Photovoltaic Modules for Indoor Energy Harvesting

This paper presents the performance of indoor energy harvesting systems based on different photovoltaic modules (monocrystalline silicon, polycrystalline silicon, amorphous silicon and polymer) and artificial electric lighting sources (spot, incandescent, fluorescent and cool white flood LED). In this concern, it is clearly proved that, maximum output power densities to be harvested from the photovoltaic module depends mainly on the spectral responses of both the light source and the module material. Herein, and from the study, experimental work, results and analysis, it is clear that monocrystalline silicon is the optimum solution for all light sources, followed by polycrystalline silicon, whenever used with spot-and incandescent lamps. On the other hand, amorphous samples were proved to be lightly sensitive to fluorescent light and cool white flood LED. Finally, polymer samples were weakly responded whenever exposed to any of the investigated light sources.


INTRODUCTION
Energy-harvesting systems that convert ambient energy to usable electrical power have emerged as a potential alternative to wired and battery power [1]. Indoor photovoltaic harvesters will soon be playing a major function in supplying energy to low operation power sensors and wireless devices, especially if photovoltaic technology can be advanced and customized for these applications. This is due to the widespread accessibility of light as an energy source inside residential and commercial buildings [2]. Thus, we concluded that by placing the solar cells behind (covered part) the tube light, we can absorb all the photons emitted from the backside of tube light and some amount of electricity can be generated with the help of those emitted photons. Furthermore, that generated electrical energy can be stowed in rechargeable batteries or a power bank. This energy can then be used to power the tube light in the absence of a power supply. Hence, energy can be successfully harvested and recycled using artificial lights [3]. The growth of a light harvesting technology that transports remarkable output power in indoor and low-level light conditions has tremendous potential for application in the area of domotics and building management systems [4].
Of major significance to energy harvesting powered devices is that the solar harvester selected will harvest sufficient energy when deployed irrespective of the light source producing the illumination [5]. Many different solar cell technologies have been progressing and optimized for energy harvesting from either natural or artificial light; the output power of a solar cell is influenced by the spectral composition of the incident light. Energy harvesting powered devices will not operate if the solar cell cannot harvest adequate energy, which may happen if the solar cell is optimized for a different light source [6]. Thus this paper studies the difference in output power of solar modules under different artificial lighting sources and aims to aid the selection of devices deployed in buildings. The four different types of solar module (monocrystalline silicon, polycrystalline silicon, amorphous silicon and polymer) were selected because they represent the main types available. Four important artificial light sources were tested: spot lamp, incandescent lamp, fluorescent lamp and white flood LED, typically encountered within buildings, for various illumination levels.

Artificial light sources
Artificial sunlight is using the light sources to simulate sunlight which is naturally produced, where the unique characteristics of sunlight are needed. The artificial light sources could be included spot bulbs, incandescent bulbs, fluorescent tubes and light-emitting diodes (Fig. 1). They cannot reproduce the exact sunlight spectrum (300-1000 nm), however, there are available commercial bulbs that approximate this spectrum to a very satisfactory degree. Feature of the proposed artificial light sources could be summarized as follows:

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Spot lamps (incandescent light bulbs with reflector), which has a reflection surface that allows light to be focused as a flood or spot.

Utilized solar modules
Many different solar cell technologies have been developed and optimized for energy harvesting from either natural or electrical light sources. The spectral composition of the incident light mainly affects the output power density of solar cells [8]. The four different types of solar modules used in this work were selected owing to a range of the types used in supplying energy to low operation power sensors and wireless devices; they are made from different materials optimized for use either indoors or outdoors. The key details of the selected four solar modules are shown in Table (

Output power densities under different illumination sources
The output power density of each solar module is measured under the four different artificial light sources; spot, incandescent, fluorescent and cool white flood LED. The output power density versus the output current as the load resistance is varied of the monocrystalline silicon, polycrystalline silicon, amorphous silicon and polymer solar modules under four different illumination sources at 700 lux are shown in Fig. (4). Table (2) illustrates the maximum output power density of the proposed solar modules under the proposed artificial light sources at 700 lux. From which, it is clear that monocrystalline silicon is the optimum solution for all light sources, followed by polycrystalline, whenever used with spotand incandescent -lamps. On the other hand, amorphous samples were proved to be lightly sensitive to fluorescent light and cool white flood LED. Finally, polymer samples were weakly respond whenever exposed to any of the investigated light sources.
To summarize the results from the tests of the solar module types under the four artificial light illumination sources: the devices exhibit their highest power density under illumination from spot and incandescent sources. These results indicate that the wavelengths at which the different solar modules are sensitive are most prevalent in spot light and incandescent light. This is as expected as spot light and incandescent light emission covers a wider spectral range than fluorescent or LED sources which emit over narrower ranges.
Comparing the results for each solar module under the different illumination sources shows that the output power density of the E-19 (monocrystalline silicon) solar module falls significantly when the illumination is changed from spot to incandescent (47% reduction), fluorescent (95% reduction) and white light LED (95.6% reduction) as shown in Fig. (4a). This indicates that the key wavelengths from which the E-19 converts the most energy lie outside of the range of emission of the fluorescent and LED sources. Likewise for 15870 (polycrystalline silicon) solar module also shows a drop in power when changing to incandescent-(55% reduction), fluorescent-(97% reduction) and cool white flood LED-(97.4% reduction) sources (Fig. 4b).
Amorphous silicon is so common, the terms 'thin film' and 'amorphous' are often used interchangeably. As amorphous silicon is less efficient than monocrystalline and poly-crystalline, amorphous modules need to have a large surface area in order to generate the same power output [9]. The E-28 (amorphous silicon) solar module shows a drop in power when changing to incandescent, fluorescent and cool white flood LED sources, but with less marked reductions of 4.5%, 27% and 32%, respectively (Fig. 4c). As well, for the E-28 the reductions are less than for the E-19 and 15870 solar modules. However the output density from the tested E-28 is nearly similar under all four tested illumination sources encountered in buildings.
Finally the Power Plastic Type 20 (polymer) solar module has lower output power densities for each light source than all the other types tested. Changing to incandescent, fluorescent and cool white flood LED lighting sources causes corresponding reductions in level of 21%, 63% and 65% from that seen with the spot source as shown in Fig. (4d).   N 2 3 4 7 -3487  V o l u m e 1 4 N u m b e r 1  J o u r n a l o f A d v a n c e s i n P h y s i 1 4 N u m b e r 1  J o u r n a l o f A d v a n c e s i n P h y s i

CONCLUSIONS
In conclusion, the solar modules generally harvest less power under fluorescent and cool white flood LED than spot, incandescent artificial light sources. The detrimental effect of upgrading to higher efficiency illumination sources on the performance of solar energy harvesting devices has been evaluated. In general cases, most power is harvested by solar modules under spot illumination sources followed by incandescent illumination sources and then fluorescent and cool white flood LED. The large difference in output power density of the monocrystalline silicon, polycrystalline silicon solar modules and polymer module between spot, incandescent and fluorescent /cool white flood light LED sources could restrict operation to just spot lighting and incandescent lighting. Indoor energy harvesting devices based on this type of solar module will perform poorly if the lighting source is upgraded to fluorescent or cool white flood light LED. The amorphous-Si solar module tested (E-28) shows a nearly similar power density output under all four tested illumination sources encountered in buildings, therefore, for general use electrical lighting sources, a solar module based on amorphous silicon will perform satisfactorily under all lighting sources.