DE10026162C2 - Process for quality control of photovoltaic cells and modules (PV modules) - Google Patents

Description

The invention relates to a method for quality control of photovoltaic cells and modules (PV modules).

Quality control of photovoltaic cells and modules len, hereinafter referred to as PV modules for short, is an emergency agile service for users of PV systems under Be consideration of guarantee services of 10 and more years. The operating behavior of a solar cell is determined by its current Voltage characteristic curve (I-U characteristic curve) shown. By measurement current characteristic curves under natural environmental conditions can the operability of a solar generator from one or more PV modules. Deviations of the characteristic curve from the theoretical expect value allow conclusions to be drawn about internal faults, cables breaks, partial shading, mismatches, etc. s. w .. Besides that current operating behavior is used to detect degrada tion phenomena but also the measurement of the stationary property Properties of peak power and series internal resistance are necessary.

The peak power is defined as the maximum power at Standard test. These standard test conditions (STC However, DIN EN 60904-3) places high demands on the test stood, which consequently leads to high costs per measurement.  

Losses in the PV modules are resisted by the series interior was described, an increase in series internal resistance shows both internal losses and aging contacts. The experimental effort for the measurement of the series interior However, the level in the laboratory is quite high as it is in any case it is necessary to remove and close the PV module to be checked bring it to a laboratory and check it there. This effort is so high that such a quality control is practicable table is not worth it, but is about the same effort it is even possible to reinstall a PV module.

The object of the invention is therefore to create a solution with the reliable quality control of PV modules much easier and with much less effort can be carried out.

This task is described in a method of the beginning ten kind solved according to the invention in that under real Um conditions at the place of use of the relevant PV module two current-voltage characteristics of the PV module at difference Licher irradiance measured in the same spectrum and two effective characteristic curves are determined from this, ba based on these effective characteristics of the series interior and the peak from at least one effective characteristic performance can be determined.

With the method according to the invention it is possible to carry out a quality control  under normal environmental conditions at the on to carry out the location of the relevant PV module itself, d. H. an expansion of the module, a transfer to a laboratory and that Compliance with exact standard test conditions is not required sary. Rather, it is possible, after measuring the two current Voltage characteristics under real environmental conditions this taking into account the environmental conditions, esp in particular the existing irradiance and ambient temperature effective characteristic curves. With the two effekti On the one hand, the series internal resistance can be used and on the other hand at least an effective one Characteristic curve are transferred to standard test conditions, wor from which the peak power can be determined.

To determine the peak power, it is advantageously provided that that in addition to measuring the current-voltage characteristics effective irradiance is determined, which is only from the Part of the solar spectrum is formed, which leads to energy conversion is used in the respective PV module, and that the Um ambient temperature is measured and from the effective characteristic line, the effective irradiance and the ambient temperature the peak power is determined. As from DIN EN 60891 is known in principle, it is possible to use a real one Ambient conditions measured current-voltage characteristic of To convert PV modules to other temperatures and insolation expected. The conversion must be carried out during the measurement irradiance, spectrum and cell temperature  be known. By describing the characteristic as effective solar cell characteristic (Wagner, Andreas: Photovoltaik Engineering, "The method of effective solar cell identification line ", Springer-Verlag Berlin, Heidelberg 1999) the Um calculation is explicitly possible by using the measured Ambient temperature above the known temperature coefficient of the PV modules the cell temperature is determined. The effec tive irradiance can be via an irradiance sor be measured with the same spectral sensitivity how the examinee owns.

To determine the effective irradiance, pre can be seen that this with the help of a reference module with ver spectral properties comparable to those of the checking PV module is measured, the reference module is oriented in the same orientation to the sun as that PV module to be checked.

It is particularly preferably provided that one from the quotient radiation intensity and associated short-circuit current Phox constant of the PV module formed at AM 1.5 conditions determined and the effective irradiance determined becomes. With known phox Constantly the PV module itself serves as a measuring device, an additional one Reference module is not required.  

To determine the series internal resistance at a first irradiance first a first current-voltage Characteristic curve measured and from this a first effective characteristic line is determined and then at a second  changed irradiance a second current-voltage Characteristic curve measured and from it a second effective characteristic curve determined, with the two determined effective Characteristic curves of the series internal resistance is determined.

From the standard DIN EN 60891 is a procedure for the conversion of measured current-voltage characteristics of PV modules from kris talline silicon on other temperatures and insolation known. A method for determining the Se internal resistance indicated. This is done by graphical Evaluation of the measured characteristic curves special working points determined and from this the series internal resistance is determined. The However, computational accuracy depends on the scale Character accuracy.

By describing the characteristic curve as an effective solar cell len characteristic are the required working points explicitly predictable, graphic inaccuracies are eliminated. Therefore, the effec tive characteristic curve determined and from it arithmetically the series resistance can be determined.

When measuring the two current-voltage characteristics is advantageous provided that this within a short period of time valles is carried out. So the measurements can be made one after the other at a distance of, for example, 1 min.  

In order to change the irradiance to possible without changing the spectrum To be able to change as simply as possible is advantageous see that after measuring the first characteristic curve over the PV A large filter is placed on the module surface caused spectral change. This can be done easily Realize at the installation site, using as a filter for example as a fine-mesh network is used. Such a network can be formed, for example, by a fly net or the like be and a mesh size of z. B. 5 mm.

To perform the procedure described above with a measuring device To be able to, for example, a known PV module Characteristic curve measuring device can be expanded with the following properties the. The previous external control computer is replaced by an in microprocessor replaced. The measuring device comes with a Provide control panel. The measurement result is shown on a display output. The algorithms for performing the measurement process rens are implemented in a non-volatile memory in the device mented.

The invention is illustrated below with reference to the drawing explained in detail. This shows in

Fig. 1 shows a current-voltage characteristic of a PV module,

Fig. 2 is an equivalent circuit diagram of a PV module as an ideal model,

FIG. 3 is an equivalent circuit diagram as a simple model,

Fig. 4 is an equivalent circuit diagram as a standard model,

Fig. 5 is an equivalent circuit diagram of a two-diode model,

Fig. 6 is an equivalent circuit diagram for the effective solar cell characteristic,

Fig. 7 shows a current-voltage characteristic curve for determining the resistance as a load of a solar module and

Fig. 8 is a measured current-voltage characteristic for series internal resistance determination.

The method according to the invention for quality control of PV modules under real ambient conditions at the place of use of the relevant PV module is based primarily on determining an effective solar cell characteristic from the current-voltage characteristic curve measured at the place of use. So-called solar cell characteristic models are required for this. The purpose of the approximation of the characteristic curves with equivalent circuit diagrams lies in the resulting explicit predictability of adaptation problems between PV solar generators and consumers. The following requirements must be made of a calculation method for adaptation tasks in the engineering area:

• - explicit calculation of the current-voltage characteristic equation U (I),
• - explicit calculation of the characteristic _equation parameters from the characteristic values I _sc , U _oc , I _pmax , U _pmax ,
• - Approximation accuracy in the range of the available measuring accuracy (state of the art: 1%).

A typical current-voltage characteristic curve with the aforementioned characteristic values is shown in FIG. 1.

There are several different equivalent circuit diagrams, which are shown in FIGS. 2 to 5. Fig. 2 shows a so-called ideal model. However, this has a low approximation quality, the following equations apply to the current I and the voltage U:

Fig. 3 shows a simple model that has a good approximation quality. The following equations apply here for the current I and the voltage U:

The numerical value for the internal resistance R _s can become negative.

Fig. 4 shows a standard model, which has a good approximation quality. The following implicit equation results for the characteristic:

An explicit solution for the voltage U is unknown.

Fig. 5, the so-called two-diode model shows. It has a very good approximation quality.

The following implicit equation results for the characteristic:

An explicit solution for the voltage U is unknown.

None of the four equivalent circuit diagrams shown ( FIGS. 2 to 5) fulfills all three predefined requirements.

However, it could be shown that the very good approximation quality of the two-diode model could also be achieved with the equivalent circuit diagram according to FIG. 3 if a negative numerical value for the series internal resistance R _{s is} permitted. Since negative resistances do not really exist, the element determined cannot be an ohmic resistor.

The element in the equivalent circuit diagram must therefore be represented by a fictitious photoelectric component, the characteristics of which correspond in a first approximation to that of a positive or negative resistor. The component should be described by R _pv (photovoltaic resistor). It should be noted that the true series internal resistance R _s should not be confused with the photovoltaic resistance R _pv .

Fig. 6 shows the equivalent circuit for the effective solar cell len characteristic.

The following follows for the effective solar cell characteristic:

Explicit form

With the introduction of the photovoltaic resistor, power calculations and calculations for part-load behavior can be carried out explicitly with the required accuracy of 1%. The reference quantity for the accuracy is the maximum power P _{max of} the examined solar generator.

To determine the four independent equation parameters R _pv , U _T I ₀ , I _ph , four independent characteristic values of the characteristic are also required. In the present case, these are the characteristic values I _sc , U _oc , I _pmax and U _pmax .

If the slope M for the open circuit voltage is also taken into account in the system of equations: (see FIG. 1):

The following five equations can be drawn up for the four parameters:

U (I = 0) = U _oc (10)

U (I = I _sc ) = 0 (11)

U (I = I _pmax ) = U _pmax (12)

From this system of equations, the equation parameters of the effective characteristic curve can be determined approximately as follows:

A prerequisite for the calculation of the four equation parameters is that the five characteristic values I _sc , U _oc , I _pmax , U _pmax , M are known with sufficient accuracy.

The four characteristic values I _sc , U _oc , I _pmax , U _pmax , can usually be determined by measurement with an error of <1%. The determination of the slope M, however, is associated with a larger error.

Since with the help of the equation systems (10), (11), (12), (13) from the four characteristic values I _sc , U _oc , I _pmax and U _pmax the equation _parameters R _pv , U _T , I ₀ , I _ph Let it be calculated and since the slope M (9) can be clearly calculated from the equation parameters with (8), there is a clear function:

M = (I _sc , U _oc , I _pmax , U _pmax ) (19)

The following general approximation functions were used for the increase M is derived, which is the calculation of the effekti ven solar cell characteristics with an accuracy of 1% equips.

With the equation constants

The following application example shows the operating point of a resistor R _L when directly connected to the PV module. We are looking for the resistor whose current I _L = 2 A flows:

The associated current-voltage characteristic curve is shown in FIG. 7.

It follows

U (I _L = 2 A) = 20.5 V

R _L = 10.25 Ω (23)

From the above, the series internal resistance can be determined as follows:
The standard DIN EN 60891 prescribes methods for converting measured current-voltage characteristics of photovoltaic components made of crystalline silicon to other temperatures and insolation. To determine the series internal resistance R _s under artificial sunlight, the following conditions must be observed:

• - At room temperature, two characteristic curves must be different irradiance (the size does not need to be known to be), but the same spectral distribution of the irradiance  be measured.
• - The temperature of the cells must be constant during the measurement be maintained (permissible tolerance +/- 2 ° C).

Two operating points U ₁ and U _{2 must} be determined from the two characteristic curves, from which the series internal resistance can be calculated. The standard gives the following procedure for determining the two operating points:

• - Definition of a current difference ΔI between short-circuit current and current in the selected working point of characteristic curve 2 (index 2 denotes the characteristic curve with the lower short-circuit current)
  ΔI = 0.5.I _sc2 (24)
• - Determine the working points U ₁ and U ₂ with (8)
  U ₁ = U. (I _sc1 - ΔI, R _{p ν 1} , U _T1 , I ₀₁ , I _ph1 ) (25)
  U ₂ = U. (I _sc2 - ΔI, R _{p ν 2} , U _T2 , I ₀₂ , I _ph2 ) (26)
• - Calculation of the series internal resistance
  

Since the spectrum for the measurement of the series internal resistance R _s does not have to be known, measurements can also be carried out outdoors under natural sunlight. The condition of the unchanged spectral distribution of the sunlight can be met by measuring the two characteristic curves within a short time interval (ΔT <1 min). The change in the irradiance without changing the spectrum is brought about by a large-area filter which is placed over the solar module surface immediately after the first measurement without a filter. A fine-mesh network is used as a filter (mesh spacing e.g. 5 mm), so the spectrum remains unchanged. Alternatively, instead of using a filter, the PV module can also be pivoted with respect to the direction of solar radiation, which results in a different angle of incidence.

The determination of the series internal resistance is shown with reference to the measurement shown in FIG. 8.

Measurement 1

Measurement 2

In FIG. 8, both the measured values and the effective characteristic curves calculated with (8) are entered to demonstrate the approximation accuracy.

Determining the current difference with (24)

ΔI = 0.5.I _sc2 = 0.398 A (30)

Working point U ₁ with (8)

U ₁ = U (I _sc1 - ΔI, R _{p ν 1} , U _T1 , I ₀₁ , I _ph1 ) = 18.38 V (31)

Working point U ₂ with (8)

U ₂ = U (I _sc2 - ΔI, R _{p ν 2} , U _T2 , I ₀₂ , I _ph2 ) = 19,663 V (32)

Series resistance

R _s = 1,067 Ω (34)

The peak power can be determined as follows. The peak power is the peak power under standard test conditions (STC).

P _pk = P _max (E ₀ , T _j0 ) (35)

Standard test conditions mean:

• - irradiance
  
• - Sun spectrum
  AM 1.5 (37)
• - cell temperature
  T _j0 = 25 ° C (= 298 K) (38)

When measuring the current peak performance under natural In addition to the irradiance, the ambient conditions also have current spectral distribution of light has significant impact kung on the photon yield. The spectral evaluation of the Solar cell sensitivity is expressed in the short-circuit current Solar cell off.

For the description of the spectral dependence entered a spectrally evaluated effective irradiance leads. The effective irradiance for a solar cell is only formed from the part of the solar spectrum that is used for Energy conversion is used in this solar cell. By analogy to the unit lux (lx) of lighting technology, where the irradiance  through human eye sensitivity tral is evaluated, spectral is proposed here Sensitivity of the solar cell through effective irradiation strength with a new unit "Photovoltaik Lux" (phox) describe. The short circuit current is a linear measure of the effective irradiance.

With AM 1.5 the following applies for the current irradiance E:

Outside of AM 1.5:

E _eff = I _SC .K _phox (40)

If the phox constant K _phox is known, the solar module to be measured itself serves as a "phox measuring device".

Determination of the phox constant K _phox at AM 1.5 on a clear day under natural environmental conditions:
AM 1.5 occurs when the zenith angle applies

With AM = 1.5 it follows

The times at which this zenith angle occurs can be determined with a known geographical location and date. Example: Dortmund (λ = 7 ° east longitude, ϕ = 51 ° north latitude) August 17, data in Central European Summer Time CEST

AM = 1.5 (am) 11 h 9 min
AM = 1.5 (pm) 16 h 4 min (43)

To determine the phox constant at AM 1.5 conditions with a pyranometer the irradiance is measured as well the short-circuit current that occurs.

If the phox constant for the module to be measured is known, if the short-circuit current is known, an additional measurement of the radiation intensity is not necessary for determining the peak power. For further calculations, the dependence on the irradiance is always related to the effective irradiance E _eff (measured in phox). To determine the peak power of a PV module, the phox constant of the module must first be known. If the phox constant is known, a current characteristic curve of the module can be measured at any time.

The current effective characteristic curve parameters can now be determined. With additionally known cell temperature T _j and the temperature coefficient c _{T of} the power, the expected values at STC can be calculated.

Conversion of measured values to STC

The peak power follows:

P _pk = I _pmax0 .U _pmax0 (47)

The following relationships can be approximately used to display the complete characteristic curve:

Some additional information is required to use relationship (46):

• - Power temperature coefficient silicon cells
• - cell temperature

The power temperature coefficient must be taken from the data sheet. If no information is available, the following value can be assumed as the standard value for crystalline silicon cells:

c _T = -0.0044 K ^-1 (49)

The systematic error of the peak power, due to the uncertainty of the temperature coefficient used, is in the range c _T = (-0.003 K ^-1 to -0.006 K ^-1 ) and is therefore less than 1%.

The cell temperature changes depending on the radiation and ambient temperature. Often there is a specification of the nominal operating cell temperature:

NOCT = T _j (E _N , T _ambN ) (50)

Test conditions for the determination of NOCT Nominal Operating Cell temperature

• - irradiance
  
• - ambient temperature
  T _ambN = 20 ° C (52)

The nominal operating cell temperature must be taken from the data sheet. If no information is available, the following value can be assumed as the standard value for crystalline silicon cells:

typical: NOCT = 48 ° C (53)

If the ambient temperature is known, the Approximately determine the cell temperature. The cell follows oil temperature depending on the irradiation.

The systematic error of peak power caused by the Uncertainty in the cell temperature determined is Range ΔT = (-10 K to +10 K) and is less than 5%.  

The measurement shown in FIG. 8 for determining the R _s was carried out at the following effective irradiance:

E _eff1 = 777 phox (55)

By covering the module with the fine-mesh network, the effective irradiance was reduced by a factor

Thus follows

E _eff2 = 309 phox (57)

The following cell temperature was determined for both measurements:

T _j = 294 K (58)

Because the two measurements within a short time interval have been carried out and the cell temperature is Mesh filter does not change abruptly, is in both measurements sang the same cell temperature.

To calculate the peak power using formulas (45), (46) and (47) the following values were used:  

Measurement 1

Measurement 2

The example case is a polycrystalline Module, year of manufacture 1991.

Quality control was carried out in February 2000.

Data sheet specifications

_Peak power P _pk

= 50 W (61)

Tolerance ± 10% (62)

Degradation losses in 10 years <10% (63)

With the guaranteed minimum power of 45 W and the degradations losses <5 W follows the permissible minimum power after 10 years ren of 40 W.

The 10-year guarantee has just been met here.

Claims (6)

1. Method for quality control of photovoltaic cells and modules (PV modules), characterized in that two current-voltage characteristic curves of the PV module are measured at different irradiance levels and the same spectrum under real ambient conditions at the location of the PV module in question, and two of these effective characteristics are determined, the series internal resistance and the peak power being determined from at least one characteristic based on these effective characteristics. 2. The method according to claim 1, characterized, that in addition to measuring the current-voltage characteristics effective irradiance is determined, which is only from the Part of the solar spectrum is formed, which leads to energy conversion is used in the respective PV module, and that the ambient temperature is measured and from the effective Characteristic curve, the effective irradiance and the reverse the peak power is determined. 3. The method according to claim 2, characterized, that one from the quotient of irradiance and associated  Short-circuit current formed under AM 1.5 conditions Phox constant of the PV module is determined and the effec tive irradiance is determined. 4. The method according to claim 1, characterized, that the measurement of the two current-voltage characteristics is carried out within a short time interval. 5. The method according to claim 4, characterized, that after measuring the first characteristic curve over the PV-Mo a large filter is placed on the surface caused spectral change. 6. The method according to claim 5. characterized, that a fine-mesh network is used as a filter.