IEEE Power & Energy Magazine - November/December 2021 - 40

the storage is still able to qualify for the solar investment tax
credit. The U.S. tax credit does not require a direct electrical
connection from the solar to the storage. Showing that
energy used to charge the storage did not exceed the energy
generated by the solar component on an hourly basis is generally
sufficient, and this can be achieved in both ac- and
dc-coupled configurations.
Facilitating additional synergies may require sharing
more physical components, such as in " dc-coupled " hybrid
resources with PV and battery technologies sharing inverters.
Employing a shared inverter can reduce total equipment
costs and facilitate the capture of clipped energy for
battery charging. Shared inverters can be conventional
unidirectional PV devices (where the battery can charge only
from the coupled PVs) and bidirectional ones (where the battery
can charge from either the coupled PVs or the grid).
Because large PV plants have numerous inverters located
by each " PV string, " storage equipment must typically be
placed by each shared inverter (Figure 4). More details on
each inverter location is shown in Figure 5.
Sharing an inverter has additional complexity and difStorage
Equipment Adjacent
to Solar Inverters
ferent operational constraints, but these challenges may be
justified by the related synergies. With PV panels becoming
increasingly inexpensive, and with bidirectional inverters
now commonly available with little or no cost premium,
dc-coupled designs with bidirectional inverters may offer
efficiency advantages while still
enabling the storage to charge from
the grid when useful or necessary.
The dc-coupled design may more
easily facilitate capturing clipped
PV energy for charging the storage
technology (Figure 6). This could
also be done by oversizing PV inverters
in ac-coupled designs and using
the plant controller to allocate the
ac energy to both the grid and storage
charging.
Tie-In Location at
Multiple Solar Inverters
figure 4. A dc-coupled plant, showing the location of storage equipment. The
dc-coupled integration of storage into existing solar is more complex, as room must
be available to place the battery equipment adjacent to each solar inverter. (Source:
NextEra Energy; used with permission.)
Structural
Considerations:
Hybrid Offers and
Opportunity Costs
A hybrid resource is flexible,
typically with advanced software
and analytics, and it may provide
a set of offers for both energy
and ancillary services to market
operators. This enables an operator
to select services with the
highest value through a market's
co-optimization of energy and
ancillary services. In distinguishing
between hybrid and colocated
resources, an important question
is why a single combined offer
may be more valuable than individual
ones from each technology.
To be treated in a fair and nonInverter
and
Transformer
DC Converter Energy Storage
System Enclosure
PV Array Site Road
figure 5. The layout of one tie-in location within a solar-plus-storage plant, showing
the location of the energy storage system equipment. (Source: NextEra Energy; used
with permission.)
40
ieee power & energy magazine
discriminatory manner, a hybrid
resource would be responsible for
providing the services that a market
clears it to provide, performing
at a level comparable to that
of a conventional resource. The
analytics and offer strategies of
a hybrid resource will need to be
quite sophisticated to ensure this
november/december 2021
IEEE Power & Energy Magazine - November/December 2021

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