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Heat pump
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Heat pump
A heat pump is a device that uses energy—generally mechanical energy, although the absorption heat pump instead uses thermal energy—to transfer heat from one space to another. The mechanical heat pump, also known as a Cullen engine, uses electric power to transfer heat by compression. Specifically, it transfers thermal energy by means of a heat pump and refrigeration cycle, cooling one space and warming the other. In winter, a heat pump can move heat from the cool outdoors to warm a house; in summer, it may also be designed to move heat from the house to the warmer outdoors. As it transfers rather than generates heat, it is more energy-efficient than heating by gas boiler.
In a typical vapour-compression heat pump, a gaseous refrigerant is compressed so its pressure and temperature rise. When the pump operates as a heater in cold weather, the warmed gas flows to a heat exchanger in the indoor space, where some of its thermal energy is transferred to that space, causing the gas to condense into a liquid. The liquified refrigerant flows to a heat exchanger in the outdoor space, where the pressure falls, the liquid evaporates, and the temperature of the gas falls. Now colder than the temperature of the outdoor space being used as a heat source, it can again take up energy from the heat source, be compressed, and repeat the cycle.
Air source heat pumps are the most common models, while other types include ground source heat pumps, water source heat pumps, and exhaust air heat pumps. Large-scale heat pumps are also used in district heating systems.
Because of their high efficiency and the increasing share of fossil-free sources in electrical grids, heat pumps are playing a role in climate change mitigation. At a cost of 1 kWh of electricity, they can transfer 1 to 4.5 kWh of thermal energy into a building. The carbon footprint of heat pumps depends on how electricity is generated, but they usually reduce emissions. Heat pumps could satisfy over 80% of global space and water heating needs with a lower carbon footprint than gas-fired condensing boilers: however, in 2021 they only met 10%.
Heat flows spontaneously from a region of higher temperature to a region of lower temperature. Heat does not flow spontaneously from lower temperature to higher, but it can be made to flow in this direction if work is performed. The work required to transfer a given amount of heat is usually much less than the amount of heat; this is the motivation for using heat pumps in applications such as the heating of water and the interior of buildings.
The amount of work required to provide an amount of heat Q to a higher-temperature reservoir such as the interior of a building, while extracting heat from a lower-temperature reservoir such as ambient air is: where
The coefficient of performance of a heat pump is greater than one so the work required is less than the heat released, making a heat pump a more efficient form of heating than electrical resistance heating. As the temperature of the higher-temperature reservoir increases in response to the heat flowing into it, the coefficient of performance decreases, causing an increasing amount of work to be required for each unit of heat being transferred.
The coefficient of performance, and the work required by a heat pump can be calculated easily by considering an ideal heat pump operating on the reversed Carnot cycle:
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Heat pump
A heat pump is a device that uses energy—generally mechanical energy, although the absorption heat pump instead uses thermal energy—to transfer heat from one space to another. The mechanical heat pump, also known as a Cullen engine, uses electric power to transfer heat by compression. Specifically, it transfers thermal energy by means of a heat pump and refrigeration cycle, cooling one space and warming the other. In winter, a heat pump can move heat from the cool outdoors to warm a house; in summer, it may also be designed to move heat from the house to the warmer outdoors. As it transfers rather than generates heat, it is more energy-efficient than heating by gas boiler.
In a typical vapour-compression heat pump, a gaseous refrigerant is compressed so its pressure and temperature rise. When the pump operates as a heater in cold weather, the warmed gas flows to a heat exchanger in the indoor space, where some of its thermal energy is transferred to that space, causing the gas to condense into a liquid. The liquified refrigerant flows to a heat exchanger in the outdoor space, where the pressure falls, the liquid evaporates, and the temperature of the gas falls. Now colder than the temperature of the outdoor space being used as a heat source, it can again take up energy from the heat source, be compressed, and repeat the cycle.
Air source heat pumps are the most common models, while other types include ground source heat pumps, water source heat pumps, and exhaust air heat pumps. Large-scale heat pumps are also used in district heating systems.
Because of their high efficiency and the increasing share of fossil-free sources in electrical grids, heat pumps are playing a role in climate change mitigation. At a cost of 1 kWh of electricity, they can transfer 1 to 4.5 kWh of thermal energy into a building. The carbon footprint of heat pumps depends on how electricity is generated, but they usually reduce emissions. Heat pumps could satisfy over 80% of global space and water heating needs with a lower carbon footprint than gas-fired condensing boilers: however, in 2021 they only met 10%.
Heat flows spontaneously from a region of higher temperature to a region of lower temperature. Heat does not flow spontaneously from lower temperature to higher, but it can be made to flow in this direction if work is performed. The work required to transfer a given amount of heat is usually much less than the amount of heat; this is the motivation for using heat pumps in applications such as the heating of water and the interior of buildings.
The amount of work required to provide an amount of heat Q to a higher-temperature reservoir such as the interior of a building, while extracting heat from a lower-temperature reservoir such as ambient air is: where
The coefficient of performance of a heat pump is greater than one so the work required is less than the heat released, making a heat pump a more efficient form of heating than electrical resistance heating. As the temperature of the higher-temperature reservoir increases in response to the heat flowing into it, the coefficient of performance decreases, causing an increasing amount of work to be required for each unit of heat being transferred.
The coefficient of performance, and the work required by a heat pump can be calculated easily by considering an ideal heat pump operating on the reversed Carnot cycle:
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