8-EEE - IJEEER - FEASIBILITY

International Journal of Electrical and Electronics Engineering Research (IJEEER) ISSN 2250-155X Vol.2, Issue 3 Sep 2012 84-93 © TJPRC Pvt. Ltd.,

FEASIBILITY STUDY OF COMMERCIAL SILICON SOLAR PV MODULE BASED LOW- CONCENTRATION PHOTOVOLTAIC SYSTEM BRIJESH TRIPATHI1,2, PANKAJ YADAV2, MAKARAND LOKHANDE1 & MANOJ KUMAR1 1 2

School of Technology, Pandit Deendayal Petroleum University, Gandhinagar – 382007 (India)

School of Solar Energy, Pandit Deendayal Petroleum University, Gandhinagar – 382007 (India)

ABSTRACT A concentrator photovoltaic (CPV) system has a potential for further cost reduction of solar Photovoltaic (PV) power as compared to flat panel PV. In this work a piecewise linear parabolic trough is designed to reflect the solar radiation with uniform intensity on the PV module receiver system.

Silicon solar PV module based CPV system is modeled and simulation is done to study the variation of output power, open-circuit voltage and short-circuit current with respect to module temperature and irradiance. These simulation results are experimentally validated using a CPV (CR ~ 8) system developed in laboratory. The results confirms that the commerically available silicon solar PV module performs satisfactorily upto ~ 8 Sun concentration.

KEYWORDS: Concentrator Photovoltaics (CPV), Current-Voltage Characteristics, Modeling, Simulation

INTRODUCTION Silicon based solar photovoltaic (PV) technology has emerged as a potential renewable energy source for future power requirements. Still the cost reduction of this technology is an important area of concern. There are several ways by which the cost of this technology can be reduced. One of them is to reduce usage of costly silicon material for solar energy conversion. The amount of costly PV material consumed/Watt of generating power can be reduced by using thinner wafer, thin-film solar PV technologies or by concentrator photovoltaic (CPV) technology. Compared to non-concentrating solar PV systems, the required area for solar PV module is reduced by the factor of concentration ratio, providing significant reduction in the cost of solar PV system. The considerable amount of research is going-on in the field of CPV systems with different optics (mirrors or lenses—Fresnel or anidolic), spot sizes and geometries, tracking strategies, cooling systems (active or passive) and cells (Si or III–V compound semiconductors, whether single or multi-junction) [1, 2]. Above all, only multi-junction III– V-based solar cells (MJ sells) have achieved an efficiency of 43.5% at 500 Suns for a triple-junction metamorphic solar cell at laboratory level as reported in [3] but they are quite expensive [4] and for bringing them to commercial level, it needs geometrical concentration ratio (CR) of greater than 500.

Generally, higher the concentration ratio, greater the accuracy needed in tracking the Sun and smaller the

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Feasibility Study of Commercial Silicon Solar PV Module Based Low- Concentration Photovoltaic System

manufacturing and installing tolerances permitted. This means that high efficiency and high concentration concepts need very accurate systems, including their manufacture, installation and Sun tracking which increases their cost.

The two remarkable exceptions where silicon was used, the Euclides system, which used lasergrooved buried contact Si solar cells made by BP Solar as mentioned by Sala et al. [5], and the back points contact Si solar cells manufactured by Amonix, Inc. as mentioned in [6]. Presently, none of these silicon cells are available in the market for anyone developing a CPV system. Low-concentration photovoltaic systems (< 20Ă&#x2014;) have gained researcherâ&#x20AC;&#x2122;s interest in recent years [7-11]. In the beginning of this decade Sala et al. [5] have shown that the efficiency of silicon based photovoltaic system increases with concentration ratio. This study shows that for silicon solar cells the optimum lies near to 5 Sun to extract maximum from silicon solar cells. Recently, an industrialization potential of silicon based concentrator photovoltaic system with an estimated cost of $ 0.5/Wp is reported by Castro et al. [12], where the group uses back contact solar cells under 100 Suns. Recently Shuetz et al. [13] have reported design and construction of ~7Ă&#x2014; low-concentration CPV system based on compound parabolic concentrators. In this paper, construction, modeling, simulation and experimental validation of CPV system fabricated by using commercially available crystalline silicon solar cells (manufactured for 1-Sun application) for CR ~8 is reported.

MODELING AND SIMULATION OF SOLAR PV MODULE/ARRAY Modeling A crystalline silicon wafer-based solar photovoltaic (PV) cell of size 125 mm x 125 mm typically produces around 2.5 Watts at a voltage of 560 mV. These cells are connected in series and/or parallel configuration on a module to produce enough high power. A PV array is a group of several PV modules which are electrically connected in series and parallel circuit to generate the required higher current and voltage. The equivalent circuit for solar PV module, having NP numbers of cells arranged in parallel and NS number of cells arranged in series, is shown in Fig. 1:

Figure 1: The general model for solar PV module

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Brijesh Tripathi, Pankaj Yadav, Makarand Lokhande & Manoj Kumar

The terminal equation for current and voltage of the array can be written below as described by Veerachary [12], Veerachary and Shinoy [13], Kim and Youn [14] and Kim et al. [15]:

(1)

where, the parameters are represented as given below: – Light generated current or photocurrent – Cell saturation or dark current – Electron charge (1.6 x 10-19 C) kB – Boltzmann’s constant (1.38 x 10-23) TC – Working temperature of solar cell (Kelvin) RSH – Shunt resistance RS – Series resistance NS – Series number of cells in a PV module NP – Parallel number of modules for a PV array The photocurrent ( expression for

) mainly depends on the solar insolation and cell’s working temperature. The is given below: (2)

where, the parameters are represented as given below: – Cell’s short-circuit current at 25°C and 1kW/m2 - Cell’s short-circuit current temperature coefficient – Cell’s reference temperature - Solar insolation in kW/m2 The saturation current of a solar cell varies with the cell temperature, which is described as:

(3) where, the parameters are represented as given below: – Cell’s reverse saturation current at a reference temperature and solar radiation

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Feasibility Study of Commercial Silicon Solar PV Module Based Low- Concentration Photovoltaic System

– Band-gap energy of the semiconductor used in the cell A – Ideality factor The cell’s reverse saturation current at reference temperature depends on the open-circuit voltage (VOC) and can be approximately obtained by following equation as given in Tsai et al. [16]: (4) Based on the theoretical model described above, the authors have simulated solar PV module, designed for ~8 Sun concentration application.

Simulation Based on the above mentioned model a computer code was generated. In this code we have taken a solar PV module having 16 cells connected in series. The parameter values are listed in Table 1.

Table 1: The parameters used for simulation under 1 Sun concentration

Module Parameter

RS

For 1 Sun

0.071

RSH (Ω)

Eg (eV)

NS

NP

A

Tc (K)

Tref (K)

K (J/K)

KI

Q (C)

ISC (A)

IRS (A)

VOC (V)

845

1.12

16

1

1.5

298

298

1.38×10-

0.65×10 -

1.602×10-

0.259

0.86×10 -

9.86

23

3

19

(Ω) 12

Using above mentioned parameters the I-V curve of modeled solar PV system was generated and is shown in Fig. 2.

Figure 2: Current-voltage characteristics of the designed solar PV module under 1 Sun, AM1.5 at 25° C

Brijesh Tripathi, Pankaj Yadav, Makarand Lokhande & Manoj Kumar

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DEVELOPMENT OF CPV SYSTEM A linear parabolic reflector was designed and constructed with commercially available glass mirrors having reflectivity of ~80%. A solar PV module was fabricated by connecting sixteen silicon cell pieces (material: mono-crystalline silicon, size: 14 mm x 64 mm, efficiency ~13%) in series cut from commercially available solar cell. The reason behind the selection of the specific size of the cells mentioned here is to solve the current handling problem of the solar cells under concentration. A typical solar cell of size 125 mm Ă&#x2014; 125 mm producing 2.5 Watts at a voltage of 560 mV would have current handling capability of around 4.5 A. This cell, when used under 10 Sun concentration may produce 45 A current by assuming a linear relationship between the current increment and CR. But if the size of the cell is reduced to 1/10th of normal size, then the current generated under 10 Sun concentration would be less than or equal to 4.5 A, then it can be easily handled without damaging the solar cell contacts. This module was tested under standard test conditions (STC) and detailed parameters are given in Table.1.

The current-voltage characteristics are modeled at module level and shown in Fig. 2. A microprocessor based one axis tracking system is developed for Sun tracking and the water cooling is employed to regulate the temperature of the solar PV module. Pyranometers were used to measure the global and direct normal irradiance (DNI). Commercially available thermocouple was used to measure the temperature of module at different concentration. The developed prototype of the CPV system is shown in the Fig. 3. Experiments were conducted under actual test condition (ATC) with continuous water flow to maintain temperature ~50-70 â °C and with global irradiance of 876 W/m2. The current-voltage values of CPV system were taken by Agilent SMU 6632B and by using multi-meters and load rheostat.

Figure 3: The constructed prototype of concentrator photovoltaic (CPV) system

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Feasibility Study of Commercial Silicon Solar PV Module Based Low- Concentration Photovoltaic System

RESULTS AND DISCUSSIONS The developed CPV system was studied by varying the number of reflecting mirrors arranged in parabolic trough. Experimental parameters were noted for 2 mirrors, 4 mirrors, 6 mirrors and 8 mirrors.

The values of parameters are listed in Table 2. The current-voltage characteristic curves are plotted in Figures 4, 5, 6 and7 for 2 mirrors, 4 mirrors, 6 mirrors and 8 mirrors respectively. From these values we could observe that the series resistance of the crystalline silicon solar cell increases from 0.715 to 0.095 Ω with increasing concentration of incident radiation. This is because of increased value of the cell temperature from 298 to 332.5 K under concentrated light. The increase in temperature also affects the open-circuit voltage (VOC) and it is observed that the VOC decreases from 9.86 to 8.24 V with increasing values of temperature. Fill factor also found decreasing from 74.58% to 66.23% with increase in concentration and temperature of the solar cell. This may be attributed to the increase in ohmic losses due to higher series resistance. The power output was increasing from 1.91 to 5.84 Watts with increase in the concentration of light but the increase is not as many folds as the number of mirrors used. This is because of following reasons: (a) the reflectivity and specific position of mirror produces lower radiation at different incident angles, (b) with increase in temperature ohmic losses increases because of higher series resistance, and (c) the voltage drops across the metallic contacts due to the increased resistance because of temperature rise. As a consequence of above mentioned reasons the efficiency of the solar PV module keep on decreasing with increasing concentration of radiation. Although it is reported in literature that the cell efficiency increases with increase in concentration but that happens only for constant temperature.

In this case the decrease in efficiency is observed because of increasing temperature.

Table 2: Various parameters for crystalline silicon solar PV module used in CPV system Parameters

λ (W/m2)

RS (Ω)

TC (K)

ISC (A)

Voc (V)

FF (%)

PMAX

1 Sun (STC)

Theory

1000

0.071

298

0.3

9.85

74.86

1.91

Experiment

1000

0.07

298

0.3

9.86

74.58

1.91

2 mirrors

Theory

1225

0.078

321

0.3

8.5

70.02

2

0.075

321

0.3

8.48

73.97

2.07

0.084

328

0.6

8.4

67.29

3.55

0.081

328

0.6

8.39

70.37

3.72

0.09

331

0.9

8.34

64.85

4.91

0.087

331

0.9

8.31

66.11

5

0.095

332.5

1.1

8.31

62.3

5.54

0.097

332.5

1.1

8.24

66.23

5.84

η (%) 7.07 7.07

Experiment

--

6.05 6.26

4 mirrors

Theory Experiment

2254 --

5.83 6.11

6 mirrors

Theory Experiment

3234 --

5.62 5.73

8 mirrors

Theory Experiment

3822 --

5.37 5.66

Brijesh Tripathi, Pankaj Yadav, Makarand Lokhande & Manoj Kumar

Figure 4: Current-voltage characteristics of the designed CPV system under 2 mirrors

Figure 5: Current-voltage characteristics of the designed CPV system under 4 mirrors

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Feasibility Study of Commercial Silicon Solar PV Module Based Low- Concentration Photovoltaic System

Figure 6: Current-voltage characteristics of the designed CPV system under 6 mirrors

Figure 7: Current-voltage characteristics of the designed CPV system under 8 mirrors The simulated results are in accordance with the experimental observations. Slight deviation is observed because of the data is collected manually. This shows that the developed model explains the

92

Brijesh Tripathi, Pankaj Yadav, Makarand Lokhande & Manoj Kumar

behavior of CPV system under actual test conditions. Further, the developed model can be used to predict the behavior of a CPV system for any value of concentration and temperature.

CONCLUSIONS In this study the design, construction, modeling and simulation of the CPV system is presented.

The experimental results validate theoretical model. The developed theoretical model is able to predict the performance of CPV system under ATC. This study shows that the commercially available silicon solar PV cells can be modified to work under low level concentration (< 10 Sun) to have higher power output without compromising with the performance of the solar cell.

ACKNOWLEDGEMENTS The authors acknowledge the financial support provided by Gujarat Energy Development Agency (GEDA) to develop CPV system by grant number: GEDA\EC:REC\March-2010/3/9174.

Authors also acknowledge WAAREE Energies Pvt. Ltd., India for providing encapsulated crystalline silicon solar PV modules for this study.

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