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

David Henry david at harmonyfarm.com
Fri May 31 18:08:59 PDT 2002 

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Bill,

I have a system with some Solar Boost 3048's and  a C40. During peak 
output at noon I consistently see a minimum 10% boost in amperage at 
array voltage vs charge voltage. Compared to a C40 in the same system 
it is 10-15%. The boost is greater during low light conditions and on 
cold clear days. The cost of the SB3048's is ~1/2 the cost of 10% 
more PV. Not a big deal but definitely still a plus. Especially if 
someone may move their system into an off grid application at a later 
date.

David

>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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>
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