Dynamic Computer Control of Closed Environmental Plant Growth Chambers. Design and Verification

Abstract

ABSTRACT CONTROLLED-environment chambers can be used to closely investigate plant responses to well-defined and -maintained conditions. This paper describes the theoretical and practical basis for the design and development of a computer-based growth chamber control system that can produce a wide range of constant or time-varying environments. Analyses of energy, humidity, and CO2 balances, expressed in terms of dry bulb temperature, dewpoint temperature, and CO2 concentration, were developed to relate the effects of external environmental conditions and internal biological and physical processes to the direct regulation of chamber conditions. These relationships were used to match cooling (air conditioners) and heating power to the maximum anticipated latent energy loads and sensible energy requirements. In practice, the theoretical control equations were transformed into digital software algorithms that suited the characteristics of the sensors and hardware in each control sub-system. Design of plant chamber control systems may be hampered by the lack of heat exchanger design information over the range of interest. Controls of outdoor plant chambers are difficult because of the large energy and mass flows relative to system capacitance. Nevertheless, this system demonstrates that feedback-feedforward control algorithms can be used to control several chambers by multiplexing gas samples through CO2 analyzers and dewpoint hygrometers. The combined system of software, sensors and control elements has been successfully used in several CO2 enrichment studies. Dry bulb temperature was regularly controlled to within 1 C, dewpoint to within 1.5 C and CO2 concentration to within 5% of desired values.