Geothermal And Passive Greenhouses – Reducing Farming Carbon Emissions

Unsustainable Abundance

One great comfort of modern life is the possibility of getting fresh produce directly from the supermarket anytime. Fresh produce is directly waiting for us at the supermarket. Outside, it might be freezing and covered in snow, but we will still find an abundance of tomatoes, lettuce, and fresh fruits.

This, however, comes at the cost of massive carbon emissions. Either these products are shipped, most of the time by plane, from the other side of the world. Or they are produced in heated greenhouses, mostly kept warm with natural gas or propane.

One option would be to forgo such luxury. This is what our ancestors used to do, especially in cold climates, relying on a diet of onion, potatoes, carrots, cheese, pickles, and salted or smoked meat during the winter. But realistically, this is not something the average consumer is willing to do.

Luckily, new designs of greenhouses relying on smarter use of energy from both the Earth and the Sun can help reduce the need for fossil fuels to grow our winter food supply.

Passive Greenhouses

Greenhouse essential design is to capture and store the sun's thermal energy. By letting sun rays enter, protecting plants from the wind, and keeping the heat in. This boosts plant productivity by lengthening the growing season and providing warmer temperatures, boosting plant growth. This is a must for producing even “basic” food like tomatoes or paprika in many parts of the US, Canada, and Europe.

Heating of greenhouse by solar radiation – Source: Hydroponics simplified

Passive greenhouses push this basic concept further by increasing the efficiency of every possible component.

Thermal Mass

The first idea is to increase the inertia of the system. The largest risk for greenhouse culture is short-lived frosts that would kill fragile plants like tomatoes. Increasing the thermal inertia of the greenhouse reduces the maximum and minimum temperature peaks.

One way to do so is to have the north side of the greenhouse not made of plastic or glass but of bricks or stone. The solid wall will retain more heat and directly absorb the sun's heat during the day. It will then re-emit it toward the greenhouse inside at night, making it more likely to stay frost-free.

This design has been deployed at a massive scale in China and is now being studied in scientific literature.

Chinese Passive Greenhouses – Source: Research Gate

To boost the effect of the wall further, a layer of insulation, often polystyrene, is applied to the outside of the wall so the re-emitted heat stays on the inside of the greenhouse.

Other designs can also add barrels of water and a very powerful heat sink, touching the north wall for even greater thermal inertia. Stones around plants and a concrete/stone floor at the greenhouse's center can also help.

Reducing losses

Another option to keep the greenhouses warmer at night is to reduce thermal losses. For this, the Chinese passive greenhouses unroll a blanket on top of the greenhouse every night. Initially done by hand, this is now more commonly done with automatic systems.

Source: Research Gate

Some more progress could be made, notably on ventilation, but in any case, this design can already operate without any fossil fuel and represents a large component of how China can feed its 1.4 billion people population.

Source: Research Gate

Geothermal Greenhouses

Ground Temperatures

Passive greenhouses are a radical improvement over the fossil-fuel heated designs common in the West, especially in the US or the Netherlands.  But in the harshest climate, it might not be enough, especially in the case of very cloudy and snowy winters, where it might be entire weeks when a greenhouse does not receive any direct sunshine.

Geothermal energy is already used to warm houses through ground heat pumps. The idea is to buy a network of pipes deep in the ground, at a level where the temperature is constant throughout the year. Then, use a heat pump to extract that heat and keep the house warm.

Source: energy.gov

What makes a ground heat pump superior to an air heat pump, at least in cold climates, is that the group will have a constantly above-freezing temperature. The exact temperature varies depending on the region, as well as the potential geothermal activity.

A Simplified Geothermal Design

For greenhouses, there is no need for a heat pump to “concentrate” the ground energy up to a temperature comfortable for humans.

Staying several degrees above frost at night is enough, with the moments when the sun shines bringing the extra degrees needed to resume plant growth. From these simple facts emerged the concept of a geothermal greenhouse.

The idea is to bury a pipe deep enough for the surrounding soil to stay warm all year round. Air from the greenhouse is then circulated in the pipe, warming it, and then re-injected into the greenhouse to keep the air inside above freezing temperature.

This strategy has been deployed in US states with cold winters, like Nebraska, to grow locally citrus, figs, or grapes.

Simple Is Beautiful

The advantage of this design is that it is very energy efficient. The need for air circulation makes it not entirely passive, as the ventilation system requires some electric power. But it is a very low energy consumption compared to any other form of heating for greenhouses.

It is also a relatively low-tech solution, with the digging required doable by one person with a backhoe or excavator and only needed once for several decades.

Lastly, the stable average temperature underground means the same system can be used to cool down the greenhouses in summer, potentially helping boost plant productivity in summer months as well, and/or reducing water consumption.

Other Geothermal Heating

The basic geothermal greenhouse is a good solution for lengthening the growth season or making frost-sensitive crops and trees viable in Nordic regions. More power geothermal heat sources might be needed to reach optimal productivity all year round.

One option is hot springs and other localized ground heat sources (60-150°C / 140-300°F). While difficult to scale everywhere, there are “dozens of greenhouse operations in the Rocky Mountain and West Coast states that are heated by medium temperature geothermal energy.” The US, but also Japan, Canada, Chile, and other regions with hot springs and volcanic activity could be good candidates for expanding this type of greenhouse.

Another option could be deep geothermal boring, digging deep enough to access rocks permanently warmed up by the Earth's geothermal activity. Unfortunately, such deep wells are, for now, very expensive. While some companies, like Eavor, are looking to solve this problem for power generation, it is unlikely to be deployed for food production any time soon.

Conclusion

Geothermal greenhouses are still a very novel concept and work best combined with the concept of passive greenhouse. Passive greenhouses are now becoming more mainstream globally, following their mass adoption in China.

So, it is likely that geothermal greenhouses will become the next step in making the food chain more localized and resilient while reducing dependency on fossil fuels.

At the time of writing this article, we could not find a publicly listed company specializing in building passive or geothermal greenhouses. This technology's “low-tech” aspect may have made it not “startup-friendly,” as very little proprietary IP could be developed.

Still, some kits are available from private companies like:

• Ceres Greenhouse Solutions (high-yield greenhouses).
• SunCatcher (Solar passive greenhouse).
• Atmos Greenhouse Systems(Geothermal “climate battery”).

It is nevertheless a key to reducing fossil fuel consumption, as well as building a more resilient economy, as the crisis hitting Dutch greenhouse production in 2022 illustrated. The country has 24,000 acres of greenhouses consuming  106 petajoules yearly, providing  21% of the peppers, 20% of the cucumbers, and 17% of the tomatoes grown in Europe.

So, this could become a new green sector yet to be developed at scale, with standardized systems to reduce costs and increase efficiency and reliability.