IEEE Power & Energy Magazine - July/August 2018 - 81

regional coordination to increase the diversity of load and
generation can be one of the most cost-effective ways to integrate renewables by reducing relative forecast errors, thermal
july/august 2018

Load

600
400
200

Dec.

Nov.

Oct.

Sept.

July

Aug.

June

0
Apr.

Seasonal Energy Imbalance Extends
Across a Large Geographic Area

Nondispatchable Generation
800

May

that the production patterns for wind and solar generally do
not match load on a diurnal basis is a well-understood problem
that can be addressed in many ways, including generator ramping, load shifting, electricity trading, and energy storage. not
as well understood is that, in very high renewable systems, systemic overgeneration will occur in some seasons and systemic
energy deficit in other seasons. for the united states, the former will occur in spring and fall and the latter in summer and
winter, as illustrated in figure 2. particularly in hydro-wind
dominated systems, even longer timescale imbalance, including interannual, can also be observed.
solving seasonal energy imbalance is arguably the main
impediment to reaching aspirational 100% renewable energy
goals called for by localities across the united states. seasonal energy imbalance is a multifaceted challenge and will
require some mix of dispatchable generators, long duration
storage, seasonal flexible load (e.g., electric fuel production),
and overbuilding and curtailing renewables. Many of the most
widely discussed solutions for integrating wind and solar, for
example, flexible loads in buildings, flexible vehicle charging,
and battery electric storage, will be ineffective for balancing
on seasonal timescales. in the scenario shown in figure 2,
electric fuel production in spring and fall plus limited thermal
generation in summer and winter are used to solve this imbalance, in addition to seasonal renewable curtailment.

overgeneration-driven renewable curtailment occurs when
nondispatchable and must-run generation exceeds load,
including storage. the quantity of curtailment that a system
experiences is highly nonlinear, and as the amount of solar
and wind on the system approaches the levels required in
deeply decarbonized systems, is projected to be significant,
as shown in figure 3.
resources that provide flexibility can derive value from
arbitraging between periods with and without curtailment;
however, diminishing returns and a long-tailed distribution of
hours with substantial curtailment make it certain that some
amount of curtailment will always be economical. ultimately,
the most cost-effective system design and the level of resulting
curtailment will depend on the relative costs between adding
additional renewable capacity versus adding resources that
provide flexibility. the lower the cost of renewables and the
higher the cost of flexible resources, such as energy storage,
the higher the optimal level of curtailment.
the u.s. ddpp high-renewables scenario, with 80%+
renewables by energy, results in curtailment in 20-40% of all
hours, depending on location. actual curtailment may be less
than these levels due to new types of loads currently unanticipated that can take advantage of electricity that is zero cost

Mar.

Energy Imbalance Has a Long Timescale
in High Renewable Systems

Renewable Curtailment Occurs
Across Many Hours

Jan.

for most of the united states today, summer electric loads
are strongly temperature dependent above 65 ˚f due to airconditioning. But below 65 ˚f, load today is only weakly coupled to temperature, except where electric heating is already
common. if a much larger share of heating across the united
states is electrified, load will have a strong dependence on
temperature across the entire range, and many more utilities
will find themselves with both summer and winter peaks.
this is already evident in places like florida with large airconditioning load but also infrequent cold snaps and high
penetration of electric resistance heating.

Feb.

New Loads Will Change
Seasonal Load Profiles of Many Utilities

ramping, and round-trip efficiency losses from storage. however, increased regional coordination will not greatly diminish the fundamental drivers of seasonal energy imbalance
common to most of the united states for example, across
north america, spring is characterized by low loads due to
moderate temperatures, while solar insolation grows with the
approach of the summer solstice, higher pressure differentials in temperate regions drive high winds, and spring snowmelt produces high hydro. therefore, regional coordination
(in the absence of a globally connected grid) can only address
seasonal imbalance to a degree.

TWh

electric vehicles in the hundreds of kilowatts or up to 1 MW for
heavy-duty transportation will be common. in terms of instantaneous demand, these power demands are more concentrated
than all but large industrial loads and will have large locational
impacts on transmission and distribution systems. Without the
development of real-time vehicle-grid communications, the
exact timing of these loads will be difficult to predict.

figure 2. The firm load versus nondispatchable generation in 2050 by month for the U.S. DDPP high-renewables
scenario.
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - July/August 2018