One topic that is often overlooked when comparing the pros and cons of various energy sources is capacity factor (CF). CF is a measure of the performance of a power source over time as a percentage of its full power potential. A power plant with a 50% capacity factor would operate at 100% power 50% of the time. Conversely, a plant that operated at 50% power 100% of the time would also have a CF of 50%.
The Grid Needs Stability
Capacity factor is important not only because it reflects the performance of a generating station, but also because a higher capacity factor represents a more stable power grid.
Imagine what happens to the grid when a natural gas turbine shuts down. The demand, or load, on the grid remains the same, but that power has to come from somewhere, i.e. another generator. Power plants with a high capacity factor are valuable because they are reliable enough such that they rarely strain the grid. Consequently a power supply with a low capacity factor would most likely require redundant and diverse backup systems to ensure that power is not interrupted during periods of low/no generation.
So what are the capacity factors of the various energy choices? According to the US Energy Information Administration (EIA), in 2007 the capacity factors were as follows:
- Combined Cycle Natural Gas Plant–11.4%
- Oil–13.4%
- Hydroelectric–36.3%
- Renewables (Wind/Solar/Biomass)–40%
- Coal–73.6%
- Nuclear–91.8%
As you can see, capacity factor varies considerably.
Expensive Fuels, Intermittent Sources, Equipment Failures
Most gas turbines sit idle during normal load condition due to the high cost of natural gas. During the peak of summer when air conditioners are running, demand (and cost) rises and the turbines fire up to cash in on the valuable power. Otherwise, it is not cost effective to run the turbines. 2009 numbers are not yet available, but it is likely that with the decline in the price of natural gas, the CF would increase.
The US currently gets more than half of its electricity from coal fired plants. At 73.6% CF, coal represents reliable base load generation. Base load refers to those power plants that are expected to operate 24/7. The missing 26.3% is primarily due to equipment problems caused by the aging and lack of maintenance on the nation’s coal fired plants.
You will notice that renewables manage just a 40% CF. While wind, solar, and biomass have the obvious environmental benefits and low/no fuel related cost, the intermittent nature of a low capacity factor means that backup generation capabilities are required to be in standby. This tends to be the expensive, and carbon emitting natural gas, or the more expensive additional wind and solar generation with massive arrays of batteries to store that extra power for the 60% timeframe when there is nothing being generated. Without base load generation, renewables alone would result in an unreliable power grid with devastating consequences for the country.
Reliable Nuclear Energy
Nuclear far exceeds any other source in capacity factor. In 1979, when the Three Mile Island event occurred, the average capacity factor of the nation’s nuclear plants was at 58.4%. At that time, regulations on the maintenance of safety equipment were far more lax resulting in frequent unplanned shutdowns. After the TMI event, the Institute of Nuclear Power Operators was formed and tighter standards were put in place. Now, with an absolute intolerance for unexpected equipment failures, nuclear plants around the country are operating at better capacity than ever before despite the ongoing aging of the nuclear fleet.
Also improved over the year was the organization of refueling outages. Every two to three years, a reactor must shut down to refuel approximately half or one third of the fuel in the reactor. In the 1970s and 1980s, refueling took months on end as there was little prior experience to draw from. In 1990, the average duration of a refueling outage was 104 days. Now, a reactor can be refueled safely, quickly, and efficiently such that the average duration is just 38 days with many plants doing the job in considerably less time.
The resulting improvement in the capacity factor of nuclear power plants since 1990 alone has supplied 230 billion kilowatt hours of added electricity every year making nuclear power the most reliable source of electricity available today.
13 Comments
Pretty cool information about energy. I’ve always though nuclear energy would end up being one of the best sources for energy once we learned a little more about how to harness the power. Disposing of nuclear waste is a little messy, but we’ve got some bright people working on those issues as well
Blake,
You’re right. Nuclear is one of the best available today. The more you read into it, the more you’ll find that there’s no such thing as “nuclear waste.” Only a valuable resource that can be recycled over and over again. The “problems” facing nuclear were solved decades ago (by those far brighter than I) and it’s politics that stands in the way.
Great to Know
The combined cycle capacity factor looks very low (did you combine simple cycle and combined cycle plants), and the renewables capacity factors look high here. Biomass has a considerably higher capacity factor than wind and solar.
Right on both accounts Lana. The combined cycle CF is so low due to the fact that they sit idle most of the year. I would imagine that once operational, they are comparable to coal in reliability, but they are too expensive to operate 24/7 hence, the low CF.
As to the renewable numbers, I agree that the Biomass numbers skew the statistics. However wind/solar data was not available without the biomass factor. For the sake of balance, I used the same source for all data points in this post. Even with biomass skewing the numbers, it still shows nuclear way out in front.
Tsinghua University in China has, up and running, High efficiency “Pebble Bed Reactors” built in modular form for maximum heat output! Tell me the truth, unshaded, and unjaundiced now, Are there better reactors than the current American designs available in the world today? That produce less weaponizable products? less dangerous waste for a given heat output? Have we been led to the garden path ? Down the propagandists trail? tricked or fooled in any way into seeing only American designs as worthy? Can a low pressure high temperature gas moderated pebble Reactor outperform the common American reactor designs? is national pride and prestige our first interest in nuclear matters? Do others have perhaps “plausible’ or even superior engineering ideas? Is it possible that the larger gene pool in Asia just might produce some innovative and superior notions? Just maybe? Re-branded, like Edison’s light bulb it may save face and a nation desperate for energy?
No.
Hey Jack!
Thanks for the article. I just had something to point out: I think the capacity factor for combined cycle natural gas plants is actually 42%. The capacity factor for conventionally-fired natural gas plants is 11.4%.
(see: http://www.eia.doe.gov/cneaf/electricity/epa/epata6.html)
When you use the waste heat in a gas-steam turbine system to generate electricity (the combined-cycle approach), your efficiency rises to upwards of 60% and power generation suddenly becomes a lot cheaper. That’s why combined cycle plants have a capacity factor of 42%.
Centralized gas turbine combustion (see http://www.naturalgas.org/overview/uses_eletrical.asp) and gas-steam turbines make up nearly all of the electricity generation not attributable to combined cycle plants. They have a capacity factor of 11.4% because these technologies really aren’t intended for base load generation; they’re better suited for distributed generation during peak load hours, as you said.
Kind of splitting hairs, but just thought I’d provide the link. (An article about global electricity production is here http://www.eia.doe.gov/oiaf/ieo/electricity.html .)
Is it true that you can reprocess spent nuclear fuel indefinitely? I thought it could only be fed through a few times. Also, do you know if closed fuel cycles (reprocessing) is outlawed in the US or just not common practice?
-Mark
Mark,
Just to clarify, Capacity Factor is independent of thermal efficiency, which is what you’re describing. Thermal efficiency is a measure of electrical energy produced divided by thermal energy of burning the fuel.
Capacity factor has to do with what % of full power-24/7 does that source produce. Regardless of thermal efficiency, if a generator on runs at 50% of the plate value (even at 100% thermal efficiency, then the capacity factor is only 50%. If the generator runs at 100% power all day every day (even at 5% efficiency) then the capacity factor is still 100% even though much of the potential energy is wasted.
You are mixing up two concepts here, the achievable capacity and the currently used capacity. Biomass and gas could provide as much capacity and reliability as a coal fired plant. It is just not called upon due to the varying load.
So you are not really mesuring potential with your numbers, but actual load.
Nuclear is actually bad because it can’t handle varying load – still you rate it at 91.8%..
I would agree that natural gas and biomass could acheive a capacity factor similar to coal, but they don’t. It’s not because they aren’t needed, it’s because they’re just too expensive to operate. Expensive fuels is as much a cause of low CF as intermittent sources or equipment failures, if not more.
As I said in the post, I would expect the CF for natural gas has improved since 2007 because the price of natural gas has dropped with the financial collapse.
Nuclear can handle varying load, the fact that it can’t is a misconception. It’s just not perferable to run a reactor at 50% power because it’s so cheap to operate, why not shut down that expensive gas turbine instead? I don’t rate it at91.8%, this is an undisputible fact courtesy of the EIA.
You state that “The missing 26.3% is primarily due to equipment problems caused by the aging and lack of maintenance on the nation’s coal fired plants.” Does this mean that new coal plants operate at a similar capacity factor as nuclear plants? Can you please provide sources or supporting evidence? Thanks.
what I need to know is which type of the capcity we should use to calculate the capacity factor of a certain unit, we have different types of capacity :
-name plate(mentioned by manufacturer)
-actual( at 50c)
-ISO(@ 15c,1.013 bar and 60% RH)
-dependable(depend on inlet temp.)
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