SSE Release 6.0 Methodology

I. Introduction

NASA, through its’ Earth science research program has long supported satellite systems and research providing data important to the study of climate and climate processes. These data include long–term estimates of meteorological quantities and surface solar energy fluxes. These satellite and modeled based products have also been shown to be accurate enough to provide reliable solar and meteorological resource data over regions where surface measurements are sparse or nonexistent, and offer two unique features — the data is global and, in general, contiguous in time. These two important characteristics, however, tend to generate very large data archives which can be intimidating for commercial users, particularly new users with little experience or resources to explore these large data sets. Moreover the data products contained in the various NASA archives are often in formats that present challenges to new users. Accordingly, NASA’s Earth Applied Sciences program has provided the means to make these data available for government and public sector usage. To foster the usage of the global solar and meteorological data, NASA supported, and continues to support, the development of the Surface meteorology and Solar Energy (SSE) dataset which has been formulated specifically for photovoltaic and renewable energy system design needs. Of equal important is the access to these data; to this end the SSE parameters are available via a user friendly web–based portal designed based on user needs.

The original SSE data–delivery web site, intended to provide easy access to parameters needed in the renewable energy industry (e.g. solar and wind energy), was released to the public in 1997. The solar and meteorological data contained in this first release was based on the 1993 NASA/World Climate Research Program Version 1.1 Surface Radiation Budget (SRB) science data and TOVS data from the International Satellite Cloud Climatology Project (ISCCP). This initial design approach proved to be of limited value because of the use of “traditional” scientific terminology that was not compatible with terminology/parameters used in the energy industry to design renewable energy power systems. After additional consultation with industry, Release 2 SSE was made public in 1999 with parameters specifically tailored to the needs of the renewable energy community. Subsequent releases of SSE – SSE–Release 3.0 in 2000, SSE– Release 4.0 in 2003, SSE– Release 5.0 in 2005, and now SSE– Release 6.0 in 2008 — have continued to build upon an interactive dialog with potential customers resulting in updated parameters using revised NASA data as well as inclusion of new parameters as requested by the user community.

The Prediction Of Worldwide Energy Resource (POWER) project was initiated in 2003 both to improve subsequent releases of SSE, and to create new datasets applicable to other industries from new satellite observations and the accompanying results from forecast modeling. The POWER web interface (http://power.larc.nasa.gov) currently encompasses the SSE data set, tailored for the renewable energy industry, as well as parameters tailored for the sustainable buildings community, and the bio–energy/agricultural industries. In general, the underlying data behind the parameters used by each of these industries is the same — solar radiation and meteorology, including surface and air temperatures, moisture, and winds.

The purpose of this document is to provide data users of POWER/SSE Release 6.0 insight into the underlying source for the solar and meteorological data sets, and to provide additional information relative to the various industry specific parameters, their limitations, and estimated accuracies based on information available to NASA at the time of this document. The intent is to provide information that will enable new and/or long time users to make decisions concerning the suitability of the POWER/SSE data for his or her project in a particular region of the globe. And finally, it is noted that SSE Release 6.0 and this document is focused primarily on parameters of interest to the renewable energy industry, although there are many parameters common to both the SSE and Sustainable Buildings components of POWER, and the underlying solar and meteorological data for all three POWER components is the derived from common data sources.

In general, the meteorology and solar radiation parameters contained in SSE Release 6.0 were obtained from the NASA Science Mission Directorate’s satellite and re–analysis research programs. Parameters based upon the solar and/or meteorology data were derived and validated based on recommendations from partners in the energy industry. Release 6.0 extends the temporal coverage of the solar and meteorological data from approximately 11 years to 22 plus years (e.g. July 1983 through June 2005) with improved NASA data, and includes new parameters and validation studies.

The remainder of this section provides a summary of the estimates of the levels of uncertainty for insolation (solar radiation), temperature, surface pressure, relative humidity, and wind speed through comparisons with ground measurement data. A more detained description of the parameters and the procedures used to estimate their uncertainties is given in the subsequent section of the document.

It is generally considered that quality ground based observational data are more accurate than satellite–derived values. However, measurement uncertainties from calibration drift, operational uncertainties, or data gaps are unknown or unreported for most ground site data sets. In 1989, the World Climate Research Program estimated that most routine–operational solar-radiation ground sites had "end–to–end" uncertainties from 6 to 12%. Specialized high quality research sites are hopefully more accurate by a factor of two.

SSE estimates have been compared to ground site data on a global basis. Radiation parameters were compared to data from the Baseline Surface Radiation Network (BSRN) (Table I.1). Meteorological parameters were compared to data from the National Climate Data Center (NCDC) (Table I.2). The SSE Release 6.0 wind speeds have been carried over from SSE Release 4 because newer data sets do not provide enough information about vegetation/surface types. The RETScreen Weather Database (RETScreen 2005) was used to test uncertainties in the SSE Release 4 wind speeds (Table I.3)

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