Optimised desalination of seawater by a PV powered reverse osmosis plant for a decentralised coastal water supply

Abstract

In the past decade desalination, especially by reverse osmosis, was established as a reliable and secure technology for the production of potable and agricultural water. The situation in Mediterranean countries as for example Spain can be described by a growing lack of water due to limited natural resources, an increasing water demand by agricultural and tourist activities especially in coastal areas. The tense situation is exacerbated by the infiltration of seawater to ground water aquifers in coastal areas thus naturally fresh water resources became salty. The installation of an efficient desalination technology requires an adequate energy supply. Whereas in most urban regions a secure, continuous energy supply is guaranteed in many rural coastal regions the lack of potable water is connected to a missing energy supply. This often provokes a situation like “without energy there is no water”. Due to the fact that almost 60% of the investment costs for energy supply systems are needed for the installation of the distribution system in rural regions decentralised conventional and renewable energy supply systems becomes an increasing importance. By the utilisation of renewable energy systems the producer is independent from any supply of fossil fuel resources as gas or diesel but has to take into account the changing energy offer by the sun or wind. But the combination of renewable energies and water production by reverse osmosis has become the key technology for decentralised water supply plants. In the past few years the Technological Institute of The Canary Islands (ITC) and the Aachen University of Applied Sciences are investigating the concept of seawater desalination by reverse osmosis supplied by renewable energies (PV). At the test field of the ITC in Pozo Izquierdo a pilot system called DESSOL was installed to demonstrate the technical feasibility of the technology. During the period of operation important results for the optimisation of the plant operation and the coordination and timing of the renewable energy system to the reverse osmosis plant operation were obtained. Thus for example a solar thermal system was integrated to the energy supply system to increase the daily water production. Thus the plant is now fed with pre-heated seawater. Actually the plant produces a medium daily amount of 3 m 3/d of potable water. This experience has served for the manifestation of the technical concept of this plant type and could be transformed to much larger drinking water production systems without loosing the efficiency of the plant DESSOL. The reverse osmosis plant with a nominal production of 10 m 3/d (specific energy consumption of 5.5 kWh/m 3) is supplied by a 4.8kWp PV generator and a 19 kWh battery back-up system. The energy system was optimised for the utilisation as energy supply for the reverse osmosis plant. The plant is operated automatically for an average of 8 h during summer and 7 h during winter time while the plant operation is adjusted to the changing energy production of the PV generator. The plant is equipped with a double flushing system with fresh water for the membranes and pumps during the daily periods when the plant is out of operation. In the full paper the principle construction details of the PV supplied reverse osmosis plant will be presented. Emphasis will be given to the description of the automatic control unit which adjusts the plant operation to the changing and discontinuous energy supply by the PV generator. The plant operation performance and the option of pre-heating the feed water will be discussed.