Stefan Wilbert

Measurement of Solar Radiance Profiles With the Sun and Aureole Measurement System

Due to forward scattering of direct sunlight in the atmosphere, the circumsolar region closely surrounding the solar disk looks very bright. The radiation coming from this region, the circumsolar radiation, is in large part included in common direct normal irradiance (DNI) measurements, but only partially intercepted by the receivers of focusing collectors. This effect has to be considered in the performance analysis of concentrating collectors in order to avoid overestimation of the intercepted irradiance. At times, the overestimation reaches more than 10% for highly concentrating systems even for sky conditions with relevant DNI above 200 W/m2. The amount of circumsolar radiation varies strongly with time, location and sky conditions. However, no representative sunshape measurements exist for locations that are now of particular interest for concentrating solar power (CSP) or concentrating photovoltaics (CPV). A new sunshape measurement system is developed and analyzed in this study. The system consists of the sun and aureole measurement instrument (SAM), an AERONET sun photometer and postprocessing software. A measurement network is being created with the presented system. The uncertainty of this system is significantly lower than what was obtained with previous devices. In addition, the spectral optical properties of circumsolar radiation are determined. As a result, the necessary information for CSP and CPV systems, and a basis for the development of modeling methods for circumsolar radiation, can now be achieved.

Validation of Two Optical Measurement Methods for the Qualification of the Shape Accuracy of Mirror Panels for Concentrating Solar Systems

The solar field is the major cost component of a solar thermal power plant and the optical quality of the concentrators has a significant impact on the field efficiency and thus on the performance of the power plant. Measuring slope deviations in the parabolic shape of the mirror panels in the accuracy and resolution required for these applications is a challenge as it is not required with the same characteristics in other industries. Photogrammetry and deflectometry are two optical measurement methods that are typically used to measure this shape accuracy of mirror panels used in CSP applications. They have been compared and validated by measuring a typical mirror panel under optimal conditions. Additionally, a flat water surface has been measured as an absolute reference object using deflectometry. The remaining deviations between the results of both methods and to the reference object are discussed and possible sources of errors during the measurement are identified. A detailed error analysis is conducted for both methods and compared to the experimental findings. The results show that both methods allow for surface slope measurement with the necessary accuracy for present CSP applications and that among the two, deflectometry exhibits advantages in speed, measurement accuracy and spatial resolution. However, for obtaining correct results several sources of errors have to be addressed appropriately during measurement and post-processing

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

For 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.

Comparison of measurement techniques for the determination of circumsolar irradiance

Depending on the present atmospheric conditions a considerable part of the direct solar radiation can be scattered into the circumsolar region closely surrounding the solar disk. This forward scattered radiation, the circumsolar radiation, is widely detected by pyrheliometers, while concentrating solar collectors can only use a concentrator specific fraction of it. This lower response to circumsolar radiation can lead to systematic overestimations of the collector performance. We use a Sun and Aureole Measurement system (SAM) combined with AERONET sun photometer measurements to determine the circumsolar radiation, and compare the results to a newly developed 2-channel pyrheliometer instrument BPI-CSR460.

Uncertainty of rotating shadowband irradiometers and Si-pyranometers including the spectral irradiance error

Concentrating solar power projects require accurate direct normal irradiance (DNI) data including uncertainty specifications for plant layout and cost calculations. Ground measured data are necessary to obtain the required level of accuracy and are often obtained with Rotating Shadowband Irradiometers (RSI) that use photodiode pyranometers and correction functions to account for systematic effects. The uncertainty of Si-pyranometers has been investigated, but so far basically empirical studies were published or decisive uncertainty influences had to be estimated based on experience in analytical studies. One of the most crucial estimated influences is the spectral irradiance error because Si-photodiode-pyranometers only detect visible and color infrared radiation and have a spectral response that varies strongly within this wavelength interval. Furthermore, analytic studies did not discuss the role of correction functions and the uncertainty introduced by imperfect shading. In order to further improve the...

Validation of Direct Beam Irradiance Measurements From Rotating Shadowband Irradiometers in a Region With Different Atmospheric Conditions

Rotating shadowband irradiometers (RSIs) are a common type of radiation sensors for measurement of direct normal irradiance (DNI) at remote sites where daily maintenance of the instruments is not feasible or practicable. Their primordial lower accuracy due to systematic deviations of the photodiode response can be improved significantly with a thorough calibration of each RSI against high precision sensors and application of suitable corrections on the raw data. With different available correction functions for the systematic errors, RSI data coincide with first class reference sensors within 2–3% root mean square deviation (RMSD) for 10 min averages of DNI and meet the annual irradiation sum within 1.5%. Such comparisons of RSI data to reference irradiances have only been published for a small number of sites. To endorse the credibility of RSI measurements, it has to be shown that these accuracies derived for certain locations are also valid at other sites with differing atmospheric conditions. Therefore, a parallel measurement campaign with six RSIs and a reference station with first class and secondary standard instrumentation has been performed in the in the extreme climate of the United Arab Emirates (UAE). The results of this comparison are presented in this paper. The stated empiric accuracy could be validated and confirmed for the UAE.

Principles of CSP performance assessment

The performance of a concentrated solar power component of Systems describes ist capacity to accomplish ist design purpose in concentrating and(or converting solar irradiance to useful energy. Generally, this can be quantified by ist Efficiency relating the useful Output of the System or component to the nergy Input or effort. In Addition, for some Standard components, key performance indicators such as specific heat loss for parabolic trough Receivers and Focus Deviation for mirrors have been established in the past decade. The assessment of the performance of a component or System is necessarily based on the measurement of ist operational characteristics, typically involving the Evaluation of energy balances of the System itsefl and/or its heat Transfer fluid (HTF) under specific well-known operating conditions. The Information on prevailing or resulting conditions can be obtained using appropriate measurement Equipment. The key measurement challenges in concentrating solar power applications are presented by high temperature and pressure, concentrated solar Radiation, as well as Special HTF´s such as thermal oil or molten salts.

Validation of spatially resolved all sky imager derived DNI nowcasts

Mainly due to clouds, Direct Normal Irradiance (DNI) displays short-term local variabilities affecting the efficiency of concentrating solar power (CSP) plants. To enable efficient plant operation, DNI nowcasts in high spatial and temporal resolutions for 15 to 30 minutes ahead are required. Ground-based All Sky Imagers (ASI) can be used to detect, track and predict 3D positions of clouds possibly shading the plant. The accuracy and reliability of these ASI-derived DNI nowcasts must be known to allow its application in solar power plants. Within the framework of the European project DNICast, an ASI-based nowcasting system was developed and implemented at the Plataforma Solar de Almeria (PSA). Its validation methodology and validation results are presented in this work. The nowcasting system outperforms persistence forecasts for volatile irradiance situations.

Atmospheric Extinction in Simulation Tools for Solar Tower Plants

Atmospheric extinction causes significant radiation losses between the heliostat field and the receiver in a solar tower plants. These losses vary with site and time. State of the art is that in ray-tracing and plant optimization tools, atmospheric extinction is included by choosing between few constant standard atmospheric conditions. Even though some tools allow the consideration of site and time dependent extinction data, such data sets are nearly never available. This paper summarizes and compares the most common model equations implemented in several ray-tracing tools. There are already several methods developed and published to measure extinction on-site. An overview of the existing methods is also given here. Ray-tracing simulations of one exemplary tower plant at the Plataforma Solar de Almeria (PSA) are presented to estimate the plant yield deviations between simulations using standard model equations instead of extinction time series. For PSA, the effect of atmospheric extinction accounts for losses between 1.6 and 7 %. This range is caused by considering overload dumping or not. Applying standard clear or hazy model equations instead of extinction time series lead to an underestimation of the annual plant yield at PSA. The discussion of the effect of extinction in Tower plants has to include overload dumping. Situations in which overload dumping occurs are mostly connected to high radiation levels and low atmospheric extinction. Therefore it can be recommended that project developers should consider site and time dependent extinction data especially on hazy sites. A reduced uncertainty of the plant yield prediction can significantly reduce costs due to smaller risk margins for financing and EPCs. The generation of extinction data for several locations in form of representative yearly time series or geographical maps should be further elaborated.

Best Practices Handbook for the Collection and Use of Solar Resource Data for Solar Energy Applications: Second Edition

This report presents detailed information about solar resource data and the resulting data products needed for each stage of a solar energy project, from initial site selection to systems operations. It also contains a summary of solar forecasting and its development throughout the last few years. The U.S. Department of Energys Solar Energy Technologies Office, project developers, engineering procurement construction firms, utility companies, system operators, energy suppliers, financial investors, and others involved in solar energy systems planning and development will find this handbook to be a valuable resource for the collection and interpretation of solar resource data. This handbook is expected to be used as a reference during each project stage.