What Heat Source can I use for a Heat Pump?

Ground source heat pumps can extract heat from three types of renewable sources: soil, rock and water. They do this by using buried collectors called ground arrays.

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Heat source #1: Surface soil

Surface soil is usually an ideal source for properties in rural areas, or any building with a fair amount of surrounding land (40m x 12m). This allows room for long and shallow trenches.

Ground array for soil: Slinky pipes or straight pipe

To extract the heat from surface soil, a network of ground collectors, consisting of a series of pipes, is buried in the ground in trenches to a depth of 1 – 2m. Typically consisting of coiled pipe, collectors of this nature are referred to as ‘slinkies’.

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Ground source slinky trenches

Heat source #2: Rock

Rock is a perfect conductor for heat energy, particularly for properties with minimal land such as tower blocks, flats, businesses or homes with limited garden space.

Ground array for rock: Boreholes

Boreholes are the collectors used to extract heat energy from rock for ground source heat pumps. To extract heat from the rock, a U-tube pipe is installed into a drilled hole measuring 60-200m deep by 150mm wide. Boreholes are a very discreet and compact solution when saving space and minimal ground disruption is a priority.

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Borehole and drill rig for ground source heat pump

Heat source #3: Water

Water is the most efficient source of heat transfer and is suitable for any property within 100m. Water sources include lakes, ponds, rivers, the sea, and even aquifers – all of which are successfully used by Kensa to provide heating and hot water to residents on the shoreline (or in boats)!

Ground array for water: Pond mats

Pond mats are typically used to extract heat from water sources, but methods such as boreholes and submerged open-loop pipework are also used to great success.

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Pond mats for ground source heat pumps

I knew that ground source was a great technology for extracting the natural heat that’s available from the environment to provide a completely green heating system. The good thing about the pipes being in the lake is that water has great conductivity meaning we are able to extract heat sustainably throughout the winter to keep the house perfectly warm.

Mr Clarke, homeowner

Where does the heat for a ground source heat pump come from?

Renewable heat taken from the ground or water is predominantly stored solar energy. This is naturally replenished through direct solar energy, and indirectly through the rain. Stored solar energy is different from geothermal heat, which is absorbed from the earth’s core by digging much deeper into the ground.

What is the right energy source for my heat pump?

The heat source you use can be determined by how much space you have around the building. This is because the area needed for ground source heat pumps will either require horizontal groundwork for soil or water, or vertical groundwork for rock.

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How much area is required for ground source heat pumps?

The space you need for a ground source heat pump varies depending on the ground collector used. A typical property using slinky pipes in trenches will require an area of land measuring around 40m x 12m. Boreholes can range from 60-200m deep and 150mm wide. And water must be within 100m to be used as a heat source.

How do heat pumps collect renewable heat?

An open loop borehole for a ground source heat pumpGround source heat pumps are designed to extract heat energy via an array of collectors – referred to as ground arrays. Ground arrays take the form of:

The two most commonly used in the UK are slinky pipes and boreholes. However, pond mats in water should be the preferred option if there is a pond, lake or any sort of water source nearby.

An open loop system is a less commonly used collector. It uses the sea or a large body of water as the source, absorbing, compressing and delivering heat to a building, and then circulating the water back into the source or a discharge area.

How does the process of collecting heat work?

Heat moves from the ground to the heat pump

A mixture of glycol and water is pumped around the pipework buried in the ground. The collectors absorb some of the energy in the ground and transfer this energy back to the heat pump. The heat pump then compresses and delivers heat into the property at its desired temperature.

Ground arrays meet the building’s heating demands

Heat collectors are sized to meet the heat needs of the building, not the heat pump. All collectors have broadly the same performance, and a building’s thermal efficiency affects heat pump performance more than the type of collector.

more on how a heat pump system works

Choosing a collector: Ground source heat pump borehole or trench?

Slinky trench vs. borehole for ground source heat pumps

As the most commonly used collectors in the UK, it’s likely that you’ll be choosing between a ground source heat pump borehole or trench. Here are the key differences between the two:

Horizontal slinky systems

Slinky trenches are usually cheaper and easier to install. They don’t need a specialist drilling contractor, and the installation work can be undertaken by a digger that is already onsite doing other work.

However, slinky pipes must have sufficient land area to collect and deliver the energy requirements of the building. Adding more loops into a limited area of land doesn’t increase the amount of energy available – the limiting factor is the land itself, not the pipes in it.

Drilled borehole systems

Boreholes on the other hand are typically a more expensive option, especially on smaller jobs – and getting physical access for the drilling rig may be a challenge on space-restricted sites.

Having said that, boreholes can usually be installed in any type of ground. The smallest of gardens meet the heat load of the property, as the depth of boreholes vary to suit the heat requirement. Boreholes can be sited under car parks and even under the buildings themselves, as unlike horizontal arrays, they are far less affected by what’s directly above them.

What’s the difference between water source & ground source heat pumps?

Water source heat pumps collect energy from places such as streams, lakes or ponds, whereas ground source heat pumps absorb heat from the ground below us.

However, in terms of the physical system itself, a water source and ground source heat pump is the same. Kensa’s ground source heat pumps can collect energy from both ground and water – such as soil, rock and even the sea.

Factors that could influence your choice of heat source

There are many things to consider when it comes to choosing a heat source and collector. Remember that Kensa can help you with anything from finding an installer, to design or commissioning support. If you’re an installer yourself, you can even use our MCS Umbrella Service.

Factors to consider when choosing a heat source include:

•  Nearby water

If you are within reasonable reach (100m) of a water source such as lake, stream, or the sea, this should be your first option for a heat source. Water has exceptional thermal conductivity and cheaper installation costs.

•  Cost

Borehole drilling is the most expensive of the ground array options, but it’s the quickest and least invasive option.

•  Geology

The ground conditions and conductivity of the underlying geology will influence the cost of any ground array.

•  Heat loss calculation

An accurate heat loss calculation for the building is absolutely essential when designing any ground array for a ground source heat pump system. If you need help with this, Kensa conducts calculations as part of our MCS service.

•  The heating distribution system

The water temperature at which the heating distribution system runs will change the efficiency (CoP) of the heat pump system. So a system using underfloor heating will normally need a slightly larger ground array than one using radiators in an identical building.

•  Efficiency of the heat pump

One of the ironies of using an ultra-efficient heat pump like the Kensa Evo model is that the ground array size needs to increase slightly. If less energy for a given heat output is coming from electricity, then more has to come from the ground array.

•  Land area

As a general rule, you need a garden area of at least 40m x 12m to install horizontal heat collectors in the soil.

•  Reducing the heat load

Surprisingly often, calculations for a horizontal array will show that the available garden area isn’t quite big enough to meet the heat load. The usual solution for this situation is to go down the borehole route, but this could add several thousand pounds to the overall installation cost.

It could be worth spending this money on insulation to reduce the heat load and energy bills. By reducing the heat load, the available garden area can become a viable heat source. It may also be possible to specify a smaller heat pump, which will further offset the insulation costs.

•  The number of buildings

Ground arrays can be shared between multiple properties in a similar fashion to district heating. Each property will have its own heat pump, and the system won’t need any metering or billing arrangements – this is known as Networked Heat Pumps.

The boreholes or slinkies can be sited where practical rather than individually in gardens. As the system grows in size, a degree of diversification can be applied – so the total area for the ground array can be reduced compared to individual systems.

•  Other possible heat sources

Less commonly, other sources of free energy can be found in existing water wells, old mines, natural springs, river water and even seawater. While these projects aren’t so mainstream, the best ones can make highly efficient and low-cost energy sources.

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What are Networked Heat Pumps?

Networked Heat Pumps are Kensa’s award-winning and pioneering approach to district and communal heat networks – without the drawbacks of traditional district schemes. The design connects ground arrays to individual heat pumps in multiple dwellings.

Traditionally, rock is utilised as the heat source via borehole collectors for use in Shared Ground Loop Arrays, however, water sources could provide significant groundwork savings and deliver even more efficient results.

See more on Networked Heat Pumps
Kensa ground source heat pumps district heating Shoebox heat pump diagram

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