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Improved underground greenhouse to grow crops in the cold season using minimal energy

Povilas S
Povilas S Nov 21, 2022
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A Walipini-type greenhouse, improved to maintain higher temperatures, have easier access to water and an additional heat source to let you grow food crops and other useful plants in the cold season in an energy-efficient way.
  • Be self-sufficient in food production all year round.
  • Save energy (especially important during the time of energy crisis).
How it works:
Underground greenhouses (also known as Walipini greenhouses) are made by digging a pit in a way that enough light would reach its bottom and covering it with a "roof" of glass or other transparent material. The plants are then grown at the bottom of the pit. This gives the benefits of thermal stability in different seasons - cool in the summer heat and relative warmth in winter. It can be used to grow crops in the cold season in latitudes that are not too far north from the equator.
I suggest the following improvements:
Using mirrors to transfer the daylight down:
Walipini greenhouses are usually made in rather shallow pits (a few feet deep). Using mirrors hung on the "walls" of the pit to transfer the daylight down would allow us to have a much deeper pit and still have enough light to grow plants in its bottom. Having a deeper pit would bring the following benefits:
Higher temperatures - at a certain depth below the surface the temperature stays constant all year round and approximately equals the mean year temperature of that location. It's hard to determine the exact depth because it depends on the precise location, its weather, and the specifics of its terrain. But approximating the data from various sources , one could expect optimal temperatures in the cold season below 10 feet.
Intersection with a geothermal heating system - deeper pit can be used for both laying geothermal pipes and making an underground greenhouse since a geothermal heating system uses the same principle - underground temperature stability to provide warmth to the living spaces in winter and cool in summer. Geothermal pipes can be laid in the bottom of the pit (between plant beds) or a bit deeper, underneath the plants. Installing geothermal pipes to fully heat the house requires quite a lot of free land space, so a part of the pit used for geothermal piping could be left uncovered to build a greenhouse in it.
The water inside the geothermal pipes absorbs heat from the soil, it is then moved upwards by a pump, heated further in the furnace until the optimal temperature, and distributed through the household heating system. Once the water gives off heat and cools down, it is moved down underground and the cycle continues.
The greenhouse part of the pit could be included in the geothermal house heating system as another "room" so that the water coming down to the pipes laid in/below the pit would be of optimal temperature to sustain a microclimate best suitable for plant growth. This can be regulated by a thermostat placed in the greenhouse.
Easier access to water - a deeper pit means a shorter distance to groundwater. One could drill one or a few small boreholes in the bottom of the pit to reach the groundwater and use a passive water acquisition system, for example, one based on capillary action or a mechanical pump to water the plants in the greenhouse. The groundwater can also be used to fill the geothermal pipes, this is known as an open loop geothermal system.
Thermally insulating the greenhouse from above:
I suggest thermally insulating the greenhouse by using two or more sheets of transparent plexiglass and leaving an air gap between them. If the greenhouse pit was deep enough, the space between the sheets would make a "second floor" where a person could fit at full height. This is convenient for cleaning the glass sheets from time to time and also, a thicker air layer means better thermal insulation. Having airtight insulation might require additional vents for CO2 and O2. Those could lead to and out of the house (to clean the air of the living spaces) or directly outside.
Another energy-efficient way to keep the temperatures optimal for plant growth inside the thermally insulated greenhouse in the cold season, apart from geothermal heating would be to use compost heating, but since the latter method is rather chaotic, it would be difficult to keep the temperatures stable and there's also a risk of fire. On the bright side - having a fire in the pit is less dangerous.





Creative contributions

Rethink the mirror placement and design

J. Nikola
J. Nikola Nov 22, 2022
I realized that underground greenhouses rely on the heat stored in the soil. Placing mirrors on the side walls made of soil could reduce the heating of the soil during the day and the release of the heat during the night. I did not calculate or find any data on this, but since these soil walls of 1.5-2.5 m height are a huge surface, I think we should definitely think about this.
Maybe your idea of the two-way mirrors could jump in here.
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Povilas S
Povilas S17 days ago
It's not necessary to cover the entire surface area of the walls with mirrors, you could use a few bigger mirrors and leave gaps between them, also, you could leave a gap between the back of the mirror and the wall to let the air circulate. The thermal insulation of the greenhouse that I talked about in the idea description should also eliminate this problem.
The "two-way" mirror I talked about in the challenge is more of a one-way mirror.
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General comments

J. Nikola
J. Nikola17 days ago
First, thank you for this cool find! I never heard of walipini greenhouses. It seems that they are great for growing all-year-round crops. However, I am not sure that all your improvements would actually improve the efficiency of the greenhouse.
Thermal insulation with a double layer of plastic film or plexiglass seems to regularly implemented in walipini construction. I tried to find out if a deeper dig would result in a bigger temperature difference, but there was no data about it, besides that digging deeper is significantly more expensive. It is true that the water would be closer to you if you dig deeper, but in some areas, where groundwaters are close or where the rainy seasons are hard, this could make you underground greenhouse flood. Nothing to be scared of, but should take into consideration. On the other hand, your improvements such as mirrors and geothermal heating would definitely help the sunlight reach the plants easier (not sure how much, but it definitely would) and keep the greenhouse warm during the cold months. You should have in mind the efficiency of the geothermal heating system and its tight connection with the soil - if you put the pipes on the greenhouse floor or a bit beneath the ground, the heating efficiency of the system could be affected. But overall, really a cool find.
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Povilas S
Povilas S17 days ago
J. Nikola The plexiglass or other transparent material is generally used for constructing "the roof" of the greenhouse, my idea is to place additional, horizontal layers of the plexiglass between the walls of the pit (see the blue lines in the cover image), ideally, the gap between the glass would be big enough for a person to stand and walk there making a "second floor" of the greenhouse. A thick air layer between the glass would better insulate the greenhouse.
My idea is for the greenhouse pit to be 4-6 m. deep. I did pretty much web research on underground depth-temperature dependence. Many sources mention the depth below 10 feet (~3m.) to be thermally stable all year round. Check these references: 1, 2, 3, 4. Many sources dealing with geothermal heating also state 10 feet to be a thermally stable depth and the upper limit of depth in which horizontal geothermal pipes are placed. The latter is perhaps due to the increased cost of digging deeper, as you mentioned, but greater depth should only be of benefit temperature-wise.
The area of the greenhouse would be only a small part of the whole area through which the geothermal pipes are spread, the rest would be covered by soil, so this shouldn't affect the efficiency of the geothermal system much.
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J. Nikola
J. Nikola17 days ago
Povilas S Okay, I didn't realize that until now, sorry. In that context, you would definitely have the problem of sunlight not reaching the lower floor. Now I get why mirrors.
That's pretty deep. I checked your sources and it seems they all tested the soil temperature at indicated depths of the densely packed soil. You would have a bit different scenario where you don't have dense soil but a greenhouse pit. The effect would be different depending on its size and the level of ventilation. If small, temperatures would probably be more stable all year round. However, if properly ventilated, the effect could be significantly reduced.
Concerning the last paragraph, I understand now. I never checked how large are the geothermal piping systems.
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