1997-07-01

Thermal Analysis and Control in the Astroculture™ Space-Based Plant Chamber 972295

Maintaining thermal balance in a space-based plant chamber has proven to be difficult to achieve, particularly if the air temperature in the plant chamber is desired to be below that of the atmosphere of the space vehicle. Analysis of the thermal condition of a plant chamber has identified three heat sources as major contributions to this serious problem. The first is the input of radiant energy into the chamber required to support plant growth. The second is via thermal conduction through the chamber walls. The last major thermal input is from the fans and other electronic components embedded inside the chamber. Design solutions to achieve thermal balance are further exacerbated by virtue of the limited power availability, volume and mass restrictions, and safety considerations.
Obviously, use of an efficient lighting system and a lighting system that does not emit energy in the infrared region of the spectrum (700 to 2000 nm) has a major impact on the capability of the temperature control system to maintain the experimentally defined temperature of the plant chamber atmosphere. Use of recently developed insulating materials having ultra-low thermal conductivity properties (low K-values) minimizes the amount of heat that must be extracted from the chamber when the air around the chamber is above that of the desired temperature of the air within the chamber.
Thermoelectric units are used as a heat pump to extract or add heat to the chamber atmosphere to maintain the temperature set-point. Proper design and dimensioning of the hot and cold side heat dissipation units (radiators) are critical for obtaining effective heat transfer rates of the thermoelectric units. These heat dissipation units also have a major impact on the efficiency of the thermoelectric units.
The temperature control system used in the ASTROCULTURE™ flight unit, that included an enclosed plant chamber, during the STS-73 mission demonstrated a capability to maintain the air temperature of an enclosed plant chamber to within ±1 °C of the desired set point. This was approximately 4 °C below the air temperature of the STS-73 middeck area. These performance data indicate that thermal control can be effectively maintained in an enclosed plant chamber using a properly designed thermoelectric unit system.

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