Heat Pumps Working Principle
Heat energy naturally moves from warmer to cooler spaces. However, a heat pump can reverse this process, absorbing heat from a cold space and releasing it into a warmer space. Heat is not conserved during this process and a certain amount of external energy, such as electricity, is required. In heating, ventilation, and air conditioning (HVAC) systems, the term "heat pump" usually refers to a vapor compression refrigeration unit optimized for high efficiency in both directions of heat energy transfer. These heat pumps can be reversible and work in either direction to provide heating or cooling of interior spaces.
Heat pumps are used to transfer heat because the higher-level energy required is less than the energy released as heat. Most of the heating energy comes from the outside environment, with only a small amount coming from electricity (or other advanced energy sources needed to run the compressor). In an electric heat pump, the heat transferred may be three to four times greater than the electrical energy consumed, giving the system a coefficient of performance (COP) of 3 or 4, compared with a COP of 1 for a traditional resistive heater, where all heat is generated by the input electrical energy. .
Heat pumps use refrigerant as an intermediate fluid to absorb the heat of evaporation in the evaporator and then release the heat in the condenser where the refrigerant condenses. The refrigerant flows through insulated pipes between the evaporator and condenser, allowing efficient heat energy transfer over relatively long distances.
Reversible heat pump
Reversible heat pumps work in either direction to provide heating or cooling to interior spaces. They use a reversing valve to reverse the direction of refrigerant flow from the compressor through the condenser and evaporator coil.
In heating mode, the outdoor coil acts as the evaporator and the indoor coil acts as the condenser. The refrigerant flowing out of the evaporator (outdoor coil) brings heat energy from the outside air (or soil, or better yet, moving water) indoors. The temperature of the vapor in the compression pump increases. The indoor coil then transfers the thermal energy, including compression energy, to the indoor air, which then moves inside the building through the air handler.
Alternatively, the thermal energy is transferred to water and then used to heat the building through radiators or underfloor heating. The heated water can also be used for domestic hot water consumption. The refrigerant then expands, thereby cooling, and absorbs heat from the outside temperature in the external evaporator, and the cycle repeats. This is a standard refrigeration cycle, except the "cold" side of the refrigerator (evaporator coil) is outside where the environment is cooler.
In cold weather, air source heat pump outdoor units require intermittent defrosting. This will cause the auxiliary or emergency heating element (located in the air handler) to activate. At the same time, frost on the outdoor coils will melt quickly due to the warmth of the refrigerant. During defrost mode, the condenser/evaporator fan will not operate.
In cooling mode, the cycle is similar, but the outdoor coil is now the condenser and the indoor coil (reaching a lower temperature) is the evaporator. This is the air conditioner operating mode we are familiar with.
History
1748: William Cullen demonstrates artificial refrigeration.
1834: Jacob Perkins built a practical refrigerator out of ether.
1852: Lord Kelvin describes the theory of heat pumps.
1855–1857: Peter von Rittinger develops and builds the first heat pump.
1945: City of Norwich electrical engineer John Sumner installs an experimental water source heat pump central heating system, using the nearby river to heat the new council administration building. The seasonal efficiency ratio is 3.42. The average heat output is 147kW and the peak output is 234kW.
1948: Robert C. Webber is credited with developing and building the first geothermal heat pump.
1951: The first large-scale installation - London's Royal Festival Hall opens with a city gas-driven reversible water source heat pump fed by the River Thames to meet winter heating and summer cooling needs.
Working principle
Mechanical heat pumps exploit the physical properties of a volatile evaporating and condensing fluid called a refrigerant. The heat pump compresses the refrigerant, causing the temperature on the heat-receiving side to increase, and releasing the pressure on the heat-absorbing side.
Simple stylized diagram of a heat pump vapor compression refrigeration cycle: 1) condenser, 2) expansion valve, 3) evaporator, 4) compressor.
The gaseous working fluid is pressurized by the compressor and circulated through the system. On the discharge side of the compressor, the now hot, high-pressure vapor is cooled in a heat exchanger called a condenser until it condenses into a high-pressure, medium-temperature liquid. The condensed refrigerant then passes through a pressure reducing device (also called a metering device). This could be an expansion valve, a capillary tube, or a device that extracts work, such as a turbine. The low-pressure liquid refrigerant then enters another heat exchanger, the evaporator, where the fluid absorbs heat and boils. The refrigerant then returns to the compressor and the cycle repeats.
The refrigerant must reach a high enough temperature when compressed to release heat through the "hot" heat exchanger (condenser). Similarly, the fluid must reach a low enough temperature when allowed to expand, otherwise heat cannot flow from the cold area of the environment into the fluid in the cold heat exchanger (evaporator). In particular, the pressure difference must be large enough so that the fluid condenses on the hot side and still evaporates in the low-pressure region on the cold side. The greater the temperature difference, the greater the pressure difference required and therefore the greater the energy required to compress the fluid. Therefore, as with all heat pumps, the coefficient of performance (thermal energy moved per unit of required input work) decreases as the temperature difference increases.
Insulation is used to reduce the work and energy required to achieve a sufficiently low temperature in the space to be cooled.
Heat transfer
Heat is typically transferred through engineered heating or cooling systems using flowing gas or liquid. Air was sometimes used, but in many cases it quickly became impractical because it required large pipes to transfer relatively small amounts of heat. In systems using refrigerants, this working fluid can also be used to transfer heat over considerable distances, although this may become impractical due to the increased risk of costly refrigerant leaks. When large amounts of heat need to be transferred, water is often used, often supplemented with antifreeze, corrosion inhibitors, and other additives.
Heat source/radiator
A common heat source or sink in small installations is outside air, as used in air source heat pumps. Fans are needed to improve heat exchange efficiency.
Larger installations that handle more heat or in small physical spaces often use water source heat pumps. Heat is derived from or displaced in a flow of water, which can carry much more heat through a given pipe or pipe cross-section than an air flow can. The water can be heated at a remote location by boilers, solar energy or other means. Alternatively, when needed, cooling towers can be used to cool the water, or the water can be discharged into a large body of water, such as a lake, stream, or ocean.
Geothermal heat pumps or ground source heat pumps utilize shallow underground heat exchangers as heat sources or sinks, with water as the heat transfer medium. This is possible because below the surface, temperatures are relatively constant throughout the seasons, and the Earth can provide or absorb large amounts of heat. Ground source heat pumps work on the same principle as air source heat pumps, but water pumped through underground pipes exchanges heat with the ground. Ground source heat pumps are simpler and therefore more reliable than air source heat pumps as they do not require fans or defrost systems and can be installed internally. Although geothermal heat exchangers require higher initial capital costs, annual operating costs are lower because well-designed geothermal heat pump systems operate more efficiently because their source temperatures are higher than winter air.
Heat pump units can be installed next to auxiliary conventional heat sources, such as electric resistance heaters or oil or gas combustion. Auxiliary heat sources are installed to meet peak heat loads, or to provide a backup system.
Application areas
There are millions of installations in the country using air source heat pumps.
HVAC
In HVAC applications, heat pumps are typically vapor compression refrigeration units that include reversing valves and heat exchangers optimized so that the direction of heat flow (the movement of thermal energy) can be reversed. The reversing valve switches the direction of the refrigerant in the cycle so the heat pump can provide heating or cooling to the building. In cooler climates, the default setting for the reversing valve is heating.
The default setting in warm climates is cooling. Since the two heat exchangers (condenser and evaporator) must exchange functions, they are optimized to operate adequately in both modes. Therefore, the SEER rating (i.e. Seasonal Energy Efficiency Rating) of a reversible heat pump is usually slightly lower than that of two individually optimized machines. For a device to receive an Energy Star rating, it must be rated at least 14.5 SEER.
Water heating
In water heating applications, heat pumps can be used to heat or preheat water for swimming pools or to heat drinking water for domestic and industrial use. Typically, heat is extracted from the outdoor air and transferred to an indoor tank, and another type extracts heat from the indoor air to help cool the space.
District heating
Main article: Drammen heat pump
Commissioned in 2011, the district heating system uses 3 systems to extract heat from the fjord, which has a temperature of approximately 8 °C, to provide a total capacity of 14 MW to homes and businesses in the town centre. A city ordinance requires many new buildings to have such heating systems.
Refrigerant
Until the 1990s, refrigerants were typically chlorofluorocarbons such as R-12 (dichlorodifluoromethane), one of several refrigerants sold under the brand name Freon (a trademark of DuPont). Due to the damage to the ozone layer caused by the release of CFCs into the atmosphere, their production is now banned or severely restricted by the Montreal Protocol in August 1987.
A widely adopted alternative refrigerant is a hydrofluorocarbon (HFC) known as R-134a (1,1,1,2-tetrafluoroethane). Heat pumps using R-134a, which replaces R-12 (dichlorodifluoromethane), have similar thermodynamic properties but have minimal ozone depletion potential and lower global warming potential. Ammonia is widely used in large systems, and the less corrosive but more flammable propane or butane are sometimes used.
Since 2001, carbon dioxide (R-744) has been increasingly used, utilizing transcritical cycling, although it requires higher operating 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 used more frequently; however, it is a powerful greenhouse gas, Will cause climate change. Stirling cycle, offering the largest number of environmentally friendly gas options.
Recent refrigerators use R600A, which is isobutane, does not deplete ozone and is less harmful to the environment. Dimethyl ether (DME) is also becoming increasingly popular as a refrigerant.
Noise
Geothermal heat pumps do not require an outdoor unit with moving mechanical parts: no external noise is produced.
Air source heat pumps require an outdoor unit that contains moving mechanical parts, including a fan that produces noise. In 2013, CEN began to formulate noise pollution protection standards for heat pump outdoor units.
In the United States, permissible nighttime noise levels were defined in 1974 as “an average 24-hour exposure limit of 55 A weighted decibels (dBA) to protect the public from all adverse effects on the health and welfare of residential areas (U.S. Environmental Protection Agency 1974) .This limit is a 24-hour day and night average noise level (LDN), with a 10 dBA penalty imposed on nighttime levels between 2200 and 0700 hours to account for sleep disturbance, and no penalty imposed on daytime levels.
Another feature of ASHP external heat exchangers is the need to stop the fan from time to time for a few minutes to defrost.
Performance considerations
When comparing the performance of heat pumps, it's best to avoid using the term "efficiency," which has a very specific thermodynamic definition. The term coefficient of performance (COP) is used to describe the ratio of useful thermal movement per input work. Most vapor compression heat pumps use electric motors as their operating input.
According to the U.S. Environmental Protection Agency, ground source heat pumps can reduce energy consumption by up to 44% compared to air source heat pumps and up to 72% compared to resistance heating.
When used to heat a building with an outside temperature of 10 °C, a typical air source heat pump (ASHP) has a COP of 3 to 4, while a resistive heater has a COP of 1.0. That is, one joule of electrical energy will allow a resistive heater to produce only one joule of useful heat, whereas under ideal conditions one joule of electrical energy will allow a heat pump to transfer three to four joules of heat from cooler to cooler locations. The place. A warmer place. Note that air source heat pumps are more efficient in hotter climates than colder climates, so when the weather is much warmer the unit will operate at a higher COP (because it has a smaller temperature gap to bridge) . When the temperature difference between hot and cold storage is large, the COP is lower (poor). During extreme cold weather, the COP will drop to 1.0.
On the other hand, well-designed ground source heat pump (GSHP) systems benefit from moderate temperatures underground because the ground naturally acts as a thermal energy storage.
When temperature differences are large (for example, when using an air source heat pump to heat a house where the outdoor temperature is 0 °C (32 °F)), it takes more work to move the same amount of heat indoors compared to mild weather. many. Eventually, due to the Carnot efficiency limit, the performance of the heat pump will decrease as the difference between outdoor and indoor temperatures increases (the outdoor temperature becomes colder), reaching the theoretical limit of 1.0 at -273°C. In practice, air source heat pumps typically achieve a COP of 1.0 when the outdoor temperature is about -18 °C (0 °F).
Additionally, when the heat pump absorbs heat from the air, some of the moisture in the outdoor air may condense and potentially freeze on the outdoor heat exchanger. The system must periodically melt this ice; this defrost translates into additional energy (electricity) expenses. When it's extremely cold outside, it's easier to heat with an alternative heat source such as a resistance heater, oil furnace, or gas furnace than to run an air source heat pump. Additionally, avoiding use of your heat pump during extremely cold weather can reduce wear and tear on your 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 area and corresponding temperature difference (between the refrigerant and air flow) directly affects the operating pressure and thus the work the compressor must do to provide the same heating or cooling effect. Generally speaking, the larger the heat exchanger, the lower the temperature difference and the more efficient the system.
Heat exchangers are expensive, require drilling for certain heat pump types or large spaces to increase efficiency, and the heat pump industry generally competes on price rather than efficiency. Heat pumps are already at a price disadvantage compared to traditional heating solutions such as boilers in terms of initial investment (rather than long-term savings), so the push for more efficient heat pumps and air conditioners is often driven by legislative measures for minimum efficiency standards. The price of electricity can also affect the attractiveness of a heat pump.
In cooling mode, the operating performance of a heat pump is described in the United States by its energy efficiency ratio (EER) or seasonal energy efficiency ratio (SEER), both of which are measured in units of BTU/(h·W) (1 BTU/(h ·W) = 0.293 W/W). A higher EER number indicates better performance. The manufacturer's literature should provide a COP to describe performance in heating mode, and an EER or SEER to describe performance in cooling mode. However, actual performance varies and depends on many factors such as installation details, temperature differences, site elevation and maintenance.
As with any equipment that relies on coils to transfer heat between the air and fluid, it's important that the condenser and evaporator coils stay clean. If dust and other debris deposits accumulate on the coils, the efficiency of the unit (in both heating and cooling modes) will be affected.
If the temperature differences remain equal, a heat pump is more effective at heating an interior space than cooling it. This is because, in heating mode, the input energy of the compressor is also converted into useful heat and is discharged to the interior space through the condenser together with the delivered heat. But for cooling, the condenser is usually located outdoors, and the dissipated work (waste heat) of the compressor must also be transported outdoors using more input energy instead of being used for a useful purpose.
By the same token, the net effect of opening a food refrigerator or freezer is to heat up the room rather than cool it down, as its refrigeration cycle dissipates heat into the indoor air. This heat includes the work dissipated by the compressor and the heat dissipated from the interior of the equipment.
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