Air temperature control

Hi! I'm new to arduino and I'm trying to design a mini semi autonomous greenhouse. I'm having trouble finding a way to heat and cool the air. The greenhouse would basically be a box of about 0.125m³ with a plant inside.

Here are some options I've found so far: - use resistors to heat the air (5W of dissipation seem enough) - use a thermo electric cooler (as showed here https://tinyurl.com/ybn8dlwb) - use a fan system (I don't really know how it would work) - use hot and cold water.

What do you think would be the best way of controlling air temperature of a box like this? Are there other options that come to your mind? Please let me know :) .

  1. Yes, that’s how heaters work.

  2. No. Peltier coolers are extremely inefficient. So much so that it is difficult to stop them just heating up the “cold” side.

  3. Yes. A greenhouse in the sun is always hotter than the outside air. Bringing in fresh air from outside will cool it.

  4. If you have access to appropriately hot and cold water, yes. Outside of an industrial plant or large building which supplies this as a service, it is not going to be very good. You don’t usually have hot water pipes in your garden and the “cold” water in the garden hose might be heated by the sun.

A fan or vent will let you cool the greenhouse as required. Insulate and plug air leaks until there is no artificial heating required.

Thank you for your reply. Well, in fact the greenhouse would be used indoors and the point is to be able to grow any plant regardless of the season, thererfore it would not be possible to rely on outside air to heat or cool the greenhouse. I just read that fans can not cool or heat the air because they just make it move.

As a result, resistors seem to be the best option to heat the air and peltier coolers (with a heat sink) the best option to cool the air inside the greenhouse.

yannhouacq: Thank you for your reply. Well, in fact the greenhouse would be used indoors and the point is to be able to grow any plant regardless of the season, thererfore it would not be possible to rely on outside air to heat or cool the greenhouse. I just read that fans can not cool or heat the air because they just make it move.

As a result, resistors seem to be the best option to heat the air and peltier coolers (with a heat sink) the best option to cool the air inside the greenhouse.

How are your plants going to get light in the dark winter months? Whatever provides the light will probably also provide most of the heating required. Greenhouses work because they trap infrared radiation which does not pass through normal glass. Air from the room can be used to reduce the temperature in the box to the ambient temperature of the room. To go lower than that you will need to actively reduce the temperature. That could be done using peltiers or by running cold water through the pipes in the box.

There are very few plants for which normal indoor temperatures are too high (especially if you don't have hot summers, or run aircon in summer). You can easily keep the temperature in your greenhouse at ambient temperatures by ventilating - something you anyway have to do to provide fresh air to the plants. Heating is more commonly needed.

Cooling is much harder than heating, especially on such a very small scale, so you really should consider what exactly you want to grow first.

ardly: How are your plants going to get light in the dark winter months? Whatever provides the light will probably also provide most of the heating required.

I was thinking about using COB leds (100W maybe), they don't seem to generate much heat.

wvmarle: Cooling is much harder than heating, especially on such a very small scale, so you really should consider what exactly you want to grow first.

That's true, first I'll be testing it with tomatoes but I'd like it to be very versatile and able to grow a very large range of plants.

0.125 m2 for tomatoes? They’re going to need quite a bit more footprint, and 1.5-2 meters vertical space, if you want to actually get them to fruiting.

100W of LEDs is a very large amount of light for such a small footprint. I’m used to seeing 100-300W/m2 so your footprint would be 12.5-37.5W. Even high-light plants like cannabis don’t need that much.

yannhouacq: I was thinking about using COB leds (100W maybe), they don't seem to generate much heat.

That's true, first I'll be testing it with tomatoes but I'd like it to be very versatile and able to grow a very large range of plants.

COBs are often used with heatsinks. I don't know how efficient they are in turning electricity into light but I bet 100W of LEDs will make a small glass box hot pretty quick. You have not said how tall the box is going to be. If you are going to grow tomatoes it will need to be quite tall. Convection is probably your best bet for keeping the inside of the box at close to the ambient room temperature. Tomatoes also need a lot of water.

wvmarle:
0.125 m2 for tomatoes? They’re going to need quite a bit more footprint, and 1.5-2 meters vertical space, if you want to actually get them to fruiting.

100W of LEDs is a very large amount of light for such a small footprint. I’m used to seeing 100-300W/m2 so your footprint would be 12.5-37.5W. Even high-light plants like cannabis don’t need that much.

Well, I was thinking about using small tomato plants that are only 45cm tall like the Bush Early Girl for example. But that’s not the point of this topic.

wvmarle:
100W of LEDs is a very large amount of light for such a small footprint. I’m used to seeing 100-300W/m2 so your footprint would be 12.5-37.5W. Even high-light plants like cannabis don’t need that much.

I read somewhere that the recommended power was from 30 to 40W per square foot. That would mean using ±85 watts in my case.

ardly:
COBs are often used with heatsinks. I don’t know how efficient they are in turning electricity into light but I bet 100W of LEDs will make a small glass box hot pretty quick. You have not said how tall the box is going to be. If you are going to grow tomatoes it will need to be quite tall. Convection is probably your best bet for keeping the inside of the box at close to the ambient room temperature. Tomatoes also need a lot of water.

Yes, I’ll try to estimate the heat that I can get from the LEDs.

0.125 m2 is 1.345 ft2. So that’d be 40-55W based on those numbers.

There are almost 11 ft2 in 1 m2.

wvmarle:
0.125 m2 is 1.345 ft2. So that’d be 40-55W based on those numbers.

There are almost 11 ft2 in 1 m2.

I’m sorry, I confused cubic centimetres and square centimetres in my original post. The box would actually be around 0.125 m³. So the footprint would be 50*50cm or 0.250 m2. I’ll correct it, thanks for noticing.

yannhouacq: ..... The box would actually be around 0.125 cm³......

Small tomatoes then :)

yannhouacq:
I’m sorry, I confused cubic centimetres and square centimetres in my original post. The box would actually be around 0.125 cm³. So the footprint would be 50*50cm or 0.250 m2. I’ll correct it, thanks for noticing.

Now it’s getting even worse.

50x50 cm (indeed 0.25m2) and 0.125 cm3 means the box is just 50 µm high. I don’t think a tomato seed fits in there.

Why don’t you just give actual dimensions?

wvmarle:
Now it’s getting even worse.

50x50 cm (indeed 0.25m2) and 0.125 cm3 means the box is just 50 µm high. I don’t think a tomato seed fits in there.

Why don’t you just give actual dimensions?

It would be 505050cm = 125 000 cm³ = 0.125m³

Still too small for tomatoes. Even if your growing medium and the root system is kept out of that box.

"Bush Early Girl" is about 1m wide by 1m tall, so you are going to need 1m³ not 0.125m³ and as @wvmarle points out that does not include the roots and soil.

You need to do some research on the plants you want to grow. Take a look at Bottle Gardens which function without automation. Another option might be perspex or polycarbonate tubes.

I've raised tomatoes in wire cages about 1ft in diameter with 4 or 5 plants per cage and gotten massive yields. I learned how from my Dad who did the same in the home gardens since 1966 when my brother told him. Dad trellised cucumbers, no white side and very crisp as they grew hanging and needed the fiber.

You need to water them regular and trim for shape. It's kind of fun to give baskets of things away, eat as many yourself, stew and freeze the rest and still have frozens when the next crop gets ripe from only 4 cages of plants.

OP get onto Youtube and find the GrowingYourGreens guy then Self-sufficient man. Most of those are adaptable. https://www.youtube.com/user/growingyourgreens

Latest new one to me is growing potatoes in 18 gallon fiber bags. One guy used trash bags with holes in the bottom. The cloth breathes better. Either way got very good yields, but those aren't tomatoes... or something like.

I'd use a radiant heater to warm the box and keep outside the box relatively cool. Indoors where the only heat source is your heater it would be up to you to not overheat the box much or for long. If it takes 10 minutes to cool 2 degrees, don't sweat it. Your automation can chase sensor reads pushing for perfect till the whole thing is wrecked, have a care about time and margins.

Before you get too far into your project, research the wavelength of the light needed for your particular plant to grow. They do not need “light”, but particular wavelenghts of light, like from the sun.

I have a customer that had to have custom made LEDs to get the combination wavelengths of light needed for his product.

Paul

Yah. Red and blue leds with maybe a UV led or two. The mix needs to change with the "season", cut the blue some and shorten ON periods when you want the plant to go through pre-winter rush to produce.

The colors that the leaves don't use are the colors the leaves reflect.

IIRC only extremophiles eat yellow and green, reflect red and blue. There is an extinction event tied to green plants being what we see.

Paul_KD7HB: I have a customer that had to have custom made LEDs to get the combination wavelengths of light needed for his product.

In commercial hydroponic growing that's quite common. Volume is high enough to make it worth while (big greenhouses; the cost of custom LEDs is easily offset by the increase in crop yield). I would expect growlight makers to have LEDs tailored to many common crops by now.