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

Bill Brooks billb at endecon.com
Wed May 29 16:30:25 PDT 2002


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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