Diesel is Wrong, Solar is Right for Backup Power

I’ve been on my emergency preparedness kick recently, and that includes power since the storms this last year have been a real pisser. Nothing like being without power for three days in the home you just bought.

I was previously looking at military surplus diesel generators. Aside of my friends stabbing me in the neck over and over again for me asking them to help me get it on and off the truck, I’m not too huge on the idea of having Yet Another Engine to take care of. On the other hand, that particular model is so popular that theres an entire MEP-004A forum dedicated to it. And, there’s the fact that I have 250 gallons of diesel sitting in a tank in my basement labeled “home heating oil – NOT APPROVED FOR ROAD USE”.

Now, assuming that the generator goes for the nominal price of $500 to $1000 (they’re getting really popular in the post-Katrina marketplace), maybe I could do better with something else. It’s not just buying the generator – it’s buying the generator, the engine rebuild kit (if required), the wiring, and doing the conversion. Military power is usually 208v rather than 120v or 240v because it’s easy to step up/down or DC to AC convert. The other side of the coin is moving parts are the enemy. Parts which don’t move tend to want to stay that way which is why I try to start the MR2 once a month or so in winter. (I’m probably being paranoid). I don’t want to deal with that in a generator. After mulling it over quite a bit I decided to investigate various things like water service powered sump pumps (dumb), power-from-phone/coax, etc. None of these seemed well thought out. Dumping more water in your yard for the sake of pumping out water from your well is stupid. If the power lines are down, phone and cable likely is too. It was tough getting away from the idea of a diesel generator or making my own nuclear reactor. While legal, your neighbors tend to frown upon it.

Finally after a bunch of surfing around on various industrial equipment liquidation sites there was another answer: solar. Solar panels, for whatever strange reason, are dirt freaking cheap. A solar battery charger might be $20. A solar panel alone, for $20 buys you a pretty big panel. So now we have ideas, the question is how does it compare to our diesel generator at 15KW and how expensive is it? There’s a few options. I tend to shy away from the battery chargers since they are rated to charge one battery. They’re cheaply built and they assume you’re also charging one battery. Putting it under load isn’t going to happen. On the other hand I could buy the end-of-the-world version. But as everyone knows if it costs $1800 online on your end of the world site you can build it for about 10% of the price.

Lets put that idea to the test. High end rectifier: $30 from Rat Shack. Low end inverter: $71 for 1.5KW.  Why a low end inverter? I want to use cheap batteries and I suspect the rectifier puts out fairly dirty power. Expect to replace this every few years if you’re using it intermittently but more on the point you might also want to buy a much bigger, nicer unit. So for $100 we’ve got the power path from the panel to the wall socket. How much are batteries? $20 a brick. Keep in mind battery acid is now a federally controlled substance and requires you sign for it, you can get it if you ask. So you can refurb those batteries if you’re not buying sealed cell. And finally how much is a fairly large solar cell? $500 new for a high end one, but since we’re buying junkyard batteries, $100 for a new, fairly nice one or $130 on ebay for enough of them to carpet the entire roof. Since the solar cells do eventually go bad, I tend to like the ebay solution of buying the rejects in bulk so I can simply replace them as the apple tree turns them into broken glass.

Now why does this system work better than the diesel? For one, I can charge the batteries off the existing grid (when it works). I can even charge them from a generator. I can charge them from a car. I can charge them from solar. I could probably even ride a bike connected to an alternator (another $20 junkyard part) and charge them. Car batteries grow on trees. In fact there’s 1.4 of them for every breathing American at the moment and they’re only going to get cheaper as Priuses start showing up in the junkyard. For two I can charge my MR2 and the lawnmower from the solar setup here. There’s no reason why you can’t just put jumper cables from the MR2 or the tractor to the batteries. For three, most of the parts are cheap or free once you buy into the rectifier and inverter. You’re looking at $200 for the whole setup, or $300 to make it nicer. If you want to go seriously high end you can spend $600 for a 6KW, 12KW peak 50A power inverter. I’m sure you can find it cheaper on ebay.

How much do you really need? The government guessdimates that you need 14.5KW a day in summertime (about 600W an hour). Doing the math you can see our low end inverter there (1.5KW sized for a reason) will run about 20 hours running full tilt assuming the batteries aren’t total crap and you have more than one. I used five of them for my math and the health of the battery is frankly a crapshoot.

Now, there’s a choice, and I’m hoping the more electrically inclined readers caught this. You could just unplug the fridge from the wall, plug it in here and forget about it. You could plug the oil burner in here and forget about it. It would probably work great for these two and it give you two more plugs for lamps. In fact that’s probably the safest way to do this. If you want to drive the house off it, you could use the incredibly dangerous male-to-male extension cord and plug it into the wall socket once you disconnect yourself from the grid. However this plan was never designed to run through the house wiring. If you do try it, you’re on your own and you’re strongly advised to buy the proper phase-matching circuits. These are spendy which is why they’re not represented here – I’m not trying to build a solar power plant. If you skip it and wire it directly anyway, you’ll blow a breaker if you’re lucky, if you’re unlucky you’ll set the wiring on fire, and if you’re REALLY unlucky you’ll be on the grid when the power comes back on and you’ll have phase mismatch and your power company will beat the living hell out of you if you live through the resulting explosion. Don’t do it. If you’re really interested, grid-tie starts at $1000.

Speaking of wiring: hows the wiring done? The batteries are dead simple and wired in parallel. You can’t possibly screw it up. You wire all the same poles together, put the charger on one side of the rail and the inverter on the other side of the rail keeping positive to positive and negative to negative and you win. You could stack a thousand batteries like this and you still win. The rub is on the solar panel side. You need, on a cloudy day, to generate a minimum of power to get the rectifier to turn on. On a sunny day this means you’ll be generating far too much power. Solar panels, therefor, are a game of wiring things in both series (to hit the minimum voltage) and parallel (to hit the minimum amps). The longer the parallel run, the more current it produces. The longer the serial run, the more volts it produces. The rectifier will turn on at a minimum of voltage and do it’s thing until you hit a maximal of voltage. In our case, that particular Sunforce product I linked to accepts up to 100 watts @ 7 amps by 12 volts. The voltage on solar panels is the game to play. Since that rectifier will only accept up to 12 volts, you wire the individual cells in series (postive-to-negative) until you hit 12 volts at sunrise. Once you’ve got a few of those, you wire the assembly in parallel (positive to positive) to the rectifier, trying not to exceed 7 amps. For whatever strange reason people seem to just make these huge strips of cells. The problem is that by doing this you waste any excess power you make – it gets converted to heat. Since we’re dealing with DC, we don’t need to worry about phase matching, but if you buy 10,000 individual cells like some of these idiots suggest, you’ll explode something in short order.

Oh, you could just skip all the solar stuff and charge the batteries from the wall socket. PECO price caps and all that.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s