Category Archives: EuroSun2008-13

Data basis and verification

The performance of the NWP model ARPS and the proposed MOS procedure is verified for the site Florianopolis, localized in the south of Brazil with 48° 3ri5’’W longitude and 27° 36’ 76’’S latitude. The measured global horizontal radiation within the period from January 2000 to June 2006 was used to calculate the daily mean values. For daily mean values, the utilized pyranometer CM11 has a measurement uncertainty of 1 % for 95 % confidence as stated in [31]. For the quality control [32], [33] the measured radiation vales have to appear in the measurement range of (0 to 1367) W/m2. If this criterion was not fulfilled for a time interval larger than 10 min, the daily mean value was rejected as recommended in [33]. From the training-validation of 6.5 years, 119 days were excluded by the quality criterion, leading to the remaining data, which appears in 53 consistent time series. To facilitate the implementation, the data blocks of the training data set were chained, rather than is accomplished a specific DWT of each block, obtaining three equal length vectors with synchronized day numbers. The vector of residuals {sA} is obtained by subtraction of the measured {H} from the forecasted daily solar radiation means {HA}. The obtained predictors sAsi_1 to sAsi-k and predictands sAsi (eqn. 3), selected from the partially reconstructed sub-signals (eqn. 2, {sAS}), have to consider the limitations of each of the data blocks to avoid uncharacteristic modification of the ANN input pattern. As to see in eqn. 3, the data of the first k days of each block are used exclusively as predictors, thus a time series with the length nj provide (n-k) training samples for the ANN. Each sample has an input vector with the pattern length k, the predictors, and one output variable, the observed predictand, of the forecast at the considered time scale. The total number of training-validation samples ntv is obtained with expression (5).

ntv = YU (nj — k ) (5)

Where j = 1…nb defines the number of data blocks obtained by the data qualification, with block individual number of training samples nj. The resulting data set is subdivided in two subsets, the training and the validation set. As recommended in Kaastra [34], the validation set, which is independent from the training set has to represent (10 … 30) % of the data. This set may be selected randomly from the data or it follows immediately the training set [34]. From the data the last year, representing a validation set of 18 % was separated. Due to hardware improvements of the measurement system [35], the validation set was not exposed to system outages, which leads to its consistence.

image113

4. Results

The average of the selected daily mean values of the measured solar radiation is 182.36 W/m2. For ARPS model simulations, based on the reanalysis dada, was obtained a RMSE of 70 W/m2 that corresponds to 38.4% of the measured average value. The maximal error of the daily mean solar radiation simulation is with 264.3 W/m2 higher than the measured average value (compare figure 3 — third chart and figure 4). If the correction is build up with data of twelve subsequent previous days (k = 12), a set of 1356 predictor vectors (sAs, (i-1) … sAs, (i-k) ) were selected. With the proposed MOS method the RMSE of the ARPS model reduces to 18.92 W/m2 for the training data set, which corresponds to 10.37 % of the measured average value of 182.36 W/m2. For the independent validation data set was obtained 9.06 % (see figure 3, fourth chart and figure 5). Worst performances were observed for the sub-signal di which contains the details of the higher frequency band (RMSE = 9.08 W/m2) and for the approximation sub-signal a1 (RMSE = 6.82 W/m2). The generalization performance of the ANN was verified with the validation data set. By arbitrary configured number of neurons in each layer with (k = 12), the best performance of the d1 sub-signal correction was observed, for 22 neurons at the first, and 12 neurons at the second hidden layer. This configuration of the neurons was used also for the other three ANN, whereby the one used for the approximation signal was configured as RNN, due to slight improvement in its performance. To access the probably higher boundary uncertainties under operation of the prediction model, it is necessary to accomplish ntv times (equation 5) the DWT for the data set having ntv predictor/predictand samples (see discussion in section 3.2). Avoiding numerical effort, the present article release only the results based on a single DWT of the data set as accomplished in

[9] .

Figure 3 — Daily mean values of the solar radiation — charts from the top to the bottom: (1) measured solar radiation H; (2) forecasted solar radiation with the ARPS model HA; (3) (H — HA); (4) (H — HA, corr), where HA, con — is the corrected ARPS forecast. The validation set appears from 2000 to 2500 days.

Models evaluation

The ordinary and residual kriging models were obtained based on the training dataset. After that, the validation dataset were used to evaluate these models. Particularly, model estimates were evaluates in terms of the ME, MAE, RMSE and the correlation coefficient.

Reference values for ground reflectivity

In order to calculate the ground reflectivity pg, in an iterative procedure values larger than the mean value of the distribution plus ag are filtered out, until convergence is achieved. The mean value of the new distribution is assigned to pg.

In the original Heliosat method a constant value ag was used, ag = 27 is a suitable value for MSG. But an approach that accounts for the dependency of ag on the sun-satellite geometry leads to even better results. The bias of the clear sky irradiance is used as a measure to quantify the influence of ag. For clear sky situations, the quality of the ground reflectivity can be evaluated directly and there is little super imposition with other effects. If ag is chosen too small, the corresponding values of the ground reflectivity will be too small as well. This results in an underestimation of the irradiance. On the other hand, if CTg is too high, an overestimation of the irradiance is the consequence. Best results were found for 15<=CTg<= 50 for different classes of solar and satellite zenith angles and the azimuth angle between sun and satellite.

Parameters influencing SSI

Clouds regularly cover about 50 % of the earth and represent the most important modulators of radiation in the earth-atmosphere system (Liou 1976). A cloud in libRadtran is characterized by its optical thickness (tc), type (cloud water or ice cloud), height of cloud top (ztop) and bottom (zbot) and the effective radius (ref) particles.

Many radiative transfer codes contain the same six atmospheric profiles corresponding to geographical and seasonal averages (Mayer, Kylling 2005; Vermote et al. 1997): Midlatitude Summer (afglms), Midlatitude Winter (afglmw), Subarctic Summer (afglss), Subarctic Winter (afglsw), Tropical (afglt) and U. S. Standard (afglus). “afgl” means Air Force Geophysics Laboratory.

The gas whose variation have a major influence on the SSI are water vapour (H2O), ozone (O3), carbon dioxide (CO2), oxygen (O2), methane (CH4), and nitrous oxide (N2O) (Vermote et al. 1997).

The attenuation of radiation by aerosols varies with its nature, density and size distribution. Following Shettle (1989), required parameters in libRadtran are: aerosol type from 0 km to 2 km altitude (haze), aerosol type above 2 km altitude (vulcan), season, and visibility (vis). The visibility is closely linked to the aerosol optical thickness (iaer) (Vermote et al. 1997). Taer represents the total extinction induced by aerosols of the medium for a given wavelength. It is sensitive to micro-physical properties of aerosols. Because these properties are difficult to assess accurately, the spectral variation of the aerosol optical thickness is usually calculated using a simplified method:

Taer A = в (Я/ Ям/" (1)

where AM = 1000 nm, в is the aerosol optical thickness at the wavelength 1000 nm and a is the Angstrom coefficient (Perrin de Brichambaut and Vauge, 1982).

When radiation reaches the earth’s surface, it can be absorbed or reflected. The intensity of the reflected radiation varies with the value of the incident radiation and the reflectance of the receiving surface. The ground reflectance is the function of the illuminating and emitting angles; the albedo is its hemispherical average. Both change with soil type and wavelength. A portion of this reflected radiation is then backscattered by the atmosphere and increases the value of the diffuse component of the SSI.

Unifying Access

The second major objective of the MESoR project is to unify and ease access to solar resource information. This builds upon experiences made within the SoDa portal, adopting mapping features from PVGIS web system. SoDa was build with proprietary software and communication protocols. As the World Wide Web evolved over recent years, the new MESoR portal builds upon open source software with a larger development community and standard web services. This will make the new portal more sustainable in terms of software development and the connection to the portal more easy and open as only widely accepted standards have to be followed.

The portal will serve as a broker to solar resource information and services. It does not contain and maintain data for itself. It just links data bases and services with a single point of entry and a common user interface. Databases and services have to be hosted by the providers. They keep control over their data and applications.

Metadata are essential to exchange knowledge between applications. They describe objects to be exchanged (e. g. a time series of irradiance, a geographical location, a date…). After a series of consultations with several bodies involved in standards, such as ISO, GEOSS (Global Earth Observation System of Systems), INSPIRE (Infrastructure for Spatial Information in Europe) and national meteorological offices, a thesaurus has been defined which is specific to solar resource. A thesaurus is a set of terms that describe the solar resource.

A prototype of the broker will be set up during the project. A new user interface has been designed. It utilises the API (application programming interface) of Google Maps. Users can therefore use the full capabilities (geographical search, maps and images) of Google to identify their sites and select the right locations or regions. As this interface is easy to use and applied already in many other applications it the user feels familiar with it. The front page of a service gives the site selection window and some descriptive information of the service, as a general description, property rights and credits, inputs and outputs descriptions. The results can be written to the browser window or saved in a specific format (e. g. spreadsheet-compatible). The available data bases can be selected by the menu on top of the page. Fig. 2 shows two sample screenshots of the current prototype (see http://project. mesor. net).

image031
Conclusions

The project MESoR together with the IEA Task 36 “Solar Resource Knowledge Management” aim at developing better guidance about the energy application of solar resource information. The benchmarking exercise will ease the comparison of different data sources and the standardised rules will make accuracy evaluations more transparent and comparable. The guide will help users in the choice of data sources and how to use the data for their specific needs.

The new broker portal aims to ease the access to solar resource data and online applications by serving as single point of entry to a variety of data sources all with a similar look and feel and common data formats. More information about the project can be found at http://www. mesor. net.

References

[1] L. Wald, (2006). Available databases, products and services. In Dunlop E. D., Wald L., Suri M. (Eds.), Solar Energy Resource Management for Electricity Generation from Local to Global Scale. Nova Science Publishers, New York, pp. 29-41.

[2] M. Suri (2007). Solar resource data and tools for an assessment of photovoltaic systems. In Jager-Waldau A. (editor), Status Report 2006, Office for Official Publications of the European Communities, Luxembourg, pp. 96-102.

[3] B. McArthur, (2004). Baseline Surface Radiation Network. Operations Manual Version 2.1.

[4] Iqbal, M., 1983. An Introduction to Solar Radiation. Academic Press, New York

[5] C. N. Long, E. G. Dutton (without year). BSRN Global Network recommended QC tests, V2.0 (available at BSRN website http://bsrn. eth. ch).

[6] B. Espinar, L. Ramirez, A. Drews, H. G. Beyer, L. F. Zarzalejo, J. Polo, L. Martin. Analysis of different comparison parameters applied to solar radiation data from satellite and German radiometric stations, Solar Energy (2008) In Press.

WRF Analysis Results

The Weather Research and Forecasting (WRF) system was developed by the National Oceanic and Atmospheric Adminstration (NOAA) National Centers for Environmental Prediction (NCEP). The current release is Version 2.2. The WRF modelling system software is in the public domain and is freely available for community use. The WRF is designed to be a flexible, state-of-the-art atmospheric simulation system that is portable and efficient on available parallel computing platforms. The WRF is suitable for use in a broad range of applications across scales ranging from meters to thousands of kilometres, including:

• Real-time NWP

• Forecast research

• Parameterization research

• Coupled-model applications

• Teaching

The WRF model is run over Europe with 3 nesting levels and using NCEP boundary conditions data. The variable validated is global solar radiation which is a direct output from WRF model. The period of data studied is the year 2005. The spatial resolution of the domain over Spain is 27km.

WRF model has been executed in analysis mode for the domain of the whole Europe with special resolution of 27km approximately and hourly temporal resolution. Three nested domain where used and the model was fed with data from NCEP global model. The period of simulation goes from 1/1/2005 to 28/2/2005.

The validation is done comparing with ground measurements from 40 stations from AEMet and variable direct output downward global solar radiation of WRF model. Normalized results of MBD and RMSD for hourly and daily resolution are presented in Fig. 6-4. The graphic shows all the stations ordered from lower latitude to higher latitude. Stations with lower latitude present a predominance of clear sky situations which results in small errors.

MBE Hourly Solar Radiation Forecasting

20

image107

25 L______________ [_________________ [_________________ Г________________ Г________________ [_______________ [_________________ [_________________ [

36 37 38 39 40 41 42 43 44

Latitude

Подпись: RMSE Hourly Solar Radiation Forecasting 0 36 37 38 39 40 41 42 43 44 Latitude

Fig. 6. Normalized MBD for hourly global solar radiation

Подпись: 20

MBE Daily Solar Radiation Forecasting

-30

36 37 38 39 40 41 42 43 44

Latitude

Fig. 8. Normaliazed MBd for daily global solar radiation

RMSE Daily Solar Radiation Forecasting

 

90

 

36 37 38 39 40 41 42 43 44

Latitude

 

image110

image111 image112

Overall the model is able to reproduce clear sky synoptic conditions. Errors are presents mainly in situations where fronts and huge cloud movements need to be reproduced. Fig. 10-13 presents a synoptic situation from the date 22/2/2005. Fig. 10. is the measurements from the satellite which represents the real situation. Fig. 12 is an instantaneous map of the global irradiance modelled. We can see as the model is missing a lot of small clouds. Fig. 11 and 12 represents measured and WRF modelled hourly global irradiance for two stations, Granada in the South of Spain and Oviedo in the north. In this date the model is getting a contradictory prediction with measurements. In Granada (lower middle part of Fig. 10.) the model is getting an overcast situation in the firs half of the day and clearsky in the other half, although measurements are the other way. In Oviedo (higher middle part of Fig. 10.) it is clearly an overcast situation and the WRF model predicts a clear sky situation.

2. Conclusion

WRF model has been validated for a whole year over Spain. The comparison has been done with ground solar global radiation measurements. Errors are from 30-98% in terms of RMSD for hourly global radiation and from 23-89% in terms of RMSD for daily global radiation. Different synoptic situations have been studied and the WRF model fed with NCEP data is not reproducing them quite well, however ECMWF model reproduce them in a much nicer way. Using ECMWF data as boundary conditions and testing other cloud parameterizations could improve results from WRF model in the future.

References

[1] Veziroglu TN and dot a. 21st Century’s energy: Hydrogen energy system. Energy Conversion and Management 2008; 49:1820-1831.

[2] Patlitzianas KD, Ntotas K, Doukas H, and Psarras J. Assessing the renewable energy producers’ environment in EU accession member states. Energy Conversion and Management 2007; 48:890-897.