Optimized reflectors for non-tracking solar collectors with tubular absorbers

We present an approach to find optimal reflector shapes for non-tracking so lar collectors under practical constraints. We focus on cylindrical absorbe rs and reflectors with translational symmetry. Under idealized circumstance s, edge ray reflectors are well known to be optimal. However, it is not cle ar how optimal reflectors should be shaped in order to obtain maximum utili zable energy for given operating temperatures under practical constraints l ike reflectivity less than unity, real radiation data, size limits, and gap s between the reflector and the absorber. For a prototype collector with a symmetric edge ray reflector and a tubular absorber, ae derive from calorim etric measurements under outdoor conditions the optical efficiency as a fun ction of the incidence angle. Using numerical optimization and raytracing, we compare truncated symmetric edge ray reflectors; truncated asymmetric ed ge ray reflectors and free forms parametrized by Bezier splines. We find th at asymmetric edge ray reflectors are optimal. For reasonable operating con ditions, truncated asymmetric edge ray reflectors allow much better land us e and easily adapt to a large range of roof tilt angles with marginal chang es in collector construction. Except near the equator, they should increase the yearly utilizable energy per absorber tube by several percent as compa red to the prototype collector with symmetric reflectors. (C) 2000 Elsevier Science Ltd. All rights reserved.