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re: search for solar-energy-storing
19 apr 1996
jonathan sawyer wrote:
>"duane c. johnson" wrote:
>>well if common sense is used it is neither cost effective nor efficient.
economies of scale and past subsidies may influence prices more than common
sense at the moment. these prices depend on biased advertising and ignorant
consumers. perhaps a lot more $4/watt pvs are being produced than rooftop
water heating solar panels now, and maybe that's a good thing. there are more
efficient and cheaper ways to heat water, eg some commonsensical $4/ft^2 bare
swimming pool tube mat heaters or a $2/ft^2 site built epdm rubber collector
or low-priced big fins in a sunspace.
>what about pv's running a heat pump?
dumb idea. but smarter than running electric resistance heaters or
electric clothes dryers with pvs...
>in my house i have a geothermal heatpump heating a 3000 gal water tank which,
>in turn, heats the house through slab heating.
how about heating the house via a low-thermal-mass plastic-glazed sunspace
with a low-power fan or motorized damper in series with two thermostats
on a day with some sun?
>cop of the heat pump is over 5.0 (50 deg to 90 deg).
the cop of john christoper's 1981 csi hq building in new hampshire is about
50 ("2% fan power, 98% sun power.") improve that with sealed containers of
water in the "rock bed" and motorized dampers vs fans or blowers and it might
have a cop of 500:1 at a lower initial cost. a motorized damper costs about
$100 and uses 2 watts when moving, 0 watts when not moving (most of the time.)
>powering this with a 15% efficient photovoltaic panels will produce over
>75% "heat" efficiency and the colder it gets the more heat you produce!.
do i have this right? 1 joule of sun falls on the pvs, 0.15 emerge from
the pvs, 0.15 go into the battery, 0.15 come out of the battery, 0.15 come
out of the inverter, 0.75 come out of the heat pump, 0.75 go into the water
tank, and 0.75 come out of the water tank, all with no additional input
of energy, at a system cost of $10/delivered watt of house heat?
now suppose we put the pvs in a sunspace and the 85% of the sun's heat that
is normally wasted with pvs ends up heating the sunspace air which flows
into the house and heats the house. this would raise the system efficiency
considerably, no? and you and your friends could sit in the sunspace and
play cards and admire your pvs on a winter day with some sun, and revel
in how independent and creative you are.
now omit the pvs, and the cost drops dramatically, while the system efficiency
remains about the same, on a day with some sun.
now add a little insulated room containing enough sealed containers of water
to store heat for a week without sun, with 10' of fin-tube pipe near the
ceiling of the little room and a water heater in a warm-water convection loop
on the floor above, and the system efficency rises again.
add a little more floorspace to the room, and you have a sauna with free heat
and a place to dry clothes.
now omit the 3000 gallon tank, plumbing, slab plumbing, pumps, heat pump,
etc, and the cost nosedives again. before long, you may end up with a simple,
practical heating system, unless you are in love with pvs and heat pumps.
>of course you also get free electricity in the summer.
"free," after that little matter of buying the pvs, batteries, inverters,
etc, etc..
>i am in the process of installing pv panels for such an operation. i priced
>the system cost of pvs and thermal panels and found the pv's to be only
>50% higher on a square foot basis.
water heating solar panels are no bargain at $30/ft^2. polycarbonate plastic
"solar siding" or a roof or a sunspace that lasts 20 years at $1/ft^2 is.
polyethylene film that costs 4 cents/ft^2 and lasts 3 years before it needs
recycling, while producing $3/ft^2 worth of heat is definitely a bargain.
pvs are not a bargain, altho putting them in a sunspace or adding an
extra layer of glazing and trickling some water over their faces before
the water goes back to your 3000 gallon tank might be a good idea.
stanford ovshinsky's amorphous pvs produce 50 peak watt/m^2 at 20 c and
45 pw/m^2 at 60 c, at a cost of about $4/pw.
nick
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