Waste energy usage and entropy economy

Energy 28 (2003) 1281–1302 www.elsevier.com/locate/energy

Waste energy usage and entropy economy Wolfgang Fratzscher a,∗, Karl Stephan b a

Martin-Luther-Universita¨t, Halle-Wittenberg, FB Ingenieurwissenschaften, D-06099 Halle, Germany b Universita¨t Stuttgart, Pfaffenwaldring 9, D-70550 Stuttgart, Germany

Abstract In this paper a novel concept is introduced for the energetic evaluation of technical systems, based on the law of entropy. Our technical systems are considered as open systems interacting with their environment. Main attention is directed to waste energy as the source for the entropy export necessary to maintain a desired state of order within the system. The entropy export results from internal and external irreversibilities. A reduction of both irreversibilities, i.e. an approach towards a reversible process management, reduces pollution on the environment through entropy export and primary energy expenditure. With that, the strategy developed here and called entropy economy, contributes to a better optimization of technological processes and thus to sustainable development. The most important means for avoidance and reduction of irreversibilities are discussed in detail and applied to definite regional objectives. As examples reveal, from a thermodynamic perspective it turns out that in some cases a considerable amount of primary energy could be saved with the currently known technical methods. Additional expenditures for devices and technical equipment are not the only obstacles for realization: opposition from the realms of economic, social and legal requirements is likely to exert a negative influence on the development of optimally integrated ‘cascading’ cycles.  2003 Elsevier Ltd. All rights reserved.

1. Some theoretical statements It is surprising that in regard to energy problems the ‘Law of energy conservation’ (First Law of Thermodynamics) is most frequently the one explicitly quoted. The Second Law of Thermodynamics (Law of Entropy) on the other hand, even though it has already been known for a long time (e.g. Refs. 1–5]), is taken neither qualitatively nor quantitatively into consideration. Thus, problems connected with energy supply are treated in the same way as problems connected with the supply of raw materials and products, for which a law of conservation applies as well. This ∗

Corresponding author. Tel.: +1-49-345-522-5535. E-mail address: [email protected] (W. Fratzscher).

0360-5442/$ - see front matter  2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0360-5442(03)00109-9

1282

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

circumstance takes the peculiar characteristics of energy insufficiently into consideration and does not take advantage of a vast body of scientific statements. This is not explicitly evident if one considers processes far above their surrounding temperature. In processes close to or even below their surrounding temperature, as well as in certain chemical processes, the restrictions caused by the sole usage of statements considering the ‘Law of Energy Conservation’ become obvious. Standard processes have then to be used in order to provide ad basis of evaluation. When applying both laws of thermodynamics to a technical system, which can normally be represented as a stationary process, the trivial statement that ‘all provided energy has to be emitted’ again follows from the ‘Law of Energy Conservation’. The Second Law of Thermodynamics states that the emitted entropy, plus the irreversible entropy production, must be higher than the entropy input. If the entropy emission is increased above that amount, then the entropy in the system is reduced. This would mean a higher state of order according to Prigogine, i.e. a restructuring takes place. That is the goal and task of technical systems. The emitted energy, crudely named ‘waste energy’, has from this point of view the task of an entropy supplier and the realization of the necessary entropy export, which can only take place in form of heat and/or mass flow. On Earth, the state of emission in these circulations has to be different from the surrounding state, if the transfer into the environment is achieved through natural processes. These irreversible and heat and/or mass transfers for system operation are therefore connected to exterior irreversibilities that have to be taken into consideration in the calculation of the energy efficiency of the system. This is the starting point for the following considerations. In essence this means that waste energy, characterized by entropy and possible export into the environment, is used for the energy valuation of technical systems. Such an operation not only delivers comparable results for all technical systems, but can be a basis for establishing a comparable and generally valid measure for the strain on the environment. A reduction of entropy export, while maintaining the same conditions in the system, gets us closer to reversibility and reduces the environmental strain. If we describe such a strategy as ‘entropy economy’, a high level of entropy economy characterizes a contribution to sustainable development [6]. In basing reservoir characteristics on the environment it is possible to record and to quantify the previous considerations by the use exergy [7]. This has the further advantage that the statements of the Second Law of Thermodynamics are presented in units of energy and that they can be compared directly with the corresponding data, e.g. with primary energy usage. 2. Avoidance of internal irreversibilities The minimum of the entropy export is achieved through a reversible management of processes in the system. This corresponds to an entropy input being directly connected to the energy output. Such a situation requires equilibrium processes, which either run indefinitely or require indefinitely large transfer surfaces, both conditions being obviously in excess of any technical requirement. The necessary deviation from the equilibrium of processes, demands for the presence of internal irreversibilities that lead to a corresponding increase in the entropy export. Table 1 shows measures and areas of exergetic quality standards for simple processes and machines, as well as for systems, taken from ‘second-law’ analyses. The quality can be defined

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

1283

Table 1 Internal quality index n Process, system Machines and devices (simple processes) Pump Compressor Expansion engine Throttling Gas turbines in combined process Heat transfer within the system Heat transfer at the end of systems Combustion Steam generators Fuel cells Chemical reactors Distillation columns, without heat transfer Distillation columns, with heat transfer Energy conversion plants Condensing Power Station Combined gas- and steam turbine plant with cogeneration Cogeneration Small co-generation plants Heat transfer devices Internal combustion engines Refuse combustion, thermal usage Refuse-to-energy power plant

n

Process, system

n

0.3 ... 0.45 0.4 ... 0.6

Hydroelectric power plant 0.7 ... 0.9 Wind power plant 0.3 ... 0.8 Solar-thermal power plant 0.2 ... 0.4 Photo-voltaic power plant 0.08 ... 0.18 Transport- and storage facilities Gas pipeline 0.95 ... 0.99 Oil pipeline 0.98 ... 0.99 Hot water network ⬇0.95 Steam network 0.8 ... 0.9 Hot water storage ⬇1 Processing plants (processing systems) Cement manufacturing ⬇0.6 Ammonia synthesis ⬇0.7 Oil distillation 0.95 ... 0.98 Gas separation facility ⬇0.95 Air separation plant ⬇0.15 Heating and air-conditioning systems

0.3 0.3 0.2 0.4 0.2 0.3

Domestic boilers Heat pumps Solar–thermal warm water production Electric heating Air-conditioning

0.85 0.7 ... 0.9 0.55 ... 0.85 0 ... 0.99 0.65 ... 0.75 0.1 ... 0.99 0.01 ... 0.5 0.6 ... 0.7 0.25 ... 0.45 0.9 0.7 ... 0.99 0.7 ... 0.95 0.05 ... 0.7

... ... ... ... ... ...

0.5 0.5 0.3 0.7 0.3 0.35

0.1 ... 0.35 0.4 ... 0.8 0.05 ... 0.2 ⬍ 0.1 0.1 ... 0.2

as the input–output ratio n and characterizes the relative interior losses caused by irreversibilities. These losses can be reduced in accordance to the measurements of the given external conditions. In this Table, data are first presented for devices and components in simple processes. Their losses are mainly due to friction. The wide spectrum in the throttling process is first defined by the position of the process in comparison to the condition of the environment. Furthermore, it has to be pointed out that the throttling process not only has to be seen exclusively as a loss process, but also as serving in the transformation from potential energy into ‘cold’ within low temperature technology. Heat transfer systems work with very different data for exergy quality. Such devices show very low qualities near ambient temperature, which is also apparent for cooling processes. Exergy efficiency improves the further the processes are from ambient temperature. This is especially true in the area of low temperatures where the least final temperature differences are used. Experience shows that combustion as a self-sustaining chemical reaction brings about a 30% loss. Heat-transfer losses in steam generators also add to this, which explains the given quality. The transformation of chemical energy, on the other hand, must be characterized with positive data for qualities in exergy. This is shown by fuel cells and chemical reactors. This also gives reasons for considerations concerning the improvement of energy transfer facilities through the inclusion of chemical processes. Within separation systems, the quality provides only inadequate information, since the concentration energy disappears almost completely in the balance.

1284

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

Data for exergy qualities within energy transfer facilities are also shown. If a combustion process is implemented in connection with heat transfer under considerable temperature differences, then these data all lie in the same range. This is also true for the comparison between a condensing steam power plant and cogeneration. It must be stressed again that the advantages of cogeneration only become clear in comparison to standard residential heating systems. Therefore, heat transfer devices show very poor exergy results. If an incineration process is implemented for refuse processing the exergy data for these systems remain the same. Hydroelectric and wind power plants can achieve high levels of exergy quality since the energy input is here in the form of mechanical energy. Data for transport and storage systems show no fundamental losses in the quality of energy. The problem with these system components consists therefore primarily in the installation costs. Material processing systems generally show good results in comparison to energy processing systems. This finding is only true from the perspective of energy estimation and disregards the material value of the products. This becomes apparent in the evaluation of a chemical reactor. This is also one of the reasons why a progressive trend is being seen in the realization of unity of material and energy processing. The low amount for air separation plants is due to the fact that the state of product usage is given through its concentration of exergy and its implementation through mechanical energy. The high amount in raw oil distillation refers to the fact that the chemical energy of products is almost completely maintained. Systems for the provision of lowtemperature heat, which serve as residential heating, work with very low exergy efficiency. That proves, as already mentioned, the advantage that can be achieved by using cogeneration systems. On the other hand, the gain that can be achieved with low energy houses through the use of appropriate insulation is becoming particularly evident. The central problem here is, of course, the careful consideration between the investment costs and the gains during operation of the system. Somewhat more detailed statements can be made about power plants because clear work production data for usage are available. When using fossil fuels, the energy efficiency is approximately the same as the exergy evaluation, since reaction heat and exergy of the fuels differ only slightly. A large number of single and multistage cycles for the production of work is shown in Fig. 1, and represents the current state-of-the art of cycles likely to be developed to technological maturity within the next decades. The top of the Carnot-curve for efficiency, which has been drawn for comparison, represents the loss for thermal power processes on the basis of fossil and biological energy sources. This loss consists of external losses (entropy export) caused by the combustion and the irreversible heat transfer from fumes to the working fluid within a cycle. The distance of the respective degree of exergy efficiency in thermal power processes from the Carnot-graph exposes the interior losses, particularly those caused by incomplete ‘Carnotisation’ (irreversible expansion and compression), and through heat transfer between the working material of multistage cycles and to the environment. The efficiency in the single-stage steam power cycle, which at the beginning of this century reached only single digit values, currently reaches 43–45% and will exceed the 50% mark after the development of more efficient construction materials in perhaps a not too far future. Pressure ratios up to 4000 would be required for such a high efficiency. Higher pressures can only be achieved through a two-stage process, i.e. through the coupling of a gas-turbine cycle with a steam power cycle, which allows at present efficiencies of up to 57% through sufficiently top

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

1285

Fig. 1. Efficiency and its limits for various energy production systems (1 to 5 fuel cells, 6 to 13 combined heat and power plants). Fuel cells: (1) Acid fuel cell 80 °C; (2) Polymerelectrolyte membrane, acid fuel cell (mobile) 80 °C; (3) Phosphoric acid, fuel cell 200 °C; (4) Carbonate melting, fuel cell 600 °C; (5) Solid matter oxide, fuel cell 1000 °C. Thermal power plants: (6) Gas turbine process 1190 °C; (7) Gas and steam turbine process 1190 °C; (8) Economical power plant (for example Staudingen) 545 °C/262 bar; (9) Bexbach II (Saarberg AG) 575 °C/290 bar; (10) Ultra-over critiCal steam power process 700 °C/375 bar; (11) Gas and steam turbine power plant with integrated coal gasification (IGCC) Puertollano 1120 °C; (12) IGCC 1190 °C; (13) IGCC development potential 1250 °C, according to [8].

cycle temperatures in gas turbines (e.g. 1190 °C). A further increase in the efficiency would essentially be possible through the prior implementation of a third thermal power cycle. Such a three-stage cycle, which has already been researched for decades, but until recently remained technically undeveloped due to extreme temperature and material problems will probably be replaced by another three-stage transformation process that is not exclusively designed around thermal cycles. This transformation system consists of a high-temperature fuel cell, in which preconverted natural or biological gas is partially converted into electricity at 1000 °C, to be then discharged after complete combustion in a gas turbine. The hot turbine exhaust fumes serve as a heat source for a steam power cycle. The total efficiency for the three cycle-process is expected to reach 68%. As Fig. 1 shows, the efficiency in fuel cells can exceed the Carnot-factor of the process temperature, since the transformation does not need the intermediate use of heat. The process temperature, which in this case no longer limits the efficiency, is only interesting in regard to the usage of waste heat. In Table 2 the development potential for efficiency in power plants are shown for the various types of systems up to the year 2015. These data provide general guidelines for possible reductions in entropy export. Using the data on waste energy, energy input can be economized, which can then lead to a reduction of environmental pollution. The development of these novel reserves is possible either through a change in technological concepts or through proposals that will be discussed in the following part, using the reduction in exterior irreversibilities as an example, i.e. the actual waste energy.

1286

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

Table 2 Efficiency of various types of power plants Type of power plants

Net efficiency (energy production) in % Current levela

Nuclear power plants 33–34 Hydroelectric power plants 80–90∗ Pump storage plants 80–90∗ Geothermal power plants 75–85∗ Wind energy converters 40∗ Solar thermal power plants 15∗ Photo-voltaic systems 10–12 ∗ Biomass–thermal power plants Incineration 15–30∗ Gasification (Natural) gas–steam turbine plants 58–60∗∗ Combined gas–steam turbine plants with integrated gasification Stone coal 46∗ Lignite 45∗ Combined gas–steam turbine plant with 42.5∗ pressurised fluid bed Fuel cell power plants PAFC (natural gas) 42∗ MCFC (pressure and G&S) Natural gas Coal SOFC (pressure and G&S) Natural gas Coal 47∗ Common cogeneration plantsc 40∗ Decentralised cogeneration plantc

Development potentialb 34–35∗∗

40∗ 24–29∗∗ 12–17∗∗ 30–40∗ 35–40∗∗ 60–62∗∗ 49∗ 49–50∗∗ 45–48∗∗

47–50∗∗ 70–80∗∗∗ 63–70∗∗∗∗ 70–80∗∗∗ 63–70∗∗∗∗

Processed according to [9]. a First usage: ∗1995,∗∗1998. b Expected first usage: ∗2000, ∗∗2005, ∗∗∗2010,∗∗∗∗2015. c In addition, power heat is also provided.

3. Reduction of external irreversibilities As already mentioned, the necessary entropy export in technical systems is first defined through entropy input which, in a stationary process, has to be re-emitted into the environment for a reversible process, in the same amount. Since there are by force irreversible processes within the system, the necessary entropy export is raised by the entropy production caused by these processes. Furthermore, entropy production arises through the emission of waste energy into the environment, because the state of emission is by definition different from the respective state of the environment. With that, waste energy can be characterized by its exergy. In doing away with

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

1287

the usage of waste energy, this exergy represents the energy loss, which shows the possible savings potential. The waste energy, characterized as external exergy loss, can be transferred in two principal types of usage: within and outside the observed process. The schematic of internal usage, often also described as primary usage, is pictured in Fig. 2. It is technically a kind of recycling or feedback and always thermodynamically associated with regenerative effects, which is why it can principally be described as regeneration. This means that the implementation can either be reduced for the same need or that the intended yield can be raised with the same input. The consequent usage of this principle leads to system configurations that can be described as an integrated arrangement. Thus, the regeneration is a part of the process and can no longer be externally recognized as a special variant thereof. The savings potential of this technique is often important, because not only external losses are reduced, but a specific reduction of other losses can also be achieved through possible variants of technological systems. For that reason, this technique is also called integration. Implementation in a second process or plant can be described as both external and secondary usage and leads to coupling production or combination. This principle is schematically shown in Fig. 3. The identification of the possibilities for improvements can be done in three ways: 1. In the balance of process I the supply for the production of WIP is expanded by WN in the waste energy usage for substituting a part of the supply to process II which WIIP produces. 2. The original systems are collectively observed as a new total system III, which has to produce WIP and WIIP. In this sense the coupling has to be considered as integration for the total system. 3. The waste energy usage is considered to be a coupled production. Mutual influence factors are introduced for the assessment of waste energy expenses on an economical basis in the simplest case, which allows for a calculation of the original process for the new conditions.

Fig. 2.

Structure of internal (primary) waste energy usage.

1288

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

Fig. 3. Structure of external (secondary) waste energy usage.

3.1. Entropy export through heat These variants of usage can be applied to all types of energy. Heat is by far the most important from a quantitative point of view and is therefore the first responsible for entropy export in technical systems. Accordingly, the following waste energy lead to entropy emissions [6]: 앫 Condensation heat emitted by power plants, (2,5...3) TW; 앫 Waste heat from commercial heating, 2 TW; 앫 Waste heat from industrial ovens and furnaces, 0,5 TW. The scale of energy has been used in order to place the given data in due perspective. These data resemble a kind of surrounding energy that cannot be exploited [7,18]. The connection between these amounts of energy and their amounts of entropy is given by the Gouy–Stodola lost-work theorem. As a comparison, it may be stated that the current world energy usage amounts to 12 TW [6]. The respective temperature is substantial for the internal and external usage of waste heat in creating so-called heat cascades by stacking heat transfer processes, that are fed and providing energy at different temperature levels. From the usability aspect of such measures, the heat users can be divided accordingly into multi- and single-level users. For the latter, neither waste heat from other processes is available nor is waste heat for heating required. Heat usage at various temperature levels is used for the purpose of material transformation in chemical technologies and in the food and luxury items industry. In the Federal Republic of Germany, about 35% of the total energy is used for commercial heating, while 25% is used for process heat. Commercial heating can be categorized as a typical single-level usage. In contrast to commercial heating as a typical use of low-temperature heat, process heat is needed in a wide range of temperatures. For instance, there are a large number of technological processes that are often

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

1289

coupled with water or aqueous solutions, in which heat is required at many different temperature levels. Such processes are considered multi-level users. As can be seen in Fig. 4, the use of process heat at corresponding temperature levels is distributed very differently throughout the individual industrial branches so that a cross-connection, already from this point of view, can be very difficult. This is further complicated by logistic considerations and by technological requirements. On the other hand, the small need for high temperature heat (iron industry) in comparison to the need for low temperature heat (particularly in the chemical industry) is apparent in this diagram. Almost 50% of the industrial heat requirements lies under 200 °C. This unfavorable situation, hindering the creation of heat cascades and regenerative heat usage is, certainly in part due to an energy politics that supports a transfer of energy-intensive, high-temperature processes abroad. A compensation for missing high temperature processes can be accomplished through the connection of regeneration with pre-processes for power production through which heat can be supplied at various temperature levels. Although it is not visible in Fig. 4, the regeneration is practised extensively in technological systems: an example is the high amount of regeneration reached in sugar factories, where a threeto five-stage evaporation of the sugar solution is carried out through regenerative pre-heating and vacuum crystallization, which is reflected by a correspondingly reduced use of low pressure steam. From these findings a thermal coupling could be introduced between high temperature processes and those that require heat at medium and low temperatures, and that finally provide their waste heat for commercial heating purposes. Such heat cascades, although optimal from a thermodynamic perspective, encounter a great number of difficulties in their realization: from the matching of the respective offers to the respective requirements, to institutional, economical, legal and social problems. An expansion of theoretically possible variants can be developed, if simple energy or heat transformation processes are considered [11,13]. The single and coupled right and left running (reversed) cycles can also be placed among the heat transformation processes. The power plants and reciprocating engines processes have already been mentioned above. Their implementation in the past, for example, has already raised the energy level of high temperature processes considerably, as the enormous reduction of specific energy usage in certain chemical processes like NH3-synthesis and metallurgy show. So-called ORC (organic-rankine-cycle) processes have been

Fig. 4. Temperature level and heat usage in different industrial branches in Germany (from [10]).

1290

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

employed for some time for the utilization of low temperature heat. From a thermodynamic perspective, the implementation of reversed-cycle processes (as heat pumps) can be useful considerably in the strategy of waste heat recovery, since various temperature levels can be achieved with their help. Steam compression heat pumps are of particular interest in the utilization of waste heat since the expansion of the condensed phase can be achieved without large throttling losses, and the relatively large evaporation enthalpy permits a low flow rate of the working fluid. On the other hand, gas compression heat pumps, which present the important advantage of a gradual heat input and output, also deserve attention. Through the use of open cycles the capital expenditure is considerably reduced through the elimination of evaporators, simultaneously reducing internal thermodynamic losses. Therefore, steam compressors are preferred to closed cycles especially at higher temperatures. However, the utilization of steam compression is possible only when certain technical conditions are met, e.g. when the steam is free of solid matter or pollutants, when it contains a low percentage of inert gas, and the working fluid has favorable corrosion and toxicity characteristics. Even so, the implementation of a compression heat pump is still more convenient than using an open cycle, even if large Roots ventilators have to be implemented for reasons of steam contamination. These compressors work with an efficiency of 50%, so that in spite of the elimination of evaporators in open cycles, temperature increases of only 12 to 15 K can be reached. However, this operational efficiency can only be reached if the compressor and the technological process are matched in such a way that the compressor can operate constantly near its maximum efficiency. Heat requirements are improving under partial load operation, but the transformation ratio drastically decreases. The efficiency of a compressor ranges from 0.9 for high quality screw compressors to approximately 0.35 for liquid entrainment compressors. In residential areas heat pumps are in competition with the heat supply of co-generation plants. The maximum COP of heat pumps in this case depends on the efficiency of the supply of electric energy. For a cogeneration plant with 30% power production, 60% heat output and an efficiency of 40%, with losses for pipes and transformations of 10%, the output threshold expressed in terms heat output over input of electrical energy reaches values around 10. Even in very modern power plants, the threshold still lies at 5.7 and at 4 in power plants with efficiencies of 45 and 50%. These performance figures can be achieved by heat pumps, even in winter, only when fed by waste heat sources of relatively high temperatures. The situation for heat pumps in this case is also not very favorable from an economical point of view. One reason is that specific investment costs for co-generation plants and heat pumps are similar, and on the other hand with co-generation plants cost reductions can be made because of the electrical energy production. Absorption heat pumps, in contrast to compression heat pumps need almost exclusively heat as driving force. The heat ratio of an AHP depends predominantly on the temperature difference between the low temperature heat and the top cycle temperature. The thermophysical properties of the working fluids used in this system causes the heat ratio to be apparently independent of the temperatures of heat sources and heat losses, and to remain between 1.4 and 1.8. At the same time there is no longer any freedom in the provided average heating temperature. Heat ratio and heating temperature are therefore far less variable for absorption heat pumps than for compression heat pumps. This disadvantage can be at least partially balanced by adopting multi-stage processes. Through this either an increase in heat ratio can be achieved, combined with a decrease in the

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

1291

average temperature of the heat supply, or an increase in the temperature level of the supplied heat from heating at the expense of a decrease in heat ratio [12]. Absorption and compression heat pumps can work with both low temperature heat intakes from the environment and with waste heat flows. The absorption heat transformer is exclusively qualified for the partial revaluation of waste heat flows. At the same time, the devalued part of heat loss has to be emitted into the environment. The absorption heat transformer is therefore placed at the end of the energy-chain and enacts a partial utilization of waste heat that otherwise could no longer be used in a technological process because of its low temperature level. The driving force for the revaluation of heat can be gained from heat power itself, i.e. there always has to be a potential difference between used waste heat and the environment. The above-mentioned correlation for the AHP is true for the heat ratio and its reachable utilization temperature. A realistically attainable single-stage heat ratio lies between 0.5 and 0.6. Waste heat flows often appear connected to mass flows, especially as fumes with high steam content. This applies, for example, to steam from evaporation processes, air from dryers, and fumes from combustion processes. Regenerative heat utilization is often not possible here due to the low dew point of the fumes, since there are not enough heat users available at this temperature level. On the other hand, utilization of the condensation enthalpy of steam is necessary for an effective waste heat usage. A solution to this problem is possible through the implementation of open heat transformation processes [13]. In this way the temperature level of the reusable heat can be raised and the capital intensity of sorption cycles is decreased through the elimination of evaporators. Besides heat utilization, dust removal of hazardous or unwanted substances can be achieved through direct contact of solutions and fumes. Considering all these facts, a supply system can be proposed that follows the principle of heat cascades and includes possibilities for heat transformation. It can be realized with today’s technology, and can be suggested as an example for a regional integrated energy system. The supply system consists of three levels: the user level, the municipal (regional) level, and the multi-regional (national) level (Fig. 5).

Fig. 5. High quality exergy supply system (from [19]).

1292

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

The user level includes private households, small users, and industries. They need light, power, and heat either in the form of residential and water heating or industrial process heat. While residential and water heating requires temperatures below 100 °C, process heat presents a wide temperature range above 100 °C. In contrast to a conventional supply system, only the industrial sector at the user level in the new system receives a fuel input produced by high temperature industrial process heat. The usage sectors of private households and small users satisfy their low-temperature heat requirements through heat networks. All usage sectors receive connections for power supply. Both the production and storage of electricity and heat in networks are achieved at the regional level. Thermal power plants based on combined gas-steam turbines are the focal point of production. Such thermal power plants can also be operated in residential areas, since they do not cause considerable emission. This is especially true if the emissions of individual domestic heaters are simultaneously avoided. Such thermal power plants produce electricity and low-temperature heat in co-generation. Since the ratio of heat and electricity produced do usually not correspond with the needs, heat pumps can be implemented in order to use otherwise superfluous energy in the heat market. At present requirements fulfilled in the low temperature heat market lead from cogeneration to the production of an abundance of power. This can be reduced by using industrial waste heat, solar heat, or geothermal energy, especially with the implementation of heat pumps. In addition to the supply from heat power plants it is possible to produce electricity from regenerative sources, such as wind and photo-voltaic processes. A further alternative energy source at a regional level is biomass, which can be produced by gasification in heat power plants and be used in lieu of natural gas. The multi-regional level provides readily-usable energy for transport to the regional level. Solid fuels like stone coal and lignite coal are gasified and are thereby made available for utilization on the regional and user levels. Oil can be transferred directly from refineries to the regional and user levels. Natural gas is transferred from its sources through long-distance gas mains, while oil is transported through pipelines. The purification of fossil fuels from contaminating elements is implemented at a regional level. Here even a basic CO2 separation may be achieved. Fundamental production components of the new supply sysstem at the regional level are combined heat and power plants, electric heat pumps, and heat operated refrigeration cycles as well as heat exchangers used as decoupling facilities for geothermal and solar heat, industrial waste heat, and condensation heat from combustion gases. Electricity and heating networks as well as heat storage facilities are fundamental distribution components. Numerical estimations for such a supply system in concrete cases of conurbation showed that savings of around 50% in primary energies as well as corresponding reductions in the pollution of the environment could be accomplished [15,16]. If these considerations are developed into practical applications, a comprehensive concept will be developed from a thermodynamic perspective that could represent an optimization of the heat network. According to Fig. 5 the industrial sectors like iron, steel and chemical industry deliver the highest temperatures and, therefore, also the highest amounts of exergy. For this reason they are primarily interesting as waste heat users, because they emit this exergy as external irreversibilities unused into the environment. These conditions are schematically depicted in Fig. 6. The high temperature heat in iron and steel industries at the usage level TNE is produced and directly transferred through combustion

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

1293

Fig. 6. Possibilities in heat cascades.

processes. They finally end up as waste heat Q0 in the environment (Fig. 6a). The required heat performance in the chemical industry was previously produced at adequate temperatures by industrial furnaces that simultaneously delivered heat through combustion processes. Regarding heat transfer a much higher temperature difference has to be achieved, which leads to correspondingly high irreversibilities (Fig. 6b). Since the development of large scale synthesis, it has become common practice to produce the average temperature heat through steam networks. Steam is produced in industrial power plants by combined production with a power generation (Fig. 6c). The power plant process—a disproportionate process—is arranged upstream of the heat supply, which generally leads to a reduction in internal irreversibilities. To further improve the entropic situation an attempt can be made not to emit industrial waste heat directly into the environment but to implement suitable combinations for commercial heating purposes using TNR. This calls for an alliance between the industry sectors and the public energy supply, and requires the installation of district heating networks. This level of waste heat enables such a solution to be realistically feasible. This has already been realized in some regions, especially in conurbations, despite the economical and legal problems connected with it. The given requirements for heat transfer, however, allow even further suggestions from the perspective of waste heat utilization. If it were locally possible, a joint venture between the steel and chemical industries could be organized so that the latter satisfies its average temperature requirements using the high temperature waste heat as the first, which would in turn allow the chemical industry to offer its waste heat to the community for the purpose of commercial heating (Fig. 6d). This would then be a fully realized heat cascade. The difficulties opposed to such a solution are obvious. One should remember, however, that a combination of various industrial branches in the form of gas networks for inflammable and technical gases has long been established. A further futuristic solution is hinted at schematically in Fig. 6e. Using suitable thermal–chemical cycles, which can be regarded as combined processes, it is qualitatively conceivable that a cycle combined with combustion processes could produce average temperature heat through the reception of environmental heat (or cooling heat). This would then be a solution that would come very close to a reversible process.

1294

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

3.2. Entropy export through matter The entropy balance of technical systems is influenced not only through heat exchange, but also through matter exchange. The entropic effects connected with this as well as the possible natural processes can also contribute to entropy export and thereby to a state of order within the system. Especially if the state of output of matter differs from the state of the environment, then natural processes will take place with a corresponding entropy increase during the transition into the state of the environment, which can be regarded as external irreversibilities of the system. In order to evaluate the energy efficiency these have to be incorporated into the system since they cause a corresponding primary expenditure of energy. Their quantitative assessment is possible with the term ‘exergy’, which immediately indicates the additional expenditure in the scale of energy. The usual method of pure energy observation is not capable of assessing all of the losses associated with this, since for example the throttling and mixture process can occur without energy effects. Entropic effects connected with the mass flow occur if this output and this environment state differ in: Temperature (T ⫽ TU), Pressure (p ⬎ pU), Composition (x⫽xU), Chemical state (g ⫽ gU). In the first case thermal energy, being free or bound to matter, can be transferred. The second case deals with mechanical energy that is lost, e.g. during throttling if the exhaust is simply channeled into the environment. In the third case during transition mixture processes take place due to diffusion. They can be explained as destruction of concentration energy. Finally the last case characterizes the chemical transition into the state of the environment. All these processes can exist, if one or more of the mass flows occur as byproducts during the operation of a technological system. If mass flows occur only after the utilization of products, the products can be different from their environment with regard to composition and/or chemical state. Different from byproducts these products can then be classified as waste material. A typical example is the refuse. In order to illustrate this, quantitative estimations of entropy productions connected with those processes and thereby the existing entropy exports for the corresponding technical systems have been carried out on a world-wide scale. For the sake of better understanding, data are given in the energy scale calculated by the Gouy– Stodola equation from the entropy [6]. 앫 Thermal entropy of fumes, 450 GW; 앫 Throttling loss in exhaust processes, 150 GW. Similar loss processes occur during the implementation of compressed air. Analyses have shown that these can be reduced up to a third of the energy input. In the case of Germany this means a reduction in entropy-export of approximately 2 MW/ K.

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

1295

Considering the usual composition of the atmosphere, one can estimate entropy productions also caused by changes in concentration through partial pressure differences. The following examples [6] will illustrate this: 앫 Concentration entropy of combustion products, 450 GW; 앫 Concentration entropy of products of air separation plants, 450 GW. Entropy production occurs also through changes in the chemical state. An entropy export of this kind [6] can result through: 앫 Chemical entropy during the oxidation of iron, 1 TW; 앫 Chemical entropy of plastic wastes, 100 GW. These data show that the entropy export connected with the transport of matter is at least one order of magnitude smaller than the entropy export due to heat transfer. The strategies for the reduction of these losses are generally known. The losses through thermal energy can be reduced through regenerative cycles and through the inclusion in heat cascades, as long as operational reasons such as corrosion, purification and economic results do not prevent them. The creation of heat networks often offers impressive examples of the reduction of the external heat intake and output. Throttling losses can be overcome through a reduction of friction or, for higher throttling losses through the implementation of expansion engines. Entropic or exergetic analysis can be employed in deciding which procedures are appropriate. In future, particular attention should be given to the reduction of losses in concentration energy through diffusion processes. Their contribution in the form of gas is not insignificant. In order to reduce these losses, an attempt can be made to either convert the substances into their liquid or solid state or to develop and use expansion engines with semi-permeable walls. The latter represents in this sense a strategic scientific and developmental task. The reduction of losses because of chemical transformations can either be achieved through prevention of reactions, which would require appropriate measures of protection and insulation and a defined storage, or through a careful management of transformations. It should be kept in mind that although a time shift is achieved in the first case, primary energy is not saved. Procedures for refuse processing are typical of the second case. In estimating entropic influences on refuse incineration plants, thermodynamic analyses of various procedures have been made. A result [17] can be seen in Fig. 7. The total exergetic efficiency is highest for smolder and incineration procedures and is comparable to a large furnace without melting of the byproducts. The thermo-select procedure shows a low total efficiency. Particularly the means required for its implementation are relatively high. It is noteworthy that considerable losses of the thermo-select process come from the energy transformation within the gas engine (estimated efficiency, 0.3). However, if a customer uses the synthetic gas produced in the process, then the resulting total efficiency is considerably better, since the pyrolytic gases represent highquality intermediate products from an exergetic point of view. This requires either the transport of gases or a local vicinity of the plant. This proves again the conclusion that a combination of the usage of matter and energy leads to favorable results from the perspective of entropy economy. Until now technological procedures for energy production that are not based on combustion

1296

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

Fig. 7.

Comparison of exergetic efficiencies in thermal procedures.

of primary energy carriers (e.g. procedures for gas production from lignite) have been proven uneconomical. It must be assumed that this is also true for the production of gas from waste, whose composition resembles lignite in many respects. However, another aspect concerning gas production from waste is noteworthy. The composition of waste and thereby the composition of hot fumes has a great influence on the ‘optimal’ steam parameters. If a gasification of waste with subsequent gas purification is technically possible, then the clean gas can be used as an energy carrier to feed a gas-steam power plant or as process gas directly within a chemical process. A significant increase in the exergetic efficiency is thereby possible. From a thermodynamic perspective, biomechanical processing, which is often considered as an alternative to thermal processing, is characterized by higher exergy losses than an exclusively thermal procedure, because biological methods produce either low temperature heat or biogases of low exergetic degree of transformation. From the perspective of technology and thermodynamics, the separation of fractions with high heating values is principally better than a thermal treatment. Through an increase in the utilization of plastics, however, the energy of waste is low and cannot be considerably increased by classifying processes. In laboratory experiments fractions had reached enthalpies of combustion of about 11 MJ/kg. This value does not considerably exceed the average values of about 9.5 to 10 MJ/kg. Interesting from an entropic point of view is the fact that the exergy of domestic refuse is larger than the enthalpy of combustion and the enthalpy of reaction (Fig. 8). As a consequence the chemical energy is not completely convertible into thermal energy by standard combustion reactions. This is characteristic for bio-energy carriers. In Table 3, some combustible materials are listed together with the reaction equations and thermodynamic parameters interesting in this context. As can be seen, the gasification of bio-energy carriers with oxygen in contrast to fossil fuels is not exothermic, but on the contrary requires energy. Surprisingly, the reaction entropy is actually so high that the equilibrium temperature of gasification is below the temperature of the environment. As it is different from other fuels, bio-energy carriers have high oxygen content. Thus, additional oxygen for gasification is not required. Gasification of bio-energy carriers could therefore be used for low temperature applications (refrigeration). Pyrolysis differs only slightly from gasification, since it also requires little oxygen, with the

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

1297

Fig. 8. Ratio of specific exergy and enthalpy of combustion for different fuels (from [18]).

Table 3 Classification of standardised exergies of reaction of combustible materials (from [14]) Reaction equation

Combustion C+O2→CO2

CH4 + 2O2→CO2 + 2H2Og C2H2 + 2.5O2→2CO2 + H2Og C6H6fl + 7.5O2→6CO2 + 3H2Og Gasification 1 C + O2→CO 2 C2H2 + O2→2CO + H2 1 CH4 + O2→CO + 2H2 2 C2H6 + O2→2CO + 3H2 C6H6fl + 3O2→6CO + 3H2 C11H22O11 + 12O2→12CO + 11H2 Pyrolysis CH4→Cf + 2H2 C2H2→2Cf + H2 C6H6fl→2Cf + H2 C11H22O11→Cf + 11CO + 11H2 TGG = ⌬Rh0 / ⌬RS0

⌬R e ⌬Rh

⌬Rh0 in kJ/mol

⌬Rs0 in J/molK

TGG in K

e∗ =

⫺393.1 ⫺284.2 ⫺285.5⬅HO ⫺241.6⬅HU ⫺927.1⬅HO ⫺802.6⬅HU ⫺1 254.0 ⫺3 131.2 ⫺5 667.1⬅HO ⫺5 181.9⬅HU

2.8 -86.5 -163.0 -44.7 -242.4 -5.4 -54.3 137.9 5 684 1 861.9

⫺140,000 3300 1750 5400 3800 147,700 2300 ⫺22,700 ⫺9970 ⫺2783

1.002 0.903 0.82 0.944 0.92 0.998 0.99 1.013 1.03 1.11

⫺110.4

89.0

⫺1200

1.25

⫺447.3 ⫺35.5

119.5 169.7

⫺3740 ⫺210

1.08 2.43

⫺136.1 ⫺711.0 898.0

355.3 702.2 3 385.2

⫺387 ⫺1000 265

1.77 1.30 ⫺0.13

74.8 ⫺226.6 ⫺49.0 857.6

80.7 ⫺58.5 250.8 2 802.7

930 3870 ⫺195 306

0.68 0.92 2.54 0.02

1298

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

exception that the equilibrium temperature is slightly above the environment temperature, which therefore yields a small, but positive exergy change. A reversible implementation of processes with such materials is not at all achievable. Thus, heat acquisition from the environment has to be organized so that ultimately the environment energy is converted into work with the aid of chemical energy. From this, interesting tasks for future development might be derived. 4. Some applications Energy distribution, performance, density, and its temporal use in different regions can vary considerably. This is, however, of decisive importance for technical applications and the economical and social implications. Energetically relevant data concerning various typical regions are summarized in Table 4. As examples data for two conurbations, a municipality and an industrial city, a rural area and a mixed area consisting of a rural area and some smaller cities are given. It is conceivable that the chosen areas differ considerably. The final energy consumptions differ by some orders of magnitude, the specific final energy consumption in kW per inhabitant by one order of magnitude, and the final energy demand density in MW/km2 by two orders of magnitude. Table 4 Characteristics of the investigated regions

Example

Characteristic data Area in km2 Inhabitants in 1000 Population density in individuals/km2 Final energy consumption in MW Specific final energy consumption in kW/inhabitanta Final energy demand density in MW/km2 Specific final energy consumption for households in kW/inhabitant Energy consumption by sector in % Industry Domestic Small users (industrial and tertiary) Transportation a

Conurbation municipality

Conurbation industrial city

Mixed areas (rural areas and smaller cities)

Du¨ sseldorf

Duisburg

Region of Wesel Spree-Neißeregion

220 600 2700 2050 3.4

220 500 2300 8550 17

560 250 450 850 3.4

1650 150 90 300 2.0

9.3 1.1

39 0.8

1.5 1.3

0.18 1.4

30 25 23 22

90 4 2 5

44 29 5 22

11 46 11 32

As annual average performance without transportation.

Rural area

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

1299

Household consumption in kW per inhabitant is of the same order for all regions. The structure of final energy consumption differs also significantly. The almost equal distribution in the administration city of Duesseldorf is different from the industrial city of Duisburg. The rural and the mixed regions also differ through the structure of their final energy consumption even though their specific data are similar. The location of these regions in Germany is indicated in Fig. 9. The differences within these regions not only have consequences for their technical structures, but particularly also for managerial decisions. This is also important for energy problems and decisions, since economical realms of requirements, such as financing models and legal implications depend on it. Both technical and economical perspectives influence the interaction between waste energy usage and waste economy essential for the creation of an entropy economy. Optimal solutions are only possible with regard to the regional situation. Implementing bio-energy sources is mainly of interest in rural regions. The geographical location of design areas has some peculiarities important for the assessment and development of energy systems, such as connections to other nearby commercial regions, the economical environment, historical developments in industry and in energy systems. Characteristic for developed regions is a widely varying demand for heat and refrigeration throughout the year and during the course of the day. This requires a highly flexible energy supply system. Exemplary solutions have been suggested for the parliament and government sector in the Berlin Spreebogen, an area in the centre of the city, and recently for a bank building in Frankfurt. They consist of several small block heating power units (co-generation plants) in modular design providing power, heat and refrigeration. Flexibility can be achieved by switching the singular

Fig. 9. Geographical location of the analyzed regional objects.

1300

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

modules on and off. Waste heat of electricity production serves for heating in winter and cooling in summer, using absorption refrigeration machines. Various heat and cold storage devices serve as a time balance, and are effective throughout the seasons. Such a relatively complicated energy supply system is obviously capable combining the necessary high flexibility with sufficient thermodynamic quality. In an industrial city energy requirements are completely different from that of an administrative municipality. The high specific energy consumption and generation permits the utilization of industrial waste heat for district heating by heating networks (see Fig. 5). Case studies for the city of Duisburg yielded similar results, Fig. 10. The primary energy available for consumption is related to the current energy supply. Thus, possible savings become visible. The implementation of modern GaS (gas and steam) power plants, and the complete coverage of the space heating market through district heating is represented by curve 1. If heating is done by heat pumps, then savings as shown in curve 2 can be achieved. Thus, energy savings of 45% compared to the present situation may be achieved due to the flexibility of such an energy supply system. In rural areas with small cities an energy supply structure has been designed that covers the commercial heating requirements in such thinly populated areas via decentralized electric heat pump systems (10%) and in the denser populated areas via a widely branched out heat network (90%). Heat networks are run from GaS (gas and steam) power plants and with large heat pumps for time compensation of the consumption. Compared to the current supply, up to 50% of primary energy usage could be saved with such a concept. In rural areas a decentralized structure in the heat market in Germany at present makes predominantly use of lignite. In the future, petroleum and natural gas will also become of importance. With today’s technology it is conceivable to replace approximately 15% of the final energy potential by the production and usage of biomass. Saving potentials for heat supply can be found in individual cases for communities or for smaller residential areas, in rural companies (eg. in breweries).

Fig. 10. Relative primary energy consumption as a function of a related electricity number in Duisburg. PE, primary energy consumption of the reference system (present system); PE∗, primary energy consumption of the system investigated; s, specific electricity consumption (electrical/heating power).

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

1301

Through the implementation of sorption heat pumps and transformation processes, favorable thermodynamic solutions can be suggested.

5. Conclusions The explicit integration of the Second Law of Thermodynamics in the evaluation of energy processes unequivocally puts waste energy into a central position. Through waste the necessary entropy export essential for the maintenance of the desired state of order is realized. The amount of entropy export is determined through the internal and the external irreversibilities. A reduction of irreversibilities not only means a cutback on primary energy but also less pollution, both in the scale of entropy. From this point of view, the reversible process is energetically the most favorable, since it can be implemented with a minimum of entropy export. Striving for this goal means, however, a reduction of irreversibilities either through slower process operation or through an increase in transfer dimensions. Thus, possible energy savings are necessarily connected to additional expenditure for the technical equipment. As shown above, from a thermodynamic perspective a large number of possibilities for the realization of this principle exist. They promise considerable savings on primary energy under definite technical circumstances. The basic problem—energy savings and additional expenditure for necessary equipment—calls also for economical categories as a solution. This also holds for many other problems in connection with the evaluation of technical systems, the assessment of cumulative effects, or external expenditures, etc. Apart from these aspects it becomes obvious that favorable entropy solutions involve also a wide range of legal questions and problems, because, as an example, the design of heat cascades and heat networks often concerns different administrative and political institutions. Regulations and laws have to be observed or are needed. In addition the costs and the duration of investments in energetic systems affect the society. Techniques for estimating such technological repercussions have been established in many countries via technology assessment. Summarizing, we can state that the social requirements, to be considered when implementing an entropy economy, often considerably restrict the thermodynamic possibilities. To overcome this problem a cooperation of all disciplines involved can open up new areas and a new level of entropy economy. By this a considerable contribution to sustainable development can be made, all the more so as raw materials and waste products necessary for energy supply are of outstanding importance among all industrial goods.

References [1] [2] [3] [4]

Gaggioli R, editor. The second law analysis. ACS Symp Series 122; 1980. Moran M. Availability analysis: A guide to efficient energy use. New York: ASME Press, 1989. Gaggioli R, editor. Efficiency and costing. ACS Symp Series; 1983. Bejan A, Tsatsaronis G, Moran M. In: Moran MJ, Sciubba E, editors. Thermal design and optimisation. New York: Wiley; 1996. p. 56-0. [5] Tsatsaronis G. Thermoeconomic analysis and optimization of energy systems. Progress in Energy and Combustion Systems 1993;19:227–57.

1302

W. Fratzscher, K. Stephan / Energy 28 (2003) 1281–1302

[6] Fratzscher W, Fratzscher W. Strategies of waste energy use. Wiesbaden: Vieweg, 2000 In German, Strategien zur Abfallenergieverwertung. [7] Fratzscher W, Brodjanskij VM, Michalek K. Exergy—Theory and application (in German. Leipzig: Deutscher Verlag fu¨ r Grundstoffindustrie, Springer, 1986 In German, Exergie—Theorie und Anwendung. [8] Pruschek R, Oeljaklaus G, Go¨ ttlicher G, Kloster R. Gas-steam power stations with high efficiency, VDI report No. 1321. Duesseldorf: VDI-Verlag, 1997 In German, Gas-Dampf-Kraftwerke mit hohen Wirkungsgraden, 1–18. [9] Dehli M. Future development in CO2-emissions in the European electricity economy, VDI report No. 1321. Duesseldorf: VDI-Verlag, 1997 In German, Ku¨ nftige Entwicklungen bei den CO2-Emissionen in der Europa¨ ischen Elektrizita¨ tswirtschaft, 559–78. [10] Geiger B, Hess H. Energy-economical data, energy consumption in the Federal Republic of Germany, VDIGET-Jahrbuch. Duesseldorf: VDI-Verlag, 1999 In German, Energiewirtschaftliche Daten, Energieverbrauch in der Bundesrepublik Deutschland, 272. [11] Bittrich P, Hebecker D. Classificiation and evaluation of heat transformation processes. International Journal of Thermal Sciences 1999;38:465–74. [12] Alefeld G, Radermacher R. Heat conversion systems. Boca Raton: CRC Press, 1994. [13] Bittrich P, Bergmann T, Hebecker D. Development of a high-temperature caloric-value-vessel, VDI report No. 1321. Duesseldorf: VDI-Verlag, 1997 In German, Entwicklung eines Hochtemperatur-Brennwertkessels, 327–340. [14] Lucas K. Supply systems in a regional energy-multi-system. In: Fratzscher W, Stephan K, editors. Strategies of waste energy use. Wiesbaden: Vieweg; 2000. In German: Versorgungssysteme im regionalen Energieverbund, in Strategien zur Abfallenergieverwertung. [15] IUTA. Data collection, assessment, and representation of heat offer, heat and cold demand as well as the long distance electrical network of the example of a region in a geographical information system. In German, Erfassung, Bewertung und Darstellung von Wa¨ rmeangebot, Wa¨ rme- und Ka¨ lte bedarf sowie des Fernwa¨ rmeleitungsnetzes einer Beispielregion in einem geographischen Informationssystem, Final report, Abschlussbericht einer IUTAStudie an das Landesumweltamt Nordrhein-Westfalen, 1996. [16] Environmental Office North-Rhine Westfalia, Umweltamt NRW: Rational use of energy and CO2-economy in Duesseldorf. In German, Rationelle Energieverwendung und CO2-Einsparung in Du¨ sseldorf, Landeshauptstadt Duesseldorf, 1992. [17] Mewes D, Tokarz A. Concepts of waste recovery. In: Fratzscher W, Stephan K, editors. Strategies of waste energy use. Wiesbaden: Vieweg; 2000. In German: Abfallverwertungskonzepte, in Strategien zur Abfallenergieverwertung. [18] Baehr HD. Thermodynamik, 11th edn. Berlin, Heidelberg, New York: Springer, 2002. [19] Hebecker D. Combustion and gasification of biogenic energy carriers. In: Fratzscher W, Stephan K, editors. Strategies of waste energy use. Wiesbaden: Vieweg; 2000. In German: Verbrennung und Vergasung von biogenen Energietra¨ gern, in Strategien zur Abfallenergieverwertung.