Perry Process also humoured me, and suggested £5k ex-VAT, ex-craneage, etc, for a 20,000l stainless-steel second-hand tank (316L) as at November 2007, so we might hope that, excluding works and insulation, for a total capacity of ~60kl (60m^3) and ~20GJ, the core thermal store might cost well under £20k, ie < £1/MJ. (Compare: ~£50+/MJ, ie ~£200/kWh and over 70kg to 50% discharge at 2007 prices, for lead-acid batteries, the RE electricity staple.)
For reference, our current CH/DHW is provided by a [archive] "Potterton Performa 24" 'instant' mains gas combi boiler rated at 24kW, SEDBUK band D, corresponding to a DHW flow of 9.8l/min raised through 35°C. The manufacturer tells me that this cannot be used to heat DHW fully or partly 'pre-heated' in a house thermal store by solar/GSHP.
Various parts of this scheme are not clear to me yet:
glazed first to get the temperature up cheaply, then vacuum to really peak up the temperature just before the water goes into storage.
See this [archive] ebuild.co.uk thread on storage heaters in turn referencing this MSc thesis on Electrical Storage Heater in the context of Storage Technologies
which led me to post this comment (lightly edited):
Suppose I ignore my normal outrage about electric resistance heating and [wasted] exergy, which does not [fully] apply if I can (say) get my magnetite to ~1400°C where it or the nichrome wire I'm heating it with melts, and I want to seasonally store PV output from summer until I need heat in winter. What volume (never might the weight for now) would I need to cover my space heating? Indeed I'll do my DHW ("flash to steam, baby") also and get rid of gas, for simplicity.
Last year we used 3MWh of gas, ie ~1E10J, for space and water heating.
So at 3893104J/K.m^3 and (say) at least a 1000°C temperature delta that gives me 3.9E9J/m^3 or somewhat under 3m^3 of magnetite to hold the energy for a seasonal store.
Somewhat less of a size issue than a buried water-filled milk tanker or three on my small plot, but ripe with its own health-and-safety issues.
And we don't really have 3MWh of electricity to spare (we generated just under 4MWh last year and used about 1.7MWh as electricity) but not utterly implausible. Nominally with a heat pump the exported electricity would cover our low-grade heat requirements over the year.
(And at those high temperatures would the store be worth tapping for a little electrical energy, eg for lighting at night, with a Seebeck device?)
Could it be done?
The density of magnetite is ~5t/m^3, so ~15t for the seasonal store. Adequate insulation would probably at least double the total volume, though possibly not the weight.
3MWh is our entire annual heat demand from gas, and if we ignored our DHW demand of ~4kWh/d or ~1.5MWh, that leaves ~1.5MWh for space heat demand or maybe ~2.5MWh for a colder year. If we could deal with a 'typical' winter with 2MWh/2m^2/10t that seems more plausible still. (I still hope for the return of a domestic instananeous heat-pump DHW system to the UK market so that we could end direct use of natural gas entirely without too much of a carbon penalty!)
This discussion merged into a [archive] storage thread on eBuild with Andrew Bissell, the CEO of Sunamp, explaining how its heat batteries work (for daily storage, eg SunampPV) and the possibilities for seasonal storage at 500kWh/m^3 with Thermochemical Materials (TCMs) that use reversible chemical reactions to store heat and can be held, charged, at room temperature to remove the need for vast insulation.
See also re-thinking DHW with SunampPV.
At 16WW in 2023, with ~1MWh/year dropped from our gas consumption by DHW diversion to a heat battery, it is just about plausible to cover the residual ~2MWh/year of space heat from a milk tanker thermal store or equivalent!
The storage volume and/or temperature could be reduced (or capacity increased) by use of a phase-change material (PCM) such as Sunamp's Plentigrade(TM).
2007-11-26 Rex Butcher wrote to me with some thoughts on this issue:
My wife and I are going to be building a new, well insulated house next year and we are considering installing quite a number of 'eco' (I hate the term) devices. Certainly, rainwater collection will be one of them, and I am looking at ways of reusing the bathwater. But I can see with bathwater, particularly during the winter, that there will be more water available than we can use.
One idea I have is to adopt a 'Japanese' style of bathing. Shower and clean outside the bath and them get into the bath, full of hot water for a nice long soak. In Japan, the whole family will use this hot soaking tub of water and at the end of the bath time, it is virtually uncontaminated. In fact, in Japan, that water will be reheated and used for a few days before being discarded and refilled. Most Japanese bathrooms have a sort-of 'in-line' heater that takes the bath water, heats it up and returns it to the bath. The bath itself is usually filled with cold water. But trying to find such a heating device in England is proving very, very difficult.
However, that is not why I started this mail.
In one section, you mention about heat banks / thermal stores, such as the TORRENT 450 'RE' (450l). I have been looking into this as I would like to install a thermal store in our new house, It seems to me that there is a great difference between the UK manufacturers and European manufacturers. I don't know who is correct, but my feeling is that Europeans have greater experience in this field. UK seems to make small(ish) tanks with only 50mm of insulation as standard, then tell Joe Public how good they are and how quickly they recycle.
Europeans seem to go with much larger tanks (800ltr/1000ltr) and have 100mm of insulation. Obviously, more insulation equals less heat loss. Have a look at [archive] http://www.tisun.com/cms/en/vertriebspartner-haendler/england-wales/ or [archive] http://www.martin-sandler.de/index.html (it is a German site but if you Google Martin Sandler and hit the Translate button!) Construction Resources sell this tank.
Regarding GSHP, we have looked into them and decided not to install one, mainly on a cost/payback issue. We do not really have sufficient space for ground loops so the only alternative would be boreholes, which as you know, are horrendously expensive. However, I did find a lower price alternative to boreholes. This company (http://www.lankelma.co.uk/) are Dutch; they bang something like scaffold poles into the ground, drop the GSHP flow/return pipes into the tube, give the tube a twist to unscrew the pointed base, and extract the tube, leaving the ground to encase the flow/return pipes. They apparently usually go 25-30 metres deep, so 4 x 25m deep holes would give a total pipe length of 200m in a very compact space with minimal disruption to the garden. I believe they are not as expensive as normal boreholes but certainly much faster.
My reply:
Yes, the fact that a 450l tank is the biggest that I have yet been able to find being marketed in the UK is not great. My feeling is that In any case I'd want at least double that or more to carry several days' heat especially in winter when there are often several overcast days in a row.
Also, the appropriate British Standard still allows over 2kWh/day of heat to escape from a ~250l tank, which seems too high for modern designs. I could not find what the TORRENT 450 tank loss was specified as: did you run across a number from them?
Yes, the Japanese-style bath is interesting. I remember signs reminding foreigners like me to remember to wash before getting in the bath when I was in Tokyo, and for reasons of hygiene if nothing else I still do some washing before filling the bath. (Actually, I'm keen on greywater heat recovery too if it can be done well to help trim the heat loss.)
PS. I have just run across consolar.co.uk which has (a) a very sensible sizing guide amongst other things and (b) stores up to 2200l.
The Rex came back to me on the greywater/bathing issue:
Thanks for the Consolar link, I was looking for it. Every manufacturer has a different system and how to find (and buy the best/most efficient) is very difficult. I happen to like the Tisun solar heating system, but I have no logical reason; likewise, the Consolar seems to use a similar solar heating system, stratifying the water. One UK company that seems to be popular is DPS, but they don't make large or well insulated tanks. However, they do use an external plate heat exchanger to heat the DHW. What are the benefits over having a coil within the thermal store? Beats me. They state that the heat exchanger can be easily replaced but why should it need to be? Guess you pays your money and makes your choice!
You are obviously familiar with the Japanese bathing method. My parents-in-law now only fill their bath about twice a week, heating it every evening and topping up as necessary. Shower, soap and scrub outside the tub, climb in for a nice submersed soak. It's great. There are soaking tubs available but they are just a different shaped bath. The best I have found is actually Australian (http://www.japanesebath.com.au/) and not so outrageously priced. I did e-mail them about the Japanese style of bath water heating but their response was that it was unhygienic and therefore dangerous to the health. Yeah, 150 million Japanese find it unhealthy too!
I have even thought about getting a Japanese heating unit next time we are there but I can foresee all kinds of connection problems, let alone a CORGI man taking a deep intake of breath and deeming it dangerous. My parents-in-law have their heater on the outside wall, adjacent to the bathroom. No larger than a desktop computer; the controller is in the bathroom. Hit the on button to fire it up, set the timer and away you go. Water flow is by convection, no pumps involved. Just an inlet and outlet into the bath. If I could get something like this at a reasonable cost, I would also insulate the bath, which is something one never sees. Don't think Japanese baths are insulated but after the bath, one covers the water with a top (sometimes floating) and usually the next evening, the water still has a bit of residual warmth.
I have even investigated Jacuzzi in-line heaters but they don't seem to provide sufficient heat to get a bath up to soaking temp in 15/20 minutes, which is the time my parents'-in-law takes. But then, a Jacuzzi heater is designed to handle a lot more water and not to get to bathing temps.
Regarding reclaiming the heat from a bath, I have also looked into it a little and found this gadget; these links; http://www.gfxtechnology.com/index.html, [archive] http://www.lowheat.iphe.org.uk/ (but I don't think anything exists yet; also see [archive] http://www.eurekamagazine.co.uk/article/9915/Keeping-in-the-heat.aspx) but I'm not sure how this kind of heat recovery works. After all, the waste heat from the bath, although considerable, will be cooler than water in the thermal store, so I guess there would have to be another heat exchange unit, low on the store, similar to the solar exchanger. Probably not really worth it. Can't afford my next idea, but a very large buried collection tank in the garden to collect warm waste water could have collector coils for the heat pump to extract the warmth. But what happens when the heat has been removed?
For grey water, there is this toy ([archive] http://h2oplc.com/grey_water.htm) from Germany I think, but it is probably more complex and expensive than necessary. If we can use the Japanese system, have a diverter on the bath outflow, almost clean water could be sent to a holding tank for reuse or directed to the the sewerage system when the holding tank is full. But as I said previously, if that water is used for toilet flushing, there will almost certainly be more than we can use so would still have quite a lot of wastage.
And then my non-expert guess as to why the suggestion of 'unhealthiness' may have arisen:
I think that most thermal stores will benefit from stratification so you get hotter water than you really deserve for the energy put in!
As for the Japanese bathing system being 'dangerous' I think the danger may be Legionella (and other bugs) that can breed in water kept below 60°C and not heated above that before use. I seem to remember the very few Japanese baths that I took being excruciatingly hot, even by my 'asbestos' standards, so the water may conceivably have been at or just over 60°C. [probably ~40°C]
I saw the GFX thing, thanks, and one of us does take showers, so that might cover some of our usage. The bath is more problematical since the cold water for heating for the bath is not being drawn when the warm water goes down the drain, of course, and also, as you said, it's not obvious that it will be warmer than much or any of the store.
My current thought is either to use it to heat the seasonal thermal store if possible, or to use the heat pump to quickly pump its heat into the house store if possible, or to use it to take the chill off a holding tank of incoming mains water destined for the DHW system (but not getting it warm enough to allow bugs to breed in the chlorinated water). Dunno. I hope to have a chat with a Man Who Really Knows [who advises the UK government].
As to cleaning the greywater enough to use in WC cisterns and stuff: that does seem like quite hard work and think it is more suited to a place where water itself is scarce, which is not (yet, often) the UK. I have a friend in Malta who uses a well in his garden (which may or may not be potable) for WCs and for cooling/heating with a heat pump AFAIK. They really do have a horrible shortage of water, with much of it being desalinated seawater using expensive electricity from inefficient oil-burning generators. I think that the UK authorities don't want greywater to sit around untreated anywhere to breed bugs, so any reasonable solution will involve its quick exit to the sewer (with minimal obstructions to prevent gumming up with the detritus, soap, hair, etc) one way or another I suspect.
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