June 25, 2019
Variable Renewable Energy: An Introduction
Policy options to increase the use of renewable energy
sources for electricity generation have drawn congressional
interest. Some renewable energy sources, such as wind and
solar, have variable supply. Electricity generators using
variable renewable energy (VRE) sources can only produce
electricity when weather conditions are right (i.e., when the
wind is blowing or the sun is shining).
In contrast, conventional energy sources used to generate
electricity are typically available when called upon.
Frequently, the energy source can be stored on-site, or
otherwise be directly accessed by generators. Examples
include coal, natural gas, water stored in reservoirs, and
nuclear fuel rods. Supplies of these energy sources are
typically sufficient to generate electricity steadily for
periods of days to months, although supplies are not
guaranteed. Extreme weather events or disruptions in fuel
transportation networks could potentially create supply
shortages. Nonetheless, many consider these conventional
energy sources to be more reliable than VRE.
Policy, technology developments, and market forces are
leading to an increased use of VRE sources for electricity
generation. Members of Congress may be interested in
potential reliability issues and possible solutions.
Wind and Solar Variability
Electricity generation from wind and solar sources is
weather-dependent, but typical patterns in wind and solar
availability exist. Figure 1 shows typical patterns for U.S.based electricity generators using wind or solar energy.
Figure 1. Generation Patterns for U.S. Wind and
daily generation patterns for photovoltaic panels can vary based on
the direction they face.
Over the course of a typical day, generation from any
particular wind turbine peaks overnight and during early
morning hours, and generation from most solar generators
peaks in early afternoon. Over the course of a year,
generation from any particular wind turbine typically peaks
in winter and early spring, though regional differences
exist. Generation from any particular solar generator
typically peaks in summer.
Demand for electricity also shows variability, typically
reaching a daily maximum point (i.e., peak demand) in the
early evening and seasonal maxima during winter and
Potential Reliability Issues
The U.S. electricity system was designed to accommodate
conventional sources that can be called upon as needed to
generate electricity, barring extreme events or regular
The variable nature of wind and solar sources requires
changes in electric power system design and operation, in
order to prevent damage to system components and
widespread power outages. These changes are sometimes
referred to as renewables integration. Generation from VRE
sources in the United States in 2018 was 8% of total
generation, of which 80% was from wind sources. Most
reliability indicators used by the North American Electric
Reliability Corporation (NERC) were stable or improving
in 2018, suggesting that VRE sources are not creating
widespread reliability issues at present generation levels.
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
Operator and the U.S. Energy Information Administration.
Notes: The y-axis represents the potential energy generation from
each individual source, from a minimum of zero to a maximum
defined by the installed capacity of the source. Actual generation
from any individual source will depend on its size, local weather
conditions, and other factors. Wind seasonal generation patterns vary
by region, so the pattern shown here may not apply in all cases. Solar
Reliability concerns over timescales of hours to days have
been a focus area for Congress. As Figure 1 shows,
maximum generation from VRE sources does not align with
typical daily peak demand. Other sources of electricity
generation or storage must quickly increase their output to
meet demand as generation levels from VRE sources fall
(e.g., when the sun sets). The capability to quickly change
output is known as ramping. Not all electricity sources are
capable of ramping, and some that are may be more costly
Variable Renewable Energy: An Introduction
to use than other sources. In areas of the country with high
levels of generation from VRE sources, concern is growing
about maintaining sufficient ramping capability.
At current levels of generation from wind and solar sources,
seasonal variability has not generally presented reliability
issues. A main reason is that many other conventional
sources are available that can generate electricity when
output from wind and solar sources is relatively low.
Projections over the coming decades generally agree that,
given current policy and market trends, more wind and
solar sources will come online and many conventional
sources will retire, increasing the share of total electricity
generation coming from VRE sources in the United States.
For example, the U.S. Energy Information Administration
(EIA) projects that wind and solar could comprise 13%33% of total electricity generation in 2050 (Figure 2)
depending on economic growth, technology development,
and other factors. Seasonal variability of VRE sources
could present greater reliability challenges moving forward
if the share of generation from these sources increased.
Figure 2. Projections of Share of Total Electricity
Generation from Wind and Solar Sources, 2020-2050
Transmission System Expansion
The transmission system is the network of over 700,000
miles of high-voltage wires and supporting equipment that
moves electricity over long distances. Some regions
experience transmission congestion when electricity cannot
be moved to where it is needed because of insufficient
transmission capacity. Congestion increases overall
electricity costs for consumers, and it can create hurdles to
balancing generation from VRE sources, particularly over
Expanding the transmission system could potentially
support renewables integration by alleviating transmission
congestion. It might address seasonal variability by
allowing electricity generated during periods of high output
to be delivered to more areas.
Smart Grid Upgrades
Smart Grid describes, in part, upgrades to the transmission
and distribution systems that could support renewables
integration. Some Smart Grid upgrades could support
integration of large-scale wind and solar projects connected
to the transmission system. Other aspects of the Smart Grid
could support integration of small-scale projects connected
to the distribution system, the network of 6.5 million miles
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.
Source: CRS analysis of data in EIA, Annual Energy Outlook 2019,
Notes: Dark line represent EIA’s reference case. Shaded area
represents the range of outcomes in EIA’s side cases. Assumptions
for EIA’s cases are described in the Annual Energy Outlook 2019.
As noted above, current levels of generation from VRE
sources have not created widespread reliability issues.
Sufficient backup capacity is usually available, and system
operators and participants are developing new practices to
address the variability of wind and solar sources.
Policy options to address reliability issues that could arise
in the future have drawn congressional interest. Some
frequently discussed potential solutions are described
below. Implementing any of these might require significant
infrastructure investment. How such investment should be
funded and what other potential costs and benefits would be
associated with such investment could be key
considerations. Other concerns, such as environmental
impacts, cybersecurity, and data privacy, may be of interest
Electricity is essentially generated as a just-in-time
commodity. Some technologies can store electric energy in
some form and then release it back to the grid, potentially
saving electricity generated during periods of high supply
and delivering it during periods of low supply. Energy
storage can be located on the transmission system or the
distribution system. Pumped hydro, in which water is
pumped to a reservoir during periods of high electricity
supply and then released to generate electricity during
periods of low electricity supply, is a form of energy
storage that has been used for many years. Other
technologies are attracting increasing interest. These
include batteries, compressed air storage, and fuels like
hydrogen. Energy storage could potentially be used to
address variability over multiple timescales, though most
energy storage technologies deployed today are usually
capable of balancing over timescales of minutes to hours.
For more information about VRE sources, potential
reliability issues, and possible solutions, see CRS Report
R45764, Maintaining Electric Reliability with Wind and
Solar Sources: Background and Issues for Congress, CRS
Report R45156, The Smart Grid: Status and Outlook, and
Variable Renewable Energy: An Introduction
CRS Report R42455, Energy Storage for Power Grids and
Electric Transportation: A Technology Assessment.
Ashley J. Lawson, Analyst in Energy Policy
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan shared staff to
congressional committees and Members of Congress. It operates solely at the behest of and under the direction of Congress.
Information in a CRS Report should not be relied upon for purposes other than public understanding of information that has
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