Heating indoor spaces by burning natural gas accounts for ~30% of the UK's total CO2 emissions. Around 23 million properties are connected to the gas network. Each 1kg of gas burned delivers ~12kWh of heat and releases ~4kg of CO2. That cannot continue in a future net-zero UK and capturing CO2 at individual buildings is completely implausible using any known technology.

Many consider that hydrogen should replace natural gas in the gas network. Technically, this is feasible. Hydrogen can be produced from electrolysis or from natural gas. In case of the latter, 'carbon-capture' methods can collect most of the resulting CO2 and pump that underground. However, distributing hydrogen through the gas network might not necessarily be the most sensible course of action in all cases. This project will answer the question about how best to use different parts of existing gas network in a future net-zero UK.

Even with carbon-capture, producing hydrogen from natural gas does cause some CO2 emissions. Typically >5% escapes. Using renewable electricity to make 'green' hydrogen via electrolysis and then burning that in boilers delivers less than 7kWh of heat into homes for every 10kWh of electricity used. By contrast, using electrically driven heat pumps can deliver 40kWh of heat for every 10kWh of electricity consumed. Although there are other advantages to producing hydrogen for heating, it remains questionable whether this is optimal in many parts of the UK.

It is very likely that a large fraction of the existing infrastructure will be used for distributing hydrogen across the country. However, some specific parts of the network could be better exploited in a different way. This project will explore the different possible uses for those parts of the gas network. All of these potential uses are motivated mainly by solving problems that would arise if heat pumping were deployed very extensively in the UK as the primary heating mechanism.

One possible future use for parts of the gas network is to feed non-potable water into properties. This water could serve as the source of low-temperature heat to support heat pumps. A new variety of heat pump turns incoming water into an ice slurry and discards the slurry to melt again later. This 'Latent Heat Pump' (LHP) can extract a lot of heat out of cold water (12L of water provides ~1kWh of heat). That heat emerges from the water at about 0C and as a consequence, the LHP can have a coefficient-of-performance (COP) >4 even when the outside air is very cold. For most air-source heat pumps, the COP falls sharply in very cold weather and, for obvious reasons, the COP matters most in very cold weather.

A second possible future use for the gas network is to serve as a return (collection) network rather than as a delivery (distribution) network. Here, the fluid returning through the gas network would be an aqueous solution of a chemical that was hydrated (mixed with water) at the property to release heat. This measure would be taken only in very cold weather. Calcium Chloride and Magnesium Sulphate are two very cheap salts that release heat when dissolved in water. There are other inexpensive substances that release large quantities of heat upon reacting with water.

Finally, if water was being conveyed in the low-pressure tiers of the gas network, the high-pressure tiers of the gas network would be free for another use. A very attractive possibility here would be to use those parts as the pressure vessel for a compressed air energy storage system. That system would simultaneously be able to assist the electricity transmission system by doing a parallel transmission from North to South at times of high North-South power traffic.

How acceptable each of these propositions is to key social stakeholders (including policy makers, prospective business, and public end-users) will be integral to their real-world viability, and so will be examined here also.