How Heat Pumps work
Heat pumps are common & efficient
Electric heat pumps are an essential part of standard household equipment:
- Refrigerators: a heat pump cools a refrigerator’s interior by moving heat from the interior and into the kitchen.
- Reverse cycle air-conditioners: a heat pump cools a room by moving heat from the room to outside the house. When you reverse the cycle, the heat pump heats the room by transferring heat from outside your home to inside the room.
Heat pumps use electricity to run a fan and compressor – and can be amazingly efficient. A high-performance residential heat pump can use one unit of electrical energy to move six times as much heat energy into your house, an efficiency of 600%.
By comparison:
- Electrical heaters have an efficiency of 100% as they can convert all the electrical energy into heat, and
- Gas heaters have an efficiency of between 50% and 95%, as some of the heat generated can escape up the heater’s flue and heat the house exterior.
A Heat pump hot water service
One use of a heat pump is to heat your hot water. You can run the heat pump when the sun is up and use electricity from photovoltaic solar panels to power the heat pump. After installing the panels and heat pump, you heat your water for free, generating no emissions.
The heat pump heats your hot water by moving heat from the air outside your house into your hot water tank. It uses a refrigerant, a liquid that boils at a low temperature, like minus 26 C, and works in a cycle as follows.
The Heat pump cycle
Consider the heat pump’s cyclic operation with its changes to the refrigerant and heat flows. (The diagram below uses the numbers of the numbered points in the following text.)
- The Refrigerant is initially a liquid at room pressure in the heat pump’s evaporator.
- The Evaporator: An electric fan heats the refrigerant by drawing air from outside the house and passing it over a heat exchanger. The air warms the heat exchanger’s metal, which boils some refrigerant, turning it into a gas. (When the outside air temperature exceeds the refrigerant’s boiling point, minus 26°C, the air can heat and vaporise liquid refrigerant—but the colder the day, the harder the fan must work.)
- The Refrigerant is now a cold gas at room pressure.
- The Compressor: A valve closes, shutting the refrigerant gas in the compressor where an electric pump compresses the gas. As the pressure builds, the gas gets hotter, just as a bicycle pump gets hot as you pump up the tyres. The gas can heat to 95 C.
- The Refrigerant is now a gas at high temperature and high pressure.
- The Condenser: The hot, pressurised refrigerant gas enters a second heat exchanger, heating the heat exchanger’s metal, which then heats the water in the water tank. As the refrigerant gas releases its heat, it condenses, returning to a liquid state. Due to the refrigerant being under pressure, it condenses at a higher temperature than its minus 26°C boiling point at room pressure in the evaporator.
- The refrigerant is now a warm liquid, still under high pressure.
- The expansion valve opens, releasing the refrigerant into the evaporator. The pressure around the liquid refrigerant drops, causing the refrigerant to vaporise and cool.
- The refrigerant is now a cold liquid at room pressure: It is back where it started in the evaporator, ready to repeat the heat harvesting cycle.
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| Refrigerant: a liquid at room pressure. (1) & (9) | Evaporator: Fan blown air boils the refrigerant (2) | |
| Expansion valve: Release the pressure (8) | Refrigerant: a cool gas at low pressure (3) | |
| Refrigerant: a warm liquid at high pressure (7) | Compressor: Pump pressurises the refrigerant gas (4) | |
| Condenser: Produces hot water & cools the refrigerant (6) | Refrigerant: a gas at high temperature & high pressure (5) |
Further information
Updated: 24 July 2024