The most effective type of heat pump is the geothermal heat pump, GHP. It doesn't create heat by burning fuel, like a furnace does. Instead, in winter it collects the Earth's natural heat through a series of pipes, called a loop, installed below the surface of the ground or submersed in a pond or lake. As you may have experienced in a cave, the temperature six feet beneath ground surface is cooler in summer and warmer in winter than the ambient temperature. Using this temperature as its source the GHP can operate within its most efficient range. In winter, fluid circulates through the loop and carries the heat to the house. There, an electrically driven compressor and a heat exchanger concentrate the Earth's energy and release it inside the home at a higher temperature. Ductwork distributes the heat to different rooms. In summer, the process is reversed. The underground loop draws excess heat from the house and allows it to be absorbed by the Earth. The system cools your home in the same way that a refrigerator keeps your food cool by drawing heat from the interior, not by blowing in cold air.
The geothermal loop that is buried underground is typically made of high-density polyethylene, a tough plastic that is extraordinarily durable but which allows heat to pass through efficiently. The fluid in the loop is water or an environmentally safe antifreeze solution that circulates through the pipes in a closed system. Earliest systems were open loop, but those could impact the groundwater supply and are not used as much today. There are two types of closed loops used to provide constant temperature to the GHP. Horizontal ground loops are usually the most cost effective when trenches are easy to dig and the size of the yard is adequate. Workers use trenchers or backhoes to dig the trenches six feet below the ground in which they lay a series of parallel plastic pipes. They then backfill the trench. Fluid runs through the pipe in a closed system. A typical horizontal loop will be 400 to 600 feet long for each ton of heating and cooling.
The vertical loop is used where there is little yard space, when surface rocks make digging impractical, or when you want to disrupt the landscape as little as possible. Vertical holes are typically 150 to 450 feet deep and contain a single loop of pipe with a U-bend at the bottom. Each vertical pipe is then connected to a horizontal underground pipe that carries fluid in a closed system to and from the indoor exchange unit. Vertical loops are generally more expensive to install, but require less piping than horizontal loops because the Earth's temperature is more stable farther below the surface.
Geothermal heat pumps (GHPs), more accurately called ground-source heat pumps, have been proven capable of producing large reductions in energy use and peak demand in buildings. Although the U.S. was once the world leader in GHP technology and market development, European markets now absorb 2 to 3 times the number of GHP units annually as do the U.S. domestic markets. In 2007 the Intergovernmental Panel on Climate Change identified the building sector as having the highest green house gas emissions, but also the best potential for dramatic emissions reductions. In their report GHPs were specifically identified as a solution that is economically feasible under certain circumstances‖ in continental and cold climates. Their report cited cases where total electricity use decreased by one third and heating energy use by 50 to 60 percent.
Tax credits for home and business owners investing in GHP systems were enacted in October 2008 through 2016 and increased in the stimulus plan of 2009. Hopefully these tax credits will help GHP achieve wider market acceptance despite its large upfront capital costs. The largest hurdle to the widespread adoption of GHP technology seems to be the capital cost for initial installation. The outside portion of the GHP system can be half or more of the overall GHP system cost (and equal to the total cost for a traditional furnace and air conditioner). The technology while economically viable, is little known or understood and has suffered from the high upfront and installation costs. If the costs of the exterior coils were excluded, GHP systems have about the same price as competitive alternatives. In addition, due to the lack of demand, there are few design and installation firms in the market.
Buildings, both residential and commercial, account for about 40 percent of primary U.S. energy consumption, 72 percent of U.S. electricity consumption, 55 percent of U.S. natural gas consumption, and significant heating oil and propane consumption in the Northeast. While industrial use of electricity has been flat for about 15 years, electrical use to power commercial and residential building has grown by more than 50 percent since 1985. U.S. resources and investment have been deployed to build the infrastructure required to generate, transmit, and distribute electricity to serve that growth. Reducing the peak electricity demands for air conditioning and heating could alleviate peak demand on the electrical grid. Buildings present one of the best opportunities to economically reduce energy consumption and limit green house gas emissions. A recent study by McKinsey & Company study performed for the Department of Energy found that reducing the consumption of energy in buildings is the least costly way to achieve large reductions in carbon emissions.
A study by the U.S. Environmental Protection Agency (EPA) comparing the major HVAC options for residential applications determined that GHPs were the most energy efficient and environmentally benign option. Yet only about 60,000 units are installed each year in the combined new built and retrofit market. This languishing of the market is attributed to several federal policy lapses. A program at the DOD ran for several years in the late 1990’s intended to increase use of GHPs in federal buildings. This program’s authority was allowed to lapse. Although DOD took the initiative to restore the program 14 months later by then much of the GHP project pipeline had diffused away. A second policy mistake damaging to federal agency use of GHPs occurred in 2005 when the Energy Policy Act defined renewable energy that counted toward agency renewable goals as power generation only, excluding thermal forms of renewable energy such as GHPs. No lobbyists were paid to identify this oversight. Federal utilization of GHPs might have created the critical mass for the market; instead it was once more forgotten.
The basics of GHP technology have changed very little over the decades but awareness, understanding, and acceptance of the systems is limited. The systems are truly misnamed, GHPs are often confused with geothermal power production, in which the extreme heat of subsurface geological processes is used to produce steam, and ultimately to generate electricity. GHPs are also sometimes confused with the direct use of geothermal heat in which greenhouses, aquaculture ponds, and other agricultural facilities are heated using lower-temperature sources such as hot springs. Ground source heat pumps can be used economically anywhere and utilize the earth stored solar energy to function. There are at least 16 manufacturers of GHPs in the United States that participate in the residential and commercial markets. The GHP market began to develop in the late 1970s, and has had its ups and downs due to the cyclic nature of the buildings industry and volatility in government and utility support and the prices of competing forms of energy. The current tax incentives and awareness of US energy consumption may serve as an opportunity for the GHP market to achieve critical mass.