On grid w/battery (was non wrenches) [RE-wrenches]

Bill Brooks billb at endecon.com
Wed May 29 21:27:40 PDT 2002


Joel,

Three humorous responses and a dis (just kidding).

1.) Unlikely

2.) Likely

3.) Thousands of Japanese and Germans can't be right. They don't care about
personal independence.

I hope your wish comes true (but it will take a lot of PV-UPSs to make it
happen) ;-)

-----Original Message-----
From: Joel Davidson [mailto:joeldavidson at earthlink.net]
Sent: Wednesday, May 29, 2002 7:21 PM
To: RE-wrenches at topica.com
Cc: Jeff Newmiller; John Berdner; Chuck Whitaker
Subject: Re: On grid w/battery (was non wrenches) [RE-wrenches]


Here are 3 not-so-blanket statements and 1 wish.
1) If I replace my SW4048 with an ST2500, my PV array will produce more
energy.
2) If I replace my SW4048 with an SB2500, my PV array will produce still
more
energy.
3) If I separate my PV production from my UPS needs, thousands of Japanese
and
Germans will agree that I did the right thing.
and 1 Wish - I wish Americans would buy at least 200 MW of grid-tie PV
annually
so wrenches would be too busy installing PV to answer their email :-)

Bill Brooks wrote:

> Jeff,
>
> Please recognize that this thread started from a discussion with Joel
about
> whether he should ditch his old inverter and install a new one. I was
clear
> in my qualifications that I was referring to systems in California with
the
> batteries located outside so their temperature tracks much more closely
with
> array temperature changes.
>
> In general, an ideal max power tracking system will outperform a constant
> voltage (temp. compensated) system. However, many qualifiers are needed to
> actually see the difference in the field. Remember, we are talking about
> battery-based systems, so some type of voltage conversion will be
necessary
> to make the inverter operate at AC line voltage.
>
> The point I was trying to make (quite unsuccessfully I might add) is that
> blanket statements are made all the time on this issue and they are
> generally based on comparisons in off-grid systems where the operating
> voltage and the array voltage have the greatest spread--especially in the
> winter when batteries voltage is low and array voltage high. No question
> that the losses in a MPPT controller are trivial compared to the operating
> point losses on the I-V curve.
>
> However, this situation becomes much more cloudy when looking at an
on-grid
> system at float voltage. I installed some of the early on-grid 54-Volt
> battery systems in North Carolina back in 1995 and they worked fairly
well.
> It is very dependent on module type and overall array characteristics. I
> have found that many 48-Volt arrays operate at a fairly low fill factor
> (around 60%) and therefore below expected operating voltage and this was a
> surprise to me. As I began to look into this issue, I found that my own
> 48-Volt system was operating only a few percent below a max power point
for
> a large portion of energy generation of the system.
>
> Since dc/dc converters in MPPT controllers for battery-based systems do
> consume some energy (3-5% energy lost), the relative benefit for these
types
> of systems must be considered in the context of their fixed-voltage,
> floated, temperature-compensated counterparts (try saying that 5 times
> fast).
>
> In Vermont, you need to consider what temperature your array is running at
> when it delivers most of its energy. That information, along with the
> selection of the PV array will work together to give you a solution that
> will either suggest a MPPT system or a constant voltage system.
>
> Bottom line, an ideal MPPT system will outperform a constant voltage
system.
> I have yet to test an ideal MPPT system on a grid-tied battery system that
> was able to show the benefits everyone is so sold on. Hopefully the MM-5
> will prove to be that device.
>
> I would much rather have a more sophisticated inverter than my old SW. It
is
> far from optimal for the task I am asking from it, but I could do worse
and
> I'm not sure how much better I can do today. Maybe SMA will solve the
issue.
>
> Never disengage your brain and simply accept what everyone believes to be
> true. Don't waste your time questioning authority, question numbers and
> statistics--they do more lieing than authorities.
>
> Keep up the good work,
>
> Bill.
>
> -----Original Message-----
> From: Jeffrey Wolfe, Global Resources [mailto:global at sover.net]
> Sent: Wednesday, May 29, 2002 12:07 AM
> To: RE-wrenches at topica.com
> Subject: RE: On grid w/battery (was non wrenches) [RE-wrenches]
>
> Bill,
>
> I think the key thing to remember here on MPPT (and other topics) is that
> the whole world is not California, and vice versus. Many areas of the
> country (such as Vermont) DO see the higher max power voltages for a good
> bit of the year, only getting into significant voltage dropoff for a short
> amount of time. We do not have a "metered test bed" but whenever we look
at
> a system with MPPT, the numbers we see at that look are boosted.
>
> Since this list goes out to all of us, we just need to make it clear when
> we're talking about the entire PV industry, and when we're talking about
the
> entire PV industry in CA. I know the difference is small, but it's really
> quite important to those of us outside CA!   :)
>
> Jeff
>
> -----Original Message-----
> From: Bill Brooks [mailto:billb at endecon.com]
> Sent: Tuesday, May 28, 2002 5:59 PM
> To: RE-wrenches at topica.com
> Subject: RE: On grid w/battery (was non wrenches) [RE-wrenches]
>
> Rick,
>
> Thank you for your clear explanation of how a dc/dc controller can work in
a
> grid-connected PV system. However, in California, there is rarely much
> difference between the power output at max power voltage (14-15 volts per
> 12V module) and the power at float voltage (13.4 volts). We can come up
with
> dozens of special cases where the MPPT clearly wins, but the lion's share
of
> the time in California it is only marginally helpful. It also costs energy
> to operate.
>
> I'm not slamming yours or any competitive product. I just see overstated
> examples like STC voltage (that happens in Alaska in the winter time--that
> was a joke by the way) of 17 volts used in the examples and I have only
seen
> that max power voltage for brief periods in the winter here. The key here
is
> that the large voltage difference does not exist.
>
> What is under development is a max power tracking effectiveness test. No
one
> can argue with the benefit of an MPPT controller for off-grid systems--the
> voltage differences can be substantial. I remain skeptical for on-grid
> battery-based systems. I hope to be proven wrong soon.
>
> Bill.
>
> -----Original Message-----
> From: Rick Cullen - RV Power Products [mailto:rick at rvpowerproducts.com]
> Sent: Tuesday, May 28, 2002 2:06 PM
> To: RE-wrenches at topica.com
> Subject: Re: On grid w/battery (was non wrenches) [RE-wrenches]
>
> Wrenches,
>
> I would like to respond to the MPPT / Float issue below. It is correct
that
> MPPT becomes inactive when the charge controller is in Float charge mode.
> This is because the controller controls battery voltage by reducing charge
> current. Since it is intentionally reducing charge current during float,
> MPPT operation is no longer needed. MPPT only operates when a non grid
tied
> system is in Bulk, which is when it is needed.
>
> But, the situation is very different when the system is grid tied. In a
grid
> tied system it is the inverter that is really controlling battery charge
> voltage in a "diversion controller" type strategy. The inverter "controls"
> battery voltage by adjusting how much power it pulls away from the
batteries
> and delivers to the grid. The inverter pulls away and delivers to the grid
> as much power as it can without allowing battery voltage to drop below the
> inverter's charge voltage setpoint. When a Solar Boost MPPT charge is
used,
> it's charge voltage setpoint should be set to be slightly higher than the
> voltage the inverter is regulating to. When this is the case, the Solar
> Boost controller will remain in bulk, working it's heart out trying to get
> to it's charge voltage setpoint which the inverter won't let happen. The
net
> effect of this is that more current and power is available for the
inverter
> to pull away and send to the grid.
>
> The comparatively low Float voltage battery backed grid tied systems
> normally reside at tends to increase output current from the MPPT
> controller. If everything was 100% efficient (which of course they aren't
> and some power is lost) output current = input current x Vpv/Vbat. To pick
> some numbers, suppose Vfloat=26.6V and the PV's are at their standard test
> conditions of Vmp=34V, then output current will increase by 26.6V/34V=1.28
> or 28% relative to input current so more current is made available for the
> inverter to extract. The key here is that larger the difference between
Vbat
> and PV Vmp, the larger the current increase will be. A system which
remains
> in Float holds battery voltage on the low side which is good for MPPT
> performance.
>
> Rick Cullen
> RV Power Products, Inc.
> Providing quality MPPT solar charge controllers
> Phone 760-597-1642 x102
> Fax 760-597-1731
>
> mail to: rick at rvpowerproducts.com
> www.rvpowerproducts.com
>
> ----- Original Message -----
> From: "Bill Brooks" <billb at endecon.com>
> To: <RE-wrenches at topica.com>
> Sent: Monday, May 27, 2002 5:39 PM
> Subject: RE: On grid w/battery (was non wrenches) [RE-wrenches]
>
> > Joel and Marco,
> >
> > With all do respect to my friends at Advanced Energy, the old SW is our
> most
> > stable inverter on the market. Joel, I think your unit could handle a
much
>
> > larger array no problem. It seems that you have fallen into the trap of
> > believing that max power tracking has a lot to offer you. While I
believe
> > there are benefits to MPPT, a properly setup battery-based system will
> > harvest 95% or more of what a MPPT controller will do. In fact, the
> Advanced
> > Energy MPPT or RV Products controllers will save little or nothing when
> > dealing with a floated battery (even so stated in RV Products' manual).
> Play
> > with your system some time and run the inverter up 4 or 5 volts (with
the
> > C-40 set to max voltage) and you will find your power begins to drop,
not
> > increase. It does depend on the array, but I would not be so quick to
> write
> > off the use of your unit. From what I remember being at your house, you
> did
> > not have a huge array. Stick with what works. The biggest issue with the
> old
> > SW was bad power factor when connected to the grid. I think you already
> > adjusted your machine.
> >
> > Bill.
> >
> >
> > -----Original Message-----
> > From: Mangelsdorf, Marco [mailto:mmangelsdorf at hei.com]
> > Sent: Monday, May 27, 2002 5:13 PM
> > To: RE-wrenches at topica.com
> > Subject: RE: On grid w/battery (was non wrenches) [RE-wrenches]
> >
> >
> > Joel and others,
> >
> > At the risk of sounding like a syncophant salesman for our friends at
> > Advanced Energy, Inc. in NH, their Multi-Mode battery-based 5 kW
> > inverter was designed to do exactly what the SW can't do: max power
> > point track.  Their inverter also uses no grid power at night to keep
> > the batteries floated.  I've been closely monitoring a number of
> > Multi-Modes in the field here in the tropics and the reports have been
> > very positive in terms of performance.
> >
> > On another point: what's the latest with Xantrex getting UL listing for
> > the SW 4.2 with the GTI?   It's my understanding that the SW/GTI has
> > been approved by CTL in oh Canada but has yet to get official UL
> > approval.  Does anybody know anything otherwise?  Has anyone had any
> > difficulty getting their new SW/GTI assembly approved by the AHJ or
> > local utility?
> >
> > aloha,
> > marco
> > ProVision Technologies, Inc.
> > Hilo, Hawai'i
> > www.provisiontechnologies.com
> >
> > >From Joel Davidson:
> >
> > The point is our battery bank has just sat there 35,000 hours keeping us
> > off the
> > PV array max power point and buying a float charge at night from the
> > grid for
> > less than 24 hours of relatively non-essential backup power.
> >
> > Of course, I'd have to change inverters unless Xantrex figures out how
> > to get the SW4048 to max power point track when on the grid and function
> > as a
> > UPS in stand-alone mode during grid outages.
> >
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