Microalgae can be used as a future energy source that can replace fossil fuel. Recently, the use of microalgae for carbon dioxide fixation through a photosynthetic process to produce high-value materials that can be used for cosmetics and pharmaceuticals has gathered significant attention. In this paper, we report the results of a study on the design and fabrication of a light-guiding plate (LGP) using a hybrid light-emitting diode (LED) and sunlight source that can be applied to a photobioreactor for the mass production of microalgae. We used the LightTools illumination design software (Synopsys, Inc.) for optical modeling and design.
First, the LED light and sunlight sources were modeled. For modeling the LED (LWH1056N, LUXPIA) light source, the intensity data measured using the LED test and measurement system (OL770 UV/VIS, Optronic Laboratories) were used. Sunlight was collected with a circular Fresnel lens, transmitted through an optical fiber (SK-80, MITSUBISHI), and finally radiated from the exit port of the optical fiber. The light illuminance distribution measured at a plane 15 cm away from the exit port was utilized for modeling the sunlight. Second, the reflective film (SY-80S, SK) was modeled by experimentally measuring its bidirectional reflective distribution function. The LGP pattern was modeled as a Lambertian scatterer with its reflectivity R and scatterer width d as the model parameters. The size of the LGP used for testing was 1,010 mm (height) × 510 mm (width) × 10 mm (thickness). Sixty LEDs were arranged at equal intervals in the direction of width on each side, and 32 sunlight sources, i.e., 32 exit ports of the optical fibers, were arranged at equal intervals in the direction of height. Two test LGPs were fabricated: one for the LED sources and the other for the sunlight sources. On the rear surface of each test LGP, LGP patterns of an equal interval (1.5 mm) were engraved along the direction of width (for the LEDs) or along the direction of height (for the sunlight sources). The model parameters for LGP patterns were determined by matching illuminance distributions obtained by optical simulations with experimentally measured distributions. The model parameters determined for the LED light and sunlight sources were R = 80% and d = 0.18 mm and R = 70% and d = 0.17 mm, respectively.
The design parameters of the LGP, more specifically the LGP patterns, comprised the maximum pattern interval, minimum pattern interval, and interval constant of the pattern interval function expressed in the form of an exponential function. These parameters were varied during the optical simulation for the LGP design. The average deviations, defined by the ratio of the standard deviation of illuminance and average illuminance, of LED- and sunlight-LGPs were calculated, and each LGP design was optimized in terms of the average deviation. LGPs with LED light and sunlight sources were fabricated using the LGP manufacturing machine (MR-CA24, Mirae LNS). The values of average illuminance and illuminance distribution uniformity of the LED-LGP were 2,933 lx and 89%, respectively, and those of the sunlight-LGP when the sunlight illuminance was approximately 111,000 lx were 8,174 lx and 90%, respectively. A hybrid-type LGP was fabricated by engraving the LGP patterns, obtained for the width direction from the LED-LGP design and the height direction from the sunlight-LGP design, along both directions on a single LGP. When only LEDs were used as the light source in this hybrid LGP, the average illuminance and illuminance distribution uniformity achieved were 2,908 lx and 89%, respectively, whereas those achieved using sunlight alone as the light source were 8,362 lx and 89%, respectively. When LEDs and sunlight were simultaneously used as the light source, the illuminance distribution uniformity was maintained at approximately 90%.
A light-level control system for the hybrid LGP was designed for maintaining the output light from the LGP at a constant value, which is the most important factor in achieving the required photon flux density for culturing microalgae. The entire control system was designed using LabVIEW (National Instruments, Inc.) and consisted of an optical sensor (LT-40S, RIXEN Tech Co.) used for monitoring the light output and a switch-mode power supply employed for controlling the duty cycle of the operation of the LEDs. When the target value of the LGP output was set to 70 mE/(m2·s), the error range of the LGP output was found to be within ±2%.