Sources of heat

Heat Trust's customer protection standards for heat networks do not specify fuel source, so any heat network no matter the fuel type can register with us. To find out more about registering with Heat Trust please see our supplier pages, or for more information for customers please see here. 

Heat networks can source their heat from any fuel or generation process; this is sometimes known as being fuel agnostic.
Typical examples of heat sources for heat networks are: gas boilers, Combined Heat and Power (CHP) plants, wasted heat (from industrial processes, wastewater treatment etc.) and biomass fuelled boilers. More heat networks are now being heated by technologies such as large-scale heat pumps (usually water or ground), geothermal sources or even underground transport. We have highlighted some examples of these in the UK below. There are also projects in development in the UK and operational internationally using waste heat from mine water, deep geothermal, data centres, greenhouses and large-scale solar thermal (often with seasonal thermal stores) to power heat networks.

Heat networks can also make use of thermal stores (large insulated tanks of hot water). These can either provide heating temporarily if the primary source of heat fails, therefore increasing the reliability of supply for customers; or they can supply heat at a time of high demand on the fuel source to help balance out demand. The time these can supply heat for depends on the size of the store, ranging from a few hours with small tanks to between seasons with large ground pits.

Gas

Currently most heat networks (90%) use gas as their primary fuel source, typically through one or more gas-fired boilers. The gas is usually sourced from the national gas grid, and of the UK’s total gas demand in 2018, about 50% came from the UK’s contintental shelf and 50% was imported. However 'biogas' from anaerobic digesters, e.g. at sewage treatment works, can also provide the gas fuel for heat networks.

Some gas boilers produce only heat, like individual property boilers, however some produce electricity (burning the gas to create steam which turns turbines) and capture the excess heat at the same time. This is known as a Combined Heat and Power (CHP) plant (which can also use fuels other than gas) and is often more efficient than generating electricity or heat independently of each other.

Wasted heat

Many industrial processes generate heat as a by-product, such as incinerators, underground transport and sewage treatment works. Heat networks can make use of otherwise wasted heat by transporting it to an end user that requires heat. This is often limited by the end-users of heat not being close enough to the source of wasted heat.

Energy From Waste (incinerator)

Waste from transport

The London Underground produces waste heat, mostly from friction of the trains on the rails. Through adding a heat pump, the temperature of the heat captured can be raised to provide heating to a heat network.

Waste heat from sewage treatment works

Some of the processes used to treat wastewater, including sewage, involve anaerobic digestion of organic matter within the waste water, which generates heat as well as producing gas.

Biomass

Biomass usually comes in the form of wood pellets burned in a furnace. These wood pellets can come from waste/off-cuts from the wood industry or virgin woodland.

Heat pumps

Heat pumps capture heat energy from either ground, water or air, and then through a pump powered by electricity increase the temperature to heat homes and buildings. This increase or ‘upgrade’ in temperature works in a similar way to how a fridge is cooled but in reverse. Heat is generated through rapidly increasing the pressure of refridgerant gasses in a contained space, this increases the temperature of the gases, which can pass to adjacent water or air, piped onwards. At the scale of heat networks, only water or ground source heat pumps tend to be used.

Water Source

Ground Source

Geothermal

In the UK the top 10 - 15m of ground is heated by the sun and acts as a thermal store. By running pipes of water through the ground at these depths, the heat will be transferred to the water, which can then go on to heat people's homes. In some places, such as Iceland, the heat from volcanic activity (and heat conducted upwards from the Earth’s core and mantle) can be captured, which have much higher temperatures.

Geothermal is different to ground source heat pumps because electric pumps aren't used to raise the temperatures, and their pipes tend to go deeper into the ground.