Quality Assurance in Solar with the use of I-V Curves

Transcription

1 Quality Assurance in Solar with the use of I-V Curves Eternal Sun Whitepaper Written by: RJ van Vugt

2 Introduction I Installers, wholesalers and other parties use performance tests in order to check on quality of incoming PV modules. This is one of the many ways to control quality and maintain a high level of quality. Main indicator is often P max which is mainly used for two issues: Verification of the production flash list Verification of the module name plate rating Instead of only looking at P max it is more interesting to look at the whole I-V curve and not only look at verification of the flash list and the name plate. Quality Assurance 2

3 Introduction II An I-V curve gives information about most of the electric properties of the PV module, not just on maximum output. I-V curves of all PV-technologies are comparable at Standard Test Conditions (AM1.5, 1000W/m² and 25 ºC). A thorough understanding of the I-V curve is needed in order to draw the right conclusions from an I-V curve. If the I-V curve is understood, it can be used to assess quality of the PV-module (both cell and module design as manufacturing). This whitepaper is intended for everyone in the solar sector who wants to have a better understanding of IV-curves and the data that is usually provided on a PV-module datasheet. Quality Assurance 3

4 I-V curve without illumination An I-V curve is the official way to plot the characteristics of a semiconductor I (A) The figure on the right shows an I-V curve without illumination. A PV-module without illumination has the same electrical characteristics as a large diode. V (V) Quality Assurance 4

5 I-V curve with illumination I The figure on the right shows a I-V curve with illumination. I (A) The I-V curve shifts as the PV-module begins to generate power. The higher the light intensity, the greater the amount of shift. V (V) Quality Assurance 5

6 I-V curve with illumination II Since the PV module is generating power, the convention is to invert the current axis. I (A) The I-V curve shows important parameters which will be discussed further on: Short Circuit Current Open Circuit Voltage Fill Factor Efficiency Series Resistance Shunt Resistance (Non-) Uniform Shading Damaged cells Mismatched cells V (V) Quality Assurance 6

7 Short-Circuit Current (I sc ) Short-Circuit Current is the current through the PV module when the voltage across the PV module is zero (when the PV module is short circuited). I (A) I sc I sc depends on: Spectrum of light source Optical properties of the PV module (light absorption) Number of photons (i.e. power of lightsource/ intensity) Area of PV module V (V) Quality Assurance 7

8 Influence of light intensity on IV-curve I sc is influenced by number of photons (intensity of light). I (A) 1000 W/m² 800 W/m² IV-curve shifts downwards because of this influence. 600 W/m² 400 W/m² V oc is slightly affected. V (V) Quality Assurance 8

9 Open-Circuit Voltage (V oc ) Open-Circuit Voltage is the maximum voltage available from a PV-module, and it occurs when the current is zero. I (A) V oc depends on: Band gap of semiconductor Amount of doping of P&N layers Material purity Light generated current. Temperature of the PV-module V oc V (V) Quality Assurance 9

10 Influence of temperature on IV-curve V oc is influenced by temperature of the PV-module. I (A) IV-curve shifts to the left because of this. I sc is slightly affected by temperature. 55 C 45 C 35 C 25 C V (V) Quality Assurance 10

11 Maximum Power Point (MPP or P max ) The maximum power point is the point on the I and V axes where the PV-module generates the most power. I (A) I mpp P max P (W p ) On the figure on the right a power axis is added and a power graph is added. P max MPP depends on V mpp and I mpp. V mpp V (V) Quality Assurance 11

12 I-V curve and time IV-curves are combined I-t and V-t curves. I (A) IV-curves can be swept forwards (start at I sc ) or backwards (start at V oc ). This means: from V oc to I sc, or from I sc to V oc. If a forward sweep differs from a backward sweep the PV-module has capacitive effects. V (V) Quality Assurance 12

13 Effiency (η) The effiency is the most commonly used parameter to compare the performance of one PV-module with the other. I (A) Pmax P (Wp) Ratio of energy output from the PV-module (P max ) to input from the sun (P in ) Pmax η = P max / P in V (V) Quality Assurance 13

14 Fill Factor (FF) I Product of I sc and V oc Fill Factor is the ratio of the maximum power from the solar cell to the product of V oc and I sc. I (A) Impp P max On the right it is visualized as the ration between the two squares. Product of V oc and I sc is the theoretical MPP point. Vmpp V (V) Quality Assurance 14

15 Fill Factor (FF) II Product of Isc and Voc Compare the lower FF on the right with the FF on the previous slide. I (A) Impp Pmax V oc and I sc are the same, V mpp and I mpp are both lower. This results in a lower P max and a lower FF. Notice the larger angles of the IV-curve with the straight I sc and V oc lines. Vmpp V (V) Quality Assurance 15

16 Characteristic Resistance (R ch ) Product of I sc and V oc R ch of a PV-module is the output resistance of the PV-module at its Mpp. I (A) I mpp Mpp R ch = V mpp /I mpp, or approximated by V oc /I sc. It is a useful parameter in solar analysis, when examining the impact of parasitic loss mechanisms. V mpp V (V) Quality Assurance 16

17 Effect of parasitic resistances Resistive effects in PVmodules reduce the efficiency of the PVmodule. The most common are series resistances (R s ) and shunt resistances (R sh ). The value of resistance depends on the area of the PV-module. To compare resistances Ωcm² is used. Series resistance is mainly caused by bad solar cell design, bad module manufacturing and wrong material choice. Shunt resistance is mainly caused by bad module manufacturing. Quality Assurance 17

18 Series Resistance (R s ) Poor conduction Three causes, mainly poor solar cell design: The movement of current through the emitter and base of the solar cell The contact resistance between the metal contact and the silicon. The resistance of the top and rear metal contacts. The bold IV-curve on the right is the IVcurve with a high series resistance. Series resistance should be as low as possible. I (A) V (V) Quality Assurance 18

19 Series Resistance (R s ) The movement of current through the emitter and base of the solar cell (see cell and module on the right). The contact resistance between the metal contact and the silicon (see cell on the right). The resistance of the top and rear metal contacts (see cell and module on the right). Quality Assurance 19

20 Shunt Resistance (R sh ) Low shunt resistance provides an alternate current path. This reduces the amount of current flowing through the solar cell and reduces the voltage. Shunt resistance should be high. Effect is bigger at low light levels: Less light generated current, impact of loss is larger. When cell has lower voltage, the impact of resistance in parallel is large. I (A) V (V) Quality Assurance 20

21 Shunt Resistance (R sh ) Low shunt resistance provides an alternate current path. If busbars and fingers are manufactured in the right way, shunt resistance is high (see cell on the right). If connections at the top (or bottom) of the module are manufactured in the right way, shunt resistance is high (see module on the right). Quality Assurance 21

22 Series & Shunt Resistance combined The bold IV-curve on the right is an IVcurve where series resistance is high, and shunt resistance is low. I (A) It is possible to approximate the series and shunt resistances from the slopes of the IV-curve at V OC and I SC V (V) Quality Assurance 22

23 Effect of uniform shading on IV-curve Uniform shading decreases the light intensity. I (A) Uniform shading leads to the same behavior as lower light intensities would. It mainly decreases I sc. V (V) Quality Assurance 23

24 Non-uniform shading & single cell failure The bold IV-curve on the right is an IVcurve where (exaggerated) mismatch losses are visualized. I (A) Losses due to nonuniform shading and non-uniform reflective coating show the same type behavior as mismatch losses. Losses due to cracked cells show the same type (but stronger) behavior as mismatch losses. V (V) Quality Assurance 24

25 Poly-Silicon vs Thin Film I sc, V oc, P max are all different. Fill Factor can be the same (not necessarily). I (A) Impp Efficiency can be the same (not necessarily). The figure on the right shows two IV curves with different I sc, V oc, I mpp and V mpp, but with the same W p and roughly the same FF. Impp Vmpp Vmpp V (V) Quality Assurance 25

26 GaAs module Small angle between V oc -line and IV-curve. Very low series resistance. A very sharp knee. Small angle between I sc -line and IV-curve. Very high shunt resistance. Sample GaAs module, explains (lower) series resistance. Very high Fill Factor (83%). Quality Assurance 26

27 Thin Film Module Both angles are larger compared to the GaAs module and the thinfilm module. IV-curve is smooth, no defects in module. The knee is rounder compared to silicon. Fill Factor OK (70%). Pmax much lower as Poly-Silicon (around 50W p instead of 250W p ). Quality Assurance 27

28 Poly-Silicon module Both angles are slightly larger compared to the GaAs module. The knee is sharp. IV-curve is smooth, no defects in module. Both cell choice (sorting and manufacturing) and manufacturing of the module could be slightly improved. Fill Factor still OK (75%). Quality Assurance 28

29 Poly-Silicon module I Both angles are larger compared to the GaAs module. The knee is sharp. IV-curve has a gradual decline, no defects in module. Both cell choice (sorting and manufacturing) and manufacturing of the module could be slightly improved. Fill Factor OK (75%). Quality Assurance 29

30 Poly-Silicon module II Both angles are larger compared to the GaAs module. The knee is sharp. IV-curve has almost no decline, no defects in module. High R sh, manufacturing of the module could be slightly improved. Fill Factor OK (76%). Quality Assurance 30

31 Poly-Silicon module III Both angles are larger compared to the GaAs module. The knee is sharp. IV-curve has almost no decline, no defects in module. High R sh, manufacturing of the module could be slightly improved. Fill Factor OK (76%). Quality Assurance 31

32 Poly-Silicon module IV Both angles are larger compared to the GaAs module. The knee is sharp. IV-curve has a large drop at 14V. Low(er) R sh. Fill Factor OK (74%). Quality Assurance 32

33 Poly-Silicon module V Both angles are larger compared to the GaAs module. The knee is sharp. Gradual drop starting at 4V. Suspected loose wire in junction box Fill Factor not OK (69%). Quality Assurance 33

34 Poly-Silicon module VI Same panel as Module V Opened junction box and taken out one wire (one string). Fill Factor not OK (69%). Only difference: Voc at 24V instead of 36V and Vmpp at 18V instead of 28V! Quality Assurance 34

35 PV-module with broken cells IV-curve is not smooth. Large angles with V oc and I sc lines. Small Fill Factor (53%). P max around 140W p Quality Assurance 35

36 Difference between good and broken module I To demonstrate how I-V curve of a broken module looks like we haven taken a mini module and measured it. The next step will be to brake it. It s a mini module, therefore lower V oc and I sc. Very smooth IV-curve High Fill Factor: 76% P max around 56W p Quality Assurance 36

37 Difference between good and broken module II Mini Module, therefore low V oc and I sc. 10 hits with small hammer. Still high Fill Factor: 75,55% P max around 54W p Drop in V mpp, from 9,3 to 9,0. Almost no drop in I mpp. Long term effects not clear. Quality Assurance 37

38 Difference between good and broken module III Mini Module, therefore low V oc and I sc. 4 hard hits hammer Drop in Fill Factor: 69% P max around 46W p Drop in V mpp, from 9,3 to 8,3. Drop in I mpp from 6A to 5,6A. Long term effects not clear. Quality Assurance 38

39 Difference between good and broken module III Mini Module, therefore low V oc and I sc compared to previous curves. 8 hard hits hammer Drop in Fill Factor: 63% P max around 41W p Drop in V mpp, from 9,3 to 8,1. Drop in I mpp from 6A to 5,0A Long term effects not clear. Quality Assurance 39

40 Difference between good and broken module III Mini Module, therefore low Voc and Isc compared to previous curves. 8 hard hits hammer Drop in Fill Factor: 63% Pmax around 41Wp Drop in Vmpp, from 9,3 to 8,1. Drop in Impp from 6A to 5,0A. Long term effects not clear. Quality Assurance 40

41 I-V curves can be used to assess quality of PV module I-V curve gives information on: Performance (P max, V oc, I sc, η and FF) Resistance of the PV-module (R s and R sh ) Improper cell and module design and/or Improper manufacturing Faulty module manufacturing Effect of parallel and series connections Non-uniform anti-reflective coating Possible micro-cracks and/or hotspots Loose wiring in junction box This information is valuable for a quality and/or product manager who has to maintain high quality throughout the year. By sampling and independently testing those samples, quality assurance of all PV-modules is within reach. Quality Assurance 41

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43 THIS DOCUMENT IS PREPARED BY ETERNAL SUN BV. SOLELY FOR THE INFORMATION OF THE CUSTOMER. IT SERVES AS BASE FOR DISCUSSION, AND FEEDBACK. THE CONTENT OF THIS DOCUMENT IS ONLY INTENDED FOR THE EYES OF THE ADDRESSEE(S). COPYRIGHT AND INTELLECTUAL PROPERTY RIGHT PROTECTION EXISTS IN THIS DOCUMENT AND IT MAY NOT BE REPRODUCED, DISTRIBUTED OR PUBLISHED BY ANY PERSON FOR ANY PURPOSE WITHOUT THE PRIOR EXPRESS CONSENT OF ETERNAL SUN ETERNAL SUN. ALL RIGHTS RESERVED. ETERNAL SUN B.V. MOLENGRAAFFSINGEL JD DELFT THE NETHERLANDS COC NL