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Author Topic: Heat Sinks  (Read 2207 times)

Sra

  • Joined Jun 2016
Heat Sinks
« on: January 02, 2017, 10:39:21 pm »
Does anyone have experience to share on installing a heat sink below a green house or poly tunnel?
I've read up on it on the web but there is a lack of actual fact as to whether it actually works!
Interested in if anyone can give advice on volume of heat sink verses area of growing space and especially anyone who has actually measured any differences having installed one.
Thx
Simon

Scotsdumpy

  • Joined Jul 2012
Re: Heat Sinks
« Reply #1 on: January 03, 2017, 09:28:37 am »
Not a heat sink per se but we have a large hot bed in our polytunnel which warms up enough to raise the temperature to prevent frost. It doubles as a propagator for early seed starting and grows all manner of early salads and carrots.

farmershort

  • Joined Nov 2010
Re: Heat Sinks
« Reply #2 on: January 03, 2017, 10:08:25 am »
Does anyone have experience to share on installing a heat sink below a green house or poly tunnel?
I've read up on it on the web but there is a lack of actual fact as to whether it actually works!
Interested in if anyone can give advice on volume of heat sink verses area of growing space and especially anyone who has actually measured any differences having installed one.
Thx
Simon

I assume you watched the "it's not easy being green" episode on the subject? I did try and get some answers out of Dick & James via twitter about the heatsink, but I got a stoney silence from them. This may just be the nature of the communication medium, but I'd previously had a nice chat with Dick about elderflower champagne... so he does respond. I also can't remember the INEBG heat-sink being in the post-tv series book... which suggests it may not have worked very well.

I have a small suspicion that if you manage to create a heatsink which works well enough to maintain good temps overnight, then you'll probably be removing too much daytime heat from the greenhouse to accomplish it.

In essence, it's just a heat exchanger in the apex of the greenhouse roof (fancy house ones are now dubbed "heat recovery units"), and then pipe work and small 12v fans to pump hot air from the heat exchanger into a heat-sink (or water pumps if you go down that route). The heat sink could be a pile of waterbutts at one end of the greenhouse, or it could be a 2000 litre submerged water/oil tank. I think INEBG rightly pointed out that there are serious environmental questions around using oil as your heatsink.

Getting heat into your heatsink would seem to be the easy bit... pumping that retained heat back into the greenhouse "seems" as simple as letting the system continue to pump around, but you'd be pumping heat back to the apex of the roof.... which is a bit daft. An improvement on this would seem to be to have a valve-switch which was perhaps on a timer circuit of some sort to switch the heat-sink-stored-heat to pump through a ground-level pipe system. It has now become a level more complicated....

As for sizing, there are equations dotted around the internet to give you these answers, but I wouldn't dream of being able to quote them accurately. You'd need to calculate the amount of heat captured by your heat exchanger (based on it's size/surface-area and efficiency), The losses through insulated pipes (U values will help here), and the temperature raise per volume of water (if using water as the heat-sink) - I'm thinking about calories.... the amount of energy required to raise the temp of 1kg of water by 1 degree C. I can't quite think how to convert the heat exchanger numbers into that system, but I'm sure there would be a way.... Then you should know how much energy the heat exchanger can produce based on average daylight and outside temp levels, and what amount of water that could heat, and to what temp.

I hope that wondering posts helps a little..

Sra

  • Joined Jun 2016
Re: Heat Sinks
« Reply #3 on: January 03, 2017, 09:46:37 pm »
I read the Dick and James book where they had a system that removed warm air from the apex into a 1m cubic heat sink (crushed glass I think). It vented back at a low level rather than back to the apex (I agree that would be pointless). They were using it to warm a 4m by 2m green house.

The theory seems fine and using solar to charge the battery for the fan means there is no running cost.

My doubt is that there is enough warmth in the apex in the winter to 'charge' the heat sink to an extent that it could make an appreciable difference over night (also with nights being longer).

Now compound that with the fact I'm considering a 20m by 4m poly tunnel. On that basis assuming a direct ratio between surface area and heat sink volume I'm looking at 10 cubic meters of heat sink!

Not a problem to dig out but a lot of upscaling of fans, batteries, and solar to run it - that or several pits. Not sure the ££ that involves stacks up - might be better to look at other options including hot beds or piles of manure ready for spreading at a later point but I would do that anyway.

Was hoping someone had done the heat sink thing and actually measured some difference as a result? I agree you could go down the route of attempting to calculate it but there are so many variables, not least the rate of heat loss from the tunnel to the outside which on a windy night could be a major factor.

Thanks for replies so far!

Womble

  • Joined Mar 2009
  • Stirlingshire, Central Scotland
Re: Heat Sinks
« Reply #4 on: January 03, 2017, 10:29:31 pm »
In essence, it's just a heat exchanger in the apex of the greenhouse roof

What does the heat exchanger do then?  Are you exchanging heat between circulating water and the air at the top of the polytunnel for instance?

If so, that might be quite efficient (water is about four times better at storing heat compared with glass), but it does add another layer of complexity. If you just had a bed of crushed glass or whatever, you can suck hot air directly from the apex, down into the glass. The air then heats the glass, and exhausts at ground level, a bit colder than it started. Then at night you continue to run the fan in the same direction. The cool air is warmed by the bed, and exhausts at Ground level, preventing freezing.

What am I missing?

Quote
I can't quite think how to convert the heat exchanger numbers into that system, but I'm sure there would be a way...

The simplest calculation on heat stored would be to assume that the bed reaches equilibrium with the hot air at the apex during the day (say 30 degC), and then gives that heat back during the night, until it reaches say 10 degC). Then, Q=m x C x (deltaT). So, for 1000 kg of glass, with a heat capacity of 0.84 kJ/kgK, you're looking at a storage 'capacity' of 1000 x 0.84 x (30 - 10) = 16,800 kJ.

To put that into context, it's the same heat output as running a 2 kW fan heater for a bit over two hours, so (as long as I haven't messed up the maths  :) ) actually that looks like quite a useful amount of heat storage?
I can probably do the calcs on the heat exchanger once I know what sort of system you're looking at.

OK, I'm a *little* bit more ready for going back to work tomorrow now. Thanks!  ;D
« Last Edit: January 03, 2017, 10:31:56 pm by Womble »
"All fungi are edible. Some fungi are only edible once." -Terry Pratchett

GribinIsaf

  • Joined Aug 2015
  • Montgomeryshire
    • Gribin Isaf
Re: Heat Sinks
« Reply #5 on: January 03, 2017, 10:39:52 pm »
Might this be of interest

Heating a Greenhouse with Compost and Manure:

https://www.permaculture.co.uk/articles/heating-greenhouse-compost-and-manure

There is also a permaculture article somewhere about using external hot beds (woodchip/manure) with a pipe exchanger to bring the heat inside.

We have also built hotbeds (fresh morse manure) inside a polytunnel with good results




Sra

  • Joined Jun 2016
Re: Heat Sinks
« Reply #6 on: January 06, 2017, 09:21:03 pm »
Thanks both

I was going to use reused wine bottles filled with water as the heat sink. Air from apex pushed by small fan into a sub terrainian insulated box - through the bottle bank then vented back at about bed height. Bottles allow good air flow around them - needed with a low power fan which won't push against a counter pressure - and they are free!

I've calculated a cubic metre would hold 502 bottles which is 288kg of glass and 376 kg of water. So Womble if you are feeling kind and could do the math to work out the storage capacity that would be great! DeltaT at 20 is optimistic in winter but not impossible.

Sra

  • Joined Jun 2016
Re: Heat Sinks
« Reply #7 on: January 06, 2017, 10:03:17 pm »
Doing a bit of math myself using figures from the internet I got about 36,000 KJ

Similarly a 20x4m polytunnel has a volume of 126 cubic m

Energy to heat air at 4 degrees (air density is in the equation) up 1 degree for that volume of polytunnel is 161 kJ?

It seems a very useful amount of heat energy to be putting back into the tunnel even if you halve the deltT value.

Certainly enough to keep overnight temps up a couple of degrees even taking rate of heat loss from the tunnel into account.

Am I missing something? Apart from the fact I need to find a hell of a lot of wine bottles!

Penninehillbilly

  • Joined Sep 2011
  • West Yorks
Re: Heat Sinks
« Reply #8 on: January 06, 2017, 10:44:26 pm »
We toyed with an idea of painting and old radiator black, having a small pump sending the heated water to a tank below ground. Never got any further with it, but now you've jogged my memory . . .

Womble

  • Joined Mar 2009
  • Stirlingshire, Central Scotland
Re: Heat Sinks
« Reply #9 on: January 08, 2017, 07:12:26 pm »
I've calculated a cubic metre would hold 502 bottles which is 288kg of glass and 376 kg of water. So Womble if you are feeling kind and could do the math to work out the storage capacity that would be great!

Apologies; I've only just seen this.

Q= {(mglass x Cglass)+(mwater x Cwater)} x (deltaT). So, for every one degree of temperature rise or fall, your store is going to either absorb or give up {(288x0.84)+(376x4.18)} = 1814 kJ

So, for each 'degree' of heat you're able to store, that's enough to run your imaginary 2kW electric heater for 15 minutes.

Energy to heat [126 m3 of] air at 4 degrees (air density is in the equation) up 1 degree for that volume of polytunnel is 161 kJ?

Yes, there or thereabouts  :thumbsup: .  However, I think the thing you're missing is that that air will rapidly give up its heat to the soil, plants, polytunnel structure etc. Also, the polytunnel will lose heat to its surroundings at a rate proportional to the temperature difference between it and the ambient air.

Precise calculations get a bit difficult at this point, but my gut feel is that you'll be able to keep the frost off that way at least? It might be worth putting a little anti-freeze in the bottles just in case we get a really hard winter though!

Personally, I think emptying 502 wine bottles sounds like the best bit of the whole project. Do give me a shout if you need a hand with that  ;) .
"All fungi are edible. Some fungi are only edible once." -Terry Pratchett

YorkshireLass

  • Joined Mar 2010
  • Just when I thought I'd settled down...!
Re: Heat Sinks
« Reply #10 on: January 09, 2017, 09:53:17 am »
We toyed with an idea of painting and old radiator black, having a small pump sending the heated water to a tank below ground. Never got any further with it, but now you've jogged my memory . . .

Possibly a different concept, but this reminded me of a system with sealed water pipes and radiators, where some were buried in hot compost piles, and then carried the heat through to heat the greenhouse. But that's sort of the reverse of what's being discussed here, right?

Womble

  • Joined Mar 2009
  • Stirlingshire, Central Scotland
Re: Heat Sinks
« Reply #11 on: January 09, 2017, 12:29:38 pm »
That would be similar to a scheme I saw online once where there was a henhouse adjoining the greenhouse, so that hot air from the sleeping hens was ducted into the greenhouse to keep it warm.

The only detail I can remember was that they'd gone to the trouble of calculating the heat output of a sleeping chicken!  :roflanim:
"All fungi are edible. Some fungi are only edible once." -Terry Pratchett

 

heating a polytunnle with waste heat ??

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