Hydrogen and electricity are competing carriers, and there may be a role for both in providing low carbon heat.
There has recently been a lot of interest in the role of hydrogen as a carrier of low carbon energy, because it produces no CO2 on combustion (or oxidation in a fuel cell). This is the first of three posts looking at hydrogen and how it might compete with electricity to provide low carbon heat. Hydrogen and electricity may also compete in transport, but that is a large subject in its own right and will need to await further posts.
This first post outlines some of the possibilities and the issues raised. The next post will compare electricity with hydrogen for heating in buildings. The third post will look at the ways they may complement each other to supply heat.
There are broadly two main sources of primary energy for low carbon heat:
- Fossil fuels with CCS, which I’ve assumed in these posts will usually be natural gas.
- Renewables, likely in practice to be mainly wind and solar.
Each of these primary energy sources can get to the energy consumer in the form of electricity or hydrogen. Wind and solar can produce low carbon electricity directly, or they can produce hydrogen via electrolysis of water. Natural gas can be burnt in a CCGT to produce electricity. It can also be processed to produce hydrogen, most commonly in a steam methane reformer (SMR). I’ve assumed here that SMRs are used, although many are looking at alternative approaches such as autothermal reforming (ATRs) which may allow for higher efficiencies and capture rates.
If fossil fuels are used CCS is required, as both CCGTs and reformers produce CO2. This means they provide low carbon energy, rather than a zero-carbon energy, as a maximum of 90-95% of the CO2 produced is captured. Any CCS built now or in the future will likely still be in use by 2050, so its capture rate must be judged against 2050 net-zero targets. In this context, the residual emissions from any large-scale use of CCS for fossil fuels are likely to be significant, and may place limits on the extent of deployment. SMRs produce different streams of CO2. Some of this is concentrated and so relatively easy to capture, some is more dilute. Both streams need to be captured for the technology to play an appropriate role in a net-zero carbon economy.
Both CCGTs and reformers also produce waste heat, which may be used, so improving the overall thermal efficiency, although applications to date have been limited.
Hydrogen can be converted into electricity using a fuel cell or CCGT (with appropriately designed turbines). This may enable use of hydrogen for electricity storage.
Electricity for building heating is likely to come from heat pumps (likely mainly air source heat pumps) as these greatly improve efficiency.
This gives a variety of routes for primary energy to low carbon end use. These are shown in the diagram below. In practice several of these may co-exist, and some may not happen at scale. The pathways shown assume natural gas cannot continue to act as a carrier of energy to individual buildings. This is because its combustion inevitably produces CO2 and very small-scale CCS for individual buildings is likely to prove impractical, for example because of the very extensive CO2 transport network that would be required.
Both fossil fuels and renewables can deliver energy as electricity or hydrogen …
Which mix of these pathways will provide the best solution? It’s not yet clear. It will depend on various factors.
Suitability for end use. Some industrial processes require high temperature heat or a direct flame, which heat pumps cannot provide. Conversely, hydrogen needs to demonstrate its safety in a domestic context, though this is likely tractable.
Consumer acceptability. This is critical for residential heating, and both hydrogen and heat pumps face potential difficulties. For example, heat pumps may be perceived as noisy, or require modifications such as installation of larger radiators which people resist.
Costs. Which route is cheaper depends on a wide range of factors, including :
- The capital costs of the equipment (e.g. CCGT or SMR, hydrogen boilers, and heat pumps)
- The costs of reinforcing, creating or repurposing grids, including the extent to which the natural gas gird can be repurposed for hydrogen, and the cost of reinforcing the electricity distribution network to accommodate demand from heat pumps.
- The cost of the primary energy, for example whether renewable energy is produced at times of low demand so might be available at a low price. If electricity from renewables is available very cheaply then resistance heating without heat pumps may make sense in some cases.
- The thermal efficiency of the processes, for example the extent to which CCS adds costs by requiring additional energy, and the coefficient of performance (heat out divided by electricity in) for heat pump, especially in winter.
- The costs of electricity storage via batteries or as hydrogen.
- Load factor for heat and electricity production.
Many of these variables are uncertain. They also vary with location and over time. The very large cost falls for renewable electricity demonstrate the need for caution in judging options on present costs.
In my next post I will take a look at how these factors may play out for building heating in the UK, and will consider the policy implications.
Adam Whitmore – 30th September 2019