normal pipe sizes must
Moreover, they do not require an electrical supply to operate and they naturally modulate The two types of thermosiphon systems are pressurized and unpressurized.
the circulation flow rate in phase with the radiation levels, while the main disadvantage of In pressurized thermosiphon units, the make-up water is from city mains or than the working pressure of the collectors and storage tank.
thermosiphon systems is that they are comparatively tall units, which makes them not very pressure units and the collectors and storage tanks must be able to withstand the working pressure. When city water is used directly, pressure-reducing and relief intermittent, a cold water storage tank is installed on top of the solar collector, makes the collector unit taller and less attractive. Another disadvantage of the passages.
In unpressurized systems, usually installed where the city water supply is
valves must be installed to protect the system because the pressure can be greater supplying both the hot water cylinder and the cold water needs of the house. This
Thermosiphon systems can be built with freeze protection devices, ranging from dump
system is related to the quality of the water used. As the system is open, extremely
valves or heaters in the bottom of the collector header for mild freeze areas to inherent
hard or acidic water can cause scale deposits that clog or corrode the absorber fluid
10
collector and the tank.
freeze resistance by using a natural circulation, antifreeze closed loop between the
11
CHAPTER 4 In active systems, a heat transfer fluid is pumped through the collectors. These are usually 4.1 ACTIVE SYSTEMS more expensive and a little less efficient than passive systems, particularly if antifreeze There are three types of systems belong in this category; 4.2 Forced circulation water heater systems: especially where there is no basement, because space is required for the additional equipment, such as the hot water cylinder.
measures are required. Additionally, active systems are more difficult to retrofit in houses,
the water, the collectors can be mounted either above or below the storage tank. Forced
In this system, a pump is used to circulate potable water from storage to the collectors
Fig. 4.1 Forced circulation water heater system
circulation systems often use a single storage tank equipped with an auxiliary water heater,
when there is enough available solar energy to increase its temperature and then return
the heated water to the storage tank until it is needed. Because a pump is used to circulate
12
but two-tank storage systems can also be used. An important feature of this configuration is the spring-loaded check valve, which is used to prevent reverse thermosiphon circulation or connected directly to city water mains. Pressure-reducing valves and pressure relief the collectors. valves are required, however, when the city water pressure is greater than the working energy losses when the pump is not running. Forced circulation systems can be used with water supplied from a cold water storage tank the water is extremely hard or acidic, because scale (calcium) deposits may clog or corrode Forced circulation systems can be used in areas where freezing is infrequent. For extreme weather conditions, freeze protection is usually provided by recirculating warm water Such recirculation freeze protection should be used only for locations where freezing occurs rarely (a few times a year), since stored heat is dumped in the process. A installed at the bottom of the collectors to provide additional protection. pump will not work and the system could freeze. In such a case, a dump valve can be from the storage tank. This loses some heat but protects the system. A special thermostat
pressure of the collectors. Forced water heating systems should not be used in areas where
that operates the pump when temperature drops below a certain value is used in this case. disadvantage of this system occurs in cases when there is power failure, in which case the For freeze protection, a variation of the forced circulation system, called the drain-down system is used. In this case, potable water is also pumped from storage to the collector make-up water supply with the normally closed valve and draining it using the two
normally open valves. It should be noted that the solar collectors and associated piping must be carefully sloped to drain the collector’s exterior piping when circulation stops.
array, where it is heated. Under a freezing condition or in the case of a power failure, the
systems drains automatically by isolating the collector array and exterior piping from the
13
4.3 Air-Water heating system:
Air systems are indirect water heating systems because air, circulated through air
Fig. 4.2 Water heater system with antifreeze loop and internal heat exchanger
collectors and via ductworks, is directed to an air-to-water heat exchanger. In the heat
exchanger, heat is transferred to the potable water, which is also circulated through the heat exchanger and returned to the storage tank. This type of system is used most often
because air systems are generally used for preheating domestic hot water and hence the auxiliary heater is used in only one tank. The advantages of this system are that air does not need to be protected from freezing or boiling, is non-corrosive, does not suffer from
handling equipment (ducts and fans) needs more space than piping and pumps, air leaks is generally higher than that of liquid systems.
are difficult to detect, and parasitic power consumption (electricity used to drive the fans)
heat transfer fluid degradation, and is free. Additionally, the system is more cost effective
because no safety valves or expansion vessels are required. The disadvantages are that air-
14
4.4 Pool heating systems
A radiant heating system is considered "open" any time the same hot water is used for both heating and domestic hot water. This type of system is very efficient because a single heat source (in this case, solar with a fossil fuel back-up) provides for all the home's hot water needs. In other words, the homeowner doesn't need two completely separate systems, the most common use of this category of system is in pool cooling. many times with overlapping mechanical components, performing separate heating tasks; as storage. In most cases, the pool’s filtration pump is used to circulate the water through solar panels or plastic pipes. For daylong operation, no automatic controls are required, because the pool usually operates when the sun is shining. If such controls are employed, they are used to direct the flow of filtered water to the collectors only when solar heat is off; thus the collectors are inherently freeze protected.
Fig. 4.3 Water heater system with antifreeze loop and external heat exchanger
Solar pool heating systems require no separate storage tank, because the pool itself serves
available. This can also be achieved by a simple manually operated valve. Normally, these Additionally, plastic pipes or tube-on-sheet panels can be used. In all cases, however, a large area is required and the roof of a nearby building can be used for this purpose.
kinds of solar systems are designed to drain down into the pool when the pump is switched
15
The pool heating load is the total heat loss less any heat gains from incident radiation. The total heat loss is the sum of losses due to evaporation, radiation, and convection. This calculation requires knowledge of the air temperature, wind speed, and relative humidity which at substantial quantities can lower the pool temperature. The addition of make-up temperature. Pools usually operate in a narrow temperature range of 24–32°C. Since the pool has a large mass, its temperature does not change quickly. The use of pool a cover will be in place. In addition, the cover may not have a perfect fit. Hence, a conservative approach should be taken when allowing for the effect of a cover.
or partial vapor pressure. Other causes of heat losses, which have a much smaller effect, are turbulence caused by swimmers, conduction to the ground (usually neglected), and rainfall, water should be considered if the temperature differs considerably from the pool operating cover reduces heat losses, particularly evaporative losses; however, when designing a solar pool-heating system, it is often not possible to know with certainty the times during which
16
CHAPTER 5 5.1 COMPONENTS OF A SOLAR HEATING SYSTEM There are three main components that make up a solar heating system Storage tank to store the solar heated water or air, potable water. Solar collector, which converts solar radiation to usable heat,
Heat exchanger module which transfers the heat from the solar collector to the
5.2 THERMAL STORAGE TANK Thermal storage is one of the main parts of a solar heating, cooling, and power generating necessary if the solar system must operate continuously. system. Because for approximately half the year any location is in darkness, heat storage is
For some applications, such as swimming pool heating, daytime air heating, and irrigation operating at night and when the sun is hidden behind clouds. Usually the design and selection of the thermal storage equipment is one of the most storage system has an enormous influence on overall system cost, performance, and such as the collector loop and the thermal distribution system. The most important functions of the storage tank are;
pumping, intermittent operation is acceptable, but most other uses of solar energy require
neglected elements of solar energy systems. It should be realized, however, that the energy reliability. Furthermore, the design of the storage system affects the other basic elements, Improvement of the utilization of collected solar energy by providing thermal system response to sudden peak loads or loss of solar input. quickly reaching high temperatures, which lower the collector efficiency. capacitance to alleviate the solar availability and load mismatch and improve the
Improvement of system efficiency by preventing the array heat transfer fluid from
17
Generally, solar energy can be stored in liquids, solids, or phase-change materials (PCM). Water is the most frequently used storage medium for liquid systems, even though the collector loop may use water, oils, water-glycol mixtures, or any other heat transfer easy to transport using conventional pumps and plumbing. For service water heating sometimes the structural mass of the building is used. a high storage capacity, based on both weight and volume. Additionally, as a liquid, it is tank, which is usually circular. Air systems typically store heat in rocks or pebbles, but be appropriate for the intended application. The lower the temperature of the fluid supplied to the collectors, the higher is the efficiency of the collectors.
applications and most building space heating, water is normally contained in some type of
medium as the collector fluid. This is because water is inexpensive and non-toxic and it has
The location of the storage tank should also be given careful consideration. The best outside above the ground or on the roof. arrays to avoid long pipe runs.
An important consideration is that the temperature of the fluid delivered to the load should
location is indoors, where thermal losses are minimal and weather deterioration will not be a factor. If the tank cannot be installed inside the building, then it should be located Such a storage tank should have a good insulation and good outside protection of the insulation. The storage tank should also be located as close as possible to the collector 5.2.1 Air system thermal storage. The most common storage media for air collectors are rocks. Other possible media include PCM, water, and the inherent building mass. Gravel is widely used as a storage medium because it is abundant and relatively inexpensive. be distributed directly to the space. In cases where large interior temperature swings can be tolerated, the inherent structure of the building may be sufficient for thermal storage. Loads requiring no storage are usually the most cost-effective applications of air collectors, and heated air from the collectors can
18
Generally, storage may be eliminated in cases where the array output seldom exceeds the thermal demand. The main requirements for gravel storage are good insulation, low air leakage, and low pressure drop. Many different designs can fulfill these requirements. these materials. Airflow can be vertical or horizontal. The container is usually constructed from concrete, masonry, wood, or a combination of
Fig. 5.1 Air systems thermal storage tank can work as effectively as a horizontal flow bed. In these systems, it is important to the opposite direction. In this way, pebble beds perform as effective counter-flow heat In this arrangement, the solar-heated air enters at the top and exits from the bottom. This
exchangers. The size of rocks used in the pebble bed range from 35 to 100 mm in diameter, depending on airflow, bed geometry, and desired pressure drop. The volume of the rock needed depends on the fraction of collector output that must be stored. For residential systems, storage volume is typically in the range of 0.15–0.3m3 per square meter of change materials and water. PCMs are functionally attractive because of their high a pebble bed. collector area. For large systems, pebble beds can be quite large and their large mass and
heat the bed with the hot air flow in one direction and to retrieve the heat with airflow in
volume may lead to location problems. Other storage options for air systems include phase volumetric heat storage capabilities, since they require only about one tenth the volume of
19
storage tank. This option has two advantages: pebble bed’s volume.
Water can also be used as a storage medium for air collectors through the use of a Water storage is compatible with hydraulic heating systems.
conventional water-to-air heat exchanger to transfer heat from the air to the water in the
It is relatively compact; the required storage water volume is roughly one third the
5.2.2 Liquid system thermal storage.
Fig. 5.2 Liquid system thermal storage There are two types of water storage for liquid systems available; Pressurized and unpressurized, the basic differentiation is the use of an external internal heat exchanger and single or multiple tank configurations. Water may be stored in copper, galvanized mains water supply. Pressurized storage is preferred for small service water heating insulated and large tanks should be provided with Pressurized systems are open to city systems, although in cases like Cyprus, where the water supply is intermittent, it is not
metal or concrete tanks. Whatever storage vessel is selected, however, this should be well suitable. Typical storage size is about 40 to 80 L per square meter of collector area. With
20
pressurized storage, the heat exchanger is always located on the collector side of the tank. Either internal or external heat exchanger configurations can be used. addition to the extra storage volume, increased heat exchanger surface (when a heat exchangers are connected in a reverse return mode to improve flow balance. one large one, if such a large-capacity tank is not available. Additional tanks offer, in
An external heat exchanger provides greater flexibility because the tank and the exchanger can be selected independently of other. The disadvantage of this system is the parasitic pump. energy consumption, in the form of electrical energy that occurs because of the additional For small systems, an internal heat exchanger–tank arrangement is usually used, which has
exchanger is used in each tank) and reduced pressure drop in the collection loop. The heat
Sometimes, because of the required storage volume, more than one tank is used instead of
the advantage of preventing the water side of the heat exchanger from freezing. However, the energy required to maintain the water above freezing is extracted from storage, thus exchanger without causing freezing or extraction of heat from storage. If necessary, this arrangement can also be used with internal heat exchangers to improve performance. cost effective than pressurized. This system, however, can also be employed in small domestic flat-plate collector systems, and in this case, the make-up water is usually supplied from a cold water storage tank located on top of the hot water cylinder. For systems with sizes greater than about 30 m3, unpressurized storage is usually more about 25°C. When the heat transfer fluid is warmed to this level, it can enter the heat
the overall system performance is decreased. With this system, a bypass can be arranged to
divert cold fluid around the heat exchanger until it has been heated to an acceptable level of
Unpressurized storage for water and space heating can be combined with the pressurized city water supply. This implies the use of a heat exchanger on the load side of the tank to cooled water is returned to the bottom of the tank so as not to distract stratification. For returns to the top. Where a heat transfer fluid is circulated in the collector loop, the heat isolate the high-pressure mains’ potable water loop from the low-pressure collector loop.
In an unpressurized system heat is extracted from the top of the solar storage tank and the the same reason, on the load side of the heat exchanger, the water to be heated flows from
the bottom of the backup storage tank, where relatively cold water exists, and heated water
21
exchanger may have a double-wall construction to protect the potable water supply from of the heat exchanger. When small pumps are used, both may be controlled by the same controller without overloading problems. The external heat exchanger provides good parasitic power consumption may be reduced by an internal heat exchanger. system flexibility and freedom in component selection. In some cases, system cost and
contamination. A differential temperature controller controls the two pumps on either side
Stratification is the collection of hot water to the top of the storage tank and cold water to the bottom. This improves the performance of the tank because hotter water is available at higher efficiency.
for use and colder water is supplied to the collectors, which enables the collector to operate 5.3 SOLAR COLLECTORS Solar collectors are the heart of most solar energy systems. The collector absorbs the sun’s light energy and changes it into heat energy. This publication describes the different types of solar collectors used for residences. It also briefly covers the solar heating systems for
which they are best suited indirectly the following;
Solar collectors heat a fluid, either air or liquid. This fluid then is used to heat directly or Water for household use Indoor spaces
Water for swimming pools
There are several types of solar collectors used for residences.
Water or air for commercial use
Air to regenerate desiccant (drying) material in a desiccant cooling system.
22
5.3.1 Flat-Plate Collectors
Fig. 5.3 A typical flat-plate collector space-heating installations. A typical flat plate collector is an insulated metal box with a percentage of the total available solar energy. The glazing allows the light to strike the be transparent or translucent. Translucent (transmitting light only), low-iron glass is a Flat-plate collectors are the most common collector for residential water-heating and
glass or plastic cover called the glazing and a dark-colored absorber plate. The glazing can common glazing material for flat-plate collectors because low-iron glass transmits a high
black because dark colors absorb more solar energy than light colors. Sunlight passes
through the glazing and strikes the absorber plate, which heats up, changing solar radiation
absorber plate but reduces the amount of heat that can escape. The sides and bottom of the collector are usually insulated, further minimizing heat loss. The absorber plate is usually
23
into heat energy. The heat is transferred to the air or liquid passing through the collector. Absorber plates are commonly covered with “selective coatings,” which retain the are often made of metal usually copper or aluminum because they are both good heat conductors. Copper is more expensive, but is a better conductor and is less prone to categories; corrosion than aluminum. Flat-plate collectors fall into two basic glazed or unglazed
absorbed sunlight better and are more durable than ordinary black paint. Absorber plates
24
5.3.1.1 Liquid Collectors
Fig. 5.4 Unglazed collectors In a liquid collector, solar energy heats a liquid as it flows through tubes in or adjacent to the absorber plate. For this type of collector, the flow tubes are attached to the absorber plate so the heat absorbed by the absorber plate is readily conducted to the liquid. pattern. A serpentine pattern eliminates the possibility of header leaks and ensures uniform flow. A serpentine pattern is not appropriate, however, for systems that must
The flow tubes can be routed in parallel, using inlet and outlet headers, or in a serpentine
25
through the collector and then flows to the house to be used for domestic purposes. the temperature drops or use an antifreeze type of heat-transfer fluid. loop” (or “indirect”) systems
drain for freeze protection because the curved flow passages will not drain completely. The simplest liquid systems use potable household water, which is heated as it passes directly This design is known as an “open-loop” (or “direct”) system. In areas where freezing temperatures are common, however, liquid collectors must either drain the water when
In systems with heat-transfer fluids, the transfer fluid absorbs heat from the collector and storage tank inside the house, transfers heat to the water. Such designs are called “closed Glazed liquid collectors are used for heating household water and sometimes for space because swimming pools are generally used only in warm weather. Because these collectors need not withstand high temperatures, they can use less
then passes through a heat exchanger. The heat exchanger, which generally is in the water
heating. Unglazed liquid collectors are commonly used to heat water for swimming pools. expensive materials such as plastic or rubber. They also do not require freeze-proofing
26
5.3.2 Air Collectors
Fig. 5.5 An air collector Air collectors are simple, flat-plate collectors used primarily for space heating. The absorber plates in air collectors can be metal sheets, layers of screen, or nonmetallic
routed between the absorber plate and the back insulation to reduce heat loss through the
The disadvantage of this strategy is that it can also increase the amount of power needed
for fans and, thus, increase the costs of operating the system. In colder climates, the air is
systems, fins or corrugations on the absorber are used to increase air turbulence and improve heat transfer.
materials. The air flows past the absorber by natural convection or when forced by a fan. Because air conducts heat much less readily than liquid does, less heat is transferred between the air and the absorber than in a liquid collector. In some solar air-heating
27
glazing. However, if the air will not be heated more than 17°C above the outdoor efficiency.
with liquid systems. Although leaks are harder to detect and plug in an air system, they are also less troublesome than leaks in a liquid system. operating temperatures are usually lower than those of liquid collectors. 5.3.3 Evacuated-Tube Collectors Air systems can often use less expensive materials, such as plastic glazing, because their
temperature, the air can flow on both sides of the absorber plate without sacrificing
Air systems have the advantage of eliminating the freezing and boiling problems associated
Fig. 5.6 An evacuated-tube collector Evacuated-tube collectors heat water in residential applications that require higher temperatures. In an evacuated-tube collector, sunlight enters through the outer glass tube,
strikes the absorber tube, and changes to heat. The heat is transferred to the liquid flowing
28
collector) covered with a selective coating. Evacuated-tube collectors are modular tubes can be added or removed as hot-water needs change. tubes, forming a vacuum. Conductive and convective heat losses are eliminated because some radiant heat loss (heat energy will move through space from a warmer to a cooler surface, even across a vacuum). However, this loss is small and of little consequence compared with the amount of heat transferred to the liquid in the absorber tube.
through the absorber tube. The collector consists of rows of parallel transparent glass
tubes, each of which contains an absorber tube (in place of the absorber plate in a flat-plate When evacuated tubes are manufactured, air is evacuated from the space between the two
there is no air to conduct heat or to circulate and cause convective losses. There can still be Evacuated-tube collectors are available in a number of designs. Some use a third glass tube
inside the absorber tube or other configurations of heat-transfer fins and fluid tubes. One eliminating the need for a separate solar storage tank. Reflectors placed behind the evacuated tubes can help to focus additional sunlight on the collector. collectors particularly useful in areas with cold, cloudy winters. These collectors are more efficient than flat plate collectors for a couple of reasons. commercially available evacuated-tube collector stores 19 liters of water in each tube,
First, they perform well in both direct and diffuse solar radiation. This characteristic,
collectors achieve both higher temperatures and higher efficiencies than flat-plate collectors, they are also more expensive.
position, the sun is only perpendicular to the collector at noon. While evacuated-tube
absorber for most of the day. For comparison, in a flat-plate collector that is in a fixed
combined with the fact that the vacuum minimizes heat losses to the outdoors, makes these
Second, because of the circular shape of the evacuated tube, sunlight is perpendicular to the
29
5.3.4 Concentrating Collectors
Fig. 5.7 Concentrating Collectors Concentrating collectors use mirrored surfaces to concentrate the sun’s energy on an to achieve high temperatures. absorber called a receiver. Concentrating collectors also achieve high temperatures, but Some designs concentrate solar energy onto a focal point, while others concentrate the along the focal line. A heat-transfer fluid flows through the receiver and absorbs heat. These collectors reach much higher temperatures than flat-plate collectors. However, concentrators can only focus direct solar radiation, with the result being that their
unlike evacuated-tube collectors, they can do so only when direct sunlight is available. The mirrored surface focuses sunlight collected over a large area onto a smaller absorber area sun’s rays along a thin line called the focal line. The receiver is located at the focal point or
30
tracking mechanisms to move the collectors during the day to keep them focused on the sun. Single-axis trackers move east to west; dual-axis trackers move east and west and north and south (to follow the sun throughout the year). In addition to these mechanical trackers, there are passive trackers that use Freon to alternative to mechanical systems. Concentrators are used mostly in commercial supply the movement. While not widely used, they do provide a low-maintenance
performance is poor on hazy or cloudy days. Concentrators are most practical in areas of high insolation (exposure to the sun’s rays), such as those close to the equator.
Concentrators perform best when pointed directly at the sun. To do this, these systems use
applications because they are expensive and because the trackers need frequent than dual-axis trackers.
maintenance. Some residential solar energy systems use parabolic-trough concentrating Most residential systems use single-axis trackers, which are less expensive and simpler
systems. These installations can provide hot water, space heating, and water purification.
31
CHAPTER 6 6.1 TECHNOLOGICAL IMPROVEMENTS AND FUTURE OF SOLAR HEATERS The efficiency of solar heating systems and collectors has improved from the early 1970s and costs have dropped somewhat. The efficiencies can be attributed to the use of low-iron,
tempered glass for glazing (low-iron glass allows the transmission of more solar energy coatings.
capable of heating 0.002 cubic meters per second of outside air. On a sunny winter day, the panel can produce temperatures up to 28°C higher than the outdoor air temperature. Transpired air collectors not only heat air, but also improve indoor air quality by directly preheating fresh outdoor air. These collectors have achieved very high efficiencies more or insulation, they are inexpensive to manufacture. All these factors make transpired air collectors a very cost-effective source of solar heat. There are other prototype cooling systems operating today. Some use heat from solar than 70% in some commercial applications. Plus, because the collectors require no glazing collectors for absorption cooling. Others are being used to renew the desiccant material in
heats the metal, and a fan pulls air through the holes in the metal, which heats the air. For residential installations, these collectors are available in 2.4-meter by 0.8-meter panels
available for homes. Called a transpired collector, it eliminates the cost of the glazing, the metal box, and the insulation. This collector is made of black, perforated metal. The sun
than conventional glass), improved insulation, and the development of durable selective Also, a new solar air collector, formerly used primarily for commercial buildings, is now
desiccant cooling systems. Desiccants, such as silica gel, naturally attract moisture. They are used to reduce humidity and the resulting cooling loads in hot, humid climates. collectors will be integrated into many applications. Solar collectors can be used for nearly any process that requires heat. As environmental
laws become stricter and the price of conventional power increases, it is likely that solar
32
CHAPTER 7 7.1 CONCLUSION Water heating is a practical application of solar energy in many parts of the world. Natural circulation systems are widely used in climates where freezing does not occur. Forced are used in climates where freezing is a problem. circulation systems using drain down or non-freezing fluids in collector exchanger loops must be taken to use auxiliary in such a way it does not drive up collector temperature. Auxiliary energy is used in essentially all systems where high reliability is wanted and care
33
REFERENCES Books, Pamphlets, and Reports S. Sklar and K. Sheinkopf, Bonus Books, Inc., Consumer Guide to Solar Energy, 160 East Illinois Street, The Fuel Savers, Chicago, IL 60611, 1991. Lafayette, CA, 1991. Brick House, Periodicals Home Energy Magazine, Berkeley. 2124 Kittredge Street, No. 95,
B. Anderson, Morning Sun Press, B. Anderson and M. Riorden, The New Solar Home Book, NH. Amherst.
Solar Energy Industries Association, Solar Industry Journal, NW. Washington DC. Solar Today, Unit G-1, 4th Floor 122 C Street, 2400 Central Avenue, Boulder, CO 80301.
34
