SURS systems use much shallower curvature than that of ‘involute’ CPCs (CPC’s designed for an acceptance angle similar to that of a flat plate collector). This was sometimes referred to as an ‘ortho-CPC’ during development as it was a straighter design. There are now two variants. Both configurations use the same mathematical formulation for the curvature as conventional CPC’s, but use different values of the variables, notably the acceptance angle and the reference tube from which the reflector curve is drawn.

In the SURS A design, which was the first version discovered, the reflector is designed to direct light to neighbouring tubes rather than the closest tube as in the CPC. Therefore,

there is crossover of radiation from adjacent reflectors, and an array of tubes is presumed rather than one. It is, in essence, a multi-absorber CPC.

Fig. 2. Three tube element of SURS A array showing involved reflectors. Each reflector side is referenced to the more distant tube in the curved V, rather than the nearets as in the CPC. Two dotted tangent lines are shown. The curve criterion is such that light passing along these lines will be reflected straight back along the incident direction. Light incident more normally to the collector aperture will be collected. Between the left and the centre tube, the light ray is collected after one bounce. Between the centre and right tubes, the light ray shown is collected after a double bounce. The double bounce incurs a second reflection loss, but this is less important if mirror reflectance is high as it is with thin glass reflectors.

In SURS A, incoming light tangential to a neighbouring absorber tube is reflected back in the same direction. Light at more normal incidence is reflected either to the neighbouring tube or one of the two reflector segments between adjacent evacuated tubes. Light can be reflected from one reflector to the other and still be collected. This is not done in a CPC. SURS A could also be designed for EW orientation of tubes but this is has not been done.

This design requires two curved reflector strips for each tube, but all reflectors are identical. They may be produced by the conventional thin glass manufacturing technique, except that the reflector backing is pressed into the desired shape before assembly, and then the mirrored glass is pressed together with the curved backing and the glue is allowed to set. It would be important to minimise optical absorption by edge supports as much as possible. There are several possible means of reflector support but this issue is not addressed in this paper.

The configuration appears to offer good water drainage compared to CPCs, because a slight gap can be left at the bottom join, but the whole panel is deeper at about 100mm. In ideal form at low concentration, SURS is as efficient as a CPC if a perfect reflector is modelled, and such SURS designs offer up to 98% in initial light collection before optical losses, similar to a ideal CPC with a gap left between cusp and absorber due to the evacuated space. If the reflectance of the mirror is not high, as with aluminium reflectors, performance may be lower than that of a CPC because of a greater tendency for double ray bouncing. However, because high reflectance thin glass can be used for SURS and not for a tightly curved CPC, effective reflectance can be higher and practical performance greater.

This means that this reflector may have a specific niche market for evacuated tubes provided the glass reflector can be manufactured at reasonable cost.

For spacings of from 16 to 20 tubes in a 1350mm width, the performance of SURS A is excellent with few rays lost in any direction, but for wider tube spacings SURS A starts to lose rays and performance drops off more rapidly than a CPC. A second configuration, SURS B, was developed to improve widely spaced tube module performance.

SURS B

This is visually and practically very similar to SURS A but the curvature criterion is different. A sketch of the collector is shown in Fig. 3.

The B curve is constructed such that light passing tangent to the nearest tube is reflected horizontally across the V. Because of symmetry, it is reflected back out tangentially to the other tube. Light incident inside these lines will be reflected to the tubes, although some rays will not be collected for certain angles. Although SURS A performance is difficult to better for 16 tube spacings or higher, the SURS B configuration appears to have a slightly higher performance than SURS A at wide tube spacings and is the collector of choice for the 14 and 12.5 tube spacings.