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Only about 1/3 of the retail cost of electricity is for generating it - most of the rest goes to maintaining the grid, billing, etc. Rural + Remote = non-generation costs being higher than in an urban environment.

May be helpful here...

Nope, what we're thinking of is using an ASHP instead of a GSHP if the quotes come back too high. We'd prefer a GSHP, but aren't prepared to pay very much of a premium for it (say £2-3k). Cooling is mandatory, whether we get a GSHP or ASHP. Apologies, that was poorly phrased. The point I was trying to convey is that the time constant for a ground loop is very long - the specific heat capacity is very high compared to the thermal conductivity. This means that ground loops are sized for the total heat demand over a significant period of time (i.e. primarily on ground thermal conductivity - it comes directly into the sizing calculation), rather than that found in a cold snap. Essentially you end up with a system with a great deal of thermal inertia on the ground loop and thus a COP unaffected by cold snaps. Nah, the reasoning that we're interested in a GSHP is independent of the reason that cooling is mandatory. I'm wary of oversizing the heat pump. Quite apart from the cost impact, the smaller the heat pump the easier it looks like it will be to control. Additionally, if modelling with PHPP using somebody competent then we shouldn't need much margin - although as noted PHPP should predict a heat load of about 2kW, and the smallest available heat pump would be 3kW, so 50% margin. What I have in mind is not dissimilar, but probably a bit simpler. Essentially the plan is to turn the thermostat up one degree when electricity is cheap (mostly from PV but also when it's windy in winter) as well as shift the time clock for the hot water cylinder around the same way, and otherwise leave it to do it's own thing. Given the location and alignment, we're quite likely to go for the 10W/m2 criteria rather than the 15 kWh/m2/year one to avoid getting too much solar gain which should also help a bit with the summer overheating.

180m2 house, planning to go the full Passivhaus route which means 10W/m2 at the design low temperature condition. It's worth noting that the ground array sizing is based on the annual heating requirements in any case, rather than the peak ones: soil has a hell of a lot of thermal mass, and the limiting factor is thermal conductivity within the soil so a cold snap really doesn't come into the sizing calculations. The ground loop sizing calculations are based on 15kWh/m2/year for heating and 10 for hot water as the total heat demand to feed into the MCS calculator. The reality is that the smallest heat pump (air or ground to water) I can find out there is 3kW, and in extreme cold conditions should you need headroom you could always use an immersion heater. That means I'm at least 50% oversized whether I want to or not, and can provide all the required heating and hot water from the heat pump down to the design cold condition without needing additional heat. Planning for warming the house up again after a heating failure during a cold snap is overkill for me - I'd just grab a couple of fan heaters and run them until it was over, and it's worth noting that the house would have a very long time constant against cooling down in that case anyway. It would probably be possible to go smaller with an air conditioner, but that would annoy me a lot. An MVHR duct heater would be a very cheap solution, but some form of active cooling is non-negotiable for my wife: she grew up in the US where they all have air conditioning and bitches about the lack of it every year. I'm not even going to try convincing her that a well-insulated house really doesn't need it. The Kensa Shoebox 3kW can be built with a passive cooling module while the Nilan Compact P Geo 3 can also do cooling. If either comes out too expensive we're likely to switch to an ASHP - but again a very small one.

The MCS sizing calculation gives a single 30m slinky or 85m of straight pipe at 1m spacing for my predicted heat load and ground conditions. Garden is 40m long, so no reason to consider more than one slinky. Trenches for the straight pipe would be bucket width and 5m apart which should be good enough. Just noted an error in my earlier statement - 3 x 14m gives me the pipe length requirement from the MCS calculation which is probably rather pessimistic as nearly half my calculated load is for hot water and I'd use it for summer cooling as well - the calculation basically assumes heating only. I'm actually assuming 3 x 20m trenches which is the longest I can go without some trees starting to be a nuisance for the digger. Still nearly 50% oversized however. Yes, although in practice depth is less critical than you'd think - the difference between 1m and 2m is pretty minimal for instance. Surface area is critical, but for a low energy house you really don't need very much of it.

One of the critical factors here is that with an air-source heat pump if you need a bit more energy you just turn the fan up. With ground source the heat has to come to the collector - either through moving groundwater or thermal conduction through the earth. Conduction will always work - it's pretty much a statement of the zeroth law of thermodynamics that energy will flow from hot to cold - but this requires a temperature gradient and as a result if you pull too much heat out for a given length of ground loop this temperature differential will get very big and you can easily freeze it up. That's very likely to be what happened here - the ground loop was too small and froze the ground up. Rather than spending the electricity to try and thaw out the ground, the GSHP was working really hard to extract heat from a source at -10°C - while the new ASHP can pull the same amount of heat from air at +5°C. That requires half the work, and hence gives a much better COP. Assuming you can get RHI would make me nervous - if for any reason you can't it's a hell of a bill to deal with, and getting it is reliant on the whim of the current government. Essentially ground source heat pumps work on the fact that there is a lot of thermal mass in the soil. That's why the earth is at ~10°C at depth - it's the average air temperature for the UK over the course of a year. Depending on your geology and how much heat you have to dump, recharging a borehole from waste heat **may** work: it works very well for the underground as they've got a load of heat from passengers & trains plus a load of cold water to get rid of, but unless you live in a triple-glazed greenhouse then you're probably going to be relying on heat coming from elsewhere. The normal route is a mixture of rain and flowing groundwater. Basically, a slinky is coiled pipe laid in a trench, "strings" will be straight pipe. To get the same performance, the slinky will need more pipe and the straight pipe will need more trenching. I'm looking at straight pipe because the loops are a bit more flexible (so easier to fit under my lawn), pumping losses will be slightly lower and it'll use less antifreeze. Again, system size is critical - my calculated heating load is about 2kW, and that means I can get away with 3 x 15 m trenches with a loop of pipe in each and still be heavily oversized.

Financially it's a no-brainer to go for ASHP. I'm planning/hoping to go for GSHP, but I've got two particular circumstances which make it viable (TBC when we get quotes! ) It's going to be a very well insulated and not enormous house, a 3kW unit with a small ground array is viable as a result which brings the cost of the heat pump unit itself a lot closer. We've got a very large back garden, and a lawn which could really do with re-laying and is more than big enough for the ground array. Couple this with the fact that it's cheaper to put some excess topsoil on top of the lawn than it would be to dispose of it and the groundworks costs should be very low. That means we can justify the couple of thousand pounds higher install cost in exchange for not having an outside unit (which I find ugly) facing the garden. I don't think it would be viable if we had to pay much more of a premium than this however. I'm not overly concerned about running/maintenance costs - Glycol should last a long time if kept cold and not exposed to UV, and a GSHP being indoors should probably last a bit longer than ASHPs so the higher unit cost should probably come out in the wash in the long term. We have gas on site at present but are planning to disconnect it - this is primarily because cooling is a hard requirement for my wife which implies a heat pump, but also because for a very low energy house the standing charges make gas a surprisingly expensive way to heat.

For a lot of industrial applications they do. I'm somewhat at a loss as to why they prefer to use "inverter drive" as a marketing term for heat pumps, but it seems to work. What the engineers call it is kind of irrelevant here - for instance the marketing blurb doesn't mention a rectifier, but that's kind of a critical component. The term "inverter" has precisely one meaning in electrical engineering, as does "converter". An inverter is a particular type of converter, and in turn there are a large number of possible types of inverter. Precisely. There are some subtleties but that's fundamentally what is going on.

No, it has a very specific meaning in electrical engineering - it's a device which converts DC into AC and that's exactly what happens in both PV and heat pump installations. And you don't run your tea or your partner though an inverter.

Ummm... the component which turns AC into DC is called a rectifier. The component which turns it back into AC is called an inverter. The key benefit of this is that it enables you to change fixed-frequency AC (50Hz) into variable frequency AC matched to the running speed you want from your compressor.

Allowing the temperature to drop and riding through a cold snap only works for intermittent cold days, although it's worth noting that a very well insulated and airtight house is likely to be significantly more comfortable if you try to do that than one built to building regulations. If that's your general plan however it just adds up to to using resistance heat - and there are ways to make it even easier than pulling out a fan heater (duct heater in MVHR, etc.). Capital cost is very low indeed. Running cost, not so much: a bill of £1200/year for heating and hot water in an exceptionally well insulated house is distinctly mediocre. With a heat pump that goes down to £400/year which is much better. It's also worth noting that fitting a heat pump is equivalent to reducing the heat losses in the house by a factor of 3. If you account for the cost of building to the much higher standard required to achieve this in other ways, then achieving any given energy consumption standard will almost certainly be cheaper if a heat pump is used rather than any other form of heating. I don't disagree with that. It doesn't make a huge difference to the maths though - for anything but the very smallest flats, it is much more efficient (both financially and in terms of lifetime energy use) to use a heat pump rather than resistive heat. It's also worth noting that I used Passivhaus numbers - it's still pretty rare for houses to be built to that standard, even most self-builds miss it and the volume housebuilders are miles off. That's deliberate on my part - if the numbers are clear for a Passivhaus, it's a no-brainer for 99% of the housing stock. The obvious parallel is a car. The average car drives 10,000 miles per year, and the average speed across the whole road network is 60 mph. That means the average car is driving for 167 hours per year - 1.9% of the year. Getting a taxi every time would have a much lower capital cost, but the running cost is significantly higher and in the vast majority of cases having your own car is considered more cost-effective.

Important thing to note here: when you make hydrogen by electrolysis only about a third of the cost comes from the electricity, there are some pretty big costs associated with the plant, storage, distribution, etc. Even if the electricity was free, you're looking at 10p/kWh for green hydrogen once the infrastructure is fully developed. In reality it'll be more expensive, since the plant costs are so high that you won't only run them when there is surplus electricity on the grid. Fag packet maths, based on the Passivhaus standard because it's easier: 15 kWh/m2/year for heating, or 10W/m2 peak load - in reality the two are usually pretty close so let's assume that the notional "well insulated" new house hits both standards. Make it a fairly large house - 200m2 to make the maths easier - and assume that hot water use is also 15 kWh/m2/year which seems to be a reasonable assumption for a reasonably efficient system. 3000 kWh/year for hot water, 3000 kWh/year for heating. 8.2 kWh/day for hot water, plus **peak** heating load of 2kW → 56.2 kWh/day, requiring a 2.3 kW heat pump to satisfy it. Round up to 3kW. COP assumed to be 3, so compared to resistive heat only the saving is ~4000 kWh/year. At 20p/kWh, that's £800/year saving from using a heat pump. Even if you oversize to a 5kW heat pump, you're looking at paying £3k for a monobloc unit → 4 year payback time. Assuming you did go for a 5kW unit, it would be running at full power for 600 hours/year to provide sufficient heating - just under 7% of the time - and the same amount again to provide hot water. If you have a more efficient or smaller house - and particularly if you can use less hot water - then the payback time will get longer. I'm really struggling to see a heat pump not making sense though.

Roof-integrated PV is surprisingly cheap. A ~10kW system would generate about 1100 kWh in April on my planned roof, and cost about £6k upfront in parts to generate 9300 kWh/year. Even at 5p/kWh that's £450 in export tariff a year, so £600 is a pretty reasonable guess for what it's worth in income/savings. Given that the cost is in the region of £100/m2 and nice tiles are likely to cost at least £50/m2, then a PV system which exports a lot starts to look very attractive for most self-builds.

I never cease to find you a source of amazement. And several other things too.

It's very simple: Stage 1 is to get everybody thinking that hydrogen is the solution - and to get them to commit to making it a fundamental part of the energy system. After all, it's new technology (so must be better) without being so unfamiliar as to be scary - so that has to be a good thing, right? Stage 2 is to blur the line between "blue" and "green" hydrogen - because both are the same in the pipes, aren't they? Stage 3 is to "discover" that "green" hydrogen is really, really expensive, and spend lots of time talking about fuel poverty. Stage 4 is to profit from the fact that your otherwise worthless natural gas reserves are now critical to the new "hydrogen economy".

Just realised I missed something important: you can achieve the same thing with two phases (say the two A phases above - wire one forwards and one backwards). However, for a big motor that isn't very helpful because it can't easily create a rotating magnetic field. With a 3-phase system, you can wire the motor stator up in the same way as the generator shown above, and it'll create a rotating magnetic field which follows the same path as the magnet in the .GIF above. Virtually all big motors do this, albeit in most case they use a squirrel cage induction rotor which lets the motor run slightly slower than the grid frequency.

OK, I'm tired so this might be a bit of a bad explanation, but it all comes down to how electricity is generated. When you move a magnet past a wire, it induces a current in the wire. In practice going in a straight line isn't practical, so you end up with a rotating magnet. This is also why electricity is delivered as a sine wave. Now it's worth noting that electricity is delivered as a current. For a domestic single-phase system, that means it arrives on the Live wire and returns to the grid down the Neutral wire - with both wires carrying the same amount of current: the number of electrons entering and leaving need to match. For a power station and transmission system that's a pain in the backside - only half your wires actually transmit useful power, and with a single phase your generator has got lots of empty space inside which could be used for something different. This is where three-phase is really helpful - as you can see from the .GIF below, the sum of all the three currents in a three-phase system is always zero. That means for instance that you can create a neutral within the generator by tying the three windings together, and the neutral wire on the supergrid pylons is the little thin one at the very top of the pylons, with all the other wires carrying power. This feeds back to why big electrical loads need to be attached to a 3-phase supply: if the loads aren't balanced then either you get a distorted sine wave or more likely you start sending a load of current down the neutral wire. Turning 3-phase into domestic single phase is easy: you send out the three phases plus the neutral wire from the transformer, and each house gets one live phase and a neutral, which brings the voltage down from "415V" (the phase to phase voltage) to "230V" (the phase to neutral voltage).

Pros: 3x the electrical power in and out of a single phase system. That's handy for things like faster car chargers, big PV arrays, etc. Some things - basically big motors - need 3-phase to work. If you're planning a big workshop that may help as the bigger tools may need it. Cons Installation cost may be bigger - depends on your circumstances. Smart meters are harder to get for it at present. Not worth spending £££££ to get, but in most cases the difference in cost should basically be down to 4 cores rather than 2 in the cable they lay, which is pretty trivial.

We've just had the quantity surveyor report back for our planned build, a ~170m2 Passivhaus which will have a heating load of <2kW. It includes the following line: How on earth did a sum of ~£500 per m2 on radiators not get noticed and queried? More worrying, is there anything else in there as dodgy?

"Construction Manager" (#1122) is on the shortage occupations list (https://www.gov.uk/government/publications/skilled-worker-visa-eligible-occupations/skilled-worker-visa-eligible-occupations-and-codes), as is "Builder" (#5319) so it will be possible to get a visa. You need to get approved as a sponsor before he can apply - that costs about £500 - and once you have it you can sponsor him to apply for a work visa.

That's probably the one area that the Mixergy tank makes any sense, and only if you don't use the external plate heat exchanger. With PV your cheap electricity will be during the day in summer and night in winter, and just using an immersion at 5p/kWh (PV export cost) is relatively expensive. Heating from the bottom (or using the plate heat exchanger) means you de-stratify, and if you've got a relatively small heat pump (matched to a well-insulated house) you're going to have lukewarm water for some hours in the middle of the day. However, for all their "smart" controls and integration with Octopus Agile they have no ability to use PV for anything but an immersion. That's a major failing, and means you've got to be willing to write your own control system to really benefit from the increased cost & complexity (which to be fair is pretty minor in the grand scheme of things).

https://www.topcylinders.com/mixergy-300-litres-standard-smart-hot-water-cylinder has it at £1150, which includes the control box.

The standard version has a single (smallish) coil at the top which is in the hottest water. That gives a worse COP compared to standard type coils, which is why they do the external plate heat exchanger which can pull in colder water. So you can use it direct with an ASHP, but are likely to see a performance hit. Having said that, charging from the top will give much faster recovery so it is probably quite a good match for a big tank/small heat pump.

Umm... if it's a Passivhaus, shouldn't PHPP give you your sizing numbers directly?

Not exactly - it's more of an issue that the mechanical inertia in a wind turbine is fairly small. Good article explaining it here - https://spectrum.ieee.org/energywise/energy/renewables/can-synthetic-inertia-stabilize-power-grids . Combined with batteries it's likely to produce a good solution. Note that it's also possible to have a synchronous wind turbine if you really want it - go for something like a doubly-fed induction generator and you can tie the main stator directly to the grid. There are reasons it isn't the preferred architecture, but it isn't a dead duck. There is an issue with response time for batteries - grid inertia produces an inherently instantaneous response, which is quite difficult given the battery chemistry. It's less challenging but non-trivial to achieve the same with non-synchronous sources like wind turbines. Having said that it's quite likely that giant flywheels may be the cheapest way to add what is effectively inertia to the grid - they're cheap up front, cost peanuts to run and reduce the system costs associated with providing the inertia a long way away from your loads.