Getting to net zero carbon

This article was originally published in issue 32 of Passive House Plus magazine. Want immediate access to all back issues and exclusive extra content? Click here to subscribe for as little as €10, or click here to receive the next issue free of charge

The UK’s commitment to becoming ‘net zero carbon’ (NZC) by 2050 raises some interesting questions. Social housing organisations, whose investment plans span thirty years, are in the front line: they need to start thinking about NZC now, so they are looking for NZC housing retrofit strategies.

Perhaps the most challenging question was posed by a distinguished academic colleague at UCL: “How can you specify net zero carbon retrofit of buildings if you don’t know the extent to which the electricity supply will have been ‘decarbonised’ by 2050?”

That question set me thinking about strategies that avoid guessing what we don’t yet know, not just about decarbonisation of the electricity grid but also about emerging technologies such as local battery storage, waste heat networks and hydrogen replacing natural gas, but instead aiming to get our housing stock ‘NZC ready’ by (say) 2040, with some sort of renewable energy top-up to get to NZC during the following years.

How much solar photovoltaic capacity would we need for that, and would there be sufficient roof space to accommodate it? Would we need community wind power as well as solar PV?

My academic colleague wasn’t impressed by this approach. He showed me analyses suggesting that, given the way generation costs are changing, it should be possible to completely decarbonise the UK electricity grid by 2050 using mostly wind power. The problem is that this needs nearly thirty times as much wind power capacity as we have at present.

That’s challenging, but if we deep retrofit the housing stock (building fabric and building services) then that figure can be reduced to perhaps fifteen times as much capacity as we have at present. And if we also improve our non-domestic buildings, we might get away with ten times as much capacity.

The lesson I took from this discussion is that we probably can completely decarbonise the electricity grid, but only if we retrofit all buildings in order to reduce demand — otherwise we won’t have sufficient power.

There is another confounding factor. Removing fossil fuels from buildings and converting them to all-electric heating, hot water and cooking, as recommended by the UK Climate Change Committee, will increase the load on the electricity distribution network beyond its current capacity.

Changing from petrol and diesel-fuelled vehicles to electric vehicles, which we charge from the grid, does the same, and making both changes at the same time will mean that in some places we will dramatically exceed distribution capacity. Local blackouts will probably be inevitable, unless investment in distribution capacity is carefully paced.

So, what about my ‘near NZC plus top-up’ strategy? I think it is still a sound approach, but we must be cautious. Replacing gas-fired boilers with electric heat pumps, and gas hobs with electric induction hobs, is the right thing to do, but not all at once. Let’s get electric vehicles (EVs) established first, so that we don’t immediately overload the grid, and to give the car industry time to develop better batteries that we can use in our homes.

Perhaps the next generation of heating appliances should be hybrid heat pumps – a mini gas-fired boiler and a small air-to-water heat pump in the same box, with the heat pump leading and the gas boiler only firing when there is insufficient heat in very cold winter air to maintain the efficiency of the heat pump, or to top-up the hot water.

Then, at the next change of appliance, we can replace the hybrids with true heat pumps, just as we are replacing hybrid cars with EVs, fifteen years on.

Having made this rather elaborate argument, I am struck that the ‘near NZC plus top-up’ strategy amounts to a retrofit approach that has been advocated for many years – fabric first. Improve the longlife building fabric to reduce demand; then improve the shorter life building services, in stages (hybrids, heat pumps, batteries) to satisfy the residual demand efficiently; then top up to NZC with solar PV, local windpower or hydrogen, when we know how much carbon is left to remove.

The NZC target raises questions to which we don’t yet have robust answers. Perhaps the last questions worth raising are the most controversial: if by 2050 all our homes (and other buildings) are all-electric, and the electricity supply is carbon-free, will they really need to have been retrofitted to a standard quite as high as passive house?

Obviously we will still want to heavily reduce demand and deliver comfortable, warm and healthy homes — but will they really need to be super-airtight? Will they really need heat recovery ventilation? I guess that the cost of electricity will be an important factor, but if heating our homes does not involve emissions, will allowing a little more heat loss (and some natural ventilation) really be a problem?

With thanks and apologies to Professor Tadj Oreszczyn at UCL Energy Institute, whose ideas I have probably misunderstood, or at best misrepresented.