FlexMax 80 and High Voc

Re: FlexMax 80 and High Voc

RWB,

Not quite sure what you are asking... The formulas for panel voltages should be:

• Voc (cold) = Tcold * Tcoeff V/°C + Voc @ 25C
• Vmp (hot) = Thot * Tcoeff V/°C + Vmp @ 25C

Using a standard BP solar BP-4175 (175 watt) mono-crystalline panel:

Max Power Voltage - Vmp 35.7 Vdc
Open Circuit Voltage - Voc 44 Vdc
Voltage Temp Coeff - Vtoc -0.16 V/°C
STC Rating - Pmp 175 Wstcdc
Max Power Current - Imp 4.9 Adc
PTC Rating 155.2 Wptc

Say -10 C for cold and +45 C for Hot.

Assume worst case temperatures... Assume Tcold=ambient temperatures for Voc (maximum array output voltage). Assume Thot is ambient temperature +35C temperature rise (little wind, hot day). Remember STC temperature is 25C, so all values have to be adjusted by -25C:

• Voc (cold) = Tcold * Tcoeff V/°C + Voc @ 25C
• Vmp (hot) = Thot * Tcoeff V/°C + Vmp @ 25C

• Voc (cold) = (-10C-25C) * -0.16 V/°C + 44 Volts = 49.6 Volts (@-10C)
• Vmp (hot) = (45C-25C+35C) * -0.16 V/°C + 35.7 Volts = 26.9 volts (at 45C ambient + 35C rise)

Note that current does not change much with temperature so can be ignored for calculating minimum/maximum controller current.

Notice that you almost need a 2:1 voltage range (Voc cold to Vmp hot)... So, if you have a battery bank that needs 60 volts to charge, plus 2 volts for controller wiring drop--your Vmp min of 62 volts would require a controller with a Voc max of almost 124 volts...

Concidering that the "standard" MPPT controller will operate only to round 140 VDC--that gives you a pretty narrow range of optimum Vmp to hit...

Note that the Power Curves for a solar panel are fairly flat at Vmp--So even if your Vmp is a bit low--for charging/equalizing in your very hottest weather--the system will still work OK.

data sheet for a crystalline silicon panel

If your required Vmp is within ~10% of your panels actual Vmp (temperature corrected)--you will not lose very much energy (10% off of Vmp would probably lose around 10% or a bit less in power output).

Running 10% off of Vmp is not great--but your panels will not spend all of their summer at 45C + 35C rise either...

If you want to estimate the other panel output characteristics based on cell temperature (and solar irradiation) you can use these numbers from the generic crystalline panel information (they are all about the same, even amorphous should be close to the numbers below--they are still all physics based on silicon). Note, the below are % voltage and current changes, not absolute like the BP Solar numbers above were:

Temperature Coefficients:

Nominal Operating Cell Temperature (NOCT) 45ºC±2ºC
Temperature Coefficient of Pmax = -0.34 %/ºC
Temperature Coefficient of Voc= -0.48 %/ºC
Temperature Coefficient of Isc= 0.017 %/ºC

Notice that the current Temp Coeff is positive "+"... The voltage coefficient and power coefficients are negative.

As for calculating power output--just measure (with your solar irradiation monitor/assume various Watts per Sq. Meter. The panels are usually rated at 1,000 Watts per Sq. Meter.

There are "edge of cloud effects" and such where your panels could output more power than expected (broken clouds can focus sunlight, panels can cool off when shaded by clouds, then bright sun hits before panels heat up, etc.).

Amorphous panels can have a rapid falloff in output efficiency in the first 6 months of sunlight--after that period, they should level off.

From the PV Watts program, you can see what they assume for generic system values/losses for derating. They use 0.77 for Grid Tied systems.

For off grid systems, I use 0.77 (panel and MPPT GT inverter efficiency--very similar for MPPT DC solar charge controllers) and a couple more:

• 0.80 for Flooded Cell Batteries
• 0.90 for AGM Batteries
• 0.85 for TSW (true sine wave inverter)
• Derating = 0.77 * 0.80 flooded cell bat * 0.85 invrt = 0.52 overall derating

You can use a program like PV Watts Website and get monthly or even hourly output data from ~20 years of measurements of solar irradiation data for various sites around the US and World. Plug in your array size (1kW is smallest number--if your system is smaller, just multiply by the size in kWatts--like 400 watts would be * 0.4kW).

Note, if you are in coastal areas or other locations with lots of humidity and/or variable cloud cover--there can be significant year to year variations (over 10%). For the US, you can use the "Red Book" and look at PDF printouts for various locations (they have a small charge with the average and high/low ranges). High deserts tend to have low year to year variability. Coastal areas tend to have higher variations.
I will stop here--because I am not quite sure where you want to take this.

-Bill

Re: FlexMax 80 and High Voc

A single axis offset with the sun is simply Cos (angle)... If you are 10 degrees off angle, you are Cos (10) = 0.98 (98% of power).

If you are Cos 20 = 0.94 (94% of power)--Cos 45 = 0.70 etc...

The Vmp is, more or less, dependent on cell temperature--so if the cells are all roughly equivalent sun and wind--the Vmp's will be very similar...

However, your big problem may be Imp and Vbatt vs Vmp of a "panel"... Normally, you want to match Vmp to Vmp for parallel connections within 10% or so--not too much energy lost. And you want to do the same thing with Imp--match them to 10% or better--Or, you will lose a lot of power in your series strings. The "worst" ideal single angle mismatch:

• InvCos 0.90 = 26 degrees

If any of your series solar panels are off plane by 26 degrees or more--you will have a greater than 10% (SWAG) power loss.

And at morning/evening with the sun low in the sky--you will have panels getting near zero direct sun--so those strings will stop producing.

To be honest, your angles (just from looking at the pictures) seem to not cause that large of power drop (at least during mid-day).

If you want to see what the ideal setup (a set of parallel panels each with their own polar orientation--such as a "12 volt panel" charging a 12 volt battery--Really Vmp>17 volts on a MPPT or PWM controller and no Imp series connection issues)--You could use the PV Watts program and calculate the average output for each panel WRT to panel tilt and azimuth (pick three panels to start with middle and two end points). And see what the "hit in production" maybe. If you want, assuming this is an off-grid system, use 0.52 as a derating factor (battery and inverter losses included).

If you need high array voltage (48 volt battery bank or for Grid Tied inverter of 200-600 VDC)--then the series current issues may be a bigger problem (but I don't think that much for such a "gentle" set of angles). (not too much is on the order of 10% losses or so maximum).

Just my two cents.

-Bill

PS: The Shade on the Carport from the home's roof is probably much more of a problem.