Exergy

Wikipedia

Wikipedia has an extensive entry on exergy, which it defines as:

"In thermodynamics, the exergy of a system is the maximum work possible during a process that brings the system into equilibrium with a heat reservoir.[1]. When the surroundings are the reservoir, exergy is the potential of a system to cause a change as it achieves equilibrium with its environment. Exergy is then the energy that is available to be used. After the system and surroundings reach equilibrium, the exergy is zero."

"Exergy is a measurable value that is decreased during the conversion of useful energy to useless energy. Therefore, exergy measures the actual potential of a system to do work. The exergy consumed to create something, a product or service, is more than the work done to create it. Exergy is the work that can no longer be done elsewhere because the economic good was made. Exergy has been described as a measure of energy quality because of these traits."

After a bit of 19th century history, it lists a bunch of mathematical formulas that are way over my head. It refers to applications of exergy in the design of chemical plants - rather than employing processes that lose as little energy as possible relative to energy inputs, exergy leads to processes that lose/destroy as little available work as possible from a given input of available work. The article then goes on to explore the application of exergy in natural resource utilization, sustainability, thermodynamic value of economic good and complex physical systems.

Comparison of energy and exergy (extract from Wikipedia)

The energy change of a process is...	The exergy change of a process is...
its ability to produce motion	its ability to produce work
conserved by the first law of thermodynamics	only conserved for reversible processes and destroyed by irreversible processes
different from zero (E=mc²)	equal to zero when at equilibrium with the environment
independent of environment parameters	dependent on environment parameters
limited by the second law of thermodynamics for all processes	unlimited for reversible processes due to the second law
a measure of quantity only	a measure of quantity and efficiency of utilization

Quality of energy types

The Wikipedia article defines energy quality as the "ratio of exergy to energy in a substance". Specifically, thermal energy and radiation cannot be completed converted to work, and therefore exergy is less than energy. Context appears to be critical - different forms of energy have varying ability to perform work depending on the specific transformation mechanism. Wikipedia uses the example of water in an internal combustion engine vs. water driving a turbine. It goes on to suggest that "energy quality" relates to "the energetic differences between substances and their propensities to perform work given a specific mechanism." It is intriguing that Fil mentioned a 6-stroke engine that injects water into the cylinders after the power stroke to convert waste heat into usable energy.

Exergy.org

This website describes the relationship of energy and the Second Law of Thermodynamics. A key concept is the "work potential of a substance relative to a reference state." A reference state is at equilibrium - it has zero motion and potential. It is not affected by the interaction under study. The environment on the surface of the Earth is often used, even though it is neither at equilibrium nor unaffected by energy transformations. Various attempts have been made to correct for these effects while still providing useful analytical results.

The site goes on to talk about physical, chemical and nuclear energy, providing formulae, tables and figures that do not explore the value of analyzing the exergy of these energy sources.

Global Exergy Reservoirs, Flux, and Anthropogenic Destruction shows a diagram of exergy flows through primary and secondary reservoirs to natural energy destruction (friction, chemical or nuclear decay, loss of solar energy through re-radiation and conversion to other forms of energy) or anthropogenic destruction ("total amount of exergy destroyed while processing or attempting to collect a resource, regardless of the useful amount of energy we extract.") Reservoirs or accumulations of exergy are included. Areas of the diagram are clickable to show additional details. The analysis of photosynthesis (The Biomass Resource) points out that photosynthesis is about 0.5-10% efficient. Again, lots of detail but little discussion of the implications. The Biomass page indicates humans use about 16TW of land productivity, 5.5TW (combined with 0.5TW of fossil fuels) are employed in agriculture, generated 0.8TW food and 0.02TW ethanol for transportation. 5TW of wood contributes to 1.5TW of energy from wood fuel. It is not clear where the remaining 16-(5.5+5)TW of human consumption goes, possibly to Industrial applications.

The last section titled Exergy Analysis talks about "tracing the useful portion of the energy flowing through a system" to uncover "primary sources of loss" and "performance relative to the theoretical ideal", without going into any details.

Exergy: Kay

James Kay has written a number of articles on [SOHO_Systems|SOHO] (self-organizing, hierarchical, open) systems and implications of exergy. His work is the best I have found so far in exploring the implications of exergy to sustainability, such as his suggestion on using narratives to analyze SOHO systems such as ecosystems, rather than traditional approaches that "focus on forecasting and a single type of entity such as a watershed or a forest community."

Exergy: Dincer/Rosen

Marc Rosen is the Dean of Engineering and Applied Science at the University of Ontario Institute of Technology (UOIT, Oshawa). Previously, he taught for 16 years at Ryerson (Fil has contacted Marc to set up a meeting). Marc, in conjunction with Ibrahim Dincer (professor of Mechanical Engineering at UOIT) published EXERGY: Energy, Environment and Sustainable Development, a " research-oriented textbook ... for advanced undergraduate and graduate students ... and as an essential tool for practitioners."

Exergy: Schneider/Sagan

Eric Schneider was co-author of a number of papers with James Kay. He has continued to be active in researching exergy and its implications.

Implications

Based primarily on various articles by James Kay, the concept of exergy provides a promising framework relating energy (sources, usage, quality/exergy) to the formation, structure and functioning of complex systems. Kay argues that a restated version of the 2nd law of thermodynamics underlies the emergence of structure and organized processes in open systems through which high quality energy flows. This allows entropy to be reduced locally, with a corresponding increase in entropy in the encompassing system that supplies the energy. Weather systems, ecosystems and life itself are examples of entropy 'running backwards' towards increased organization, effectively 'stealing' entropy from the surrounding environment by dissipating energy and reducing energy gradients. Ultimately, the energy comes from the sun (with a small dose of geothermal energy from the earth itself). Overall, disorder is increasing, but pockets of order emerge where energy flows and other environmental conditions allow.

I have trouble relating exergy to sustainability. I had hoped that exergy would provide insights into how we can more effectively use energy. Matching energy quality to the task can open up design possibilities, but I am not sure we can develop an 'exergy target' that designers can use to evaluate how far they are from maximum energy effectiveness. Exergy calculations are very context specific. In the case of a lake where the water column is clear and sunlight is captured by bottom-dwelling vegetation, solar energy is passing through the water column but cannot be used by phytoplankton because their growth is constrained by a lack of free phosphorus. The energy of the sun measured in the water quality is some percentage of the incident solar radiation on the lake, but the exergy is zero. There is no capability in the system to turn the solar energy in the water column to work - that happens at the bottom of the lake. If enough phosphate-rich run-off enters the lake, phytoplankton can suddenly 'bloom' - the exergy of sunlight in the water column is now high.

Kay also does not provide specific guidelines on which structures/processes are sustainable and which are not. The best I could find (Ecosystems 2000) was "Species which survive in ecosystems are those that funnel energy into their own production and reproduction and contribute to autocatalytic processes with increase the total exergy degradation of the ecosystem." One could argue that human systems are just another example of the increased structure and organized behavior that we see in nature. Like photosynthesis and hurricanes, our attempts to use (dissipate) energy are consistent with the imperative of the 2nd law. Even our use of fossil fuels could be seen as an example of the 'Release' phase of Holling's four-box model (Ecosystems 2000 - you need to log into the Wiki to see the diagram), although the timescale of the 'Reorganization' phase far exceeds the likely lifespan of the human species.

One implication is that our actions are really not that different from other species. There is no "lost world" of sustainability. If beavers had our technological capabilities, they would likely be as destructive as we are. I think 'rates' are critical - using fossil fuels no faster than they are replenished would not be a problem. What I do not see is any obvious 'rate limiter' in nature. Perhaps 'balance' is a key concept. Species that co-evolve enter a state of 'dynamic balance' where neither side has the clear upper hand, a theme that Gribbin described in Deep_Simplicity. It is possible that success brings its own (perverse) reward. The first North Americans met large animals that had not evolved with humans and were therefore easy prey. Diamond argues that the lack of large animals put early North Americas at a disadvantage, preventing them from evolving out of a hunter-gather existence. In contrast, Eurasians had numerous large animals that were easily domesticated.

Kay's ideas on using narratives to explain complex systems ( Ecosystems 2000) is appealing. People are attracted to stories, not dry formulae and esoteric descriptions. Telling stories may seem accessible to the typical designer. Also, we can tap into expertise in developing scenarios.

It might be worthwhile looking at where entropy is increasing to compensate for the reduced entropy created by self-organizing systems. Our attempt to dissipate energy/degrade exergy to create order must be balanced by greater disorder elsewhere. At the moment, I am not able to come up with any good examples.

John's comments (e-mail of January 18th) have been posted to the Exergy discussion page.