Automatic Quality Control of Daylight Measurements: Software for IDMP Stations. BENOIT MOLINEAUX and PIERRE INEICHEN. 1 Introduction. The earlier ...
Automatic Quality Control of Daylight Measurements: Software for IDMP Stations BENOIT MOLINEAUX and PIERRE INEICHEN
1 Introduction The earlier quality control procedure was found to be limited in its ability to detect certain significant instrumental errors. Involving mostly the same tests based on geometrical relations and model predictions, the method described here was developed with the experience of two years of daylight data acquisition. The computer program based on this method allows in addition to choose the validation limits in order to be able to increase the precision of the tests. This appears useful as the desired precision of the data bank usually depends on the final use which is made of the data. The program reads an input file based on the ASCII format defined earlier and creates an output file in the same format with AQC flags. Finally, the visualization of the comparisons between measured and predicted values is possibly the greatest attribute of the program due to the nature of the errors encountered. For example, if a sensor goes out of alignment the sequential plotting of the graphs enables to rapidly detect when the problem arrived and with what order of magnitude.
2 The AQC tests Three types of test are used. The first is the geometrical relation between global, direct and diffuse irradiance or illuminance components. Eeg = Eed +Ees·cosθz
(1)
θz is the solar zenith angle. The second is based on comparing measured data with predictions from the empirical models. Although these models have been extensively validated, the predictions have a non zero uncertainty which must be taken into account. The AQC flags generated using this method can therefore not be considered as exact but rather as a very good indication as to the validity of the data. The general trend of the models is to define two parameters, sky clearness and sky brightness (respectively ε and ∆), based on the sole measurements of global and direct (or diffuse) irradiance. Depending on these two parameters, irradiance on tilted surfaces, normal beam illuminance, illuminance on horizontal and tilted surfaces can be predicted with a root mean square error ranging from 5 to 20%. It appears essential to achieve a reliable validation of global and direct (or diffuse) irradiance as these are used as input parameters to predict all other parameters. Global irradiance, measured from a fixed instrument, is the less likely to be erroneous, diffuse irradiance may be measured with a fiuxed or mobile instrument. Direct irradiance is the most delicate to measure precisely and it is therefore the first to be considered doubtful if the three are not compatible according to eqn (1). If only two of the three are available, the third is computed from the other two. A third parameter used by the luminous efficacy models is the height of precipitable water content, W [cm], which is computed from the dew point temperature, Tdp. However, if Tdp is not available, the program will ask for an average value of W. (Model precision is not significantly deteriorated when using a constant value of W = 2cm with Geneva data.) AQC - B. Molineaux - P. Ineichen
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With validated illuminance measurements it becomes possible to test the sky scans by comparing scan integrals with illuminance measurements. This is the third category of AQC tests based on the following relations. Diffuse illuminance on a horizontal plane is measured, it can also be computed by integrating the 145 luminance measurements evenly distributed over the sky dome : Evd =
2 π 145 scan ·sinh i i ∑ np i =1
(2)
scani measured luminance at point i if the center of the point is more than 6o away from the sun, otherwise scani = 0 [cd/m²] hi angular height of the point above the horizon np number of points more than 6° away from the sun, = 144 2π number of steradians within the sky dome (½ sphere) seen by the horrrizontal plane. Diffuse illuminance on a tilted plane can be obtained from relations (3) and (4): Evd =
π 145 scan ·sinh i pi ∑ np i =1
(3)
Evd = Evg - Evs ⋅ cosθz
(4)
hpi angular height of point i above plane, θz : solar zenith angle scani luminance measured at point i if in field of view of the plane and more than 6° away from sun, otherwise scani = 0 [cd/m²] np number of points for which scani ≠ 0 π number of steradians within the ½ dome (¼ of sphere) seen by the vertical plane. Note: distance between points is approximately 12°, each point covers about an 11° angle. The choice of eliminating points within 6° of sun implies only one point is eliminated, representing direct illuminance. Minor differences may be introduced, depending on the weather, due to the fact that direct illuminance is actually measured within a 5.7° angle. Time duration of scan (about 30s for the PRC Krochmann and 2.5 mins for the EKO sky scanner) is another possible source of disagreement if weather is rapidly changing. The following table gives a list of the AQC tests, Fig 1 gives their order of application. No.
Test
Parameter(s) being tested
Default validation limit
1.
Eed, Ees→Eeg
Eeg, Eed, Ees
100 W/m²
2.
Eeg, Eed→Ees
Eeg, Eed, Ees
100 W/m²
3.
Eeg, Ees→Eegn/e/s/w
Eegi, Eeg, Ees
100 W/m²
4.
Eeg, Ees→Evg
Evg, Eeg, Ees
10 klux
5.
Eeg, Ees→Evd
Evd
10 klux
6.
Eeg, Ees→Evs
Evs
10 klux
7.
Evg, Evd→Evs
Evs
10 klux
8.
Eeg, Ees→Evgn/e/s/w
Evgn/e/s/w & Eeg, Ees
10 klux
9.
Evgn/e/s/w, Ees→scan integrals
sky scans
10 klux
Table 1: The AQC tests AQC - B. Molineaux - P. Ineichen
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Each test refers to either a geometric relation (test 1, 2, 7 and 9) or a model (other tests). Each model is symbolized by the input and output data. The tests are only applied if the concerned parameters are all available. If either tests 1 or 2 is failed, Ees is flagged inconclusive and the procedure is continued with a value of Ees derived from Eeg and Eed. Eeg and Eed will then be flagged according to the results of tests 3, 4 and/or 8, see Fig. 1. Test 8 is a combination of the irradiance tilted surface model and the luminous efficacy model applied to the diffuse and direct components to predict global illuminance on a vertical plane. This combination gave best results with our Geneva data [1]. The validation limits are absolute values and are plotted on the graphs displayed by the program, see Fig. 2. Absolute values were chosen as most representative after a close statistical study of the differences between modelled and measured values. The default values given are derived on a ± 3RMSD basis observed with 2 year’s instantaneous data from Geneva [1]. Integrated hourly data should accept more precise validation limits.
3 The AQC procedure The following figure shows the different paths followed by the program depending on the availability of the different parameters and their success or failure in passing the AQC tests.
Figure 1: Order of application of the AQC tests
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Symbol Evg ? i, ii, iii Y, N, P, F 1, 2, 3... A B C F*
Meaning Question, is Evg available? 1, 2 or all 3 of Eeg, Eed & Ees are available Yess, No, Pass, Fail: result of previous question or AQC test Numbered AQC test, as in Table 1 and Fig. 2 All data is inconclusive From here on Eeg & Eed are validated Test 9 can be applied if Evgn/e/s/w and Evs are validated If test 1 or 2 is failed, Ees is recalculated from Eeg & Eed
These tests are only applied if the data read from the input file are not flagged «9» or «999», if the values are positive and if they have passed exceeding value tests (based on the solar constant). The procedure is not applied if global irradiance is less than 5 [W/m²] or if the sun is below the horizon. 4 Using the AQC.EXE software 4.1 Input data files: The input files must be ASCII with white space separators, based on the CIE format defined in [2]. The program can treat one or more white spaces but not blank lines between entries or records. Each value must be followed by its AQC flag, whether tested or not. The program recognizes the beginning of the data list as the first time the station identificator (eg. USA2 or CH1) appears in the file, the identificator must therefore not appear in the preceding comments and is asked to the user before attempting to read data from the file. Version 1 of this program will fail if comments are added after the station identificator appears (such as suggested by R. Seals). The program carries out a series of coherence tests on the data read from the input file. You will have to quit execution if there is an incoherent value found in the record header (day number > 365 or 366, latitude > 90° etc) or in the file header. Negative irradiance, illuminance or luminance values found in the input file will be reduced to 0 and flagged inconclusive in the output file (unless there was a missing value flag). All calculations call upon the solar altitude. The solar altitude is computed using the time found in the file, if the altitude is also found in the file and disagrees with the computed value by more than 2°, the program is interrupted and a message appears on the screen. The execution may be continued upon pressing any key. 4.2 Output data files: The output file reproduces the same sequence of data as the input file with AQC diagnostics. The number of characters per line is limited to 80 and there will be only one white space separator between entries (the number of entries per line may vary, depending on the size of the entries). The name and path of the input and output data files is chosen by the user. 4.3. AQC flags: Only the last digit of the quality control flags will be modified by this program. The flag must contain at the most 3 digits, no attention is paid to the first two digits (if these are different from 0, they will be copied from input to output file) and only in the case of a 9 (missing data) will any attention be paid to the last digit of the flags found in the input file. If the flag contains only one digit, this is considered as the last AQC - B. Molineaux - P. Ineichen
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of the three digits. If the flag found in the input file exceeds 999 the program will be interrupted and an error message will appear, any key will set the program running again. The AQC digit found in the output files will be either «9» (missing data), «1» (test is inconclusive), «2» (data is validated) or «3» (data found questionable). Due to the nature of the tests and their order of application many instrumental errors will actually fall in the category «inconclusive» as the program, comparing two quantities, does not know which of the two is questionable. 4.4 Display of Results: a. VGA monitor: If you are using a VGA monitor a number of graphs can be drawn on the screen while the program is running, based on 9 comparisons between modelled/calculated and measured values. The following figure is an a example of the screen obtained with our data from March 1993. If an error is detected when reading a record from the file, the program is interrupted and the error message appears flashing on the bottom line of the screen. The time and date displayed are those for which the error was detected. The time is true solar.
Figure 2 Graphical display, research class stations
OPTIONS: An option is given for the screen to be cleared at the beginning of each new day, enabling the user to spot the days for which the data may be questionable. The graphs may also be drawn without generating output files, enabling a first visualization of the quality of the data. The program may be interrupted at any stage and will give a visualization of the statistical results obtained so far. For general class stations the second line of graphs is replaced by four separate graphs differentiating the results obtained concerning the illuminance measured on the four vertical surfaces.
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b. All monitors: A final table is available to both CGA and VGA monitors and shows the summary of the comparisons between calculated and measured values. The RMSD and MBD (Root Mean Square Difference and Mean bias Difference) are given for the 9 comparisons between computed and measured values according to the AQC tests. It is important to note that these results are given before application of the models as AQC tests. The proportion of validated, questionable, inconclusive and missing values is also given. An example of such a table is given below:
Figure 3: Table of results, March 1993, Geneva data
The first four lines of numbers give the results and the number of points concerned by the tests (see line above, Test:). Results are given before application of the models as AQC tests. The next five lines of numbers include missing data and refer to the parameter listed in the line, Var:. The columns NESW concern the four vertical surfaces together. The sky scans are compared to five illuminance values (4 vertical, 1 horizontal).
5 References [1] B. Molineaux, P. Ineichen, Quality Control Procedure for Illuminance, Irradiance and Luminance Measurements. IEA Task XII Expert Meeting France Lyon (Jan. 1993). [2] CIE TC-3.07, Guide to Recommended Practice of Daylight Measurement A CIE Publication, Vienna, Austria. August 1994.
6 Acknowledgments The software presented here was developed with sponsorship from the Swiss Federal Energy Office and the University of Geneva. Finally, the authors would greatly appreciate if a copy of the final table displayed by the program, summarizing the results obtained with different sets of data, could be sent back to them. AQC - B. Molineaux - P. Ineichen
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