Greenhouses, at the most fundamental level, are simply buildings designed to protect tender or out-of-season plants from damaging cold. A basic greenhouse is a simple structure with walls and roof made of some sort of transparent material which retains energy from exposure to the sun within the structure. An even more rudimentary greenhouse-type structure is a cold frame, which is essentially a simple clear glass or plastic cover placed over plants that may be in the ground or in a raised bed.
Today, greenhouses are employed all over the world to extend growing seasons and produce fresh, nutritious food even in extreme climates. Greenhouse structures can be broadly grouped into four distinct designs, including tall Venlo greenhouses of the Netherlands, passive solar greenhouses of China, low-cost Parral greenhouses of the Mediterranean region, and gutter-connected polyethylene houses of India and African countries.
Traditional greenhouses use some principles of passive solar heating to trap heat from the sun and keep plants warm, but in high altitudes or northern climates, there may not be enough solar energy to overcome the bitter cold. Geothermal energy, on the other hand, is heat from within the Earth that has been present since the original formation of the planet. It is contained in the rocks and fluids beneath the earth's crust, distributed through layers of magma several thousand miles thick. To a greater or lesser degree, geothermal energy can be accessed from any point on the planet. It’s freely available, pollution-free, and low-tech.
How Does a Greenhouse Use the Earth’s Natural Heat?
Beneath the earth’s crust is a layer of thousands of miles of liquid magma (ranging from 1200° to 2300°F) and below that is another layer of magma under so much pressure that it remains solid. This enormous store of heat helps keep Earth’s crust from becoming permanently frozen by dispersing the heat to the surface. Atmospheric temperatures can overtake the natural warmth of soil in some regions and seasons, but just a few feet below the surface, the earth maintains a near-constant temperature, in contrast to the summer and winter extremes of the ambient air above ground.
Greenhouses sunk into the ground or into a hill on the north side can take advantage of this free heat, using the principle of the “thermal flywheel effect.” Thermal flywheel describes the tendency of a material to remain at a given temperature, regardless of the insulating value of surrounding matter. (A concept similar to Newton’s first law of motion). In practical terms, the flywheel effect means a structure’s average internal temperature can remain constant much longer than would otherwise be expected.
How Does a Greenhouse Use the Sun’s Natural Heat?
Greenhouses inevitably use some elements of passive solar heating, simply by trapping solar energy within an enclosed structure. Still, some greenhouse designs are designed purposefully to take advantage of the sun’s power more directly and efficiently. Translucent material that allows light through without reflecting it away allows the sun to heat the interior as it provides necessary light for the plants to thrive. Other elements of passive solar design include the structure’s orientation, layout, insulation, thermal mass and ventilation.
