Effect of passing clouds on the dynamic performance of a CSP tower receiver with molten salt heat storage

The present paper explores the off-design performance of a CSP tower receiver due to the passage of clouds over the solar field. A quasi steady-state performance model is considered in the first part of the paper, accounting for the optical performance of the solar field and receiver only, yielding the expected incoming heat flux distribution on the aperture plane of the receiver and without further considerations about the thermal performance of the receiver itself. The twofold aim of this analysis is to identify the type of cloud that brings about the worst operating conditions of the receiver and to define an aiming strategy that produces the most homogeneous distribution of the incoming heat flux, this being a indirect metric of reduced thermal stress on this component. It is observed that larger clouds have a more negative impact on the heat flux distribution whereas the effect of smaller clouds is easily compensated for by other uncovered areas of the field. Large differences in performance arise when considering different aiming strategies, what leads to the selection of one of these as the optimum one given that it manages to evenly distribute the heat flux over the receiver surface. This limits the number and intensity of hot spots without affecting the overall power production. The optimum strategy is based on pointing the heliostats towards different aiming spots strategically located on the receiver surface, depending on the distance between tower and heliostat. The second part of the paper presents an in-house code developed to study the transient thermal performance of the receiver. The outcome of this second analysis confirms the results of the previous optical study: the selected optimum strategy leads to a 1% higher efficiency of the receiver. Also, the temperature distribution of the larger shadows is more harmful than when several smaller clouds pass over the solar field, both for the larger temperature gradients in the receiver and for the production of molten salts. This is also confirmed in the last part of the paper where different patterns of combined cloud passages are explored in regards to the charge/discharge process of the thermal energy storage system.