In most of North America, growing year-round in a greenhouse requires substantial heating and cooling. Glass and thin plastic are horrible insulators, making traditional greenhouses notoriously inefficient structures.
Even though greenhouses collect vast amounts of solar energy during the day—usually too much—they lose all heat very quickly at night due to the lack of insulation. Thus, an unheated greenhouse will normally dip down to the outdoor temperature at night. In climates with freezing winters, that makes growing difficult, if not impossible.
With more interest in producing local food, growers are looking for ways to do it sustainably by reducing the fossil fuels that it takes to grow year-round. Net-zero energy greenhouses, which produce as much or more energy than they consume, could be the solution.
The first step in creating an independent and abundant year-round greenhouse is to design an energy-efficient structure. Passive solar greenhouse design is the typical go-to strategy for those looking to grow more sustainably year-round. Passive solar greenhouses incorporate a series of design principles that aim to maximize the use of free solar energy and reduce the reliance on fossil fuels.
Elements of a Passive Solar Greenhouse
Passive solar greenhouses have been around, growing abundantly, for several decades. They vary in almost every way imaginable, but they do share some basic principles.
The first is to orient most of the glazing materials (translucent materials like glass or plastics) toward the sun. For greenhouses in the Northern Hemisphere, most of the glazing should face south, which is the direction of the sun year-round.
For a rectangular greenhouse, this means one of the long sides should run north to south to maximize the area of south-facing glazing relative to the footprint.
The north side of the greenhouse plays a small role in collecting light and heat. (The exception is very cloudy climates, like the Pacific Northwest, where cloud cover diffuses incoming light.)
This wall can be insulated much like a home, with rigid foam insulation is installed between wall studs, to reduce unnecessary heat loss. In this way, the greenhouse becomes more like an energy-efficient shed than a leaky and non-insulated greenhouse.
Other principles of passive solar greenhouse design include insulating the foundation of the structure and angling the south-facing glazing area so that transmits the maximum amount of light during the day.
Storing Solar Energy
In some climates, a passive solar greenhouse can grow year-round without any additional heating or cooling. For those in harsher climates, however, additional systems are required to maintain sufficient temperatures for good year-round growth.
There are numerous options in this arena, from simple water barrels used as thermal mass to advanced heat exchangers. Most rely on the simple fact that most greenhouses collect far too much heat during the day, even in winter.
Typically, the grower must vent this excess heat outside, essentially wasting it. Smart greenhouse owners, on the other hand, utilize this free solar energy to heat the greenhouse at night.
Most sustainable heating/cooling methods in solar greenhouses are thermal storage methods: they store excess heat from the greenhouse when it is plentiful during the day, and use it for heating at night and through colder periods.
The oldest of these strategies is integrating thermal mass, materials that passively warm up during the day and slowly radiate this heat back into the greenhouse when the temperature drops. Water is the most common. By stacking barrels of water along the north wall of the greenhouse, a grower can create a cheap and highly effective thermal “battery” to stabilize temperatures.
Other thermal storage strategies include a ground-to-air heat exchanger, which uses the thermal mass of soil. Often called a climate battery, a ground-to-air heat exchanger circulates air through a network of pipes buried in the soil.
The system transfers excess heat from the greenhouse air to the soil, which stays a warm and constant temperature (also, remember that the earth is a stable temperature deep underground).
When the greenhouse requires heat on winter nights, the system draws this heat back into the greenhouse. Thermostats operate the fans so the system only works when the greenhouse gets too hot or too cold.
Growers can also become more creative, integrating a range of sustainable systems that provide fossil-free heating and cooling. Compost heaters, rocket mass stoves, even saunas and hot tubs are some of the many strategies back-yard gardeners have used to grow year-round. Which system is right, however, depends on your climate, greenhouse size, desire for automation, available resources, and mainly, personal preference.
Once you have created an energy-efficient greenhouse and equipped it with proper heating and cooling systems, the final step of net-zero energy greenhouse is supplying, or offsetting, electricity usage.
Not all greenhouses require electricity. Some growers forgo it and use thermal mass and passive systems for climate control. Instead of ventilation fans, solar vent openers provide air circulation and cooling without electricity.
Going this route is an excellent option for growers in milder climates or those who may only want to grow cold and hardy crops through the winter. The major downside is limited control and hands-on operation as electric fans, back-up heaters, lights, and automated irrigation systems are not possible.
Most growers who plan to grow year-round, however, rely on the helpful automation of fans and other electric appliances to facilitate.
Exhaust fans are usually the most useful electric equipment as overheating the greenhouse is very easy. (On a related side note, try at all costs to avoid electric heaters, which are extremely energy-intensive to run and make it difficult to create a net-zero energy structure.)
To supply these power loads, one can integrate a solar panel system into the greenhouse. The specifics of installing a solar photovoltaic (PV) system depend on your goals and the power requirements of the greenhouse.
Like a home PV system, there are options: connecting the panels to the grid (grid-tied), adding battery backup (grid-tied with battery backup), or creating an off-grid system with battery storage.
Most growers who want to be completely disconnected from the grid opt for an off-grid system with batteries. With the proliferation of small solar panel kit systems, owners of small greenhouses with limited electric demand can quite easily generate and store enough energy to operate an exhaust fan and low-power equipment.
For larger systems, this is usually much more expensive than grid-tied, given the added cost of an extensive battery bank. An important consideration early on is whether your home is a better location for the panels. Houses usually have more space for panels, allowing for a larger and more cost-effective system, and can be eligible for tax credits.
They still allow for a net-zero energy greenhouse, since the power generation is the same, and can supply power for your home at the same time.
Read More About Growing in Greenhouses
Putting it All Together
Creating a net-zero energy greenhouse usually depends on these three strategies: designing and building a highly efficient greenhouse that maintains more stable temperatures to begin with, adding sustainable climate control, and, if applicable, off-setting energy usage with a solar panel system (see: Solar Greenhouse).
With this triple-threat combination, gardeners can enjoy homegrown fruits and vegetables year-round. A net-zero energy greenhouse is source of local, nutritious, and sustainable food.