Upper limits to the mean annual optical efficiency of solar mono-tower systems

This study presents an analysis of the theoretical upper limits to the mean annual optical efficiency of the Light Collection and Concentration (LCC) subsystem of a solar tower system, which is the subsystem composed of the heliostat field and the active surfaces of the receiver that capture the concentrated solar radiation sent by the heliostat field to transform it into thermal energy. The “ideal” LCC subsystem is modelled gradually, by incrementally introducing into the model factors that limit the mean annual optical efficiency of any LCC subsystem and whose influence in the optical efficiency cannot be reduced to zero, starting with the cosine factor, followed by the atmospheric transmittance, and then by the product of the receiver intercept factor and a factor related to the shading of the heliostat field by the receiver. At each degree of complexity of the ideal LCC subsystem model, the optimal configuration of the “ideal” LCC subsystem that achieves the highest mean annual optical efficiency in terms of heliostat field shape and receiver size is identified for a selected range of latitudes, tower heights and heliostat field sizes or total mirror areas. Thus, the upper-limits of the mean annual optical efficiency of the LCC subsystem of solar towers are obtained as a function of the specified parameters. The results obtained show that at a latitude of 30° North, for tower heights between 200 and 300 meters and heliostat fields with mirror areas between 1.3 km² and 2.5 km², the upper limit of the mean annual optical efficiencies vary between 79.2% and 74.7%, with the highest upper limit corresponding to the combinations of the tallest towers and smallest heliostat fields. This strongly indicates that the mean annual optical efficiency of actual solar mono-tower systems of commercially realistic heliostat field sizes and tower heights will hardly reach 70% at any location on Earth.