Thursday, May 19, 2011

Green Tech Oy Vey!

Sometimes you just want to crawl under a rock, particularly when your DC-AC power inverter's low voltage alarm goes off for the N-th time in the middle of the night and wakes up your neighbors. Such is the plight of this greentech do-it-yourselfer.

Let me explain.

Since early 2008, I've been playing with a mini-solar power system that I set up at my house. It started out with just one panel, one deep-cycle battery and a cheap 400W inverter from Radio Shack but has grown to 7 panels, 3 deep cycles, and a 600W pure sine wave inverter (needed to convert DC from the battery into AC used by appliances). At peak sun, it can generate 35-40 amps -- not nearly enough for a family of four to live off the grid, but enough to keep the batteries charged and power a handful of small gizmos, the sort that come with a "wall wart" AC converter (e.g. modem, router, phone rechargers, laptop recharger, printer, even a small desktop computer + monitor with good sun).

I need to keep the inverter turned on around-the-clock because I am using a timer to control when the connected appliances are on, so they don't drain the batteries overnight (there's no need for your wireless router to be on at 3am, at least not most nights). The use of a timer necessitated the pure sine wave inverter, since the timer needs a nice-looking wave form to keep track of time accurately. The cheaper inverter produces a 'modified sine wave' (actually a square wave) -- poopy from the point of view of the timer and some appliances like computers/monitors, which will be noisier with it.

Sounds peachy so far. What's the problem?

Because space is tight inside the house, I've had my entire set up outside: panels (duh), batteries, inverter, etc. Handily, I can connect the inverter to an indoor 6-in-one power strip via an extension cord running under a door. Weather is mild year-round in the Bay Area, so it's not too rough on the batteries, which are protected from rain & sun. However, my batteries have grown decidedly less happy over the past three years (and yes they are protected by a charge controller to avoid over charging). My upgrade to the 600W inverter which draws more baseline current than the cheapo 400W unit has meant that there is more load on the batteries even when it's not powering any appliances.

Enter the low-voltage alarm.

A handy feature of the inverter is that it will sound an alarm when the battery charge drops below a certain point (10.7V). This is to warn you about the situation before it shuts down, which it will do when the battery voltage drops even further (10V), to avoid damaging the batteries.

Sometimes, due to cloudy/cool days or lots of indoor appliance use, the batteries don't sufficiently charge during the day to make it through the night (even when the only load on the system is the inverter itself) and the alarm will trip at some random point between 2am-6am. Other times, such as when I'm working late, I will temporarily extend or disable the timer and will forget to re-enable it later, meaning the appliances will be continuously running and likely will drain the batteries before morning sun or on the following night.

Then, the low-voltage alarm trips...

Now this alarm screams like a banshee, and it annoys the bejeebers out of our neighbors (who often have windows open at night). We, on the otherhand, don't hear it because we sleep with our double-paned windows closed and often have an air filter running, too. The alarm went off in the wee hours this AM, and zapped our neighbors from their slumber yet again. I'm truly sorry for this and have another wine bottle peace offering at the ready.

But there is good news.

This morning, after the latest 'incident', I moved the inverter indoors out of neighborhood airspace. This meant a less than optimal battery-to-inverter connection (via 10 AWG wire which fits under the door, unlike the 4 AWG wire I had been using). So the next time the inverter blasts it will just be us that are annoyed.

I had been researching ways to disable the inverter's alarm, but as far as I can tell, this is not possible (and probably not a good idea to do so).

Ideally, the batteries + inverter should be kept in a more controlled environment, like inside the house, allowing them to hold their charge better and keeping the inverter out of earshot of neighbors. I could also ditch the batteries altogether and invest in a grid tie to PG&E's power, but I'm (1) trying to keep costs down and (2) like the fact that I can run without any grid dependency.

Someday we'll buy a real solar installation, probably from my friends at Sun Power, and my jury-rigged solar install will be a thing of the past.


1 comment:

  1. Read about one family's experiences living off the grid in the woods of Maine:
    The Pleasures and Pitfalls of Off-the-Grid Solar (NYTimes, 18 Jan 2012)