Pulse Width Modulation Control of LEDs Can Maintain Targeted Light Levels

LEDs are becoming more popular for use in indoor growing operations and supplemental lighting in greenhouses to promote growth. The capital cost of LED lights is high and may slow adoption of LED lights. The light output of LEDs can be adjusted by manipulating the duty cycle (duration of power ‘on’ compared to ‘off’ during a given, very short cycle). That may result in energy savings, improving the economic feasibility of LEDs. Our goal was to see if the light output from LED light bars can be adjusted automatically to maintain the photosynthetic photon flux (PPF) at or above a specific target level in the presence of variable sunlight. Four sensors measured the PPF levels of sunlight and the PPF underneath three LED bars. Using a datalogger, the LED light bars were controlled in three ways: 1) LED bar on at full power for 14 hr/day, 2) LED bar on at full power when the sunlight was below a specified PPF level (for 14hr/day), and 3) an LED light bar controlled with a pulse width modulation (PWM) board to adjust the duty cycle of that light bar, so that the combined amount of PPF below the light bar (from sun plus LEDs) did not drop below a specific PPF target level. The target PPF level was changed daily to 50, 100, 150, 200, or 250 µmol·m^-2·s^-1. The PWM-controlled LED bar successfully maintained target PPF levels in response to shading or cloud cover. Compared to the control treatment, PWM control of the LEDs based on PPF reduced power use by 20 to 92%, depending on daily light integral (DLI) and target PPF.  With a target PPF of 250 µmol·m^-2·s^-1, PWM control of the LEDs reduced power use by 20% on a day with a DLI of 2.5 mol·m^-2·d^-1 and 53% when the DLI was 8.5 mol·m^-2·d^-1. With a target PPF of 50 µmol·m^-2·s^-1, power use was reduced by 90 to 92%. Energy savings were less when the LEDs were simply turned on or off based on PPF (0 to 60%, depending on DLI and target PPF). Controlling LED lights using PWM can assure that the lights are only on when needed and at the minimum power needed to provide adequate. The potential energy savings of using PWM control of LED lights to maintain a specific minimum target PPF will lower the operating costs of using LEDs and make them more economical.