How hot PV panels?

khotton · October 2007

Just how hot do PV panels reach above ambient air temperature? I was assuming that a nominally rated 12V panel like the KC130TM would (with 4 in series 17.6Vp x4 = 70.4V) provide sufficient voltage to properly charge a 48V battery bank. However after reading a recent post by BB (Bill), I am not sure.

I may change my plan to use 3 in series Evergreen 180W panels (25.9Vp x3 = 77.7V). However, this option is leaving me searching to low-profile roof mounts (TwoSeas shows no product for 3 Evergreen spruce panels -- what are my mount options?)

Kevin

BB. · October 2007

Re: How hot PV panels?

Depends on the panels and the temperature... For example:

Module: Evergreen ES-180-RL
Max Power Voltage - Vmp 25.9 Vdc
Open Circuit Voltage - Voc 32.6 Vdc
Voltage Temp Coeff - Vtoc -0.1141 V/°C
STC Rating - Pmp 180 Wstcdc
Max Power Current - Imp 6.95 Adc
PTC Rating 158.4 Wptc

Using Xantrex's calculator (quick place to look up solar panel specs.):

Set the average high to 95F, the Vmp=20.766 for Evergreen 180W panel

The equation should be:

[25.9 Vmp (STC) - 20.766 Vmp (95F ambient)] / [-0.1141V/°C] = 45°C rise over STC ratings (45 °C rise over 25 °C cell temperature test conditions)

From a Siemen's data sheet:

Determined under standard test conditions (STC): Irradiance = 1000w/m² cell temperature = 25 °C; solar spectral irradiance per ASTM E892 (Air Mass = 1.5).

or 77F+(45C*1.8°F/°C)=158°F (70°C) solar panel temperature in a 95F ambient (using Xantrex's rule of thumb)...

I think somebody measured some panel temperatures in hot weather--anybody remember them?

To compare with Xantrex's numbers, I have seen my BP 4175 panels Vmppt voltage (from my inverter--10 panels in series--so just multiply/divide my numbers by 10) set at ~289 Volts on a 85+ degree day. My panel was predicted (using the Xantrex solar array sizing tool) to have an output voltage of 285 volts on a 95F day (even though my spec. Vmp from BP would suggest that it would be 357 volts on an STC day).

On recent cooler days (~70F, mounted flat on my pitched roof, very light winds)--I have been seeing ~319 volts:

[35.7 Vmp (STC) - 31.9 Vmp (70F ambient)] / [-0.1141V/°C] = 33°C rise over STC ratings (60F rise).

A 70°F day (today's date) seems to indicate that I have a 70+60=130°F panel temperature on my roof on a clear, sunny, 70°F day with little wind (assuming my Xantrex inverter is calculating Vmppt correctly, and that my wiring loss is minimal--12 gauge, 20', 5 amps)...

So, getting a 60-77°F rise (my numbers, and Xantrex's number?) seems to give pretty consistent results for my home.

Pending better answers from other's on this board--I am still pretty convinced that the numbers (for Vmp and Voc) that Xantrex uses in their solar array / inverter sizing tool seems to give pretty accurate worst case numbers (based on their user selectable ranges)...

So, for a 95°F average summer day, the Evergreen panels (predicted by Xantrex) will have a Vmp=20.766 volts...

You have a 48 VDC battery system--assuming that you have flooded cells and want to equalize on hot days, you would need about 60-62 VDC. And if you are using a MPPT controller, you will need a ~2 volt minimum drop for the controller to function:

62V+2V/20.766=3.08 panels--really close but will probably work for all but the very hottest days... (3x20.766=62.39 volts)...

If you assume that you only need 60 VDC for equalization (or less), then three panels in series should be fine. Also, if it is windy in your location and/or you have good airflow under the panels (to help keep them cool), this will also help too...

But, you are, for example, in Colorado? So, you have to worry about cold weather boosting the panel temperatures too... If your area gets down to -4°F, then your Evergreen panels can get a Voc=37.7 volts (Voc-string=3x37.7=113.1v, 4x37.7=150.8 volts--may exceed controller max-voltage) on those cold/sunny days... So, you have to make sure that your Voc string rating does not exceed the controller's maximum voltage too...

Lots of typing--Did I make things more confusing?

-Bill

crewzer · October 2007

Re: How hot PV panels?

You can interpolate an estimate from the KC-130’s spec sheet. The panel temp spec at NOCT is 47 C. NOCT specs are based on 20 C ambient temperature and 80% insolation. Under these conditions, the cell temp is 27 C above ambient.

Assuming a linear response, the gain under full insolation would be ~34 C above ambient. So, if you live someplace where temps reach, say, 35 C (95 F), module cell temps could reach ~70 C, or 158F! Poor alignment and/or a good breeze will reduce this gain.

A separate but related problem is mounting location. Mounted close to a roof, the array will get even hotter due to higher ambient temperature and poor ventilation behind the modules.

A perhaps “perfect” solution for using KC-130 modules and a step-down MPPT controller for a 48 V battery system is to use five modules wired in series. The string’s temperature corrected Voc remains below 140 V down to -40 (F or C), and the Vmp in even hot environments is high enough (>65 V In bulk mode, typically ~75 V) for any 48 V battery application.

HTH,
Jim / crewzer

khotton · October 2007

Re: How hot PV panels?

Thanks so much for your replies. I am a mechanical engineer, but have managed to acquire basic understanding of electrical power systems (and of course PC computers).

I currently live in Broomfield, Colorado but this system is for use in Mexico (near San Miguel de Allende). Climate there is semi-arid (like Colorado) and high elevation 6900-ft, Latitude is 21-degN. I don't think it ever gets above 95F or below 32F (probably never below 40F if the sun is up).

I would like to mount the panels on a nearly flat (cement roof -- used for rainwater catchment). So I was looking for low-profile roof mounts (but again did not find any TwoSeas model for 3 Evergreen 180W panels).

Kevin

BB. · October 2007

Re: How hot PV panels?

One strong recommendation is that you have at least some pitch (is it 5 degrees minimum--Jim, Niel, anyone?) so that dust/dirt/leaves/rain keep the panels reasonably self cleaning--as well as allowing better air circulation to keep the panels cool in hot weather (cooling is less of a requirement--just more of a recommendation).

Dead flat panels are probably going to require constant cleaning (monthly or more often?).

-Bill

crewzer · October 2007

Re: How hot PV panels?

I believe the minimum recommended tilt angle is 15 degrees... this allows for convection ventilation behind the array and for some amount of "self cleaning" on the front...

For an off grid home at 21 N latitude, I'd set the tilt angle at 21 degrees minimum, and perhaps as high as ~36 degrees (latitude + 15). The latter would reduce summer energy harvest, but it would improve winter harvest....

HTH,
Jim / crewzer

niel · October 2007

Re: How hot PV panels?

i agree with bill that you should have some pitch to the pvs to minimumly allow dirt to wash off in the rain and the increased angle allows for more cooling too. if you will go that far why not put them pitched for the average angle aimed south for that location? you will gain more usable power doing this too. as a mechanical engineer this shouldn't be too difficult for you to fabricate yourself as even i had made some mounts for myself from aluminum angle and using stainless steel nuts, bolts, and washers to fasten it all together.

khotton · October 2007

Re: How hot PV panels?

I think you might have misunderstood me. I will have a flat roof, but I plan on tilting the panels toward the south, probably a bit higher than the latitude -- maybe 25-degrees -- to slightly bias output during the winter months at the expenses of some reduction of summer power.

Kevin

How hot PV panels?

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