Green Heat Transfer Fluids
Bio-derived glycols offer an alternative to oil- or natural gas-derived heat transfer fluids.
Glycol-based heat transfer fluids and antifreezes are widely used in industrial applications ranging from process cooling to solar thermal heating and HVAC. Propylene glycol and ethylene glycol are the most commonly used glycols due to their relatively reliable nature and established properties.
Traditionally, propylene glycol and ethylene glycol are derived from petroleum or natural gas and do not offer a “green,” or renewable solution for heat transfer fluid applications. In recent years, glycols have been produced from crops and crop waste such as corn and corn stalks as well as glycerol, a byproduct of biodiesel plants. These fluids are designed to fill a demand for greener products. The bio-derived glycols offer alternatives to oil- or natural gas-derived heat transfer fluids while maintaining quality and efficiency.
Types of Bio-Derived Glycols
Two main types of bio-derived glycols are sold commercially today. The first is a bio-based glycol that is identical to the typical propylene glycol or 1, 2-propanediol. It has the same properties except that it is made from a biological source such as crop waste or corn.
The second type is trimethylene glycol or 1, 3-propanediol, which is similar to propylene glycol but has a few different properties that are improved from normal propylene glycol. Like propylene glycol, it is approved for engine coolant usage and is registered with the NSF International Nonfood Compounds as acceptable for incidental food contact as a heat transfer fluid. This “green propylene glycol” offers less greenhouse gas emissions of CO2 and lower non-renewable energy consumption than typical propylene glycol production.
Comparing ethylene glycol, propylene glycol - both petroleum-derived and bio-derived - and the “green propylene glycol” can be done by looking at the properties that are most important in the processes they are used in. For instance, glycols often are used in HVAC processes with heat exchangers, pumps and sometimes boilers, as well as solar thermal hot water heaters, where higher temperatures can come into play, and geothermal heat pumps, with wells that go deep into the ground. The three most important properties in these processes would be thermal conductivity, viscosity and thermal stability, or thermal degradation. These three properties will affect heat transfer, the pumping power and the fluid life/system life.
The data is covered in depth online at www.process-cooling.com. However, a few general statements can be made:
- The thermal conductivity of ethylene glycol is best, followed by propylene glycol and the green propylene glycol, which have very similar numbers. Ethylene glycol is well documented to have better thermal conductivity than propylene glycol. However, it is a toxic material.
- The viscosity data shows that ethylene glycol has the lowest viscosity profile over the temperature range. The green propylene glycol has a lower viscosity than propylene glycol at lower temperatures and a similar viscosity at the higher temperatures.
- Thermal stability of the glycol fluids is an indicator of how durable the heat transfer fluid is. Looking at the stability of the glycol chemistries, ethylene glycol degrades the quickest under constant temperature and constant pressure, meaning it might not be the best choice for constant high temperature applications in boiler systems or solar thermal hot water systems. The green propylene glycol has the slowest thermal degradation.
Each of these features help the green propylene glycol warrant a look for applications where the user would like a renewable fluid. PC