Power block off-design control strategies for indirect solar ORC cycles

The performance of a 5MW(el) indirect ORC cycle coupled to linear solar collectors with different technologies is assessed, aiming at evaluating the effect of different control strategies on annual electricity output. Two different solutions are considered for solar collectors: a state-of-the-art parabolic trough collector with Therminol VP1 as heat transfer fluid (HTF), reaching 390 degrees C as maximum temperature within the solar field, and a cheaper Linear Fresnel Reflector (LFR) with Therminol 55, limited to an operating temperature of 310 degrees C. A simplified procedure is firstly proposed in order to identify the organic fluid that guarantees the highest performance under design conditions. Toluene is the selected working fluid in a saturated regenerative Rankine cycle configuration. After fluid selection, a more detailed analysis involving turbine sizing and piping estimate is carried on in order to set optimal on-design parameters such as the evaporating pressure of the working fluid. Finally, yearly electricity production is calculated taking into account off-design performance of all plant components as a function of the effective solar radiation. Two different off-design control strategies are considered for the turbine, namely sliding pressure and constant pressure at the turbine inlet. The levelized cost of electricity (LCOE) is computed for both cases. For high temperature collectors the LCOE results respectively about 180 (sic)/MWh with partial admission and 175 (sic)/MWh with sliding pressure off-design control strategy. LFR technology leads to similar LCOE when its specific cost is about half than the parabolic trough collector. (C) 2015 The Authors. Published by Elsevier Ltd.