Converting insolation values

[quintonpereira]

Firstly, thank you for your reply to my first post :http://forum.pvsyst.com/viewtopic.php?f=22&t=1397&p=0&e=0&sid=d750c9a4e98f51edeac50844eb3d05a1#p3934

My second and more practical question may appear simple, but I am highly interested in the accuracy of data.

Our SCADA systems are currently connected to analogue thermopile pyranometers. Our daily generation reports consist of a 15 minute time interval and since we do not have dataloggers, what we see are instantaneous values obtained throughout the day.

We want to accurately convert W/sq.m to kWh/sq.m everyday to calculate PR. My questions are:

1. Do we directly multiply the instantaneous values with 15 minutes and add up the values for the day?

2. Is this time sufficient or do we choose to then log 1 minute or (any other suggested) intervals?

3. Do we then integrate and find the area under the curve, if so what mathematics are involved? What are the limits of the integral?

4. What back-end calculations does PVsyst do in this regard?

5. How do we reduce the error in our observed values?

Thanks again for your replies in advance.

[Bruno Wittmer]

Hello,

you might want to have a closer look at the International Standard IEC 61724 "Photovoltaic system performance monitoring - Guidelines for measurement, data exchange and analysis".

You will find there useful information on monitoring the irradiance.

The sampling interval for the irradiance is specified to be 1 min. or less, so I would say you sampling interval of 15 min. is too large.

The standard also explains how to integrate or average your data according to the sensor and value you are measuring.

If you want to measure hourly data that you want to import in PVsyst, the values should be representative for the hour that follows the time label. If for example you measure a value for 15:00, it should reflect the data from 15:00 to 16:00.

For minimizimg your statistical errors, you need to keep the sampling interval small. To keep systematic errors small you should carefully calibrate your sensors and measure at a place that is representative for your whole array. If you can afford a redundant measurement this will also reduce the uncertainties.

The standard also gives guidelines on how to check the quality of your measured data.