The primary objective of this study was to develop a fiber-optic hybrid day-lighting system for mobile application such as military shelters in order to cut energy use and the use of fossil fuels. The scope included the design, development, and testing of a hybrid lighting system that is capable of producing about 16,000 lm output with design challenges including light-weight, compactness, and optics that can tolerate a high tracking misalignment. The designed system is comprised of two subsystems: the solar collector and the solar hybrid lighting fixture (SHLF). The solar collector, consists of a housing, a structural stand (tripod), a dual axis tracking system, Fresnel lenses, secondary optics, and fiber-optic cables. The collector is a telescoping aluminum box that holds eight 10-in diameter Fresnel lenses, which focus sunlight onto eight secondary optics and deliver uniform light to the fiber-optic cables. The secondary optics have filters to block UV/IR. The optics has been designed to have a high half-acceptance angle of 1.75 deg and can accommodate a tracking accuracy of 1.50 deg or better. This novel SHLF consists of two components: a solar fiber-optic system and a light emitting diode (LED) system. The fiber-optic cable is coupled to an acrylic light diffusing rod that delivers the sunlight into the room. During sunny periods, the solar fiber-optic lighting could provide full illumination level. In order to keep the same level of lighting during cloudy periods, the LED portion of the light fixture can supplement the output of the SHLF. A compact, light-weight prototype system was built and tested. The results showed that the system's output per lens for the 20 ft cable was about 1750±50 lm at a global solar illuminance of 115,000 lx. The total system was capable of delivering 14,000 lm of sunlight.
More than 400 × 109 kWh of energy  is used annually to provide artificial lighting for buildings, which costs about $40 billion annually (the largest energy end-use sector in the nation) at an average rate of $0.1/kWh. While some of this lighting is needed in the early morning or late evening hours, most of the electric lighting is used during daylight hours. There is an opportunity to save at least half of this energy considering cloudy periods. Even though conventional daylighting employing windows and skylights has helped reduce electricity use, cannot take full advantage of sun-light. For building shapes with a large area-to-perimeter ratio, conventional daylighting is not really an option for much of the building because many rooms will be too far from exterior walls or the roof. Also, for buildings with more than one story, skylights cannot be effective for floors other than the top floor.
It has been recognized that a promising alternative to conventional daylighting systems is to transport light deep inside a building using optical fibers. Several efforts [2–7] have studied the feasibility of daylighting systems using optical fibers. There are similar technologies on the market today using this technology, such as Himawari  and Parans Solar . Himawari's technology was developed in the 1980s and did not achieve any market penetration since it was cost-prohibitive. Parans Solar has been successfully offering a fiber-optic daylighting system that is meant to be fixed on a flat rooftop. More than a decade ago, Gorthala  built a prototype passive fiber-optic daylighting system (U.S. Patent No. 6,299,317) that incorporated a sunlight collector with a primary and a secondary concentrator in tandem, plastic optical fibers and a passive dual-axis solar thermal tracker. The significance of the technology is that the dual axis tracker uniquely orients itself toward the sun without the need for electric motors or electric power input. The prototype built was a functional prototype to demonstrate the proof of concept. However, this technology is not suited for mobile military applications.
Several military services are exploring the use of alternative and renewable energy technologies to generate power at forward-deployed locations and reduce petroleum-based fuel demand. For example, the Air Force Research Laboratory has created the Renewable Energy Tent City—a collection of various deployable shelters powered by solar and fuel cell generators; the Marine Corps Systems Command is developing the Deployable Renewable Energy Alternative Module—a module towed by a vehicle that would employ solar, wind turbine, battery, and generator technologies; the Army Research Laboratory is working with universities and private sector firms to develop a processor that converts tires into energy and recyclable products for potential use at forward-deployed locations.
The new daylighting system being developed is meant for mobile military applications. Current expeditionary shelters measure 32 × 20 ft and have been using 300 W of fluorescent lights, which are being replaced by light emitting diodes (LEDs). Typically, eight such light fixtures are employed in a shelter yielding a total output of 16,000 lm.
MILSPEC standards MIL STD-1472F and MIL-PRF-44259E specify illuminance levels for different tasks. General office work requires an illuminance of 755 lx (70 ft candles or lm/ft2) and a minimum illuminance of 540 lx (50 ft candles or lm/ft2). According to, “MIL-PRF-44259E 3.4.10 Illumination,” an individual light shall provide a minimum illumination of 140 ft candles (1500 lx) when measured 18 in from the surface of the assembly at the midpoint of the assembly using a cosine corrected photometer probe when tested. The technology being developed can be adopted for building spaces that are underground or do not have windows, but require daylight. This technology can also be used for temporary shelters used in disaster relief operations.
Research and Design Methodology
The overall objective of this study was to develop a solar hybrid lighting system employing fiber-optic cables for mobile military shelter application, which required that the system be light-weight that can be carried and installed by two soldiers, compact, rugged, and reliable. The design of the solar hybrid lighting system can be broken down into key individual components: the optics for sunlight collector, the collector housing, the tripod/structure, the tracking system, the fiber-optic cables, and the solar hybrid light fixture (SHLF). A schematic of the solar hybrid lighting system is shown in Fig. 1. Key research questions/aspects are:
What is the optimum design (in terms of efficiency) for the optics that maximizes sunlight collection for transporting through fiber-optic cables?
What materials are needed for various components of the system to withstand high temperatures?
How to design the system to be light-weight, compact, and mobile?
How to design the system to operate reliably under mobile conditions?
How to deliver the sunlight/artificial light optimally to the shelter space?
These and other research parameters were considered in the design and development of the overall system. The research/design approach for each of the key components of the hybrid lighting system is presented below:
where A1 is the inlet aperture area, A2 is the exit aperture area, n1 is the refractive index of the medium at the inlet, n2 is the refractive index of the medium at the inlet, θ1 is the inlet half-acceptance angle, and θ2 is the exit half-acceptance angle.
The concentration factor as a function inlet half-acceptance angle for a specified half-exit angle of 42.0 deg is shown in Fig. 3. For instance, for an inlet half-acceptance angle of 1.75 deg and an exit half-acceptance angle of 42.0 deg (the fiber-optic cable half-acceptance angle), the maximum C is about 480 assuming air (n1 = n2) as the medium at the inlet and the exit. The overall concentration factor of a two-stage concentrator or lens system is still a function of inlet and exit half angles. To illustrate the importance of a secondary lens in a two-stage system, a concept of a single lens fiber-optic light concentrator is shown in Fig. 4. As shown in Fig. 4, edge-rays (from Fresnel lens) including a normal ray, A, and off-axis rays, B and C, are able to enter the optical fiber, but off-normal rays D and E do not enter the fiber for the specified inlet tolerance angle, the lens parameters, and the fiber parameters. Referring to Fig. 5, however, the extreme edge rays, D and E, are able to enter the fiber with the addition of a secondary lens employing the principle of total internal reflection (TIR).
As shown in Fig. 5, a two-stage lens system accommodates a larger sun angle tolerance. The present two-stage optics configuration consists of a 10 in diameter Fresnel lens as the primary optic that focuses sunlight onto the secondary optic, which is based on TIR principle. The use of a large diameter Fresnel lens minimizes the cost of the system in terms of labor in assembling the collector. It is the key feature of this design. Also, acrylic Fresnel lenses are light-weight. The secondary optic's purpose is to couple light into the fiber-optic cable within its acceptance angle even at a low tracking accuracy, which means a high angle of acceptance. Also, it is important for the TIR lens to provide uniform illumination into the optical fiber.
The initial design premise was that the optics system should concentrate light into 0.5 in diameter cable and maintain full functionality with sunlight angle error of by ±1.75 deg. This is a great challenge since most systems that have been proposed have an inlet angle of about ±0.5 deg. Using these constraints, a variety of Fresnel lens and secondary lens geometries were examined employing raytracing analysis by photopia software. A summary of output efficiencies at 0 deg and 1.75 deg sun angle entrance offset are shown in Table 1. For each shape, an analysis was performed for the revolved geometry as well as faceted geometry with a hypothesis that a faceted design will result in uniform illuminance without hot spots. The conical/CEC combination with a faceted design was ultimately chosen because of its superior performance under both operating conditions. A prototype of a faceted conical/CEC secondary made from acrylic is shown in Fig. 6. Details of the faceted secondary lens design and analysis are presented in another publication .
The present design primarily includes an acrylic secondary lens considering its light weight. However, considering the low operating temperature of 82 °C for acrylic, new optical silicone materials have been evaluated since their operating temperatures are as high as 200 °C. Prototype lenses from the following optical silicone materials were fabricated using a compression molding process:
Dow Corning Sylgard 184.
Dow Corning MS 1002.
Figure 7 shows each of the liquid silicone rubber test lenses that were evaluated. Samples were evaluated on the basis of visual optical clarity (is there yellowness/cloudiness?), temperature stability, light transmission, and form rigidity as compared to acrylic (Fig. 7).
Evaluation of the test lenses indicate that Dow Corning MS 1002 provides the best optical clarity of all silicone samples examined. However, transmission still needs to be improved to attain the targeted 92% transmission in the visible spectrum. Light transmission comparison tests under outdoor field conditions of 110,000 lx (10,000 ft candles) resulted in an output of 2700 lm (Table 2) for the Dow-Corning Silicone MS-1002. The output is lower than that of the acrylic secondary lens by 25%. Therefore, acrylic was the final choice.
Secondary lens holder: The purpose of the lens holder assembly is to securely position and protect the secondary optic and an IR/UV filter, while maintaining a good optical connection to the fiber-optic cable. An IR/UV filter is an essential component for filtering IR, which can attribute to melting of the acrylic lens/fiber, and filtering UV, which can degrade the plastic optical fiber. An adequate heat dissipation is a primary design consideration of the lens holder. A previous design iteration caused overheating of the fiber-optic cable at the mate with the secondary lens. Because of this, heat dissipation at this point was an absolute necessity.
A major part of the design analysis included thermal modeling of both the lens holder and cable face. Ultimately, the design goal was to maximize convective cooling through the lens holder in order to maintain the cable face at safe operating temperatures. Results of thermal analysis led the design to include a series of convective cooling fins surrounding the lens–cable interface. Figure 8 shows the thermal modeling results. The maximum temperature at the fiber–lens interface reached about 88.7 °C (192 °F) without fins. However, the temperature decreased to about 60 °C (140 °F) with fins.
The lens holder design assembly was created as a four-part system comprised of a filter holder, secondary optic holder, cable holder, and cable ring. The assembly effectively houses the UV/IR filter and secondary lens while providing a method to mate the cable and the secondary optic. Two versions of the finished lens holder assembly are shown in Fig. 9. Version 1 has a plastic lens holder while version 2 has an aluminum lens holder to promote increased heat dissipation. The current lens holder design has been functioning effectively during on-sun field testing; no cable face or lens damage has occurred.
Fiber-optic cables: The fiber-optic cable is responsible for transporting light from the secondary lens to the hybrid light fixture in the shelter. At a typical specification of 1% loss/ft and a bulk retail price of $5/ft for 0.5 in (12.7 mm) cable, it has a significant cost and an efficiency impact on the system. For a fiber-optic cable, the half acceptance angle is defined by the refractive index of both the core and the cladding. A plastic optical fiber cable with an acrylic core (11.4 mm diameter), Teflon cladding, and a polyethylene jacket having a half acceptance angle of 42 deg was considered in the study. After revisions of the secondary lens design, the cable core with a 13.2 mm diameter was found to be more suitable.
Coupling of the fiber-optic cable to the bottom of the secondary lens proved to be a critical process in the system. The effect of improving the optical coupling was investigated through two means: highly polishing the fiber-optic cable ends and applying an optical coupling/index matching gel between the mating surfaces. The index matching gel reduces light losses due to Fresnel reflection at the lens–fiber interface. If mated correctly, the transition losses can by mitigated to less than 1%. Figure 10 shows the index matching gel applied to the end of the fiber-optic cable. For testing purposes, the cable is polished in a multistage process, where the final sanding is done to 5000 grit finish and is touched up with plastic polishing compound.
Collector housing: For a mobile application, the collector housing should be light-weight, rugged, and compact. To meet these requirements, a telescoping housing constructed from thin-walled aluminum (3/32 in thick sheets) was developed as shown in Fig. 11. The telescoping feature reduces the depth of the collector to almost half prior to setup and enables easy shipping. The design is weather and impact resistant, and includes rivet-mounted aluminum angle reinforcements on the interior of the housing, a 1/8 in thick aluminum base mounting plate to increase rigidity, and an 3/32 in clear acrylic sheet to protect optics. All exposed seams are weather protected with silicone sealant to prevent moisture damage to optical components.
Supporting structure: In order for the hybrid lighting system to be portable, it must have a lightweight supporting structure that is easily mobile, but can allow the system to maintain its integrity under loading. It was decided that a conventional aluminum tripod-styled structure would be reasonable for this application since it can be folded up, convenient for storage and transportation. The design includes horizontal structural supports and extended vertical mast to raise the height of the collector, and the feet of tripod can be secured to the ground with stakes or be weighed down with sand bags to prevent it from any potential tipping caused by wind loading. The built prototype tripod is shown in Fig. 12.
Dual-axis tracking system: In designing the dual-axis tracking system, the team tested several tracker controller products and ultimately selected the Heliotrack V3.3  dual axis solar tracking controller and sun sensor. The controller is powered by a 12 V battery source and provides the user with ability to manually drive the tracker's motors—this is particularly useful during collector mounting/dismounting process. The controller utilizes an adjustable sun sensor that can be hard mounted to the collector. Three positioning screws allow the sensor plane to be adjusted so that it rests perpendicular to the collector plane. It comes preprogrammed with the ability to auto return to “morning” orientation and provides variable tracking intervals from 1.8 s to 532 s. Field experimentation has shown that 31 s is an optimal tracking delay for the current optical configuration. An image of the controller and sun sensor is shown in Fig. 13.
Two types of solar tracking designs were considered. Initially, a conventional tip-tilt style was tested. This type of tracker uses one linear actuator for the North–South alignment and one for the East–West alignment. The system is preferred because it offers a low cost and a reasonable accuracy. An off-the-shelf mechanical setup was ordered from Windy Nation (Fig. 14) and was used as the preferred tracking system for a majority of the testing/development process with the Heliotrack controller. One shortcoming of the tip-tilt style tracker is that there is a limited space below the collector for fiber-optic cable and lens holders. The tracking range must be limited to prevent interaction between structural/mechanical components and fiber-optic cable.
To improve tracker capabilities, a precision azimuth-altitude tracker from small power systems was obtained. The unit was modified to be integrated into the current tripod and is compatible with the Heliotrack controller. The rotating azimuth motion provides significantly more clearance for lens holders and cables beneath the collector housing and eliminates interference issues that the tip-tilt mechanics presented at extreme angles. The precision obtained from the rotational gear tooth assembly has also eliminated the over-shooting issue that was occasionally occurring with the previous linear-actuator tip-tilt design. A photo of the current azimuth-altitude prototype tracker is displayed in Fig. 15.
Solar hybrid light fixture: The SHLF (shown in Fig. 16) is integral part of the solar hybrid fiber-optic lighting. A detailed design and analysis of the SHLF is considered for another publication. The SHLF consists of two primary components. The first component provides illumination from the solar collector focused into the fiber-optic cable. The cable couples to an acrylic rod in the lighting fixture, also using index-matching gel, and the rod disperses the light uniformly. In order for this to work, total internal reflection of the input light to the acrylic rod is disrupted by using white paint strips (this can also be accomplished by cutting grooves into the acrylic rods). Due to fluctuations in solar illumination and lighting requirements, the fiber-optic lighting component is supplemented by an LED component of lighting. The two LED drivers, designed based on the requirements highlighted in the designated MIL-Specs (MIL STD-1472F and MIL-PRF-44259E), have an efficacy of around 100 lm/W and are meant to supplement the solar illumination. A photosensor on the SHLF adjusts the output of the LEDs to provide the necessary light output needed based on the fiber-optic illumination. The SHLF has male and female power connections and can be plugged in to make a chain of up to 12 light fixtures.
A full prototype system of the fiber-optic hybrid day-lighting was built and tested. The final prototype is light weight with the solar collector 40 lbs and the tripod below 50 lbs. Each hybrid light fixture weighs about 7 lbs. Overall, the system can be setup in the field by one person, and it becomes easier and can be setup in 20 min with two people.
A fully assembled hybrid luminaire, SHLF, had photometry measured in an integrating sphere (Fig. 17). The 2 m diameter integrating sphere has the capability of measuring total light output in lumens for full-scale light fixtures. The results were then compared to the Jameson's LED and fluorescent fixtures currently used in military shelters. Table 3 shows the results for the light fixtures. The SHLF performed in the middle for power and lumens but had the best efficacy and power factor.
The next test was to measure illuminance at 18 in from the light fixture. The experimental setup was shown in Fig. 18. The illuminance in foot-candles was then measured with a light meter at 18 in from the midpoint to determine if the light output would meet the specification for 140 ft candles. The first results were lower with the luminaire only being able to reach 125 ft candles. However, with the luminaire output being 1841 lm, it is evident that the luminaire is putting out enough light to hit the spec but the light is not being directed properly. To meet the specifications, design modifications are planned for the light fixture.
The system's overall performance was tested through spring and summer and was setup on a building rooftop where it can receive sun from between 8 AM and 6 PM unobstructed. One of the tests involved the evaluation of the fiber-optic cable temperature at the interface between the secondary lens and the fiber-optic cable. This involved operating the prototype during clear, sunny conditions for at least 3 h and removing the fiber-optic cable quickly from the secondary lens holder, and taking an image with an IR camera. An IR image corresponding to an outdoor ambient temperature of about 90 °F is shown in Fig. 19. The highest observed temperature for the fiber-optic cable was 129 °F, which was below the maximum operating temperature of 140 °F.
The overall in-field performance of the hybrid lighting prototype was assessed by measuring the fiber-optic light output employing a specialty integrating sphere (shown in Fig. 20) suitable for 0.5 in diameter cable. Different lengths of fiber-optic cable to verify the losses in the cable over certain lengths, as well as to verify the output were tested. For this testing, a cable short enough to mate with the secondary lens in order to estimate mating losses was used, and its output was measured. Outputs from a 22 ft cable and a 36 ft cable were measured.
The measured fiber-optic light output for different cable lengths is shown in Fig. 21. Also, theoretical output, which is based on the assumption of 1% loss of lm/ft for the acrylic fiber-optic cable, is shown in Fig. 21. In addition, the experimental and theoretical output values are also presented in Table 4.
The results show that the system's output for the 20 ft cable remained in the range of 1750±50 with a global lux conditions of 115,000 on clear days during long-term testing. Previously, there had been issues with the fiber-optic cables overheating with the prototype secondary lens holder. However, the new aluminum secondary lens holder with fins at the mate between the secondary lens and the fiber-optic cable has worked well in preventing overheating issues.
The concept of a portable fiber-optic hybrid daylighting technology was demonstrated through a prototype system. The solar hybrid light fixture is unique, and its functionality has been illustrated. The secondary lens design with fins proved to reduce the overheating of the fiber-optic cable. Testing between two design iterations has shown an increase in luminous output of the system and shows that there is still room to improve. The total system light output was about 1400 lm which is lower by 12.5% from the target. However, this can be easily supplemented by the LED portion of the SHLF. In the future, more effort will be dedicated to improving the light output, designing a more lightweight system, developing a new SHLF with a blue-blackout feature and consumes less power.
This work was performed at Steven Winter Associates, Inc. under the Department of Defense SBIR Phase II project. The authors would like to thank John Sullivan, the project manager at Natick U.S. Army Soldiers Systems Center for his guidance and strong support of this project. In addition, the authors would like to thank Tom Reynolds, the group leader at Natick U.S. Army Soldiers Systems Center, for his continued support of this project.
U.S. Department of Defense (Contract No. W911QY-13-C-0031).