Heat pump

Heat energy naturally transfers from warmer places to colder spaces. However, a heat pump can reverse this process, by absorbing heat from a cold space and releasing it to a warmer one. This process 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. That is, heat pumps able to provide heating or cooling to the internal space as required.

Heat pumps are more efficient for heating than resistance heaters because most of the energy they release comes from the ambient environment, and only a fraction from the externally-supplied energy required to run the device. 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 work like refrigerators, inside-out. They 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.[5]

The simpler heat pumps tap the atmosphere as heat source; for better performance and greater energy flow, ground water or geothermal energy will be tapped, but this requires more expensive installation. The heat can be released directly into the air (this is simpler and cheaper), or through the water plumbing of central heating or to provide domestic hot water. Heat pumps take advantage of low temperature underfloor heating, because COP can be higher when the temperature difference is lower.

Milestones:

• 1748: William Cullen demonstrates artificial refrigeration.
• 1834: Jacob Perkins builds a practical refrigerator with diethyl ether.
• 1852: Lord Kelvin describes the theory underlying heat pumps.
• 1855–1857: Peter von Rittinger develops and builds the first heat pump.[6]
• 1928: Aurel Stodola constructs a closed loop heat pump (water source from lake Geneva) which provides heating for the Geneva city hall to this day.
• 1945: John Sumner, City Electrical Engineer for Norwich, installs an experimental water-source heat pump fed central heating system, using a neighboring river to heat new Council administrative buildings. Seasonal efficiency ratio of 3.42. Average thermal delivery of 147 kW and peak output of 234 kW.[7]
• 1948: Robert C. Webber is credited as developing and building the first ground heat pump.[8]
• 1951: First large scale installation - The Royal Festival Hall in London is opened with a town gas-powered reversible water-source heat pump, fed by the Thames, for both winter heating and summer cooling needs.[7]

Mechanical heat pumps are essentially a refrigerator turned inside out and oversized. To deal with the bigger flow of energy, pumps or fans are required where a refrigerator need only passive exchangers.

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.

A simple stylized diagram of a heat pump's vapor-compression refrigeration cycle: 1) condenser, 2) expansion valve, 3) evaporator, 4) compressor

A fictitious pressure-volume diagram for a typical refrigeration cycle

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.[9]

It is essential that the refrigerant reaches 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.

There are millions of domestic installations using air source heat pumps.[10] They are used in climates with moderate space heating and cooling needs (HVAC) and may also provide domestic hot water.[11] The purchase costs are supported in various countries by consumer rebates.[12]

District heating

Heat pumps can be integrated in district heating systems, especially if these are operated with low temperatures.

Heat pumps can also be used as heat supplier for district heating. Possible heat sources for such applications are sewage water, ambient water (like sea, lake and river water), industrial waste heat, geothermal energy, flue gas, waste heat from district cooling and heat from solar heat storage. In Europe, more than 1500 MW were installed since the 1980s, of which about 1000 MW were in use in Sweden in 2017.[13] Large scale heat pumps for district heating combined with thermal energy storage offer high flexibility for the integration of variable renewable energy. Therefore, they are regarded as a key technology for smart energy systems with high shares of renewable energy up to 100% and advanced 4th generation district heating systems.[13][14][15]

Until the 1990s, the refrigerants were often chlorofluorocarbons (CFCs) 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 chlorofluorocarbons cause to the ozone layer if released into the atmosphere.[19]

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.[20] 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.[21]

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.[22][23] 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.[24] Dimethyl ether (DME) has also gained in popularity as a refrigerant.[25]

As quite similar criteria shall be fulfilled by working fluids applied to heat pumps, refrigeration and ORC cycles, several working fluids are applied by all these technologies and can be sorted into the same thermodynamic classification category based on the shape of their saturation curve.

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 European Committee for Standardization (CEN) started work on standards for protection from noise pollution caused by heat pump outdoor units.[26] 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.[27] 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 air source heat pumps (ASHPs) 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.[28][29]

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 United States Environmental Protection Agency (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.[30] 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.[31]

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, to produce one joule of useful heat, a resistance heater needs one joule of electrical energy, while a heat pump under conditions where its COP is 3 or 4 would require only a 0.33 or 0.25 joules of electrical energy, the difference being taken from the cooler place. Note the diminishing returns: increasing the COP from 1 to 2 halves the required energy (50% saving), then increasing it from 2 to 3 saves only a (1/2 - 1/3)= 1/6 (17%) more, going from 3 to 4 saves 8% more, etc. Improving COP to high numbers comes at a price that is quickly not worth it.

Also 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.[32]

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:

${\displaystyle COP_{\text{heating}}={\frac {\Delta Q_{\text{hot}}}{\Delta A}}\leq {\frac {T_{\text{hot}}}{T_{\text{hot}}-T_{\text{cool}}}},}$

${\displaystyle COP_{\text{cooling}}={\frac {\Delta Q_{\text{cool}}}{\Delta A}}\leq {\frac {T_{\text{cool}}}{T_{\text{hot}}-T_{\text{cool}}}},}$

where

• ${\displaystyle \Delta Q_{\text{cool}}}$ is the amount of heat extracted from a cold reservoir at temperature ${\displaystyle T_{\text{cool}}}$,
• ${\displaystyle \Delta Q_{\text{hot}}}$ is the amount of heat delivered to a hot reservoir at temperature ${\displaystyle T_{\text{hot}}}$,
• ${\displaystyle \Delta A}$ is the compressor's dissipated work.
• All temperatures are absolute temperatures usually measured in kelvins or degrees Rankine.

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).[36] 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.

By definition, all heat sources for a heat pump must be colder in temperature than the space to be heated. Most commonly, heat pumps draw heat from the air (outside or inside air) or from the ground (groundwater or soil).[37]

The heat drawn from ground-sourced systems is in most cases stored solar heat, and it should not be confused with direct geothermal heating, though the latter will contribute in some small measure to all heat in the ground. True geothermal heat, when used for heating, requires a circulation pump but no heat pump, since for this technology the ground temperature is higher than that of the space that is to be heated, so the technology relies only upon simple heat convection.

Other heat sources for heat pumps include water; nearby streams and other natural water bodies have been used, and sometimes domestic waste water (via drain water heat recovery) which is often warmer than cold winter ambient temperatures (though still of lower temperature than the space to be heated).

A number of sources have been used for the heat source for heating private and communal buildings.[38]

Wikimedia Commons has media related to Heat pumps.

• Heat pump (engineering) at the Encyclopædia Britannica
• Practical information on setting up geothermal heat pump systems at home
• IEA Technology Collaboration Programme on Heat Pumping Technologies