Heat transfer fluids: Full steam ahead?
In the search for higher CSP plant efficiency, heat transfer fluids used continue to play a key role. Synthetic oil remains the HTF of choice for most - but how likely is it that OEMs will move towards air and direct steam in the future? And what are the main constraints?
By Andrew Williams
According to leading solar energy consultant Arik Ring, for power tower technology, the 'quest' for high temperature is on - with Solgate emerging as the first high temperature brayton cycle demonstration project using air. Solar brayton cycle technology is now also being actively promoted by Aora, Abengoa and DLR, as well as CSIRO in Australia and Wilson Solar Power in the US.
Mr Ring believes that air-based HTFs 'make no sense' for parabolic troughs, but admits that the use of water as a HTF may have some advantages in such systems, including less need for heat exchanges, a possibly higher working temperature and access to a cheap working fluid.
He notes, however, that dual phase water and steam is difficult and costly to handle. “Heat exchanges for water cost more and water is corrosive. Moreover, evaporation may cause pitting and high pressure inside the receiver pipes - it has yet to be proved on a commercial site," he adds.
Meanwhile, Greg Glatzmaier, who leads the CSP Heat Transfer Fluids & Thermal Storage Project at the National Renewable Energy Laboratory (NREL) believes that for parabolic troughs, synthetic oil will remain the heat transfer fluid of choice for the near future.
He highlights that its use is well-established and demonstrated on a commercial scale. He motions to Abengoa Solar’s new Solana plant in the US, which will operate with this heat transfer fluid and will use molten salt as the storage fluid.
"For power towers, BrightSource Energy is building the Ivanpah Solar Complex in California and will use direct steam as the heat transfer fluid in their three receivers. I see direct steam and molten salt as being the heat transfer fluids for power towers in the near future," he says.
"I’m not aware of any planned commercial plants that will use air as the heat transfer fluid," he adds.
Apart from the well-documented examples of Solarlite and Novatec Solar, Glatzmaier notes two other companies that are pursuing 'line-focus' DSG: Hitite Solar Energi in Turkey and Areva in France. Hitite uses a parabolic trough reflector that pivots around the receiver pipe so the pipe does not move. This arrangement eliminates the need for the ball joints that allow movement. Areva uses a Linear Fresnel reflector with a pipe for the receiver. This geometry also does not require the receiver to pivot or move.
"The main issue is to prove DSG is superior in overall techno-economic analysis. There are improvements in engineering, but no breakthroughs," adds Ring.
Although NREL hasn't yet looked into the use of 'subcritical' gases as HTFs, Glatzmaier's view is that the main constraint to the use of air as the HTF in large-scale volumetric receivers is the low volumetric heat capacity, which requires the design to have very high flow velocities through the piping and volumetric receiver. The benefit is that there is essentially no limit on temperature for air so these systems can operate at very high temperatures.
Meanwhile, Ring argues that, for Brayton Cycle at least, heat transfer to a gas is 'inferior' compared to liquid, because such systems are very sensitive to reductions in pressure. This means that they must be operated efficiently, using materials less prone to pressure drop. "Brayton Cycle needs high temperatures; therefore it needs expensive materials," he says.
Seeking out answers
So, how close are these problems to being resolved? According to Craig Turchi, Senior Engineer at NREL's CSP Program, direct steam generation in power towers is currently operating on a commercial scale in Spain (Abengoa), and BrightSource will soon have the first commercial direct steam power tower in the U.S. However, he is not aware of any planned commercial parabolic trough power plants that use DSG or any planned commercial solar plants that will use air as the HTF.
"I know that air HTF has and is being investigated in research laboratories in Europe, including DLR in Germany," he says.
Meanwhile, Ring points out that Aora uses air as a HTF for its innovative 'tulip system' at two working sites in Israel and Spain. Moreover, he says, Southwest Solar has a demonstration unit in Arizona and Abengoa are currently building a large-scale project near Seville.
So, even though mineral or synthetic oil looks set to remain the HTF of choice for some time yet, there are now clear indications that air and DSG based systems are gaining ground and may emerge as an increasingly viable option in the future.
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