VOICES | NIGEL BANKS We need to focus on short-term emissions Aiming for net zero carbon in new homes means rethinking the upfront carbon in building services engineering. Ilke Homes Nigel Banks looks at what this means for engineering design if targets are to be met W NIGEL BANKS is R&D director at ilke Homes. The modular housebuilder has committed to only build zero carbon homes with zero energy bills, at zero additional cost by 2030, while reducing the wholelife emissions of its products by 75%. hat is clear from COP26 is that we all need to deliver significant emissions reductions this decade. As designers, this means understanding better the emissions resulting from our designs and, potentially, challenging some of our preconceived ideas of what delivers low carbon or zero carbon buildings. Having looked at what this means in new homes recognising that re-use of existing structures is also critical my initial analysis of what produces the lowest emissions in the short term, over the next 20 years (upfront emissions, plus 20 years of operational emissions), gave me some surprises. I felt it would be useful to share these initial findings, which Im sure many of you will instinctively think are incorrect but I invite you to run the numbers yourselves and really challenge yourself on what will save carbon emissions in the next 20 years. Use heat pumps Hopefully, everyone is aware that Heat pumps have a huge impact on how much extra upfront carbon we should spend to save heat National Grid history and future energy scenarios for the carbon intensity of electricity generation (gCO2/KWh) Figure 1: The UK electricity Grid is rapidly decarbonised; heat pumps now produce extremely low carbon heat that will soon be zero carbon. Source: National Grid FES2021 Data Workbook the electricity Grid has significantly decarbonised and that a grid-connected heat pump delivers very low and, increasingly, close to zero carbon heat (see Figure 1). We cant continue burning natural gas, and green or blue hydrogen wont be here in any scale in the next decade (or two). Heat pumps, however, have a huge knock-on impact on how much extra upfront carbon we should spend to save heat, as saving heat energy wont save much carbon in the 20 years of using a heat pump. Dont go too far on fabric For example, a U0.8Wm-2K-1 triple-glazed window has higher upfront emissions than a U1.2Wm-2K-1 double-glazed window. With a gas boiler, this upfront carbon pays back in two years but in an air source heat pump- (ASHP-) heated new home, it never pays back (Figure 2a). There is also a limit on how far to go with wall insulation. Adding 60mm of mineral wool to an externally insulated wall improves the U-value from 0.18 to 0.15, but this upfront carbon never pays back with ASHP. However, the upfront carbon pays back in 14 years with a gas boiler (see Figure 2b). Bio-based materials may not help The chart in Figure 3, from a recent IStructE paper, really helped me understand why sequestered carbon from biobased materials is not included in upfront carbon. Cutting down a tree adds to emissions today, and it is the tree that replaces the one cut down that gradually sequesters carbon. Trees typically have a 50-year harvesting cycle, so other bio-based materials with a faster, or annual, harvesting cycle (hemp, straw) may be more helpful in the short term. Dont connect to gas CHP district heating unless you know the heat you get is already below 0.04kgCO2e/kWh. Gas combined heat and power-led heat networks are now worse than natural gas boiler-heated ones. Not many heat networks will use heat pumps this decade and even if they do, the network losses may mean a local heat pump is lower carbon, especially if there is a lot of new pipework to lay (upfront carbon) to connect you up. 14 December 2021 www.cibsejournal.com CIBSE Dec21 pp14-15 Nigel Banks.indd 14 26/11/2021 14:27