IEEE Power & Energy Magazine - July/August 2015 - 43

Operational
Challenges
of Achieving 50%
Renewables
Overview
Wind and solar energy create a
number of challenges for electric system operators, which
are magnified at the level of
penetration needed to achieve
july/august 2015

Megawatts

resources. It includes some solar thermal resources
with energy storage, in-state wind, and out-of-state
wind, in addition to some in-state solar. This scenario
tests whether a more diverse portfolio can cost less
than the large solar scenario due to reduced integration challenges, despite the higher initial cost.
All resources, including rooftop solar, are assumed to be
compensated at the cost of installing and maintaining the systems (including a market-based equity return). In addition to
these four 50% scenarios, the study analyzes two scenarios
that serve as reference points against which to compare the
costs and operational challenges of the 50% scenarios:
✔✔ The 33% scenario meets a 33% goal in 2030, representing an extension of the resource portfolio that is
already expected to be operational to meet the state's
current 33% RPS in 2020.
✔✔ The 40% scenario meets a 40% goal in 2030 by relying mostly on large, utility-scale solar PV resources.
The geographic scope of the analysis is a combination
of California's three largest balancing authority areas [the
California Independent Service Operator (CAISO), Los
Angeles Department of Water and Power, and the Balancing
Area of Northern California], serving over 95% of the state's
electric load.
Figure 1 shows the mix of renewable resources considered
for each of the described scenarios. All high-renewable scenarios include the resources expected to be online to meet
California's current 33% renewable target by 2030. Moving
from 33% to 50% renewables, the large solar, small solar,
and rooftop solar scenarios add a resource mix that consists
of 80% solar PV resources and 20% wind (with the type
and location of the solar resources varying by scenario).
The diverse scenario includes a broad mix of technologies.
In addition to the resources added to meet the renewable
target (defined as utility-procured production divided by
retail sales), the study assumes
27,000
that 7,000 MW of customerowned solar PV resources
25,000
are installed by 2030 under
California's net energy meter23,000
ing policies, enough to meet
21,000
approximately 5% of total load.

a 50% renewable grid. In particular, wind and solar generation have three key limitations in electric system operations:
✔✔ variability: their output varies from moment to moment, creating a need for balancing services on various time scales
✔✔ uncertainty: their output cannot be predicted with any
certainty in advance
✔✔ concentration: their output is concentrated during a
limited number of hours of the year when the solar or
wind resources are abundant.
These limitations create challenges in designing power
systems to rely on wind and solar for a large proportion
of energy supply. The variability and uncertainty of wind
and solar energy have been the subject of numerous studies. Electric system operators address these challenges
through the procurement of additional operating reserves,
such as regulation or frequency responsive reserve for very
short time frames, or load following reserves for 5-60min time periods.
Concentration is an even larger challenge for energy
system design at a higher penetration. Serving half of the
electric load with wind and solar requires these resources
to provide well above half of the total supply during many
hours. Indeed, the study finds that "overgeneration," which
occurs when total energy supply exceeds the system's ability
to ability to absorb it, is the single largest operational challenge under 50% renewables.
California's renewable integration challenges have been
illustrated in the CAISO's widely circulated "duck chart''
(see Figure 2). So named because of its superficial resemblance to a water fowl, the chart highlights the changes that
occur to California's daily "net load" profile (hourly electric
load minus "must-run" resources) over successive years as
more solar is added. Whereas in 2013 the net load shape is

2013

19,000
17,000
15,000

Significant Change
Starting in 2015

13,000
11,000

Net Load

2015

Increased
Ramp
Potential
Overgeneration

2020
0 1 2

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

figure 2. CAISO's duck chart has been used to illustrate the need for increased upward
ramping capability under a higher-solar future (CAISO, www.caiso.com).
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http://www.caiso.com

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