Heat pumps or hydrogen: how do we decarbonise our heating?

By Nicola Terry, Blogger: ‘Energy Thoughts and Surprises’

 

You may have heard stories in the media about trials of hydrogen gas as a low carbon heating solution. However, CCF recommends heat pumps for zero carbon heating for most homes. Why? What is wrong with the hydrogen?  Well there are a number of reasons we think hydrogen is not a sensible option.  Firstly, there is no sustainable source that can supply at the scale required, and secondly, even if there were, getting it into our homes will take decades and we do not have that long.

Hydrogen is not available in nature as a gas, at least not in bulk. (Actually our farts contain quite a high proportion of hydrogen, but this is not a practical supply for home heating).  The main sources of hydrogen are colourfully categorised as brown, green and blue. Green is the best.

  • Green hydrogen: made from water, using renewable electricity. This is effectively zero carbon, but expensive. Making in bulk needs lots of electricity.
  • Brown hydrogen: made from methane, by a chemical process which extracts the carbon in the form of CO2 – exactly what we are trying to avoid.
  • Blue hydrogen: made the same way as brown except that the CO2 is captured and stored – although in practice it is difficult to capture 100%.

Blue hydrogen has at best a third the carbon emissions of natural gas.

The processes to make hydrogen from methane are well developed and could be scaled up fairly easily. The processes for capturing and storing the carbon are reasonably well developed, though we have done very little storage so far. Still, blue hydrogen is not as clean as we need. It is impossible to capture all the carbon and even if you did, there is always some methane leakage at the gas well and on the way to the processing plants. Methane is a major greenhouse gas, with a global warming potential 56 times that of CO2 over 20 years [1]. Estimates of the GHG emissions of blue hydrogen vary but 90 gCO2e/kWh is typical – this is about half that of natural gas. Some estimates are more favourable – about a third. So taking this route only reduces emissions by two thirds, at best [2].

Green hydrogen will need five times more wind farms and solar panels than heat pumps.

We have long known how to make hydrogen from water by using renewable electricity but the efficiency has been very poor, until recently. Now, after a lot of research effort we are up to about 80% which is not bad. Further development is still useful, though, to bring down the temperature needed. This would make it easier to run the process intermittently, when there is spare power available cheaply. At the moment, we only have spare power very occasionally, but this will be more common as we increase our wind and solar power generation.

The real trouble with this approach is that, in comparison with heat pumps, our boilers are terribly inefficient. This chart shows the difference. With boiler efficiency around 86%, compared to heat pumps 280% (a conservative estimate) it is better to use the hydrogen in a power station to generate more electricity for driving heat pumps than it is to use it to fuel your boiler. If we were to run our boilers on green hydrogen from electricity, rather than using the electricity directly in a heat pump we would need five times as much electricity overall – that means five times more wind turbines and solar panels. You could argue that this does not matter, as long as we can build them cheaply enough. However, even wind turbines and solar panels have environmental impacts.

There are other potential sources for clean hydrogen. To my mind, the most interesting approach uses sunlight directly rather than electricity. However, there is a lot more development needed to make this method efficient and scalable.

nicola.qeng-ho.org/resources/pathways-to-zero-carbon.pdf

The existing gas mains will need only minor upgrades for hydrogen.

After making the hydrogen, the next step is to get it into our homes. In fact this is not a problem as it turns out only a few parts of the gas mains system would need replacing. Hydrogen reacts with metal alloys, including high performance steel and this can lead to cracking. However, a large proportion of our gas mains are made of plastic or other materials which are hardly affected. Also hydrogen has a lower energy density than natural gas but it makes up for this by flowing faster.

All gas appliances and industrial plant would need to be converted to run on hydrogen instead of natural gas.

The problem is not with the mains but with our boilers and other gas appliances. They would all need converting or replacing. This is because hydrogen ignites and burns differently from natural gas [3]. When I was a child, we had a similar conversion, from town gas (mostly a mix of carbon monoxide and hydrogen) to natural gas. The problem is bigger now because we have a lot more appliances and a lot more kinds of appliances – every one of which needs slightly different handling. Safety is critical and we cannot afford to make mistakes.

Converting the gas grid to hydrogen cannot start until about 2040.

Converting the gas grid needs to be done a segment at a time. For each segment, we need to shut off the gas, convert the appliances, and turn it on again. To make this practical, all the equipment needs to be ‘hydrogen ready’, so that an engineer can do the adjustments or fit the new parts quickly. Some manufacturers are already selling hydrogen ready boilers and it seems likely that the government will be require all new gas boilers to be hydrogen ready in the mid 2020s [4]

However, boilers last a long time, so unless we opt to replace them before this we would have done normally, we cannot start to convert the gas grid until about 2040. This is too late. We need to substantially reduce our emissions by 2030 to be on target for limiting global heating – as shown in this chart from the Committee on Climate Change Sixth Carbon Budget Report.

www.theccc.org.uk/publication/sixth-carbon-budget/

There is a place for hydrogen – but not much for home heating.

In summary, hydrogen cannot be produced sustainably at sufficient scale to replace all our gas home heating, and even if it could, it would take too long to convert the gas grid to hydrogen. However, there certainly is a place for hydrogen in the zero carbon economy. It will be needed for some industrial processes that currently use gas, where heat pumps are not suitable (for example where high temperature heat is needed). It will be used to replace coking coal in making steel. It can also be used by power stations when wind and solar are not sufficient. This is feasible as long as the total requirement can be met by green hydrogen.

Hybrid heat pump/gas boiler solutions could be used with either hydrogen or biogas.

Assuming the whole gas grid is converted, which may never happen, then it may be that hydrogen is used alongside heat pumps as a hybrid solution for some homes. However, if most homes use heat pumps then it is more likely that the remaining gas demand can be met by zero carbon biogas sources. It would be reasonable to produce enough biogas for 5-10% of our heating, but definitely not all of it [5].

The heat pump solution is available now. It just needs scaling up.

Heat pumps are an easier solution because they can run on renewable electricity, they are very efficient and they are available now – though manufacturing needs to be scaled up and also we need to train lots more installers. For some homes, switching to heat pumps will be difficult without improving energy efficiency first. However, insulation and draught proofing solutions are also available now.

Heavy lobbying from the oil and gas industry has effectively stalled government policy.

Unfortunately, the government is subjected to heavy lobbying from the oil and gas industry and the gas boiler industry who stand to lose an awful lot of business as we move over to heat pumps instead. Their campaign aims to persuade elected officials that the switch to heat pumps will be unpopular and expensive. However, this does not mean that the hydrogen approach is better. There has been some eye -opening research on this, based on interviews with civil servants and industry consultants.

“A specific concern raised was that a small number of high profile stories about poor performance of low carbon heating systems could affect wider public opinions on overall heat decarbonisation. ‘All it takes is a couple of horror stories or something to go wrong to really change public perception quite quickly and certain newspapers publishing certain things can (laughs) really shift the dial. So it’s a massive concern for us and that’s for all the options.’ [civil servant]” See Is the gas industry promoting uncertainty as a delaying tactic? and Disruptive and uncertain: Policy makers’ perceptions on UK heat decarbonisation.

Fortunately, we do not have to wait for our government to decide for us!

[1] Methane oxidises in the atmosphere fairly quickly and over 100 years, which is the timeframe most often quoted, it is ‘only’ 20 times as bad as CO2. However, given the urgency of cutting emissions, 20 years is more appropriate and over the shorter time frame it is 56 times worse. See here for a table of global warming potentials.

[2] For a recent estimate, see Hydrogen on the path to net-zero emissions from the Pembina institute, a Canadian non-profit clean energy think tank. They estimate 20-35 gCO2e/MJ which means 72-126 gCO2e/kWh.  This includes upstream emissions such as methane leakage at the well. For comparison, the emissions of natural gas are about 200 gCO2e/kWh, again including upstream leakage, as well as the CO2 from combustion. Pembina assume 80% to 90% capture rate but this is not the only reason the emissions are high. You need more methane to start with, to make hydrogen with the same amount of energy, which increases the upstream emissions and process carbon. Plus, hydrogen has such a low density you have to compress it to store or transport it anywhere and that takes energy too.

[3] It is possible to mix a proportion of hydrogen with methane without changing the burn characteristics too much.  Trials have shown that 20% is safe – though this is 20% by volume, which is only 7% be energy value. See also Why is the hydrogen level set at a maximum of 20%?

[4] Industry is pushing for a ban on ‘conventional’ gas boilers in 2025. For example see CBI calls for ban on gas boilers from 2025 to ‘accelerate progress’ to net zero emissions target.

[5] Biomethane is a versatile fuel that will be valuable for industry as well as home heating. The CCC recommends that use for home heating should be restricted to hybrid heat pump systems The Net Zero Technical Report says ‘Our previous analysis has emphasised the importance of limiting bioenergy use in buildings to biomethane produced from anaerobic digestion and other niche uses (as part of hybrid heat pumps systems in off gas homes and local heat networks). Our scenarios restrict the use of bioenergy to biomethane and serving peak demand in off-gas buildings with hybrid heat pumps.’

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