More PV Panels vs Running Generator

 This thread is related to my other thread, titled "More Batteries vs
Running Generator".  In this thread, I'd like to talk about the point
where it's more economical to run the generator vs purchasing
additional PV panels.

This seems to be another area where I don't see anyone actually
looking at system sizing from an economics standpoint.  For example,
normally a system is sized by the average daily load, but doing that
does not take into account the depletion of energy caused by cloudy
days.

With the PV capacity calculated to the average load, you'd in theory,
never be able to recharge the battery banks after cloudy weather,
without aid of a gas generator.  In essence, using this method of
sizing calculation, no matter how large a battery bank you have, every
cloudy days worth of energy is eventually being replaced back into the
battery bank with expensive gas generator created energy!  In this
case, economically, there is no reason to ever have a battery bank
with more than 1 day of average capacity!  Think about it!

For optimum economics, the same type of analysis that I talked about
in the "More Batteries vs Running Generator" thread needs to be
applied to PV sizing as well.  In other words, how much extra PV
capacity can be added to offset generator run time, in the most
economic ratio?

In the "More Batteries vs Running Generator" thread, I established an
example average lifetime cost per kWh of $0.567, for a Honda EG3500,
including purchase, maintenance, and fuel costs.  I will stick with
that example for this thread.

To establish the costs of the PV generated energy, I'll assume using
the discount "off-spec" PV panel deals which run about $2.50 a watt,
have a true life of 40 years, Peak Sun Hours of 4.5, and that the
panels end up giving me effectively 75% of the rated capacity of the
panels, over the life of the panels.  Using these assumptions, over
the PV life, the cost is $0.038 per kWh.  Wow, that's damn cheap!

So, how do we determine the point it's cheaper to run the generator
than increase the PV capacity further?  To complete the analysis, I
guess, we would need to determine/estimate (some how!) the number of
times a year that there is multiple extended cloudy days.  But how to
get this information?

For cloudy day examples, I again will use the example of an average 40
times a year that there is extensive clouds extending more than one
day, ten times a year with extensive clouds extending more than two
days, and 3 times with extensive clouds extending more than 4 days.

We also have to make some sort of assumption of the average days of
full sun we will have between these days of cloudy weather.  This will
give us our "recovery days" that the PVs have to make up for the
cloudy days.  Again, I'm not sure exactly how this information can be
found, but it has to be available somewhere!  In this example, I'll
assume there is on average of 4 days between spells of cloudy weather.

So, lets do some calculations, assuming, once again, an average daily
energy demand of 10 kWh a day, and assuming the "base" PV array is
rated at 2.5Kw:

40 times yearly of >1 days clouds, with 4 days on average to recover
would require 25% increase in capacity to totally offset all generator
runtime.  This would cost an amortized cost of $62.50 per year, and
would offset 400 kWh a year of generator run time costing $226.80
yearly.  Extra PV is a good deal here!

10 times yearly of >2 days clouds, with 4 days on average to recover
would require 50% increase in capacity to totally offset all generator
runtime.  This would cost an amortized cost of $125 per year, and
would offset 200 kWh a year of generator run time costing $113.40
yearly.  PV does not seem to pay here, but it's real close, and
probably best to pay the slight premium for the extra PC capacity!

4 times yearly of >3 days clouds, with 4 days on average to recover
would require 75% increase in capacity to totally offset all generator
runtime.  This would cost an amortized cost of $187.50 per year, and
would offset 120 kWh a year of generator run time costing $68.04
yearly.  PV does not pay here to extend it out to 3 days, and running
the generator is cheaper, long-term, for that extra day.

So there you go.  In our example, adding up to an additional 50% of PV
capacity above the average system demand seems to be economically the
best deal.  In this example, adding 50% extra PV capacity would offset
$385.56 of generator run-time (all 1 and 2 days, and 2/3 of the 3
days), leaving only $22.68 in yearly generator run-time.

Any comments?

--
BRENT - The Usenet typo king. :)

Fast Times At Ridgemont High Info http://www.FastTimesAtRidgemontHigh.org
Voted #87 - American Film Institute's Top 100 Funniest American Films

 
Well, it's all in your assumptions, then isn't it?  $5-10/watt
installed costs, 20-year lifetimes, etc, and you'd have more
traditional costs of around 30 cents per KWHR.


Put up a pyrometer and a datalogger and do the analysis for your exact
location (that's what I'm doing!).  Or see if you can find the data
online somewhere.

Alternately, get the generator and see how much you end up using it.
You might end up doing something like:  When the battery bank drops to
25%, use the generator to charge it to 75%.


So you are assuming 4 peak solar hours per day, and anything else goes
to making up your deficit?

--
William Smith
ComputerSmiths Consulting, Inc.    www.compusmiths.com

 On Mon, 15 Dec 2003 08:20:07 -0500, William P.N. Smith <> wrote:


I don't know who ever decided to use 20 years as the lifetime of PV
panels, but it's a crock IMO.  The warranties are that long, and no
manufacture warranty something anywhere close to it's true estimated
lifespan.  If we did, we should be calculate replacing inverters and
charge controllers every 2 years, and batteries every year!  I use 40
years for PV, wiring and mount lifespan, and I'll bet on that number
as being closer to the real world results.

You are right though, the balance of system will add to these costs.
Let me use my own current system as an example:

PV array output rated at 372 Watts (40 year life)
$25  PV mounts (40 year life)
$40  PV/charge controller wiring (40 year life)
$50  Charge controller (20 year life)
$40  Battery/controller wiring
$130 Batteries (7.5 year life, just replaced my first set)
$20  Battery/Inverter wiring (40 year life)
$50  400 Watt inverter (20 year life)
$50  fuses/disconnects (40 year life)
$200 Ah meter (20 year life)
----
$1468 over the 40 year system life or $3.95/W for balance of system
costs, or a system total of (assuming using $2.50/W off spec panels)
$6.45/W, or a cost of $0.441 per kWh, based on my average electrical
demand of 1 kWh daily.  Sound right?  Many of these items economically
scale better on a larger system, of course.  For example, I am way
"over-batteried" in my current system, my Ah meter price is also out
of wack with the rest of the system costs, and my charge controller,
fuses, disconnects could handle a much larger system/array.


That's great for current information, but not long-tern trends, no?
What happens if your measurements occur during an exceptional year?  I
would seem better to compare you measurements to that of a long-term
measuring station that's close to you, and then interpolate the long
term data using the differences you measured at your site.


That's the plan.  80% DoD the generator goes on, 20% DoD, generator
off.

--
BRENT - The Usenet typo king. :)

Fast Times At Ridgemont High Info http://www.FastTimesAtRidgemontHigh.org
Voted #87 - American Film Institute's Top 100 Funniest American Films

 
I was just pointing out that traditional methods of costing out solar
power systems show them to produce power at somewhere around 30 cents
per KWHR, and your original cost estimate of 3.8 cents was a bit low.
You've since proved my point.  8*)


Well, your data collection starts the day you put the pyrometer up, so
start as soon as possible.  "Fortunately" my permitting process took a
couple of years more than I had anticipated, and I'll have some four
years of data by the time I build my new house, but IMHO, data from
the exact site is a lot more reliable than data from "over there
somewhere", due to local microclimates and such.

On The Other Tentacle, you mentioned that you are out in the desert
somewhere, so you probably want to know what the nearest long-term
data looks like for a sanity check.  Correction factors for collection
efficiencies, wiring losses, inverter and battery losses, etc may
swamp any precision you can get anyway, so your best data collaction
device may be the system you put up, as it's a perfect model of
itself!  8*)  There's nothing wrong with a SWAG and upgrading in the
future as you discover deficiencies...

--
William Smith
ComputerSmiths Consulting, Inc.    www.compusmiths.com

 

Somehow you have to factor in the real world.  Hailstorms happen, weinds
& tornadoes  happen, and a 20 yr warranty is worthless if the company
goes under.  How many of these companies have been in business for 40
years?

--
free men own guns - slaves don't
www.geocities.com/CapitolHill/5357/