How does a shallow geothermal heat pump work?

Last month we talked about what exactly shallow geothermal energy is and why it is worth using it. This time we will take a look at how a shallow geothermal heat pump works and compare an air water heat pump and a water water heat pump to see which one is more efficient and what are the main differences between these two popular choices.

How does a heat pump work?

Each shallow geothermal heat pump basically consists of three main parts:

• Collectors or heat exchangers, which use a heat transfer medium to obtain energy from an energy source – either air, water or the earth’s crust.
• A heat pump or a hybrid system to which collectors send the medium and where its energy is converted into usable energy that heats or cools the building
• A distribution system that ensures that the building is evenly heated or cooled.

The second law of thermodynamics says that heat naturally moves from a warmer to a colder place. In theory, this means that in winter the building could simply be heated with a system that would allow the heat to naturally pass through it from the ground into the building, as groundwater and the earth’s crust are warmer than winter temperatures. However, in practice, the process is not so simple, as you want to heat the building to a higher temperature than 10 to 13 °C, which is the usual temperature of the aforementioned sources.

So how do we heat a house to 21 ºC and still draw heat from a source that has “only” 10 ºC? Usually with a little help of compression.

Compression in this case means compressing the gas, and thermodynamics tells us that the gas heats up when we reduce its volume and compress it.

Compression heat pumps, therefore, contain a heat transfer medium that evaporates even at low temperatures. When it heats up in heat exchangers, it changes its state of matter and reaches the compressor as a gas, not as a liquid. The compressor compresses it and consequently heats it to the desired temperature. The warm or hot gas then transfers heat to the heating system. With the removal of heat, the heat transfer medium gradually condenses and changes its state of matter back to liquid. It enters the heat exchangers through an expansion valve, which lowers the refrigerant’s pressure and thus allows the refrigerant to boil and the liquid to cool. The heat transfer medium is now ready to repeat the heating cycle and is sent back underground.

For the best possible energy efficiency, it’s crucial to minimize the amount of external energy required for compression or any other heating method used by the heat pump.

The efficiency of the pump depends largely on the heat source. As we shall see, heat pumps that draw heat from the air are in principle the least efficient, followed by heat pumps that draw from the earth’s crust. Heat pumps that use water as a heat source are generally the most efficient.

But before we move on to the air water and water water heat pumps, let’s take a quick look at hybrid devices, which are an increasingly popular solution in the market for energy-efficient cooling and heating systems.

How do hybrid devices differ from standard heat pumps?

Hybrid devices can both heat and cool at the same time, as the device combines a geothermal heat pump and a chiller. As a result, devices such as Menerga RewaTemp enable optimal simultaneous production of heating and cooling energy with the help of renewable energy sources.

The advantages of such devices are:

• Simultaneous production of heating and cooling medium and ensuring the reduction of the required energy input.
• Actual parallel operation – without switching between cooling and heating mode as reversible heat pumps do. Such operation ensures minimal energy consumption of the device.
• Optimal automatic adaptation to the cooling and thermal energy needs of the building, which is possible with the help of automated regulation.
• The integration of the cooling unit, heat pump and domestic hot water heating system in one device ensures a minimal electric power connection.

How does an air water heat pump work?

These heat pumps use outside air as a heat source. The heating process is usually similar to the above description of the compressor heat pump, except that the heat exchange medium is heated by the air and not the water or the earth’s crust. When it comes to cooling, the process is reversed, as the heat exchange medium removes heat from the air inside the building and releases it into the air outside the building, thus cooling the interior.

Air water heat pumps are commonly used for low-temperature heating via radiant floors or radiators. The efficiency of these heat pumps is greatly influenced by the temperature of the heating water we want to achieve. They are quite energy inefficient when heating water to high temperatures, especially during winter.

You may be wondering how heat pumps that use air as a heat source can even heat a building during winter.

Even at sub-zero temperatures, air contains some heat that is useful for heat pumps. Therefore air-to-water pumps can still work and warm up a place when the outside temperature is down to -15 °C. The problem arises if the temperature drops below -15 °C, because then the heat pump cannot get enough heat from the air and is not able to heat the building.

Despite the fact that air-to-water heat pumps are capable of heating even when the temperature is below 0 °C, this does not mean that they are energy efficient. Quite the opposite. At sub-zero temperatures, they are not significantly more efficient than conventional heaters, so they are not a good choice for colder environments and higher altitudes.

How does a water water heat pump work?

A water water geothermal heat pump uses water and its energy as a heat source. A water source can be a lake, pond, river, stream, well, or a borehole that extends to groundwater. The latter is the most common source of heat of a water water shallow geothermal heat pump.

A water water heat pump also uses water as a medium to heat buildings. It converts the energy obtained from the initial water source to a higher temperature and then sends it to the building’s distribution system which delivers it to the radiators or floor heating.

As mentioned in the first article on shallow geothermal energy, the main advantage of groundwater is its constant temperature, which ranges between 7 °C and 12 °C even on cold winter days. This constant temperature means that a water water heat pump can successfully heat the building even on the coldest days. An air water heat pump is not always capable of this or consumes huge amounts of energy for sufficient heating.

Another big advantage of using a water source, especially groundwater, shows itself during the summer or on hot days when the heat pump can passively cool the building. This is also known as natural cooling, which simply lets colder water from the water source through the building’s cooling system and thus cools it.

When installing a water water heat pump, you must pay attention to which water source you choose. Surface water sources can freeze, which prevents the heat pump from doing its job. The chosen water source also shouldn’t be too small, otherwise the operation of the heat pump affects its temperature too much and it becomes energy inefficient. Therefore, you should always consult the experts who are going to install the heat pump when choosing a suitable water source.

Running water sources such as rivers and streams are particularly interesting, but rare choices because of location and legislation limitations. These water sources can be very energy efficient, as the water is constantly changing and the outside temperature or operation of the pump does not affect its temperature too much. Furthermore, a liquid water source is unlikely to completely freeze during winter, unless you live in a really cold area.