The key to using ground geothermal energy is to access ground that’s deep enough to escape the effects of surface temperatures. In many parts of the world, that’s as little as 4’ deep. Even in very cold climates, soil that maintains a steady temperature of 50°F through the seasonal cycle is typically found around 10’-12’ deep. Geothermal greenhouses are heated by pumping air through the ground during the winter, where the warmer soil and water around the pipes can then heat the air before its pumped back into the greenhouse.
Heat pump designs can be either horizontal, or vertical, depending on the conditions of the soil. This is determined by the soil's quality, and how close it is to the bedrock below. Horizontal and vertical designs will also have different depths, a wide, horizontal loop will only need around 10 feet of depth, while a narrow vertical loop will require between 200 and 300 feet.
