I I I I

Finned surface

Finned surface

Transparent cover plate

Absorber plate

Insulation corrugated surface ib)

corrugated surface ib)

Porous matrix Insulation

Pourous matrix <c>

Figure 16.9 Typical air collector designs. (a) Finned surface. (b) Corrugated surface. (c) Porous matrix.

Transparent cover plate

Finned absorber plate

■ Insulation

Transparent cover plate

Absorber plate

Insulation

Transparent cover plate

Porous matrix Insulation

Pourous matrix <c>

Figure 16.9 Typical air collector designs. (a) Finned surface. (b) Corrugated surface. (c) Porous matrix.

about 3 ft wide by 7 ft tall, although there is no standard size. Collectors may have internal manifolds or they may be manifolded externally to form collector arrays (Figure 16.10). Internally manifolded collectors are easily connected together, but only a small number can be hooked together in a single array and still have good flow distribution. Small arrays (5 to 15) are often piped together with similar arrays in various series and parallel arrangements to give the best compromise between nearly uniform flow rates in each collector, and as small a pressure drop and total temperature rise as can be attained. Externally manifolded collectors are easily connected in balanced arrays if the external manifold is properly designed. However, these types of arrays require more field connections, however, have more exposed piping to insulate, and are not as neat looking.

The overall performance of a collector array, measured in terms of the collector array efficiency, may be quite a bit less than the collector efficiency of the individual collectors. This is due primarily to unequal flow distribution between collectors, larger temperature rises in series connections than in single collectors, and heat losses from the connecting piping. A good array design will minimize these factors together with the pumping requirements for the array.

Concentrating collectors provide relatively high temperatures for applications such as air conditioning, power generation, and the furnishing of industrial or

Transparent cover plate

Finned absorber plate

■ Insulation

Transparent cover plate

Figure 16.10 Examples of collectors hooked in parallel, (a) Internally manifolded, (b) Externally manifolded.

process heat above 250°F (121°C). They generally cannot use the diffuse or scattered radiation from the sky and must track so that the sun's direct rays will be concentrated on the receiver. The theory is simple. By concentrating the sun's rays on a very small surface, heat losses are reduced at the high temperature desired. An important point to make is that concentrating collectors do not increase the amount of energy above that which falls on the mirrored surfaces; the energy is merely concentrated to a smaller receiver surface.

A typical parabolic trough-type solar collector array is shown in Figure 16.11. Here the concentrating surface or mirror is moved, to keep the sun's rays concentrated as much as possible on the receiver, in this case a tube through which the coolant flows. In some systems the tube moves and the mirrored surfaces remain fixed.

This type collector can be mounted on an east-west axis and track the sun by tilting the mirror or receiver in a north-south direction (Figure 16.12a). An alternative is to mount the collectors on a north-south axis and track the sun by rotating in an east-west direction (Figure 16.12b). A third scheme is to use a polar mount, aligning the trough and receiver parallel to the earth's pole, or inclined at some angle to the pole, and tracking east to west (Figure 16.12c). Each has its advantages and disadvantages and the selection depends upon the application. A good discussion of concentrating collectors is given in Ref. 8.

Fully tracking collectors may be a parabolic disk with a "point source" or may use a field of individual nearly flat moving mirrors or heliostats, concentrating their energy on a single source, such as might be installed

Figure 16.11 Typical parabolic trough-type solar collector array (Suntec, Inc.).

Figure 16.11 Typical parabolic trough-type solar collector array (Suntec, Inc.).

Figure 16.12 Trough-type collector arrangements for sun tracking, (a) N-S horizontal E-W tracking, (b) EW horizontal N-S tracking, (c) Polar axis E-W tracking.

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