Thermal store

[Tim Lamb]

I know this has been done to death previously but I am about to visit a friend who is contemplating a *grand design*.

National grid want their usual arm and 3 legs to connect so alternative sources are under consideration. The site is on a steep slope so a basement thermal store would be do-able.

Very roughly, how large a store of water would be required to provide dhw and under floor heating for say 200m2, constructed to something better than current regs.?

The idea would be to heat this with off peak electricity possibly supplemented with power from a PV array. Nice unshaded South facing roof.

I suppose tank life and replaceability is a consideration.

regards

[Tim Lamb]

In message , Tim Lamb writes

Thinking on.... I suppose dhw would be best kept separate as you don't want to heat the big store all summer.

regards

[polygonum]

I would have thought that you *would* want to heat the big store all summer. (Isn't that when the most energy is collectable?) But using solar heated water rather than electricity.

[Dave Liquorice]

If you get enough insulation in you need aircon to keep the interior at sensible temperatures. Just from the waste heat from appliances and the humans.

Thought you said NG wanted body parts for a connection. So implying an off grid place.

Wouldn't it be better to use solar thermal directly rather than go through PV's that have a much lower effciency?

The thermal store going in here will have as heat inputs, solar thermal sized for summer DHW, wood burner (something around 8kW flat out to water), existing oil boiler and immersion heater. The store will be something around 250 to 300l but that is really sized for getting maximum benefit of the solar thermal during the summer. A store sized to provide space heating from itself would require a huge solar thermal system to get it hot even in summer so you'd need to use other heat sources just for DHW and you don't need space heating in summer...

If you do want a store big enough to handle space heating you need to do the heat loss calculations for the building then it's simple maths to find out how many litres of water you need to hold that amount of energy with say a 20C temp range for the store (80C max to 60C min).

[John Rumm]

Do it in energy terms, and its easy enough to work out a ballpark.

Take a cold day, and work out the heatloss from the house making allowances for air changes etc and the level of insulation[1]. You will come out with a figure of a few kW no doubt. Multiply that up and convert to Joules you need to feed in to maintain the temperature inside for a day. Work out your energy demands for hot water, and add to the total. You can knock some off for heat wasted by appliances and people. Once you have a figure, and a useful store temperature differential of say 40 degrees (i.e. 80 deg C actual storage temperature). Divide that and the SHC of water into your energy requirement to come out with a number of litres.

(the answer will be "lots" unless you can get the energy requirement for heating down to very low worst case figures)

[1]

[John Rumm]

Big tall stores will stratify quite nicely...

[Tim Lamb]

Electricity connected, gas next door.

Umm.. presumably target temperature would need to be quite high to maximise the offtake heat. PV through an immersion would always provide useful heat whereas direct solar?

Yes. No hope of wood burner. Oil could be done but better to pay for a gas supply.

I think you could go lower for an underfloor system if dhw was separate.

regards

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[Tim Lamb]

Yes. I was just hoping for a ball park figure to see whether the concept is practical.

I'll see if her architect has come up with some drawings for rough dimensions.

regards

[Dave Liquorice]

With a store temp range of 20C you need about 1000l for every 90MJ (25kWhr) of energy.

[Tim Lamb]

Ah!

So if the store temp could drop to say 30C (underfloor heating) something like 60kWhr/1000l.

2 or 3 cu.m in a basement ought to be possible.

regards

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[Steve Firth]

We have a 300 litre thermal store with solar thermal panels, a log burner and an lpg boiler. There's also an (unused) 3kw immersion heater. It's too soon to give costs - we need to run it through winter. However monitoring the thermometer shows that the store stays above 60C through the day even when used for both underfloor heating and six showers per day.

Ambient temperatures have been above 20C so far. The floor area is 280 sq m.

The store has thermostatic mixers on the hot water and UFH circuits. These mix water from top and bottom of the store and ensure that the temperature in the store is maintained.

[John Rumm]

If you store at 80, and can drop to 30, then that gives you:

3000 x (80 - 30) x 4200 = 630MJ to play with, which is the equivalent of a 25kW boiler running for 7 hours. Probably way more than you need...

[John Rumm]

You can run the UFH from lower taping on the store than than those used for the DHW. Alternatively use blending valves to mix water from different levels to achieve the desired output temperature.

[Tim Lamb]

er.. Wouldn't you just blend stored hot water with returning heating water.

I understand what you are saying about stratification but if the store needs to be the size of a young swimming pool, tall/thin is not going to happen.

I am simply trying to get hold of how practical such a system might be.

regards

[Tim Lamb]

Is this Hampshire or Italy? The site in question is a fairly exposed bit of East coast Suffolk:-)

300l sounds tiny so your winter results should be interesting.

Last Autumn we rented a *cabin* in Suffolk which had a wet underfloor system and a store similar in size to yours heated using off peak. As many have said, underfloor is slow and not really suited to interval occupation. Once up to temperature, it seemed OK but I don't know how much full price power was used to boost the store output.

regards

[Tim Lamb]

Lets pause this. I'll report back when I have some proper figures.

regards

[Steve Firth]

harry wrote: [snip]

This is complete and utter shit. The efficiency of photovoltaics is less than that of decent solar thermal arrays.

[Tim Lamb]

Yebbut.

If the thermal store is hot after over night off peak how much useful heat would you get out of solar thermal?

regards

[Tim Lamb]

I suppose a photocell controlling the immersion heater would do.

I am sure the design will incorporate adequate insulation. I don't want to go making waves about exotic heating systems if they are not practical. Very large thermal stores would need future proofing to allow replacement without major re-building work. The site slopes and I envisage a basement accessed at rear ground level from the rear.

I can see Steve managing his set up but I struggle a bit with the thought of a late 60's widow and anything seriously complex.

[Steve Firth]

Tim Lamb wrote: [snip]

Not much management to do. The control pack does it all.