How to impact a crop's vertical temperature profile

As part of the monitoring project of Kas als Energiebron (Greenhouse as energy source), the vertical temperature profile in the tomato greenhouse of Gardener's Pride was analyzed at the beginning of 2018. The greenhouse is equipped with 200 μmol/m2/s SON-T lighting, a 14 m3/m2/hour Ventilation Jet system, a blackout screen and an energy screen. What is the impact of the use of the Ventilation Jets, vertical circulation fans and tube temperature on the vertical temperature profile?

The 24-hour average temperature was 0.5°C higher at the top than at the bottom of the crop. This difference was mainly caused during the day period, when it was 1.2°C warmer at the top.

Ventilation Jets decrease temperature difference

During illumination with closed screen(s), without the use of the Ventilation Jet and a heating pipe, the temperature was lower than 35°C and the air temperature at the top was 0.5°C higher than at the bottom of the crop. Nights when the Ventilation Jets were running, the temperature at the top was 0.2°C higher on average than at the bottom. The use of the Ventilation Jets thus reduced the vertical temperature difference by approximately 0.3°C. This is because the Ventilation Jets blow cold air from the top of the screen into the greenhouse.

The use of only the lower vertical fans for air circulation (so no input of cold and dry air) did not appear to influence the vertical air temperature profile with closed screens. In other words, the vertical air temperature profile (temperature difference between top and bottom of the crop) was the same for the times when the vertical fans were on or off. However, the air temperature at lamp height was not measured so that no statements can be made as to whether these fans will send the lamp heat downwards.

Increased tube temperature

Increasing the tube temperature to 45-50°C meant that the temperature at the top was 1°C lower than at the bottom of the crop (lighting at 75%, the Ventilation Jets on, and screen(s) closed). The use of the higher pipe therefore made it warmer at the bottom than at the top. The vertical temperature difference was changed by 1.5°C due to the warmer tube.

The outcome of this research can be translated into useful knowledge for cultivation. Firstly, the use of only the lower vertical circulation fans did not influence the temperature difference between top and bottom of the crop. The Ventilation Jets ensure that the temperature of the top of the crop is about 0.3°C lower compared to not using jets. Finally, measuring the air temperature at different heights can result in a better understanding of the influence of controls on the vertical temperature profile. In this experiment the tomato temperature was also measured, the effect of air and tomato temperature on growth time was analyzed, as well as the accumulation of gases in the greenhouse with and without a minimal ventilation opening. These results will be published in a report this summer.

Source: Kas als Energiebron