Water and Energy Transport for Crops under Different Lighting Conditions

When high-intensity discharge (HID) electric lamps are used for plant growth, system inefficiencies occur due to an inability to effectively target light to all photosynthetic tissues of a growing crop stand, especially when it is closed with respect to light penetration. To maintain acceptable crop productivity, light levels typically are increased thus increasing heat loads on the plants. Evapotranspiration (ET) or transparent thermal barrier systems are subsequently required to maintain thermal balance, and power-intensive condensers are used to recover the evaporated water for reuse in closed systems. By accurately targeting light to plant tissues, electric lamps can be operated at lower power settings and produce less heat. With lower power and heat loads, less energy is used for plant growth, and possibly less water is evapotranspired. By combining these effects, a considerable energy savings is possible. To assess potential energy savings, the authors examined the costs of water and energy transport of cowpea and soybean under four different lighting conditions: overhead metal halide lamps at low light (350 umol/m²/s), overhead high pressure sodium lamps at high light (870 umol/m²/s), an overhead close-canopy light-emitting diode (LED)-based system at low light (300 umol/m²/s), and an intracanopy LED-based system at low light (300 umol/m²/s). The authors used data from published and ongoing studies in the system analysis. When comparing the different lighting conditions, different amounts of total biomass were produced but similar ET rates were found, so lighting condition was not found to impact ET. Overall, intracanopy, LED-based lighting provided the lowest energy costs.