Research in Greece has been developing more efficient control methods for supplemental lighting, taking advantage of the dimmability of light-emitting diode (LED) grow lights (HortScience). The work compared 14 hours per day of full power supplemental LED lighting and turning the LEDs on, at full power, only when the ambient photosynthetic photon flux (PPF) dropped below a specific threshold, and adjusting the duty cycle of the LEDs so that the LED lights provided only enough supplemental photosynthetic photon flux to reach a preset threshold which was adjusted daily from 50 to 250 μmol·m−2·s−1.
Turning the LED lights on at full power and off was not practical since at times this resulted in the lights going on and off too frequently. Adjusting the duty cycle of the LED lights based on photosynthetic photon flux measurements underneath the light bar provided excellent control, with 5-minute averages typically being within 0.2 μmol·m−2·s−1 of the threshold. Continuously adjusting the duty cycle of the LED lights reduced electricity use by 20% to 92%, depending on the PPF threshold and daily light integral (DLI) from sunlight.
Simulations based on net photosynthesis (An) − PPF response curves indicated that there are large differences among species in how efficiently supplemental PPF stimulates An. Adjusting the duty cycle of the LEDs based on PPF levels assures that supplemental light is provided when plants can use that supplemental light most efficiently. Implementing automated duty cycle control of LED grow lights is simple and low cost. This approach can increase the cost effectiveness of supplemental lighting, because of the associated energy savings.
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