S06 - Session O2 - Optimizing supplemental lighting control in greenhouses based on plant physiology, variable electricity price, and sunlight prediction.

Authors: Shirin Afzalii, Sahand Mosharafian, Marc Van Iersel *, Javad Mohammadpour Velni

The electricity costs associated with supplemental lighting control in greenhouses can be high and how supplemental lighting is controlled affects profitability. To minimize the cost of electricity for supplemental lighting, we developed a lighting control approach that accounts for plant physiological responses and variable electricity prices. Plant physiological responses are incorporated into the control algorithm, based on the electron transport rate of the crop. Previous work indicated that lettuce requires a total electron transport rate over a 24-h period (daily photochemical integral, DPI) of 3 mol·m -2 ·d -1 and the algorithm assured that this target was reached each day. Supplemental lighting was provided preferentially when electricity prices are low and while sunlight levels are low (since that results in the greatest increase in electron transport rate from the supplemental lighting). The algorithm predicted sunlight at 15 minute intervals, using Markov-based prediction and then made decisions on how much supplemental light to provide. This control approach was implemented in a greenhouse with dimmable LED fixtures, providing precise control of how much supplemental light was provided. Our control approach was compared to a control, where supplemental light was provided to meet a fixed threshold (supplement plus sun light), which also assured that the plants had a DPI of at least 3 mol·m -2 ·d -1 . Our method reduced electricity costs by 4.2% and 33.8% during winter and spring growing cycles, respectively. Cost savings were much lower in winter because of the low sunlight levels, which required that much of the required light was provided by the LED fixtures. Both methods resulted in similar crop growth. We thus conclude that our method can result in significant cost savings, without negative impacts on crop growth.