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Jack Flower PhD candidate, Centre for Doctoral Training in Future Power Networks and Smart Grids |
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Dr Graeme Hawker 17 October 2018 |
The Committee for Climate Change (CCC) has, in its 2018 review of UK progress towards meeting 2030 and 2050 carbon targets, continued to highlight the ongoing difficulties and lack of progress in decarbonising the heat sector in the UK. The CCC, along with others, continues to strongly promote the uptake of heat pumps as a major part of the solution, based on the dual benefits of such devices. Firstly, through the greater-than-unity performance of such devices allowing more heat energy to be extracted than electricity is consumed; and secondly, by enabling heat demand to be met from low carbon electricity sources such as renewables or nuclear power, displacing what is at present carbon-intensive gas, oil or solid fuel consumption.
In a recent IPPI blog post We Were Promised Heat Pumps! A View on the Electrification of Heat Dr. Nick Kelly identified the various stumbling blocks that have slowed the uptake of such devices. In addition to these issues, better recognition of the extreme variety of UK heating demands and existing heat systems needs to be made in order to properly determine the contribution heat pumps can make to the nation’s energy mix.
National models used to set policy (such as UK-TIMES which was notably used by the UK government to inform the 2017 Clean Growth Strategy), are generally configured to use national averages, or to summarise sectors through a small number of representative cases. While this considerably reduces modelling complexity it can overlook this diversity and miss key outliers in the system.
What contribution can be made from the domestic heating sector?
If we propose taking a domestic property with an existing heating system (be that gas, fuel oil, or electrical heaters), we can then determine the cost of replacing that heating system with an electrical air-source heat pump (EASHP). While the initial cost to do so might be very high, we may expect this outlay to be recovered over the lifetime of the device through fuel savings. If that also leads to a measurable reduction in carbon emissions, then we may use the two values to calculate the ‘marginal abatement cost’ (MAC), or the amount of money we must invest to produce a certain level of emissions reduction, normally described in £ per tonne of CO2.
Deriving such values is an important method in allowing policymakers to prioritise different measures in national energy policy, in order to determine where public money may be spent most efficiently. If we identify a negative MAC value, this means that (over the lifetime of the device in questions) we save money at the same time as reducing emissions – the Holy Grail for energy policy! Such analyses underpin, for example, the controversial ban on incandescent light bulbs, on the basis that fluorescent and LED lighting substantially reduces energy consumption and energy bills, ultimately saving the consumer money while also reducing carbon emissions – theoretically, at least, a win for both the bill payer and the environment.
Because a MAC value is highly dependent on the fuel savings that are made, we can already identify that this is going to vary considerably between different households – those with greater annual energy demand will save more money on fuel, and those that are currently using higher-carbon fuels (such as fuel oil or solid fuels) will be displacing a greater volume of carbon for each amount of heat delivered.
If we instead assume that the benefit from heat pumps is similar everywhere, we may misrepresent the contribution such measures may make to our national carbon emissions. This means that if we more deeply inspect the diversity and character of UK domestic properties, we come out with a more nuanced view of the road ahead, and can construct clearer policies which fit the energy system we have.
At present most GB households are heated using natural gas directly fed by the gas distribution networks. The distribution of gas-heated households by heat energy demand is shown in graph a), in contrast to electrically-heated households in graph b). This illustrates that although natural gas is broadly the lowest cost heating source in the UK, there remain a large number of low-demand households relying on electrical heating – either due to the small cost saving that a gas boiler would provide for such low consumption, or due to being in households away from the gas grid. Properties at the left hand end of the scale represent smaller households, often with low rates of occupancy, or may include larger but fuel-poor residences.
The MAC values that can be derived for the replacement of existing heating sources with heat pumps is shown in graph c). This is highly sensitive to the level of heat demand, with the cost rising significantly for low heat demand households.
For households that are conventionally electrically heated with high heat demand the MAC is negative, meaning some of these households in GB would potentially benefit from overall cost savings if they adopted heat pumps. For the lower demand households, it is not a cost effective abatement option to replace conventional electrical heating with heat pumps.
Similar considerations apply to households using fuel oil, but at lower demand levels heat pumps may be seen as an attractive option as an alternative to renewing the existing system when it reaches the end of its operational life. This supports policy mechanisms which intervene in the market at the point where a home-owner is looking to replace an ageing system which is perhaps costly to run and inefficient, or even defective.
But what does this mean for people and their homes?
The widespread adoption of the combination boiler has meant that households have saved space previously taken up by bulky hot water storage tanks that are no longer in use. In parallel, household size has decreased over the years, from an average of 2.91 people at the time of the expansion of the gas grid (1971), to around 2.35 people in 2011.
This decrease has resulted from a large increase in the proportion of one person households, which almost doubled between 1971 and 1998. In the future, will people in smaller households value space saving and flexibility over bulky low heat heating options such as an air-to-water EASHP system, which in most cases is not a like-for-like replacement of the gas central heating system?
Another consideration is that in the UK, households generally heat every room in a household using a wet central heating system. Continuous efficiency improvement of the GB housing stock is anticipated, as well as the adoption of ‘sophisticated’ home energy management systems and controls. Could this lead to a significant reduction in energy consumption to fulfil comfort requirements and also lead to a change in consumer behaviour? How could graphs (a) and (b) change in the future and, if the distribution of heat energy demand changes, what will be the most cost effective options in graph (c) for a diverse range of households?
Another unresolved issue is that building standards do not currently require new buildings to be capable of being retrofitted with hot water storage, and many are currently being designed under the assumption that combination gas boilers will be used throughout the building lifecycle, which is clearly not compatible with our long-term carbon emissions.
How to achieve a decarbonized future
It remains the case, then, that heat pumps are a key technology towards decarbonising the heat sector in the UK, but that we cannot rely on the projected cost savings to be applicable to all households. In turn, we must look at a wider selection of alternatives: heat networks, waste heat and – perhaps controversially – identifying households where existing use of conventional electric heating should be continued.
The optimal heating technology is one of localized decisions which is impacted by many other factors such as; external climate, existing heating option of a household and its access to different energy networks, the building characteristics (type, efficiency, size), the number of occupants and prosperity of a household in terms of the ability to afford the capital costs for an alternative heating option, and consumer behavior. All of these factors are dynamic and are considerably spatially diverse throughout the UK, and therefore, recognition of this spatial diversity – and how it relates to fuel poverty - is crucial for setting future heat policy.
Tags: Energy Blog