Controlled Environment Agriculture

Our Best Practices

Commercial Light Integral Control

demonstration greenhouse

More than 25 years of research in the production of lettuce and research into the marketing of produce has convinced our research program that the KEY to acquiring and retaining long-term customers for a commercial production facility involves ONE simple concept:

Produce a consistent, high-quality crop at a predictable and steady rate 365 days per year.

This means that you need to grow the same number of saleable product every day and the key to this is in the control of the total amount of light the plants receive. More than any other environmental variable, the total sum of light received by the plants in a 24-hour period determines the rate that the plants grow and thus the amount of produce available for sale. We have found that the major customers of commercial production facilities will not accept decreased production in the winter due to poor natural growing conditions nor will they accept produce that has been damaged by too much light during the summer.

Q: What is the key to producing a consistent, high-quality crop year-round?

A: Light integral control.

Q: How can you obtain light integral control?

A: Use LASSI.

Research at Cornell University Daily Light Integral Control (DLI) in CEA using LASSI

DLI: Daily Light Integral, sometimes called the "light sum," is the number of photosynthetic photons (400 to 700 nm) received in one day, usually expressed as mols of photons received 6.02 x 1023 photons.

CEA: Controlled Environment Agriculture.

LASSI: Light And Shade System Implementation, a name invented by Alice Chiu (Tuson), while a Master of Engineering graduate student in Cornell's CEA program in 1996.

Motivation: A major concept arising from initial CEA research at Cornell was that vegetative growth is proportional to the DLI sum over a plant's growth period, and consistent growth timing can be achieved only by providing plants the same light integral every day. Nature does not provide consistent DLI values, as shown in the graph below (fourteen years of outdoor DLI for Ithaca, NY.) The average value for the earth below the clouds is 26 mols of PAR per day. In Ithaca (where the winters are very dark...ask anyone that lives here!), the average is 24.5 moles per day but as shown in the graph below is a result of very bright summer days and very dark winter days.

demonstration greenhouse

When plants are grown in a greenhouse, the DLI is the sum of natural (sun) light, as well as supplemental light used on darker days. Movable shades must also be considered to prevent over-shooting the DLI goal, which can cause physiological disorders such as lettuce tip burn, as well as providing less controlled crop timing. These needs motivated creation of a computer-based control algorithm that can forecast the solar DLI by sunset and operate supplemental lights and movable shades in a way that achieves the DLI goal every day, delays supplemental lighting into the electricity night rate period as much as possible, and does not use supplemental lighting frivolously early in the day, requiring movable shades be deployed later in the day to prevent DLI overshoot.

Details: The LASSI algorithm contains two parts and makes decisions on a one-hour time step. The first part is a series of eight rules, based partly on a prediction of solar DLI by sunset, that determines whether supplemental lights should be operated starting immediately, or whether lighting can be delayed for at least one more hour. The second part is two rules that determine whether shades must be closed immediately, or whether deployment can wait until the next hour. Example results for DLI = 17 for a year, with and without LASSI control, are shown in the graph below. Control is very consistent during darker times of the year, when supplemental lighting can be used during the day and at night. During brighter times, a small daily overshoot can not be retrieved later that day. However, averaging over three days for control can bring three-day averages very close to the DLI goal, and plants respond suitably to three-day averages.

demonstration greenhouse

The horizontal line shows the 17 mol target with the actual light levels hovering around the setpoint with LASSI control using a 3-day average. The dots are the natural sunlight.

Extension: The LASSI algorithm has been appended with a second algorithm to control CO2 concentration - see graph of results below. Added CO2 makes light more efficient and creates a virtual DLI. Testing shows supplemental lighting cost can be reduced by nearly half with proper CO2 and supplemental light control. Both algorithms are patented through the Cornell Center for Technology, Enterprise, and Commercialization (CCTEC).

demonstration greenhouse

LASSI with DLI (without 3-day averaging) and CO2 control.

Interested in obtaining a stand-alone light controller that uses LASSI? Contact Professor Louis Albright at: albright@cornell.edu

L. Albright modified by M. Brechner
29 August 2011