5.1. Overview of Solar Thermal Fluids

Solar thermal fluids (or heat-transfer fluides - HTF) come in six primary groups:

• Oil-based
• Water-based
• Molten salts
• Air
• Refrigerants
• Silicones

Each type of heat transfer fluid has advantages and disadvantages with respect to different types of solar thermal energy conversion systems. Oil, water, or molten salts can all be used in Parabolic Trough and Linear Fresnel collector systems, while only molten salt and water (oil is excluded here) in addition to the option of air can be used in a power tower system. Parabolic trough systems are the most widely installed type of system worldwide, at 90% of systems installed by the end of the year 2013 (source: iea.org). Refrigerants and silicones are rarely used in flat plate systems and are not used in concentrating systems for various reasons discussed later.

Oil-based fluids come in three categories; synthetic hydrocarbons, paraffin hydrocarbons, and aromatic refined mineral oils. Water based fluids can be either pure water or a water and glycol mixture with either ethylene or propylene glycol which are types of “antifreeze”. Molten salts are nitrate (ionic) salts that are only available in concentrating systems due to the high temperature requirements of the fluid. Air is standard air, comprised mostly of nitrogen and oxygen, useful for specific applications such as a drying industrial process or low grade heating of building spaces. Refrigerants are commonly used in refrigerators, air conditioners, and heat pumps, but are not commonly used in solar thermal systems today. Silicones can be either synthetic or organic, but are also not commonly used in solar thermal systems today.

There are seven key properties of a thermal fluid for solar application that must be understood before engaging in design work or decision-making regarding thermal fluid performance and/or selection. The properties include:

• Maximum temperature is the highest temperature before the fluid begins to break down or decompose. The hottest parts of a system are where this maximum temperature is most probable, and therefore must be carefully designed for those key points in the system. The critical locations include the center of any absorbing tubes in the solar collectors or at the auxiliary heater/boiler.
• Freezing temperature is the coldest temperature before the fluid changes phase into a solid, and cannot be pumped, potentially causing irreversible damage to system components. In the case of each fluid, additional properties such as the expansion of the fluid when it freezes are important when determining the potential impact of freezing.
• Density is the mass per unit volume and is temperature dependent. Density can change drastically for some fluids as they heat up. For example, hydrocarbon oils can have a ~30% decrease in density when increased from 25 degrees C to 390 degrees C.
• Steam pressure is the pressure required to prevent the fluid from changing state to a gas. At higher pressures, the temperature at which the heat transfer fluid changes phase from a liquid to a gas (steam) is also higher.
• Specific heat is measured in units of energy (J) per mass (kg) temperature (K) and represents the amount of energy required to raise the temperature of the heat transfer fluid per unit mass. For example, if a fluid had a specific heat of 2300 kJ/kgK, that fluid would require the supply of 2300 kJ of energy to raise one kg of the fluid by one degree Kelvin (Celsius). This parameter is very useful for energy and mass balance calculations.
• Enthalpy is measured in units of energy (J) per mass (kg), representing the energy contained in the fluid under specific conditions such as temperature.
• Viscosity is a fluid’s resistance to shear stress; essentially a measure of how much pumping force is required to move the fluid in a constrained environment. Viscosity can change greatly with temperature for some fluids. Lower viscosities are preferred to reduce pumping costs.

Learn more about the types and properties of common heat-transfer fluids in the following readings: