CSP: HOW TO REDUCE THE USE OF WATER
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CSTP plants normally use water for three main purposes:
- Steam generation. This water is normally recycled but there may be losses through evaporation or leakage.
- Cooling. In all thermal power plants the steam that comes out of the turbines needs to be cooled so that it condenses back to water. This is normally done by evaporation of water in cooling towers. Sea water may also be used for cooling and this function may be combined with desalination of sea water.
- Cleaning solar mirrors. Mirrors do need to be cleaned periodically and this is sometimes done using jets of water.
Since CSP works best in sunny deserts, there may naturally be a problem in finding enough water for these purposes—and there is a corresponding incentive to find ways of running CSP plants with less use of water. The following three sections consider aspects of this problem.
Minimising the use of water for steam generation
In principle, it should be possible to reduce losses from evaporation or leakage to very low levels so that, apart from the initial supply of water needed for steam generation, very little water would be required.
It appears to be possible to drive gas turbines via the direct solar heating of air, and this would of course eliminate the need for water for steam generation. The Israeli company Aora is using this technique with power towers.
Other options for reducing or eliminating the use of water in solar power plants include:
Minimising the use of water for cooling
It is feasible and economic to use air for cooling in thermal power plants and, worldwide, about 30 GW of generating capacity uses this kind of 'dry cooling'.
Dish/engine systems are designed standardly to use dry cooling. It appears that most CPV systems operate without the need for water for cooling.
Dry cooling can be done in two main ways:
- In a direct dry system, the steam is condensed directly by air in a heat exchanger. The cooling air is blown through the heat exchanger using fans.
- With the indirect ('Heller') dry cooling system, water is used to transfer heat from the steam turbines to cooling towers where the heat is dispersed into the air. The water is recycled. Steam is normally condensed by mixing it directly with cooled water from the cooling towers, and then some of the mixture is returned to the steam cycle and the rest is used again to transfer more heat away from the steam turbines.
With the indirect system it is possible to use natural convection in the cooling towers, thus avoiding the cost of fans, the energy they consume, the noise they produce, the need for maintenance, and the risk that they may break down.
With dry cooling, there is a loss of about 5% in power-plant efficiency. But if fresh water is scarce and expensive, dry cooling may be the cheaper option. It appears that power towers, which achieve higher levels of concentration and higher temperatures, are the type of CSP technology that is best-suited for dry cooling.
Where it is available, sea water may be used for cooling. This obviously reduces the need for fresh water. A valuable bonus is that the cooling function may be combined with the desalination of sea water.
Minimising the use of water for cleaning mirrors
There is at least one system for cleaning mirrors without the use of water. NOVATEC BioSol AG have designed a robot that 'crawls' along the flat mirrors of a Fresnel mirror type of CSP plant and removes dust and debris using brushes. There is an illustration on the last slide (number 14) of their company fact sheet. Incidentally, Novatec's Fresnel CSP plants use air for cooling, not water.
Another option for cleaning mirrors is the use of compressed air. Damp cloths or sponges may also be effective, using a minimum of water. Condensation in semi-arid deserts may be helpful in this regard.
Yet another option is to use self-cleaning glass for the mirrors (see, for example, Pilkington Activ™ and Bioclean™). In the first "photocatalytic" stage, a special coating reacts with daylight to break down organic dirt. That may be removed using relatively little water in a second "hydrophilic" stage. As before, condensation may be helpful here.
These things may be used singly or in combination.
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Last updated: 2011-01-05