How do heat pumps work
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Well, the first thing we are going to see is the air-to-air heat pump. These are the simplest types of heat pumps. They often look a lot like a standard air conditioning unit with one unit located outside and one inside.
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| air to air heat pump |
These can function as heaters only, or the most popular option is to have a device that can provide both heating and cooling via a reversing valve. We’ve covered how reversing valves
work in our previous video tutorial, click here to watch that.
There are several ways to set up a reversible heat pump, but I'll show you a simple and typical example.
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| Heat pump diagram, how a heat pump works |
The main components that we will have are the compressor, the reversing valve, the internal exchanger, an expansion valve with non-return bypass, a two-way filter drier, a sight glass, another expansion valve with a check valve and a return valve. bypass, then we have the outdoor heat exchanger. We also have a controller and several temperature and pressure sensors around the system.
In heating mode, the refrigerant leaves the compressor as high-pressure, high-temperature steam and passes through the reversing valve. The reversing valve is placed in heating mode so that the refrigerant passes through and goes to the indoor unit. Cold air is blown over the indoor unit heat exchanger to remove some of the heat energy and provide heating to the room, as the heat is removed, the refrigerant condenses. After giving up some of its energy, the refrigerant comes out as a slightly cooler high-pressure liquid. Then the refrigerant goes to the expansion valve and bypass, the expansion valve closes so the liquid refrigerant goes through the check valve, through the filter drier and sight glass, then through the valve expansion because the check valve returns to this side prevents flow.
As the refrigerant passes through the expansion valve, the refrigerant expands in volume and becomes a half-liquid, half-vapor mixture. This volume expansion reduces temperature and pressure. We have covered how thermal
expansion valves work and how electronic
expansion valves work
The refrigerant then goes to the outdoor heat exchanger. Here, a fan blows outside air over the coil, adding heat to the cold refrigerant. The refrigerant boils at a very low temperature, and as it boils, it removes heat energy.
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| Boiling point of the refrigerant |
As an example we know that water releases heat energy in the form of vapor when it boils and we know that it boils at 100°C (212°F) well if we look at some common heat sources of heat pump chillers R134a has a point boiling point of -26.3°C (-15.34*F) and R410A has a boiling point of -48.5°C (-55.3*F). Therefore, it is very easy to extract thermal energy even at very low outside temperatures. We've covered how coolants work in a previous video, again links below for that.
Thus, the refrigerant captures the heat energy from the outdoor air and leaves the outdoor heat exchanger as slightly superheated steam at low pressure and temperature and returns to the reversing valve. The reversing valve then diverts this to the compressor to repeat the cycle.
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| Heat pump diagram - In cooling mode |
If the system is switched to cooling mode, then the system acts as a normal split air conditioner. The compressor forces the refrigerant vapor at high pressure and temperature to the reversing valve, the reversing valve diverts it to the outdoor unit. The outdoor unit fan blows ambient air through the heat exchanger, this air will be at a cooler temperature to discharge the heat energy of the refrigerant. The refrigerant condenses losing its thermal energy. It then comes out as a high-pressure, low-temperature liquid refrigerant.
It then goes to the expansion valve but this is closed so that the refrigerant passes through the check valve, through the sight glass and then through the two-way filter drier. The next check valve is then closed so that the refrigerant passes through the expansion valve. As it passes through the expansion valve, the refrigerant changes to a half-liquid, half-vapor mixture, causing a drop in pressure and temperature. It then flows through the indoor heat exchanger and a fan blows the hot indoor air onto the cold coil, causing heat to be transferred from the air to the refrigerant and thus the refrigerant boils and carries that heat away. The refrigerant leaves the indoor unit in a slightly superheated state at low pressure and low temperature and flows through the reversing valve. The valve diverts this back to the compressor to repeat the cycle.
Air/water heat pumps
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| Air to water heat pump: how heat pumps work |
These units work similarly but without the reversing valve. The high pressure and high temperature vapor refrigerant leaves the compressor, but this time it goes to a plate heat exchanger. On the other side of the plate heat exchanger, the water circulates in a hot water tank. Cold water enters the heat exchanger from the tank, and as it passes through the heat exchanger, it absorbs heat from the hot coolant, so it will come out much hotter and return to the hot water storage in the tank. As the refrigerant gives up its heat to the water, it condenses and leaves the heat exchanger as a high-pressure, low-temperature liquid. We covered how heat exchangers work in our previous tutorial, click here
The refrigerant then passes through the filter drier and sight glass and then to the expansion valve. The expansion valve causes the refrigerant to become part liquid, part vapor, and at low pressure and temperature. It then passes through the outdoor heat exchanger where ambient outdoor air boils the refrigerant, the refrigerant then exits as a low pressure, low temperature, slightly superheated vapor and is drawn back to the compressor.
The hot water tank supplies hot water to the building's radiators, sinks and showers.
geothermal heat pump
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| Geothermal heat pump: how heat pumps work |
There are two main types of ground source heat pumps, which are horizontal or vertical type. They both work essentially the same way, what varies is how they access heat in the ground. We'll cover when to use the different types as well as the pros and cons in our next article on heat pumps, this article just focuses on how they work.
Geothermal energy can be used to heat air or water. In the air type system, the heat pump can also have a reversing valve and then provide heating or cooling.
In either case, the outdoor heat exchanger may be a plate heat exchanger with the coolant passing through one side and a mixture of water and antifreeze cycling through the other side. The antifreeze and water mixture is pumped around pipes in the ground. This will allow you to capture heat energy in heating mode and bring it to the heat exchanger, the refrigerant on the other side of the heat exchanger absorbs the heat as it has a very low boiling point so when it boils it releases the heat and can then be use inside the building.
In the air type system, there may be a reversing valve. This will allow the cooling system to extract unwanted heat from the building and transfer it to the antifreeze-water mixture. This water will then be pumped around pipes in the ground and transfer heat to the ground, making it cooler and ready to capture more heat.
Water source heat pump
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| Water source heat pump - open and closed circuit type |
Water source heat pumps come in two main variations. Closed and open loop. The closed loop takes thermal energy from a pond or river and sends it to the heat exchanger. The open loop draws water from an aquifer or river and pumps it to the heat exchanger.
In a closed loop type, a mixture of water and antifreeze circulates around the pipes to collect heat energy and take it to the heat exchanger where the refrigeration system will absorb the energy and use it for heating. Alternatively, it will dump unwanted heat from the buildings into the antifreeze water mixture to cool the building. So the unit works in the same way as a ground source heat pump.
In an open loop type, water is drawn through a pump and sent directly to the heat exchanger. The heat exchanger then extracts heat energy from the water or rejects unwanted heat back into the water. The water then passes through the heat exchanger and returns to the source, some distance away.
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