Frequently asked questions - United Kingdom

A closed loop ground source energy system is an installation that can be used for sustainable heating and cooling of your house, apartment or business premises. A ground source energy system consists of an above-ground part, with the heat pump (HP) as the main component, and an underground part formed by one or more closed plastic (PE100 quality) loops installed in drilled bore holes, also called ground heat exchangers. These ground source heat exchangers are usually placed vertically, but can also be installed in horizontal fashion.

The underground part of a closed ground energy system (BTES) consists of one or more plastic (PE100) loops, also known as ground heat exchangers (GHEs). These GHEs form a closed circuit in which a fluid circulates, exchanging energy with the ground through conduction. No groundwater is extracted or other fluids exchanged with the ground in this process.


Open ground energy systems, also known as ATES (Aquifer Thermal Energy Storage), are ground energy systems that actively extract and inject groundwater to utilise the heat and cold from it.


Due to the differences between the two types of systems, they are suitable for different types of locations and projects. Generally, ATES are used for larger-scale projects (such as schools or hospitals), while closed loop is better suited for smaller and medium sized projects such as schools, apartments and individual houses. 


In closed loop, cold and heat are stored and extracted within the same ground volume (using the same GHE), while ATES typically have separate (injection and extraction) wells that need to be spaced at least several tens to hundreds of meters apart. As a result, the underground space requirements (thermal effects) are generally smaller for closed loop than for ATES. However, to achieve the same capacity, more boreholes are required for closed loop compared to ATES. When considering the use of an ATES, it is important to consider the quantity (availability of suitable aquifers) and quality (geochemistry) of the available groundwater at a specific project location. 

The practical drilling depth of vertical closed ground energy systems in the current practice in the Netherlands is typically between 40 to approximately 350 meters. Projects that involve drilling deeper than 500 meters fall under the Dutch Mining Laws in the category of "geothermal energy."


Differences between the two categories manifest in factors such as the temperature range produced, the applicable legislation, the required initial investment costs, the requirements and specifications of the necessary above-ground installation, and the type of location and project for which both can be deployed.

The underground part of a closed ground energy system consists of one or more ground heat exchangers (GHE), which are pipes made of PE100 (polyethylene) connected at the bottom with a U-shaped coupling, forming a closed circuit. The GHEs are inserted into a borehole (approximately 15 cm in diameter) drilled to the required depth in the ground.


The borehole around the loop is backfilled to ensure stability and thermal contact with the formation in which it is drilled. If necessary, the borehole is filled with "grout" (a liquid clay/sand mixture) or gradually filled with fine gravel and impermeable clay. The purpose of applying grout or gradually adding clay is to prevent possible vertical mixing of groundwater between the penetrated aquifers over the depth of the bore-hole. 


The ground loop is connected to a heat pump (HP) located above ground in the building via a circulation pump. The circulation pump circulates water (or a mixture of water and antifreeze) through the ground heat exchangers and the heat pump. The heat pump extracts heat from the circulating medium on the evaporator (source) side. The cooled medium is then returned to the ground loop, and as it is now cooler than the ground, it absorbs heat again. This process repeats as long as the heat pump is producing heat for the building. When cooling the building, the system operates in reverse. The heat is released to the ground via the circulating medium in the loop.


The system as a whole, is energy-efficient, sustainable, and circular, as it efficiently converts low-temperature ambient energy into a high-temperature usable temperature within the building and can store seasonally produced cold and heat in the ground. The required electrical energy for the circulation pump and the heat pump can be generated with renewable energy. In summary, dependant on how the electricity for the system is generated, there is little or no CO2 emissions during heating and cooling!

Yes its energy efficient in a sense that it uses a small amount of Primary energy (paid for electricity) to provide a large thermal output by raising the temperature of (free) available ambient energy.


A Ground Source Heat Pump (GSHP) is capable of producing both heating and cooling for a building using the ground to extract heat from when in heating and disposing of heat to the ground when in cooling. Electrical energy is used to run the heat pump compressor and a circulation pump. The ratio between the total generated thermal power by the heat pump (kW) and the consumed electrical power (kW) is the Coefficient of Performance (COP).


For example, a HP that delivers 10 kW of thermal energy (heat or cold) and consumes 2 kW of electrical power to do this has a COP (Co-efficient Of Performance) of 5. The higher the COP, the greater the savings on primary energy (electricity) used and the more energy is extracted/injected to the ground.


The COP depends on the type of HP and the how hard the HP has to work. If the temperature difference between the source and load side of the HP is small, the amount of work required is low, resulting in a high COP. If the temperature difference is large, the COP will be lower, and also the capacity (kW) of the HP will also decrease. If a building is poorly insulated, for example, the heat pump will have to work harder and longer to achieve the desired indoor temperature, significantly reducing efficiency.


In addition, a well-designed Borehole Heat Exchanger (BHE) side of a GSHP is crucial to ensure high efficiency (good source temperature conditions) over a very long period, as the BHE will have a life span of more than 50 years.


When in heating operation, the HP extracts heat from the circulation medium circulating in the BHE, causing the ground temperature around the loops to gradually decrease. To maintain the heat flow from the ground to the circulating medium, the circulating medium will continuously be cooled by the HP. To prevent the fluid temperature from dropping too low over time, which would require the HP to work harder and reduce its efficiency, an adequate number of borehole meters should be installed based on a proper design.


The overall average annual efficiency of a closed ground energy system, considering all operating conditions, is generally expressed as a "Seasonal Performance Factor" (SPF). In the Netherlands, SPF values for closed ground energy systems typically range between 3.5 and 6.0, making a well-designed system significantly more energy efficient than a gas boiler.

A ground-coupled heat pump is sustainable because it utilises both the available renewable ambient energy and the seasonally stored cold and heat in the ground, which is released during heating and cooling processes in the building. During heating, heat is extracted from the ground, and the generated cold can subsequently be used for cooling the building. In this way, the ground acts as a heat/cold battery.


To reap the full benefits of a GSHP system lifespan the ground heat exchanger (GHE) should be designed and constructed for at least fifty years of use. The heat pump itself, as well as other components such as valves and pumps, generally have a lifespan of fifteen years. If the system is deactivated, the original ground temperature will naturally recover through heat conduction from the surroundings or due to the flow of groundwater.


The lower primary energy consumption of a ground-coupled heat pump helps reduce CO2 emissions. Especially when compared to conventional heating and cooling systems based on fossil fuels, the use of a ground-coupled heat pump can significantly decrease the carbon footprint of a building. When the electricity used for the GSHP is generated from fossil free sources such as solar or wind, there are no CO2 emissions.

GSHP systems are economically attractive primarily due to their energy savings, their low maintenance requirements, and their very long lifespan. Another significant financial advantage is that a well-designed ground energy system uses relatively less electricity compared to, for example, an air-source heat pump, which helps alleviate the strain on the electrical grid.

In summary, well-designed and properly installed systems save costs and maintenance while providing annual financial benefits. An added benefit is that an efficient system can limit cost increases for end-users even in the face of rising energy prices, making energy costs manageable. The higher upfront costs are often mentioned as a drawback. However, this can also be seen as an investment in the value of the property.

To improve the market position of closed ground energy systems in renovation projects, new concepts are being developed. One example is the use of closed collective sources for housing blocks or apartment buildings, which helps reduce investment costs.

Lastly, a ground energy system contributes in achieving climate goals and reduces our dependence on imported gas. As an end-user, you require less energy, making you less reliant on commercial market players. While not directly financial, these are significant advantages from a value creation perspective.

A hybrid heat pump system refers to a configuration where the GSHP the heat pump is combined within a single installation with another heating or cooling system such as a gas boiler or a dry air cooler. In such a setup, the closed loop ground source energy system will meet the heating and cooling demand for the majority of the year. However, during peak loads in winter, the gas boiler can be utilised to cover peak demands.


This approach ensures comfort levels are maintained in all conditions while optimising energy savings. Additionally, the required amount of borehole meters for the underground connection of the closed loop system is reduced, which helps limit the initial installation costs of the closed loop system. A hybrid heat pump system serves as a practical intermediate step towards a fully electric solution.

If the number or total length of the ground heat exchangers (GHE) in a closed ground energy system is insufficient in relation to the energy loads the GSHP has to provide to the building, the system will not function properly. The heat pump (HP) will have to work harder, resulting in increased electricity consumption and decreased efficiency. In the worst case scenario, the system thermal capacity (kW) is reduced and it may not be able to provide enough heat (or cold) to bring the building's temperature to the desired (comfort) level.

Therefore, both the above-ground and underground aspects of the system need to be carefully designed and documented in an integrated design process. To allow others to understand and judge the design and the choices made, the information sources used should be traceable, and the assumptions upon which the design is based should be clearly explained.

The design process starts with accurately calculating or estimating the expected energy demand and distribution over the year (heating, cooling, and domestic hot water) and deciding whether this all covered by the GSHP system or a hybrid approach is more suitable.

The next step is to determine what building load temperature requirements there are (heating and cooling) what energy efficiency is required.

Once the above information is available and of acceptable quality to the designer, site conditions, available drilling area and local geology need to be looked into. The borehole wellfield (depth, positioning and number of loops) is modelled and temperature results are compared to design requirements (annual temperature envelope, long term temperature development, temperatures under peak loading).

In summary, to ensure the functionality of a GSHP system over a long period (>30 years), a complete and well-documented design that meets the requirements set by the project is an absolute requirement.

Closed ground energy systems provide in a large part of Europe generally, - on an annual bases - more heating than cooling to the buildings they serve. As a result, with long-term (> 50 years) use of a closed loop system, the soil around a ground heat exchanger (GHE) will experience a net cooling effect.


For an individual closed loop systems, this cooling in the ground is compensated to some extent by the local climatic conditions, the natural heat conduction from the surrounding soil and, if applicable, local groundwater flow. These parameters should already be considered in the design of a closed loop system.


However, if multiple separate closed loop installations are located close to each other they can thermally interact. The cooling of the ground around the GHE of one system can affect the temperature of the ground around the GHE of another system. This phenomenon is called thermal interference.


Why can thermal interference cause problems?

Simply put, a colder ground makes it more difficult to extract energy from the ground, causing the heat pump to work harder than initially anticipated and reducing the efficiency of the closed loop ground source energy system over time.


If therefore a GSHP system is designed as if it is the only system in the area, without considering thermal effects caused by other systems, the design may be inadequate and long term temperatures can be significantly lower than predicted.


New installed systems can, and should, take into account the effects of existing closed loop system during the design phase. However, for previously installed systems, it is not possible to consider systems that were installed later.


Therefore, in the Netherlands, there are regulations regarding the maximum allowed interference at a closed loop ground source energy location caused by the installation of other closed loop systems. This protects existing systems, but the downside is that it can be very challenging or even impossible for new systems to be implemented.


Preventing such problems in densely populated areas in terms of ground energy (e.g., new residential areas) requires, similar to spatial planning, some organization of the subsurface. In some cases, it may be cost-effective to have a ground energy plan prepared in advance, ensuring that all residences have the opportunity to utilise ground energy. It should be realised that thermal effects may take several years (5-10) to become apparent.


In addition to thermal interference between closely located closed loop systems, interference can also occur between well based energy systems (ATES) and closed loop systems in some cases. ATES often have spatial separation between the warm and cold wells, placing a relatively large demand on the ground. In many open well type systems, hydrological and thermal effects are still measurable in the used aquifer at considerable distances (>100m) from the source locations.


The lateral spread of thermal effects in the vicinity of closed loop systems is generally much smaller. However, when a closed loop system is located relatively close to an ATES source (within the thermal influence area), there can still be mutual influence.


In such cases, the "first come, first served" principle applies, and it is the responsibility of the party planning to install a system later to demonstrate that there will be no negative impact on the existing system.

Our clients include commercial entities such as drilling and installation companies, real estate and energy companies, developers, and construction firms. We also work with government entities such as municipalities, provinces, and national government.


When considering ground-coupled heat pump systems, it is crucial to have a good understanding of their long-term performance from the outset. As a certified company, Groenholland has the knowledge and experience to design closed ground energy systems of any size to meet the requirements and desires of our clients.


How does a project become successful?
One of the conditions for success is an indoor distribution system that is suitable for low-temperature heating and high-temperature cooling allowing the GSHP to be highly efficient.


Another important factor for success is an early stage involvement in the project, allowing integration and sequencing with building plans and mechanical services.


Space issues relating to available drilling positions are often conceived as show stoppers, however there are numerous successful projects where the ground heat exchanger has been installed beneath the building. In short, a perceived lack of space does not have to be an insurmountable obstacle!

Read more about our references for further information!