As part of its plan to get to net zero carbon emissions by 2050, the UK Government seems to have subscribed to the currently dominant view that electric heat pumps are far more efficient than direct gas/hydrogen for home heating, with Coefficients of Performance (COPs) of up to 4. So we should go for them and stop using gas for heating. The government has set a target of 600,000 heat pump installation p.a. by 2028, up from around 35,000 p.a. now. However heat pumps are much more expensive than gas boilers, and, despite lobbying pressure, the government won’t say when exactly gas use for heating will be banned!
So the debate on heating options is far from over, and as I have noted in earlier posts, there are other views on all this. A long standing one concerns the possible role that large Combined Heat and Power (CHP) cogeneration plants could play for high efficiency energy conversion (80% or more) supplying heat to homes via community-scale heat nets. CHP plants can have COP equivalents of up to 10 or more, depending on the temperature required, and arguably can supply heat more reliable over the year than heat pumps e.g. in winter when air/ground temperatures are low. And large CHP units with large heat stores linked to district heating (DH) networks can also add a lot of system flexibility, dealing with peaks and troughs in heat demand, with CHP plants being able to vary the ratio of heat to power produced to match varying renewables output and varying demand. Heat of course can be stored for some time without major losses and is one of the cheaper energy storage options.
The trouble with the CHP/DH idea is that, so far, many of the CHP/DH systems on the continent, where there are a lot, and the fewer number in the UK, have been run on fossil gas and we don’t want to use fossil gas any more. Biomass and waste is also used, but there may not be enough sustainable biomass to make a major contribution to heating and there can be toxic emission issues with waste combustion. And, arguably, it’s claimed that it wouldn’t make sense to use expensive renewably produced green hydrogen to run CHP plants - green gas will always be more expensive than the green power used to make it. In any case, it is argued, if we do have green power it’s better to use that direct in a heat pump- big or small. But, then again, some say green hydrogen will get cheaper, hopefully getting down to €5/kg by 2025, and it does have the attraction of being easily storable, unlike electricity. And we also have the gas grid- why not repurpose it for hydrogen for heating homes either directly or in mini CHP fuel cells. With biogas as a possible addition. Or even Ammonia.
The debate continues. Some say heat nets are centralised and invasive and use a lot of materials for piping. Some even say what’s wrong with small electric fires and also other more directed personal heaters. Others however see heat nets as part of a process of decentralising energy generation, with heat distribution via networks, including local nets. Going for the green max, some look to solar-fed district heating, with community-scaled heat stores, as developed in Denmark. However, that now seems to have been overtaken by large heat pumps there and in Norway…
Big heat pumps certainly do have their attractions, with some also now being deployed in the UK, including some using ground/geothermal heat sources. But they can only be sited in some locations. Same with another approach, the direct use of mine heat. And of course, CHP/heat nets are not really viable in most low-population rural areas So, for heating, we may end up with a mix of mainly large and small heat pumps, along with some green hydrogen/ biogas-fired CHP/DH in some urban areas- assuming that all the green biogas, H2 or NH3 available hasn’t been used for vehicle/aircraft fuel.
However, it’s still early days for much of this, with a lot of uncertainty- and some intriguing biogas from grass and waste heat/hydrogen production cross-over ideas. And even the idea of using people’s heat energy! Plus of course a lot more building insulation, making any sort of heating less necessary. Or, at the other extreme, there’s heat from nuclear CHP plants- assuming they can be built near to cities where there is enough of a heat load, as some think is possible for the proposed new breed of Small Modular Reactors. Seems a long shot, given the safety and security issues, though SMRs might find a role for power and heat supply in isolated communities in remote, cold, locations.
All in all, a fascinating area, with its fair share of controversy and some quite conceptually stretching thermodynamic views. For example, some say that, since the heat they rescue would otherwise be wasted, the COP for CHP plants is in theory infinite. Though couldn’t you say something similar for heat pumps- since they make use of ambient heat that would otherwise not be used.? But that’s low grade heat from outside of the system boundary and you have to use high grade power to pump it up to higher temperatures. Whereas, while tapping-off heat from an intermediate stage of the steam turbines in a CHP plant does reduce its electrical power output, it can deliver high grade, high temperature heat. Ah, but using that requires expensive and invasive pipework. But installing the heat exchangers for domestic ground source heat pumps is also pretty costly and disruptive too- and you have to strengthen the power grid to meet the new very peaky heat demand that used to be met by gas. So the pipe versus wire debate continues!
The long running Claverton Energy Group (CEG) expert group has been discussing the CHP/DH and heat pumps issues again of late- you can get access by subscribing to CEG at https://claverton.groups.io/g/main And also watch out for the books on CHP/DH that have been commissioned by the IoP from two leading members of the group, which will hopefully offer a clear way through the expanding and sometimes confusing and controversial green heating field.
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