Decarbonising heat – the race to replace gas

20. April 2018 | News

The UK needs to find a low-carbon alternative to the use of natural gas in homes. Although electricity is an obvious contender, the size and seasonality of gas demand is encouraging the Government to think again and consider the feasibility of converting the current gas grid to hydrogen instead. Whatever the final decision, the choice will have far-reaching impacts on both the European power and gas markets.

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Decarbonising heat – the race to replace gas

The UK needs to find a low-carbon alternative to the use of natural gas in homes. Although electricity is an obvious contender, the size and seasonality of gas demand is encouraging the Government to think again and consider the feasibility of converting the current gas grid to hydrogen instead. Whatever the final decision, the choice will have far-reaching impacts on both the European power and gas markets.

The UK’s energy system has, in many respects, been built around gas. Almost half of final energy demand in the UK is used to provide heat (760 TWh) and approximately 70% of heat across the domestic, industry and service sectors is provided by natural gas. Little wonder therefore that today the gas network supplies more than twice the energy provided by the electricity grid and 85% of British households are directly connected to the gas network.

Ever since gas street lighting was introduced in London in 1814, gas’ role in powering the UK has risen seemingly inexorably. Following more than two centuries of investment, the UK’s gas distribution networks currently measure roughly 280,000 km in length, to say nothing of the 7,600-km high-pressure transmission network and the investments made by end-users in gas-fired appliances and equipment.

However, this love affair with gas is on a collision course with efforts to reduce greenhouse gas emissions. The use of natural gas for heating accounts for 14% of the UK’s total greenhouse gas emissions and decarbonising heat is therefore essential to meet the Government’s emissions commitments.

Change don’t come easy

What will replace it? Initial planning on how the UK would meet its emissions targets concluded that electrification was the answer. However, as the real-world challenges of electrifying heating demand have become more apparent, policy makers have increasingly been looking for alternatives.

Electrifying heating demand entails many different challenges, but we’ll focus on just one here, which is starkly illustrated in Figure 1 overleaf. This chart shows synthesised half-hourly demand for space and hot water heating in Great Britain against the national demand for electricity. Peak half-hourly heat demand is more than six times the size of peak electricity demand and, unsurprisingly, varies markedly between the cold winter and hot summer months. The bulk of this heat demand is demand for gas – about 80% of the heat demand shown would be produced using natural gas and, since even the most efficient gas boilers are only around 90% efficient, the actual quantity of gas being used exceeds 80% of the heat demand shown. Replacing the gas demand implied by this chart using electricity would imply an enormous increase in the quantity of electricity generation capacity required – capacity which, let us not forget, needs to be low-carbon.

Admittedly, this chart, which looks at estimated demand profiles for 2010, is likely to exaggerate the scale of the challenge. Heat pumps ought to generate at least two to three times as much useful heat as the power they consume and, by 2050, improvements in insulation and heating controls (along with a warming planet) should help reduce the absolute heating requirement. In addition, thermal storage like hot water tanks, could help to smooth demand across the day. Yet despite these caveats, it’s impossible to escape the simple truths that heat demand is both very large and very seasonal relative to electricity demand. Replacing gas with electricity, which is notoriously costly to store and transport, won’t come cheap.

The burning question

What are the alternatives to a massive expansion in electricity generation and grid capacity – and how might it affect the UK’s trading partners? It is worth noting at the outset that the electrification of seasonal heat demand is hardly unprecedented. Norway already uses electricity extensively to heat its buildings, which contributes to large seasonal fluctuations in electricity demand. The system can afford to do so because its hydro reservoirs provide relatively cheap seasonal storage. To some extent, greater interconnection between Norway and Britain combined with investments to maximise the scope for seasonal storage could help to provide greater seasonal low-carbon capacity, which Britain and other countries will need if heating demand is to be electrified.

However, the British government is also seriously exploring the scope to limit the need for electrification by decarbonising the gas grid directly. One means of doing so is to replace natural gas with so-called ‘green gas’, methane created either from the digestion of biomass or the processing of dry organic material or landfill waste. However, even under the most optimistic assumptions, the available supply of the necessary inputs is likely to limit feasible production to 20% of current gas demand, making this at best a partial solution. Even then, it’s debatable whether using the limited green gas available to decarbonise low-temperature heat would make sense, .

convert to hydrogen, which emits no greenhouse gases at the point of combustion. However, hydrogen itself is merely an energy carrier and the environmental and cost implications of a switch to hydrogen would therefore depend on the supply chain used to produce it. Electrolysis using low-carbon electricity would be a sound approach from a climate perspective, but hardly a low-cost approach, since it too would require significant expansions in generation capacity. Although the use of hydrogen to store energy might help smooth out seasonal fluctuations in demand and thereby reduce the peak capacity requirement, the losses resulting from the electrolysis process would significantly increase the absolute amount of electricity required.

Alternatively, hydrogen could be produced using natural gas or other fossil fuels, but these processes generate significant emissions and, without the use of CCS, would likely increase overall emissions. If combined with CCS however, this approach would secure a future for the Norwegian offshore gas industry, potentially increasing both demand for gas and creating new demand for both hydrogen production and carbon storage.

However, the challenges to hydrogen conversion don’t stop with the supply chain. Conversion would also trigger the need for significant investments in both the network and in end-use appliances. Most of the existing gas distribution network, and all of the transmission network, consists of metal piping that is unsuitable for transporting hydrogen. Only those elements of the network constructed of plastic (in use since 1970) would not need replacing as part of a hydrogen conversion. By the early-2030s, only around a third of the existing distribution network, concentrated mainly in urban areas, will be plastic and therefore could be saved. Similarly, existing end-user appliances would at the very least need to be retrofitted, if not replaced entirely, to safely operate using hydrogen.

Turning up the heat

Despite these challenges, there are several projects looking to assess and pilot the use of hydrogen in Britain. £9m was recently awarded to the H21 project to test the viability of a future hydrogen conversion and the British Government has recently announced a Call for Evidence on the options to decarbonise heat.

One way or another, the UK’s historic reliance on gas in the home will end. Given the size of the UK’s future energy import requirements, the choice of what replaces it could have an enormous impact on the future demand for power and gas in Europe.