Solar radiation ground measured data quality assessment report

ANALYSIS AND ASSESSMENT OF MEASURED
RADIOMETRIC DATA: TECHNICAL REPORT
November 20, 2012

ANALYSIS AND ASSESSMENT OF MEASURED RADIOMETRIC DATA Page 3 of 53
TECHNICAL REPORT:
ANALYSIS AND ASSESSMENT OF RADIOMETRIC DATA MEASURED
DATE:
November 20, 2012
AUTHORS:
IrSOLaV (INVESTIGACIONES Y RECURSOS SOLARES AVANZADOS S. L.).
CUSTOMER:
XXXX

INDEX
1 INTRODUCTION........................................................................................................................................... 5
2 METHODOLOGY .......................................................................................................................................... 7
3 QUALITY ANALYSIS................................................................................................................................... 10
3.1 Statistical Summary............................................................................................................................... 11
4 GRAPHICS.................................................................................................................................................. 25
4.1 Daily charts ............................................................................................................................................ 25
4.2 Weekly charts: global solar irradiance................................................................................................... 34
4.3 Weekly charts: direct normal irradiance (DNI)....................................................................................... 43
5 CONCLUSIONS .......................................................................................................................................... 52
6 REFERENCES............................................................................................................................................ 53

1 INTRODUCTION
The main objective of this report is to analyze the solar resource measured in the ground
meteorological station XXX, selected to host a solar thermal power plant. The solar resource
analysis applies to a site with the following geographical coordinates: Latitude XX, Longitude XX,
XXX meters of altitude, and located in XXX, hereinafter referred to as IrSOLaV 50MW CSP
PROJECT.
Global solar radiation is defined as the solar radiation received on a horizontal surface in a solid
angle of 2π steradians. As a result of the interaction of sunlight with the atmosphere, the global solar
radiation (G) is the sum of the direct component (B) (which has not interacted with atmospheric
components and therefore has not changed in its angle of incidence) and the diffuse component (D)
(result of atmospheric dispersion processes and it can be assumed to come from all points of the
sky and has no predominant direction). These three components, (G, B and D) are related to each
other by using the following expression, where ϴ is the zenith angle.
BBG  cos (1.1)
The particular characteristics of the interaction of the sunlight with the atmospheric components
force to make the measurements of solar radiation with various instruments and/or procedures,
depending on the component to be measured.
The most reliable and comprehensive recommendations to make the measurement of solar
radiation are established by the BSRN (Baseline surface Radiation Network) (McArthur, 1998). This
institution recommends that the measurements of the three components have to be done with a
configuration based on the use of a pyranometer to measure global horizontal solar irradiance, and
one with a shading device for the diffuse irradiance. Finally, the direct normal irradiance must be
measured with a pyrheliometer mounted on a solar tracker with two axes. Thus, by measuring the
three components independently allows using procedures for quality assessment of the
measurements based on the interrelationship between the three components (see Eq. 1.1).

The main errors in the measurement of solar radiation can be grouped into the following categories:
systematic errors of the measurement (such as a poor calibration of the equipment), errors by poorly
maintenance (dirty sensor domes, or presence of obstacles), and or malfunctioning of the solar
tracker.
This report presents an analysis of the quality of the measurements of the three components of solar
radiation based on the recommendations of the BSRN.

2 METHODOLOGY
Three components of the solar radiation (global, diffuse and direct) are measured in the station of
IrSOLaV using two pyranometers and one pyrheliometer. The period of the measurements is 1
minute and the temporal reference is Local Time. In particular, this study analyzes the solar
radiation data measured by the station since June 5, 2011 to May 7, 2012.
Before proceeding to the quality analysis of the measurements, we used an expression to transform
the temporal register from Local Time to True Solar Time (TST), which is a temporal reference
independent of the site where the measure have been acquired. The change to true solar time is
performed by two corrections; the first one takes into account the difference in longitude between
the meridian of the observer and the meridian of the temporal reference.. The second includes
various effects through the equation of time. We must point out that in the specific case of this
station the equation of time has not been applied due to the appreciation of a temporal shift in the
measurements if this value was used. Besides, many changes in the temporal shift of the clock have
been observed.
Once the temporal reference has been transformed to true solar time, comparisons are made and
the measured data is assessed using the following categories of filters levels:
1. Checking the time reference of the records;
2. Calculation of hourly values, daily and monthly averages;
3. Quality analysis with physical filters;
4. Quality analysis with cross component filters.
5. Quality analysis when the solar tracker is off under clear sky conditions.
The verification of the temporal reference of the records is checked to have certain that solar
irradiance is measured correctly between sunrise and sunset. This check is done visually and it uses
a model of clear sky. Graphics are plotted each day for the following components: direct normal and
global horizontal irradiance of clear sky, global horizontal and direct normal irradiance and diffuse
measurements. To estimate the values of clear sky, the model used is the ESRA (European Solar
Radiation Atlas) and the aerosol values used are the Linke Turbidity index provided by SODA
(Beyer et al., 1996, Dumortier, 1999, ESRA 2000a, ESRA 2000b). The graphs of the solar irradiance
components of ESRA clear sky model provide information of great interest. In addition, it allows the
visualization of the moments of sunset and sunrise, besides we can compare the measurements
with the values of the model in clear sky days. Accordingly, it is worth mentioning that the values of
clear sky model have uncertainty associated with the uncertainty of the Linke turbidity index

fundamentally. However, the comparison is useful in terms of the profile shape of solar irradiance
during the day as well as the relationship between direct and global irradiance for each day. Thus,
both the shape and the relationship between the components are comparable in the days of clear
sky conditions.
The quality analysis with physical filters refers to the verification of the recorded values of the
different components of the solar radiation, taking into account physical sense and not exceeding its
value, therefore, limits physically possible. Table 1 presents the physical limits imposed on each
component of solar radiation according to the recommendation of the BSRN.
Table 1: Physical limits of the solar radiation component
Parameter Minimum
Flag for
Minimum
Maximum
Flag for
Maximum
Global
Irradiance (GHI)
-4 2 1.2 2
1.5(cos ) 100 /SC zI W m   3
Diffuse
Irradiance (DIF)
- - 700 W/m
2
13
Diffuse
Irradiance (DIF)
-4 2 1.2 2
0.95(cos ) 50 /SC zI W m   4
Direct Normal
Irradiance (DNI)
-4 2 SCI  5
Direct Normal
Irradiance (DNI)
- - DNI Clear Sky 6
ISC: Solar constant (1367 Wm
-2
), ɛ: eccentricity of the orbit, ϴz: zenith angle
The quality analysis of component cross filters is used to check that the measured data meets the
interrelationship between the three components (GHI, DIF and DNI). Failure to pass these filters
establishes a supposition that any of the components were poorly measured or that the solar tracker
doesn’t points to the sun properly. The next table shows the conditions imposed on the cross
components analysis.

Table 2: Conditions for the cross component
Parameter Conditions Limits Flags
cos z
G
D B 
2
75º, cos 50 /z zD B W m    1 ± 8% 7
cos z
G
D B 
2
75º 93º, cos 50 /z zD B W m     1 ± 15% 8
D
G
2
75º, 50 /z G W m   < 1.05 9
D
G
2
75º 93º, 50 /z G W m   < 1.10 10
The next procedure relates the three components but using a more tight procedure. This test is
based on the comparison of instruments which measure the same variables. The next table defines
the limits for this procedure:
Table 3: Conditions for the second group of cross component filters
Parameter Lower Limit Upper Limit Flags
B·cos z (G-D)-50 W/m
-2 (G-D)+50 W/m
-2
11
G-D B cos z - 50 W/m
-2 B cos z + 50 W/m
-2
12
The next procedure (4.1) applied relates the diffuse component (DIF or D) and global extraterrestrial
irradiance (Gext) using the diffuse index defined as:
Kd=
A higher limit of 0.6 is given to this filter and in case it is not fulfilled the flag number 14 is activated.
The next procedure makes use of clearness index (Kt) which is defined as the quotient between
ground measured global solar irradiance (GHI or G) and extraterrestrial solar irradiance (Gext). In this
procedure we establish the next condition for the activation of flag number 15:
If Kt is lower than 0.2 and D/G is lower than 0.9 then flag 15 is activated
The flag number 16 uses the same variables as the last filter but with the following conditions:
If Kt is higher than 0.5 and D/G is higher than 0.8 then flag 16 is activated

The last filter (Flag 17) used is named as the tracker off filter and it is used to detect when the solar
tracker is not working correctly. First, the global solar irradiance (Sum SW) is estimated from
measured diffuse solar irradiance and measured direct normal irradiance using the expression
which relates the three components (Eq. 1.1). Then the following condition is established using clear
sky global irradiance (Gcclear) estimated with the model of ESRA and monthly climatological Linke
Turbidity values from SODA:
For D > 50W/m2
,
If (Sum SW)/Gcclear>0.85 and if D/(Sum SW) the the tracker is not properly following
the sun.
This last filter only works under clear sky conditions.
Besides this filters, we have estimated direct normal irradiance (Ibest) from measured GHI and DIF
using the following expression:
Where Kd0 is defined as:
Kd0=
And is the angle of solar altitude.
3 QUALITY ANALYSIS
Based on the methodology of analysis developed in the previous section, the result of this statistical
analysis is presented. On the other hand, daily measures are plotted together with the estimation of
global and direct normal radiation for a clear sky day. The profiles of solar radiation to clear skies
have been estimated with the model of ESRA, as indicated in the methodology section. The daily
charts are provided in individual files for each day separate from this report. The format of the files is
PNG (Portable Network Graphics).

3.1 Statistical Summary
3.1..1 Box-Whisker Diagrams
In this section, we can see that the box-whisker plots show the principal moments of the distribution
of each component of the solar radiation, GHI (Figure 1), DIF (Figure 2) and DNI (Figure 3). Box-
Whisker diagrams were made with the average 60 minute records of each component of solar
radiation measurement. In these charts you can check the distribution of the population measured,
using as parameters the following distribution: the median, represented by the red dotted line, the
percentiles 25% and 75%, represented by the upper and lower edges of the boxes blue,
respectively, and outliers, represented by red crosses.
Box-Whisker diagram of the global solar radiation (Figure 1) shows a median with a profile very
similar to the horizontal global radiation on a clear sky day, as expected.
Figure 1: Distribution of global radiation. Box-Whisker representation.
Regarding diffuse irradiance (Figure 2), we must point out that the outliers belong to values when
the solar tracker was not working properly.

Figure 2: Distribution of diffuse radiation. Box-Whisker representation.
Finally, Box & Whisker diagram of direct normal radiation (Figure 3), as expected, it shows a
medium with a very similar profile of normal direct radiation on a clear day. However, there are many
outliers in the sunrise and sunset.

Figure 3: Distribution of direct radiation. Box-Whisker representation.
3.1..2 Statistics from the filtering
Below we show the contour plots of the flags which have been obtained after analyzing the data with
the filters presented previously. These diagrams show the distribution of the flags in percent, which
have been detected for each month and hours.

Figure 4: Flag contour diagram 1 (%). Percentage of data which is OK
Figure 5: Flag contour diagram 7 (%)

Figure 12: Flag contour diagram 17 (%). Solar Tracker Off.
In the next graphics, the flag values detected individually for each hour and each month is plotted.
The flag number 0 indicates that the sun elevation is lower than 0. In green color we indicate that the
registration of the three components of solar irradiance is correctly measured. With the red color we
indicate that some or all of the solar irradiance components (GHI, DIF or DNI) are suspicious to be
wrong or are wrongly measured.

Figure 13: Flags for each hour and day. Month: June. Year: 2011
Figure 14: Flags for each hour and day. Month: July. Year: 2011

Figure 15: Flags for each hour and day. Month: August. Year: 2011
Figure 16: Flags for each hour and day. Month: September. Year: 2011

Figure 17: Flags for each hour and day. Month: October. Year: 2011
Figure 18: Flags for each hour and day. Month: November. Year: 2011

Figure 19: Flags for each hour and day. Month: December. Year: 2011
Figure 20: Flags for each hour and day. Month: January. Year: 2012

Figure 21: Flags for each hour and day. Month: February. Year: 2012
Figure 22: Flags for each hour and day. Month: March. Year: 2012

Figure 23: Flags for each hour and day. Month: April. Year: 2012
Figure 24: Flags for each hour and day. Month: May. Year: 2012

4 GRAPHICS
Here we show some samples from daily and weekly graphics to explain the most important errors
that have been detected in the measurements.
4.1 Daily charts
Here some daily charts are shown to explain individual cases where errors have been detected.
First we show a figure to explain the information which is presented in the daily graphics. The
variables which are included are the following: global horizontal irradiance (GHI) in black color (
), global extraterrestrial irradiance (Gext) in red color ( ), direct normal irradiance
(DNI) ( ), clear sky direct normal irradiance (Bcclear) ( ), diffuse irradiance (DIF) (
), clear sky diffuse irradiance (Dcclear) ( ), estimated global horizontal irradiance
(Sum SW) obtained from measured diffuse irradiance and direct normal irradiance using the
following equation DIF+DIRcosθ ( ), estimated direct normal irradiance (Ibest) obtained from
measured global horizontal and diffuse irradiance ( ) and clear sky global horizontal irradiance
(Gcclear) ( ). Besides, the flag code indicates which is the quality control value detected for
each one of the three magnitudes of solar irradiance measured. The green color in this number
indicates that the measure is right and the red color indicates that the measurements are suspicious
to be wrong or are wrong. Downside another row indicates if the solar tracker is working properly.
The code 0 with green color in the background indicates that the solar tracker is working ok and in
red color with code 1 we indicate that it is not working properly. The malfunctioning of the solar
tracker is only identified when there is a clear sky day condition.

Figure 25: Example to explain the meaning of the codes representing the quality flags
In the next figures we show days when a malfunctioning of the solar tracker has been detected:
Figure 26: Julian Day: 161, Year: 2011

As mentioned before, the Solar Tracker off flag only works for clear sky days. However, as we can
observe in the next figures, when the solar tracker is off and there are within a particular day clear
sky conditions and cloudy conditions for certain hours, the trackeroff flag doesn’t work properly for
the hours with cloudy sky conditions. However, the other quality flags detect that there are errors in
the measurements when the clear sky conditions are not present and the trackeroff flag fails
detecting a malfunctioning in the solar tracker. These facts can be observed from the following
figures:
Another advisable thing to point out from the daily graphics is that when global horizontal irradiance
(GHI) reaches the level of global clear sky (Gcclear), measured direct normal irradiance (DNI) doesn’t
reach the value of clear sky direct normal irradiance (Bcclear). This fact can be observed visually from
the next graphics, because this “possible” underestimation is not detected by the flags due to the
fact that in some days the absolute difference between measured direct normal irradiance (DNI) and
estimated one (Ibest) from GHI and DIF is lower than 100W/m2
. From the next figures, it can be
observed visually the days when there is a difference in measured and estimated direct normal
irradiance, however the flags detect that the measurements are right.

However, other days show moments when the cross components quality checks filters detect an
error in the measurements, related mainly in what we suppose is a non-properly measurement of
DNI as can be observed from the next figures:

However we can’t affirm with 100% of certain that the failing of cross component filters is due to not
properly measuring of DNI. Although the data-logger measures the three variables at the same time
assigning the same temporal reference we can observe in the next figures a deviation of the GHI
measurement to the left (non-symmetric measurements) compared with clear sky global irradiance,
which in consequence makes the flags detect errors in the measurements.
Figure 39: Detection of errors in the measurements due to not properly registering the temporal
reference of GHI
4.2 Weekly charts: global solar irradiance
Here we present the weekly plots:

Figure 40: Global Solar Radiation: Week 1-2011 Figure 41: Global Solar Radiation: Week 2-2011

Figure 50: Global Solar Radiation: Week 11-
2011
Figure 51: Global Solar Radiation: Week 12-2011

Figure 52: GHI: Week 19-2011 Figure 53: GHI: Week 20-2011

Figure 88: GHI: Week 19-2012
4.3 Weekly charts: direct normal irradiance (DNI)
Next we present weekly charts of hourly DNI for independent days.
Figure 89: DNI: Week 1-2011 Figure 90: DNI: Week 2-2011

Figure 137: DNI: Week 19-2012

5 CONCLUSIONS
As a summary and conclusions, we sum up that:
 The solar tracker was not working properly until August 2011, so measurements of
diffuse and direct normal irradiance for June and July 2011 are bad and should be
removed consequently from the database to avoid any misuse.
 The temporal reference should be clarified and corrected. From the analysis we have
concluded that equation of time is already being applied to measurements.
 There are many values which don’t qualify to pass the cross component filters. This could
be for the following reasons:
o Errors in the calibration constant of the instruments.
o The diffuse pyranometer is not correctly being shaded.
o Pyrheliometer is not correctly pointing the sun disc. We have observed that when
global horizontal irradiance reaches clear sky level direct normal irradiance
doesn’t reach clear sky DNI estimated using the same Linke Turbidy values.
Besides, higher values of DNI compared with measured ones are estimated using
diffuse and global measured irradiance. However, it is difficult to say if the high
DNI estimated from measurements of DIF and GHI is due to not measuring
properly DNI or it is used to not making a proper register of solar irradiance.
o The shaded and unshaded instrument needs corrections of its measured values
due to Infrared loss. However, the magnitude of this error in the measurements is
estimated to be lower than 10W/m2
[Dutton EG et al].
o Installation of a GPS to correct the temporal deviations of the data logger clock is
highly recommended.

6 REFERENCES
Beyer, H. G., Wald, L., Czeplak, G. and Terzenbach, U., 1996. Solar radiation maps for the new
European solar radiation atlas ESRA. Proceedings of: The 1996 EuroSun Congress, Freiburgh
(Germany).
Dumortier, D., 1999. The European Solar Radiation Atlas and the Satellight web server.
Proceedings of: 2n Workshop on satellites for solar energy assessments, Golden (USA).
ESRA, 2000a. The European solar radiation atlas. Vol. 1: Fundamentals and maps. Edited by:
Scharmer, K. y Reif, J. Les Presses de l'Ecole des Mines, Paris (France).
ESRA, 2000b. The European solar radiation atlas. Vol. 2: Database and exploitation software.
Edited by: Scharmer, K. y Reif, J. Les Presses de l'Ecole des Mines, Paris (France).
McArthur, L. J. B., (Report 1998). Baseline Surface Radiation Network (BSRN). Operations Manual
V1.0. WMO/TD-No.879.
Dutton EG, Michalsky JJ, Stoffel T, Forgan BW, Hickey J, Nelson DW, et al. 2001. Measurement of
Broadband Diffuse Solar Irradiance Using Current Commercial Instrumentation with a
Correction for Thermal Offset Errors. Journal of Atmospheric and Oceanic Technology. Mar
1;18(3):297-314.

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