The Impossible Dream? Why Renewables Won't Reduce CO2 Emissions by Much

Or so the story goes. But new research shows that it is not quite so simple. As the proportion of renewable energy penetrating the electricity grid grows, the reduction of CO2 emissions drops sharply. By the time wind power (and, by analogy, solar) reaches about 20 percent of the grid, the savings in CO2 emissions are negligible, of the order of a few percent.

The result seems counter-intuitive -- surely the more renewable energy, the greater the reduction of CO2 emissions, and less threat of global warming. But the reason for this finding can be found on the miles per gallon sticker on the windows of new cars. The mileage for highway driving is always greater than that for city -- stop and go -- driving. When we touch the brake pedal, we change the engine speed. The lower mileage for city driving means less efficiency from the gasoline, and more pollution per mile driven.

For example, the Toyota Camry, the best-selling auto in the U.S. for years, has a highway rate of about 32 miles per gallon, and about 22 for city driving. If we plotted these two numbers on a graph, we would see the mpg gradually decreasing as the proportion of city driving increased. In the upper left-hand corner of the graph would be long-distance haulers, who stay on the interstates and don't stop between fill-ups. At the bottom right-hand corner would be taxis, which rarely venture out of town. There would be a smooth curve connecting the two points.

In the same way, when back-up electricity (mostly natural gas power plants) -- for the times the wind doesn't blow and the sun doesn't shine -- is ramped up and down, there are more CO2 emissions compared to when the back-up is running full blast. Result -- much of the emissions savings from using wind power or solar is lost.

In analogy to the Camry graph, when just a few wind turbines or solar collectors are part of the grid, their effect on the grid is close to negligible. Their output displaces fossil fuels on a one-to-one basis, and therefore there are CO2 savings. (There are, of course, CO2 emissions associated with the steel, copper, aluminum, and rare earths needed to build the turbines, but these are relatively small).

As more and more windmills and collectors enter the grid, the curve of CO2 savings resembles the Camry curve. Natural gas turbines - back-up - have to be turned on and off to account for the variability of Mother Nature. This switching generates far more CO2 than having the gas turbines running continually.

So when we draw the curve of CO2 emissions of wind and solar, it resembles the Camry curve I mentioned above. In the upper left-hand corner the grid penetration is very small, indicating few turbines or collectors. CO2 savings are close to 100 percent. The curve of CO2 savings gradually decreases, dropping to a few percent when the grid penetration is around 20 per cent.

Since most of us have not seen a gas turbine turned on and off, let me draw an analogy. Almost all of us have started a power lawn mower. Depending on its condition, we usually see a cloud of smoke - pollution - when it starts up after a few pulls of the cord. Even if we turn off the mower and start again, there will be a smaller cloud.

Why gas and not coal or nuclear as back-up? The latter two take hours to turn on and off, and are designed to run continuously. Depending on their design, gas turbines can be ramped up and down within minutes.

Proponents of renewables sometimes claim that if the wind has ceased in one area, it probably is blowing in a nearby location. In that way, at least theoretically, wind power variability can be averaged out, obviating the need for any back-up. Would that this were true. The German company E.On Netz measured wind power output over the entire country, from the North Sea to Bavaria in the south. They found that output varied by a factor of about a thousand over a day or two, showing that the averaging analogy does not hold much water.

I suppose that when the wind is howling in Oregon and the air is dead still in Manhattan, power in principle could be transported cross-continent. But most of the energy would be lost in the process.

What about energy storage? This could reduce or eliminate wind and solar variability. Electricity has to be supplied instantly and without fail to ratepayers - the goal for utilities for loss of service is one hour a year.

Although the Federal government, over the years, has spent billions on this subject, it has very little to show for it. Batteries, like those used in autos like the Chevrolet Volt, would be far too expensive to handle renewable variability. The cheapest battery per unit energy stored, the lead-acid version, was developed by the Frenchman Planté in the 1880s. So we have made little progress in bringing the price down in over a century.

Pumped storage is used in about 17 places in the U.S. When excess energy is generated from any source, renewable or not, it is used to pump water into a reservoir on top of a hill or mountain. When energy is later needed, the water flows down through a turbine, much like a hydro dam. But this form of energy storage is completely dependent on local geography. No hill and no space for a reservoir, no pumped storage. While data is not completely available, pumped storage supplies only a tiny fraction of potential renewable energy storage.

How about molten salts, another form of energy storage? The technology goes back at least four decades and perhaps farther. The idea is to store heat energy in certain salts, and use that heat to boil water, generate steam and run a turbine, much like a fossil fuel plant. Again, the cost to supply storage this was for millions of consumers is prohibitive, although yet another experiment using salts is planned for Nevada.

Is the graph I have developed based on some abstruse computer models, similar to those of the Intergovernmental Panel on Climate Change, purporting to predict the world's climate a century from now? In a word, no. There are experimental results from Colorado, Texas, Germany, Ireland, Estonia and other countries bolstering my argument. While there are uncertainties in the data from which I developed the graph described above, the general trend is clear: The more penetration of the grid by renewables, the less CO2 saved.

In a recent article in Bloomberg News, Prof. Severin Borenstein, head of the University of California (Berkeley) Energy Institute, and one of the most prominent academic advocates for renewables, threw overboard all the arguments for renewables except one - saving CO2. More jobs (as envisioned by the President's stimulus package)? Most of the renewables jobs paid for by the stimulus ended up in China. And dare I mention the word Solyn - forget it. Cheaper than fossil fuels? This was based on projections of fossil-fuel fired electricity prices rising year by year. The development of hydraulic fracturing and the drop in natural gas prices put an end to that argument. Prof. Borenstein still clung to the idea that renewables would reduce CO2 emissions and then alleviate global warming. The data is now in, and his last argument fails.

Governments around the world, from Spain to Germany to the U.S., have poured billions of dollars into subsidizing and requiring renewables. They may have saved some CO2 from the initial introduction of these systems, but the purpose was not to build one or two windmills or solar collectors, but to have them displace most, if not all, fossil fuels. As I demonstrate, the word of the play Man of La Mancha (based on tilting at windmills), it truly was an impossible dream.

Comment Matthew Shapiro
5.17.12 A couple of comments. First, there are many thousands of MW of new pumped storage potential now proposed in the United States, muchof whichis developable withinreasonable cost parameters. Second, the question of the impact of renewables has to be taken in a larger context of how the grid and its generation base can and should be operated. If we are to seriously consider a major contribution of renewables in the coming decades, we need to consider a different kind of "dispatch order" than has traditionally been the case (baseload, thenintermediate,then peaking, with renewables attached like icing).The new order would reflect greater flexibility and technological advances thanthe old order is based on. Here is a possible "new order" to dispatch that would allow renewables to make their full impact: (1) energy efficiency plus demand response programs; (2) variable and intermittent wind and solar, integrated on a wide area basis to maximize diversity benefit; (3) bulk storage in theform of pumped storage and CAES as a bridge to the lesser flexibility of (4) combined cycle gas, followed by minimal use of (5) gas peakers. Existing hydro would serve as baseload to intermediate generation. New nuclear would notbe needed; coal would likely be completely phased out.

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