5. Integration of a Desalination Plant into a Concentrating Solar Power Plant

This chapter addresses the description and thermodynamic analysis for the integration of desalination plants into the power cycle described in Chap. 4. The systems chosen for this study combine a Concentrating Solar Power plant using parabolic-trough collector technology for electricity generation with various desalination plants, giving rise to what is known as a parabolic-trough concentrating solar power and desalination (PT-CSP + D) plant. The description of the PT-CSP plant, based on the Andasol-1 (Blanco-Marigorta et al., 2011) commercial plant, is detailed in Chap. 4, showing all the model equations. The desalination technologies selected to combine with the PT-CSP plant were multi-effect distillation (MED) and reverse osmosis (RO), as discussed in Chap. 1. On one hand, the simultaneous production of water and electricity using an RO plant connected to a CSP plant seems the simpler option. On the other hand, the integration of a low-temperature MED (LT-MED) plant is an interesting alternative because it allows replacement of the conventional power-cycle condenser by using exhaust steam as the thermal energy source for the desalination plant. However, to satisfy demand, while providing a certain performance, the LT-MED plant inlet temperature should be around 70 °C (corresponding to 0.031 bar absolute), meaning that the steam does not completely expand through the turbine and therefore the power-cycle efficiency is low compared with a stand-alone electricity-generating plant. This is the reason why another alternative to the MED plant, MED with thermal vapour compression (TVC), is considered. In this case, the steam expands completely in the turbine until it reaches the permitted value for the condenser conditions. However, part of the steam circulating through the turbine is extracted and used as high-pressure steam; this, together with the low-pressure steam coming from one of the MED effects, generates the inlet steam required in the first stage of the desalination plant. Moreover, in this study, a new concept of CSP + MED plants is evaluated (which, until now, has not been studied in published works), a thermally fed LT-MED plant with steam coming from a thermocompressor (LT-MED + TVC). In this case, the low-pressure steam (the entrained vapour) used by the thermocompressor comes from the exhaust steam of a PT-CSP plant instead of one of the MED effects. In each of the systems studied, desalinated water production is evaluated as well as the power and efficiency of the dual thermal solar power and desalinated water cycle.