Heat pump

For theoretical background, see Heat pump and refrigeration cycle.

A heat pump is a machine or device that effectively "moves" thermal energy from one location called the "source," which is at a lower temperature, to another location called the "sink" or "heat sink", which is at a higher temperature. While compressor-driven air conditioners and freezers are technically heat pumps, the class includes many other types of devices, and the term "heat pump" usually implies one of the less-common devices in the class that are not dedicated to refrigeration-only.

During the operation of a heat pump, some of the thermal energy must be transformed to another type of energy in the process, before reappearing as thermal energy in the sink. The heat pump uses mechanical work, or some source of thermodynamic work (such as much higher-temperature heat source dissipating heat to lower temperatures) to accomplish the desired transfer of thermal energy from source to sink.[1] In the classical thermodynamic sense, a heat pump does not actually move heat, which by definition cannot flow from cold to hot temperatures. However, since the effect of the device in moving thermal energy is the same as if heat were flowing (albeit in the incorrect direction with regard to temperature difference), the "heat pump" is named by analogy.

A heat pump always moves thermal energy in the opposite direction from temperature, but a heat pump that maintains a thermally conditioned-space can be used to provide either heating or cooling, depending upon whether the environment is cooler or warmer than the conditioned-space. When pumps are used to provide heating, they are used because less input from a commercial-energy source is required than is required for newly-creating thermal energy by transforming heat-free sources of energy (for example, electricity) or low-entropy sources of energy (for example, a gas flame) directly into the required heating. This is because the heat pump utilizes some thermal energy from the environment for part of the delivered-heating, increasing the "efficiency" of the process. In cooler climates, it is common for heat pumps to be designed only to provide heating.

Even when a heat pump is used for heating, it still uses the same basic refrigeration-type cycle to do the job (merely changing operation so that the warm end of the device is inside the conditioned space, heating it). Such heat pumps, which always provide heating of spaces, may be found in climates that never or rarely require cooling.

For the class of "reversible-cycle heat pump" devices designed to work in either thermal direction, the device simply operates in a way that changes which coil is the condenser, and which coil is the evaporator, rather than physically turn the device around. Such a switch in function is normally achieved by a "reversing valve." Reversible-cycle heat pumps are often seen in providing building-space heating in high latitude climates that are much warmer than comfortable in one season, but colder in another season. In heating, ventilation, and air conditioning (HVAC) applications, the term heat pump normally refers to a vapor-compression refrigeration device that includes a reversing valve and optimized heat exchangers so that the direction of thermal energy flow may be changed without loss of efficiency. Most commonly, when used in heating, heat pumps draw heat from the air or from the ground.[2]

Contents

Overview

Heat pumps have the ability to move thermal energy from one environment to another, and in either direction. This allows the heat pump to effectively bring thermal energy into an occupied space, or to take it out. In practice, this is always done in the opposite direction of a temperature gradient. A heat pump works in the same manner as an ordinary air conditioner (A/C), which itself is a type of heat pump. In the warming mode for a space, a heat pump effectively reverses a refrigeration unit so that the warm radiator is inside the space, rather than outside.

In classical thermodynamics, heat is defined as a movement of energy in the direction of a thermal gradient, and in this strict sense a "heat pump" is misnamed, since by definition, classical heat cannot be moved or pumped from colder to warmer temperatures. In fact, some of the energy moved by heat pumps is moved in the form of thermodynamic work, and often mechanical work (a narrower definition of work), and not as heat. However, since the effect is somewhat the same (thermal energy disappears in one place and reappears in another), the device gained its name by loose analogy.

A heat pump uses an intermediate fluid called a refrigerant which absorbs heat as it vaporizes and releases the heat when it is condensed. It uses an evaporator to absorb heat from inside an occupied space and rejects this heat to the outside through the condenser. The refrigerant flows outside of the space to be conditioned, where the condenser and compressor are located, while the evaporator is inside. The key component that makes a heat pump different from an air conditioner is the reversing valve. The reversing valve allows for the flow direction of the refrigerant to be changed. This allows the heat to be pumped in either direction.

Operating principles

Since the heat pump or refrigerator uses a certain amount of work to move the refrigerant, the amount of energy deposited on the hot side is greater than taken from the cold side. One common type of heat pump works by exploiting the physical properties of a volatile evaporating and condensing fluid known as a refrigerant. 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 like an expansion valve, capillary tube, or possibly a work-extracting device such as a turbine. The low pressure, liquid refrigerant leaving the expansion device 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.

In such a system it is essential that the refrigerant reach a sufficiently high temperature when compressed, since the second law of thermodynamics prevents heat from flowing from a cold fluid to a hot heat sink. Practically, this means the refrigerant must reach a temperature greater than the ambient around the high-temperature heat exchanger. Similarly, the fluid must reach a sufficiently low temperature when allowed to expand, or heat cannot flow from the cold region into the fluid, i.e. the fluid must be colder than the ambient around the cold-temperature heat exchanger. 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 heat moved per unit of input work required) decreases with increasing temperature difference.

Insulation is used to reduce the work and energy required to achieve and maintain a lower temperature in the cooled space.

Due to the variations required in temperatures and pressures, many different refrigerants are available. Refrigerators, air conditioners, and some heating systems are common applications that use this technology.

Heat sources

Many heat pumps also use an auxiliary heat source for heating mode. This means that, even though the heat pump is the primary source of heat, another form is available as a back-up. Electricity, oil, or gas are the most common sources. This is put in place so that if the heat pump fails or can't provide enough heat, the auxiliary heat will kick on to make up the difference.

Geothermal heat pumps use shallow ground (which is often at a constant temperature not too far below "shirt-sleeve temperature") as a heat source and sink, and water as the heat transport medium. They work in the same manner as an air-to-air heat pump, but instead of indoor and outdoor coils they use water pumped through earth materials as a heat transfer medium. These are environmentally-friendly and a cheaper alternative in the long run due to lower operating cost.

Solar Assisted Heat Pumps use thermal waste energy from water source heating and cooling systems as "fuel" for a Thermal HVAC system. This is a new technology which uses the energy from the water in holding tanks and a refrigerant to water heat exchange system. The tanks serve as thermal flywheels and thermal buffers, as needed. In this configuration, the water in the middle tank serves as the “fuel” for the system. This fuel is pumped into the cold heat exchanger where the heat in the water is extracted and transferred to warm up the cold refrigerant. The cold water is then pumped into the cold tank. On the opposite side, the hot water is heated by way of the hot heat exchanger and the heated water is put back into the hot tank to either be rejected or used further in other heat exchange processes. In most cases water returns from the zone where work is being done to the neutral tank .[3]

Applications

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 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 the cooler climates the default setting of the reversing valve is heating. The default setting in warmer climates is cooling. Because the two heat exchangers, the condenser and evaporator, must swap functions, they are optimized to perform adequately in both modes. As such, the efficiency of a reversible heat pump is typically slightly less than two separately optimized machines.

In plumbing applications, a heat pump is sometimes used to heat or preheat water for swimming pools or domestic water heaters.

In somewhat rare applications, both the heat extraction and addition capabilities of a single heat pump can be useful, and typically results in very effective use of the input energy. For example, when an air cooling need can be matched to a water heating load, a single heat pump can serve two useful purposes. That is, a heat pump domestic water heater located in the living area of a home could cool the home, reducing or eliminating the need for additional air conditioning. This installation would be best-suited to a climate that is warm or hot most of the year.

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 was discontinued in 1995 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). R-134a is not as efficient as the R-12 it replaced (in automotive applications) and therefore, more energy is required to operate systems utilizing R-134a than those using R-12. 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. 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. 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 are now exploiting the R600A which is isobutane, and does not deplete the ozone and is friendly to the environment.

Dimethyl ether (DME) is also gaining popularity as a refrigerant.[4]

Efficiency

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 to work input. Most vapor-compression heat pumps use electrically powered motors for their work input. However, in most vehicle applications, shaft work, via their internal combustion engines, provide the needed work.

When used for heating a building on a mild day of say 10 °C, a typical air-source heat pump has a COP of 3 to 4, whereas a typical electric resistance heater has a COP of 1.0. That is, one joule of electrical energy will cause a resistance heater to produce one joule of useful heat, while under ideal conditions, one joule of electrical energy can cause a heat pump to move much more than one joule of heat from a cooler place to a warmer place.

Note that the heat pump is more efficient on average in hotter climates than cooler ones, so when the weather is much warmer the unit will perform better than average COP. Conversely in cold weather the COP approaches 1. Thus when there is a wide temperature differential between the hot & cold reservoirs the COP is lower (worse).

When there is a high temperature differential on a cold day, e.g., when an air-source heat pump is used to heat a house on a very cold winter day of say 0 °C, it takes more work to move the same amount of heat indoors than on a mild day. Ultimately, due to Carnot efficiency limits, the heat pump's performance will approach 1.0 as the outdoor-to-indoor temperature difference increases for colder climates (temperature gets colder). This typically occurs around −18 °C (0 °F) outdoor temperature 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. In other words, when it is extremely cold outside, it is simpler, and wears the machine less, to heat using an electric-resistance heater than to strain an air-source heat pump.

Geothermal heat pumps, on the other hand, are dependent upon the temperature underground, which is "mild" (typically 10 °C at a depth of more than 1.5m for the UK) all year round. Their COP is therefore normally in the range of 4.0 to 5.0.

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. 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, as it is 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.[5]

In cooling mode a heat pump's operating performance is described 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, temperature differences, site elevation, and maintenance.

Heat pumps are more effective for heating than for cooling if the temperature difference is held equal. This is because the compressor's input energy is largely converted to useful heat when in heating mode, and is discharged along with the moved heat via the condenser. But for cooling, the condenser is normally outdoors, and the compressor's dissipated work is rejected rather than put to a useful purpose.

For the same reason, opening a food refrigerator or freezer heats 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:

COP_{\mathrm{heating}} = \frac{\Delta Q_{\mathrm{hot}}}{\Delta A} \leq \frac{T_{\mathrm{hot}}}{T_{\mathrm{hot}}-T_{\mathrm{cool}}}
COP_{\mathrm{cooling}} = \frac{\Delta Q_{\mathrm{cool}}}{\Delta A} \leq \frac{T_{\mathrm{cool}}}{T_{\mathrm{hot}}-T_{\mathrm{cool}}}

where

COP and lift

The COP increases as the temperature difference, or "lift", decreases between heat source and destination. The COP can be maximised 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 radiators require high water temperatures, affecting the choice of heat pump technology.

Pump type and source Typical use case COP variation with output temperature
35 °C
(e.g. heated screed floor)		 45 °C
(e.g. heated screed floor)		 55 °C
(e.g. heated timber floor)		 65 °C
(e.g. radiator or DHW)		 75 °C
(e.g. radiator and DHW)		 85 °C
(e.g. radiator and DHW)
High-efficiency air source heat pump (ASHP). Air at −20 °C[6] 2.2 2.0 ‐ ‐ ‐ ‐
Two-stage ASHP air at −20 °C[7] Low source temp. 2.4 2.2 1.9 ‐ ‐ ‐
High efficiency ASHP air at 0 °C[6] Low output temp. 3.8 2.8 2.2 2.0 ‐ ‐
Prototype transcritical CO2 (R744) heat pump with tripartite gas cooler, source at 0 °C[8] High output temp. 3.3 ‐ ‐ 4.2 ‐ 3.0
Ground source heat pump (GSHP). Water at 0 °C[6] 5.0 3.7 2.9 2.4 ‐ ‐
GSHP ground at 10 °C[6] Low output temp. 7.2 5.0 3.7 2.9 2.4 ‐
Theoretical Carnot cycle limit, source −20 °C 5.6 4.9 4.4 4.0 3.7 3.4
Theoretical Carnot cycle limit, source 0 °C 8.8 7.1 6.0 5.2 4.6 4.2
Theoretical Lorentzen cycle limit (CO2 pump), return fluid 25 °C, source 0 °C[8] 10.1 8.8 7.9 7.1 6.5 6.1
Theoretical Carnot cycle limit, source 10 °C 12.3 9.1 7.3 6.1 5.4 4.8

One conclusion is that while current 'best practice' heat pumps (ground source system, operating between 0 and 35 Celsius) have a COP of normally around 4, no better than 5, the maximum achievable is (see under Carnot-cycle) 12. This means that in the coming decades, the energy efficiency of top-end heat pumps is likely to at least 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.

Types

The two main types of heat pumps are compression heat pumps and absorption heat pumps. Compression heat pumps always operate on mechanical energy (through electricity), while absorption heat pumps may also run on heat as an energy source (through electricity or burnable fuels).[9] 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. Although an absorption heat pump may not be as efficient as an electric compression heat pump, an absorption heat pump fueled by natural gas may be advantageous in locations where electricity is relatively expensive and natural gas is relatively inexpensive. A natural gas-fired absorption heat pump might also avoid the cost of an electrical service upgrade which is sometimes necessary for an electric heat pump installation. In the case of air-to-air heat pumps, an absorption heat pump might also have an advantage in colder regions, due to a lower minimum operating temperature.ROBUR heat pumps comparison

A number of sources have been used for the heat source for heating private and communal buildings.[10]

Heat sources

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).[11] The heat drawn from the ground 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. 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).

Air-source heat pumps

Air source heat pumps are relatively easy (and inexpensive) to install and have therefore historically been the most widely used heat pump type. However, they suffer limitations due to their use of the outside air as a heat source or sink. The higher temperature differential during periods of extreme cold or heat leads to declining efficiency, as explained above. In mild weather, COP may be around 4.0, while at temperatures below around −8 °C (17 °F) an air-source heat pump can achieve a COP of 2.5 or better, which is considerably more than the energy efficiency that may be achieved by a 1980's heating systems, and very similar to state of the art oil or gas heaters.[12] The average COP over seasonal variation is typically 2.5-2.8, with exceptional models able to exceed 6.0 in very mild climate, but not in freezing climates.[13]

Air source heat pumps for cold climates

At least two manufacturers are selling heat pumps that maintain better heating output at lower outside temperatures than conventional air source heat pumps. These low temperature optimized models make air source heat pumps more practical for cold climates because they don't freeze to a stop that quickly. Some models however, defrost their outdoor unit electrically at regular intervals, which increases electricity consumption dramatically during the coldest weeks. In areas where only one fossil fuel is currently available (e.g. heating oil; no natural gas pipes available) these heat pumps could be used as an alternative, supplemental heat source to reduce a building's direct dependence on fossil fuel. Depending on fuel and electricity prices, using the heat pump for heating may be less expensive than fossil fuel. A backup, fossil-fuel heat source may still be required for the coldest days.

The heating output of low temperature optimized heat pumps (and hence their energy efficiency) still declines dramatically as the temperature drops, but the threshold at which the decline starts is lower than conventional pumps, as shown in the following table (temperatures are approximate and may vary by manufacturer and model):

Air Source Heat Pump Type Full heat output at or above this temperature Heat output down to 60% of maximum at
Conventional 47 °F (8.3 °C) 17 °F (-8.3 °C)
Low Temp Optimized 14 °F (-10 °C) -13 °F (-25 °C)

Ground source heat pumps

Ground source heat pumps, which are also referred to as Geothermal heat pumps, typically have higher efficiencies than air-source heat pumps. This is because they draw heat from the ground or groundwater which is at a relatively constant temperature all year round below a depth of about thirty feet (9 m).[14] This means that the temperature differential is lower, leading to higher efficiency. Ground-source heat pumps typically have COPs of 3.5-4.0 at the beginning of the heating season, with lower COPs as heat is drawn from the ground. The trade off for this improved performance is that a ground-source heat pump is more expensive to install due to the need for the drilling of wells or digging of trenches in which to place the pipes that carry the heat exchange fluid. When compared versus each other, groundwater heat pumps are generally more efficient than heat pumps using heat from the soil. Ground sources tend to accumulate cold, which is a significant problem if ground water is stagnant and they have been designed to be just big enough. One way to fix cold accumulation, is to use ground water to cool the floors on hot days. Another way is to make large solar collectors, for instance by putting plastic pipes just under the roof, or by putting coils of black polyethylene pipes under glass on the roof, or by piping the tarmac of the parking lot. The most cost effective way is to put a large air to water heat exchanger on the rooftop.

Heat distribution

Heat pumps are only highly efficient when they distribute produced heat at a low temperature, ideally around or below 32 °C (90 °F). Normal steel plate radiators are no good: they would need to have four to six times their current size. Underfloor heating is the ideal solution. When wooden floors or carpets would spoil their efficiency, wall heaters (plastic pipes covered with a thick layer of chalk) and piped ceilings can be used. Both systems have the disadvantage that they are slow starters, and that they would require extensive renovation in existing buildings. The alternative is a warm air system in which water runs through a ventilator driven water to air heater. Such a thing can either complement floor heating during warm up, or it can be a quick and economical way to implement a heat pump system into existing buildings. Oversizing them reduces their noise. To efficiently distribute warm water or air from a heat pump, water pipes or air shafts should have significantly larger diameters then in conventional systems, and underfloor heaters should have much more pipes per square meter.

Solid state heat pumps

Main article: Magnetic refrigeration

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.[15] 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.

Main article: Thermoelectric materials

Solid state heat pumps using the Thermoelectric Effect have improved over time to the point where they are useful for certain refrigeration tasks. Commercially available technologies have efficiencies that are currently well below that of mechanical heat pumps, however this area of technology is currently the subject of active research in materials science.

Main article: Thermoacoustic hot air engine

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

History

Milestones:

See also

References

  1. ^ The Systems and Equipment volume of the ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA, 2004
  2. ^ Air-source heat pumps|url=http://www.nrel.gov/docs/fy01osti/28037.pdf
  3. ^ Jim O'brien - Inventor, Triea Systems (invalid reference - not WP:RS)
  4. ^ http://www.mecanica.pub.ro/frigo-eco/R404A_DME.pdf 101110
  5. ^ BSRIA, "European energy legislation explained", www.bsria.co.uk, May 2010.
  6. ^ a b c d The Canadian Renewable Energy Network 'Commercial Earth Energy Systems', Figure 29. . Retrieved December 8, 2009.
  7. ^ Technical Institute of Physics and Chemistry, Chinese Academy of Sciences 'State of the Art of Air-source Heat Pump for Cold Region', Figure 5. . Retrieved April 19, 2008.
  8. ^ a b SINTEF Energy Research 'Integrated CO2 Heat Pump Systems for Space Heating and DHW in low-energy and passive houses', J. Steen, Table 3.1, Table 3.3. . Retrieved April 19, 2008.
  9. ^ http://www2.vlaanderen.be/economie/energiesparen/doc/brochure_warmtepomp.pdf
  10. ^ Homeowners using heat pump systems
  11. ^ "Heat pumps sources including groundwater, soil, outside and inside air)" (PDF). http://www2.vlaanderen.be/economie/energiesparen/doc/folder_warmtepomp.pdf. Retrieved 2010-06-02. 
  12. ^ EnergyIdeas.org, "Product & Technology Review: Acadia Heat Pump", Table 1, Dec 2007.
  13. ^ "the IPCC 4th Working Group III report" (PDF). http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-chapter6.pdf. Retrieved 2010-06-02. 
  14. ^ Earth Temperature and Site Geology, http://www.geo4va.vt.edu/A1/A1.htm
  15. ^ Guardian Unlimited, December 2006 'A cool new idea from British scientists: the magnetic fridge'
  16. ^ Banks, David L.. An Introduction to Thermogeology: Ground Source Heating and Cooling. Wiley-Blackwell. ISBN 978-1-4051-7061-1. 

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