Do present weather observations (e.g. rain, fog, snow fall) in weather file have any impact on simulated performance of PV solar panels and solar collectors ?

As is often the case, solar radiation for a site is estimated in weather files by the Zhang-Huang and Perez models (in the EP weather converter). However, if in addition there are present weather observations in the weather file, this would probably require EnergyPlus to calculate kind of superposition of effects, modifying the Zhang-Huang-Perez solar radiation estimates according to temporary rain, fog, snow fall etc. (?) It seems, however, that present weather observations (obscurations) do not have an influence on simulation results re. performance of solar panels/collectors, and that present weather observations in weather files will not add information which improves simulation results. Is this correct ? If not, how does EnergyPlus utilize these additional weather observations and calculate effects ?

I don't think on general principles that EnergyPlus should incorporate climate modeling capabilities, as that could easily lead to double-counting and likely misuse (site-specific effects like shading from neighboring buildings and wind reductions due to terrain-factors excepted). I'm not so concerned about certain parameters on the weather file not being used in deriving solar radiation. That doesn't mean their effects are ignored, just that they're expressed in other parameters, chiefly the Sky Cloud Cover. When there's rain, fog, snow, etc., the Sky Cloud Cover should be 0. You might argue that Cloud Cover 0 + Rain, Fog, etc. should reduce Solar Radiation even more, but how much more? You'd need to get some good measured solar plus weather station observations and then add those additional terms to the regression which, if you're using Z-H, already has 4 (Cloud Cover, Temp Rise, Wind Speed, and RH). I have no idea how much would be the improvement, if any.

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Joe Huang

answered 2015-03-05 19:20:07 -0500

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(1) So this seems to confirm that if present weather observations are omitted in the weather file, it will not affect solar radiation estimates (correct ?) (2) How is Sky Cloud Cover defined in the Z-H model ? originally I expected that Overcast=100% sky cover corresponding to CC=1 and Clear Sky = 0% sky cover correspondig to CC=0 This is necessary to know if custom weather input files are created and more generally when interpreting simulation results.