Chapter 3

Solar Thermal Energy Collectors

Sun’s heat energy is a diffuse energy. It is always first collected and then concentrated. In residential systems, simple and cheap solar panels are used to collect the solar heat energy below 60°C. Residential panels for heat collection are referred to as flat plate collectors.

In utility scale systems, solar heat energy is required to be concentrated at high temperature level in the range 70°C–80°C at the collectors. The utility panels are, therefore, called concentrators.

Solar energy collectors are special kind of heat exchangers that transform solar radiation energy into internal energy of the transport medium. The major component of any solar system is the solar collector.

The solar collector absorbs the incoming solar radiation, converts it into heat, and then transfers this heat to a fluid (usually air, water, or oil) flowing through the collector. The solar energy, thus, collected is carried from the circulating fluid either directly to the hot water or space conditioning equipment or to a thermal energy storage tank, from which it can be drawn for use at night and/or cloudy days.

Solar collectors can be used in a large variety of applications. The following are the main areas of applications

1. Solar water heating: It includes thermosiphon, integrated collector storage systems, air systems, direct circulation, and indirect water heating systems.
2. Solar space heating systems: This includes both water and air systems.
3. Solar refrigeration: It includes both adsorption and absorption systems.
4. Industrial process heat systems: They include both low temperature (air and water based) applications and solar steam generation systems.
5. Solar desalination systems: They include both direct (solar stills) and indirect systems (conventional desalination equipment powered by solar collectors).
6. Solar thermal power generation systems: They include the parabolic trough systems, the power tower or central receiver systems, and the parabolic dish systems (dish/Stirling engine).

A Stirling engine is a heat engine operating by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work.

Similar to the steam engine, the Stirling engine is traditionally classified as an external combustion engine, as all heat transfers to and from the working fluid take place through the engine wall. This contrasts with an internal combustion engine where heat input is by combustion of a fuel within the body of the working fluid. Unlike a steam engine’s (or more generally a Rankine cycle engine’s) usage of a working fluid in both its liquid and gaseous phases, the Stirling engine encloses a fixed quantity of permanently gaseous fluid like air.

The amount of heat energy produced by a solar collector depends on the type of collector, its working surface direction towards the sun, meteorological conditions of the location, and many other factors.

3.1 TYPES OF SOLAR COLLECTORS

The collectors that are being marketed to utilize thermal energy from the sun can be subdivided into the following categories.

3.1.1 Flat Plate Collectors

Flat plate collectors are the most common type. They are also referred to as non- concentrating collectors and have the same area for intercepting and for absorbing solar radiation. A typical flat plate collector is shown in Figure 3.1.

Figure 3.1 Flat plate collectors

It has five important parts:

1. Dark flat plate absorber of solar energy: The absorber consists of a thin absorber sheet (of thermally stable polymeric materials such as aluminium, steel, or copper to which a black or selective coating is applied) because of the fact that the metal is a good heat conductor. Copper is more expensive, but is a better conductor and less prone to corrosion than aluminium. In locations with average availability of solar energy, flat plate collectors are sized approximately 0.5 to 1 square foot per gallon of daily hot water use.
2. Transparent cover: This allows solar energy to pass through, but reduces heat losses.
3. Heat-transport fluid (air, antifreeze, or water): To remove heat from the absorber, fluid is usually circulated through tubing to transfer heat from the absorber to an insulated water tank.
4. Heat insulation backing: Often backed by a grid or coil of fluid tubing.
5. Insulated casing: It is made of a glass or polycarbonate cover.

When solar radiation passes through a transparent cover and impinges on the blackened absorber surface of high absorptivity, a large portion of this energy is absorbed by the plate, and then transferred to the transport medium in the fluid tubes to be carried away for storage or use. The underside of the absorber plate and the side of casing are well insulated to reduce conduction losses. The transparent cover is used to reduce convection losses from the absorber plate through the restraint of the stagnant air layer between the absorber plate and the glass. It also reduces radiation losses from the collector as the glass is transparent to the short-wave radiation received by the sun, but it is nearly opaque to long-wave thermal radiation emitted by the absorber plate. For solar water heating systems in home and solar space heating flat plate collectors are the most common type of solar collector used.

3.1.1.1 Flat Plate Air Collectors

Schematic arrangement of a typical flat plate air collector is shown in Figure 3.2. It uses air as the heat transport medium. Air flat plate collectors are used mainly for solar space heating. The absorber plates can be made of metal sheets, layers of screen, or non-metallic materials. The air flows past the absorber by using natural convection or a fan. Since air does not conduct heat as easily as liquid, air collectors are typically less efficient than liquid collectors.

Figure 3.2 Flat plate air collectors

3.1.1.2 Flat Plate Liquid Collectors

These collectors use liquid as the heat transport medium. Liquid flat plate collectors heat liquid as it flows through tubes in or adjacent to the absorber plate as shown in Figure 3.3. The simplest liquid systems use household water that is heated as it passes directly through the solar collector and then flows to the house. Solar pool heating uses liquid flat plate technology, but the collectors are typically unglazed. The liquid tubes can be welded to the absorbing plate, or they can be an integral part of the plate. The liquid tubes are connected at both ends by large diameter header tubes.

Figure 3.3 Flat plate liquid collectors

3.1.2 Concentrating Collectors

By using reflectors to concentrate sunlight on the absorber of a solar collector, the size of the absorber can be dramatically reduced, which reduces heat losses and increases efficiency at high temperatures. Another advantage is that reflectors can cost substantially less per unit area than collectors. This class of collector is used for high-temperature applications such as steam production for the generation of electricity and thermal detoxification. These collectors are best suited to climates that have an abundance of clear sky days, and therefore, they are not so common in many regions. Stationary concentrating collectors may be liquid-based, air-based, or even an oven such as a solar cooker.

One such collector is a parabolic dish reflector, which is shown in Figure 3.4.

Figure 3.4 Parabolic reflector

3.1.2.1 Stationary Concentrating Collectors

These collectors are operated in a stationary mode for applications like air conditioning. Stationary concentrating collectors use compound parabolic reflectors and flat reflectors for directing solar energy to an accompanying absorber or aperture through a wide acceptance angle. The wide acceptance angle for these reflectors eliminates the need for a sun tracker. This class of collector includes parabolic trough flat plate collectors, flat plate collectors with parabolic boosting reflectors, and solar cookers. The development of the first two collectors has been done in Sweden. Solar cookers are used throughout the world, especially in the developing countries.

3.1.2.2 Tracking Concentrating Collectors

In the case of high temperature applications, like solar electric generation tracking, the sun is necessary. Heliostats are tracking mirrors that reflect solar energy onto a fixed target.

3.1.3 Comparison of Collectors

Two important performance parameters used for comparison of solar collectors are as follows:

1. Temperature range required for various range and
2. Collector concentration ratio

A collector is defined as concentrating collector if its absorber (fin) area A_d is smaller than the aperture area A_a and if reflective surfaces are used to reflect a portion of the incident sunlight into the absorber [Collector concentration ratio (CCR) is the ratio of A_a/A_d]. It is also used as a measure for classifying collectors. Since this ratio approximately determines the operating temperature, such method of classification is equivalent to classifying collectors by its operating temperature range. A concentrating collector with a low concentration ratio (2 ≤ CCR ≤ 5) can be designed in such a way so that its absorbers intercept a major portion of the incident sunlight not only at one fixed attitude but also within certain range of sun angles. This acceptance range may be as wide as 40 to 60°. Such collectors can then be operated in a stationary mode (i.e., tracking the sun is not required). The collector that have high concentration ratios (i.e., CCR >10), precise tracking is essentially required. Table 3.1 gives comparative features of a few important solar collectors. Brief description of few solar collectors is also presented.

Table 3.1 Comparative Features of a Few Important Solar Collectors

3.2 CONFIGURATIONS OF CERTAIN PRACTICAL SOLAR THERMAL COLLECTORS

Undoubtedly, there are many different ways that solar energy can be applied, but there are also many different methods for collecting the solar energy from incident radiation. The following are the list of some popular types of solar collectors.

3.2.1 Flat Plate Collectors

Flat plate collectors are the most common solar collector for solar water-heating systems in homes and solar space heating. A typical flat plate collector is an insulated metal box with a glass or plastic cover (called the glazing) and a dark-coloured absorber plate. These collectors heat liquid or air at temperatures less than 90°C.

Flat plate collectors are used for residential water heating and space heating installations.

3.2.1.1 Liquid Flat Plate Collectors

Liquid-based collectors use sunlight to heat a liquid that is circulating in a ‘solar loop’. The fluid in the solar loop can be water, an antifreeze mixture, thermal oil, etc. The solar loop transfers the thermal energy from the collectors to a thermal storage tank. The simplest liquid systems use potable household water, which is heated as it passes directly through the collector and then flows to the house. The type of collector selection depends on how hot the water must be and the local climate. Unglazed collectors are typically used for swimming pool heating.

3.2.1.2 Air Flat Plate Collectors

These are used primarily for solar space heating. The absorber plates in air collectors can be metal sheets, layers of screen, or non-metallic materials. The air flows past the absorber by using natural convection or a fan. Because air conducts heat much less readily than liquid does, less heat is transferred from an air collector’s absorber than from a liquid collector’s absorber, and air collectors are typically less efficient than liquid collectors. The thermal energy collected from air-based solar collectors can be used for ventilation, air heating, space heating, and crop drying.

The most common air and liquid-based solar thermal collectors are as follows:

1. Glazed flat plate solar thermal collectors
2. Unglazed flat plate solar thermal collectors
3. Unglazed perforated flat plate solar thermal collectors
4. Back-pass flat plate solar thermal collectors
5. Batch flat plate solar thermal collectors
6. Solar cookers
7. Evacuated (vacuum tube) flat plate solar thermal collectors
8. Concentrating (flat plate collectors with flat reflectors)

3.2.2 Glazed Flat Plate Collectors

Glazed flat plate collectors are shown in Figure 3.1. They are very common and are available as liquid-based and air-based collectors. These collectors are better suited for moderate temperature applications where the demand temperature is 30°C–70°C and for applications that require heat during the winter months. The liquid-based collectors are most commonly used for the heating of domestic and commercial hot water, buildings, and indoor swimming pools. The air-based collectors are used for the heating of buildings, ventilation air, and crop drying

3.2.3 Unglazed Flat Plate Solar Collectors

Unglazed flat plate collectors account for the larger proportion of collector installed per year of any type of solar collector in many countries. Because they are not insulated, these collectors are best suited for low temperature applications where the temperature demand is below 30°C. By far, the primary market is for heating outdoor swimming pools, but other markets exist including heating seasonal indoor swimming pools, pre-heating water for car washes, and heating water used in fish farming operations. There is also a market potential for these collectors for water heating at remote and seasonal locations like summer camps. Unglazed flat plate collectors are usually made of black plastic that has been stabilized to withstand ultraviolet light. Since these collectors have no glazing, a large portion of the sun’s energy is absorbed. However, because they are not insulated, a large portion of the heat absorbed is lost, particularly when it is windy and not warm outside. They transfer heat so well to air (and from air) that they can actually ‘capture’ heat during the night when it is hot and windy outside.

3.2.4 Unglazed Perforated Plate Collectors

The key to this type of collector is an industrial grade siding or cladding that is perforated with many small holes at a pitch of 2–4 cm. Air passes through the holes in the collector before it is drawn into the building to provide preheated fresh ventilation air. Efficiencies are typically high because the collector operates close to the outside air temperature. These systems can be very cost effective, especially when they replace conventional cladding on the building because only incremental costs need be compared to the energy savings. The most common application of this collector is for building ventilation air heating. Other possible components for this system are: a 20–30 cm air gap between the buildings, a canopy at the top of the wall that acts as a distribution manifold, and bypass dampers so that air will bypass the system during warm weather. Another application for this collector is crop drying. Systems have been installed in South America and Asia for drying of tea, coffee beans, and tobacco.

3.2.5 Back-pass Solar Collectors

Air-based collectors use solar energy to heat air. Their design is simple and they often weigh less than liquid-based collectors because they do not have pressurized piping. Air-based collectors do not have freezing or boiling problems. In these systems, a large solar absorber is used to heat the air. The simplest designs are single-pass open collectors. Collectors that are coated with a glaze can also be used to heat air for space heating.

3.2.6 Batch Flat Plate Solar Thermal Collectors

In ancient days, water tanks that were painted black were used as simple solar residential water heaters. Today, their primary market is for residential water heating in warm countries. Modern batch collectors have a glazing that is similar to the one used on flat plate collectors and/or a reflector to concentrate the solar energy on the tank surface. Because the storage tank and the solar absorber act as a single unit, there is no need for other components. On an area basis, batch collector systems are less costly than glazed flat plate collectors but also deliver less energy per year.

3.2.7 Flat Plate Collectors with Flat Reflectors

It is found that the addition of reflector on collector increases the solar yield on the collector and the overall thermal performance of the collector. The enhancement in the solar yield on the collector is about 44% in winter and 15% in summer conditions, which is consistent with more hot water demand in winter.

A variation of flat plate collector is shown in Figure 3.5.This simple reflector can markedly increase the amount of direct radiation reaching the collector. This is in fact a concentrate because the aperture is larger than the absorber plate.

Figure 3.5 Flat plate collector with flat reflection

3.2.8 Evacuated Tube Collectors

Conventional simple flat plate solar collectors were developed for use in sunny and warm climatic conditions. However, their benefits are greatly reduced when conditions become unfavourable during cold, cloudy, and windy days.

Furthermore, weathering influences such as condensation and moisture will cause early deterioration of internal materials resulting in reduced performance and system failure. Evacuated heat pipe solar collectors (tubes) operate differently than the other collectors available on the market. These solar collectors consist of a heat pipe inside a vacuum-sealed tube, as shown in Figures 3.6 and 3.7.

Figure 3.6 Typical evacuated tube solar collectors

Figure 3.7 Schematic evacuated tube solar collectors

Evacuated tube collectors can achieve extremely high temperatures (75°C–180°C), making them more appropriate for cooling applications and for commercial and industrial applications. However, evacuated tube collectors are more expensive than flat plate collectors, as their unit area costs about twice than that of the latter. Evacuated tube collectors are efficient at high temperatures.

The collectors are usually made of parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin is covered with a coating that absorbs solar energy well, but which inhibits radiation heat loss. Air is removed, or evacuated, from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss.

Vacuum (also ‘evacuated’) tube solar collectors are amongst the most efficient and most costly types of solar collectors. These collectors are best suited for moderate temperature applications where the demand temperature is 50–95°C and also for very cold climates such as in the farthest northern part of Canada. Similar to the glazed flat plate solar collectors, applications of vacuum tube collectors include heating of domestic and commercial hot water, buildings, and indoor swimming pools. Due to their ability to deliver high temperatures efficiently, another potential application is for the cooling of buildings by regenerating refrigeration cycles. Vacuum tube solar collectors have a selective absorber for collecting sunlight that is in vacuum-sealed tube. Their thermal losses are very low even in cold climates.

3.3 MATERIAL ASPECTS OF SOLAR COLLECTORS

Flat, corrugated, or grooved plates, to which the tubes, fins, or passages are attached. The plate may be integrated with the tubes.

3.3.1 Absorber

The following are the types of solar flat plate absorbers that are most frequently used.

1. all copper plates are with integrated water passage (roll bond type). These plates can also be made of aluminium.
2. all copper (copper tube on copper sheet).
3. copper tube or aluminium fin
4. iron or steel
5. plastic (polymers)

3.3.1.1 Absorptive Coatings

Many varieties of absorptive coating are being used, ranging from flat black paint to baked enamel. Flat black absorber coatings have high absorptivity.

Specification requirement of an absorber coating for a flat plate collector is as follows:

1. It must not degrade under ultraviolet exposure.
2. It must withstand temperature up to 200° C.
3. It must withstand many temperature cycles over ±40° C.
4. It must withstand many cycles of low to high relative humidity.
5. It must not chalk, fade, or chip.
6. It must not be so thick that heat conduction through the paint to the metal absorber is impeded.

3.3.2 Glazing

One or more sheets of glass or other diathermanous (radiation transmitting) material is used as transparent covers. Following are its important functions:

1. It must reduce convective losses from the absorber plate.
2. It must suppress radiative heat losses from the absorber plate.
3. It must protect the absorber from the elements and from excessive UV exposures. A glazing material must be resistant to UV radiation.

Glass meets the entire abovementioned requirements and also compatible with the general requirement of longevity.

The following are the specification requirement of glazing materials:

1. They must be reasonably impact resistant.
2. Thin or no tempered glass panes are questionable because of the risk of damage from hail, birds, and vandalism.
3. Plastic materials of low tensile strength (i.e., Teflon) are not advisable.
4. They must be resistant to significant temperature shock.
5. Sudden rain will cause rapid overall limb changes. A leaf on a stagnant collector can cause high localized thermal stresses.

Thus, heat tempered glass is absolute necessity for outer collector glazing. Generally, plastic glazing can easily withstand the temperature shocks. Non-UV inhibited plastic materials are not acceptable.

Teflon (high transmittivity) and polyvinyl fluoride (PVF, Tedlar) are known to withstand UV radiation and is often used to protect other materials underneath from UV radiation.

3.3.2.1 Areas of Practical Applications Attentions

1. High cost
2. Longevity (is ultraviolet resistivity, thermal stability, corrosion resistance)
3. Black chrome coating is used in several high quality collectors. They are reasonable in cost and longevity. It can be used on copper, aluminium and steel absorbers.
4. Absorptivity is 92% to 95% in the visible spectrum and 10% to 20% in the infrared spectrum.
5. They must be able to withstand wind conditions.

3.3.2.2 Glazing Materials

1. Glass and fiberglass meet these requirements.
2. Tedlar used alone cannot serve the purpose.
3. Tedlar when bonded to the fiberglass, it acts as a good glazing material.
4. Optical rating must not change during its service life.
5. This requirement can hardly be met by any plastic glazing materials.
6. Fiberglass partially serves the purpose.

3.3.3 Insulation Shell

A solar flat plate collector must be insulated against excessive heat losses on its back side and on its edges as follows:

1. Back side – 3.5 inch of fiberglass insulation or 2 inch of foam insulation.
2. Side – 1 inch of fiberglass or 0.5 to 0.75 inch of foam insulation.

The following are the specifications to be met by insulating materials

1. It must withstand the maximum collector stagnation temperature rating (200°C) without damage. Foam materials shrink due to excessive heat.
2. The maximum stagnation temperature must not cause evaporation or sublimation of substances in the insulating materials such as the binder of the fiberglass.

Special fiberglass materials are available that have quite satisfactory outgassing rate.

3.4 CONCENTRATING COLLECTORS

As discussed in Section 3.1.2, they usually have concave reflecting surface to intercept and focus the sun’s beam radiation to a smaller receiving area, thereby increasing the radiation flux. In other words, concentrating solar collectors use shaped mirrors or lenses to provide higher temperatures than the flat plate collectors.

However, each method of concentration has following drawbacks:

1. The mirror requires a clean smooth reflecting surface, because dust particles could scatter light away from the receiver or the light could be partly absorbed by a thin dirty film.
2. Smooth surface because contour error can also result in missing the receiver.

One criterion for the selection of a specific concentrator is the degree of concentration and hence temperature that is to be achieved. As a rule, concentrating energy on a point produces high to very high temperature; while on a line, it produces moderate to high temperature. Non-focusing concentrators produce low to moderate temperature.

Concentrating collectors are of various types and can be classified in many ways. They may be as follows:

1. Based on means of concentration: reflecting type use mirrors or refracting type use Fresnel lenses.
2. Based on reflecting surfaces used: parabolic, spherical, or flat.
3. Continuous or segmented.
4. Based on the formation of the image: imaging or non-imaging.
5. Imaging concentrator may focus on a line or at a point.
6. On the basis of collector concentration ratio or operating temperature range.
7. By the type of tracking.

The parabolic dish reflector (see Fig. 3.12) utilizes the point focus. The parabolic trough (see Fig. 3.10) is an example of line focus optics. Concentrating collectors are available in different configurations, which are discussed in the following sections.

3.4.1 Compound Parabolic Solar Collectors

These are non-imaging concentrators as shown in Figure 3.8.

Figure 3.8 Compound parabolic solar collector

3.4.2 Fresnel Solar Thermal Collectors

In order to deliver high temperatures with good efficiency, a high performance solar collector is required. Systems with light structures and low cost technology for process heat applications up to 400°C could be obtained with parabolic trough collectors. It can effectively produce heat at temperatures between 50°C and 400°C. They are made by bending a sheet of reflective material into a parabolic shape. A metal black tube, covered with a glass tube to reduce heat losses, is placed along the focal line of the receiver (see Fig. 3.9a). When the parabola is pointed towards the sun, parallel rays incident on the reflector are reflected onto the receiver tube. It is sufficient to use a single-axis tracking of the sun, and thus, long collector modules are produced.

Figure 3.9 Fresnel trough collector (a) parabolic (b) linear

Linear Fresnel reflector (LFR) technology relies on an array of linear mirror strips that concentrate light on to a fixed receiver mounted on a linear tower. It can be imagined as a broken-up parabolic trough reflector (see Fig. 3.9a), but unlike parabolic troughs, it does not have to be of parabolic shape, large absorbers can be constructed and the absorber does not have to move.

A representation of an element of an LFR collector field is shown in Figure (3.9b). The greatest advantage of this type of system is that it uses flat or elastically curved reflectors that are cheaper when compared to parabolic glass reflectors. Additionally, these are mounted close to the ground, thus minimizing structural requirements. One difficulty with the LFR technology is that the avoidance of shading and blocking between adjacent reflectors leads to increased spacing between reflectors. Blocking can be reduced by increasing the height of the absorber towers, but this increases the cost.

3.4.3 Parabolic Trough Solar Thermal Collectors

Parabolic troughs are devices that are shaped like the letter ‘U’, as shown in Figure 3.10.

Figure 3.10 Parabolic trough reflector

Systems with light structures and low cost technology for process heat applications up to 400°C could be obtained with parabolic trough collectors. They can effectively produce heat at temperatures between 50°C and 400°C. They are made by bending a sheet of reflective material into a parabolic shape. A metal black tube, covered with a glass tube to reduce heat losses, is placed along the focal line of the receiver (see Fig. 3.10).

The troughs concentrate sunlight onto a receiver tube that is positioned along the focal line of the trough. Sometimes, a transparent glass tube envelops the receiver tube to reduce heat loss. Parabolic troughs often use single-axis or dual-axis tracking. In rare instances, they may be stationary. Temperatures at the receiver can reach 400°C and produce steam for generating electricity. In California, multi-megawatt power plants were built using parabolic troughs combined with gas turbines.

3.4.4 Cylindrical Trough Solar Collectors

Since linear translation does not introduce defocusing of the concentrated radiation, the aperture of a cylindrical trough need not track at all to maintain focus. However, as indicated in Figure 3.11, a high-rim angle cylindrical trough would have a focal plane not a focal line. To avoid a dispersed focus, cylindrical troughs would have to be designed with low rim angles in order to provide an approximate line focus. The advantage of cylindrical mirror geometry is that it need not track the sun in any direction as long as some means is provided to intercept the moving focus.

Figure 3.11 Cylindrical trough solar collectors

3.4.5 Parabolic Dish Systems

A parabolic dish collector is similar in appearance to a large satellite dish as shown in Figure 3.12, but has mirror-like reflectors and an absorber at the focal point. It uses a dual-axis sun tracker.

Figure 3.12 Parabolic dish solar thermal collector

It is a point-focus collector that tracks the sun in two axes

1. Concentrating solar energy onto a receiver located at the focal point of the dish.
2. The dish structure must track fully the sun to reflect the beam into the thermal receiver.

The receiver absorbs the radiant solar energy, converting it into thermal energy in a circulating fluid. The thermal energy can then either be converted into electricity using an engine–generator coupled directly to the receiver, or it can be transported through pipes to a central power conversion system. Parabolic-dish systems can achieve temperatures in excess of 1,500°C. Because the receivers are distributed throughout a collector field, like parabolic troughs, parabolic dishes are often called distributed receiver systems.

The following are the important advantages of parabolic dish reflectors:

1. They are the most efficient of all collector systems as they are always pointing the sun.
2. They typically have concentration ratio in the range of 500–2,000, and thus are highly efficient at thermal energy absorption and power conversion systems.
3. They have modular collector and receiver units that can either function independently or as part of a large system of dishes.

This type of solar thermal energy concentrator is primarily used with parabolic dish engine, which is an electric generator that uses sunlight instead of crude oil or coal to produce electricity.

3.4.6 Heliostat Field Solar Collectors

Heliostat is a mirror-based system that is used to continuously reflect sunlight onto a central receiver as shown in Figure 3.13. The collected solar energy is then converted into electrical power. Generally, it is a two-axis solar tracking flat mirror that reflects sunlight onto a fixed receiver or target. Furthermore, the geometry between the sun, mirror, and receiver are constantly changing throughout the day.

Figure 3.13 Heliostat filed solar collector

Heliostats have a potential of being the truly lowest cost solution to all the power one needs. These systems can be used for domestic heating, electricity, and lighting. The realization that all flat plate solar collectors have large amount of expensive collector area and still delivering only low grade temperatures have compelled the researchers to look for a better solution. In this type of collector, a flat absorber efficiently transforms sunlight into heat. To minimize heat escaping, the plate is located between a glazing (glass pane or transparent material) and an insulating panel. The glazing is chosen so that a maximum amount of sunlight will pass through it and reach the absorber.

A heliostat uses a field of dual-axis sun trackers that direct solar energy to a large absorber located on a tower. To date, the only application for the heliostat collector is power generation in a system called the power tower. A power tower has a field of large mirrors that follow the sun’s path across the sky. The mirrors concentrate sunlight onto a receiver on top of a high tower. A computer keeps the mirrors aligned so the reflected rays of the sun are always aimed at the receiver, where temperatures well above 1,000°C can be reached. High-pressure steam is generated to produce electricity.

3.4.6.1 Working of Practical Solar Heliostat

A practical solar heliostat is a mirror that makes precise movements up or down and left or right to reflect sunlight onto a fixed spot. As the sun marches across the sky, the heliostat adjusts its position, so the spot of reflected light remains stationary on the target. The relative spherical angular position of the thermal receiver to the heliostat is inputted to the computer. The computer solves the spherical trigonometry problem and commands the heliostat motor drive system such that the mirror is positioned angularly exactly halfway between the sun and the thermal receiver. Because every heliostat in an installation is in a unique position relative to the thermal receiver, each heliostat receives unique position commands.

When multiple practical solar heliostats reflect sunlight onto a single thermal receiver, the concentrated heat of the sunlight can be used to produce hot water or steam. Relatively cold water flows through the thermal receiver and is outputted as hot water. Although such a system can generate temperatures capable of melting steel, the temperature of the water is raised to just within a degree of boiling. For the purpose of heating and cooling a commercial building, this temperature uses the heat in sunlight at the highest efficiency and lowest cost. A higher temperature would mean more heat loss, as well as a more expensive system to withstand the higher temperature and pressure.

Figure 3.14 Heliostat electric generating plant

The mirrors on practical solar heliostats have minimal reflection loss, so each heliostat reflects approximately its area in sunlight: about 1 kW of heat per square meter. As a frame of reference, a typical electric space heater produces 1.5 kW of heat. If 100 practical solar heliostats, each with 2.2 m² of mirror area, direct sunlight onto a single thermal receiver, the sunlight will be converted into 220 kw of heat.

With practical solar method of heat storage and heat distribution, 220 kw is more than enough energy to supply all of the heating needs of a 10,000 square foot commercial building. Using absorption chillers powered by hot water, it also has sufficient energy to supply the building’s cooling needs.

However, the sun never shines at night, and often does not shine during the day. Energy storage is a fundamental requirement in any serious solar energy application. It is extremely expensive to store electrical energy, even though billions of dollars have been spent improving battery and fuel cell technology. The reverse is true for thermal energy. Hot water can be stored cheaply in a thermally insulated tank. As the volume of water and energy stored increases, the cost and losses of thermal energy storage drop rapidly.

3.4.6.2 Advantages and Disadvantages of the Heliostat Solar Tower Power Plant

A few advantages and disadvantages of the heliostat solar tower system in comparison with the other three concentrating solar power technologies under development are summarized here.

3.4.6.2.1 Advantages

1. Although the heliostat solar tower approach to solar power production is not as commercially developed as the solar parabolic trough system, it is more commercially developed than either the parabolic dish–Stirling engine or linear Fresnel systems.
2. Since the heliostat solar tower system produces steam to generate electricity with a conventional Rankine steam cycle, this system can be hybridized. In other words, it can be designed to use a fossil fuel (typically natural gas) as a supplementary fuel, allowing electricity to be generated when the sun is not shining.

3.4.6.2.2 Disadvantages The heliostat solar tower system produces a fluid temperature greater than that of the single-axis tracking, parabolic trough, and linear Fresnel system, but less than that of the two-axis tracking, parabolic dish–Stirling engine system. Thus, it cannot achieve efficiency for conversion of electricity from thermal energy as high as that of the parabolic dish–Stirling engine system.

3.5 PARABOLIC DISH–STIRLING ENGINE SYSTEM

The major parts of a parabolic dish–Stirling engine system are as follows:

1. Solar dish concentrator: Parabolic dish systems that generate electricity from a central power converter collect the absorbed sunlight from individual receivers and deliver it via a heat-transfer fluid to the power conversion systems. The need to circulate heat-transfer fluid throughout the collector field raises design issues such as piping layout, pumping requirements, and thermal losses.
2. Power conversion unit: The power conversion unit includes the thermal receiver and the heat engine. The thermal receiver absorbs the concentrated beam of solar energy, converts it to heat, and transfers the heat to the heat engine. A thermal receiver can be a bank of tubes with a cooling fluid circulating through it. The heat transfer medium usually employed as the working fluid for an engine is hydrogen or helium. Alternate thermal receivers are heat pipes wherein the boiling and condensing of an intermediate fluid are used to transfer the heat to the engine. The heat engine system takes the heat from the thermal receiver and uses it to produce electricity. The engine–generators have several components; a receiver to absorb the concentrated sunlight to heat the working fluid of the engine, which then converts the thermal energy into mechanical work; an alternator attached to the engine to convert the work into electricity, a waste-heat exhaust system to vent excess heat to the atmosphere, and a control system to match the engine operation to the available solar energy. This distributed parabolic dish system lacks thermal storage capabilities, but can be hybridized to run on fossil fuel during periods without sunshine. The Stirling engine is the most common type of heat engine used in dish–engine systems.
3. Tracking system: A parabolic dish system uses a computer to track the sun and concentrate the sun’s rays onto a receiver located at the focal point in front of the dish. In some systems, a heat engine, such as a Stirling engine, is linked to the receiver to generate electricity. Parabolic dish systems can reach 1,000 °C at the receiver, and achieve the highest efficiencies for converting solar energy to electricity in the small-power capacity range.

3.6 WORKING OF STIRLING OR BRAYTON HEAT ENGINE

After the array of mirrors focuses the sunlight, the concentrated sunlight then heats up the working fluid to temperatures of around 750°C within the receiver. The heated high temperature working fluid is then used in either a Stirling or Brayton heat engine cycle to produce mechanical power via rotational kinetic energy and then electricity for utility use with an electric generator. An example of a Brayton cycle used to produce electricity for a parabolic dish power plant is shown in Figure 3.15. In the cycle, the concentrated sunlight focused on the solar fluid heats up the compressed working fluid of the cycle, i.e., air, replacing altogether or lowering the amount of fuel needed to heat up the air in the combustion chamber for power generation. As with all Brayton cycles, the hot compressed air is then expanded through a turbine to produce rotational kinetic energy, which is converted to electricity using the alternator. A recuperator is also utilized to capture waste heat from the turbine to preheat the compressed air and make the cycle more efficient.

Figure 3.15 Schematic of solar electric generation

3.7 SOLAR COLLECTOR SYSTEMS INTO BUILDING SERVICES

Although the operating costs of solar energy conversion system are generally low, the initial cost of purchasing and installing such system are high when compared to fossil fuel or other conventional energy system. Hence, the choice between solar energy systems against a conventional one must be cost effective on a long term basis (based on economic evaluation).

Since the availability of solar energy is intermittent and unpredictable it is rarely cost effective to have energy demands from solar energy alone. A solar system able to meet all the energy demands under the worst operating conditions for a long period would be greatly oversized.

The most economical way is to have

1. The solar system meets the basic energy demand while operating at its full capacity.
2. Let an auxiliary or backup system carry the peak load and unusual load.

The right proportion of solar versus auxiliary energy supply is to be determined by economics. Schematic representation of a typical space heating system with air collectors is given in Figure 3.16. Dampers is indicated for the solar rock-bed charging mode. The main components of the building heating systems are as follows:

1. Air handling unit: a fan and two motor-driven dampers.
2. Heat storage unit (rock bed)
3. Temperature control system
4. Solar collectors

Figure 3.16 Schematic of solar air heating system

Depending on the position of dampers A and B, three modes of system operation can be achieved.

1. Dampers A and B open: This is the normal day time solar heating mode. The storage unit is bypassed. If the temperature sensor in the top of the collector array is below a necessary limit required for space heating, the auxiliary furnace is automatically turned on.
2. Damper A open and damper B closed: This mode is used whenever solar heat is collected but no space heating is required at the same time. The fan blows the solar heated air through the rock bed for thermal storage.
3. Damper A closed and damper B open: This mode is used during cloudy periods or during the night hours. The return air from the building is now pulled through the rock bed, where it picks up solar heat. The auxiliary furnace is activated automatically if the temperature is insufficient to meet the demand.

3.8 SOLAR WATER HEATING SYSTEMS

Most solar water heating systems have two main parts: a solar collector and a storage tank. The most common collector is called a flat plate collector. It consists of a thin, flat, rectangular box with a transparent cover that faces the sun mounted on the roof of building or home. Small tubes run through the box and carry the fluid – either water or other fluid, such as an antifreeze solution – to be heated. The tubes are attached to an absorber plate, which is painted with special coatings to absorb the heat. The heat builds up in the collector, which is passed to the fluid passing through the tubes.

An insulated storage tank holds the hot water. It is similar to water heater, but larger in size. In the case of systems that use fluids, heat is passed from hot fluid to the water stored in the tank through a coil of tubes. Solar water heating systems can be either active or passive systems.

1. The active systems, which are most common, rely on pumps to move the liquid between the collector and the storage tank.
2. The passive systems rely on gravity and the tendency for water to naturally circulate as it is heated.

3.8.1 Active Solar Water Heating Systems

The active water systems that can be used to heat domestic hot water are the same as the ones that provide space heat. A space heat application will require a larger system and additional connecting hardware to a space heat distribution system.

3.8.1.1 Parts of Water Heating Systems

There are five major components in active solar water heating systems:

1. Collector(s) to capture solar energy.
2. Circulation system to move a fluid between the collectors to a storage tank
3. Storage tank
4. Backup heating system
5. Control system to regulate the overall system operation

A typical active water heating system that exhibits effectiveness, reliability, and low maintenance is shown in Figure 3.17.

Figure 3.17 A typical hot water system

It uses distilled water as the collector circulating fluid. The collectors in this system will only have water in them when the pump is operating. This means that in the case of power failure as well as each night, there will be no fluid in the collector that could possibly freeze or cool down and delay the start-up of the system when the sun is shining. This system is very reliable and widely used. It requires that the collectors are mounted higher than the drain back tank or heat exchanger. This may be impossible to do in a situation where the collectors must be mounted on the ground. The fluids that are circulated into the collectors are separated from the heated water that will be used in the home by a double-walled heat exchanger. A heat exchanger is used to transfer the heat from the fluids circulating through the collectors to the water used in the home. The fluids that are used in the collectors can be water, oil, an antifreeze solution, or refrigerant. The heat exchangers should be double-walled to prevent contamination of the household water. The controller in these systems will activate the pumps to the collectors and heat exchanger when design temperature differences are reached. The heat exchanger may be separated from the storage tank or built into it. The systems that use antifreeze fluids need regular inspection (at least every 2 years) of the antifreeze solution to verify its viability. Oil or refrigerant circulating fluids are sealed into the system and will not require maintenance. A refrigerant system is generally more costly and must be handled with care to prevent leaking any refrigerant. This hot water system can be used for heating swimming pools and spas. Lower cost unglazed (no glass cover) collectors are available for this purpose.

3.8.2 Active Solar Space Heating

The active solar space heating system can use the same operational components as the domestic water heating systems as shown in Figure 3.18, but ties into a heating distribution system that can use heated fluids as a heat source.

Figure 3.18 Typical space heating system

The distribution system includes hydronic radiator, floor coil systems, and forced air systems.

The fluid that is heated and stored (typically water) and can be distributed into the house heating system in the following ways:

1. Air distribution system: The heated water in the storage tank is pumped into a coil located in the return air duct whenever the thermostat calls for heat. The controller for the solar system will allow the pumping to occur if the temperature in the solar heated water is above a minimum amount needed to make a positive contribution to heating the home. An auxiliary heater can be used in two ways. It can add heat to the solar storage tank to maintain a minimum operating temperature in the storage tank at all times. In this case, the coil from the solar system will be located at the air handler supply plenum rather than in the return air duct. The auxiliary heater can also be a conventional furnace that will operate less often due to the warm air entering the air handler from the solar coil in the return duct.
2. Hydronic system with radiators: The heated water is circulated in series with a boiler into radiators located in the living spaces. Modern baseboard radiators operate effectively at 140°C. Solar heating systems can very often reach that temperature. Using the solar system’s heated water as the source of water for the boiler will reduce the boiler’s energy use, particularly if it senses the incoming temperature and will not operate when that temperature is above the required distribution temperature.
3. Hydronic system with in-slab heat: The solar heated water is pumped through distribution piping located in the floor of the home. Lower temperatures are used in this type of system (the slab is not heated above 80° in most cases). The auxiliary heat can be connected in series with the solar system’s heated output water or it can be connected to the solar tank to provide a minimum temperature.

The space heating system, like the domestic water heating system, must be backed up by an auxiliary heating system. It is not practical to size a solar system to provide a home’s entire heat requirement under the worst conditions. The system would become too large, too costly, and oversized for most of the time. The storage system should be sized to approximately 1.5 gallons of storage for each square foot of collector area.

3.9 PASSIVE SOLAR WATER HEATING SYSTEMS

A passive solar water heating system uses natural convection or household water pressure to circulate water through a solar collector to a storage tank or to the point of use. Active systems employ pumps and controllers to regulate and circulate water. Although passive system is generally less efficient than active systems, the passive approach is simple and economical.

Passive water heating systems must follow the same parameters for installations as that of active systems – south facing non-shaded location with the collector tilted at the angle of our latitude. Since the storage tank and collector are combined or in very close proximity, roof structural capacities must accommodate the extra weight of a passive system.

3.9.1 Types of Passive Water Heaters

Two types of passive water heaters are batch and thermosiphon systems.

3.9.1.1 Batch System

The batch system is the simplest of all solar water heating systems, as depicted in Figure 3.19. It consists of one or more metal water tanks painted with a heat absorbing black coating and placed in an insulating box or container with a glass or plastic cover that admits sunlight to strike the tank directly. The batch system’s storage tank is the collector as well. These systems will use the existing house pressure to move water through the system. Each time a hot water tap is opened, heated water from the batch system tank is removed and replaced by incoming cold water. The piping that connects to and from the batch heater needs to be highly insulated. On a cold night, when no one is drawing hot water, the water in the pipes is standing still and vulnerable to freezing. In many applications, insulated polybutylene piping is used because the pipe can expand if frozen. The water in the batch heater itself will not freeze because there is adequate mass to keep it from freezing.

Figure 3.19 A typical schematic of batch domestic water heating system

Since the tank that is storing the heated water is sitting outside, there will be heat loss from the tank during the night. This can be minimized by an insulating cover placed on the heater in the evening. The most effective use of a batch water heater is to use hot water predominantly in the afternoon and evenings when the temperature in the tank will be the highest.

3.9.1.2 Thermosiphon Systems

The thermosiphon system uses a flat plate collector and a separate storage tank that must be located higher than the collector as shown in Figure 3.20. The collector is similar to those used in active systems.

Figure 3.20 Thermosiphon system

The storage tank located above the collector receives heated water coming from the top of the collector into the top of the storage tank. Colder water from the bottom of the storage tank will be drawn into the lower entry of the solar collector to replace the heated water that was thermosiphoned upward. The storage tank may or may not use a heat exchanger. The thermosiphon system is more costly and complex than the batch system. In our area, it is best to use an indirect system (one that employs a heat exchanger). In that case, antifreeze can be used in the system eliminating freeze ups.

3.10 APPLICATIONS OF SOLAR WATER HEATING SYSTEMS

The following are a few industrial applications of solar water heaters.

1. Hotels: bathing, kitchen, washing, laundry applications
2. Dairies: ghee (clarified butter) production, cleaning and sterilizing, pasteurization
3. Textiles: bleaching, boiling, printing, dyeing, curing, ageing, and finishing
4. Breweries and distilleries: bottle washing, work preparation, boiler feed heating
5. Chemical/bulk drugs units: fermentation of mixes, boiler feed applications
6. Electroplating or galvanizing units: heating of plating baths, cleaning, degreasing applications
7. Pulp and paper industries: boiler feed applications, soaking of pulp.

3.11 ACTIVE SOLAR SPACE COOLING

Solar space cooling is quite costly to implement. It is best to use a solar system that serves more than just the cooling needs of a house to maximize the return on investment and not leave the system idle when cooling is not required. Significant space heating and/or water heating can be accomplished with the same equipment used for the solar cooling system. Active solar absorption cooling system is presented in Figure 3.21 in which (T) represents the sequence of flow.

Figure 3.21 Schematic of solar absorption cooling system (T represents the sequence of flow)

Heat from solar collectors separates a low boiling refrigerant in a generator that receives the pressurized refrigerant from an absorber. Solar heat can also be used in the evaporation stage of the cycle.

3.12 SOLAR AIR HEATING

Solar-heated air can be used for drying most crops that require warm air. Solar heated air is ideal for drying delicate foods since it will not burn or risk potential damage from high temperature steam heat. Solar heat is non-polluting and best of all, it incurs no fuel costs.

Existing commercial drying operations can be converted to utilize solar heat by installing our system to remove heat from the building’s metal roof or wall. We remove heat from under the metal panels, add the duct, and connect the ducts to the intake of the drier fans. The system then removes the heated air from the underside of the panels and passes the air to the drying chamber.

Simple sensors are installed in the air flow and use thermostatic controls to turn off the incoming air flow when the temperature is not high enough for solar heating. The existing system then operates, as it always has, burning high-cost fuel but serving the drying process.

For some in-field applications, one can use a ground-mounted polymer system that is low cost and very transportable. In new building, metal roofs and walls are integrated with the building’s structure. By trapping air into a confined space which has sunlight hitting it, the trapped air is heated up naturally by the solar power of the sun. This type of natural heat transfer is also used for solar water heating. The hot air rises, and this is a key component to solar heating with convection. Inside houses, the coldest air is closest to the floor and the warmest is up high along the ceiling. As the warm air at the top of the room cools, it drops lower, and as the cooler air is warmed up, it rises. In order to use this process naturally in houses and particularly to make use of the warming power of solar energy, means has to be found or create a way to have the cool air go into a space that is warmed up by the sun. That same space usually allows the warm air to escape once it has reached a certain point.

In many cases, all you need is some sort of confined area to direct and control airflow. For example, if you have a sunny window, you could put a piece of black fabric, wood, plastic, or metal against that window frame to trap the air in for a short period of time. There needs to be a space between the black material you choose to use, and the glass window pane itself. Usually, this space is at least a few inches, but there can be space as much as 5–6 inches between your window glass and the black material.

It is a solar thermal technology in which the energy from the sun, solar insolation, is captured by an absorbing medium and used to heat air. Solar air heating is a renewable energy heating technology used to heat or condition air for buildings or process heat applications. It is typically the most cost effective out of all the solar technologies, especially in commercial and industrial applications, and it addresses the largest usage of building energy in heating climates, which is space heating and industrial process heating.

Solar air collectors can be commonly divided into two categories:

1. Unglazed air collectors or transpired solar collector (used primarily to heat ambient air in commercial, industrial, agricultural and process applications).
2. Glazed solar collectors (recirculation types that are usually used for space heating).

3.13 SOLAR DRYERS

Solar dryers can be utilized for various domestic purposes. They also find numerous applications in industries such as textiles, wood, fruit and food processing, paper, pharmaceutical, and agro-industries.

3.13.1 Advantages

Solar dryers are more economical when compared to dryers that run on conventional fuel or electricity. The drying process is completed in the most hygienic and eco-friendly way. Solar drying systems have low operation and maintenance costs.

Solar dryers last longer. A typical dryer can last 15–20 years with minimum maintenance.

3.13.2 Limitations

1. Drying can be performed only during sunny days, unless the system is integrated with a conventional energy-based system.
2. Due to limitations of solar energy collection, the solar drying process is slow in comparison with dryers that use conventional fuels.
3. Normally, solar dryers can be utilized only for drying at 40°C–50°C.

One well-known type of solar dryer is shown in Figure 3.22.

Figure 3.22 A rice solar dryer

It was designed for the particular requirements of rice but the principles hold for other products and design types, since the basic need to remove water is the same. Air is drawn through the dryer by natural convection. It is heated as it passes through the collector and then partially cooled as it picks up moisture from the rice. The rice is heated both by the air and directly by the sun. Warm air can hold more moisture than cold air so the amount required depends on the temperature to which it is heated in the collector as well as the amount held (absolute humidity) when it entered the collector.

A rock-bed dryer is shown in Figure 3.23. In this dryer, air drawn by natural convection through an air inlet (A), circulates the heat collected by the primary solar energy collector (B), throughout the drying chamber (C), which is packed with limestone rocks of relatively uniform diameter. The heat would then stratify across the rock bed but, since rocks are poor thermal conductors, temperature differences would slowly disappear when air is not moving through the rock bed. Thus, samples positioned above the rock bed can continue drying during the night. This type of a solar dyer requires very little maintenance. Solar heated air can be used for drying most crops that require warm air. This air is ideal for drying delicate foods since it will not burn or risk potential damage from high temperature steam heat. Solar heat is non-polluting and best of all, it incurs no fuel costs.

Figure 3.23 Rock-bed solar dryer

Existing commercial drying operations can be converted to utilize solar heat by installing system to remove heat from the building’s metal roof or wall. Heat is removed from under the metal panels, add the duct, and connect the ducts to the intake of the dryer fans. The system then removes heated air from the underside of the panels and passes the air to the drying chamber.

Sensors are installed in the air flow and use thermostatic controls to turn off the incoming air flow when the temperature is not high enough for solar heating. The existing system then operates on, as it always has, and auxiliary standby conventional fuel system. Solar-heated air can be used to dry

1. Crops, timber, distillers grains, and textiles
2. Tea, coffee, beans, tobacco, etc.
3. Food for dehydration or processing
4. Sludge, manure, and compost

3.14 CROP DRYING

Controlled drying is required for various crops and products, such as grain, coffee, tobacco, fruits, vegetables, and fish. Their quality can be enhanced if the drying is properly carried out. Solar thermal technology can be used to assist with the drying of such products. The main principle of operation is to raise the heat of the product, which is usually held within a compartment or box; while, at the same time, passing air through the compartment to remove moisture. The flow of air is often promoted using the ‘stack’ effect that takes advantage of the fact that hot air raises, and therefore, it can be drawn upwards through a chimney, while drawing in cooler air from below. Alternatively, a fan can be used. The size and shape of the compartment varies depending on the product and the scale of the drying system. Large systems can use large barns, while smaller systems may have a few trays in a small wooden housing.

Solar crop drying technologies can help reduce environmental degradation caused by the use of fuel wood or fossil fuels for crop drying and can also help to reduce the costs associated with these fuels and hence the cost of the product. Improving and protecting crops also have beneficial effects on health and nutrition.

3.15 SPACE COOLING

The majority of the developing countries lies within the tropics and have little need of space heating. However, there is a demand for space cooling. The majority of the world warm climate cultures have again developed traditional, simple, elegant techniques for cooling their dwellings, often using effects promoted by passive solar phenomenon.

There are many methods for minimizing the heat gain. These include situating a building in shade or near water, using vegetation or landscaping to direct wind into the building, good town planning to optimize the prevailing wind and available shade. Buildings can be designed for a given climate; domed roofs and thermally massive structures in hot arid climates, shuttered and shaded windows to prevent heat gain, open structure bamboo housing in warm and humid areas. In some countries, dwellings are constructed underground and take advantage of the relatively low and stable temperature of the surrounding ground. There are as many options, as there are people.

Solar heating by convection is a natural process that involves trapping air and letting it warm up before releasing it back into a given space. Convection heating is often used as a solar heating source because the two naturally go hand in hand (see Fig. 3.24). Various types of solar air heating or ventilation systems available in the market are shown in Figure 3.25.

Figure 3.24 A typical air heating system

Figure 3.25 Various types of solar air heating systems

Type 1 is a very simple construction: ambient air passes from a glazed or unglazed collector directly into the room to provide ventilation and heating. Applications include vacation cottages (dehumidification) and large industrial buildings requiring adequate ventilation.

Type 2 circulates room air to the collector. The heated air rises to a thermal storage ceiling from which it is conveyed back into the room. This system uses natural convection and is well suited for apartment buildings.

Type 3 is particularly suited for retrofitting poorly insulated buildings. Collector heated air passes through a cavity between an outer insulated wall and an inner facade. This creates a buffer that considerably reduces heat loss via the facade of the building.

Type 4 is the classical solar air heating system and is commonly used. Collector heated air is circulated through channels in the floor or in the wall. Heat is radiated into the room with a time delay of 4 to 6 h. The advantage of this system consists in the large radiating surfaces, which provide for a comfortable climate. Systems with forced ventilation (fans) provide the best efficiency and thermal output. They may be used in buildings with large surfaces, which serve as radiation sources.

Type 5 is an advanced version of type 4; room air is circulated through separate channels of the storage. Thus, heat can be stored for a long period of time and released when it is needed. However, this type is rarely used as investment costs are rather high.

Type 6 combines a solar air collector and, via a heat exchanger, a conventional heating system. Thus, common radiators and floor or wall heating components may be used. This system can also provide domestic hot water and is particularly suited for retrofitting and for buildings in which heat has to be transported over long distances.

3.16 SOLAR COOKERS

Solar cooking is a technology that has been given a lot of attention in recent years in developing countries. The basic design is that of a box with a glass cover. The box is lined with insulation and a reflective surface is applied to concentrate the heat onto the pots. The pots can be painted black to help with the heat absorption. The solar radiation raises the temperature sufficiently to boil the contents in the pots. Cooking time is often considerably slower than conventional cooking stoves, but there is no fuel cost.

People use solar cookers primarily to cook food and pasteurize water, although additional uses are continually being developed. Numerous factors including access to materials, availability of traditional cooking fuels, climate, food preferences, cultural factors, and technical capabilities affect people’s approach to solar cooking.

Many types of cookers exist. Simple solar cookers use the following basic principles:

1. Concentrating sunlight: A reflective mirror of polished glass, metal, or metalized film is used to concentrate light and heat from the sun into a small cooking area, making the energy more concentrated and increasing its heating power.
2. Converting light to heat: A black or low reflectivity surface on a food container or the inside of a solar cooker will improve the effectiveness of turning light into heat. Light absorption converts the sun’s visible light into heat, substantially improving the effectiveness of the cooker.
3. Trapping heat: It is important to reduce convection by isolating the air inside the cooker from the air outside the cooker. A plastic bag or tightly sealed glass cover will trap the hot air inside. This makes it possible to reach similar temperatures on cold and windy days as on hot days.
4. Greenhouse effect: Glass transmits visible light but blocks infrared thermal radiation from escaping. This amplifies the heat trapping effect.

With an understanding of basic principles of solar energy and access to simple materials such as cardboard, aluminium foil, and glass, one can build an effective solar cooking device. The basic principles of solar box cooker design and identification of a broad range of potentially useful construction materials are continuously developed. These principles are presented in general terms so that they are applicable to a wide variety of design problems. Whether the need is to cook food, pasteurize water, or dry fish or grain, the basic principles of solar, heat transfer, and materials apply. The application of a wide variety of materials and techniques as people make direct use of the sun’s energy is continuously under development.

3.16.1 Types of Solar Cookers

All the solar cookers are subdivided into four configurations:

1. Solar cooking boxes (see Fig. 3.26) are well insulated boxes with a double glass lid and a cover with a reflector on the inside. Solar cooking boxes keep the food warm in the afternoon and evening. They are presently the most successful type of solar cookers in the world.
   

   Figure 3.26 Box-type solar cooker

2. Reflector cookers concentrate the sun’s radiation by a more or less parabolic reflector into a focal region, where the cooking vessel is fixed. Success in disseminating solar reflector cookers has only been reported from China, as shown in Figure 3.27.
   

   Figure 3.27 Reflector-type solar cooker

   Reflective materials are used to concentrate light and heat from the sun into a small cooking area, making the sun’s energy more concentrated and, therefore, more powerful, resulting in the fastest cooking times of all cooker designs.

   Parabolic cookers require more precision to focus the sunlight on the cooking vessel and are, therefore, the most complex design to build. If the sunlight is not focused exactly on the cooking vessel, the food will not cook efficiently.

3. Solar steam and convection cookers use vapour or hot air as heat transfer medium. Water is evaporated or air is heated up mostly in flat plate or vacuum collectors and then led in a piping system to the cooking vessel. Collector and cooking place can be separated and thus cooking in the shadow is possible. Most steam and convection cookers have a low efficiency and a high price, and further, they require relatively much effort in manufacturing.

Heat storage solar cookers do overcome the most important disadvantage of solar cooking in general. They allow cooking after sunset and some of them even in the morning. They collect the solar energy by high efficiency flat plate or vacuum tube collectors or concentrators and store the heat in a solid heat storage block or a liquid storage medium in a tank. They always are heavy and bulky; further, they are complex constructions, and generally, very expensive.

3.16.2 Advantages

Solar energy cooking has a variety of advantages, out of which the most important are as follows:

1. Cooking with solar energy saves fuel wood and/or chemical fuels.
2. Cooking with solar energy is clean and healthy and reduces health problems related to kitchen smoke.
3. Solar cooking enables individual families to do without commercial fuels, and thus, money can be saved.
4. Solar cooking saves time and effort that would otherwise be spent in collecting fuel wood.
5. Food cooked in box-type solar cookers cannot burn and does not have to be stirred or watched.
6. Food cooked in box-type solar cooker is cooked gently so that more of the nutrients and flavour of the food are conserved than when cooking on the fire.

3.16.3 Disadvantages

The following are some disadvantages related to the principle of solar cooking.

1. Solar cooking requires good weather with relatively steady sunshine.
2. Solar cooking cannot completely replace the conventional wood, gas, or kerosene fire.
3. Solar cooking is only possible during the daytime and not in the mornings and evenings (except with storage-type solar cookers).
4. Most types of solar cookers require industrially manufactured components. These can easily be destroyed, and it is difficult or impossible to repair or replace them with local material.
5. Some solar cooking boxes do not attain high temperatures. This requires long cooking time.
6. Boiling, roasting, and grilling require high temperatures, and thus, it is only possible in a few types of solar cookers
7. Some reflector-type solar cookers demand understanding, skill, and almost constant attention when handling and cooking with them.
8. The person doing the cooking has to stay out in the sun to avoid the risks of being dazzled or burnt.
9. Generally, families that need solar cookers mostly cannot afford them.

3.17 SOLAR POND

One of the best ways of harnessing solar energy is through solar ponds. It is basically a pool of water that collects and also stores solar energy. The peculiarity of the solar pond is that it has layers of salt solutions of differing concentrations, and thus, different densities to a certain depth. Once this depth is reached, then water with uniform, high salt concentration is obtained. The solar pond is a relatively low technology and low cost approach for harvesting solar energy. To develop a solar pond, pond is filled with three layers of water as shown in Figure 3.28.

Figure 3.28 Solar pond

1. The top layer is cold and has relatively little salt content.
2. Next is the intermediate insulating layer that has a salt gradient that maintains a density gradient. It is this density gradient that helps in preventing heat exchange with the natural convection of water.
3. The bottom layer is hot up to 100°C and has a high salt content.

It is because of these different salt contents in the different layers of water that the different layers have different densities. With the different densities in the water, the development of convection currents is prevented, which would have transferred heat to the surface of the pond, and then to the air above. Without these convection currents, heat is trapped in the salty bottom layer of the solar pond, which is used for heating of buildings, industrial processes, generation of electricity, and other purposes. In addition to the abovementioned uses, solar ponds can also be used in water desalination and for storage of thermal energy.

In this system, a large salty lake is used as a plate collector. With the right salt concentration in the water, the solar energy can be absorbed at the bottom of the lake. The heat is insulated by different densities of the water, and at the bottom, the heat can reach 90°C, which is high enough to run a vapour cycle engine; at the top of the pond, the temperature can reach 30°C. There are three different layers of water in a solar pond: the top layer has less concentration of salt, the intermediate layer acts as a thermal insulator, and finally, the bottom layer has a high concentration of salt. These systems have a low solar to electricity conversion efficiency, less than 15% (having an ambient temperature of 20°C and storage heat of 80°C). One advantage of this system is that because the heat is stored, it can run day and night if required. Further, due to its simplicity, it can be constructed in rural areas in developing countries.

3.17.1 Advantages of Solar Pond

There are many advantages of using a solar pond to meet the energy requirements of a place.

1. The greatest advantage lies in the fact that it has a low cost per unit area of collection and also an inherent capacity for storage purposes. In addition to this, it is possible to easily construct solar ponds over large areas with which it is possible for the diffusion of solar resources to get concentrated on a grand scale.
2. Not only is a solar pond a great source of generation of electricity, it produces many environmental advantages when compared to the use of other fossil fuels for producing electricity. With a solar pond, the greatest advantage to the environment is that the heat energy is provided without the burning of any fuel, which reduces pollution.
3. Another advantage is that because there is no use of conventional energy resources for creating electricity in solar ponds, conventional energy resources are conserved. Further, the third advantage of solar ponds to the environment is that it is coupled with desalting units that are used for purifying contaminated impaired water while the pond itself is the receptacle for waste products.

SUMMARY

• Solar thermal systems have the flexibility of being used for off-grid applications also. It includes solar water heaters (using both flat plate collectors and evacuation tube collectors), solar mass cooking, and comfort cooling applications.
• Solar collectors (or solar thermal collectors) are devices or systems designed to capture and use solar radiation for heating air or water and for producing steam to generate electricity.
• Flat plate collectors are the most common and widely used style of solar thermal collector for domestic hot water applications.
• For solar water heating systems in home and solar space heating, flat plate collectors are the most common type of solar collector used.
• When high temperatures above 120°C are required, such as for steam production, concentrating collectors are often used.
• A concentrating collector uses mirrors to concentrate the sunlight onto an absorber tube or panel, allowing much higher temperatures to be reached. Such collectors normally require 1 or 2 axis tracking to follow the sun and ensure optimal reflection angle.
• Collector Concentration Ratio (CCR) is the ratio of Aa//Ad. A collector is defined as concentrating if its absorber (fin) area Ad is smaller than the aperture area Aa. It is also used as a measure for classifying collectors. Since this ratio approximately determines the operating temperature, such method of classification is equivalent to classifying collectors by its operating temperature range.
• Heliostat is a mirror-based system used to continuously reflect sunlight onto a central receiver.
• Solar cooking technology has been given a lot of attention in recent years in developing countries. Solar cookers are primarily used to cook food and pasteurize water, although additional uses are continually being developed.
• Solar pond is basically a pool of water that collects and also stores solar energy. The peculiarity of the solar pond is that it has layers of salt solutions of differing concentration and thus, different densities to a certain depth. Once this depth is reached, then water with uniform, high salt concentration is obtained. Solar pond is a relatively low cost technology for harvesting solar energy.

REVIEW QUESTIONS

1. What are solar collectors? Give their classification and compare them based on construction and area of applications.
2. With neat sketches, discuss important parts of any flat plate solar collector. Further, discuss material aspects of individual parts.
3. Give schematic representation of an air-type flat plate solar collector system integrated with an auxiliary system for space heating and explain its working.
4. What are the main components of a flat plate solar collector? Explain.
5. Compare flat plate, parabolic dish of collectors to be used for a solar thermal plant with respect to (i) temperature, (ii) concentration ratio (iii) suitability, and (iv) cost.
6. State clearly the difference between the distributed collector system and central receiver system in solar thermal applications.
7. How are solar air collectors classified?
8. What are the main applications of a dryer?
9. Why orientation is needed in concentrating-type collectors? Describe the different methods of sun tracking.
10. What are the advantage and disadvantages of concentrating collectors over a flat plate collector?
11. Explain the considerations in installing a flat plate collector system with reference to the geographical location and the angle of tilt.
12. State the function of the following
    1. Solar thermal collector
    2. Central receiver
    3. Draw a diagram of a distributed central receiver solar thermal power plant and explain the arrangements of solar heliostats.