Improved Strategy for a Constant Daily Light Integral in Greenhouses

At Cornell University a continuous-mode greenhouse pilot-plant is producing lettuce at a constant rate throughout the year. Plants of all ages, floating on nutrient solution, are present in the greenhouse at any time. They move from the starting point to the harvest point in 24 days, all under the same environmental conditions, repeated day after day throughout the year. The key to the success of such an operation is a constant daily light integral, which can be achieved only through shading and supplemental lighting. Since the potential loss of yield from insufficient lighting (in winter) and the damage from leaf tip burn (LTB) from excessive light (in summer) are high, a rather tight light control is required, which is currently achieved with a rule-based strategy labelled C. A new strategy N, which tracks a reference light trajectory, is now introduced and compared, via simulation, with strategy C, as well as with a reference strategy R, which assumes the availability of a perfect 1-day-ahead forecast of total daily solar radiation. The results show that all strategies are rather robust to variation among years and among locations (within the US). Strategy N out-performs strategy C on average by about 7 $ m−2 yr−1 and does almost as well as strategy R (1 $ m−2 yr−1 difference). All strategies utilise at least some of the economic information contained in the performance criterion, with strategy N more than the others. The application of strategy N to sizing of the control equipment and to systems with insufficient equipment capacity, is demonstrated.