The enerMENA Meteorological Network – Solar Radiation Measurements in the MENA Region

International audienceFor solar resource assessment of solar power plants and adjustment of satellite data, high accuracy measurement data of irradiance and ancillary meteorological data is needed. For the MENA region (Middle East and Northern Africa), which is of high importance for concentrating solar power applications, so far merely 2 publicly available ground measurement stations existed (BSRN network). This gap has been filled by ten stations in Morocco, Algeria, Tunisia, Egypt and Jordan. In this publication the data quality is analyzed by evaluating data completeness and the cleanliness of irradiance sensors in comparison for all of the stations. The pyrheliometers have an average cleanliness of 99.2 % for week-daily cleaning. This is a 5 times higher effort than for Rotating Shadowband Irradiometer (RSI) stations which even have a slightly higher average cleanliness of 99.3 % for weekly cleaning. Furthermore, RSI stations show a data completeness of 99.4 % compared to 93.6 % at the stations equipped with thermal sensors. The results of this analysis are used to derive conclusions concerning instrument choice and are hence also applicable to other solar radiation measurements outside the enerMENA network. It turns out that RSIs are the more reliable and robust choice in cases of high soiling, rare station visits for cleaning and maintenance, as usual in desert sites. Furthermore, annual direct normal and global horizontal irradiation as well as average meteorological parameters are calculated for all of the stations

HiConPV - hochkonzentrierende PV am Solarturm

Im Rahmen eines EU-Projektes wurde ein neues hochkonzentrierendes PV-System bis zum Prototyp-Stadium entwickelt, Komponenten erfolgreich getestet und System- und Marktanalysen durchgeführt. Das System kombiniert Parabolspiegel bzw. Heliostate mit großflächigen, modularen III-V Solarzellen, die für 1000-fache Konzentration ausgelegt sind. Mittelfristig werden für die HiConPV-Technologie bei Anlagengrößen um 1 MW Installationskosten von etwa 1 €/W prognostiziert und damit Stromgestehungskosten unter 0,10 €/kWh erwartet

PRECISE MEASUREMENTS OF SOLAR BEAM IRRADIANCE THROUGH IMPROVED SENSOR CALIBRATION

Solar beam irradiance is measured for solar energy applications due to two reasons: to determine the available amount of the solar resource and energy yield at sites selected for an installation or to evaluate efficiency and proper operation of a concentrating solar power plant. However, the requirements on data and devices are different for both applications. In this paper, the equipment and methods used at DLR for improved solar beam irradiance measurements are presented with reachable accuracy for both applications. Different thermal irradiance sensors are compared among each other and to an absolute cavity radiometer. The sensors are calibrated with the absolute cavity radiometer to get utmost accuracy for precise field measurements. Besides, they are subsequently used for the calibration of RSP sensor heads for remote automatic weather stations, which reach a mean accuracy of 3%

Hochpräzise Messung solarer Direktstrahlung infolge verbesserter Sensorkalibration

Hochpräzise Messungen der solaren Direktstrahlung sind für Anwendungen der konzentrierenden Solartechnik eminent wichtig: Bei Wirkungsgradmessungen können Messungen in Standardpräzision bis zu 80% der Messun-sicherheit ausmachen beziehungsweise für eine Standortevaluierung unter Verwendung einer nicht angemesse-nen Messausrüstung leicht zu Fehlentscheidungen führen, die Kosten in die Höhe treiben oder erfolgversprechende Projekte verhindern. Deshalb wurde am DLR eine entsprechend hochpräzise Messausrüstung angeschafft und adäquate Sensoren zur Standortevaluierung getestet und qualifiziert

Soiling of irradiation sensors and methods for soiling correction

Access to exact solar irradiation data is indispensable for planning and dimensioning of applications as e.g. solar power plants. The expectable amount of yearly solar irradiation has an over-proportional impact on the financing and therefore has to be known very precisely. For this reason, DLR performs investigations at PSA to improve the accuracy of irradiation data, which are measured at several locations in southern Spain and Morocco. Devices as Rotating Shadowband Pyranometers (RSP) as well as thermal pyranometers and a pyrheliometer are used. Besides well-documented and known technical aberrations, soiling of the sensors is an important source for underestimation of the measured irradiation, especially at offside stations where daily maintenance is not possible. In this paper, we present soiling characteristics of these sensors and a corresponding method for its correction

Corrections for Rotating Shadowband Pyranometers for Solar Resource Assessment

Solar irradiation data is scarcely available in regions suitable for solar energy applications. For the design of solar power plants, accurate but affordable measurements are indispensable. Accurate sensors are little appropriate for remote stations due to soiling, high power consumption and elevated costs. Rotating shadowband devices represent a prospective alternative. This paper describes new corrections for the systematic deviations of a Rotating Shadowband Pyranometer (RSP) with an integrated temperature probe. The correctional functions were developed on data from 23 different RSPs over an entire year. An average reduction of the root mean square deviation of direct normal irradiation from above 6.5% to below 2.5% was achieved

Best Practices for Solar Irradiance Measurements with Rotating Shadowband Irradiometers

Large-scale solar plant projects require diligent solar resource assessments. For concentrating solar technologies the focus of the resource assessment lies on direct beam irradiation. Unfortunately, high accuracy irradiance data are scarcely available in regions which are attractive for solar energy applications. Satellite data can only be used in combination with ground data to estimate inter-annual variability and long-term mean values. Hence, new ground measurements have to be collected for solar plant projects. Ground measurement data usually show significantly higher accuracies than satellite derived irradiance data, when general guidelines regarding site selection and preparation, instrument selection and maintenance and data quality monitoring are respected. These best practices for Rotating Shadowband Irradiometers (RSIs) are presented in this document. Appropriate irradiance sensors for ground measurements must be selected in consideration of general surrounding conditions for equipment and maintenance to gain and maintain the necessary accuracy over the entire operation period. Thermopile instruments like pyrheliometers as specified in ISO standard 9060 [ISO9060 1990] are severely affected by soiling [Pape2009] and also require expensive and maintenance- intensive support devices such as solar trackers and power supply. Thus, the uncertainty of resource assessment with pyrheliometers depends heavily on the maintenance personnel and cannot be determined accurately in many cases. Due to their low soiling susceptibility, low power demand, and comparatively lower cost, Rotating Shadowband Irradiometers (RSI) show significant dvantages over the thermopile sensors when operated under the measurement conditions of remote weather stations. RSIs are also known as RSP (Rotating Shadowband Pyranometers) or RSR™ (Rotating Shadowband Radiometers). Here we use the notation RSI to refer to either instrument measuring irradiance by use of a rotating shadowband following the decision of the International expert group in IEA Solar Heating and Cooling Task 46, subtask B. The initially lower accuracy of RSIs, which can yield deviations of 5 to 10 % and more, is notably improved with proper calibration of the sensors and corrections of the systematic deviations of its response. Main causes of the systematic deviations are the limited spectral sensitivity and temperature dependence of the Si-photodiode commonly used in most RSIs. Besides the systematic deviations of the sensor response, a significant contribution to the measurement inaccuracy originates from the sensor calibration at the manufacturer, where no corrections are applied. For proper calibration however, the proposed corrections need yet to be considered in the calibration procedure. While well documented standards exist for the calibration of pyrheliometers and pyranometers ([ISO9059 1990], [ISO9846 1993], [ISO9847, 1992]) they cannot be applied to RSIs and no corresponding standards exist for RSIs This document contains RSI specific best practices for the following tasks: - Requirements on the selection of a location for a measurement station - Installation, operation and maintenance of a measurement station, including the case of remote sites - Documentation and quality control of the measurements - Correction of systematic errors & instrument calibration: procedure and frequency Also the performance and accuracy of RSIs are described

Long-term behavior, accuracy and drift of LI-200 pyranometers as radiation sensors in Rotating Shadowband Irradiometers (RSI)

Rotating Shadowband Irradiometers (RSI) are frequently used for solar resource assessment at remote sites due to their significantly higher robustness for soiling, their lower power and maintenance requirements and their cheaper acquisition and operation in contrast to pyrheliometer on tracker systems. The primordial lower accuracy of their photodiode sensor, usually a LI-200 pyranometer from LI-COR Inc., is mainly caused by restrictions of their spectral sensitivity and temperature dependence. However accuracy is notably increased by application of corrections to the raw sensor response. Thus, finally a coincidence of DNI measurements from RSIs with high-precision pyrheliometer measurements within 15 W/m2 (root mean square deviation for 10 min averages) for actual values, less than 3% for daily DNI and within approximately 1.5% of the monthly and annual sum is reached. Within this contribution, the long-term behavior of the LI-COR sensor is examined with regard to the drift of the photodiode sensitivity. This is analyzed from recalibrations of 30 sensors after one to four years and from long-term studies lasting from one to several years. If a significant drift appears, the corresponding uncertainties can be reduced through recalculation of the previous measurement data for the total measurement campaign. Furthermore, studies about the coincidence and deviation of the responses for global, diffuse and direct irradiance of RSI measurements between several individual RSIs and to reference measurements from high-precision thermopiles are presented for different time resolutions

Monitoring of Mirror and Sensor Soiling with TraCS for Improved Quality of Ground based Irradiance Measurements

Meteorological stations for solar irradiance measurements are mainly utilized for resource assessment of possible sites for future solar power plants and for thermal efficiency calculation and control of operating power plants. These stations consist of a solar tracker, two pyranometers and a pyrheliometer (MHP) for irradiance measurements. The accuracy of the MHP instrumentation is usually specified to be better than 2% if cleaned on a daily basis. However, soiling frequently exceeds other error influences significantly, reducing irradiance values and accuracy. Due to the high sensitivity to soiling shown by pyrheliometers, especially DNI measurements are affected. Reductions of measured DNI values exceeding 25% in only a few weeks are not unusual. In order to improve this situation, the soiling level of each individual sensor can be determined by following a special sequence of sensor cleaning and brief breaks combined with a close examination of the sensor responses. This allows for an approximate post processing correction of the irradiance data measured since the last cleaning (if recent). The corrections applied are cross- checked by means of an improved version of the TraCS asset. It can be used to control the sensor soiling correction procedure. The TraCS's improvement consists in rotating the mirror within its plane with the pyrheliometer thus scanning its surface instead of just viewing the same small spot on the mirror. Hence, a better accuracy of the mirror soiling level is achieved by deriving more reliable average values. Finally, the results of an examination of sensor soiling rates at several meteorological stations set up in the MENA region and cleaned following the described protocol is presented. This gives an idea about the range of regional differences in soiling rates to be expected in the North African region