Look to the deserts
German particle physicist Gerhard Knies calculated that in a mere six hours, the world’s deserts receive more energy from the sun than humans consume in a whole year. According to his estimate, an 8,100-square-mile stretch of Sahara Desert – an area the size of Wales – could power all of Europe. Concentrated solar power (CSP) plants are just one method of harnessing the sun’s energy and their popularity comes from their impressive capability to store that energy. Here, Clive Jones, managing director of thermal fluid specialist Global Heat Transfer, discusses best practice for CSP plants when handling and maintaining thermal fluids.
The building of a CSP plant is a major undertaking, both technically and financially. By avoiding emergency maintenance, CSP facilities extend operational times, allowing them to provide more energy for longer periods.
Routinely running chemical analysis of heat transfer fluids in a system is critical to maximising the profitability of CSP plants. Thorough testing allows engineers to monitor the condition of a heat transfer fluid and mitigate any degradation.
The type of heat transfer fluid most commonly used in CSP plants is a eutectic mixture of biphenyl and diphenyl oxide (BDO). These fluids need to be able to work as efficiently as possible at high temperatures. For example, Global Heat Transfer’s BDO thermal fluid Globaltherm Omnitech works up to temperatures of 400 degrees Celsius.
Due to the chemical structure of heat transfer fluids, operating at high temperatures for long periods of time causes degradation. This is a normal process in the life of a thermal fluid. By testing regularly under working conditions, plant managers can monitor a system’s health and CSP plants can continue to run efficiently.
As heat transfer fluids degrade, they produce corrosive oxidation by-products and a mixture of heavier and lighter fractions as a result of thermal cracking. The formation of heavier hydrocarbon chains results in carbon deposits called coke. These heavy-ends increase the kinematic viscosity of the fluid, which leads to a reduction in heat transfer and overall efficiency. This is because thermal fluid with heavy ends requires more energy to push through a system; a liquid with a higher viscosity has a higher flow resistance. The more energy required, the more it costs the CSP plant.
If left untreated, heavy-ends lead to solid carbon baking onto the internal surfaces of the heat transfer system. This coating results in a poorer flow rate and results in a potential fire risk due to the creation of hot spots. It is easy to detect heavy-ends by measuring the mass of solids in a thermal fluid sample.
At the other end of the spectrum, the build-up of lighter hydrocarbon chains is called light-ends. The proportion of light-ends in the heat transfer fluid can be established by measuring flash point temperatures. The lower the flash point temperature, the higher the proportion of light-ends and the higher the flammability risk posed to the heat transfer system and CSP plant as a whole.
CSP plant engineers need to regularly sample heat transfer fluids and conduct chemical analysis to monitor any changes in fluid condition and manage their decomposition. This increases sustainability and helps maximise the life of both the heat transfer fluid and system.
Sampling needs to be conducted when the thermal fluid is in operation – this means when it is hot and in circulation in the system. This allows a representative sample to be taken. However, it does also mean it can be a delicate operation, which the CSP plant staff might not be qualified for. This is when a specialist should be called in to carry out the relevant thermal fluid tests to ensure the safety of staff and infrastructure.
Quarterly sampling is necessary to detect any short-term changes in the status of the fluid. This acts as a means of predicting future degradation and allows companies the time to take appropriate action to avoid unnecessary downtime and high costs. A strategic thermal fluid maintenance plan is crucial for healthy and efficient heat transfer systems in CSP plants.
When sampling live BDO thermal fluids, engineers should first carry out a risk assessment. Those carrying out the sampling, need to wear the appropriate personal protective equipment (PPE) and the task needs to be performed in a closed system to prevent the loss of volatile components that may be present in the heat transfer fluid.
When handling heat transfer fluids, potential eye hazards are the first thing that must be taken into consideration. Risks include hot fluids and chemicals splashing, as well as exposure to dust and gas. Best practice dictates that before entering a hazardous environment, an engineer should put on safety spectacles. A suitable facemask must cover these spectacles to protect the eyes against any adverse vapours or splashes. The mask also protects the wearer’s skin.
On entering any facility, it is compulsory that all relevant staff wear a hard hat to prevent any form of head injury. An engineer’s uniform should also include a disposable filtering face piece or respirator – half or full-face – an air-fed helmet and breathing apparatus.
Other protective equipment should include disposable Supertex coveralls and high-visibility clothing. Hazards to hands and arms include abrasion, temperature extremes, cuts and contact with chemicals. When taking a live sample of a heat transfer fluid, heat-rated gauntlets should be worn.
As well as the routine safety measures that should be taken when handling any heat transfer fluid, CSP plant managers need to understand the dangers associated specifically with BDO thermal fluids. Anyone handling BDO fluids needs to avoid accidental ingestion, inhalation and contact with skin or eyes. This includes changing clothing that may have had prolonged contact with a BDO fluid.
To maintain efficient heat transfer systems, CSP plant engineers need to test BDO thermal fluids on a regular basis. Due to the dangers that eutectic mixtures of BDO pose to human health, it is crucial that a specialist company with experience of handling such fluids is the one to carry out the sampling. Global Heat Transfer has over 25 years of experience in this area of thermodynamics.
The potential of concentrated sunlight as an effective method of energy generation is impressive, if still expensive. CSP plant managers need to be aware that proactive maintenance is a viable way to keep heat transfer system expenses down. This method negates the risks and heavy costs associated with severe thermal fluid degradation. By employing a specialist with experience when it comes to handling BDO thermal fluids, a CSP plant can keep heat transfer systems running safely and efficiently.