Low temperature heating system for greenhouses based on enclosed water curtain and liquid foam insulation

Abstract

A greenhouse water curtain heating system allows heating of the greenhouse by low-temperature water, which can be obtained from residual waste heat sources. The water curtain can be applied on the outside of the greenhouse roof or enclosed between two foils. But also enclosed water curtains suffer from high heat losses, which limits the integration of low temperature waste heat sources. An effective way of reducing the heat losses is to combine the water curtain system with retractable liquid foam enclosed between two foils. But until now, a systematic evaluation of the thermal performance of such system combination is still lacking. Thus, this study aims to fill in this research gap by evaluating the heat transfer characteristics of a double-foil greenhouse roof section installed with a combined water curtain and liquid foam system. Experimental tests have been conducted under a wide range of temperature scenarios in a climate chamber where the heat loss and heat gains from the water curtain is measured. The results have been compared with the data in the existing studies. This study revealed that combining the water curtain system with liquid foam, reduces the heat losses by half compared to using just the water curtain: the heat loss coefficient was reduced from 4.4 W·m−2·K−1 down to 2.0 W·m−2·K−1. The heat losses through the roof using the combined system are also lower than the heat losses from a double foil greenhouse with other heating systems. An average water curtain temperature of 5.1 °C above the inside greenhouse temperature can compensate for the heat losses through the roof at an outdoor temperature of −19 °C. Based on the study results, recommendations for market implantation of this technology are provided. This study confirms energetic benefit of combining water curtains and the liquid foam technology.