Once my first raised bed garden is happy producing food I started planning the next raised bed. It was to be an 18 inch wide and up to 16 feet long bed in the Mittleider tradition. However, while watching youtube videos about people using the Mittleider method, I became inspired to try and extend my growing season -perhaps even year round by using the earth as an energy storage system (aka, a battery).
Each gram of wet soil can hold 0.35 calories of heat energy. At depths from 4ft to 6ft, the soil is far enough down to be insulated from the air above and maintains a year-round temperature from 55 to 60 degrees F. Many residential and commercial buildings utilize this almost free energy and feed it into heating and cooling systems that would normally be pulling from less optimal sources such as trying to cool air from outside during the summer time or trying to heat cold air during the winter.
I started researching methods of accessing this geo energy. After digging around on youtube some more, I found several systems that blow air through the ground via tubes and back into the greenhouse. The air would be heated by the sun and then some of that energy would be transferred into the ground where it can be used during the night. Air, however, is a poor conductor of energy at about 1/4th the specific energy as water and less than the soil itself. So I wondered if there was a better way.
Water has one of the highest specific heat properties of any medium. At first I thought about pumping water through a radiator and warming or cooling air via that method. However, it occured to me that the system could be much simpler. During warm days, a common method of cooling the plants is to water them. The plants, via the roots then become cool. The same principle should be possible with heat as well. If the soil temperature is maintained, we may be able to keep the plants happy. Can water be used to heat/cool the soil directly? Yes!
An efficient way of heating businesses and homes has been in-floor radiant heating. The method pumps hot or cold water through PEX tubing in the flooring system. The warm/cool floor then heats or cools the ambient air in the room.
To make a long discovery process short, I decided to combine these methods. I will pump water into the earth and cool a 12 ft by 18 inch bed directly at the root level. To insulate the bed, I’ll put a double layer 6mm green house film around it. The hope is that I can keep the temperature around 50-60 degrees all year (+20 degrees in the winter). Can it be done? Let’s find out.