How Geothermal Heat Pumps Work
Geothermal energy has been a heating and cooling source for thousands of years. Cavemen took advantage of this natural resource by living in earth-sheltered dwellings, but the actual technology base for extracting geothermal energy only has been available for 20 years.
The word geothermal literally means earth plus heat. Geothermal energy is the world's largest energy resource and drawing from it is considered being environmentally responsible. As a renewable resource, it can be used in ways that respect rather than upset our planet's environmental balance.
Soil and near-surface rocks have almost a constant temperature from geothermal heating. Like a cave, the ground temperature is warmer than the air above it during the winter and cooler than the air above it in the summer. Geothermal heat pumps take advantage of this marvel by exchanging heat with the earth, either by moving heat into the earth or absorbing it from the earth through a ground heat exchanger. Because they do not use the fluctuating outside-air temperature, geothermal heat pumps are capable of using less energy. A geothermal heat pump's efficiency is expressed as its coefficient of performance (COP). The COP rating is merely a ratio of total energy output to total energy input. For geothermal heat pumps, the COP can be as high as 4.0. This means that for each $1 spent on electricity for the geothermal heat pump, the unit produces $4 worth of electrical heat output. The higher the COP, the more efficient the unit.
Geothermal heat pumps use electricity efficiently to transfer thermal energy from one location to another. The system does not convert electricity to heat; rather, it uses electricity to move thermal energy between the building and the ground, and conditions the building or process to a higher or lower temperature according to heating or cooling requirements. Buildings or processes that use geothermal systems generally consume less energy compared to traditional heating and cooling systems.
Geothermal energy also delivers environmental and economic benefits. According to the Environmental Protection Agency (EPA), geothermal heat pumps are one of the nation's most efficient and least-polluting heating, cooling and water-heating systems available. Geothermal energy also uses local resources, which minimizes or eliminates energy transportation costs.
In addition, geothermal heat pump systems have few maintenance requirements. When properly installed, the underground components are virtually worry-free. They use fewer mechanical components, making them durable and reliable. Underground piping often has 25- to 50-year warranties, and geothermal heat pumps themselves typically last more than 20 years. Because components within the heat pump are easily accessible, maintenance can be performed on a timely basis. Piping is underground or underwater, so little maintenance is required. Occasional cleaning of the heat pump's heat exchanger coil on open-well systems and regular changing of the air filter is about all that is necessary to keep the system operating efficiently.
The ground heat exchanger in a geo-thermal heat pump system is made up of a closed- or open-loop pipe system. Most common is the closed loop, in which high density polyethylene pipe is buried horizontally 4 to 6' deep or vertically 100 to 400' deep. The pipes are filled with water or an environmentally responsible anti-freeze solution that acts as a heat exchange medium. In the winter, the lower temperature fluid absorbs heat from the earth and carries it into the building for distribution to each heat pump heating air or water. In the summer, the process reverses and the heat pump transfers heat from the building and deposits it into the cooler ground.
Air delivery ductwork distributes heated or cooled air. Because it moves air through the heat pump system, the unit that contains the indoor coil and fan is sometimes called the air handler. The air handler contains a large blower and filter. If the indoor coil is replaced with another water-to-refrigerant heat exchanger, the same theory also may be used to heat or cool water.
A water-to-water heat pump works in much the same way as a water-to-air heat pump. The water-to-water heat pump contains a source input and output to the ground source or ground water heat exchanger but, rather than giving up or absorbing heat from the air, it gives up or absorbs heat from water. This design may be used in high volume water heating or cooling.
To use the earth as a heat source or heat sink, a series of pipes - commonly called a loop - are buried in the ground. The loop can be buried vertically or horizontally. It circulates fluid (either water or a mixture of water and antifreeze) that extracts heat from or adds heat to the surrounding soil. Different loops may be broken into four variations:
- Vertical-loop system.
- Horizontal-loop system.
- Pond-loop system.
- Open-loop system.
Vertical-Loop System. This system has a closed-loop piping design that includes vertically buried piping (50 to 400' deep) connected via headers to bring fluid to and from the heat pump. A vertical-loop design usually is employed when soil conditions are not conducive to trenching, the land area is limited, or the soil is rocky. The pipe layout may be arranged as a direct-return or reverse-return design.
Horizontal-Loop System. Also a closed-loop design, this system has long horizontal loops buried 4 to 6' below the surface that bring fluid to and from the heat pump. The horizontal system uses a number of horizontal trenches that range from a single pipe to multiple pipes. Like the vertical closed-loop system, the horizontal configuration may utilize a direct- or reverse-return design.
Pond-Loop System. Also referred to as a slinky loop, this system uses an existing pond or body of water as a heat exchanger. The slinky loop is a flattened, overlapped, circular coiled, closed-loop heat exchanger. The loop may be installed in a horizontal or vertical fashion.
Open-Loop System. Ground water from a well is used to exchange heat in an open-loop system. The ground water is pumped from the well into the geothermal heat pump, where heat is extracted or rejected and then returned to an aquifer.
For the geothermal system to work, the ground-source heat exchanger must be attached to a heat pump. The geothermal heat pump circulates water through pipes buried in a continuous loop. The continual circulating fluid absorbs or gives up heat from the building or process and transfers it into the earth. Because of this functional system design, little electricity is used and the system has a trivial effect on the environment.
As fossil fuel supplies run out, it is obvious that other sources of energy must be developed. A solution to this problem lies in developing renewable energy sources. The efficient application of geothermal technologies will make geothermal energy one of the largest energy resource bases available.
Industry Puts Heat Pumps to UseA heat pump transfers heat from natural heat sources in its surroundings to a building or industrial application. Relatively few heat pumps currently are installed in industry. But, as environmental regulations become stricter, industrial heat pumps can become an important technology to reduce emissions, improve efficiency and limit the use of groundwater for cooling.
According to IEA Heat Pump Centre, Netherlands, to ensure the sound application of heat pumps in industry, processes should be optimized and integrated. Through process integration, improved energy efficiency is achieved by thermodynamically optimizing total industrial processes. An important instrument for process integration is pinch analysis, a technology to characterize process heat streams and identify heat recovery possibilities. Such possibilities may include improved heat exchanger networks, cogeneration and heat pumps. Pinch analysis is especially powerful for large, complex processes with multiple operations.
Industrial applications vary in the type of drive energy, heat pump size, operating conditions, heat sources and application type. Heat pump units generally are designed for a specific application. Major types of industrial heat pumps include the following.
Mechanical Vapor Recompression Systems (MVRs). These are classified as open or semi-open heat pumps. In open systems, vapor from an industrial process is compressed to a higher pressure and temperature and condensed in the same process giving off heat. In semi-open systems, heat from the recompressed vapor is transferred to the process via a heat exchanger. Because one or two heat exchangers are eliminated (evaporator and/or condenser) and the temperature lift generally is small, MVR system performance is high. Typical coefficients of performance (COP) range from 10 to 30. Current MVR systems work with heat-source temperatures from 158 to 176°F (70 to 80°C) and deliver heat between 230 and 302°F (110 and 150°C). Water is the most common working fluid although other process vapors also are used, notably in the chemical/petrochemical industry.
Closed-Cycle Compression Heat Pumps. Most heat pumps work on the principle of the vapor compression cycle. The main heat pump system components are the compressor, expansion valve and two heat exchangers, which are referred to as the evaporator and condenser. The components are connected to form a closed circuit. A volatile liquid, known as the working fluid or refrigerant, circulates though the components. Currently applied working fluids limit the maximum output temperature to 248°F (120°C).
Absorption Heat Pumps. Although not widely used in industrial applications, some absorption heat pumps have been used to recover heat from refuse incineration plants, notably in Sweden and Denmark. Current systems with water/lithium bromide as a working pair achieve an output temperature of 212°F (100°C) and temperature lift of 149°F (65°C). COPs typically range from 1.2 to 1.4. The new generation of advanced absorption heat pump systems will have higher output temperatures and higher temperature lifts.
Heat Transformers. These units have the same main components and working principle as absorption heat pumps. With a heat transformer, waste heat can be upgraded without using external drive energy. Waste heat of a medium temperature (i.e., between the demand and environmental level) is supplied to the evaporator and generator. Useful heat of a higher temperature is given off in the absorber. All current systems use water and lithium bromide as a working pair. These heat transformers can achieve a delivery temperature up to 302°F (150°C) with a typical lift of 122°F (50°C). COPs under these conditions range from 0.45 to 0.48.
Reverse Brayton-Cycle Heat Pumps. These pumps recover solvents from gases in many processes. Solvent-laden air is compressed and expanded, then it is cooled through expansion. Solvents condense and are recovered. Further expansion with the associated additional cooling, condensation and solvent recovery takes place in a turbine, which also drives the compressor. Industrial heat pumps mainly are used for:
- Heating and cooling process streams.
- Water heating for washing, sanitation and cleaning.
- Steam production.
In process water heating and cooling applications, many industries require process water in the range of 104 to 194°F (40 to 90°C). They also can have significant hot water demands for washing, sanitation and cleaning purposes. These demands can be met by heat pumps. Heat pumps also can be a part of an integrated system that provides both cooling and heating. Mainly, electric closed-cycle compression heat pumps are installed in industry, but a few absorption heat pumps and heat transformers also are in use.
Heat pumps are used extensively in industrial dehumidification and drying processes at low and moderate temperatures (maximum 212°F [100°C]). The main applications include drying pulp and paper, various food products, wood and lumber. Heat pump dryers generally have high performance characteristics and often can improve the quality of dried products when compared to traditional drying methods. Because drying is executed in a closed system, odors from the drying of food products are reduced. Both closed-cycle compression heat pumps and MVR systems are used in industry.
Evaporation and distillation are energy-intensive processes, and most heat pumps are installed in these processes in the chemical and food industries. In evaporation processes, residue is the main product; vapor is the main product in distillation processes. Most systems are open or semi-open MVRs, but closed-cycle compression heat pumps also are applied. Small temperature lifts result in high performance with COPs ranging from 6 to 30.