“Hydrogen versus Heat Pumps – thinking through the science of a hydrogen economy”

Looking at the energy use, cost and carbon cost of two different options.

An increasing number of people are talking about replacing gas boilers with electrically-driven heat pumps. Separately, we hear on the media about the long-awaited “hydrogen economy” arriving at the end of the decade. So an obvious question is… should I hold off on a heat pump and just stick with gas because it will be replaced by hydrogen?

Suitable for A level (ages 16-18), advanced Key Stage 4 (ages 14-16) and people who want to refresh their school science knowledge.

Skills: Costing, Comparisons and Chemistry.

Here we ask a few questions that get us to think about the science.

Hydrogen – fuel of the future

Ultimately hydrogen will be derived from electrolysis of water using renewable electricity, so called “green hydrogen”. Today hydrogen is created mainly from steam methane reformation (see second box below), a process which uses natural gas and releases carbon dioxide into the atmosphere (“grey hydrogen”). Some hydrogen is created from coal and heavy oils which is even more carbon intensive. Some groups (the Climate Change Committee, CCC [1]) advocate a middle approach (“blue hydrogen”) to bridge the gap before green hydrogen volumes become sufficient. That approach is one where a substantial portion of the carbon dioxide emissions in steam methane reformation are trapped and not released. It is claimed by others that the carbon dioxide intensity of blue hydrogen could be similar to that of simply burning natural gas [2].

Hydrogen combustion

At point of combustion a pure hydrogen gas burns in air to release water vapour:

2H2 + O2 → 2H2O

The heat released from combustion is 286 kJ/mol. This assumes liquid water is produced and is called the higher heating value or gross calorific value or Gross CV (another figure exists for the lower heating value or Net CV where the energy required to vaporise the water is subtracted – this figure is not recommended because efficiencies above 100% can be quoted when some of the water is condensed).


H2 has molecular weight 2.016 g/mol (2 * 1.008) so the energy in 1 kg of gas is 142 MJ/kg (as 1 MJ/kg is identical to 1 kJ/g)

Comparing with methane. The heat released from combustion of methane is 890 kJ/mol. Methane is CH4 with molecular weight of 16 g/mol (12.011 + 4 * 1.008 = 16.043). The energy in 1 kg of methane is 55.5 MJ/kg.

Question:

If we don’t have huge quantities of renewable electricity, is it better to heat our homes from burning green hydrogen (derived from electrolysis of water) or from running a heat pump?

Electrolysis is at best around 80% efficient (although by 2030 this is hoped to increase to 85%).

If we had a completely efficient electrolysis we need 142 MJ/kg so an 80% efficient system we need 177 MJ/kg. We could assume that the best efficiency for combustion of H2 is 95% (Gross CV), assuming it will be similar to the best gas boilers today. We’ll assume negligible pipeline losses. To deliver 1 kWh of heat, that will require 3.8 MJ of hydrogen (1 kWh = 1000 W * 3600 s = 3.6 MJ) which is 0.027 kg. We know that it takes 177 MJ/kg of electricity to produce green hydrogen so 0.027 kg H2 will require 4.72 MJ of renewable electricity (1.31 kWh).

Or we could use a heat pump. With a coefficient of performance of say 3.8, we need just 0.26 kWh of electrical energy to move 1 kWh of heat into our home. Assuming losses in the electricity grid are around 10% then we would need 0.29 kWh of renewable electricity (we didn’t need to add the transmission and distribution losses to the electricity for electrolysis as hydrogen will probably be generated at scale in close proximity to low carbon sources of electricity).

So the renewable generator needs to generate 4.5 times more electricity to allow you to use green hydrogen in the home than for you to use a heat pump.

Question:

If green hydrogen were available today, what would be the difference in carbon dioxide emissions between burning H2 and running a heat pump?

We’ve just seen that we require 4.5 times more electricity to deliver the same heat in a home. This feels like an easy question to answer. To check we can assume we are using onshore wind power for both the electrolysis and for the heat pump (the IPCC uses a median value of 11 gCO2e/kWh [3] – this is not quite zero because it accounts for the embedded carbon in construction, maintenance and decommissioning).

Electrolysis requires 1.31 kWh of electrical energy and we’ve assumed the best case of no transmission and distribution losses when we produce hydrogen right next to the wind farm, so the total emissions figure is 0.014 kgCO2e.

A heat pump requires 0.26 kWh at point of use which would be 0.0029 kgCO2e. We then add to that the transmission and distribution losses in the UK grid for 2021 (0.01879 kgCO2e/kWh [4]) of 0.0049 kgCO2e to give us a total emissions figure of 0.0078 kgCO2e.

So we emit twice the carbon dioxide for electrolysis to produce hydrogen to give us the same heat as running a heat pump.

Steam Methane Reforming

Steam methane reforming is described by

CH4 + H2O ⇌ CO + 3H2 (Energy In ΔH = 206 kJ/mol)

Often a water-gas shift reaction is used to generate more hydrogen

CO + H2O ⇌ CO2 + H2 (Energy Out ΔH = -41 kJ/mol)

So 1 mol of CH4 produces 3 mol H2 requiring 206 kJ and a further 1 mol H2 generating 41 kJ. Efficiencies are around 70% so let’s assume 1 mol CH4 requires 294 kJ and generates 29 kJ of usable heat. The net 265 kJ required is assumed to come from combustion of methane on site. Methane combustion gives 890 kJ/mol. Allowing for a 95% efficiency here gives 800 kJ usable heat per mole. So approximately we can say that we need a third of a mole of CH4 in addition to our one mole of CH4 feedstock to produce 4 mol H2.

Without trapping the carbon dioxide produced this process would generate 1.33 mol of CO2. In other words, 4 mol H2 (4 * 2.016 g H2) releases (1.33 * (12.011 g + 2 * 15.999 g)) = 58.5 g CO2. That is 7.26 kg CO2 released for every 1 kg H2 burnt.

In the example, 1 kWh of heat was produced from burning 0.027 kg H2. So 1 kWh of heat results in 0.196 kg CO2 released during hydrogen production. This excludes Well-to-tank emissions and embedded carbon in the process plant.

Aside:

For comparison, what are the carbon dioxide emissions from burning methane?

It might be tempting to consider what the carbon dioxide savings are from switching from natural gas to green hydrogen. To generate the same 1 kWh heat with a gas boiler using methane, then we also need 3.8 MJ of methane (assuming 95% efficient gas boiler) which is 0.068 kg. Published carbon dioxide intensity value for UK grid natural gas is 2.538 48 kgCO2e/kg [4] giving 0.173 kgCO2e for 1 kWh heat. However to that we need to add the Well-to-tank (WTT) carbon dioxide emissions to account for production of the fuel (extraction, refinement and transport) which is 0.434 428 92 kgCO2e/kg [4]. This amounts to an additional 0.030 kgCO2e. The carbon dioxide (equivalent) emissions to provide 1 kWh heat is 0.203 kgCO2e.

An idealised analysis of steam methane reforming (assuming 70% efficiency of thermal processes and 95% efficiency for heat released from combustion) would suggest a carbon intensity for the best “grey” hydrogen of 0.230 kgCO2 (with 17% increase to approximate the WTT or Well-to-tank emissions), slightly worse than from simply burning methane directly. It is quite easy to see that not only standard grey hydrogen could be worse than methane but that even blue hydrogen, depending on how effective the carbon capture is and how much methane escapes (methane being a potent greenhouse gas), may have a high carbon intensity [2].

Question:

What would be the difference in running costs between burning H2 and running a heat pump?

We’ve seen that we need 4.5 times more electricity to make green hydrogen than needed to run a heat pump to get a similar level of warmth in the home. But economies of scale could mean that the premium for green hydrogen is much lower than 4.5.

It is expected that by 2030 the retail price of green hydrogen will drop to €5–7/kg [5]. We needed 0.027 kg of green hydrogen to deliver 1 kWh of heat in our home costing between 11p and 16p (assuming £/€ exchange rate of 0.85).

If the electricity unit price is in the range 15p/kWh to 20p/kWh, then the heat pump running cost would be 4p to 5p (using 0.26 kWh as would be shown by the electricity meter).

So we would expect running the heat pump to cost about a third of the cost of burning green hydrogen.

Any other things we should know?

Burning hydrogen has a slightly higher flame temperature than burning methane (look up adiabatic flame temperature in an encyclopedia). This means that more oxides of nitrogen (NOx) could be released. These pollutants are a hazard to human health and contribute to poor air quality in our cities already. Furthermore, it is quite possible that the burners in a boiler would have to be slightly redesigned for hydrogen combustion and for limiting the NOx emissions. So if we want to leave our boilers alone and are waiting for pure hydrogen to replace methane then there’s a chance that, even if the natural gas grid becomes a pure hydrogen gas grid, we would be required to change our equipment to burn it. Of course it is not certain that the gas grid would be used for pure hydrogen: perhaps a more likely scenario is that the percentage of hydrogen in the gas mix in the grid is increased to a level that is still acceptable for the operator and for end-users (and the methane part of the mix is ultimately derived from non-fossil sources such as biogas).

So if there is little point in burning hydrogen to heat our homes, what would hydrogen be used for? Hydrogen is already heavily used in the chemical industry, refineries and for metal processing (one of the biggest uses is for ammonia production for fertilizers). In addition, hydrogen has the potential to be used across industry and in the transportation sector in vast quantities for processes that are very difficult to electrify directly or where high temperatures are required. One promising new use is to replace coal in steel production (“green steel”). See the CCC presentation [1] for an overview of the role of hydrogen in the UK in the future.

[1] Bell, K., Bellamy, O., Joffe D., “Unpacking the Sixth Carbon Budget – The transition for energy”, Climate Change Committee (CCC) Presentation, December 2020, https://www.theccc.org.uk/wp-content/uploads/2020/12/Unpacking-the-Sixth-Carbon-Budget-the-transition-for-energy.pdf, see p.16.

[2] Howarth, R. W., Jacobson, M. Z., “How green is blue hydrogen?”, August 2021, Energy Science & Engineering, Wiley Online Library, https://doi.org/10.1002/ese3.956, https://onlinelibrary.wiley.com/doi/10.1002/ese3.956

[3] Intergovernmental Panel on Climate Change (IPCC) 2014 carbon intensities: https://www.ipcc.ch/site/assets/uploads/2018/02/ipcc_wg3_ar5_annex-iii.pdf, p.7

[4] UK Government GHG Conversion Factors for Company Reporting, June 2021, https://www.gov.uk/government/publications/greenhouse-gas-reporting-conversion-factors-2021

[5] Jovan D. J., Dolanc G., “Can Green Hydrogen Production Be Economically Viable under Current Market Conditions”, December 2020, Energies 2020, 13, 6599; doi:10.3390/en13246599, http://www.mdpi.com/journal/energies