greenhouse heating heat buffer

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Mhl
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by Mhl » 10/05/13, 11:19

Hello,

I am currently building a small greenhouse to winter my plants and prevent them from cluttering the veranda. For heating, I thought at first to ferment manure (possibly straw) to produce heat. Information taken, I then opted for the installation of a water reserve on the ground in order to have a thermal buffer. It seems that it is not very effective. I finally discover on this forum that it is more interesting to bury this reserve of water.

Cortejuan uses dipping resistors to heat this water. Also, I would like to know the model he used to raise the temperature of the water to 17 ° C. I am considering instead to opt for a photovoltaic panel. [/ B]
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cortejuan
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by cortejuan » 10/05/13, 13:34

Hello,

First you need to know if the purpose of the thermal buffer you are considering is to save paying energy (electricity or gas) or if the buffer is used to restore the daytime heat, the night (or both).

In the first case, the interest of burying the water reserve is to avoid using the space dedicated to plants in the greenhouse. In addition, the heat leaks are recovered in the soil so the solution is optimal but it needs a heat exchanger (radiator + pump + thermostat) because if the tank is buried, the heat exchange with the greenhouse will be very weak in front of the nocturnal losses through the windows of the greenhouse.

If it is to play on the thermal buffer, the interest of diving resistors is null except if like me, you benefit from a tariff type EJP: The resistors heat the water at night (in normal period) and the heat is returned during the day (in the EJP period).

The must is a stove / boiler wood (rather outside for convenience, security and for the sake of saving space in the greenhouse).

I bought one but it is perfectly feasible and with a wheelbarrow, you heat 2000 l of water to 50 degrees, the calories will be restored over time and depending on the temperature that you want to maintain, you can maintain your frost-free greenhouse for at least a week.

cordially
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Mhl
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by Mhl » 10/05/13, 14:37

The main goal is to maintain the frost-free greenhouse where the interest of the thermal buffer achieved by a reserve of water that is heated. The choice of the electric is mostly related to the situation of the garden in the city. Why photovoltaic? Because it's greener and especially because the garden is populated with shrubs of all kinds and I'm not too much time to get back to earthworks for the passage of electrical wires.

The idea of ​​resistance seems good to me, but rather than choosing a model at random and fumbling, I preferred to know the one you used. Based on this information, the surface of my greenhouse, its water supply and solar panels available on the market, I would try to find a suitable resistance.
And if I feel that it will not work, I will come back to my initial idea, not elegant, but effective.
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cortejuan
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by cortejuan » 10/05/13, 17:18

Re,

Oh yes, I understood ...

concerning the resistances used, they are simple immersion heaters used to heat any liquid, they are available everywhere. I installed two of 350 W each so low power to maintain the water at about 10 degrees during periods of extreme cold. The problem, but others on this site will answer you better than me, the problem is the surface of your sensors: if you consider that you need 700 W power, you will need several sqm of capture area so that will cost you a lot. Why not opt ​​for a solar thermal collector? You can make it yourself and you will heat your water more efficiently. The low surface area photovoltaic is ideal for not lighting for heating.

cordially
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Mhl
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by Mhl » 13/05/13, 11:25

Hello,

Thanks for the information.

I have planned the solar collector that I will manufacture myself. For lack of time, I opt temporarily for photovoltaics. This panel will be used later for lighting a shed.

Kind regards.
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cortejuan
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by cortejuan » 06/11/13, 16:23

Hello,

some news from the front. I remember my system: a greenhouse of 31 m3 of volume and 15,6 m2 of ground surface. solar accumulation heating and outdoor wood stove. the heat is stored in a reserve under the greenhouse (900 liters) and outside (2000 liters thermally insulated). The heat is conveyed by a pipe system to radiators of cars equipped with their fans. The pumps used are low consumption (electric car pumps).

Last winter, the system worked without the addition of the stove (bought at the end of winter). The absence of sunshine forced me to work with propane gas so as not to overload the electrical installation supplying my heat pump. So big gas consumption (about 60 liters) knowing that the set temperature is 10 degrees.

The temperatures of the water reserves are regularly down to ten degrees so ideal for it does not freeze but unsuited to my wish to maintain 10 degrees in the greenhouse.

The contribution of the stove has changed everything. Its power is about ten kW.

Here are some numbers:

One hour of heating is 6 degrees gained on the temperature of 2000 liters of water is a stored energy of 13.9 kWh, sufficient to maintain the temperature in the greenhouse during the whole night when the temperature falls to 3 or 4 degrees to outside.

So in short, an outbreak of one hour is a temperature of 10 degrees during 12 hours, so it's perfect.

BUT, it's not so simple because in fact experience shows that it is necessary for it to work well (the system does not run all the time) that the temperature of the reserve is the highest possible. In my case, given the pipes and the tanks in polypropylene and polyethylene I limit the temperature to 40 degrees.

Knowing that I lose at most ten degrees per night (night in the vicinity of 0 degrees) I recharge my stove every 3 days.

When it is very cold (-10 degrees) I will recharge every day.

So it's a little presence but as the stove is outside, overheating is no danger except that the water arrives in the tanks at 70 degrees (still acceptable given the material).

So my conclusions

in the solar energy plan, the system is good if the sunny periods are important, in period of cloudy or foggy weather it is a cat. So the greenhouse is not the solar trap as I thought. It would require much more efficient heat exchangers than my car radiators.

The system is however valid in the spring and autumn because it closes the daytime temperature rises and restores the night which makes it possible to raise the setpoint around 15 degrees.

It is illusory to want to heat a greenhouse with low temperature heat transfer fluids (from 10 to 20 degrees) because the radiation to the sky is such that it requires a high instantaneous power (related to the temperature of the stored water).

A fortiori, the installation of water tanks to see larger containers will absolutely not guarantee the non-freezing of plants because the power delivered instantly is ridiculous compared to the losses by radiation / conduction (unless the greenhouse is super isolated ( conduction AND radiation)).

That's it for my project just before winter.

At the fruit production level, the maintenance of the winter temperature, the mild temperature in the spring (thanks to the sun) and the regular brewing allowed me to harvest 150 big fruits of the passion during the summer.

cordially
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Mhl
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by Mhl » 12/11/13, 10:09

Hello,

To improve the "performance" of my future greenhouse, I will dig so that it is more or less buried. The windows will be tilted so that the sun's rays strike them perpendicularly around the winter solstice (or perhaps rather in January-February, often the coldest time in the plains).
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cortejuan
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by cortejuan » 12/11/13, 15:49

Hello,

tilting the windows is a good idea, the so-called dutch greenhouses are so designed, however, if this technique improves the capture of solar calories, it exposes a little more the greenhouse to the sky vacuum during the night.

For my part, I just acquired a temperature recorder to use multiple sensors. I'm starting to do my first tests and it's really interesting. I will compare with and without opaque curtain at night. My first results are very amazing but working in research, I know that every time (or almost) where I got very unexpected results, there was a mistake of manipulation somewhere ...

As soon as I confirm my results, I post the temperature graphs for those interested.

cordially
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Mhl
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by Mhl » 12/11/13, 16:10

I am not sure because in our regions, in winter, the sun's rays make an angle of about 20 ° (more in the southern regions), or walls inclined at 70 °. If the northern part consists of an insulating wall, the upper part of the greenhouse can be obscured during the coldest periods and thus limit radiation to the sky.
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cortejuan
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by cortejuan » 12/11/13, 16:20

It is true that if the Dutch who are excellent gardeners have opted for this type of configuration is that it must be effective ...


Good luck in your work. I will follow with interest

cordially
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