Modelling and optimisation of solar organic rankine cycle engines for reverse osmosis desalination

The objective of this paper is to model and optimise solar organic rankine cycle (ORC) engines for reverse osmosis (RO) desalination using currently available solar thermal collectors. The proposed systems are intended to be potentially attractive for remote areas without (or with very high cost) access to the public electricity grid.In this study the ORC system is modelled using the Aspen Plus process simulator, with the required inputs from other programmes to model Reverse osmosis and thermal solar plants in the same modelling platform. The first part of this paper analyzes a comprehensive list of candidate working fluids for the ORC desalination application, and a selection is made of the most interesting fluids according to the type of solar collectors used in each case. The optimal operating temperature is calculated for the solar ORC integrated with the RO plant that optimises the global ORC-thermal solar plant efficiency. The second part of the paper deals with the applicability of the system obtained. Two case studies were examined from Almería and Barcelona that can be considered representative of two different levels of solar radiation characteristic of the Mediterranean area of Spain. In these case studies the area of the solar field collectors was calculated, considering both brackish and sea water desalting applications for a handling capacity of 15 m3/day. An economic estimation is also reported comparing the present results with those of an equivalent photovoltaic-RO plant. The technical-economic results obtained for the two locations suggested the adequate thermal solar technology to be that represented by the PTC collector system. The use of an equivalent photovoltaic system to generate electricity to drive the RO desalination system had a higher cost than the optimised solar ORC–RO system specially when using the best solar thermal technology.