New SOLAR PV Product

[MountainDon]

Alright! I just found out about a relatively new product that could be indispensable to some of us with solar PV systems, completed or simply in the works.

It's from Apollo Solar and is called the Low Temperature Overvoltage Bypass™ It will cut one panel out of the series array when the temperature dips below a preset point.

For those who do not immediately grasp the significance of this, let me supply a little background info. The output voltage of PV panels increases with decrease in temperature and conversely the output voltage drops with an increase in temperature.    Why is this significant? 

Frequently it is desirable to connect PV panels in series, increasing the panel system voltage and reducing the size of the cabling as compared to a parallel connected circuit. This is especially important to me as my PV panels cannot be located right near the cabin. Too many trees. Charge controllers all have upper input voltage limits. Mostly they burn out when that threshold is exceeded. 

Let's say you've calculated your system requirements and 525 watts worth of panels would fulfill your needs. Three 175 watt panels would work perfectly. I'm looking at three BP SX3175N panels, 175 watts, 24 VDC nominal, 36.1 working (Vmp) volts at 4.85 amps (Imp). 3 x 24 = 72 VDC. High enough voltage, low enough amps to make for reasonable sized cables from the panels to cabin. Not all charge controllers can handle 72 VDC input but some like the Outback MX 60 are rated to 72 VDC nominal input. However there is an absolute maximum input of 150 VDC. At that point the unit may become toast. But wait, you say! 36.1 x 3 is only 108.3. What's the big deal?

The big deal is there's another voltage rating that's important. There's what's called open circuit voltage, or Voc. Voc is what the panel voltage can spike to under certain conditions, such as no load, or very cold temperatures. When the suns direct rays first strike the panel on a cold morning the voltage can easily reach Voc. The Voc for these BP panels is 43.6. 43.6 x 3 = 130.8. But wait, you exclaim again, that's nearly 20 volts under the MX60A's rated maximum.!    There's a cushion. No big deal. ?

Well, if things are just right, or wrong, the voltage can spike above the Voc.  You'd think it's not supposed to going by the spec sheet, but it can. Panels are generally rated at 20 degrees C (68 F) A nice cold crystal clear winter morning in the NM mountains would be ripe for one of those times. It's generally advised to limit the Voc of a series string to 75% of the max rated voltage of the charge controller. You might get away with it, but you might not. At $500 for an MX60 that's a big gamble; too big for me.

[The MX60 actually shuts down at 141 volts. However if the voltage reaches 150 the unit will be damaged.]

So that's where the Low Temperature Overvoltage Bypass™ could pay for itself. Most of the time your system would be producing max voltage and power. When it gets real cold your safeguard kicks in and disconnects a panel. As soon as things warm up you're running at full power again. BTW, It's the panel temp that's measured, not air temp, and the panels warm up nicely after a little use. The failsafe mode defaults to safe with one panel cut out.

It's an answer I've been looking for. 
Available at...
http://www.affordable-solar.com/apollo.low.temperature.overvoltage.bypass.htm
and
http://thesolar.biz/solar_panels.htm
Scroll down 3/5 the page, under the PV panels ... 

There's a 10 degree C and a 0 degree C model. (50 F and 32 F) though the linked firm carries only the 50.    I lean towards wanting the 32, but have no research to validate that.