Heat pumps are becoming increasingly popular, but when it comes to choosing between a ground source heat pump and an air source heat pump, there is quite a lot of confusion. In this article, we will highlight the advantages of opting for a ground source heat pump and will address some of the common misconceptions.
The elephant in the room
Before we delve into the benefits of having a ground source heat pump, let’s first address the elephant in the room: capital expenditure or CAPEX.
Ground source heat pumps (GSHPs) have a reputation for being significantly more expensive than their air source (ASHP) counterparts, as you not only have to invest in the heat pump itself, but also in an expensive geothermal source. While it is true that GSHPs typically involve higher upfront costs, especially for single-family homes, this is not always the case.
- Increasingly, in neighbourhoods or city centres, strict noise regulations apply during certain hours. In some cases, the noise from an ASHP may require additional noise mitigation measures. These can be quite costly, particularly for larger projects, which reduces the difference in investment cost between the two systems.
- The largest portion of the investment is usually the geothermal system. For larger projects, or for fifth generation district heating, this system can be centralised, which lowers the investment cost for each individual user.
That said, if a GSHP still turns out to be more expensive than an ASHP in your particular situation, we have the rest of this article explaining other reasons why a geothermal solution may still be the better choice.
Advantages of a ground source heat pump
The advantages of a ground source heat pump can be divided into two categories: individual reasons (why you, as a building owner, would choose a GSHP) and collective reasons (why we, as a society, should support the use of GSHPs). All the reasons are listed below, starting with the individual ones. The final two points, which concern the urban heat island effect and the issue of grid congestion, are collective reasons.
Higher efficiency
It is often said that GSHPs have a higher efficiency than ASHPs and therefore have a lower operating cost. This reasoning is not entirely straightforward, so we will discuss the efficiency claim separately from the argument about operating costs, which is covered below.
The efficiency of any heat pump is determined by the temperature lift. This refers to the temperature difference between the heat source (either the ground or the air) and the required supply temperature, which is typically between 35°C and 55°C. The greater this difference, the lower the efficiency.
Since the ground temperature is usually higher than the air temperature, a ground source heat pump does indeed achieve higher efficiency. This higher ground temperature can be explained by the fact that the ground, in contrast to the outside air, is not simply used as a source of heat but acts more like a heat battery. By storing heat in the ground during the summer months through cooling (as discussed further below), the ground temperature increases. This stored heat can then be used in winter to provide more efficient heating.
The higher efficiency of GSHPs is also reflected in the seasonal coefficient of performance, or SCOP, which is officially provided in all datasheets. In the past, the efficiency difference was quite significant, although today the gap has become smaller. However, a performance difference still remains because heat transfer from a fluid to the refrigeration cycle inside the heat pump is easier than from air. Furthermore, ASHPs require a defrosting cycle, which also impacts performance, as discussed later.
!Let op
One should be careful when comparing SCOP values for different technologies. The default condition of B0/W35, under which the efficiency of the heat pump is usually stated, often underestimates real-life performance. This is because, if the borefield is sized correctly, the average fluid temperature will generally be much higher than 0°C.
In certain areas, such as cities or regions affected by climate change, the statement that the ground is on average warmer than the air does not always hold true. In practice, it is possible for an ASHP to achieve a higher average SCOP than a GSHP. However, this does not necessarily mean that it is cheaper to operate.
Operational cost
Nowadays, more and more electricity suppliers are moving away from a single fixed electricity price towards dynamic tariffs, where the price can vary from hour to hour. This shift is driven by the increasing share of intermittent electricity sources, such as solar and wind energy.
Electricity prices tend to rise when demand is high but production is low. This typically occurs in winter, when there is limited sunlight and periods of low wind speed. During these colder conditions, an ASHP consumes more electricity than a GSHP, resulting in higher electricity costs.
In the milder periods of the year, such as spring and autumn, when the air temperature is similar to the ground temperature, the difference in efficiency between the two systems may disappear. However, since electricity prices are not critical during these periods, this becomes less relevant. Therefore, even if seasonal efficiency appears to be similar, the operational cost of a GSHP will be lower because its efficiency remains higher during the most critical and costly periods.
Defrosting cycle
ASHPs extract energy from the air. However, when it is very cold outside, moisture in the air can freeze onto the heat exchanger, effectively blocking it. To prevent this, ASHPs have what are known as defrosting cycles. During these cycles, the unit periodically reverses its operation to heat the heat exchanger and melt or evaporate any frozen moisture.
This defrosting process consumes electricity and can lead to significantly lower seasonal efficiency, especially in colder climates. Furthermore, during the defrosting cycle, the ASHP cannot supply heat to the building. Any heat demand during this period must therefore be met by a buffer tank, an electric resistance heater, or a backup ASHP in the case of larger systems.
Since GSHPs operate with a fluid in their primary circuit, this defrosting issue does not occur.
!Let op
This behaviour does not apply to traditional chillers and only affects heat pumps. Chillers only release heat into the environment, which means the temperature in the heat exchanger is always higher than the air temperature, so freezing does not take place.
Effective cooling
We have already mentioned the advantage of having seasonal thermal energy storage when working with a GSHP. This also contributes to improved cooling performance.
First of all, ground source heat pumps offer the possibility of using passive cooling. In this mode, no compression is required, and the cold geothermal fluid is used directly to cool the building. This results in what is often referred to as near-free cooling. In warmer climates, however, it is also possible to rely on active cooling. In this case, the heat pump uses the compressor to extract heat from the building, similar to how an ASHP operates.
The main difference is that, with a GSHP, this heat is not released into the environment but is instead stored in the ground for use in winter. This creates a more efficient overall system. In addition, the efficiency of active cooling with a geothermal system is generally higher than with an air-based system, as ground temperatures are usually lower than ambient air temperatures during the cooling season.
!Let op
In practice, both active and passive cooling can be combined when using a geothermal system. More information on this topic can be found hier.
Lifetime and maintenance
When calculating the total cost of ownership (TCO) for both an ASHP and a GSHP, it is important to take into account the expected lifetime and maintenance costs. ASHPs typically have a lifetime of between 15 and 20 years, whereas GSHPs can last for around 25 years.
The reason is straightforward. ASHPs are installed outdoors and are therefore exposed to various weather conditions, such as wind, snow, rain and hail, which contribute to wear and tear. The repeated freezing and defrosting of moisture on the heat exchanger also accelerates material degradation, making this type of heat pump more prone to leaks. Additionally, when an ASHP is installed near the coast, the aggressive and corrosive air containing sea salt must be considered, as it can significantly shorten the system’s lifespan.
When this shorter lifetime is taken into account, the potentially higher initial cost of a GSHP becomes noticeably lower over time. Moreover, the geothermal part of the investment can easily last up to 50 years, provided it is accurately designed (see below).
Aesthetics and noise
Since GSHPs are installed entirely indoors, there is no visible unit outside the building. Although subjective, this often results in a more premium appearance. In addition to the aesthetic advantage, GSHPs are also quieter than air source systems, as there is no fan required to move air over the heat exchanger.
Over the years, ASHPs have become reasonably quiet, but if you live in a neighbourhood where many are in use, the sound can still be noticeable.
Urban heat island effect
The reasons above were individual ones: why should you, as a building owner, choose a GSHP? In addition to these personal benefits, there are also collective advantages, where choosing GSHPs is more beneficial for society as a whole than a widespread reliance on ASHPs. The issue of grid congestion will be discussed next, but first, let us focus on the urban heat island effect.
The urban heat island effect, as shown above, is the phenomenon where city centres and densely built neighbourhoods experience significantly higher temperatures than rural areas, with differences of up to 5 to 10 degrees Celsius on average. This is mainly due to materials such as concrete and road surfaces, which retain heat and gradually warm up the surrounding environment. Various sources of heat within the city, such as vehicles and heat pumps operating in summer, contribute further to this issue.
When every individual house owner installs an ASHP, the outside air will only increase in temperature, making summers even warmer and forcing your neighbours to also install a cooling system, making the outside air even warmer. In addition to that, since it is now warmer outside, more heat enters the building, so the ASHP has to work harder, pumping more heat into the environment. It is a negative, vicious cycle.
With (collective) GSHPs, this heat is not dumped into the environment, but stored in the ground. This can help balance the temperature swings in the outside air, since from summer to winter, only energy from the ground is used, instead of using the city air as a heat or cold dump.
!Let op
In regions where there is a significant cooling demand, dumping all the heat solely into the ground can cause problems in the long term. Therefore, it is advised to use some form of regeneration to cool down the ground in winter, so it can cope with the higher cooling demand in summer. This highlights the importance of correct design of the geothermal borefield.
Grid congestion
With the push towards electrification of our industries, transportation, and HVAC systems, the electricity grid is under considerable stress. Taking into account the intermittent nature of solar and wind electricity production, some regions may face congestion problems, where it is no longer possible to connect new buildings to the electricity grid or where the maximum allowed power is limited.
The situation described above applies to the Netherlands. As you can see, there are already quite a number of regions where there is a problem with the available capacity on the electricity grid.
When choosing between an ASHP and a GSHP, this is an important aspect to consider. At critical times, as already discussed above, a GSHP is more efficient than an ASHP, reducing the peak demand on the electricity grid and allowing for more connections. As a society, if we want to move towards renewable heating and cooling, opting for a GSHP is more beneficial than an ASHP when grid infrastructure investments are taken into account.
The importance of design
All the advantages mentioned above, from seasonal thermal energy storage to the lower operating costs of a GSHP, are of course dependent on a well-designed geothermal source. It is important to simulate using the correct assumptions for the building load (more information on that topic can be found hier), to account for interference between nearby buildings, to accurately model the borehole thermal resistance and the hydraulic behaviour of the borefield (see this article for more information), and most importantly, to use state-of-the-art software such as GHEtool Cloud to simulate your borefield.
Conclusie
This article has discussed the advantages of choosing a ground source heat pump instead of an ASHP, despite the sometimes higher initial investment cost. Both individual benefits, such as longer lifetime, improved aesthetics, reduced noise, and lower operating costs, were considered alongside broader societal advantages, including the mitigation of the urban heat island effect and relief of grid congestion.
It is important to note that, in order to fully realise the potential of a GSHP, the geothermal borefield must be accurately designed. Software tools such as GHEtool are essential for achieving this.
Referenties
- Bekijk onze video over dit artikel op onze YouTube pagina hier.
Nederlands 
English (UK) 
Deutsch