Energy efficient supplemental lighting controls in greenhouses

Abstract

Electricity for supplemental lighting is a major cost of year-round crop production, so increasing efficacy of lighting is of primary concern to growers. Market opportunities may support projects with high energy use intensity, which again highlights the need for energy efficient technologies. In greenhouse production, where electric light is supplemented against a broad-spectrum background, only supplemental wavelengths with the highest photosynthetic efficiency justify the cost of electric lighting, and plant growth is largely correlated to the overall quantity of light received. Any reduction in the quantity of supplemental light, while maintaining minimum instantaneous and integrated lighting targets, will coincide with reduced electricity costs for growers. Modulating the intensity of supplemental light can be accomplished at high temporal resolution using computational control algorithms with intensity modulated LED fixtures. The current research compares the accuracy of two lighting control strategies, intensity modulation and binary off/on. Intensity modulation varies supplemental light intensity, balancing supplemental against ambient light to meet an instantaneous light intensity threshold, while binary control switches lights on at full power. Each strategy is combined with the Lighting & Shade System Implementation (LASSI) control algorithm, developed at Cornell University (Albright et al., 2000) which was originally developed for binary control of High Intensity Discharge lights. Binary control is expected to accurately provide a target DLI, with some overshoot on cloudy days that become sunny. Modulation control is expected to precisely meet a minimum threshold DLI while minimizing overshoot of supplemental lighting. Networked microcontrollers were set up to control LumiGrow 650e LED fixtures which were deployed in a greenhouse for 34 days during winter to compare binary vs. intensity modulation control. Overall, this project lays the groundwork for incorporation of additional software into dynamic greenhouse lighting systems.