Are hybrid heat pumps the solution for low carbon heating?

The Committee on Climate Change has just published their view on the role of hydrogen in a low carbon economy [1]. Their recommendation for people currently on gas is to insulate your home as much as possible and then install a hybrid heat pump/boiler system. You can get hybrid systems now, only burning methane gas rather than hydrogen. The idea with a hybrid is that the heat pump supplies all the heat most of the time, but with a boiler for backup when demand is very high or electricity supply is expensive. In theory, a hybrid system should supply around 90% of the heat from the heat pump - however, this is only the case when the system is configured correctly. In field trials the proportion of heat from the heat pump has ranged from 96% down to 30% [2]. No wonder the RHI subsidy requires that hybrid heat pumps have metering to actually measure how much renewable heat is supplied through the heat pump [3]. If hybrid heating systems are to play their part in a sustainable future we have to learn how to manage them properly.


The pure electric scenario strains the grid, the pure hydrogen one is more expensive.
The CCC favours the hybrid approach over pure hydrogen or pure heat pumps:

• A pure electric solution (heat pump only) would put more strain on the electricity supply system - both generation and distribution - on cold winter days when heat demand is high.
• A pure hydrogen system would require more hydrogen than we can generate from low carbon sources so it would need carbon capture and storage (CSS) too. However CSS is not actually as low carbon as the other low carbon solutions so it would be more expensive to provide a zero carbon solution.

CCC commissioned research from Imperial College London (ICL) to compare the different pathways [4]. They found that a fully electric heating system requires a great deal more reliable power plant than a hybrid or gas based system, most of which would be hardly every used. This is expensive. However the hydrogen gas solution requires a lot of new infrastructure too. Overall, ICL found the hybrid solutions costs slightly less than the all electric solution and a lot less than pure hydrogen in the zero carbon case, as shown in the chart below.

Comparison of costs in different scenarios for decreasing carbon emissions. The pure hydrogen scenario is much higher costs in the zero carbon case. Data from [4]

The hybrid solution is supposed to be more acceptable to consumers.
Although the hybrid solution is the least cost in all cases, there isn't that much difference from the all electric. The CCC prefers the hybrid not just on the grounds of cost but also on consumer acceptability. The CCC says [1]:

• A hybrid heat pump can be retrofitted around the existing boiler, making it part of an upgraded, smart heating system. This retrofit can be done alongside improvements to energy efficiency of the building, leading to dramatic cuts in both emissions and fossil fuel consumption while retaining high performance and potentially improving comfort levels.
• Retaining the boiler means that the heating system would provide equivalent performance to existing heating systems, and would not require changes to radiators. This more incremental approach to switching to heat pumps is likely to be considerably more acceptable to the public than replacing the boiler with a heat pump.

But the hybrid system is much larger than a conventional boiler.
However, neither of these statements is entirely true. Firstly, a hybrid system is likely to be much larger than a normal gas boiler and you still need both inside and outside components of the heat pump. The table shows the size of some products from Daikin. The hybrid system is more than twice the height of the standard boiler and twice the depth. However it is still smaller than a high temperature heat pump. Size was a big issue when I investigated replacing our boiler at home with a heat pump - Why I haven't installed a heat pump (yet).

System	HxWxD
Condensing gas boiler(24 kW) [5]	400 x 590x 256mm
Hybrid boiler (indoor component) [6]	1075 x 450 x 405
High temperature heat pump (indoor component) [7]	705 x 600 x 695

Hybrid heat pumps can give equivalent performance but if you treat them like a boiler they use gas like a boiler.
Normal heat pumps can heat up to about 55C; gas boilers are normally configured to heat up to 70C but you should not need this except in cold weather. With a pure heat pump system you normally have to fit larger radiators to get enough heat at the lower temperature. A high temperature heat pump can run with your standard radiators but the efficiency is not quite so good. With a hybrid system you can run the heat pump normally and get top-up heat from the boiler.

The ICL analysis was based on a model where the heat pump provides heat up to 55C and the boiler tops up to 65C when necessary. 89% of the heat requirements were met by the heat pump, on the basis that the controller only activated the gas when strictly necessary.

This chart shows the ratio of heat supplied by heat pumps in hybrid systems from field trials reported last year. You can see that some performed very well but others did not. The 89% requirement is certainly achievable but only 3 cases did so, out of 14. There are several possible reasons for poor performance. A big one is the scheduling: heat pumps are good for gentle continuous heat but we normally configure our gas boilers for a burst in the morning and again in the early evening. If you configure a hybrid system the same as a gas boiler you will probably use gas most of the time. You might not even notice. You will see more gas and less electricity on your utility bill but in terms of overall cost there might not be much difference - at least with current flat pricing regimes.

Proportion of heat supplied by boiler and heat pump in field trials [2]

It is doubtless true that a hybrid system will be more acceptable for most householders than a pure heat pump because of the peace of mind of the boiler backup. However you still need to find somewhere to put the larger unit, and you still need to configure it like a heat pump and not like a gas boiler, if we are to achieve lower emissions.

Both scenarios have their risks. We need to learn how to manage them.
The hybrid heat pump strategy runs the risk that poorly configured systems will require a great deal of gas. However there is no guarantee that a heat pump system is configured correctly either - if not it will use more power than intended, especially in cold weather at peak times. Both strategies have risks but I think the hybrid strategy is more flexible and potentially resilient.

To make hybrids work well we need excellent control and monitoring systems, good practice from installers and savvy consumers. Our need to reduce carbon emissions is urgent so we had better get on it.

Whatever happens though, reducing demand with good insulation and heating controls is a step in the right direction.

[1] Hydrogen in a low carbon economy (CCC) Nov 2018
[2] Hybrid Heat Pumps (Element Energy, for BEIS) Dec 2017
[3] Factsheet: do I need metering for the RHI (Ofgem) Sep 2017
[4] Analysis of alternative UK Heat Decarbonisation Pathways (Imperial College London) Aug 2018
[5] Gas condensing Boiler Daikin D2CND
[6] Hybrid heating system Daikin
[7] Why I haven't installed a heat pump (yet). (This blog)