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Piece-meal Solar PV and MPPT Controllers

                        


Joined: May 23, 2011
Posts: 34
Location: Big Island, Hawaii
i'm piecing together my power system (Solar PV with microhydro one day down the line) as i get the funds/find the deals. For a long time i've been wary of the newer type of charge controlers (Maximum Power Point Trackers), because 1) they are much more expensive 2) they have shorter warranties in general (2 years vs 5). Recently thou, as i've been researching more, it seems like a MPPT makes a lot of sense. Being able to use 24v panels in a 12v system is FANTASTIC, and being able to wire in series for boosted voltage is also great. 24v panels are about 1/2 the price of 12v panels per watt.

quick numbers:
60 amp MPPT controller - $500
2 240 watt 24v panels - $980
vs
60 amp PVM controller - $200
4 120 watt 12v panels - $1600

2 240w 24v panels in series to produce 48v can run through the same wire as a single 120w 12v panel - so that's 1/4 panel wiring cost right there, not to mention simplified installation.

anyway. Any permies using MPPT in their off-grid solar system? can you report on longevity/reliablity? recommend a brand or model?

Anyone have experience using MPPT for microhydro? i've got about a 1/4 mile run from hydro potential. it would simply be impossible to get any decent amperage that far at 12v. of course the microhydro would require a dedicated controller, and thousands of feet of wiring, not to mention the cost of turbines/penstock etc...probably cheaper to just get more solar panels? still up in the air on this one...

currently i've got about 400 amp hours of battery and 3 60w 12v panels. Plenty for lights, water pump, and a computer charge, and even light power tool usage on sunny days (cutting firewood with an electric chainsaw anyone?). but i'm soon gonna be adding a DC fridge to my system and it's 400w daily draw is going to put me right at the edge of what my system can handle during the sunny months. i've also got a much larger water pump (21 amp draw v. 7 amp draw on my current pump) i'd like to install and use for irrigating (my main tank has about 4 feet of head, my gravity tank only about 12 feet - very little pressure!). and for these luxuries, i gotta have the POWER!

i switch my inverter on only when it's in use, 3-4 hrs a day. i'm working on getting everything possible to be DC. anyone got a reccomendation for a small inverter with VERY LOW idle load?

Aloha and Mahalo!


Big Island, Hawaii, 2,000 ft elevation, 200+ inches yearly rainfall.
Ua Mau Ke Ea O Ka Aina I Ka Pono
Len Ovens


Joined: Aug 26, 2010
Posts: 1282
Location: Vancouver Island
    
  15
Kaiwiki McCoy wrote:i'm piecing together my power system (Solar PV with microhydro one day down the line) as i get the funds/find the deals. For a long time i've been wary of the newer type of charge controlers (Maximum Power Point Trackers), because 1) they are much more expensive 2) they have shorter warranties in general (2 years vs 5).
Even 5 is pretty short. The panels are rated 25yrs plus. Properly cared for batteries (even lead) can last 10 yrs... nickel/iron can last much longer. Makes the charger sound like the weak link.



Recently thou, as i've been researching more, it seems like a MPPT makes a lot of sense. Being able to use 24v panels in a 12v system is FANTASTIC, and being able to wire in series for boosted voltage is also great. 24v panels are about 1/2 the price of 12v panels per watt.

quick numbers:
60 amp MPPT controller - $500
2 240 watt 24v panels - $980
vs
60 amp PVM controller - $200
4 120 watt 12v panels - $1600

2 240w 24v panels in series to produce 48v can run through the same wire as a single 120w 12v panel - so that's 1/4 panel wiring cost right there, not to mention simplified installation.


The only thing I can say about it is that a 12v panel is more tolerant of shading than a 24v panel and parallel connected panels are more shade tolerant than series connected panels. So for a clear sky site, the higher voltage is better. A site that has more trees may do better with the lower voltage... Then again the higher voltage may do well enough in a shorter time to offset the problem. Any one cell is .5v (rough guide) so the higher the voltage of the panel, the smaller each cell has to be. One blocked cell cuts the output from all the others that are in series with it. The ideas behind The MPPT charger are easy enough to understand.... I wonder if one could be made at home but longer lasting.
                        


Joined: May 23, 2011
Posts: 34
Location: Big Island, Hawaii
On the warranty issue: i think the MPPT's might have a shorter warranty because they've only been around for 10-15 years or so, vs PWM chargers which are much older (40+ years?) or single-stage chargers which should last until the metal rusts away. I'd still expect a MPPT to last for a decade or longer (solid state electronics are robust), but they are much more complex than PWM controllers. Solar power systems are SO much more resiliant (when well designed and carefully installed) than gas generators or any other electric power system that i know.

as far as i know, most (modern) panels have build-in blocking diodes that circumvent the problem of older panels re: shading. Without the blocking diode, a little shading on one cell would essentially shut down the entire panel, and any other panels connected in series with it. modern panels use a blocking diode that isolates the shaded cells and maintains the performs of the rest of the cells. i hadn't heard about the shading diffrences between 12v and 24v...just keep them unshaded, no problems.

i won't attempt to build a MPPT controller, and i'm pretty sure anyone short of an electrical engineer would spend more time and money trying to build one that just buying a warrantied version. MPPT controlers "assign" a DC voltage to the panels (and this changes with changing conditions, hence the "Tracking"), then invert that to AC, then re-invert it to 12/24/48v DC to feed the batteries. the control circuitry must be pretty complex. a homemade PWM controller might be a fun project, but the commercial versions are cheap and reliable, and i'd rather be watching the chickens...

go solar!

Aloha!
Len Ovens


Joined: Aug 26, 2010
Posts: 1282
Location: Vancouver Island
    
  15
Kaiwiki McCoy wrote:
as far as i know, most (modern) panels have build-in blocking diodes that circumvent the problem of older panels re: shading. Without the blocking diode, a little shading on one cell would essentially shut down the entire panel, and any other panels connected in series with it. modern panels use a blocking diode that isolates the shaded cells and maintains the performs of the rest of the cells. i hadn't heard about the shading diffrences between 12v and 24v...just keep them unshaded, no problems.


The panels I have seen shading problems with (even with bypass diodes) have been higher voltage (100v +). These panels are used with the inverter per panel setups where the inverter sits on the panel. These inverters give real time performance data out per panel and a shadow the size of my hand on one of those panels is 50 to 80% drop in output for that panel. My 12v panels have long skinny cells and hardy blink for a shadow that size... less than 10% loss. 24v panels would have more cells than a 12v panel, but so long as a good part of all cells is not shaded they will perform well. That is, they should handle branch shadows quite well.

Also remember I am 49.5 deg north and in the winter when I need the most of the sun, I am more likely to have to deal with branch shade. No that doesn't mean I should move But I do have to think more about where I put my panels.


i won't attempt to build a MPPT controller, and i'm pretty sure anyone short of an electrical engineer would spend more time and money trying to build one that just buying a warrantied version.


Fair enough... I wouldn't build one either, if I could or not.
Kevin Pegg


Joined: Apr 02, 2012
Posts: 9
Location: Smithers, BC Canada
I have 3 MPPT controllers on my off-grid system. They will increase power output, I didn't datalog my old controllers enough to say how much, but I'd estimate in the range of 25% improvement. I had hoped to replace the 20 year old controller on my Bergey XL.1 wind turbine but my old unit is too low a voltage to benefit from MPPT.

MPPT controllers are becoming common as it's harder and harder to find low voltage solar panels - the bulk of the market is focused on larger grid-tie deployments with the solar panels common today in the 35-40 VDC range open circuit. Using a higher voltage solar panel on a lower voltage system just throws away power.

Older early-generation Outback MX60 controllers I've replaced with the Midnite Solar units, mainly because a) I've learned to really dislike Outback products from experience in this market, and b) the Midnite controller has several specific features that I wanted - higher input voltage (up to 300 VDC) as well the controllers are networked so I can look at their performance on my computer. They have an online component but not finished yet. This controller has a 5 year warranty and a unique "end of warranty tune-up" where you can send them the controller & $125 6 months before end of warranty and they will replace all wearable components and fully check it out, and then give an additional 2 year warranty.

http://www.midnitesolar.com/products.php?menuItem=products&productCat_ID=21&productCatName=Charge%20Controllers

Morningstar also makes a smaller and more restrictive unit for simpler applications. Top quality units haven't had a failed morningstar anything for many many years. Used in lots of industrial applications.
http://www.morningstarcorp.com/en/sunsavermppt
http://www.morningstarcorp.com/en/tristar%20mppt

MPPT controllers are WAY more sophisticated and complex and expensive than older Pulse Width Modulated (PWM) or very old on/off shunt style.

As for microhydro, yes the same Midnite classic controller above will work for microhydro (or wind). It will let you increase your transmission voltage dramatically, reducing wire size significantly. 12V is very restrictive and really only suited to very short wire runs ie 50'. With MPPT you could go as high as 300 VDC transmission and have the controller reduce the voltage to battery voltage right at the batteries. The 12V side of things becomes the restriction - so limited to 60A @ 12 VDC. Multiple controllers can be tied together to act as one unit if need more than 60A.

And in case anyone is curious, I don't work for any of these companies, but I am in this industry and have set up hundreds of remote people with off-grid power systems over the last 2 decades. I am very fussy for quality product as that $100 controller that fails may cost me $2,000 in travel to replace it. Just like any industry, there's a lot of junk out there these days. I recall that a decade ago with a bad batch of Trace C40 controllers, cost me a small fortune when there was a bad batch of them out there, all installed of course in the most remote locations possible. If your system is mission-critical, invest in spares!

Kevin

 
 
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