IEEE Power & Energy Magazine - November/December 2021 - 39

have operational strategies that reflect synergies from each
technology's unique strengths, the relative shares of which
will depend on the mix that provides the greatest value. In
other words, the relative size of each constituent part can be
tuned to maximize net benefits, and it is common for the sum
of the sizes of the parts to exceed the maximum amount of
power that can be injected into the grid at any given time.
Thus, the optimal sizing and use of the parts may differ
through time in both the optimal initial design and future
refurbishments to better align with evolving markets and
value streams.
Additionally, physical considerations for hybrid resources
depend on the point of coupling with the grid. By sharing
transmission infrastructure, hybrid resources can reduce
total project costs (through joint interconnection facilities
and transformers), increase the ease and speed with
which the systems are integrated,
and improve efficiency (Figure 1).
In many market regions, the interconnection
queue position is a valuable
resource. Interconnection study
processes impose significant delays,
and upgrades are expensive, making
it difficult to build multiple, separate
projects in a timely manner.
Hybrid resources that include
variable RE and storage technologies
can actively optimize their transmission
utilization through coordination.
RE production that would otherwise
be curtailed due to transmission limitations
can be harnessed to charge
the storage. With PV panels becoming
more inexpensive, additional
solar capacity may also be designed
into hybrid resources (increasing the
dc/ac ratio, meaning PV panels can
produce more power than inverters
can deliver to the grid). This can enable
hybrid resources to charge the
storage with energy that is internal to
them (Figure 2).
Shared transmission infrastructure
is the dominant point of coupling
for " ac-coupled " solar-plusstorage
hybrid resources (which
use separate PVs and battery inverters),
but the resources can also
benefit from sharing a physical site
and communications equipment
(Figure 3). Such a configuration is
flexible in the design, operations,
and maintenance of a plant. It can
involve joint market participation,
metering, and telemetering, and
november/december 2021
Production With
Low dc/ac Ratio
PV
Inverter
AC Grid
Battery
DC-dc
Charge
Controller
(a)
DC Bus
(b)
figure 1. Examples of hybrid resources with shared
transmission and interconnection infrastructure: (a) a " dccoupled "
design, where the PVs and storage share inverters,
and (b) an " ac-coupled " design, where PVs and storage
have separate inverters.
Clipped Energy From
High-dc/ac System
ac Injection Limit
Additional Shoulder
Production With
High dc/ac Ratio
Clipped Energy
Recovered Using a
dc-Coupled Battery
0246 810121416182022
Hour of Day
figure 2. Charging storage with a hybrid resource's clipped energy.
Babcock Ranch Storage: 10 MW/4 h
Tie-In Location to
Existing Solar
Storage Equipment Area
figure 3. An ac-coupled project where the storage and PV solar have their own
inverters. The ac-coupled integration of storage into solar is relatively simple, as the
storage equipment can be placed adjacent to the solar collection sub, with little
impact on operations. (Source: NextEra Energy; used with permission.)
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IEEE Power & Energy Magazine - November/December 2021

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