D. Mills*, G. L. Morrison**

*School of Physics, University of Sydney, Sydney Australia.

E-mail: d. mills@physics. usyd. edu. au

**School of Mechanical and Manufacturing Engineering
University of New South Wales, Sydney Australia 2052
E-mail: g. morrison@unsw. edu. au

In this paper, an evacuated tube collector system for domestic hot water and space heating has been designed using novel optics. The objective was not to gain increased performance as compared to a CPC using a similar reflector material, but to allow the possible use of glass reflector and also to deliver several practical benefits. The optical system is based upon a novel arrangement of reflector optics in which reflectors adjacent to a tube in the array reflect solar radiation only to neighbouring tubes. In this arrangement, the tubes are mounted above the optics for better diffuse radiation collection, and can protrude above snow in winter situations. The reflector arrangement uses shallower curvature than for conventional CPCs, potentially allowing the use of thin glass reflector. Build up of dirt can be avoided by providing a gap in the bottom of the reflector shape which allows rainwater to wash out the reflector system. performance results are reported in the paper along with the authors’ calculated optical performance.

Introduction

The reflector system uses a novel non-imaging reflector which may be regarded as an alternative reflector to the CPC (Compound Parabolic Concentrator) (Winston, 1975)commonly used with evacuated tube domestic solar water heaters in Europe. It is now called SURS (Solahart Unique Reflector System).

When approached by the manufacturer to assist with optical design for a new evacuated tube module, the authors were aware that a well designed CPC cannot be improved upon in optical terms for a given set of materials. However, the CPC has some disadvantages. It cannot use higher reflectance glass reflector materials because curvatures requires are too tight; drainage cannot occur at the lower part of the reflector, so that streaks of dirt can form from dried channelling water; and the evacuated tubes become covered by snow in winter conditions.

A new variation non-imaging reflector design was developed to overcome these disadvantages. Unlike a CPC, it is not strictly a collector which performs maximally (Mills, 1995), but it is very close in practical performance to a CPC when using the same reflector, and because of less extreme curvature, it has the option of using thin glass reflectors for higher performance. At this point, commercial versions have not yet used the glass option, but this is a possibility for the future.