Advise sought for large off grid PV system; 480 volt 3-phase [RE-wrenches]
Ray Walters
walters at taosnet.com
Wed Mar 28 17:56:53 PDT 2007
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HI Larry;
It always helps for us to have enough info on the project. 500 kwh/
day is unbelievable for a house, but makes sense for 50 to 100
houses. Air conditioning is tough on solar, I've used a propane fired
ammonia type air conditioner (Lennox) off grid to cut power. I would
definitely be looking at evaporative cooling, ceiling fans, and
aggressively limiting solar gain in the summer (especially to the
west). Also, I lived for years in Austin, Tx with 100 plus
temperatures and high humidity the "old way": fans ,screened windows,
and big shade trees. Just going to evap. cooling would cut summer
loads by 75%.
According to my old Siemens Design guide, for over 20 Kwh/ day it is
more economical to go to a diesel- hybrid system, and at 250 Kwh/ day
or more, all diesel (no solar) is the best choice. I too have
experience in village scale work (mostly water pumping in Morocco)
but full refrigerated type air conditioning really needs to be
thought through, especially in a desert climate where evap cooling
works well for far less energy.
I would consider sizing the battery for just one to two days of
storage since it will be a hybrid system with auto start. Thats still
going to be an enormous battery bank. I would consider a
distributed system like the sunny island with smaller battery banks
and inverters at each building "grid tieing" to the main system. That
may allow you to drop the voltage and use regular inverters for a lot
of the system without taking a hit on voltage drop. I looked into big
480 3 phase inverters before, and only found grid tie, no off grid
stuff. You definitely want to check with Outback, they're stuff can
be setup in 3 phase, I just don't know about 480, but transformers
can deal with that. As one design option, I would think about
reducing the voltage on the distribution system, go with multiple
intertied systems, and step the generator 480 down to charge the
batteries through off grid inverters. Reduce those Air Con surge
loads, and you probably don't need that 480. Those gennys may even be
reconfigurable to a lower voltage.
Obviously a project of this scope needs serious engineering: starting
with load reduction. I'm with Todd, unless its a village of over 100
people thats just too much juice being used. Air Conditioning is not
essential tp life, ask most of the rest of the world living without it.
R. Walters
SolarRay.com
US Peace Corps, 88 -90
On Mar 27, 2007, at 5:58 PM, Walt Ratterman wrote:
>
> Larry:
>
> Walt Ratterman here. I might be able to give you some assistance,
> but I
> don’t want to hold you up either. I do a lot of this kind of work,
> but
> generally it is overseas. Specifically, we are doing fairly large
> solar /
> diesel hybrid systems in Rwanda to provide electricity for
> laboratories and
> emergency lighting, and water pumping.
>
> I am getting ready to get on a plane in the morning to Burma to
> work on some
> much small PV systems for clinics there, but I will have email on
> and off
> for a bit. If you still need assistance after the others respond,
> let me
> know.
>
> Walt Ratterman
> wratterman at sunenergypower.com
>
>
>
> -----Original Message-----
> From: larry crutcher [mailto:larry at starlightsolar.com]
> Sent: Tuesday, March 27, 2007 1:59 PM
> To: RE-wrenches at topica.com
> Subject: Advise sought for large off grid PV system; 480 volt 3-phase
> [RE-wrenches]
>
>
> RE-Wrenches,
>
> I am designing an off grid PV system and am looking for input and
> recommendations from experienced installers. Although I have years of
> building battery based systems, this is a big one. The customer has
> been
> running 60kW diesel generators 24/7 for about 2 decades. They want
> a PV
> system with batteries that will allow the generators to be shut down
> during the winter and low load periods. Summer loads can be above
> 500kWhs per day but winter drops to less than 100kWhs which I can do
> with PV's.
>
> Power details: The site has 480 volt, 3-phase Y distribution and 6
> transformers. There are only two 480 volt distribution circuits
> leaving
> the generator building. I have failed to locate power equipment that
> will produce 480 volt 3 phase for off grid use. Telecom equipment does
> not look suitable. I have found inverters to produce 60 kW @ 208
> volt 3
> phase (Exeltech) and a 75 kva step up transformer for 480Y. I don’t
> like
> the transformer idea. The generator must be available at any time to
> take over loads. I am thinking of transfer switching @ 480 volts. I
> really would like automation for generators, battery charging, and
> transfer switches.
>
> One generator is scheduled for rebuild and it will have auto-start
> added
> to it. The site has room for several large ground mount arrays of PV.
> Wind power does not look viable, < 5 m/s @ 30 meters. I am in a desert
> location that has very high ambient temperatures above 115°. A
> building
> for the batteries and inverters will be climate controlled to 80° F.
>
> So, how would you propose a solution for this type of site? Do you
> know
> where I can get 480 3-phase inverters?
>
> Next post I'll ask about batteries, BIG batteries. No, even bigger!
>
> Larry Crutcher
> Starlight Solar
>
>
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R. Walters
Solarray.com
NABCEP # 04170442
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