Experimental evaluation of a multi-skid reverse osmosis unit operating at fluctuating power input

Abstract

A reverse osmosis (RO) desalination unit, composed of three identical skids connected in parallel and using an energy recovery system, has been designed and manufactured for operation at variable conditions. The main goal of the current experimental investigation is the evaluation of such a system for operation at variable power input and the increase of fresh water production as a function of the power input conditions.An RO desalination unit can be powered by renewable energy sources such as wind and solar energy. However, the power that derives from these sources can strongly fluctuate, since it depends on the weather conditions and/or available solar radiation. As the available power production fluctuates, the water feed and the production of permeate also fluctuate. Therefore, flexibility concerning the RO unit is required, in order to operate efficiently, ensuring the good quality (low salinity) of the produced fresh water. A configuration of three identical sub-systems using an energy recovery unit of axial piston motor – APM type has been chosen, in order to ensure flexibility at variable load operation. The number of the sub-units that operate depends on the available power. The total RO unit capacity is 2.1m3/h of fresh water. All the components have been equipped with the appropriate instrumentation for detailed experimental testing.The laboratory experimental testing uses frequency inverters in order to simulate the variable power input. The power input has been monitored through the control of the high pressure pump frequency. The RO unit has been tested for seawater salinity of 37,500ppm, and for different seawater temperature. An efficient performance of the RO unit is achieved due to the on/off switching of the three sub-units, keeping an acceptable fresh water quality at both part and full load operation.The application of such flexible RO unit has been then examined and demonstrated in three different case studies, using local weather data. The power input originated from photovoltaics (PV) of capacity ranging from 10 up to 20kWp, showing that it is suitable to combine RO with renewable energy sources with strong intermittent power production.