IEEE Power & Energy Magazine - November/December 2021 - 38

Many experts were initially perplexed by the rapidly
growing popularity of hybrid resources, which combine
multiple technologies into a single entity.
and innovate as systems change and new technologies
emerge. Admittedly, policy incentives may partially contribute
to the near-term popularity of hybrids among the
developers and customers of power plants, particularly in
the United States, where storage components can qualify
for solar investment tax credits when charged from solar
energy. However, there are many other factors behind the
increasing interest in hybrid resources. The focus here is on
the broader, long-term drivers of hybrid resources beyond
the influence of policy incentives.
The interest in hybrid resources reflects logical responses
that emerge from increasing complexity and risk. Such concepts
also appear and evolve in other industries and within
nature itself. They are motivated by practical considerations,
including the following:
✔ replicating known subsystems and design constructs
✔ logical preferences for robust risk mitigation strategies
in the face of a complicated and unknowable future
✔ important drivers toward decarbonization, electrification,
digitalization, and decentralization.
This article focuses on large-scale hybrid resources connecting
to the transmission system. However, the concepts
apply to other emerging resources, including microgrids,
aggregated distributed resources, various types of resource
portfolios, and the general concept of virtual power plants at
both the distribution and transmission levels. The evolutionary
pressure behind these trends is to collectively relieve
limiting factors and gain a competitive advantage by reducing
risk and creating upside opportunities, thereby increasing
the likelihood of long-term success and profitability.
These factors may be physical (sharing equipment), structural
(more easily satisfying rules and processes), financial
(insuring against uncontrollable impacts to projected value
streams), and technological (retaining flexibility and
optionality to alter power plants more easily). These categories
are not mutually exclusive but provide a useful structure
for our discussion.
Definition of a Hybrid Resource
While the term hybrid is often used loosely, we define
hybrid resources as power plants that 1) consist of multiple
energy technologies (generation, storage, and others) and 2)
are viewed and operated in a system (and a power market,
if applicable) as a single resource. This simple definition is
consistent with recent work by the California Independent
System Operator (ISO), which states that hybrid resources
are " a combination of multiple different generation tech38
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nologies that are physically and electronically controlled by
a single owner/operator and scheduling coordinator behind
a single point of interconnection that participates in the ISO
markets as a single resource and is optimized by the ISO in
the market as a single resource. " A similar definition is provided
by a forthcoming report from the U.S. Department of
Energy. This contrasts with the related concept of colocated
resources, which include 1) but not 2) and are separately
described by the California ISO as " a combination of multiple
different generation technologies behind a single point
of interconnection that participates in the ISO markets as
different resources. "
Hybrid resources can consist of many different technologies
and components, but recent and proposed installations
are dominated by projects that combine renewable energy
(RE) and storage resources. By adding storage-particularly
to solar plants in regions where high and growing solar penetrations
have shifted energy needs into the evening hours-
hybrid resources can retain higher capacity value than RE
components alone. They can also use the storage component
to shift energy and ancillary services to more valuable evening
periods while avoiding transmission-related hurdles for
such shifts. Projects combining batteries with solar photovoltaic
(PV) and wind resources are the most common, but
hydropower-plus-battery developments have recently been
deployed, as well. Such RE-plus-storage hybrids are sometimes
called near-firm renewables, and they are successfully
being commissioned as alternatives to new natural gas
power plants in some situations.
In an ideal case, hybrid resources are allowed to provide
services at the point of interconnection, with complete independence
in their operations, offers, innovations, modifications,
and upgrades. But can these attributes be accurately
valued, optimized, and modeled? In this article, we explore
these questions and, specifically, the practical issue of why
an owner/operator may prefer to be a hybrid resource rather
than multiple stand-alone ones. We close with a brief discussion
of how understanding hybrid resources and their contributions
toward innovation and the evolution of the grid and
market services may be critical stepping stones on the path
toward future energy systems.
Physical Considerations
Physical considerations for hybrid resources reflect the characteristics
of the constituent technologies as well as those
associated with their points of common coupling. Projects
that combine technologies with different characteristics will
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IEEE Power & Energy Magazine - November/December 2021

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