Small Solar Sizing Exercise

[hojpoj]

I had created this post to try and encapsulate everything I could think of related to this project.  Obviously I will have missed something along the way that may be addressed later.  Hopefully, though, I've provided enough information to get a W.A.G. from the more experienced folks here whether I can expect to achieve a reasonable level of success with the system components I've identified.


Background:
Living near the Chesapeake Bay in Virginia, we are subject to some interesting weather patterns.  Freezing rain, freakishly powerful thunderstorms, Nor'Easters, and Hurricanes all have adverse impacts upon our grid power.  Given that our neighborhood has a number of large old oak trees and overhead utility services, downed limbs or trees have the potential for disrupting electrical service for hours, days, or even weeks (in the case of Hurricane Isabel).  In fact, there are times where we have had blue-sky power outages that lasted for over 10 hours.  It seems that the substation servicing our branch of the grid is a little flaky. 

My wife uses a CPAP to control her sleep apnea (and to my great relief, her snoring).  During the last power outage we had her CPAP wasn't operating, the bedroom air was stagnant (couldn't open the window due to driving rain), and needless to say I wasn't getting any sleep.  I had *just* bought a Honda eu2000i portable generator so that we could run a fridge or freezer during a power outage, and switch over to running her CPAP on the quiet mode during the night, if necessary, but I didn't have any gasoline handy, and I sure as hell wasn't getting up to go out in those storms to try and fire it up. 

While my house has a built-in manual transfer switch and a 220 plug for a generator, I didn't feel that our typical power usage necessitated getting a large (and more importantly, noisy) generator that would see minimal use.  If we found ourselves in an extended outage scenario, my parents (who live 20 minutes away) have a 5500W generator that I could borrow.  Typically they get power back quickly (or don't lose it at all since they have underground utilities), so the Honda generator should be able to get us through the short term. 

Now, I could easily gin-up a battery backup for just the emergency times, but since I've been wanting to do a general battery backup system for awhile, I might as well make it able to be used *all* the time, instead of just emergencies.

Project Purpose:
Create a renewable energy system capable of powering my wife's CPAP machine.  Additional available power for air circulation or lighting is considered a bonus.  As the system isn't truly necessary, this is viewed as a learning opportunity.

About the CPAP
CPAP Model:  _____
Power Supply:  12V, 5A

As this unit includes an optional humidifier (electric resistance heat), the actual draw of the unit when humidification isn't required would be less than 5A (probably appreciably so).  However, I can't seem to find my dang Kill-A-Watt meter to determine what the actual draw is, therefore I'll assume the full 5A, and allocate the reserve amount for air circulating fans (12V muffin fans?) and LED lighting.
   
As for tying the CPAP to a battery, the manufacturer offers a cigarette lighter plug for the unit- I plan to get one to avoid any warranty or compliance issues.

Math:
Load: 5A @ 12VDC (60W)
Load Duration: 8 hours (Would really only be ~7, but am including buffer)

Battery Capacity used: C[used] = 40 Amp-Hours  (5A * 8Hr)

Solving for Required Battery Capacity:
Assume that for any given night the battery never falls below 80% capacity in order to maximize battery life, and that the 20% discharge is equivalent to the Battery Capacity used by the CPAP (40 A-hr)

C[bat] * 0.2 = 40 A-hr
C[bat] = 200 A-hr

Typically Recommended Charge Values:
Bulk Charging = 0.25C = 50A
Absorption/Acceptance = 0.05C = 10A
Float = 0.01C = 2A

Values were grabbed from some website I failed to bookmark.  Numbers I saw seemed to vary, so I went with the more conservative values I saw.  This was just for my own edification, as any Renewable Energy source available to me is unlikely to meet even the 0.05C figure.

Recharging Efficiency
Just a back-of-the-envelope assumption that charging efficiency is about 75%.  So if the battery capacity used is 40A-hr, then:

Required Amp-hours to recover:  C[rec] * 0.75 = C[used]
C[rec] = C[used] / 0.75
C[rec] = 53.33 A-hr

RE Sources Available:
Wind: Surrounded by trees in a suburban neighborhood
Hydro:  No moving water
Solar: Only viable option

Even still, my location is by no means optimal for solar.  The best spot I have is above the sun room of my house, as casual observation shows it is the sunniest spot on my property.   During the summer months the surrounding trees don't shade it, and during the winter there are only deciduous trees along the southern exposure.  I expect there will still be some shading issues, but I will neglect them for now.  I am hoping to at least get a marginal boost in light exposure by the white roof of the sunroom (kind of like the boost one gets from having snow on the ground). 

I will assume for now that on any given day there is only 5 hours of power production at the rated panel capacity.  I know this underestimates for summer conditions, and overestimates for winter conditions, but I'll use it as a happy medium. 

Back to Recharging:
For a 5-hour window, 53.33 A-hrs would require a 10.67 Amp input… bugger.

So for a 12V system, 10.7 A would require a panel that could supply 128.4 Watts… argh.

Summary of System Requirements:
Battery Capacity = 200 A-Hr
Charging system capable of handling over 10A
Solar Panel capable of supplying over 120Watts

And this, folks, is where the compromises come into play.  As I've already sunk a fair amount of money into a generator, I cannot justify spending an equivalent amount for just the CPAP.  After reading through a number of posts and searching about, I had identified the following items for use:

Battery Bank:
SAM's Club Golf Cart 6V Battery.  QTY: 2, ~$90 apiece. (http://www.samsclub.com/sams/shop/product.jsp?productId=prod3590220)

As I'm only going off the information provided on the website, it looks like these are about 170 A-hr capacity?  For those with experience, please correct me on the rating, the listing on the website really sucks.

This value falls well short of the 200 A-Hrs I was looking for, but I'm having difficulty finding any better capacity for nearly the same financial outlay.

Solar Panel:
Grape Solar 100W 36-Cell Monocrystalline Panel: QTY: 1, $189 Shipping included
(http://tinyurl.com/8y33pmx)
As someone pointed out on another thread, Costco is carrying these.  Given that the shipping is included, the price beats out just about anything I can find elsewhere.

Once again, falling short of the calculated requirement by about 28W.  It's looking like I may have to cut out that reserve from the 5A, and find the real CPAP power draw without the humidification.

So, for the bare essentials we're looking at about $370 invested.  But as with anything, the accessories are where you REALLY spend the money.

Charge Controller:
This is where I'm at a crossroads. 

I would like to go with an MPPT solution, but the $200+ for a basic MPPT goes a bit beyond what I can justify for this project.  Given my site, I could really use every percentage point boost in efficiency.

Another option would be taking a crapshoot on one of the Chinese MPPT units on eBay.  The costs for these seem to range from $40 at the lowest (most questionable) end, to about $110 for a kit that also includes a remote digital readout that gives A-hr values and all that good stuff (I'm an engineer, I like data ).  The ampacity breakpoints seem to be in multiples of 10, though I think I've seen a 15A unit somewhere out there. I recall someone saying that these Chinese boxes can actually be good, or crap as is the case for a lot of things from China.  How many people have had experience with these?

While I have some familiarity with the Arduino platform, I found the amount of detailed information  to be a tad lacking (for my tastes) on Tim Nolan's arduino mppt project (http://www.timnolan.com/index.php?page=arduino-ppt-solar-charger).  I suppose my level of discomfort is more attributed to my lack of understanding of power electronics, particularly when the discussion turns towards FETs (I'm an M.E., if someone can come up with a good analogy for FET operation I am all ears).  I kinda understand transistors, but by no means grasp their nuances.

The next thing I was looking at was experimenting with paralleled DC buck converters discussed here (http://www.fieldlines.com/index.php/topic,146685.0.html).  Heck, it's an experimental system, why not experimental charge control?  I presume that I'll still need a separate Low-Voltage Disconnect capability, which brings me to…

The next option was the Ghurd Gold Standard.  I could forsake any attempts at increased charging efficiency, and just go with the bulletproof dump controller.  This, of course, would also require me to procure an appropriate dump load.  Does it require a second Ghurd controller reconfigured to get LVD operation?

The very last option would be to get some bang-bang controller like the Harbor Freight unit.  But let's be honest, if I'm spending this much money, why would I buy something that's just as likely to fry my equipment?

Miscellaneous stuff I bought, have, or will need to get:
This looked purdy, so I bought one: http://tinyurl.com/6tglvtq
Battery disconnect switch (Need)
Battery Cart (Have something to modify)
Battery Interconnects (Have)
Hydrometer (Have)
Freestanding Solar Panel Mount (Need)
Wiring (Probably have everything)

Depending on the charge controller selected, I may use an Arduino (I have several) to create a datalogging system.  I believe the only parts I don't have are good current sensors.  I also have a wide variety of electronic components, but all of it's intended for low-power microcontroller stuff (I don't think anything is rated for more than 5A).

Mechanical parts aren't much of an issue since I have a reasonable scrap pile, a small flux-core wire feed welder, and a benchtop lathe and mill. 

Attached is an image of my house using the NOAA solar calculator (http://www.esrl.noaa.gov/gmd/grad/solcalc/)

[Mary B]

Golf cart batteries come in different sizes with 220AH being more common. MPPT will not be a huge gain under 500 watts, the extra cost won't offset the price, you could pick up a second panel or a larger panel instead.

[hojpoj]

I know that below a certain wattage MPPTs don't provide much benefit, I just didn't realize that it was that high a value.  Thanks for clearing that up.

As for the golf cart batteries, I suppose this'll warrant a trip to SAMs to see if I can put eyes on one and check the ratings for m'self.  Certainly would be nice if rating is more than I thought.

[DamonHD]

Hmm, can't say I agree.  I'm happy with the MPPT on my <200W 12V system.

(And indeed on my 5kWp+ grid-tie.)

Rgds

Damon

[DanG]

The key is not buying twice - finding a system voltage and sticking with it, not having a string of retail purchases each only an incremental upgrade so at some point in the future the system you usie has a hidden cost of three or four times of the hardware in use.

[hojpoj]

The key is not buying twice - finding a system voltage and sticking with it

I wholeheartedly agree with this sentiment, and I'm well aware of the pitfalls of upgrade paths.  I believe it always would be 12V, the only variable being the charging method.  I suppose for now I'd be better off getting the bigger panel without MPPT, and should the need arise to expand the storage system I could get one to improve charging (and probably stick another panel in series to help cut down line losses).

Thanks for all the feedback.

[Mary B]

Damon MPPT works fine but the payback isn't there on the price of the controller for under 500 watt systems is what I meant to say. C40 charge controller under $100, MPPT in the same range $450+. Tired, had 20 people in the house all week for the 4th of July holiday! Now to clean for a week 

[Bruce S]

hojpoj;
Welcome aboard.
I'll weigh in on a couple things that should help.
The CPAC machine, I have a neighbor that uses the same type (not sure of your model) which includes the humidifier.
I helped them setup their smallish PV system, it contains 60W of Harbor Freight panels a morningstar sunsaver 6 solar controller and two 220Ahr Sam's batteries that are now 2 years old.
His CPAC machine runs just fine on a Vector 300W MSW inverter, even with the humidifier on, he does not have the direct 12Vdc connector.
We tested units all the way down to a 140watt unit but that was pushing it and did not have the humidifier on at that time.
Just as a test, we connected it to my kill-a-watt unit and let it run over night, without the humidifier on, the instant running was 120Vac at 0.352 and at 5AM this morning the accumulated was a mere 2.46Amps which would be on a 12Vdc system about 25A .
The unit with the humidifier on really pushes the unit up instead of the 0.5Amp @ 120Vac its startup is up at nearly a full Amp which surprised all of us, but understandable since the heater needs to come up to temp.

When you do go to Sam's to buy the batteries, they will take ANY old battery as a core, we used two little 12 5Amp SLA out of an old UPS.

Voltages, depending on if you go with 12Vdc or above, the other thing you will need to look into is the wire sizes, the higher voltages the smaller wire you can use. So you will need to know how far the battery bank and assorted "stuff" will be from the need.

IF you go with the GHurd controller; there is already a provision on the MB for changing it from 12Vdc to 24Vdc. AND GHUrd can help with sizing the and possible shipping the correct dump load as well.
Personally I have 3 units and I use another set of batteries as the dump instead resistor banks and my units are 12V NiCd banks.
YES normally you will need a second one built specifically for LVD. You could also request the KIT to already be built and set for the battery type, there is an extra charge  .
His website is www.ghurd.info . Nice guy most of the time 

IF you have a 90Amp MIG welder, you can use 1/4' angle iron IF you go slow. I grabbed one from H.F. when they were on sale and had a nice 20% off coupon, works okay for little stuff, and tacking repairs.
IF you're looking to save money here and there, I would recommend the GHurd unit to start with at ~$20.00 it well worth it and they work.

Hope this stuff helps
Bruce S

[XeonPony]

Damon MPPT works fine but the payback isn't there on the price of the controller for under 500 watt systems is what I meant to say. C40 charge controller under $100, MPPT in the same range $450+. Tired, had 20 people in the house all week for the 4th of July holiday! Now to clean for a week 

That is bad info my friend! the mppt is the break point for my 200w system with out it the system was an epic failur the second I put the mppt on it started to crank out 1.5Kw hours a day from my average of 500 at best befor the mppt.

My solar noon is only a short burt due to trees so all the morning and evening sun was lost and that is where the mppt saved the system! Since I built a manual tracking system I've goten it to near 2+ Kw hourse a day befor I upgraded the entire system.

MppT is worth every cent when you NEED the power .

Now for the CPAP a friend of mine uses one as well, it uses a switch mode suply that will take nearly any thing with out issue, though with the adapter you gain in efficiency due to less conversion losses, my friend did this and on the 12v dc system it ran in his RV great.

[hojpoj]

Thanks Bruce, that was quite helpful.  Particularly on how to avoid the core charge... makes me wish I hadn't taken some batteries to the scrappers a few months back    As for the little welder, pre-heat helps a lot when you're making bigger welds, though I still haven't gotten the knack of not blowing through thin stuff like tubing.

The CPAP's one of the Philips Respironics units, though I forgot the specific model and keep neglecting to go back and update that piece.  I know that they've got the Car adapters that cover pretty much the whole line, though.  For much of the year the humidifier isn't necessary, just the wintertime.  The other part is that when you first start the CPAPs, the air pressure is low, but will ramp up after about 45 minutes to the 'actual' setpoint.  I've had a knock-off kill-a-watt meter hooked up to it for the past few nights, but haven't gotten to check it aside from when I first hooked it in.  On 120VAC it was drawing about 0.3 Amps, though had a few momentary excursions to 0.8 A (humidifier wasn't even hooked in).  I'll need to check it once its gotten up to the real pressure to see what its power draw is.

Fortunately I already know where stuff would be located, the sun room that a prior owner added on actually encapsulates one of the master bedroom windows.  The run from the batteries to her night stand wouldn't even exceed 7 feet.  The system would always be 12V so as to be compatible with all the cheapo automotive stuff out there.  The bummer for a solar panel is that even if I make a good wall penetration, the panel-to-battery run will be ~25' (not including service loops).  Not sure I have any heavy stranded wire in that length, might need to check and see if I still have those old welder cables... hmmmm...

After doing a rough pricing-out, the wife sorta cringed at the whole system cost.  Though she was amenable to the idea of the battery backup, in spite of its cost.  The solar charging would have to wait until the finances  recover from some other issues.  Thus, in the short term I may just make it a grid-charged battery bank, with the CPAP always connected to it so that unexpected power failures overnight don't force me out of bed to start up a generator.  For that, I suppose I'll need to find a good smart charger that I can leave connected 24/7.  I'd like to set it all up so that adding the solar input at a later time will be easy, so I'll still probably set it up with some small buss bars, disconnects, etc.  Once I get enough time to draft it all up, I'll post a schematic to be sure I've captured all the important bits- particularly from a safety standpoint.

As for the battery charger, any recommendations?  I know that Battery Tenders used to have a good reputation, but some reviews I've seen recently have been less-than-stellar.  Was hoping to find something adequate (will charge batteries without cooking them) for less than $65, more justification would be needed beyond that price point.

Cheers,
Eric

[Bruce S]

Best I can say about battery chargers is to maybe pickup a multi-set timer and run one off that?
I had one setup that way to keep a few backup car batteries topped off during non-use. I picked up a dual-stage one from HF for about $20 let it top off the batteries, then put the timer on it set to lower setting charge stuck it out the output side of the timer and left it alone.
Stayed that way until I was ready to use the battery.
I do this with motorcycle batteries during the winter months too, just shorten the timer on time.

Since you are on-grid like I am, you could make use of some free if you can get them  , car batteries to bide your time while your funds build back up.
Would also allow you to fine tune your sparky skills and test everything as you go.
For long term deep cycles are best, but proof of concept , a free car battery can't be beat, an 850CCA battery will test out right at about 74Ahr.

Best of Luck

[hojpoj]

Eh, I figure I'll be better off buying the right batteries the first time so I'm not kludging stuff later.  Only free batteries I've gotten are free for a reason.  Plus, I have the OK to buy good ones now, anyways, was just holding off on doing it until I get the hardware specced out 

The idea here is to lay the groundwork for the inevitable expansion to RE charging (solar, make a pedgen of the recumbent exercise bike?) so that stuff isn't scabbed on.  If I build the infrastructure properly, then (theoretically) I should be able to substitute the grid charger for solar until such time as the funds are available to buy *that* equipment.  I'd only anticipate a few months delay 'til getting a panel, anyhow (being a D.I.N.K. has its benefits).

Could use that time to build the system monitor 

[Bruce S]

Do a search for Norm on this forum, his pedgen stuff is some of the best out there.
His posts can also point you to a website he uses to keep in shape.
Remember to have a little fun with it too!!

Cheers;
Bruce S