PV Output Calculation Steps

calum h · Jun 7, 2015

Hi guys,

For my current dissertation project at The University of Strathclyde, I am attempting to build an Excel model which calculates PV output for given direct and diffuse solar radiation levels. I have attached below the calculation steps (within figure and as attachment) that I was taught to use as a student. However, as this material was never examinable I am not 100% sure of how to apply them, or if they are even correct in the first place.

Two specific questions I have at the moment: (1) are all trigonometric calculations performed in degrees rather than radians, and; (2) is the definition of surface azimuth correct?

The Excel model I have developed certainly isn't happy! My typed formula seems fine (after checking countless times), so I thought I better check that the calculation steps are correct.

If someone is able to provide just a small piece of help on this, I would really appreciate your help.

Thanks in advance,

Calum.

Gavin A · Jun 7, 2015

We have developed an excel spreadsheet to do these calcs, however I'm currently a little tipsy, so... the bit that seems to be missing is anything about the proportion of the sky that's covered for diffuse radiation, so eg if there's a tree to the north, this will block x% of the sky for the diffuse radiation.

Also this seems to be a formula to calculate instantaneous output, which is fine, but is that what you're trying to calculate? Beyond those observations I'm not going to work through the calcs for you, sorry, that's a bit too much like hard work for a sunday night, but they look about right.

I can pass this by our expert in such things for comment though, I know he's regularly trying to update and perfect our formulas so he'll probably be interested.

The Solar King · Jun 8, 2015

Calum

Who is your Tutor/Prof etc? There are some really good guys in the academic sphere involved with solar in both Glasgow and Edinburgh. Have you looked for any support either through SiSer or SSEG?

BruceB · Jun 10, 2015

I do not know those formulae, but I have done plenty of maths and spreadsheet work. To pick up some of your points:

- In principle it does not matter whether your angles are in radians or in degrees, the formulae will still work if coded correctly as the angles are inputs not outputs.

- However because you have written it with various "90-" it is clear that the angle inputs should be in degrees.

- Be aware that Excel defaults to radians so to get the sine of an angle in degrees it is "=sin(radians(angle_in_degrees))"

- Whether you have surface azimuth defined correctly is arbitary. The important thing is that surface azimuth and solar azimuth have the same direction of reference. Whether it is north or south or anything else does not matter, but for "cos(w)" to work and =1 when the panel is pointing at the sun choose an appropriate reference.