Energy News: Not All Megawatts Are Created Equal – An Examination of Electric Generation Capacity Factor
By Michael D. Vanden Berg
The Utah Geological Survey recently published the fourth edition of Utah’s Energy Landscape, a comprehensive, visually-based publication detailing Utah’s diverse energy portfolio. The highlight of this new edition was the exponential growth in Utah’s utility-scale solar electric generation capacity. In 2015, 166 megawatts (MW) of new utility-scale photovoltaic solar capacity was installed in southwestern Utah and 601 MW was under construction and expected to be complete by the end of 2016.
By the time this article is published, 767 MW of solar capacity will be operational, surpassing the installed capacity of all other renewable energy sources (wind, hydroelectric, geothermal, and biomass) combined. The new solar capacity accounts for about 8.6 percent of Utah’s total electric utility capacity of about 8,900 MW, the vast majority of which comes from fossil fuels (coal accounts for 54 percent and natural gas accounts for 35 percent). However, not all megawatts of generation capacity are created equal when comparing fossil fuels to renewable energy resources.
The capacity factor (CF) of an electric power plant is the ratio of its actual output over a period of time to its potential output if the plant could operate 100 percent of the time. The CF for a power plant is calculated by dividing the actual amount of electricity generated by the plant by how much electricity the plant could have generated during the same amount of time at 100 percent capacity. For example, if a coal plant with a nameplate capacity of 100 MW generated 1,200 megawatt-hours (MWh) in one day, it would be operating with a CF of 50 percent (1,200 MWh / [24 hours x 100 MW]). Using electricity data collected by the U.S. Energy Information Administration, we can calculate the CF for various electric plants and fuel types employed in Utah and observe how the CFs change throughout the year.
The chart (Average annual capacity factors for Utah power plants by fuel type) displays the average annual CF for different fuel types in Utah for a three-year period (2013– 2015). The first observation that stands out is the difference between base-load electric plants and “peaker” plants. Base-load plants produce the lowest-cost electricity and are designed for maximum efficiency. Coal-fired power plants in Utah essentially run full time (CF of about 75 percent) and provide important base-load power to users in Utah and surrounding states. No power plants, even base-load plants, run at 100 percent capacity due to unexpected equipment failures or routine maintenance. Landfill gas and geothermal operations also have high CFs, 82 percent and 62 percent, respectively, and are run as base-load plants, but only account for 1.3 percent of Utah’s total electric generation. Combined-cycle natural gas (CF of about 40 percent) and hydroelectric (CF of about 28 percent) power plants are often used as “peaker” plants. These operations can be rapidly brought online to provide power at times of high demand, like during hot summer days when air conditioners are running full steam.
The CF for the above-mentioned power plants can all be manually changed depending on the amount of electricity needed at the time. In the case of hydroelectric plants and depending on the availability and quantity of stored water, operators can quickly increase the amount of water running through the plant’s turbines. In contrast, the CF for wind and solar installations is entirely dependent on external factors, for example the sun only shines during the day and the wind only blows at certain times. Utah’s two utility-scale wind farms (a third was added in 2016, but generation data are not yet available) operate at an average CF of 24 percent, and preliminary data from Utah’s new solar installations indicate that these plants operate at an average CF of about 20 percent, far below that of a typical base-load power plant.
The graph (Average monthly capacity factor in Utah by fuel type) displays the average monthly CF for Utah power plants for 2014 and 2015. This graph nicely highlights the seasonal changes in CF depending on the fuel type. For example, base-load coal plants show large dips in generation during the spring and fall when electricity demand for heating and cooling is generally lower.
Natural gas plants run at their highest capacity during the summer months to supply needed electricity for air conditioners and run lowest during the winter months. Hydroelectric plants can boost capacity in the spring as snow melts and runoff increases, and solar enjoys an expected increase during the longer summer days. In general, wind capacity is highest in the spring and lower in the late fall to early winter. Geothermal plants are most efficient during the winter months when they can take advantage of greater differences between the hydrothermal water and air temperatures, and are less efficient in the summer months. Geothermal and solar installations are perfect compliments, providing peak energy at opposite times of the year, a situation in which Utah can take full advantage.
In summary, caution needs to be used when speaking about the nameplate capacity of power plants and then comparing these numbers across different fuel types. In reality, 1 MW of coal capacity can generate about three times more electricity than 1 MW of solar capacity. It is still a remarkable achievement for Utah to have nearly 767 MW of new solar capacity, but with only a 20 percent CF, most of that added capacity can never be used. For this reason, it is essential to have a diversified fleet of electric power plants in the state to provide a stable and reliable supply of electricity to all Utah citizens.
For more information on Utah electricity or other energy-related information, refer to the Utah Energy and Mineral Statistics website at http://geology.utah.gov/resources/energy/utah-energy-and-mineral-statistics.
Survey Notes, v. 49 no. 1, January 2017