Oils and other synthetic liquids are commonly used in CSP plants, as they have a much wider working temperature range.
Heat transfer fluids carry the heat to the storage tank and then to the steam generator. As a result, it is important for good fluids to have a low viscosity and high thermal capacity. Water, synthetic oil, and molten salt can be used as a heat transfer fluids.
Concentrating solar-thermal power (CSP) technologies can be used to generate electricity by converting energy from sunlight to power a turbine, but the same basic technologies can also be used to deliver heat to a variety of industrial applications, like water desalination, enhanced oil recovery, food processing,
Heat can be transferred in three ways: by conduction, by convection, and by radiation.
- Conduction is the transfer of energy from one molecule to another by direct contact. …
- Convection is the movement of heat by a fluid such as water or air. …
- Radiation is the transfer of heat by electromagnetic waves.
The plants will use organic oil as the heat-transfer fluid and molten salt as the storage fluid.
CSP technologies use mirrors to reflect and concentrate sunlight onto a receiver. The energy from the concentrated sunlight heats a high temperature fluid in the receiver. This heat – also known as thermal energy – can be used to spin a turbine or power an engine to generate electricity.
The heat transfer fluids used in CSP technologies include air, water, molten salts, glycol based, glycerol based and synthetic oils which can transfer heat effectively. Of these air and water are not being used now-a-days as air upon heating will increase by volume and so the heat exchanger to be installed should be of larger size for efficient heat transfer which also increases the investment cost to a greater extent. Water on high temperatures will get oxidized quickly that can encourage the materials of the absorber tube to react and can cause corrosion in the inner parts of the tube. Molten salts have the tendency to get solidified when it reaches high temperature. The other fluids available are used for different operating temperatures. Glycol based fluids are used for applications below 175ºC and synthetic fluids for applications above 400ºC. The basic idea for the CSP-PT plant to use heat transfer fluids in their operation.
For the CSP plants to be operated at colder regions water cannot be used as it freezes at 0°C. So there should be some HTFs selected with anti-freezing properties and the fluid lifetime should be for over 20 years. Adding antifreeze agents to water will have negative impacts on system performance by increasing the boiling point thereby resulting in higher power consumption as the viscosity increases. Also the heat transfer efficiency gets reduced because the thermal conductivity and specific heat decreases by introduction of anti-freeze agents. When taking the effect of corrosion, the salts are corrosive and its effect cannot be protected by corrosion inhibitors. The glycols and alcohols without corrosion inhibitors are corrosive as glycol produces acid on oxidation which will result in lower pH value due to high temperature. This leads to acid formation which is corrosive in nature. So pH buffers should be used to maintain HTF in neutral and proper corrosion inhibitors should be used.
The corrosion can be controlled by design and operation such as selection of materials, temperature limit and exposure to oxygen. The selection of fluids as HTF is also important to select by considering the usage of corrosion inhibitors and purity level of fluid to be used. It is necessary to maintain metals in the passive state rather than active state which reduces the corrosion rate and increases the life time of materials. When considering the health, safety and environmental aspects of heat transfer fluids it should not be toxic. From the literatures viewed and among the HTFs that are in old practice alcohols and glycols are classified as moderately toxic. Alcohols have flammability and due to fire safety concerns it is avoided. Among the glycols, propylene glycol is formally approved by Food and Drug Administration because of the advantages like freeze protection, non-corrosive, relatively efficient heat transfer, and no adverse health & safety effects and less cost. The HTFs with higher concentration should not be used in CSP plant as the load increases and so it has to be diluted with distilled or de-ionized water and the minimum concentration achieved can be 20 to 25% to the maximum concentration of 60 to 65%. The HTFs should not be over diluted for example, when propylene glycol is over diluted leads to corrosion and bio-fouling (growth of algae or micro-organisms along the surface of material) which causes unpleasant odor.
III. CRITERIA FOR SELECTION OF HTF From the above literature surveys, the criteria for selecting the heat transfer fluids are selected. The heat transfer fluids play an important role in indirect mode of power production where it delivers heat to the water when comes in contact inside the heat exchanger. The steam that is generated from the heat released by heat transfer fluids is then sent to the turbine for power production. The turbine will operate only when there is sufficient rated pressure of fluid is passed through it. So the desired pressure and temperature has to be obtained when the heat is released from the HTF in heat exchanger.
There are certain criteria’s for a suitable HTF to be selected and those are as follows.
- High operating temperature
- Stability at high temperature
- Low material maintenance and transport costs
- Non-corrosive
- Safe to use
- Low vapor pressure
- Product life cycle
- Low freezing point
- Low viscosity
HEAT TRANSFER FLUIDS USED The heat transfer fluids mainly used in concentrated solar power are based on the selection criteria specified above and few HTFs are listed down with their properties that are collected from different literatures. Out of the heat transfer fluids used in present days, phenyl-naphthalene has been considered as the best on evaluating its performance
