Solar thermal dump load under wood pile

[acresswell]

I'm planning to fit a log boiler, 2000 litre accumulator tank and solar thermal panels on the roof. I'd like to fit as large an area of solar thermal as possible, to extend the number of months where the log boiler is redundant (and therefore minimise the volume/mass of logs I have to shift) but I'm aware that if I fit too many solar thermal panels then in the middle of summer I run the risk of boiling/exploding the accumulator tank or some of my pipework, so I'm thinking about a thermal dump under the wood pile. I've got to move the wood pile anyway while we do the rebuilding work, and I'd naturally be happy if my logs dried out a bit more  (to further minimise the mass of logs I have to shift)
 
I'm wondering about lifting the wood pile off the floor on scaffold tube (with the scaff tube resting on bricks, to give decent airflow through the logs).  I could then feed some PEx underfloor heating pipe down the centre of the scaff tube. The scaff tube should be a decent conductor, but how about the air space between PEx and scaff tube? I suppose I could try filling it with oil, but it could get messy. I considered using copper, but 3 concerns: 1) physically robust enough unless protected by scaff tube?  2) freeze resistance? 3) risk of setting up an electric cell if putting iron,zinc and copper in close proximity.

The "PEx in scaff tube" should certainly be physically robust but can anyone foresee any other problems? 
How about freeze resistance? (I'd rather avoid having to drain down every winter because it'll introduce oxygen into the rest of the system every time I refill it and because I'm busy enough doing other things!)
Any guess at the output per 6m length of galvanised scaff tube?
Any better suggestions?
Does it make sense to control it simply by having a thermostat set at 95 degrees C at the bottom of the thermal store?

All suggestions appreciated,

Adrian

[martin W]

Hi,

you would need to used something other than PEX to dump the heat under the store. You could use PEX to connect to the store to some finned copper pipe (or such like) that is under the wood store. I just don't think the pex would dump enought heat in a small area as you are likely to have unse the wood pile.

Will PEX survive use at 95°C plus? How are you going to protect the thermal store if the logg boiler over heats it? I woudl consider the heat dump to run from the store so the solar is always heating and the dump only dumps heat from store (however it got too hot).

Would leaf pile, etc under store be at risk of smoulding / catching fire if heated (in contact) with copper at 95°C. (I would not have thought so).

[Alan]

Why not have a slab of concrete on top of say Kingspan insulation with the Phex pipe in the concrete. It would be mechanically robust to just store the wood on top. With adequate pipe insulation on the flow and return pipes it should survive the winter.
You could make a wood / metal frame to allow a bit of air circulation. But heat will rise from the slab anyway.
You do not mention how many tubes / size you are fitting.
It is better to have far to much pipe in the under slab heating system instead of not enough, You just control the heat input on a hand valve. ( not enough pipe and you cant control Nowt. )

The system will not be fail safe. If power fails / pump goes wrong ?
Now is the time to ponder more.

Regards

Alan

[Baz]

You will need a lot of pipe to get the necessary surface area. Consider old single radiators which are often available on ebay. Plastic pipe is asking for mice to chew it. Unless you have a concrete floor to the woodshed you should stack on pallets anyway to keep it off the ground. It might do to use the radiators vertical as section dividers which in summer will be fully surrounded by logs.

Consider the ecenomics. Excess tubes seems attractive but are expensive.
I did some rough calculations and as best as I remember:
30 x47mm tubes 'earns' £70 during the 5 months of summer. Then for the other 7 months they 'earn' £50 (cheap rate electricity equivalent). 30 tubes is just enough for 40Litres x 4 people average in summer. If you get a second batch of 30 tubes it is mostly wasted in summer but helps in winter however at a cost of say £500. That's 10% payback so not bad. Beyond that I suggest looking carefully at whether spending the money on insulation is actually better. Also bear in mind the danger of damage due to stagnation events with excess capacity and maintenance costs.

[dhaslam]

It is good to have  some a good bit of  excess solar heat and in practice all it will do in summer is  keep the store warm.  In the Spring and Autumn you will be able to avoid running the boiler in sunny weather.   At present my 850 litre store is at 38C it only gains a few degrees  on a sunny day  from sixty  47mm tubes.   Some heat goes back to the DHW cylinder and occasionally the underfloor heating can come on.     If you do need to dump heat you should  dump heat from the store at about 60C so you could still use plastic pipe at that temperature.     

If you want to have warm  air to dry wood it would be  better to use  warm air from a black painted roof, possibly the woodstore itself. If you make a narrow channel under the roof  you should be able to collect very warm air that can be circulated by fan through the wood.   

[EccentricAnomaly]

Also bear in mind the danger of damage due to stagnation events with excess capacity...

Do you consider stagnation more likely or more damaging with excess capacity? In either case, why?

[Baz]

Once the store is hot the input to the panel could be 90? so with oversized panels isn't there a more significant chance that it will boil even with a high pump rate. Dhasam is obviously better able to advise with his practical setup so dumping at a low temp would be the solution.

[acresswell]

The new log pile will be about 10m long and 0.5m deep.  It’s definitely a pile, not a shed...  no roof structure, though I’m likely to throw a tarp over the very top (not sides).  It’ll be leaning against a retaining wall, with a concrete base (which is part of the structure of the retaining wall), so I could easily add a “screed” to the top of the concrete and incorporate pipework in there.

Thank you for your responses...

Martin W... I’m definitely planning on running the dump from the thermal store. I doubt that we’ll ever overload the thermal store by firing the log boiler unnecessarily but, as you say, it’ll form a nice little protection against that event. I doubt that leaves will start to burn at 95-100degreesC... when trying to determine the biomass of soil or leaf litter we would regularly heat samples to 150-200degreesC to evaporate any water (with no sign of any smoke), before heating them to nearer 800degreesC to burn off the carbon content. I do take your point about whether the PEx will cope with water at 95degrees C.  I could fit a mixing valve to reduce it to something like 60-70degrees C, though obviously the hotter the water in the dump, the more effectively it will dump.

Alan...  not sure why you’re recommending using insulation under the pipes/concrete. Surely the idea is to lose as much heat as possible.  I don’t care where the heat goes!  I certainly agree that it’s better to put too much pipe than too little... not sure that I would bother with the hand valve, since when the heat dump is needed I’m happy to lose as much heat as possible as quickly as possible without restriction. I agree that if power fails then the heat dump (on a pumped circuit) won’t work, but the pump to the solar panels will also have stopped, so the problem won’t be too much heat reaching the store!  My other backup-plan would be to pour hot water down the drain, therefore cooling the tank via the DHW coil (but I’d rather avoid doing this if possible)

Baz... I certainly get your point about mice and PEx.  I’m concerned that rads not may cope with freezing but I’ve got some rads in the house which will become redundant at the time of the building work, so it’s an attractive proposition! I wasn’t planning to over-spec by as much as doubling the number of tubes... maybe just 25-50% more.  I’m already planning on low-U-values, and it’ll be difficult to improve on this without compromising on living space.
dhaslam... I’m certainly hoping that, like you, the thermal store will take most of the solar capacity and save it for a rainy day.  This heat dump is only to act as a backup for when we have a whole month of hot weather and the store gradually gets hotter and hotter. I’d be reluctant to start dumping valuable heat when the thermal store was only at 60degreesC since this could happen just through normal use of the log boiler (though I can use a mixing valve to reduce the temp to 60degrees C even if the tank is warmer). 

Baz... I see what you mean about the risk if the input to the panels is at 90-95 degrees C. Maybe I should start dumping when the bottom of the tank is at 80degrees C or so...?

Any other thoughts?  Especially on avoiding problems with freezing? I don’t want to waste heat in winter by having to keep warm water in the dump over-winter, but to keep the system simple I’d prefer to treat it as just an ordinary heating circuit (no separate header tank, etc) with different controls.

Thanks,
Adrian

[Countrypaul]

I may be completely off the mark here, but why can't you add antifreeze to your heating circuit to prevent the freezing up issue? Obviously the antifreeze would have to be "safe" in the event of contaminating the potable water but that is common and used in solar circuits anyway. What is in your solar circuit to prevent it freezng up?

Not sure what strength you would need or the consequences on reduced heat capacity of the circulating water, but I'm sure others can comment.

Paul

[Countrypaul]

On second thoughts, a 2000L TS might beexpensive to treat with antifreeze.

Paul

[SimonHobson]

Whilst it won't help with the wood burner (which would therefore need it's own overheat protection), is there milage in diverting the solar circuit through the dump instead of the store coil ? That way the dump is part of a circuit that'll be a) fairly small in volume, and b) already dosed with antifreeze. All that should be needed is a three port valve to divert the flow, and a means of controlling it (do solar controllers sometimes have an output suitable for this ?)

[acresswell]

Simon, that sounds like a great idea. It wouldn't need to be the solar controller that triggered the change... just a thermostat on the thermal store.  If it's the log boiler overheating the thermal store then the change of flow in the solar circuit won't make anything worse! (I

I can see 3 options for the diversion -
1) when thermostat activates, solar circuit is diverted to only go through the dump load (not the thermal store).
2) when thermostat activates, solar circuit is diverted so that it goes first through dump load and then secondly through the thermal store.
3) when thermostat activates, solar circuit is diverted so that it goes first through thermal store and then secondly through the dump load.

I know it's late (or should that be very early?) and my brain isn't working properly.  For some reason I my instinct is that 2) is better, but I can't explain why. Just exhaustion, or am I right?

Thank you!

[SimonHobson]

Simon, that sounds like a great idea. It wouldn't need to be the solar controller that triggered the change... just a thermostat on the thermal store.  If it's the log boiler overheating the thermal store then the change of flow in the solar circuit won't make anything worse!

Indeed. The downside of course is that you would have two separate overheat protections - one for solar, one for woodburner.

I can see 3 options for the diversion -
1) when thermostat activates, solar circuit is diverted to only go through the dump load (not the thermal store).
2) when thermostat activates, solar circuit is diverted so that it goes first through dump load and then secondly through the thermal store.
3) when thermostat activates, solar circuit is diverted so that it goes first through thermal store and then secondly through the dump load.

I know it's late (or should that be very early?) and my brain isn't working properly.  For some reason I my instinct is that 2) is better, but I can't explain why. Just exhaustion, or am I right?

I'd suggest 1 as being by far the simplest - once the store is up to max safe temp, then you might as well just dump your solar altogether. On the other hand, it does depend on the capacity of your dump and what your priorities are. Ie, if your lump of concrete under the woodpile gets very hot, what do you want to do - keep heating it and eventually have your solar overheat, or keep putting heat into the store and deal with it elsewhere ?
2) may actually cool the bottom of the store - if solar input is not too high, then you could cool your solar loop below the store temp before putting it through the solar coil. Thus you will be throwing away heat you might actually want later - but only from the bottom of the store.
3) may continue to put heat into the store, depending on your dump capacity and flow rates.

If you wanted fail-safe-ish then you could arrange for the diverter to go to the dump load by default, and rely on it motoring over to heat the store. That way the valve is exercised regularly and you shouldn't find it seized when it is needed.


On the other hand, if you wanted to protect the store from both solar and woodburner (and avoid needing two different dump loads) then you could either put an extra coil in the top and run the dump load on it's own loop, or use a plate heat exchanger (which of course then needs two pumps, one for the store side, one for the load side). Whether this is enough for an uncontrolled input like the woodburner I don't know.

[acresswell]

Now more awake...

I reckon 3) will probably suit me the best.  The water returning to the solar panel will still be cooler than without the dump load (and therefore the water passing back from panel to thermal store should be cooler)

I don't like the idea of 1) because the dump load may not dump sufficiently.  There'll be a fairly small thermal capacity of water in the dump load and although it'll dump some heat under the  logs it'll be important to make use of any remaining thermal capacity in the thermal store.  Even if it's only raising the temp of the 2000 litre store from 90degreesC to 99.99999degreesC over a period of 12 hours, I reckon that's equivalent to an extra 2KW dumpload for that 12-hour period...  giving time for the sun to go in and/or some heat to be removed via DHW

I may actually want the dump load to continue working for a short time after the sun has gone in, so that the tank cools to a "normal hot" temperature overnight and is then able to accept heat from the panels the following day.  I won't be worried about the heat loss if the tank temperature is already above the normal (desirable) range.

I'm not too stressed about the risk of the boiler overheating... if it starts to heat then the fan is stopped, reducing the oxygen flow and rate of combustion.  I'm also fairly confident that we won't bother to load and fire the boiler unless the tank has already cooled considerably and therefore has enough spare thermal capacity.

Thanks,

Adrian