Department of Natural Resources and Agricultural Engineering, Agricultural University of Athens, Greece
Received Date: May 17, 2017; Accepted Date: May 22, 2017; Published Date: May 24, 2017
Citation: Karavas CS, Papadakis G (2017) Renewable Energy Driven Small-Scale Sea Water Reverse Osmosis Desalination Systems: A Survey. J Fundam Renewable Energy Appl 7: e115. doi:10.4172/2090-4541.1000e115
Copyright: © 2017 Karavas CS, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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In recent years, water and energy constitute two of the most important concerns of the worldwide agenda. These resources comprise essential commodities for the preservation of life on Earth and the human survival. At the dawn of the 21st century, there are more than 2 billion people without access to fresh water . Taking into consideration that 97% of the earth’s water comes from the oceans, an attractive solution to the lack of potable water is seawater desalination . The main desalination processes are the thermal and the membranes ones. The membrane processes, such as electrodialysis and primarily reverse osmosis (RO), have been implemented not only on desalination but generally on water treatment and now have more global acceptance than thermal processes . RO is a process where the feed water enters a semi-permeable membrane under pressure, larger than the osmotic pressure, and the membrane produces fresh water and concentrate solution .
Water separation processes such as desalination require energy to produce fresh water and the minimum theoretical energy for desalination is 1.06 kWh/m3 . However, desalination plants consume higher amounts of energy per unit volume of fresh water produced. The continuously increase of the fossil fuels prices and their environmental impact as compared to the advantages of renewable energy and corresponding renewable energy systems (RES), makes the combination of RES with desalination units a sustainable and economically viable solution . There are numerous studies for desalination plants powered by renewable energy technologies such as RO desalination units using solar energy, wind energy and thermal energy. Most of them focus on the quantity of the water production from the desalination unit with the aim to minimize the specific energy consumption (kWh/m3) and the total investment cost.The Renewable Energy Systems Group (RES Group) at the Agricultural University of Athens (AUA), (www.renewables.aua.gr), focuses on three major issues to SWRO desalination technology powered by RES.
1. Investigation of small-scale SWRO desalination units under full and variable load operation.
2. Autonomous small-scale SWRO desalination systems.
3. Energy Management Systems (EMS) utilizing computational intelligence approaches for SWRO desalination systems.
In the past two decades, various SWRO desalination systems were developed in many areas in the world such as islands and regions, which face significant shortage in drinking water supply. Most of them use energy recovery devices (such as pressure exchangers and turbine systems) in order to reduce the energy requirements of the RO plant since more than 90% of the input energy to a RO desalination system is wasted in the brine line. Excellent results have been attained to the specific energy consumption when small-scale SWRO desalination units are combined with energy recovery devices . Specifically, a small-scale SWRO desalination unit with energy recovery device can present more than 80% reduction in its energy consumption compared to a same conventional desalination system (without energy recovery device) . Historically, SWRO desalination systems were designed and developed for steady power input (nominal load operation). However, various studies have shown that lower specific energy consumption can be reached when a SWRO desalination unit operates at variable load conditions (over a range of input pressure to the membranes) by keeping constant the energy recovery ratio . A comparative analysis have been experimentally investigated between two SWRO desalination units equipped with different energy recovery devices (hydraulic one and a rotary piston pump) and the results showed that the specific energy consumption presented a reduction of about 16% for both desalination units when operated in variable load conditions .
Coupling RES with desalination units leads to environmental benefits and constitute an important factor to the economic and social development of remote areas . Several research and demonstration studies have been carried out about the implementation of RES, such as solar energy through photovoltaics (PV)  and solar thermal energy through solar collectors [13-15], in small-scale SWRO desalination plants. Recently, a directly PV driven stand-alone SWRO desalination system (i.e., without the use of any conventional energy storage system) was experimentally investigated under variable load operating conditions and its unit cost of water produced was compared to the cost of transported water in one typical island of the Cyclades complex, Greece. The results showed that the SWRO desalination system led to a considerably lower cost of the produced fresh water and the total monetary gain was more than 60% . This system exploited most of the available PV energy production potential and only a small portion of PV energy was not utilized. This small portion of nonexploitable PV energy comprises of the PV produced energy that is not sufficient to operate the desalination unit (minimum required energy) as well as the energy that could be produced but it is not consumed because the desalination unit operates at nominal load. In other words, it was found that the incorporation of conventional battery storage is not justified for operating the RO unit during daytime. However, in order to take advantage of all the produced energy by PVs and ensure a continuous SWRO system operation during daytime without sharp variations due to the variable PV power generation, short-term energy storage systems can be used .
The integration of new desalination plants and the variable energy production of RES require a cost effective planning strategy in order to design the optimum energy supply system. Except of stand-alone desalination systems, another energy supply system for desalination systems can be the autonomous microgrids . Microgrids are usually small-scale integrated systems and mostly employed in remote or isolated communities . Microgrids enable distributed generation and can cover important needs in the area that they serve namely potable water and electricity . The proper operation of a microgrid requires advanced energy managements systems (EMS) in order to manage and operate the SWRO desalination system, satisfy the water and electrical demand, secure microgrid’s economical operation and keep the energy interchange between production and consumption stable. Several EMS have been developed based on computational intelligence techniques such as fuzzy logic  and fuzzy cognitive maps [22,23].
The RES Group has focused its efforts on developing Decentralized EMS (DEMS) based on multi-agent systems (MAS). MAS emerge as a solution to decentralized coordination problems as the coordination between the agents is succeeded in the absence of a central controller. A Multi-agent DEMS (MAS-DEMS) can optimize the electricity supply to desalination plants and can operate the desalination plant under variable and intermittent conditions. A MAS-DEMS was compared with a centralized EMS (CEMS) for an AC polygeneration microgrid and the results showed that the MAS-DEMS can perform satisfactorily and resulted in lower net present cost than the CEMS . Recently, the RES Group designed, built and experimentally investigated of a DC microgrid, which incorporates short-term energy storage systems, in order to electrify a small-scale SWRO desalination unit. A MASDEMS was implemented in the DC microgrid to ensure the optimum microgrid’s operation. Furthermore, the agents communicate and cooperate with each other in order to take decisions about the energy production and the energy storage. The agents also decide about the fresh water production by determining the operation point of the desalination unit with the use of computational intelligent technique (Fuzzy Cognitive Maps). A schematic diagram of AC and DC microgrid incorporating SWRO desalination unit is presented in Figure 1.
This work was conducted within the framework of research project “Development of Energy Management Systems for the Design and Control of Autonomous Hybrid Systems and Microgrids, incorporating Computational Intelligence Techniques”, funded by the Scholarship Foundation of Greece in the context of “RESEARCH PROJECTS FOR EXCELLENCE IKY / SIEMENS”.