What Heat Source can I use for a Heat Pump?

Jump to:

• Heat source #1: Surface soil

• Heat source #2: Rock

• Heat source #3: Water

• What is the right energy source for my heat pump?

• How much area is required for ground source heat pumps?

• Factors that could influence your choice of heat source

• What are Shared Ground Loop Arrays?

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.

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?

Ground 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:

• U-tube pipe in boreholes
• Slinky pipe or straight pipe in trenches
• Pond mats in water
• Open loop systems in aquifers or larger water sources.

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.

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

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 new 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 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.

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.