A methodology to identify a target DLI to control supplementary lighting

Plants need photosynthetically active radiation (PAR) to perform photosynthesis. The total amount of PAR the crop receives in one day is called the daily light integral (DLI). During winter, supplementary lighting is used to raise the DLI in the greenhouse. Since lighting needs energy, it should be used efficiently. As the DLI has an important influence on the growth and production of the crop, growers need a profound knowledge concerning the DLI their crop needs. The identification of a target DLI is a valuable tool to control supplementary lighting. Typically, in literature, the DLI for plants to achieve a high yield, maintaining a high quality, is investigated. However, in practice, this DLI cannot always be achieved using the supplementary lighting installed in the greenhouse. In this paper, a methodology is proposed to determine a target DLI throughout the period when supplementary lighting is applied. The target DLI is calculated taking into account the PAR transmittance of the greenhouse, weather data and the properties of the installed lighting. As a case study, this methodology was used to determine a target DLI throughout the period in which Belgian growers typically grow their vegetables using supplementary lighting. It is applied to a model of a greenhouse growing tomatoes. Using computer simulations, a traditional illumination scheme applied in Belgian greenhouses, using fixed hours to illuminate the crop, is compared to an illumination scheme which aims to approach the target DLI. This leads to possible energy savings, although also the total amount of PAR decreases, which will probably affect the yield.