I am new in this solar power PV world. I can't seem to find a consistant answer to this question.

The FACTS:

Off Grid Garage
48v Battery Bank (4x2 SBS C11s)
MX-60 Charge Controller, 72' from the panels.
4 Matrix PW1650-165 panels (12v or 24v)
Dallas TX (10 days below freezing average)

The QUESTION:

Which would be the better wiring solution --
A - 2 in series x 2 parallel at 24v each
B - 4 in series at 12v each
C - 4 in series at 24v each
or
D - It ain't going to matter

Thanks in advance for any experienced advice,

Glenn

A) would allow the smallest wire size , lowest cost and lowest losses , this assumes 4 total wires home runned , two strings , each with there own wires

B) would have less total wire , but need more expensive larger wire , only two wires , 4 in a single string

C) No can do , you will exceed the maximum input on the MX-60

D) Then why ask

using the Homepower wire spreadsheet , you can use 12 wire on option "A" with 1.5% wire losses , a 500 ft roll is 30 bucks at Home depot , 10 gauge , is 1% , I think thats 42 dollars roll at Home depot and would be my choice , both can sneak into 3/4" conduit


"B" would require 6 wire to be under 1% losses , thats about 100 bucks 500ft roll and the conduit size bumps up ... don't try pulling 6AWG thru smaller than 1" conduit ...

So as you can see is a balance of cost (wire and conduit) vs losses.

Thanks for the quick response.

Bigger Wire and Higher Voltage = Less loss. I got it.

Any experience about the effects of parallel vs serial with regard to actual field operation?

I was wondering about:
a) with regard to the effects of individual panel performance loss over time, is serial or parallel better;
b) are there significant advantages to higher voltage strings and their ability to make power with less light in the morning and evening;
c) which would perform better in a "clouds are rooling by" kind of a day;
d) are there factors in the combinations listed greater than the questions I am asking that I should be considering?

Glenn

Glenn,

Here is my understanding:

Series

Assuming an inverter that can use 36-48V and four 12V panels in series. At normal output you're looking at 48V. If one panel gets fully shaded you're down to 36V but still working. Two fully shaded and you're inverter drops offline because you're at 24V. Also shaded panels can affect output due to losses from current flowing through the shaded cells. Partially shaded panels may still produce depending on the shade/panel.

Parallel

Assuming an inverter that can use 24V and four 24V panels in parallel. If a panel is shaded that entire panel's output is lost because it can't meet the voltage of the other panels. However you can have all but one panel shaded and still be producing. You can set up arrays pointing different directions and it will work. Parallel arrays can be grown a single panel at a time as finances permit depending on the wiring.

Series/parallel

Assuming an inverter that can use 48V and four 24V panels in two sets of two making 48V. If a panel is partially or fully shaded that entire set is no longer making 48V so both panels stop producing. However it would require shading at least one panel in each set before the inverter would stop producing. A single 48V series string (4x12V)can easily be grown into two strings depending on the capacity of your inverter.

This link makes excellent reading on why shading is bad:
http://www.fsec.ucf.edu/pvt/Resources/p ... GTESTS.PDF

Sean

Sean,

You're right, that is interesting reading. Thanks for the info. I am going to change the orientation of the panels in my setup.

In case there is any interest, I'll give a brief summary of my solar project.

On a bet with some buddies of mine I am attempting to provide Central Heat and Air to my newly built 26x36x10 garage for under 10k. Their side of this is that solar will never be an option in TX if you can't run the A/C and mine is they just aren't smart enough. We had put back a few by then.

I've got the approach mapped out, parts are arriving, and construction is under way. Due to the cost constrant I'm going to have to get every last watt out of this rig so that's where I'm hoping to gain a little experience from the folks in this forum.

Thanks again for the info,

Glenn

I surmise from your info that you have built a smallish, very well insulated and roof vented one car garage that you plan on air conditioning with a under performing 400 watt AC unit that can only run when the sun is really out and shining.

Otherwise, feggit about it - AC and solar electric generation really don't mix and isn't a feasibly sound practice.

Oh come on low Larry. You sound like my buddies.

It's 26'x36'x10' (936 sqft). It is pretty well insulated and it does have a commercial 10' baffled and dampened ridge vent. The Roof is 4/12 pitch so that the panels can lay flat on the roof and one side is oriented due solar south. I'm looking at 1.5 ton of A/C. Since it's just a garage I'm only planning on holding it to the low 80s in the summer and around 60 in the winter.

From the standpoint of going out and commercially procuring a standalone solar A/C system I agree with you 100%. It ain't going to happen. What I am trying to do is put together a system using free labor (mine with a little help from my friends); the internet for research and price shopping; and a good day at the tradeoff table, where I am continually balancing leading edge vs low cost, that will do it.

I have been fortunate to find some pretty good deals on components so at the moment I've only gone through about 1/2 of the budget and I have all of the components for a working system -- though by my calculations it doesn't have enough power to operate the system in the summer or winter without switching to grid power. We will see as the installation progresses.

Like you most people just shake their head. But if solar is going to be practical to me, even as a hobby, at the end of the day the result has to make my life better and has to be somewhat affordable. Hopefully I'll come up with a winning combination.

Glenn

Its just basic math .. your AC is going to use about 3kWh while running , your PV system will get what 500 net wHr in peak sun (with correct tiilt) ?

So best case your looking at 1-1.5 hours run time of the AC on PV power alone per day ... unless its insulated like an walk in cooler you don't have the energy required to cool the building. Try runnin gthe AC in the main house for only 2 hours a day to get a feel for what your proposing

You can get much more involded to calculated your btu load based on the building design and losses , you also can have a 3 to 1 range on the efficiency of the AC itself (seer 10 to Seer 16.5 for todays options)

I have about 30kHw day generation on my house and it doesn't cover the AC load (8 year old , well insulated home) in the summer

My previous house I built , that cost-no-object energy efficient everything (r40 ceilings , double insulated walls , double insulated windows , radiant barrier , Seer 14.5 Heat pump) took the full output of my system to net even , and that system did about 25 kWr /day

While I think its great your doing a solar system , you have goals that don't match the needs of your project

There's only one way where solar may work in this application, but it's probably outside of your budget, and doesn't even involve costly solar panels :
http://www.shec-labs.com/press/releases ... 0press.htm

http://www.shec-labs.com/SolarConcentra ... trator.htm

There you go - a sun operated Stirling engine to be used for your AC compressor and blower. Don't delay, give 'em a call today ...

Thanks for the analysis. It reaffirms some of what I considered. Not even sure that I could make it with conventional A/C systems. I selected a TRANE Ground Source GSUJ for several reasons. Not the least of which was the fact that I found one for $150. I actually bought a cheaper unit on Ebay for $110 but it is just 1-ton and less efficient. In operation the 1.5 ton unit uses 10.7A plus 1.2 for the blower and pump. <http://www.trane.com/Residential/Products/HeatPumps/GroundSource/GSUJ-SVX01A-EN.pdf>

That's right at your 3KW number and assuming I get lucky and hit on the 1.5 hour per day it's still in the running. There is budget left for 8 more panels. That's up to 4.5 hrs/day in operation. Remembering that I'm only shooting for around 80deg I think it will be close. This year, which was cool, never had a night stay above 80. so if we assume night hrs at 50% duty for 12hrs, that will give me 15 minutes of run time per hour during the day. An HVAC guy told me that generally they try for 10 minutes every 30 but they are trying to keep the temp at 72, a full 10 degrees lower. I know I'm screwed on days when the sun doesn't shine, as I have precious little reserve. But it is a garage A/C not a space station CO2 Scrubber and I do have I house to live in.

That's my basic math. I know it's on the edge. The good news is that I have some instrumentation built in the design. The MX-60 has some data. There is an electric meter on the A/C unit. A thermal couple on the water inlet (manual read though) and a temp/humidity meter in the room that displays current and max that I will use to collect data. The microcontroller used to switch the inverter on and off can record usage time for both Grid and Inverter. This should allow me to test with the 4 panels and extrapolate the 'how many really' number.

This project will take a little time to get done but I must admit it is fun. This weekend is fun with backhoes as I bury 1000' of pipe 10 feet deep. Mounting the panels will be next unless, like Jed, I hit oil. Then I'll be back to a fossil fuel guy. Then again, maybe I'll stay with solar, I think they have a rebate program in Beverly Hills.


Glenn