Propylene Glycol vs Ethanol: A Case for Ethanol in Geothermal Systems

Propylene Glycol and Ethanol are both environmentally friendly, readily biodegradable, and safe to handle when used as geothermal fluids. Propylene Glycol-based fluids are often the default fluid of choice when filling a geothermal closed-loop system. However, there are many benefits that come with the use of Ethanol-based heat transfer fluids. Here are two different example scenarios outlining the possible difference between the use of a Propylene Glycol-based fluid vs. the use of an Ethanol-based fluid.

(Note: The price of Propylene Glycol-based fluids is largely driven by the price of crude oil. Even though Propylene Glycol may be produced from Glycerin (commonly referred to as bio-based PG), the majority of Propylene Glycol on the market is crude derived. Conversely, the price of Ethanol-based fluids is determined by agriculture markets (primarily corn) which historically has proved to be far more stable than oil markets.)

Example Scenario 1: Geothermal System designed for Propylene Glycol (GlycoChill+ P Series)

Cost: Propylene Glycol is often added to a geothermal system at 25-30% concentration. Although the cost of Propylene Glycol fluctuates, it could easily be twice the cost of Ethanol, or more. At lower temperatures, the fluid is more viscous and may become harder to pump. Larger pumps will be needed, costing more in electricity to power them (the inherent nature of glycols).

Maintenance: A fluid with a good inhibitor package will be necessary to prevent biofouling. Propylene Glycol is a complex sugar, and bacteria feeds off of it if not properly maintained. The fluid will need to be checked annually to make sure the useful lifespan of the inhibitors hasn’t been used up or “spent”.

Example Scenario 2: Geothermal System designed for Ethanol (Exo Endosol)

In terms of viscosity and heat transferability, Ethanol is more comparable to water. Lower cost and less maintenance are what make Ethanol-based fluids an attractive alternative to using Glycol.

Cost: Since Ethanol has greater heat transferability, a system designed properly with an Ethanol-based geothermal fluid may cost less to install, as fewer wells may be needed. Ethanol is often used at 20-25% concentration. This means that the total volume of concentrate product needed is usually less than a system designed for Glycol. The cost of Ethanol may be half the cost of Propylene Glycol, which can add up to a significant cost savings for the initial fill.* Additionally, savings are generally recognized on the maintenance costs. Since Ethanol is less viscous at lower temperatures, the fluid is easier to pump and requires less pumping power.

Maintenance: Since Ethanol is an alcohol, bacteria does not grow in it, so biofouling is not an issue in an Ethanol-based system.

*Note that hazmat fees may apply on larger volumes.

The Use of Ethanol in Geothermal Systems

The fact that Ethanol is flammable is a legitimate safety concern concerning its use as a heat transfer fluid. How big of a concern it is may be better determined on a case-by-case basis. When used in a geothermal closed-loop system, it is both diluted (often to 20-25%) and primarily underground. Both of these factors help reduce the risks associated with a flammable product.

Often a geothermal system is designed for use with a specific fluid, and the type of fluid to use would be specified by the engineer of the system. If your system is designed for use with Propylene Glycol (including larger pumps and different materials of construction used), you may not be able to use an Ethanol-based fluid in your system. Ethanol-based fluids also can’t be mixed with Glycol. Before using an Ethanol-based fluid, check local building codes.

If you have further questions about the use of Ethanol-based fluids in geothermal systems, reach out to a representative.