[Greenbuilding] Solar electric configuration details

[Lawrence Lile]

I've been considering putting together some solar electric equipment and
want to think it through out loud on the list and get some ideas. 

The other part of the EPACT tax credit I didn't mention is a 30% rebate,
up to $2000, on solar electric systems.  This is gravy, folks! One could
imagine investing up to $6000 in solar electric equipment to maximize
this tax rebate and accomplish what I wanted to do anyway.

So I said, "What size system and components could I put together to make
a $6000 system?  What could I accomplish with it?"

Here are my criteria:

1. Despite my Ebay'ing for used cells, the Finance Queen (my wife)
insists we must only buy new equipment.  I'll be installing it. 

2.  The goal is to have an uninterruptible, glitch free power source for
key equipment in the house:  the heating system, a few lights in the
tornado shelter, the computer, and possibly a freezer.  I'm most likely
to have power outages as a result of ice storms, when the freezer can
coast. Power Glitches can happen on the grid anytime.  High winds can
also happen, but are usually so local that power gets restored within a
day, when the freezer can also coast. We are in the Midwest, 800 feet
above high tide, safely away from Hurricane country but smack in Tornado
Alley. 

3. The house will be on the grid 99% of the time.  The other 1% will be
due to natural disasters.  I have never experienced a power outage more
than 24 hours where I live.  The chance of a longer power outage is
slim, and the chance that the longer outage coincides with a very cloudy
day is even slimmer, since most storms pass through and bring clear
weather the next day.  I could get by with 24 hours storage. 

4. The worst windstorm in memory hit St Louis last summer, where power
was lost up to 7 days, however most of those days were probably sunny.
I predict Global Warming will bring heavier storms more frequently.  I'm
still thinking 24 hours is pretty good. 

5. I've got a #6 cable roughed in from my roof to the area where the
batteries will go.  That's good for 55 amps, or 1320 watts at 24V.
Plenty of room to expand.  

6. When I total my loads, sans freezer, I get about 420 watts peak and
2KW per day usage. I am afraid the freezer might double the size of the
required system and blow the budget altogether. 

7. I come up with 8 batteries at 88 amp-hours each

8. I calculate 6 solar panels at 130 watts each for a total of 780 watts
peak.  At 4.5 hours per day average sun, that's about 3.5 KWHR/Dy before
inverter and charger losses, or about 2.5KWHR/day after losses. 

9. I'm looking at Kyocera 130 watt solar panels.  Any experience with
them?

10.  I'm looking at an Outback MX60 charge controller, because of it's
MPPT characteristic and expandability. Anyone tried one?

11. Haven't picked out an inverter yet, but I'll probably be using
something in the 1000W-1500W range.  I probably won't go with the real
high end units like Xantrax.  I think my loads can tolerate a cheaper
modified sine wave type unit. 

12.  If we get a week of cloudy days, we might exceed the storage
capacity of the batteries.  I want to cook up something that will kick
in a grid-powered battery charger if the batteries go below 40% charged.
This is the part I am stumped on.  I could build something triggered by
battery voltage in the electronics shop, but considering the success
rate of my homebrew electronics projects, I'd rather buy something off
the shelf that can do this task.  Anyone know of something that could
accomplish this task?

13. The total bill for this kit of parts would be about $6000 and
generate $102 worth of juice every year.  That's a 58 year payback,
which really stinks.  After the rebate it's only 39 year payback, which
still stinks. It might produce 5% to 10% of my electric needs.  This
project would have to be worth it from power quality and backup
considerations to make it worth doing economically.  Is this nuts? What
do people think?  



 
 
Lawrence Lile, P.E., LEED AP