Background and history of Australian solar data
The earliest solar radiation data produced for Australia came from the CSIRO in the mid-1960s. Named after their author, the Spencer Tables used completely clear skies to calculate irradiation levels for architectural and engineering surfaces.
By contrast, the data used for the first four editions of the Australian Solar Radiation Data Handbook (ASRDH) are all statistical values (long-term means and frequencies of occurrence) that account for the reduction, particularly in beam radiation, caused by actual transient cloud and turbidity conditions.
Early editions used an isotropic sky model where the diffuse radiation came uniformly from the full hemisphere of the sky. The reality of the sky being much brighter around the sun meant that the tabulated values for northerly windows and roofs were 20 per cent greater than old models had suggested.
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A fully revised Third Edition was published by the Energy Research and Development Corporation (ERDC) in Canberra, incorporating the best available anisotropic sky algorithms for interpreting global, diffuse and direct solar measurements into the required architectural and design tables for a wide range of fixed and sun tracking surfaces.
The edition incorporated isorad (contour) maps of capital city hinterlands, prepared from satellite measurements of reflected radiation used to infer how much was reaching the ground anywhere over the Australian land mass. This provided site-specific information for the first time for approximately 100 km in every direction from the city centres.
Unfortunately, the Third Edition had to re-use the much less reliable pre-1986 data from the Australian Climate Data Bank. Industry pressure eventually convinced the Federal Government that this was unsatisfactory – especially for Adelaide, Brisbane, Canberra and Sydney, which each had only three years’ worth of solar records in 1986. Subsequently, substantial inroads were made into the enhancement of the accuracy of solar measurement at the BOM, despite constrained funding support.
A major enhancement project for the ASRDH commenced in July 2002 under a grant from the Australian Greenhouse Office’s Renewable Energy Industry Development Program and was completed by the end of 2005.
Optimal solar radiation data
The competitiveness of solar system design is constrained by the accuracy and reliability of the designed output. The more precision that is available in calculations, the less conservative and expensive the solar system can be designed to meet a known energy demand.
Seasonal and diurnal patterns are important in determining the value of the output for grid-connected systems and storage sizing for stand-alone systems.
Clean and accurate sun-tracking instruments are crucial, and redundant measurement is a technique of quality assurance commonly undertaken. Here, there are usually coincidental measurements of global, diffuse and direct-beam, along with solar altitude a°. These readings can then be compared with the trigonometric function: Global = Diffuse + (Direct Beam) x Sin a°.
Wherever that equation produces a dissonance, the cause needs to be identified and the data and the instruments adjusted accordingly.
New data sources
In 2010, the BOM released gridded global horizontal irradiance (GHI) and direct normal irradiance (DNI) data, in watts per square metre. This hourly data accounts for virtually the whole of Australia, and is derived from satellite imagery processed by the BOM.
The Exemplary Australian Solar Energy Atlas (EASEA) was developed to visualise the GHI and DNI data on a computer screen. Further, the extraction of time series for individual locations was perfected so as to allow the extraction of hourly data, with the data interpolated minute-by-minute to values at exactly on-the-hour local standard time.
Two data sources – the GHI and DNI hourly data released by BOM and a ‘clear sky’ model and data – provide the foundations for ASRDH Fifth Edition.
While the newest edition of the ASRDH covers nearly 100 sites, there is no longer any practical constraint on the number that can be treated this way except for the limitations on the number of BOM sites (over 1,000) from which the surface data can be accessed to generate the complementary values published for the non-solar weather elements.
Comprehensive tools to describe Australia’s solar resource can be seen to serve as an upgrade to the current solar radiation resource stream, with the purpose of reducing the uncertainty in plant peak and average output estimation and benchmarking by increasing the number of sites.
As such, they represent a key underpinning of the expansion of solar and other renewable energy infrastructure investments now being planned.
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