SOLFORRENEW · A comprehensive framework for high-resolution assessment and short-term forecasting of the solar resource for renewable energy applications

Periodic Report Summary 1 - SOLFORRENEW (A comprehensive framework for high-resolution assessment and short-term forecasting of the solar resource for renewable energy applications)

The overall research objective of the SolForRenew project is the leverage of the current methods for solar resource assessment and forecasting in renewable energy applications with Numerical Weather Prediction (NWP) models. The public Weather Research and Forecasting (WRF) model is being used as test-bed. This research will provide an improved comprehensive framework for fine-scale solar resource assessment and forecasting. It is expected to contribute for a smoother implantation of the renewable energies into the electricity market. During the outgoing phase, spanning the first 24 months of the project, Dr. José A. Ruiz-Arias (hereinafter, the researcher) has gone through with a research stay in the Mesoscale Prediction Section (http://www.mmm.ucar.edu/sections/mesoscale-prediction.php) of the Mesoscale and Microscale Meteorology (MMM) Division at the National Center for Atmospheric Research (NCAR) Earth System Laboratory (NESL), in Boulder, USA. The research stay was conducted under the supervision of Dr. Jimy Dudhia. The particular research objectives during this reporting period include: 1. Improvement of current assessment methods of Global Horizontal Irradiance (GHI) and Direct Normal Irradiance (DNI) in NWP models, a. by developing a parameterization of aerosol optical properties to account for the aerosol extinction in surface solar irradiance and, b. by implementing a statistical terrain parameterization that allow including non-resolved terrain-related effects in surface solar irradiance at coarse spatial scales. 2. Improvement of current forecast methods for GHI and DNI in NWP models, a. by improving cloud nowcasting and, b. by improving AOD forecasting. Description of the work and main results A versatile parameterization of the shortwave aerosol optical properties (AOP) for NWP models has been developed and implemented in the WRF model. It is required to account for the aerosol extinction effects in surface solar irradiance. If aerosols are not considered, the computed surface solar fluxes may show a bias of up to 20% when they are compared against ground observations. The task was initiated with a thorough study of the currently available aerosol optical depth data sources with a twofold objective: i) to delimitate the data sources that best match NWP requirements and, ii) to assess their potential to improve DNI and GHI assessment in NWP models. The study concluded with the evaluation of the daily Level-3 Moderate-resolution Imaging Spectroradiometer (MODIS) AOD dataset at global scale against ground observations from the Aerosol Robotic Network (AERONET), and the proposal of models and methods to reduce its inherent regional bias. Results have been published in two research papers in high-impact international scientific journals [1, 2]. In addition, a control study has been conducted that has served as a baseline to calibrate the skills of the AOP parameterization. The control study, published in a research paper in a high-impact international scientific journal [3], consisted of a clear-sky shortwave closure evaluation of the skills of three different solar radiation schemes of choice in WRF. Results were validated against high-quality solar irradiance ground observations in the contiguous US. The AOP parameterization has been described in a research paper and submitted for its publication in a high-impact international scientific journal [4]. The parameterization will be publicly available in the next major WRF release (version 3.6) scheduled by April, 2014. The steps for a comprehensive regional validation of the methodology have been initiated in two different regions: the contiguous US and Spain, using two different data sources for regional aerosol optical depth data. In regards to the non-resolved terrain effects in surface solar fluxes, an additional issue is the sun must be tracked very precisely, particularly at sunrise and sunset, to best capture the terrain horizon effects. However, the conventional NWP methods to compute SW fluxes are so slow that, in practice, they cannot be called at every model integration time step. Therefore, a fast updating scheme for surface solar fluxes is being developed. A preliminary version was included in the last WRF minor release (v3.5.1, http://www.mmm.ucar.edu/wrf/users/wrfv3.5/updates-3.5.1.html) and it will be updated in the next major release (version 3.6) scheduled by April, 2014. The AOP parameterization is required for precise computation of DNI and DIF irradiances. The fast updating scheme is required for high-frequency computation of solar fluxes. The two of them will benefit the assessment of non-resolved terrain effects. This task is being undertaken during the return phase. For the improvement of the current forecast skills, two parallel actions are being conducted. On the one hand, a novel approach for cloud nowcasting has been initiated in collaboration with Dr. Tom Auligné at NCAR. It is based on near-real-time assimilation of satellite retrievals in the form of cloud fraction. The NWP model is then used to advect the cloud fraction values and predict their values in future times (up to six hours ahead). From the cloud fraction data, surface solar fluxes can be estimated. A first prototype system has been already deployed and the first validating experiment has been initiated. On the other hand, the skills of the Monitoring Atmospheric Composition and Climate (MACC, http://www.gmes-atmosphere.eu/) model at predicting aerosol optical depth and its hypothetical beneficial impact in clear-sky surface solar irradiance prediction are being evaluated using the AOP parameterization. The benefits will be evaluated from the perspective of a solar power plant, considering how the solar forecast is helpful for the operation of the plant and extrapolating the benefits to the electric market when solar resource is forecasted at large scale. Expected results The project will result in a comprehensive and publicly available tool, the WRF model, to evaluate and forecast the DNI and GHI solar irradiances at multiple spatial scales and forecasting horizons. With the research conducted in this project, the WRF model will become a powerful tool for renewable energy applications demanding a means to evaluate the solar resource potential for solar plants feasibility studies and, more importantly, solar forecasts for an improved operation of the solar plants. At a larger spatial scale, the use of solar forecasts with the WRF model will lead to the minimization of the power grid fluctuations issues caused by the solar resource intermittency, and it will end up with the maximization of the penetration of solar energy production technologies in the power grid. The developments undertaken within this research framework will result in an improved version of WRF for solar resource assessment and forecasting, WRF-Solar, and be freely available to the tens of thousands of registered users of WRF. Thus, the SolForRenew research advances will impact a huge number of applications, not only in Europe, but also around the world for years to come. References [1] Ruiz-Arias, J.A., Dudhia, J., Gueymard, C.A., Pozo-Vázquez, D., 2013 “Assessment of the Level-3 MODIS daily aerosol optical depth in the context of surface solar radiation and numerical weather modeling” Atmospheric and Chemistry Physics, Vol. 13, pp. 675-692. doi: 10.5194/acp-13-675-2013 [2] Ruiz-Arias, J.A., Dudhia, J., Lara-Fanego, V., Pozo-Vázquez, D., 2013 “A geostatistical approach for producing daily Level-3 MODIS aerosol optical depth analyses” Atmospheric Environment, Vol. 79, pp. 395-405. doi: 10.1016/j.atmosenv.2013.07.002 [3] Ruiz-Arias, J.A., Dudhia, J., Santos-Alamillos F.J., Pozo-Vázquez, D., 2013 “Surface clear-sky shortwave radiative closure intercomparisons in the Weather Research and Forecasting model” Journal of Geophysical Research: Atmospheres, Vol. 118, pp. 1-13. doi: 10.1002/jgrd.50778 [4] Ruiz-Arias, J.A., Dudhia, J., Gueymard, C.A., 2013 “A simple parameterization of the shortwave aerosol optical properties for the calculation of surface direct and diffuse irradiances in a numerical weather model” Geoscientific Model Development, Submitted. Contact Information: José A. Ruiz-Arias, jararias at ujaen.es David Pozo Vázquez, dpozo at ujaen.es

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