Clive Jones, managing director of heat transfer fluid specialist for concentrated solar power thermal storage applications, Global Heat Transfer, explains how solar plant managers can extend the lifespan of their solar technology.
According to Solar Power Portal, during 2019 and 2020, planners identified more than 10GW of new solar power site capacity in the UK. Solar farms will allow the UK to generate and supply large amounts of affordable, renewable energy to the grid. However, reaping the full benefits of this energy source requires engineers to proactively monitor and maintain the system and the thermal fluid that transports and stores energy.
In recent years, solar power has become a low-cost energy because research has reduced manufacturing, maintenance and installation costs. Despite the cloudy days we have in the UK, solar is still a practical investment option. There are many solar farms appearing across the country, including a 350MW solar farm that has been approved for construction on the North Kent coast.
Concentrated solar power (CSP) farms collect sunlight by reflecting it onto a receiver that contains heat transfer fluid (HTF). HTF is heated by the sun and used to turn water into steam to drive a turbine and generate electricity. Thermal oils can easily store the sunlight’s energy to allow constant power, irrespective of the type of weather. There are four different types of CSP — parabolic trough, solar power towers, dish systems and Linear Fresnel reflectors.
Choosing a thermal fluid
Heat transfer fluids in a CSP should be specifically designed to perform at the correct temperature for extended periods of time. A parabolic trough includes hundreds of mirrors that reflect the sunlight into one concentrated point. This point can reach temperatures of over 400°C for extended periods of time. If the fluid cannot withstand this, the rate of thermal cracking is increased and the fluid’s lifespan is reduced. Converting thermal energy to electricity is more efficient at high temperatures, so the HTFs must tolerate these scorching conditions.
The opposite issue can occur with cold temperatures, so engineers should select a fluid that is able to operate at low temperatures without freezing. Solar plant managers should ensure the fluid has a freezing temperature lower than their ambient condition. One good solution is a eutectic mixture of diphenyl oxide and biphenyl.
This HTF can perform in vapour and liquid phases, so it is thermally stable at high temperatures and has a low viscosity, reducing friction and the amount of energy used to pump it round the system. In solar applications, synthetic oils are common, but some applications may require the use of mineral-based oils, which operate at similar temperatures to the common synthetic oils.
Some farms that experience extreme temperatures may use a molten salt heat transfer media that can withstand temperatures of up to 600°C. However, the high freezing point of 120-220°C means that the fluid requires anti-freeze methods, increasing operation and maintenance costs.
Monitoring the solar technology
Considering the location, size and application requirements of the solar farm is only the first part of the journey. If well maintained and serviced by fluid monitoring solutions, solar panels can last for 25 to 30 years. External damages, such as a cracked component, are easy to spot, but the same cannot be said about thermal fluid management. Once a thermal fluid enters the system, the manufacturer can no longer see its condition, so while the system seems to be operating effectively, the fluid’s condition may be deteriorating.
Heat transfer fluids operate at high temperatures for extended periods of time, causing the thermal fluid to degrade over time by a process called thermal cracking. Thermal cracking occurs when bonds in the hydrocarbon chains break, producing shorter chained light ends that can boil and ignite at lower temperatures, which reduces the flash point of the thermal fluid.
When light ends are generated, both system and workforce safety are severely compromised. Regular sampling and analysis can help engineers to monitor the flash point temperature – if the temperature has dropped it is highly likely that there are light ends contained in the fluid. Engineers can improve flash point management by installing a light-ends removal kit that will remove volatile light ends, ensuring a more cost effective, clean and safe heat transfer system.
Cracking also creates carbon that leads to fouling – the formation of sludge that reduces the efficiency of the system. The sludge builds up and ultimately causes the system to be flushed, drained and the fluid replaced to maintain high levels of efficiency and safety. Unfortunately, this maintenance and flushing can be an expensive and time-consuming process, especially if it is unexpected.
Degradation will vary depending on the application, so regular testing is the best way to maximise fluid lifespan. Technicians can accurately monitor fluid condition by taking regular samples from a hot, circulating system and sending it to the laboratory for analysis. Solar plant managers can easily monitor the test results to make sure their system is working at an efficient rate and not impacting energy generation.
Regular testing can allow solar power managers to carry out proactive maintenance to extend the lifespan of the fluid, while decreasing downtime and the frequency of the fluid being replaced. Consider this example, a test tells the manager that the HTF has a decreased flash point. An engineer could use a light ends removal kit to remove the volatile light ends, reducing the risk of overheating and fire.
Working with a thermal fluid specialist to implement a thermal fluid lifecycle maintenance plan allows plant managers to closely monitor fluid lifespan and receive support in sampling, analysis and maintenance.
Larger solar plants are often located in remote, isolated areas, so it might be more difficult for third parties to regularly visit the site to offer engineering support. By installing a remote condition monitoring system, plant managers can continually monitor fluid conditions, sharing real-time data with the cloud that engineers can access from any location. Engineers will be alerted if the system detects any potential issues that could impact productivity, enabling them to act quickly and offer remote support or plan a visit to the site.
As the solar power industry continues to expand in 2021, the demand on HTF will rise. Once the solar farm is set up, HTFs must be regularly monitored, tested and maintained to get the most out of the initial investment.