ENTROPY AND SUSTAINABILITY

To the Editor: The fourth paragraph of John Hill’s letter (September 2008) requires some comments.
He wrote, “The first stated goal, solar energy, which creates no entropy, is sustainable by any definition. It takes the forms of solar heating, solar absorption cooling, photovoltaic power generation, and wind power generation.”

Regardless of the definition, solar photovoltaic power generation is sustainable only when the sun is shining. Hydroelectricity is a form of solar energy because the sun drives the hydrologic cycle. Only the hydropower form of solar, with storage of water behind dams, has a degree of sustainability—that is, until the dam silts up.

He also wrote, “Most of the wind turbines, connected to a grid, generate 1.5 to 2 MW day and night; they do not require energy storage.”

Grand Coulee Dam: A sustainable form of solar power.

Wind power can hardly be termed sustainable, even though it can sometimes produce power at night. A daily record of wind speeds will show there are significant periods when the wind speed is too low to generate electricity. When the wind is not blowing, we still need fossil fuels, stored solar energy, and hydro or nuclear power to keep the lights on.

Theoretically, no form of energy is sustainable. However, for humans sustainability is related to time. For example, by using fast breeder reactors, the world has enough nuclear fuel for tens of thousands of years. This gives us plenty of time to find possible alternative sources of energy.

H. DOUGLAS LIGHTFOOT
BAIE D’URFE, QUEBEC

To the Editor: As a retired M.E. designer of industrial machinery for nearly 50 years, with a modicum of heat-transfer experience, I found John L. Hill’s letter on sustainability to be most compelling.

Mr. Hill’s argument is correct. It seems to me that we should, in our struggle to broaden our energy resources and minimize environmental impact, strive to focus on entropy as the best measure of long-term sustainability. As a first step, we need to make the concept understandable to, say, a newspaper reporter.

JOHN HOAGLAND
 LONGMEADOW, MASS.

A LITTLE CAUTION, PLEASE

To the Editor: This is in reference to the write-up (“No Renewable Bottleneck”) in the News & Notes Section of the February 2009 issue.

The brief write-up talks about the work of Elaine Hart and Mark Jacobson of Stanford University. The impression that one gets is that this is a full-blown detailed study, which is not the case. It appears that the work is based on a single-day simulation of the electric grid and is not even the worst case scenario. Please see the authors’ Web site (http://www.stanford.edu/~ehart/) and see the cautionary note that Ms. Hart has posted.

While I appreciate ASME bringing in the news on the important aspects of energy generation and reliability, it would be prudent to provide the cautionary information as well.

GOPAL SISTLA
SCHENECTADY, N.Y.