Effect of cyclic heating and a thermal screen on the nocturnal heat loss and microclimate of a greenhouse

Experiments were carried out in a small heated greenhouse, in which roses were grown. Hot-air heating was applied only during the night. The hot air was distributed to the crop via perforated polyethylene sleeves placed on the ground between the rows of plants. To determine the effect of a thermal screen on the energy consumption and on the greenhouse microclimate under cyclic heating, a horizontal 20%-aluminised thermal screen was automatically deployed every night between 18:00 and 06:00, at a height of about 2.5 m above the ground. The screen was deployed during two of the four weeks of data collection. The air temperature in the greenhouse at night was usually maintained at 16–18 °C by an on–off controller. The thermal screen did not reduce the heat loss from the greenhouse because it was relatively small in area and only 20% of its area was covered by reflective aluminised material. Yet, despite this, it kept the canopy temperature slightly higher than without a screen. Two different models were used to determine the global heat transfer coefficient from the greenhouse: the first assumed a quasi-steady-state heating condition; the second used a transient approach in which heat storage in the greenhouse air and crop was taken into account. It was found that there was a small difference between the results of the two models. A simple model for calculating leaf temperature is offered and used for calculating the temperature of an upper leaf. The model and experimental results are in good agreement even under unsteady heating.