Area solar radiation - input

Discussion created by ampatzi on Aug 23, 2011
Dear All,

I am in need for some advice on the use of the Area Solar radiation tool.
I am trying to understand how the algorithm works by analysing the outputs when using different sets of input values for the beam atmospheric transmissivity (t) and the diffuse to global ratio (D/G).

Since Ie (extraterrestrial) is known (depending on LAT) and t is provided as input to the model, then B (beam) can be calculated as B=t*Ie. Since D/G is also provided as input, then B/G= 1-D/G.
From these you can get G, and then G-B=D.

I am running annual solar analysis with monthly outputs in Wh/m2. The results do not make any sense. The outputs provided for D, G and B do not correspond to 'real averages' although the average D/G for the 12 months is indeed confirmed, as it does the relationship between D, B and G (D+B= G). Nevertheless when calculating the t from the output B and the known Ie, I get something else than what I have given as input for t. Does some 'auto-balancing' between the two input parameters take place within the software? Why is 't' altered? When trying anything else than clear conditions the results are severely scaled down:

for my location average annual values for t and D/G are 0.3 and 0.6 respectively. The outputs confirm the D/G=0.6 but the resulting t equals 0.10! (keep in mind that the tool varies these parameters throughout the year, but the average from the 12 months should equal to the input provided).

I also run a solar analysis using t=1 and D/G=0.1, trying to get a value for G as close to Ie as possible. The resulting annual total for G is 2943kWh/m2, around 125% of the actual Ie for this location...

Nothing of this makes sense to me. I would appreciate any help from someone who has experience with this tool. Thank you all in advance for reading this!