Solar Power Generation

[PRincez]

Hi there,

I've been trying to solve a puzzle and this relates to solar energy generated by a PV panel (kWh/day). There are so many formula and equations out there for PV energy generated however i'm not sure which is which and which is correct!

I've tried to use basic electrical knowledge to work out the value; Energy generated per PV panel (kWh/day) = panel wattage(w) x efficiency(%) x average insolation (kWh/m2/day) x average hours of sunlight(hr). I've used the NASA website to retrieve information of the location using coordinates - so getting the parameters for the calculation is not an issue.

Can someone with vast knowledge and experience of soar power generation please put me through this.

I'll appreciate any info provided and or pointers etc. Thanks!

 

[bilalk]

If accuracy is not an issue, I usually go by the rule of thumb for a quick feel.
The key is to go by the total nameplate capacity of the generator and not dive into efficiency calculation of each individual module.

Note: the following calculation is valid for northern areas such as Toronto, Chicago, Mains, Montreal, etc...and depend upon your location you may vary the amount of daylight hours per day and the amount of months per year.

Total generated pv power by a pv generator = nameplate capacity of the generator x 10 hours/day for 90 days. This is the amount of energy generated in three summer-months. Then for remaining 9 months of the year I use 50% (just 5 hrs/day as an average) of full daylight availability.

Therefore, the total calculation may look like this....I am using an example of 500kW nameplate capacity of a pv generator:

Total pv generation in the three summer-months = 500kW x 10hrs/day =5000kWh/day x 90days = 450,000 kWh.
Total pv generation in the winter-months = 500kW x 5hrs/day = 2500kWh/day x (9x30 days) = 675,000 kWh.
Adding both quantities: 450,000 + 675,000 = 1,125,000 kWh/year.

if the incentives are, say $0.30 cent per kWh generated, then you would earn $337,500 per year. (very good investment, isn't it )
Obviously in Tuscon AZ, you would go with a higher averages, for example: for summer months you may go with 15hrs/day of sunshine for a total of 8 months/year, and for the winter months you may go with 10hrs/day for 4 months.

 

[omegamanltj]

Remember your nameplate capacity is based on the assumption that the panels are receiving the direct sunlight at the full angle. This will not be the case in practice unless you have a sophisticate tracking system. To be conservative, I would use something like 60% of the nameplate capacity,less if your roof is not direct southern facing (assuming you are in northern hemishpere). Obviously that's pretty rough and not good enough for a commercial project , but just a rough idea if you are a homeowner.

 

[bilalk]

omegamanltj:
I agree with you. It is safe to derate the nameplate capacity with at least 20 to 40% in order to account for location and weather conditions. and yes, it is a ballpark figure, but why not also good enough for I&C applications?

 

[LionelHutz]

Geeze, first rule in engineering is to always work the units. Starting with the power from the sun as kWh/m2/day you'd just multiply by the m2 of the panels to get the kWh of sun power in a single day. Then multiply by the panel efficiency to come up with the output of the panels per day.

Right now, you have W x kW x hours / m2 / day as the units. A complete miss-match of units.

 

[omegamanltj]

Hi Bilalk,
I am not a professional solar designer, I just assumed you would have to go into a lot more detail if you were doing a large commercial installation, as the owner, would probably want a lot more clarity on the expected payback time and rate of return.
One thing i have noted with my own roof mounted system at home, the output seems to have deteriorated over the years and its is only about 4 years old.
The first couple of years we were averaging about 8 kwhr perday it mid summer from a 1.5 kw system, but this summer we averaged about 6 kwhr per day. I know the weather is variable day t day and year to year, but my gut feeling is that the performance of the panels has dropped off.
Lloyd

 

[racookpe1978]

yes, solar PV panel performance drops significantly (17% - 25%) the first few years, but the decrease seems to stabilize after that at around 75% of the original nameplate claimed rating at solar noon.

Still, solar power is only effective 6-8 hours per day, even averaged over the entire year.

To even begin thinking about solar, you MUST report your latitude (this affects solar exposure levels each hour of the day and day of year) and geographic area (cloud, humidity, and atmospheric clarity levels vary greatly at regions of the same latitude) and how you are considering mounting the solar panels.

Solar water heating is often cost effective. Solar PV panels? Rarely a payoff if grid power is available. Sometimes marginally useful if you are far from a grid with only very, very limited electrical needs.

 

[PRincez]

LionelHutz

Thanks for your answer.Are you saying that for a 200w PV panel for example, the energy generated is E= Insolation value (kWh/m2/day) * surface area of the panel only? the number of hours of sunshine is not taken into account? Please advise. Thanks.

 

[itsmoked]

That's correct PRincez because the Insolation value already accounts for the entire "solar day"(all the hours of the day).

Keith Cress
kcress -

http://www.flaminsystems.com

 

[bilalk]

Hi,
I agree, of course for anything beyond coffee chat stage, you may want to conduct some more accurate calculations.
On another note, I wonder if the declining performance output of Omegamaneltj's pv system may be attributed to the PID phenomenon in solar panels. PID stands for Potential Induced Degradation, you may read more about it in the following link:

http://solarenergy.advanced-energy.com/upload/File/White_Papers/ENG-PID-270-01 web.pdf

Because the system is still new (just 4 years old) so I believe it is still under warranty. you may ask the developer to inspect, verify and test your system and fix the PID if needed to.

 

[LionelHutz]

No, that gives you the amount of solar energy that could reach the surface of the panel in a day.

Of course, you also have to consider the way the insolation was measured - flat to the ground, pointed to the sun at solar noon or??? Will your panels emulate the way it was measured by this NASA data? The data is not much use if the data is for the energy hitting the ground and your panels are pointed towards the sun.