Heat pump

Overview

Heat energy naturally transfers from warmer places to colder spaces. However, a heat pump can reverse this by absorbing heat from a cold space and releasing it to a warmer one. Heat is not conserved in this process and requires some amount of external energy such as electricity. In heating, ventilation and air conditioning (HVAC) systems, the term heat pump usually refers to vapor-compression refrigeration devices optimized for high efficiency in both directions of thermal energy transfer. These heat pumps can be reversible, and work in either direction to provide heating or cooling to the internal space.

Heat pumps are used to transfer heat because less high-grade energy is required than is released as heat. Most of the energy for heating comes from the external environment, only a fraction of which comes from electricity (or some other high-grade energy source required to run a compressor). In electrically-powered heat pumps, the heat transferred can be three or four times larger than the electrical power consumed, giving the system a coefficient of performance (COP) of 3 or 4, as opposed to a COP of 1 for a conventional electrical resistance heater, in which all heat is produced from input electrical energy.

Heat pumps use a refrigerant as an intermediate fluid to absorb heat where it vaporizes, in the evaporator, and then to release heat where the refrigerant condenses, in the condenser. The refrigerant flows through insulated pipes between the evaporator and the condenser, allowing for efficient thermal energy transfer at relatively long distances.

Reversible heat pumps

Reversible heat pumps work in either direction to provide heating or cooling to the internal space. They employ a reversing valve to reverse the flow of refrigerant from the compressor through the condenser and evaporation coils.

In heating mode, the outdoor coil is an evaporator, while the indoor is a condenser. The refrigerant flowing from the evaporator (outdoor coil) carries the thermal energy from outside air (or soil) indoors. Vapor temperature is augmented within the pump by compressing it. The indoor coil then transfers thermal energy (including energy from the compression) to the indoor air, which is then moved around the inside of the building by an air handler.

Alternatively, thermal energy is transferred to water, which is then used to heat the building via radiators or underfloor heating. The heated water may also be used for domestic hot water consumption. The refrigerant is then allowed to expand, cool, and absorb heat from the outdoor temperature in the outside evaporator, and the cycle repeats. This is a standard refrigeration cycle, save that the "cold" side of the refrigerator (the evaporator coil) is positioned so it is outdoors where the environment is colder.

In cold weather, the outdoor unit of an air source heat pump needs to be intermittently defrosted. This will cause the auxiliary or emergency heating elements (located in the air-handler) to be activated. At the same time, the frost on the outdoor coil will quickly be melted due to the warm refrigerant. The condenser/evaporator fan will not run during defrost mode.

In cooling mode the cycle is similar, but the outdoor coil is now the condenser and the indoor coil (which reaches a lower temperature) is the evaporator. This is the familiar mode in which air conditioners operate.

History

Milestones:

Operating principles

Mechanical heat pumps exploit the physical properties of a volatile evaporating and condensing fluid known as a refrigerant. The heat pump compresses the refrigerant to make it hotter on the side to be warmed, and releases the pressure at the side where heat is absorbed.

The working fluid, in its gaseous state, is pressurized and circulated through the system by a compressor. On the discharge side of the compressor, the now hot and highly pressurized vapor is cooled in a heat exchanger, called a condenser, until it condenses into a high pressure, moderate temperature liquid. The condensed refrigerant then passes through a pressure-lowering device also called a metering device. This may be an expansion valve, capillary tube, or possibly a work-extracting device such as a turbine. The low-pressure liquid refrigerant then enters another heat exchanger, the evaporator, in which the fluid absorbs heat and boils. The refrigerant then returns to the compressor and the cycle is repeated.

It is essential that the refrigerant reach a sufficiently high temperature, when compressed, to release heat through the "hot" heat exchanger (the condenser). Similarly, the fluid must reach a sufficiently low temperature when allowed to expand, or else heat cannot flow from the ambient cold region into the fluid in the cold heat exchanger (the evaporator). In particular, the pressure difference must be great enough for the fluid to condense at the hot side and still evaporate in the lower pressure region at the cold side. The greater the temperature difference, the greater the required pressure difference, and consequently the more energy needed to compress the fluid. Thus, as with all heat pumps, the coefficient of performance (amount of thermal energy moved per unit of input work required) decreases with increasing temperature difference.

Insulation is used to reduce the work and energy required to achieve a low enough temperature in the space to be cooled.

Heat transport

Heat is typically transferred through engineered heating or cooling systems by using a flowing gas or liquid. Air is sometimes used, but quickly becomes impractical under many circumstances because it requires large ducts to transfer relatively small amounts of heat. In systems using refrigerant, this working fluid can also be used to transfer heat a considerable distance, though this can become impractical because of increased risk of expensive refrigerant leakage. When large amounts of heat are to be transferred, water is typically used, often supplemented with antifreeze, corrosion inhibitors, and other additives.

Heat sources/sinks

A common source or sink for heat in smaller installations is the outside air, as used by an air-source heat pump. A fan is needed to improve heat exchange efficiency.

Larger installations handling more heat, or in tight physical spaces, often use water-source heat pumps. The heat is sourced or rejected in water flow, which can carry much larger amounts of heat through a given pipe or duct cross-section than air flow can carry. The water may be heated at a remote location by boilers, solar energy, or other means. Alternatively when needed, the water may be cooled by using a cooling tower, or discharged into a large body of water, such as a lake, stream or an ocean.

Geothermal heat pumps or ground-source heat pumps use shallow underground heat exchangers as a heat source or sink, and water as the heat transfer medium. This is possible because below ground level, the temperature is relatively constant across the seasons, and the earth can provide or absorb a large amount of heat. Ground source heat pumps work in the same way as air-source heat pumps, but exchange heat with the ground via water pumped through pipes in the ground. Ground source heat pumps are more simple and therefore more reliable than air source heat pumps as they do not need fan or defrosting systems and can be housed inside. Although a ground heat exchanger requires a higher initial capital cost, the annual running costs are lower, because well-designed ground source heat pump systems operate more efficiently because they start with a warmer source temperature than the air in winter.

Heat pump installations may be installed alongside an auxiliary conventional heat source such as electrical resistance heaters, or oil or gas combustion. The auxiliary source is installed to meet peak heating loads, or to provide a back-up system.

Applications

There are millions of domestic installations using air source heat pumps. They are used in climates with moderate space heating and cooling needs (HVAC) and may also provide domestic hot water. The purchase costs are supported in various countries by consumer rebates.

HVAC

In HVAC applications, a heat pump is typically a vapor-compression refrigeration device that includes a reversing valve and optimized heat exchangers so that the direction of heat flow (thermal energy movement) may be reversed. The reversing valve switches the direction of refrigerant through the cycle and therefore the heat pump may deliver either heating or cooling to a building. In cooler climates, the default setting of the reversing valve is heating.

The default setting in warmer climates is cooling. Because of the two heat exchangers, the condenser and evaporator, must swap functions, they are optimized to perform adequately in both modes. Therefore, the SEER rating, which is the Seasonal Energy Efficiency Rating, of a reversible heat pump is typically slightly less than two separately optimized machines. For equipment to receive the Energy Star Rating, it must have a rating of at least 14.5 SEER.

Water heating

In water heating applications, a heat pump may be used to heat or preheat water for swimming pools or heating potable water for use by homes and industry. Usually heat is extracted from outdoor air and transferred to an indoor water tank, another variety extracts heat from indoor air to assist in cooling the space.

District heating

Commissioned in 2011 this district heating extracts heat from a fjord whose temperature is around 8 °C using 3 systems giving a combined capacity of 14 megawatts to town center residences and businesses. A city ordinance mandates this heating system for many new buildings.

Refrigerants

Until the 1990s, the refrigerants were often chlorofluorocarbons such as R-12 (dichlorodifluoromethane), one in a class of several refrigerants using the brand name Freon, a trademark of DuPont. Its manufacture is now banned or severely restricted by the Montreal Protocol of August 1987 because of the damage that CFCs cause to the ozone layer if released into the atmosphere.

One widely adopted replacement refrigerant is the hydrofluorocarbon (HFC) known as R-134a (1,1,1,2-tetrafluoroethane). Heat pumps using R-134a replaced R-12 (dichlorodifluoromethane) and have similar thermodynamic properties but with insignificant ozone depletion potential and a somewhat lower global warming potential. Other substances such as liquid R-717 ammonia are widely used in large-scale systems, or occasionally the less corrosive but more flammable propane or butane, can also be used.

Since 2001, carbon dioxide, R-744, has increasingly been used, utilizing the transcritical cycle, although it requires much higher working pressures. In residential and commercial applications, the hydrochlorofluorocarbon (HCFC) R-22 is still widely used, however, HFC R-410A does not deplete the ozone layer and is being used more frequently; however, it is a powerful greenhouse gas which contributes to climate change. Hydrogen, helium, nitrogen, or plain air is used in the Stirling cycle, providing the maximum number of options in environmentally friendly gases.

More recent refrigerators use R600A which is isobutane, and does not deplete the ozone and is less harmful to the environment. Dimethyl ether (DME) has also gained in popularity as a refrigerant.

Noise

A ground source heat pump has no need for an outdoor unit with moving mechanical components: no external noise is produced.

An air source heat pump requires an outdoor unit containing moving mechanical components including fans which produce noise. In 2013, the CEN started work on standards for protection from noise pollution caused by heat pump outdoor units. Although the CEN/TC 113 Business Plan outset was that "consumers increasingly require a low acoustic power of these units as the users and their neighbours now reject noisy installations", no standards for noise barriers or other means of noise protection had been developed by January 2016.

In the United States, the allowed nighttime noise level was defined in 1974 as "an average 24-hr exposure limit of 55 A-weighted decibels (dBA) to protect the public from all adverse effects on health and welfare in residential areas (U.S. EPA 1974). This limit is a day–night 24-hr average noise level (LDN), with a 10-dBA penalty applied to nighttime levels between 2200 and 0700 hours to account for sleep disruption and no penalty applied to daytime levels. The 10-dB(A) penalty makes the permitted U.S. nighttime noise level equal to 45 dB(A), which is more than is accepted in some European countries but less than the noise produced by some heat pumps.

Another feature of ASHP external heat exchangers is their need to stop the fan from time to time for a period of several minutes in order to get rid of frost that accumulates in the outdoor unit in the heating mode. After that, the heat pump starts to work again. This part of the work cycle results in two sudden changes of the noise made by the fan. The acoustic effect of such disruption on neighbors is especially powerful in quiet environments where background nighttime noise may be as low as 0 to 10dBA. This is included in legislation in France. According to the French concept of noise nuisance, "noise emergence" is the difference between ambient noise including the disturbing noise, and ambient noise without the disturbing noise.

Performance Considerations

When comparing the performance of heat pumps, it is best to avoid the word "efficiency", which has a very specific thermodynamic definition. The term coefficient of performance (COP) is used to describe the ratio of useful heat movement per work input. Most vapor-compression heat pumps use electrically powered motors for their work input.

According to the US EPA, geothermal heat pumps can reduce energy consumption up to 44% compared with air-source heat pumps and up to 72% compared with electric resistance heating. The COP for heat pumps range from 3.2 to 4.5 for air source heat pumps to 4.2 to 5.2 for ground source heat pumps.

When used for heating a building with an outside temperature of, for example, 10 °C, a typical air-source heat pump (ASHP) has a COP of 3 to 4, whereas an electrical resistance heater has a COP of 1.0. That is, one joule of electrical energy will cause a resistance heater to produce only one joule of useful heat, while under ideal conditions, one joule of electrical energy can cause a heat pump to move three or four joules of heat from a cooler place to a warmer place. Note that an air source heat pump is more efficient in hotter climates than cooler ones, so when the weather is much warmer the unit will perform with a higher COP (as it has a smaller temperature gap to bridge). When there is a wide temperature differential between the hot and cold reservoirs, the COP is lower (worse). In extreme cold weather the COP will go down to 1.0.

On the other hand, well designed ground-source heat pump (GSHP) systems benefit from the moderate temperature underground, as the ground acts naturally as a store of thermal energy. Their year-round COP is therefore normally in the range of 3.2 to 5.0.

When there is a high temperature differential (e.g., when an air-source heat pump is used to heat a house with an outside temperature of, say, 0 °C (32 °F)), it takes more work to move the same amount of heat to indoors than on a milder day. Ultimately, due to Carnot efficiency limits, the heat pump's performance will decrease as the outdoor-to-indoor temperature difference increases (outside temperature gets colder), reaching a theoretical limit of 1.0 at −273 °C. In practice, a COP of 1.0 will typically be reached at an outdoor temperature around −18 °C (0 °F) for air source heat pumps.

Also, as the heat pump takes heat out of the air, some moisture in the outdoor air may condense and possibly freeze on the outdoor heat exchanger. The system must periodically melt this ice; this defrosting translates into an additional energy (electricity) expenditure. When it is extremely cold outside, it is simpler to heat using an alternative heat source (such as an electric resistance heater, oil furnace, or gas furnace) rather than to run an air-source heat pump. Also, avoiding the use of the heat pump during extremely cold weather translates into less wear on the machine's compressor.

The design of the evaporator and condenser heat exchangers is also very important to the overall efficiency of the heat pump. The heat exchange surface areas and the corresponding temperature differential (between the refrigerant and the air stream) directly affect the operating pressures and hence the work the compressor has to do in order to provide the same heating or cooling effect. Generally, the larger the heat exchanger, the lower the temperature differential and the more efficient the system becomes.

Heat exchangers are expensive, requiring drilling for some heat-pump types or large spaces to be efficient, and the heat pump industry generally competes on price rather than efficiency. Heat pumps are already at a price disadvantage when it comes to initial investment (not long-term savings) compared to conventional heating solutions like boilers, so the drive towards more efficient heat pumps and air conditioners is often led by legislative measures on minimum efficiency standards. Electricity rates will also influence the attractiveness of heat pumps.

In cooling mode, a heat pump's operating performance is described in the US as its energy efficiency ratio (EER) or seasonal energy efficiency ratio (SEER), and both measures have units of BTU/(h·W) (1 BTU/(h·W) = 0.293 W/W). A larger EER number indicates better performance. The manufacturer's literature should provide both a COP to describe performance in heating mode, and an EER or SEER to describe performance in cooling mode. Actual performance varies, however, and depends on many factors such as installation details, temperature differences, site elevation, and maintenance.

As with any piece of equipment that depends on coils to transfer heat between air and a fluid, it is important for both the condenser and evaporator coils to be kept clean. If deposits of dust and other debris are allowed to accumulate on the coils, the efficiency of the unit (both in heating and cooling modes) will suffer.

Heat pumps are more effective for heating than for cooling an interior space if the temperature differential is held equal. This is because the compressor's input energy is also converted to useful heat when in heating mode, and is discharged along with the transported heat via the condenser to the interior space. But for cooling, the condenser is normally outdoors, and the compressor's dissipated work (waste heat) must also be transported to outdoors using more input energy, rather than being put to a useful purpose. For the same reason, opening a food refrigerator or freezer has the net effect of heating up the room rather than cooling it, because its refrigeration cycle rejects heat to the indoor air. This heat includes the compressor's dissipated work as well as the heat removed from the inside of the appliance.

The COP for a heat pump in a heating or cooling application, with steady-state operation, is:

where

Coefficient of performance (COP) and lift

The COP increases as the temperature difference, or "lift", decreases between heat source and destination. The COP can be maximized at design time by choosing a heating system requiring only a low final water temperature (e.g. underfloor heating), and by choosing a heat source with a high average temperature (e.g. the ground). Domestic hot water (DHW) and conventional heating radiators require high water temperatures, reducing the COP that can be attained, and affecting the choice of heat pump technology.

One observation is that while current "best practice" heat pumps (ground source system, operating between 0 °C and 35 °C) have a typical COP around 4, no better than 5, the maximum achievable is 8.8 because of fundamental Carnot cycle limits. This means that in the coming decades, the energy efficiency of top-end heat pumps could roughly double. Cranking up efficiency requires the development of a better gas compressor, fitting HVAC machines with larger heat exchangers with slower gas flows, and solving internal lubrication problems resulting from slower gas flow.

Depending on the working fluid, the expansion stage can be important also. Work done by the expanding fluid cools it and is available to replace some of the input power. (An evaporating liquid is cooled by free expansion through a small hole, but an ideal gas is not.)

Compression vs. absorption

The two main types of heat pumps are compression and absorption. Compression heat pumps operate on mechanical energy (typically driven by electricity), while absorption heat pumps may also run on heat as an energy source (from electricity or burnable fuels). An absorption heat pump may be fueled by natural gas or LP gas, for example. While the gas utilization efficiency in such a device, which is the ratio of the energy supplied to the energy consumed, may average only 1.5, that is better than a natural gas or LP gas furnace, which can only approach 1.

Magnetic

In 1881, the German physicist Emil Warburg put a block of iron into a strong magnetic field and found that it increased very slightly in temperature. Some commercial ventures to implement this technology are underway, claiming to cut energy consumption by 40% compared to current domestic refrigerators. The process works as follows: Powdered gadolinium is moved into a magnetic field, heating the material by 2 to 5 °C (4 to 9 °F). The heat is removed by a circulating fluid. The material is then moved out of the magnetic field, reducing its temperature below its starting temperature.

Thermoelectric

Solid state heat pumps using the thermoelectric effect have improved over time to the point where they are useful for certain refrigeration tasks. Thermoelectric (Peltier) heat pumps are generally only around 10-15% as efficient as the ideal refrigerator (Carnot cycle), compared with 40–60% achieved by conventional compression cycle systems (reverse Rankine systems using compression/expansion); however, this area of technology is currently the subject of active research in materials science. A reason why this is popular is because it has a "long lifetime" as there are no moving parts and it does not use potentially hazardous refrigerants.

Thermoacoustic

Near-solid-state heat pumps using thermoacoustics are commonly used in cryogenic laboratories.