Garden & Greenhouse July 2017 Issue - Page 24

FEATURESTORY by Lindsey Schiller Using Water to Control Greenhouse Temperatures Year round greenhouses, or even those used for season extension, face the challenge of extreme temperature swings. The structure collects exces- sive amounts of heat during the day when the sun is out, causing them to easily overheat. However, they quickly lose this heat at night due to a lack of insula- tion, resulting in over-cooling or possibly freezing. Many growers turn to heating and cooling to maintain a stable temperature – ventilating during the day and heating the greenhouse at night. While reli- able, these strategies can be costly and unsustainable. Thermal mass materials present a natural alternative to evening out temperature swings maintaining a suit- able growing environment without expensive heating and cooling. Thermal mass materials are dense materials which store heat. They absorb thermal energy dur- ing the day, either from direct light or the heat of the greenhouse, and re-radiate this heat back into the greenhouse when the temperatures drop at night. Water is the most commonly used thermal mass in greenhouses for two reasons: it has the highest 24 heat capacity per volume of any of readily available material, and it is cheap. The only needed component is a storage container – an abundant commodity in our plastic-laden society. By stacking several large drums of water in a greenhouse, a grower can create a ‘water wall’ -- a large and low-cost thermal battery for the green- house. The disadvantage with this low-cost climate control strategy is primarily that water takes up considerable space in the greenhouse which could otherwise be used for grow- ing. Incorporating water walls often requires building a big- ger greenhouse or taking up some of your existing grow- ing room. Thus, water walls are most commonly used in large backyard greenhouses or small-scale commercial greenhouses – structures large enough to accommodate the extra space easily. Other disadvantage is a lack of precise control. Un- like a ventilation fan or heater, which you can set to operate at a specific temperature, ther- mal mass can have a varying and unpredictable effect. To absorb heat during the day, water walls rely on passive solar energy. Thus, they can have are dependent on the outdoor climate, and will have limited effect during periods of cold and cloudy weather. They are particularly well suited for struc- tures which do not rely on electricity or supplemental heating, such as off-grid or passive solar green- houses. Tips for Using Water as Thermal Mass Place Containers on the North Wall In the winter months, the goal of a water wall is to absorb as much light and heat during the day, so that it can store this thermal energy for use as heating at night. To this end, water walls should be exposed to light in the winter to absorb as much heat as pos- sible during the day. July 2017