PrintEnergy Conservation Potential
To understand why energy conservation is even an option (let alone a necessary one), we have to go back to a simple fact about energy conversion: You want some, you waste some. Burning coal in a power plant or gasoline in a car; or heating water using solar energy – all of these things are “energy conversion processes.” We are taking energy that is in one form (like lumps of coal) and going through chemical, mechanical or other conversions to turn that energy into things that we want (like lighting and transportation). While these conversion processes have done great things for people and society, they aren’t perfect. The energy that goes into a conversion process is always greater than the energy that comes out of a conversion process. This is basically a consequence of the second law of thermodynamics, which says that it’s impossible to harness all energy in one form (like a lump of coal) to do useful work in another form (like light a room or make toast).
Technically, the second law of thermodynamics applies only to conversion processes involving heat – which basically describes the vast majority of the world’s energy technologies. These technologies burn fossil fuels in a generic process called a “heat engine.” Internal combustion engines in cars, industrial boilers, and power plants are all examples of the “heat engine” principle. In the 1800s a physicist named Sadi Carnot figured out that there was a theoretical limit to how efficient a heat engine could get, depending on the type of work it was asked to do and the conditions under which the heat engine operated.
What Carnot figured out, in today’s terms, is that a heat engine dependent on steam (like a power plant) could not get much more efficient than about 50%. A heat engine that is driven directly by combustion gases, like a car or truck engine, cannot get much more than 25% efficient. This means that in the most perfect of all circumstances, it’s impossible for a simple power plant to waste less than about 50% of all the fuel that’s put into it. Cars are even worse – they are set to waste about 75% of all the fuel that we pump into them.
Of course, there are some clever ways that we can push the efficiency of energy conversion processes. Combined cycle power plants, for example, are able to capture some of the waste heat from combustion and use that heat to drive a second turbine for power generation. But even these types of plants generally don’t achieve efficiencies of more than about 65%. Wind and solar are “inefficient” as well – a wind turbine might capture about 50% of the potential energy in the wind that hits the turbine, and solar photovoltaic cells are generally able to capture only about 20% of the solar energy that hits their surfaces. (On the other hand, energy from the wind and the sun are free once the equipment is built and installed, so maybe the “efficiency” is not as important in the case of wind and solar.)
Watch the following video about Sankey Diagrams (6:23)
In this EcoWest presentation, we break down energy trends in the US and western states by using a graphic known as a Sankey diagram. Energy flows through everything, so it's only fitting to use this type of flowchart to depict our complex energy economy. Sankey diagrams are named after an Irish military officer who used the graphic in 1898 in a publication on steam engines. Since then, Sankey diagrams have won a dedicated following among data visualization nerds. The graphic summarizes flows through a system by varying the width of lines according to the magnitude of energy, water, or some other commodity.
One of the earliest and most famous examples of the form illustrates Napoleon's disastrous Russian campaign in the early 19th century. Created by Charles Joseph Minard, a French civil engineer, the graphic depicts the army's movements across Europe and shows how their ranks were reduced from 422,000 troops in June of 1812, when they invaded Russia, to just 10,000 when the remnants of the force staggered back into Poland after retreating through a brutal winter. Data visualization guru Edward Tufte says it's probably the best statistical graphic ever drawn.
Sankey diagrams created by the Lawrence Livermore National Laboratory depict both the source and use of energy. The boxes on the left show the nation's power portfolio, and the lines moving to the right show where that energy ends up, with the width varying by the magnitude of the flow. This graphic, using 2008 data, shows that petroleum in the transportation sector accounts for the biggest overall energy flow. In 2008, more than half of electricity generation came from coal, followed by nuclear and natural gas.
Here's the next year's data. Total energy use actually fell slightly as the US economy fell into the recession, but the overall pattern of the flows remained the same. In all of the energy diagrams, you'll notice that a significant share of energy is rejected. A good example of rejected energy is waste heat from power plants. The greater the percentage of rejected energy, the less efficient the system is.
Here's 2010. It's worth noting that in this sequence of slides, the size of the rectangles does not vary according to the amount; it's only the thickness of the lines that change from year to year. Here's 2011, the most recent version. Between 2010 and 2011, the thickness of the coal line decreased as the nation shifted toward natural gas. Besides being used to fuel power plants, natural gas is used directly in homes, businesses, and factories.
Here's another Sankey diagram for US energy flows that was created by the Department of Energy. This version includes some interesting facts and statistics in the margin. Now let's shift to energy flows in the 11 western states. First off, it's worth noting that the region's energy economy is heavily influenced by California, which accounts for 41% of the total energy flows.
Here's the picture for California. As you might expect, petroleum used in the transportation sector dominates the system in a state that is known for its car culture and also home to major transportation hubs. Looking at the electricity generation box, you can see that natural gas now provides the biggest share of the state's power portfolio, but nuclear, hydro, and geothermal are also major contributors. There's barely any coal use for power generation within California, but you'll notice that the state also imports a fair amount of energy from other states, including coal-fired power plants in the southwest.
It's a totally different story in Wyoming, where virtually all of the electricity generated in the state comes from coal. Some of that power is also exported to other states. Compared to California, far less energy flows into the transportation sector in this sparsely populated state. As with the national slides, it's important to note that the rectangles don't change size from state to state. That means the width of the flow lines are not comparable from slide to slide; they merely show within a single state how the energy flows are divided.
It's no surprise that Wyoming, home to the Powder River Basin coal deposit, is so heavily reliant on coal, but so are some other inland states such as New Mexico and Utah, both of which export some of that electricity. Colorado is heavily dependent on coal, but natural gas is also critical, and about 6% of electricity generation comes from wind, a higher fraction than any other western state. Montana also uses lots of coal, but hydropower makes up nearly one-third of the power portfolio.
In Arizona, the Palo Verde nuclear power plant, the nation's largest, accounts for 27% of the state's electric generation, although some of that power is exported to places like California. In Nevada, natural gas is the top source for power plants, while geothermal accounts for 9%. Coal may be king for electricity generation in many states in the Intermountain West, but it's hydropower that dominates the power portfolios in the Pacific Northwest.
Here's Idaho, which imports a good deal of its electricity from surrounding states. In Oregon, hydropower dams account for 64% of electricity generation, while in Washington state, it's 71%. You can download more slides and other resources at ecowest.org.
The tales of inefficiencies in modern energy systems are almost too numerous to count, and we haven’t even talked about the ways in which people choose to use energy. The graphic below provides a nice summary of how much energy is wasted in the United States. The figure is called a “Sankey diagram” and it traces the flows of energy (from left to right) through all sectors of a country’s economy. (The example in the graphic below was produced by a US government laboratory, so it naturally focuses on the United States.) The left-hand side of the Sankey diagram shows all of the energy inputs to a nation’s economy and how much of each is used. The box for “petroleum” is larger than the box for “solar” because the US economy uses a lot more petroleum than it does solar energy. From each individual resource, you can trace the various paths, showing how much of that energy resource is used in different sectors of the economy. For example, coal is used for power generation and is used directly in industrial and commercial boilers as well. As indicated by the width of the path, the vast majority of coal in the US economy is used for power generation. The quantities of coal used for industrial and commercial boilers are much smaller. All the way over at the right, you can see two boxes – one is labeled “Energy services” and the other is labeled “Rejected Energy.” The Energy Services box tallies up all of the coal, oil, gas, solar and other resources that we actually harness for doing useful things. The Rejected Energy box measures how much of those resources is lost due to inefficiencies in our energy conversion systems. As you can see, the US is now 32% efficient, and if we switched to an all-electric economic system with the highest efficiency generators, we could approach something closer to 50% efficiency.
Activate Your Learning
According to the Sankey diagram above, what are the two biggest sources of wasted energy in the U.S. economy?