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NETWORK / 41 / NOVEMBER 2016 the gases produced during SMR, transporting the CO₂ by road or rail, and then storing the gas in geological formations deep below the earth's surface – us- ing yet more energy pumping it in – where in theory it cannot leak into the atmosphere. Hydrogen storage To meet the changing seasonal demand for gas, H21 says excess hydrogen manufactured in summer would be stored for winter by piping it 60 or more miles to Hull where there are seven salt caverns with a volume of 400,000m³ each, the perfect vessels for storing 854,000MWh of compressed hydrogen. This convenient quirk of geology is not going to be available in other regions. It's also worth remembering that most hydrogen is used near the site of its production because of concerns that it can make many metals brittle, which complicates the design of pipelines, transportation containers and storage tanks. Gas infrastructure Switching gas sources would require the wholesale conversion of the gas infrastructure. When this was last done, in the change from town gas to North Sea gas in the 1960s, it took a decade and cost more than £100 million. More than this, transporting hydrogen through the gas network is a big challenge. For the same kWh per volume, it would be necessary to increase the pipe capacity by a factor of three. So, where, for example, 100 homes require 150,000m³ of natural gas, they would now need 450,000m³ of hydrogen. Replacing gas appliances The combustion control process for today's gas appliances differs from what would be needed if they were fuelled by hydrogen. As retrospective conversion of the technology is not feasible, all gas condensing heating boilers, gas fires, gas cookers and gas-driven tumble driers would have to be replaced – with technology that no manufacturer has yet developed. If such appliances were to be manufactured in low volumes – for the UK market only, for example – they would be far more expensive than appliances made in large volumes. Set-up costs For one city alone, the Leeds City Gate proposal forecasts that set-up costs will be £2,054 million, followed by ongoing annual costs of £60 million for CSS, £31 million for SMR and salt cavern management, and £48 million for an SMR efficiency loss at 30%. Expensive for consumers An example of the readiness to gloss over true costs is worry- ingly evident in the statement made by H21 Leeds City Gate that if it were "funded by the current UK regulatory business plan it would have negligible impact on customers' total gas bills". In fact, in addition to the considerable expense of replac- ing gas appliances (estimated at £1,053m for the Leeds area), running costs would be much higher. To heat homes, the en- tire manufacturing process for a hydrogen network will require almost twice the amount of natural gas. A typical household gas bill would rocket from about £525 per year to £1,100. Instead of looking towards a complicated and costly hydrogen network, it would be wiser to focus on adding a modest proportion of hydrogen to the existing gas supply. Tests have shown that the majority of gas appliances could work if there was 5% hydrogen content in the gas mix and ongoing tests are promising with a 10% mix. To enhance the economic effectiveness of this, we should be producing hydrogen from excess power generation by renewable sources such as wind farms and large solar PV fields. Better still we can use renewable sources to power the process that creates synthetic methane (CH4), which can be injected into the grid. Power-to-gas uses wind and solar electricity to split water into hydrogen and oxygen by electrolysis. The hydrogen is then mixed with carbon dioxide from a biogas system and converted, using a microbiological process, into methane gas. The methane may then be fed into grid or stored. This technology is scalable and can provide unrivalled energy storage capacity – an advantage over hydrogen, which is voluminous and expensive, and over electricity, which suffers standing losses. There's living proof of this. At Viessmann's head office in Allendorf, the methane produced by power-to-gas is being fed back into Germany's natural gas grid. This is a major part of a combination of measures, known in Allendorf as Efficiency Plus, which has reduced the consumption of oil and gas by 60% and cut CO₂ emissions by a massive 80%. The biological methanation process at Allendorf runs under a moderate amount of pressure and at relatively low temperatures. It directly processes the carbon dioxide contained in the raw biogas, meaning the CO₂ doesn't need to be present in high concentration or purified form. This is significant because it opens up new procurement paths. Smaller sewage treatment and biogas plants, in which no biogas purification is performed, can now also be considered as CO₂ sources. Increasing our capacity to capture CO2 that would be produced by natural plant decomposition and use it constructively must surely be an added benefit. All we need now is for policymakers to see the sense in this. N Christian Engelke, technical director, Viessmann UK Power-to-gas: Storing excess power in the gas grid Excess power Electrolysis Methanisation Power/gas grids Producing hydrogen from excess power Micro-organisms and CO 2 act on the hydrogen, converting it into synthetic methane Methane is injected directly into the natural gas grid Excess power is converted into methane using electrolysis and CO 2 . The methane can be transported and stored in the grid, and can be converted into power as required. Power (GW) Excess power (Two-week period) Oxygen Diaphragm Hydrogen Bacteria at work Power cabel Gas pipe H 2 CH 4