Low carbon heat technologies

Farms require heat for a range of purposes including grain and vegetable drying, produce chilling, controlled livestock environments and the farmhouse.

Most, like many other rural businesses and homes, are not on the mains gas grid. Heating therefore has to be sourced through a range of fuel types – oil, bottled/tanked gas, diesel, biomass and electricity. The impact of using fossil fuels impacts farm greenhouse gas emissions. Investment in renewable energy technologies, for heat and electricity, has led to significant reductions in manufacturing costs – the justification for the removal of Feed in Tariff (2019) for electricity production and the Renewable Heat Incentive (2021).

Renewable heat technologies have some way to go to replace the fossil fuels used for this purpose on farms. The Climate Change Committee (CCC)/ Element Energy Report outlined a number of heating options and deployment levels in their balanced pathway in the UK’s Sixth Carbon Budget (see Table 5). Some of these technologies will be more appropriate for rural areas.

Heating and chilling are energy intensive processes, and demand can be highly seasonally variable. Hence, heat storage, transfer and recovery are increasingly important, particularly where intermittent renewables are involved. Possible farm applications are discussed below, although it should be noted that uncertainty of long-term government support for heat decarbonisation makes it difficult for farm businesses to make investment decisions.

Heat pumps

A heat pump extracts natural heat from a medium using ‘reverse-refrigeration’ processes. The medium is typically air (air source heat pump – ASHP) which has a varying temperature, ground (GSHP) or water (WSHP) whose temperature varies less than air.

A heat pump uses electricity to concentrate heat from the medium by using a vapour compression cycle. Heat pumps increase the temperature received from air/ground/water by compressing a refrigerant gas which heats up because the heat energy is concentrated into a small space. A heat exchanger transfers the heat to the heating circuit of the building. When the high-pressure gas gives up its heat, pressure is released through an expansion valve, whereupon it becomes very cold. Heat is then transferred to the cold coolant via the source.

ASHPs are typically ‘air to water’ (heat transferred in a hot water circuit) and less commonly ‘air to air’.

Heat pump efficiency

All heat pumps have a ‘coefficient of performance (COP)’, i.e. the ratio of useful heat energy produced to the amount of electrical energy consumed. For example, a COP of 3 means that the heat pump will supply 3 kilowatt-hours (kWh) of heat for every 1 kWh of energy that it uses.

A fossil boiler has a circuit temperature typically set at a default of 65 ^oC-70^oC and could be sized using a set of relatively rough parameters since extra energy could be easily obtained by burning more fuel. With the low-grade heat of an ASHP which is around 35^oC-50^oC the design approach is different.

Retrofitting heat pumps

If an ASHP is retrofitted to a building, larger radiators may be required. Installers have to balance a number of factors to size the heat pump and optimise the installation. These include taking into account local climate, the heating circuit flow rate and temperature, the heat transfer from emitters (radiators/underfloor heating) and building losses from poor insulation and draughts.

A study of air and ground source heat pumps by Fraunhofer found that those retrofitted to buildings worked perfectly well with radiators, although systems with the highest efficiencies had the lowest flow temperatures (~35^oC) and (typically) underfloor heating.

Cost of heat pumps

For an average UK dwelling, 2021 ASHP costs vary between £8000 – £18000 and are cheaper to install than GSHP or WSHP.

The cost of heat pumps is still significantly more than a gas boiler. It is envisaged that costs will come down as more are installed. For example, Octopus Energy has highly ambitious plans to drive the cost down to parity with boilers and have built an R&D centre to do that, as well as training installers and further optimising the technology.

The UK also ranks below the European average on gas prices and above it on electricity prices, a factor which further discourages electrification unless some self-generation is available.

Policy change and heat pumps

The UK government has now confirmed that gas boilers will be banned from new housing in 2025, and all households will no longer be able to buy gas boilers from 2035. To encourage this transition to low carbon heating a Boiler Upgrade Scheme has been launched, which will offer UK households at least £5,000 towards the costs of installing a new air source heat pump from April 2022. It should be noted that the initial grant budget will be limited to £450m – equivalent to providing around 90,000 homes with a grant.

Ground source heat pumps

A ground source heat pump extracts ground heat through a system of pipes and a heat exchanger (powered by electricity) to the heat pump. It can be installed using a borehole or in shallow trenches. Because the ground acts as a large thermal store (both a heat sink and a heat source), the ground temperature varies much less than that of air and thus the COP variance is much smaller.

A ground source heat pump is very efficient: it can deliver 3 to 4 kW of heat for every 1 kW of electricity it consumes. For example, the COP of a GSHP with access to a ground temperature of 10^oC (typically constant ground temperature at 10m depth) will be significantly higher than that of an air source heat pump with access to -5^oC from the ambient air.

This concept of utilising the earth as a heat sink by putting waste heat (e.g. from solar thermal, heat pump) from warm weather into the ground and retrieving it during times of cold weather works well with ground source heat pumps. It has been marketed by ICAX as ‘Interseasonal Heat Transfer’ and used for both heating and cooling.

Water source heat pumps

A water source heat pump can extract heat from an underground aquifer or from a river or lake. Again, water does not have the temperature variability of air. Professional advice is required to assess the geology and potential environmental considerations, as well as the heat pump design and installation.

The Wales and West ‘Freedom’ project converted 75 houses to use a combination of heat pump and gas boiler, coupled with a smart controller which predicted the user’s needs. By switching between gas and electricity, this avoids electricity use at peak demand and can benefit from time-of-use pricing (i.e. utilising electricity at the cheapest times).

Further resources (including case studies) on heat pumps are available from many sources, including the Ground Source Heat Pump Association (GSHPA) and the Heat Pump Association, as well as from podcasts such as BetaTalk – The Renewable Energy and Low Carbon Heating Podcast.

On farm heating/cooling options

A heat/chill heat pump system can be used to provide low carbon heat for a range of on-farm agricultural processes, for example, grain or grass drying, space heating, daffodil drying, wood chip drying or any other type of agricultural product drying. The process can also be used ‘in reverse’ to chill products like potatoes or milk. The low-grade heat recovered can be supplemented and used for heat applications such as hot water and space heating.

At its Eastern AgriTech Innovation Hub, NIAB is trialling renewable energy technologies and has installed an ASHP for both heating and cooling a large polytunnel. Dyson Farming’s large strawberry glasshouse at Carrington, Lincolnshire is powered by renewable energy from the adjacent digester, as well as being supplied with waste heat from the site’s combined heat and power (CHP) plant. It could potentially supply CO₂ in future.

Ventilation heat exchangers in livestock barns are an important way to recover heat from ventilated air. Such systems are less common in the comparatively temperate UK climate, as payback is faster in colder temperatures and with well insulated buildings.

Championing the Farmed Environment (CFE) have compiled a list of free resources for UK farm building efficiency which includes information on heating. An excellent ammonia case study on heat recovery from air scrubbing is available at the Pig & Poultry online forum.

Many farms take a ‘whole farm’ approach to reducing emissions, so their measures do not fall neatly into ‘decarbonising heat’, ‘building soils’ or ‘renewable energy’. Two case studies which illustrate this are Stephen Temple’s Norfolk dairy farm and that of Wyn Evans below.