IEEE Power & Energy Magazine - November/December 2021 - 71

organization integration. More transmission results in lower
costs for customers and more access to lower-cost generation.
The studies are some of the first to evaluate how wind, solar,
storage, transmission, and DERs all play essential, unique, and
complementary roles in providing consumers with reliable and
affordable electricity. Wind and solar tend to produce at opposite
times, so they complement each other. However, the best
wind and solar resources are generally in different locations,
so transmission helps to aggregate them and deliver a reliable
mix of power to customers. Transmission also allows cancellation
of local weather-driven variations in wind and solar output
with opposite variations in other regions. This provides a more
constant supply of power.
The model builds energy storage to help meet reliability
needs, increase transmission capacity by absorbing excess
generation, and provide for deficiencies when wind and solar
output is low. DERs help control the peak electricity demand
required by the utility-scale grid. Together, along with some
flexible capacity resources that provide energy when needed,
these resources are forecast by the model to provide a reliable,
efficient, and clean portfolio.
The model builds transmission that allows efficient dispersion
of low-cost resources across a large market of customers.
This works to reduce the systemic nature of the volatility
of both supply and demand. This works even when DERs
are present because these resources contend with volatility
in supply. Figure 3 shows the installed generation capacity
for the two studies, indicating increased variable renewable
energy over time. Comparing Figure 2 with Figure 3 demonstrates
that, as variable renewable energy levels increase, the
model builds significant transmission to maintain reliability
and reduce cost.
Many of the same transmission upgrades were built across
all four scenarios of both studies, indicating that these investments
are likely needed regardless of future trends in renewable
costs or carbon reductions. The model also used battery storage
100,000
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2020
2025 2030 2035
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(a)
2040 2045 2050
to increase the utilization of transmission lines, demonstrating
that storage is a transmission complement, not a substitute.
The Peaking Challenge: Getting to 100%
Many analyses, such as NREL's Renewable Electricity Futures
study, have forecast a dramatic increase in the costs of servicing
load as systems approach 100% variable renewable energy.
This is caused by the seasonal mismatch of supply and demand
for renewable energy. This increase in costs is a special case
of the more general problem of meeting peak demand that has
been part of the planning process since the beginning of the
electric power system. The problem is caused by the low utilization
of peaking power plants, which results in a high levelized
cost of energy.
Figure 4(a) shows a load duration curve for California in
2013. It demonstrates that a small fraction of the total load is met
by a large fraction of the capacity. This means that the system
must build and operate large amounts of capacity that provide
little energy. Figure 4(b) shows the incremental capacity factor
of units providing an increased fraction of load, where the
capacity factor is the amount of energy produced by a plant in
a year divided by how much that plant would produce running
at full output all hours of the year. As an increasing fraction of
the demand is met, the remainder occurs during fewer hours of
the year, meaning power plants serving that load run less. As the
capacity factor declines, fixed costs of capacity must be recovered
across fewer units of energy delivered.
The peaking challenge ultimately limits the economic
contribution of renewable energy even in " overbuild " scenarios,
where lower-cost wind and solar allow for significant
amounts of curtailed energy. Because of the seasonal mismatch,
much of the residual load associated with the last 10%
of energy supplied occurs during periods of hot weather with
low wind or cold days with low solar. As a result, the incremental
capacity factor of renewable energy resources added to
meet this demand is low (because most of the energy supply
StrongCarbon_HighSolar (SCHS)
StrongCarbon_HighWind (SCHW)
WeakCarbon_HighSolar (WCHS)
WeakCarbon_HighWind (WCHW)
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figure 2. Transmission buildout in the (a) ACEG and (b) CCSA studies for the four main scenarios in each. The higher the
decarbonization requirements, the more transmission required. BAU: business as usual; CE: clean electricity.
november/december 2021
ieee power & energy magazine
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BAU
BAU-DER
CE
CE-DER
Total Interstate Transmission
Added (GW-Miles)
Total Interstate Transmission
Added (GW-Miles)

IEEE Power & Energy Magazine - November/December 2021

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