The topic of decarbonisation of heat has been a topic of intense research in the past five years, owing to the fact that it is such a key issue in the reduction of carbon emissions. Commitments made from the UK in the Paris agreement, followed by legislation to achieve, initially an 80% reduction by 2050, that has now evolved into being zero-carbon in the same time frame. The possibilities of how this might be achieved have been widely discussed, with four main strategies currently representing potential options for future decarbonised supply:
- Use of lower carbon gas;
- Electrification through the use of heat pumps;
- District heating or heat networks;
- Hydrogen gas network.
It is noted that these are the current possible options and that further technological advancements are expected (Ofgem, 2016, p6). However, striving for net-zero by 2050 does not allow the possibility to rely on future developments, with such a significant problem presently posed.
It has been explicitly stated across academic literature that energy efficiency will have to be a key consideration in any realistic strategy to achieve net-zero and meet the current emission targets; where energy efficiency is considered to be the reduction of energy to provide the same service. The Committee on Climate Change (CCC) released a report in May 2019 detailing how the UK could viably build towards a net-zero greenhouse gas emission economy, stating how measures to improve energy efficiency are essential to reduce energy demand in any credible strategy (CCC, 2019a). This importance is further clarified when, in 2015, the Association for the Conservation of Energy found the UK to possess one of the most inefficient housing stocks when compared with fifteen other European countries, going on to state that 21 million of the 26 million UK households still have a poor level of energy efficiency; homes classified into Bands D, E, F and G on an Energy Performance Certificate (Guertler, Carrington and Jansz, 2015). However, this has been recognised, with the UK government launching its Clean Growth Strategy in October 2017, specifying that "all fuel poor homes are to be upgraded to Energy Performance Certificate (EPC) Band C by 2030" (HM Government, 2019, p63).
The abundance of research into future UK heat scenarios typically conclude that a basis of reduced energy demand will be necessary for any decarbonisation strategy. For example, when (Lowes, Woodman and Clark, 2018) reviewed scenarios for decarbonised space and hot water heating, building on work from Winskel (2015), they found energy demand would need to reduce by approximately 20% regardless of the methods implemented to then meet this demand – with this modelling working towards an 80% reduction by 2050. When Chaudry et al. (2015) investigated the significant uncertainties surrounding the future of the UK heating industry, key messages were obtained; neatly concluding "Energy demand reduction is essential for meeting emission targets" (Chaudry et al., 2015, p628).
Given that improved energy efficiency is considered an essential measure in the delivery of zero-carbon buildings, as well as it being a cost effective, low-regret measure, this study works on the basis that it will act as a prerequisite to new heat supply technologies. Hence, analysing possible options, including modelling future scenarios, will consider a reduced future demand.
Use of lower carbon gas
This option would involve integrating biologically produced gases into the existing gas network, that currently supplies approximately 80% of buildings in the UK (DECC, 2013). These are produced via the natural decomposition of organic waste, releasing a gas composed of mainly methane and carbon dioxide. It can be considered a renewable resource as essentially the carbon dioxide absorbed from the atmosphere during its formation is then released when the gas is burned to release energy. When purified and cleaned to meet pipeline standards, converting it to biomethane, it can then be injected into the current national gas grid without any changes to the network or household applications (Ofgem, 2016). The chemical properties allow this blend of gasses to be used identically to natural gas, hence providing an incredibly convenient way to decarbonise supply. Although as of 2018, biomethane only accounted for 0.4% of supply in the national grid (BEIS, 2019).
However, despite this, literature presents an inconclusive view on its role in the decarbonisation of UK heat supply. Eyre and Baruah (2015) suggest there is scope for a much greater degree of reduction to heat demand; hence providing the opportunity for bio-energy in the gas grid to play a significant role. In 2017, Anthesis and E4tech reported that bioenergy presents the possibility for 108 TWh/annum in 2050 via renewable gas, equivalent to approximately 25% of the UK's heat demand (Anthesis and E4tech, 2017), yet does rely on development of bio synthetic natural gas which is currently unproven. On the other hand, Dodds and McDowall (2013) found that bio-methane injection only has a small role in future scenarios, with a maximum 6% of total UK gas supply available, mainly due to biomass being more appropriate used elsewhere in the economy, clearly not providing a sufficient reason to persist with the current gas network.
Lowes, Woodman and Clark (2018) neatly summarise some of the issues surrounding the feasibility of biogas playing a major role in future heating systems. The main issue remains that using bio-resources in industry in high-temperature processes is considered a more efficient use of the material. Other considerations worth taking into account include the fact that using arable farmland for purpose grown energy crops, which is most common in the UK, goes against the policy of using farmland for food production, while it has also been shown to be a remarkably expensive way to reduce heating emissions.
It is worth noting that while countries such as Sweden and Finland have been able use biomass extensively to reduce heating emissions (Hanna, Parrish and Gross, 2016), this has come naturally through their large forest resources and highly developed wood and paper industries.
While there may be a role for biogas, particularly due to the benefit of using the current gas network, it is unlikely that it will play a significant role in the long-term future of UK heating system. Clearly, another more wide-reaching solution will be necessary.
An alternative could be to use hydrogen in the place of natural gas, a substance that does not release carbon dioxide; combusting to form water while releasing energy instead. Generally, hydrogen has not featured in heat decarbonisation studies with it not being considered a feasible, commercially-viable technology. Yet it appears to be maturing into credible zero-carbon alternative to natural gas; mainly based on recent studies demonstrating it could be delivered in the existing gas network (Dodds et al., 2015). The H21 Leeds City Gate report (Northern Gas Networks et al., 2016), was a detailed desk-based investigation into the conversion from natural gas to a pure hydrogen network. This report highlighted how the modern polyethylene pipes increasingly used in the gas distribution network are suitable for hydrogen transportation, with the UK currently halfway through a project to replace its iron gas pipes. While it goes on to state how this option would have a low impact to consumers as well as negate the need for a significant upgrade to the electricity network or extensive development of heat networks, several sizeable issues remain.
The first being how to source the hydrogen, with the true carbon footprint of hydrogen depending on how it is obtained and delivered. The above-mentioned report proposed producing hydrogen via steam methane reformation, which in itself requires substantial quantities of natural gas as feedstock. Not only does this process release carbon dioxide, but it was found that, because of its inefficiency, 47% more natural gas would be needed to provide the sufficient hydrogen for heating in Leeds. However, as Nikolaidis and Poullikkas (2017) note, "CO2 emissions of steam reforming can be strongly reduced by CO2 capture and storage (CCS), through which CO2 is captured and injected in geological reservoirs or the ocean." This technology is very much still developing but the possibility does appear to exist. The other option would be to obtain hydrogen through the electrolysis of water, that will not only place a huge demand on electricity, but also then depend on the future decarbonisation of electricity production. Currently, neither method is refined enough for large scale, low cost production.
Safety of hydrogen in buildings also remains a concern, with risk of hydrogen ignition greater than that for natural gas (Dodds and Demoullin, 2013), who go on to state that new hydrogen sensors and meters would be an essential requirement, in addition to meticulous appliance testing. However, an advantage of this conversion to the consumer would be the identical user experience of a hydrogen boiler to a gas boiler – requiring no behavioural change. This might make the necessary large-scale conversion more welcomed.
Overall, there is acceptance that hydrogen can provide the necessary zero-carbon heat supply the UK demands, yet requires government to pledge significant investment into development and refinement of the technology, with considerable uncertainties remaining. As Dodds et al. (2015) concluded, "there is a need for the academic community to include this option into future assessments and for policymakers to consider these technologies."
In contrast, heat networks, or district heating, is a well-developed option for future heat supply, having been widely established in other European countries already. For example, 64% of Danish households are already supplied by district heating systems (Dansk Fjernvarme, 2019). As a result, they have also been appropriately investigated in UK low carbon scenarios.
Heat networks work by centralising the generation of heat and hot water, before then distributing this to individual buildings. The greatest benefit of this system is its flexibility to utilise a wide range of sources for heat supply, such as; fossil fuels, biomass, waste heat, geothermal or electricity. Once the system is in place, it is much easier to switch to a low-carbon system without the difficulty of justifying intense disruption to the consumer. Furthermore, they can be easily integrated into existing wet-based heating systems most common in the UK, with only a direct replacement of a boiler to a heat exchange unit required (CCC, 2019b).
Research by Imperial College London for the Committee on Climate Change (2018) found the main barrier towards their deployment being the significant capital cost, resulting in the implementation likely being restricted to high heat density areas, typically in urban areas, for them to be economically viable. However, 40% of the total heat demand in the UK does currently come from such areas (Imperial College London, 2018), with the research concluding they have a significant role to play if the UK is to reach net-zero by 2050, considering them to be the most cost-effective solution in many areas. Earlier research (Element Energy Ltd, 2015) was also supportive of their role, shown by modelling various scenarios for the development of district heating to 2050. In the 'central' scenario where existing barriers have been overcome, they find that heat network can potentially provide 81 TWh by 2050, equivalent to 18% of total heat demand, while also being responsible for an abatement of 15.1 MtCO2. It goes on to model a 'maximum' scenario which envisages a serious policy push. In this case, potential supply increases to 111 TWh, roughly 25% of total demand, and carbon emission savings rise to 20 MtCO2/yr.
The literature appears to conclusively agree that heat networks will have a significant role to play in the UK's low carbon future, particularly in new-build developments as well as high-density heat areas where this strategy would have the capability to provide cost-effective low carbon heat. However, clearly another solution must be found for those areas where a heat network is not a viable approach.
Currently, the most feasible technology for achieving a decarbonised heart supply is through highly efficient heat pumps that use an electrical input to extract heat from sources such as the air, ground and water before transferring it inside for use. Despite the high efficiency – typically producing 3kW of thermal energy for every 1kW of electrical energy consumed (Blundell, 2019) – widespread deployment of this option would certainly place increased demand on the national grid, as well as relying on the continued evolution towards a low carbon electricity system.
Low carbon scenarios often expect heat pumps to have a major role in the supply of decarbonised heat. Dolman, Abu-Ebid and Stambaugh (2012) suggested that in 2050 that heat pumps will supply 35%-65% of total UK heating, with these scenarios achieving 90%+ reduction in emissions relative to 1990 levels. Additional research (Imperial College London, 2018) has shown that to achieve 0 MtCO2/yr emissions from heating in 2050, the two most cost-effective means of doing this involve a substantial heat pump uptake, with heat pumps delivering almost 85% of heat in these scenarios. However, it is worth noting that the cheapest of these scenarios is considered a 'hybrid pathway' using hybrid heat pumps, which combines the use of an electric heat pump with a tradition gas boiler to deliver heating (where reinforcement of electricity networks may not be required).
This investigation worked on the basis that all homes off the gas grid were already to be converted to heat pumps, as previously recommended by the Committee on Climate Change (CCC, 2016), where they are already a cost-effective, low-carbon alternative to oil or electric central heating. Typically, these would be found in less densely populated areas, hence upgrading the national grid would have less impact. As a next step, it could be beneficial to consider those areas with a high potential for solar PV, as network upgrades may be required regardless (MacLean et al., 2016).
A key factor to the consumer when converting to a heat pump system is the change to the behavioural patterns regarding heat. Compared to a typical gas boiler which can be switched on and off whenever heat is required, a heat pump works best if it continuously running, maintaining a comfortable temperature. The user experience from heat pumps was investigated (Caird, Roy and Potter, 2012), where the majority of the 83 users surveyed were very satisfied. 75% reported that the heat pump was an improved method of heating, although in almost all cases the heat pump was used in a home off the gas-grid, thus not replacing a gas boiler, while several stated how a major benefit was the constant and whole home warmth provided.
This survey went on to establish that the heat pump was most effective in properties with high energy efficiency, highlighting the need for energy efficiency to develop hand in hand with new technologies. The article concludes that likely reasons for some dissatisfaction would arise from lack of consumer understanding about their operation and poor-quality installation, with inappropriate design and sizing. However, the ability to meet peak demand is a legitimate concern with this technology, with electricity bring much harder to store than gas, justifying the aforementioned integration of hybrid heat pumps (Imperial College London, 2018), as way to overcome this barrier.
Predictably, and similar to some of the other options discussed, the key barriers are considered to be a significant initial cost and the disruption the conversion would necessitate, particularly when considering upgrades to the electricity system. To the consumer, the cost of a heat pump is over twice as much as a new gas boiler, while also presenting the problem of a large spatial requirement for the outdoor unit, explaining the slow uptake in the UK – only 20,000 units sold per annum (Etude, 2018). Yet these have not been such issues in countries such as France and Italy, where annual sales exceed 180,000; countries that, like the UK, have a substantial natural gas network. This has been achieved through information campaigns, technical standards and fiscal incentives (CCC, 2016).
All the evidence suggests that heat pumps will have a major role in a zero-carbon future, with the importance now turning to how that is logistically achieved. There is a need to examine and quantify how their deployment would affect electricity demand, considering the impacts this would have on electricity system at a local and regional level, to assess whether this is a feasible option for a consumer.
Whether it is required to operate electric heat pumps or to mass produce hydrogen via electrolysis, clearly the decarbonisation of heating relies heavily on a well-established, capable, low-carbon electricity system. Luckily, progress has been much stronger in that sector with evidence suggesting this is very realistic target. Emissions in the power sector are now at just 65 MtCO2, 68% below 1990 levels, with low carbon generation making up 54% of this in 2018 (CCC, 2019c). It is worth noting progress will be hard to sustain with coal-fired power stations now almost completely phased out.
However, when considering future energy systems, there is a need to, not only consider the electrification of heating, but also the electrification of vehicles; both dramatically increasing the demands on the system. Sales of electric vehicles nearly doubled from 2017 to 2018 globally, while the UK's electricity demand grew by 3.7% in the same period (CCC, 2019c).
A couple of studies have investigated the affects this might have on the electricity sector at a national level. (Baruah et al., 2014) found that "High electrification of heating and transport services, which are two major fossil fuel consumers in the UK, increases annual electricity consumption and peak electricity load by 35% and 93%, respectively, by 2050." The study goes on to state that demand management measures such as vehicle-to-grid (V2G), in which electric vehicles can discharge stored energy back to the grid at peak times, is a viable way to reduce peak load to acceptable levels. It is worth mentioning this study didn't account for significant improvements in both building energy efficiencies and heat pump performance, and therefore reasonable to suggest the actual impacts will be considerably lower.
This need for demand management in such a scenario was verified when the National Grid published a report in 2018 investigating future energy scenarios, modelling how the UK can meet increased demand, while also being compliant with climate targets (target being an 80% reduction by 2050). In all scenarios, offshore wind generation and solar capacity markedly increase, hence requiring a significant gas-fired back-up capacity in today's market. It recognises V2G as just one way to achieve a greater flexibility, also mentioning development of smart appliances to enable much greater control (National Grid, 2018).
These studies show how nationally, we can develop our electricity grid to a low-carbon system capable of managing future needs. Yet, there appears a need to explore the impacts on a possible consumer, questioning if the use of heat pumps through electrification currently provides a viable heating option to buildings presently supplied by the gas network.
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