The Pros and Cons of various climate control method in passive solar greenhouses.
STANDARD GREENHOUSES require excessive heating and cooling to grow year-round in most climates, making it hard to grow fresh, local food in the off-season. The Year-Round Solar Greenhouse is a gardener’s guide to designing and building an energy-efficient greenhouse that is abundant, sustainable and self-sufficient. The first step in this process is creating an insulated energy-efficient structure, based on the principles of passive solar.
However, even an energy-efficient greenhouse requires some form of climate control typically. These strategies range from cheap and simple (e.g. water barrels stacked inside) to automated and complex (e.g. a solar hot water system). We delve the most common options for renewable heating / cooling methods in section 2 of the book, explaining their basic design as well as pros, cons and best applications.
The most common climate control method in passive solar greenhouses is thermal mass. Thermal mass materials are dense materials capable of storing large amounts of thermal energy (heat). Simply put, they sit in the sun and heat up during the day and then slowly release this heat at night. Water is the oldest and most common. Several large water barrels stacked on the north side of a greenhouse can dramatically reduce temperature fluctuations and help extend the season.
While water is cheap and simple, it also has some practical challenges. Barrels take up space in a greenhouse that could otherwise be used for growing. Secondly, water must be in direct sun to absorb significant amounts of heat during the day – an impediment for many winter climates.
A new variation of using thermal mass is emerging, using an abundant free resource under every greenhouse: the soil underground. The earth can be used to store thermal energy and stabilize temperatures in the greenhouse, just like water. Unlike water, though, it does not take up space in the greenhouse. Moreover, it is plentiful – providing much more heat storage than water – and stays a consistent temperature year-round. The soil a few feet underground stays a fluctuates much less than the air temperature, staying between 40-50 F for most of North America.
A Ground to Air Heat Transfer system – also called a climate battery -- allows the greenhouse to ‘tap into’ this large source of thermal mass. Here’s how it works: when the greenhouse heats up during the day, fans pump hot air from the greenhouse through a network of pipes buried underground. As the hot air travels through these pipes, heat is transferred to the soil. After traveling underground, the cooler, drier air is then exhausted back into the greenhouse. In this way, the system cools the greenhouse by transferring heat in the soil.
When the greenhouse gets too cold, the system also serves a heating function. The same fans circulate air underground. At these times, the soil is warmer than the air, and warmer air is transferred to the greenhouse. In this way, a climate battery both heats and cools the greenhouse depending on the air temperature inside the greenhouse. Because heat is transferred from the greenhouse to the soil, and then back again, we often refer to the system as a ‘self-heating greenhouse.’ The sun provides the energy (heat); the greenhouse collects the heat; and soil stores that heat until it is needed.
In addition to a heater / cooler, the GAHT system is also acts as de-humidifier. During the day, air in a greenhouse is hot and humid. When the GAHT system pumps humid air through the soil, the air reaches the dew point. Water condenses out of the air, and percolates into the soil via perforations in the pipes underground. This phase change, from water vapor to liquid, is a major driver of the GAHT’s cooling power during the day. It also reduces the risk of molds, diseases and pest infestations by keeping reducing the humidity of the greenhouse. Effectively, a GAHT system takes water out of the greenhouse air and transfers it the soil, where it is accessible to plants roots.
GAHT systems and their variations have been around for decades. They are becoming increasingly popular as a cheap and sustainable way to heat and cool greenhouses as people discover the greenhouses’ immense potential to collect solar energy (heat), and the soil’s capacity to store this heat over long periods. They can be installed in greenhouses of any design or scale, and work well in most climates. (There are situations in which they are not the best suited, like if you cannot dig underground or have a very hot climate.) In general, this basic and time-tested system allows the greenhouse to maximize use of solar energy, creating efficient and renewable climate control for year-round growing.