Steps to Net Zero: Step 3 – Eliminate Fossil Fuels

In our recent blog (Steps to Net Zero: Step 2 - Operational Energy), we saw the importance of designing for minimal energy demand and consumption. In step 3 to Net Zero, we investigate how to eliminate fossil fuel use.

Heat demand currently represents over 40% of UK energy consumption by the end-user, with the vast majority coming from fossil fuels such as natural gas. Meanwhile, the National Electrical Grid is rapidly decarbonising. In 2014, the carbon emissions factor of grid electricity was 495 grams of CO₂ for each kWh of electricity generated. In 2020, this is set to reduce to below 150g CO₂e/kWh and is on course to halve again in the next decade as more offshore wind farms come online. This allows for the electrification of heat and heralds the end of fossil fuel combustion as the primary source of heating.

However, the transition to all-electric heating brings a number of challenges which must be overcome:

• Capacity: The National Grid is already running at near full capacity. If we were to simply switch all our heating over to electric there would not be enough capacity to meet demand. Not to mention the increasing strain electric cars are putting on demand. It’s therefore vital that we reduce heat demand at point of use as outlined in step 2. A maximum installed heating capacity of 10 W/m² should be targeted.
• Demand response: Another big issue is when this demand is required. Heating energy is typically concentrated into a few small peaks a year when it is cold outside. We need to look at ways of spreading these, and other demand on the Grid, out. We can do this through storage and scheduling of non-time dependent loads.
• Limits on renewable energy: The UK has limits on the amount of renewable electricity it can produce and with all sectors looking to access more renewable energy, demand reduction is vital. This is why we need strict energy intensity targets for buildings.
• Domestic hot water: This is important as it can be a significant load concentrated in short duration. Storage and demand reduction are therefore key.
• Operating costs: Although the electrification of heat offers significant carbon savings it does raise a few issues around operating costs and fuel poverty as electricity is typically four times more expensive then natural gas. Direct electric heating should therefore be avoided unless demands can be reduced to Passivhaus levels or lower. In most cases, heat pumps offer a viable solution as they operate at efficiencies of three or four times that of direct electric heating. This has the duel benefit of reducing costs and demand on the grid.
• Lowering system temperatures: Heat pumps bring their own problems as they require lower system temperatures (30-50^oC) to achieve efficiencies. This is fine in new, well insulated, modern buildings but more of a concern in older, uninsulated buildings.
• Energy networks: Minimising demand and system temperatures creates opportunities to establish heat sharing networks. This is particularly useful in mixed-use developments where there is different heating (and cooling) demand profiles, offering opportunities for load shifting and heat recovery/sharing. 5^th Gen networks employ distributed heat pumps to transfer heat around ambient heat loops with user rejecting or taking heat out of the network as required.
• Existing heat networks: These present a particular challenge as we have been installing these around the country for the last decade supplied by gas-fired CHP engineers. They are typically designed at much higher temperatures then are ideal for heat pumps and it is not viable to simply replace them all in the short term. A potential solution will be to replace the traditional gas-fired CHP plant with staged heat pumps; with a number of heat pumps in series using different refrigerants to bring the water up to temperature in stages whilst maintaining efficiencies.

The biggest challenge though is the fact that at least 70% of the buildings we operate in 2050 have already been built today and the vast majority have gas fired heating systems operating at 60-80^oC. As it stands, in order to reach the UK Government’s 2050 net zero target, we must refurbish 17,500 homes, and millions of square feet of non-domestic buildings, per week.

Electrification and heat pumps will require substantial retrofit of these buildings to allow for the lower temperatures and reduced demands. A potential alternative solution under consideration for existing buildings, is the conversion of the current natural gas network to hydrogen. This would enable a rapid decarbonisation of the Grid, but further development is still needed. However, there is little guidance from the Government on the viability of this option.

All-electric solutions, therefore, seem to be a compelling, readily available and a relatively simple means of delivering net zero carbon heat and when considered in conjunction with zero emission vehicles, there is a potential to also significantly improve air quality.

Case study - Wells House

Wells House is a mixed-use development in central London. The development covers 14,444 m² and comprises office spaces along with retail and a nightclub.

Approaches used

Wells House is combustion-free on-site (air quality neutral). Space heating, cooling and DHW is provided by electric heat pumps. The system chosen consists of 4-pipe heat pump/chillers providing heating and cooling, with heat recovery from the cooling process. Water to water heat pumps boost the temperature of the LTHW to produce DHW. In addition, the development is enabled for a future connection to a district network.

Results of an operational energy model of the building demonstrate that the development could exceed the performance of a similar office project based on fossil fuels by up to 25%.

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