Variable Renewable Energy: An Introduction

Variable Renewable Energy: An Introduction

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June 25, 2019
Variable Renewable Energy: An Introduction
Policy options to increase the use of renewable energy
daily generation patterns for photovoltaic panels can vary based on
sources for electricity generation have drawn congressional
the direction they face.
interest. Some renewable energy sources, such as wind and
Over the course of a typical day, generation from any
solar, have variable supply. Electricity generators using
particular wind turbine peaks overnight and during early
variable renewable energy (VRE) sources can only produce
morning hours, and generation from most solar generators
electricity when weather conditions are right (i.e., when the
peaks in early afternoon. Over the course of a year,
wind is blowing or the sun is shining).
generation from any particular wind turbine typically peaks
in winter and early spring, though regional differences
In contrast, conventional energy sources used to generate
exist. Generation from any particular solar generator
electricity are typically available when called upon.
typically peaks in summer.
Frequently, the energy source can be stored on-site, or
otherwise be directly accessed by generators. Examples
Demand for electricity also shows variability, typically
include coal, natural gas, water stored in reservoirs, and
reaching a daily maximum point (i.e., peak demand) in the
nuclear fuel rods. Supplies of these energy sources are
early evening and seasonal maxima during winter and
typically sufficient to generate electricity steadily for
summer.
periods of days to months, although supplies are not
guaranteed. Extreme weather events or disruptions in fuel
Potential Reliability Issues
transportation networks could potentially create supply
The U.S. electricity system was designed to accommodate
shortages. Nonetheless, many consider these conventional
conventional sources that can be called upon as needed to
energy sources to be more reliable than VRE.
generate electricity, barring extreme events or regular
maintenance requirements.
Policy, technology developments, and market forces are
leading to an increased use of VRE sources for electricity
The variable nature of wind and solar sources requires
generation. Members of Congress may be interested in
changes in electric power system design and operation, in
potential reliability issues and possible solutions.
order to prevent damage to system components and
widespread power outages. These changes are sometimes
Wind and Solar Variability
referred to as renewables integration. Generation from VRE
Electricity generation from wind and solar sources is
sources in the United States in 2018 was 8% of total
weather-dependent, but typical patterns in wind and solar
generation, of which 80% was from wind sources. Most
availability exist. Figure 1 shows typical patterns for U.S.-
reliability indicators used by the North American Electric
based electricity generators using wind or solar energy.
Reliability Corporation (NERC) were stable or improving
in 2018, suggesting that VRE sources are not creating
Figure 1. Generation Patterns for U.S. Wind and
widespread reliability issues at present generation levels.
Solar Sources
Over timescales of one second or less, electricity must stay
within narrow ranges of voltage and frequency.
Conventional sources can support these requirements as a
natural consequence of the alternating current (AC) power
they produce. Wind and solar sources do not produce the
same kind of electricity directly. The electricity they
generate is converted to AC power for the grid by a device
called an inverter. Federal regulations established since
2016 require inverters to help meet some voltage and
frequency requirements of the electric power system.

Source: CRS, based on data from the California Independent System
Reliability concerns over timescales of hours to days have
Operator and the U.S. Energy Information Administration.
been a focus area for Congress. As Figure 1 shows,
Notes: The y-axis represents the potential energy generation from
maximum generation from VRE sources does not align with
each individual source, from a minimum of zero to a maximum
typical daily peak demand. Other sources of electricity
defined by the instal ed capacity of the source. Actual generation
generation or storage must quickly increase their output to
from any individual source wil depend on its size, local weather
meet demand as generation levels from VRE sources fall
conditions, and other factors. Wind seasonal generation patterns vary
(e.g., when the sun sets). The capability to quickly change
by region, so the pattern shown here may not apply in al cases. Solar
output is known as ramping. Not all electricity sources are
capable of ramping, and some that are may be more costly
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Variable Renewable Energy: An Introduction
to use than other sources. In areas of the country with high
Transmission System Expansion
levels of generation from VRE sources, concern is growing
The transmission system is the network of over 700,000
about maintaining sufficient ramping capability.
miles of high-voltage wires and supporting equipment that
moves electricity over long distances. Some regions
At current levels of generation from wind and solar sources,
experience transmission congestion when electricity cannot
seasonal variability has not generally presented reliability
be moved to where it is needed because of insufficient
issues. A main reason is that many other conventional
transmission capacity. Congestion increases overall
sources are available that can generate electricity when
electricity costs for consumers, and it can create hurdles to
output from wind and solar sources is relatively low.
balancing generation from VRE sources, particularly over
daily timescales.
Projections over the coming decades generally agree that,
given current policy and market trends, more wind and
Expanding the transmission system could potentially
solar sources will come online and many conventional
support renewables integration by alleviating transmission
sources will retire, increasing the share of total electricity
congestion. It might address seasonal variability by
generation coming from VRE sources in the United States.
allowing electricity generated during periods of high output
to be delivered to more areas.
For example, the U.S. Energy Information Administration
(EIA) projects that wind and solar could comprise 13%-
Smart Grid Upgrades
33% of total electricity generation in 2050 (Figure 2)
Smart Grid describes, in part, upgrades to the transmission
depending on economic growth, technology development,
and distribution systems that could support renewables
and other factors. Seasonal variability of VRE sources
integration. Some Smart Grid upgrades could support
could present greater reliability challenges moving forward
integration of large-scale wind and solar projects connected
if the share of generation from these sources increased.
to the transmission system. Other aspects of the Smart Grid
could support integration of small-scale projects connected
Figure 2. Projections of Share of Total Electricity
to the distribution system, the network of 6.5 million miles
Generation from Wind and Solar Sources, 2020-2050
of wires, supporting equipment, and meters that deliver
electricity to most end users.
Smart Grid components include phasor measurement units
(PMUs) that can monitor reliability status over timescales
of microseconds (current technology makes measurements
over a few seconds); advanced metering infrastructure
(AMI), sometimes known as smart meters, that can
communicate with the grid in almost real-time (current
technology does not communicate with the grid); and data
communication and data processing components.

Energy Storage
Source: CRS analysis of data in EIA, Annual Energy Outlook 2019,
Electricity is essentially generated as a just-in-time
https://www.eia.gov/outlooks/aeo/data/browser/.
commodity. Some technologies can store electric energy in
Notes: Dark line represent EIA’s reference case. Shaded area
some form and then release it back to the grid, potentially
represents the range of outcomes in EIA’s side cases. Assumptions
saving electricity generated during periods of high supply
for EIA’s cases are described in the Annual Energy Outlook 2019.
and delivering it during periods of low supply. Energy
storage can be located on the transmission system or the
Potential Solutions
distribution system. Pumped hydro, in which water is
As noted above, current levels of generation from VRE
pumped to a reservoir during periods of high electricity
sources have not created widespread reliability issues.
supply and then released to generate electricity during
Sufficient backup capacity is usually available, and system
periods of low electricity supply, is a form of energy
operators and participants are developing new practices to
storage that has been used for many years. Other
address the variability of wind and solar sources.
technologies are attracting increasing interest. These
include batteries, compressed air storage, and fuels like
Policy options to address reliability issues that could arise
hydrogen. Energy storage could potentially be used to
in the future have drawn congressional interest. Some
address variability over multiple timescales, though most
frequently discussed potential solutions are described
energy storage technologies deployed today are usually
below. Implementing any of these might require significant
capable of balancing over timescales of minutes to hours.
infrastructure investment. How such investment should be
Additional Analysis
funded and what other potential costs and benefits would be
associated with such investment could be key
For more information about VRE sources, potential
considerations. Other concerns, such as environmental
reliability issues, and possible solutions, see CRS Report
impacts, cybersecurity, and data privacy, may be of interest
R45764, Maintaining Electric Reliability with Wind and
as well.
Solar Sources: Background and Issues for Congress, CRS
Report R45156, The Smart Grid: Status and Outlook, and
https://crsreports.congress.gov

Variable Renewable Energy: An Introduction
CRS Report R42455, Energy Storage for Power Grids and
Ashley J. Lawson, Analyst in Energy Policy
Electric Transportation: A Technology Assessment.
IF11257

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