Electricity generation and water desalinisation - powered by heat collected in highly saline lakes.
K. naveen kumar
10h71a0224
Introduction:-
- The sun is the largest source of renewable energy and this energy is abundantly available in all parts of the earth. It is in fact one of the best alternatives to the non-renewable sources of energy [1].
What is solar pond & working principle:-
A solar pond is a body of water that collects and stores solar energy. Solar energy will warm a body of water (that is exposed to the sun), but the water loses its heat unless some method is used to trap it. Water warmed by the sun expands and rises as it becomes less dense. Once it reaches the surface, the water loses its heat to the airthrough convection, or evaporates, taking heat with it. The colder water, which is heavier, moves down to replace the warm water, creating a natural convective circulation that mixes the water and dissipates the heat. The design of solar ponds reduces either convection or evaporation in order to store the heat collected by the pond.
- Working principle:
The solar pond works on a very simple principle. It is well-known that water or air is heated they become lighter and rise upward. Similarly, in an ordinary pond, the sun’s rays heat the water and the heated water from within the pond rises and reaches the top but loses the heat into the atmosphere. The net result is that the pond water remains at the atmospheric temperature. The solar pond restricts this tendency by dissolving salt in the bottom layer of the pond making it too heavy to rise
- Energy from salt lakes
Any lake absorbs heat from the sun. Normally, heat is lost as warm water rises to the surface and cools by evaporation. But water is a VERY poor conductor of heat and if this circulation can be stopped, the heat can be trapped in the bottom of the lake. A salt lake, (ideally about 3m deep), managed so that the water on top is of (relatively) low salinity and the water on the bottom is of very high salinity, will not circulate to release heat because the water on the bottom is so heavy with salt it cannot rise. The deeper water gets very hot - to over 100 degrees in the right circumstances - 80 degrees is common in the tropics. In Southern Australia 60 degrees C is easily achievable - even in winter. The main management problem is to extract heat at the right rate so the lake does not boil or 'turn over' and lose its heat.
Getting the energy out of the pond
Energy, in the form of hot water, is extracted by circulating fresh water in pipes laid on the bottom of the pond.
This picture of the Solar pond at Pyramid Hill (near Kerang in Northern Victoria) shows the mesh of pipes running down the wall of the pond. These run across the bottom of the pond and up the other side. Fresh water (or radiator coolant) is circulated through them, and is heated by the saline pond water. This hot fluid is then used to heat glass houses, boil refrigerant in a rankine engine to make electricity, or (in the case of Pyramid Hill) used to heat air to flash dry gourmet salt products.
Electricity from solar ponds !
To create electricity from the solar pond hot water is also a well understood technology. The convertor is called an 'Organic Rankine Cycle Engine' an ORC engine for short - named after a 19th century engineer. This is the same process used to extract energy from deep ocean temperature gradients off Japan. There are several more sophisticated technologies than the Rankine engine - but this is the easiest to describe and build.
A Rankine engine is used to convert waste heat from a solar pond (or any other source - such as industrial waste heat) into energy to drive a rotating shaft - which (most often) is used to drive a conventional alternator to make electricity.
The Rankine engine works by using the hot water to evaporate a low boiling point chemical, such as those used in refrigerators. That vapour then becomes a high pressure gas which can be used to drive a turbine and produce electricity. Once used, the gas is recirculated, cooled, condensed and recycled - same as in a refrigerator.
In fact, a low cost version of a Rankine engine can be build using 'off the shelf' industrial scale air conditioner parts. The units at Alice Springs and Birdsdville were built this way.
Advantages:
- The heat storage is massive, so energy can be extracted day and night - hence it is a source of 'base load' solar power - no batteries or other storage needed !
- Solar ponds can have very large heat collection area at low cost.
- The major production potential is during peak electrical power demand (and price) in mid summer
- The technology and scientific principles for collection and extraction of heat and its conversion to electricity are well understood and well documented in scientific papers.
- Any qualified engineer would be able to build one of these systems (being a refrigeration specialist would be useful if you wanted to build a Rankine engine)
- Process heat :
Conclusion:
Solar ponds can be effectively used as replacements inindustries that use fossil fuel to generate thermal energy. Solar ponds can be used for process heating, refrigeration, water desalination, production of magnesium chloride, bromine recovery from bittern, enhancement of salt yield in salt farms. It will be the future energy source.
Salt lakes can be used to produce ENERGY - surely one of the most valuable commodities in our community. There are many parts of Australia being using this procedure.
Categories:
TECHNICAL ARTICLES(session-2)
