Powering Rooftop Equipment with Rooftop Generation [RE-wrenches]

Tue Jul 27 23:22:59 PDT 2004

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Mark,

Need to make sure all the safeties are tied into the fan circuit after
the inverter. Typically, on anything larger than about 2000 cfm (code
varies) the fan needs to shut down on temp and smoke, and depending upon
climate, may want to shut down on freeze protection and others. So these
will all need to tied in after the A/B drive, not before it as they
probably are now.

So far as avoiding a hole, not a big deal. They probably bring the
conduit up through the curb (violating the ductwork, etc.), and do not
touch the roof already.

I can see where you probably would need a disconnecting means at some
location off the roof. A shunt trip would probably be acceptable in most
jurisdictions.

The A/B drives may also have a problem with the speed of change of the
DC source. Yes, I know the grid is back there, instantly. But my time
with an ABB drive/converted to an inverter/Charge controller taught me
that the PV DC side changes really fast, and that this throws off
typical modern VFD type equipment. Not saying it's a problem, but I'd
watch for it pretty closely.

Also need to look at what the minimum power draw is (could be about 1/12
of full, if it's a good variable speed system), and compare this to PV
input rating.

Good luck, sounds like fun.

Jeff
Jeffery D. Wolfe, P.E.
NABCEP Certified Solar PV Installer(tm)
Global Resource Options, LLP, Solar  Wind  Energy Solutions
A Woman-Owned Limited Liability Partnership
601 Old River Road, Suite 3, White River Junction, VT 05001-9030
Saratoga Technology + Energy Park, 10 Hermes Drive, Malta, NY 12020
800-374-4494 x107 Toll Free     802-295-4415 x107 Phone
802-295-4417 Fax
Jeff at GlobalResourceOptions.com,   www.GlobalResourceOptions.com

-----Original Message-----
From: Mark Robinson [mailto:Mark at TheEnergyGrid.com] 
Sent: Monday, July 26, 2004 4:24 PM
To: RE-wrenches at topica.com
Subject: Powering Rooftop Equipment with Rooftop Generation
[RE-wrenches]

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  I'm looking for a way to install PV systems in commercial
establishments (like grocery stores) that need to avoid roof
penetrations. I'm thinking of using PV to power rooftop equipment with
an Allen Bradley Motor Speed Controller and a Nextek box. I've got an
idea... If there are any holes in it (big enough to throw a wrench
through), I figure you guys will be happy to point them out. 

    The motor speed controller usually takes AC, rectifies it to DC,
then inverts it back to AC at variable frequency to a variable speed
motor. The motor speed controller increases efficiency of the motor
because it reduces power to it when full load is not needed (VAV
Blowers, manufacturing processes, etc).

    I'm connecting the solar DC and the AC that used to power the
equipment to the Nextek power supply. The Nextek power supply takes all
the solar DC available and couples it to the DC bus in the Allen Bradley
unit (bypassing the rectifier section).  At night, the Nextek supply
makes DC from the utility just as the Allen Bradley would have and
continues to support the A/B DC bus. 

    We get no roof penetrations (assuming no-penetration solar mount)
for wiring, increased efficiency on the roof, and a very easy system to
install.
Additionally, the rooftop power stays on in a daytime power failure
(plus battery backup could be added for a nighttime power failure). 

Possible problems...

    PV Shutoff - are we going to have to run a PV shutoff down the side
of the building? Would this reg apply where we're not supplying the
grid? 

Other problems/thoughts/ideas?

Mark Robinson

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