Fresh water supplies are severely limited in many parts of the world, e.g., Central America, Gulf States of the US, Africa, Middle East, Southern Asia, Australia and Mediterranean Europe. Fortunately, they are close to large saltwater bodies with plenty of sunlight available year round. The sunlight can be utilized to desalinate saltwater to provide fresh water for drinking, sanitation and industry. Sustainable engineering is achieved by harmoniously utilizing nature’s abundant and free provisions (solar power, wind, sea water, etc.) in a way that is both beneficial for us humans and for the environment.
In this setup, sunlight is concentrated, using mirror arrays that follow the sun (heliostats), and directed at a collector tank containing saltwater which is heated beyond boiling point, the steam is condensed in condensing tanks where the freshwater is stored before being pumped to other storage tanks for supplying to customers. There minerals can be added to enhance drinking water.
This system can be setup on the sea on a platform that is floating or fixed at about 30m (100ft) of depth, or on land with the saltwater being pumped to the location. The condenser tanks are located about 30m below the water surface, where the water is very cool, or if on land they can be placed underground at about 10m depth with the soil providing insulation from the heat.
For the same amount of energy supplied, saltwater evaporates faster than freshwater due to the former’s lower heat capacity. As the saltwater is heated beyond boiling point, the steam is directed downward to the condenser tanks. In another tank, superheated seawater is passed through an orifice or expansion valve, which produces steam that is also directed to the condenser. The remainder hot concentrated saltwater (brine) from this tank is used to store heat for heating seawater at night in a countercurrent heat exchanger, thus enabling desalination at night. Some of the hot brine is also used for producing sea salt.
To lower costs, the mirrors can be flat (instead of parabolic) and the collector tanks can be made longer/wider to provide more collector surface area. The collector tank has glass side walls, glass domed roof and graphite/steel flooring and vertical graphite corrugated walls for added wetted surface area for heat transfer. The graphite may be backed with stainless steel for added strength. Graphite is a good conductor of heat, resistant to wear, environmentally friendly and its black color enhances heat absorption. Photovoltaic solar panels are used to generate electricity (with power conditioners and battery) to run the pumps, controls and other electrical equipment. Thus, this system can be made totally self-reliant with no external power required (except for back up diesel generator sets).
The plant has very few moving parts, and the long service life and continuous output with minimal maintenance can offset initial setup costs. Components dont have to be very precise as other solar plants, thus keeping costs very low. A refrigeration heat exchanger system running on solar power can improve the condenser output.