Hybrid PV/Battery/Generator [RE-wrenches]

[Antony Tersol]

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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