IEEE Power & Energy Magazine - September/October 2017 - 66

As long as the battery is charged by the time the owner arrives
for the daily commute, why not let an EV earn some money
while it is sitting idle?
energy storage resources will need to review their energy and
power relationship to improve regulation performance.

Integrating Renewables:
What Is the Role for Energy Storage?
The last decade has seen tremendous growth in wind and
solar generation in response to significant cost reductions,
customer demand for clean energy, and favorable tax incentives and other public policies. While increasing the amount
of renewables on the grid is generally seen as a good thing, the
variability of wind and solar generation can pose challenges
to grid operation, including impacts on controlling frequency
and voltage fluctuations that can affect grid stability, new
sources of forecasting error and stochasticity, increased steepness of ramping events, as well as market impacts that affect
traditional resources used to supplement renewables.
To better understand these challenges, PJM commissioned a 2012 study to evaluate the specific impacts associated with a large portion of system energy generated from
renewables. The study's main conclusion was that the PJM
system, with adequate transmission expansion (up to US$13.7
billion) and additional regulation reserves (up to an additional 1,500 MW), would not have any significant reliability
issues operating with up to 30% of its energy provided by
wind and solar generation. The study found that PJM's large
geographic footprint provides significant benefit for integrating wind and solar generation because it greatly reduces the
magnitude of variability-related challenges. The study made
some recommendations and suggestions:
✔✔ determine reserve requirements based on forecasted
levels of wind and solar production
✔✔ improve intraday unit commitment
✔✔ improve resource flexibility
✔✔ operate active power controls on wind and solar plants.
Storage technologies can help grid operators address
some of these challenges, specifically improving resource
flexibility and implementing reliability services traditionally
provided by central station steam generators. As an independent grid operator and competitive wholesale market operator, PJM is seeking to properly define and value these services so that energy storage has the ability to provide them in
the same or similar manner as traditional resources.
When storage is colocated or even electrically coupled
with wind or solar, it can help to make wind and solar more
dispatchable and economical by reducing curtailments and
providing services that wind or solar couldn't provide on their
own. For example, an innovative solar and storage generator at
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Hopewell Valley Central High School in New Jersey participates in the PJM energy market (solar) and the regulation market (storage). By combining an 876-kW rooftop and parking
lot solar canopy system with a 1-MW/580-kWh lithium-ion
battery, the system can provide more value than just energy
revenue. During normal operation, the output from the unit is
sold in the PJM energy markets. The battery modulates output
power to also participate in the PJM regulation market, including at night when the solar is not generating. In addition, the
combined resource has the ability to provide backup power to
the high school during an emergency situation or other grid
outage. This application benefits to community as the school
will serve as a place of refuge during emergencies. It is among
the first fully integrated combined solar and storage units to
participate in PJM markets.

Challenges: Where Are We
Going from Here?
FERC issued a notice of proposed rule making (NOPR) in
November 2016 to remove barriers for energy storage re--
sources to participate in wholesale markets. This NOPR, and
the industry comments that followed, highlighted some of the
key challenges and opportunities related to the integration of
energy storage resources in organized wholesale electricity
markets. We highlight a few here.

Modeling
Energy storage resources have complex characteristics. They
can act as generation, load, or transmission; they can ramp
up or down extremely fast; they have no limit on number of
starts per day, no minimum loading requirement, and very
minimal start-up cost; they are energy limited and runtime
depends on a state charge.
To recognize these characteristics, new "storage" models
are desirable to accurately reflect the physical operational
characteristics of energy storage devices. This could include
parameters such as state of charge, upper and lower charge
limits, and energy charge and discharge rate. Advanced storage modeling inevitably poses challenges to the complexity and computational solve time of the security constrained
unit commitment process of ISOs/RTOs. Nevertheless, as
more and more storage connects to the grid, these models
will be critical for optimizing the dispatch of the system.

Economic Valuation
As the primary grid-scale bulk storage technology, pumped
storage hydro often requires a long lead time to build, as long as
september/october 2017

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - September/October 2017