Monday, March 13, 2017

Artificial heated lagoons for rain

 With colder sea temperatures than land temperatures (eg Western Cape in summer, UAE, etc), cold air from the sea does not hold a lot of moisture and when it blows onto land and heats up, relative humidity decreases. Also this cold air does not easily rise by convection to form rain. One of my ideas is artificial lagoons with solar energy reflected into them to heat the water.
With deep ocean one has a limitless supply of heat from the water (evaporation cools the surface, but there is plenty of heat from the water to heat it up again), but the surface can only be heated to about the sea temperatures below the surface. About 43% of energy from the sun is of visible light frequency. Visible light has the distinct property of being able to penetrate seawater to some depth, whereas the infrared generally gets absorbed quickly. So if you have shallow pools with dark bottoms you suddenly have 43% of the solar energy available in a shallow pool that normally would have heated a huge body of water a little, but there is very little energy that can be used from the water to heat the surface when it cools from evaporation. As I am looking for high temperatures I have to think about shallow pools rather than deep sea. With a shallow pool the energy comes mainly from the solar energy (not the water). If one wants high temperatures (with evaporation and radiation the shallow pool can cool quickly), one needs ways of getting solar energy to the pool or lagoon, etc. So my proposal has been shallow lagoons made by bulldozing out of the sand and with mirrors to reflect solar energy into the lagoon. Say the sun provides 700 W of energy per square metre of water surface. Eventually the temperature of the lagoon water gets so high (at about 26.5 deg C) that all the 700 W per sqare metre is needed for evaporation and so mirrors will be needed for more energy. My graph below shows the 700 W straight line (0.7 kW), the evaporation rate on a 1 sq metre surface (upper curve kg/hour) and the power needed for the evaporation (lower curve kW). Not correct to have kW and kg/hour on the vertical axis, but it works out. The conditions are a 40% relative humidity, a 3 m/s wind over the surface and an atmospheric pressure of 100 kPa.

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