Greenhouse solar heating in the Argentinian northwest

Abstract

In the present paper a low cost solar active water heating system is proposed to increase the average night temperature and avoid freezing problems inside greenhouses. The system collects the excess of energy in the greenhouse during the day, and returns it at night. The most important part of the system is a plastic bag made of two thin polyethylene films vertically hung inside the greenhouse, that works as a solar collector during the day and as a heat exchanger at night. The films are soldered in such a way that the water introduced at the top of the bag falls by gravity following a zig-zag path. The warm water is stored in a pond directly built in the ground and waterproofed with a polyethylene film. The system has been designed taking into account the meteorological conditions in Salta where 12 freezing days are expected each year with minimum temperatures down to −8°C. A prototype was built in a 600 m 2 greenhouse covered with a single polyethylene film. It was located in the Province of Salta, Argentina, with a 24.5° South latitude and an altitude of 1200 m. Tests were performed for three years beginning at the 1992 winter, and a satisfactory thermal and operational behavior was obtained. With outside temperatures down to −6°C at the end of the night, the system was able to keep a temperature of 2°C inside the greenhouse.