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Heat Transfer Fluids used for CSP (Concentrated Solar Power Systems)

Concentrated solar power (CSP) systems use mirrors or lenses to concentrate a large area of sunlight onto a small area.

This concentrated light is then used as a heat source for a power plant. CSP systems can be used to generate electricity, produce process heat, or provide cooling.

In order to transfer the heat from the concentrated sunlight to where it will be used, a heat transfer fluid (HTF) is used. The HTF is circulated through the solar collector where it is heated by the sun and then flows to a heat exchanger where it transfers its heat to water or another fluid.

The hot water or fluid then produces steam which drives a turbine to generate electricity.

There are several different types of heat transfer fluids that can be used in CSP systems, each with its own advantages and disadvantages.

What properties should a heat transfer fluid have?

An HTF (heat transfer fluid) has to have a number of properties in order to be suitable for use in a CSP system.

HTFs must be carefully engineered to transmit heat easily, operate in the desired temperature range, circulate well within a confined space, and be suitable with containment materials.

This is what an ideal HTF will look like:

High conductivity to improve heat transfer

The HTF should have a high thermal conductivity in order to transfer heat easily from the solar collector to the heat exchanger. A high thermal conductivity means that the HTF can transfer heat quickly and efficiently.

This is important because the HTF needs to be able to transfer the heat from the solar collector to the heat exchanger while the fluid is moving at a high velocity.

Low viscosity to reduce fluid pressure loss and necessary pumping power

HTF should have a low viscosity in order to reduce fluid pressure loss and the necessary pumping power. Low viscosity means that the HTF can flow easily and does not require a lot of energy to pump through the system.

This is important because the HTF needs to be able to circulate easily through the solar collector and heat exchanger.

Low working pressure to minimize tube wall thickness, reducing temperature gradients and strains on the wall.

The HTF should have a low working pressure in order to minimize tube wall thickness.

This reduces temperature gradients and strains on the wall. Heat exchangers are typically made of metal tubes with thin walls, so it is important that the HTF can circulate through the system without causing too much strain on the wall.

Allows direct thermal storage with high heat capacity.

The HTF should have a high heat capacity in order to allow direct thermal storage. This means that the HTF can store a large amount of heat energy, which can be used to generate electricity even when there is no sunlight.

This is important because it allows the CSP system to generate electricity around the clock, even at night or on cloudy days.

Broad temperature range of operation

A good HTF should be able to operate over a broad range of temperatures. This means that the HTF can be used in a variety of CSP systems, from low-temperature systems that use water as the heat transfer fluid, to high-temperature systems that use molten salt.

This is important because it allows the HTF to be used in a variety of CSP systems.

Low melting temperature to eliminate field freezing and the requirement for costly heat tracing components in ducts.

A low melting temperature is important for HTFs used in high-temperature CSP systems. This means that the HTF can be used in systems that use molten salt as the heat transfer fluid.

Molten salt has a high melting point, so a low melting point HTF is necessary to transfer the heat from the solar collector to the heat exchanger.

High thermal stability

High thermal stability is important for HTFs used in high-temperature CSP systems. This means that the HTF can withstand the high temperatures found in these systems.

CSP systems can reach temperatures of over 1000 degrees Celsius, so it is important that the HTF can withstand these temperatures without breaking down.

Low volatility to minimize loss

As the HTF circulates through the system, it will come into contact with the hot solar collector. The HTF should have low volatility in order to minimize loss. Volatility is the tendency of a fluid to vaporize at high temperatures.

If the HTF vaporizes, it will be lost from the system and will not be available to transfer heat. This will increase the cost of the system and decrease its efficiency.

Non-toxic, non-corrosive, and chemically stable

The HTF should be non-toxic, non-corrosive, and chemically stable in order to avoid any damage to the system components. This is important because the HTF needs to be able to circulate through the system without causing any damage.

Suitable for use with commonly available materials

The HTF should be compatible with commonly available materials, such as metals and plastics. This is important because the HTF needs to be able to circulate through the system without causing any damage.

Low cost and availability

The HTF should be low-cost and readily available. This is important because the HTF needs to be affordable and easy to find.

All of these desired characteristics are dependent on the type of HTF. HTFs that use pressurized gases or two-phase systems like water/steam doesn’t have to worry about temperature restrictions because those systems don’t have an upper or lower limit.

The minimum operating temperature for liquid HTFs is dictated by the melting point of the HTF. By simplifying HTF systems and removing the requirement for heat tracing elements, operating temperatures can be reduced to or below ambient standard temperatures, which can save money.

Increased power conversion efficiency can be achieved at greater operating temperatures. This is vital for future cost reductions.

There are several factors that influence the maximum working temperature (Tmax), including the HTF’s thermal stability limit and thermal disintegration, as well as the vapor pressure or corrosion rate of the materials containing the HTF.

High-temperature peaks can emerge in some receiver sites, which can contribute to the commencement of the HTF breakdown reactions, hence system designs should account for both average fluid working temperatures and maximum film temperatures.

To evaluate the HTF’s thermal stability limit, thermogravimetric analysis (TGA) is used to track the weight of materials as they rise in temperature. Tmax is commonly defined as the point at which a sample loses 3% of its weight at a heating rate equal to 10K/min and is used to compare compounds in a laboratory setting.

Cost is an important factor to consider when selecting a commercially successful HTF.

Many substances with fascinating physical qualities can be substantially more expensive, thus engineers must strike a balance between acceptable physical properties, raw material costs, and the cost of compatible construction materials.

What are the types of heat transfer fluids?

Steam, synthetic oil, and molten salts are the most common commercial HTFs. High-temperature fluids like liquid metals (like sodium), molten glass, pressurized gases, supercritical fluids (like supercritical carbon dioxide), and solid particle suspensions are the focus of current research and development activities.

Receivers and power cycles will be able to operate at higher temperatures thanks to this enhanced high-temperature HTM.

Thermal Oils

Thermal oils, also called heat transfer oils, are hydrocarbon-based liquids that are used as heat transfer fluids (HTFs).

In the early stages of CSP trough plants, synthetic oil was used as HTF to avoid the high-pressure demands and liquid-vapor phase shift of water. It is still common for parabolic trough plants to employ synthetic oil as their HTF.

Because synthetic oils have better thermo-physical qualities (lower viscosity and higher thermal conductivity) and are less flammable than traditional mineral oils, they are often favored over mineral oils.

Therminol-VP1, Dowtherm A, and Diphyl are some of the brand names given to the eutectic mixture of biphenyl and diphenyl oxide (DB/DPO), the most popular synthetic oil.

For an HTF, it’s the most thermally stable organic substance available, and it also has a low viscosity. It is combustible because it is an organic fluid, and has been the cause of certain CSP plant fires.

This material can be used at temperatures up to 400 degrees Fahrenheit without severe thermal degradation. The current oil-trough technology has a major drawback in that it can only operate at a maximum temperature of 350°F.

Researchers explore new formulae to raise the maximum working temperature of thermal oil from time to time because it is a well-known HTF. Coatings and/or additives have been used by researchers to prevent breakdown reactions that occur at 400ºC.

In order to boost the thermal conductivity, even nanoparticles have been included.

These advances have yet to be tested commercially, perhaps because the risk of utilizing an unknown fluid is greater than the tiny gains that have already been reported.

Hydrogen formation is believed to rise over time due to the aging of DB/DPO in parabolic trough installations (above 10 years). This is an important issue nowadays, especially given how many commercial plants use thermal oil.

The formation of hydrogen reduces the vacuum inside the trough tube enclosure, which is necessary for proper heat transfer.

The gas will function as insulation, lowering thermal conductivity and, as a result, lowering solar collector efficiency. Much has been published on hydrogen formation in thermal oil in the literature.

Molten Salts

Molten salts have been used for a variety of purposes, including as a coolant in nuclear power plants and as an electrolyte in batteries.

As heat transfer fluids (HTFs), molten salts are liquids at high temperatures that can store a large amount of thermal energy. They are also non-flammable, have high thermal conductivity, and are chemically stable.

The most common molten salt pairs used in CSP systems are calcium nitrate-potassium nitrate (Ca(NO3)2-KNO3, or CAN), and sodium nitrate-potassium nitrate (NaNO3-KNO3, or NaK).

Other mixtures that have been investigated are eutectic mixtures of potassium nitrate and sodium nitrate (KNO3-NaNO3), as well as chlorides, such as sodium chloride (NaCl) and potassium chloride (KCl).

The main advantages of using molten salts are their high heat capacity and thermal conductivity. These properties allow for longer heat storage periods and higher thermal efficiency.

Another advantage is that molten salts can be used at very high temperatures, up to 704°F. This is much higher than the maximum temperature possible with oil-based HTFs (400°F).

The main disadvantages of molten salts are their high cost and corrosiveness. Molten salts can be corrosive to some materials, such as metals. In order to prevent this, the use of corrosion-resistant materials, such as stainless steel, is necessary.

The high cost of molten salts is due to the fact that they are not as easily available as oil or water. They must be mined and then processed in order to be used as an HTF.

Molten salts are the most promising HTF for CSP systems because of their high heat capacity and thermal conductivity. However, their high cost and corrosiveness are significant drawbacks that must be overcome.

Organic Rankine Cycle (ORC) Fluids

The Organic Rankine Cycle (ORC) is a technology that uses an organic fluid, such as a hydrocarbon or fluorocarbon, as the working fluid.

The ORC is a Rankine cycle, which is similar to the familiar steam Rankine cycle, but with the organic fluid instead of water. The organic fluid is vaporized by heat from the CSP system, and the resulting vapor is used to turn a turbine.

The main advantage of ORC systems is that they can use a variety of fluids with low boiling points, such as hydrocarbons and fluorocarbons. This allows for the use of lower temperature heat sources, such as waste heat from industrial processes.

ORC systems are also relatively simple and have a low capital cost. The main disadvantage of ORC systems is their low efficiency.

ORC systems are promising for CSP applications because of their versatility and low cost. However, their low efficiency is a significant drawback.

Inorganic Fluids

Inorganic fluids are those that do not contain carbon atoms in their molecules. Water is the most common inorganic fluid, but other fluids, such as ammonia (NH3) and sodium (Na), have been investigated for use as HTFs.

Water has a high heat capacity and is readily available. It is also non-toxic and has a low cost. However, water has several disadvantages as an HTF.

Firstly, water boils at 212°F, which is much lower than the temperatures necessary for CSP (600-1000°F). As a result, water must be pressurized in order to prevent it from boiling.

This increases the cost and complexity of the system. Secondly, water is corrosive and can damage materials such as metals.

Ammonia has a boiling point of 240°F, which is higher than water but still lower than the temperatures necessary for CSP. Ammonia is also corrosive and can damage materials.

Sodium has a boiling point of 1412°F, which is much higher than the temperatures necessary for CSP. Sodium is also corrosive and can damage materials.

Inorganic fluids have some advantages, such as their high heat capacity and low cost. However, their corrosiveness and low boiling points are significant drawbacks.

Gases

HTF for CSP plants has also been investigated for air and other gases (CO2, H2, and He). It is much easier and safer to operate and maintain classic HTF with pressurized gases than it is with unpressurized hydrogen tetrachloride.

There are numerous advantages to using gaseous HTFs in power block turbines, such as reduced heat exchanger requirements (and the associated heat losses).

Higher efficiency Brayton cycles, which can be paired with traditional Rankine cycles to achieve a combined cycle efficiency of more than 50%, can be used with solar-heated gases that are expanded directly by the sun’s heat.

Gases, on the other hand, have a number of limitations, including poor heat transfer. Poor heat transmission causes difficulties with receiver designs.

Because of their low density and poor heat transfer, coupled with the need for large surface areas for efficient heat exchange, cost-effective thermal storage is more difficult to achieve.

In order to achieve adequate efficiency, gases require high pressures, which necessitate the use of thick wall structural parts and high pumping power requirements.

Inert gases, such as CO2, are another possible HTF that is being reconsidered. From an environmental and safety standpoint, pressurized CO2 has advantages. It is non-flammable and non-toxic, and it is readily available at a low cost. Helium has also been studied in CSP plants for HTFs.

Its key advantages are that it is inert and has a substantially larger specific heat capacity than air.

However, because it contains CO2, it must be utilized in a closed cycle (the fluid is recirculated from the solar receiver to the turbine, then condensed and returned to the receiver), thus leaks are a big issue.

Supercritical Fluids

Supercritical fluids (SCFs) are fluids that exist above their critical temperature and critical pressure. SCFs have some attractive attributes for use as HTFs in CSP plants.

For example, they have high thermal conductivity and heat capacity, which results in improved heat transfer. In addition, SCFs can be used at very high temperatures without decomposition.

However, SCFs have some disadvantages. They are very difficult to control and can be corrosive. In addition, they require high pressures, which can lead to problems with leaks.

Particle Suspensions

Using solid particles to transport heat is another technique that can achieve temperatures in excess of 1000ºC and substantial heat fluxes in the receiver.

CSP systems using advanced higher temperature cycles (as an alternative to liquid metals) and thermochemical cycles and processes in which the particles are the reactive medium can benefit from these new HTMs.

Because the heated solid particles are stored directly, no intermediate heat exchangers are needed between the heat transfer media and storage.

Particle HTFs have many potential advantages over other heat transfer fluids. For example, they can achieve very high temperatures (in excess of 1000ºC) and provide high heat fluxes in the receiver.