What does 1 mega Watt relate to in terms of volume Jon, also if external I figure you'd need a real load of insulation to keep it warm even though the large volume helps with that.

J

Your figures suggest a water tank with about 200mm of insulation. Wouldn't the cost of the tank would be the major factor, not the insulation? Cylindrical would be a more efficient shape, a sort of blind gazebo, perhaps. Handy for if you're hoping to collect a lot of solar heated water ...

How long, exposed to winter incl. wind and rain chill, to decline
a) from 60oC to 35oC, the extremes of simple solar harvesting and retrieval
b) from 60oC to 5oC, ditto but resorting to heat pumping for retrieval
c) from 95oC to 35oC, the extreme of simple retrieval, but harvesting near-summer solar using large areas of expensive high tech collectors running at low efficiency
d) from 95oc to 5oC, ditto but resorting to heat pumping for retrieval?

Could this store be installed underground thus reducing heat loss

Thanks jon - just to get an idea of the class of such storage - sounds about typical of best-possible insulated tank type storage - 3 months max therefore requiring coupling to large high tech collectors to top up the store from weedy winter insolation. True inter-seasonal, 6-8 months, remains the holy grail!

But would those stores retain useful heat right thro the heating season?

Posted By: Jeff Norton (NZ)3 months in the depth of winter

That means Jun/Jul/Aug for you(?) but may I call it Dec/Jan/Feb?
Useful heat's needed for Feb, so the store has to be fully charged 3 months max earlier i.e. end-Nov. Can Nov solar input achieve that? What if solar input is only sufficient to keep it fully charged, given continuous standing loss, until say end-Sept?

Posted By: Jeff Norton (NZ)only envisaged 18 degC background heat from thermal store

Does settling for reduced rm temp extend the duration of the store, when the loss that limits endurance is standing loss, not the useful heat that's taken out?

Posted By: fostertom

Posted By: Jeff Norton (NZ)3 months in the depth of winter

That means Jun/Jul/Aug for you(?) but may I call it Dec/Jan/Feb?
Useful heat's needed for Feb, so the store has to be fully charged 3 months max earlier i.e. end-Nov. Can Nov solar input achieve that? What if solar input is only sufficient to keep it fully charged, given continuous standing loss, until say end-Sept?

Posted By: Jeff Norton (NZ)only envisaged 18 degC background heat from thermal store

Does settling for reduced rm temp extend the duration of the store, when the loss that limits endurance is standing loss, not the useful heat that's taken out?

Yes Jun/Jul/Aug but Dec/Jan/Feb is fine..

I target 18 degC WHO temps to maintain healthy home etc.. which 90% of the time that is fine for sleeping and active living etc and spot heat for higher temperatures (i.e. log fire) for passive living like watching the box/reading etc, spot heat bath room when required.

The thermal store has all summer to build energy and with some of my calc's Nov has a slight solar surplus so doesn't tap the heat store but Dec goes into deficit so requires heat from store to maintain 18 degC internal temp. Jan & feb require energy from store and then March goes into slight surplus again. May onwards starts rebuilding thermal store in earnest.

Jon,

It would be worth you looking up some of the home made largish thermal stores shown on the “builditsolar dot com” site; this came up through a related discussion on the pre-insulated pipes for use with stores site away from the house. Which you would also have to consider. There is a detailed project called “Solar Shed - - Thermal Storage tank”, that only 500 gallons. It might be that if you’ve a will to do this, you would build it rather than import the basics and convert it.

Very large vertical cylinders are marketed for example by Solar Bayer from 4000lit upto the 12m high 58,000 lit. Prices following the usual 1 £ or 1 Euro per litre. So cringingly expensive.

If we take your size as 10 x 8 x 8 ie shed size volume is 18122 lit, let’s call that 20,000 lit. This contains 840 MJ for a 10 deg temp drop, that’s the energy it will give up for each 10 deg drop. It does not matter if that is from 70 deg down to 60 deg or from 50 deg down to 40 deg, it’s the same. The problem occurs when converting that into usable heat within the home as the power delivered will be better when starting from a higher temp than a lower temp. Thus, you will get that transfer of energy quicker (more energy in less time = better power) when the water is hotter with respect to the ambient room temperatures. Basic principal of a heat exchanger.
If we have a low energy house than consumes 5 kW in an hour (5 kWhr) that’s 5000 J/s for the hour. 840000000 / 5000 = 168,000 seconds this is 47 hours. If the heat is only on for 10 hours per day, this will last 4.7 days. So if we can drop the temperature by 40 deg it would last 4 times longer = 18.7 days. The problem would be that when it’s down to 30-40 deg it will not easily transfer heat to the home. Underfloor heating operating at lower temps will help this, but as the heat load is still 5000W and UFH whilst operating at lower temp and smaller temp drops, does require a higher vol flow rate, so it consumes volume faster. Thus if you need 5000W then regardless of the delivery mechanism, the depletion time will be the same.

Cheers, Mike up North

I got sucked in longer than I intended. I'll turn in when they call Ohio, I said ... then, let's just see what Indiana does ...