Ed Davies

Yep, that's about right. To add to @JSHarris's reply, one of the key things about a phase-change-material store is that most of the heat is stored at the same temperature (58 °C in the case of the Sunamp). This means that rather than have a very hot store (say 80 °C) in the case of a DHW cylinder with the resulting extra heat loss and need to mix the water down to a usable temperature the leakage from the Sunamp is less and the output water is already at a safe temperature for distribution. At the other end of the scale, the temperature of the water cylinder as it discharges heat will drop to the point (around 40 °C) when it's no longer much use but still losing more heat if it's not recharged. A Sunamp will continue to give nearly 58 °C water all the time until it's nearly completely discharged.

At the risk of going way off-topic, we don't any more. The Ordnance Survey grid is now defined in terms of the European Terrestrial Reference System which is, in turn, defined in terms of the International Terrestrial Reference System (via a transform which takes account of the drift of the European continent north eastwards at about 2cm/year). The ITRF is basically the coordinate system which forms the axes of the WGS84 ellipsoid.

Thanks, does to me too, now. Odd, could there be caching problems for individual posts in a forum like this, even though new posts are showing?

kW. But why so small? By my calculations 6 kW is about the minimum for one person who doesn't use a lot of electricity. The marginal cost of PV these days means it's silly to scrimp on it so it seems sensible to try to at least make your renewable generation over the winter average at least your consumption then leave any generator to deal with the case where there's a long lull. In other words, use the generator to cover short-falls in battery capacity rather than short-falls in average generation. Me too. The OP says “100% off grid” but nothing about the stated circumstances indicate they want more than just electricity. It's a matter of taste whether you consider, e.g., oil deliveries being “on grid” but just to save the cost of an electrical connection an oil boiler seems the obvious means of space and DHW heating. If you have sufficient PV and/or wind for the winter then there should be plenty of spare for the shoulder months (spring and autumn) so oil consumption should be pretty low.

Are you sure? For me know, even after a page refresh, it still reads:

Please fix this typo, people are already confused enough about these units.

Something else: to have anything like reliable power you'll need a mix of sources, storage and some way to control what power is used when. This is not terribly difficult but not the sort of thing that typical electricians, etc, have any clue about. Individual suppliers of PV, wind turbines, batteries, inverters and so on will tell you about their individual bits but none of those will have the “big picture” of how your system operates. You'll need one person who does have that big picture and also understands the details of how to set up each of the parts so they work together properly. There are very few people with a suitable track record to qualify them for that job so your best bet is for that person to be you. I'm thinking here of a chap in Aberdeenshire who built a very nice wooden house which was off-grid for very much the same sort of connection-cost reasons. He had a mix of sources of information with different people commissioning different parts and giving conflicting advice. For reasons which aren't clear to me he was advised by one supplier to remove one of the diversion dump loads on his battery and for other reasons which are also unclear this resulted in a fire in his utility room. Luckily it was contained and the damage was fairly limited. But it was evident from his description that he didn't really understand quite basic aspects of the system so was easily mislead.

There are some things to be said for being single.

I'm doing a “no-combustion” house. Meaning it doesn't burn stuff like wood, oil or gas and doesn't use mains electricity (a lot which comes from burning stuff). Well-insulated not-so-large house (~= 0.1 W/m²·K, good airtightness) with sufficient but not excessive windows. 6 kW of PV. 10 kWh of lithium iron phosphate batteries. 6 or 8 20x47 mm solar thermal panels (I've bought 8 but may only be able to fit 6 or 7 while allowing fire escape from the two bedrooms). Probably a small wind turbine but not decided what sort yet. Large (10m³) thermal store inside the house for space heating bordering on inter-seasonal storage. I'm a big believer in mixing wind and PV. There are lots of dull days and lots of lulls in the wind [¹] but it's very rare for a dull lull to last more than a day or so. Will that be sufficient “without a very large compromise in terms of living”? I hope so but ask me in a few year's time. I suspect 9 months of the year will be easy, December, January and February will be comfortable with a bit of forethought - look at the forecast and the state of charge before doing washing or cooking something that takes a long time. We'll see. OK, a backup generator's likely but I'd consider it a fail if it's needed more than a handful of times a year. [¹] Although I'd have made a bit more progress on the build this year if there'd been a few more lulls.

For the 12 days from 2018-11-21 to 2018-12-03 the night usage was 417 units (82459 - 81988). That's equivalent to a steady 1.45 kW. That actually seems quite low to me.

3 kW into the hot water tank, 3 kW into the small NSH, 4 kW into the big one would be 10 kW.

How many are just the tube broken and how many is it the heat pipe as well? Heat pipe just pulls out the top. £15 for just the tube, £25 for tube and heat pipe on the Navitron site at the moment (for 47 mm tubes that yours look like, but I'm not sure).

I'm fairly bored with this conversation; until we know why they've limited PV charging it's all just speculation. My guess is still that it's just bone-headed software. Still, if this were needed: Just connecting them in series would do it, I think. If the DHW flow is left to right then the left-hand one will discharge first and only when it's fully solid and its output temperature drops below the PCM transition temperature will the second begin to discharge (ignoring the small amount of energy stored as sensible heat once the PCM is all in one or the other phase and any tiny losses in the pipes between them). You'd want to prioritise feeding any available PV to the rightmost box which is below the required charge level (50% or whatever) and when that's full or there's more PV power available than its immersion element can take then you charge the ones to its left. Thinking about it, this sort of system might work well in a (badly designed) house with multiple bathrooms/kitchens which are well separated so there are long pipe runs from any central heat source. Have a small Sunamp near each bathroom/kitchen cluster to give quick hot water most of the time and prioritise charging them where possible but have a larger unit in the utility room or where ever for the main store to tide you over dull days. Is there a use for very small PCM units to go under sinks to provide immediate hot water until hot water arrives from the main source. For showers and the like which take a relatively large amount of energy once started they might not even need a separate charge circuit but could perhaps take it from the supplied hot water if their phase change temperature was a bit lower than the central source. They could be built into washing machines forming part of the weight those have for damping anyway.

Do we have any actual evidence that this thing about not accepting PV until discharged to some silly amount (50 or 90%) has anything to do with protection from the PCM overheating? That seems to be just an assumption being made here. Perhaps there's another reason why they're insisting on these deep discharges. Perhaps it's a good reason but if it is then indeed that's a serious problem but it seems more likely to me that somebody just hasn't thought through the controller design sensibly. Edit to add: my suspicion is that they decided on the 50% discharge so that any source they called for heat from, like an ASHP, would have a nice long run to work efficiently and they simply forgot that PV only works when it works.

Yes. But clearly putting the immersion right in the goo is cheaper and fine for most people most of the time. If you want to do something odd (as many of us will) then the best bet is using the low-power circuit to charge with an external Willis heater or whatever. Nothing's lost.

According to the Sunamp manual the maximum constant heat source flow temperature is 85 °C. Linked from le-cervaux's post . Table 4.2a. Strangely, their section of solar thermal doesn't mention limiting to this.

Out of interest, why outside? So long as it's more than 600 mm from the bath or shower (i.e., outside the zones) it can be inside. Does a zone overlap the obvious place for it just inside the door?

This is more or less what I have in mind though with additional sources and sinks for the heat. Would that ever have problems with starting from cold, like the electric element does? I.e., would you have to keep the flow rate down when there's solid PCM round the pipes or would it be sufficient to simply keep the flow temperature not above 70 or 75 °C or whatever the limit is? I assume that'd be safe but it'd be nice to be sure.

I think their aim's badly off in this respect. Sure, they want to move beyond the early-adopter/beta testers infesting forums such as this but still they're going to be selling for quite a while to people who are willing to do something a bit unusual who will naturally take an interest in the details. Their design should reflect that.

I'm severely disappointed, @JSHarris, that you haven't made your own triangular blinds that reef like a boat's sail at the bottom.

Yep, been thinking about doing this change (solar thermal to PV on the gable end) so worth bringing up anyway. I'm wondering if there are any plastic (UPVC perhaps) strips made for conservatories or greenhouses which could just clip in between the panels to just overlay the panels' frames. Probably screwed as well, of course.

It's not north facing, but faces a bit north of east, bearing 073° IIRC, so will tend to get sun early morning. In summer it'll get sun for a few hours before the main roof gets much so breakfast, etc, can come off these panels rather than cycling the batteries. It's a gable end so not much eave shading and not a place I'll walk past much. Plants will, indeed, need control around the house but about 8 metres away is the neighbour's fence. His cattle will stop much growing other than whins which don't grow that high and the ground slopes down a bit anyway. In general, I think there's a lot to be said for having PV pointing well off south to flatten the generation peak. Facing a bit round to the west for grid-connected would help with the evening consumption peak for example. As we move towards more renewables just maximizing generation becomes less of a priority compared with getting the timing right to minimise storage and transmission.

Also, when you have very variable weather, opportunities to generate at different times of day are good.

Yes, off-grid so no DNO to worry about. Only problem will be that it's at the opposite end of the house to my main electrical stuff so I might think about “grid-tie” to my own mini-grid.

Excellent idea, thanks. Yes, 40% more expensive than timber cladding (per m²) but generates some electricity. PV panels £38.80/m², 155 W/m²; timber cladding £27.85/m², 0 W/m² so that's £0.07/W ?