By Andrew Williams

Many CSP plants, in particular those employing parabolic trough and power tower technology, concentrate sunlight onto a heat transfer fluid (HTF), which is used to heat steam in a standard turbine generator. 

Until now, the majority of installations have tended to use some form of mineral or synthetic thermal oil as the principal HTF.  However, many in the industry are beginning to seek out alternative HTFs that can be used at the higher operating temperatures necessary to improve power cycle efficiency.  Amongst those currently under consideration are pressurized gas, air and ionic liquids. 

So what advantages, if any, do these alternatives provide – and which CSP technologies could potentially make use of them?

Breaking the temperature barrier

One major advantage of using air as a heat transfer fluid is that it facilitates the operation of some CSP technologies at far higher temperatures than might otherwise be possible. 

This is particularly true for power towers, where mineral oil cannot be used as a heat transfer fluid due to its low maximum working temperature (around 400°C / 752°F) - making it ineffective for use in power towers, which are typically designed for operation at temperatures above 500°C (932°F).

“Using pressurized air on a tube or close volumetric receiver it is possible to drive a Brayton cycle (or a combined cycle) on a solarized gas turbine at temperatures up to 1000ºC (1832°F), increasing the efficiency of the system compared to typical Rankine cycles at 550ºC (1022°F),” says Jesus Fernandez of the Solar Concentrating Systems Division at CIEMAT.

Other advantages of using air as the HTF include its sheer availability, as well as the fact that it can be taken from the atmosphere for free or at negligible cost.  Moreover, leakages do not cause environmental problems in the same way that molten salt or mineral oil spills might.

Daunting dimensions

However, the use of air is not without its downsides - a major one being that it must either be pressurized or fed through very large pipes.

“The unpressurized air tower constructed by DLR in Julich is only a 1MW plant, and the pipes they use to transport the air are a few feet in diameter.  If you scale that to a much larger plant, the pipe size becomes too large to be practical,” says Joseph Stekli of the Office of Solar Energy Technologies at the US Department of Energy.

In light of this, large-scale operators have little choice but to use pressurized air, which requires thick wall piping, in turn leading to increased piping costs.  It is also likely that operators would need either a compressed air storage tank, and/or an onsite air compressor, both of which add further costs.

Another disadvantage is the low heat transfer rate of air.  Although this can be overcome using a variety of engineering solutions, they all require an increase in the costs attached to one or more components of a CSP plant.

“These disadvantages do not necessarily outweigh the advantages, as we have yet to see a large scale plant built using air in order to judge how the cost of a plant using air as the HTF would compare to the cost of more traditional designs,” says Stekli.

Best technology fit

At this stage, companies have proposed using pressurized air in both parabolic trough and power tower technology. 

“Due to the limited number of companies promoting linear fresnel technology right now, I not aware of anyone proposing the use of this technology with air, but I do not see why air could not be used in linear fresnel technology as well,” says Stekli.

For Fernandez, air would be mainly used in a volumetric or pressurized tube receiver driving a combined-cycle plant (upper brayton cycle and lower rankine cycle).

“In this way, hybridization with natural gas is easy to implement,” he says.

A key advantage of using pressurized gas over molten salt and mineral oil is that operators do not have to worry about the HTF freezing in the pipes.  This eliminates the need to install costly freeze protection systems and substantially reduces the risk of a possibly catastrophic incident occurring in a plant. 

A second advantage is that, in the same way as air, pressurized gas has the potential to be used at much higher temperatures, enabling increased cycle efficiency. "A pressurized gas, such as air or CO², could operate at the high temperatures (around 1000^oC / 1832^oF) of today's most advanced power generation cycles, such as Brayton or supercritical Rankine,” says Joseph Stekli of the Office of Solar Energy Technologies at the US Department of Energy.

No clear cost advantage

In terms of financial outlay, it is not currently clear that there is a cost advantage to using pressurized gas.While it is true that the HTF, as long as CO2 or air is being used, is obviously cheaper, and operation at higher temperatures generally increases turbine efficiency, effectively lowering the price of an entire CSP plant, there are also some cost increases attached to the use of pressurized gas.To begin with, much thicker walled piping must be used due to the increased pressure.

“This is not such a big deal in a tower plant, but in a parabolic trough plant, where there is literally miles of HTF piping, that thicker piping presents a huge cost increase over the traditional piping used,” says Stekli.

Secondly, pressurized gases do not have as high a thermal conductivity as either mineral oil or molten salt. According to Stekli, this means that operators need to install reflectors over a greater surface area to transfer the same amount of heat as other HTF's.

To some extent, this can be mitigated through the use of more innovative designs, but in general the solutions involve more reflective surface area, and the cost increase depends on the solution.All the solutions, however, involve varying degrees of cost increase in the reflector.

These are just an example of some of the drawbacks to using pressurized gases, but it furthers the point that some of the pros to their use have to be measured against their weaknesses. And as a large-scale CSP plant using pressurized gas has not been built, "it is not really clear what the total cost advantages may or may not be,” says Stekli.

Part II of this two-part series will explore the potential of pressurised gas and ionic liquids as alternative heat transfer fluids.

To respond to this article, please write to:

Andrew Williams: awilliams@csptoday.com

Or write to the editor:

Rikki Stancich: rstancich@csptoday.com