Which energy source to choose for your heat pump: air source, water source, ground source or geothermal water source?

Choosing the right energy source has a significant impact on its efficiency, installation costs and monthly costs. Choosing between an air source heat pump, a water source heat pump, a ground source heat pump, and a geothermal source heat pump is, therefore, one of the key factors when deciding about a particular method of heating and cooling.

In this article, we will look at what each of these energy sources requires to be suitable for use and how efficient your heat pump should be when using them.

1.) What energy sources do heat pumps use?
2.) Choosing an air source heat pump
3.) Choosing a ground source heat pump
4.) Choosing a water source heat pump
5.) Do you need a water storage tank?

What energy sources do heat pumps use?

A heat pump should operate automatically without almost any manual work by its user. However, in order to do that, it has to be properly installed and its installation varies from an energy source to energy source.

Heat pumps can usually use three types of energy sources:

• air,
• water,
• land or the energy of the earth’s crust.

Heat pumps using a ground source or a geothermal water source are the most efficient ones, as these two energy sources are always available and their temperature is always higher than 0 °C. Heat pumps that use these two geothermal energy sources are called geothermal heat pumps.

Many countries have rather favourable areas for the exploitation of shallow geothermal energy sources and the demand for such heat pumps is growing. The use of geothermal energy sources is quite specific and we will discuss it in more detail soon.

But let’s start with the energy source that always surrounds us – air.

Choosing an air source heat pump

Air source heat pumps still dominate the market. But they have quite a few shortcomings.

When using outside air as an energy source that heats or cools a building, you must pay attention to:

• System efficiency- Heat pumps that use air for heating and cooling are usually not very efficient – especially when temperatures drop below zero or on hot summer days when they rise above 30 °C.Ž
  That’s when such heat pumps consume a lot of additional energy for satisfactory heating or cooling, which of course increases the monthly costs.
• Noise. If the heat pump consists of an outdoor and an indoor unit, you must be careful that the fan in the outdoor unit does not cause too much noise and becomes an annoyance.
  Due to large temperature fluctuations, air is an inefficient energy source during some periods. That’s why it’s a good idea to see if a water source, ground source or a geothermal water source heat pump is a viable option for your building.

Choosing a ground source heat pump

Ground source heat pumps are extremely efficient, as the temperature of the ground at a suitable depth is relatively constant throughout the year – between 10 °C and 13 °C.

Ground’s energy can be used by installing either a horizontal or a vertical system. A ground source heat pump always uses a closed circuit loop with an energy transfer medium such as water or brine.

A closed loop system means that the medium, which transfers the energy of the ground, circulates through pipes or heat collectors. It goes into the ground and then back to the heat pump on the surface. On its way through the pipes, it heats up or cools to ground temperature, and then transfers this energy to the heat pump (heating) or from the heat pump to the ground (cooling). The process of heating or cooling lasts as long as the building needs heating or cooling.

If we want our heat pump to be efficient, we must ensure that the pipes in which the medium circulates are made of a material that conducts energy as efficiently as possible.

Collector pipes can be laid into the ground in two ways:

Horizontally

The horizontal placement of the collectors is generally the most cost-effective option for houses and buildings that have a larger area of ​​land around the building.

The advantage of a horizontal system is that it is not necessary to drill deep into the ground. The pipes are laid in shallow trenches, about 1.2 to 1.6 meters below the surface. Although the ground there does not yet have a constant temperature, the fluctuations are still significantly smaller than the fluctuations in the outside air temperature.

The most common layouts are:

• a parallel layout, where the loops run side by side at the same depth,
• a two-layer layout, where two pipes are laid at different depths so that one layer is below the other. 

Optimal ground energy efficiency is achieved by looping the pipes at a distance of approximately 0.7 meters.

Vertically

A vertical system is suitable for more densely populated areas or for larger buildings, which would require a vast surface area for a suitable horizontal system and would therefore have a significant impact on the environment.

Vertical collector pipes are inserted into wells that can be from 30 to 150 meters deep -in some cases even deeper. Such collector pipes are also called geoprobes.

A geoprobe consists of two or four tubes with a diameter of approx. 10 -16 cm. The two tubes are connected at the bottom of the geoprobe and form a closed U loop. The geoprobe enables the operation of a ground source heat pump in relatively stable temperature conditions, which is important for high energy efficiency and long service life.

The space between the walls of a well and each geoprobe is filled with a well-conducting material that simultaneously fixes the geoprobe and enables close contact with the surrounding soil. By circulating the heat medium in the loops of the geoprobe, the heat pump efficiently utilizes the energy potential of the ground (approx. 10 °C). This energy can be raised up to 65 °C by compressing and expanding the transfer medium in the heat pump.

Choosing a water source heat pump

Water is a very efficient heat pump energy source. It is possible to use nearby surface water sources, such as a pond or a lake (if the legislation allows it) or groundwater, which is an even more efficient source due to the relatively constant temperature.

Surface water sources

If we want to use surface water as an energy source, it is necessary to take into account the volume or flow and depth of the source, as not all water sources are suitable.

It is recommended to use a closed loop system, where the pipe loops are laid at least 2.5 meters below the water level. This avoids water freezing around the pipe, which would prevent heat transfer and cause the heat pump to operate inefficiently.

A geothermal water source

Groundwater is an excellent energy source, as it has a relatively constant temperature, and at the same time is a good energy transfer medium.

To reach groundwater, you need a few 10 or even 100 meters deep well. The exact depth depends on the geological conditions and the groundwater level.

A geothermal water source heat pump usually uses an open system. This means that the groundwater is pumped through a collector pipe from one well to the heat pump and, after the energy is transferred, it is returned to the groundwater via another well.

Before using groundwater, it is necessary to perform flow measurements and groundwater analysis, as the water must be suitable for such use.

Do you need a water storage tank?

Let’s see when we need a water storage tank when using water as a heat pump’s energy source.

 A water tank allows for a smaller number of compressor starts and a more even heating water temperature, so it is recommended to have one, but it’s not always mandatory.

However, you do need a water storage tank in the following cases:

• When using an air to water heat pump, as the water tank serves as a heat source when you have to defrost the evaporator.
• When each room has its own thermostat connected to the floor heating distribution box. Without a water storage tank, the water flow in the system may be too low, which hinders or even disables the operation of the heat pump.
• When radiator systems are regulated by mixing valves because you want to control the heat supply to an individual part of the building. This is also a case when the water flow may be too low.
• When the heat pump is installed outside the building and contains all the vital components – a so-called monobloc heat pump. If such a heat pump is not frequency-controlled, you need a water storage tank inside the building.
• When the heat pump is connected to a system combined with a solid fuel boiler or solar collectors. In this case, the heat from all sources should be combined in the water storage tank, and the heat pump should control the temperature of the water in the storage tank. If other sources do not provide enough heat, the heat pump is switched on to reheat the water, otherwise, it is idle.
• When the heating or cooling hydraulic system does not provide sufficient flow through the heat pump.

We hope that the article helped you understand why different energy sources are important, especially when installing the heat pump and aiming for maximal energy efficiency.

In the next and last article of the series on heat pumps and geothermal energy, we talk about sanitary water heat pumps.