|
|
re: off-shelf and off-grid fridge??????
15 feb 2000
anthony matonak wrote:
>well, i've got just an average off the shelf fridge that takes about
>1.6kwh a day... four days of use (without sun) would require 1.6kwh*4
>= 6.4 kwh, batteries can be as much as 80% efficient...
moreso, i've heard (close to 100%), if they aren't fully charged.
but we might say that 4 days of use without electricity requires
storing a certain amount of coolth, say 3x1.6kwhx4days = 19.2 kwh
or 65k btu with a cop of 3. about 454 pounds or 57 gallons or 7 ft^3
of ice melting. more insulation would help.
>and inverters run some 80 to 90% so make that 8 kwh * 1.25 = 10kwh.
that's $1070 at $64/kwh for $85 trojan t105 220ah 6 volt batteries
which wear out after a few hundred 60% discharge cycles, or $14.25
for 57 25 cent 1 gallon frozen milk jugs, but where can we put them?
rich komp writes that batteries should last their design lifetime if
the average discharge depth is 30%. figure 5.6 on page 57 of his book
practical photovoltaics shows a typical motive power battery with about
4,000 cycles at a 30% discharge depth and 1,000 at 60%. the curve shows
the log of the number of discharge cycles decreasing linearly with
increasing depth.
>...if you add another few inches of insulation to the fridge you
>could cut (some say) power use to some 60% (2/3)...
why not? seems like that needs to be done very carefully to avoid rusting
the box via condensation through air that leaks under the insulation...
>...if you don't mind burning out your panels a little faster you might
>try 2x mirrors to double their production (at least in the cold winter
>months) to cut in half the number of panels...
what kills panels? high temperatures, i suppose, with non-eva potting,
and concentration and current densities low enough that metal migration
doesn't kill them. we might estimate the max panel temp and duration in
hours per year, and predict that panels without cooling and panels with
would have the same lifetime at the same time and temperature. this works
for other semiconductors with similar activation energies.
>of course, if it gets cold enough in your parts you could just freeze
>blocks of ice at night and use them in the fridge during the day.
we might add salt to make lower temp ice and put them in the freezer,
or keep the freezer filled with moist salty frozen foods to minimize
the cost of batteries. wal-mart sells 20 oz. coleman brite-ice packs
for $1.27 each, about $24 per kwh, cheaper and longer-lasting than
batteries, with a larger and more desirable surface-to-volume ratio
than milk jugs. i wonder what their melting temp is.
if the powered-up freezer makes 0 f, we might fill the jugs with a
15% brine with that freezes at 12.4 f, and keep most frozen foods
frozen on cloudy days, with a thermostat and small fan to circulate
freezer air through the fridge compartment. it would also be nice
to leave the light connected.
nick
|
|
