Building a water warmed by greenhouse

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Registration: 16/05/15, 23:11

by adrien22 » 16/05/15, 23:22


I plan the construction of a greenhouse against a wall to be built in a formwork block filled with concrete (size of the greenhouse 4x7.5m, situation: BElgique)
I still hesitate to dig a tank of +/- 3000litres, or even more, under the greenhouse and to pass a pipe in the wall to collect the excess heat and store it in the water of the tank via a coil and then operate in reverse at night (to smooth the day / night temperature variation).
According to your experience, what do you think?

I wonder if the concrete wall (1.5m * 7.5m) will be able to absorb and restore heat fairly quickly. Otherwise I can always go to a radiator system.

I also want to winterize citrus there.

Thank you

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by Rabbit » 17/05/15, 11:13

If you put the tank under the greenhouse, no need to circulate the water overnight. if the diameter of the conduits is sufficient, the water will circulate automatically by thermosyphon when the greenhouse is cooler than the water.

As for burying the pipe under the concrete. The idea is good but the heat transfer will be low as long as the water from the tank will not be at 70 ° c. Concrete is relatively insulating. On the other hand the concrete could be painted in matt black to absorb the heat in spring (low sun) and to restore it at night.

I would say that it will require a large diameter duct which will run along the bottom of the exterior wall of the greenhouse (interior side). It is to make a heat curtain thus blocking the cold of the walls and allowing to heat. And a recuperator with heat circulator at the top of the greenhouse. It would be to collect heat during the day.

An anti-freeze must also be provided. Not too toxic, maybe alcohol. To have.

Remember to provide insulation for your water tank.
By the way the water from the tank can be used in summer to water the plants. The reserve is reconstituted in autumn and winter in anticipation of spring.
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by Did67 » 17/05/15, 13:55

I think there are two ideas, which don't necessarily hybridize happily:

- the "big wall" is supposed to serve as stock; it will be a poor sensor, because the temperature will not rise much; it will diffuse in the concrete

- the tank you want to use as a thermal buffer, will only store well if the temperature difference between the water it contains and that leaving the wall is sufficient [the heat flow stored is proportional to the surface of the exchanger but also to the difference in temperature; so if you want to store enough with a small temperature difference, you will need an exchanger with a large area] ...

From there, two ideas:

a) why buried the cistern? [the place, no doubt ???]; because once "filled" with lukewarm water, it would be an excellent thermal buffer ...

b) above all, why use the wall as an ineffective collector (even painted black; let us understand: it will capture all the incident solar energy on its surface; but it will dilute it in its mass, suddenly, the water of the circuit which crosses it will remain at relatively low temperature at its outlet and will reload the tank quite badly]

I think that a network of pipes painted in black, or better, a battery of recovery radiators painted in black will have another more interesting performance ... Or even a black sheet with behind a copper coil welded on it ... Some something very light (so it will heat quickly, without storing much; so some incident energy will cause a big heat!)

You can of course combine "buffer wall" (painted black, which "accumulates" the heat of the day, prevents overheating during the day - it "releases" the cool during the day = absorb calories - and restores the night) AND a system thermal buffer which is neither more nor less than a DIY low-end solar water heater: a cistern, with a coil to restore / absorb the calories from and to the water and black painted recovery radiators to absorb solar calories / return them to the greenhouse air at night

The liquid tampon can be used over time, over weeks or even months depending on the volumes in question. The wall is a shorter term storage, over a few days.

The wall is passive. You "suffer" and do not control anything (except to put a curtain or a veil in summer, to avoid overheating - plants need the night temperature to drop to optimize growth!).

The cistern is an "active" buffer: you can manage the flows, either by slaving (solar pumping with regulation - in summer, you only store by programming correctly, without restoring at night - no need! Or failing that pumping, by valves, to manage the thermosiphon if the height differences are sufficient and the circuit simple enough - if it is convoluted, the thermosyphon may not be sufficient especially if the height difference is small].

But this is just an opinion ... based on a few basic physical principles.
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by adrien22 » 21/05/15, 20:39

I consulted a doc on the heating batteries and unless using a very oversized one with a large air flow, the power released is too low for water temperatures of less than 20 ° C. In addition, the installation is quite heavy. I think I will turn to installing a canvas on the roof of the greenhouse for cold nights ... the best energy is that which we do not consume!
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by antoinet111 » 21/05/15, 21:26

Hi, I work a lot in greenhouses, there are many reflective fabrics to limit the diffusion and heat loss, it's like a shade cloth but with aluminum sheets inside.

it is not very expensive and durable.
horticultural suppliers have choice and good advice. (especially in regulation)
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I vote for the writing of concrete post and practicality.
Down the talkers and ceiling fans!


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