Journal of Energy and Power Engineering 5 (2011) 356-360

Comparison of Different Solutions for Blocking Diode Applications in a Photovoltaic Panel under Varying Ambient Conditions H.C. Neitzert1 and A. Astone1, 2 1. Department of Electrical Engineering (DIIIE), Salerno University, Via Ponte Don Melillo 1, Fisciano 84084, Italy 2. Telecom Italia Mobile (TIM), Via Pascoli 9, Trieste 34129, Italy

Received: June 22, 2010 / Accepted: October 30, 2010 / Published: April 30, 2011. Abstract: We characterized a crystalline silicon based mini-module under varying ambient conditions, developed a PSPICE model for this panel, including temperature and irradiation dependence and applied this model to the simulation of the impact of a blocking diode under different shadowing conditions. Different blocking diodes were examined, like germanium and silicon homojunction diodes and silicon Schottky diodes and compared to “intelligent” diodes, consisting of operational amplifiers with MOSFET switches. The simulations indicate a strongly reduced power loss in a panel integrating the new “intelligent” blocking diodes even when compared to silicon Schottky diodes, as the best performing traditional blocking diodes. Key words: Blocking diode, bypass diode, photovoltaics, circuit simulation, shadowing, intelligent diode, MOSFET.

1. Introduction A long lasting discussion regarding the sense of the integration of blocking or bypass diodes in solar panels [1, 2, 3] did so far not come to a conclusive answer. For example it has been reported that bypass diodes have been responsible for failures in solar battery charging stations [4]. Under some ambient conditions, however, in particular under partial shadowing conditions of solar panels or solar fields or under non homogeneously distributed temperature conditions, without the use of this diodes, large power losses are measured due to non matching currents or voltages [3]. On the other hand is the voltage drop of conventional diodes not negligible. With the development of low-cost analogue circuitry and MOSFET’s with extremely low on-resistance values, the realisation of “intelligent” diodes with almost no power losses Corresponding author: H.C. Neitzert, professor, research fields: solar cells, nanocomposite based sensors and reliability of optoelectronic components. E-mail: [email protected].

becomes viable [5]. Regarding the use of traditional single diode solutions for blocking diode applications, different types of devices are employed. Mostly silicon based pn-diodes or Schottky diodes are used. In the case of high temperature environmental conditions, also SiC blocking diodes have been proposed [6, 7]. Operation in a extremely wide temperature range between -170 ℃ and 270 ℃ has been achieved in this case [6] and good reliability during accelerated lifetime testing has been reported [8]. This is for example of interest for space based photovoltaic systems. In the case of terrestrial systems, however, low-bandgap semiconductors should be more interesting because of the lower forward bias voltage drop. Historically germanium and selenium have been preceding the nowadays dominating silicon as material for rectifying diodes. May be in the future, germanium can gain again some importance for blocking diode applications in PV panels.

Comparison of Different Solutions for Blocking Diode Applications in a Photovoltaic Panel under Varying a Mbient Conditions

As a first step, a questionnaire has been sent to a number of solar panel producers, asking if they use blocking or bypass diodes within their solar panels and what type of diodes they use. The responses, summarized in Table 1, indicated that only few producers integrated blocking diodes into their solar panels, while some more integrated bypass diodes. Most popular diodes were silicon Schottky diodes because of the lower voltage drop as compared to silicon pn-diodes, as mentioned before. Subsequently a commercial crystalline silicon photovoltaic mini-module (see Fig. 1) has been characterized under varying ambient conditions. In particular, we measured at ambient temperature between 30 ℃ and 60 ℃ and used three different irradiance values (illumination with a halogen lamp). It should be noted that under standard irradiation conditions (AM1.5G) a conversion Table 1

efficiency of 12.13% has been measured for the mini-module. In Fig. 2 for example the current-voltage characteristics at different temperatures under irradiation with a halogen lamp with an incident irradiance of 2105 W/m2 is shown. 2.1 Characterization of a Silicon PV Mini-Module For a single silicon PV cell of the mini-module, based on the measurements that are partially shown in Fig. 2, a PSPICE model (see Fig. 3) has been developed, that accounts also for temperature and irradiance variations. 0.1 30 (°C) 0.05 Current (A)

2. Results and Discussion

0

-0.05

Use of blocking and bypass diodes–interviews.

Manufacturer Solar-Fabrik United Solar Ovonics Corp. First Solar LLC Solar Integrated Technologies

Blocking diode Bypass diode Diode type no yes 80SQ045 optional

yes

Schottky

no

no

-

no

no

-

yes

module dipendent

Shell Solar B.V. no

357

Current measured

-0.1

Current simulated 0

0.1

0.2

0.3 Voltage(V)

0.4

0.5

0.6

Fig. 2 Measurement and simulation of the temperature dependent (between 30 ℃ and 60 ℃ in 10 ℃ steps) current-voltage characteristics under irradiation with 2,105 W/m2 of a single diode of the above shown mini-module.

3

girrad

V Irr

Rs

1

D

Rsh

I

2

1 3

2

Fig. 1 Characterized silicon mini-module.

Fig. 3 PSPICE model of single cell, taken into account the solar irradiation condition and the temperature behavior.

358

Comparison of Different Solutions for Blocking Diode Applications in a Photovoltaic Panel under Varying a Mbient Conditions

2.2 Characterization of Different Types of Blocking Diodes

100 Selenium

1N5818

10-1 10-2

2.3 Comparison of Power Losses for Different Blocking Diodes

OA81

-3

10

10-4 1N4007

10-5 10-6

Experiment

10-7

Simulation

10-8 0.4 0.5 0.6 0.7 0.3 Voltage (V) Fig. 4 Comparison of the measured and simulated forward bias current-voltage characteristics at room temperature of 4 different commercial diodes. 0

0.1

0.2

0.10 0.09

Current

0.08 0.07

Experiment Simulation

0.06 0.05

60°C

0.04 0.03 0.02 0.01

30°C

0

0.05

0.1

0.15 0.2 Voltage(V)

0.25

0.3

12% 10%

10,73 9,96

60°C 30°C

OA81 1N4007

8%

Selenium

6%

1N5818

5,42

4%

3,62 2,71

2,18

1,81

2%

Subsequently a simulation of the power losses at 30 ℃ and 60 ℃, when using the 4 different blocking diodes has been performed. The results as a function of the number of series connected silicon solar cells are shown in Fig. 6 for a given irradiance. The use of

0.5

Fig. 5 Measurement and simulation of the temperature dependent current-voltage characteristics of the 1N5158 Schottky diode between 30 ℃ and 60 ℃ (in 10 ℃ steps).

Power Loss

Subsequently we characterized 4 different types of possible blocking diodes: a conventional high power silicon diode (1N4007), a silicon based Schottky diode (1N5818) and additionally 2 different “historical” low-bandgap semiconductor diodes (AEG selenium rectifier) and a germanium diode (OA81). It should be noted, that the OA81 diode is a low power device and has been only included because we did not have access to commercial germanium power diodes. Other types of high power germanium diodes could, however, in the future be an interesting alternative to conventional silicon diodes. In Fig. 4 the measured and simulated forward bias diode current-voltage characteristics at room temperature are displayed. The superior performance of the Schottky diode as compared to the silicon pn-diode is clearly seen. As expected we find that also the selenium diode has a lower voltage drop for a given current value when compared to the silicon pn-diode. The germanium diode (OA81) suffers from the high series resistance and has therefore the worst behavior of the 4 investigated diodes under high forward bias current conditions. For every diode type also a complete characterization at 4 different temperatures has been done. As an example in Fig. 5 the current-voltage characteristics between 30 ℃ and 60 ℃ (in steps of 10 ℃) of the silicon Schottky diode are shown and compared to the PSPICE simulation. A good agreement between measured and simulated characteristics has been achieved.

1,61

0,40 0,80

0,32

0,26

0,53 0% 1 2 3 4 5 6 Number of modules in series connection Fig. 6 Power loss as a function of the number of series connected solar cells for different types of blocking diodes at two different ambient temperatures (30 ℃ and 60 ℃).

Comparison of Different Solutions for Blocking Diode Applications in a Photovoltaic Panel under Varying a Mbient Conditions

2.4 Intelligent Diode with Low Voltage Drops A very strong further reduction of the power loss (see Fig. 7), has been achieved using a combination of a low power operational amplifier (type “OP290”) in combination with a low on-resistance MOSFET (type “IRF630”), working as “intelligent” diode with low own power consumption and very low voltage drop in forward bias direction. The circuit of the “intelligent”

2.0 1.8 1.6

1,61

30°C

1N5818

60°C

1.4 Power Loss (%)

silicon Schottky diodes (type “1N5818”) instead of silicon pn-diodes (Type “1N40007”) resulted in a factor of three lower power losses as well at 30 ℃ and at 60 ℃ ambient temperatures.

359

MOSFET + OpAmp

1.2 1.0 0,80

0.8 0.6

0,53 0,40

0.4 0.2

0,10

0.0 1

0,09

0,06

0,05

0,32 0,03

2 3 4 5 Number of modules in series connection

0,26 0,03 6

Fig. 7 Power loss as a function of the number of series connected solar cells for a Schottky diode and an “intelligent” diode at two different temperatures.

Fig. 8 Configuration of an “intelligent” blocking diode inserted in a PV module.

blocking diode and the insertion into the solar panel are shown in Fig. 8.

3. Conclusions A comprehensive simulation of the impact of the blocking diode choice on the power losses of a silicon photovoltaic mini-module has been done. The simulation parameters of the silicon mini-module and

of the investigated blocking diodes have been obtained by extraction of the device parameters from electrical measurements under different temperatures and irradiation conditions. An appreciable reduction of the power losses have been obtained using a silicon Schottky diode instead of a silicon pn-diode. Also the use of a “historical” low-bandgap selenium rectifier resulted in reduced power losses as compared to the

Comparison of Different Solutions for Blocking Diode Applications in a Photovoltaic Panel under Varying a Mbient Conditions

360

silicon pn-diode. Simulation showed, that a strong further improvement can be achieved when, instead of the silicon Schottky blocking diode, an “intelligent” diode, consisting of an operational amplifier in combination with a power MOSFET is used.

[4]

[5] [6]

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[2]

[3]

S.J. Durand, Attaining a 30-year photovoltaic system lifetime: The BOS issues, Progress in Photovoltaics 2 (2007) 107-113. J.C. Wiles, D.L. King, Blocking diodes and fuses in low-voltage PV systems, in: Proc. of the 26th IEEE Photovoltaic Specialists Conf., Anaheim, 1997, pp. 1105-1108. A. Abete, E. Barbisio, F. Cane, P. Demartini, Analysis of photovoltaic modules with protection diodes in presence of mismatching, in: Proc. of the 21st IEEE Photovoltaic Specialists Conf, Kissimmee, 1990, pp. 1005-1010.

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[8]

C. Greacen, D. Green, The role of bypass diodes in the failure of solar battery charging stations in Thailand, Solar Energy Materials & Solar Cells 70 (2001) 141-149. Available online at: http://www.icbase.com/pdf/ONS/ONS24940605.pdf. E. Maset, E. Sanchis-Kilders, J. Jordan, J. Bta. Ejea, A. Ferreres, J. Millan, et al., High temperature SiC blocking diodes for solar array, in: Proc. of the 2009 Spanish Conf. on Electron Devices, Santiago de Compostela, 2009, pp. 443-446. E. Maset, E. Sanchis-Kilders, P. Brosselard, X. Jordà, M. Vellvehi, P. Godignon, 300 ℃ SiC blocking diodes for solar array strings, Materials Science Forum 615-617 (2009) 925-928. E. Maset, E. Sanchis-Kilders, J.B. Ejea, A. Ferreres, J. Jordan, V. Esteve, et al., Accelerated life test for SiC schottky blocking diodes in high-temperature environment, IEEE Trans. on Device and Material Reliability 9 (2009) 557-562.