The environmental push for renewables and mandates to force them into existence are rightly facing some serious headwinds. The American Renewable Energy Production Tax Credit Extension Act of 2011 foundered in Congress, and more states are experiencing significant power rate increases to cover renewable energy production costs. While renewables are generally not ready for prime time in large quantity on today’s power grid, that doesn’t mean environment concerns ought to be trashed, especially when a more effective off-the-shelf solution is available.
Wind Energy vs. Natural Gas
Suppose we weigh the environmental pros and cons of a decision to add a new generation resource. The two “finalists” are a 100-MW wind farm and a 100-MW combined cycle gas plant. Our goal in selecting a new plant is to offset carbon dioxide (CO2) emissions from an existing 100-MW coal-fired plant that is nearing retirement.
Currently, our target coal plant operates at less than 67% of the time because it is “dispatched” or operated to conform to load requirements. Our plant runs about the same rate as the entire U.S. coal fleet. It normally runs at very high availability rates—in excess of 90%—but is “scheduled” to match consumer load requirements. Our prospective gas plant can easily take the place of the retiring coal unit, as the new gas unit will perform with similar characteristics. It can be sited flexibly and is fueled by a combustion process that can be operated to match the consumer load requirements at any time.
In contrast, our prospective wind resource’s schedule is based on the operator’s best estimate of future weather patterns. The primary challenge of wind is the intermittency of the supply. Unless it is backed up with another peaking resource (usually gas turbines), we cannot use the wind resource by itself as a complete coal replacement.
Intermittency: Wind Energy’s Major Drawback
The obvious allure of wind energy is the promise of electricity with lower emissions of CO2 and other contaminants. So, how well does wind energy perform? The national average output efficiency for wind is slightly less than 30%. This means that a 100-MW wind farm will average only 30 MW of output. This might come in the form of near 100% output for 30% of the time and zero output during nearly 70% of the time, with numerous iterations in between. But over time the average will be about 30 MW. Thus, the ability of wind farms to displace coal plants is “energy limited” because of the intermittent actual output.
Although our target coal unit averages 67 MW in output, the maximum replacement that we can get from the wind resource is approximately 30 MW, which is about a 45% displacement. Consequently, we are unavoidably left with an average 37 MW of coal-fired generation that must operate when the wind is not blowing.
Wind energy generation tends to be strongest during off-peak periods (probably about 30% of the time)—the same periods when the coal plant is most likely to be scaled back to the minimum load. And if the wind energy comes on during these periods, the coal plant cannot be further reduced unless it is taken completely off-line, which would subsequently incur restart costs and ramping challenges. Therefore, the wind farm will most likely displace some other resource, such as gas or possibly hydro. Obviously, the carbon reductions from wind energy will be much lower, or even nonexistent, in this circumstance. So the range of coal displacement that wind energy can achieve is somewhere in the range of 20% to 45% and averaging 30%.
Natural Gas’s Efficiency Provides an Environmental Edge
Now let’s consider the gas alternative. Natural gas combustion produces about 45% fewer CO2 emissions than coal combustion. Furthermore, gas units are more efficient than their coal counterparts because they use less fossil fuel to generate the same amount of electricity. When the increased efficiency is accounted for, a combined cycle gas plant can be expected to produce about 60% less CO2 than a coal plant. Because gas can run essentially all of the time and can easily ramp up or down to match consumer loads, it’s a natural operational substitute for coal. Therefore, if we were to install a new 100-MW combined cycle gas-fired plant instead of the 100-MW wind farm in our earlier example, there would be a net 60% CO2 reduction compared to coal.
With the same installed capacity, a combined cycle gas turbine can provide net CO2 reductions that are greater than wind energy sources—probably about double the environmental benefit. In addition, a new wind farm can be expected to have an installed cost that is about double the price of a new combined cycle gas turbine.
As we gather increasing real-world data on renewables’ performance, it is apparent these resources have a long way to go before becoming viable contributors to the world’s energy needs. Still the environmental community can take heart there is an alternative that’s ready to go at half the cost.
— Kimball Rasmussen (firstname.lastname@example.org) is president and CEO of Deseret Power based in South Jordan, Utah. His recent works that are available at no cost upon request include A Rational Look at Climate Change, A Rational Look at Green Jobs, and A Rational Look at Renewable Energy.