In this article, we’re going to be discussing heat pumps, the different types, and how they work. Coming up: How heat pumps work, air to air heat pumps, air to water heat pumps, ground-source heat pumps, water source heat pumps, as well as animations and systems schematics for each of these. I just want to take a moment to thank our partner Danfossfor sponsoring this article. A critical aspect of heat pumps is how energy-efficient they are, and Danfoss has everything you need to make sure your heat pump is running at what they call 360degree energy efficiency.
These can either work as a heating-only device or, alternatively, the more popular choice is to have a device that can provide both heating or cooling by making use of the reversing valve. We’ve covered reversing valves in our previous article. There are a few different ways to configure a reversing valve heat pump. I’ll show you a simple, typical example. The main components we’ll have in this type of system are the compressor, the reversing valve, the indoor heat exchanger.
An expansion valve with a non-return valve bypass, a bi-directional filter drier, a sight glass, another expansion valve with a non-return valve and bypass. Then we have the outdoor heat exchanger. We also have a controller and a number of temperature and pressure sensors around the system. In heating mode, the refrigerant leaves the compressor as a high-pressure,high-temperature vapor and passes to the reversing valve. The reversing valve is positioned in heating mode, so the refrigerant passes through this and heads to the indoor unit.
Cool air is then blown over the indoor unit’s heat exchanger to remove some of the thermal energy and provide heating to the room. As heat is removed, the refrigerant will condense into a liquid. Having given up some of its energy, the refrigerant leaves as a high-pressure, slightly cooler liquid. The refrigerant then comes to the expansion valve and bypass. In this mode the expansion valve is closed, so the liquid refrigerant passes through the non-return valve. It then passes through the filter drier and sight glass and then passes to the second expansion valve.
It will then pass through this expansion valve because the non-return valve on this side is preventing flow in that direction. As the refrigerant passes through the expansion valve, the refrigerant expands in volume and turns into a part-liquid, part-vapor mixture. This expansion in volume reduces the temperature and pressure. We’ve covered how thermal expansion valves and electronic expansion valves work in great detail in the previous article.
A fan is blowing outside ambient air over the coil and adding heat to the cold refrigerant. The refrigerant boils at a very low temperature, and as it boils it will carry away the thermal energy. As an example, we know that water will carry thermal energy away as steam when it boils, and we know that it boils at 100 degrees Celsius or212 degrees Fahrenheit. Well, if we then look at some common heat pump refrigerants, R134a for example, has a boiling point of minus 26.3 degrees Celsius or minus 15.34 degrees Fahrenheit. Refrigerant R410a has a boiling point of minus 48.5 degrees.
Celsius or minus 55.3 degrees Fahrenheit. So, it’s very easy to extract thermal energy even at very low outdoor temperatures. We’ve covered how refrigerants work, also in the previous article. So the refrigerant picks up the thermal energy from the outside air and leaves the outdoor heat exchanger as a low-pressure, low-temperature, slightly superheated vapor, and then heads back to the reversing valve. The reversing valve then diverts this to the compressor to repeat the cycle.
If this system is then switched into cooling mode, the system then acts like a normal split air conditioner. The compressor forces the high-pressure, high-temperature vapor refrigerant into the reversing valve The reversing valve diverts this to the outdoor unit. The fan of the outdoor unit blows ambient air across the heat exchanger. This air will be at a cooler temperature, so it carries the thermal energy of the refrigerant away. The refrigerant condenses as it loses its thermal energy. Having given up some of its energy, the refrigerant leaves as a high-pressure, slightly cooler liquid.
It then heads to the expansion valve, but this is closed, so the refrigerant passes through the non-return valve. It then passes through the sight glass and the bi-directional filter drier. The next non-return valve is then closed, so the refrigerant passes through the expansion valve. As it passes through the expansion valve, the refrigerant changes to a part-liquid, part-vapor mixture, which causes it to drop in pressure and temperature. It then flows into the indoor heat exchanger. And in here, a fan blows the warm indoor air over the coil.
This causes the heat to transfer from the air into the refrigerant, and so the refrigerant boils and takes its heat away. The refrigerant leaves the indoor unit in a low-pressure, low-temperature, slightly superheated state and flows into the reversing valve. The valve diverts this back to the compressor to repeat the cycle. Air to water heat pumps: These units work in a very similar manner to air heat pumps but without the reversing valve. The high-pressure,high-temperature vapor refrigerant leaves the compressor, but this time it heads into a plate heat exchanger.
On the other side of the plate heat exchanger, water will have been cycled through a hot water storage tank. Cooled water enters the heat exchanger from the tank, and as it passes through the heat exchanger, it absorbs heat from the hot refrigerant. The water will then leave a much hotter temperature and flow back to the hot water storage tank to repeat this cycle. As the refrigerant gives up its heat to the water, it will condense and will then leave the heat exchanger as a high-pressure,lower-temperature liquid. We’ve covered how heat exchangers work in our previous article.
The refrigerant then passes through the filter drier and the sight glass, and then into the expansion valve. The expansion valve causes the refrigerant to become a part-liquid, part-vapor state. It’ll be at a low temperature and pressure. It then passes through the outdoor heat exchanger, where the outdoor ambient air causes the refrigerant to boil. The refrigerant then leaves at a low-pressure, low-temperature, slightly superheated vapor and is then sucked back into the compressor to repeat the entire cycle again.
The hot water tank then provides hot water to the radiators, sinks, and showers within the building. Ground source heat pump: There are two main types of ground source heat pumps, that being the horizontal and the vertical type. Both essentially work the same, it’s just how they access the heat in the ground that varies. We’ll look at when to use the different types as well as the pros and cons of these in our next article. In this article, we’re just gonna focus on how they work. The ground source can be used for heating air or water. In the air type system.
A heat pump can also have a reversing valve and then supply either heating or cooling. In both cases, the outdoor heat exchanger can be a plate heat exchanger with the refrigerant passing on one side and a mixture of water and antifreeze cycling on the other side. The water and antifreeze mixture is forced by a pump around the pipes within the ground. This will allow it to pick up the thermal energy in heating mode and bring this to the heat exchanger. The refrigerant on the other side of the heat exchanger absorbs the heat because it has a very low boiling point.
As it boils it carries the heat away, which can then be used within the building. In the air type system, there can be a reversing valve. This will allow the refrigeration system to pull unwanted heat out of the building and transfer this into the water-antifreeze mixture. This water will then be pumped around the pipes in the ground, and it will transfer the heat into the ground, thus returning cooler, ready to pick up more heat. Water source heat pumps: Water source heat pumps come in two main variations, closed and open loop.
The closed-loop sends a mixture of water and antifreeze to collect thermal energy from a pond or river and transfers this through the heat exchanger. The same water is then sent around again to repeat the cycle. Open-loop pulls in freshwater from an aquifer or from a river and pumps this into the heat exchanger to collect the heat. Once it passes through, it is then released back into the same water source. In a closed-loop type, water and antifreeze mixture cycles around the pipes to collect the thermal energy and bring this to the heat exchanger.
The refrigeration system will then would absorb the energy and use it for heating. Alternatively, it will dump the building’s unwanted heat into the water-antifreeze mixture to provide cooling for the building. The unit then works the same as a ground source heat pump. In an open-loop type, the water is pulled in via a pump and sent directly to the heat exchanger. The heat exchanger then pulls the thermal energy out of the water, or it dumps the unwanted heat into the water.