Heat transfer fluid and how it affects performance of a GCHP system

Heat transfer fluid is used to move energy to and from the ground to a heat pump in any ground coupled heat pump system. In a hot climate energy is transferred from the building to the ground by heating water to a temperature higher than the ground temperature. Fluid temperatures from the heat pump can reach temperatures as high as 90° to 100°F (32° to 38°C), In a cold climate, however...

energy is taken from the ground by chilling the fluid to a temperature several degrees lower than the temperature of the ground around the pipe. When the ambient ground temperature starts at 45° to 50°F (7° to 10°C), that means the the fluid temperature has to approach, or even drop below 32°F (0°C) to be able to absorb energy from the cold ground. It means antifreeze must be added to the water to prevent if form freezing in the heat exchanger. 

IGSHPA guidelines recommend the addition of antifreeze whenever you expect the water temperature from the ground to drop below about 42°F (6°C). How much antifreeze should be added to water in the GHX to ensure it doesn't freeze? IGSHPA guidelines recommend enough antifreeze should be added to protect the fluid from freezing to a temperature approximately 15°F (8°C) lower than the coldest expected operating temperature. 

This means that even if your system is designed to operate at a minimum temperature of 35°F (2°C), the fluid should not freeze at a temperature higher than 20°F (-7°C). If the lowest temperature you anticipate is 30°F (-1°C), the fluid should be protected to about 15°F (-9°C). So why not simply protect the fluid to 0°F (-18°C) and not worry about making a mistake on the minimum design temperature of the system?

Adding antifreeze to water changes the density and viscosity of the mixed fluid. Changing the temperature of the fluid changes the density and viscosity of the fluid.  Changing the density and viscosity of the fluid changes the characteristics of how the fluid flows in a pipe. It will change:

• How much energy is needed to push the fluid through a pipe. A more viscous fluid (such as cold syrup) is more difficult to pump than hot water...it requires a larger pump and will use more energy.
• How the fluid flows through a pipe. Imagine a river flowing across the flat prairie...along the shore the water may be barely moving...friction is holding the water near the shore...and most of the flow occurs near the center of the river. Now imagine a river flowing down a steep mountain pass...gravity pulls even the water along the shore down the hill. The same thing happens inside a pipe. When more power pushes the water through a pipe, even the water along the pipe wall is forced to move...and the increased flow increases how quickly heat transfers from the water through the pipe to the dirt surrounding the pipe. 

The type of antifreeze most commonly added to water includes ethanol (grain alcohol), methanol (wood alcohol) and propylene glycol. Each of these fluids change the characteristics of the mixed fluid differently, depending on how much antifreeze is added and the temperature the system operates at. The changes in the fluid must be taken into account when determining the size of GHX piping used in the system, number of boreholes or trenches required, size of the circulation pump and overall system efficiency. 

In other words, a system designed to work well with water, may or may not work well if ethanol is added to the fluid, or propylene glycol, or methanol. The GHX may operate perfectly if 25% propylene glycol is added to the water, but may fail if 35% propylene glycol is added.  

Other characteristics of the fluid must be considered when specifying the type of antifreeze that is added, including:

• Cost of fluid
• Health and safety regulations regarding flammability of the fluid at the percentage specified
• Pumping power and cost over long term operation
• Environmental regulations in some jurisdictions about the use of different types of fluids that could potentially come into contact with an aquifer or water in lakes or rivers
• Potential for the mixed fluid to promote bacteria growth

The selection and percentage of fluid specified can have an impact on the overall system cost (and the decision a building owner makes) and long term system efficiency.