How can you choose the best heat transfer fluid for your solar thermal energy storage system?

It depends on a lot of things. What is the desired operating temperature? What are the limits in viscosity (think pumping power requirements)? Will this fluid be used in a thermal driving head design? Is it compatible with the material the piping is made of? How large of an expansion tank is needed? Etc.

Heat Transfer Fluid (HTF) is used to transfer the solar energy from the solar Field to the Solar Thermal Energy Storage (STES). The parameters for correct HTF selection are: ? Thermodynamic properties: operating temperature, operating pressure, freezing point and vapor pressure ? Transport properties: viscosity as well as density, heat capacity and thermal conductivity ? Health & Safety parameters: flashing points, auto-injection point, toxicity Molten salts have freezing temperatures far above ambient, which can cause issues. More stable fluids are: synthetic, organic and silicon-based fluids, up to 400oC or higher. Currently, the most common HTF for parabolic collectors is organic fluid, a mix of biphenyl and diphenyl oxides.

The most common HTF used in CSP industry are air, CO2, water, molten salt, glycol based, glycerol based, synthetic organic oils and silicon based fluid which can transfer heat with a very good efficiency. There are several technologies related with CSP and the used HTF system is different. The most common technologies are: Linear Fresnel Solar Collector: This technology uses water, molten salt, glycol based and organic fluid as HTF. Parabolic Trough Collector: . This technology uses water, glycol based, organic fluid and silicone basis as HTF. Central Tower: This is a type of solar furnace to receiver on the top of a tower to focus sunlight using movable mirrors or heliostats. This technology uses water, molten salt, air and CO2 as HTF.

-The parameters that have to be taken into account for a correct HTF selection are several. Following the most important: -Thermodynamic properties like, the operating temperature, operating pressure, freezing point and vapor pressure give the ranges of parameters over which the fluid will convey heat -Transport properties such as viscosity and density indicate the pumpability of the fluid. - Collector and heat exchangers. Health & Safety parameters as flashing points, auto-injection point, toxicity, oxidation define the mitigation level -Air and CO2 have an important thermal expansion in volume in a closed space. It can increase the pressure with a consequence of possible explosion, so it is considered useful in a low temperature range.

Either the molten salt and the syntetic/organic oil have a freezing points close or above the ambient temperature. In these cases, the system should be equipped by some equipment that can mitigate this effect (e.g. Natural Gas, diesel.) that should be available in emergency cases. Another new generation HTF that was developed recently is the silicone-based fluids intended especially for parabolic trough CSP plants. This can withstand temperatures up to 425 °C for long periods, and retain their low viscosity even at -40 °C. The high thermal stability and good heat transfer property makes the silicon based HTF very attractive. However is one of the most expensive element on the market at the time and was never employed in a commercial plant.

Concentrated Solar Power (CSP) is becoming more important in the last 20 years, not only for electricity generation but also heat for the industry. STES is one of the most important parts of the CSP industry and it is a challenge to maintain acceptable cost with good efficiency. The technologies are very different and strongly depend on the final application, either solid (like concrete, stones, thermal particles, etc.), liquid (like molten salt, glycol based, glycerol based, synthetic oils, water, etc.) and gas (like steam, air, CO2, etc.). -Sometimes the HTF and the STES are using the same fluid, this increases the overall efficiency due to the fact of avoiding intermediated exchange system. If possible it is the best solution.