expected vs. actual PV Watts

I am attempting to use SAM to compare expected output from a solar array to actual production. The array has a weather station located on site, so I am able to get total radiation values from the weather station. I am trying to pass this into the C# wrapper for the SCC module "pvwattsfunc" to get the snapshot value of the expected ac power.

For the snapshot time I am attempting to test with, the calculated poa value comes back as 251.3582 W/m2, which seems reasonable given the measured sol value of 263.4 W/m2 from the weather station. The expected ac power, however, returns as 167,139.1 W vs the measured output from the array at that time of 45,500 W. The total system nameplate capacity is 858kW.

I don't have reason to believe that the array is actually performing that poorly, but rather than since I'm new to PVWatts and SAM that I am doing something wrong with my input parameters. I put the weather station solar value into both the current beam and the previous step poa value, although I don't think that the previous step should matter since I'm just taking one value at a time.

Any suggestions or recommendations would be appreciated. My input parameters from code are listed below for reference.

Thanks in advance,

Mikkel

One correction - my values were low because I had thought the production value was already ac watts, when in fact it was Wh for a 15 minute period. Of course, that means that now my values are generally higher than predicted by the SAM module.

One question I still have is if there is an explanation as to why the poa value differs so significantly from the measured radiation value. Should I be putting my whole radiation value in to the beam radiation parameter? The actual break out between beam and diffuse will obviously vary quite a bit depending on cloud cover, etc., but the weather station I get data from does not break out the components of the radiation.

Dear Mikkel,

I'm glad you sorted out the main reason your results were lower than you expected.

As you know, the pvwattsfunc module requires both a beam (direct normal) and diffuse horizontal radiation value for each time step. It calculates the resulting POA radiation from those values (and from sun and array angles calculated from the time and location data). I suspect that your assigning the total radiation to the beam variable is causing the difference you describe.

You might want to try using a model like the NREL DISC model (which I discuss in this post ) to estimate the beam and diffuse components of the total radiation value, and assigning those values to the beam and diffuse inputs to see if you get results closer to what you expect.

Best regards,
Paul.

Paul,

I was able to incorporate the DISC model that you recommended into my code to break out DNI and DHI values from my GHI measurement. For the most part, I think it is working well, but I see some anomalies as I get to the end of the day with the first day's data that I'm testing with. Specifically, I get quite a few POA values that are zero when the measured GHI is still higher than zero. In one time step, the POA value actually goes negative - which I don't think should be possible. The DNI and DHI values passed in for that timestep are both positive, so I'm a little bit at a loss as to why the POA would suddenly drop below zero, then come back again in the next step.

Any ideas as to what may be going on there? I have attached a spreadsheet called 'TestData20130927.xlsx' to the original message that shows the output from the time span in question.

Thanks in advance,

Mikkel