2018 ASHS Annual Conference

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. Intensity of photosynthetically active radiation (PAR) is expressed in quantum units, or photosynthetic photon flux density (PPFD), which is the number of photons occurring per square meter per second (umols m-2 s-1). The daily light integral (DLI) (mols m-2 d-1) is the sum of PPFD occurring in a day and is commonly used to express crop specific light requirements. 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 mechanisms and LED fixtures. The current research compares the efficiency of two lighting control strategies, intensity modulation and binary off/on. Intensity modulation varies supplemental PPFD, balancing supplemental against ambient light to meet an instantaneous threshold, while binary control switches lights on at full power. Each strategy is combined with the Lighting & Shade System Implementation (LASSI), developed at Cornell University (Albright et al., 2000). The LASSI algorithm strives to meet a target daily light integral, making lighting decisions based on expected solar DLI, the potential for supplemental lighting, the season and the time of day. The algorithm takes effect by delaying supplemental lighting each day for a number of hours determined by the season and the insolation occurring to the current time. 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 manage LumiGrow 650e LED fixtures, and the project lays the groundwork for incorporation of additional software into dynamic greenhouse lighting systems.