Solana's Technology

There are several ways to create electricity using the sun’s energy. Two common methods are concentrating solar power and photovoltaic. Both applications have their appropriate place for helping meet the future energy needs of Arizona.

Solana will use concentrating solar power (CSP) technology with storage capability. Unlike a photovoltaic solar plant, which uses sunlight to produce electricity, CSP uses the sun’s heat.

Solana’s parabolic mirrors focus the sun’s heat on a heat transfer fluid. The fluid can reach a temperature of 735 degrees Fahrenheit. To produce electricity, the hot fluid transfers its heat energy to water, creating steam. The steam is then used to run conventional steam turbines. The heat energy in the fluid also can be stored and used at a later time to generate electricity.

The Plant
Large thermos-like buildings containing molten salt will be located next to the steam boilers. At select times, instead of immediately creating steam, the heat transfer fluid will heat the molten salt. Then, if electricity is needed when the sun is not shining, the fluid can be heated by running it through the hot salt instead of through the mirrors. Using this process, electricity can be made from heat energy that was created up to six hours earlier.

1) Parabolic mirrors heat the heat transfer fluid.   2) Hot fluid returns from the solar field.   3) The hot fluid transfers its heat energy to water, creating steam.   4) Steam is used to drive a turbine, creating electricity. 5) The hot fluid also heats molten salt. 6) If the sun is not shining, the fluid can be heated by the molten salt. 7) The fluid is sent back to the solar field.

Concentrating Solar Power

Parabolic mirrors focus solar energy onto a heat transfer fluid. To produce electricity, the heated fluid transfers its heat energy to water, creating steam. The steam is then used to run conventional steam turbines. The heat energy in the fluid also can be stored and used at a later time to generate electricity.

Solar trough technology uses long rows of mirrors which track the sun from east to west in order to best focus sunlight onto the receiver pipes (heat collecting element). The receiver pipes are filled with fluid that is heated by the sun's energy, much like a huge magnifying glass. The heated fluid is then sent to a heat exchanger where steam is created, and that steam is then used to turn a turbine. Large insulated tanks filled with molten salt can be used with concentrating solar power to store the heat from the fluid. This stored heat can then be used to produce energy during periods of low or no sun, including the evening hours.

Storing Energy

Using thermal storage allows energy provided by the sun to be distributed over a longer window of time. In the example above, the stored energy is used to provide heat for the electricity generation well into the night. APS' peak summer demand occurs in the early evening, often as late as 7 p.m., long after the sun has provided its most intense radiation. Thermal energy storage allows the solar trough to supply electricity when energy is most needed by APS customers.

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