Wind Power Has Great Potential

The letter by Frits de Wette (Claims for Wind Power Greatly Overblown, APS News May 2007) makes a few good points but completely ignores the literature that evaluates the wind energy potential, grid integration, and technology in the US and in Europe. For example, the on-shore wind electric potential of the US has been analyzed in considerable detail[1]. Taking into account restrictive land use constraints and economics, the potential is over 1200 GWavg, about 90 percent of which is located in the Great Plains. This potential is understated by about 30 percent since maximum wind turbine tower height was assumed to be 45 m, and towers as high as 100 m are currently being used. This also ignores the US offshore wind electric potential which is very conservatively estimated at about 100 GWavg. Compared to the total US generated electrical power of 440 GWavg (2005), there is clearly the possibility that wind generated electricity could make a substantial contribution to the power supply in the US, especially if energy efficiency and conservation were taken seriously.

Wind turbines are designed to produce power locally at the least possible cost for a given wind regime, which results in a capacity factor (the ratio of average power to maximum power) of about 30 percent. However, it is possible to design wind turbines [2] with a much higher capacity factor. For example, a capacity factor of about 50 percent is possible if the cost of electricity increases by about 10 percent, and even higher capacity factors are obtainable but at an ever-increasing cost. Moreover, if large premiums are paid for wind-generated electricity, as is the case in Germany, then capacity factors of 15-20 percent are tolerable.  Since maximizing profitability is the only consideration for the wind turbine owner, low capacity factors are a perfectly reasonable choice. 

Good wind resources are usually located far from consumers, and large amounts of intermittent energy are not easily handled by utilities, so that transmission and storage issues must be acknowledged and overcome if intermittent wind energy is ever to contribute significantly to demand. This analysis [3] has been done, and one can conclude that it is technically and economically feasible to transform intermittent wind energy to a reliable power source for distant consumers by combining large-scale wind turbine arrays with high voltage transmission lines and compressed air energy storage (CAES).

CAES is based on gas turbine technology and uses compressed air stored in underground structures (solution mined salt caverns or porous rock in a stratigraphic or structural trap) as the storage medium. This is a proven technology, with plants operating in the US and Germany, and is the lowest cost utility scale storage technology available.

Such an approach to wind energy integration has been taken by a group of Iowa utilities, who are building a 268 MW CAES plant with underground porous rock storage. More details are available at

Current wind energy development relies on utilities to provide transmission and back-up for the intermittent power, often without compensation.  This is the reason behind the hostility on the part of some utilities and system managers to intermittent renewable energy, and it is evident in the E.ON report cited by de Wette. Of course, demanding anything without compensation is a good way to make bitter enemies. While it may be justifiable for small numbers of wind turbines on a grid, this should not be expected to continue.

It is unfortunate that renewable energy advocates continue to use hand-waving arguments to justify, or simply ignore the difficulties with, integrating large amounts of intermittent renewable energy on the grid, and that skeptics refuse to examine the issues carefully. One can show [4] that it is possible to power a modern industrial economy using intermittent renewable energy, and that it is not technical or economic limitations, but our lack of imagination that prevents us from taking this approach.

Alfred Cavallo
New York, NY

1. Elliott, D.L. et al., 1991, An Assessment of the Available Windy Land Area and Wind Energy Potential in the Contiguous US, PNL-7789, Pacific Northwest Laboratories, Richland, WA.
 2. Cavallo, A.J., 1997, Wind Turbine Cost of Electricity and Capacity Factor, J. Solar Energy Eng, 119, 312-314.
 3. Cavallo, A.J., 1995, High Capacity Factor Wind Energy Systems, J. Solar Energy Eng, 117, 137-143.
 4. Cavallo, A.J., 2007, Energy, 32, 120-127.

Why Not Go Where the Winds Really Blow?

With reference to the letters on wind power in the March and May issues of APS News: while it is wonderful to generate electricity (inefficiently) in our own back yards, WHY NOT GO TO WHERE THE WINDS REALLY BLOW? Among sailors, the winds of the far southern hemisphere are well known, particularly at  >40 degrees S. latitude. They are known as the furious forties, or even more so the frightful 50’s, or smashing 60’s. Locations falling into this category would be such as (the infamous) Cape Horn, southern New Zealand and Cape of Good Hope, S. Africa. There are also numerous isolated islands, more like rocks, in the southern oceans, e.g. there is a solitary rock about 300 km south of Cape Horn–just imagine the winds. Locations as above have few calm days where the wind drops to only about 60 km/hr.
The next question is how to move the energy extracted to populated areas, particularly in the case of the more remote of the above sites. Perhaps underseas very high-voltage cables are even currently available; if not, then perhaps this would be a worthwhile area for engineering research. Alternately, possibly the energy could be transported in another form–say electrolysis to generate hydrogen, and maybe that could even be converted to methane or propane. Also, energy-consuming industries might be relocated to near these surplus electric generation areas; e.g. aluminum metal extraction.

Russell W. Dreyfus
Sarasota, FL

US Qualifies as a “Rogue State”

I applaud the emphasis on the human dimension in Elizabeth Turpen’s Back Page article (APS News, April 2007). But she concentrates on the possibility of pure fissile material becoming available, primarily to terrorists but also to “rogue states”. The crucial human dimension, however, is the feelings, of both governments and the people, in countries which have not yet made nuclear weapons but might do so. We should recognize those who have decided not to do so and give them and their countries honor and thus enhance their legitimate national pride.

Associated with that is the human dimension in the United States government which persists in arrogant and non-scientific attitudes. Most scientists argue that the Anti-Ballistic Missile program will not work. The US Senate failed to ratify a test ban when it has been shown that in every postulated scenario the US would be safer with such a ban than without. The military still maintains a stockpile of 10,000 nuclear weapons when 100 should be enough to scare anyone. It must be recognized that in much of the world these three counterproductive actions are enough to classify the USA as a “rogue state”. Many scientists and others overseas, with considerable justification, argue that the USA is, in effect, the principal violator of the Nuclear Non-Proliferation treaty, earlier of sections 4 and 6 and recently of sections 1 and 2. 

During the cold war between the USSR and USA there were many personal scientific interchanges, in some of which I participated. I and many others have suggested that these personal interactions were crucial in keeping the war cold. Where now are the daily interactions between Iran and the USA? A former minister in the Iranian government recently told me privately that whereas most Iranians were not interested in nuclear energy as recently as 5 years ago, now 70% of the people would vote for a strong supportive position, as a matter of national pride. We need to help them find national pride in peaceful activities for the benefit of the region and the world, instead of the more warlike uses which we, unfortunately, have taught them.

Richard Wilson
Cambridge, MA

Premature Praise for California

In his essay, “Climate Change is all about Energy” that appeared as the Back Page in the May 2007 issue of APS News, Drew Shindell writes, “Primarily through mandating more efficient use of energy, California has held its per capita energy use roughly constant since the early 1970s. During this same period, per capita energy use has gone up ~50%, nationwide.”

This last statement is not correct. Since the early 1970s, the nation’s per capita energy consumption has remained relatively constant–apparently similar to California’s (see Inasmuch as California’s energy consumption has pretty much tracked that of the rest of the nation it might be a bit premature to sing praises to that state’s regulatory policies.

John Scofield
Oberlin, OH

Drew Shindell replies:
In my essay on climate change, I referred to California’s successful energy efficiency program. I should have made clear, however, that the efficiency gains that greatly diverge from the national average were in the electricity sector. California’s per capita electricity use has been roughly constant since the 1970s, while the US as a whole has seen per capita electricity use increase by ~50%. As stated in the essay, the remarkable success in California is primarily due to gains in building, appliances, and utilities. These could be replicated nationwide, and indeed some other states and cities are adopting similar regulations. As electricity generation is the single largest contributor to US greenhouse gas emissions, California demonstrates that increased efficiency can lead to substantial reductions in global warming emissions, with ancillary benefits such as reduction of air pollution and reliance on imported oil. Clearly other sectors of the economy that contribute to overall energy use, such as transportation and industrial emissions, also need to be addressed.

Inconsistency is an Acceptable Price for Democracy

I’m not sure what point Michael Lubell is trying to make with his column about term limits in the May, 2007 issue. Very few of the members of Congress who made term-limit pledges actually honored those pledges in 2006. Instead, those who were replaced failed to get re-elected for a litany of reasons from rampant corruption to lack of character to anger over the war. Mr. Lubell laments the loss of consistency in policy as a result of the election and, recalling the term-limit pledges and the bills passed twelve years ago after the last major “change” election, imagines that the new members are there because of term limits. What actually happened was that we had an election, in which the voters chose new representatives because of the less than satisfactory performance of the old ones. Is he lamenting our democratic political process because of the lack of consistency it occasionally engenders? Or is he, as I suspect, saying that mandatory term limits should be approached with caution? The latter is a perfectly good argument to make, but I must confess a lack of dismay that some of the new members still need to be educated about competitiveness issues and the need for science funding, when I weigh that minor inconvenience against the value of being able to change our government when change is needed. I, for one, will not mourn the “consistency” of the old Republican majority. We should all gladly suffer a little inconsistency and uncertainty about our pet issues, such as science funding, to preserve the means of peaceful change and accountability that elections provide. And I can think of few worse examples to illustrate the dangers of mandatory term limits than the 2006 elections. From where I sit, the 2006 elections demonstrated the benefits of turnover in government, if anything at all.

Andrew Puckett
Newport News, VA

Need a Comprehensive View of Climate Change

I read Drew Shindell’s Back Page (APS News, May 2007)  with interest and appreciate his discussion of the physics of climate change and its relation to energy consumption. It stimulated my interest while at the same time increasing my frustration at ever hoping to gain a comprehensive view of the dynamic equilibrium that results in the climate we have. Lay articles as well as those written for the general physics community seldom (never in my experience) deal with more than one or a few of the mechanisms that affect our climate; and then usually not with enough quantitative (or even ball-park semi-quantitative) detail to allow evaluation of their relative importance.  

 For example, if an author quotes the amount of carbon emitted into the atmosphere per person per year, I would also like to know how much carbon is absorbed per acre per year of forest, agricultural land, or surface water. Knowing the fractional imbalance in the emission would be an important part of a quantitative understanding.

Likewise, although everyone talks about carbon dioxide, very few talk about methane or ozone like Shindell did. But he gave no quantitative hint about the relative importance of the latter two relative to carbon dioxide.

Water vapor in the air is an infra-red active absorber; why is it not discussed? Is it important or not? If the surface temperature of Earth rises, then the rate of evaporation of water should increase. Will this lead to a run-away effect in global warming or will there be an increase in cloud cover, reflecting sunlight, that reduces insolation?

I sure would appreciate a more comprehensive article on climate equilibrium than the usual one.

Aare Onton
Saratoga, CA

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Editor: Alan Chodos
Contributing Editor: Jennifer Ouellette
Staff Writer: Ernie Tretkoff