The role of energy storage for mini-grid stabilization

48 pagesMini-grids may be designed to operate autonomously with or without connection to a central grid. While operating autonomously, they cannot rely on the central grid to provide stabilization to control the line voltage and frequency, balance supply and demand of power and manage real or reactive power. Energy storage can provide stabilization in a mini-grid as follows: when the system works autonomously, storage provides or absorbs power to balance supply and demand, to counteract the moment to moment fluctuations in customer loads and unpredictable fluctuations in generation. When grid connected, energy storage systems also can provide ancillary services to improve power quality such as voltage and frequency regulation, harmonic filtering, and fault clearing (i.e. supply of short circuit current). This is named the power use of energy storage, contrary to the usual energy use of energy storage

An OGC Web Processing Service for assessing the quality of solar radiation measurements

International audienceA service is presented that assesses the quality of measurements of daily global irradiation or means of global irradi-ance acquired by a ground station. Measurements are checked against models resulting in a measure of plausibility. This on-line service obeys the WPS (Web Processing Service) standard of the Open Geospatial Consortium. It is free of use and can be integrated into routine operations and Web portals thanks due its interoperability capability

Estimating the photosynthetically active radiation under clear skies by means of a new approach

International audienceThe k-distribution method and the correlated-k approximation of Kato et al. (1999) is a computa-tionally efficient approach originally designed for calculations of the broadband solar radiation by dividing the solar spectrum in 32 specific spectral bands from 240 to 4606 nm. This paper describes a technique for an ac-curate assessment of the photosynthetically active radiation (PAR) from 400 to 700 nm at ground level, under clear-sky conditions using twelve of these spectral bands. It is validated against detailed spectral calculations of the PAR made by the radiative transfer model libRadtran. For the direct and global PAR irradiance, the bias is −0.4 W m−2 (−0.2 %) and −4 W m−2 (−1.3 %) and the root mean square error is 1.8 W m−2 (0.7 %) and 4.5 W m−2 (1.5 %). For the direct and global Photosynthetic Photon Flux Density, the biases are of about +10.3 µmol m−2 s−1 (+0.8 %) and 1.9 µmol m−2 s−1 (−0.1 %) respectively, and the root mean square error is 11.4 µmol m−2 s−1 (0.9 %) and 4.0 µmol m−2 s−1 (0.3 %). The correlation coefficient is greater than 0.99. This technique provides much better results than two state-of-the-art empirical methods computing the daily mean of PAR from the daily mean of broadband irradiance

How close to detailed spectral calculations is the k-distribution method and correlated-k approximation of Kato et al. (1999) in each spectral interval?

International audienceThe k-distribution method and the correlated-k approximation of Kato et al. (1999) is a smart approach originally designed for broadband calculations of the solar radiation at ground level by dividing the solar spectrum in 32 spectral bands. The approach is a priori not suited for calculation of spectral irradiance. Nevertheless, this paper evaluates its performance when compared to more detailed spectral calculations serving as references for the spectral intervals no. 3 [283, 307] nm to 26 [1613, 1965] nm for clear and cloudy situations. The evaluation is based on numerical simulations. The clearer the sky, the greater the root mean square error (RMSE) in all bands. In the spectral intervals no. 3 and 4 [307, 328] nm, the irradiance is underestimated by large – approximately −90 % and −17 % in relative value -because the wavelength interval is large with respect to the absorption by ozone and a single value of ozone cross section is not enough for each interval. For each spectral interval from no. 5 [328, 363] nm to no. 18 [743, 791] nm, and for both global and direct radiation, the bias and the RMSE are less than 1.5 % of the irradiance in the corresponding interval under clear skies and may amount to 3 % in cloudy conditions. For greater wavelength intervals no. 19 to no. 26, the relative bias and RMSE show a tendency to increase with wavelength and may reach 8 % and 7 % for global and direct under clear skies respectively, and 11 % and 15 % under cloudy skies

A new approach for estimating operationally the spectral distribution of surface solar irradiance: preliminary results

International audienceThe k-distribution method and the correlated-k approximation of Kato et al. (1999) are a smart approach designed for calculations of the broadband solar radiation at ground level by using only 32 spectral bands. This communication presents a preliminary assessment of the performance of this approach compared to more detailed spectral calculations for the spectral intervals no. 3 [283, 307] nm to no. 26 [1613, 1965] nm for clear and cloudy situations. For spectral intervals no. 5 [328, 363] nm to no. 26, the relative errors are less than 5% of the irradiance in the corresponding interval. The irradiance is strongly underestimated in the spectral intervals no. 3 and no. 4 [307, 328] nm. It is found that if necessary, errors may be accurately corrected with simple models computed only once

Analysis of the influences of uncertainties in input variables on the outcomes of the Heliosat-2 method

International audienceThe Heliosat-2 method, which employs satellite images to assess solar irradiance at ground level, is one of the most accurate among the available operational methods. Its input variables have uncertainties which impact on the final result. The General Law of Uncertainty Propagation is employed to analyze the impact of these uncertainties on a single pixel with Meteosat-7 inputs in various stages, beginning with the sensitivity coefficients and the changes induced in the clear-sky index (KC) by each independent variable. Once these coefficients are known, the partial combined standard uncertainty (CSU) is calculated for KC from each independent variable and albedo. Finally, the total CSU of KC is calculated. All of the results are in agreement and show that the most influential variables in the uncertainty of estimation of cloudy skies are, in this order, the Linke turbidity factor (54% of KC value), terrain elevation (33%), the calibration coefficient of the satellite sensor (13%) and the ground albedo (5%). What causes the initial uncertainty in the ground albedo is its variation over time and the difficulty in assessing it from a reflectance time-series for mixed clear and cloudy skies. The Linke turbidity factor is the most influential variable on the width of the uncertainty interval, not only because of its own uncertainty (17% in this study), but because it is also used in numerous intermediate calculations. For clear skies, the partial CSUs are considerably lower, except for ground albedo (5% also)

High spatial resolution solar atlas in Provence-Alpes-Cote d'Azur

International audienceConsidering the spatial resolution of satellite-based databases of surface solar irradiation such as HelioClim-3, the geographic scale of solar mapping that can be derived is approx. 1 / 5 000 000. This geographic scale is suitable for continent or country solar mapping. Nevertheless, customers' feedback notably states that there is a need of spatial resolution improvement. In this context of spatial resolution improvement of solar resource estimation, the project solar atlas in Provence-Alpes-Côte d'Azur (PACA), finalized in December 2010, aims at increasing spatial resolution of HelioClim-3 to derive 250 m resolution solar maps. The solar atlas accounts for the global, direct and diffuse on inclined plane or in normal incidence irradiations and their temporal variations intra and inter-annual, on a monthly basis, since February 2004. The Digital Elevation Model SRTM-V4 is used to get topographic data with a spatial resolution of approximately 100 m and a localization accuracy of typically 10 m. This topographic data is used to estimate high resolution terrain effects on HelioClim-3 derived solar radiation fields, taking into account orographic effects: * Effects of optical path length variations due to terrain elevation; * Shadow effects on direct and circumsolar diffuse radiation computed from local horizon estimation; * Horizon effects on isotropic diffuse radiation with the sky view factor estimation. These orographic phenomena are indeed the source of significant local spatial variations in solar radiation values and potentially discriminating in choosing the location, the calculation of profitability or the sizing of photovoltaic or thermodynamic solar power systems. Solar mapping with a geographic scale of 1 / 3 000 000 or more does not represent properly these local phenomena. In order to establish an accurate mapping at 250 m with associated uncertainty using HelioClim-3 data, procedures of calibration has been established with meteorological ground stations. The global horizontal irradiation is calibrated thanks to the Météo France meteorological network comprising, in PACA, about 30 automatic ground stations delivering at least daily global irradiation on horizontal plane. A strict data quality control has been applied to these ground data to guarantee their quality as reference ground-based daily irradiation data. The diffuse and direct irradiations are calibrated thanks to three specific ground stations dedicated expressly to the project, delivering one year of simultaneous 10-min global and diffuse irradiations on the horizontal plan and direct irradiation on normal incidence. Statistical analysis of direct and cross-validation estimation errors after calibration with the pyranometric ground stations has been made to establish the uncertainties associated with the solar atlas. The root mean square error of monthly irradiation estimation is about 5% (c. 5 kWh / m2) for global, direct and diffuse components on horizontal plane, without significant bias error. The accuracy of monthly direct normal irradiation is about 10% (c. 15 kWh/m2). Data from the solar atlas in PACA is available for free at www.atlas-solaire.fr. In particular, a Web Map Service (WMS), based on the Open Geographical Consortium standards, has been set up to deliver maps from the solar atlas in an interoperable and standard way

Photovoltaic Forecasting: A state of the art

International audiencePhotovoltaic (PV) energy, together with other renewable energy sources, has been undergoing a rapid development in recent years. Integration of intermittent energy sources as PV or wind power is challenging in terms of power system management in large scale systems as well as in small grids. Indeed, PV energy is a variable resource that is difficult to predict due to meteorological uncertainty. To facilitate the penetration of PV energy, forecasting methods and techniques have been used. Being able to predict the future behavior of a PV plant is very important in order to schedule and manage the alternative supplies and the reserves. In this paper we presented an overview aiming at a classification attending to the different techniques of forecasting methods used for PV or solar prediction. Finally, recent new approaches that take into account the uncertainty of the estimation are introduced. First results of these kind of models are presented

User's Guide to the MACC-RAD Services on solar energy radiation resources

The European Earth observation programme GMES (Global Monitoring for Environment and Security), now Copernicus (the European Earth Observation Programme) since December 2012, aims at providing environmental information to support policymakers, public authorities and both public and commercial users. A systematic monitoring and forecasting of the state of the Earth's subsystems is currently under development. Six thematic areas are developed: marine, land, atmosphere, emergency, security and climate change. A land monitoring service, a marine monitoring service and an atmosphere monitoring service will contribute directly to the monitoring of climate change and to the assessment of mitigation and adaptation policies. Additional GMES services will address respectively emergency response and security-related aspects. The pre-operational atmosphere service of GMES is currently provided through the FP7 projects MACC and MACC-II (Monitoring Atmospheric Composition and Climate). MACC combines state-of-the-art atmospheric modelling with Earth observation data to provide information services covering European Air Quality, Global Atmospheric Composition, Climate, and UV and Solar Energy. Within the radiation subproject (MACC-RAD) existing historical and daily updated databases for monitoring incoming surface solar irradiance are further developed. The service will meet the needs of European and national policy development and the requirements of (commercial) downstream services (e.g. planning, monitoring, efficiency improvements, integration into energy supply grids). The SOLEMI and the HelioClim 3 databases operated by respectively DLR and ARMINES and its subsidiary Transvalor have been specifically developed in several national, European and ESA projects to fulfil the requirements for long-term databases and NRT services. On its transition process from the precursor services HelioClim and SOLEMI the following User's Guide intends to summarize existing knowledge, which has been published only in a scattered manner. Part A 'Users' Expectations' describes the communities of users, their expectations and gives an overview of the compliance of the MACC RAD service with those. In Part B 'The legacy HelioClim 3 and SOLEMI databases', the current databases HelioClim 3 and SOLEMI as well as the methods used to convert satellite images into solar surface irradiance are presented. The quality of the retrieved irradiances is discussed. An overview of the operations and workflow is presented for the creation, updating and monitoring of these databases. Part C 'The new HelioClim 4 database' describes the new Heliosat 4 method and the new HelioClim 4 database and provides an overview of the operations and the workflow. Part D 'Quality control of estimates of irradiance' discusses the means to control the quality of the elaboration of the products and to assess the uncertainty of the estimates of irradiance. Part E 'Delivering products' is devoted to the supply of HelioClim 4 products. The products are defined. A prototype of a means to access the HelioClim 4 products is presented. It is intended to update this User's Guide regularly following the realisation of the MACC RAD service line

On quality control procedures for solar radiation and meteorological measures, from subhourly to montly average time periods

International audienceMeteorological data measured by ground stations are often a key element in the development and validation of methods exploiting satellite images. These data are considered as a reference against which satellite-derived estimates are compared. Long-term radiation and meteorological measurements are available from a large number of measuring sta- tions. However, close examination of the data often reveals a lack of quality, often for extended periods of time. This lack of quality has been the reason, in many cases, of the rejection of large amount of available data. The quality data must be checked before their use in order to guarantee the inputs for the methods used in modelling, monitoring, forecast, etc. To control their quality, data should be submitted to several conditions or tests. After this checking, data that are not flagged by any of the test is released as a plausible data. In this work, it has been performed a bibliographical research of quality control tests for the common meteoro- logical variables (ambient temperature, relative humidity and wind speed) and for the usual solar radiometrical variables (horizontal global and diffuse components of the solar radiation and the beam normal component). The different tests have been grouped according to the variable and the average time period (sub-hourly, hourly, daily and monthly averages). The quality test may be classified as follows: * Range checks: test that verify values are within a specific range. There are two types of range checks, those based on extrema and those based on rare observations. * Step check: test aimed at detecting unrealistic jumps or stagnation in the time series. * Consistency checks: test that verify the relationship between two or more time series. The gathered quality tests are applicable for all latitudes as they have not been optimized regionally nor seasonably with the aim of being generic. They have been applied to ground measurements in several geographic locations, what result in the detection of some control tests that are no longer adequate, due to different reasons. After the modification of some test, based in our experience, a set of quality control tests is now presented, updated according to technology advances and classified. The presented set of quality tests allows radiation and meteorological data to be tested in order to know their plausibility to be used as inputs in theoretical or empirical methods for scientific research. The research leading to those results has partly receive funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement no. 262892 (ENDORSE projec