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ac pv panels
3 dec 1995
tom gray   wrote:
>nick pine writes:
>>solar panels with built-in inverters that plug into wall sockets
>>to lower electric bills should be coming quite soon. (charlie collins) kindly forwarded the following from, paraphrased below with a few comments...

solar design associates is now developing an ac solar module with solarex,
owned by amoco. the ac module is a dc module with a small integrated inverter.

solarex calls the product "powerwall." the first modules are designed for 
curtain walls, the shear wall systems used on modern office buildings. these
modules replace the glass or other sheet material used in curtain walls, eg
in framing manufactured by kawneer of norcross, georgia. a residential roof
product is also in the works.

powerwall modules are available in ac and dc models, in sizes up to 53" x 87",
[32 ft^2--why not 4' x 8'?] with outputs up to 250 watts.

an ideal solar system should harvest both heat and electricity from the same
aperture [although pv power falls off as temperature rises...] virtually every
building has thermal as well as electrical energy needs, and roof area is
limited... in 1979, sda developed a combined, flat-plate, pv/thermal collector.
the device worked well. unfortunately, no company was willing to produce it.
now they are revisiting this device... 

[i wonder if it still has a thermal output?]

in 1980, existing pv modules (~ 4 ft^2) were not large enough to integrate
well with building systems. too many connections were needed to make a
significant electrical output. sda presented the case to all major pv makers
for building a large-area module of 25 to 30 square feet, which could be
integrated into a building skin to form the structure and weathering surface.
every manufacturer thought they were crazy - except one.
sda began working with mobil to develop a 24 ft^2, glass-superstrate pv module
which could be sold with or without a frame. now the rest of the industry is
following suit. custom module manufacturers in europe now offer pv modules of
3 square meters and larger, designing them to architect's specifications for 
direct building integration.
dc outputs of pv arrays impose serious limits on the electrical use. most
electrical loads require ac power, and 30-40% of the cost of a pv system has
been used in the transition from the dc pv module to the load. such systems
were complex and required special dc-rated components not readily available.
the obvious answer was to create an ac pv module. sda is now doing that now. 

>    my model is simple - let's say the house of your dreams would cost 
>    $100,000 us.


>    let's be really depressing, and estimate the cost of the same house,
>    after designing in "energy autonomy", to be $150,000 to $250,000.

houses need siding and roofs, right? suppose we use thin polycarbonate plastic
in place of siding and roof, with an airspace underneath, to collect some
sun. no sheathing, no tarpaper, no shingles. less labor, since these plastic
sheets come in very large pieces and rolls... so to my mind, a solar heated
house can be less expensive to construct than a conventional house. i wouldn't
make the roof out of pv panels, yet... but let's say you did, and it cost
another $50-150k... 

>    now, over your lifetime, will you ever have to pay a total of $150,000
>    for energy?  no way in hell!  even if your "energy bills" ran $200 per
>    month, you could pay them for 62 years, and be under the extra $150,000.

yes, although that is future money, and some economists argue that energy
costs rise faster than inflation. 

>    but...


>    most people have a mortgage, and pay interest on their home loan.
>    the way mortgages work, people end up paying them off in either
>    30 years, or 15 years.  let's be optimistic, and say that you can
>    get a 6% interest rate on the loan... 
>                            15-yr model         30-yr model
>        cost of house       $100,000            $100,000
>        monthly payment         $843.86             $599.56
>        number of months        180                 360
>        total payments          $151,894.80     $215,841.60
>    gee, isn't that nice?  it seems that financing the silly
>    roof over your head is not economical.

in what sense, exactly? it seems to me that deciding to buy a house is
different from deciding to invest in a stock or put money in a savings
account. people live in houses. it's hard to live in a bank account,
no matter how much interest the bank pays. mortgage "payback" is not
the same as investment "payback."

when i talk about solar economics, i'm thinking that most people are making
decisions about where to invest money, or whether to borrow money to do 
something that has a monetary return. i suppose that most people would rather
put money in a bank, rather than buy a solar system, if the bank deposit pays
equally well, and has a lower risk and takes less time to manage.

>    so, if we pay no interest...

i'm not quite following you here. i thought you were talking about 6%. if you
put up the cash out of your pocket up front (is that what you are talking
about?), you forgo the opportunity of putting it in the bank and earning some

>    we can afford the extra cost of investing in stuff that "pays us back"
>    by reducing or eliminating our long-term need to pay bills to the power
>    company, water utility, sewer utility and such.

my original point with tagdi was that if a "solar heater" allows you to avoid
paying the oil dealer for 1 gallon of oil per square foot of "heater" per year,
about 60 cents, where i live, the "solar heater" has to be very inexpensive to
build, or it has to serve more than one purpose, to make this kind of narrow
economic sense to the owner.


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