Abstract
The thermal effectiveness approach, widely used for steady-state heat exchangers, has limitations when applied directly to solar thermal plants, where operating conditions vary continuously. In particular, the requirement to calculate instantaneous effectiveness reduces its usefulness in design tasks. This study proposes a simplified expression that relates thermal effectiveness (e) to the number of transfer units (Ntu?) and the process thermal load (Qp?) for solar thermal plants using parabolic trough collectors. The expression was derived from a dataset of a total of 168 optimal plant designs—previously obtained by minimising total cost under varying thermal loads, inlet and outlet temperatures, and seasonal conditions (summer and winter). The resulting correlation reproduces the expected e–Ntu? behaviour observed in conventional heat exchangers, while also accounting for the influence of thermal load and seasonal effects. In the case study, plants operating at lower loads achieved higher effectiveness, with seasonal differences being significant: maximum values reached about 0.60 in summer and 0.50 in winter. The proposed relationship can support preliminary sizing of solar thermal networks, providing a fast alternative to detailed simulations, with future work aimed at extending its validation to different climatic and operational contexts.
