abstract

In this paper, the coupling between the low-temperature solar organic Rankine cycle (ORC) and seawater and brackish water reverse osmosis desalination units has been carried out. Four substances have been considered as working fluids of the solar cycle (butane, isopentane, R245fa and R245ca). With these four fluids the volumetric flow of fresh water produced per unit of aperture area of stationary solar collector has been calculated. The former has been made with the optimized direct vapour generation (DVG) configuration and heat transfer fluid (HTF) configuration of the solar ORC. In the first one (DVG), working fluid of the ORC is directly heated inside the absorber of the solar collector. In the second one (HTF), a fluid different than the working fluid of the ORC (water in this paper) is heated without phase change inside the absorber of the solar collector. Once this fluid has been heated it is carried towards a heat exchanger where it is cooled. Thermal energy delivered in this cooling process is transferred to the working fluid of the ORC. Influence of condensation temperature of the ORC and regeneration’s process effectiveness over productivity of the system has also been analysed. Finally, parameters of several preliminary designs of the low-temperature solar thermal driven RO desalination are supplied. R245fa is chosen as working fluid of the ORC in these preliminary designs. The information of the proposed preliminary designs can also be used, i.e., for the assessment of the use of thermal energy rejected by the solar cycle. Overall analysis of the efficiency of the solar thermal driven RO desalination technology is given with the results presented in this paper and the results obtained with the medium temperature solar thermal RO desalination system presented by the authors in previous papers. This work has been carried out within the framework of the OSMOSOL and POWERSOL projects.

Introduction

OSMOSOL project [1] is a Spanish government-funded project for the technological development of the solar thermal driven reverse osmosis (RO) desalination. Among the specific objectives of the project, the preliminary design of solar desalination systems of capacities between 100 m3 /day and 1000 m3 /day was proposed where a solar thermal heated organic Rankine cycle drives the high pressure pump of the RO unit. Besides that, in the POWERSOL project [2], development of a small scale shaft power generation technology (50–۵۰۰ kW) for electricity generation and fresh water production by brackish water or seawater reverse osmosis desalination is being carried out. Solar thermal driven RO desalination is a renewable energy driven desalination technology that has not been studied in depth neither from a theoretical (e. g. design proposals) nor experimental point of view, as it was concluded from the specific bibliographic review already done and published by the authors [3,4] and confirmed by Ghermandi and Messalem [5] in a subsequent review work about status of solar driven (thermal and photovoltaic) RO desalination.