The importance of only building those buildings that are really needed was explained in part 8. Making sure that buildings keep out winter cold and summer heat – that is, improving their thermal performance – was dealt with in part 9.
Nevertheless, supplemental heating and cooling, hot water, and energy for cooking, will always be needed: this final part looks at how these can be provided without fossil fuels.
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| Installing a heat pump. Photo from the Phyxter web site |
The most common sources of such energy are fossil fuels and biomass, for the most part directly combusted on site, in buildings. This results in high levels of emissions – and, in much of the world, hazardous fumes associated with cooking indoors. (See part 9 and Appendix 3, in the PDF version).
These emissions and indoor pollutants can only be dealt with by changing the energy source. Decarbonising cooking, hot water and space conditioning means switching out all of that fossil fuel and biomass for other, greener, sources of energy – ideally, relayed by electricity.
Notwithstanding recent findings on the toxicity of gas stoves, cooking with gas is much cleaner than cooking with biomass, and gas is widely pushed as a comparatively clean-burning “transition fuel” for poor countries. But in my view, the case for gas in such circumstances is often over-stated, with a view to providing fossil fuel companies with a new source of revenue. The best alternative to biomass is electricity – for example, in an electric pressure cooker or induction stove.
In this post, however, I will focus on space conditioning and hot water. I will outline the main alternative technologies: heat pumps (section 10.1) and district heating and district cooling (section 10.2). I will then assess the potential role of hydrogen (section 10.3). I will discuss how such technological changes can be approached in a socially just way (section 10.4). Finally there are some Conclusions (section 10.5), from both this part and the whole series.
Crucially, heat pumps, district heating and (to a lesser extent) district cooling, are very mature and established technologies. They have industrial manufacturers and supply chains ready to expand, to fill an enormous potential – and necessary – market.
Local installation expertise is also crucial, and that varies enormously: in the UK, for example, a historical failure to train sufficient numbers of engineers, fitters and other tradespeople is a massive bottleneck. Nevertheless these technologies exist – they are not technofix dreams.
That expansion in turn requires the right support from states, both domestically and internationally: direct funding, regulation, technological transfers, and skills training.
10.1. Heat pumps
Apart from when they are used in industrial Combined Heat and Power (CHP) plants and similar settings, heat pumps are powered by electricity. So decarbonising heating and cooling for buildings with heat pumps is additionally dependent on decarbonising electricity – whether it is locally generated, or comes from a national or regional grid.
A heat pump is essentially a device for transferring thermal energy from one place to another.
And because it simply transfers existing thermal energy, it can be used for space heating, hot water and space cooling, simply by changing the direction of heat flow.
Heat pumps use electrical energy to pump a refrigerant (F-gas) through pipes, say from the outside of a building to the inside. The refrigerant transfers thermal energy in the process, and then loops back to do the same all over again.
There are a few different kinds of heat pumps: they vary e.g. by the source of heat and by typical output temperature. Most heat pumps used in buildings are air-source (ASHP) or ground-source (GSHP). When heat pumps are used in district heating and cooling systems (see below), the source (or sink) for heat might be water (sea, lakes, rivers, water treatment networks) or sewage.
Usually, water serves as an intermediary (“hydronic”) carrier of thermal energy, once the heat has been transferred. So most heat pumps send hot water through hot water pipes and radiators, or into underfloor heating pipes, as does a gas boiler: these are air-to-water heat pumps or “wet heating systems”. There are also air-to-air (A2A) heat pumps that send out hot air (or cold air). A heat exchanger heats water directly, to provide hot water.[1]
Even when it is very cold outside, heat pumps are able to extract thermal energy from the air, from the ground, or from a water source. In the case of space cooling, an air-source pump simply transfers heat from the interior to the outside.
Historically, ground-source pumps have had better performance, but have been more expensive. However, air-source heat pumps are now almost or just as good.
Crucially, heat pumps are massively more efficient at providing thermal energy than gas boilers. This is because they transfer thermal energy that already exists in the surroundings, rather than producing heat from scratch through combustion.
You therefore get more energy usefully transferred in the form of thermal energy (that is, heat gained or expelled) than you use in the form of electricity to power the process. Heat pumps function as if they were more than 100% efficient.
This effective efficiency of a heat pump is known as its Coefficient of Performance (COP). For example, a heat pump with a COP of 2 will transfer 2kW of heat for every 1kW of electrical power used – effectively as if it were 200% efficient.
The COP depends on environmental conditions. When the “thermal gradient” is steeper, and the difference greater between the interior target temperature and the exterior ambient temperature, a heat pump has to do more work, and the COP is lower – but typically the COP ranges between 2 and 4.
A “SCOP” is a seasonal COP, meant to quantify year-round efficiency in a given locale for a given heat pump.[2] In the UK, any heat pump on sale after March 2016 is meant to have a minimum SCOP of 2.5, i.e. should be effectively 250% efficient or more over the course of an average year.[3]
The typical difference in efficiency between a gas boiler and a heat pump is illustrated below. Here, the blue “C” values stand for the quantity of energy consumed (“energy in”), and the red “U” values stand for useful thermal energy transferred (“energy out”).
The arrow from C→U shows the efficiency of the energy conversion. According to LETI, 85% is the average efficiency of gas boilers installed in the UK, although a typical new boiler has an efficiency of 90% or more.
Home energy consumption and use are widely measured in kilowatt-hours (kWh). One kWh means 1,000 Watts of energy used for one hour. For a sense of scale: the power of a typical electric kettle is 3 kilowatts (3 kW); it holds 1.7 litres of water, and typically takes about 230 seconds to boil 1.7 litres of cold tap water. Boiling a full kettle therefore uses about 0.2 kWh.
The average gas-consuming household in the UK used ~12,000 kWh of gas in 2020, according to the BEIS.
So, returning to the graphs above: in the case of a typical gas boiler, if you burn 100 kWh-worth of gas, you will get 85 kWh of useful thermal energy out. For the same 85 kWh out, you only need 34 kWh-worth of electricity powering a SCOP 2.5 heat pump – only 34% of the energy consumed by a gas boiler!
Heat pumps in the UK are effectively about 3x more energy-efficient than a gas boiler.[4]
That picture is complicated, however, by the price of electricity. In the UK, electricity presently costs at least 4x as much per kWh than gas does (see here). In 2021, the price of electricity for households was 5.59x that of gas. The reason for the disparity is that gas receives an effective subsidy, and – perversely – electricity consumption receives a much higher environmental levy than gas consumption does. The result is that, for UK households, the energy efficiency gains of heat pumps are outweighed by the operational costs – and that is on top of the higher upfront costs of installing a heat pump.
Heat pumps are vastly more efficient than gas boilers when it comes to reducing greenhouse gas emissions. In the UK, to get the same amount of heat, about one-third of the volume of emissions are produced. And, as electricity is decarbonised, that volume of emissions from electricity will go down, without any changes to the heat pump.[5]
Here is the upshot:
🔥 The efficiency gain alone of heat pumps means that the energy consumption per unit of useful heat out (or transferred) goes down massively.
🔥 Even while the emissions intensity of the national grid is not much below that of a gas boiler (see footnote 5), the efficiency of a heat pump alone drives down emissions very effectively. And as the grid is decarbonised, so are heat pumps.
🔥 This means that, so long as the source of electricity is decarbonised, it is not necessary – for purposes of decarbonisation alone – to reduce the amount of heating or cooling required, by adding insulation and other thermal efficiency improvements.
🔥 Reducing the amount of required supplemental heat or cooling nevertheless has its own benefits – in terms of security of thermal comfort, and energy efficiency. Efficiency savings across the energy system also means that less infrastructure is required, which produces savings in the associated embodied emissions, materials, and land-use.
🔥And finally, depending on where you are in the world: as far as your average existing building is concerned, the input-output efficiency gains of a heat pump, compared to fossil fuel or biomass, will likely reduce your overall energy consumption for space conditioning far more than most retrofit options, apart from the very deepest retrofits on offer.
Something to beware of, however, is that if they break or are poorly installed, heat pumps can leak their refrigerant – usually, one form or another of F-gas. F-gases tends to have high global warming potential (GWP), so heat pumps that use refrigerants with lower GWP are preferable. Heat pumps will potentially move from F-gases to propane, CO2 (!), or other coolants in the future.
One very important thing, additionally, is the lifespan of a heat pump. Recent estimates range from 15 years to 25 years.[6] The consensus by researchers is that the lifespan of the average heat pump is now longer than it was in the past – and that heat pumps generally last a few years longer than gas boilers.
Heat pumps, in any case, are expensive– much more costly than a gas boiler. That cost will go down as the market for them expands and (presumably) benefits from economies of scale, and becomes more competitive. But the mainstream uptake of consumer-scale heat pumps will for the foreseeable future require substantial financial assistance from governments.
10.2. District heating & district cooling
District heating (DH) provides hot water and space heating to many buildings from a centralised source, by pumping hot water through insulated pipes.
Buildings in a heat network receive hot water from the network into a heat interface unit, which uses a heat exchanger to deliver hot water to taps, and space heat via radiators, underfloor heating and air-handling systems.
District heat is a familiar and established technology in many countries, although less common in the UK.[7]
Like electricity, the fuel source for district heat is flexible and “hot swappable”. It can be generated from a mixed variety of sources: residual heat recovered from industrial processes; fossil fuel, waste (trash), or biomass combustion; or renewables-powered heat pumps. Heat pumps are the main option for decarbonised district heating and cooling networks. [8]
Similar principles to district heat can also be applied for district cooling systems.
District cooling deposits heat – for example into a river, so that the network’s water is cooled. Whereas heat networks transfer heat, district cooling is said to transfer “coolth”.
District heating and cooling networks are best suited to feeding buildings and homes clustered close together, in towns and cities and suburbs. They are not suitable for isolated dwellings.
District heating and cooling networks can be combined to increase efficiency, as district heating and cooling (DHC). London’s Olympic Park has a DHC network.
Cooling networks are less physically efficient than heat networks, and they have greater upfront costs, so building-based cooling may be more practical and economical.
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| Above: a design drawing of the Olympic Park Energy Centre in Stratford, London, with a combined cooling and heating system with a biomass boiler. Top: circulating pumps suspended above the heating network at the park |
“Swappability” means that a network can switch between different sources or sinks of heat depending on the time of the year, for efficiency. The Helsinki DH network is warmed by seawater in the summer, and sewage water in winter.
These systems are generally amenable to gradual shifts in the energy mix. This “future proofs” district heating and cooling, as opposed to locking homes and other buildings into just one fuel source for heat.
Additionally, heating and cooling networks combine well with thermal storage to buffer demand. Examples of thermal heat stores are water tanks, tanks of molten salt, boreholes (rocks) and aquifers (water). Coolth can also be stored: for instance, Paris has a district cooling network that includes 30 MWh of ice storage.
Heat and coolth storage are an important way to buffer fluctuations in use – and crucially, these thermal stores reduce dependence on electricity and carbon-intense sources of “dispatchable” power during periods of peak use.[9]
I mentioned the high cost and comparatively short useful life of a consumer-scale heat pump. If you are looking to reorganise the way that heating and cooling are delivered to buildings, it seems absurd to swap one piece of short-lifespan heating equipment for another. District heating and district cooling for the most part get rid of that concern.
When complex heating or cooling equipment is placed in centralised facilities, they can be serviced, replaced, and updated as needed, but the capital cost and the hassle of that do not devolve onto individual households.
District heating and cooling also imply a materials saving, on the embodied materials and emissions of manufacture, compared to using millions of consumer heat pumps.
Sure, there will be the odd pipe or pump that needs fixing across the network of buildings supplied – but that is minor, compared to wholesale replacement of heat pumps every 15-25 years. I see little reason – from the position of use-values – to have heat pumps all over the place, whenever district heating and cooling are the more efficient option.[10]
To summarise: in my view, district heating powered by decarbonised heat pumps should be the priority, wherever they are feasible within the necessarily urgent timeframe of decarbonisation. Heat pumps in individual homes are next best, for example for isolated dwellings – and can function for cooling, too, if need be.
10.3. Hydrogen
You often hear about the potential for hydrogen to displace gas for home heating in a “green” way – and the fossil fuel industries are pushing this.
A hydrogen-based energy system would use hydrogen as an energy store (like a battery), or an energy carrier (like natural gas). Energy is applied to produce the hydrogen in a chemical reaction – either from fresh water or methane (CH4). The hydrogen is stored or transported, and then is either directly combusted to release energy again (as would be the case for home heating), or its chemical energy is released through a “redox” reaction in a fuel cell (e.g., in a car). Either way, the direct waste product is just water (and maybe some nitrogen oxides).
It sounds good, but there are numerous substantial problems. Not least is the source of energy.
Almost all hydrogen used at present (~95%) is produced from fossil fuels: coal (“black” hydrogen), lignite (“brown” hydrogen), and methane (“grey” hydrogen).
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| How hydrogen is produced. Source: LETI (2021), Hydrogen. A decarbonisation route for heat in buildings? |
Much is pinned on generation from methane with carbon capture and storage (CCS),“blue” hydrogen. However, a recent life-cycle analysis by Robert Howarth and Mark Jacobson found that the total emissions associated with the production of blue hydrogen are only 9%-12% less than for grey hydrogen. This is because methane is also used to power the CCS, which means that vented and fugitive methane emissions are higher.[11]
Yet, even purely “green” hydrogen – produced using electrolysis of water, powered by electricity from renewables – is not a viable or desirable replacement option for heating in homes, according to a recent review of the scientific literature focused on the UK.[12]
Additional problems include: sourcing fresh water for electrolysis; the necessity of pressurised storage; the fact that water freezes below zero– a problem for fuel cells; hydrogen has a tendency to chemically “embrittle” storage tanks; and the cost and location of natural sources of platinum and iridium, which are used in fuel cells.[13]
One of the main arguments for hydrogen is as both a medium-term and “interseasonal” store of energy, to buffer fluctuating flows of renewable power – an alternative to mechanical stores of energy such as reservoirs. However, this argument does not apply to hydrogen for heating.
A recent report by the House of Commons Science and Technology Committee concluded that, in the UK, hydrogen “does not represent a panacea” in the path to “net zero”. They reckon that hydrogen will likely only have “specific but limited roles to play across a variety of sectors to decarbonise where other technologies – such as electrification and heat pumps – are not possible, practical, or economic.”
For some time the idea of a green hydrogen economy has looked like a ploy by the fossil fuel industry and their friends in government, to maintain dependence on a centralised fossil-fuel infrastructure, while retaining market access for “blue” and “grey” hydrogen – with a branding arsenal of cutesy colours like “pink”, and “turquoise” hydrogens.
There is still talk, in the UK at least, of making gas boilers “hydrogen ready”. However, as far as I can tell, the thermal efficiency of new domestic hydrogen-combustion boilers is the same, or a bit worse, than a gas boiler. The UK’s Climate Change Committee (CCC) “assume that the efficiency of hydrogen and gas appliances is identical” (or indeed, worse: ~80% in 2020 for residential boilers, or ~86% for non-residential boilers).
There has also been talk, including from the CCC, of “hybrid” heat pumps featuring a hydrogen boiler for “back-up” power.
In terms of efficiency, more consequential than the poor performance of hydrogen boilers are the enormous energy efficiency losses upstream of final hydrogen combustion. The illustration below, from the Hydrogen Science Coalition, is instructive. For “green hydrogen”, each unit of useful heat out of a domestic hydrogen boiler requires about 6x more electrical power going in upstream than a SCOP 3.0 heat pump – and that’s assuming a hydrogen boiler with 90% efficiency.[14]
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| Source: Hydrogen Science Coalition |
More than anything, the energy inefficiency of the hydrogen supply chain means that, even with an inflow of entirely renewables-generated hydrogen, a hydrogen-based heat network fed to individual homes could needlessly dominate demand for electricity, and – in a commodified market for energy – likely cost much more for end-consumers than the alternatives. All while keeping the door open to nastier flavours of hydrogen.
Additionally, very recent evidence points to new concerns over the risks of hydrogen gas itself as an “indirect greenhouse gas” in the stratosphere – where it increases the warming effect of methane. In light of those findings, updated guidance from the CCC suggests that “greater attention may need to be given to hydrogen leakage and its role, offsetting some of the benefits of a hydrogen-based economy.”
Indeed, the UK CCC now (2023) say that hydrogen for home heating looks like a needless drain on a finite resource – green hydrogen. With a clear supply squeeze in the pipeline, they give every indication of wanting to “narrow the space” for hydrogen dependency, by further diminishing hydrogen’s role in home heating, or removing it entirely.
For all the above reasons, and with additional concerns around safety, campaigns against hydrogen – and against the fossil industries pushing it – have been gathering force.
In the UK, the government’s formal position remains that it will pursue community trials for hydrogen as a means of home heating.
However, recently (July 2023), plans for one such trial have been successfully defeated by residents of Whitby, Merseyside. After that defeat, energy minister Grant Shapps said that hydrogen for home heating in the UK as a whole now looks “less likely” – good news, and a considerable victory for campaigners.
Even more recently, there has been some indication that geological sources of hydrogen gas may be economically available (“white hydrogen”). That could be good, to the extent that it would provide a ready-made source of clean-burning chemical energy, side-stepping the energy-inefficiencies of hydrogen production, along with the associated greenhouse gas emissions.
However, the environmental and social side-effects of white hydrogen extraction remain unclear. All the drawbacks listed above would also still apply: the problems with storage, the indirect greenhouse effects of hydrogen itself, and the material constraints in the fuel cell supply chain – and the likelihood that any continuation of a hydrogen economy simply maintains the economic viability of dirty sources of hydrogen.
For home heating, and not just in the UK, it is clear that, compared to heat pumps and district heating, hydrogen combustion is a dead end.
10.4. Just transitions
So, heat pumps combined with district heating and cooling, are the best ways to decarbonise space conditioning and hot water. Heat pumps in individual homes are the next best thing. Hydrogen for heating buildings is a dud.
Decarbonising electrical grids will require ultra-high voltage DC lines, and diverse systems of power storage so that electrical power can be effectively dispatched on demand, instead of relying on fossil-based sources of energy, and the dead-end of hydrogen.
Green electricity can and should come from distributed, community-controlled mini-grids. Nevertheless, it seems that national, regional, or at least local, grid access will be necessary for most people, in order to buffer fluctuations in local green production and end-demand. This is the case for home heat.
Alongside the full decarbonisation of energy and heat, I think that we should be fighting too for the decommodification of essential electricity, globally. I would like to see public ownership of 100% renewable power-generation, power-storage, heating and cooling systems, and the electricity system – all managed democratically in the public interest, locally and nationally. And I think that all households should have “universal basic energy”, free at the point of use, to cover an agreed quota of energy needs.
Decarbonising transport, manufacturing industry, and the economy as a whole means that everything presently powered by something other than electricity needs to be transferred onto electricity.
This is “energy transition” as normally conceived, and it brings with it the need for a massive expansion of the generation of electricity, wherever fossil fuels presently function. This “crowding in” to electricity is one reason why the in-built efficiencies of heat pumps are so important – providing much more useful energy out per energy in than the alternatives.
But we should also want to massively expand access, globally, to socially-necessary use-values powered by green electricity: to power basic, and more than basic, needs.
At the world scale, I have referred throughout to the 2018 paper by Arnulf Grubler and his colleagues, modelling a “contraction and convergence” Low Energy Demand (LED) scenario for world energy usage.
In their LED scenario, global access to electricity and standards of living would increase, while global total energy consumption would go down to about 60% of where it is today, reaching ~245 EJ/year in time for 2050.
I pointed out that the authors envisage the global north narrowing on its current per capita residential floor area of 30m2. They see the global south’s mean per capita floor area rising from 22m2 in 2020 to 29m2 in 2050, and being more evenly distributed. (See part 6.)
All of that indoor living space for ~9.2 billion people in 2050 also needs to be habitable – so they see it built or retrofitted to be energy and thermally efficient.
They have the world total for useful thermal energy (that is, roughly, the heat energy used in the world’s buildings) in 2020 at ~12,200 terrawatt-hours (TWh) – that’s 12,200,000,000,000 kWh. But that useful energy comes from ~19,200 TWh of final energy: that is, ~40% is lost between energy-in (mostly coal, gas and biomass) and energy-out (mostly heat, in leaky buildings).
Grubler et al project, in 2050, a world total of useful thermal energy available at ~5,500 TWh, and final energy at ~4,400 TWh (16 exajoules). That is, mostly electricity goes in, and ~25% more useful thermal energy comes out (or is transferred), worldwide. How is that done? Heat pumps and insulation.
In any case, all of the world’s buildings – and especially homes – should be able to perform their essential functions of providing shelter and comfort, including sufficiently warm or cool interior space.
Plainly, principles of contraction and convergence should mandate significant constraints on consumption by the world’s largest consumers.
Just looking at space conditioning: even with dramatic efficiency savings from heat pumps, space conditioning should be constrained – if only to limit the quantity of embodied emissions, and the quantities of materials like rare earths, needed to produce solar panels, wind turbines and the like.
That in turn means rationalising the use of buildings to socially-necessary and egalitarian ends, and improving the thermal performance of those buildings, where that is able to constrain lifecycle emissions over (say) a 30-year period of building use.
In part 6, I went into the enormous build-outs in new buildings floor area forecast by the IEA and other international organisations– and pointed to the enormous unmet need right now in global housing. Decent housing is a basic need, and must be expanded as a matter of political urgency. So operational inefficiencies and inadequacies of buildings cannot be replicated.
That means updating and strictly enforcing building codes. New, aggressive thermal efficiency standards should be applied to new buildings internationally – covering fabric efficiency, and energy systems such as heat pumps.
That is especially important in locations of rapidly expanding building stocks. States should mandate the use of heat pumps for supplemental space conditioning and hot water; and/or provide, and be helped to provide, district heating and cooling as a municipal service.
Operational energy is increasingly being regulated, as I mentioned in part 6. Viet Nam and Papua New Guinea are moving solidly in the right direction, according to the UNEP, as are the countries involved in the Caribbean Regional Energy Efficiency Building Code (CREEBC), and the EU, Colombia, Lebanon, Maldives, Montenegro, Panama and Vanuatu.
Those regions that do need to construct more new buildings over the coming decades have – or should be allowed to have – all the benefits of “late developer advantage”. They are in a position to bake in high operational efficiencies in new buildings, to secure standards of thermal comfort, and mitigate the danger of fuel poverty, from the get-go.
Yet those (poorer) countries forecast to experience the most rapid and significant increases in population and building stocks over the coming decades – Nigeria, Bangladesh, India – have weaker measures in place to tackle operational or embodied emissions.
In many such places, there is also already a dramatic under-supply of adequate housing, and a lack of affordable housing. For example, 54% of Nigeria’s urban population are presently defined as living in slums, informal settlements or inadequate housing. Nationwide, even now, Nigeria has 16-20 million too few homes for its population.
Evidently, Nigeria needs many millions of new residential buildings. These need to be hyper energy-efficient operationally, and especially thermally. They need to be flood resilient, and need to be fed with stable infrastructures of electricity, heating and cooling, that are equally sustainable, and cheap if not free at the point of use.
With regard to the world’s existing buildings, there are important conversations to be had globally about the best way, politically, to bring buildings’ operational energy and operational emissions into an appropriate “contraction and convergence” pathway.
Heat pumps alone bring enormous energy efficiency savings compared to fossil fuel boilers and biomass combustion, as I have shown above – whether they are used in individual buildings, or as part of district heating and cooling networks.
Depending on the thermal performance of existing buildings, energy conversion savings like these may be the largest efficiency saving available, even when compared to quite substantial retrofit improvements to fabric efficiency. Fabric improvements are often challenging, can entail disruption, and carry risks if done badly.
Yet, when thermal performance is poor, significant but shallow fabric improvements, e.g. draught-proofing and additional loft insulation, can often be made easily. Such measures can be enough to greatly improve thermal comfort while avoiding the downsides.
Different pathways will be appropriate according to location and circumstance. Different countries and regions will have different baselines to work from. One important consideration will be how to pace decarbonising space conditioning and hot water, alongside any necessary improvements to the thermal performance of existing buildings through fabric retrofit.
10.5 Conclusions
There are no immovable social, political or economic constraints on the people of the world to solve whatever problems they collectively choose to solve. Among those should be sustainable energy and sustainable buildings for all – to address the real needs people have, like decent housing.
But those problems cannot be solved through a system centred on the profit motive alone.
In this series, I have outlined where greenhouse gas emissions come from in the built environment, globally – and many of the means available for decarbonising it. I have also situated decarbonisation in the context of a “contraction and convergence” approach to international development. In this last post I have addressed all this from the perspective of home heating and cooling.
Just transitions are essential in relation to heating and cooling. They are also essential with respect to the built environment as a whole, globally. This means decarbonising both embodied emissions, and operational emissions.
In my view, all of that requires the global economy to be rebuilt, and made autonomous from the capitalist drive for profit – oriented instead on providing for essential human needs.
That requires an enormous political effort on the part of the working class, globally. It means freeing national economies from the directive control of the capitalist class. And it means steering the economies of the world in another, more liberatory, direction.
The End
🔴 Go to Contents and Introduction
Download the whole series as a PDF here
[1] You can read a useful overview here, from a 2021 report by the UK’s Department of Business, Energy and Industrial Strategy (BEIS) on the electrification of home heating
[2] You can read more on this from the UK’s Carbon Trust here
[3] Here is some test data from 2021, from the UK Department for Business, Energy & Industrial Strategy (BEIS).
[4] See here for a similar example from LETI
[5] When natural gas is burned in a gas boiler for home heating, on average it directly releases 180 grams of CO2-equivalent emissions for every kilowatt hour of energy consumed (180g CO2e/kWh): these are its “scope 1” emissions. For gas consumption in the UK, an additional 31g CO2e/kWh are associated with extraction, refining and transportation (“scope 3” emissions), including vented and fugitive emissions. (These are the 2022 estimates assembled by the BEIS.) So the total emissions factor for natural gas combusted in UK homes is ~211g CO2e/kWh.
Note that, for methane’s “scope 3” emissions – specifically, the warming effects of deliberately vented and fugitive methane – BEIS use a 100-year global warming potential (GWP) of 25 (ie, 25g CO2e per g methane emitted). However, in the view of many experts, a 20-year GWP for methane is more appropriate – in which case, the IPCC recommends a GWP of 84-87.
In any case, by those 100-year emissions factors, a gas boiler burning 100 kWh-worth of natural gas in the UK, to deliver 85 kWh of heat, is responsible for ~21.1 kg CO2e of emissions.
Heat pumps, on the other hand, are usually powered by electricity from the national grid, which has an emissions intensity (indirect “scope 2” emissions) of about 190 gCO2e/kWh for power generation (2022 figure, BEIS). (This fluctuates greatly and varies regionally: see the National Grid’s “Future Energy Scenarios” 2022 data workbook, and here.) Additional (“scope 3”) emissions of ~18 gCO2e/kWh are associated with the electricity transmission and storage network (for example through energy lost between generation and end-consumption). So that adds up to an emissions intensity of ~208 gCO2e/kWh in 2022 – not that much different from burning gas directly.
However, 85 kWh of heat transferred by a heat pump in the UK, will on average be powered by just 34 kWh of electricity. That means only ~7 kg CO2e of emissions, in 2022 – so about 33% the emissions from a gas boiler, for the same amount of useful heat energy transferred.
[6] The UK’s Climate Change Committee (CCC) said (in 2020) that – as with gas boilers – the lifespan of a domestic air-source heat pump is generally around 15 years, and that a ground-source heat pump lasts perhaps 20 years before it needs to be replaced. The UK’s Energy Saving Trust (2021), said much the same. The IEA (2022) estimates a 17-year lifespan for the average gas boiler, 15 years for air-to-air heat pumps, and 18 years for air-to-water heat pumps. One 2013 study, citing earlier data, suggested that “30 years […] is a standard estimated lifetime for GSHP systems”.
Jan Rosenow, an energy systems researcher with the Regulatory Assistance Project (RAP), says that heat pumps can indeed last longer than 15-20 years, if they are properly maintained. The RAP base their projections for the cost of heat pumps on a 20-year timespan of operation.
Meanwhile, the heat pump industry in the UK says that recent technological developments mean that the lifespan of a new heat pump is now 20-25 years. Publicly-accessible evidence for that seems to be thin on the ground. However, one manufacturer estimates that 80-90% of their heat pumps last longer than 20 years – dependent on proper installation, “reasonable conditions”, regular servicing, and prompt repair when problems arise.
[7] This section draws on research commissioned by the UK’s Climate Change Committee
[8] Alongside the heat source, heat networks can also be classified by temperature. Early DH networks in the late 1800s and early 20th century tended to be steam-based, with network temperatures over 120°C. Second and third generation networks were hot water-based. More recent (4th generation) DH technologies tend to use lower temperatures, of ~60°C or below. Building-scale heat pumps can be used to “top up” the temperature of incoming water from a heat network.
[9] The modelling for 2015 research for the CCC indicated that periods of peak heat use nevertheless required “dispatchable” sources of heat, in the form of combustion – even when the “baseload” heat source is fully renewable. The authors model a remarkable 35% of annual heat “demand” for such networks coming from gas combustion. One would assume lower temperature baseloads could mitigate that need. And, contrary to those 2015 assumptions, two heating sites in Helsinki apparently now use heat storage to meet peaks of heat dispatch, and for the most part forego boiler use entirely.
[10] Notably, a recent paper in the journal Applied Energy surveyed options for decarbonising heating systems in the UK, and found that district heating fed by heat pumps would be the cheapest option overall because of its economies of scale – about 11% cheaper than fitting heat pumps in every home.
[11] Jacobson and Howarth estimated that emissions from “blue” hydrogen production are ~486-500 gCO2e/kWh, against grey hydrogen’s ~550 gCO2e/kWh. Note that – correctly, in my view – they used a 20-year global warming potential (GWP) of 86 for methane, instead of the more usual 100-year GWP of 28-36. They think that the emissions associated with blue hydrogen could be reduced to ~200 gCO2/kWh, if the CCS was powered solely by renewables. But that’s still barely less than the emissions factor for natural gas (see above) – and would only come after enormous build-outs in infrastructure.
[12] See here for more on this from Fiona Harvey in the Guardian.
[13] Youtuber Sabine Hossenfelder has a really good explainer video, from which I can only conclude that the notion of a “hydrogen economy” of any scale is absurd. She points out that green hydrogen production looks likely to remain very expensive for a while. Without very steep carbon tariffs and regulation in place, why would anyone use renewables to make hydrogen instead of using methane, or instead of storing green energy by other means?
[14] The paper that I cited on district heating gives a smaller differential than the Hydrogen Science Coalition estimate, but still a large one: x4. Either way, hydrogen does terribly compared to a heat pump, just on energy efficiency grounds, before you factor in emissions. LETI make similar comparisons in their own report.
The UK 6th Carbon Budget (6CB) (2020) recommended that only surplus (“curtailed”) renewable power be used for hydrogen production. But it still saw hydrogen as a plausible supplier of home heating. The 6CB’s middle-road “balanced net zero” (BNZ) pathway envisaged it being used in 11% of homes by 2050. However, all of those boilers would be “hybrid” heating systems, in which most heat still came from heat pumps. The 11% translates to ~2.2% of year-round domestic space heating UK-wide.
Absolutely great series of articles. More chance of herding a bunch of cats on fire than implementing the conclusions but lays out the roadmap for Armageddon + 1 en route to utopia.
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