The importance of transfer fluid to solar success

Concentrated solar power (CSP) plants use reflectors to concentrate sunlight onto a receiver which contains heat transfer fluid that is heated and used to convert water to steam.

The steam drives a turbine, generating electricity. In addition, the heat transfer fluid can store energy from sunlight, providing consistent power despite intermittent supply.

There are four types of CSP – parabolic trough, solar power towers, dish systems and Linear Fresnel Reflectors.

Heat transfer fluid in a CSP must be designed to work at the appropriate temperature for prolonged periods of operation in solar applications. Because it is more efficient to convert thermal energy to electricity at high temperatures, the fluid must be able to withstand this.

In parabolic trough solar generation, the hundreds of mirrors reflecting light into a concentrated area means the fluid has to work at over 400°C for extended periods. Thermal fluid should be made to operate at low temperatures without freezing.

A eutectic mixture of diphenyl oxide and biphenyl provides the best of both worlds. It can perform in both vapour and liquid phrases, so is thermally stable at high temperatures and has a low viscosity, reducing frictional flow and energy needed to pump it through the system. 

Molten fluoride, chloride and nitrate salt, such as Omnistore, can be used as heat transfer fluids as well as for thermal storage. However, its high freezing point of 120-220°C requires antifreeze methods, increasing operation and maintenance requirements and costs.

A solar panel can operate efficiently for 25 to 30 years, but only if well serviced and maintained. While external damage to a solar plant may be easy to identify, the heat transfer fluid cannot be seen, which makes it difficult to monitor its condition.

When operated at high temperatures, thermal fluids degrade over time by a process called thermal cracking. Thermal fluid degradation can lead to problems with heat transfer systems.

During cracking, the bonds in the hydrocarbon chains break, producing shorter chained light ends, which reduces the flash point of the thermal fluid. Light ends boil and ignite at lower temperatures, reducing the flash point of thermal fluid and creating a fire hazard. Also, cracking produces carbon that leads to fouling.

There is a standard degradation curve for each fluid and the higher the operating temperature, the faster this process takes place. The best way to maximise fluid lifespan is regular testing.

Technicians can check fluid condition by sending a sample from a live system for analysis. Solar plant managers can monitor the degradation process and ensure it doesn’t impact energy generation.

Test results can inform a proactive, preventative maintenance plan to help ensure a healthy system, while reducing downtime and decreasing the chance that the fluid will need replacing.

If testing shows the flash point has decreased, engineers could use a light ends removal kit to remove volatile light ends. For large solar plants, the safest approach is a third party expert. As competition for the world’s largest solar farm heats up, large quantities of thermal fluid will be needed. It is vital that thermal fluids are proactively monitored and maintained, so that bigger really does mean better.

Clive Jones is managing director of Global Heat Transfer