Large residential system [RE-wrenches]
Darryl Thayer
daryl_solar at yahoo.com
Tue Dec 9 23:25:00 PST 2003
Hello, I have wanted to give my solution since you
posted the problem however I have been to busy. I
wish I had the job, I like these odd ball projects.
Here is what I would do.
Inverters
I would get rid of the SWs and replace with a Outback
Quad stack VFX3648. I would use the stack to power
both panels. In doing this I would reduce inverter
parasitic losses and increase the start current. ( I
am currently starting a 130 amp 240 volt surge with an
Outback Quad stack). Plan this with a Hub 10 so that
if the load exceeds the 14,400 watts you can simply
add inverters to meet the load. If it is deemed that
a larger system is needed the Outback stack can be
increased to 8 inverters with confidence. This is up
to a 28.8 kW system that under low power acts like a
single inverter in idle.
(Note Outback may be able to go to 12 inverters using
two Hubs)
Generator
The generator is key in this process, because the RE
system can’t be meet the peak demand for very long. I
would change the generator to one of those from Canada
that gives 48 nominal volts DC which is capable of
battery charging and as battery replacement. Such
that if the load is large enough the generator will
pick up the DC load and the inverters will process
this load. I would size this generator to carry at
least 7.5 kW for the heat pump, and another 5 kW for
hot tub and other pumps, and 7.5 kW for the
miscellaneous residential load. ( One of my
customers has a large house and has many parties, he
has a non cooking, non heating or cooling load of 5 to
7.5 kW) This is then a 20 to 25 kW Diesel generator
with as I said 48 volt nominal DC output.
Solar array
I would increase the solar array to 10 kW. This would
allow powering the heat pump from solar and no
generator in the swing seasons. I would look at the
average full sun hours and the typical load. For
example, assume the RE system is to pick up the heat
pump for up to 25% operation or 6 hrs per day, or for
5kW for a 5 ton heat pump is 30 kWh per day. If
there are three full sun hours per day then 10 kW of
solar gives 30 kWh per day.
Batteries
The battery system should have a least about two day
capacity or 60 kWh at 33 % DoD or 180 kWh total
capacity at 48 volts or 3.6 kAh of capacity. I think
you are stuck with flooded cells, and a maintenance
person needs to check these regularly. I have had
good luck with C&D batteries for larger loads, also
Surrette. Yes, do this with large 2-volt cells. I
think the sealed batteries are great for occasional
cycling, never in parallel. Your system sound like
the batteries are cycling batteries and need to be
flooded. The amount of cycling I envision for your
system you could use either lead calcium or lead
antimony. The lead calcium would use far less water
than the lead antimony.
--- Gary Higbee <gary at windstreamsolar.com> wrote:
> Wrenches,
>
> I've taken on straightening out a residential system
> I did not design or
> install, and would appreciate your thoughts. The key
> thing here is not money
> so much as very solid performance. The owners want a
> hands-off system and it
> must be very reliable. Show up, turn everything on,
> and no problems. Sure,
> there can be maintenance contracts, but we want
> things to work very well.
>
> THE HOUSE
> This is a high-end off-grid place on a square mile
> of land in a warmish part
> of the Cascade range in Oregon. The house is
> periodically visited, and when
> the owners are there the loads can be very
> high--they occasionally entertain
> dozens to hundreds of guests. Cooking and water
> heating are propane, but
> there's a 5 ton heat pump that is run in the summer,
> the well pump, a hot
> tub circulating system, sewage pump, and plenty of
> assorted house
> loads--lots of CF lights (yea!), TV's, satellite
> internet, radio telephone,
> computer and such.
>
> THE EXISTING SYSTEM
> Currently there are two SW4024's, a pitifully small
> (400 Ah) and fried
> battery bank, a 40kW (gasp, gross overkill) diesel
> generator, 2.5kW of PV,
> and an incomplete Harris PM unit.
>
> LOADS
> Other than not running the heat pump do any of you
> know how to reduce the
> startup surge of the thing? I did measurements
> yesterday, and had a start
> surge of 55 amps per leg and 20 amps per leg
> running. I know we're starting
> a compressor here, so maybe there's not a whole lot
> that can be done to
> soften the start...
>
> BATTERIES
> We do plan to install a much larger battery bank. It
> appears that
> maintenance-free is quite important (unless these
> folks want to contract
> someone to check and fill flooded cells). OK, so
> what is the best large
> sealed cell we can get? I don't want a bunch of
> parallel strings, so we need
> big or low-voltage cells. This is a big one, as I've
> got to get this
> component ordered in the next few days--in
> preparation for a several-hundred
> person gathering in a couple weeks.
>
> INVERTERS
> Though the loads are generally fairly low, when the
> owners are at the house
> they can skyrocket. There are two separate AC load
> panels, and currently the
> dual SW inverters feed both. We have separate feeds
> from the power shed to
> each panel. I contemplate adding another two SW's
> and breaking out the main
> panels so we feed each panel from each set of two
> SW's. In this
> configuration we'd be running two stacked sets of
> SW's (a total of four
> SW's) from the same large battery bank. Conversely
> we could dump the current
> SW's and go with a large Outback system, or use the
> existing SW's and add a
> set of the new SW plus units.We could also keep the
> existing SW's, and add
> four Outback units for the second panel, though I'm
> concerned about mixing
> inverters on the same battery bank. Thoughts?
>
> GENERATOR CONTROL
> We're using what I think is a repackaged GenMate,
> that used to be sold by
> Wrico (a manufacturer of generator systems).
> Currently the unit has
> intermittent reset problems, which means that if the
> generator does not
> start on the first round the controller doesn't
> respond to the inverter's
> request to give it another go. I have verified that
> the restart command is
> making it out of the SW, and that the controller is
> not responding to a
> second try some of the time. I'm told that this unit
> is no longer
> manufactured, and that it is also very reliable. The
> contractor here tells
> me to "just fix it." I have other generator control
> units out there on the
> same brand of generator, and they work fine. I also
> have several Wrico
> generators without the control units (i.e., directly
> inverter-driven), and
> they work fine, too. Any particulars on generator
> control units?
>
> GENERATOR
> I'd love to see the 40kW diesel unit leave, and be
> replaced by something
> that makes sense for this system--maybe in the 12kW
> range. This big machine
> seems very out of line. Any of you in the West have
> a commercial prospect
> that could use such a generator? This is a
> longer-term issue, as the above
> are critical items to the operation of the house.
>
> SUMMARY
> This is an interesting system, which I have
> inherited. I want so much for
> things to work well, and greatly appreciate and
> value your wisdom.
>
> Thank you!
>
> Gary
>
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Gary Higbee (gary at windstreamsolar.com)
> (541 )607-1818 (Eugene)
> (541) 954-3881 (Cell)
> Solar, wind, and hydro site analysis and system
> design
> Components dealer and installation assistance
> Energy Trust of Oregon contracted system inspector
> ~ WindStream Solar (www.windstreamsolar.com) ~
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
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