Study of a passive solar heating greenhouse crop grow gutter

Abstract

Water-filled transparent polyethylene tubes have been used as a passive solar system in greenhouses in Greece. In this work, the performance of a passive solar heating system used to serve as a crop grow gutter is studied. The experiments were performed during autumn and winter of 2007 in two greenhouses, located at the University of Thessaly near Volos, Greece, cultivated with a tomato crop. One greenhouse was used for control and plants were grown in rockwool slabs located in a commercial grow gutter, while in the second greenhouse the rockwool slabs were located over the new grow gutter. The gutter consisted of a water-filled polyethylene tube with a diameter of 0.45 m combined with two perforated tubes (diameter of 0.2 m) on the top, in a distance of 0.25 m between them, to serve as gutter-channel for drained nutrient solution collection. The perforated tubes had holes of about 0.01 m every 0.50 m and greenhouse air was pumped to inflate the tubes and mix the greenhouse air. Several parameters inside and outside the greenhouse were recorded in order to set up and evaluate a numerical model working as basis for further parametric studies. The experimental results showed that the new passive solar heating grow gutter resulted in about 7 to 15% heating energy saving. The perforated tubes' air escaped with a temperature of 1 to 2̈C higher than the average greenhouse air temperature, increasing locally the air temperature at plants' level, indicating heat dissipation from the water-filled tubes. The configuration of the water-filled PE tube, combined with the perforated tube is simulated using the Fluent CFD code, incorporating external User Defined Function source code for the non-trivial boundary conditions. The experimental measurements obtained are used for model validation; while a small parametric study is performed altering the operational conditions of the proposed system in order for conclusions to be drawn and possible optimizations to be recommended.