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Solar uncertainty: measuring the power from the sun

Solar uncertainty poses a problem to our industry. Geographical location, weather conditions all add to the problem. Clive Lee explains the Kipp & Zonen solution…

For solar energy resource assessments, site prospecting, or monitoring the generating efficiency of solar power plants it is necessary to measure the amount of solar radiation arriving at the surface in that location. In the first two cases, it is usually integrated over days, but for efficiency and performance ratio monitoring real-time data is needed.

When using solar radiation data to calculate bankability, efficiency, profitability, compliance with contractual targets and return on investment it is essential to know the uncertainty. Is it ± 2% or ± 5%? It can make a big difference to the bottom line.

But, what affects these measurements, what are the sources of error, how uncertain are they, and what does it mean for the user of the data?

How much solar energy is out there?
The radiative power of our sun is approximately 63 MW per m² of its surface and its mean distance from earth is about 150 million km (1 astronomical unit, 1 AU). The Direct Normal Irradiance (DNI) at the top of the Earth’s atmosphere at this distance is termed the ‘Solar Constant’ and the current accepted value is 1,360.8 ± 0.5 W/m2 (NASA, 2008). Actually, it isn’t constant and varies 0.1% due to solar activity cycles (nominally 11 years) and because of the Earth’s elliptical orbit around the sun, it is 6.6% higher in January (closest, Perihelion) than in July (farthest, Aphelion).


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