By Joni Sliger, Energy Fellow
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| "Photoshop art created from two NREL-Image Gallery photos of sunset view of electrical power towers combined with wind machines." Credit: NREL and Raymond David (Photo Illustration) |
Hailed as the world’s largest machine, our nation’s transmission and distribution
infrastructure is a critical component of our electricity system. These power
lines carry electricity from every electric power plant to every electricity
consumer. Investing in this infrastructure is necessary for the transition to a
clean energy future. This blog series explores the issues surrounding
transmission, both nationally and in the Pacific Northwest, and considers
various options for meeting our transmission needs in the 21st
century. This first post offers a basic introduction to transmission in the
United States.
What is
Transmission?
Transmission,
in the electricity world,
refers to the flow of electricity from its generation source, like a power
plant or a wind farm, to a local substation, typically over long distances. The
infrastructure that carries this electricity is the transmission system. At the
substation, the transmission system connects to the distribution system, which
carries electricity on to consumers. One may view a map of the nearly 200,000 circuit miles
of transmission lines in the U.S. on the Energy Information Administration’s U.S. Energy Mapping System.
As electricity travels on a power line, some of it is lost
due to inefficiencies in the system. For example, power lines may
unintentionally produce heat, which uses up some of the electricity on the
line. Heat losses are a common problem: consider the remarkably inefficient incandescent light bulb, which loses 90% of the electricity it draws to
unintentionally producing heat. The energy lost in transmission or distribution
is called a ‘line
loss.’
In the U.S., 5 to 6% of the electricity generated is lost due to line losses nationally.
Inside Energy provides an interactive graphic of the line losses in each state; Wyoming loses the least power at only
2.2%, while Idaho loses the most at 13.3%. Losses vary depending on whether a
state has more transmission lines or more distribution lines. Transmission
lines travel farther distances than distribution lines, but they have fewer
line losses, because they operate at a higher voltage.
High-voltage lines can carry more electricity to
consumers, ensuring more sales. However, the voltage is too high for household
use, so transformers reduce the voltage at the substation and
along the distribution system.
To sum up, transmission and distribution lines carry power
from a generating source to a user. Transmission lines carry power farther
distances but are more efficient, because they operate at a higher voltage than
distribution lines.
What are the
Problems with Transmission?
With almost 200,000 miles of transmission lines in the U.S., one may be surprised to
hear that a lack of transmission capacity is a major constraint to bringing
renewable energy sources online. Unfortunately, our current transmission system
suffers significant limitations.
Time plays two roles in affecting transmission systems.
First, the passage of time decreases the efficiency of transmission systems.
Over time, transmission lines suffer physical wear and tear and become less
efficient. Additionally, new technologies are discovered, so the lines become
outdated. Most transmission lines in the U.S. are based on 1950s technology.
Replacing these lines and associated infrastructure with newer technologies is
often referred to as ‘modernizing the grid.’
Second, time affects the demands on the transmission system.
In the U.S., electricity must be kept operating at 60 Hertz, the
frequency of choice for our systems. Grid operators constantly monitor the system
to ensure its frequency is stable. If the frequency changes too much, the system crashes, causing brownouts or blackouts. As consumers draw power off the grid, electricity must be
available to replace it, so that operators can balance the grid. If more electricity
is available than consumers need, producers have to throw it away. Advances in energy storage technologies, like modern battery systems, may soon enable
producers to save this excess energy and avoid the use of ‘peaking plants.’
Peaking plants operate only when demand is at a high, even if only for a few
hours or days a year; with such limited use, the construction of peaking plants
is very expensive relative to the energy they produce. Without energy storage,
the transmission system cannot take advantage of changes in energy demands.
Transmission lines can only carry so much electricity at one
time. Their capacity varies with the age and efficiency of the technology of
the line; newer lines can operate at higher voltages, which allows them to
carry more power, while suffering fewer line losses, as noted above. With only
a limited amount of capacity available, generators have to compete to get their
electricity to the market for sale. A congested transmission system, where too little transmission capacity is available,
creates a bottleneck that constrains electricity from getting to where it is
needed. Financing the deployment of new sources requires access to a market, so
bottlenecks are a significant impediment to possible development.
Where transmission lines are located affects whether
generators face a bottleneck or not. Unfortunately, most transmission lines are
currently located far away from the best sites for renewable energy
development. As my colleague, Amy Schlusser blogged previously, we primarily built our transmission system to
carry power from large, fossil fuel-fired power plants to power-hungry urban
centers. In rural areas, where the potential to develop renewable energy is often greatest, there is often
a lack of available transmission or capacity. In short, our transmission lines
are built in the wrong places for a future powered by renewable sources.
To bring more renewable energy sources online, the most
obvious solution is to build more transmission lines. This is extremely expensive
and faces challenges in siting. Concerns include conflicts with local private property owners
(and NIMBYism) as well as conflicts with other needs, such as environmental
regulations to preserve wilderness and protect wildlife, such as sage-grouse.
Other options exist though, such as more distributed generation and offshore energy
development, as I’ll discuss later on in this series. Before discussing possible
solutions, however, this series will next look deeper in how the transmission
system operates, particularly in the Pacific Northwest, and explore open-access
transmission policies, pancaked rates, balancing areas, and more.
