It was an engineering problem that had bugged Zhibin Yu for years but now he had the perfect chance to fix it. Stuck at home during the first UK lockdown of the Covid-19 pandemic, the thermal engineer suddenly had all the time he needed to refine the efficiency of heat pumps: electrical devices that, as their name implies, move heat from the outdoors into peoples homes.
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The pumps are much more efficient than gas heaters, but standard models that absorb heat from the air are prone to icing up, which greatly reduces their effectiveness.
Yu, who works at the University of Glasgow, UK, pondered the problem for weeks. He read paper after paper. And then he had an idea. Most heat pumps waste some of the heat that they generate and if he could capture that waste heat and divert it, he realized, that could solve the defrosting issue and boost the pumps overall performance. I suddenly found a solution to recover the heat, he recalls. That was really an amazing moment.
Yus idea is one of several recent innovations that aim to make 200-year-old heat pump technology even more efficient than it already is, potentially opening the door for much greater adoption of heat pumps worldwide. To date, only about 10% of space heating requirements around the world are met by heat pumps, according to the International Energy Agency, or IEA. But due to the current energy crisis and growing pressure to reduce fossil fuel consumption in order to combat climate change, these devices are arguably more crucial than ever.
Since his lockdown brainstorming, Yu and his colleagues have built a working prototype of a heat pump that stores leftover heat in a small water tank. In a paper published in the summer of , they describe how their design helps the heat pump to use less energy. Plus, by separately rerouting some of this residual warmth to part of the heat pump exposed to cold air, the device can defrost itself when required, without having to pause heat supply to the house.
The idea relies on the very principle by which heat pumps operate: If you can seize heat, you can use it. What makes heat pumps special is the fact that instead of just generating heat, they also capture heat from the environment and move it into your house eventually transferring that heat to radiators or forced-air heating systems, for instance. This is possible thanks to the refrigerant that flows around inside a heat pump. When the refrigerant encounters heat even a tiny amount in the air on a cold day it absorbs that modicum of warmth.
A compressor then forces the refrigerant to a higher pressure, which raises its temperature to the point where it can heat your house. It works because an increase of pressure pushes the refrigerant molecules closer together, increasing their motion. The refrigerant later expands again, cooling as it does so, and the cycle repeats. The entire cycle can run in reverse, too, allowing heat pumps to provide cooling when its hot in summer.
The magic of a heat pump is that it can move multiple kilowatt-hours of heat for each kWh of electricity it uses. Heat pump efficiencies are generally measured in terms of their coefficient of performance, or COP. A COP of 3, for example, means 1 kWh of juice yields 3 kWh of warmth thats effectively 300% efficiency. The COP you get from your device can vary depending on the weather and other factors.
Its a powerful concept, but also an old one. The British mathematician, physicist and engineer Lord Kelvin proposed using heat pump systems for space heating way back in . The first heat pump was designed and built a few years later and used industrially to heat brine in order to extract salt from the fluid. In the s, members of the British Parliament discussed heat pumps when coal stocks were running low. And in the years following the -74 oil crisis, heat pumps were touted as an alternative to fossil fuels for heating. Hope rests with the future heat pump, one commentator wrote in the Annual Review of Energy.
Now the world faces yet another reckoning over energy supplies. When Russia, one of the worlds biggest sources of natural gas, invaded Ukraine in February , the price of gas soared which in turn shoved heat pumps into the spotlight because with few exceptions they run on electricity, not gas. The same month, environmentalist Bill McKibben wrote a widely shared blog post titled Heat pumps for peace and freedom in which, referring to the Russian president, he argued that the U.S. could peacefully punch Putin in the kidneys by rolling out heat pumps on a massive scale while lowering Americans dependence on fossil fuels. Heat pumps can draw power from domestic solar panels, for instance, or a power grid supplied predominantly by renewables.
Running the devices on green electricity can help to fight climate change, too, notes Karen Palmer, an economist and senior fellow at Resources for the Future, an independent research organization in Washington, D.C., who coauthored an analysis of policies to enhance energy efficiency in the Annual Review of Resource Economics. Moving towards greater use of electricity for energy needs in buildings is going to have to happen, absent a technology breakthrough in something else, she says.
This video illustrates the principle behind heat pumps. CREDIT: THIS OLD HOUSEThe IEA estimates that, globally, heat pumps have the potential to reduce carbon dioxide emissions by at least 500 million metric tons in , equivalent to the annual CO2 emissions produced by all the cars in Europe today.
Despite their long history and potential virtues, heat pumps have struggled to become commonplace in some countries. One reason is cost: The devices are substantially more expensive than gas heating units and, because natural gas has remained relatively cheap for decades, homeowners have had little incentive to switch.
There has also long been a perception that heat pumps wont work as well in cold climates, especially in poorly insulated houses that require a lot of heat. In the U.K., for example, where houses tend to be rather drafty, some homeowners have long considered gas boilers a safer bet because they can supply hotter water (around 140 to 160 degrees Fahrenheit), to radiators, which makes it easier to heat up a room. By contrast, heat pumps tend to be most efficient when heating water to around 100 degrees Fahrenheit.
The cold-climate problem is arguably less of an issue than some think, however, given that there are multiple modern air source devices on the market that work well even when outside temperatures drop as low as minus 10 degrees Fahrenheit. Norway, for example, is considered one of the world leaders in heat pump deployment. Palmer has a heat pump in her U.S. home, along with a furnace as backup. If it gets really cold, we can rely on the furnace, she says.
Innovations in heat pump design are leading to units that are even more efficient, better suited to houses with low levels of insulation and potentially cheaper, too. For example, Yu says his and his colleagues novel air source heat pump design could improve the COP by between 3 and 10%, while costing less than existing heat pump designs with comparable functionality. They are now looking to commercialize the technology.
Yus work is innovative, says Rick Greenough, an energy systems engineer now retired from De Montfort University in the U.K.. I must admit this is a method I hadnt actually thought of, he says.
And there are plenty more ideas afoot. Greenough, for instance, has experimented with storing heat in the ground during warmer months, where it can be exploited by a heat pump when the weather turns cool. His design uses a circulating fluid to transfer excess heat from solar hot-water panels into shallow boreholes in the soil. That raises the temperature of the soil by around 22 degrees Fahrenheit, to a maximum of roughly 66 degrees Fahrenheit, he says. Then, in the winter, a heat pump can draw out some of this stored heat to run more efficiently when the air gets colder. This technology is already on the market, offered by some installers in the U.K., notes Greenough.
But most current heat pumps still only generate relatively low output temperatures, so owners of drafty homes may need to take on the added cost of insulation when installing a heat pump. Fortunately, a solution may be emerging: high-temperature heat pumps.
We said, Hey, why not make a heat pump that can actually one-on-one replace a gas boiler without having to really, really thoroughly insulate your house? says Wouter Wolfswinkel, program manager for business development at Swedish energy firm Vattenfall, which manufactures heat pumps. Vattenfall and its Dutch subsidiary Feenstra have teamed up to develop a high-temperature heat pump, expected to debut in .
In their design, they use CO2 as a refrigerant. But because the heat-pump systems hot, high-pressure operating conditions prevent the gas from condensing or otherwise cooling down very easily, they had to find a way of reducing the refrigerants temperature in order for it to be able to absorb enough heat from the air once again when it returns to the start of the heat pump loop. To this end, they added a buffer to the system: a water tank where a layer of cooler water rests beneath hotter water above. The heat pump uses the lower layer of cooler water from the tank to adjust the temperature of the refrigerant as required. But it can also send the hotter water at the top of the tank out to radiators, at temperatures up to 185 degrees Fahrenheit.
The device is slightly less efficient than a conventional, lower temperature heat pump, Wolfswinkel acknowledges, offering a COP of around 265% versus 300%, depending on conditions. But thats still better than a gas boiler (no more than 95% efficient), and as long as electricity prices arent significantly higher than gas prices, the high temperature heat pump could still be cheaper to run. Moreover, the higher temperature means that homeowners neednt upgrade their insulation or upsize radiators right away, Wolfswinkel notes. This could help people make the transition to electrified heating more quickly.
A key test was whether Dutch homeowners would go for it. As part of a pilot trial, Vattenfall and Feenstra installed the heat pump in 20 households of different sizes in the town of Heemskerk, not far from Amsterdam. After a few years of testing, in June they gave homeowners the option of taking back their old gas boiler, which they had kept in their homes, or of using the high temperature heat pump on a permanent basis. All of them switched to the heat pump, says Wolfswinkel.
In some situations, home-by-home installations of heat pumps might be less efficient than building one large system to serve a whole neighborhood. For about a decade, Star Renewable Energy, based in Glasgow, has been building district systems that draw warmth from a nearby river or sea inlet, including a district heating system connected to a Norwegian fjord. A Scandinavian fjord might not be the first thing that comes to mind if you say the word heat but the water deep in the fjord actually holds a fairly steady temperature of 46 degrees Fahrenheit, which heat pumps can exploit.
Via a very long pipe, the district heating system draws in this water and uses it to heat the refrigerant, in this case ammonia. A subsequent, serious increase of pressure for the refrigerant to 50 atmospheres raises its temperature to 250 degrees Fahrenheit. The hot refrigerant then passes its heat to water in the district heating loop, raising the temperature of that water to 195 degrees Fahrenheit. The sprawling system provides 85% of the hot water needed to heat buildings in the city of Drammen.
That type of thing is very exciting, says Greenough.
Not every home will be suitable for a heat pump. And not every budget can accommodate one, either. Yu himself says that the cost of replacing the gas boiler in his own home remains prohibitive. But its something he dreams of doing in the future. With ever-improving efficiencies, and rising sales in multiple countries, heat pumps are only getting harder for their detractors to dismiss. Eventually, says Yu, I think everyone will switch to heat pumps.
Chris Baraniuk is a freelance science journalist and nature lover who lives in Belfast, Northern Ireland. His work has been published by the BBC, the Guardian, New Scientist, Scientific American and Hakai Magazine, among other publications.
This article originally appeared in Knowable Magazine, an independent journalistic endeavor from Annual Reviews. Sign up for the newsletter.
This article is about details of the most common type of heat pump . For more general information, see heat pump
Heat pump on balcony of apartment
An air source heat pump (ASHP) is a heat pump that can absorb heat from air outside a building and release it inside; it uses the same vapor-compression refrigeration process and much the same equipment as an air conditioner, but in the opposite direction. ASHPs are the most common type of heat pump and, usually being smaller, tend to be used to heat individual houses or flats rather than blocks, districts or industrial processes.[1]
Air-to-air heat pumps provide hot or cold air directly to rooms, but do not usually provide hot water. Air-to-water heat pumps use radiators or underfloor heating to heat a whole house and are often also used to provide domestic hot water.
An ASHP can typically gain 4 kWh thermal energy from 1 kWh electric energy. They are optimized for flow temperatures between 30 and 40 °C (86104 °F), suitable for buildings with heat emitters sized for low flow temperatures. With losses in efficiency, an ASHP can even provide full central heating with a flow temperature up to 80 °C (176 °F).[2]
As of about 10% of building heating worldwide is from ASHPs. They are the main way to phase out gas boilers (also known as "furnaces") from houses, to avoid their greenhouse gas emissions.[3]
Air-source heat pumps are used to move heat between two heat exchangers, one outside the building which is fitted with fins through which air is forced using a fan and the other which either directly heats the air inside the building or heats water which is then circulated around the building through radiators or underfloor heating which releases the heat to the building. These devices can also operate in a cooling mode where they extract heat via the internal heat exchanger and eject it into the ambient air using the external heat exchanger. Some can be used to heat water for washing which is stored in a domestic hot water tank.[4]
Air-source heat pumps are relatively easy and inexpensive to install, so are the most widely used type. In mild weather, coefficient of performance (COP) may be between 2 and 5, while at temperatures below around 8 °C (18 °F) an air-source heat pump may still achieve a COP of 1 to 4.[5]
While older air-source heat pumps performed relatively poorly at low temperatures and were better suited for warm climates, newer models with variable-speed compressors remain highly efficient in freezing conditions allowing for wide adoption and cost savings in places like Minnesota and Maine in the United States.[6]
Technology
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Air source heat pumpAir at any natural temperature contains some heat. An air source heat pump transfers some of this from one place to another, for example between the outside and inside of a building.
An air-to air system can be designed to transfer heat in either direction, to heat or cool the interior of the building in winter and summer respectively. Internal ducting may be used to distribute the air.[7] An air-to-water system only pumps heat inwards, and can provide space heating and hot water.[8] For simplicity, the description below focuses on use for interior heating.
The technology is similar to a refrigerator or freezer or air conditioning unit: the different effect is due to the location of the different system components. Just as the pipes on the back of a refrigerator become warm as the interior cools, so an ASHP warms the inside of a building whilst cooling the outside air.
The main components of a split-system (called split as there are both inside and outside coils) air source heat pump are:
Less commonly a packaged ASHP has everything outside, with hot (or cold) air sent inside through a duct.[10]
These are also called monobloc and are useful for keeping flammable propane outside the house.[3]
An ASHP can provide three or four times as much heat as an electric resistance heater using the same amount of electricity.[11] Burning gas or oil will emit carbon dioxide and also NOx, which can be harmful to health.[12] An air source heat pump issues no carbon dioxide, nitrogen oxide or any other kind of gas. It uses a small amount of electricity to transfer a large amount of heat.
Most ASHPs are reversible and are able to either warm or cool buildings[13] and in some cases also provide domestic hot water. The use of an air-to-water heat pump for house cooling has been criticised.[14]
An internal view of the outdoor unit of an air source heat pump A: indoor compartment, B: outdoor compartment, I: insulation, 1: condenser, 2: expansion valve, 3: evaporator, 4: compressorHeating and cooling is accomplished by pumping a refrigerant through the heat pump's indoor and outdoor coils. Like in a refrigerator, a compressor, condenser, expansion valve and evaporator are used to change states of the refrigerant between colder liquid and hotter gas states.
When the liquid refrigerant at a low temperature and low pressure passes through the outdoor heat exchanger coils, ambient heat causes the liquid to boil (change to gas or vapor). Heat energy from the outside air has been absorbed and stored in the refrigerant as latent heat. The gas is then compressed using an electric pump; the compression increases the temperature of the gas.
Inside the building, the gas passes through a pressure valve into heat exchanger coils. There, the hot refrigerant gas condenses back to a liquid and transfers the stored latent heat to the indoor air, water heating or hot water system. The indoor air or heating water is pumped across the heat exchanger by an electric pump or fan.
The cool liquid refrigerant then re-enters the outdoor heat exchanger coils to begin a new cycle. Each cycle usually takes a few minutes.[11]
Most heat pumps can also operate in a cooling mode where the cold refrigerant is moved through the indoor coils to cool the room air.
As of tech other than vapour compression is insignificant in the market.[15]
Usage
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ASHPs are the most common type of heat pump and, usually being smaller, are generally more suitable to heat individual houses rather than blocks of flats, compact urban districts or industrial processes.[1] In dense city centres heat networks may be better than ASHP.[1] Air source heat pumps are used to provide interior space heating and cooling even in colder climates, and can be used efficiently for water heating in milder climates. A major advantage of some ASHPs is that the same system may be used for heating in winter and cooling in summer. Though the cost of installation is generally high, it is less than the cost of a ground source heat pump, because a ground source heat pump requires excavation to install its ground loop. The advantage of a ground source heat pump is that it has access to the thermal storage capacity of the ground which allows it to produce more heat for less electricity in cold conditions.
Home batteries can mitigate the risk of power cuts and like ASHPs are becoming more popular.[16] Some ASHPs can be coupled to solar panels as primary energy source, with a conventional electric grid as backup source.[citation needed]
Thermal storage solutions incorporating resistance heating can be used in conjunction with ASHPs. Storage may be more cost-effective if time of use electricity rates are available. Heat is stored in high density ceramic bricks contained within a thermally-insulated enclosure;[17] storage heaters are an example. ASHPs may also be paired with passive solar heating. Thermal mass (such as concrete or rocks) heated by passive solar heat can help stabilize indoor temperatures, absorbing heat during the day and releasing heat at night, when outdoor temperatures are colder and heat pump efficiency is lower.
Replacing gas heating in existing houses
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Good home insulation is important.[18] As of ASHPs are bigger than gas boilers and need more space outside, so the process is more complex and can be more expensive than if it was possible to just remove a gas boiler and install an ASHP in its place.[3][19] If running costs are important choosing the right size is important because an ASHP which is too large will be more expensive to run.[20]
It can be more complicated to retrofit conventional heating systems that use radiators/radiant panels, hot water baseboard heaters, or even smaller diameter ducting, with ASHP-sourced heat. The lower heat pump output temperatures means radiators (and possibly pipes) may have to be replaced with larger sizes, or a low temperature underfloor heating system installed instead.[21]
Alternatively, a high temperature heat pump can be installed and existing heat emitters can be retained, however as of these heat pumps are more expensive to buy and run so may only be suitable for buildings which are hard to alter or insulate, such as some large historic houses.[22]
ASHP are claimed to be healthier than fossil-fuelled heating such as gas heaters by maintaining a more even temperature and avoiding harmful fumes risk.[18] Also by filtering the air and reducing humidity in hot humid summer climates they are said to reduce dust and allergens and mould health risk.[23]
In cold climates
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The outdoor unit of an air source heat pump operating in freezing conditionsOperation of normal ASHPs is generally not recommended below 10 °C.[24] However ASHPs designed specifically for very cold climates (in the US these are certified under Energy Star[25]) can extract useful heat from ambient air as cold as 30 °C (22 °F), however below 25 °C electric resistance heating may be more efficient.[24] This is made possible by the use of variable-speed compressors, powered by inverters.[25] Although air source heat pumps are less efficient than well-installed ground source heat pumps in cold conditions, air source heat pumps have lower initial costs and may be the most economic or practical choice.[26] A hybrid system, with both a heat pump and an alternative source of heat such as a fossil fuel boiler, may be suitable if it is impractical to properly insulate a large house.[27] Alternatively multiple heat pumps or a high temperature heat pump may be considered.[27]
In some weather conditions condensation will form and then freeze onto the coils of the heat exchanger of the outdoor unit, reducing air flow through the coils. To clear this the unit operates a defrost cycle, switching to cooling mode for a few minutes, heating the coils until the ice melts. Air-to-water heat pumps use heat from the circulating water for this purpose, which results in a small and probably undetectable drop in water temperature;[28] for air-to-air systems heat is either taken from the air in the building or using an electrical heater.[29] Some air-to-air systems simply stop the operation of the fans of both units and switch to cooling mode, so that the outdoor unit returns to being the condenser such that it heats up and defrosts.
Noise
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An air source heat pump requires an outdoor unit containing moving mechanical components including fans which produce noise. Modern devices offer schedules for silent mode operation with reduced fan speed. This will reduce the maximum heating power but can be applied at mild outdoor temperatures without efficiency loss. Acoustic enclosures are another approach to reduce the noise in a sensitive neighbourhood. In insulated buildings, operation can be paused at night without significant temperature loss. Only at low temperatures, frost protection forces operation after a few hours. Proper siting is also important.[30]
In the United States, the allowed night-time noise level is 45 A-weighted decibels (dBA).[31] In the UK the limit is set at 42 dB measured from the nearest neighbour[32] according to the MCS 020 standard[33] or equivalent.[34] In Germany the limit in residential areas is 35, which is usually measured by European Standard EN .[35]
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 is especially powerful in quiet environments where background night-time 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.[36][37]
By contrast a ground source heat pump has no need for an outdoor unit with moving mechanical components.
Efficiency ratings
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The efficiency of air source heat pumps is measured by the coefficient of performance (COP). A COP of 4 means the heat pump produces 4 units of heat energy for every 1 unit of electricity it consumes. Within temperature ranges of 3 °C (27 °F) to 10 °C (50 °F), the COP for many machines is fairly stable. Approximately TheoreticalMaxCOP = (desiredIndoorTempC + 273) ÷ (desiredIndoorTempC - outsideTempC).[citation needed][38][better source needed]
In mild weather with an outside temperature of 10 °C (50 °F), the COP of efficient air source heat pumps ranges from 4 to 6.[39] However, on a cold winter day, it takes more work to move the same amount of heat indoors than on a mild day.[40] The heat pump's performance is limited by the Carnot cycle and will approach 1.0 as the outdoor-to-indoor temperature difference increases, which for most air source heat pumps happens as outdoor temperatures approach 18 °C (0 °F).[citation needed]Heat pump construction that enables carbon dioxide as a refrigerant may have a COP of greater than 2 even down to 20 °C, pushing the break-even figure downward to 30 °C (22 °F).[citation needed] A ground source heat pump has comparatively less of a change in COP as outdoor temperatures change, because the ground from which they extract heat has a more constant temperature than outdoor air.
The design of a heat pump has a considerable impact on its efficiency. Many air source heat pumps are designed primarily as air conditioning units, mainly for use in summer temperatures. Designing a heat pump specifically for the purpose of heat exchange can attain greater COP and an extended life cycle. The principal changes are in the scale and type of compressor and evaporator.
Seasonally adjusted heating and cooling efficiencies are given by the heating seasonal performance factor (HSPF) and seasonal energy efficiency ratio (SEER) respectively. In the US the legal minimum efficiency is 14 or 15 SEER and 8.8 HSPF.[25]
Variable speed compressors are more efficient because they can often run more slowly and because the air passes through more slowly giving its water more time to condense, thus more efficient as drier air is easier to cool. However they are more expensive and more likely to need maintenance or replacement.[23] Maintenance such as changing filters can improve performance by 10% to 25%.[41]
Refrigerant types
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Impact on decarbonization and electricity supply
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Heat pumps are key to decarbonizing home energy use by phasing out gas boilers.[19][11] As of the IEA says that 500 million tonnes of CO2 emissions could be cut by .[64]
While heat pumps with backup systems other than electrical resistance heating are often encouraged by electric utilities, air source heat pumps are a concern for winter-peaking utilities if electrical resistance heating is used as the supplemental or replacement heat source when the temperature drops below the point that the heat pump can meet all of the home's heat requirement. Even if there is a non-electric backup system, the fact that efficiencies of ASHPs decrease with outside temperatures is a concern to electric utilities. The drop in efficiency means their electrical load increases steeply as temperatures drop.[citation needed]
As wind farms are increasingly used to supply electricity to some grids, such as Canada's Yukon Territory, the increased winter load matches well with the increased winter generation from wind turbines, and calmer days result in decreased heating load for most houses even if the air temperature is low.[65]
Heat pumps could help stabilize grids through demand response.[66] As heat pump penetration increases some countries, such as the UK, may need to encourage households to use thermal energy storage, such as very well insulated water tanks.[67] In some countries, such as Australia, integration of this thermal storage with rooftop solar would also help.[68]
Although higher cost heat pumps can be more efficient a study concluded that for the UK "from an energy system perspective, it is overall cost-optimal to design heat pumps with nominal COP in the range of 2.83.2, which typically has a specific cost lower than 650 £/kWth, and simultaneously to invest in increased capacities of renewable energy generation technologies and batteries, in the first instance, followed by OCGT and CCGT with CCS."[69]
Economics
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Cost
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As of buying and installing an ASHP in an existing house is expensive if there is no government subsidy, but the lifetime cost will likely be less than or similar to a gas boiler and air conditioner.[70][71] This is generally also true if cooling is not required, as the ASHP will likely last longer if only heating.[72] The lifetime cost of an air source heat pump will be affected by the price of electricity compared to gas (where available), and may take two to ten years to break even.[70] The IEA recommends governments subsidize the purchase price of residential heat pumps, and some countries do so.[73]
Market
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In Norway,[74] Australia and New Zealand most heating is from heat pumps. In heat pumps outsold fossil fuel based heating in the US and France.[73] ASHPs can be helped to compete by increasing the price of fossil gas compared to that of electricity and using suitable flexible electricity pricing.[19] In the US air-to-air is the most common type.[75] As of over 80% of heat pumps are air source.[11] In the IEA appealed for better data - especially on air-to-air.[73]
Maintenance and reliability
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It is thought that ASHP need less maintenance than fossil fuelled heating, and some say that ASHPs are easier to maintain than ground source heat pumps due to the difficulty of finding and fixing underground leaks. Installing too small an ASHP could shorten its lifetime (but one which is too large will be less efficient).[76] However others say that boilers require less maintenance than ASHPs.[77] A Consumer Reports survey found that "on average, around half of heat pumps are likely to experience problems by the end of the eighth year of ownership".[78]
History
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Modern chemical refrigeration techniques developed after the proposal of the Carnot cycle in . Jacob Perkins invented an ice-making machine that used ether in , and Edmond Carré built a refrigerator that used water and sulfuric acid in . In Japan, Fusanosuke Kuhara, founder of Hitachi, Ltd., made an air conditioner for his own home use using compressed CO2 as a refrigerant in .[79]
In Thomas Midgley Jr. discovered dichlorodifluoromethane, a chlorinated fluorocarbon (CFC) known as freon. CFCs rapidly replaced traditional refrigerant substances, including CO2 (which proved hard to compress for domestic use[80]), for use in heat pumps and refrigerators. But from the s CFCs began to lose favor as refrigerant when their damaging effects on the ozone layer were discovered. Two alternative types of refrigerant, hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs), also lost favor when they were identified as greenhouse gases (additionally, HCFCs were found to be more damaging to the ozone layer than originally thought). The Vienna Convention for the Protection of the Ozone Layer, the Montreal Protocol and the Kyoto Protocol call for the complete abandonment of such refrigerants by .
In , amid international concern about the effects of chlorofluorocarbons and hydrochlorofluorocarbons on the ozone layer, scientist Gustav Lorentzen and SINTEF patented a method for using CO2 as a refrigerant in heating and cooling. Further research into CO2 refrigeration was then conducted at Shecco (Sustainable HEating and Cooling with CO2) in Brussels, Belgium, leading to increasing use of CO2 refrigerant technology in Europe.[80]
In the Japanese company Denso, in collaboration with Gustav Lorentzen, developed an automobile air conditioner using CO2 as a refrigerant. They demonstrated the invention at the June International Institute of Refrigeration/Gustav Lorentzen Conference.[81] After the conference, CRIEPI (Central Research Institute of Electric Power Industry) and TEPCO (The Tokyo Electric Power Company) approached Denso about developing a prototype air conditioner using natural refrigerant materials instead of freon. Together they produced 30 prototype units for a year-long experimental installation at locations throughout Japan, from the cold climate of Hokkaidō to hotter Okinawa. After this successful feasibility study, Denso obtained a patent to compress CO2 refrigerant for use in a heat pump from SINTEF in September .
Manufacturing
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Demand for heat pumps increased in the first quarter of the 21st century in the US and Europe, with governments subsidizing them to increase energy security and decarbonisation. Europeans tend to use air-to-water (also called hydronic) systems such as to radiators, rather than air-to-air systems more common elsewhere. This may help European manufacturers to defend their home market. However, as they are not high-tech and are similar to aircon Asian countries such as China made three-quarters of heat pumps in .[82]
See also
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References
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IPCC reports
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