What’s a Natural Battery in a Greenhouse?
Almost any kind of dense mass absorbs heat as it is exposed to sunlight. Think of the painfully hot sidewalks of midsummer you would dance across as a child, trying to skip from one patch of cool green grass to the next. Even shallow puddles could be uncomfortably hot!
The denser the mass, the more solar energy it absorbs, ergo the more energy that can be stored. The color of the mass affects the ability of the mass to absorb the energy, i.e., the speed at which it “charges.” In a sunken greenhouse, then, dark earthen walls absorb heat during the daytime, charging them with thermal energy. Overnight, as the air temperature cools, the flywheel effect causes the earthen walls to discharge their stored heat, thus keeping the ambient temperature within the greenhouse warm enough to protect the plants it holds.
To protect plants during even the coldest night of winter, the key is to have enough energy stored to keep the greenhouse temperature above the limit where plants will be damaged - generally not below 45°, though this depends on the specific crops being grown.
An easy way to increase thermal mass is to add barrels of water, painted flat black, along a wall where it will receive sunlight. Besides adding more “batteries” to the equation, these water barrels can also be used for plant irrigation, eliminating plant shock.
Is Insulation Necessary in a Geothermal Greenhouse?
Using geothermal and solar energy alone isn’t the most efficient way to take advantage of the free energy provided by nature. It’s important to keep the heat where you need it most: inside the structure where your plants are! In this case, insulation is the key. Greenhouses developed in the 17th century used glazing (glass) to cover greenhouse frames, and the term glazing now refers to any type of transparent or translucent material used for that purpose.
Glass is no longer the preferred material for glazing - while it allows the most light through, it is heavy, expensive, and fragile, and compared to double layered options, is actually a relatively poor insulator. Instead, rigid plastic panels or flexible plastic sheeting are used most often. Double layers of glazing with a dead-air space between the layers are among the most effective insulators. For example, BTL Liner’s ArmorClear products offer customizable, flexible, anti-condensate covers suitable for both hobbyists and commercial greenhouse operators.
In extreme climates, where nighttime temperatures are consistently well below freezing, additional insulation in the form of insulated shutters or panels can be installed over the glazing at night. This is usually not required for most climates.
What are Walipini Greenhouses?
Walipini greenhouses gained popularity from an agricultural project in 2002 where volunteers traveled to LaPaz, Bolivia to help farmers design and build affordable greenhouses that worked in the cold Andean Plateau. The prospect of an affordable greenhouse project rapidly gained popularity among DIY’ers in the US and elsewhere and today details and efficiencies continue to be improved among hobbyists.
In simple terms, a Walipini is a rectangular hole in the ground 6’ to 8’ deep, covered by a double layer of translucent plastic sheeting. In concept, it’s not much different than a sunken greenhouse, but it offers some advantages (and disadvantages) from the more traditional pit design.
The orientation of a Walpini to the sun is critical, since it’s sunk completely into the ground. The longest side of the rectangle must face the winter sun (South in the Northern Hemisphere, North in the Southern) and a thick wall of compacted earth on the opposite side provides support for an angled roof. The roof seals the hole, allows sunlight to enter, and provides an insulating airspace.
Walipinis are built deeper underground than pit greenhouses to take greater advantage of geothermal heat. At 8-12 feet deep, geothermal energy can do more than ensure a minimum temperature in the greenhouse. There’s natural variation, of course, but as you dig deeper, it’s possible to find soil temperatures as high as 75°, which sounds pretty toasty.
