Earth Notes: GSHP: UK Domestic Ground-Source Heat Pumps

How to get a heat pump for your house (in Great Britain) and the Boiler Upgrade Scheme.

2012: Revisiting GSHP for Our Home

As of the start of 2012 we're about carbon-neutral on energy. The excess exports from our PV compensate for our gas combi. We have reduced demand (ie increased energy efficiency) greatly. But I'd like to stop directly burning fossil fuel (ie gas) directly, soak up more energy solar locally to reduce grid demands, and rely on decarbonisation of the electricity grid to green us further.

All that implies some sort of heat-pump plus solar DHW.

Justin Broadbent of Isoenergy visited (2012-01-23) and he pointed out that a heat-pump would be unlikely to save money (but we ought to wait until June to see the RHI details), that an ASHP might get us an SPF of 3 if done right but that a GSHP with borehole(s) might cost the same and have an SPF of 4 (combi style with ~200l thermal store with direct solar inputs), and that it might cost us something like £18k all in.

An important assertion was that we could probably run our rads in the low-40s(C) more constantly (in contrast to the low-duty-cycle boiler cycling we get now) if we plan slightly ahead and leave more time for rooms to warm up; doing so would help maintain the heat-pump's SPF.

(JB also thought that our ex-airing cupboard could probably accommodate a ~200l tank/store if I wanted to put one there, and rightly reminded me that we also ought to consider upgrading remaining rads to more-efficient newer ones and lagging all visible pipework if making this sort of expenditure.)

I had been concerned that no GSHP boring machine would be able to get to us, but it seems as though that is not true, and a 100m borehole might well do us.

GSHP Boring Costs (2012)

A very rough breakdown of GSHP costs was suggested:

• £5k for drilling 100m borehole + (40mm) pipes, grout and header
• £7k for GSHP SPF4 (Seasonal Performance Factor) 'combi' unit (for hot water and space heating with radiators) including ~200l thermal store with entry points for solar thermal to enable some space-heating support in shoulder months and on bright days
• £1k glycol
• £1k plumbing and commissioning
• £3k other work/labour time on site
• £1k on upgrades to older radiators, and other improvements.

This does not include any solar thermal costs.

Rough CO2 and Running Cost Comparison

Taking halfway between the harsh 2010 (~6MWh gas) and mild 2011 (~4MWh gas) and indeed hoping for further efficiency improvements with new glazing, etc, average demand from gas has been ~14kWh/d apparent, or ~11kWh/day actual allowing for our ~80% (or lower) gas combi efficiency. Tank losses with the GSHP might be ~1kWh/day so taking us to ~12kWh/d total demand, and an SPF of 4 would take us to close to 1.1MWh electricity import.

(Our 2011 gross electricity demand was ~1.5MWh, and net exports ~2MWh, so we could easily accommodate an extra 1.1MWh imports and still be a low-use household and net exporter, and with no separate gas/heat demand at all, so firmly carbon-negative ignoring embodied energy costs.)

5MWh of gas is currently ~£200 and 950kgCO2.

1.1MWh of ("100% green") electricity is currently ~£140 and ~500kgCO2.

So at an SPF of 4 we might save a tiny bit of money and a little under half a tonne of CO2 each (moderate-heating-season) year.

Assuming a system life similar to PV of (say) 20 years, that implies £900/y to save ~450kg/y (ie ~£2/kgCO2/y), compared to the PV of somewhat under £1/kgCO2/y, so it's expensive.

Adding Solar PV/T

Adding enough PV/T to remove 500kWh of DHW heat demand (approx one third) might avoid 125kWh of electricity demand and generate up to another 500kWh, saving possibly another 200--300kgCO2/y, and likely changing the economics (£/kgCO2/y) for the better.

2007: A Carbon-neutral London Home for 50 Grand?

(2011 note: we were about energy carbon-neutral with a little over 5kWp of PV and lots of insulation and conservation around the house for probably somewhere under half the £50k figure.)

When I visited the Energy Solutions Expo I started thinking about use of heat-pump technology for heating.

I had 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 (now see Alto Energy), 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 neutralise 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 availability 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.

Tip-Top-CoP: A Cautionary Note

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 pasteurise 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!)

Another Cautionary Note

In March/April 2010, after the government had announced its RHI (Renewable Heat Incentive) consultation and outline plans, a friend doing some building anyway was considering replacing his existing oil-fired central heating (with radiators) with GSHP (or ASHP).

One problem is that most of the systems, especially GSHP, have a low maximum temperature (before an ordinary immersion heater is used) of ~55°C that requires radiators to be replaced with larger units. Basically they are more suitable for underfloor heating (UFH) in very well-insulated homes requiring too much upheaval to be practical, not being direct drop-in replacements for an existing combi, at least from my friend's point of view.

The Eco-Cute ASHPs with high (>60°C) water temperatures at decent CoP are either unavailable in the UK (such as the Hitachi 'instant' unit) or don't have the correct MCS accreditation for the RHIs (the Sanyo units).

Another problem is in getting the installers (etc) to behave professionally, actually come out and visit rather than waffle over the phone, etc, etc, to the point where my friend was disheartened enough to put off even thinking about it for another year, at least until the RHI levels are fixed.

Milk Tanker Going Down!

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.

GSHP Review Needed

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?

Here is a good start: the Energy Saving Trust 2010 heat-pump field trial.

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.

A Real System in France, Since 2010

Read a case study of a real-life domestic GSHP in western France over several years!

Ground Space Required

Viessmann's site offers the following simple rule of thumb for example:

Ground source heat pumps are impractical for some domestic installations, because of groundwork and access issues. Slinky pipes require an area of land approximately twice as large as the total floor area of the house, in order for them to collect sufficient heat, without freezing the ground. If a large area of land is not available, bore holes would need to be drilled.

If using boreholes a starting point is to assume 15m/kW required depending on ground type, with a depth of up to 100m being common (eg good for ~7kW max, at ~£25+/m circa 2010), and there is a limit on how close the boreholes can be, no less than a few metres apart.

For us with a heated floor area of 76m^2 and total heat demand of ~6MWh/y, a 100m borehole supporting up to 6kW--7kW might be more than enough, indeed 60m for 4kW might do at a pinch, though I'd rather over-specify a bit rather than risk the ground cooling and thus the CoP dropping too far.

Direct Generation of Electricity From Ground-Source

2014-05-26: I have been considering supplementing the PV input to my (~4W) off-grid powered SheevaPlug server with direct thermal generation in winter when PV is at its least good; I already have a little bit of wind (my MotorWind array) for this, but thermo-electric generation (TEG) might have an even better anti-correlation vs PV and also have the advantage of being silent and having no moving parts.

After a bit of Internet searching I found and prodded two (German) companies, Thermalforce.de (see their "m902 Geothermal probe") and Quick-ohm.de (see their QC-127-1.4-6.0 device) who have very politely humoured me thus far.

A bit of rumination and back-of-an-envelope calculation from these assumptions:

• Efficiency of electrical output maybe ~1% at ~8K mid-winter London air temperature (~2°C) differential from ground temperature (~10°C).
• Rule of thumb round here for boreholes for ground-source heat-pumps of 15m/kW of required thermal output, or ~70W/m.

suggests that a device with a 1--2m heat-pipe might be able to extract <0.7W or (say) ~0.5W of electrical energy mid-winter, radiating and/or convecting away ≥70W to the air (note: the QC-127-1.4-6.0 'cold side' is 40mmx40mm ignoring likely necessary heat-sink), and ignoring details of converting to and transmitting in a form usable by the server.

Note (fact alert): the Met Office says that (quoted 2014-05-26) "Mean annual temperatures vary from about 11°C in central London and along the south coast to about 9°C over higher ground well inland" which should be close to my year-round ground temperature at reasonable depth, and "January is the coldest month, with mean daily minimum temperatures varying from over 3°C in London and along the coast to about 0.5°C over the higher ground" which means that I probably have somewhat less differential to play with than in my initial assumptions above. The horrors of a mild maritime climate!

Thermalforce.de is suggesting a unit made up of the following:

• hp1800 2 pieces (8mm copper pipe water/glycol 1.7m-long heat-pipe with condenser, 100W throughput, ~0.08K/W)
• hp602, 4 pieces (copper heat-pipe adapter, half-pipe)
• kk101160 (heatsink, aluminium)
• sp01 (spacing block for heatsink)
• TEG 127-200-28, 2 pieces (5.1Voc and 1W @ delta-T of 100K, ~60mV/K, possibly 10mW/K output)

which looks like I might hope to get ~100mW continuous output in the depths of winter (I'd need 40 to power my SheevaPlug)! By comparison, PV is lucky to get 1 sun-hour per day equivalent at that time, so a capacity factor of perhaps 4% or less, so 100mW of thermal-electric generation (TEG) is maybe worth ~2.4W of PV, and possibly a little more since a battery might not be required with TEG, the ground's thermal mass itself being storage.

Note that according to weather data (for nearby EGLL/Heathrow) from degreedays.net, over the last 36M (1121d) the number of days below a baseline of 5°C, ie at which the above device might hope to generate something of use, is 287 or about 95 days per year, and provides good (not complete) coverage over the Nov/Dec/Jan winter PV low.

Good Questions To Ask

Ice Energy had some good questions to ask any potential heat-pump supplier, such as:

• How many have you sold?
• How long have you being doing it?
• How many dedicated staff do you have for this technology?
• Do you have out-of-hours cover for breakdowns, eg at Christmas?
• How long is the warranty, and is it externally bonded/backed?