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The Energy on the Grid


Two posts ago, I introduced you to the national energy grid. Last post I described how The Grid is organized. This post will describe some of the major energy flows along The Grid.

Figure 1 shows the major sources of electricity generated in the USA in 2017. Almost 1/3 of it was generated by burning natural gas, about 1/3 by burning coal, and about 1/3 was generated from other energy sources. Nuclear was the largest of the other sources, accounting for

Figure 1. Data source: U.S. Energy Information Administration 2018.

about 20% of total demand. Nuclear, hydro, wind, solar, and geothermal are the sources that do not emit carbon dioxide by burning fuel, so they are desirable from climate change and pollution perspectives. Together, they accounted for about 36% of total demand. Wind and solar are intermittent sources, and that has important implications for grid reliability. Together they account for about 7% of total demand.

(Click on chart for larger view.)





Figure 2. Historical and Projected Changes in Electricity Demand. Source: North American Electrical Reliability Corporation 2017.

I described NERC, the North American Electric Reliability Corporation, in the last post. To ensure grid reliability, NERC attempts to estimate changes in demand The Grid will have to meet. Figure 2 combines historical and projected changes in demand over rolling 10-year periods from 1990 through 2027. It shows the change in demand during summer (light blue) and winter (dark blue). (In the South, demand for electricity is highest in summer, but in the North, it is highest during winter.) The columns show the change in GW (gigawatts, billions of watts) and the blue lines show the percentage compound annual growth rate.

Well, interpreting this complex graph as a little challenging, so let’s unpack it. First of all, it is a graph of change, not overall demand. So, demand for electricity has grown over every 10-year period, and it is expected to continue to do so. Second, the chart shows average annual change during each 10-year period, not cumulative change over the whole period. Third, the last period for which the data is all historical is 2007-2016. Starting with 2008-2017, some of the data is historical, some of it is projection. By 2017-2026, all of the data is projection. Fourth, the rate of demand growth accelerated in the decades starting around 2004. But fifth, the increase in demand is projected to slow in the future, in both the raw number of gigawatts and in the compound annual growth rate.

Bottom line here: The Grid is projected to have to satisfy increased demand for electricity, although the rate of growth is projected to slow.

Figure 3 shows historical additions and retirements in generating capacity supplied to The Grid, by fuel. You can see that for each type of generation, in most years, some was added and some was retired. Over the span of the chart, the net result has been a decrease in coal and nuclear generation, with an increase in natural gas, wind, and solar. Figure 4 shows similar data projected into the future. The projection shows a continuation of the trend: net retirement of coal and nuclear generating capacity, net addition of natural gas, wind, and solar. NERC projects that more natural gas generating capacity will be added than any other kind.

Figure 3. Historical Changes in Generating Capacity. Source: North American Electrical Reliability Corporation 2017.

Figure 4. Projected Future Changes in Generating Capacity. Source: North American Electrical Reliability Corporation 2017.









Figure 5. Source: Department of Energy 2017.

Figure 5 shows some of this data in a form that is a bit less wonkish. It shows the location, size, and type of generating station retirements on The Grid from 2002-2016 . Triangles represent power plants owned by independent power generators, while circles represent power pants owned by vertically integrated electric utilities. Grey icons represent coal burning plants, blue icons represent natural gas burning plants, and green icons represent nuclear plants. The size of the icon represents the plant’s generating capacity. Look at the concentration of gray icons in the eastern part of the country! The retirements all occurred in a  14-year period.

Many of these plants were old and inefficient, and many of them were large spewers of GHGs and other pollutants. So, from an environmental perspective, their retirement may be good news. It doesn’t take a rocket scientist, however, to see that the retirement of so many plants represents a significant transition on The Grid.

Why does this matter? Because we are looking at reliability here, not climate change. We haven’t quite developed enough information to understand the implications yet, but we will by the end of the series of posts. At this point, we can simply say that coal-based and nuclear generating stations have proven very reliable, and they fit into The Grid nicely. NERC has concerns about the reliability of natural gas, wind, and solar generating stations for the supply of bulk electricity.

Now, what about geography?

Most electricity is generated within the NERC region where it is consumed. The flow between NERC regions is comparatively small, but because The Grid has to be so finely balanced, it is important. It flows in sometimes surprising directions. The direction is determined by many factors, including the availability of transmission lines with unused capacity, historical patterns of energy consumption, and the cost of the electricity. Inexpensive electricity generated at a distance is sometimes substituted for more expensive electricity generated locally.

The flow of energy over The Grid is shown in Figure 5 at right. The map is from 2010. The regions shown in it differ slightly from current NERC regions, and they use different names. However, it was the best representation I could find. On this map, “Midwest” = the MISO Region, “Central” = the SPP Region, “TVA” = the SERC-N Region, and “Mid-Atlantic” is roughly the PJM Interconnection Region. Let’s look a bit more closely at the map.

A region in Northern Illinois served by Commonwealth Edison belongs to the Mid-Atlantic Region, but is physically separated from it. The largest power flow in the nation occurs from this region to the rest of the Mid-Atlantic Region. This represents power that is generated by highly efficient coal and nuclear generating stations operated by Commonwealth Edison. They can’t be cycled on and off easily, so during periods of slack demand (at night) they export large amounts of power at low prices.

The second largest flow occurs from the Southwest into California. As a single state, California imports more electricity than any other.

The Midwest Region is a net exporter of power. It receives power from Manitoba and Commonwealth Edison, but it distributes even more to the TVA and Central Regions. In doing this, it participates in a counterclockwise flow from Manitoba, through the Midwest and the South, and eventually to the Mid-Atlantic Region.

The Central region is a net importer of electricity. It receives inflows from the Midwest, keeps some of it, and distributes less than it receives to Texas and the Gulf.

The amount of energy available to any region, therefore, depends mostly on the generating capacity within the region, but also on the amount it receives from other regions. The transmission of energy between regions depends not only on the need for it, but also on the availability of transmission capacity.


Department of Energy. 2017. Staff Report to the Secretary on Electricity Markets and Reliability. Downloadedm 2018-05-19 from https://www.energy.gov/sites/prod/files/2017/08/f36/Staff%20Report%20on%20Electricity%20Markets%20and%20Reliability_0.pdf.

North American Electrical Reliability Corporation. 2017. 2017 Long-Term Reliability Assessment. Downloaded 4/27/2018 from https://www.nerc.com/pa/RAPA/ra/Pages/default.aspx.

Source: “Electricity tends to flow south in North America.” Today in Energy. EIA, http://www.eia.gov/todayinenergy/detail.cfm?id=4270.

U.S. Energy Information Administration. “U.S. Electricity Generation by Source, Amount, and Share of Total in 2017.” Frequently Asked Questions. Downloaded 4/28/2018 at https://www.eia.gov/tools/faqs/faq.php?id=427&t=3.

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