U.S. patent application number 12/316354 was filed with the patent office on 2009-07-02 for evaluation method.
This patent application is currently assigned to SMA Solar Technology AG. Invention is credited to Peter Drews.
Application Number | 20090171597 12/316354 |
Document ID | / |
Family ID | 39437406 |
Filed Date | 2009-07-02 |
United States Patent
Application |
20090171597 |
Kind Code |
A1 |
Drews; Peter |
July 2, 2009 |
Evaluation method
Abstract
The invention is directed to an evaluation method for
determining a power reduction due to ageing of at least one
photovoltaic module at constant radiation intensity with a
measurement of an electric variable that may change after a period
of time such as cell current, cell voltage and/or cell power,
without additional sensors for measuring the radiation intensity,
with the following steps fixing at least one class k that can be
compared year by year and within which a daily power curve or
portions of the daily power curve of the photovoltaic module can be
compared to each other due to the radiation intensity and the
outside temperature to be expected as well as the radiation time,
the class k corresponding to a defined period of a year, measuring
energy output values Ek,n that can be compared year by year from a
comparable power curve Pk,n (t) through the variable delivered by
the photovoltaic generator so that energy differences are
determined on the basis of the classes k, n being the respective
year, indicating a power reduction of the photovoltaic module with
respect to one or several previous years from the energy output
value E k,n directly delivered by the photovoltaic module by
calculating the difference with respect to the energy outputs Ek,i
of the years i to be compared, data of comparable days having a
comparable power curve P(t) of the photovoltaic module being
observed over several years.
Inventors: |
Drews; Peter; (Kassel,
DE) |
Correspondence
Address: |
PYLE & PIONTEK, LLC
221 N LASALLE STREET , ROOM 1207
CHICAGO
IL
60601
US
|
Assignee: |
SMA Solar Technology AG
Niestetal
DE
|
Family ID: |
39437406 |
Appl. No.: |
12/316354 |
Filed: |
December 11, 2008 |
Current U.S.
Class: |
702/60 ;
702/64 |
Current CPC
Class: |
Y02E 10/50 20130101;
H01L 31/02 20130101; H02S 50/10 20141201 |
Class at
Publication: |
702/60 ;
702/64 |
International
Class: |
G01R 21/00 20060101
G01R021/00; G01R 21/06 20060101 G01R021/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 1, 2008 |
EP |
EP 07024889.3 |
Claims
1. An evaluation method for determining a power reduction due to
ageing of at least one photovoltaic module at constant radiation
intensity by measuring an electric variable that may change after a
period of time such as cell current, cell voltage and/or cell power
without additional sensors for measuring the radiation intensity
with the following steps fixing at least one class k that can be
compared year by year and within which a daily power curve or
portions of the daily power curve of the photovoltaic module can be
compared to each other due to the radiation intensity and the
outside temperature to be expected as well as the radiation time,
the class k corresponding to a defined period of a year, measuring
energy output values Ek,n that can be compared year by year from a
comparable power curve Pk,n (t) through the variable delivered by
the photovoltaic generator so that energy differences are
determined on the basis of the classes k, n being the respective
year, indicating a power reduction of the photovoltaic module with
respect to one or several previous years from the energy output
value E k,n directly delivered by the photovoltaic module by
calculating the difference with respect to the energy outputs Ek,i
of the years i to be compared, data of comparable days having a
comparable power curve P(t) of the photovoltaic module being
observed over several years.
2. The evaluation method as set forth in claim 1, characterized by
acquiring and/or filing a daily curve of the power P(t) of the
generator as well as of the energy output E of the day for the
generator, in particular in a data bank, determining a daily power
curve Pk,n(t) the course of which is comparable at least with a
daily power curve Pk,i(t) from previous years i and filing the
daily output curve Pk,n(t) found and the energy output Ek,n in a
data bank.
3. The evaluation method as set forth in claim 1, characterized by
forming daily classes kj, which are allocated to fixed time
intervals of a day.
4. The evaluation method as set forth in claim 1, characterized by
forming and evaluating a first derivation of the function P(t).
5. The evaluation method as set forth in claim 1, characterized in
that only energy output values are compared which come from
measurement periods comprising only one daily power peak.
6. The evaluation method as set forth in claim 1, characterized in
that data with comparable power curve P(t) of the photovoltaic
module are observed over several years.
7. The evaluation method as set forth in claim 4, characterized by
determining comparable days in one step by determining and
evaluating the number of zero crossings of the first derivation
P'(t) is determined and evaluated.
8. The evaluation method as set forth in claim 4, characterized by
determining comparable days in two steps by a. evaluating the first
derivation P'(t) of the function P(t) using a first criterion and
b. evaluating the daily power curve P(t) using a second
criterion.
9. The evaluation method as set forth in claim 8, characterized by
the first criterion for evaluating the first derivation P'(t) by
the fact that the extreme values of the first derivation P'(t) will
not exceed an upper limit value P'max and will not fall below a
lower limit value P'min or that the area included by the curve of
the first derivation P'(t) will not exceed a fixed limit value.
10. The evaluation method as set forth in claim 8, characterized by
the second criterion for this evaluation of the daily power curve
P(t) by the fact that the daily energy output Etag is formed and
that this daily energy output lies above a fixed limit value or
that the maximum of the daily power curve P(t) lies within a
tolerance band Pabs_min and Pabs max.
11. The evaluation method as set forth in claim 1, characterized by
specifying limit values in the first year of measurement and
verifying the limit values in the following years.
12. The evaluation method as set forth in claim 1, characterized by
acquiring at least one average value .DELTA.Emittel,n from the k
energy output difference values .DELTA.Ek,n for the year n as a
measure for power reduction.
13. A solar plant with means for carrying out the measurement
method as set forth in claim 1.
Description
1. FIELD OF THE INVENTION
[0001] The invention relates to an evaluation method for
determining a power reduction due to aging of at least one
photovoltaic module at constant radiation intensity by measuring an
electrical variable that may vary after a while such as cell
current, cell voltage and/or cell power without additional sensors
for measuring the radiation intensity.
[0002] The power of solar cells or of photovoltaic modules is
subject to aging. The loss of efficiency ranges from 10% to 20%
over a period of time of 20 years.
[0003] For mounting, solar cells are combined into modules,
so-called solar or photovoltaic modules or solar panels. After 20
years, a solar module only has 90% to 80% of the power
indicated.
2. DESCRIPTION OF THE PRIOR ART
[0004] From a study "Pratt R. G. et al: "Power of a 4 kW
amorphous-silicon alloy photovoltaic array at Oakland Community
College, Auburn Hills, Mich." XP010750513" it is known to record
the efficiency and the energy output of the plant over a short
period of time. It appears from the study that the mere comparison
of e.g., the energy output in the same month of e.g., two
consecutive years does not allow inference on the aging of the PV
modules since the values characterizing the energy output such as
radiation intensity and temperature differ too much.
[0005] Methods are known, which are based on artificially
accelerated aging tests. These tests are only performed for certain
conditions such as temperature and irradiation and are thus usually
allocated safety factors. Hence, the efficiency a manufacturer
guarantees after a certain time is usually less than the actual
efficiency drop. For such tests, additional sensors are moreover
utilized for sensing for example the temperature or radiation
intensity.
[0006] In practice, the power drop of the solar module is very
difficult to follow over time. It is difficult to locate whether
the power drop is within the limits indicated by the manufacturer
since certain general conditions are needed for this purpose such
as a certain outside temperature, precise sensors or calibratable
radiation sensors and the like.
BRIEF SUMMARY OF THE INVENTION
[0007] It is the object of the invention to find a method of the
type mentioned herein above by means of which a long-term power
reduction of solar modules in an installed solar plant can be
readily determined with high accuracy.
[0008] This object is solved by the following method steps: [0009]
fixing at least one class k, which is comparable on a year-to-year
basis, within which a daily power curve or portions of the daily
power curve of the photovoltaic module can be compared to each
other on the basis of the radiation intensity and the outside
temperature to be expected as well as of the radiation time, said
class k corresponding to a defined time range, [0010] measuring
energy output values Ek,n that can be compared on a year-to-year
basis from a comparable power curve Pk,n (t) by the variable
delivered by the photovoltaic generator so that energy differences
will be determined on the basis of the classes k, n being the
respective year, [0011] indicating a power reduction of the
photovoltaic module with respect to one or more previous years of
the energy output value Ek,n immediately delivered by the
photovoltaic module by calculating the difference from the energy
outputs Ek,I of the comparable years i, [0012] data of comparable
days having a comparable power curve P(t) of the photovoltaic
module are being observed over several years.
[0013] Further advantageous implementations of the invention are
characterized in the dependent claims.
[0014] Thanks to the measurement method of the invention, it is
possible to determine very precisely a long-term power reduction
without the need of additional sensors.
[0015] The invention relies on the observation that comparable days
in terms of radiation intensity and temperature of the solar plant
can be found over several years, these days allowing a reliable
statement with regards to power reduction thanks to their
comparability. In order to ensure comparability in terms of solar
radiation, it is possible to compare over the years days in which a
power curve P(t) is almost identical for example. Since the
radiation intensity and the radiation time fluctuate in the course
of a year, the year is divided in several periods, i.e., in several
classes k. k may for example be equal to 52 so that the comparison
may be made weekly. The actual comparison then only occurs within
one class k over the years n.
[0016] Accordingly, the invention relies on the idea consisting in
observing the energy output of the solar plant over several years.
Actual data are compared with the values in previous years. This
allows locating in which way the energy output decreases over the
years.
[0017] In every year, the days must be found at which the energy
output of the solar plant is comparable with values of previous
years in order to be capable of making a reliable statement in
terms of power reduction.
[0018] Accordingly, for each year n, there may be a maximum of k
energy values. These energy values can be compared to the energy
values of a previous year, for example to those of the previous
year or of the first year. The difference between the energy values
is for example a measure of the power reduction. This difference
can be normalized. The energy amount of the previous year may serve
as a standard.
[0019] Accordingly, the invention allows observing over the years
power reduction within one class. Thanks to this accurate
measurement, a solar plant can thus be connected for a longer time
to a grid without maintenance works. It is also possible to locate
early increased power reduction so that photovoltaic modules can be
exchanged in time.
[0020] To measure the energy output, a variable of the generator
such as the generator current, the generator voltage or the
generator power can be measured.
[0021] In an advantageous embodiment of the measurement method of
the invention, there are provided at least two classes k, which are
distributed over the year and within which the daily power curves
or the portion of the daily power curve are comparable on the basis
of the radiation intensity and the outside temperature as well as
of the radiation time. This allows for taking into consideration
the seasonal fluctuations of the radiation intensity and of the
outside temperature as well as of the irradiation time to be
expected.
[0022] Sensing or filing a daily curve of the power P of the
generator as well as of the energy output E of the photovoltaic
generator of the day is particularly advantageous, a day power
curve P(t) being determined, the course of which is comparable to
at least one day curve Pk,n(t) from previous years. Accordingly, a
measurement of the day curve of the power of the solar generator
P(t) as well as of the daily energy output Etag, i.e., of the two
variables is advantageous. These values can be recorded and filed
in a data bank, for example by means of a data logger. Within each
class k, one day power curve Pk,n(t) the course of which is
comparable to the day power curves Pk,i(t) from previous years can
be determined for each year n. This also means that at most k
energy output values Ek,n are determined for each year, said energy
output values being compared to the energy output values Ek,I of
the previous years i, for example the first year values Ek, 1 so
that at the most k values are determined for the energy differences
.DELTA.Ek,n. At least one of these energy differences .DELTA.Ek,n
related to an energy output value Ek,i of a previous year, for
example of the first year Ek,1 is used to indicate the power
reduction of the solar generator.
[0023] Accordingly, within each class, one day or one measurement
period is determined the power curve P(t) and the energy output E
of which is comparable to measurements performed in earlier
days.
[0024] It is advantageous that a number ktmax of day classes kj is
formed. The day classes correspond to time periods that are
associated with fixed time intervals and that are distributed over
the daily sunshine period to be expected. This comparison then
occurs between measurement intervals comprising both the same k and
kj classes. One class may also be a class having a same pair of
coefficients (k, kj) or a same tuple. In the variant widened to
include the day classes, there is a maximum of k*kj energy values
E((k,kj),n), which are compared to the energy values of the
previous years. Differences and averages can be calculated
here.
[0025] In an advantageous embodiment of the invention, comparable
days are determined within one class in two steps at most, namely
by calculating and evaluating the first derivation Pk'(t) of the
function Pk(t).
[0026] The first derivation Pk'(t) of the data host Pk(t) is hereby
calculated and it is checked whether certain limit values have been
respected. If these limit values are exceeded or not reached, it
can be assumed that this day was cloudless.
[0027] Alternatively, the first derivation Pk'(t) is calculated and
evaluated by testing a curve area included therein for fixed limit
values. In a first step, the first derivation Pk'(t) then enters an
evaluation method which yields values that are not allowed to
exceed or fall short of imposed limit values.
[0028] Using a plausibility criterion, it can be clearly determined
in a second step whether the day is really cloudless. One
plausibility criterion may for example be measurement data from the
data host Pk(t) lying within tolerance bands such as a tolerance
band for a certain region or a tolerance band for summer days and
one for winter days. This means that this plausibility criterion is
applied to the data host P(t) for example.
[0029] An alternative plausibility criterion is applied to the
energy output Ek,tag. The energy output Ek,tag is not allowed to
fall short of an imposed limit value either.
[0030] As an alternative or in addition thereto zero crossings of
the first derivation Pk'(t) are advantageously evaluated in another
implementation. A day is clearly cloudless if there is only one
zero crossing. This evaluation method can be performed in only one
step.
[0031] It is further sensible to impose limit values in the first
year and to verify these limit values in the following years.
[0032] In another advantageous developed implementation of the
method of the invention there is provided that at least one average
value .DELTA.Emittel is calculated from the k energy difference
values .DELTA.Ek,n for the year n as a measure for power reduction.
By calculating the average, it is possible to make a very precise
statement about the aging condition of the solar cells. The
advantage of this way of proceeding is that quotients or percentage
values obtained thereby, meaning daily and weekly values, can be
averaged both for the k and for the kj classes. One thus obtains a
very wide statistical basis, which ensures good accuracy of the
values.
[0033] A particular benefit is obtained if a daily power curve is
determined, which substantially comprises a daily power maximum.
This case occurs if the day is cloudless. In this variant of the
invention, only energy values from measurement periods are compared
in which there was no shadow. Shadowing can be recognized with the
methods mentioned herein above.
[0034] In this variant, time periods of shadow are recognized by
evaluating the change in luminosity occasioned by passing clouds.
In principle, one locates the change in luminosity by calculating
the derivation of the power generated by the solar plant with
respect to time. High values of such a derivation are evaluated by
an evaluation algorithm which calculates whether the time range of
the measurement was influenced by clouds. An effect of benefit is
obtained by implementing an evaluation standard through fix or
adaptive threshold values. An evaluation standard with amount
averages, quadratic averages or other values can be utilized. An
advantage is obtained if a time range with shadowing is recognized
by comparing the amount of power or energy generated in the time
range with a comparative value. If there are significant negative
differences, there were shadows. Such a comparative value can be
generated from a model of radiation on a cloudless day. A
comparative value can also be calculated from the radiation values
of previous days, in particular if these corresponding time
intervals were recognized to be cloudless. An effect of benefit is
obtained if a comparative value is calculated from values of
previous years that have been stored.
[0035] An exemplary embodiment will be described in closer detail
with reference to the drawings, additional developed
implementations of the invention and advantages thereof being
described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] In said drawings:
[0037] FIG. 1a shows a matrix for several years 1 through n as well
as several classes 1 through k showing energy outputs,
[0038] FIG. 1b shows a matrix comparable to that of FIG. 1a, this
matrix now showing the energy differences,
[0039] FIG. 2a shows a measured curve of the power P of a
photovoltaic generator for a cloudless day,
[0040] FIG. 2b shows a schematic course of the curve shown in FIG.
2a,
[0041] FIG. 2c shows the course of the first derivation for the
function shown in FIG. 2b,
[0042] FIG. 3a shows a measured gradient of the power P of the
photovoltaic generator for a day with passing clouds, is FIG. 3b
shows a schematic curve of the power shown in FIG. 3a for the day
with passing clouds,
[0043] FIG. 3c shows the course of the first derivation for the
function shown in FIG. 3b,
[0044] FIG. 4a shows the measured course of the power P of the
photovoltaic generator for a cloudy day,
[0045] FIG. 4b shows the schematic curve of the power P for this
day with overcast sky,
[0046] FIG. 4c shows the curve of the first derivation for the
function shown in FIG. 4b.
DETAILED DESCRIPTION OF THE INVENTION
[0047] FIG. 1a illustrates a matrix with energy output data for
several years 1 through n, which are indicated in the matrix lines
as well as for several classes 1 through k, which are indicated in
the columns. The formulae below the matrix indicate the energy
differences for each class k and for the respective year n. For
each year n, one then has k energy values E.sub.k,n. These energy
values passing clouds can then be compared to the energy values
E.sub.k,l of the year before i, of any previous year or of the
first year. The difference .DELTA.E.sub.k,n=E.sub.k,i-E.sub.k,n is
taken to measure the power reduction. This difference can be
normalized. The energy output of the year before i can for example
serve as a standard so that the power reduction can be expressed by
.DELTA.E.sub.k,n/E.sub.k,i=(E.sub.k,i-E.sub.k,n)/E.sub.k,l As a
result, a power reduction of the photovoltaic modules can be
observed within one class over several years.
[0048] Accordingly, in FIG. 1a, .DELTA.E.sub.1,n through
.DELTA.E.sub.k,n signify energy differences for each class.
.DELTA.E*.sub.l,n means normalized energy differences for each
class; this will be explained in closer detail later.
[0049] In accordance with the invention, the method is based on the
measurement or on the acquisition of the daily curve of the power
of the solar generator P(t) as well as on a daily energy output
Etag. These data can be stored in a data bank.
[0050] Distributed over the year, there are several classes k. Two
classes k are required though. This is beneficial in order to take
into account a seasonal radiation intensity as well as a seasonal
dependent outside temperature.
[0051] Within each class k, a power curve Pk,n(t) is determined for
each year n, said curve having a course that is comparable with the
daily power curves Pk,I of previous years i.
[0052] I.e., for each year, a maximum number k of energy output
values E.sub.k,n are ascertained, which are compared to the energy
output values E.sub.k,I of the previous years, for example with the
energy output values E.sub.k,l, so that a maximum number k of
values is determined for the energy differences
.DELTA.E.sub.k,n.
[0053] At least one of these energy differences .DELTA.E.sub.k,n
related to an energy output value E.sub.k,l of a previous year i,
for example of the first year value E.sub.k,l, is used to indicate
the power reduction of the photovoltaic generator.
[0054] Preferably, the energy outputs are related to one day. I.e.,
within each class one ascertains a day the power curve P(t) and the
energy output E of which are comparable with measurements performed
in earlier days. For each year n, one then has k energy values
E.sub.k,n, which relate to one day. These daily energy values
E.sub.k,n can then be compared with the daily energy values
E.sub.k,n-1 of the year before or with the energy values E.sub.k,l
of the first year or of any year.
[0055] In FIG. 1b there is shown another matrix which includes the
energy differences for several years 1 through n as well as for all
classes 1 through k. Below the matrix, there is shown a measurement
graph showing the energy differences .DELTA.E related to the years
1 through n.
[0056] From these variables, mean values from all the k energy
differences .DELTA.(E.sub.k,n) can be acquired for one year n,
namely with the formula:
.DELTA. E mittel , n = 1 k m = 1 k .DELTA. E m , n ##EQU00001##
[0057] For normalized energy differences, the above mentioned
formula takes the following form:
.DELTA. E mittel , n * = 1 k m = 1 k .DELTA. E m , n *
##EQU00002##
[0058] Then, the values .DELTA.E.sub.mittel,n or
.DELTA.E*.sub.mittel,n indicate a yearly average value for power
reduction of the photovoltaic generator in the respective year
n.
[0059] It is also possible to consider quadratic averages or an
average to the power of p, i.e., .DELTA.(E.sub.k,n).sup.p or
.DELTA.(E.sub.(k,kj),n).sup.p. As a result, a significant change in
the values is possible through which the ageing process of the
cells can be normalized. An effect of benefit is obtained if p is a
constant comprised between 2 and 6. Herein after, location is
described for comparable days.
[0060] The FIGS. 2a, 3a and 4a show different daily power curves
P(t) for different weather conditions. In FIG. 2a, the measured
power curve is based on a cloudless day. In FIG. 3a, a measured
curve of the power P of the solar generator is shown as a function
of the time for a day with passing clouds, the sun irradiating
periodically the photovoltaic generator through holes in said
clouds. In FIG. 4a, the measured curve of the power P of the solar
generator relates to a very cloudy day or to a day with constant
weak solar radiation.
[0061] As shown in FIG. 2a, the radiation power of the sun
increases at sunrise. About noon, it reaches its maximum peak.
Toward sunset, the radiation falls toward zero again. FIG. 2a
accordingly illustrates the measured curve of the power P of a
photovoltaic generator or of one or more photovoltaic modules as a
function of time t for a cloudless day. As can be seen from the
curve, there is only one daily maximum with respect to power P.
There is no power break due to passing clouds.
[0062] In FIG. 3a it can be seen that there are strong power
fluctuations. The intensity of the radiation of the photovoltaic
modules, which has changed because of the passing clouds, can be
seen clearly.
[0063] If a day generates a curve as shown in FIG. 4a, the day is
for example cloudy or rainy and the solar radiation quite low.
Typically, this may be a winter day.
[0064] Accordingly, the FIGS. 2a, 3a and 4a show typical measured
power curves
[0065] P(t) for days with different weather conditions, these being
in discrete form, i.e., they constitute effective measured
variables. For simplicity's sake, these functions are shown
schematically or as continuous functions in the FIGS. 2b, 3b and
4b. The discrete measurement data can also be transformed into
continuous functions through appropriate interpolation methods.
[0066] Preferably, in a first step, the first derivation P'(t) of
the power curve P(t) is formed and evaluated for each day, as shown
in the FIGS. 2c, 3c and 4c.
[0067] FIG. 2b shows the cloudless, sunny day. A plurality of
measurement results are filed in a data bank over the day.
[0068] These results are shown in the curve through measurement
points on the curve. The curve has a day maximum peak, which is
typically about noon. The area below the curves corresponds to the
curve integral or to the energy.
[0069] FIG. 3b schematically shows the power curve for passing
clouds. A kind of harmonics, which are generated by the periodical
shadowing through the clouds, are superimposed on a basic curve,
which corresponds to the curve in FIG. 2b.
[0070] In order to be capable of determining a power reduction of
the solar module with very high accuracy, data having a comparable
power curve P(t) of the photovoltaic module or of the solar plant
are preferably observed over several years. As they are comparable,
it is possible to make a reliable statement with respect to power
reduction of the photovoltaic module. In principle, completely
cloudy days as illustrated in FIG. 4a or 4b or days with passing
clouds as shown in the FIG. 3a or 3b are in principle suited for
comparison. A comparison with cloudless days is however preferred,
i.e., a curve as shown in FIG. 2a or 2b is compared year by
year.
[0071] The measurement method preferably uses the power curve P(t)
as well as the energy output E that corresponds to the area
enclosed by the curve. This area is hatched in the FIGS. 2b, 3b and
4b. Both are evaluated. Through this measurement method, additional
information such as outside temperature or radiation data is not
needed. Sensors are not needed either since the power data are
measured from the variable of the photovoltaic module that has been
delivered. A voltage, a current or both can be measured. It is also
possible to directly measure the power.
[0072] The FIGS. 2c through 4c show the first derivation P'(t) of
the functions shown in the FIGS. 2b through 4b.
[0073] FIG. 4b shows an example for the curve of the power of the
photovoltaic generator as a function of time (t) for a day with
overcast sky. In FIG. 4c, there is for example shown the associated
first derivation with respect to time. As opposed to a cloudless
day, the maximum power Pmax can however be significantly less.
[0074] In a first step, the function P'(t) is formed.
[0075] The first derivation P'(t) can be evaluated in different
ways. The evaluation clearly indicates whether the day is cloudless
or not, as shown in FIG. 3c.
[0076] In a second step, one then analyzes and makes certain
whether a cloudless day has indeed been found. For this purpose,
the power curve P(t) or its first derivation P'(t) is evaluated.
Preferably, two evaluation steps are utilized in order to reliably
acquire a comparable cloudless day.
[0077] In one of the steps, the first derivation P'(t) is
evaluated. For this purpose, there are two possibilities of
evaluating the first derivation P'(t). A first possibility is based
on the fact that the evaluation method is based on analyzing a
maximum for P'(t). If, as shown in FIG. 2c, the maximum value P'max
is for example below an imposed limit (upper dashed line) or if the
minimum value P'min is above an imposed limit P'min (lower dashed
line), it is supposed that the day is cloudless.
[0078] As shown in FIG. 3c, the first derivation of the power curve
for a day with passing clouds has a much higher maximum value P'max
but also a much lower minimum value P'min than the first derivation
of the power curve for a cloudless day shown in FIG. 2c. In the
FIGS. 2c, 3c and 4c, the upper limit P'max and the lower limit
P'min are also shown as dashed lines. Such limit values can also be
defined for certain regions. This is possible because average
radiation values are known in principle for all the regions in a
country. Since radiation values are not only known for regions but
also e.g., for certain cities, fine-tuning is possible. These limit
values are advantageously acquired and fixed for e.g., a cloudless
day in the first year the plant is in operation. Then, verification
is performed in the course of the years. Thus, even long-term
climatic changes in a region due to climate change can be taken
into consideration.
[0079] As shown in FIG. 4c, the values of the first derivation of
the power curve for an overcast day are also below or above the
imposed limits. Another criterion can be readily used to
undoubtedly and automatically locate a cloudless day. This is
advantageous because a completely cloudy day yields a daily power
curve P(t) that is similar to that of a cloudless day.
[0080] A second possibility of evaluating the first derivation
P'(t) is described herein after. In this variant of the evaluation
method, P'(t) is also formed from the power curve P(t) for each
day.
[0081] For each day, the integral is for example determined
I = 1 Tag .intg. Tag ( P ' ( t ) t ) 2 t ##EQU00003##
If the value I acquired lies below a maximum allowable limit I_max
so that I<I_max, it can be assumed that the day is cloudless and
this day can be included in calculating the power reduction. This
limit is obtained from typical radiation values and depends for
example on the geographical situation. The corresponding day is
then fixed according to the same pattern as in the previous example
of the evaluation method of P'(t).
[0082] The integral I is a measure for the area included in the
first derivation P'(t). A comparison between the FIGS. 2c and 4c
clearly shows that the area enclosed by the curve is significantly
smaller on a cloudy day. This area is determined by the integral I.
This means, if I<I_max, the day may also be cloudy. Therefore,
it is appropriate to perform an additional evaluation step.
[0083] In another possibility of evaluating the first derivation
P'(t), only the zero crossings of the P'(t) are taken into
consideration. If the day is for example cloudless, the number of
zero crossings of the curve P(t) is equal to 1. This zero crossing
takes place at the time of power maximum, as shown in FIG. 2c. If
more than one zero crossing is located, as is illustrated in FIG.
3c, it can be assumed that the day is not cloudless.
[0084] Herein after, the second step of evaluating the power curve
P(t) is described in closer detail.
[0085] Since in the first step it is at first only supposed that
the day is cloudless or not, this must be confirmed in a second
step.
[0086] There are different variants to achieve this. The first
possibility is to evaluate the daily power curve P(t) in the second
step.
[0087] The first method for evaluating the daily power curve P(t)
consists in determining the daily energy output Etag and in
comparing it with an imposed minimum value. If the daily energy
output Etag exceeds this minimum value, it is certain that the day
is cloudless.
[0088] The second evaluation method in the second step consists in
evaluating the extreme values of the power curve P(t). For this
purpose, an absolute value of the power Pabs is acquired from the
host of data P(t) measured within one day. It may for example be
the maximum value Pmax of the power P(t) for the day observed or
also an average of several power maxima. If this value Pabs lies
within a tolerance band ranging from Pabs_min to Pabs_max, then it
may well be a relatively cloudless day. Indirectly one also
considers the radiation intensity and the duration without the need
for an additional sensor.
[0089] This method can be even further improved by using the
measurement or the measurement values of the temperature of the
modules and/or of the outside temperature.
* * * * *