Energy Matters Forums

[seawatch]

Hi, my name is Bill. I have a grid connected 1.5kW system using 8 panels Suntech STP 185S and an Aurora PVI-2000 Inverter. I am situated on the NW Coast of Tasmania, 250 metres from Bass Strait. The system has been
in operation for 8 months and has produced an average of 6.6kW/day, sofar. Close to predicted by EM.
I have been keeping a log of most of the variables on display and I'm trying to relate the daily output to the Solar Exposure readings that are produced by the Bureau of meteorology under "climate data"
There are a number of such measuring stations in the vicinity and all show a high degree of consistency.
When plotting the BOM solar exposure in kW/sq M against my system's daily output in kW/day,there does appear to be a degree of linear relationship at the high and low levels but a lot of scatter in mid range.
My question:- What is known about the relationship between these two variables? Does the relationship change with the season.

[cw nsw]

Hi Bill - welcome

Interesting question, I've been tracking my output figures (10 months) against the BOM stats too, but tbh, not quite sure, what I'm reading/interpreting.

I haven't found any BOM statements about the relationship between the variables - so figure the next option is to phone BOM in Sydney - my nearest office -- 4 hrs away. Have you got a 'local' BOM person you can ask?

[Gordon-Loomberah]

Hi Bill, and welcome to the EM forums

My question:- What is known about the relationship between these two variables? Does the relationship change with the season.

I would think a seasonal variation could be related to the positioning of your panels- how close to N and at what tilt. The season(s) where the panels are more directly facing the sun would tend to give a higher production, proportionally. Keep in mind that the BOM climate data is an average over many years, individual years could show significant variation, particularly in areas subject to the El Nino and La Nina weather variations.

[cw nsw]

http://www.bom.gov.au/climate/data/

Gordon, in responding to Bill, I was referring to the data available via link above (new BOM service) and assume that Bill was referring to the same - I'm tracking local solar exposure data monthly

[Gordon-Loomberah]

Oh, thanks, I haven't seen those pages before. Still, the PV angle vs season comments hold. Regarding the intermediate scatter, I wonder if it is related to local cloudiness variations, as the intermediate value days would tend to be when there is some cloud about. Unless you were at the same location as the recording station, some scatter could be expected... although I assume it would at least partially smooth out as the clouds drifted by, if you werent in a location above which orographic clouds consistently formed.

I know with my solar radiation sensor, the results of cloudy days show higher irradiance between the clouds due to reflection, edge effect etc, and deep minimums when under thick clouds. Actually, I've been thinking about a thread showing some graphs of the theoretical solar radiation vs actual for days with differing amounts and types of clouds.

[seawatch]

Thanks for your responses. Just some additional information.The BOM Solar exposure results are published daily. We have at least 4 stations within 20km radius and results are very similar. I'd imagine the station 1.5 kM away is a good representation of the solar exposure experienced at my panels. There is a slight mismatch in timing though. BOM results run from MN to MN whilst my Aurora inverter sums Watts from 07:50 to 07:50 the next day but amount of radiation before this time is very small.
My panels are mounted approx 5 degrees West of North at an inclination of 17 degrees with no shadows of structures or trees.

[seawatch]

Hi cw nsw,
I have written to BOM as I'd expect them to be able to answer this but they said:- Answers of a technical nature about solar panel output is not something we do. BOM provides the data only.
I"m aware that Solar Exposure (global) is the sum of energy in the direct sunlight beam and energy in the diffuse radiation but I would have expected solar panels do something similar unless their spectral response characteristics is very different.

[cw nsw]

Hi Bill - I've got my nearest station's (8km away), daily update page filed as a "passive bookmark" for now - with a theoretical plan to start daily comparisons July 1.

at end June I'll look at my 12 month figures, below are my Jan - Apr 2011 monthly average figures:

daily solar exp kWh/m JAN 8.00, FEB 6.70, MAR 5.60, APR 4.80

daily ave output kWh JAN 11.19, FEB 9.32, MAR 7.87, APR 6.77

When I first looked at the station information a couple of months ago, it basically reflected the bellcurve that I expect my system output to have. Figures above indicate a seasonal relationship.

[seawatch]

Thanks cw, for your figures. A plot of your monthly average results does follow an almost perfect straight line relationship.Perhaps I was expecting too much to get that outcome out of daily results.
My guess is that your system is somewhat larger than my 1.5 kW. I was hoping to be able to paste a copy of my graph in this document but it won't take it.

Regards
Bill

[cw nsw]

bill

don't think we can post graphs here - I might put a 12 month graph on my blog at end June, link for BOM page is also there too, for anyone else wanting to compare the numbers.

my system is 2kw, link to my pvoutput page is also on blog. anyway here is Jul - Dec 2010 figures (mean annual ave daily s/e is 5.15). For the 6 months below it was 5.48 with daily kWh for those 6 months 8.10.

Jul 3.00 & 4.45
Aug 3.70 & 5.81
Sep 4.80 & 8.17
Oct 5.90 & 9.55
Nov 6.70 & 9.64
Dec 7.80 & 11.00