Technical and economic comparison between PV-RO system and RO-Solar Rankine system. Case study: Thirasia island

This paper presents the technical characteristics as well as an economic comparison of two autonomous desalination systems. The first system concerns the laboratory experimental results of a direct coupled PV-SWRO desalination system without batteries equipped with a hydraulic energy recovery device of the Clark pump type. The PV system consists of 18 Arco-Solar monocristaline PV panels, with total peak power of 846 W. These panels are directly connected to a brushless DC motor of 5 10 W, powering a rotary vane pump as low pressure pump. The clark pump utilizes the high pressure brine to intensify the feed pressure to the RO desalination system. The desalination system has a capacity of 0.1 m(3)/h. The specific energy recovery of this system has been experimentally found to be in the range of 3.8-6 kWh/m(3) depending on the feed water temperature and the feed water salinity. The second system is an organic solar Rankine system for RO desalination. The heat produced by the solar collectors' field is transformed to mechanical power through an organic Rankine cycle process. The shaft power produced drives the high pressure pump of the RO unit, initiating the desalination process. The prototype system consists of vacuum tube solar collectors 90 m(2), a Rankine engine of 100 kW and an RO unit of 0.3 m(3)/h. The energy recovery system of the RO unit consists of a pair of axial piston pumps (APP) manufactured by Danfoss. One of them operates reversely, as turbine, exploiting the hydraulic energy of the brine stream. The aim of the economic analysis is not only to estimate the cost of each system but also to compare them on the same basis, in order to allow a meaningful comparison. Hence, each system cost is determined having utilised the same economic assumptions like interest, operating expenses, etc. Moreover, these desalination systems can provide sufficient amounts of potable water for the local needs of a small island, like Thirasia, (Cyclades, Greece) at a cost as low as 7.77 (sic)/m(3), very close to that of water hauling.