Novel feedwater preheating system for parabolic trough solar power plant

Abstract

Several improvements in the referenced parabolic trough power plant (PTPP) model (Andasol II) have been implemented by APROS software. The main advantage of this novel feedwater preheating system is to improve electricity production and evening operation hours at a low cost for modifying an existing PTPP in Spain. This in turn leads to less dependence on fossil fuels during the evening and cloudy periods. The reference model is improved by an increase in the collector loops in the reference solar field (SF) model to 208 loops rather than 156 loops to increase the absorbed thermal power, increasing the capacity of the thermal storage system (TSS) from 1017 MWth h to 1,360 MWth h to improve the night operating period, replacing the Feedwater/Steam circuit with a Feedwater/HTF circuit using five shell and tube counter-current heat exchangers that are used instead of condenser heat exchangers. The HTF enters the fifth low-pressure preheater with 128 kg/s and 393 °C in the daytime and 377 °C at night. The main advantage of this improvement is to use the entire generated steam within the steam turbine and not to heat the feedwater in the feedwater circuit. In addition, the control circuits are modified and added new controllers with new limitations for controlling the new operating conditions. This development is the first attempt in the field of research to use a Feedwater/HTF circuit in the PTPP in order to enhance performance. This improvement (optimization 1) in turn leads to a raise in the total HTF mass flow and the quantity of stored molten salt, as well as increasing the mass flow of HTF to the power block (PB). According to optimization 1, no steam will be taken out of the HP and LP-turbine toward the feedwater circuit. Therefore, the feedwater fed through new preheaters is heated by thermal oil instead of steam extractions. The predictions obtained from this optimization are compared to the simulated results that were previously extracted from the reference model on clear and cloudy days for evaluation of the behaviour of the optimized PTPP. It should be mentioned here that it can be used the same turbine and generator in the actual power plant to achieve this increase in the performance of the power plant, according to the technical data of manufacturers. Finally, an economic study of the cost was conducted for referenced and the optimized PTPP depending on the levelized energy cost (LEC). The findings reveal that the levelized energy cost of the optimized PTPP with storage energy is about 10.5% lower than that of the referenced PTPP.