How do you heat your home or provide hot water for your business or family? Do you use electricity to drive electric heating systems to generate heat? Or do you use LPG, gas, or other systems that need fossil fuel you store on-site to generate heat? Either way, you’ve probably considered alternative methods before or, since you’re reading this, you’re considering them now?
And it’s easy to see why you would when you consider the ever-increasing heating costs. Another problem is that many of the heating systems we mentioned earlier rely on fuels that cause harmful emissions and pollution. This is an especially relevant consideration because to reach the UK’s carbon emissions targets, we’ll need to reduce our emissions by 95% over the next 30 years.
One of the best ways to do this is by using ground source heat pumps for heating. Because they rely on renewable energy, they’re far kinder to the environment and, as an additional advantage, they’re efficient too. Let’s look at how ground source heat pumps work and how you can calculate how efficient they are.
What Is a Ground Source Heat Pump?
A ground source heat pump is a heating system that’s able to extract solar energy stored in the ground and compress it to increase its temperature. In this way, ground source heat pumps not only provide renewable energy but can provide you with all your hot water and heating consistently.
How Does a Ground Source Heat Pump Work?
Now that you’ve seen what ground source heat pumps are, let’s dig a little deeper into how they work. The best analogy to illustrate how they work is by thinking of your fridge. The heat pump in your fridge transfers heat out of it and disperses it at the back of the fridge using a heat exchanger.
So, at their core, ground source heat pumps harness natural heat stored in the ground by using underground water pipes that absorb the heat, then compressing it using the heat pump, and providing efficient heating to your home. Let’s now look at the process ground source heat pumps use in more detail.
Step 1: Ground Heat
The entire process starts with the sun heating the surface of the ground. This heat then transfers deeper into the subsoil. Because this process takes place slowly over time, the ground temperature doesn’t experience as many fluctuations and variations as air temperature due to changing weather.
In fact, in the UK, the ground temperature stays stable at between 8°C and 12°C throughout the year, irrespective of the season or weather. Ultimately, this means that ground heat can provide a reliable and efficient source of heating year round.
Step 2: Heat Absorption by the ground loop
During the next step of the process, the system absorbs the ground heat mentioned above. For this, it uses the ground loop, which is a network of pipes installed underground. Based on your specific needs, requirements, available land, and ground conditions, you can lay these pipes out in various configurations.
Either way, these pipes contain a cold mix of water and anti-freeze, which is constantly pumped through them. Because heat naturally flows from hot to cold, heat then transfers from the warmer soil to the colder fluid. In this way, the fluid becomes warmer as it circulates under the ground.
Step 3: Heat Transfer in the Evaporator
Transferring the heat from the fluid mentioned above is the next step. The heat pump which comprises four separate components that each perform a specific function does this. The first of these is the evaporator, which is, using the example above, a heat exchanger similar to the one you’ll find in your fridge.
The heated fluid in the ground loop is pumped into the evaporator. Here, the evaporators separation plates separates the heated fluid from a refrigerant. These plates permit the transfer of heat from the heated fluid to the refrigerant, which is a cold mixture of liquid and vapour. As this mixture heats up, it starts to boil and, as a result, turns into a gas. This gas then has the same pressure as the original refrigerant, but its temperature is raised slightly.
Step 4: Increasing the pressure using the Compressor
Once the refrigerant is turned into a gas, its fed into the next component of the heat pump, the compressor. The compressor is an important part of the entire system. For one, it acts as a pump so that fluid can circulate through the system. Second, it heats the refrigerant sufficiently by using pressure to provide enough heat that will then be transferred to the building.
As mentioned, the compressor increases the pressure of the gas significantly. As a result, its temperature also increases substantially. It’s important that only gas enter the compressor, as any liquid can damage it. That’s why the evaporator plays a vital role in the heat pump system.
Step 5: Heat transfer in the condenser
With its temperature increased, the gas then flows into the condenser. It’s a heat exchanger similar to the evaporator and also has a set of heat transfer plates that separate the gas from water. As before, and because heat flows from hot to cold, the heat transfers to the water. This heats the water to a temperature capable of providing a home or building with heating or hot water.
Coefficient of Performance
It’s worth mentioning that the amount of heat the condenser puts out depends on the heat pump’s Coefficient of Performance or COP. This COP indicates the efficiency of the heat pump. In other words, it shows you how much heat you’ll get from the heat pump in relation to the energy you put in. For example, if, for instance, a heat pump has a COP of 2, you’ll get 2 kW of heating energy for every 1 kW of electrical energy to run the pump.
To calculate the coefficient of performance of a heat pump, you’ll need two figures:
• The heat output of the condenser or Q.
• The energy supplied to the compressor or W.
You will divide the condenser's heat output (Q) by the power supplied to the compressor (W):
COSP = Q/W
The answer to this equation will show you the relationship between the energy output and energy input, or, in simpler terms, how much heat the heat pump will put out for every unit of energy used. This means that, the higher the COP, the more heat you’ll get for the electrical power you supply, which, in turn, means that the system will be more cost-efficient.
For example, a 1 kW heat pump with a COP of 3.5 will heat almost 40 litres of water in an hour, while a 1 kW heat pump with a COP of 2 will heat only about 24 litres.
When calculating the COP for a heat pump, there are some things you need to keep in mind. For one, because you express COP without units, you’ll need to use the same unit for both input and output energy when doing the calculation.
Also, you should distinguish COP from COSP, which, in contrast, shows you the relationship between the amount of energy and the amount of work done. For example, CSOP will show you how much energy it takes to produce 1 kW of heat.
Step 6: Repeating the cycle
When the heat transfers from the refrigerant to the water during the above process, the refrigerant becomes colder and goes back to its liquid form. However, before the pump can pump it back to the evaporator to start the process over, the liquid needs to cool down sufficiently to allow it to absorb heat from the fluid coming from the ground loop. This is where the expansion valve comes in.
It reduces the pressure of the liquid and it decreases the liquid’s temperature. As this happens, the refrigerant turns into a mixture of liquid and vapour again. Once done, the refrigerant is ready to start the process again.
Step 7: Heat Distribution
The distribution system collects and stores the water that was heated by the condenser. . From here, the distribution system can pump the water throughout your home or building through a network of pipes for underfloor heating or to radiators. You can also use the stored water for taps, showers, and baths.
The Bottom Line
Ground source heat pumps are excellent at heating your home or building in a way that’s kinder to the environment, lighter on your pocket, and more efficient. Hopefully, this post helped illustrate how they work and and how you can calculate the efficiency of a heat pump.