Open-loop Ground Source Heat Pumps - Sustainable heating and cooling from water within the ground

As the UK seeks to decarbonise and employ low carbons alternative, such as heat pumps. One technology that can help to accelerate this process is open-loop ground source heat pumps.

1. What is an Open-Loop System?

An open-loop ground source heat pump is a type of ground source heat pump, extracting heat from an aquifer, upgrading it to higher temperatures within a refrigerant cycle and then delivering it to buildings where it can be used for heating or hot water generation. The process can also work in reverse to provide cooling, taking heat from the buildings and storing that heat back into the ground.

To access the aquifer, a pair of wells is drilled – also known as boreholes - one to serve as an abstraction borehole using a submersible pump and the other to reinject the water back into the ground. For large systems, multiple pairs of wells can be drilled.

2. How do they differ from other ground source heat pumps?

A more common type of ground source heat pump uses closed-loop boreholes. A cooled water-based solution (often mixed with antifreeze) is circulated within a closed circuit of pipes under the ground. Heat is collected from the soil via conduction and transferred via the closed circuit to the heat pump.

In open-loop systems water is pumped from depth and passed through the heat exchanger of a heat pump. Heat from the aquifer is transferred to the heat pump’s refrigerant circuit. The water – that is now cooler – is then returned to the ground. This means there is no water consumed by the process or mixed with any chemicals – the only difference is the water returned to the ground is slightly colder than it was when abstracted, or for a system used for cooling purposes, the water will be slightly warmer at the end of the process.

3. Where can they be implemented?

In the UK, there are many large aquifers close to the surface. A good example is the London Aquifer. Whilst the surface of London is typically a layer of clay, below this there is a thick layer of chalk, a very porous rock that holds lots of water, ideal for an open- loop system. The site geology is a key factor in suitability for open-loop.

4. Who are they most suitable for?

The main advantages of an open-loop system are:

High COP over the year – as the water is abstracted from a source below ground, it is a constant temperature of around 12°C throughout the year, meaning achieving COPs of 3-4 from such a system is likely. This benefits sites where high temperatures are required, for example retrofits, where an ASHP would struggle to match the open-loop performance in Winter.

Low surface footprint – compared to the equivalent output of an air source heat pump, less space is needed. A pair of wells, one for abstraction and one for rejection are required normally about 100m apart. The boreholes are accessed with belowground chambers with a manhole cover. The heat pump can be housed within existing boiler plantrooms.

Low cost at scale – for larger systems an open-loop system is often more cost effective than a closed-loop system, as whilst the boreholes that are drilled may be deeper, far fewer are required. This means that for retrofit heating systems with limited space an open-loop system is often the perfect choice – providing the geology is suitable.

5. How can their challenges be overcome?

The drilling technology required is well established and the water abstracted from the ground generally requires very little filtration to be suitable heat extraction.

One challenge that open-loop schemes face is uncertainty about their yield until a borehole is drilled and tested. To mitigate against this risk, historic records of nearby boreholes can be researched and the geology examined for suitability. Whilst an initial exploratory well can be drilled it can be best to drill a full size well, as this will provide more certain information about the yield and also can be used as part of the system later.

To ensure resilience of supply, each borehole can be equipped with a submersible pump, so that in the case of pump failure the flow direction can be reversed.

It is important to keep each well sufficiently distant from one another so that they do not hydraulically or thermally influence each other. An approximate distance of 100m is required, and on most sites, this is possible to find.

The main challenge to open-loop schemes can be the high upfront cost – however for a well considered scheme the superior performance over time can make them cheaper over the lifetime of the system than alternatives.

A licence from the Environment Agency is required prior to drilling. However, the process is well established, and the technology is looked upon favourably, especially where heat is being abstracted from aquifers that may be suffering from warming.

6. Is an Open-loop ground source heat pump the right choice for your project?

Open-loop ground source systems are best considered where a large demand is present, geological conditions suit abstraction and enough separation can be found between the boreholes. If the site is suitable, this technology can provide discrete, efficient heating/cooling with low operating costs.

Frequently Asked Questions About Open-Loop Ground Source Heat Pumps in the UK

What is an Open-Loop System?

An open-loop ground source heat pump extracts heat from an aquifer, upgrades it via a refrigerant cycle, and delivers it to buildings for heating or hot water. It can also provide cooling by storing building heat back into the ground. Water is drawn from an abstraction borehole and returned via a reinjection borehole.

How do they differ from other ground source heat pumps?

Unlike closed-loop systems, which circulate water in sealed pipes underground, open-loop systems pump water directly from an aquifer through a heat exchanger. The water is returned to the ground slightly cooler (or warmer if used for cooling) without chemicals, offering efficient heat transfer and minimal resource consumption.

Where can they be implemented?

Open-loop systems are suitable where large aquifers are near the surface, such as the London Aquifer. Site geology is crucial; porous rocks like chalk store sufficient water for heat extraction, making the technology ideal for suitable UK locations.

Who are they most suitable for?

Open-loop systems are ideal for sites with high heating demands, retrofits requiring high temperatures, and areas with limited space. They offer high COPs (3–4), a low surface footprint, and cost advantages at scale compared to closed-loop or air source systems.

How can their challenges be overcome?

Challenges like uncertain aquifer yield, pump failure, and high upfront costs can be mitigated by drilling exploratory wells, using multiple submersible pumps, ensuring borehole separation of around 100m, and carefully assessing geology. Licences from the Environment Agency are required, but technology is established and efficient.

Is an Open-Loop Ground Source Heat Pump right for your project?

Open-loop systems are suitable for projects with large heating or cooling demands, appropriate geology, and enough space for borehole separation. When conditions are right, they provide efficient, low-cost, and low-carbon heating and cooling solutions.