Off-grid update... [RE-wrenches]

[Bruce Roush]

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Hi Guys,
Christopher's advice is all good. I just wanted to add a few thoughts:

The reason why the array can be configured for a higher voltage than the 
batteries is the Outback MX60 (and other) MPPT (Maximum Power Point 
Tracking) Charge Regulators. These devices allow you to capture 
significantly more power from the same array as when compared to a 
non-MPPT Regulator. The increase in output is epecially good for off 
grid systems because the extra yeild is higher when it is most needed; 
In the winter when the array is cold and the battery tends to hover in a 
more discharged state. This benefit is there even if the array is wired 
at the same nominal voltage as the array.
 
If wiring the array for a higher voltage, be sure to look at the open 
circuit voltage of the array corrected for the coldest temp expected at 
the site of installation, so that the upper voltage limits are not 
exceeded. 

I would not recommend having the batteries at 24 volts unless you have 
some 24 volt loads you don't want to replace. The reason is that the 
MX60 is a current rated device, so you can get a much "bigger" regulator 
out of an MX60 when it is wired to a 48 volt battery. (60A x 24V = 1440 
watts vs. 60A x 48V = 2880 watts).

Bruce Roush
Solar Depot, Petaluma


cfreitas at outbackpower.com wrote:
> 
> Welcome back to the off-grid world Jim,  
> 
> There really isn't any difference in the installation requirements for a
> 12, 24 or 48 vdc system these days other than the ampacity of the
> conductors.  Most wire, breakers, etc have maximum voltage ratings that
> are well above the voltages encountered in a 12, 24 of 48 vdc system.
> The 50 vdc limitation is rarely utilized by anyone to their benefit
> these days - and when it was previously it was probably being done
> incorrectly.  
> 
> One change to be aware of is that using multiple different voltages on
> the same system is now pretty common - for example wiring the PV array
> for a higher nominal voltage (such as 48, 60 or even 72 vdc) but the
> battery staying at 24 vdc.  This improves the performance and lower the
> total cost in many situations.  It also means you don't need to wrestle
> with such large wires from the PV array.  
> 
> The "non-load break disconnects" I think is a reference to bolted
> connections or Anderson type connectors for servicing the battery or
> system. Not a big deal - you still need a real load break rated DC
> disconnect for the battery in residential/commercial installations (but
> not RVs or boats ?!?)
> 
> The other issue is that ground fault protection (GFP) is now readily
> available and must be included on roof-top mounted PV arrays on houses.
> This was always required but was not often enforced previously as there
> was not listed hardware available to meet this requirement. Now days its
> easy to include and is worth doing.    
> 
> Christopher Freitas
> OutBack Power Systems
> cfreitas at outbackpower.com
> 19009 62nd Ave NE 
> Arlington WA 98223 USA 
> Tel 360 435 6030
> Fax 360 435 6019
> www.outbackpower.com 
> 
> 
> >> -----Original Message-----
> >> From: Jim Duncan [mailto:ntrei at earthlink.net] 
> >> Sent: Thursday, February 24, 2005 2:08 PM
> >> To: RE-wrenches at topica.com
> >> Subject: Off-grid update... [RE-wrenches]
> >> 
> >> 
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> >> 
> >> Wrenches, 
> >> I'm planning my first off-grid install since the late '90s 
> >> and I know a 
> >> lot has changed both equipment wise and with the NEC. 
> >> I thought that since a 48V system was considered being able 
> >> to operate 
> >> at above 50V so was considered not low voltage. Now the NEC 
> >> refers to 
> >> (24) 2V cells the same as a 12 or 24 V system. Is there no 
> >> difference in 
> >> the equipment, disconnect rating, ampacities, sizing etc?
> >> Also what are some examples of 690.71 "Non-load break bolted 
> >> or plug in 
> >> disconnects..." in the battery container? 
> >> Thanks as always
> >> Jim Duncan
> >> North Texas Renewable Energy Inc
> >> Fort Worth, Texas
> >> ntrei at earthlink.net
> >> 817.917.0527
> >> **************
> >> There is one thing stronger than all the armies in the 
> >> world, and that is an idea whose time has come. Victor Hugo
> >> 
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