Less PV's & track - or buy more PV's ?

[MagicValleyHPV]

I just bought a single Sanyo 186w bifacial PV. I had originally figured on two, but decided to pose this question before I buy anymore, if any.  Assuming perfect tracking, with this single 186w Sanyo feeding a Morningstar MPPT 15a controller into a 24v battery system, how much total WH difference can I expect compared to two 186 Sanyos laying fixed and horizontal on my motor home roof?

I'm a metal fabricator so I have the means and the tools to build a tracker.

[independent]

I don't know if this helpful or not. There is a guy who writes books on solar and installing on campers and such in Oz. His name is Collyn Rivers, he's pretty much the guru there. Read two of his books and found some valuable info. Anyway, he's stopped using tilt up panels on his vehicles and installations (guessing that's where they are going). Found the increase in total electricity gain not to be worth it. Mind you he is in Australia and they have lots of sun. Guessing you've done a energy audit. I think the gain is about %30 with a tracker.

[dave ames]

 hi magic,

 we may need to rethink the system. the morningstar mppt controllers are great but are not open format controllers. they run as nominal in/out mppt. ie: 12 volt nominal pv in to 12 volt out, or 24 volt in to 24 volt out ... .... .. - with a 24 volt system it will be trying to run that 54 volt nominal pv module at about 36 volts (trying to run it as a 24 volt nominal module) in effect turning that 186 watt pv module into a ~130 watt pv module. and it won't take much of a drop in temperature to put our voc over the controller max voltage limit.

http://www.morningstarcorp.com/en/support/item.cfm?ItemId=192

could maybe do a true open format controller like the apollo or outback?


just one guys thoughts,
 
dave

[MagicValleyHPV]

Thanks, Dave,

The liink doesn't specify MPPT or PWM. Could it be referring to PWM?

Quoted from the SS MPPT manual (page 23):

:High Voltage Strings and Grid-tie Modules

 Another beneft of TrakStar MPPT technology is the ability
to charge 12 Volt or 24 Volt batteries with solar arrays of higher nominal voltages. a 12V battery bank can be charged
with a 12 V, 24 V, or 36 V nominal off-grid solar array. Certain grid-tie solar modules may also be used as long
as the solar array open circuit voltage (Voc ) rating will not exceed the ss-Mppt 75 V maximum input voltage rating at
worst-case (coldest) module temperature. the solar module documentation should provide Voc  vs. temperature data.
Higher solar input voltage results in lower solar input current for a given input power. High voltage solar input
strings allow for smaller gauge solar wiring. this is especially helpful for systems with long wiring runs between
the solar array and the ss-Mppt."

Before I purchased the controller, I quized Morningstar about this application, and they said fine, as long as the Voc didn't exceed the controller's 75v input limit.  

[MagicValleyHPV]

Dave,

This graph is on page 39 of the SS MPPT manual (the Sanyo 186Bifacial states a Vmp of 54.8:

[dave ames]

 cool!

thanks for that correction. that ss trakstar mppt has been out since '08!.. so looks like i'm behind in the reading, we have downloaded the manual and datasheet for updating the bio-computer.

looks like your in good shape with that module with a watchful eye on cold temp corrections.

as for the second module or solar tracker?

for the sheer "cool factor"  a tracking array can't be beat. in a no fuss practical way two modules on a fixed mount is probably the way to go though.

cheers, dave

[MagicValleyHPV]

Dave, I sure hope so, my tab is starting max-out. The second graph (also on page 39) is for 12v storage. Applying the same module, the efficiency drops from 97% to about 92%. I have enough pieces already gathered to experiment - we'll see what happens. -kenny-

[MagicValleyHPV]

I don't know if this helpful or not. There is a guy who writes books on solar and installing on campers and such in Oz. His name is Collyn Rivers, he's pretty much the guru there. Read two of his books and found some valuable info. Anyway, he's stopped using tilt up panels on his vehicles and installations (guessing that's where they are going). Found the increase in total electricity gain not to be worth it. Mind you he is in Australia and they have lots of sun. Guessing you've done a energy audit. I think the gain is about %30 with a tracker.

Great resource, thank you.

From: http://www.campertrailers.org/collyns_page.htm  Collyn says,

"In my experience it is not worth arranging for tilting or tracking systems in latitudes less than about 27 degrees.  Adding about 20% more module capacity will make up for any loss."

Since I'm closer to 44 degrees latitude, tracking 'may' indeed prove beneficial if I decide against another rather expensive module. I do, however feel that on less than optimum days (such as major overcast), additional module surface area would be a significant advantage.

I definitely need to purchase at, one of his books.  

[ghurd]

A single-axis tracker could well be worth the effort if the battery load and capacity can effectively absorb the extra power.

I seriously doubt a dual-axis tracker would be cost effective for most people, especially considering the reliability issues of the actuators etc.
The cost of the actuators, etc, for the 2nd axis, would usually be better spent on more PVs.
G-

[hydrosun]

The advantage of a tracker is greatest in the Summer when the arc of the sun is farther.  That is why trackers were first used on panels for pumping water for irrigation. It spreads out a consistent level of  power over a longer period.  It gains less in the winter. So if you have plenty of power in the summer and need more in the winter you'd be better off spending the extra money on solar panels and a mppt that gains more when the panels are cool. If you are only using the rv in the summer the mppt will gain little when the panels are hot.  So a manual tracker for the summer and an electronic tracker for the winter has the best payback.
 The percentage gained someone quoted is from an array angled and facing south. When the panels are horizontal  they are at the worst angle in the winter. In the summer panels at your latitude would  be at about a shallow 30 degree angle so less is lost when flat.  So if low output in the winter is the problem then you'd want  to be able to manually adjust the angle to face the winter sun.
I have a tracker on my own home  system because I have a hydro in the winter and wanted more output from my panels in the summer. It spreads out the power  over a longer period and doesn't peak at noon. When we are cooking dinner after 5 pm we are still getting near full output instead of pulling power from the batteries that would have to be refilled the next morning.  For most systems the cost of solar panels have come down in price to make it cheaper to add panels instead of the cost and potential of problems of a tracker. The ability to manually adjust the angle seasonally is still worth while. Or just to fold down when driving and then prop up when parked.
Chris

[Striider]

Hydrosun - great post!  You have me thinking about angling my (stationary) panels a bit more towards the evening sun.

[GaryGary]

Hi,
The NREL PVWatts calculator will do 1 or 2 axis tracking in addition to a fixed array.
It takes into account your location and local weather.
It gives monthly output, so you can see both the when and how much benefit tracking has.

http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/

Seems like it would be worth running both cases for your area.

Just as a quick check for my area (Billings, MT)

Fixed, horizontal 2KW array    Output:  2146 KWH/yr,  July 302 KWH, Dec 58 KWH
Two axis tracked 1KW array   Output:  1869 KWH/yr,  July 228 KWH, Dec 93 KWH

I just used 1 and 2 KW arrays for easy numbers to plug in -- you can enter whatever your exact panels are for your case.

Horizontal is good in summer, but very poor in winter -- especially at higher latitudes.

Seems like an array that stows flat and just tilts up to a fixed tilt with fixed azimuth might be a reasonable compromise?

Gary

[Ungrounded Lightning Rod]

From: http://www.campertrailers.org/collyns_page.htm  Collyn says,

"In my experience it is not worth arranging for tilting or tracking systems in latitudes less than about 27 degrees.  Adding about 20% more module capacity will make up for any loss."

Since I'm closer to 44 degrees latitude, tracking 'may' indeed prove beneficial if I decide against another rather expensive module. I do, however feel that on less than optimum days (such as major overcast), additional module surface area would be a significant advantage. 

Note that (if you have sufficient storage that getting your power in a shorter time near noon is not a problem) the tradeoff between more panels fixed (or tilted seasonally) and fewer panels on a tracker is a matter of the relative cost of the panels and the tracker.

Panel prices have been dropping drastically and (with the rumored price war as China tries to drive the competition out of business) can be expected to continue to drop.  Trackers are low-production-volume mechanical devices and I suspect their prices have not been dropping in step, if at all.  So the tradeoff is changing to "more panels, no tracker" in more (or essentially all) situations.