5.2. Applications of Various Fluids

Cons and pros of various heat-transfer fluids

Most notably, molten salts bring a high level of corrosion with them. Additionally, molten salts must be kept above their freezing temperature in pipes running to and from collectors or must be drained back from the system components into a holding tank which is configured to deal with the solidification (freezing) of the salt. There has long been a search for molten salts that remain liquid at room temperature, as described briefly in the Advanced Heat Transfer and Thermal Storage Fluids article from the reading. Such salts are anticipated to be low in cost and their high thermal stability (resistance to flammability) makes them very desirable for high temperature applications where alternatives such as oil are highly flammable.

Water-based fluids, such as glycol solutions, degrade over time and must be changed every 3-5 years. If a glycol fluid is subjected to very high temperatures, such as stagnation temperatures when a system is at capacity and the load is not using the heat in the summer, this degradation speeds up, further reducing the life of the fluid. In part, this is one driver for solar air conditioning systems as a way to use excess heat in the summer, increasing the life of the fluid while using free fuel (solar radiation). Pure water is subject to freezing in the winter, but is ideal (very low cost) for locations that do not experience freezing temperatures as well as systems that are equipped (adding additional capital cost) with a drainback tank to hold the water in a thermally controlled space during the times when the collector temperature is below freezing.

Oils are a great fluid for concentrating systems because of their high boiling point (>300 degrees C). With relatively low costs, low freezing points, and high thermal capacity compared to water or air, oils are the best choice for most concentrating systems and are used worldwide.

Some systems use refrigerants, which was discussed briefly above. Chlorofluorocarbon (CFC) refrigerants, such as Freon, have been used historically in some solar thermal systems, but have been phased out due to the negative effect of CFCs on the earth’s ozone layer when CFCs are vented to the environment (either intentionally or accidentally). The benefit of refrigerants is a low boiling point enabling the leveraging of the fluid’s phase change as well as a high heat capacity. CFC refrigerants can be replaced, with some system modifications, by methyl alcohol, ammonia, and more. Research is ongoing in this field.

Silicones is another group of fluids discussed briefly above. Silicones are still rare in solar applications. These fluids require more energy to pump than alternatives due to a high viscosity and they also tend to leak easily through microscopic holes in a solar thermal system. Silicones are interesting and will easily be researched further because of their noncorrosive nature and long lifetime (compared with the 3-5 years of oil).

Applications

Flat plate collectors that are intended for domestic hot water and space heating typically use a water-based heat transfer fluid, either pure water if the location is either not subject to freezing temperatures or if the system contains a drain-back tank, or a water/glycol (antifreeze) mixture.

Parabolic trough collectors typically used an oil based heat transfer fluid, though water and molten salt can both be used as well. When water is used, it is used to directly generate steam for use in a turbine or other steam application. Oil can achieve a higher temperature before boiling, thus increasing the efficiency of the collector.

Power tower systems often use molten salt as a fluid of choice. This is because the molten salt does not need to be pumped through a long series of tubes to pass by each collector. The fluid simply resides in the central boiler tank where all of the power tower mirrors are aimed. When the sun sets and the heat is used up, the molten salt freezes (solidifies) in the boiler tank, ready to be melted again when the sun comes up. This greatly reduces the complexities associated with the fluid solidifying inside a series of tubes across a large area with parts that may or may not be easily subjected to solar radiation or other heat sources to melt the salt the following day. Air is typically only used in flat plate collectors, but can also be used in power towers. Air is a good choice for applications such as food/textile drying or space heating.

Please look through the following presentation at the IEA CSP Workshop - it provides additional summary of application of different heat transfer fluids in various solar thermal systems: