Yunus A. Cengel

Energy, Entropy and Exergy Concepts and Their Roles in Thermal Engineering

Energy, entropy and exergy concepts come from thermodynamics and are applicable to all fields of science and engineering. Therefore, this article intends to provide background for better understanding of these concepts and their differences among various classes of life support systems with a diverse coverage. It also covers the basic principles, general definitions and practical applications and implications. Some illustrative examples are presented to highlight the importance of the aspects of energy, entropy and exergy and their roles in thermal engineering.

Economic evaluation of geothermal power generation, heating, and cooling

Economic analysis of a typical geothermal resource shows that potential revenues from geothermal heating or cooling can be much larger than those from power generation alone. Geothermal heating may generate up to about 3.1 times and geothermal absorption cooling 2.9 times as much revenue as power generation alone. Similarly, combined power generation and heating may generate about 2.1 times and combined power generation and cooling about 1.2 times as much revenue as power generation alone. Cost and payback period comparisons appear to favor power generation, followed by district heating.

The use of the galerkin method for radiation transfer in an anisotropically scattering slab with reflecting boundaries

Abstract Radiation transfer in an absorbing, emitting, anisotropically scattering, plane-parallel medium with diffusely reflecting boundaries is solved by application of the Galerkin method. With this approach, the radiation heat flux, angular distribution of radiation intensity, and the divergence of the radiation heat flux anywhere in the medium can be determined highly accurately. For optical thicknesses up to about 10, exact results are also readily obtainable if sufficient number of terms are considered in the expansion. Numerical results are presented for representative cases.

Is bigger thermodynamically better

Abstract Mixing, in general, is an irreversible process, and some entropy is generated and thus some exergy is destroyed during such a process. Therefore, combining two systems thermodynamically that are at different states may yield a system that is larger in size, but much smaller in exergy content or “usefulness”. In this paper we consider some mixing processes, and show that getting bigger is not necessarily better by examining the effect of mixing on exergy destruction.

Radiation transfer in an anisotropically scattering plane-parallel medium with space-dependent albedo ω(x)

Abstract A method of analysis is presented for solving radiation-transfer problems involving space-dependent albedo ω(x) for an absorbing, emitting and anisotropically scattering plane-parallel medium with reflecting boundaries. The albedo is represented in terms of Legendre polynomials in the form ω(x) = ΣRr=0DrPr(x/L), where x is the optical variable, L is the half optical-thickness of the slab, Pr(x/L) are the Legendre polynomials and Dr are known expansion coefficients. The effects of spatial variation of albedo on the reflectivity and transmissivity of a medium having a slab geometry are examined for the cases of both forward and backward anisotropic scattering over a wide range of system variables. The effects of ω(x) on the angular distribution of radiation are also shown for some representative cases.

Exergy Analysis of a Solar Heating System

Exergy destruction associated with the operation of a solar heating system is evaluated numerically via an exergy cascade. As expected, exergy destruction is dominated by heat transfer across temperature differences. An energy analysis is also given for comparison of exergy cascade to energy cascade. Efficiencies based on both the first law and second law of thermodynamics are calculated for a number of components and for the system. The results show that high first-law efficiency does not mean high second-law efficiency. Therefore, the second-law analysis has been proven to be a more powerful tool in identifying the site losses. The procedure used to determine total exergy destruction and second law efficiency can be used in a conceptual design and parametric study to evaluate the performance of other solar heating systems and other thermal systems.

Integrals involving Legendre polynomials that arise in the solution of radiation transfer

Abstract The integrals involving the product of the Legendre polynomials with the exponential integral function and the exponentials arise in the solution of equation of radiative transfer by a suitable expansion in the space variable in terms of the Legendre polynomials. The success of such methods of solution depends on the availability of rapidly converging analytic expressions for such integrals. In this work, analytic expressions are presented for some of such integrals.

Examining the merging and splitting processes in daily life in the light of exergy

Abstract When two thermodynamic systems at different states are mixed, the exergy contend of the combined “bigger” system may actually be smaller than the exergy content of either of the two systems. Therefore, from the second-law point of view, mixing of systems should be avoided unless the systems being mixed are nearly at the same state. In this paper, we examine the merging and breaking up of families, companies, and states using the entropy generation and exergy destruction associated with various mixing processes of thermodynamic systems as a guide. In analogy to thermodynamic systems, we present arguments that the more dissimilar are the items being merged, the larger the destruction of the figure of merit or exergy. Therefore, forcing very dissimilar things into a unity may create highly destructive situations. Also, things that are similar in some aspects and dissimilar in other aspects should be combined only partially, involving the similar aspects only. The individual items should maintain their individuality in regard to the dissimilar aspects to avoid destruction. It is also pointed out that breaking up of countries, companies, and even families with irreconcilable differences may sometimes be the best thing to do, and each part of the whole may be much better off after the break-up.

Thermal Storage in the Walls of a Solar House

Residential winter thermal energy storage features water encapsulated into 3-in. (7.6-cm) diameter plastic pipes, mounted into conventional stud wall cavities of a house. With an air solar collector, solar-heated air can be passed through the stud cavities, heating the water. During the discharge mode, this water loses its heat directly to the house, and the radiating walls allow the residents to feel warm even at lower interior air temperatures. Empirical and theoretical components performance data are reported for the waterwall thermal energy storage unit. The interaction between a large air-heating solar collector and the waterwall thermal energy storage is considered for the winter heating mode. The collector-storage thermal integration analysis is detailed for a charging flow rate of 40 cfm per cavity. A simpler but reasonably accurate integration analysis is illustrated for 10 cfm cavity flow rate. Performance parameters indicate that the waterwall thermal energy storage approach is very compatible with a solar air heater.

Radiation heat exchange between electronic components on a circuit board and the walls of its enclosure

Radiation heat transfer between rectangular electronic components on a printed circuit board and the walls of its enclosure is studied analytically using a Monte Carlo method. The radiation heat transfer between the electronic components and the cover is determined for the cases of diffuse and specular surfaces with constant properties, and for diffuse and specular surfaces with variable temperature and direction-dependent properties. The radiation interchange between the components and the cover of the enclosure are determined and presented for various dimensionless parameters and surface emissivities in tabular and graphical forms. The radiation heat transfer, in general, is found to be comparable in magnitude to natural-convection heat transfer at operating conditions encountered in practice. It is shown that radiation can serve as an effective heat transfer mechanism for the cooling of electronic components in sealed enclosures cooled externally.