Molten salt: how to avoid the big freeze
Heat tracing systems are a critical feature of CSP plants that use molten salt as a heat transfer fluid
The blistering sun that beats down on Fuentes de Andalucía, southern Spain, can make for a pretty torrid working environment. But even the most scorching midsummer rays are not hot enough for the molten salt that forms the lifeblood of the Gemasolar power tower plant there.
Gemasolar is famous for being the first CSP plant in the world that can provide round-the-clock solar power, thanks to a thermal energy storage tank that can feed the turbine for up to 15 hours after the sun has set. It does this by heating molten salt from 270ºC up to around 565ºC.
The salt’s heat can be used in place sunlight for many hours before it cools, but there is a catch: if its temperature drops below 240ºC, it will freeze.
The frozen salt could block pipes and prevent the whole system from working, putting the plant into a coma that it will be difficult, and expensive, to wake from. Furthermore, the salt would shrink as it cools, potentially stressing pipes and increasing the risk of failures.
To prevent that from happening, Gemasolar, which is operated by Torresol Energy and was developed as a joint venture between the Spanish engineering companies Sener and Cobra, is equipped with 12 km of mineral insulation heating cable and a heat tracing system.
The principle of heat tracing systems is relatively simple: you lag pipes and tanks so as to minimise heat loss and throw in a monitoring and heating capability so any areas that start to cool down dangerously can be heated up again.
In practice, of course, technologies such as Gemasolar’s Akotrace system are a little more complex.
Developed by Ako, a Spanish company with a 30-year pedigree in temperature regulation and control, Akotrace features three separate layers of equipment. The first is made up of heat cables, heaters, probes and lagging, with several layers of redundancy to prevent failures.
Heat transfer fluid
Above that are control and power cabinets that deliver energy to the parts of the CSP heat transfer fluid system that might need warming up. These again can be configured for redundancy, and placed around the solar field if required.
The third layer of the system is a communications network that uses a fibre-optic ring to connect up all the cabinets and provide an Ethernet connection to the plant’s distributed control system. This even allows the system to be managed remotely over the internet, if needed.
The levels of redundancy throughout the system are needed to make sure the systems will never fail. And they have become invaluable, not just in plants such as Gemasolar but also in ones where molten salt is used as a heat transfer fluid (HTF) across the entire solar field.
Back in 2001, a report for the US National Renewable Energy Laboratory (NREL) by Kearney & Associates showed it would make sense to use molten salt as a HTF provided the operating temperature of the system could be kept above 400ºC.
A plant would need to divert about 4% of the solar energy it collected into freeze protection heating, said the ‘Engineering Evaluation of a Molten Salt HTF in a Parabolic Trough Solar Field’ study, but could expect a 3% to 7% performance improvement including that deduction.
Thereafter, though, the engineering issues associated with keeping salt in its molten state were long considered an important challenge.
In 2007, as part of an NREL trough workshop, Doug Brosseau, Greg Kolb and Bob Bradshaw of Lockheed Martin’s Sandia National Laboratories asked: “Are we crazy? Are we really serious about flowing molten salt through miles and kilometres of thin-wall steel tubes?”
Freeze recovery
Nevertheless it seems the potential problems, which the trio identified as including frequent freeze-ups, freeze recovery and damage on expansion, have been fully overcome.
Ako, which is the leader in a market of less than half a dozen or so heat tracing system suppliers, now claims to have installed Akotrace systems in more than 25 CSP plants.
These include Gemasolar and other important Spanish installations including the Samcasol, Valle 1 and 2, and Andasol III 50MW parabolic trough plants.
Sonia Martínez Llorens, head of communications at Ako, confirms the company will also be providing the heat tracing systems for Abengoa’s 280MW parabolic trough Solana Generating Station at Gila Bend in Arizona, USA, which will open next year with six hours of storage.
“Using tracing to heat up the fluid is essential,” she says.
Jesus Fernández-Reche, of the Spanish Centre for Energy, Environmental and Technology Investigation’s Almeria Solar Platform, adds: “It would be a problem if the salts freeze in the HTF circuit.
“However, with these systems to melt the salts again the likelihood of freezing is approximately zero. And as long as they remain in the liquid phase their useful lifespan should equal that of the plant itself.”