Inspired by this question:
Given the current UK (2014) generation mix, would changing a house from gas to electric heating increase or decrease its carbon footprint? The original question was about resistive storage heating, but I'd be interested in a 3-way comparison that included heat pumps as well.
As we're looking at carbon footprints, it's important that the scope of the consequences we look at (aka our system boundary), goes out to the global level, and to the long term. We would get different answers with a smaller scope: if we only looked at short-term impacts, for example.
In Britain, given the current generation mix, any required additional electricity production will come from either coal or gas, depending on total level of grid generation, the forecastability of the additional demand, and the relative prices of coal and gas. It will also depend on the efficiency of heaters.
And for gas, the carbon impact will depend on the amount of additional gas leakage from the network, and the source of the gas.
Relative global warming potential for different supplies:
To calculate short-term relative impact, ignoring the wider implications for the grid and generating infrastructure, simply divide those by the approximate efficiency (Seasonal performance factors, SPF, for heat pumps):
Example calculation:
Coal as the marginal producer, with air-source heat pump. Relative carbon footprint = 1 / 200% = 0.5.
If you have a supplier that increases its investment in clean energy when its revenue increases (i.e., in the UK, that's Ecotricity - no connection except as a very happy customer), then the hierarchy of lowest carbon footprint will depend on those patterns. In this case, resistance heating with storage heaters starts to look very attractive, because the bill income enables higher investment in renewables; because coming onto the market now are smart controllers that will switch the heaters on and off to match exogenously varying renewables such as PV and wind, which enables higher integration of those supplies into the grid. In those cases, then the likely hierarchy, from lowest to highest carbon footprint, considering the long-term infrastructure impacts as well as the short-term marginal emissions, will be something like:
Gas heating comes at the bottom, even if the supplier will inject biogas or synethetic gas into the network, because the network leakages of gas have a significant global warming impact.
If you have a supplier that just takes electricity and gas as they come, from the network, and their revenue doesn't change the pattern of their infrastructure investment, and if gas is the marginal electricity fuel, then the likely hierarchy, from lowest to highest carbon footprint, will be something like:
but the position of gas heating will depend on the coal/gas blend in the electricity mix: The more coal, the higher that gas heating will come in that list.
There are big questions about how smart controls will play with heat pumps. I've yet to see anything convincing, but it is imaginable that combining storage with heat pumps will allow such systems to provide valuable balancing services to the grid, in just the same way as resistance heating with storage does.
There are also, to my knowledge, no estimates of the impact of a marginal increase in gas consumption, on gas leakage rates. But that gas leakage can have a huge impact on the carbon footprint, because the gas is methane, and that has a very high global warming potential, molecule-for-molecule, relative to carbon dioxide.
As the Danes have discovered, the optimal system configuation is one that considers technology choices at the level of the system, rather than dwelling by dwelling. From that perspective, district heating offers the optimal solution, for several reasons:
Ultimately, clean district heating speeds up the complete decarbonisation of heating and of electricity supply, so has the lowest long-term carbon footprint. At the moment, the next-best long-term option is probably smart-controlled resistance heaters going into storage heaters.
