frost-free (?) geothermal greenhouse (earth battery) in Central Europe

Hey all,

I’ve been reading all sorts of helpful and inspiring stuff on this forum for ages, and now I finally have something to share that might be of interest to others.

This spring, we finally started building our geothermal greenhouse!
Our goal is to keep it frost-free, allowing us to massively extend the growing season and grow plants like citrus, passion fruit, and other hardy subtropicals.

I’d like to share the setup and construction process with you as we go along. I’d also love to hear your thoughts and ideas, especially when it comes to the nitty-gritty details of installing the fan and some of the other ideas we have in mind to improve the overall performance. I will upload more photos and explanations of our set-up along the way.


The concept works best in areas with year-round sunshine. It is more experimental in areas like ours in Central Germany (Latitude: 51° 40′ 0″ N), where winters are mild but mostly cloudy. Officially, we are located in a USDA Zone 7b climate, though it's usually more like 8ish (8a-8b) as we are located in a valley near a major river... generally, winters are not that cold but mostly grey. We lack the sunshine during winter that would allow year-round tropical farming in a (well-insulated) geothermal greenhouse. On the other hand, our winters are usually quite mild. The few exceptionally cold nights (very rarely down to -15°C) are accompanied by bright, sunny weather which heats up the system.

The few existing prototypes of these types of greenhouse in Europe look promising. I found some videos of geothermal greenhouses in Norway, Czech Republic and the UK. Please let me know if you know of others in similar climates/winter conditions!


Our greenhouse measures 7 × 12 m, with a height of 4.20 m. It’s built using wooden Gothic arches made of spruce, treated with a copper solution.
It is covered with 10mm polycarbonate sheets. We bought the greenhouse as a kit from Latvian company Garten Dream who were super nice during the whole process.

We added 8 automated windows installed along the North and South side at the bottom. On the East and West side, two large swinging doors (2,30m each) allow for easy access. On top of those doors, there are two large windows on either side. Together with an electric fan, this should allow for proper ventilation and heat reduction in summer.

For the "earth battery", we dug 10 trenches (starting at ~1,20m depth with a slope down to ~1,60m) under the area where the greenhouse now stands. Through those trenches we laid drainage pipes (16cm Ø) that run across the whole length of the greenhouse. The hot, moist air of the day will be sucked into these pipes from under the roof by a ventilator through a 25cm Ø pipe.

For the fan, I am considering either a PrimaKlima EC Ø250mm (max. 1450m³/h) with integrated temperature/speed control or a Blauberg Centro-MZ 250 L (max 900m³/h). The estimated air volume of the greenhouse is roughly 250m³, so these fans should be able to move the entire volume 4-5 times per hour at maximum capacity (which will only be needed on sunny days where we have enough solar energy from the roof of our house). During nights, the fans will run at around 15-30% of their capacity to slowly move the stored heat from underground back into the greenhouse.

The best explanation I could find about how exactly to install the electronics that will regulate the fan, is this video from a dude who built a geothermal greenhouse in Mongolia! Any other tipps on which parts to instal and how exactly to wire everything? I am looking for a cheap, robust (no wifi etc.) solution, idealy something that allows me to simply plug in the ventilator and some easy to understand controls for fan speed and temperature settings (I imagine that the fan should start running once temperatures reach more than 20°C AND also once temperatures drop below, say, 7°C, though we will have to figure out the best settings during this first winter).

Additional heating will be supplied by a kind of Jean Pain-style powered floor heating that runs under a little terrace in a corner of the greenhouse where we would like to chill in winter and start seedlings in spring. The terrace is made of a ≈20cm deep bed of sand though which we layed a total of ≈60m of a thick HDPE water pipe (black) and a bunch of smaller PE floor heating pipe (red). We will plaster the surface of the terrace with some nice reddish sand stone slabs (all these materials we sourced second-hand from our area). The pipes will be filled with water and via a small circular pump which will pump the water through a spiral that sits in a large compost pile that will be stacked outside leaning to a section of the North wall of the Greenhouse (for the compost, we can source huge amounts of wood chips from a friend who runs a tree care company + infinte amounts of straw/manure mix from our neighbours' cattle farm).

For irrigation, we want to fetch any rainwater from the greenhouse roof + the roof of a nearby shed (another ≈30sqm). All water will be directed inside the greenhouse. First into a ≈4000L basin which sits below ground level between the concrete foundation along the North wall and a second wall that we built using the leftover hollow  stones with which we built the foundation of the greenhouse. The basin will be covered with a HDPE liner. We imagine to use this basin also for aquaponics in the future.

At the bottom of that basin, there will be a pump which can pump water to the top of a "tower" made of 3 black IBC totes of 1000L each that sit on each other and a connected via pipes. This way, all three IBC containers should always be filled to the max. The lowest IBC sits inground, the top being level with the surface of the greenhouse floor. That lowest IBC will be connected to a pump that will then feed the drip-hose irrigation system.
In total, that will be roughly 7000 litres of water inside the greenhouse acting as some proper thermal mass.

Another feature that I would like to mention: The main path way through the greenhouse is a sunken path. The North wall of this path is an old concrete foundation of fence that used tu run along the site where the greenhouse now sits. Instead of removing all that concrete (which wiuld have been quite a hustle but ultimately would have made all the earth works much easier), we thought let's make use of it. So it now stablizes the main growing bed and the parth that tuns long that bed. It will also add some more thermal mass.
The path way as such will be the lowest point of the greenhouse, thus acting as a cold sink in winter. We found some really nice inspiration in a video by someone who built a large geothermal greenhouse in the US (can't find the exact video, but this is the channel). Kinda coyping their idea, we burried three 220L rain water barrels under the path way. The top of the barrels is open. The bottom sits above one of the underground drainage pipes which we connected to those water barrels. In summer, the barrels will be invisible as they are covered by stone slabs. In winter, we will exchange the stone slabs with some repurposed metal light shaft covers. That will allow the cold air, that will already accumulate at the floor of the path way, to be sucked into the underground tubing system.

Here are some photos of the construction process, will upload more along the way.

Grateful for any comments of suggestions