Hybrid PV/Battery/Generator [RE-wrenches]

[Joel Davidson]

If the purpose of the PV system is to offset utility power consumption, then
use a battery-less PV system. I'm not saying this because John Berdner is right
(he is). I have lived with an older SW4048 inverter on my grid tie PV system
with battery backup for 4 years. The system works fine (that's installer lingo
for no callbacks).

Define your UPS needs and serve them separately. I am still a strong believer
in separating zero-time power needs (computer, security, life support, phasers,
etc.) from other needs (lights, refrigerator, well pump, TV unless something
good is on, etc.)

I don't like combustion generators automatically turning on and off. We've all
heard stories about generators running without oil, cars with auto-start warmup
in Minnesota starting up in gear and driving through the wall, etc. The test of
a good UPS is that the least technical adults (or precocious pre-teens) should
be able to turn on lights, heat, water and communications during a power outage
in the dark without hurting themselves. A few well-marked switches are
appreciated.
Switch #1 - Turn this swith on first.
Switch #2 - Turn this switch on after turning on Switch #1.
Etc.


Antony Tersol wrote:

> I have a question regarding possible configurations for a large
> residential power system.  Following is description of situation and my
> analysis thus far.  Any comments, feedback, experience with such a
> system, generator/inverter recommendations?
>
> Thanks, Antony
>
> We have a residential client interested in a PV system who already
> decided on having generator backup as well.  They have critical loads
> that they want always provided, and power occasionally is out for days
> when trees take out the lines (they are also concerned about possible
> longer-term outages - earthquake or other disasters taking out regional
> power).  It seems logical to use batteries with a generator to buffer
> the demand.  If the batteries run low, the engine generator runs at full
> power—its most cost- and fuel-efficient mode of operation—until they are
> charged.  Without batteries, the generator will be running continuously
> at night even if the only load is extremely small (that blinking 12:00
> on the vcr clock that no one ever set).
>
> 1. If we go with straight grid-tie batteryless PV, we would need a
> transfer switch downstream, so that the PV wouldn't be running in outage
> situation when generator is on.
>
> grid------panel------transfer switch--------------main panel-----load
>             |           |
>         inverter    inverter
>             |       |      |
>            PV   battery    generator
>
>
>
> advantages: a. relative simplicity
>         b. generator can power the entire main panel
>         c. PV doesn't take efficiency hit from batteries
> disadvantages: a. PV useless when power is out
>         b. redundant inverters?
>
> 2. If we use PV with battery, and multiple small inverters (~5kw):
>
> grid--------------main panel---------inverter-------sub panel------load
>                         |            |     |
>                         |       battery   generator
>                         |            |
>                         |           PV
>                         |
>                         |------------inverter-------sub panel------load
>                         |            |     |
>                         |       battery   generator
>                         |            |
>                         |           PV
>                         |
>                        etc.
>
> advantages: a. PV functional during outages, minimizing generator needs
>         b. redundancy, so that even if one inverter fails, still have power
>         c. using same inverters as likely in other smaller systems, so our
> technical knowledge better leveraged (better for us and the client)
> disadvantages: a. increased complexity - need sub-panels and need to
> feed generator into each inverter
>         b. loss of efficiency because of battery losses
>
> 3. If we use PV with battery, and one large inverter (10 to 20 kw):
>
> grid--------------main panel---------inverter-------sub panel-----load
>                       |              |     |
>                       |           battery   generator
>                    sub panels       |
>                                    PV
>
>
> advantages: a. PV functional during outages, minimizing generator needs
>
> disadvantages: a. loss of efficiency because of battery losses
>         b. no redundancy, if inverter fails, no power
>         c. large inverter unique to large systems, so our experience and
> knowledge limited.
>         d. only sub panel connected to inverter has backup power
>
> Parameters:
>
>         House will have 400 amp service
>         Typical load expected to be 40-70 amps (@ 120v), 80-100 amps max
>         PV system 10-20 kwp
>
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