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In solar tower plants, where a rotationally symmetric field of heliostats surrounds the tower, an axisymmetric secondary concentrator such as a compound parabolic concentrator (CPC) or a tailored concentrator or a cone is the obvious choice. For locations at higher latitudes, however, the reflecting area of the heliostats may be used more efficiently if the field of heliostats is located opposite to the sun as seen from the tower. Then the field is asymmetric with regard to the tower. In the case of an asymmetric field, an axisymmetric concentrator necessarily has a concentration significantly lower than the upper limit. Furthermore, the area on the ground from which a tilted axisymmetric concentrator accepts radiation is an ellipse, including also heliostats very distant to the tower producing a large image of the sun. For these reasons we investigate asymmetric secondaries. From the shape of the edge ray reflectors constructed for rays in the central south north plane we conclude that a skew cone reflector might be appropriate for the field, and optimize its free parameters by means of ray tracing. Asymmetric concentrators may increase the concentration by up to 25% at the same efficiency compared to optimized axisymmetric CPC or cone reflectors.

We derive the theoretical upper limit for concentration of direct solar radiation at low latitudes with nontracking concentrators from the projected solid angle sampled by the apparent motion of the sun, for the case where the energy efficiency is referred to the energy incident on the entrance aperture. based on the fact that the solar radiation is not uniformly distributed within this projected solid angle and that the apparent solar motion is known, we derive the optimal acceptance as a function of direction and time, which means rejecting the lower density radiation and switching off the device when losses would be higher than gains. Just as a device may gain concentration by rejecting radiation from certain directions, it can also gain by not operating at all, thus avoiding losses at certain times. Trough-type systems, which have translational symmetry, cannot be ideal nontracking concentrators, but for low losses they perform only slightly worse than general three-dimensional concentrators.

Parabolic trough type solar power plants could reach higher efficiencies by using secondary reflectors, which increase the concentration of the solar irradiance onto the absorber tube. Recently, compact secondary concentrators have been proposed which are constructed according to the edge ray principle; they are composed of an involute part and an edge ray reflector with a form close to a straight line. Using these reflector forms as a starting point, we optimize numerically various secondary concentrators and investigate their performance by means of raytracing, taking into account reflectivity losses, shading and the effective solar angular distribution, i.e. the sunshape. We investigate the performance of secondary concentrators with the outer part approximated by a straight line. The entire characteristic curve of collection efficiency versus concentration was investigated, for both a pillbox sunshape and a Gaussian sunshape. It is found that the secondary reflector significantly improves the concentration and is essentially optimal in a wide class of shapes, even at efficiencies close to unity.

The Bayerisches Zentrum fur angewandte Energieforschung Bayern (ZAE) performs an in situ test of eight different solar domestic hot water systems in Oberpfalz, Bayern. We present results of the evaluation of the collector loop from one system. This system has been chosen because due to the relatively low load the variability of the collector loop temperature is expected to be insufficient for testing purposes. The system under test consisted of the collector field and the piping from the cellar to the roof. First, we use a dynamic collector model which ignores the extra pipes and thus implicitly takes into account the pipes by modified parameter values. It is shown that this model can be inaccurate if the input data such as the irradiance and the inlet temperature do not vary sufficiently and independently from each other. Second, we use a model which explicitly takes into account the pipes, using the overall loss coefficient and the total specific heat capacity of the pipes as new parameters. It is shown that it is possible to identify the values of these additional parameters. The overall accuracy improves with the extended model. The models without and with pipes differ mainly in dynamic effects associated with the energy stored in the pipes during stops of the collector loop pump.

We present an approach to find optimal reflector shapes for non-tracking solar collectors under practical constraints. We focus on cylindrical absorbers and reflectors with translational symmetry. Under idealized circumstances, edge ray reflectors are well known to be optimal. However, it is not clear how optimal reflectors should be shaped in order to obtain maximum utilizable energy for given operating temperatures under practical constraints like reflectivity less than unity, real radiation data, size limits, and gaps between the reflector and the absorber. For a prototype collector with a symmetric edge ray reflector and a tubular absorber, we derive from calorimetric measurements under outdoor conditions the optical efficiency as a function of the incidence angle. Using numerical optimization and raytracing, we compare truncated symmetric edge ray reflectors, truncated asymmetric edge ray reflectors and free forms parametrized by Bezier splines. We find that asymmetric edge ray reflectors are optimal. For reasonable operating conditions, truncated asymmetric edge ray reflectors allow much better land use and easily adapt to a large range of roof tilt angles with marginal changes in collector construction. Except near the equator, they should increase the yearly utilizable energy per absorber tube by several percent as compared to the prototype collector with symmetric reflectors. [C] 2000 Elsevier Science Ltd. All rights reserved.