Parabolic Troughs have dominated Concentrating Solar Power [CSP] until recently, but several companies are vying to replace them. Will the upstarts succeed, or will incumbency and improvements to trough technology ward off the competition?
The technology which can deliver power when it is needed at a reasonable price should triumph. Photovoltaic [PV] technologies are rapidly producing price reductions, and can be used almost anywhere, but only produce power when the sun is shining. In contrast, CSP which is still cheaper than PV enables inexpensive thermal storage, with the promise of dispatchable power to compensate for the variability of other renewable power sources and demand. Dispatchability assures CSP with storage a place in the eventual energy mix.
Heat Transfer Fluids
The ability and efficiency of a technology to accommodate thermal storage (and provide dispatchability) is a function of the heat transfer fluid and working temperature.
Three heat transfer fluids have been demonstrated to date: Steam (in power towers and troughs) mineral oil (in most parabolic trough plants,) and molten nitrate salts (in power towers.) The working temperature for steam is limited by the potential for corrosion. Molten salts and oil break down at high temperatures, with molten salt and steam capable of achieving the highest temperatures (about 565° C for nitrate salts.)
Lower temperature steam is also the working fluid for Ausra, a company working to commercialize the Compact Linear Fresnel Reflector (CLFR) geometry. CLFR breaks up a trough into a series of narrow, nearly flat, reflectors saving on the high cost of carefully focused troughs. Ausra recently announced that they were refocusing on becoming a technology and materials provider, rather than building solar farms on their own. An industry observer who prefers to remain anonymous thinks that this will mean the end of the company for practical purposes, since the process heat market is very difficult to sell into, and few companies are willing to back expensive, untried technology, especially from a third party vendor.
Oil is commonly used as the heat transfer fluid in parabolic trough systems because it does not freeze at night (nitrate salts freeze at 220° C) and operates at lower pressure than steam. According to Bill Gould, Chief Technical Officer of Solar Reserve, such systems have peak operating temperatures of 375°C. Solar Reserve is working to commercialize the nitrate salt/power tower combination which was demonstrated at DOE's Solar Two in the late 1990s, for which Bill Gould was the project manager.
60% NaNO3 and 40% KNO3 by weight.
Has very low vapor pressure, but begins to decompose around 600 °C
$90-$160/kWe (trough); $30-$55/kWe (tower)
The best established thermal storage system is two-tank molten salt, according to Greg Glatzmaier, a Senior Engineer II on the National Renewable Energy Laboratory's (NREL) CSP research team. Pressurized steam or oil have also been used, but at higher cost per kWh. Pressurized steam is only practical for short term buffer storage, according to Greg Kolb, a Distinguished Member of Technical Staff National Solar Thermal Test Facility.
Commercial projects using oil as a heat transfer fluid and molten salt for thermal storage include Nevada Solar One and Solar Millennium's (SMLNF.PK) Andesol parabolic trough plants. Solar Millennium is currently the only pure-play publicly traded CSP company I'm aware of.)
According to Gould and Glatzmaier, the thermal storage systems systems at the Andesol plants suffer 7%-10% round-trip energy losses in heat exchange. If molten salt is also used as the heat transfer fluid, then there is no need for heat exchangers, and no such heat loss. The lower working temperature of these plants also requires much more salt and larger tanks to effectively store the same amount of electricity as for a power tower, once the lower temperatures and efficiency losses are taken into account..
Gould calculates that a trough plant will require three times as much molten salt (along with larger tanks to store it) as a power tower to store an equivalent amount of energy. With additional information from Glatzmaier, I calculate that, to store the equivalent of 1 kWh of electricity at a trough plant requires approximately $90-160 of capital cost, compared to about $30-$55 at a tower, with the variability arising from the commodity price of salt, which is mainly used as fertilizer.
The Shape of Things to Come
In terms of configuration, many experts see long term advantages in power towers. Nate Blair, a Senior Analyst at NREL says the underlying efficiency advantage of towers arising from higher working temperatures will lead to more power from a similar investment in hardware. A Rankin cycle turbine will operate at about 37% efficiency for troughs, or 41% for a tower, meaning a tower can produce approximately 8% more electricity from the same amount of heat.
The combination of energy storage using molten salt, no heat transfer losses, and the thermal efficiency of power towers, point to power towers with molten salts as the working fluid as the long-term favorite.
There are challenges. Only parabolic troughs are a proven, bankable technology. Dr. Leitner estimates that it will cost between $500-$700 million to commercialize a new technology. Solar Reserve plans to overcome this barrier with a performance guarantee from United Technologies (NYSE:UTX) up to the value of the contract, or $200 million, but in the current financial climate financing remains difficult.
SkyFuel has plans to use the innovative reflective film ReflecTech in a hybrid of parabolic trough and CLFR configuration called a Linear Power Tower [LPT]. By increasing the diameter of the receiver they hope to reduce heat loss and allow the salt to stay molten for longer periods. ReflecTech enables relatively inexpensive, large parabolic mirrors to be used in the CLFR configuration, with 10 mirrors, each about 3 meters wide focused on each receiver. This should achieve 85x magnification, sufficient to reach temperatures comparable to those in a power tower.
SkyFuel hopes to commercialize the LPT incrementally, by first testing it as part of existing parabolic trough plants using oil as the heat transfer fluid. Might the parabolic trough triumph by incorporating the advantages of power towers?
DISCLOSURE: The author has a long position in UTX.