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GHPs use the ground as a moderate-temperature heat source during the winter and a heat sink during the summer. They draw renewable (yes, renewable) thermal energy from the ground during the winter to heat buildings, and reject heat from buildings back into the ground in summer, thus replenishing the heat drawn from the ground during the previous season.
As for efficiency, it’s a lot easier to reject heat from a building to the ground (~55˚F) compared to outside air that can be in excess of 100˚F on a hot summer day. And in winter, it’s easier to recover heat from the ground (~55˚F) compared to outside air that can be <40˚F. Consider the ground as a readily available renewable storage battery for heat that thin air (in the case of standard air-to-air heat pumps) simply cannot provide.
In a recent article, author Eric Woodroof, Ph.D. said, “GHPs reduce the kilowatt-hours required for air conditioning. When you also consider that when a utility promotes GHP applications (as a demand-side management method), the utility will have reduced demand during peak periods, requiring less generation plants and less pollution.”
It is true that GHPs have higher installation costs than traditional air-to-air heat pumps, because of the cost of drilling vertical boreholes (not “wells”), or excavation for horizontal closed loop systems of pipework that comprise ground heat exchangers. GHPs also require expert, qualified design and installation of ground loops to achieve their full energy efficiency and savings potential. But ground loops are guaranteed to last over 50 years, posting a small fraction of life cycle cost. Indeed, in many cases, ground heat exchangers will outlive the buildings that they serve.
Woodroof provides an excellent analysis of GHP efficiencies, citing an example of a 5-ton air-to-air heat pump, “which would move 5 x 12,000 Btuh, which equals 60,000 Btuh. If the air-to-air Seasonal Energy Efficiency Ratio (SEER) is 10, that means we use ~6 kW every hour we run the air-to-air heat pump. In contrast, a GHP would have a SEER of 20 during the summer, which means you would only need ~3 kW. Thus, the GHP reduces demand by ~3 kW, reducing emissions and helping the utility shave peak demand during the summer.”
Woodroof continues, “In the winter, the SEER of the GHP drops from 20 to 13.65 (COP = 4), meaning that the unit will draw 4.4 kW to move 60,000 Btuh. 4.4 kW equals about 15,000 Btuh of input energy, with the remaining 45,000 Btuh coming from the earth. The total fuel/energy usage is still less than conventional sources (fossil fuels) because the GHP gets ~75 percent of the energy from the earth (~45,000 Btuh), which avoids fuel that could be going into a natural gas-fired heater/boiler.”
Most people think of renewable energy as easy-to-measure electricity (kilowatts) for the grid. But, GHPs produce renewable energy measured in Btu that are consumed without the transmission grid. Renewable thermal energy (Btu) produced by GHPs is now being recognized by governments as a compliance measure under state mandates that require utilities to buy electricity from renewable power generators like wind and solar. Maryland and New Hampshire passed laws last spring recognizing GHPs as a renewable resource that qualifies for utility Renewable Energy Credits, just like wind- and solar-generated electricity.
Those credits are earned according to the electricity use avoided by GHPs compared to standard HVAC systems. New metering devices can measure the temperature differential (ΔT) of incoming and outflowing fluid through a GHP, then accurately count the number of Btu produced by the earth. A simple conversion to kilowatts equals the renewable electricity equivalent production of GHPs. Reduced energy use through the deployment of GHPs ultimately means less pollution from coal- and natural gas-fired power plants.
According to Oak Ridge National Laboratory Buildings Technologies Research and Integration Director Patrick Hughes, Ph.D., “GHPs capture a distributed, thermal form of renewable energy that is available everywhere. GHPs use the only renewable energy resource that is available at every building’s point of use, on-demand, which cannot be depleted (assuming proper design of the heat exchanger) and is affordable in all 50 states.”
Regarding GHPs’ use of electricity, Hughes says, “Although GHPs consume electrical energy, they move three to five times more energy between the building and the ground than they consume while doing so.”
The distributed thermal renewable resource offered by GHPs is already at the load, unlike the vast majority of wind and solar power generation resources that require costly and difficult-to-site transmission lines. And with GHPs, Hughes says, “The renewable resource is available on demand, unlike wind and solar, which may or may not be available when needed.”
Given both the energy that GHPs recover from the ground in winter, and their recycling of heat to the earth in the summer months, the thermal energy tapped by GHPs is indeed renewable. With proper system design and consideration of soils and other factors, GHPs have been proven to save from 40 to 70 percent on heating and cooling bills (including hot water heating). Those numbers can only get better with new units now being manufactured that promise to deliver even more renewable energy from the earth. And collectively, GHPs offer a 24/7, 365 solution to intermittent renewable power production from wind and solar sources.
GHPs can provide both renewable energy and dramatically raise the efficiency of our power grid while reducing energy consumption in buildings of all kinds in most locations around the country.
Publication date: 10/29/2012