A carbon-neutral London home for £50,000?
When I visited the Energy Solutions Expo I started thinking about use of heat-pump technology for heating.
I have already estimated that as of 2007 I could just about eliminate our net household electricity use over the year with 4kWp of grid-tie solar PV to cover our ~7kWh/day consumption. We might even be slight net exporters. We could eliminate that portion of our 'carbon footprint' and over the course of ~25 years we'd probably recover the outlay of maybe £28k total (~£7 per installed Watt-peak (Wp)).
We use about 10kWh/day of mains natural gas for cooking and hot water, and at least about the same again in winter for central heating. We're going to try to trim that a bit with improved insulation and avoiding overheating the house (eg we keep the central thermostat between 15°C and 18°C typically and the radiator/hot water temperature 60°C-ish), but it might be possible to eliminate almost all of the 'gas' power (ie its 'carbon footprint') with GSHP technology. For every 3--4kWh of energy extracted this way about 1kWh of electricity is needed to drive the pump mechanism itself with a typical CoP (Coefficient of Performance) of 3 to 4. With care, that might come from our PV (again, using the grid as 'storage'), at maybe about an extra 1kWp (mean ~2.5kWh/day) extra PV to cover each ~10kWh/day of heat pumping.
At the 'e2' show I spoke to Cool Planet at their stand, but they don't normally do domestic-sized systems. I contacted some other GSHP providers in the UK, but, for example, one does not do replacement systems (eg to supplement an existing domestic hot water (DHW) and central heating system).
I had more luck with Ice Energy Heat Pumps Ltd, and spoke to Ben Hall who was very helpful. I quote his very rough estimate of what I might need given some details about my property:
I would estimate that with a floor area of 180sqm to be heated in a reasonably well insulated house you would need a 7kW output heat pump. This would require 2 x 60m deep boreholes. Our Greenline HT+C7 GSHP would provide all heating via radiators or UF heating and all DHW to a max temperature of 65 Degrees C.
We offer a design supply and commission service we do not install the systems ourselves. The borehole drilling company you chose would install the ground loops whilst a plumber of your choice would take care of the internal install.
The design supply and commissioning of this system including the heat pump, the external pipework all manifolds and all deliveries to site would be approx £7000. You would also receive a grant of £1500 from N power.
One company who have carried out the drilling for many of our clients is Jackson Drilling who are based in Somerset.
I then followed up with a question of expected equipment lifetime/maintenance, and Ben Hall replied:
The life span of 25+ years that we state is based on the experience of IVT who have been manufacturing and installing the heat pumps that we sell in Sweden since the late Sixties. The 5 yr parts and labour warranty we supply is extendable and transferable (should you move house), however people don't tend to extend the warranty because the Heat Pump is so reliable. This is due to the very few moving parts involved. Therefore when one does fail it tends to be within the first week of operation and is due to something being overlooked in the manufacturing process (very rare obviously)
The GSHP we sell requires Zero scheduled maintenance for its entire life. Therefore one of the main benefits is that as well as reduced running costs, no further outlay on maintenance is necessary.
Supposing that we wanted to keep this entirely carbon neutral over the course of a year. That might be accomplished by total install costs of possibly £9k for the heatpump, boreholes, plumbing, etc, after all grants, and possibly another £14k of extra solar PV to cover the electricity used. So for (say) £23k we could probably neutralise all our gas use, a little less than the £28k estimate to neutalise mains electricity use (other than the heat-pump usage covered above).
If those numbers are at all accurate, then for £50k we could possibly eliminate the carbon footprint of our small London house for power, with relatively little maintenance cost and effort over a probable 25+ year lifetime, though see the caveats below.
This is only possible if we can pump excess PV energy into the local grid and get it back later; otherwise the demand for heat, etc, does not match the availablity of solar PV at all well. This also means that if there is a power cut we lose heating just as we do now with our 'combi' gas boiler. (If power cuts were an issue then PV could be used to power a battery bank to drive the heating (and presumably other essentials such as a fridge and lighting); eg at 12V about 500Ah per day of backup required might do.)
Note that is also possible to add solar thermal to help with DHW, depending on overall system design. In effect, this would displace a little of the solar PV described above for driving the heat-pump.
Brian Mark of Fulcrum commented 2007/11/12:
We are designing thousands of code [4] sustainable homes [and] level 5&6 zero carbon homes using various ground source heat pump systems but the technology and how the energy use and carbon emissions are calculated are very confusing, for instance to supply Domestic Hot Water you need a supply temperature from the heat pump of at least 70°C and no normal ground source system can improve much on a seasonal Coefficient of Performance of 1.8 for this. This means that for DHW generation the CO2 emissions are worse than gas if the heat pump is connected to the Nat. Grid though you could use renewables to supply the electricity but if you are relying on PV panels they will not produce anything like enough in the winter and you will be relying on a grid connection anyway.
There are not accreditation standards yet for agreeing manufacturer's claims of COP for heat pumps so there are a lot of misleading claims from heat pump system suppliers at the moment...
We continued the discussion by phone and he said that unless you get a CoP of 2.2 or better then in terms of atmospheric CO2 production it is better to burn mains natural gas for domestic heating including DHW. It seems that the claims of a CoP or 3 or 4, even for surface GSHP and low-temperature space heating are probably unsupportable. And boreholes are especially problematic since continuous heat flow into them from surrounding soil may allow the temperature to drop several degrees below the normal undisturbed 11°C mean, further lowering the CoP.
(One supplier countered by saying that they obtain a good CoP by keeping the tank at 50°C, which is also safer as regards scalding, but raise the temperature to 65°C once per week to pasturise the water.)
As of January 2008 the French government has tied tax breaks to specific CoP targets (>=3.3 at specified temperatures), which seems one way to tackle the snake-oil element.
My hope to 'store' excess summer solar PV generation in the National Grid (by displacing other non-baseload generation) and get it back in the winter may not be realistic because the winter peak-load requirement may require more carbon-intense generation (eg coal) and thus the cubic metres of natural gas burn that I defer in the summer may not be replaced with natural gas burn in the winter to power my heat-pump.
(We also discussed some extra gotchas in DHW, like Legionella safety, recovering heat from daytime waste/grey water overnight with heap pumps, and to put the solar-thermal input to a stratified DHW storage tank at the bottom for best energy transfer, etc... A very interesting chat!)
One interesting idea that Brian offered is 'simply' to take a 20,000l-ish second-hand stainless steel milk road tank, and bury it (well insulated) full of water (and maybe corrosion inhibitors) in the garden. In the summer, energy from solar thermal capture is pumped in, and in the winter pumped out again for space heating and DHW, and much less energy is required to pump the energy this way. Indeed, any excess RE year round, such as the dump load for a wind-turbine or even solar PV, that would otherwise be discarded, could be captured in the thermal store. This is much more efficient then trying to generate and store energy as electricity, for example, when all that is needed is heat. I have to do the sums for the energy capacity vs our energy usage and so on, but this looks like an interesting alternative to GSHP. Milk is good for you!
See my system design sketch.
What we really need are some good reviews of different GSHP products from start to finish in terms of cost, efficiency, install difficulty, long-term characteristics, and even customer service.
A good 'controlled' study is hard to do, in part because it's not truly a mass-market product (yet) and finding two near identical sites for comparison is well-nigh impossible.
No supplier is going to be offering free installs to 'prove' their product, and if they did, would you trust the reviewer?
A number of respected people and organisations have said that in the UK there is not enough competition in the market and consequently there are too many snake-oil salesmen and shoddy installers.
Incidentally, Ice Energy has some good questions to ask any potential GSHP supplier, such as:
Need help? Don't know your solar from your loft insulation? Try ExNet's energy advice: free to get you started, then there for the tricky questions later too!
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