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Author Topic: heating a polytunnle with waste heat ??  (Read 3121 times)

kris

  • Joined Jun 2012
heating a polytunnle with waste heat ??
« on: October 03, 2015, 08:13:10 pm »
 hi all   haven't been on here for a while  I have a large poly tunnel  im looking to heat to get a jump on growing this year as im pushing my market garden  sales im increasing the outdoor beds  and using the polytunnle all year round if  I can  the question I have  are ?

 how much heat  would I need in the Poytunnle in terms of KWH ?
 I know the heat would depend on the size of the tunnel  my smallest one which I want to heat all year round  is 30ft wide x 90 ft long ?
   what temp do I have a chance to achieve ?
 what temp would or should I try and aim for to be able to push the growth?
 any advice welcome ?

as for the  waste heat we have some available to us on the farm  around 22khw thermal  on a continuous basis  that currently is just dissipated to the  air  I want to use it


any cropping advice
Voss Electric Fence

cloddopper

  • Joined Jun 2013
  • South Wales .Carmarthenshire. SA18
Re: heating a polytunnle with waste heat ??
« Reply #1 on: October 03, 2015, 08:32:31 pm »
I guess it depends on the crops /temperatures you're  after .

Perhaps my system may be of help to you.
 
 I use  scrap candle wax to re cast home made 8 inch long  3" dia candles to heat our small 12 x 8 foot glasshouse at night or durin g the day if it is frosty .
The flame is about 1 & 1/4 inches long . Each candle last five  or six days if just used during the hours of darkness , the stumps , if any are added to the melting pot again.
 
 Last winter using a that single heating candle 9 set in a steel pot sat on bricks away from combustibles in the glasshouse  I over wintered 45 geraniums in six big tubs , two bay trees in pots , grew a few salad onions , lettuce & radish .

 I did the germination's in a heated bed away from the glasshouse & grew them as individual station sown plants in individual bottomless pots. Then planted them out once I'd hardened them off a bit into the glasshouse tubs when they were about 1 & 1/2 " tall .

 I did it like this because the only time you need greater heat is for the germination period & it saves space in the glasshouse .
Strong belief , triggers the mind to find the way ... Dyslexia just makes it that bit more amusing & interesting

Womble

  • Joined Mar 2009
  • Stirlingshire, Central Scotland
Re: heating a polytunnle with waste heat ??
« Reply #2 on: October 03, 2015, 08:47:00 pm »
Hi Kris,

Here's my engineer's reply!

Using waste heat is certainly possible, and commercial polytunnels are sometimes heated using waste heat from power stations. However, the key thing is the temperature your waste heat is supplied at.  For example, 22 kW of heat supplied at 40 degC would be wonderfully useful. However, 22kW of heat supplied at 2 degC above ambient would be a waste of time unless you could magnify it by using something like a heat pump (unlikely to be cost effective).

The other thing is how you transport that heat. Could the hot air be blown directly into the polytunnel, or would you need some sort of secondary transport system (e.g. waste air heats water, water circulates under soil to keep it toasty). If you do go for a water system, be careful of legionella infection in systems circulating at below 60degC.

Overall, the temperature you'll be able to achieve in the PT will be an equilibrium between the heat you can supply, and how fast the polytunnel loses that heat to the surroundings (sorry if that's a totally obvious point). There will be a building heat loss calculator out there somewhere that you can use to estimate the heat loss (and hence the required heat input) at any given internal air temperature above ambient.

As for what to actually grow, and what temperature to grow it at, I'm sorry I haven't a clue!!  ;D
"All fungi are edible. Some fungi are only edible once." -Terry Pratchett

kris

  • Joined Jun 2012
Re: heating a polytunnle with waste heat ??
« Reply #3 on: October 03, 2015, 09:06:57 pm »
 hey thanks for the replys so far  the  waste heat is hot water at around 80 degrees c  it has some distance to travel but would be insulated pipes  my plan was to do a two fold heat plan the  first is via a heat exchanger which I have already  the second would be a take off pipe from the exchanger to run surface pipe on the ground  up and down the tunnel possible between rows to warm the soil up as well  I would be using a circulation pump before it goes back to the heat source in a circuit

  any idea what temps I might be able to achieve  would 15 oc be poss that way  would be able to grow most crops

kris

  • Joined Jun 2012
Re: heating a polytunnle with waste heat ??
« Reply #4 on: October 03, 2015, 09:24:01 pm »
does anyone have any further advice on what maybe to plant  or start off very early

pgkevet

  • Joined Jul 2011
Re: heating a polytunnle with waste heat ??
« Reply #5 on: October 04, 2015, 07:00:21 am »
I have no idea what temps you might hit/maintain over winter; thats a mix of insulation and distribution. If you managed to keep the soil at or above 15C and the air above 5C with enough 'daylight' length then i;d guess there's a lot that would overwinter and early start if you can keep parasites away. When I was into growing exotic plants and extending seasons on other stuff in the same houses then it was always uncontrollable whitefly that ravaged cucumbers and melons, peppers and aubergines that I added to my exotics. from the commercial viewpoint i's what will sell. so presumably the common lettuce and salads make a simple sensible aim.
But if you don't want to be sensible (me) then with 15C overwinter and some daylight extending lamps and say 8 feet of headroom then I'd be back to growing bananas, my pink grapefruit and lemons wouldn't need to be in tubs, I'd be looking at doing more exotic hanging baskets and flower tubs with an overwintering service examples like canna lillies early, strelitzia, plumeria, tea, pineapple, staghorn fern and the like.
I gave all my exotics away when we moved here (apart from th citrus) but there's a couple of pics from the past for inspiration:








Womble

  • Joined Mar 2009
  • Stirlingshire, Central Scotland
Re: heating a polytunnle with waste heat ??
« Reply #6 on: October 04, 2015, 07:58:12 am »
hey thanks for the replys so far  the  waste heat is hot water at around 80 degrees c

Wow, that's not waste heat, that's er, heat, and enough for a typical domestic house!!

OK, just thinking aloud instead of doing the morning crossword, and this is worth exactly what you paid for it  ;).......

The hardest bit of a heat loss calculation is finding a reliable value for the heat transfer coefficient. I couldn't find anything for polytunnel plastic, and even a single glazed window is tricky (lots of references quoting without units, or taking the window frame into account). However, single glazing is typically quoted at 5 to 6 W/m2K, and since most of the resistance to heat loss is actually in the transmission of heat in the air either side of the window, and not the glass itself, that's probably a reasonable figure for plastic too (I told you this would be worth only what you paid for it!  ;D)

So, since we're just thinking aloud still, let's go with 6 W/m2K and see what happens:

Your PT is 30ft x 90ft, so that's 10m x 30m close enough. If we assume it's shaped like half a beer can laid on its side, the area of plastic is roughly pi x r-squared (you get a full circle area by combining both ends), plus 0.5 x pi x diameter x length.  That gives you plastic area = 3.14 x (5^2) + (0.5 x 3.14 x 10 x 30) = 80 + 470 = 550 m2, near enough.

If you want to maintain 15degC in your PT even when it's say -5 degC outside, that gives you a temperature difference of 20degC.

So, Q=U x A x temperature difference = 6 x 550 x 20 = a heat loss of 66 kW, which is more than you have available  :(.

To be more useful then, what could you perhaps achieve?  If you have 22 kW available on a continuous basis, then dT=Q/(UA) = 22,000/(6*550) = a 6.7 degC boost compared with ambient temperatures.  So, not exactly tropical during the winter, but enough to keep most things alive during the winter I'd have thought, and surely enough to extend your growing season to almost year round?

If it was me, I'd put up an internal partition in the PT, to make the heated area smaller and easier to heat. I'd maybe even do a tunnel in a tunnel arrangement, which would give a nice hot portion of the PT to grow weird and wonderful plants in, and then a still-a-bit heated main section outside that.

Again, I hope that's helpful. If anyone else fancies telling me where my calcs are all wrong, go right ahead - I have only just got out of bed after all!   :)



"All fungi are edible. Some fungi are only edible once." -Terry Pratchett

kris

  • Joined Jun 2012
Re: heating a polytunnle with waste heat ??
« Reply #7 on: October 04, 2015, 09:37:36 am »
thank you that very helpfull  at that temp i would still be able to  encourage seedlings to push on    the waste heat is possible then  just may need to make a smaller area inside to be hotter  what about insulating the tunle  bubble wrap and so on  would it add
 my plan for plants was  more salad crops  and berries   i will have lighting in there  to extend the day length
  i like the idea  of a tunnle inside a tunnle but not sure how practical that would be  worth looking at
 











bloomer

  • Global Moderator
  • Joined Aug 2010
  • leslie, fife
  • i have chickens, sheep and opinions!!!
Re: heating a polytunnle with waste heat ??
« Reply #8 on: October 04, 2015, 09:44:19 am »
Tunnel in tunnel plan is easy lay pipe on beds as planned use blue mdpe pipe to make hoops over the bed and cover in agricultural fleece, which is cheap as chips.

Without heating a normal poly tunnel is 5-10 degrees above outside temp, the fleece tunnel adds at least 5 degrees to that if not more and thats without your assignable heat source.


pgkevet

  • Joined Jul 2011
Re: heating a polytunnle with waste heat ??
« Reply #9 on: October 04, 2015, 09:49:45 am »
How about making inside working easier with say 1/3 or 1/2 of the PT length covered with bubblewarp and then another layer of PT plastic and an internal partition (say simple frame work with plastic each side as double glazing).. that ought to give Wmble some recalculations to do specially if the heat is distributed biased to the insulated section....
Oh, and we're still stuck with some of yesterdays telegraph crossword....

kris

  • Joined Jun 2012
Re: heating a polytunnle with waste heat ??
« Reply #10 on: October 04, 2015, 03:48:22 pm »
 i thought about double layering the cover with  bubble wrap in between  but  costs involved  are high
 if the poly tunnel is naturally 5-10 degrees above our door temp normally  without adding heat
what would it be with the heat still 5-10 or more ???

 been having a chat through with my dad lol  we are planing to just do it  and see what temperatures we can do over winter e.g through dec jan
 going to  have a different  prorogation room  before planting out any way
  i want to do some hydroponics in the tunnle that im planning to heat   ive recently seen another small scale farmer who does market garden veg  and just discussed hydroponics with him  its always something i wanted to do  his crops are impressive to say the least  the yields   

 ps still doing the crossword from 1990's

kris

  • Joined Jun 2012
Re: heating a polytunnle with waste heat ??
« Reply #11 on: October 04, 2015, 05:15:27 pm »
Had a thought  after chatting with a local allotment holder  if I cover my poly tunnle in a layer of bubble wrap then another layer of plastic would this insulate the tunnel more using less heat to achieve a higher temp is this possible ? I found a load of cheap bubble wrap large bubbles  apart from poping it by rolling over it lol  thought it may help bring the heat requirement. Down


ScribbleUk

  • Joined Sep 2012
Re: heating a polytunnle with waste heat ??
« Reply #12 on: October 06, 2015, 11:01:06 am »
I have seen a similar suggestion before about polytunnel insulation on another site.  I would have thought that it would help retain some heat if the heat source was inside the polytunnel, but would have the opposite effect of preventing heat building up during the day as the extra layers of polythene and bubble wrap filter out too much sunlight.

I think a "greenhouse inside a polytunnel" would be the best idea.  Creating a relatively small insulated space that could be temperature controlled.  You could have several variations to experiment with - a literal greenhouse, coldframes, mini-cloches, etc to see what works best.  They would be modular, and could be easily removed depending on the needs of the season.

I assume your heat source is free, but I've read of folks using hot beds, water storage, bricks, composters, etc as free sources of heat as well.

Of course, before you insulate everything it makes sense to plug as many gaps as possible to stop heat escaping in the first place through door frames, etc.  But then you have the problem of poor ventilation and possible disease and mould occurring due to lack of air flow.

An interesting challenge :-)

cloddopper

  • Joined Jun 2013
  • South Wales .Carmarthenshire. SA18
Re: heating a polytunnle with waste heat ??
« Reply #13 on: October 07, 2015, 10:14:42 pm »
Area of half a cylinder pi d h /2 plus pi r2   ????? .. is that what you eventually did ?

 Don't forget that you'll also loose some  infrared & UV due to the use of extra poly sheeting .

 I consider that running a pipe of hot water on the ground  would give some conduction of heat into ground in the immediate vicinity , that the radiated & convected heat would warm up the plants around it so trapping the heat as still warm air is the way to go but be aware as has already been said about condensation .. would a cheapie dehumidifier with a pipe drainage help or would it be too expensive .
 
Rather than run a massive internal second skin perhaps make small concreted in post and rope parallel lines up either side of the center path way and simply drape  fleece or bubble warp over  the taut ropes .  if you make it so that the sheets are say 8 foot wide have a few inches of over lap over the next sheet you should be able to open us a short section at a time and still retain heat whilst working in the opened up section.
it would be less of a volume to heat and thus you should be able to attain a greater temperature in the smaller enclosures .

Come warmer weather these smaller 8 foot wide sheets should be much easier to roll up on a former such as plastic water' drain  pipe or a round wooden pole than having to cope with a massive single sheet in one go .
Strong belief , triggers the mind to find the way ... Dyslexia just makes it that bit more amusing & interesting

 

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