Author: Clive Jones, managing director, Global Heat Transfer “One thing I feel sure of [...] is that the human race must finally use direct sun power or revert to barbarism." This quote from a 1914 issue of 'Scientific American' comes from the inventor, engineer and solar pioneer Frank Shuman, whose illustrious career boasts accolades from attaining more than 60 patents and building the world’s first solar thermal power station in Egypt. Shuman’s revolutionary work in solar thermal power generation provided the basis for today’s concentrated solar power (CSP) plants. Here Clive Jones talks about fulfilling Shuman’s dream with thorough heat transfer fluid maintenance for CSP plants. Heat transfer fluid maintenance and analysis are essential operations that need to be conducted on a regular basis. Unfortunately, some plant managers do not realise that there is a problem until it is too late.

The trough

One system for large-scale solar thermal power generation is the parabolic trough method. Thermal fluids play a vital role in this process and maintenance is paramount to ensure efficient and safe generation of energy. Parabolic trough power plants use curved, mirrored troughs to reflect direct sunlight onto a central glass pipe containing fluid - called the receiver, absorber or collector. Thermal heat transfer fluid then passes through the receiver and becomes incredibly hot. This is used to heat water to boiling point and the steam then drives a turbine generator, converting mechanical energy to electrical energy. In this process, about a third of the heat energy is converted to electricity. With parabolic solar thermal generation, fluids have to work for prolonged periods of time at up to 400°C. This is because at high temperatures, thermal energy can be converted to electricity more efficiently. Synthetic oils are typically used in these applications because they suffer degradation at a slower rate to that of natural oils, although some applications use mineral-based oils such as Omnipure.

The right fluid for the job

A heat transfer fluid’s thermodynamic attributes will vary according to operating conditions. At high temperatures, a thermal fluid will experience chemical degradation. The freezing point of thermal fluid must be lower than ambient conditions. Alternatively, the temperature of the thermal fluid needs to be kept above ambient temperature to stop the heat transfer fluid from freezing. For example, some products freeze at 12°C, which is higher than you might expect. Thermal fluids in solar thermal applications need to have a stable chemical composition at high temperatures to reduce the effects of degradation, as well as a low viscosity. This helps reduce frictional flow and the amount of energy needed to pump the fluid through the system. Molten fluoride, chloride, and nitrate salt can also be used as heat transfer fluids as well as for thermal storage in solar thermal power plants. A significant problem with the use of molten salt concerns its high freezing point of 120-220 degrees centigrade, which varies with the salt used. This requires innovative freeze protection methods and means that systems using this heat transfer fluid also have higher operation and maintenance requirements and costs. As well as carefully selecting the right type of fluid for the job, regular monitoring needs to be undertaken so as to establish the condition of the thermal fluid. The best way to get the most out of fluid and system is to test thoroughly and regularly. Regular representative fluid analysis and a proactive, preventative maintenance plan ensures a healthy system, while reducing down time and decreasing the amount of costly thermal fluid changes needed. Problems with heat transfer systems start when fluids are left for prolonged periods of time without correct supervision or maintenance. Due to their chemical structure, thermal fluids degrade with age or under improper operating conditions. Thermal cracking and oxidation cause molecules in the oil or fluid to break down, which produces solid carbon. If left, this carbon builds up and clogs pipes. At this stage, maintenance activities are relatively easy to conduct, because the carbon is still soft and can be flushed by using thermal cleaning products. At temperatures of 400 degrees, there is also the risk of light ends developing. Light ends are another aspect of heat transfer fluid degradation that solar plants need to be aware of. The formation of short-chained hydrocarbons, or light ends, are denoted by a decrease in flash temperature, which represents a potential fire risk. This is because light ends have lower boiling and ignition temperatures. Flash temperature represents the proportion of flammable decomposition products in a thermal oil.

Routine laboratory testing of open and closed flash temperatures

The development of light ends needs to be monitored by routine laboratory testing of open and closed flash temperatures, because poorly maintained heat transfer systems pose a danger to staff and infrastructure. Blocked pipes and light ends eventually lead to breakdowns and costly repairs or replacements, not to mention the added expense associated with flushing the system and refilling. In addition, disposal of old fluids has to be carried out by qualified professionals in accordance with environmental regulations. This can be extremely expensive if unplanned, hence the need to have a comprehensive maintenance contract in place. By monitoring heat transfer fluids on a regular basis it is possible to detect problems and to take preventative actions that minimise degradation and oxidation, keeping solar thermal power generation applications efficient and cost-effective. Ideally, any plant using heat transfer fluids should create a robust maintenance plan that contains regular system flushing, thermal fluid top-up and careful flashpoint and fouling management. Shuman feared that fossil fuels alone were not enough to sustain man’s growing need for energy. In today’s world, where energy infrastructure is meant to support 7.2 billion people, sustainable energy alternatives have become an imperative. Luckily, with the correct care and attention when it comes to heat transfer fluids, thermal power stations continue to grow as a viable source of renewable energy.