What's the greenest space/central heat (CH) for a typical UK suburban property along with DHW (domestic hot water) in 2008, after insulating to the max?
Assuming that you don't have masses of land for wood or even solar collectors, then mains natural gas at ~0.19kgCO2/kWh of heat is currently probably just about best, and a lot better than plain old electrical resistance heating (eg immersion water heater and convection/fan space heaters) at a UK grid average of ~0.43kgCO2/kWh, with other fuels such as oil and coal somewhere in between on cost and CO2.
But suppose that you don't want to be burning a fossil fuel for heat, or you just feel uncomfortable about the energy-insecurity and price of gas, and you don't have space for the collectors of a ground-source heat-pump (GSHP) for example because you live in a flat, what then?
In CO2 (ie 'global warming') terms alone, if the heat-pump system has a CoP (Coefficient of Performance, ie how much kWh heat you get out for each kWh of electricity in) consistently at or above ~2.3, then the heat-pump solution beats gas.
Why 2.3? Because that's the ratio of the CO2 emissions of electricity/gas per kWh.
(A belt-and-braces approach suggests purchasing the electricity to run the heat-pump on a 100%-renewable tariff to help continue to help 'green' the grid rather than just making everyone else's supplier a little 'browner', or better still microgenerate as much possible from local PV/wind.)
A GSHP solution can almost certainly manage that CoP of 2.3 or better, year round, especially if used for underfloor radiant (low-temperature) heating. It's a bit harder if DHW is being produced as well, eg over ~45°C. In the long-run the UK's grid electricity is likely to get 'greener' (ie less CO2 per kWh) and natural gas more expensive, so if you have the money and the space for the collectors and the system is installed 'right' in a well-insulated and efficient house, GSHP is almost certainly a good thing.
ASHP is likely to have a lower CoP than GSHP, especially in winter when most used/needed, and is thus a dicier proposition right now.
Why? Because in winter when you most need the heating (especially space heating) air temperature is likely to be significantly lower than ground temperature, making the system work harder and less efficiently. A typical ground temperature might be ~8°C or higher year round, whereas, although air temperatures are rarely below zero in most of the UK, in winter and at night the air might well be close to freezing. And indeed at air temperatures of about -20°C or lower typical ASHP won't operate at all and you need a backup heat-source such as inefficient electric resistive heating.
There is a propect of R744(CO2)-refrigerant-based AHSP for DHW with a CoP of ~4 from an external air temperature of ~0°C, which would certainly be ahead in CO2 terms of even the best condensing gas boiler for DHW, but most ASHP on the market won't get near to that, especially in winter, and you may be needing to replace an entire 'combi' system implying further compromises.
Combining with solar thermal may work especially well even in the UK, with no space-heating requirement at all for (say) 6 months, and solar meeting almost all DHW demand for the same period.
Note that at a CoP of 4 (or higher) then in terms of overall efficiency in terms of heating/DHW kWh from available insolation it may be better to use solar PV (20%+ efficiency) in conjunction with ASHP to provide heating (pulling in extra power as needed from the grid), given the ability to export all excess in times of low demand (eg summer), which cannot be easily done with solar thermal/DHW systems.
For example, in the UK using solar PV and G83/1 grid-tie inverter technology, and assuming a normal single-phase supply, with minimial red-tape up to 16A (3.7kWp) of generation can be connected. Ignoring other electricity demand and limitations for the moment, that corresponds to nearly 16kW of heat availability, comparable to a small (gas) boiler.
More pertinently perhaps, on a reasonably-pitched south-facing London roof that might require ~25m^2 of roof space and produce ~2.7kWh/day of electricity thus 10kWh of heat, which might well cover all DHW needs for a small family. (Even on our east-west facing roof surfaces, generating ~1.4kWh/day, that might generate the equivalent of ~6kWh/day DHW which might just suffice.)
It may of course be possible to get permission from the DNO (Distribution Network Operator, that runs the electricity cables to your house) and the electricity supplier upstream to connect more than 3.7kWp of PV, if you have sufficient roof space.
On very dull/cloudy/cold days you would be importing electricity to heat your DHW (eg instead of gas), but over each month (even December), you should be approximately neutral or better, exporting in summer and displacing fossil-fuel burn elsewhere on the grid. (When using ASHP for space heating you'd always be importing for that unless you employed a huge and well-sited PV array.)
One important point in choosing an ASHP could be its refrigerant, since the 'wrong' sort in a domestic system upon leaking/disposal (a few kg but at up to the green-house equivalent of several tonnes of CO2) could easily exceed years of fossil-fuel-fired CO2 flue output. So, from that point of view, good refrigerants with low GHG/CO2(e) impacts include: R744 (CO2 itself), R717 (NH3/ammonia, not a GHG), and R290 (propane).
Again, like for GSHP, we could do with good independent reviews in UK conditions since these systems are so expensive that you can't just try two or three, ripping them out until you get it right, and the technology is complex.
Here is a good start: the Energy Saving Trust 2010 heat-pump field trial.
In private communications with one of the ASHP manufacturers 2008/08, I was told that the UK domestic boiler replacement market is ~1.5 million/year, so everyone is keen to be in it with a good range of products, but that the CO2/R744 systems were proving very tough to make economically.
Ice Energy has some good questions to ask any potential heat-pump supplier, such as:
Carbon and cash savings were better than I expected, although the ASHP still does not represent a good investment without subsidy. But the government's proposed Renewable Heat Incentive will provide very substantial cash payment to encourage a rapid take-up of these devices.
A retrofit of a four-bedroom house in Toddington, built in 2000, saw the house fitted with a non-condensing boiler rated at 23kW, supplying radiators with water at 70°C and with an output of 8.4kW. The hot water demand was 140 litres per day. The boiler was replaced with an ASHP based on a house heat loss of 8kW at an ambient temperature of -3°C. Some radiators were upgraded and were supplied with water at 55°C to provide heating at 8.4kW. The average COPh over the 2007/8 winter was 3.5. The savings, compared to the gas boiler, were 48% less CO2 emissions and a 38% reduction in running costs.
The machine that serves this site is powered by local off-grid solar PV; draw is ~1W.
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