In the arid and semi-arid areas that lack both freshwater and sea water, desalination of groundwater becomes an essential method to produce safe water for domestic use. A groundwater desalination system should have several desirable features such as sustainable energy source, high recovery ratio, high water output per energy unit, and sufficient space of land. Due to the high demand for sustainable production and environmental protection & conservation, a new system for driving reverse osmosis has been proposed and it uses a solar-Rankine cycle.
Expanded steam is used as the working fluid against a power piston, which activates a piston pump to pressurize the salty water and force it to pass through a semi-permeable membrane. The forces between the two pistons are equalized by a countering crank mechanism. The batch mode is chosen in favor of the continuous flow to allow maximum energy recovery and reduced concentration polarization within the surrounding area of the reverse osmosis membrane. The factors that are considered in the entire system include the size and efficiency of the crank mechanism, the energy losses, and the overall performance during the separation at the RO membrane.
The level of salinity in the groundwater and soil continues to be an issue of concern across many arid regions in the world. In those arid and semi-arid regions, major areas of land are found above the brackish groundwater. Due to the absence of adequate annual rainfall in some areas, the groundwater can be desalinated in order to provide fresh water for the people living in those regions. Some examples of countries that utilize desalination systems include Egypt, Australia, Israel, India, Morocco, Jordan, the United States of America, and United Arab Emirates.
Just like seawater, groundwater desalination process involves the use of energy. The conventional source of energy to power a desalination plant is the fossil fuels. However, the demand to supply an adequate amount of clean and safe water should be accompanied by minimal environmental impact. This is reflected by the use of clean energy in order to improve the efficiency of desalinating groundwater by taking the advantage of ‘free’ renewable energy sources such as solar.
There are many solar powered desalination processes that have been studied and some of them have been implemented. Most of these technologies lay on two categories; thermal distillation and non-thermal semi-permeable membrane separation methods such as reverse osmosis (RO) and electrodialysis. The distillation method by use of a single solar panel can produce a few liters of water per sq. meter while multiple solar panels can generate tens of liters per sq. meter.
Reverse Osmosis (RO) is typically thought to be the most energy efficient technique of purifying water. It does not require heat, instead, it needs mechanical work as the initial input. Generally, many RO solar plants use the photovoltaic (PV) arrays to power the RO system. This PV technology is effective and straightforward to implement, though it does not have the benefits of economies of scale. However, the biggest system ever is said to have produced 76 cubic meters on a daily basis. Consequently, the desalination plants when powered using fossil fuels can generate up to 100,000 cubic meters of fresh water on a daily basis.
Instead of using the PV technology, a solar-thermal power plant can be utilized to produce energy to drive reverse osmosis process. Unfortunately, only a few similar plants have been established so far. One of the renowned plants was constructed in El Hamrawin in Egypt more than 30 years ago and reportedly generated about 130 liters of water per day per sq. meter of solar collector when desalinating brackish water with a salinity level of 3,000 ppm. The plant utilized the Rankine cycle and the Freon-11 as the running fluid. However, this working fluid is capable of depleting the ozone, and therefore its use was banned later. Read more on ro-system.org
There are specific calculations that have been presented concerning Rankine cycle-RO systems. These ones are anticipated to use a variety of alternative working fluids such as siloxanes. However, siloxanes are hard-to-find, meaning other toxic working fluids such as toluene and benzene could be used. The modern solar-thermal power plants that are used today are based on steam Rankine cycle and can produce electricity that is fed into the grid. Such plants are found in the Mojave Desert, California.