Many parts of the world, e.g., Africa, Middle East, Southern Asia and Australia, have severe shortages of fresh water for agriculture, let alone for drinking. Many of these locations are close to large saltwater bodies with plenty of sunlight available year round. Thus, saltwater can be directly used for agriculture by combining greenhouses with concentrated solar power to desalinate saltwater to provide fresh water. Solar photovoltaic solar panels can be added to run the associated electrical system such as refrigeration units, pumps, fans, lighting, etc. to make the greenhouse units fully self sustaining. 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.
The greenhouses can be set up in empty lots near the seashore or further inland where seawater can be piped in (using solar powered water pumps). Cool water from a depth of 30m (100ft) is used for this application. A small-to-medium sized concentrated solar array that follows the sun (heliostat) is used to focus the sunrays onto a solar collector heat exchanger (SCHE) that consists of a network of graphite/stainless steel pipes where water is brought to near boiling temperature. The graphite is a good conductor and its black color helps absorb heat. The hot-water is sent to a graphite evaporator grid consisting of graphite pipes open on the top that is placed near the ceiling of the triangular-shaped glass roof, allowing the hot-water to circulate slowly causing much water to evaporate near the ceiling. A fan increases evaporation of water. There are two networks of condenser coils (steel or copper), that circulates cold sea water, one near the ceiling (just below the graphite evaporator) and the water that condenses on the outside of the condenser tubes is collected in gutters and directed towards the soil. The other condenser is placed a few inches below the surface of the soil so that the soil surface is cooled and allows the water-vapor to condense as dew on the soil. The steep walls of the glass-ceiling allow condensation to flow into gutters fed into the soil. A refrigerated heat exchanger system powered by solar power can improve the fresh water output of condensers and regulate temperatures within the greenhouse.
The return water from the seawater condenser coils and the graphite evaporator grid are mixed and added to fresh incoming water in a mixer before supplying to the solar collector heat exchanger. This cuts seawater intake, regulates discharge water salinity and reuses the heat of the hot water. A variety of crops can be grown in this setup to turn the desert into productive land.
The plant has very few moving parts, and the long service life and continuous output with minimal maintenance can offset initial setup costs. The easily procured components do not have to be very highly precise keeping costs very low.