Solar thermal collectors and applications

Sun tracking concentrating collectors

Energy delivery temperatures can be increased by decreasing the area from which the heat losses occur. Temperatures far above those attainable by FPC can be reached if a large amount of solar radiation is concentrated on a relatively small collection area. This is done by interposing an optical device between the source of radiation and the energy absorbing surface. Concentrating collectors exhibit certain advantages as compared with the conventional flat-plate type [59]. The main ones are: 1

collector the cost per unit area of the solar collecting surface is therefore less than that of a FPC.

5. Owing to the relatively small area of receiver per unit of collected solar energy, selective surface treatment and vacuum insulation to reduce heat losses and improve the collector efficiency are economically viable.

Their disadvantages are:

1. Concentrator systems collect little diffuse radiation depending on the concentration ratio.

2. Some form of tracking system is required so as to enable the collector to follow the sun.

3. Solar reflecting surfaces may loose their reflectance with time and may require periodic cleaning and refurbishing.

Many designs have been considered for concentrating collectors. Concentrators can be reflectors or refractors, can be cylindrical or parabolic and can be continuous or segmented. Receivers can be convex, flat, cylindrical or concave and can be covered with glazing or uncovered. Concentration ratios, i. e. the ratio of aperture to absorber areas, can vary over several orders of magnitude, from as low as unity to high values of the order of 10 000. Increased ratios mean increased temperatures at which energy can be delivered but consequently these collectors have increased requirements for precision in optical quality and positioning of the optical system.

Because of the apparent movement of the sun across the sky, conventional concentrating collectors must follow the sun’s daily motion. There are two methods by which the sun’s motion can be readily tracked. The first is the altazimuth method which requires the tracking device to turn in both altitude and azimuth, i. e. when performed properly, this method enables the concentrator to follow the sun exactly. Paraboloidal solar collectors generally use this system. The second one is the one-axis tracking in which the collector tracks the sun in only one direction either from east to west or from north to south. Parabolic trough collectors (PTC) generally use this system. These systems require continuous and accurate adjustment to compensate for the changes in the sun’s orientation. Relations on how to estimate the angle of incidence of solar radiation for these tracking modes are given in Section 3.2.

The first type of a solar concentrator, shown in Fig. 6, is effectively a FPC fitted with simple flat reflectors which can markedly increase the amount of direct radiation reaching the collector. This is a concentrator because the aperture is bigger than the absorber but the system is stationary. A comprehensive analysis of such a system is presented in Ref. [60]. The model facilitates the prediction of the total energy absorbed by the collector at any hour of the day for any latitude for random tilt angles and azimuth angles of the collector and reflectors. This simple enhancement of FPC was initially suggested by Tabor in 1966 [61].

Other important studies on this area were presented by Seitel [62] and Perers et al. [63].

Another type of collector, already covered under the stationary collectors, the CPC is also classified as concen­trator. This, depending on the acceptance angle, can be stationary or tracking. When tracking is used this is very rough or intermitted as concentration ratio is usually small and radiation can be collected and concentrated by one or more reflections on the parabolic surfaces.

As was seen above one disadvantage of concentrating collectors is that, except at low concentration ratios, they can use only the direct component of solar radiation, because the diffuse component cannot be concentrated by most types. However, an additional advantage of concen­trating collectors is that, in summer, when the sun rises well to the north of the east-west line, the sun-follower, with its axis oriented north-south, can begin to accept radiation directly from the sun long before a fixed, south-facing flat - plate can receive anything other than diffuse radiation from the portion of the sky that it faces. Thus, in relatively cloudless areas, the concentrating collector may capture more radiation per unit of aperture area than a FPC.

In concentrating collectors solar energy is optically concentrated before being transferred into heat. Concen­tration can be obtained by reflection or refraction of solar radiation by the use of mirrors or lens. The reflected or refracted light is concentrated in a focal zone, thus increasing the energy flux in the receiving target. Concen­trating collectors can also be classified into non-imaging
and imaging depending on whether the image of the sun is focused at the receiver or not. The concentrator belonging in the first category is the CPC whereas all the other types of concentrators belong to the imaging type.

The collectors falling in this category are:

1. Parabolic trough collector;

2. Linear Fresnel reflector (LFR);

3. Parabolic dish;

4. Central receiver.

Solar thermal collectors and applications

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