Energy Saving Achieved by Retractable Liquid Foam between Double Polyethylene Films Covering Greenhouses

Heating is a major cost component in greenhouse crop production. A retractable liquid foam insulation technology was developed recently in Canada to reduce energy use for greenhouse heating. The system dynamically generates and injects liquid foam into the cavity between two polyethylene films in the greenhouse roof. Several experiments were conducted in southwestern Ontario, Canada, from December 2007 through May 2008 to evaluate the energy saving potential of this technology. The liquid foam system was installed in two independent greenhouses; in one of them, the system was shut off to serve as control. The foam was used only during the night for greenhouse insulation. In December 2007, no crop was grown inside the greenhouses. During the first five months of 2008, tomato (Lycopersicon esculentum) and sweet pepper (Capsicum annuum) plants were grown inside both greenhouses. Natural gas consumption for heating, and inside microclimate and outside climatic conditions (air and leaf temperature, humidity, solar radiation, and wind speed) were recorded. Heat loss from the roof with or without liquid foam was calculated. During December 2007, the liquid foam reduced the heat loss through the roof by 31%. During 2008, the heat loss was reduced by 29%, 36%, and 42% for January, February, and March, respectively. A larger reduction in roof heat loss (higher energy saving) was recorded during spring nights: 62% and 58% in April and May 2008, respectively. Therefore, the liquid foam showed a good energy saving potential (29% to 62%), but its energy saving decreased with low outside temperature. Further improvement in liquid foam formulation may be needed to increase its energy saving potential during the winter under low outside temperature conditions. Finally, liquid foam insulation resulted in higher greenhouse air temperature and plant leaf temperature; this should benefit plant growth and development and promote early yield.