## 4 Renewable Energy in Desalination  
Despite rapid increases in membrane based desalination facilities, the use of fossil fuel based energy creates serious concerns regarding the sustainability and cost of obtaining water from desalination. In order to make desalination perfectly aligned with objectives of PIS, the carbon footprint of desalination plants must be reduced. Furthermore, the energy consumption of the process must be lowered to make it affordable for less fortunate communities as well. So far, the largest expansion of desalination facilities has been recorded in energy rich regions and/or regions of the developed world including Spain, Australia, and North America. This is due to the high energy demand of traditional desalination processes. Reverse osmosis is the most economical desalination technology available at a commercial scale at the moment, but still the energy consumption of this technology is more than double the minimum theoretical energy requirements of the process [36]. This scenario has attracted a large interest in the development of desalination based on renewable energy resources.  
The cost of renewable desalination is in the same range as that for the traditional resources [37]. The types of renewable energy sources suitable for desalination mainly include solar energy, geothermal energy, and wind energy. The selection of renewable energy sources and applied desalination techniques (RO, thermal, EDs (electrodialysis), etc.) depends upon several factors, mainly including the type of renewable energy sources available in the area, the nature of local water (seawater or brackish water), production capacity, remoteness of the area, etc. [38]. Currently, most of the renewable based desalination facilities are based upon RO and are driven by solar and wind energy [39]. These plants have capacities ranging from a few cubic meters to several hundred cubic meters per day. The specific energy consumption of these units for brackish water ranges from 0.9 to 29.1 kWh/m\({}^{3}\) and 2.4 to 17.9 kWh/m\({}^{3}\) for brackish and seawater, respectively [40]. Solar and wind energy sources, however, do not allow continuous operation without the use of storage batteries. To overcome this drawback, renewable energy desalination has been integrated with conventional energy sources including grid electricity, diesel generators, etc. The second largest beneficial form of renewable energy based desalination is electrodialysis, which has mainly been operated with wind energy. MD represents  
Figure 7: Mass and waste intensities and overall water recovery factor with duration of treatment.  
another emerging player in the field. MD plants driven with solar energy and geothermal energy have been installed in different regions across the globe.  
Membrane based renewable desalination is expected to emerge as an interesting sector in the future to reduce carbon emission and desalination costs, particularly in remote regions with abundant sources of saline water and renewable energy but poor infrastructure of electricity. The key factor for further growth of renewable energy desalination is the development and progress in more efficient, cheap renewable energy systems, and improved control and logic systems to regulate the energy output. Moreover, the techno-economic feasibility of membrane based desalination technologies driven with renewable energy sources at large scale needs to be investigated.