Low-Concentration Output Power Enhancement from Photovoltaic Panels *Ivan Adidharma Audwinto, Chan Hoy Yen, K. Sopian and Saleem H.Zaidi Solar Energy Research Institute(SERI) The National University of Malaysia (UKM) Universiti Kebangsaan Malaysia, 43600 UKM, Bangi Selangor, Malaysia l MALAYSIA IvanOu.1991@yahoo.com http://www.ukm.my Abstract: Burgeoning energy requirements are causing unsustainable growth of fossil-fuel based energy resources. Rapid depletion of these carbon-based resources coupled with green house emissions will lead to widespread economic disparities and unpredictable environmental disasters. Harnessing of sunlight for electricity generation offers the only economically and environmentally sustainable solution to global energy requirements. Silicon is almost 26 % of Earth, therefore, Si based PV technology is the most logical solution to our energy needs. Optical output of as- manufactured solar panels can be significantly enhanced using low sunlight concentration based on simple geometrical optics. These techniques are attractive due to their ruggedness inexpensive construction, lack of cooling requirements, and capability to increase output by ~ 40-50 %. Preliminary work on Si solar panels in 10 W p to 150 W p range has demonstrated enhanced output in ~ 30-50 % range for concentrations in 3x-5x range. A systematic study, based on several types of reflectors on mono-facial, bifacial, and back contact solar panels, is being carried out in order to evaluate their respective performance in terms of temperature coefficients, enhancement, and cost. This investigation will help identify the most economical, non-tracking approach for geometrical optics based output enhancements, and create opportunities for commercialization through advances in panel designs. Key words: PV, CPV, Low-Concentration technology, Concentrator system 1 INTRODUCTION The ability and infrastructure to generate, distribute, and effectively consume electricity determine a society s educational, social, and economical well being. Near universal access to electricity, through transmission grids connected to large scale power plants, has been largely responsible for economic prosperity and intellectual development of the industrialized world. This centralized electricity distribution model is expected to be replaced by small scale de-centralized micro-grids connected to an array of distributed energy generators. Continuing maturation of renewable energy generation and distribution technologies has been the driving force behind this paradigm shift. A critical feature of this paradigm shift lies in the concept of energy generation at the point of use. For third world countries, this paradigm shift offers unique opportunity to achieve economic progress in much of the same way as telecommunication has evolved from telephone lines to wireless. Crystalline silicon photovoltaic (PV) technology will play a leading role in this transition to small-scale distributed networks. Silicon forms almost 26 % of the Earth s crust, it is uniformly distributed across the globe, and is free from economics of resource-depletion. Silicon is also backbone of integrated circuit (IC) electronics; therefore, IC-based infrastructure advances will continue to be accessible to the PV industry. This paper investigates simple methods aimed at harvesting sunlight to extract additional power from industrially produced solar panels. Simple geometrical optics based schemes can be used to enhance panel output power. These methods fall into the category of concentration photovoltaic (CPV) systems. CPV systems fall into two categories: imaging and non-imaging.; all are based on geometrical optics considerations. Imaging optics is based on lenses; Fresnel lens with very high ISBN: 978-1-61804-175-3 125
concentrations is a good example of this [1]; imaging CPV systems is not the focus of this work. Several types of non-imaging, reflective CPV systems including parabolic [2], Fresnel mirrors [3], and heliostat types [4] have been investigated. In this work, Fresnel mirror method has been employed to investigate output enhancement using one through four mirrors. For such low concentration systems, the total sunlight incident on the panel surface is the sum of direct sunlight plus n* times the direct sunlight, where n is the number of mirrors. For such system, based on mirror angle and dimensions, sunlight tracking requirements can be relaxed [5]. The objective of this investigation is to evaluate the sizing effect in generating power enhancement in solar panels with output powers varying from ~ 6 W p to 150 W p using low-cost reflectors [6]. Such output enhancement methods are attractive for terrestrial applications where space considerations limit solar panel usage. These methods are also attractive for bifacial solar panels where suitably placed reflectors at the rear side of the panel can enhance output power 2 EXPERIMENTAL SETUP 2.1 Outdoor experiment (New Mexico) Solar panels in varying power range were used with two to four mirrors in Fresnel configuration (Fig. 1); for the experimental work reported here, no tracking was used. Experimental data was acquired under peak sunlight conditions. For mono-facial solar panels, reflectors are only in front of the panel (Fig. 1-a), for bifacial solar panels, reflectors can also be placed at the rear side of the panel to create additional enhancement (Fig. 1-b). Figures 1 (c) and (d) show pictures of two-mirror bifacial solar panel and four mirror mono-facial solar panel respectively. These low (< 10 W) panels were fabricated at our laboratory using 16 % efficient bifacial and monofacial solar cells. Higher power panels were purchased from industrial manufacturers. All panels were manufactured with mono-crystalline Si solar cells except the 50 W p panel, which was fabricated from polycrystalline Si solar cells. Simple Ag/glass mirrors were used as reflectors; size and angle of the mirrors (c) (d) Figure.1 Fresnel mirror based CPV configurations for mono-facial, bi-facial, two mirror (c), and four mirrors (d) concentration systems, were chosen to ensure 100 % overlap on the PV panel. Data acquisition program was based on electronics loads power supply. A LabVIEW computer interface was used to vary resistance across the panel and measure panel current and voltage. Current as a function of voltage was measured to calculate maximum panel power. Relevant panel parameters including output (P max ), maximum voltage (V m ), maximum current (I m ), open-circuit voltage (V oc ), short-circuit current (I sc ), series resistance (R series ), shunt resistance (R shunt ), and fill factor (FF) were measured. 2.2 Outdoor experiment (Malaysia) A similar setup was used in this experiment. Two PV panels based on polycrystalline silicon solar cell were used in a different ranging of power as shown in figure 2 below. The data taken without using mirror and by using only one mirror then lastly by using four mirrors attached at both sides of the panel. Figure.2 Polycrystalline silicon PV panel with different range of power 6.91W and 4.93W The law of light reflection playing its role in order to get an optimum amount of light fallen on top the panel surface, figure 3. ISBN: 978-1-61804-175-3 126
Figure.3 Diagram of maximum reflective light from the mirror to the surface of PV panel 3 EXPERIMENTAL RESULT 3.1 Outdoor experiment (New Mexico) Figures 4-5 show typical measurements from solar cell panels with reflectors varying from one to four. Most of the large solar panel data was collected for one mirror. Figure 6 summarizes the raw data plotted in Figs. 4-5 for two and one mirror reflector respectively. Power enhancement in ~ 20-27 % range is measured from a single reflector, in ~ 32-37 % range from two reflectors reflector, and ~ 45 % from four reflectors. No significant panel size-related effect has been observed. (c) (d) Figure.4 LIV data from 4-cell solar panel with no mirror, one mirror, two mirrors (c), and four mirrors (d). Figure.6 Pie-chart summarization of the LIV data in Figs. 2 & 3 for panel powers < 100 W p and for panel powers > 100 W p. Solar panels with less than 100 W p were tested with two reflectors and higher power panel with one reflector. 3.2 Outdoor experiment (Malaysia) Table 1 shows the dataa taken from two PV panels with using different quantity of mirror (0-4 mirrors) and figure 7 shows the graph of the increment power produced by the PV panels. Both of the panels were fabricated in our laboratory with using a silicon base polycrystalline solar cell after being cut in small pieces and reconnected it in parallel as shown in figure 2. The experiment was done in peak time while the sun emits a maximum solar irradiance. The smaller size panel which formed by using 22 small pieces of cell was able to produce 6.91W of power without using reflector. And after adding mirror one by one it managed to increase the power to 7.54W, 7.73W, 7.86W, and 9.13W respectively. As for a larger panel (24 cells) panel can only produce 4.93W of power to 8.073W after adding four mirrors. Even though it has a smaller power, it was able to get a better power increment after using the reflector as shown in figure 8. Table 1 Outdoor data collection of polycrystalline solar panel (22 cells and 24 cells) after using reflector from 0-40 mirrors Panel 22 cells (Polycrys stalline) 24 cells (Polycrystalline) Concentrator (mirrors) 0 1 2 3 4 0 1 2 3 4 P max 6.91 7.54 7.73 7.86 9.13 4.93 5.11 6.62 7.01 8.073 (c) (d) (e) (f) Figure.5 LIV data from ~ 50 W p panel without and with one mirror, ~ 100 W p solar panel without (c) and with one mirror(d), and ~ 150 W p solar panel without (e), and with one mirror (f) V m 8.32 8.08 8.56 8.56 8.49 9.22 9.36 9.25 8.81 9.51 I m 0.83 0.93 0.96 0.98 1.08 0.55 0.55 0.67 0.80 0.849 V oc 11.34 11.37 11.3 11.27 11.39 12.74 12.45 12.55 12.39 12.73 I sc 1.03 1.11 1.44 1.16 1.28 0.67 0.69 0.80 0.96 1.075 (FF) 59.29 59.84 60.1 60.22 62.55 57.44 59.57 61.37 59.30 58.99 ISBN: 978-1-61804-175-3 127
Pmax, W 10,00 8,00 6,00 4,00 2,00 0,00 6.91W No.Mirror power panel 4.93W power panel 1 2 3 4 5 1 2 6,91 7,54 7,729 7,86 9,13 0 3 4 4,93 5,11 6,62 7,01 8,073 Table 2 Air-cooling of Low CPV systems Panel Description Power with no Air Cooling (W p ) No mirror 6.88 One mirror 7.82 Two mirrors 8.32 Four mirrors 9.04 Power with Air Cooling (W p ) 4.2 Outdoor experiment (Malaysia) Relative Improvement (%) 7.40 7.6 4.86 4.86 9.12 9.61 10.00 10.62 Figure.7 Graph of power increment after adding more reflectors (0-4 mirrors) Figure.8 Pie chart summarization of the data in table 6 (percentage of power enhancement) of 22 cells panel, and 24 cells panel 4 ANALYSIS OF RESULT 4.1 Outdoor experiment (New Mexico) Solar panel output can be enhanced with simple Fresnel reflector type concentration systems. As sunlight concentration increases, the output enhancement is not linear. This is due to increased temperature of the panel due to concentrated sunlight illumination [7]. Therefore, solar panel cooling is required in order to realize maximum output. In simple experiments, a fan was used to flow air across the panel surface. Table-2 summarizes simple air cooling results for 10 W p panels with 0, 1, 2, and 4 reflectors. It appears that cooling is more effective at higher sunlight concentrations. Malaysian Meteorological Department reported that the characteristic climate features in Malaysia are uniform temperature, high humidity, and copious rainfall. It is extremely rare to have a full day with completely clear sky even during periods of severe drought. This rapid change in climate will affect the data reading of PV system. On the other hand, it is also rare to have a stretch of a few days with completely no sunshine and this is the advantage of using PV system technology as to generate electricity from sunlight energy. Unlike the experiment in New Mexico which has a stable result in power increment. In Malaysia, with having a rapid changingg weather it is hard to get a constant and continuous stable data (Figure 9). Even on a clear sky, a sudden coming cloud can t be avoided. The cloud will block the direct incident light to the panel and reflector and reduce the amount of light absorbed. And also, reflector is hardly reflecting the diffuse light because diffuse light cannot focus light that is not perpendicular to the reflector. This will cause an uneven distribution of reflected light on the PV panel s surface. Figure.9 The percentage of power enhancement in respect with the number of reflector used for 22cells and 24 cells polycrystalline PV panel with having a non linear increment in power under Malaysia climate condition during peak time ISBN: 978-1-61804-175-3 128
5 CONCLUSION A flat plat PV concentrator system using single crystalline silicon solar cell with different rating of PV module (10W,50W,70W,100W, and 150W) were being tested under a clear sky day to achieve the maximum power enhancement percentage at New Mexico. And another experiment was conducted in Malaysia with using two different sizes of polycrystalline solar PV panels. In this experiment we built a reflector with a larger area than the area of the PV module. By using this low-concentrator technology we might be able to enhance the electrical power by more than 25%. But in the other condition, it might as well increase the temperature of the PV module which will reduce the open circuit voltage, V oc along with the reducing of the power output and fill factor. Base on the data taken from all the PV panel, after using two concentrator it was able to increase the power output of the system by ~35% while by using one concentrator it can enhance the power by ~25%. Looking at this result, it seems that a different size of PV module area doesn t affect much to the increment of the percentage power enhancement. This conclusion is simply for the area which has a stable weather and climate condition such as New Mexico. But for Malaysia which is rare to have a clear sky without any interruption of the sudden cloudy and rapid changing in humidity to get a constant data. Even under such of climate, we still can get good increment in power output by the PV system. Where the 22 cells was able to increase up to 32.13% of power after using four mirrors and the 24 cells was able to enhance 63.75% of power. References: [1] Stone K. Garboushian, V. Hayden, H. Field, 19 th European PVSEC, Paris, 2004. [2] Aril Rabl, Solar Energy 18, 93 (1975). [3] F.H. Klotz, Proceedings of the 16th European Photovoltaic Solar Energy Conference, pp. 2229 (2000). [4] D. J. Alpert, R. M. Houser, A. A. Heckes, W. W. Erdman, Solar Energy Materials 21, 131 (1990). [5] www.ceimat.es. [6] I. S. Hermenean, I. Visa1 A. Duta, and D. V. Diaconescu, International Conference on Renewable Energies and Power Quality (ICREPQ 10). [7] John A. Duffie, William A. Beckman in Solar Engineering of Thermal Process, John Wiley & Sons,(1990). ISBN: 978-1-61804-175-3 129