PERFORMANCE LIMITS OF HELIOSTAT FIELDS

Geometric and thermodynamic arguments are used to derive upper limits on the performance of a solar energy collection system, consisting of an axisymmetric heliostat field, a solar tower, secondary optics and a black receiver. Performance limits on collected power, concentration, and work output are presented. Performance of tower systems with seve ral secondary optics options is compared: tower-top Compound Parabolic Concentrator (CPC), Tailored Edge-Ray Concentrator (TERC) approximate d by a cone, and Cassegrainian with ground-level CPC or Compound Ellip tic Concentrator (CEC). Optimized ray tracing is used to generate the design parameters of the secondary concentrators that yield the highes t optical efficiency. The results show that the tower-top Cone provide s the best performance regarding both concentration and efficiency, ex cept for very large fields. The Cassegrainian designs come in second, but become equal and even better than the Cone for large fields. The r esults for the Cassegrainian are sensitive to the value of the reflect ivity, due to the additional reflections incurred. The choice of a CEC is better than a CPC for the terminal concentration in a Cassegrainia n system, but the difference is small. The suitability of the differen t design options for high-temperature solar applications is discussed. The recommendations regarding optical configuration depend on field s ize, as well as on application-specific constraints.