Maximizing a solar day?

rgk1 · June 2011

I always see calculations that into account the average number of hours of good sunight for a given location. At what point will a solar panel begin to produce usable voltage and current? I guess what I am asking is does the voltage and current slowly increase as the suns energy gets stronger and more direct and drop off as the sun begins to set? I can see where a tracker would maximize solar harvest, but would a MPPT controller with say 48 volts of solar panels and 12 volt output/battery bank/inverter begin to produce enough usable output to start charging the batteries earlier in the day till later in the afternoon as opposed to a 12 volt (17v or so) panel?

icarus · June 2011

Re: Maximizing a solar day?

It is related to the angle of inciidence. The closer the PV is to being perpendicular to the sun, the higher the (relative) performance will be. For example, my panels get full sun right after sunrise, and are oriented to do so, since they are shaded after ~3pm. The are also oriented to get maximum gain ~ 11 am instead of noon for the same reason. Other factors to consider is local climate conditions. Early morning fogs or late afternoon haze a will have an effect as well. Low sun angles, nearer to sunrise/sunset will decrease out put considerably, as will higher PV temps.

So, I guess there is no easy answer.. A mechanical tracker will help, but it has been suggested that for the expenses of a tracker, you are better off getting additional PV for the price. (unless you need absolutely the best harvest with a limited space/budget like for water pumping.)

Tony

rgk1 · June 2011

Re: Maximizing a solar day?

I am mainly trying to better understand how the panels operate and whether I would see any benefit of using 2 panels in series for a higher voltage in hopes of gaining more total amperage at 12v over the course of the day. Under theoretical ideal conditions would a 34 volt array (2 - 17 volt panels in series) reach 17 volts and useful current before a single 17 volt single panel would?

BB. · June 2011

Re: Maximizing a solar day?

No--not really... The panels do not generate very much energy until they have reached Vmp anyway... From what I have seen, basically weak sunlight (sunrise/sunset on panel).

Once you have weak sunlight on panel and Vmp is reached--then any more sun will make more current available.

To put it in some numbers:

• sunlight on an average day ranges from 32 000 to 100 000 lux
• TV studios are lit at about 1 000 lux [i.e. 1000 lumens per square metre]
• a bright office has about 400 lux of illumination
• At sunset and sunrise, ambient outdoor light is also about 400 lux (if the sky is clear).
• moonlight represents about 1 lux
• starlight measures a mere 0.00005 lux

Lets say that "useful power" from an array is 0.1% (1/1,000) of the array ratings. A 1,000 Watt array producing 1 watt.

100,000 Lux * 0.1% = 100 Lux

So--for 1-4 watts of a 1,000 watt array, you would need the amount of light, sun rise/set or moderately bright office.

If you think that 1% of rated power (10 watts from 1,000 watt array), then bright TV studio lighting level.

Realistically, depending on your controller you may have some losses too... An MPPT controller uses a fair amount of power (call it 2.5% or so) just to turn on its electronics... So, even if you really needed those couple of watts from your 1,000 watt array--it would still need to generate ~25 watts just to turn in the MPPT charge controller on (PWM have less Tare Losses).

I sometimes check my 3.5 kW array on my GT inverter--with Vmp=354 volts, I never seem to get enough power to turn the GT inverter on with less than 320 volts or so (i.e., ~0+ watts of generation). (just checked (~320 volts @ 0.1 amps and ~8 watts to grid at sun set). And my GT 3.3 Xantrex Inverter will operate in MPPT mode down to ~200 VDC... So it is not finding any additional power much below Vmp array voltage.

Unless you have special requirements (like a solar powered calculator in office)--I would not worry about series/parallel.

Sending power > 10-20+ feet from Panel to Charge controller and > 400 watts--then I would look at series connections on a MPPT controller.

-Bill

rgk1 · June 2011

Re: Maximizing a solar day?

Looks like I will stick with a single panel for my small system. I guess some things just don't work the way i think they would. But thats why I'm here. Someone has the knowledge and experience to make the newbie understand. Thanks again.

solarix · June 2011

Re: Maximizing a solar day?

Your question concerns the voltage / current relationship in a PV panel.
The typical panel will produce a higher voltage (Voc) when no current is flowing, thus in the morning, even with little sunlight, the voltage comes up quickly until it is high enough to push your load (charge batteries) as no current is flowing. Once the panel voltage is higher than your batt voltage, current then starts to flow and a little power is produced. Then the current gradually increases as the sun gets a better incident angle and the panel voltage comes down a little until at midday it gets close to Vmp and Imp. The sequence is then reversed until nightfall.
Actually, this assumes you have a MPPT type controller, if you have a PWM controller - it ties the panel voltage to the battery voltage.

ggunn · June 2011

Re: Maximizing a solar day?

rgk1 wrote: »

I guess what I am asking is does the voltage and current slowly increase as the suns energy gets stronger and more direct and drop off as the sun begins to set?

Current yes, voltage no.

Voltage comes up to maximum with very little light and stays constant except for temperature effects until the light goes away. Voltage is the highest shortly after dawn when cell temperature is low.

Current is proportional to insolation (the strength of the sunlight) and is highest when the sun's rays are perpendicular to the surface of the module(s).

Power is the product of current and voltage and is therefore roughly proportional to current. Power is the number you are interested in, and it doesn't matter whether you have a high voltage low current or low voltage high current array. With a higher voltage lower current system, though, you can use smaller (cheaper) wire.

I recently did a theoretical comparison of a 20 degree tilted 180 azimuth fixed array with one the same size with the same tilt and azimuth but single axis tracking to be located near Dallas, and PVWatts predicted about a 28% increase in production for the single axis tracker over the fixed array.

Of course, you have to figure in the cost of the tracking equipment and the additional space you will need to avoid shading effects in your analysis to decide if it makes economic sense to use tracking gear.

sub3marathonman · June 2011

Re: Maximizing a solar day?

BB. wrote: »

Realistically, depending on your controller you may have some losses too... An MPPT controller uses a fair amount of power (call it 2.5% or so) just to turn on its electronics... So, even if you really needed those couple of watts from your 1,000 watt array--it would still need to generate ~25 watts just to turn in the MPPT charge controller on (PWM have less Tare Losses).

I sometimes check my 3.5 kW array on my GT inverter--with Vmp=354 volts, I never seem to get enough power to turn the GT inverter on with less than 320 volts or so (i.e., ~0+ watts of generation). (just checked (~320 volts @ 0.1 amps and ~8 watts to grid at sun set). And my GT 3.3 Xantrex Inverter will operate in MPPT mode down to ~200 VDC... So it is not finding any additional power much below Vmp array voltage.

-Bill

Now I'm wondering if at those low lighting conditions if the Enphase individual panel inverters would do better than a MPPT charge controller and central GT inverter? And when, if ever, does the central GT inverter overtake the Enphase efficiency? (I haven't done a search for this information, so please forgive if it has already been discussed.)

BB. · June 2011

Re: Maximizing a solar day?

I would not go to a lot of trouble/money to spend for a micro inverter over central inverter...

In the grand scheme of things--On average, properly sized arrays+inverters probably only vary in output by ~1-2% maximum (i.e., ~96-98% efficiency range for GT inverters).

At the low end of the power range (sunrise/sunset), Enphase seems to do a good job on minimal power required to run the inverter (tare loading--or power required to run the inverter electronics). You may gain a little more power generation during those periods--but again, a significant increase in efficiency at a time of day when little power is being generated anyway is not a lot of power (8 watts vs 16 watts extra for an hour a day from a 3.5 kW array--just a wild guess/example).

Other things, panel orientation/tilt, moving vent pipes/trees/sources of shading during the day, etc. are going to make a bigger difference.

-Bill

nsaspook · June 2011

Re: Maximizing a solar day?

Here are some charts from my system from today and yesterday (cloudy) showing the quick rise in voltage but slow rise in current/power.
Time are in GMT.
With my small off-grid system I see almost no gain in power from adjusting the array in the morning when the PV power is going to recharge the battery, the sun light is just too weak to make much current. Late in the day when the controller is in float and loads are running moving it generates a few more watts but a second array with more evening tilt seems to be a better way to spend the extra money.

The last image shows todays recharge to float cycle.

Maximizing a solar day?

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