CSP: HOW TO REDUCE THE USE OF WATER

Back to Concentrating Solar Power

CSP 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.

There have apparently been experiments to test the idea that gas turbines may be driven by solar heating of air, and this would of course eliminate the need for water for steam generation. We currently have very little information about this possibility.

Other options for reducing or eliminating the use of water in solar power plants include:

• The use of dish/engine systems for generating electricity.
• The use of PV to generate electricity in CSP plants. Relevant news reports are marked CPV on our News page.

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'.

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.

Links:

• The economics and the future of water conserving power plant cooling (Zoltán Szabó)
• Towards greener and more economical power cooling (Zoltán Szabó)
• Ambient air acts as heat sink in indirect dry cooling system (András Balogh and Joseph Budik, Engineer Live)
• EGI Contracting Engineering Co. Ltd (Heller-Forgo dry cooling system)
• SPX Cooling Techologies:
  • Air cooled condensers
  • Indirect dry cooling
• GEA Power Cooling Inc:
  • Air cooled condenser
  • Heller indirect dry cooling systems

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 .

Back to Concentrating Solar Power