Influence of the Heating Method on Greenhouse Microclimate and Energy Consumption

Abstract

The influence of the heating system method on greenhouse microclimate was investigated overnight, using an experimental greenhouse tunnel with a tomato crop. The heating system consisted of plastic heating pipes located close to the gutter holding the growing substrate and an air heater, located 2·6 m above the ground. All the measurements were performed during two periods: (a) a period when only pipe heating was used; and (b) a period with pipe and air heating together. Heating pipes only could maintain the desired inside air temperature up to a temperature difference between inside and outside air of 10 °C, whereas this difference was increased to 15 °C with the addition of the air heater. Energy consumption with a system combining heating pipes and an air heater was 19% higher. The use of the air heater enhanced the vapour pressure difference and thus the crop transpiration. For both cases crop temperature was lower than air temperature but this difference was larger with the air heater and resulted in an increase in crop aerodynamic conductance. The use of the air heater for dehumidification purposes was also investigated. It was shown that with the air heater, although the mass transfer conductance to the cover was higher, condensation flux was smaller which resulted in less condensation at the inner surface of the cover. These results indicate that the use of a mixed system is favourable in greenhouse tunnel conditions since the use of the air heater, although increasing slightly the energy consumption, improves the control of both air temperature and humidity, particularly by keeping the inside air dew point temperature lower than the cover temperature and preventing the occurrence of condensation on the plastic films.