The most common heat transfer fluid (HTF) used in parabolic trough concentrated solar power (CSP) plants is a synthetic organic mix of 73.5% diphenyl oxide and 26.5% biphenyl (BDO). This HTF is used to transfer the heat from the solar collectors to the power cycle. Like all HTFs, solar fluids degrade over time and so it is unclear how often they need to be sampled in order to assess their rate of degradation.
HTFs are a costly investment and condition based monitoring is needed to assess their integrity and to help preserve their functionality. The current article aimed to assess the optimal sampling frequency for solar HTFs based on published literature.
HTF manufacturers commonly recommend that a HTF is sampled and chemically analysed on an annual basis when a HTF is operating near to its upper operating temperature. Alternatively, it is advised that a fluid is sampled and analysed every 2 years if it operates more than 20°C below its upper operating temperature .
However, it must also be acknowledged that the frequency that a fluid is sampled may be defined by the plant insurer who may specify the number of samples per year that need to be taken from the HTF system . This requirement may not be too focused on the overall condition of the fluid, but may be more to do with preventing potential fire risks.
Outside of the recommendations from the fluid manufacturer and the requirements of the insurance policy, there are data from mineral fluids to show that the condition of a fluid is better maintained when it is sampled more frequently . Study results showed that parameters relating to HTF condition inversely correlated with sampling frequency whereby increases in sampling frequency were associated with better overall HTF condition. The implication of this finding is that a mineral HTF should be assessed as frequently as is feasibly possible. It was also shown that missing a sample was associated with worse overall HTF condition.
Interestingly, however, there is no published data to indicate the optimal sampling frequency for a synthetic eutectic BDO fluid. Some data can be extracted from the talk ‘Heat transfer fluid life time analysis of diphenyl oxide/biphenyl grades for concentrated power plants’ by Lang and Lee at ‘SolarPACES’ in 2015 .
Degradation of CSP BDO fluids
Well-known BDO-based HTFs include brands such as Dowtherm A, Therminol VP-1, Globaltherm® Omnitech and Diphyl. Lang and Lee developed models to study the thermal degradation of Dowtherm A and compared this with a non-branded DBO fluid.
The model incorporated three key factors – fluid temperature, impurity concentration (organic and inorganic) and concentration of thermal degradation by-products. Based on a combination of laboratory data, used to determine a theoretical rate of degradation, and normalized data based on real HTF degradation in CSP plants, the authors concluded that the non-branded HTF had a higher rate of degradation (1.7 to 2.4 times) than the branded BDO HTF (Figure 1).
How frequently should a BDO HTF be sampled?
The data in Figure 1 was re-analysed to measure the time, in years, between percentage degradation points (i.e. 0 to 9%; see Table 1). Based on these numbers the sampling frequency can be assessed for each percentage degradation point and Table 1 shows a similar sampling frequency for both BDO HTFs. So, irrespective of a fluids being non-branded or branded, the primary factor that dictates sampling rate is the rate of thermal degradation. Table 1 also reveals that sampling rate markedly increases above a 1% degradation rate with fluids going from being sampled once every two years up to £1% degradation rate and then being sampled two to four times per year when >1%.
There is little debate that the condition of a HTF needs to be routinely monitored (i.e., sampled and then chemically analysed to assess its condition) . Indeed, it is important to gain a true representative sample of the HTF whilst it is hot and circulating in the HTF system. This should be performed by a reputable supplier along with a well-designed maintenance plan, such as the Thermocare® programme offered by Global Heat Transfer Ltd, which will help sustain the life of the HTF. To date, there has been very limited or no scientific data on the sampling rate of synthetic fluids. HTF manufacturers suggest a HTF should be sampled once per year, but this is general advice from all fluid manufacturers. The data from Lang and Lee  sampled data from BDO HTFs commonly used in CSP plants. So, the current analysis is new and extremely important. Indeed, fluid manufacturers can use this information to update their safety data sheets. Plant operators should consider a maintenance plan based on the results of fluid testing and insurers can use this information to ensure CSP plant operators use testing to underpin the safety of their plants.
Findings showed that the sampling frequency of BDO HTFs is an important consideration as it guides sampling regimes for both virgin and used BDO HTFs. It also highlights the importance of maintenance plans and using the results of condition based monitoring to devise interventions to limit the percentage degradation of a fluid. Maintenance plans are also a mechanism whereby cost savings can be achieved by plant operators as maintaining a lower percentage degradation rate means fewer samples per year. It also has the potential to extend the operating life of the BDO HTF, which sustains the operating life of the plant and reduces system wear and tear.
The author would like to acknowledge the writing support provided by Red Pharm communications, which is part of the Red Pharm company (please see @RedPharmCo on Twitter).
Chris Wright is a research scientist, graduating from the University of Leeds in the UK with a BSc and PhD. His research focuses on the use and maintenance of heat transfer fluids in manufacturing and processing, which includes food, pharmaceutical, specialist chemicals and solar sectors.
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