1 Introduction
It is a known fact that as much as one fifth of worldwide power consumption is allocated by lighting alone [
1]. This shows the relative importance of energy saving capabilities of lighting to the grid and sustainability in the long run. Lighting can be categorized into general lighting and specialized lighting [
2]. General lighting includes all of the personal and direct lighting requirements such as indoor and outdoor applications where the light is used for the direct and apparent need to provide illumination whereas specialized lighting refers to indirect applications such as led backlit systems for displays etc. For over 60 years, the most dominant and popular lighting system has been the fluorescent lamp. These lamps were known for their high energy saving capabilities, reduced heating effects and most importantly, their wide spectrum of applications in the industry. These devices work by emitting ultraviolet radiation which is produced by an electric field. The electric field causes high energy electrons to flow through a medium, preferably a mixture of argon and mercury. The constant bombardment of these light weight electrons with that of the heavier atoms of mercury and argon produces an excitation on an atomic level which releases light in the ultraviolet spectrum. Due to the apparent reason that ultraviolet is no use for illumination in the human visual spectrum, a coating of fluorescent material is lined along the tube, which emits the visible spectrum on contact with ultraviolet rays [
3]. Fluorescent lamps come in a variety of types specific to its application. Figure 1 illustrates the range of fluorescent lamps and their applications in the industry. Compact fluorescent lamps or CFLs, as the name says it, are used for applications where the size of the light is an impor-tant issue CFLs have grown an interest among general households for both their attractive spiral and oddly shaped designs and most importantly, their range of sizes. However, CFLs are still far off from the efficiency that is required for cheaper and effective power consumption levels. A highly efficient CFL bulb has the capability to convert about 63% of the total energy into ultraviolet rays. The overall efficiency is further reduced to a maximum of 28% due to losses in the conversion of ultraviolet to visible in the phosphor [
4]. Another drawback of CFLs is that it takes a while for it to reach full brightness or reach full efficiency. This makes them unfit for applications where light is needed right away. Also, their spectral distributions cannot be altered or varied according to specific requirements, in other words, control them as a dimmer [
5].
A more favorable lighting solution according to many experts is the light-emitting diodes. These devices are said to last up to 10 times as long as CFLs which translates to 10 to 15 years operation time. LEDs are also said to work better as dimmers and are generally harder to break, but these diodes can only propagate light in one direction and are, therefore, said to be better applicable for spot lights and similar applications. However, the recent LEDs address the issue of its one directional propagation with the use of diffusers to spread the light along an Omni-directional path [
6,
7]. LEDs are generally seen as the greener, more efficient and effective upgrade to the traditional CFL lighting system. Though they do not offer a highly significant energy saving capability in relative terms, they do, however, offer a high operation time and reduced effects to human health conditions due to ultraviolet exposure in the CFLs. They also offer better resistance to environmental conditions. The working principle behind the LED is also far direct than the CFL in general. They rely on a p-n junction semiconductor that produces visible light when connected to a forward biased circuit by moving the holes and electrons together. This causes a burst in energy that is emitted as photons. It is important to note that LEDs unlike CFLs do not emit polychromatic light. However, this is made possible by the combination of the three primary monochromatic colors emitted simultaneously which combines to form white LEDs [
8,
9]. One interesting characteristic of LEDs is the ease of controlling their spectral power distributions using regulated electric circuits. This feature allows the same color of an LED when compared to other sources of light, like CFLs, to have a much different spectral power distribution produced by metamerism [
10]. It is this feature that makes LED lights more vibrant and beautiful when compared to other sources. In terms of pure efficiency with regard to luminosity, CFLs are said to produce 90 lumens/watt as compared to 79 lm/W by the LEDs. However, recent innovations have led to increased efficiencies of more than 130 lm/W in LEDs [
11,
12]. But, due to the time dependence of CFL luminosity efficiency, it would take a while for it to reach its full efficiency of 90 lm/W.
2 Methodology and experimental setup
The main objective of this paper is to prove the energy saving capabilities of LEDs and CFLs on relative terms; however a number of other experiments were also conducted to widen the scope or coverage. The solar cell used was a standard DIP glue epoxy encapsulated PV solar cell with a mean efficiency of 15%, a maximum voltage of 3V and a maximum current of 0.33A. Initially, the aim of the experiment was to prove the variation of the intensity, distance and location of light entering the solar cells on the voltage and current induced in the cells.
Figure 2 (a) and (b) shows the general configuration of all experiments. The effect of variation of distance was investigated by positioning the solar cells directly below the light source and measuring the voltage and current produced and then varying the distance between the light source and the solar cells. The theory behind this is to prove that the relationship follows the inverse square law applied to the variation of photonic energy with distance. Therefore, the energy received should be inversely proportional to the square of the distance between the source and the point of interest [
13]. The effect of the light intensity of solar cells was investigated by covering specific areas of the solar cells using an opaque material such as cardboard. This makes it possible to control the intensity of light that enters the cells.
A set of configuration has been chosen for this experiment. At 1st configuration, the solar panel was 0% covered. The solar panel was 25% covered at 2nd configuration. At 3rd and 4th configuration, the solar panel was 50% covered and 75% covered respectively. For each configuration, the resulting current and voltage produced by the solar panel were measured. This should exhibit a direct proportionality between intensity of light and power generated [
14].The effect of the location of light entering the PV was investigated using the same cardboard sheet to cover all but a specific location in the cells. In this case, the resulting current and voltage produced by the solar panel were measured at the center of the solar panel by making a hole at the cardboard through which light can pass. Current and voltage produced by the solar panel were also measured on the side where lines end, side with long lines and the corner of the solar cells by covering rest of the part of the solar panel using cardboard. For all experiments, namely, the effect of intensity, the effect of distance and the effect of location, a set of different light sources were used.
Table 1 lists the different light sources used for both sets of experiments and Fig. 2 (c) illustrates the full experimental setup. The above mentioned experiments are emphasized mainly on the solar cells itself. However, this data can also be used to determine the characteristics specific to the light sources alone by using the output power of the solar cell to determine the relative efficiencies of LEDs and CFLs.
One may ask why solar cells should be used for this when there are other more specific tools for this assessment. The reason for this is that the use of solar cells enhances or qualifies these experiments for future research on indoor power harvesting via solar panels. These experiments may also serve as an initial benchmark for future improvements on solar cell limitations and provide a more integrated view of characteristics specific to the solar cells, the light source and their combined correlations.
This can be done by using the set of data for the 0% covered condition of the experiment for intensity of light. By observing the data, the light source that emits the maximum photonic energy should theoretically produce a corresponding maximum power in the solar cells. By measuring this power in terms of current and voltage relative to the power generated via another light source, for example, the CFL light source, a sense of which has a better efficiency can be determined using the input power together with the output power to each light source. It should now be theoretically possible to determine which light source has better efficiency while both light sources possess equal wattage.
3 Results and comparison
The effect of distance upon the power generated has a clear indication of reduction in overall power with increase in distance. This shows clear evidence of the inverse square nature of dependence as illustrated in Figs. 3 and 4. The experiment used both voltage and current obtained via a multi-meter connected to the circuit to determine the power generated. Figure 4 depicts the variation of cell voltage and cell current. The overall power generated may be optimized to the maximum power point (MPP) or specifically, the values of current and voltage that yield the maximum power output. For ease of operation, a maximum power point tracker (MPPT) may be used to optimize the power output. Typically, this can optimize power gains by 10% to 15%. Among the light sources, light source 3 (incandescent) shows the ability to stand out in terms of power generation varied with distance. According to Lumen Coalition, a standard incandescent light bulb such as the one used in this experiment, with a wattage of 100 watts produces about 1600 lumens of light in the visible spectrum [
15]. This luminosity is higher than that of the other light sources (refer to Table 1). This trend is highly significant in the results obtained in this paper where the other light sources are compared to the voltage generated corresponding to each light source (see Fig. 3).
The effect of intensity of light upon the power generated shows apparent evidence of increasing power with decreasing shading via the cardboard sheet. The results, as displayed in Figs. 3 to 6, show that the 0% covered state generates the maximum power, with the minimum power corresponding to the 75% covered state. PV cells generally face the problem of shading of solar energy which usually leads to inconsistent power generation and low system efficiency [
16]. This may be overcome with the use of Fresnel lenses which use a prismatic structure that concentrates light into the PV cell. Fresnel lenses are said to be one of the best choices due to its compensations such as reduced volume, reduced weight and also, cheaper production costs enabling it to effectively increase energy density in the solar energy field [
17]. Furthermore, it is also observed that light source three (incandescent) generated the highest power in the PV cells for all shading states. With reference to Table 1, it is observed that an increase in luminosity improves the power generation in the PV cells, with light source 3 (incandescent) exhibiting the highest power generation. This finding is relevant to other research conducted on PV cells as these cells usually exhibit a high sensitivity to incandescent light sources regardless of their high inefficiencies. A typical incandescent light bulb only uses 10% of its total wattage where the remaining 90% is dissipated to the surrounding as heat energy [
18,
19].
The effect of location of the solar cell on power generation, according to Figs. 7 to 10, show a clear trend of consistent changes of power generated among all light sources with different location changes. This shows that the results are significant and accurate. Furthermore, it is observed that the largest power is generated when the PV cell is placed directly below the area that corresponds to the center and the sides with long lines (Figs. 7 and 9). This information provides useful insight into which areas of a PV cell to focus more light on, or in other words, highly sensitive areas of the cell. The lowest power generated is found to be at the sides where lines end corresponding to Fig. 8 and the corner section of the PV cell as illustrated in Fig. 10. This may be caused by the non-uniformity in the PV, the blocking of light due to chamfered corners, and etc. The comparison of the LED and CFL was done by taking both light sources 5 and 6 corresponding to the LED followed by the CFL light source of equal wattage. The objective of this comparison is to determine a relative energy saving capability of the LED to the CFL light source. This research did not take into account the shape, diameter and incandescent equivalent wattage and its influence on the light source. Therefore, it was assumed that they would not contribute a significant change. The luminosity however, has the ability to produce a higher PV cell power generation as observed through the experiment for the effect of light intensity. Additionally, overheating of the PV cell was assumed to cause no change to the overall efficiency of the cell due to short time of operation of the experiments. It was also assumed that the wattage of each light source, namely, light sources 5 and 6 to be the actual input power to the light source and that no additional power was drawn from the power source. The output power was taken as the maximum current/voltage generated in PV cells the instant the bulb was switched on. The efficiency of the solar cell was assumed to be 100% in order to cater for a more significant result although this result would not invalidate the research due to the fact that only the relative power efficiencies of both CFL and LED were actually taken into account in order to conclude based on which is better. Table 2 lists the efficiencies of both light sources with its input power assumed to be constant at an upper limit of 12W. The minimum efficiency of the PV was taken as 15% with its maximum efficiency set at 100%. This was done to increase a constant factor among both efficiencies to provide a clear distinction. However, only the ratio of both efficiencies was used to determine the relative efficiencies. As seen via Table 2, the LED light source is nearly 3.7 times as efficient as the CFL light source. This registers higher energy saving capabilities than our literature as a typical LED is said to be nearly 1.2 times more efficient in power consumption than a CFL of the same wattage [
20].
4 Conclusions
The results obtained in this paper are significant and in line with theoretical models and relationships. The first experiment, namely the effect of distance upon the power generation follows closely to the inverse square relationship. It is also observed that the luminosity of light shows a salient trend in the voltage plot against the distance (see Fig. 3). According to this plot, the highest power generation shows a strong dependence on the luminosity of light or the bulb used with higher luminosity shifting the line upwards. The second experiment, namely, the effect of intensity (Figs. 3 to 6) also shows a strong dependence on the relationship that the intensity of light is directly proportional to the power generated, and therefore, it is proposed in this paper that the ability of light to fall perpendicular to the PV cell would generate the highest power optimized by the intensity of light. Solar tracking or the use of Fresnel lenses may optimize these phenomena [
17,
21]. The third experiment, the effect of location upon power generation (Figs. 7 to 10) shows that PV cells do not have uniform properties throughout and that there are sections that are more capable of capturing light. This allows for more focus on these specific locations in the PV cell so that the maximum efficiency may be obtained. The energy saving capability of LED vs. CFL is emphasized in the last section of this paper. LEDs are observed to produce a much higher level of PV power generation. Based purely upon the wattage, an LED was observed to produce a power generation 3.7 times that of a CFL, given all other factors are kept constant. Therefore, it is concluded that a CFL is 3.7 times less power efficient than an LED based light source of equal wattage.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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