Design for an improved temperature integration concept in greenhouse cultivation

The ability of crops to tolerate temperature deviations from the average set point could play an important role in energy saving greenhouse climate regimes. This principle is used in the so called temperature integration procedure, which is based on empirical knowledge and uses fixed maximum and minimum temperatures. More dynamic flexible boundaries depending on the underlying crop processes would probably increase the potential for energy saving in greenhouses. Therefore, our aim was to improve the temperature integration concept by introducing dynamic temperature constraints. Processes with a fast temperature response (e.g. photosynthesis or stress) were decoupled from developmental processes with a slow response time. A,modified temperature integration procedure was designed combining the usual long-term integration over several days and fixed boundaries for daily average temperature with short-term integration over 24 h with flexible temperature limits. Because the optimum temperature for canopy photosynthesis rises with increasing concentration of atmospheric CO2, this aspect was included in ventilation control. Because plants react not only to extreme temperatures but also to their duration, a dose concept was applied to stress-related temperature constraints: The desired mean temperature for the subsequent 24 h was calculated once in 24 h. Within this 24 h cycle, temperature set points for heating and ventilation were optimised in relation to the fast crop processes. The temperature regime was tested by simulations. Greenhouse climate, energy consumption and dry matter increase were simulated for complete: years and different parameter settings for tomato as model crop. With the modified regime compared with regular temperature integration, with the same +/-2 degreesC long-term temperature bandwidth 4.5% (normal secure settings) or up to 9% (extreme settings) more energy could be saved (on a yearly basis). Crop gross photosynthesis could increase by approximately 2.5%. (C) 2003 Elsevier Science B.V. All rights reserved.