In this age of technology and automation, energy demand is gradually increasing to meet day-to-day needs. Satisfying this energy demand without degrading the environment is a critical concern of the scientific community^[1-2]. Sources of alternative or "free" energy, such as the integration of solar photovoltaic (PV) systems within the building envelope or the usage of modular equipment such as wireless sensor networks, have attracted increased attention as they provide new options for reducing fossil fuel consumption and greenhouse gas emissions^[3-5]. This concept has tremendous potential in the development of next-generation self-powered devices, such as intelligent sensors, which communicate wirelessly, thereby cutting down maintenance cost and ensuring flexibility of building management systems without requiring the relocation of pre-existing infrastructure^[6].

A solar system can be integrated within a building in several ways. Some solar systems are added to the building after construction, whereas some are added to replace building elements, thereby serving multiple functions. In addition to outdoor solar energy collection, another promising alternative is indoor energy harvesting. Indoor energy harvesting systems utilize available energy, i.e., light, from the lamps inside buildings^[6-7]. In buildings, the illumination system accounts for approximately 9% of electricity use in residential houses and 40% of electricity use in commercial settings^[8]. In energy harvesting applications, PV devices receive lower levels of illumination (10-10000 times less than outdoor direct sunlight exposure) at different indoor orientations and tilt angles compared with optimized outdoor conditions^[2]. In a typical office environment with fluorescent lighting, approximately0.1 W/m² can be produced using a crystalline silicone (c-Si) PV module, which translates to 0.5 W/m² of interior lighting available for energy harvesting using a PV conversion efficiency of 20%^[7]. While the actual amount of interior light will vary from building to building, this is still a substantial amount of energy that can be harvested.

The study of indoor energy harvesting is important because the electrical illumination spectra (typically from fluorescent lighting or halogen lamps) is significantly different from outdoor conditions under standard test conditions (STCs) (i.e., 1000 W/m² irradiation, an air mass (AM) spectral coefficient of 1.5 G, and temperature of 25 ℃)^{[2, 6]}. As different lamp types utilize different methods of illumination, the spectrum content of the emitted light varies according to the type of lamp. Fluorescent and halogen lamps generally have more light energy concentrated in the longer wavelength region (visible to infrared) and significantly less light energy distributed in the shorter wavelength region.

Devices that are powered by energy harvesting systems will not operate if the selected solar modules are unable to harvest sufficient energy^[9]. One of the solutions to this is to concentrate the input light source onto the receiving area of a PV module such that the PV module can convert indoor irradiation into electrical energy optimally. The performance baseline for comparison is a stationary solar module, which operates at a fixed orientation. These systems typically display low specific efficiency for low balance system and minimal operational and maintenance cost^[10]. Another attractive option for improving the specific efficiency of a PV system is the use of a solar concentrator system, where optical concentrator elements concentrate on input irradiation and guide the isolated light onto a target solar module. Optical concentration offers an attractive approach in cost reduction as it replaces expensive solar cell semiconductors with less costly optical elements. Low-concentration methods generally increase the insolation by other expressions and the concentrators that are utilized include compound parabolic concentrators (CPC), V-through concentrators, and flat planar concentrators. Flat planar reflectors or concentrators comprise reflecting surfaces (i.e., mirrors) that increase the solar collection area and increase the energy yield of a solar PV system. Mirror reflector systems have the advantage of being inexpensive compared with V-trough and parabolic reflectors. In fact, flat planar reflectors are widely utilized in both thermal and photovoltaic solar systems due to their simple geometry and low-cost implementation^[11]. By introducing flat planar reflectors, the high installation cost of conventional PV systems can be reduced.

This study primarily aims to modify the PV system by adding a reflector element and to integrate solar collectors into lighting systems as well as optimize the distance between the solar module and the mirror. An indoor PV testing station is equipped with a halogen lamp as the illumination source, which is one of the most-commonly used light source in commercial buildings. Subsequently, a systematic study is conducted on the influence of distance between the solar panel and the reflector for light harvesting in a controlled environment. For experimental setup, the energy is harvested with a polycrystalline PV device and channeled to power a light bulb.

First, the apparatus was assembled, as shown in Fig. 1. A solar module and a light source were placed on the testing table. A light bulb was connected to the solar panel via crocodile clips to act as the load. The arrangement of all equipment was then placed at fixed distance and position to achieve constant output data, and the distances between the equipment was carefully measured and aligned.

Fig.1

Fig.1 Experimental setup

The light source was switched on until it was stabilized inside the opaque enclosure, and the initial voltage of the solar module was measured and recorded. A reflective mirror (12.7 cm× 17.3 cm) was placed between the light source and solar module. The reflective mirror was inclined with an 11° tilt angle to reflect irradiation from the light source and focus the image onto the solar module. The distance x between the mirror and solar module was initially set at 10.0 cm, and the voltage and current values were measured by using a multimeter. x was then adjusted to 15.8, 20.0, 25.0, and 30.0 cm. The output voltage and current values were recorded to calculate the output power density of the system. Changes in brightness of the bulb were observed throughout the experiment.

An opaque enclosure (Fig. 1(b)) was prepared to protect the experiment setup and shield the measurements from the influence of ambient light. This minimized disturbances from the surrounding environment, such as the irradiation of light source other than the experimental light source (e.g., fluorescent lights and daylight in the room).

Solar cells are solid state devices that convert light energy directly to electrical energy without the use of moving parts^[5]. When PV modules are exposed to illumination, they behave similar to a current source with a voltage limiter. They are characterized by three main parameters: a maximum power point (MPP), an open circuit voltage (V_oc), and a short circuit current (I_sc). In this study, the polycrystalline photovoltaic module, a type of c-Si, was selected as the key element in the light energy harvesting apparatus as it can absorb light over a wide range of wavelengths, including the infrared region. These modules are readily available and deliver power conversion efficiency of approximately 20%^[6].

The distance x was defined as the length between the mirror and the solar module. This parameter plays an essential role in the optimization process as it affects the output voltage and the brightness of the bulb. As the distance between mirror and solar panel increases, the output voltage decreases. This is due to lower levels of irradiation that reach the solar panel via the reflective mirror. Hence, the manipulation of distance indicated that at 10.0 cm the solar panel provided the highest output voltage and the brightest LED bulb illumination compared with those observed at other distances. Furthermore, the mirror's surface area played an important role in receiving light from the source. Previous research has reported that the concentrator's surface area must be larger than the solar panel to ensure the reflected light covers the entire PV module uniformly^[12]. Thus, a mirror of 219.7 cm² was selected to maximize the efficiency of the smaller solar panel with a measured area of 87.0 cm².

A multilayer filter in the form of white paper was used together with the light source to adjust the luminance level and to achieve the desired illumination. The level of irradiation decreased when paper layers were added onto the light source. The luminance level reduced as the number of papers increased, which in turn inversely affected the brightness of the bulb. Four pieces of paper yielded maximum effect, in which the bulb did not light up in the absence of a mirror. The light bulb lit up again in the presence of a mirror, whereas the four pieces of paper remained in front of the light source.

Table 1 demonstrates the effect of the distance between the solar module and mirror (x) on voltage and current generation. Without the mirror, only 2.12 V was generated, and it was insufficient to power the light bulb. When the mirror was located at a distance shorter that 10 cm, low voltage values were obtained and the bulb remained off. This was due to a shadowing effect on the solar module as it was blocked from the experimental light source, which was located at 10 cm above the reference point. When x=10 cm, the bulb lit up brightly, meanwhile 2.34 V and 8.0 μ A were recorded at this distance. This indicated that the mirror worked as a concentrator to focus the irradiation from the light source and reflected it onto the solar module. Thus, approximately 10% in incremental voltage was recorded, which was in agreement with the findings reported by Refs.[13-14], where employing concentrator will improve the solar panel performance.

表 1

Table 1 Measured values under controlled indoor conditions x(cm) Voltage Voc(V) Current Isc(μA) Bulbobservation Without mirror 2.12 - Off 10.0 2.34 8.0 Bright 15.8 2.21 8.0 Dim 20.0 2.18 7.0 Dim 25.0 2.15 7.0 Dim 30.0 2.13 6.0 Off Note: x indicates the distance from solar module to mirror.

Table 1 Measured values under controlled indoor conditions

However, when x>10 cm, not all irradiation emitting from the light source was captured and reflected by the mirror to the solar module. Hence, as x increased from 10.0 to 25.0 cm, the bulb was dimly lit and the light was off when x=30.0 cm.

Table 2 tabulates a comparison of commonly used low-concentration reflector system performance with the aim to improve the voltage of solar panels and ultimately yield higher output power. From the table, the CPC displayed the highest increment in voltage compared with other type of concentrators. The voltage increased from 3.85 to 4.85 V^[15]. However, the design was not self-sustained. According to the design concept, two DC motors with power rating of 6 and 40 W respectively were required to rotate the CPC and base. The maximum power generated by the unit was 1.141 W at noon.

表 2

Table 2 Comparison of low-concentration reflector system performance Type of reflector system Performance improvement Reference Compound parabolic concentrator (CPC) 26% increment in voltage Without CPC: 3.85 V at 12 pm With CPC: 4.85 V at 12 pm [15] Aluminum foil 2.4% increment in voltage Without aluminium foil: approx.20.5 V With aluminium foil: approx.21 V [16] Silvered glass plane mirror (SGP mirror) 4.8 % increment in voltage Without SGP mirror: approx.20.5 V With SGP mirror : approx.21.5 V [16] Mirror 6%-10% increment in voltage Without mirror: 0.515-0.510 V With mirror: 0.520-0.530 V [17] Mirror 10% increment in voltage Without mirror: 2.12 V With mirror: 2.34 V This study

Table 2 Comparison of low-concentration reflector system performance

Khan et al.^[16] adopted a relatively simple design, where two pieces of reflectors were attached to the solar panels at a desired angle. Nevertheless, the performance was not satisfactory as it showed lower than 5% improvement in voltage. A previous study in Ref.[17] similarly used a mirror to improve the voltage of a solar module. A maximum of 10% increase in voltage was reported in both studies. This suggests that mirrors are a viable and economical optical element capable of boosting voltage, which in turn enhances the performance of the solar module. However, the results in Ref.[17] were reportedly affected by outdoor weather. Thus, they may not be useful for application at a different location under different conditions.

This study considered the variability of sunlight. Therefore, a lamp was selected as the light source. The proposed experimental design of the solar module and mirror configuration described herein can be installed on building structures near artificial light sources in shopping complexes, schools, and commercial buildings, where the lights are turned on for a long period of time. Fig. 2 proposes a simple schematic of a photovoltaic reflector system. As presented in the figure, the minor addition required to retrofit a photovoltaic reflector system in a room for indoor energy harvesting will not disrupt activity in the room. Mirrors have been suggested to allow natural light redirection which improves light penetration and distribution within a building that can offer energy cost savings and improved user comfortability^[18], and can be implemented alongside an indoor energy harvesting module. However, the visual and aesthetic impact of the addition of multiple mirror and solar panel modules on the interior design of building have not been assessed and it should be explored in the future.

Fig.2

Fig.2 Potential application of a photovoltaic reflector system for indoor energy harvesting

In this example, a slim solar module strip can be attached to the wall, immediately above a window. A reflective mirror is placed between the light source and solar module and inclined to reflect irradiation from the light source onto the PV module. The position of the reflective mirror is calibrated according to the methodology presented in Section 2. By optimizing the position of the mirror, the system can harvest indoor irradiation from the nearby light source and provide electrical energy to the building or individual devices, which can lead to lower utility costs.

Ambient light in buildings, particularly commercial and office spaces, are a consistently available energy source, as daytime activities require the presence of comfortable illumination levels from indoor light source. This study identified the optimum distance between an indoor light source and a PV solar collector to generate maximum output power. A mirror was used as a concentrator and placed in between the indoor light source and the PV solar collector. Subsequently, the performance of the solar collector was improved by 10% increment in voltage. The optimized system can be used for indoor energy harvesting. From a practical perspective, the solar module and mirror can be installed in buildings to collect indoor light energy and convert it into electrical energy to decrease the utility costs by reducing overall building power consumption.