Saturday, September 27, 2014

Powering a House

I did some preliminary assessment on the not-yet-smart-ish home's electrical system to get a better understanding of exactly what I'm working with.  One of the things that's nice about having worked on variety of projects over the years is that you run into familiar sights from time to time.  I should've known that DC power systems will look similar regardless of whether it's for a solar powered car or power generating kites, but it still was a pleasant surprise to peel back the layers and be presented with some vaguely familiar.

Main Inverter and Charge Controllers for the house
As the house is completely off-grid, it's basically a closed system consisting of batteries for energy storage, bunch of solar panels and a backup diesel generator for power generation, and an inverter to supply 110V AC for the house.  Even some of the names were familiar - Trace Engineering, which built the inverter used here, is owned by Xantrex whose chargers we used extensively back in the solar car days.

Lead Acid Battery Pack
The battery pack has nominal voltage of 48V, consisting of 3 parallel strings of 8 x 6V batteries.  I couldn't read any battery pack model numbers besides the maker (Deka) but the size of it looks to be about 300~350 AHr each.  So, across the 3 parallel strings, the rated capacity is probably just around 1000 AHr, giving it ~48kWhr for the entire battery pack (or enough to run a decently sized hair dryer for 24 hours!)

Measuring the voltages of the individual batteries showed some dead cells - since these are pretty old, beat-up, abused batteries, it's not surprising.  Some of the dead cells are on the top batteries from the two parallel strings closest to the battery connector - I'm guessing the relatively lower resistance due to shorter cabling probably resulted in imblanaced current draw.  I'll eventually have to replace them (with something like these - reading through some articles, the general recommendation seems to be to shoot for a single string of 2V batteries sized for the right capacity) but for the time being, I think just wiring them in a way to remove the dead cells and changing this to a 2 parallel strings while we try to get better power consumption numbers might be a way to go.  My rough guestimate is that we currently have ~24kWhr or usable storage capacity in these batteries.

It looks like there's enough solar panels for approximately 2kW of generation capacity.  Using the Solar Irradiance figures (put in California / San Jose / South South West / 53 degrees) shows a figure of 3.48 in December (least sun) and 6.63 in June (most sun), so we're generating about 7kWhr / day in an average sunny day in December, and 13kWhr / day in an average sunny day in June.  Assuming 24kWhr battery capacity, this means that it would take 3.5 days of full sun in December and 2 days of full sun in June to fully charge the battery pack even with degraded batteries and using only 2 strings in parallel.

To me, this seems like the solar panels are quite undersized for the batteries we have - it's interesting to see the trade-offs and engineering decisions made in a world of completely different economics.  When the system was put in place - probably 10+ years ago - solar panels were quite expensive and were likely to be the most expensive part of the system.  In comparison, batteries were cheaper and diesel at near $1 / gallon was probably a viable backup option in case of multiple days of cloudy weather.  In those days, it probably made sense to size the solar panels for about the average daily energy consumption around equinox.  This puts more cycle in the lead acid batteries, but that probably was an acceptable tradeoff to lower the cost of the solar panels.  And if it rained for a week or you get less sun during the winter, you'd just run the generator a bit to make up the difference

In 2014, you can buy ready to be installed solar panels for well under than $1 / watt while battery price has gone up and diesel price has quadrupled to $4+ / gallon.  These days, I think it makes much more sense to oversize the solar panels so that you put minimum number of cycles into (relatively) expensive batteries, and size the batteries to cover several days worth of energy consumption even with cloudy weather to minimize diesel generator having to kick in.

It's quite amazing just how far the solar module prices has come down, and how that completely changed the sizing of various components in the off-grid system.  And given this, for this house, the priority is probably to add more panels for generating capacity - probably at least triple the current capacity.

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