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 CPAPCPAP 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/6tglvtqBattery 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/)