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Geothermal: How it Works Overview
IntroductionGeothermal Heating and Cooling Systems provide space conditioning -- heating, cooling, and humidity control. They may also provide water heating -- either to supplement or replace conventional water heaters. Geothermal Heating and Cooling Systems work by moving heat, rather than by converting chemical energy to heat like in a furnace. Every Geothermal Heating and Cooling Systems has three major subsystems or parts: a geothermal heat pump to move heat between the building and the fluid in the earth connection, an earth connection for transferring heat between its fluid and the earth, and a distribution subsystem for delivering heating or cooling to the building. Each system may also have a desuperheater to supplement the building's water heater, or a full-demand water heater to meet all of the building's hot water needs.
Geothermal Heat PumpThe geothermal heat pump is packaged in a single cabinet, and includes the compressor, loop-to-refrigerant heat exchanger, and controls. Systems that distribute heat using ducted air also contain the air handler, duct fan, filter, refrigerant-to-air heat exchanger, and condensate removal system for air conditioning. For home installations, the geothermal heat pump cabinet is usually located in a basement, attic, or closet. In commercial installations, it may be hung above a suspended ceiling or installed as a self-contained console.
Distribution SubsystemMost residential geothermal systems use conventional ductwork to distribute hot or cold air and to provide humidity control. (A few systems use water-to-water heat pumps with one or more fan-coil units, baseboard radiators, or under-floor circulating pipes.) Properly sized, constructed, and sealed ducts are essential to maintain system efficiency. Ducts must be well insulated and, whenever possible, located inside of the building's thermal envelope (conditioned space).
Geothermal heating and cooling systems for large commercial buildings, such as schools and offices, often use a different arrangement. Multiple heat pumps (perhaps one for each classroom or office) are attached to the same earth connection by a loop inside the building. This way, each area of the building can be individually controlled. The heat pumps on the sunny side of the building may provide cooling while those on the shady side are providing heat. This arrangement is very economical, as heat is merely being transferred from one area of the building to another, with the earth connection serving as the heat source or heat sink only for the difference between the building's heating and cooling needs.
Water HeatingMany residential-sized systems installed today are equipped with desuperheater to provide domestic hot water when the system is providing heat or air conditioning. The desuperheater is a small auxiliary heat exchanger at the compressor outlet. It transfers excess heat from the compressed gas to a water line that circulates water to the house's hot water tank. In summer, when the air conditioning runs frequently, a desuperheater may provide all the hot water needed by a household. It can provide four to eight gallons of hot water per ton of cooling capacity each hour it operates. A desuperheater provides less hot water during the winter, and none during the spring and fall when the system is not operating.
Because the heat pump is so much more efficient than other means of water heating, manufacturers are beginning to offer "triple function," "full condensing," or "full demand" systems that use a separate heat exchanger to meet all of a household hot water needs. These units cost-effectively provide hot water as quickly as any competing system.
Types of SystemsGeothermal systems use the earth as a heat source and heat sink. A series of pipes, commonly called a "loop," carry a fluid used to connect the geothermal system's heat pump to the earth.
Closed and Open LoopsThere are two basic types of loops: closed and open. Open loop systems are the simplest. Used successfully for decades, ground water is drawn from an aquifer through one well, passes through the heat pump's heat exchanger, and is discharged to the same aquifer through a second well at a distance from the first. Generally, two to three gallons per minute per ton of capacity are necessary for effective heat exchange. Since the temperature of ground water is nearly constant throughout the year, open loops are a popular option in areas where they are permitted. Open loop systems do have some associated challenges:
Closed loop systems are becoming the most common. When properly installed, they are economical, efficient, and reliable. Water (or a water and antifreeze solution) is circulated through a continuous buried pipe. The length of loop piping varies depending on ground temperature, thermal conductivity of the ground, soil moisture, and system design. (Some heat pumps work well with larger inlet temperature variations, which allows marginally smaller loops).
Horizontal LoopsHorizontal closed loop installations are generally most cost-effective for small installations, particularly for new construction where sufficient land area is available. These installations involve burying pipe in trenches dug with back-hoes or chain trenchers. Up to six pipes, usually in parallel connections, are buried in each trench, with minimum separations of a foot between pipes and ten to fifteen feet between trenches.
Vertical LoopsVertical closed loops are preferred in many situations. For example, most large commercial buildings and schools use vertical loops because the land area required for horizontal loops would be prohibitive. Vertical loops are also used where the soil is too shallow for trenching. Vertical loops also minimize the disturbance to existing landscaping.
For vertical closed loop systems, a U-tube (more rarely, two U-tubes) is installed in a well drilled 100 to 400 feet deep. Because conditions in the ground may vary greatly, loop lengths can range from 130 to 300 feet per ton of heat exchange. Multiple drill holes are required for most installations, where the pipes are generally joined in parallel or series-parallel configurations.
Slinky LoopsIncreasingly, coils -- overlapping coils of polyethylene pipe -- are used to increase the heat exchange per foot of trench, but require more pipe per ton of capacity. Two-pipe systems may require 200 to 300 feet of trench per ton of nominal heat exchange capacity. The trench length decreases as the number of pipes in the trench increases -- or as Slinky coil overlap increases. (Illustration below shows a slinky coil in a pond)
Pond LoopsPond closed loops are a special kind of closed loop system. Where there is a pond or stream that is deep enough and with enough flow, closed loop coils can be placed on the pond bottom. Fluid is pumped just as for a conventional closed loop ground system where conditions are suitable, the economics are very attractive, and no aquatic system impacts have been shown.
ConclusionGeothermal heating and cooling systems can be connected to the earth in a variety of ways -- all thoroughly field proven. However, high performance requires the use of experienced professionals who understand local conditions.
Details and Diagrams
IntroductionRefrigerators and air conditioners both contain heat pumps. In a refrigerator, heat is moved from the food storage sections and discharged to the kitchen air. Air conditioners work the same way --they move heat from the inside of a building and discharge it to the outside air. Conventional or air source heat pumps differ from those in a refrigerator or air conditioner because they are reversible -- they can concentrate heat from the outside air and move it inside to provide warmth, as well as move heat out of the building to provide cooling. To do this, air-source heat pumps (and central air conditioners) need a large outside unit to exchange heat with the outdoor air.
Geothermal Heating and Cooling SystemsIn a geothermal heating and cooling systems, the heat pump is connected to the building by a distribution system -- most commonly air ducts. And the heat pump is connected to the earth through a series of pipes called a "loop". The system exchanges heat with the earth, meaning that no noisy or unsightly outdoor unit is needed.
Vapor Compression CycleAll heat pumps use a vapor compression cycle to transport heat from one location to another. In heating mode, the cycle starts as the cold liquid refrigerant within the heat pump passes through a heat exchanger (evaporator) and absorbs heat from the low-temperature source (fluid circulated through the earth connection). The refrigerant evaporates into a gas as heat is absorbed. The gaseous refrigerant then passes through a compressor where it is pressurized, raising its temperature to over 180 degrees F. The hot gas then circulates through a refrigerant-to-air heat exchanger where the heat is removed and sent through the air ducts. When the refrigerant loses the heat, it changes back to a liquid. The liquid refrigerant cools as it passes through an expansion valve, and the process begins again. Although heat pumps are complex internally, they are marvels of compact design for reliability. Some include features such as additional heat exchangers for water heating, and microprocessor-based automatic controls and protection devices.
EfficiencyGeothermal heat pumps are much more efficient than air source heat pumps because earth temperatures are much more uniform through the year than air temperatures. Not only are earth temperatures more constant, but also the range of temperatures in ground water is rather small in the United States, varying from upper 40s to upper 70s nationwide.
To further improve efficiency, many manufacturers use variable-speed, electronically-controlled motors on the duct system fans. Depending on unit size, manufacturers may opt for reciprocating, inertia, rotary, or scroll compressors -- all of which are hermetically sealed and mounted in the indoor cabinet. Some advanced heat pumps feature two-speed or variable speed operation while others feature dual compressors to vary output capacity and match loads.
The Benefits Speak for ThemselvesTens of thousands of homes are being built or retrofitted with geothermal heating and cooling systems every year, because of the advantages these systems offer: economical operation, noise reduction, and product quality. Initially reserved for the most expensive homes, now these systems have become affordable options for thousands of low and moderate income housing units because of the system's low life cycle costs compared to all other alternatives in almost every region of the country.
Geothermal Heating and Cooling Makes SenseGeothermal systems are efficient, environmentally sensitive, comfortable, and economical. Operating savings often provide paybacks of considerably less than five years -- sometimes less than two years. In addition, electric utilities such as BGE are so convinced of the value of this technology for their customers that they offer design assistance, referrals, or financial incentives to defray the first cost increment of geothermal systems.
The key is that geothermal heat pumps use electricity to move heat, not to generate it by the burning fuel or using electric resistance elements. Indeed, the U.S. EPA has found that no other technology with more favorable operating efficiencies and economics than emerging geothermal heat and cooling systems.
An Important Renewable Energy TechnologyThe U.S. Environmental Protection Agency has concluded that well-designed and properly installed high efficiency geothermal heat pump systems produce less environmental harm than any other alternative space conditioning technology currently available. On a full fuel cycle basis, emerging geothermal systems are the most efficient technology available, with the lowest CO2 emissions for minimum greenhouse warming impact. Overall, the EPA found emerging geothermal heating and cooling systems to have the lowest environmental cost of all technologies analyzed -- including air-source heat pumps and natural gas furnaces.
Geothermal Heat Pump Systems Work!No existing space conditioning technology offers greater comfort, economy, or environmental benefits than the geothermal heat and cooling systems now available for residential and commercial installations. Over 250,000 installations are in place in the United States today, and the number is rapidly increasing. More than 95 percent of all geothermal heat and cooling customers are completely satisfied with their systems.