Exergy: Dincer/Rosen

EXERGY: Energy, Environment and Sustainable Development, Ibrahim Dincer and Marc A. Rosen, Elsevier, 2007, 454 pages. It is a 'dense' book, full of formulas - it will take considerable time to digest the material.

Chapter 1. Thermodynamic Fundamentals

This chapter covers the intersection of energy, entropy and exergy. Each of these domains are used in other fields (such as statistics and information theory). Some forms of energy are entropy-free (shaft-work <mechanical work, ignoring friction?>) and some systems involve energy but no exergy (air at ambient pressure). The chapter covers the first law of thermodynamics, entropy and irreversibility (work can be fully converted to heat, but heat cannot be completely converted to work due to degradation of energy quality), the second law of thermodynamics, and exergy. Energy analysis typically involves energy balances and efficiencies, using the first law of thermodynamics. These calculations are most useful in equilibrium systems. Exergy analysis also incorporates the second law of thermodynamics and focuses on where energy quality (the ability to do work) is degraded. Exergy is a critical concept in understanding systems that are not at equilibrium.

Chapter 3. Exergy, environment and Sustainable Development

This chapter describes the implications of energy usage on sustainability and recent work incorporating the concept of exergy. Environmental concerns caused by energy usage include:

• global climate change
• stratospheric ozone depletion
• acid precipitation

A long list of potential solutions are touched on, including recycling, efficiency, renewable energy and energy storage. Numerous barriers exist to promoting renewable and advanced energy technologies. The authors believe that the second law of thermodynamics and exergy are key concepts in understanding the environmental impact of energy usage. Three areas are explored:

• order destruction and chaos creation: environmental damage through increased entropy, <reversing increased complexity and organization found in SOHO systems>
• resource degradation: exergy consumed to refine metals, direct loss of exergy from reactive resources (e.g., fuels), reduced through efficiency and use of external exergy resources (solar, <geothermal?>)
• waste exergy emissions: can destabilize the environment (pollution, waste heat) or interfere with other systems (CO2 and other greenhouse gases)

The concept of constrained vs. unconstrained exergy is introduced. Exergy that is constrained or controlled can be used to perform useful work, while unconstrained exergy can have uncontrolled impacts on the environment. <This concept shows potential for further elaboration to define exactly how the exergy is constrained.>

The authors explore the history of sustainable development along four dimensions: societal, economic, environmental and technological. Sustainability is characterized as maintaining environmental capacity for future generations. Some measures include:

• rates of renewable resource usage compared to resource regeneration
• rates of renewable resource substitution compared to use of non-renewable resources
• rates of pollution compared to assimilative capacity

Industrial ecology is discussed as adopting process form biological systems to "de-link ... consumption from depletion in immature industrial systems", such as:

• waste cascading (reduce exergy lost in waste, reduce exergy cost of using virgin resources)
• resource cycling (similar to waste cascading, but intended to be 'closed-loop')
• increased energy efficiency
• renewable energy use

Exergy can be a useful analytical tool to improve sustainability since energy is conserved while exergy is lost in any irreversible process. Exergy analysis of systems can identify systemic changes (rather than 'end-of-pipe' fixes), highlight unexpected areas for improvement (energy efficiency does not take into account differences in energy quality tied to energy gradients) and suggest upper limits for energy improvements.

Renewable energy sources have both strengths (lower cost variability, straightforward implementation, less environmental degradation and pollution, advantageous in developing countries) and challenges (diffuse, not fully accessible, intermittent, regionally variable). The ability to scale down renewable energy can provide flexibility compared to centralized power generation.

Various tools are listed, including Life Cycle Assessment, Environmental Impact Assessment, ecological footprint, Sustainable Process Index, Material Flux Analysis, risk assessment and exergy analysis. Exergy can be a useful common sustainability quantifier in that exergy:

• is dependent on parameters of the system and reference state
• captures quantity and quality of energy flow caused by work, heat and chemical interaction with other systems
• incorporates chemical energy based on difference from equilibrium reference state
• reflects value of process products (sum of exergy of all inputs)
• measures environmental cost of emissions through energy required to bring emissions into equilibrium with the reference state
• can incorporate 'non-energetic externalities' (labor, capital), if conversion values can be agreed upon

The chapter ends with a summary of an exergy analysis of a coal-fired electrical generating station. Although overall energy and exergy efficiencies are very close, component analysis shows energy efficiency of steam generation is 95%, but exergy efficiency is only 50%. In comparison, the energy efficiency of the condensers is higher than the exergy efficiency, since the temperature differential is low so that little exergy is lost,.

Chapter 11. Exergy Analysis of Steam Power Plants

This chapter was referenced in chapter 3. It includes numerous formulas and tables for both coal-fired and nuclear generation stations, as well as diagrams showing the energy and exergy flows. Exergy analysis of the coal-fired plant show 84% of the irreversibility is in the steam generation process (roughly equally split between combustion and heat transfer). Various improvements are analyzed based on simulations and data from the Ghazlan Power Plant.

Chapter 19. Exergetic Life Cycle Analysis

The chapter is one of the few that compares different processes, such as the production of gasoline and hydrogen from fossil fuels (oil, natural gas) and renewable energy (solar, wind). A 'capital investment efficiency factor' to incorporate the required capital investments in different technologies: it is the ratio of gain from technology to the required investments. At present, generating hydrogen from natural gas is requires a significantly lower capital investment than from wind or solar energy, due to the low cost of gas. However, even small increases in the cost of natural gas or decreases in the cost of hydrogen can equalize the capital investment efficiency factors. Other findings are:

• production of electricity from wind or solar energy to mitigate greenhouse gases (GHG) and airborne pollutants (AP) is less costly than using these energy sources to produce hydrogen
• substituting gasoline with hydrogen from natural gas reduces GHG and AP emissions 5-fold, significantly less than hydrogen from wind or solar power

Chapter 20. Exergy and Industrial Ecology

(TBD)