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re: solar pv system on new house?
27 oct 2002
sylvia steiger wrote:
>...my investigations have also shown that the traditional
>solar cells --> battery storage --> appliances as needed doesn't work
>well for high-wattage items like an electric stove or heater; even if
>you put the money into buying enough panels and batteries to have enough
>power, the drawdown is so big when they are used that you'll need to
>replace those expensive batteries practically every week. (okay, i'm
>exaggerating a little ...)
trojan batteries are rated for total "lifetime energy storage." they
wear out after say, 1000 100% charge-discharge cycles or 2000 50% cycles,
and so on, approximately. they actually store more lifetime energy with
fewer and deeper discharges, eg 30% vs 5%, and they have a finite shelf
life, even if never discharged. you need more batteries to store the same
amount of energy with shallower discharges, and batteries take up space
and up-front and ongoing money. most pv people tend to overpopulate and
underdischarge batteries in an uneconomical way.
>our land in the country, where we are not living now but hope to rsn,
>has very reliable grid service. however, being 15 miles out of town,
>i'm assuming if it ever fails it will be under the worst possible
>conditions, such as a blizzard or ice storm that would keep us from
>being able to get into town. reliable backup power could literally mean
>the difference between life and death in that case.
you might have just a few batteries, say enough for 12 hours for critical
loads, and a generator to charge them or operate critical loads by itself
if needed. that gives triple redundancy: grid, batteries, and generator.
>my original thought was to use a wind generator to power the high-demand
>appliances. i've been told that won't work well because you can't
>channel the wind-produced ac right into a house ac system. i still
>don't understand why but since everyone knowledgable has given me the
>same answer, i have to assume they are probably right.
i wouldn't write off wind power quickly, if you have wind. it's cheaper
than pv, closer to $1 than $5 per peak watt, especially with a tree or
a barn for a tower, and it can work on cloudy days or at night. it does
require more maintenance than pvs. you might either get a small dc turbine
and use an inverter, or get a larger one with an ac induction motor which
can match grid power if present and match your inverter if not. or maybe
make "wild ac" and rectify that to make dc. this can be more efficient than
a 60 hz system, with a varying vs fixed prop speed.
>so our current plans, when we build, are to use everything renewable we
>can -- belt and suspenders, so to speak. big south-facing windows and
>lots of thermal mass to minimize heating needs.
i'd suggest "lots of insulation," eg a house built with structural insulated
panels (sips), ie glued plywood-foamboard-plywood sandwiches.
water can be excellent thermal mass, cheaper and more compact than masonry,
storing 3x more heat by volume, easy to insulate and move around, fairly
easy to heat with the sun.
big south windows can lose lots of heat at night and on cloudy days.
part of the south wall could be simple thermosyphoning air heaters which
lose no heat at night, or you might supply daytime heat for the house from
a low-thermal-mass sunspace. you might have a hydronic slab and trickle
warm water between a dark metal roof and a single layer of polycarbonate
greenhouse roofing (eg "dynaglas") over that, with a higher temp heat
store for cloudy days. part of the metal roof might be covered with
unisolar pv "field-applied roofing laminate," with water trickling
over that.
>and a diesel generator to supply the high-wattage power needed for electric
>stove and water heater.
start the diesel whenever you want to make a cup of tea? :-) you might heat
water or the house with its "waste heat," which might be 80% of the fuel's
heating value...
>...biodiesel sounds absolutely wonderful and feasible for us to make on
>our own... assuming we are able to establish a regular source of waste
>vegetable oil (wvo), process it into biodiesel ourselves, and store it
>safely until needed, we should be able to meet our housing and mobility
>needs with no use of nonrenewable resources!
sounds like more ongoing work than sun and wind power. and less
independent, with this "regular source" requirement. and you will
be adding co2 to the atmosphere.
nick
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