Preliminary estimation of the receiver tube wall temperature and the performance of CSP plants in off design-conditions by means of a simplified dynamic model

This work deals with the development of a fast and robust simulation tool to preliminary estimate the receiver tube wall temperature and the performance of small-scale Linear Fresnel Reflectors (LFRs) solar fields. Under the assumption of uniform solar flux, the proposed 1 D dynamic model includes a more detailed heat transfer analysis between the fluid and the internal wall of the receiver tube by implementing effective correlations covering all the possible flow regimes. The model is then applied to assess the performance of a LFRs solar field developed under the EU funded project “Innova Microsolar” with varying ambient and operating conditions. Results show that: (i) the buoyancy driven flow enhances the heat transfer of about one order of magnitude compared to the assumption of a constant Nusselt number in laminar flow condition; (ii) it is possible to correlate with more than 95% of confidence over the entire range the receiver tube wall temperature to a newly defined parameter called Load Temperature which is based on input data provided by sensors usually installed in these kind of plants; (iii) the proposed model improves also the estimation of the collected thermal energy by the fluid in the receiver tube during the warm-up phase of about 0.8% compared to the reference Forristall model.