Solar Model vs. Actual Solar Generation

When I wanted to know if our solar PV system would make any money for us under the Feed In Tariff program through the Ontario Power Authority, i could either listen to sales reps or calculate it. Some sales reps "guaranteed" that we'd make money, others said that it would pay for itself without over promising. I was skeptical of the former guys even though I knew our system would pay under the FIT program with $0.802/kWhr. I used RETScreen and PVWatts to do my modelling. I posted my results in an earlier post. Essentially what I found was that each month saw good generation, higher in the summer than December.
I've plotted my cumulative generation against the RETScreen model of our 4.9 kW system. What I found was the generation was greater than modelled during the autumn of 2010, much lower than the model during the winter, and it seems that March and April are fairly closely modelled.
The plotted data below shows several data sets :

• the blue curve is the results from the datalogger. The PV system was online on 24 August, but I didn't have the datalogger until 01 November so I simply read the meter at the end of each day until the datalogger was installed
• the open brown circles are the Guelph Hydro electricity meter readings
• the RETScreen model is the purple curve with triangles
• our household consumption [load meter] are the red closed circles

The chart is plotting cumulative kWhr. Broad observations of model accuracy should be made on multiple years of data, but we can still see how the model compares. So, what do we see???

• over the autumn our PV system was generating about 2/3rds of our consumption but after the dark and snowy days after 10 December our electricity consumption remained constant or slightly increased, however our panels virtually flat-lined. Our panels cover much of our south-facing roof but it is still not enough to supply our home's consumption. Technically we are not using any of the electricity we are producing - it is metered and sent directly onto the grid. But it would be nice to know that we are contributing what we are using albeit at different times of the day. 
• the panels generated more than the house consumed between 17 March and 27 April. The panels generated 745 kWhrs and the house load was 595 kWhrs [25.2% generation over consumption].
• the Guelph Hydro meter continues to be slightly below the datalogger. I wish they were the same! It could be that the datalogger is recording total generation which is occasionally greater than the inverter rating of 5 kW, whereas the Guelph Hydro meter is recording what is actually going out onto the grid. Or maybe not! Maybe they are measuring the same thing but one of them is wrong! Fortunately the difference is small.
• the RETScreen model seems to be approximating the generation starting around 15 February. Indeed the PV generation may be very slightly higher than the model.
• my financial calculations were based on RETScreen and PVWatts models, after mid-December we are less than my predicted generation [and therefore revenue]. I hope we have a nice bright summer to get close to the model again so we make our forecasted revenue!

Our loan costs from 24 August 2010 to 27 April 2011 was $2,646 and our revenue was $2,354. This is a deficit, however we also went from August to the darkest part of the year and we are just coming back into the sunnier season. I am confident that the low revenues of December and January will be more than offset by this summer's generation. We only need 30 days of over 20 kWhr to make up the $310 deficit, so I am not worried.

April 2011 Revenue and Cost Results

The chart below shows an update of our daily revenue and loan. The horizontal line at $10.70 is our loan payment [we pay biweekly]. The individual points are the daily revenue [we get paid by Guelph Hydro bi-monthly].
My observations for April, or at least to 28 April, shows several things :

• a large scatter above and below our loan payment line, but the days where it is below $10,70 is not so dismally low as December and January
• the brightest day of April was the 15th which rivals the bright days in March. Even though the panels would have been a little warmer than in March, and the reflectance from the ground lower since the snow has melted, perhaps it is the longer day and higher sun that yielded more energy
• late April has been very cloudy and dismal here in Guelph, so revenue is on average lower than our costs
• final observation is that Chris Duke looks out his office window in April a lot and wishes for sunnier days. This has been wetter and cloudier April then other years [even the amount of fields with the corn sown is behind normal years]

April 2011 Generation Results

The bright sunny days of March have led to rather dreary days of April. This has been very interesting results for our solar power generation. I don't think there was one day in April of totally bright cloud-free days, though I'll check that later. In the mean time I am posting my PV generation to date. Given that the next few days that end April will be very cloudy and wet, I don't expect anything interesting, so I post this a few days before then end of April.
However, a quick note about extremely windy days today and yesterday. There was unconfirmed tornadoes in Kitchener, 20 km to the west of us yesterday, and very strong gusts here in Guelph. Our panels are still on the roof - that's a good thing. I was very impressed that our solar system installer [Merlyn Power]  called this morning to see if we still had panels on our roof or if there was any damage. Very considerate and consciousness of him.
So, the results:

• the middle of March was spectacularly bright as you see in my 03 April post. This continued into early April but mid to end of April were relatively cloudy with lower kWhrs
• 20 April was only 5 kWhrs which was a very rainy and dark day
• cloudy with sunny periods was yielding about 15 kWhrs

 

Daily Revenue From System to March 2011

My previous entry shows the energy generation. This entry and figure below shows the daily revenue from the system and the loan payment. I’ve converted the bi-weekly loan payment to daily values [$10.70/day]. It appears the loan was roughly in the middle of the data cloud until December where the generation was far below the loan payments. This period created a deficit which I will show in forthcoming posts.
I will revisit the Power Generation figure I last posted for further discussions, but I wanted to show that December and January had very few days where the PV system paid its way whereas this changes in March with the cold, clear, sunny days. I write this now wishing for many more bright March days but after a few days of nice warm weather reaching up to 10C, I look out the window to snow falling. A centimeter has just fallen in the past 10 minutes but it will apparently be 14C and rainy tomorrow. This snow won't last, but we may have many cloudy days until Easter.

Autumn and Winter Generation Results

Our solar PV generation had many interesting aspects over this winter. Now that spring has come and there is no longer snow on the panels, we are able to see many influences of cloudy weather, snow-covered panels, and the brightening days of spring. Also, we have gone through the fall and winter seasons and are able to see some trends. I’ll discuss autumn accumulated energy and its trend, impact of snow coverage, winter results, and energy trends in the early spring. I’ll show the results of RETScreen model, and finally I’ll analyze the instantaneous generation results. I’ll cover these all over the next several blog entries which I hope to post over this week.
Today’s entry is the daily power generation and revenues. Recall, out system is 4.9 kW of SolarWorld 245 watt panels and a Fronius 5100 string inverter [5,000 watts]. There are 20 panels on two strings. The panels are on a 40 degree slope and azimuth of 135 degrees [SE]. We have a microFIT contract at $0.802/kWhr
Daily Electricity Generation
I’ve plotted out the daily generation from 24/08/2010 [day system was grid-tied] to end of March 2011. The plot is shown below and my observations follow.

Solar PV electricity generation at 243 Farley Dr, Guelph, ON, to 31 March 2011.

1. Autumn daily generation varies considerably due to clouds: The maximum generation during the first week was between 26 and 28 kWhr but then less than 25 kW until mid November. The maximum values decreased into December to a maximum of 16 kWhr. There are several factors that affect the amount of power generated besides the clouds. The snow on the ground reflects sun onto the panels. The sun angles change the amount of time and intensity of light that strikes the panels. Colder temperatures make the panels more efficient. The most cloudy days would be producing the lowest values in the chart between August and mid-November – between 2 and 3.8 kWhrs per day on the cloudiest days. On the sunniest days the average hours of light is 4.9 hours [24 kWhr/4.9 kW of panels = 4.9 hrs] and on the cloudiest days it is 0.41 hours. 
2. Snow cover means little generation: Observe the data from 22/11 to 7/01. This month and a half period saw considerable number of days with either partial of full snow coverage on the panels. Many of these days were 0.3 to 0.8 kWhrs only! Ground mounted systems or panels that are accessible with a long broom could sweep off some of the snow to improve generation during these periods. Our panels are too high to reach. Brushing off the panels may result in scratching and damage.
   The snow was melted from the panels in early January. Several days generated over 10 kWhr, but were the snow returned mid-January. 
3. Cold, clear days increased generation: The bright clear days, the highly reflective snowy ground, and the cold temperature resulted in very high output. The peak output was up to 35 kWhrs on several days. The panels were putting out up to 5200 watts, that is 260 W from panels rated at 245 W. This really surprised me. These days were crystal clear blue sky, between -5C and +5C with a gentle breeze. The panels would have remained cold thereby increasing their electrical efficiency. This was very pleasing to see after the dark low energy days of December were not so long ago.