cost per kWh [RE-wrenches]

[Bill Brooks]

-----Original Message-----
From: John Berdner [mailto:jberdner at sma-america.com]
Sent: Monday, June 02, 2003 4:00 PM
To: billb at endecon.com; RE-wrenches at topica.com
Subject: RE: cost per kWh [RE-wrenches]


Bill:

I am really surprised by your data.
It does not agree with any data / comparisons I have seen to date.
Most grid tied with battery based systems I have seen generate far
lower performance numbers.
I believe this is the more common experience for the people I talk
with. 

We are monitoring an SW system and a Sunny Boy system - both on arrays
you guys swept with the daystar.
We just calibrated the sensors on both system to ~ 1% so the error is
about +/- 2% for our the measurement.
We see about a 25% improvement in energy capture (how much energy the
system acutally produces) and another 20% or so for keeping the battery
charged at night.
Overall we are seeing just a little less than double the levelized
>>net<<  energy output (Net Kwh/kw) from the systems we are monitoring.
Normally I expect to see a minimum 10 to 15% improvement due to MPPT
but perhaps your array voltage is low and happens to be just at the
battery voltage. 
I would not expect this to be the case year round so it will be
interesting to see how they work over the seasons. 
This could also be due in part to the low fill factor of the array. 
As the fill factor goes down the effectiveness also decreases. 

Regards,

John Berdner

>>> "Bill Brooks" <billb at endecon.com> 6/2/2003 02:57:03 PM >>>
Todd,

It would help more all things were equal (which they rarely are in the
real
world). My two arrays are identical size and identical orientation.
However,
one array (the one with the SunnyBoy) has a lot of morning shade.

When looking at the times when the arrays are both fully illuminated,
the
SunnyBoy produces about 4.5% more energy than the Trace SW. Looking at
the
operating points I found that they were both operating very close to
maximum
power point for their respective arrays. For instance, today the SW
was
running at 52.2 Volts (temp compensated from 53.6V) and the SB was
running
at 261 Volts (exactly 5 times greater with exactly 5 times more modules
in
series)--side thought--how is a max power tracker going to help me?

The difference in performance is measured by a Campbell Data Logger
with two
separate Ohio Semitronics PC-5 watt transducers. Data is taken every
second
and averaged over a 15 minute period.

The difference in performance is due to several factors. The SW has a
C-40
charge controller in series with the array and the power loss through
the
controller is 1%. Voltage drop is definitely higher with the SW given
the
lower voltage--charts suggest 2% as the difference in voltage drop
given the
wiring in the system. Finally, the conversion efficiency is higher in
the SB
likely causing the remaining 1.5% difference in energy delivery. If the
SB
is running at the published 94% under these conditions, the SW would
be
operating at 92.5%. Not bad for an old dog SW with nearly 5 years on
it.

Night-time losses for the SW are measured at a continuous 40-45 Watts.
The
SB is one big donut (no losses) at night.

For what they are, I'm impressed with both machines. If backup is not
valued
or desired by the customer, the SunnyBoy looks awefully good. If you
want
backup power, the SW is not that big of a penalty in performance. It
certainly does not warrant a separate inverter for battery backup. Is
the
the SW optimized? Only from the point that I have adjusted to the PWM
angle
to get higher efficiency. Max power tracking would have marginal impact
on
my array and nighttime shutdown would save about 0.5 kWh per day.

Qualifying statement: this is very new data and calibrations have not
been
carefully checked recently. I do believe the numbers are very close to
reality. Another qualifyer is that my unit is an old ETL-listed
workhorse. I
have not personally tested the GTI or the current UL-listed version to
see
how their conversion efficiencies and night-time tare losses compare.
My
results may not be indicative of how a new SW and SB would compare. I
just
have a new SB. The new SW could be better or worse.

That's all for now. I'll keep you posted if I learn anything new. I
just
love my Max Power Point Tracking temperature compensation probe on my
SW
inverter. It is highly reliable.

Bill.


-----Original Message-----
From: Todd Cory, Mt. Shasta Energy Services
[mailto:toddcory at finestplanet.com] 
Sent: Saturday, May 31, 2003 2:08 PM
To: RE-wrenches at topica.com 
Subject: Re: cost per kWh [RE-wrenches]


Just curious Bill on the difference in delivered kWh was between your
2
systems.
This helps define (in real terms) what the penalty is in having
battery
backup
in a grid tied system.

Todd

***snip***

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