Solar thermal collectors and applications

Compound parabolic collectors

CPC are non-imaging concentrators. These have the capability of reflecting to the absorber all of the incident radiation within wide limits. Their potential as collectors of solar energy was pointed out by Winston [44]. The necessity
of moving the concentrator to accommodate the changing solar orientation can be reduced by using a trough with two sections of a parabola facing each other, as shown in Fig. 4.

Compound parabolic concentrators can accept incoming radiation over a relatively wide range of angles. By using multiple internal reflections, any radiation that is entering the aperture, within the collector acceptance angle, finds its way to the absorber surface located at the bottom of

the collector. The absorber can take a variety of configur­ations. It can be cylindrical as shown in Fig. 4 or flat. In the CPC shown in Fig. 4 the lower portion of the reflector (AB and AC) is circular, while the upper portions (BD and CE) are parabolic. As the upper part of a CPC contribute little to the radiation reaching the absorber, they are usually truncated thus forming a shorter version of the CPC, which is also cheaper. CPCs are usually covered with glass to avoid dust and other materials from entering the collector and thus reducing the reflectivity of its walls.

These collectors are more useful as linear or trough-type concentrators. The acceptance angle is defined as the angle through which a source of light can be moved and still converge at the absorber. The orientation of a CPC collector is related to its acceptance angle (Oc, in Fig. 4). Also depending on the collector acceptance angle, the collector can be stationary or tracking. A CPC concentrator can be orientated with its long axis along either the north-south or the east-west direction and its aperture is tilted directly towards the equator at an angle equal to the local latitude. When orientated along the north-south direction the collector must track the sun by turning its axis so as to face the sun continuously. As the acceptance angle of the concentrator along its long axis is wide, seasonal tilt adjustment is not necessary. It can also be stationary but radiation will only be received the hours when the sun is within the collector acceptance angle. When the concen­trator is orientated with its long axis along the east-west direction, with a little seasonal adjustment in tilt angle the collector is able to catch the sun’s rays effectively through its wide acceptance angle along its long axis. The minimum acceptance angle in this case should be equal to the maximum incidence angle projected in a north-south vertical plane during the times when output is needed from the collector. For stationary CPC collectors mounted in this mode the minimum acceptance angle is equal to 47°. This angle covers the declination of the sun from summer to winter solstices (2 X 23.5°). In practice bigger angles are used to enable the collector to collect diffuse radiation at the expense of a lower concentration ratio. Smaller (less than 3) concentration ratio CPCs are of greatest practical interest.

These according to Pereira [45] are able to accept a large proportion of diffuse radiation incident on their apertures and concentrate it without the need of tracking the sun.

A method to estimate the optical and thermal properties of CPCs is presented in Ref. [46]. In particular, a simple analytic technique was developed for the calculation of the average number of reflections for radiation passing through a CPC, which is useful for computing optical loses. Many numerical examples are presented which are helpful in designing a CPC.

Two basic types of CPC collectors have been designed; the symmetric and the asymmetric. These usually employ two main types of absorbers; fin type with pipe and tubular absorbers [47-50].

Practical design considerations such as the choice of the receiver type, the optimum method for introducing a gap between receiver and reflector to minimise optical and thermal loses and the effect of a glass envelope around the receiver are given in Ref. [51]. Other practical design considerations for CPCs with multichannel and bifacial absorbers are given in Refs. [52] and [53], respectively, whereas design considerations and performance evaluation of cost-effective asymmetric CPCs are given in Ref. [54].

The characteristics of a typical CPC are shown in Table 3.