Optimisation of CO2 and Temperature in Terms of Crop Growth and Energy Use

In current greenhouse climate control, temperature set points follow a pre-set trajectory based on absolute or solar time parameters, adapted only to instantaneous and daily radiation. CO2 is supplied during a well defined period of the day until a maximum concentration is reached. However, the rate of CO2 supply is strongly limited by the heat demand, since flue gases are the most commonly used source of CO2. Interactions between effects of light, temperature and CO2 concentration on photosynthesis and crop growth are usually not taken into account. This study aims to make more efficient use of temperature and CO2 by developing an optimised climate control system in which temperature and CO2 are deployed such that energy use is minimised while maintaining crop production. Firstly, the diurnal temperature course resulting in a predefined daily mean value was optimised while minimising the heat demand. Most of the time, this resulted in higher day temperatures and lower night temperatures. Secondly, using the heat storage tank, the partitioning of the CO2 associated with the daily heat demand was optimised, aimed at maximised photosynthesis. Simulation results showed that in the optimised climate control system 7% less energy was used and 2.5% more production was realised than in a standard climate. Testing the optimised climate control in a greenhouse experiment showed that in a sweet pepper crop 6% more energy could be conserved compared to that in the standard climate with similar production levels and fruit quality between the climate control treatments.