PV Energy Payback Time

 This subject was a lively thread for awhile. I happened to come across what
seems like a nice presentation on this subject. It claims 2-3 years now and
less in the future.

http://www.ecotopia.com/apollo2/knapp/PVEPBTSlides.pdf

  A quick check - the slides seem to be about payback in terms of the costs to
produce the PV cells vs the output from them; i.e. how many watts does it
take to make them vs how many they produce.

NOT what end users are interested in (generally).  Myself and most others
are interested in $$$'s vs $$$'s saved.

 Redmondite wrote:

While this may be true, ones personal economic payback period will vary
wildly between individuals and therefore can not be calculated except by
the individual interested or someone working with their data.

Some examples.

Let's say you own a farm in the boonies. It has no utilities. The
electric company will run power lines out to your farm for $50,000.
You work the numbers, shop around, and find you can buy a solar
power system for $50,000 that will provide the electricity you need.
In this case, the solar power system pays for itself immediately on
installation. Economic payback period, Zero days.

Let's say you live in the city and already have electric power.
Electricity costs you 5 cents/kwh because your power is supplied
from a hydroelectric plant providing electricity cheaply. If you
buy a system at $4/watt-produced and you get an average of some
5 hours of sunlight a day then your system will generate (5 x 365)
1825 watts a year or 1.825 kwh. Priced at 5 cents per kwh this
is 9.125 cents a year the solar will produce. It would take at
least ($4/$.09125) 43.8 years to reach economic payback. The panels
are only warranted for 25 years and the rest of the equipment
will likely need replacing at least two or three times over this
period. Economic payback period would be a minimum of 44 years
and quite possibly never (depending on your maintenance costs).

So, just in those two examples you have payback period ranging
from Zero days to Infinite days. I think that covers the spectrum.

Anthony

 
However, the horrible, awful, practical fact is that you can put in an
evil fossil-fuel (or bio-fuel) system of equivalent or greater capacity,
_and_ buy the fuel to run it, for a whole load less money than the solar
system, at current prices. Then your payback is immediate _and_ you
don't have to spend a whole $50K you may not have to begin with. Indeed,
very few solar systems can afford to be "pure" - every person I know
with one (NE USA - not the best solar weather, here) owns and uses a
genset. I know there are a few folks on this group that don't, and while
I'd love to join you, I simply cannot afford to.

I'm facing a need (due to high powerline costs, of about $15K) to do my
own power, and because "reducing my loads" works fine for lighting, but
not fine for running multi-horsepower lathes (which is the main point of
the shop building), I'm looking pretty quickly at diesel (a new, clean,
relatively quiet one), with a few batteries and a small inverter so the
genset doesn't run all the time when loads are light, and perhaps a
panel or two to help with battery maintenance. Getting the equivalent
power from solar alone would cost $50K or more, while I expect to be out
and done for under $10K, which leaves me $5K for fuel and maintenence
before it costs as much as the powerline, and another $35K in fuel and
maintenence before I get to the price of the pure, or even pure mostly,
solar system (not counting its maintenance). At the current price of
fuel, diesel power actually costs somewhat less than the power from the
local power company. I'll pull heat off the (liquid-cooled) motor as
well, and get more use from my fuel.

If cheap solar ever comes to pass, the economics might change, but cheap
solar seems to be something that's always being announced as coming in
the next two years (please invest in our company...), and it never gets
closer than 2 years. While it is true that cell prices today are perhaps
1/2 of what they were 10 years ago, the cost of the system is still
quite high (it feels like we spend more on widgets to make them
code-compliant, higher-quality inverters than were available 10 years
ago making better power, etc).

~10 years ago I was (landlessly) considering solar power for my eventual
land, subscribed to Home Power for a while, looked at various things,
shelved it due to lack of land. When I got land, I figured the powerline
cost at somewhat less than 1/2 what the power company is quoting me, and
with considerably less tree impact than the power company wants, so I
did not really get serious about it again until this fall, when I got
the power company proposal and gagged. So I looked more carefully into
the current state of the solar market. For somehing that will provide my
power at a price I can pay within the next few months, mostly diesel
ends up winning hands down, even with a fairly expensive, really solid
diesel genset.

--
Cats, Coffee, Chocolate...vices to live by

 [reply to economic payback comparison; my summary: "diesel is cheaper"]

Sounds like a fairly small system.  If you were putting in something
bigger, I would suggest you look at a microturbine too.  Diesel
gensets generally need maintenance about every 800 hours of operation,
while a Capstone needs only an air filter change at 8000 hours.
For 24/7 operation this is "every month" vs "every year", which
can be a pretty big deal.

Integrated heat-and-power microturbine systems can be as much as
80% efficient, but your heat load needs to be about twice your
electrical load.  Diesel and natgas reciprocating gensets have a
slight edge here if you need more power than heat (but again, the
maintenance costs go up).

If you were in California in either of the major air quality
management districts, they would restrict you to <200 hours per
year on a diesel genset.  This again pushes things heavily in
favor of the microturbine (or for residential use, PV).  PV is
also, of course, much quieter -- probably not a consideration if
you plan to run heavy duty lathes. :-)
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W)  +1 801 277 2603
email: forget about it   http://web.torek.net/torek/index.html
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