There has been relentless pressure over the last 25 years to reduce UK CO2 emissions. Measures initially concentrated on electricity generation, where the most substantial change has been the closure of our coal-fired power stations and the massive investment in renewables and inter-connectors to Europe. Carbon emissions in the electricity sector were reduced until 2020, but have started to rise again. This is partly the consequence of the retirement of the oldest nuclear plants. There is very little scope for reducing the sector emissions further, since renewables require secure back up from either gas or coal-fired generation.
There is now pressure to reduce the emissions from two other large contributors – domestic energy use and transport. In both cases the primary policy is to increase electricity usage via heat pumps and electric vehicles. However, the power has to come from low-carbon sources - renewables or nuclear.
Hydrogen with Natural Gas
In the domestic sector, it is proposed that where heat pumps are not a convenient option, that hydrogen could supplement natural gas [or even replace it] as hydrogen produces no CO2 when burned. To reduce CO2 emissions significantly, the hydrogen must be produced using low-carbon electricity via electrolysis. Very little hydrogen is produced by this process currently, but it is the cleanest. New nuclear plants will not come on stream for many years, so wind power is the great hope for this low carbon electricity generation, but its output is very erratic output. It has therefore been proposed that excess electricity produced occasionally at times of high wind speeds (that currently cannot be stored), could be used to produce hydrogen that could be stored and made available at times of low renewables output. There would be a double benefit – the use of zero-carbon hydrogen to replace natural gas and the better utilisation of peak wind output. Some industries, such as glass and chemicals could also transfer to hydrogen,
The concept is essentially sound and is being considered on a Europe wide basis. but the practical limitations provide very serious challenges. ENTSO-E, the organisation that represents all the grid operating companies in Europe, published a substantial report in June 2022 examining prospects up to 2050 and the use of hydrogen for balancing the HV Grid. The general implications are as follows.
Substantial increase in offshore wind generation with many new developments with 200+ turbines. They forecast by that installed capacity would therefore have to be doubled by 2050. Partly, this is a consequence of the steps in the process. Electrolyser efficiency is about 70% and the thermal efficiency of any turbine about 50%, so the combined system can recover only about one third of the energy in the original electricity.
A massive increase in the production of electrolysers some of which are proven but some are experimental and will need years of development.
Sites would have to be found where electrolysers could be installed – ideally near to the turbines to minimise energy losses. This is a challenge if most wind developments are offshore.
Large scale hydrogen storage facilities have to be developed - preferably in geographically separate areas – but the only options appear to be in old salt caverns or under the sea for the largest quantities.
Pipelines have to be constructed to transfer the hydrogen from the electrolyser facilities to the storage facilities.
Industrial customers have to be recruited who can utilise the hydrogen as a replacement for natural gas. In many cases this will necessitate investment in new capital equipment.
Equipment that can use a mixture of hydrogen and natural gas has to be perfected for the commercial and domestic sectors or in some cases for burning pure hydrogen.
Turbines have to be developed and commissioned that can generate electricity with hydrogen to replace some CCGTs.
Customers will require supplies to be as secure as current natural gas, but since wind power has long periods of low output, low carbon backup supplies such as nuclear will almost certainly have to be provided to ensure that large scale users will suffer no interruption to supplies during periods with low wind speeds.
Extra transmission lines and grid strengthening will be required to handle extra generated power long way from the potential customers. Balancing the grid will become increasingly difficult as renewables equipment increases and synchronised generation is retired.
Projects are under way. A hydrogen production/storage scheme is being developed in the north west for industrial customers, based on Stanlow - the Hynet Facility. It will have an ultimate capacity of about 30,000 tonnes H2 in nineteen new leached out salt caverns, with a total energy content of 1.3 TWh – enough for a small number of companies for several weeks.
Hydrogen Production Rate
It is claimed that 100MW of electrical generation can produce 40 tonnes H2/day. To fill a single cavern requires about 1500 - 2000 tonnes, so that a 1000 MW generation plant could produce 400 tonnes per day and take five days to fill one cavern, but since the load factor of wind turbines is typically 35%, it would require an installed capacity of 3000 MW – perhaps 600 turbines for this one hydrogen project.
The challenge for the domestic sector is many times greater. Average daily domestic consumption of natural gas is about 850 GWh - 26000 tonnes. Consumption of natural gas can reach 3TWh in a day at peak winter demand, so a facility the size of Hynet would be exhausted in only half a day. Multiple large storage facilities would be necessary to replace our natural gas supplies.
The use of substantial storage of hydrogen is a logical development but will not be the complete answer. The problem still remains that hydrogen – or any other fuel – has to be converted to heat or electrical energy at a prodigious rate to meet the short term needs of the domestic sector in sub-zero weather. A further complication is that whatever plant is used will be producing peak output for only a few weeks of the year but for the rest of the time will be unused.
When it is recognised that in parallel with hydrogen projects, renewable electricity is also promised for powering domestic heat pumps; for recharging electric vehicles and generally for replacing gas and coal, the scale of the challenge becomes apparent.© Paul Spare CEng FEI FIMechE
3 December, 2022