Heating Your Building

The UK’s electrical grid is progressively decarbonising which means that left alone heat will become responsible for a larger and larger proportion of the total UK CO₂ emissions. To decarbonise heating in buildings a mixture of strategies will likely be called upon, dependent on the type, age and location of any particular building. The efficient use of electricity to generate heat is the clearest route to decarbonising our heating systems which can be seen at the moment. Building owners and designers considering a new heating system should investigate the use of heat pumps and understand the implications to their project to assess their feasibility.

Heat pumps come in lots of different variants, utilising a wide range of refrigerants and operating at a range of temperatures and efficiencies. The most common variants of heat pump discussed relate to the source of heat they use rather than the machines themselves, either ground source (GSHP) or air source (ASHP).

Heat pumps move heat from one place to another using a refrigerant. When the refrigerant is expanded from a liquid to a gas in an evaporator coil the refrigerant takes in heat energy from its particular heat source (typically air blown over an outdoor coil in an ASHP or the water circulated around a “ground loop” of pipework in the case of a GSHP). Electrical energy operates a compressor and the refrigerant is compressed and at high pressure it condenses becoming liquid and giving up its heat energy in the heat pump’s condenser coil. The heat is used to produce hot water for use in heating systems.

Heat pumps operate at their most efficient when producing a low temperature of heating water say around 45oC, although there are machines which can provide relatively high flow temperatures around 60oC at useful efficiencies.

Heat pump efficiencies are often referred to as coefficients of performance (COP) and because the temperature of their heat source (i.e. the air or ground) varies throughout the year. The seasonal coefficient of performance (sCOP) is frequently used to best represent the efficiency of a heat pump operating throughout the year. Typical sCOP for heat pumps range between 2 and 4 with GSHP usually capable of achieving higher sCOP, ASHP typically perform at the lower end of the range. This means that for each 1kWh of electrical energy consumed heat pumps can produce between 2kWh and 4kWh of heat. Great! The efficient use of electricity means that a heat pump with a COP of 3 can generate heat with a 370% reduction in CO₂ emissions compared to a gas boiler.

Heat pumps are currently comparatively expensive to purchase compared to gas boilers of similar power and this remains a big consideration in individual feasibility assessments prepared for many building owners. Minimising heat losses within any building as an initial step towards achieving net zero carbon verification will also help ensure smaller heat pumps are needed minimising capital costs. ASHP are typically the lowest capital cost heat pumps and GSHP are typically the highest capital cost heat pumps. GSHP also need significant ground works for their ground loops which add cost, however they can usually be discretely located within buildings, whereas ASHP usually have some externally located parts to ensure access to external air.

The running cost comparisons of heat pumps compared to gas boilers are very sensitive to small changes in either the unit costs of gas and electricity and the sCOP of the particular heat pump. Currently running an ASHP will typically cost more than a gas boiler whereas a GSHP will run at a very similar cost. The comparative cost of gas and electricity is another reason why minimising heating demands is important in order to maximise the feasibility of heat pumps.

At the time of writing the non-domestic renewable heat incentive (RHI) scheme is months off closing to new applicants. However, the domestic renewable heat incentive (RHI) remains available until early 2022. Where eligible domestic ASHP and GSHP systems, installed by MCS registered installers, are registered for the scheme the government will pay owners 10.85p/kwh (ASHP) and 21.16p/kWh (GSHP) for 7 years which can assist calculations of financial feasibility.

The scale of our current dependence of natural gas suggests that any transition to low and zero carbon heating will take some time. There is emerging research into the feasibility of decarbonising the gas grid – Keele University has set up a pilot scheme. This research involves injecting hydrogen into the gas grid. The consensus view at present is that existing gas-fired plant can operate safely with 20% hydrogen injected into the gas supply. This is the proportion in the Keele University pilot scheme. That does mean however that 80% of the gas supply remains natural gas (methane), so this will not be the final solution to the decarbonisation of our heat supply. There are two ways to view developments in this area:

1. Hydrogen injection into gas grids could potentially prolong heating related carbon emissions.
2. Hydrogen injection into gas grids could act as a stop gap measure to support a future nascent hydrogen economy

However, this technology is in its infancy and any steps which can be taken to reduce or stop burning natural gas should be prioritised wherever possible.

District heating may play a future role in the decarbonisation of heat within densely occupied areas. It may be particularly helpful in supplying low carbon heat to existing buildings with high heat demands and into which it is difficult to integrate local heat pumps. District heating will only be low carbon, if generated by centralised low carbon technologies, such as large-scale heat pump installations. Accurate assessment of heat losses from the distribution pipe network and energy consumption of pumps and controls would have to be accounted for in the overall assessment of the carbon benefit of a district heating system.

To reduce carbon emissions now and in the coming decades new heating systems should seek to utilise low carbon sources of heat such as electric heat pumps and to stop utilising natural gas where possible.