Growth of higher plants In closed environments requires a great deal of energy for lighting systems. Even the most efficient lights deliver only about a quarter of the energy they use as useful radiation for plant growth (photosynthetically active radiation or PAR). The remainder of the energy, as well as most of the PAR, ends up as waste heat which must be removed from the plant growth chamber. The thermal control system (TCS) which does this job can require a significant amount of volume, mass and power. Efficient and effective design of the TCS is therefore important to the overall feasibility of the plant growth chamber, either for terrestrial or aerospace purposes.
As part of the Early Human Testing Initiative being conducted by the Crew and Thermal Systems Division at the Johnson Space Center, a plant growth chamber has been designed and built which has instruments for research and is outfitted for human testing. A novel feature of the TCS is a liquid cooled coldplate mounted above the high pressure sodium lamps which serves as a reflector or luminaire. The coldplate is coated with a white coating which selectively reflects short wavelength light required by the plants while absorbing longer wavelengths.
During the design phase of the plant growth chamber, several development tests were conducted. The focus of this paper is one such test which studied the lighting and thermal control systems using a full size light box with coldplate which supports a 1.4 m2 growing area. The test stand offered the capability to test several different barrier types below the bulbs, various air flowrates over the lights and various liquid flow rates and temperatures in the coldplate. The test stand had instruments to determine how much heat was removed by the air and by the coldplate as well as how much thermal radiation was transmitted to the plant level.
Test results indicate that about half of the electrical energy put into the lamps was removed by air passing through the light box and another third was removed by the coldplate. Average light levels greater than 2000 μmol m-2 s-1 were obtained using eight 400 Watt high pressure sodium lamps (measured 20 cm below the light box).