Let’s talk about greenhouses. In terms of access to year-round, happy vegetables -- a well-designed greenhouse has few rivals. A greenhouse harnesses the power of the sun and the laws of convection. Thermal energy is trapped within the building, cycling as warm air is pushed upwards and cooler air is warmed by the soil and surfaces. This provides a happy, warm, safe place for plants to thrive. Harsh weather, pests, trampling feet, and plant diseases alike can all be prevented within a greenhouses’ safe walls. For a family looking to start providing their own food, a greenhouse is a boon like few others. However, even the sturdiest greenhouse cannot protect plants from freezing temperatures if they are not heated in some manner. Energy to heat an extra building can be a serious chunk out of your wallet. Luckily, humans have been passively, sustainably heating their greenhouses for centuries -- and we’ve gotten pretty good at it.
A greenhouse can come in practically any size and shape. Even a window raised on a few 2x4’s can be used as a cold frame to protect plants from frost and help raise their ambient temperature. Attached greenhouses are found connected to a main building, typically a home or barn with electricity/water hookups. This placement makes connecting lights, heaters, fans, and accessing water much easier. You’ll often find attached greenhouses using one of the home’s exterior walls as one of their own. This solid mass provides opportunities for thermal energy to be stored within the material during the sunny day and released back into the greenhouse during the cool night. This wall can be brick, plaster, or even an earthen wall like in the case of sunken greenhouses. Other greenhouses stand on their own. These can come in practically any shape: gothic arches, a-frames, etc. Typically, the size of the space is primarily decided by how much you intend to grow -- it’s important to remember that the more space you have, the more space there is to heat. The shape of your roof is also important, your goal is to push as much air back downwards while still allowing for proper ventilation and circulation.
No matter what your greenhouse looks like, heat is incredibly important for happy plants. Thermal energy fuels greenhouses. This is the heat and energy that comes from the sun’s rays penetrating the greenhouse’s glass walls. Thermal energy can be stored, and reused, in a variety of ways. The law of convection means that heat will move into cooler spaces when they come into contact. This transfer of energy is how greenhouses store and release energy in the transition from day to night. To make the most of the excess thermal energy during the day, thermal batteries can make a huge difference as the temperature drops at night. Some materials absorb more heat than others. A popular choice for a thermal battery is dark-colored barrels of water, or a sunken heat sink filled with mulch/rocks/bricks/masonry that can store energy during the day. At night, when the greenhouse begins to cool, heat is released from the batteries, keeping the ambient temperature from dropping too low.
Not only can you directly harness the sun’s heat – but the Earth itself is another fantastic source of heat and thermal energy. Underneath the first layers of dirt on the Earth’s surface, the soil retains a fairly standard temperature throughout the year. This heat is generated from the Earth’s molten core and can be accessed by only going a few feet underground. This is called geothermal energy and can be harnessed in a variety of ways. One of the simplest, and oldest ways to do this is by placing your entire greenhouse a few feet underground. These sunken greenhouses can be formed by excavating a cliffside or sinking your greenhouse’s foundation into the dirt itself. Exposure to open, warm soil keeps your greenhouse warm. When temperatures drop at nighttime, or in the winter, heat is released from the soil into the greenhouse air. Other, more advanced designs, involve the addition of a heat pump. A heat pump pulls air from the greenhouse and moves it through a series of tunnels underneath the Earth. The surrounding soil warms the air before it’s pumped back into the greenhouse space.
These passive forms of heating are fantastic, sustainable ways to keep your greenhouse warm and happy. But what if a little more power is in question? A typical homestead will need to power more than just a heater for their greenhouse. Machinery, lights, fans, vents, pumps, and any other kind of powered equipment will need access to energy. Of course, standard electric hookups or gas/wood powered energy is always acceptable, but these are expensive options, and not entirely sustainable. Other options, like wind or solar energy, are renewable, self-sustaining, and only cost as much as their physical equipment requires.
