October 28, 2021

Pilot project demonstrates efficiencies of geothermal heat pumps

Imagine a heating system that can heat a home in the winter, cool it in the summer, and supply domestic hot water all year-round while operating at effective efficiency levels of 300-400 percent.

The idea sounds far-fetched, especially in a state where a heating system’s considered efficient if it extracts 90 percent of the heat value from its fuel source. But a demonstration project being conducted by the Bangor Hydro-Electric Co. has shown that geothermal heat-pump systems (or GHP, for geothermal heat pump) can deliver heating performance with substantial efficiency.

According to Calvin Luther, Bangor-Hydro’s manager of residential marketing, the utility launched a heat-pump demonstration project in autumn 1993. The pilot effort, authorized by the Maine Public Utilities Commission in October 1993, was designed “to examine the effectiveness of GHP systems in Maine’s cold climate and to measure customer acceptance of their use,” Luther said.

Instead of creating heat through a fuel-combustion process, relied upon by oil and propane systems, a heat pump moves heat from one location to another. Although this new method of space heating (and cooling) may seem like a new technology, the concept has been used in refrigerators and air conditioners for decades.

Using several interconnected components, a refrigerator circulates a refrigerant through a heat exchanger, absorbing unwanted heat from within and depositing it outside the appliance. Likewise, an air conditioner draws hot outside air through a fin-coil heat exchanger containing a refrigerant liquid, removes some heat, and dumps it inside, cooling the home in the process.

According to Mark Paradis, a Bangor-Hydro product research specialist, a geothermal heating-and-cooling system works on similar principles. The system can move thermal energy stored in the earth to heat a home during the winter, and it can air-condition a home in the summer by transferring unwanted indoor heat to the ground.

Actual work on Bangor-Hydro’s pilot project began in autumn 1993, when the company started installing demonstration systems in several Bangor-area homes. Since then, the utility has installed 11 systems at nine residential customer sites.

According to Paradis, the basic components of a geothermal heating-and-cooling system include a compressor (the “heart” of the system), two heat exchangers (one to absorb and another to exhaust heat), a refrigerant liquid, a blower or pump, an expansion valve, and a reversing valve, if cooling is desired.

The Bangor-Hydro demonstration project is comprised of two types of systems: open loop water-source and closed-loop ground source:

An open-loop water-source heat pump transfers heat energy to and from a drilled water well, depending on its mode of operation (heating or cooling). A typical water-source heat pump requires about nine-10 gallons of water to operate properly. If sufficient flow exists, the system could use a customer’s water well already drilled and in use for domestic purposes. If not, a new well would have to be drilled.

In Maine, groundwater temperature remains fairly constant year-round at about 45-50F. When water is pumped from the ground to the heat pump, it passes through a metal heat exchanger that also contains a separate pipe filled with an environmentally friendly HCFC refrigerant liquid (R22). This action causes heat to be absorbed by the refrigerant, turning it into a gas. The refrigerant vapor is then drawn into the electrically powered compressor, where its volume is physically reduced, causing its temperature to be raised to more than 200F.

After leaving the compressor, it flows to another heat exchanger (condenser), where it “gives up” much of its heat to the air within the home. Cooled refrigerant (now a liquid) then travels back to an expansion valve and onto the first heat exchanger (evaporator), where it repeats the cycle, absorbing heat from the groundwater again. Cooled groundwater discharged from the heat pump is returned to the bottom of the well or to the surface, where it is reabsorbed naturally.

Conversely, during the summer, the direction of refrigerant flow through the heat pump is reversed, allowing the system to absorb unwanted heat from inside the home and transferring it to the water in the ground. All systems installed by Bangor-Hydro use a forced-air system to deliver comfort to the home.

A closed-loop ground-source system is very similar to its water-source cousin. As part of the project, Bangor-Hydro has installed two direct-expansion or DR-type closed-loop heat pumps.

Paradis explained that one system contained 1,200 feet of soft copper piping placed at the bottom of a large pit 6 feet below ground level. The other system required three shallow water wells to be drilled and cooper tubing inserted in the earth.

He then said that “the refrigerant liquid (pushed along by the compressor) flows in the copper tubing, absorbing heat from the ground, which is then removed indoors as needed. In the summer, the operation is reversed, and excess heat is removed from the home and transferred to the ground.”

Paradis also explained that the homeowner need not worry about soil and water contamination should a leak develop. “In either system (water- or ground-type), the refrigerant is under pressure. Should it escape, it would immediately boil, change to a gas, and travel up through the soil to the atmosphere.”

Paradis stated that “regardless of a customer’s home or property, a heat-pump system can be designed and installed to do the job.” He said that “customers with a small lot and inadequate space for lengthy trenches (needed for some ground-source systems) can have a water-source or vertical ground-source system installed. Likewise, those with more acreagre can choose from among several different configurations, depending on their needs and family economics.”

So far, the Bangor-Hydro project has yielded some outstanding results. Paradis mentioned the one of the test systems (a horizontal DX ground-source system) has achieved an effective operating efficiency of more than 450 percent and that water-source systems were consistently providing heat at efficiency levels ranging from 270-300 percent. “These performance levels mean that customers can now own an electrically powered space-conditioning system able to provide year-round comfort at annual operating costs equivalent to (and at times) less than oil-and-propane options.”

“Because the purchase-and-installation cost of these systems are slightly higher than an oil-or-propane boiler/furnace, we see the greatest potential for this type of system to be new construction,” Luther said. “It can be planned into the home from the start, and equipment needed for drilling wells or digging trenches or pits will already be on-site, resulting in cost-sharing that can lower the system’s overall final cost.”

Despite this fact, given the low operating cost of the system, Luther noted that when costs are compared over the expected life of the system (20 years or so), a heat pump can compete cost-effectively with other fossil fuel systems.

“Customers I have spoken with are interested in this technology, and those that are involved in our pilot project have reported that they are satisfied with the comfort their heat pumps provide,” Paradis said. “I believe that acceptance and wider use of these types of systems will explode, especially when word gets out that they can heat, cool, and heat domestic hot water while operating at effective efficiency levels that are out of this world.”

Have feedback? Want to know more? Send us ideas for follow-up stories.

comments for this post are closed

You may also like