Manufacture of a greenhouse tunnel market gardening

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Alain G
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by Alain G » 09/05/12, 17:49

Hi Dede!

I am in Quebec and I have no problem with the pool overheating!


The radiator is sufficient, it is the ventilation which is too weak but the system is well balanced for my use of summer 2011.

You couldn't do without heating the earth!
N / A! : Mrgreen:

This year I am replacing the swimming pool which will be isolated from the ground and on the perimeter which will be made of wood, so I could if necessary divert the heat into 2 large plastic tanks of 1000 liters which I recovered at work.
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by dedeleco » 09/05/12, 18:28

I was not talking about the swimming pool, but the greenhouse, not cooled enough, if we don't want to lose the irrigation water by evaporation, with strong ventilation.

In the Mediterranean region, in summer the sea and the swimming pools do not need to be heated, the air and the sun are enough, sometimes 25 to 26 ° C, for the sea !!!

In the 83, I don't need hot water in summer, 20 ° C and more is enough for me, over 3 months !!!

The greenhouse of 60m2, receives 20 to 30KW solar in midday, to evacuate to avoid overheating at 60 ° C and more.
Also, a single radiator of 1 to 2KW is insufficient to refresh the greenhouse, closed sealed.

It takes 10 times more in the greenhouse, or even 20 times, taking into account that the difference in T is more reliable than for radiators in winter at 50 to 60 ° C in it.
A buffer of 1 to 2 m3 is also not enough on consecutive hot days.

In Quebec, I don't know the difference with Nîmes or the Mediterranean, probably more continental, with the cooler nights and stronger day to night contrast ????

A swimming pool averages day and night and is therefore not overheated, but in the Mediterranean, water is heated in the sun to 25 to 30 ° C per heat wave.

With Mistral the evaporation of the dry Mistral wind and the effect of the wind going up deep water, can cool the sea to 16 ° C in the middle of July!

The land is free and stores heat or cold in large quantities without difficulty !!!!
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by Alain G » 09/05/12, 19:11

There is a plastic film placed on the ground and the feet of the plants go through a hole to avoid evaporation and weeds.

Image
Image

The pool rises a few times at 30 ° C! :D
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bleusideral
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by bleusideral » 10/05/12, 09:05

XNUMX-XNUMX-XNUMX!
exciting your little debate! good in theory there are interesting things to do, but forgive me I am a bit lost on the practical side ....
could you suggest me what i should put in place, with a pti diagram for example, to, most importantly, refresh the greenhouse in summer with the indications that i gave? because on Saturday the trench will be made and quickly filled up, I really would not be wrong. Unfortunately I can not go deeper, because very clay soil below the plant (reported) with lots of pebbles and rocks ...
yesterday a superb day, in the morning saturated humidity in the greenhouse, misting on the glasses! , afternoon, certainly untenable over 40-45 °! the few strawberry plants from last year still in place were stunted!
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by dedeleco » 10/05/12, 14:21

For the Canadian well, econology gives all the means to calculate.
Complex physics which I try to explain in simple order of magnitude, even sketched.
In general Canadian wells are undersized !!!

Here 60m2 of greenhouse with at least 20kW (60 to 1kW / m2 reduced to 20 with the inclination of the sun, losses or evaluated by saying that 1/3 of the surface inclined towards the sun receives these 1kW / m2 fully) 10 a.m. and 18 p.m., i.e. 8 hours of intense sunshine that heats
It is therefore necessary to evacuate at max 8kWh / m2 or 60m2 480kWh per day at most by very strong heat wave.

Tables with cloudy days give an average of 4 to 5 kWh / m2 in the Mediterranean region in summer.

So you have to evacuate 300 to 480 kWh per day, from the greenhouse to the Canadian well, or in the underground tube.

However, it is an exchange of circulating air from the greenhouse to solid, plastic and then earth.
What limits the most in the short term is the low conductivity of the air, 0,026W / m ° C compared to that of a typical solid like clay 1W / m ° C. which goes from 1,28 to 0,52 with humus !!
View table thoroughly conductivity, specific heat and diffusivity in:
http://fr.wikipedia.org/wiki/Diffusivit%C3%A9_thermique
(my hobby horse, very useful)

So air is about 30 times less conductive than most common non-metallic materials.

Fortunately, the circulating air, by convection or circulator has a turbulent flow, mixing everything, so that heat circulates quickly at the speed of this air multiplied by its thermal capacity.
But it stays against any surface a thin viscous boundary layer, typically no turbulence around the cm from 0,1 to 1cm depending on the turbulence and the air speed. The thickness decreases slowly with the speed of turbulence. Also taking about 0,3cm is a simple choice to locate and calculate orders of magnitude and avoid factors of 10 errors otherwise !!!.

These 0,3cm of insulating air on an exchanger, radiator, Canadian well tube, conduct heat with the flow of 0,026W / m2 for 1m of air thickness and therefore for 0,3cm 1 / 0,03 = 333 times more or 0,026 / 0,003 = 8,66W / m2 approx. C, more reliable value at low air speed and higher at high speed, as with mistral.

But that fixes the actual order of magnitude to be retained approximately 10W / m2 ° C

So if we want to dissipate the heat with a difference of 10 ° C, we have a flux around 100W / m2 = 0,1KW / m2, 10 times less than the power provided by the sun for the same surface, (Canadian well case with air at 30 ° C to earth at 20 ° C) and if we have a radiator in winter it is water inside at 70 ° C around 20 ° C (max) or 50 ° C difference therefore 5 times more, or 500W / m2.
Commercial radiators have several layers of air circulating by convection to give more, and those of very efficient cars, have a multiplication of the surface very finely divided and a high speed of air in between very close walls, (mm , reducing the thickness of insulating air to less than a mm, therefore with a flow 10 times higher).

Also to evacuate the 20KW to a cold earth with 0,1KW / m2 you need 20 / 0,1 = 200m2 of tube surface in the Canadian well.
So the largest possible and longest possible tube.
This explains why most Canadian wells are undersized.
Water circulation is more efficient, but not miraculous.

So the mini mini needs a tube surface at least equal to that of the greenhouse, and ideally 5 to 10 times more.

A usual 100mm tube has a surface of 0,1xpixL = 0,314m2 / m in length and therefore 1m2 per 3,18m of tube, which shows that the mini mini for the surface of the greenhouse 60m2 approximately 191m of tube and 3 times more for 200m2 of exchanger.

It is a lot, and it is necessary that if we put the tubes close to each other they are spaced more than the length of diffusion of heat over the duration of the heat wave, for clay, about 1mm for 1second, 100mm for 10000 = 100x100 = 2,7h and 1m after 1000x1000s = 1million seconds or 11 days, 57 which can be taken as a heat wave duration in Nîmes in my opinion except in 2003.

So you need tubes with the largest possible diameter, given the price, and maximum length possible, and spaced 1m or even 0,5m if soil with lots of humus, (grooved increases the surface, but poses drainage problems of condensation, huge and a real cowry to evacuate, (otherwise appalling rot)), to be evacuated with a slope of a few% towards suitable collectors to be cleaned often.
All the water evaporated in the greenhouse will end up in the tubes, so at 40 ° C, huge, m3 of water, to put back in the greenhouse !!!
Essential point to be observed with great care, a flow by gravity is fundamental !!!


If undersized, the greenhouse will cool a little but not enough as desired !!!!

A car radiator can evacuate tens of KW by heating the water to the max (80 ° C, even 100 ° C, 6 to 8 times more flux, in the opposite direction to that of the engine), but the heat must be evacuated towards the earth which conducts much less, but 40 times more than air.

Finally, the second step is to evaluate the behavior of the earth thus heated, by this air, in the pipe, which is due to thermal diffusion in the earth, to dissipate this heat more and more slowly over time:
http://fr.wikipedia.org/wiki/Diffusivit%C3%A9_thermique
and especially :
http://fr.wikipedia.org/wiki/Conduction_thermique
essential, but of a higher and higher level when reading.
So i give some the crucial guiding idea:
1) the heat penetrates more slowly as the square root of time:
D = about 1mm2 / s to a factor of 10
either 1mm in 1s 100mm in 100x100 = 10000s = 2,7h and 1m in 1000x1000 = 1million seconds, or 11 days, 7
this to a square root factor of 10 equal to 3 close !!! depending on the material except metals.

The figure shows this slowdown at the root of the penetration time and also of the heat flow, strong at the start and slowing down, which can give the impression that it does not work at all in a long time !!

Image
So the heated volume is like the volume of earth over this thermal diffusion length.
But also the heat flow where the evacuated heat flow decreases like the square root of time !!!

and therefore the efficiency of the Canadian well decreases similarly, strong at the beginning but much less good 11 days later, the well becoming very hot.

This is inevitable. If not too deep the earth is cooled at night to take an average between day and night.
A period day, as oscillations between day and night, gives a time gives on 24x3600 = 86400s a diffusion distance of
http://fr.wikipedia.org/wiki/Diffusivit%C3%A9_thermique
with Omega pulsation = 2xPixT and T the period here equal to 1 day = 86400s
and the diffusion D = 1mm2 / s = 10 ^ -6.m2 / s
delta = rac (2D / Omega) = rac (DxT / pi) = 0,166m broadcast over approximately 8h.

Thus, we average the temperature between day and night to a depth of about one to two times this depth, about 20 to 30cm (less if it is humus with thick grass on it, significantly more insulating)

So it’s good to put a about half of the shallow pipes at 20 to 30cm to use the night cold which cools the earth to the average on a day (30 ° C per heat wave) and avoids overheating the soil, never cooled quickly to a greater depth of 1 m in less than 11 days.
Given the shorter broadcast length for day to night, we can put them, these pipes closer together, about 30 to 40cm, as long as we want to cool the day with the cold of the night.
The cooling time is close to that spent for heating, in diffusion.

Long pipes thus must be separated between shallow and deep, to use them as needed, with uniform slope of effective condensation flow in large vessels, (as for rain and even recovery pump for watering)

There is much more about econology, in particular a very detailed and fundamental thick Swiss thesis.

But it is fundamental to think that it is often undersized and condensation a horror to be evacuated !!!
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bleusideral
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by bleusideral » 10/05/12, 15:10

Thank you Dedeleco for this brilliant and detailed debate !! breathtaking : Shock:
Like what a business that is believed to be easy at the start is nevertheless a headache ...
for my part I am really forced to do the simplest, with the means at hand and quickly.
So I'm just thinking of "doubling" my drain and using it back and forth for cooling my greenhouse.

Image


without forgetting that I have two openings which will be automaitsées thereafter, an opening of approximately 1m², the other (door) 1.8m² according to my calculations, to which I will blur the solar radiation in summer by a veil style winter veil, suspended from the ceiling and sliding by two wires stretched over the entire length.
there are some who lime the tarp, I can't, I'm in a subdivision, with large plots of course, but still subject to visual rules, I had a lot of trouble and constraint to submit my project to municipal services. the trees were the asset, but the exposure to the south is relatively clear by having a sun at its zenith in summer.
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by dedeleco » 10/05/12, 16:21

I found the precise, detailed thesis on econology giving everything on the Canadian well to read thoroughly:

http://www.unige.ch/cyberdocuments/thes ... front.html
http://www.unige.ch/cyberdocuments/thes ... _body.html
https://www.econologie.com/forums/post206157.html#206157

and a summary that does not consider all the problems, especially for a greenhouse:
https://www.econologie.com/le-puits-cana ... -1827.html

Otherwise I forgot to talk about the air flow required, to evacuate 20KW and with a given air capacity on:
http://fr.wikipedia.org/wiki/Diffusivit%C3%A9_thermique
it is necessary :
1,3x1 = 1,3KJoule / m3 and 10 ° C of air heating
to evacuate 10KW = 20KJ / s it takes 20 / (1,3x10) = 1,54m3 / s of flow !!!!
or 5538m3 / h a very high value !!!
If you heat 5 times more, with the furnace greenhouse at 70 ° C, you need 5 times less.

Which is only possible with circulators on pipes in parallel to reduce pressure losses.

It also requires a lot of power to power these circulators with one or more photovoltaic panels, hundreds of Watts.

Also this will be undersized too, and the greenhouse will be overheated, less but still enough.


Otherwise the tube length is too small and therefore insufficient in result.

Try to wedge in the trench a maximum of tubes about 20 to 30cm from each other in the ground, to have a maximum of exchange surface and take advantage of the cold nights (phase shift day night).
for example 4, 6 or even 8 times 48m !!
Each of the pipes well inclined, rather smooth, if not stink to be cleaned often !!!!


Otherwise, aeration will lose a lot of irrigation water, but evaporation will cool effectively.

A solution not to overheat as much, is an identical flysheet, (which changes nothing in the visual display except for the summer without wind) especially towards the south, with strong air circulation between the two plastics (convection, plus fan) so as not to exceed the T of the ambient air below.
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Re: Manufacture of a greenhouse tunnel greenhouse




by Perrine » 08/10/20, 10:28

Hello,

In this autumn period, I too am setting up my tunnel greenhouse. With a lower level than yours, I try to keep it simple and efficient. But I find it very interesting to read you, and I learned a lot of things, thank you all!
While inquiring about the stores where I could buy the necessary material, I quickly came across this site: spam. It is a small business but sells products (including protective tarpaulins and greenhouse tarpaulins) with very good value for money. Has anyone heard of it?

Have a nice day everyone,

Perrine
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Re: Manufacture of a greenhouse tunnel greenhouse




by thibr » 27/04/21, 20:36

other solution
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Perrine
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Re: Manufacture of a greenhouse tunnel greenhouse




by Perrine » 07/05/21, 12:47

Hello,

In this Autumn period, I too am setting up my tunnel greenhouse (found here: https://www.shopix.fr/477-serre-de-jardin). With a lower level than yours, I try to keep it simple and efficient. But I find it very interesting to read you, and I learned a lot of things, thank you all!

Have a nice day everyone,

Perrine
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