1. Introduction

Nowadays, lighting in buildings accounts for a large part of total electricity consumption. The artificial lights being used are mainly LEDs as they have a high performance and long life. However, artificial light causes eye fatigue, lack of concentration, and discomfort for users ⁽¹⁾. Active daylighting systems using renewable energy sources to illuminate buildings is a solution to these problems.

Daylighting systems are designed to collect and concentrate sunlight on a light guide module to transmit light deep into buildings. Using solar energy to illuminate buildings has advantages of improving the quality of light and reducing the estimated power consumption by lighting 50-80% ⁽²⁾. A typical daylighting system consists of three parts, a solar collector, a transmission part. and a distribution part ⁽³⁾. Some systems have been commercialized, such as the Himawari Solar Lighting System from La Foret Engineering Company, Ltd. in Tokyo, Japan, or the SP4 Fiber Optic Daylighting System, Parans. A lot of research also introduces new methods for daylighting systems ^(4-8). These systems use optical fibers to guide sunlight into the building. Since most of the systems use silica optical fibers (SOFs), these systems become cost-prohibitive, therefore, some systems use plastic optical fibers (POFs) to reduce the cost to a suitable price for users ⁽⁷⁾. POFs have an optical loss of 0.1dB/m. This loss is much larger than the SOFs’ loss of 0.03dB/m. However, the transmission part in daylighting systems has a length of fewer than 20-30 meters in most cases. Therefore, POFs can be used in daylighting systems if the performance requirement is not too high. Moreover, since the core diameter of POFs is much larger than that of SOFs, the sunlight’s coupling efficiency of POFs is much higher than that of SOFs. This higher coupling efficiency is enough to compensate for POFs’ lower transmission efficiency. The disadvantage of the daylighting system using POFs is its high sensitivity to heat ⁽⁸⁾. However, most of the studies on POFs daylighting system have been focusing on improving convergence, improving efficiency, and cost reduction. Reducing the influence of temperature on plastic optical fibers is mainly addressed by the application of filters. Xue et al. ⁽⁶⁾ showed that the POFs cannot work under very high temperatures. They proposed a solution to solve the heat problem with a concentration ratio of 278 and an acceptance angle of 3.4°. Sedki et al. ⁽⁸⁾ showed a new heat filtering device using triple heat filters. The fiberoptic daylighting system of Kandilli et al. ⁽⁹⁾ proposed the use of filters to protect POFs from heat damage. The filter does not transmit the infrared or ultraviolet part of sunlight, which helps to decrease the temperature. Similarly, researchers have been using filters only to solve the heat problem in POFs. However, if only filters are used, the heat problem is not eliminated completely. Since the beam is focused to a point, even if filters are used, a hotspot is produced on the fiber bundle. This hotspot not only generates a heat problem but also produces unequal (non-uniform) light distribution by the optical fiber bundle. In our method, a concave lens was applied with short lengths of SOFs bundles together with a filter. The concave lens produces uniformly distributed concentrated light to each optical fiber in the bundle without creating any hotspots. Uniform distribution will produce the same light intensity in each of the optical fiber in the bundle and will thereby reduce heat on the focused point. SOFs bundles observe heat and cool down before the light goes into POFs. SOFs are more resistant to heat and will absorb some heat to protect the POFs. SOFs bundles work as an optical heat discharger. This is a major advantage of our method. In this paper, the heat issue of daylighting systems was focused upon. Using a prototype of the system, the heat problem of POFs was evaluated. Finally, it was concluded that the efficiency of the heat filter for a daylighting system using POFs was not significant enough to solve the POFs heat issue alone.

2. Prototype

In this section, we fabricated a prototype of a daylighting system to measure the temperature on the POFs. The daylighting system has a similar structure with the one in reference ⁽⁹⁾. A Fresnel lens with a size of 25 cm x 25 cm and a focal length of 30 cm was used to collect sunlight. The focused sunlight passes through an infrared filter with a diameter of 5cm, then is collimated with a plano-concave lens lastly converging on the optical fiber bundle. The optical fiber bundle consists of two parts. The SOFs bundle part was placed at the output of the plano-concave lens, then it was matched with the POFs part by index matching gel to minimize the optical loss between the two different materials. The POFs work as a transmission component. The structure of the daylighting system is shown in Figure 1.

An SOFs bundle was designed to capture all the light at the output of the plano-concave lens, as shown in Figure 2. The 61 SOFs with a diameter of 2 mm and a length of 18 cm are stacked next to each other and forming a hexagonal structure. Each SOF is polished to reduce attenuation and scattering. A hexagonal frame was used as a fixed frame. POFs are arranged in a similar manner, and the core diameter of POFs is 2mm. Therefore, to match the two different types of optical fibers, they need to be fixed and the air gap between them needs to be eliminated using index-matching gel, which minimizes the optical reflection loss.

A dual-axis daylighting system was used for temperature measurement. The system has an error of ±1° for sun-tracking and it includes two Fresnel lenses so various cases can be measured under the same conditions at the same time. allowing for more reliable results when comparing systems.

3. Results and analysis

The temperature measurement test of the daylighting system was carried out on the same day to produce objective results in evaluating the effectiveness of the use of a secondary optical element for reducing the temperature on the focal area. Experiments under actual weather conditions were conducted. The experiment site was located at Hanoi. The solar radiation was 850-1000W/m2 with the ambient temperature of 35°C measured at midday.

The size of the output beam was measured by placing a receiver at the output of the concave lens. The receiver is placed parallel to the lens at the focal point to capture the image of the output beam. The diameter of the output beam was 1.5 cm (Figure 4). So, the SOFs bundle were designed to collect all of the sunlight at the focal point of the daylighting system.

Temperature measurements were collected by a K-type thermocouple having a measurement range of 0°C to 1100°C with a tolerance of ±1.5°C. The thermocouple was located at the center of the POFs. The sensor was connected to a computer by a microcontroller. The temperature was measured continuously for 4 hours at an interval of 1 minute. The actual setup of the temperature measurement system is shown in Figure 5.

POF’s maximum exposure temperature given by the manufacturer is 70°C. So, the daylighting system works stably if the temperature on POF’s bundle is less than 70°C. The temperature variation of the POF’s bundle without a secondary optical element is measured and shown in Figure 6. The average temperature (Tavg) is 200 °C and the maximum temperature (Tmax) is 266°C. So, if the secondary optical component is not added, the average temperature of 200°C on the POFs will be melted.

Based on the result of the temperature measurement of POFs at the focal point, it was found that the daylighting system using a Fresnel lens without a secondary optical component cannot be used as a POFs light guide. In the second test, a UV-IR was used filters to eliminate IR and UV radiation from a high-intensity light source. IR blocking reduces the heat on the POFs. The filter has a diameter of 5cm and thickness is 2mm.

The temperatures of POFs when using the UV-IR filter is shown in Figure 8. The filter allows light with a wavelength of 380 nm to 700 nm to pass through. Compared to the temperature at POFs without using a filter, the maximum temperature decreased from 266°C to 174°C. However, the experimental result showed that the temperature difference between the POFs using a filter and the POFs without using a filter is not significant. The result of temperature at the POFs shows that it is not possible to use POFs for a daylighting system if only protected by filters.

A daylighting system with more filters can reduce the heat more, however, the optical efficiency decreases. Using a plano-concave lens is a more effective way to reduce the temperature. The heat reaches a maximum at the center of the receiver. The plano-concave lens gives uniform light at the surface of the receiver, also giving uniform heat distribution over the surface of the receiver. Figure 9 shows the temperature in the POFs bundle when a UV-IR filter and a plano-concave lens are applied together. The maximum temperature was reduced to 117°C. The temperature quickly reaches a state of saturation and remains stable throughout the measurement. This is explained by the uniform distributed heat on the fiber making a better heat transfer.

Another way to reduce the temperature of POFs is to use an absorbing material, such as SOFs. The SOFs bundle was introduced before the POFs to reduce the heat problem. The SOFs are more resistant to heat, can operate at temperatures of up to 1000°C, and have a very high thermal shock resistance. The SOF will absorb and dissipate some of the heat to protect the POFs. Figure 10 shows the temperature of the POFs bundle when combining the filter, the plano-concave lens with the SOFs. The maximum temperature was reduced to 56.25°C and the average temperature is 51°C. The temperature difference is also reduced due to material absorption. The system with a stable temperature at the matching position also reduces elasticity to improve coupling efficiency.

This temperature is safe for POFs bundles at an ambient temperature of 35°C. If the ambient temperature is higher a solution is needed to reduce the temperature on the POFs bundle and this can be achieved by reducing the system concentration ratio or increase the volume of the SOFs bundle. The solution depends on the actual conditions to achieve the best effect.

4. Conclusion

In this study, the experimental temperature analysis of a daylighting system using POFs has been presented. It has been observed that the temperature of POFs when they are combined with a secondary optical element is lowered by 79% compared to a POFs without an SOE under the same input conditions. The tempeature was able to reduced futher by inserting a UV-IR filter and a small size silica optical fiber bundle in front of POFs bundle. The maximum temperature of the focused sun-light on POFs with our designed components was measured to 56 °C, while the temperature on POFs without any components was 266 °C. Since the POF’s maximum operating temperature is 70 °C, our method is a promising candidate for the temperature control of practical POFbased daylighting systems. The temperature can be decreased further by increasing the length of the SOF bundle or by adding heat sinks for the SOF bundle.