Using grid tie inverter with backup system
Norman Webb
inverter needs a supply voltage in order to operate so when the grid power
goes down you are left with all that power on the roof with no where to go.
If you have a back up power supply (either petrol generator or
solar/inverter) will the grid tie sync to these type of power supplies.
The main power grid can soak up any power generated by your grid tie system
but what about a small generator??
My electronics engineering 101 has long since left me, but my guess is that
the two systems should sync.
Any thoughts out there?
Norman Webb
wmbjk...@citlink.net
Russ in San Diego
I don't think this plan will work out very well. It's possible that
some pure non-battery grid-tie inverters might eventually synch to a
generator (assuming a VERY stable generator), but this might not do
good things for that generator, particularly if your PV system is
producing more power than your loads are consuming. The inverter will
try to push the excess out to "the grid", which is just your
generator. I'm not sure, but I suspect the generator might not like
having its "load" provide a higher voltage than it is trying to
produce.
There are grid-interactive inverter/chargers out there, from Xantrex
and others, that can do what you want -- but you will need batteries
-- and probably not a whole lot of batteries, either. I've got a
Xantrex SW-4048 running to a bank of 24 T-105 batteries, but you could
cut that down to 8 (or fewer, if you use higher voltage batteries, or
a lower DC voltage inverter). The SW will happily push excess to the
grid, or it will just charge the batteries if the grid is down. It
can even automatically fire up a backup generator (if you've got one)
when necessary.
Ron Rosenfeld
<russ...@gmail.com> wrote:
>Xantrex SW-4048 running to a bank of 24 T-105 batteries, but you could
>cut that down to 8 (or fewer, if you use higher voltage batteries,
or same voltage but higher capacity batteries.
In your system, you could use something like 8 Surrette 6CS21P's and have
similar capacity, better tolerance to deep discharges, and only have to
maintain a single string instead of three strings.
Of course, the Surrette's will cost you over $8,000 whereas you can
probably get 24 of the T105's for less than 1/2 that (or less if you are
getting them surplus, used). But at some point in your life, the decreased
workload of the Surrette's (fewer cells to maintain, better longevity) may
make the extra cost worthwhile.
> or
>a lower DC voltage inverter). The SW will happily push excess to the
>grid, or it will just charge the batteries if the grid is down. It
>can even automatically fire up a backup generator (if you've got one)
>when necessary.
--ron
Russ in San Diego
> On Thu, 17 Jul 2008 09:43:42 -0700 (PDT), Russ in San Diego
>
The OP's question was about being able to continue operating his grid-
tie inverter when the grid is down.
I was trying to provide a minimal cost solution for him. Thus, the
suggestion about using fewer batteries -- all he needs is a battery
bank that can provide /some/ capacity at the requisite voltage. It
would only give him a brief period of backup power, but would be
adequate to keep operating without the grid, while the sun is
shining.
As for me, my T-105 battery bank has worked very nicely over the last
7 years. I rarely dip into it. It was sized to allow me disconnect
from the grid long-term, if necessary -- but I've remained connected
the whole time. The batteries only provide me with power during the
occasional grid disruption at night -- or allow my PV system to
continue operating during daytime disruptions.
Ron Rosenfeld
<russ...@gmail.com> wrote:
>The OP's question was about being able to continue operating his grid-
>tie inverter when the grid is down.
>
>I was trying to provide a minimal cost solution for him. Thus, the
>suggestion about using fewer batteries -- all he needs is a battery
>bank that can provide /some/ capacity at the requisite voltage. It
>would only give him a brief period of backup power, but would be
>adequate to keep operating without the grid, while the sun is
>shining.
>
>As for me, my T-105 battery bank has worked very nicely over the last
>7 years. I rarely dip into it. It was sized to allow me disconnect
>from the grid long-term, if necessary -- but I've remained connected
>the whole time. The batteries only provide me with power during the
>occasional grid disruption at night -- or allow my PV system to
>continue operating during daytime disruptions.
Hey, if your T105's are working well for you, that's fine. And I'd agree
they might be appropriate for light usage during a power outage if
sufficient solar resource will be available.
But so far as the OP's request for backup power during a grid outage, I
don't think either of us can make an intelligent recommendation without
more information. Some of the things we don't know are power usage, length
of blackout, weather conditions at the time (is the sun likely to be
shining during the blackout), how long a blackout he wants to prepare for,
etc.
For example, we have a 2nd home in NH where we spend a good deal of the
winter. A winter power outage might be due to widespread blizzard and
icing conditions and might last four or five days or longer. Depending on
how many of our kids are visiting at the time, we might be using as much as
50 kWh/day! And this would be at a time when the solar resource is
virutally nil. Battery backup with solar panels is not very practical in
that situation.
--ron
Ed
>
> But so far as the OP's request for backup power during a grid outage, I
> don't think either of us can make an intelligent recommendation without
> more information. Some of the things we don't know are power usage,
> length
> of blackout, weather conditions at the time (is the sun likely to be
> shining during the blackout), how long a blackout he wants to prepare for,
> etc.
>
> For example, we have a 2nd home in NH where we spend a good deal of the
> winter. A winter power outage might be due to widespread blizzard and
> icing conditions and might last four or five days or longer. Depending on
> how many of our kids are visiting at the time, we might be using as much
> as
> 50 kWh/day! And this would be at a time when the solar resource is
> virutally nil. Battery backup with solar panels is not very practical in
> that situation.
> --ron
Try looking at it from a different angle... Grid tied with battery backup
only.....
hook up the 24 hours needed circuits to the auto switch over circuitry
(fridge, a few outlets, computer, etc)
As a back up for longer use... hook up the generator to the other circuits
you may need while you are home (early evening stuff... hot plates, hot
water if youdon't have solar thermal, etc with another set of switch over
circuits) Also hook it up to a battery charger that can charge your solar
batteries if you don't get enough sun.
Hooking up a generator directly to grid tied inverter will not be a good
idea for 2 reasons... one: the freq will not be stable enough and worse,
generators don't like to be fed excess power
Ron Rosenfeld
You probably should be addressing this to the OP. For our setup in NH,
we'll just use a propane generator for backup. We don't even have the
necessary exposure to set up for grid-tied PV (or the space/zoning/etc for
wind).
In our Maine home, everything runs from PV and/or wind.
--ron
busdweller
Do a google for a unit called Sunny Island I have'nt installed one yet
but im sure it on one of my future jobs.
Peace along the way
Dennis the bus dweller N.Y.
j...@nrgmanager.com
recently we installed just what you are thinking of (i think). in the
caribbean power generation goes offline frequently and the businesses
need to stay online. so a battery backup works fine. rule of thumb
outback is good for 4000watts or less and then xantrex does well for
above that. if you need a really big system you can parrallel these
units. at any rate when the power is on the batteries get charged,
then when the power drops off the batteries are routed to a subpanel
that power the critical loads. critical is key, because when u go to
bats you can drain them real quickly. power outages in the states last
say 5 minutes in the islands they last 30mins to hours so the setup we
just did is set up for 3 hours of runtime for computers phones fax and
lights&fans, basically keeps them in business.
and lastly always make sure you set up for a possible future renewable
upgrade, ie wind/solar or a non renewable generator this way if the
power company drops offline for days you just plug in that upgrade.