Worth reading: Low carbon heating choices
The Centre for Alternative Technology’s Zero Carbon Britain scenario mainly uses heat pumps for heating, but it’s not the only option being championed for a zero carbon future. Joel Rawson looks at the potential of some other technologies in the frame.
This article first appeared in the autumn 2021 issue of Clean Slate, the magazine of the Centre for Alternative Technology (CAT), cat.org.uk.
One of the biggest challenges we face in a zero carbon transition is moving away from the fossil fuels that most homes still rely on for heating. In the Centre for Alternative Technology (CAT)’s Zero Carbon Britain report, heat pumps replace gas or oil boilers or electric heating in most homes, plus a sliver of biomass heating for some rural areas.
But other options are being promoted, including hydrogen, heat networks and direct electric heating of various kinds. Can these offer the same level of carbon saving? And what are the implications for infrastructure to support them?
There’s already much more about heat pumps on the CAT website (at https://cat.org.uk/heat-pumps), so I won’t go into much detail here, but the key thing to keep in mind when comparing with other options below is that a well-specified heat pump should give three or four units of heat for every unit of electricity consumed.
There’s a useful case study on the website of Trystan Lea, one of the researchers for the energy modelling in CAT’s latest Zero Carbon Britain report. Monitoring of Trystan’s own home heat pump system shows how with the right choice of radiators the annual electricity-to-heat ratio is 3.9. And that’s without high levels of insulation. Through a careful choice of tariff, in 2020 they bought electricity at about 11p per unit (kWh), so the effective heating cost was less than 3p per unit. This is about two-thirds as much as they’d pay to heat with gas, but perhaps the most important thing is that the carbon emissions are about 80% less than they’d be on gas. You can read more at https://trystanlea.org.uk/heatpump2020
The main barrier is the higher upfront cost, but this should drop with investment and mainstream adoption, as happened with solar PV and wind farms. A new ‘Clean Heat Grant’ is due to replace the Renewable Heat Incentive (RHI) in April 2022. The grant will focus almost entirely on heat pumps (with £5,000 for air source heat pumps), with biomass supported in a few special cases.
Are there other zero carbon options?
Various different heating systems come up in questions to CAT’s information service and in the media, and here I run through some of the key points to bear in mind. I’ll be adding more detail to the CAT website, to help people make balanced choices. I’d also like to hear if you’ve had either good or bad experiences with any of these.
Any system can be mis-sold, and a potentially good option could be sized incorrectly, or have the wrong choice of controls, or be overpriced. Salespeople may make over-optimistic claims about carbon savings or running costs, and perhaps even disingenuous comparisons with other systems.
Of course, insulation and related measures are the most important investment whatever the heat source. We must reduce nationwide heat demand so that’s its feasible to move as quickly as possible to only zero carbon energy sources. To complement the Clean Heat Grant, we’re waiting to hear if there will be a replacement for the withdrawn Green Homes Grant, which was stymied by administrative problems. See CAT’s information service web pages for updates, and for more on insulation and retrofit plans.
A heat network, or district heating, takes heat produced at a central point and pipes it around many buildings. Clearly this isn’t something you can just choose, as it depends on planning across a number of homes. It can be a great solution, especially for denser housing. However, the carbon intensity of heat networks will vary, depending on where the heat comes from. It could be a by-product of industrial processes, from a shared heat pump, from true geothermal energy, from anaerobic digestion of agricultural and domestic green waste, or incineration of municipal mixed waste.
The potential for a heat network could be identified and developed as part of a zero carbon action plan for a region or town. If you think it could work for you and your neighbours, you could spur action by getting involved in local zero carbon action planning. CAT’s Zero Carbon Hub and Innovation Lab is working to support communities and councils, so check online to see how they could help your community.
There’s currently a fair amount of promotion of ‘hydrogen-ready’ boilers, which seems to be led by the fossil fuel industry. At the moment only ‘grey hydrogen’ is being produced, from the steam reformation of natural gas. So it’s still a fossil fuel and leads to higher carbon emissions than gas. The main industry proposal is for ‘blue hydrogen’, made by the same process but adding in a not yet invented way of capturing and storing the emitted carbon. There are questions about how this will be achieved.
‘Green hydrogen’ is made from the electrolysis of water using electricity from renewable sources. CAT’s Zero Carbon Britain model allocated excess electricity to make green hydrogen and other synthetic fuels. These are important for certain purposes – such as manufacturing, larger vehicles and planes, and running ‘peaker plants’ at times of high grid demand.
Using hydrogen to heat homes raises questions, given how valuable it is for those other uses and the difficulty of making it. A heat pump can use 1 unit of electricity to give 3 or 4 units of heat. Whereas via hydrogen, 1 unit of electricity might give only 0.6 units of heat (if hydrogen generation is 70% efficient and the boiler 90% efficient).
Overall, the heat pump would then be 5 or 6 times as efficient as hydrogen at delivering heat from renewable electricity. In theory it could all be zero carbon, but if hydrogen is more than an occasional top up option it will demand far more electricity generation (and so several times as many wind and solar farms).
CAT’s Zero Carbon Britain model avoided ‘silver bullets’ – including technologies not yet developed and with uncertain timescales. The current proposals for hydrogen do have a silvery look. We know
that heat pumps will work and will reduce carbon emissions, but a ‘hydrogen-ready’ boiler might never actually reduce carbon emissions.
In theory, direct electric heating (such as an electric boiler, radiator or underfloor heating) has the potential to be ‘zero carbon’, and a careful choice of tariff can reduce the running costs a bit.
However these can’t be more than 100% efficient, and therefore consume 3 or 4 times as much electricity as a heat pump for the same heat demand. As well as much higher running costs, there’s the issue again of the infrastructure needed to generate all that electricity.
We therefore treat with caution any claims for these sorts of electric heating as an energy saving measure. A little electric heating might be part of a heat pump system, to top up in the coldest weather, but we really want to minimise the use of direct electric heating.
The direct electric options mentioned above deliver heat in the same way as water based radiators or underfloor heating. The claim for ‘far infrared’ heating is that because it heats people and surfaces rather than the air, the electricity use will be much less while delivering the same thermal comfort.
However, at the moment it’s hard to find a detailed breakdown of performance – it’s something we’re continuing to pursue. Comparisons need to be evaluated carefully, for example they might be with a heat pump that has a lower electricity-to-heat ratio than should be achievable with proper design.
Anecdotally, some have found that far infrared works well in a bathroom, but they’ve preferred warm air heating elsewhere. But this doesn’t mean it couldn’t work in some circumstances, and we need better data to make fair comparisons.
I’d really like to hear more about what our readers, or your contacts, have found in real-life examples. In particular, data about the performance of far infrared options will help us to make better comparisons with what we know is possible with a heat pump. You can contact me on firstname.lastname@example.org or via our website: https://www.cat.org.uk/info