RENEWABLE TECHNOLOGY FEATURE KEEPING SOLAR SHINING
All around the world, nations are building vast solar plants to harvest and store energy
from the sun. Clive Jones, managing director of heat transfer fluid specialist, Global Heat Transfer, explains how solar plant managers can extend the lifespan of solar technology
S
olar farms provide a way to generate and supply large amounts of renewable energy to the grid. Not only is it clean, flexible and
affordable, but research into the field has driven solar costs down, making it a low-cost power generation technology. Even in the UK, a nation not renowned for its sunny weather, developments are underway on a 900-acre farm on the coast of north Kent. 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 Fresnal reflectors.
THERMAL FLUIDS IN SOLAR The 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. With parabolic trough solar generation, for example, the hundreds of mirrors reflecting light into a concentrated area means the fluid has to work at over 400˚C for extended periods. The thermal fluid should also be designed to operate at low
temperatures without freezing. The freezing point must be lower than that of ambient conditions or it must be kept above ambient temperature to prevent 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 the energy needed to pump it through the system. In solar applications, synthetic oils like Globaltherm Omnitech are common, but some applications use mineral based oils like Globaltherm Omnipure. 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 means anti-freeze methods are needed, increasing operation and maintenance requirements and costs.
LIFESPAN OF SOLAR TECHNOLOGY Correct fluid choice for the location, size and application requirements of the plant is just the first step. 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. Its invisibility, however, does not mean it can be ignored. When operated at high temperatures, thermal fluids degrade over time
by a process called thermal cracking. If left without correct supervision or maintenance, 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. As well as this, cracking produces carbon that leads to fouling – the
formation of sludge that reduces the efficiency of the system. Eventually, the system will need to be flushed and drained and the fluid replaced in order to keep operation efficient and safe, an expensive and time- consuming process. There is a standard degradation curve for each fluid, and the higher
the operating temperature the faster this process takes place. However, degradation will vary from application to application, so the best way to maximise fluid lifespan is regular testing. Technicians can check the condition of the fluid by taking a sample from a live system and sending it to a laboratory for analysis. In this way, solar plant managers can monitor the degradation process and make sure 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. For example, if the testing shows the flash point has decreased, engineers could use a light ends removal kit to remove the volatile light ends. For large solar plants, the safest approach is to consult a third party expert on a thermal fluid lifecycle maintenance plan. This can include things like 24-hour engineering support, sampling and analysis and maintenance. As the competition for the world’s largest solar farm heats up, large
quantities of thermal fluid will be needed. And, once the solar plant is established, it is vital that thermal fluids are proactively monitored and maintained. Global Heat Transfer has the production capacity to deliver the high volumes of fluid needed for solar farm installations and has the ability to deliver to remote locations in tight timeframes.
Global Heat Transfer / ENERGYMANAGEMENT
www.globalhtf.com ENERGY MANAGEMENT | WINTER 2020 31
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