Flat Plate Collectors

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The flat plate collectors are based on two important principles: a black base that absorbs the solar radiation better than any other color and a glass lid that is needed to keep the heat in. Figure 7.2 shows the cross-section of the most commonly used flat plate collector. Its surface should be located perpendicularly to the solar radiation direction for the maximum solar energy gain.

Here the sun's rays go through the glass cover and the air layer to warm up the black metal plate which in turn warms the water. Unfortunately, the ordinary metal plate is also warmed up. The heat insulation lagging keeps most of the heat inside the sandwich. With the heat in the water, it has now to be moved to where good use can be made of it. The simplest method for achieving this water movement is shown in Fig. 7.2, the ""thermo-siphon" system. Its operation is based on the simple fact that hot water will rise to settle above a quantity of cooler water.

As the collector heats up, the water in it rises out at the upper pipe and pushes its way into the top of the tank. This hot water then displaces some of the cold in the tank, pushing it down and out of the bottom. This heat-induced circulation is completed as the water, being pushed down the pipe, comes round the bottom and back into the collector.

Different types of solar collectors are given in Fig. 7.3. Among these, the most primitive is unglazed panels which are most suitable for swimming pool heating where it is not necessary for the collectors to raise the temperature of the water to more than a few degrees above ambient air temperature, so heat losses are relatively unimportant.

Tank

Tank

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Fig. 7.2 Flat plate collector cross-section (Howell 1986)

Fig. 7.2 Flat plate collector cross-section (Howell 1986)

Flat plate collectors are the main stay of domestic solar water heating, These are usually single glazed, but may have an additional second glazing layer. The more elaborate the glazing system, the higher the temperature difference that can be sustained between the absorber and the external wall. It is necessary and is usual that the absorber plate should have a black surface with high absorptivity. In general, most black paints reflect approximately 10% of the incident radiation. On the other hand, flat plate air collectors are mainly used for space heating only. These type of collectors are connected with photovoltaic panels for producing both heat and electricity. Evacuated tube collectors in Fig. 7.3 are in the form of a set of modular tubes similar to fluorescent lamps. The absorber plate is a metal strip down the center of each tube. A vacuum in the tube suppresses convective heat losses.

In practice, most often the collectors do not move, and therefore, they must be located such that during one day the maximum amount of solar radiation can be converted into solar energy. For this reasons, fixed collectors must be located to face south (north) in the northern (southern) hemisphere. This implies that for given a latitude there is a certain angle which yields the maximum solar energy over the year. As a practical rule, for low latitudes the angle of the collector is almost equivalent to the angle of latitude, but increases by 10° at 40°N and 40°S latitudes. All these arrangements are for flat-surfaced collectors. Typical temperatures that can be achieved by flat plate collectors vary between 40°C and 80°C depending on the as-

LINE FOCUS 50-150 °C RISE EVACUATED TUBE 10-150 "C RISE POINT FOCUS >100 "C RISE Fig. 7.3 Various collectors tronomic, topographic, and meteorological conditions. In a flat plate collector, the energy incident on the surface cannot be increased and all that can be done is to ensure that surface absorbs as much as possible of the incident radiation, and that the losses from this surface are reduced as far as possible. Figure 7.4 shows a flat plate collector.

Some of the incident radiation is lost by reflection but for a blackened surface about 95% of the radiation will be absorbed. The heat losses from flat plate collectors are shown in the same figure. In general, in these collectors the lower surface usually has an insulating layer of material such as several centimeters of glass wool. The heat can be lost through the conduction, convection, and radiation mechanisms.

Flat plate collectors are usually roof mounted and their tracking of the sun is not possible. They are subject to many external events such as frost, wind, sea spray, acid rain, and hail stones. They must also be resistant against corrosion and significant temperature changes. Low-temperature flat plate collectors are able to raise the water temperature up to boiling point in the summer, provided that they are double-glazed and the water circulation is not fast enough to carry away the heat quickly. These may be only a few square meters in area. In order to collect enough solar energy to supply the winter demand, the collectors would have to cover a large area and in such cases the solar energy production during the summer would not be wholly exploited. This means a wastage of the capital expenditure.

Fig. 7.4 a-c. Flat plate solar collectors (Dunn 1986)

7.4.2 Tracking Collectors

Logically, in order to collect the maximum radiation for each unit surface area of collector, it is necessary to direct the collector surface at right angles to the direction of direct radiation. Continuation of maximum benefit by the collector during a day is possible by keeping the collector surface perpendicular to the incident solar radiation throughout the hours of daylight. It is, theoretically, simple to show that a tracking (moving) collector compared to the horizontal collector at the same site will collect n/2 times more energy per day. However, in practice this factor is around 1.5 times. Of course, the more the direct radiation, the better is the energy generation from the sun's radiation.

7.4.3 Focusing (Concentrating) Collectors

If high temperatures are needed then the collector surface is manufactured as a curve for focusing (concentrating) the solar radiation at certain points by a mirror or lens. Mirrors are cheaper to construct than lenses. The mirror collectors may have spherical parabolic or linear parabolic shapes as shown in Fig. 7.5. In a parabolic mirror solar radiation is concentrated at a point and, therefore, the concentration ratio is approximately 40,000 whereas the concentration for a one-dimensional device of a linear parabolic system is around 200.

So far as the lens collectors are concerned there are single surface or equivalent Fresnel types as shown in Fig. 7.6. Although in flat plate collectors diffuse solar radiation also makes a contribution in the radiation collection, the concentrating collectors focus the incident sunlight on the collector surface, leaving the contribution of diffuse radiation aside.

Another disadvantage of the concentrating collectors is that they must track the sun in order to obtain the optimal benefit. Concentrating collectors rise the temperature of the heater up to 300- 6000 °C. These collectors must be aligned with sufficient accuracy to ensure that the focus coincides with the collector surface. The greater the degree of concentration, the more accurate is the alignment required.

Line focus collectors concentrate the solar radiation onto a pipe running down the center of a trough, and are mainly used for generating steam for electricity generation. To get the maximum benefit, it is necessary that the trough is pivoted to track the sun's movement in any direction. Point focus collectors as shown in Fig. 7.7 also track the sun but in two dimensions and these also generate steam for conversion into electricity.

If the solar radiation is concentrated through mirrors or lenses then temperatures in excess of the boiling point of water may be reached. It is possible to use such high temperatures through steam production for mechanical work, for instance, for water

Fresnel Plate Collector

Paraboloid mirror Point focus

Linear Paraboloid mirror Line focus

Fig. 7.5 a,b. Parabolic mirror concentrator of solar radiation

Paraboloid mirror Point focus

Linear Paraboloid mirror Line focus a b

Fig. 7.6 a-d. Single surface and equivalent Fresnel lenses

Fig. 7.6 a-d. Single surface and equivalent Fresnel lenses

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Fig. 7.7 Line and point focus by moving mirrors pumping or electricity generation. These are named high-temperature collectors. Most often parabolic mirrors are used for solar radiation concentrations. As shown in Fig. 7.7 all the sun's rays directed parallel to the axis of such a mirror will be reflected to one point.

It is necessary that the mirror tracks the sun, otherwise slightly off-axis solar beams will make inconvenient reflections, and the intensity of the radiation concentration onto a point or line will be weakened. In the line focus form the sun's radiation can be concentrated on a small region running along the length of the mirror. For the maximum focusing of the sun's radiation, it is necessary to tract the sun in an elevation that is only up and down. However, in the point focus form, the sun's radiation is reflected on a boiler in the mirror center. For optimum performance, the axis must be pointed directly at the sun at all times, so it needs to track the sun both in elevation and in azimuth (Chap. 3).

Another technology of centralized electricity generation is solar-thermal power. These are produced by using large mirrored troughs to reflect the sun's rays onto an oil-filled tube, which in turn superheats water to produce the steam that drives an electricity-generating turbine. Since the mid-1980s, about 350 MW of these solar energy systems have been installed across three square miles of the southern Califor-nian desert and these are enough to supply electricity to 170,000 homes. Especially, in areas of extensive pollution control, solar-thermal electricity substitution is required urgently for the pollution reduction. In order to produce sufficient energy, solar-thermal electricity production is only practical in areas where there are intense direct sunlight conditions such as the arid regions of the world. Another way of harnessing solar power is to use an array of mirrors to concentrate, or focus, sunlight onto water flowing through a metal pipe. The resulting steam can then be used to drive a turbine.

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