Computerized control of the greenhouse climate has increased the importance of air distribution and mixing. This report reviews the fluid mechanics of air flow through ventilation inlets and external pressures imposed by winds and applies the analyses to suggest methods of inlet control that improve traditional greenhouse ventilation. The suggested improved control has been implemented in a five-section research greenhouse on the Cornell University campus and has improved climate control significantly during ventilation. Potential pitfalls in implementing the improved control methods are discussed.
One of the deterrents to the commercial adoption of controlled-environment agriculture (CEA) on a broad scale is the significant energy cost for lighting and thermal environmental control. Advances in energy conversion technologies, such as internal combustion engines (ICs), microturbines and fuel cells, offer the potential for combined heat and power (CHP) systems, which can be matched with the needs of CEA to reduce fossil-based fuels consumption. A principal concept delineated is that an integrated entrepreneurial approach to create business and community partnerships can enhance the value of energy produced (both electrical and heat). Energy production data from a commercial dairy farm is contrasted with energy use data from two greenhouse operations with varying energy-input requirements. Biogass produced from a 500-cow dairy combined with a 250-kW fuel cell could meet nearly all of the energy needs of both the dairy and an energy-intensive 740-m2 CEA greenhouse lettuce facility. The data suggest CEA greenhouses and other closely compatible enterprises can be developed to significantly alter agriculture, as we have known it.
Several polyethylene and fabric row cover materials, and clear and black polyethylene mulch, were evaluated in a 2-year field study. For cucumbers [Cucumis sativus (L.)], visible wilting and slowed growth rates of young transplants exposed to cold nights were minimized when grown under row covers that maintained high humidities and higher air and soil temperatures than in the exposed controls. Early cucumber yields were increased 2- to 6-fold by the use of covers. In contrast, tomatoes [Lycopersicon esculentum (Mill.)] showed no significant early yield increases, but a 63% reduction in early yield in 1985 under a perforated clear polyethylene cover. The frequency and duration of daytime air temperatures exceeding 35C had a negative impact on tomato fruit size, quality, and percentage marketable. For cucumber, the relationship between cumulative degree days (during the covered interval) and biomass, early, and total yields was linear (r2 between 0.70 and 0.82) with positive slope. Tomato yields could not be accurately predicted using this approach, but correlations were improved (for the 1985 data set) by using modified degree-day formulas incorporating a negative high-temperature factor.
Although zero runoff subirrigation (ZRS) technology has great promise to manage fertilizer inputs while improving production efficiency in greenhouse operations, high initial investment costs and inadequate technical background are major impediments for initiating the change. In a world of uncertainty, greenhouse operators face the challenge of making an optimal investment decision to satisfy environmental compliance expectations and meet the companies' financial goals. Using Monte Carlo simulation, cost risk was analyzed to compare the relative risks of investing in alternative ZRS systems for greenhouse crop production. An investment model was defined for greenhouse production with alternative ZRS systems. Each cost variable was allowed to vary based on a probability distribution. Random numbers were generated to determine parameters for the probability distributions for the uncertain variables. The simulation process was repeated 300 times for each production model. Simulation results showed that among the four ZRS systems studied (ebb-and-flow benches, Dutch movable trays, flood floors, and trough benches), the Dutch movable tray system returned the highest average profit for small potted plant production and the flood floor system returned the highest average profit for large potted plant and bedding crop flat production. Risk of the production models were compared by the variability of simulation results. The Dutch movable tray system is the least risky for small potted plant production, and the flood floor system is the least risky for large potted plant and bedding crop flat production. Despite its low initial costs of adoption, the trough bench system was least competitive as a ZRS technology for a greenhouse operation because of the relative low profitability and high risk of production due to volatile profitability.
`Ostinata' Butterhead lettuce (Lactuca sativa L.) was used to study lettuce production at varied shoot (air) and root (pond) temperatures. A floating hydroponic system was used to study the influence of pond temperature on lettuce growth for 35 days. Pond water temperature setpoints of 17, 24, and 31 °C were used at air temperatures of 17/12, 24/19, and 31/26 °C (day/night). Pond temperature affected plant dry mass, and air temperature significantly affected growth over time. Maximum dry mass was produced at the 24/24 °C (air/pond temperature) treatment. Final dry mass at the 31/24 °C treatment did not differ significantly from the 24/24 °C treatment. The 24 °C pond treatment maintained market quality lettuce head production in 31 °C air. Using optimal pond temperature, lettuce production was deemed acceptable at a variety of air temperatures outside the normal range, and particularly at high air temperatures.
CO2 enrichment increases efficiency of light utilization and rate of growth, thereby reducing the need for supplemental lighting and potentially lowering cost of production. However, during warmer periods of the year, CO2 enrichment is only possible intermittently due to the need to vent for temperature control. Previous research investigated the separate and combined effects of daily light integral and continuous CO2 enrichment on biomass accumulation in lettuce. The current research was designed to look at the efficiency with which lettuce is able to utilize intermittent CO2 enrichment, test the accuracy with which growth can be predicted and controlled, and examine effects of varying CO2 enrichment and supplemental lighting on carbon assimilation and plant transpiration on a minute by minute basis. Experiments included application of various schedules of intermittent CO2 enrichment and gas exchange analysis to elucidate underlying physiological processes. Same-day and day-to-day adjustments in daily light integrals were made in response to occasional CO2 venting episodes, using an up-to-the-minute estimate of growth progress based on an integration of growth increments that were calculated from actual light levels and CO2 concentrations experienced by the plants. Results indicated lettuce integrates periods of intermittent CO2 enrichment well, achieving expected growth targets as measured by destructive sampling. The gas-exchange work quantified a pervasive impact of instantaneous light level and CO2 concentration on conductance and CO2 assimilation. Implications for when to apply supplemental lighting and CO2 enrichment to best advantage and methods for predicting and controlling growth under intermittent CO2 enrichment are discussed.