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"Solar Two successfully demonstrated efficient collection of solar energy and dispatch of electricity, including the ability to routinely produce electricity during cloudy weather and at night. In one demonstration, it delivered power to the grid 24 hours per day for nearly 7 straight days before cloudy weather interrupted operation."

From CSP Technologies Overview (US Department of Energy).

One of the attractions of CSTP is that it is possible to store solar heat in melted salts (such as nitrates of sodium or potassium) so that electricity generation may continue through the night or on cloudy days. This has been demonstrated in the Solar Two power tower and in the parabolic trough type of CSTP plant. There is more information on the NREL page about solar thermal storage.

A related point is that CSTP plants can use gas power as an alternative source of heat as a backup when there is no sun.

With the use of heat storage and gas as a backup source of heat, it is possible for a CSTP plant to deliver any combination of base-load power (background supplies that do not respond to variations in demand), intermediate-load power, and peaking power (power that can respond quickly to peaks in demand). This means that CSTP can deliver power that can be delivered when it is required, known as 'dispatchable'. This flexibility is particularly valuable for power engineers in matching supplies to demand that may vary from moment to moment.

An important point about storage of solar heat is that, although it costs money and means some loss of energy, it can mean a more profitable power plant overall:

  • One reason is that the wholesale price of electricity varies through the 24 hours of each day and if some solar power can be time-shifted from the daylight hours to the early part of the night, it may fetch a higher price than it would otherwise do.
  • Another reason is that, without heat storage, the steam turbines and generators of a CSTP plant are idle at night and not "earning their keep". With heat storage, turbines and generators for a given area of solar collectors can be smaller and cheaper and they can run for longer in each 24-hour period. There is some discussion of this point in Solar without the panels.
  • A third reason is that heat storage (perhaps with the use of gas or biofuels as backup sources of heat) can help to make CSTP into a source of electricity which is available on demand or "dispatchable". This in itself is valuable in helping to match electricity supplies with demands for electricity which vary from moment to moment (see also Solar without the panels).

If price support is provided by some such mechanism as feed-in tariffs, some of these economic benefits may be disguised. But when feed-in tariffs are phased out, these benefits of heat storage will be clear to see.

Professor David Mills of Ausra makes the point that there is actually rather a good fit between the times when CSTP plants can produce most electricity and the times when people need the electricity. In hot climates, most electricity is produced when there is heavy demand for air conditioning during the day and least electricity is produced when people are asleep. It is only necessary to shift some of the power output for a few hours to the evening period when people are still up and about.


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