2019 ASHS Annual Conference
A Greenhouse Lighting Control System That Delivers a Consistent Daily Light Integral (DLI) and Reduces LED Fixture Energy Consumption
A Greenhouse Lighting Control System That Delivers a Consistent Daily Light Integral (DLI) and Reduces LED Fixture Energy Consumption
Tuesday, July 23, 2019: 3:00 PM
Cohiba 1-3 (Tropicana Las Vegas)
Growers have typically relied on two strategies to illuminate their greenhouses with high-pressure sodium (HPS): continuous irradiation over a desired photoperiod, or threshold control that switches lights on and off at a defined solar intensity. Under these scenarios, fixtures usually deliver light at a constant photosynthetic photon flux density (PPFD) and the resulting daily light integral (DLI) may vary considerably. The superior dimmability of LEDs allows for variable supplemental PPFD throughout the day. We utilized a software system and algorithm to integrate information from solar light sensors and provide crops with a consistent DLI. The algorithm achieves a minimum preset DLI by averaging solar light values from three consecutive days and adjusting fixture intensity. Past research reports that a similar algorithm could maintain a DLI within 10 percent of a target on 87.5 percent of days (Albright, 2000). Data from five greenhouses (winter 2018-2019) demonstrated that achieving a target DLI depended on: the number of light sensors per unit area of growing space, amount of supplemental light available, maximum and minimum PPFD, and photoperiod length. Under ideal conditions, LED fixtures supplemented solar light to within 2 percent of the target DLI on greater than 90 percent of days. Compared to a 100-Watt outdoor pyranometer threshold, the DLI control system lowered energy consumption by as much as 65 percent. In conclusion, sensor-actuated LEDs provide value that goes beyond the estimated 10 to 27 percent efficacy savings associated with using LED rather than HPS fixtures.