SPAM: Re: SPAM: Re: UI w/SB50 controllers , was C40's and SW's [RE-wrenches]

Bill Brooks billb at endecon.com
Tue Feb 4 15:12:02 PST 2003


Kirk,

I think an I-V curve tracer can be helpful to visualize this issue. I have
an AstroPower array on my home and the fill factor is quite a bit lower than
Siemens and Kyocera arrays tend to be. It is oversimplifying the situation
to say "the max power voltage is 60 Volts and the battery is at 54 Volts so
I have a 10% loss." You will notice the current is much higher at 54 Volts
so you much calculate the power difference, not the voltage difference.

I was honestly quite surprised to learn this on my own system and was forced
to change my previously held positions on the subject. It's worth the time
to play around with this stuff and get a good feel for it. I wish I had more
experience with the SB50 to better understand how well they are doing the
job. It would be really nice if DOE and CEC and others that had money would
actually do side-by-side testing for these types of issues for us and
provide us with data, but alas that is dreaming.

Bill.

-----Original Message-----
From: Kirk Herander, VSE [mailto:kirk at vtsolar.com]
Sent: Tuesday, February 04, 2003 2:32 PM
To: RE-wrenches at topica.com
Subject: SPAM: Re: SPAM: Re: UI w/SB50 controllers , was C40's and SW's
[RE-wrenches]


Bill,

Yes, I agree climate ie. module operating temps make a big difference in
performance. All I really know from personal experience is that MPPT is
worth the investment in my cold climate with the modules I primarily use,
Astropower, Siemens, and Kyocera.

Kirk
----- Original Message -----
From: "Bill Brooks" <billb at endecon.com>
To: <RE-wrenches at topica.com>
Sent: Tuesday, February 04, 2003 4:51 PM
Subject: RE: SPAM: Re: UI w/SB50 controllers , was C40's and SW's
[RE-wrenches]


> Kirk and Graham,
>
> I would have to disagree with Kirk on the flat statement to get rid of the
> C-40. It has proved to be the most reliable PV charge controller to date
and
> the benefits are marginal at best for the SB50. Only in very cold climates
> with high irradiance does the SB50 clearly win. For California and other
> mild climates, I would be willing to bet that the SB50 is the same or
worse.
> Heresy, I know--but show me some good ANNUAL data to prove otherwise.
Until
> I see it I'm going to stay with the C-40.
>
> Max Power Tracking is seldom perfect. I also have no data to say that the
> SB50 does it well. Until I see an official test I will reserve judgement
and
> remain sceptical--no personal vendeta against RV Products--just 15 years
of
> Max Power Tracking experience that says that many don't work very well.
They
> almost always have errors on the high voltage side which is tremendously
> unforgiving in a PV system. My fixed-voltage system consistently operates
> from a worst case of about 8% lower power up to spot on max power point.
It
> makes a lot of difference what kind of array you are running and how high
> the fill factor is. Many modules are running fill factors in the low 60%
> range, so a fixed voltage system can actually outperform a max power
tracker
> that is loosing 5% in conversion efficiency. Temperature compensation is
> absolutely critical to all battery-based systems--particularly with AGM
> batteries.
>
> Moral of the story--don't jump to conclusions--it all depends. MPT
> controllers make sense: (1) if it works!!!, (2)On higher fill factor
arrays,
> and (3) On stand-alone applications (not floated). I can't be dogmatic in
> either direction--several factors must be considered.
>
> Bill.
>
> -----Original Message-----
> From: Kirk Herander, VSE [mailto:kirk at vtsolar.com]
> Sent: Tuesday, February 04, 2003 12:28 PM
> To: RE-wrenches at topica.com
> Subject: SPAM: Re: UI w/SB50 controllers , was C40's and SW's
> [RE-wrenches]
>
>
> I have a dual SW5548 system with 4.8 kw array, divided into 4 sub-arrays,
> each controlled by a Solar Boost 50 MPPT controller, utility-intertied  in
> combo w/ a sealed AGM battery bank. I have set both bulk & (float)
voltages
> on the inverters to the batteries recommended float voltage of roughly
52.5
> vdc. I trust that this voltage is acceptable to keep the batteries well
> maintained, and I want to maximize the delta between panel MPPT voltage
and
> battery voltage, so there is more voltage to convert to current and
> therefore more power to send to the grid.
>
> In the middle of a cool and sunny Vermont day last fall, I measured an
> efficiency of 91% when comparing DC PV power into the Solar Boosts vs. AC
> power sent to the grid. The gains to the grid using MPPT vs. the C40 are
> impressive, so I would suggest getting rid of the C40 altogether. It is
not
> appropriate for a battery-based UI system.
>
> Kirk

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