Storing Wind Energy: Nitty-gritty

Q: Can wind energy by stored? How much wind or solar power does it take to produce the equivalent of a gallon of gas? How does that compare to five-10 years ago and to projections two or three years out? Is there a way to compare efficiencies of carbon-based fuels to solar and wind?

Asked by Mark Kalkus, ’84, JD ’88, Littleton, Colo.

While modernizing the grid will improve its overall end-use efficiency, an efficient grid means little unless the power sources connected to it are efficient and cost-effective as well. Wind and solar energy efficiencies—and capacities, subsequently—have improved dramatically in the past 30 years, and are becoming more cost-competitive with fossil fuels.

Wind Power

In full wind, a modern 2.3 megawatt wind turbine—a massive 300-foot giant with 150-foot arms—generates more than 8,000 megawatt-hours in a year, enough to power more than 700 American homes. The energy generation capability of wind turbines has increased dramatically since 1981: a single 2.3 megawatt turbine today produces more energy than 180 turbines from the early 1980s. The cost of wind power has declined significantly. Several technological innovations have been especially important in this development. Turbines are larger, and longer blades translate to more wind energy harnessed and more power production. In 1981, the average rotor diameter was 10 meters. By 2009, it had increased to 82 meters.

Courtesy juwi Wind USA

Rotor hubs are more flexible, which minimizes damage during gusty winds. This allows turbines to keep spinning when there are powerful winds, which increases the amount of time they are generating electricity.

Construction materials are stronger, enabling turbines to reach higher into the sky where the winds are more powerful and there is more energy to be extracted.

These and other turbine improvements have resulted in an increase in efficiency from 20.5 percent to 40 percent. These larger, more efficient turbines and their widespread expansion have increased total energy generated from 45 MWh in 1981 to more than 8,000 MWh—or 2 percent of the U.S.'s energy supply—in 2009. A recent U.S. Department of Energy analysis shows that wind could feasibly meet 20 percent of U.S. energy demand by 2024.

It takes a typical modern turbine 2.2 minutes to produce the same amount of energy in a gallon of gas. In 1981, it took 71.5 minutes to achieve that same feat. A 35-fold improvement in 29 years—20% by 2024 is looking feasible indeed.*

Solar Photovoltaics

Solar photovoltaics, or PV, have a similar efficiency story, except their energy-generating capabilities typically align with the afternoon peak electricity demand. In sunny areas, the most efficient PV arrays convert 19 percent of the sun's energy to electricity. Land area is a valid concern when considering the drawbacks to PV, but rooftops are often overlooked. According to Nanosolar, if we covered all 12 billion square meters of rooftops in the United States with the most efficient PV technology, they would provide 25 percent of U.S. electricity supply. To provide all of our electricity, we would need just 0.4 percent of the nation's land area.

NREL via Solid State Technology

Photovoltaic efficiencies have improved in recent years, dramatically reducing the price of solar power. In 1980, the power generated by PV cost $21.83/W (in 2002 dollars). In 2009, the first large-scale PV panel manufacturer, appropriately named First Solar, announced that they were able to manufacture photovoltaics for less than $1/W, which is widely considered to be the price that will enable widespread solar power adoption.

One of the largest PV power plants in the United States is the DeSoto Energy Center in Florida. The 25-MW, 90,000-panel, 180-acre plant can generate the quivalent energy in one gallon of gas in 27.5 seconds; however, one average 275-watt panel needs 687 hours of sunlight to produce that much energy. An average U.S. household would require approximately 24 solar panels to generate all of its electricity, and the energy required to manufacture the panels in the first place would be paid back within one to three years.†For a more thorough discussion on PV, see this answer from the SAGE archives.

Projections for the Future:

There is no direct way to convert between fossil fuel and renewable energy resource efficiency; however, all sources are being used more efficiently today. In fossil fuel power plants, waste heat is captured and used to power another turbine. In buildings, thicker windows, building orientation, structural materials, and ventilation systems are only a few of the ways in which energy efficiency is improving. And, of course, we can't overlook wind's progress: today, one turbine can do what 180 could in 1980, saving land area, building materials and transmission line construction. So while there's no unit to measure efficiency between energy systems, efficiency is increasing for them all individually.

This improved efficiency may pave way for an energy supply comprised entirely of renewable energy, said Professor Jacobson in a recent Scientific American article. "Intermittency problems can be mitigated by a smart balance of sources," he writes. Geothermal energy supplies base load, while wind, solar, and other renewables are utilized depending on their availabilities. As grid and renewable energy efficiencies continue to improve, a sustainable energy future becomes more and more feasible. Efficiency is indeed the ultimate renewable energy resource.

Notes:

*One gallon of gas contains approximately 125,000 Btu's, and one Btu is equivalent to 2.9307107 x 10 ^-7 MWh, or megawatt-hours. Thus, one gallon of gas embodies the same amount of energy as .0366388 MWh. A typical modern wind turbine generates 2.5 MW of electricity and has a capacity factor, or efficiency, of 40%, meaning that when we average things out, a 2.5 MW turbine will be generating 1 MW of electricity at any given time, and 1 MWh in one hour. In 1981, an average turbine generated .15 MW of power, and had a capacity factor of 20.5%, meaning it would generate .03075 MWh in one hour, and would take 1.2 hours, or 71.5 minutes to generate the amount of energy in a gallon of gas.

†The average U.S. household consumes .92 MWh in one month, the equivalent of 25 gallons of gas. 24 2.8 square meter solar panels assuming a high efficiency of 19.2%.

LAUREN KUBIAK, '10, expects to receive her master's degree in earth systems in 2011.