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Forbes.com

 

The Real Problem With Renewables

May 11, 2010

By Robert Bryce

Hint: It’s physics.

The growing oil spill in the Gulf of Mexico has, predictably, resulted in a new chorus of voices calling for increased use of renewable energy sources. But over the past five decades renewables have actually been losing market share.

In 1949 nearly 91% of America’s total primary energy came from coal, oil, and natural gas. The balance came from renewables, with hydropower being a dominant contributor. By 2008 the market share for coal, oil and natural gas, along with nuclear, had grown to 92.5% of total primary energy in the U.S. with the remainder coming from renewables.

Given the raging hype over renewable energy sources, those numbers, which are readily available from the Energy Information Administration, are remarkable. Over the past six decades tens of billions of dollars have been spent on renewable and alternative energy schemes such as wind energy, solar energy, corn and other biofuels, and electric cars. All have aimed at cutting our hydrocarbon use. And yet only nuclear power, which went from zero to about 8.5% of the U.S. primary energy over that time frame, has managed to steal significant market share from coal, oil and natural gas.

In other words, despite these huge investments, renewables’ share of the energy market has been shrinking. What’s happening? While conspiracy theorists may want to believe that Big Oil, Big Coal and Big Nuclear are stifling the growth of renewables, the simple truth is that coal, oil, natural gas and nuclear can satisfy the Four Imperatives: power density, energy density, cost and scale.

The Four Imperatives provide a simplified way to analyze the physics and math that rule our energy and its delivery, the latter better known as power. Before going further we must differentiate between energy and power. If you recall your high school physics, the definitions are straightforward: Energy is the ability to do work; power is the rate at which work gets done. Put another way, energy is an amount; power is a rate. And rates are more telling than amounts.

The first of the Four Imperatives, power density, is the most telling of the rates. Power density refers to the energy flow that can be harnessed from a given unit of volume, area or mass. Common metrics of power density include: horsepower per cubic inch, watts per square meter and watts per kilogram. And given the current infatuation with renewable energy sources like wind and solar, the essential metric for power density is watts per square meter (W/m2), which shows how much power can be derived from a given piece of real estate. It is also the metric that exposes the inherent weakness of sources like corn ethanol, wind energy and solar energy. If a source has low power density, then it will likely require too much real estate, material or space to provide the power that we demand at prices we can afford or in the vast quantities that the world needs.

The production of corn ethanol is a loser for many reasons. Just a quick glance at corn ethanol’s power density--just 0.05 W/m2--shows why the fuel makes no sense from a physics standpoint. Corn ethanol’s low power density is inherent in all biomass, which leads us to the second of the Four Imperatives. Energy density refers to the quantity of energy that can be contained in a given unit of volume, area, or mass. And the low energy density of biomass--corn, switchgrass, wood, etc.--makes it difficult to produce sufficient amounts of energy without occupying huge swaths of land.

Now let’s consider the power density of wind energy, which is about 1.2 W/m2, and solar photovoltaic, which can produce about 6.7 W/m2. Both sources are superior to corn ethanol (nearly everything is), but they are incurably intermittent, which makes them of marginal value in a world that demands always-available power. Nor can they compare to the power density of sources like natural gas, oil and nuclear. For instance, a marginal natural gas well, producing 60,000 cubic feet per day, has a power density of about 28 W/m2. An oil well, producing 10 barrels per day, has a power density of about 27 W/m2. Meanwhile, a nuclear power plant like the South Texas Project--even if you include the entire 19 square-mile tract upon which the project is sited--produces about 56 W/m2.

Simple math shows that a marginal gas or oil well has a power density at least 22 times that of a wind turbine while a nuclear power plant has a power density that is more than 8 times that of a solar photovoltaic facility. Those numbers explain why power density matters so much: if you start with a source that has low power density, you have to compensate for that low density by utilizing more resources such as land, steel, and ultra-long transmission lines. Those additional inputs then reduce the project’s economic viability and its ability to scale.

That can be understood by comparing the land use needs of a nuclear plant with those of a wind energy project or a corn ethanol operation. The two reactors at the South Texas Project produce 2,700 megawatts of power. The plant covers about 19 square miles, an area slightly smaller than the island of Manhattan. To match that output using wind energy, you’d need a land area nearly the size of Rhode Island. Matching that power output with corn ethanol would require intensive farming on more than 21,000 square miles, an area nearly the size of West Virginia.

Environmental groups and many politicians in Washington insist that the U.S. must lead the effort to develop renewable energy sources, with wind, solar and biomass being the lead components. But doing so will mean replacing high-power-density sources that are reliable and low cost with low-power-density sources that are highly variable and high cost.

The ugly oil spill in the Gulf may continue growing in size. In response the Obama administration may approve more projects like Cape Wind, the controversial offshore wind energy project in Massachusetts. And Congress may pass another energy bill that gives yet more mandates and subsidies for renewables, but try as it might, Congress cannot repeal the laws of physics.

Original Source: http://www.forbes.com/2010/05/11/renewables-energy-oil-economy-opinions-contributors-robert-bryce.html

 

 
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