Heat Pumps

Summary

A heat pump is a thermal-energy exchanger that transfers energy from one medium to another to provide heating or cooling. This technology is rapidly developing as a renewable-energy technology to provide moderate space heating and cooling or water heating in residential, commercial, and institutional settings. The benefit of the heat-pump design over conventional energy systems is that it condenses and uses existing thermal energy, rather than fossil fuels, to generate new thermal energy. As a renewable-energy technology, heat pumps rely on the relatively constant temperature just below the earth's surface to act as a source or sink for thermal energy.

Definition and Basic Principles

In the simplest sense, a heat pump takes in thermal energy at a low temperature (the source) and releases it at a higher temperature (the sink), working against the temperature gradient using a small amount of electricity or other energy. This thermal energy is carried between two spaces by a liquid refrigerant. The temperature at which a liquid will change into a gaseous state is related to the atmospheric pressure it is under. For example, water will normally change state from liquid to gas at 100 degrees Celsius (212 degrees Fahrenheit) at sea level, or under 1 atmosphere of pressure. However, under one-tenth of normal atmospheric pressure, water will boil at 98 degrees Fahrenheit.

Thermal energy naturally moves down a temperature gradient, from a warmer medium to a cooler medium, in accordance with the second law of thermodynamics. This law characterizes the trend of the universe toward disorganization, or entropy, and explains how heat is transferred to or from the heat-pump system. While the refrigerant in a heat pump is in the sink, or evaporation stage, it absorbs ambient heat, which turns the low-pressure liquid into a higher-energy gas. This gas is then compressed to a higher pressure, and it releases that energy into the sink and condenses into a high-pressure liquid. These are the basic principles under which heat pumps, refrigerators, and air-conditioning units operate.

Background and History

The vapor-compression cycle was first used by French engineer Nicolas Léonard Sadi Carnot in 1824. Then in 1832, American inventor Jacob Perkins was the first to demonstrate a compression-cooling technology that used ether as a refrigerant. But it was in 1852 that Scottish engineer William Thomson, better known as Lord Kelvin, conceptualized the first heat-pump system, dubbed the “heat multiplier.” He recognized that this early predecessor of the air-source heat pump could provide both heating and cooling. Thomson estimated that his machine could generate an equal amount of heat as that produced through direct heating but using only 3 percent of the energy required.

In the early nineteenth century, skeptics maintained that electricity would never be a practical energy source for generating heat in large amounts. Just a handful of scientists looked at the possibility of converting electrical energy to mechanical energy as a means of generating heat, and then pumping that heat from a lower temperature to a higher temperature. But in 1927, British engineer T. G. N. Haldane demonstrated that the heat pump could operate both as a heating and a cooling apparatus using a vapor-compression-cycle refrigerator. When it functioned normally, the refrigerator provided cooling, but when operated in reverse, it could provide heat.

How It Works

There are two main types of heat pumps: vapor-compression pumps and absorption pumps. Vapor-compression heat pumps are the more common of the two.

Vapor-Compression Pumps. The vapor-compression refrigeration cycle removes heat from the air or water inside a building and enables this energy to be transferred to another medium. Vapor-compression pumps are filled with a heat-conducting refrigerant, such as Freon, and include four main components: a motor, a compressor, an evaporator, and a condenser. The motor uses electricity to draw the refrigerant in its vaporous form from the evaporator through the compressor, where it is compressed to increase pressure and temperature. The vapor then passes on to the condenser for cooling and condensing. At this point, the vapor surrenders much of the thermal energy it contains to the air or liquid it is designed to heat and returns to a liquid phase. The liquid refrigerant then continues through the expansion valve to the evaporator. The expansion valve allows only as much refrigerant to pass through as will vaporize. As this occurs, the pressure of the refrigerant falls rapidly and lowers its boiling point. The lower boiling point allows it to vaporize and accept thermal energy from the air or liquid. The refrigerant then moves on to the compressor to repeat the process.

Absorption Heat Pumps. Absorption heat pumps operate using thermal energy, rather than electrical energy, and function like an air-source heat pump. Just a small amount of electricity may be required by the solution pump. Fossil fuels, solar energy, or another energy source may be used to power the pump. Absorption heat-pump components consist of an absorber, a solution pump, a generator, an evaporator, a condenser, and an expansion valve. While vapor-compression heat pumps use only one fluid, the refrigerant, absorption heat pumps require a working fluid and an absorbent. The working fluid must have the ability to vaporize under operating conditions, and the absorbent must be able to absorb this vapor. Most systems employ either water as the working fluid and lithium bromide as the absorbent or ammonia as the working fluid and water as the absorbent. Inside the evaporator, low-pressure vapor from the working fluid is absorbed by the absorbent, resulting in the production of heat. The solution pump then increases the pressure of the resulting solution, and the heat added to the system by the external heat source causes the working fluid to boil out of the solution. The vaporous working fluid is cooled and condensed back into liquid form in the condenser, and the absorbent is diverted back to the absorber by the expansion valve. As in the vapor-compression heat pump, heat is accepted by the working fluid inside the evaporator and released inside the condenser; however, in an absorption heat pump, heat is also released inside the absorber.

Applications and Products

Many people unknowingly use heat pumps every day. The refrigerator is probably the most common example of a heat pump. The refrigerant inside this type of heat pump is selected to have a vaporizing temperature that is below the desired temperature inside the refrigerator, so that heat from inside the refrigerator is captured by the refrigerant following the temperature gradient. The refrigerant flows to the evaporator, where it turns into a gas. But the refrigerant must also have a condensing temperature that is higher than room temperature, so that the refrigerant draws thermal energy away from the condenser and changes to a liquid, releasing the heat into the air surrounding the refrigerator. Water chillers, air conditioners, and dehumidifiers are other common forms of heat pumps that operate similarly. All of these types of heat pumps are designed to allow heat to move in only one direction.

Three kinds of heat-pump systems can be used for indoor space heating and cooling: air source, water source, and ground source. Since they must perform both heating and cooling, these systems are designed both to produce and receive thermal energy.

Air-Source Heat Pumps. Air-source systems may be used in place of central air-conditioning systems as a means of cooling indoor air. These are typically installed on rooftops or other building areas where access to circulating air is available. In addition to the four main components of a heat pump, air-source heat pumps also have a four-way reversing valve that is capable of reversing the flow of refrigerant through the heat pump while maintaining the same direction of flow through the compressor. The expansion valve in an air-source heat pump is also modified to control bidirectional flow of refrigerant. When it is performing space cooling, an air-source heat pump operates exactly like an air conditioner. Refrigerant under pressure leaves the compressor at a higher temperature than the outside air, causing heat to follow the temperature gradient from the coil to the outside air as the refrigerant condenses. When the refrigerant passes out of the condenser, it is in a pressurized liquid form, and the expansion valve releases only as much refrigerant as can be vaporized when it receives thermal energy from the indoor air. The pressure of the refrigerant falls as it passes through the expansion valve and its temperature decreases, causing it to receive more heat from the indoor air and vaporize the remaining refrigerant. Once the pressure of the refrigerant has completely fallen, the refrigerant passes through the reversing valve and is directed back to the compressor to repeat the process. If the direction of the reversing valve is switched to allow for heating, the process moves in the opposite direction. Air-source heat pump systems may be connected to water-heating systems to provide hot water during the cooling cycle.

One limitation of air-source heat pump systems is the need for relatively mild winter temperatures, between 25 and 35 degrees F. Otherwise, supplemental heating may be required. One solution may be to include an electric heating device on the air outflow inside the building.

Water-Source Heat Pumps. Water-source systems transfer thermal energy to and from water, rather than air. During the winter, a fossil-fuel-powered water heater produces heat that is condensed by the heat pump and distributed throughout the building by water-filled pipes. In warmer temperatures, the indoor air heats the water in the pipes and the warm water is pumped up to an evaporative cooling tower outside the building, typically located on the roof. The water inside the pipes remains at a steady temperature of between 60 and 90 degrees F.

Since water is more heat conductive than air because of its higher specific heat, these distribution systems require only 25 percent of the energy that would be required for an air-filled duct distribution system, and because water is denser than air, water-filled pipes take up less space inside a building than ducts. For these reasons, water-source systems are typically preferred in large, multistory buildings. Because many such buildings also have sprinkler systems, the distribution pipes may be integrated into the sprinkler system to reduce overall costs.

Ground-Source Heat Pumps. Ground-source heat pumps take advantage of the relatively consistent temperature underground or beneath the surface of bodies of water such as lakes and ponds as a source or sink for thermal energy. Ground-source systems include three major components: the Earth connection subsystem, the heat-distribution system, and the heat pump. The Earth connection subsystem, or “loops,” consists of pipes buried approximately six to ten feet below the Earth's surface. Loops may be buried underground in the soil, placed in an underground aquifer, or located in a lake or pond. Loops may be oriented vertically or horizontally, based on the available space and the difficulty associated with vertical drilling or excavation. Earth connection subsystems may be open or closed to the surrounding soil, groundwater, or surface water. The heat-distribution system consists of air-filled ducts or fluid-filled pipes that transport thermal energy through a building, either to or from the heat pump at any given time.

Unlike air-source heat pumps, ground-source heat pumps are not exposed to fluctuating outdoor temperatures because the temperature below the Earth's surface remains relatively consistent year-round. Consequently, ground-source systems may be more appropriate where both heating and cooling is needed and seasonal temperatures fall below 25 degrees F to 35 degrees F. Supplemental heating systems may still be required to maintain desired indoor temperatures during extreme temperatures. Where loops are submerged in ponds or lakes, it is important to ensure that freezing will not occur.

Air cooling and water heating may be achieved simultaneously by connecting the heat-distribution system and heat pump to the hot-water system inside a building. Instead of transferring thermal energy from the heat-distribution system into the ground, this energy can be put to use for water heating. In this way, energy use is being reduced from both space cooling and water heating.

Rising energy prices and awareness about the environmental impacts of energy use have contributed to the popularity of heat-pump systems. Heat-pump systems offer a potentially lower-emission option for space heating and cooling and water heating because of their relative efficiency under suitable conditions and their reliance on electricity as an energy source. Where prevailing climatic conditions are moderate and electricity is a lower-emission energy source than natural gas, heat-pump systems reduce greenhouse gas emissions from residential, commercial, or industrial buildings. Since the heat pump is using existing thermal energy rather than attempting to generate it from another source, such as fossil fuels, heat-pump systems require energy inputs only to operate the motor and compressor. For this reason, heat pumps require only 20 kilowatts (kW) to 40 kW of electricity to generate 100 kW of thermal energy; some highly efficient heat pumps use even less electricity for a similar heat output.

While it is often easier and more cost effective to incorporate a heat-pump system into a new building design, retrofits can also be tailored to existing site and building characteristics. Electric heat-pump systems tend to be most appropriate for single-family dwellings, while engine-driven systems can be developed for larger condominium-style, commercial, or institutional buildings.

Industrial Applications. Industrial applications of heat pumps are currently limited, but heat pumps may be used for space heating, process heating and cooling, water heating, steam generation, drying, evaporation, distillation, and concentration. Waste heat is typically captured for space heating, and recapture is minimal; however, heat reuse is common in drying, evaporation, and distillation.

Industrial heat pumps may use conventional heating from fossil fuels or waste-heat capture to provide space heating inside greenhouses or other facilities. These systems typically employ vapor-compression heat pumps.

Operations that require hot water for cleaning typically require water temperatures of between 100 degrees F and 190 degrees F. Heat-pump systems for these applications may simultaneously provide space cooling and water heating. Vapor-compression heat pumps are typically employed, but absorption heat pumps may be adapted to these uses. Vapor-compression heat-pump systems can also provide steam at various pressures for process water heating at temperatures between 210 degrees F and 390 degrees F.

Pulp and paper, lumber, and food processing industries may use heat pumps to provide drying and dehumidification at temperatures up to 210 degrees F. In these applications, heat pumps have demonstrated superior performance and product quality compared with conventional drying systems. Drying systems are typically closed and produce fewer odor emissions.

A number of types of heat pumps are feasible for industrial applications, including mechanical vapor-compression systems, closed-cycle mechanical heat pumps, absorption heat pumps, heat transformers, and reverse-Brayton-cycle heat pumps.

Heat pumps can provide valuable energy savings and lower costs associated with industrial energy use by capturing waste steam that is at a temperature too low to be useful for heating and increasing its temperature, allowing for reuse. The exact dollar value of energy savings can be calculated based on the temperature lift, the increase in temperature achieved by the heat pump. Achieving a higher lift requires a more powerful pump, which in turn requires a larger amount of energy.

Careers and Course Work

The design, manufacturing, and installation of heat-pump components and technologies often requires an academic background in mechanical engineering. Specific courses required may include thermodynamics, heat transfer, fluid dynamics, mechanical design, thermal systems design, control systems, and manufacturing processes. Professional licensure, achieved through a recognized engineering association, such as the National Society of Professional Engineers, may be required for some positions. Regional, state, or federal requirements may dictate that an accredited professional engineer must approve the installation of ground-source heat-pump systems.

Many technical and trade schools offer programs geared toward installing renewable energy technology systems. Engineering systems and energy technology programs teach applied skills for ground-source heat-pump installation. Careers in heating, refrigeration, and air conditioning may require either a bachelor's degree in mechanical engineering or a technical diploma in a related field. Diploma programs in heating, refrigeration, and air conditioning enable individuals to install, service, maintain, and upgrade heat pumps and systems for these applications. Additional certificates may be required. A diploma in plumbing may be required for installing, servicing, or upgrading heat-distribution systems involving water-source or ground-source heat pumps. Where drilling is required, as in the case of an Earth connection subsystem, a diploma in drilling may be required for installing loops.

While many university or technical programs provide the foundations for understanding heat pumps and associated systems, industry associations offer additional, specialized training and accreditation. It is important to check regional, state, or federal requirements for professionals in the specific field of interest.

Social Context and Future Prospects

Tighter environmental regulations for building construction and industrial processes are forecast to increase the use of heat-pump technologies to reduce energy use and greenhouse-gas emissions. However, it is important that the source of electricity used to power the heat pump does not generate more greenhouse-gas emissions on a life-cycle basis than the fossil fuels required to generate the same amount of heat in a conventional energy system. One advantage of heat-pump technologies is that they permit switching to electricity produced from renewable-energy sources, such as hydro or wind, as these energy sources become available, while a heating system that relies solely on fossil fuels, such as natural gas, does not. Therefore, as more renewable energy is supplied to the grid, greenhouse-gas emissions associated with heat-pump systems will continue to fall.

Residential, Commercial, and Institutional Buildings. Heat pumps can decrease building energy consumption by up to 70 percent in cold climates, such as those of Nordic countries. Leadership in Energy and Environmental Design (LEED) and net-zero-emissions facility design have led to the incorporation of heat pumps into green-building design. Retrofits to existing buildings and advanced heat-pump systems will present new challenges to heat-pump system designers and installers. For example, in member-countries of the international Organisation for Economic Co-operation and Development (OECD), new housing is not projected to grow rapidly, and the majority of opportunities for heat-pump systems are expected to be retrofits to existing buildings.

Historically, heat-pump applications have been limited by high initial cost, difficulty optimizing retrofitted heat-pump systems in existing buildings, and inadequate performance of air-source heat pumps in seasonally variable climates. Technological advancements in heat-pump engineering have made significant progress toward eliminating these issues. While the growing popularity of ground-source heat-pump systems has led to an increase in the number of installations and decreasing system and installation costs, air-source heat pumps continued to dominate into the 2020s.

Ultrahigh efficiency heat pumps are being developed to achieve space cooling using a fraction of the energy of the heat pumps available in the 2010s. As the standard of living continues to rise in the developing world, demand for space cooling is forecast to increase; thus the market for ultrahigh efficiency heat pumps is projected to grow considerably as the technology matures.

Industry. The applicability of heat pumps in the industrial sector depends largely on the temperatures of the heat source and sink and the temperature lift required. Temperature lift is influenced by the design of the heat pump and the properties of the refrigerant. While previously developed refrigerants, such as Freon, performed well in technological applications, the longer-term environmental consequences of their use were unacceptable. Research has turned to developing improved refrigerants that are capable of producing greater temperature lift for industrial applications. The International Energy Agency has focused research on developing refrigerants capable of producing temperature lift in the range between 170 degrees and 300 degrees F.

Bibliography

Egg, Jay, and Brian Howard. Geothermal HVAC: Green Heating and Cooling. New York: McGraw, 2011. Print.

Herold, Keith E., et al. Absorption Chillers and Heat Pumps. Boca Raton: CRC, 1996. Print.

Kavanaugh, Stephen P., and Kevin Rafferty. Ground-Source Heat Pumps: Design of Geothermal Systems for Commercial and Institutional Buildings. Atlanta: Amer. Soc. of Heating, Refrigerating, and Air-Conditioning Engineers, 1997. Print.

Langley, Billy C. Heat Pump Technology. 3rd ed. Upper Saddle River: Prentice, 2002. Print.

McMullan, Andrew. Industrial Heat Pumps for Steam and Fuel Savings. Washington: Dept. of Energy, 2003. Office of Energy Efficiency & Renewable Energy. Web. 3 Mar. 2015.

Radermacher, Reinhard, and Yunho Hwang. Vapor Compression Heat Pumps with Refrigerant Mixtures. Boca Raton: CRC, 2005. Print.

Silberstein, Eugene. Heat Pumps. Clifton: Thomson, 2003. Print.

Xin-rong Zhang, and Hiroshi Yamaguchi. Transcritical CO₂ Heat Pump: Fundamentals and Applications. Wiley, 2021.