HTF is key to concentrated solar power (CSP) success because it carries the heat from the sun collected in the solar field to the power block.
Heat transfer fluids (HTF) vary from oil and water to molten salts. HTF is key to concentrated solar power (CSP) success because it carries the heat from the sun collected in the solar field to the power block, where it is transformed in electricity.
CSP plants are very similar to any other thermoelectrical plants in the way they produce electricity. They all take a source of heat which goes through a turbine to generate electricity. The only difference is that in the case of a CSP plant, the heat is acquired by collecting it from the sun – rather than burning gas or coal. HTFs are key to collecting the energy from the whole solar field and transporting it to the power block.
There are very diverse types of HTF starting with water, which is free (other than the cost of being de-ionizing), heavy oil and molten salts. In a new article by CSP Today the three main transfer fluids are compared and evaluated in different situations and technologies.
Water can prove unstable and difficult to manage at high temperature/high pressure situations. According to Arnold Leitner, author of the article and Founder and Former CEO of SkyFuel, “using water as a HTF is problematic as its pressure increases significantly with temperature.” Evaporating water to useful steam conditions for a steam turbine generation (550 degrees Celsius) requires pressurizing the evaporator to about 160 Bar. Using water as transfer fluid may be worth it when a steam turbine is used as it avoids efficiency losses and extra costs in the exchanger.
Transfer oils have been a preferred source for CSP developers to get around the high pressure issue. The problem with heavy oils however, is that the hydrocarbon breaks down quickly if heated to within 400 degrees Celsius. Therefore oil limits the temperature parabolic troughs can operate at.
Molten salt is a mixture of nitrate salts (mainly sodium nitrate NaNO3 and potassium nitrate KNO3 at a 60-40 ratio which melts when heated above 230 degree Celsius. According to Leitner “Molten salts can be used in parabolic troughs, but it is so much easier to get to higher temperatures if the concentration ratio can be increased from about 80x for a line concentrator such as parabolic trough to about 1,000x in a power tower.” A further challenge for molten salts being used with parabolic trough is the risk of a freeze event of the salt in the miles of receiver length.
However, whilst HTFs make an important cost component for a CSP plant in the receiver tubes and header pipes, the cost of HTF hits the scale much more significantly when it is used in very large amounts as heat a storage medium. Currently only molten salt is used for that purpose.
This article was shared by CSP Today as part of CSP TODAY LATAM 2012 (Antofagasta, 17-18 July) For more information and to read the whole article please visit www.csptoday.com/latam/en-content7.php