There is an ongoing debate about what constitutes clean energy. You even hear about “clean coal” in those discussions (I will get back to that later). We need to expand this debate to include “reliable” as part of the discussion. I define reliable as an energy source that is available to meet electricity demands any time of the day or night. If we define clean as a source that minimizes solid wastes and does not emit CO2, then electricity generated from nuclear power plants may be the only widely available source that qualifies as clean and reliable.
Let me explain with both words and graphs. First, everybody recognizes that the demand for electricity is not constant; we tend to use less at night than in the daytime. What most people don’t recognize is that even late at night, we still use 75% of the electricity consumed during normal working daytime hours (see figure 1). The data shown in figure 1 is important because it shows that even late at night, we consume a lot of electricity. That means a clean and reliable source of energy has to produce power all night long. Since we have no capacity for direct electricity storage on this scale, solar power doesn’t qualify as a reliable source of electricity for meeting our baseload requirements.
Wind is the other clean, renewable energy source, but it also has reliability problems. The “cut-in” wind speed (the speed when turbines can start producing usable power) is typically between 7-10 Mph. For most sites across the country, the wind speed varies considerably with the hour of the day, with minimum speeds between 1:00 and 5:00 AM. In addition to the hourly variations in wind speed, there are seasonal changes that affect the amount of energy available from wind power. For example, the National Climatic Data Center has collected data for 30 years and their information shows that the peak wind speeds for Fresno, CA are lowest in the winter months, and highest in the spring and summer. Although not an ideal wind site, the wind profiles for Fresno are similar to profiles at other sites around the country. The energy produced by these systems is not constant, and during many hours of the day they drop below the cut-in threshold. Figure 2 shows average hourly wind speed data for Fresno, California in July of 2009. As shown on this plot, the average wind speed does not cross the “cut-in” threshold until just after 3:00 in the afternoon. The wind speed allows energy generation during the peak demand hours of 5:00 – 8:00 PM, but would contribute nothing to the power requirements between 3:00 AM and 4:00 PM. The fact that wind power systems don’t burn any fuel while they are operating qualifies them as clean energy sources, but they can’t be relied to meet baseload electricity demands. There has been a lot effort to develop grid scale energy storage systems (massive batteries) so that power produced by wind turbines when the wind blows could be used at other times of the day, but none of that effort has produced a viable sulution. For the foreseeable future, wind will remain an attractive and clean way of helping to meet peak power needs, but won’t contribute to the supply of baseload electricity.
Now, what happens when we overlay some of the information we now have about wind energy availability on top of the electricity demand graph and see what conclusions can be drawn. The data on electricity demand came from the California ISO website referenced in end note 1, and the wind data came from the National Oceanic and Atmospheric Administration’s National Data Center referenced in endnote 3. The scale for measuring power demand is VERY different than the scale for measuring wind speed, so each data stream is plotted against a separate vertical axis in Figure 3. The absolute values in the two plot lines aren’t important, but the trends certainly are very critical, and very clear when plotted together. Although the wind begins to blow hard enough to make power just before peak demand begins, there is a significant time period where the demand is increasing from baseline levels and when there is no recoverable wind energy. From this data, the average wind energy is July is unable to produce power for 50% of the day, and that low point in energy production coincides with high, steady demand during productive daylight hours. In addition, the average wind strength is greatest during the time of day that demand for electricity is dropping rapidly. Clearly, this does not represent a reliable source of power. When you factor in the understanding that solar power does not create any electricity at night, and only minimal power during the first and last three hours of daylight when the sun is at a low angle in the sky and the inevitable conclusion is that the two cleanest sources of energy are not reliable sources. In fact, an energy production strategy that focused on wind and solar would require constructing a completely redundant system of other energy sources to pick up the load when both the wind and sun are absent. That doubling of the installed capacity to provide reliability would raise electricity rates to unaffordable levels.
Now, back to coal. Coal certainly produces reliable energy. The fuel source is abundant in the US and burning it in a traditional fossil plant produces dependable energy on demand, 24 hours a day. Coal’s availability and reliability is why it is used to produce 30% of our electricity. There is a lot of talk about “clean coal”, but that talk centers on capturing and storing CO2. Currently, a 1,000 MW(e) coal plant burns about 3.4 million tons of coal per year when operating at a 90% capacity factor. Burning that coal requires 32 million tons of air for combustion and produces 8.8 million tons of CO2 (yes, with the amount of air consumed, a coal plant puts out more tons of CO2 than the tons of coal burned). That is a huge amount of CO2 to try and capture for storage, but let’s assume it can be done. Does that make coal clean? Not by a long shot. In addition to the CO2 produced from burning that much coal, the waste stream also includes 685,000 tons of fly ash, bottom ash and slag. Toxic materials originally present in the coal are concentrated in this ash when the carbon burns off. The GAO reported that between 2000 & 2006, the power industry deposited more than 124 million pounds of six toxic pollutants (arsenic, chromium, lead, nickel, selenium, and thallium) in ash sludge ponds. A major example of what can go wrong with ash storage can be seen in the 2008 failure of the retention pond for TVA’s Kingston Coal Plant in Tennessee. The official estimate is that 5.4 million cubic yards of sludge were released from the 84-acre “containment area”. The spill covered surrounding land with up to six feet (1.8 m) of sludge as shown in Figure 4. The EPA’s approved clean-up plan now estimates the cost of the clean-up to be $268.2 million. On March 6, 2009 Duke University submitted a Survey of the Potential Environmental and Health Impacts in the Immediate Aftermath of this Coal Ash Spill. That survey showed enrichment of trace elements (including strontium, arsenic, barium, berrylium, cadmium, chromium, lithium, lead, mercury, nickel, and thallium) in samples of sediments and river water. Despite the fact that 26 reporting facilities cited spills or unpermitted releases from a total of 35 surface ash impoundments within the last 10 years, the EPA does not directly regulate coal combution residue disposal in surface impoundments or landfills. The exact number of CCR surface impoundments at utility coal fired power plants is not known, and no industry organization or government agency tracks this information.
If we are consistent with the definition of “clean” energy as a source that minimizes solid waste and CO2 production then coal power can’t be called clean, even if methods for carbon capture and storage are worked out.
Now, let’s compare the operation of a nuclear power plant that produces the same amount of electricity as the coal plant described above, 1,000 MW(e) constantly for 90% of the year. With the same overall power output, the nuclear plant only consumes 19 tons of nuclear fuel, compared to 3.4 million tons of coal required for the fossil fuel plant. Perhaps more importantly, the effluent from that nuclear plant’s operations is also only 19 tons. Since nuclear fuel does produce its power by a burning process, no air is needed, no oxygen is consumed and the 8.8 million tons of CO2 produced by a comparable coal plant is avoided. Yes, the waste requires careful management, but that has always been assured. The used fuel is placed in massive welded steel containers that are surrounded with reinforced concrete structures that are constantly monitored. This focus on safe nuclear waste management in the United States explains why there has never been a release of spent fuel into the environment here. Even under the severe accident conditions like those at Three Mile Island, all of the spent fuel has been contained. When compared to the annual storage of 685,000 tons of ash wastes from a similarly sized coal plant that are stored as sludge behind earthen dams, it is easy to understand that there is a significant difference in the cleanliness of nuclear plants compared to coal.
So, where can you get clean electricity in a reliable way? Based solely on the facts, nuclear power is a resource that must be considered. It provides reliable power 24 hours a day, emits no CO2 from power production, produces minimal solid wastes that are safely and securely managed and does not require any new technology to deploy more plants today. When contrasted with other clean technologies it is far more reliable and the cleanliness of nuclear far surpasses other reliable sources of electricity like coal. Let’s protect the environment and assure ample supplies of electricity to meet our growing baseload demand by increasing the size of the fleet of nuclear power plants. This can, and should be done in parallel with expanding the use of other clean energy sources that can help meet peak energy demands.
 From: http://energybible.com/wind_energy/wind_speed.html a source of definitions and background information
on energy production.
 From the NOAA’s National Data Center at: www.ncdc.noaa.gov/oa/mpp/digitalfiles.html#DIG
 From the Energy Information Administration at: http://www.eia.doe.gov/cneaf/electricity/epm/table1_1.html
 From GAO report: “Coal Combustion Residue: Status of EPA’s Efforts to Regulate Disposal” available at:
 From EPA fact sheet available at: http://www.epa.gov/region4/kingston/FINAL_TVA_EECA_Fact_Sheet051810.pdf
 From Duke University testimony to the Subcommittee on Water Resources and Environment, U.S. House of
Representatives available at: http://www.ejrc.cau.edu/Statements%2010-27-09/Attachment%202_Duke%20Study.pdf
 Derived from EIA data on energy generated by nuclear plants at http://www.eia.doe.gov/emeu/aer/txt/ptb0902.html and EIA data on used fuel discharged by nuclear plants at http://www.eia.gov/cneaf/nuclear/spent_fuel/ussnftab3.html