Energy and the Environment

The objective of this chapter is to review a modern transformation in the teaching, research and practice of energy engineering: the increasingly important roles played by thermodynamics (especially the second law) in problem formulation, modeling and design optimization. This methodology is known as thermodynamic optimization, or entropy generation minimization (EGM) and was first recognized in a 1982 book [1]. The most recent review [2] shows that the use of this method is expanding at an accelerated pace, and that it has recently acquired alternate names such as finite time or endoreversible thermodynamics. In this chapter we illustrate the application of the method through examples selected from refrigeration.

Energy and materials saving considerations, as well as economic incentives, have led to efforts to produce more efficient heat exchange equipment. Common thermal-hydraulic goals are to reduce the size of a heat exchanger required for a specified heat duty, to upgrade the capacity of an existing heat exchanger, to reduce the approach temperature difference for the process streams, or to reduce the pumping power. The first two objectives translate to an increase in the average heat flux of the heat exchanger, or the encouragement of high heat fluxes. In the case of systems with a specified heat dissipation, the goal is to cool the device, or accommodate a high heat flux, at moderate temperature difference. Implicit in these objectives, energy reduction (improvement of first law efficiency) and temperature difference reduction (improvement of second law efficiency) are important to global environmental protection.

There are many occasions when the optimization of a finned array can be profitable from the standpoint of either minimum weight for a prescribed energy transfer or for enhanced energy transfer from a given weight of material. Kraus et al. [1], following the assumptions posed Murray [2] and Gardner [3], showed that for individual longitudinal fins of rectangular profile (and others) and in finned arrays composed of these fins, conditions of heat flow and temperature excess at any point on a fin are induced by similar conditions at the fin base. In particular, it has been shown that, for a single fin, there is a linear transformation which maps conditions from the fin tip to conditions at the fin base and it was demonstrated that this T matrix could be used to obtain a new parametization, called the input admittance that would completely describe the performance of a fin. The conventional fin efficiency was abandoned and it was proposed that single fins and finned arrays should be characterized by this single, yet important, parameter. The input admittance was further shown by Kraus and Snider [4] to be the ratio of the heat entering the base of a single fin or a finned array to the temperature excess at the base of the fin or the finned array.

There is a worldwide interest in using pollution prevention methods to eliminate or lessen air, water, land and thermal pollution problems. Pollution prevention is designing processes that do not create pollution in the first place.

Heat exchangers play an essential role in pollution prevention and in the reduction of environmental impact of industrial processes, by reducing energy consumption or recovering energy from processes in which they are used. They are used: (1) in pollution prevention or control systems that decrease volatile organic compounds (VOCs) and other air pollutant emissions; (2) in systems that decrease pollutants in wastewater discharges, the amount of the discharge and thermal pollution; and (3) used to recover energy in facilities that incinerate municipal solid waste and selected industrial hazardous wastes. Heat exchangers are also used in the heating, cooling and concentration of process streams that are part of many other pollution prevention or control related processes.

In this paper, first presented is background information on the role of heat exchangers, their types, and a discussion of environment pollution problems. Next, the role of heat exchangers is outlined in the prevention and mitigation of the following pollution problems: air pollution from VOCs, sulfur oxides (SO_x), nitrogen oxides (NO_x); water pollution from industrial processes, thermal pollution, and land pollution resulting from municipal solid wastes or industrial hazardous wastes. Specific Research and Development needs for environmental heat exchangers are then summarized in the paper. It is hoped that this paper will challenge the heat transfer engineering community to further enhance the role of heat exchangers for pollution prevention and global sustainable development.

Conventional design of systems for environmental cleanup and for energy production relies to a great extent on prior art. Successful designs that meet regulatory requirements as well as technical and economic needs for efficiency and cost have traditionally been produced by trial and error.

Thermal radiation heat transfer figures prominently in combustion systems. In perhaps all of these applications the impact of the transport on the environment is clear. Depletion of non-renewable energy resources is a concern. Industrial combustion utilization is under increasing pressure to reduce the exhaust pollutant emissions. The generation of pollutants in a flame is clearly dependent on the temperature history of the related chemical species. Therefore, treatment of the radiation transfer becomes critical not only to the accurate determination of the fluxes to the load, but also the prediction of local temperatures in the flame with the attendant impact on pollutant formation. Treatment of radiative transport from the gas phase in a combustion system is particularly challenging due to the strong spectral variation of the gas radiative properties (absorption coefficient, K _η. The absorption spectra of gases consist of many thousands of spectral lines because gases emit and absorb electromagnetic radiation only at discrete frequencies where the corresponding photon energies match the quantum changes in energy of the gas molecules.

During the last decade, thermal design and simulation of a wide range of materials processing and manufacturing (MPM) operations has taken a more prominent role. Careful consideration of transport phenomena has been driven by the desire to improve product quality, decrease production costs, reduce energy requirements, decrease adverse environmental effects, increase product functionality and reduce time to market. Many materials processing and fabrication operations involve heating using either directed (laser, concentrator) or diffuse (high temperature fossil fuel-fired furnaces, electrical resistance, fossil fuel-fired heaters, etc.) sources. The choice of the heating method is usually dictated by the process, temperature level, economic and other considerations. For example, the type of material (i.e., opaque or semitransparent) and its spectral and directional radiation characteristics influence the process and requires careful matching of the radiation characteristics of the “source” and the “target.

Over the past thirty years, plastics have become among the most common and preferred materials for the manufacture of products ranging from packaging films to structural components. Largely because of the very large molecular weights of the molecules comprising them, plastic melts can be very viscous and consequentially difficult to process into finished goods. Industrial devices, such as shown in Fig. 1, have been developed to forcefully melt and mix different types of plastics, or plastics and additives, either in batches or continuously in devices commonly referred to as screw extruders. Ingredients are introduced into these devices in pelletized or flake form. The pellets and flakes melt as a result of contacting heated surfaces and viscous dissipation in strong shear flows. The molten blend can subsequently be forced through a die to produce tubes or fibers, for example, or into a mold to produce large parts such as automotive dashboards. Polymer recycling is an area of growing interest. For example, polyethylene (PE) and polystyrene (PS) blends are major constituents of plastic waste, which respectively consist of about 60% and 15% by weight of all recycled plastics [2]. Creating PE / PS blends and other types of plastic blends with favorable microstructures and thereby attractive mechanical properties is important for reducing plastic wastes. Unfortunately, present-day processing techniques most often provide blends with deficient properties due to the droplet microstructures that typically result [3] or strong dependencies between microstructures and composition.

The beginning of the Third Millennium marks a substantial change in the philosophy as well as practice of all the functions of the traditional power industry. While no one can predict the future, one can understand the forces and trends shaping it. It is already evident that the old structure of vertically integrated power systems has been replaced by a system comprised by chaotic equilibria which may be described by nonstationary differential equations which are very hard to master. To put it otherwise — turbulence and insecurity are unavoidable. This transition whose bellwether is information technology implies creative destruction, restructuring of existing industries and is forming network organizations.

This chapter presents some challenging questions regarding the perceived impact of energy technologies on the environment and in particular the environmental impact of energy storage technologies. The questions raised are related to what it seems to be a too slow and possibly speculative reaction of the scientific community in addressing topics linked to global environmental problems. Hence, under public pressure and some times under panic created by popular mass media sources, such as the scare of depleting natural resources, top level executive managers and politicians are forced to take decisions subject to a great deal of uncertainty. It is not unusual for executives to take decisions under uncertainty conditions, however in the particular case of energy resources the not too far history since the first oil-price crisis in 1973 demonstrated the vulnerability of the present open society to unproven and unsubstantiated predictions of upcoming “catastrophes”. The consequent allocation of limited resources to avoid such “catastrophes”, which quasi-scientific prophecies predict, can adversely affect the scientific community‘s ability to provide correct and scientific answers and enhance our understanding of the natural phenomena involved. The next section deals with perceptions of energy related global environmental problems, followed by an introduction to energy storage technologies. The contrast between thermodynamic and techno-economical optimization methods, and the consequent information relevant to environmental impact are presented for a Compressed Air Energy Storage system, which was selected as a convenient example because of its hybrid characteristics and design flexibility, and because of the author‘s familiarity with this technology. The detailed thermodynamic and techno-economical analysis corresponding to this technology is presented by Vadasz [1].

Pumped Energy Storage (PES) is defined as a system whereby energy is stored by pumping for later use. The most common version of PES is the pumping of water from a lower reservoir (natural or artificial) to an upper reservoir at a higher elevation, where water is stored and later converted into electrical energy by letting it fall through hydroelectric turbines.

In any power generating or refrigeration cycle, heat has to be discharged. This is also true in many chemical and process cycles, internal combustion engines, computers and electronic systems. Most of the energy contained in the fuel of a modern automobile engine or fossil-fired power plant is rejected to the environment in the form of heat.

An altered form of the transient technique used by Schultz and Jones [1] for measuring heat transfer coefficient has been successfully applied in a steady flow cascade with the step change in temperature created by plunging a prechilled blade into the hotter cascade flow. Results obtained from thin film sensors mounted on the surface of a turbine rotor blade show excellent agreement with theory in the leading edge region and on the pressure surface. In the suction surface trailing edge zone, 30% to 100% chord, theory predicts severe separation whilst the sensors show this flow regime occurring far later, i.e. 70% to 80% chord and with lesser magnitude. The technique has general application for determining the convective heat transfer coefficient in most flow fields at moderate cost and with times between successive measurements being of the order of 20 minutes.

Thermo-fluid dynamics plays an important role in power plant operations. A typical fossil-powered or nuclear power plant uses a working fluid as a medium to transport heat around the plant system. As heat is added to or rejected from the fluid, thermal gradients develop in the system and, under certain conditions, natural circulation can be induced. In another situation, hot and cold fluids may come in contact with each other, creating thermal stratification, which can cause thermal fatigue on the piping materials. As a further example, fluid is also discharged from power plants into the surrounding reservoir or estuaries, the thermal effluent dispersing over a wide area and possibly impacting on the ecological system around the power plant.

The production of synfuels from coal consists of three major process steps:

Research in the area of convection in porous media has been reviewed by Nield and Bejan [1]. The topic is important in areas including the insulation of buildings and equipment, energy storage and recovery, geothermal reservoirs, nuclear waste disposal, chemical reactor engineering, and the storage of heat-generating materials such as grain and coal. As well, there are a number of geophysical applications. In [1], Chapter 11 is explicitly devoted to geophysical aspects, and geophysical applications are also implicitly involved in many other sections of the book.

Groundwater contamination is one of the most severe technical problems of this century. Environmental cleanup is expensive, time-consuming, and of uncertain outcome in several cases.

The paper presents some hydrodynamic aspects of groundwater flow as a result of location of radioactive repositories in fractured rocks formations. Such repositories are planned in Sweden in fractured granite aquifers. High and low level radioactive waste repository located in aquifers are possible sources of environment pollution.

The Montreal Protocol [1–3] was the beginning of a new period for the air-conditioning and refrigeration industry owing to the continuing debate on environmental issues such as global wanning, the depletion of the ozone layer and energy efficiency. The aim of the Protocol is to replace traditional refrigerants which are CFCs and HCFs. The import of CFCs was already banned in 1996 while imports of HCFCs will be progressively restricted, with complete phaseout early in the next century. Although numerous studies have revealed various possibilities, the search for more efficient, cheaper, environmentally friendly, and safe refrigerants is most probably a never-ending one.

The concept of a heat pump, where heat is transferred from a low temperature to a high temperature via a vapour compression cycle that requires power input, is widely applied in the South African commercial sector. The main application is in the form of water chillers and air conditioners used for cooling in buildings. The associated heat that is produced in these cycles is usually purely a by-product that has to be disposed of. In some cases, mostly in split-type office air conditioners, the cycle can also be reversed so that the heating effect can be used efficiently in wintertime to heat the building.