Upper limits to the annual optical efficiency of solar tower systems

This study presents an analysis of the theoretical limits to the annual optical efficiency of the Light Collection and Concentration (LCC) subsystem of a solar tower system. The upper limits examined range from highly theoretical to relatively realistic. The concept of an “ideal” LCC subsystem is introduced as a vehicle to consider incrementally the different factors that influence the optical performance of the subsystem, starting with cosine and attenuation, progressively increasing the complexity of the model to include the spillage and the effect of the shadow cast by the receiver. At each degree of complexity, and for specified latitude, clear-sky DNI and total heliostat mirror area, the optimal configuration that achieves the highest annual optical efficiency in terms of field shape, tower height, and receiver envelope is characterized. The results obtained strongly indicate that the average annual optical efficiency of real conventional (mono-tower) solar tower systems will hardly reach more than 70% in any location on Earth for realistic heliostat field sizes and tower heights. The study concludes with a discussion of the opportunities for further improvements to state-of-the-art of LCC design, based on the limits observed.