Bi-annual Sun Tracking for Solar PV Module Support Structure: Study and Implementation

Transcription

1 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, Bi-annual Sun Tracking for Solar PV Module Support Structure: Study and Implementation Prabodh Bajpai, Member IEEE, Vaishalee Dash, N.K. Kishore, Senior Member IEEE Department of Electrical Engineering Indian Institute of Technology Kharagpur, Kharagpur West Bengal, INDIA Abstract The solar photovoltaic (PV) is considered to be the one of the most promising energy source in many applications, due to its safety and high reliability. But the initial high cost of the PV arrays and the large area necessitate their optimum use. One of the solutions that optimize the array performance is to adjust the array tilt such that the incident insolation on the array is maximized. The best results are obtained when the movement of the sun is tracked both on a daily and seasonal basis. However as this scheme is costly and involves complex control, alternative solutions have been sought for. In the most simple way, the PV panels are inclined at the latitude angle of the site, throughout the year. However it is observed that a decrease of 15 in the tilt angle in summer and an increase of 15 in winter eances the solar energy received on the panels significantly. This work includes comparative study of sun tracking methods used to increase solar energy collection in a period of time. Optimal tilt angles and corresponding monthly solar insolation values have been compared among fixed tilt angle, bi-annual tracking (sun tracking twice a year) and single axis tracking. Further, implementation of bi-annual sun tracking mechanism in IIT Kharagpur campus through a variable tilt angle module support structure has been discussed. I. INTRODUCTION The term solar photovoltaic means the direct conversion of sunlight into electrical energy using solar PV cells. Such solar PV cells are usually manufactured from thin films or wafers of semiconductor materials like silicon, gallium arsenide, cadmium telluride or copper indium diselenide. They are capable of converting incident solar energy into direct current, with efficiencies varying from 3 to 31%, depending on the technology, the sunlight spectrum, temperature, design, and the material of the solar cell. The low conversion efficiency of these materials produces a small voltage in these solar cells. Hence these PV cells are connected in series to form PV modules which can generate higher voltages. The modules are then connected in the required series-parallel combination to form PV array according to the requirement of the load. Some attractive features of these solar PV arrays is the nonexistence of movable parts, very slow degradation of the sealed solar cells, flexibility in the association of modules (from a few watts to megawatts), and the extreme simplicity of its use and maintenance. The power obtained from PV is pollution free and there is no waste product. These PV modules have a large lifetime and some may last more than 30 years. However, solar energy is inconsistent and is available only during the day time as long This work was supported in part by the research project VDA-I under grant by Vodafone Essar-IIT Centre of Excellence in Telecommunications (VEICET), IIT Kharagpur. as the sun shines. Thus for utilizing this power during the rest of the day the energy must be stored in storage devices like batteries, ultacapacitors and fuel cells. This provision of energy storage makes PV power based system more reliable and can be used in a variety of applications like telecommunications, satellites, water pumping and in many remote areas where power supply from grid is neither feasible nor economical [1][2]. For proper installation of a PV array, it is important to understand the site characteristics and tilt requirements for the array. Research in this area includes determining the most optimal tilt angles for installing a PV array. The tilt angles are calculated based on maximizing the total insolation received on the panels [3]. The most optimal tilt angle varies throughout the year. However, it is observed that a considerable amount of solar energy is received when the PV panels are tilted at an angle equal to the latitude angle of a site and an increase in the tilt angle by 15 in winter months and a decrease by 15 in summer months boosts the amount of solar energy received on the panels [3]. Experiments show that output power of the PV array changes at different times of the day for different tilt angles. It is observed that the maximum amount of solar energy is received when the panels are oriented towards the south (for PV panels in the Northern Hemisphere) and vice versa [4]. This paper compares the solar energy received on a south facing PV panel for a fixed tilt angle configuration with biannual tracking and single axis tracking schemes. Bi-annual sun tracking mechanism is studied and analyzed in this work before implementation. The bi-annual tracking scheme for the PV module support structure is installed in the laboratory. Section II describes the optimal tilt angle variation over a year for maximum solar insolation at Kolkata 22 34' 10" N, 88 22' 10" E. In section III, single axis, two axis and bi-annual sun tracking mechanisms are explained. Analytical study and results of bi-annual sun tracking method is presented in section IV and V respectively. Implementation of this mechanism is discussed in section VI. Section VII concludes the work. II. OPTIMAL TILT ANGLE FOR SOLAR PV The spectral distribution of the sun closely matches that for a blackbody at 5800 K. The solar spectrum corresponds to wavelengths within the ultraviolet UV region (7%), visible region (47%), and infrared IR region (46%). However the total solar flux striking a collector is a combination of direct-beam

2 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, radiation that passes in a straight line through the atmosphere to the collector, diffuse radiation that is scattered by molecules and aerosols in the atmosphere, and reflected radiation that is bounced off the ground or other surface in front of the collector. These three components constitute the total solar insolation [5]. As a preliminary part of the study, the most optimal tilt angles for installing the PV panels are calculated for all the months in a year by taking the mean monthly solar insolation values for Kolkata [6] located at N, which is close to Kharagpur located at N. Therefore the optimal tilt angle values are almost equal for Kolkata and Kharagpur. The most optimal value of the tilt angle is expressed as [3] sun shines directly over the Tropic of Capricorn in the Southern hemisphere. Kolkata being in the Northern hemisphere receives reduced insolation and hence the panels have to be more inclined from the horizon to receive the maximum insolation β opt = tan 1 2S 2S + D cot α ρ R (1) where S is the mean direct insolation on a horizontal plane, R is the mean global insolation on a horizontal plane, D is the mean diffuse insolation on a horizontal plane and ρ is the ground reflectance ratio ( 0.2 ). The altitude angle is represented as α in Fig.1 and is expressed as ο α = 90 φ ± δ (2) whereφ is the latitude angle of the site i.e. Kolkata (22.56 N) and δ is the declination angle expressed as 360 δ = ο sin ( n 80) (3) 365 where n represents the number of days from the beginning of the year.i.e. n=33 for 2 nd February Fig.1. A collector receiving the solar radiation The values of optimal tilt angles were calculated using (1) to (3) and Fig. 2 shows the variation of the optimal tilt angles throughout the year. The value of the most optimal tilt angle is more during winter months as during this period of time the Fig. 2.Variation of optimal tilt angles for the different months in a year III. SUN TRACKING The sun's position in the sky varies both with the seasons as well as time of the day as the earth moves around the sun. The need for tracking arises as all solar powered equipments work best when pointed at or near the sun. An efficient tracking mechanism produces more power over a longer time than a stationary array with the same number of modules. Manual or automatic adjustment of tilt angles can be done depending on the tracking scheme employed. The first two types of tracking schemes discussed below track the sun by a robust control mechanism using motors and gears [7]. The last tracking scheme is a manual adjustment of the tilt angle of the PV module support structure. This work involves a comprehensive study and analysis of the bi-annual sun tracking method and also its implementation. A. Single- axis Sun Tracking (SST) scheme Single-axis tracking for PV modules is generally implemented with a mount having a motorized tracking mechanism that rotates the PV array from east to west at a regular interval throughout the sunshine period. It provides about % increase in the solar energy received by the panels as compared to the fixed PV panels. When the tilt angle of the mount is set equal to the local latitude (called a polar mount), the tilt angle becomes an optimum one for annual solar energy collection. If such a polar mount rotates about its axis at the same rate as the earth rotates, i.e. 15 /h, then the centerline of the module always faces directly into the sun. The predicted value of clear-sky beam radiation at the earth s surface is [1] km I B = Ae (4) where A is apparent extraterrestrial flux, k is a dimensionless factor called optical depth and m is air mass ratio.

3 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, A = sin ( n 275) (5) k = sin ( n 100 ) (6) m = (7) sin α where α is the altitude angle of the sun and n is the day number. The total insolation on the module surface is the summation of the direct and diffuse component. The reflected component of the radiation is ignored in the present study as it constitutes a minor fraction of the total insolation received by the panels. The direct and diffuse component of insolation are expressed in (8) and (9) respectively and shown in Fig.3. I BC = I B cosθ (8) where θ is the incidence angle between the sun s rays and normal to the collector. However in a polar mount θ is equal to solar declination angleδ. ο 1 + cos(90 α + δ ) I DC = CI B (9) 2 where I BC is the direct-beam radiation on the collector and I DC is the diffuse radiation on the collector. δ is the solar declination angle and α is the altitude angle. B. Two axis Sun Tracking (TST) scheme Two axis sun tracking refers to tracking the sun both in azimuth and altitude angle such that the panels are always directly pointing at the sun. This scheme provides an additional 5-10 % increase in solar energy as compared to single axis tracking. However the mechanism involved is complex and increases the cost of installation of the panels. The dual axis tracking is done by two linear actuator motors, which aim the sun within one degree of accuracy. During the day, it tracks the sun from east to west. At night it turns east to position itself for the next morning sun. [8] Fig.3. Solar radiation striking a collector is a combination of direct beam, I BC, diffuse beam, I DC, and reflected beam, I RC. C. Bi-annual sun tracking (BST) scheme Bi-annual tracking is a method of tracking the movement of the sun twice in a year. The tilt angle of the PV panels is changed two times in a year to maximize the solar energy received by them. During the summer months the tilt angle is kept at latitude angle-15 and in winter months it is kept at latitude angle+15. The advantage of this method is that it is simple and involves change in the tilt angle only two times in a year. Moreover, it does not need any complex control mechanism, or motorized and maintenance intensive system for tracking the sun. The two major components of the solar insolation received by the panels is the direct and diffuse radiation. These components roughly give an idea of the total insolation striking on the PV panels. The reflected component of the radiation is ignored in the present study as it constitutes a minor fraction of the total insolation received by the panels. Considering only the direct-beam and diffuse radiation, the total insolation striking the collector surface as in [1] is I = I BC + I DC (10) where I BC is the direct-beam radiation on the collector and I DC is the diffuse radiation on the collector. The direct-beam radiation on the collector as shown in Fig.3 is expressed as I BC = I B cosθ (11) where, cos θ = cos α cos (φ S φ C ) sin β + sin α cos β (12) where θ is the incidence angle between the sun s rays and normal to the collector, α is the altitude angle of the sun, φ S is the solar azimuth angle, φ C is the collector azimuth angle, β is the collector tilt angle and I B is the clear-sky beam radiation at the earth s surface as shown in Fig.1 and Fig.3. The diffuse radiation on the collector is expressed as 1 + cos β I DC = CI B (13) 2 where C is the sky diffuse factor and expressed as 360 (14) C = sin ( n 100 ) 365 The expressions discussed above help in determining the total insolation received on a panel at a particular tilt angle. IV. ANALYTICAL STUDY FOR BI-ANNUAL SUN TRACKING Kharagpur lies on the latitude of ( N) and longitude of ( E), located in the West Midnapore district of the state of West Bengal. The latitude angle of the Kharagpur location is approximated to 22.5 for calculations and implementation in the proposed bi-annual sun tracking mechanism. All PV modules are considered to be south facing. The data shown in Table: I are the insolation values for a collector surface fixed at a tilt angle equal to the latitude of the location (Kharagpur) i.e and for two other tilt angles, i.e. latitude angle+15 (37.5 ) and latitude angle-15 (7.5 ). The mean monthly insolation values at the different tilt angles are

4 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, calculated using (10) to (14). For single axis tracking, the mean monthly insolation values are calculated using (4) to (9). From Table:I it can be inferred that the solar energy on the collector surface is higher in winter months for a tilt angle equal to latitude+15 (37.5 ), and in summer months for a tilt angle equal to latitude -15 (7.5 ), when compared to the fixed tilt angle configuration equal to the latitude angle. However, the single axis tracking provides further higher values of insolation compared to all fixed tilt angle configurations. Therefore, by combining the values of insolation for the fixed tilt angle configuration equal to latitude angle, latitude angle+15 and latitude angle-15, higher solar insolation values can be achieved over a year as shown in Table: II. This can be implemented through a bi-annual sun tracking mechanism. TABLE I MONTHLY SOLAR INSOLATION AT DIFFERENT TILT ANGLE MONTHS MEAN MONTHLY TOTAL SOLAR INSOLATION (kwh/m 2 ) FIXED TILT ANGLE LATITUDE LATITUDE+15 LATITUDE-15 SINGLE AXIS TRACKING Jan Feb Mar Apr May Annual average Jun Jul Aug Sep Oct Nov Dec Annual average of solar insolation from fixed tilt configuration equal to latitude angle (I FTC ) = 4.94 kwh/m 2. Annual average of solar insolation from single axis sun tracking (I SST ) = 5.57 kwh/m 2. Annual average of solar insolation obtained from bi-annual sun tracking (I BST ) = 5.11 kwh/m 2. The annual average of the solar insolation obtained through bi-annual tracking is higher compared to the fixed tilt angle method. Comparing the fixed tilt and bi-annual tracking configuration, the percentage increase in the solar energy obtained is (I BST - I FTC )/ I FTC = ( )/4.94 = 3.5%. Similarly, the annual average of the solar insolation obtained through single axis tracking is higher compared to the fixed tilt angle method. Percentage increase in the solar energy obtained is (I SST - I FTC )/ I FTC = ( )/4.94 = 12.75%. TABLE II MONTHLY SOLAR INSOLATION VALUES IN BI-ANNUAL SUN TRACKING MONTHS MEAN MONTHLY TOTAL SOLAR INSOLATION (kwh/m 2 ) January 5.14 February 5.56 March 5.89 April 6.15 May 5.86 June 4.85 July 4.22 August 4.41 September 4.31 October 4.91 November 4.94 December 5.05 Annual average 5.11 V. ANALYSIS OF BI-ANNUAL SUN TRACKING The mean monthly total solar insolation values for a fixed tilt angle (22.5 ), latitude+15 (37.5 ) and latitude-15 (7.5 ) configuration of a collector is illustrated in Fig.4. The figure clearly indicates higher insolation values at latitude-15 during the summer months (April, May and June) as compared to fixed tilt angle configuration. Similarly, the insolation values are more at latitude+15 during the winter months (November, December and January) as compared to fixed tilt angle configuration. During the equinoxes, i.e. on 21 st March and 21 st September the insolation values are very close for all the three configurations. Fig.4. Comparison of mean monthly insolation values for fixed tilt angle, latitude+15 and latitude-15 configuration of a collector Latitude+15 Latitude-15 Latitude+15

5 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, Fig.5. Comparison of mean monthly insolation values for fixed tilt angle and single axis tracking of a collector Fig.6. Comparison of mean monthly insolation values for fixed tilt angle and bi-annual tracking of a collector the equinoxes the increase in solar energy is less compared to that during summer months (April, May and June) and winter months (November, December and January). The annual average of the solar insolation obtained through single-axis tracking is 12.75% more as compared to fixed tilt angle configuration. The mean monthly total solar insolation values for a fixed tilt angle configuration and bi-annual tracking is presented in Fig. 6. The solar insolation values obtained through bi-annual tracking is more than that in the fixed tilt angle configuration for all the months except during the spring equinox (March), where it is slightly less and during autumn equinox (September) where it has the same value as the fixed tilt angle configuration. A comparison of mean monthly total solar insolation values for fixed tilt angle and bi-annual tracking method along with their annual average values is illustrated in Fig. 7. The figure clearly indicates that the annual average of the insolation obtained in case of bi-annual tracking method is more (3.5%) as compared to fixed tilt angle configuration. VI. IMPLEMENTATION OF BI-ANNUAL SUN TRACKING The bi-annual sun tracking method is implemented by designing the PV module support structure with the adjustable tilt angles. The layout and side views of the physically installed module support structure for implementation of bi-annual sun tracking are shown in Figs. 7, 8 and 9. The structure tilt angle needs to be adjusted only twice in a year. The module support structure is installed in the laboratory. One support structure can hold four modules (shown as M 1, M 2, M 3, M 4 ) and suitable arrangements are made for changing the tilt angle of the structure to different values as shown in Fig. 8. The pictures shown in Fig. 9 and 10 are illustrating the side views of module support structure inclined at (latitude-15 ) and (latitude+15 ) respectively from horizon. The support structure is made to face true south direction. M m N Modules tilted at 7.5 M 2 W E 1.37m 1.62 m Modules tilted at S M3 M 4 Parapet Fig.7. Comparison of mean monthly insolation values for fixed tilt angle and bi-annual tracking method along with their annual average values Fig.5. indicates the mean monthly total solar insolation values for a fixed tilt angle configuration and single axis tracking. The single axis tracking increases the solar energy output significantly in the summer and winter months. During Cement pedestal structure 0.7m 0.2m Latitude-15 (7.5 ) Latitude+15 (37.5 ) Fig.9. Layout of module support structure for implementation of bi-annual sun tracking

6 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, Fig.9. Side view of module support structure inclined at from horizon VII. CONCLUSION The bi-annual tracking method necessitates the change in the tilt angle of the collector twice in a year, latitude-15 for October to March and latitude+15 for April to September. It provides a 3.5% increase in the solar energy received on the PV array compared to that received on the array tilted at an angle equal to the latitude angle of the site. The bi-annual tracking has been implemented using tilt angle adjustment, which is very simple and economic.this method does not involve any motorized moving part or complex control scheme. On the other hand, the single axis tracking method provides 12.75% increase in solar energy received on collector compared to fixed tilt arrangement of array inclined at latitude angle. However, single axis tracking requires complex control scheme with maintenance intensive components like dc motors, controllers and gear boxes. These components have to be highly reliable because a failure in any one of these components creates a failure in the entire system and also increases the cost of the system. ACKNOWLEDGMENT The authors are thankful to the authorities of IIT, Kharagpur for their encouragement and permission to present the paper. REFERENCES Fig.10. Side view of module support structure inclined at from horizon [1] Gilbert M. Masters, Renewable and efficient electric power systems, A John Wiley & Sons, Inc., Publication [2] Felix A. Farret, and M. Godoy Simoes, Integration of alternative sources of energy, A John Wiley & Sons, Inc., Publication [3] Abd El-Shafy A. Nafeh, Evaluation of the optimum tilt of a PV array using maximum global insolation technique, International Journal of Numerical Modelling: Electronic Networks, Devices and Fields Volume17, Issue 4, Pages: ,2004. [4] V.Meksarik, S.Masri, S.Taib and C.M Hadzer, Study the effective angle of photovoltaic modules in generating an optimum energy, Power Engineering Conference, PECon Proceedings. National, [5] vol., no., pp , Dec Solar spectrum available at www. pvcdrom.pveducation.org. [6] Solar Radiation Hand Book (2008), A joint Project of Solar Energy Centre, MNRE & Indian Metrological Department availbale at [7] Chang Ying-Pin; Shen Chung-Huang;, "Effects of the Solar Module Installing Angles on the Output Power," 8th International Conference on Electronic Measurement and Instruments, 2007, ICEMI '07, vol., no., pp , Aug July [8] M. Serhan and L. El-Chaar, Two axes sun tracking system: comparsion with a fixed system, International Conference on Renewable Energies and Power Quality, ICREPQ 2010 available at