Data logger for individual strings?

Re: Data logger for individual strings?

What sort of system?

Off-Grid, Grid-Tied, New, Existing, Wattage per string/system, etc.?

The math is pretty well known--and using sites like PV Watts do a very good job of estimating average system output for the year (hourly, monthly, yearly) based on installation angle, tracking options, and local "average" weather.

Is this a "demonstration project"? Does the logging system need to be "full" system--or can you simply install a few small panels at differing angles and tracking systems to demonstrate the options/variability.

In general, in my humble opinion, measuring/comparing solar options is a game of numbers and long term averages. Gathering information over a few days or even one year can be misleading. Depending on where you live, you can have +/- 10-20% variability in output month over month or year over year (see the PDF/data for your location to learn about variability over ~20 years based on long term monitoring).

Obviously, there are long term data logging systems out there--but the larger/more complex they are, the more they cost and (many times) the more power they require.

And, with the data you gather, it needs to be compared over the long term to see/understand what would be best for a particular location (i.e., two real-time meters on two arrays--being read by somebody wondering through an exhibit at 10am on an arbitrary day would not be able to make a valid judgment as to which mounting variant would be best).

If you want to log/measure for your location--perhaps using small panels (they can even be very small--the size of a postage stamp) to gather the information and use a small logger to measure short circuit current--which is pretty much proportional to solar energy captured.

Would be cheaper/easier and less complex than monitoring a GT system (current*voltage to account for MPPT) or the issues with an off-grid system (which normally uses less than 100% of maximum available power once the batteries are fully charged).

Or just get a weather station and mount the solar irradiance sensor(s) next to the panel(s).

Lastly, the resultant data needs to be compared with actual needs/pricing of power...

For example, Solar Guppy (in Florida) finds his easterly biased array produces more power than his westerly (or southerly?) biased array because of (for his location) typical evening clouds/thunderstorms vs clear morning weather.

However, for example in California, a Grid Tied system is required to use Time of Use metering/billing.

So, for us in California, a system that it biased more towards the west may generate more $$$ credits for our bill even though it generates less total power over the year.

With Time of Use and Tied Pricing (another California fun thing where the more power you use, the higher your rates become)--measuring "accurate" solar output vs total production becomes less interesting and the overlaying of utility billing makes the whole set of calculations more complex (how many people know their kWhr usage--by the hour, monthly tiered billing, over a year). As seen below--for somebody using AC in the summer vs off-peak no A/C user--there is a 7:1 pricing ratio ($0.61 per kWhr down to $0.08 per kWhr between Peak Summer maximum tier vs winter off-peak minimum tier).

Solar Grid Tied power can really affect (and hopefully save) money for a heavy summer power user here.

And, we are probably within a couple years here of "Smart Metering"... Which will give some access to hourly (or for businesses, 15 minute) usage data--but also change the rates every 24 hours...

Oh well, much of the above probably does not apply to Florida--end of rant...

-Bill

PS: Example of TOU/Tiered pricing for Northern California new solar user:

[FONT=Fixedsys]    
Total Energy Rates $ per kWh)  PEAK  PART-PEAK  OFF-PEAK  
 Summer        
  Baseline Usage             $0.29265   (R)  $0.14432   (R)  $0.08447   (R) 
    101% - 130% of Baseline  $0.30841   (R)  $0.16008   (R)  $0.10023   (R) 
    131% - 200% of Baseline  $0.43690   (I)  $0.28857   (I)  $0.22872   (I) 
    201% - 300% of Baseline  $0.55568   (I)  $0.40735   (I)  $0.34750   (I) 
    Over   300% of Baseline  $0.61792   (I)  $0.46960   (I)  $0.40974   (I) 
 Winter     
  Baseline Usage               $0.10018   (R)  $0.08848   (R) 
    101% - 130% of Baseline    $0.11595   (R)  $0.10424   (R) 
    131% - 200% of Baseline    $0.24443   (I)  $0.23273   (I) 
    201% - 300% of Baseline    $0.36321   (I)  $0.35151   (I) 
    Over   300% of Baseline    $0.42546   (I)  $0.41375   (I) 
    
Total Meter Charge Rate ($ per meter per day)  $0.25298     
Total Minimum Charge Rate ($ per meter per day)  $0.14784     


.....


3.  TIME PERIODS:  Times of the year and times of the day are
defined as follows: 
Summer (service from May 1 through October 31): 
Peak:  1:00 p.m. to 7:00 p.m.  Monday through Friday 
Partial-Peak: 10:00 a.m. to 1:00 p.m. 
    AND 7:00 p.m. to 9:00 p.m.  Monday through Friday 
    Plus 5:00 p.m. to 8:00 p.m.  Saturday and Sunday 
Off-Peak:    All other times including Holidays. 
Winter (service from November 1 through April 30):  
Partial-Peak:  5:00 p.m. to 8:00 p.m.  Monday through Friday 
Off-Peak:    All other times including Holidays. 
Holidays:    Holidays for the purposes of this rate schedule
are New Year’s Day, President’s Day, Memorial Day, 
Independence Day, Labor Day, Veterans Day, Thanksgiving Day, 
and Christmas Day.
The dates will be those on which the holidays are legally observed. 

DAYLIGHT SAVING TIME ADJUSTMENT:  The time periods shown above
will begin and end one hour later for the period between the 
second Sunday in March and the first Sunday in April, and for 
the period between the last Sunday in October and the first 
Sunday in November. [/FONT]

Re: Data logger for individual strings?

Depending on voltage and current--A "Watts Up" or "Doc Watson" amp*hour/Watt*hour meter might be nice if your Vmp/Voc is under 60 volts...

A "problem" with measuring the outputs of two pairs of panel sets for "optimum" output on a Grid Tied system is that a PWM/MPPT controller does not absorb an "optimum" amount of power from the panels every day. After the batteries are charged to 80-90% State of Charge--the controller reduces the amount of current/power from the panels as the batteries "top off"...

So, to a large degree then, the amount of power you will see from the panels (in any orientation) is limited by the amount of the power the batteries (as decided by the charge controller) needs to recharge the batteries.

If you wanted to "properly" perform a test of optimum panel angles on an operational Off-Grid system--you need to keep the batteries below ~80% state of charge and/or place a load on the system to ensure the controller is pulling 100% of the available panel power--which sort of mucks up your system operation (requiring you to possibly "waste" power and storage to make a valid test).

Your better bet would be to take a couple small wattage panels, mounted to the side of the big arrays, and log them (either with a "load" and a Watts UP or that little solar irradiation logger, or some other method. You can set it up to log the maximum amount of output from the panel and integrate the results over the entire day (either in a spread sheet or a cumulative logging meter).

Only then will the results be independent of your off-grid daily loads and controller operations.

-Bill