The greenhouse and growth room environment is controlled by computer software and hardware supplied by the Q-Com Corporation. Some custom modifications in the software were required to allow for various new control strategies.
The greenhouse temperature is maintained at 24ºC (75ºF) during the daytime and at 19ºC (66ºF) at night. The ventilation fans are staged and operated in the following sequence: one, two and four fans. The evaporative cooling pad is turned on when the outdoor temperature exceeds 24ºC (75ºF). The operation of the shade curtain and the supplemental lighting system is dictated by a newly developed control algorithm (see references: Albright, 1998; Albright et al., 2000). The photoperiod experienced by the lettuce plants varies from day to day depending on the amount of solar radiation received. No negative effects on lettuce growth and development caused by daily changing photoperiods have been observed. The carbon dioxide concentration in the greenhouse air is only increased to 1000 ppm when no or very little ventilation is required for temperature control. Based on the interaction between the daily light integral and the carbon dioxide concentration, the target daily light integral is adjusted downward according to the amount of time during the day the carbon dioxide concentration was maintained at 1000 ppm. The temperature of the nutrient solution is controlled with a temperature sensor and a control valve in each nutrient solution heating loop. The dissolved oxygen concentration in the nutrient solution is manually controlled by adjusting a flow meter. A level sensor is used to control the nutrient solution level in each pond. The composition of the nutrient solution used in each pond is checked by the fertilizer injection system once every four hours for the duration of an entire hour. The pH of each of the four nutrient solutions is maintained at 5.8 ± 0.2 and the electrical conductivity at 1200 ± 50 µS-cm-1.
The growth room temperature is maintained at 24ºC (75ºF) continuously by a room air conditioning unit. The water-cooled lighting system is operated continuously. The recirculating chiller is outfitted with several safety features (high temperature, low flow, and low water level in the reservoir) that will discontinue the operation of the unit as well as the lamps inside the growth room to prevent overheating of the lamp bulbs. The custom-made lamp bulbs are specially fabricated to operate at lower than normal (without water-cooling) temperatures without reduction in light output. The carbon dioxide concentration inside the growth room can be maintained at 1000 ppm continuously. The composition of the nutrient solution used in the growth room (the same composition as the solution used in the greenhouse) is checked daily and adjusted manually. The pH of the solution is maintained at 5.8 ± 0.2 and the electrical conductivity at 1200 ± 50 µS-cm-1.