The objective of this project is to develop the technology base for a solar plant lighting system for space-based plant growing. The technology implements an innovative concept of dividing the ambient solar radiation into two spectral components: photosynthetically active radiation (PAR) used for plant growth and non-PAR spectra for electrical power. Applications of this solar plant lighting system include orbit/transit flight, lunar colony, and Mars colony. The specific tasks currently being performed are developing the design concept of solar concentrators for the various applications, conducting photovoltaic (PV) electric power generation tests using GaSb cells with the non-PAR solar spectra, and developing fabrication and test prototypes of key components (hull penetration port, cable connector, optical switching device, and light distribution panel).

The solar plant lighting system has the potential for generating electric power while providing plant growing light without any penalty to the plant growth activity.

Figure 1. GaSb Cell (12.5 mm 10 mm: Made by
JX Crystals) Mounted on the PV Cell Module

The electricity can be stored in power storage devices such as batteries or fuel cells and can be used to provide electric lighting when solar light is not available. Conversion of the non-PAR solar spectra can be accomplished by using low-bandgap PV cells. Candidate PV cells include Si (_bg = 1.11 micrometers [µm]), InGaAs (lbg= 1.55 µm), and GaSb (_bg = 1.8 µm). Recently, PV power generation experiments using the GaSb cells were conducted. In this experiment the concentrated solar radiation was divided by the selective spectral reflector into the PAR component (400 nanometers [nm] < < 700 nm) and the non-PAR component ( > 700 nm). The PAR spectra are transmitted to the plant lighting laboratory and the non-PAR spectra are transmitted to the low-bandgap solar cell for conversion to electricity. Figure 1 shows the GaSb cell (12.5 mm 10 mm, made by JX Crystals) mounted on the PV cell module.

Figure 2. Transmission of the PAR Spectra and PV
Power Generation for the Non-PAR Spectra

The PV power experiment was conducted using 20-inch concentrators and the following tasks were accomplished: (1) characterized performance of the PV cells when operated with the non-PAR solar spectra; (2) evaluated the effect of solar concentration on the PV conversion efficiency; and (3) measured the relationship between the PAR delivered by the lighting cable and the electric power generated. A solar test for transmission of the PAR spectra and PV power generation for the non-PAR spectra is shown in figure 2.

Figure 3. The Lightguide Inlet Receiving the
PAR Spectra From the Cold Mirror

The PV cell module at the focal point of the concentrator reflects the PAR spectra onto the inlet of the 10-meter (m) lightguide cable (see figure 3), which then transmits the PAR inside of the laboratory. The PAR light emitted at the outlet end of the lightguide is clearly visible in figure 2. The GaSb cell mounted on the PV cell module was generating electric power. To the best of our knowledge, this test is the first successful demonstration of the feasibility for transmission of the solar PAR spectra and the PV power generation using the solar non-PAR spectra. Based on the voltage-current (V-I) characteristics taken during the solar experiments, the maximum cell efficiency was calculated to be about 15 percent. This means that 15 percent of the non-PAR radiation directed to the GaSb cell was converted to electric power. Our preliminary analysis indicates a transmission efficiency for the PAR spectra over the 10-m lightguide to be approximately 65 to 70 percent.

Key accomplishments:

• Conducted a series of experiments to separate solar spectra into the PAR and the non-PAR components.
• Demonstrated, for the first time, the feasibility of generating electric power using the non-PAR solar spectra while transmitting the PAR spectra to the location for the plant growth.
• Quantified the non-PAR-based PV generating efficiency.
• Quantified the PAR transmission efficiency.

Key milestones:

• Develop design concepts of solar concentrators for the orbit/transit flight, lunar colony, and Mars colony.
• Fabricate test prototypes of key optical components.
• Develop a design of the engineering model of the optical components to be built and tested in Phase II.

Contact: C. Guidi (Cristina.Guidi-1@ksc.nasa.gov), XA-C, (321) 867-7864
Participating Organization: Physical Sciences Inc. (T. Nakamura)