CPD PROGRAMME | HYDROGEN effects despite extensive testing and analysis. A frequently voiced concern is the small molecular size of hydrogen compared with methane. The HyDeploy team notes17 that hydrogen molecules have a kinetic diameter of around 280 picometres compared with 380 picometres for methane and that detectable leaks on a network or installation typically happen through hairline cracks of 100-500 micrometres around a million times bigger than the molecules of both methane and hydrogen. Output from the Hy4Heat programme concluded that leaks are of a similar order for both hydrogen and methane, and that a good system in methane is a good system in hydrogen. The likelihood is that the UKs future energy policy will embrace a mix of technologies and energy sources. Each will offer particular benefits as well as specific detrimental impacts or challenges in use, and there are areas in the UK that are likely to always be beyond the reaches of the gas network. The use of hydrogen for combustion can deliver a wide range of output temperatures and, once a network is fully commissioned, it can provide a reliable and cost-effective means of supplying heat energy. Condensing hydrogen gas-fired boilers in development have been tested in field trials and shown to be performing safely and at least as efficiently as the natural gas boilers that they would displace. The HyDeploy project is examining the social acceptance of hydrogen, as the level of popular support is an important consideration when evaluating the feasibility of employing hydrogen in the transition to net zero. The ultimate success will be measured by the environmental impact of system and operational carbon emissions. The reuse of the existing gas distribution network limits the additional embodied carbon that would otherwise be produced when creating an extensive new nationwide network and, together with the promised availability of plentiful zero-carbon hydrogen to fuel the nations heat demands, provides a strong case for the further development of a truly green hydrogen gas network. Tim Dwyer, 2020. See page 69 for references. THREE SHADES OF H2 Hydrogen can be extracted from fossil fuels and biomass, from water, or from a mix of both. Hydrogen production is typically characterised by one of three conceptual colours.18 Grey hydrogen is currently the most prevalent, produced from processes that utilise natural gas while generating significant CO2 emissions. Blue hydrogen signifies that the carbon emissions from production are subject to capture, utilisation and storage (CCUS) currently adding a significant cost. Green hydrogen is generated through electrolysis by renewable energy sources that have (practically) no carbon emissions. Although grey hydrogen is currently cheapest (world production cost between 0.6-2 per kg19), forecasting must account for the volatility of gas prices and vagaries of regional prices. Producing enough hydrogen for the UKs heating needs alone would require 8 million tonnes of hydrogen a year, up from the annual 0.74 million tonnes made today (which is almost entirely used by industry).20 Mass production will require large volumes of cheap green electricity, as well as practically feasible opportunities for CCUS. Hydrogen economy Natural gas extraction Natural gas transmission network Methane reformer Figure 2: A 28kW wall-hung hydrogen gas boiler that is part of the programme to develop new system appliances (Source: Baxi) Balancing renewables on the grid Hydrogen as industrial feedstock Figure 3: The hydrogen economy envisaged for the UK (Source: Hy4Heat21) Power to gas Hydrogen transmission network Pressure reduction station Low-pressure gas distribution network Carbon sequestration Long-term hydrogen storage (eg, underground salt caverns) CCS transport infrastructure Hydrogen for heat in buildings Hydrogen for cooking Hydrogen for heating Hydrogen for transport Hydrogen for power Other sources of hydrogen (eg, coal gasification) 68 November 2020 www.cibsejournal.com CIBSE Nov20 pp66-69 CPD 170 Baxi v2.indd 68 23/10/2020 16:42