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Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Ecological Economics, Volume 69, Issue 10, 15 August 2010, Pages 1904–1917
This paper presents a societal level exergy analysis approach developed to analyse transitions in the way that energy is supplied and contributes to economic growth in the UK, the US, Austria and Japan, throughout the last century. We assess changes in exergy and useful work consumption, energy efficiency and related GDP intensity measures of each economy. The novel data provided elucidate certain characteristics of divergence and commonality in the energy transitions studied. The results indicate that in each country the processes of industrialization, urbanisation and electrification are characterised by a marked increase in exergy and useful work supplies and per capita intensities. There is a common and continuous decrease in the exergy intensity of GDP. Moreover for each country studied the trend of increasing useful work intensity of GDP reversed in the early 1970s coincident with the first oil crisis.
Fundamental changes in patterns of energy supply and use occurring since the onset of the industrial revolution are commonly referred to as the “energy transition”. The energy transition has led to alterations in the structure of the energy supply and has entailed a significant growth in overall energy use. It has involved a shift from a solar based energy regime exploiting products of photosynthesis, wind, and water power, to an increasing reliance on fossil fuels. These shifts are linked to the emergence of new energy conversion systems and changes in the energy service demands of final users (Smil, 1991 and Podobnik, 2005). Historically, the energy transition has been accompanied by an increase in primary energy demand and per capita energy use. The energy systems of all four industrialized countries in our study underwent such a transition. Evidence indicates that today's industrializing countries are following a similar path (Gales et al., 2007 and Marcotullio and Schulz, 2007), while industrialized nations reconsider the structure of their energy supply systems in light of concerns about energy security and climate change and progress in ‘clean’ energy and energy efficient technologies. Our work in this paper provides evidence for an additional reason to seek efficiency improvements as a means of stimulating sustainable output growth. Studies analysing long-term trends in energy use typically focus on the quantities of input categories such as total primary energy supply (TPES), which denotes the volume of primary energy inputs into socioeconomic systems, or final energy consumption, the amount of energy supplied to end users in industry and households (e.g. Bartoletto and Rubio 2008; Warde, 2007, Gales et al., 2007, Kander, 2002, Haberl et al., 2006 and Krausmann and Haberl, 2002). Exergy analysis deepens this analysis to enable consideration of the quality of energy inputs as well as the breakdown and efficiency of energy use; both important and dynamic characteristics of evolving socioeconomic systems. Exergy (or useful energy or available work) denotes the ability of energy to perform work and is formally defined as the maximum amount of work that a subsystem can do on its surroundings as it approaches reversible thermodynamic equilibrium. Exergy provides a measure of energy quality. Exergy is usually quantified and measured in energy units (Joules). Unlike energy, which cannot be consumed (a consequence of the first law of thermodynamics), exergy is consumed and lost during any conversion process (Ayres, 1998). In order to provide useful work1 such as heat, light or mechanical power, one or more conversion processes are required and according to the second law of thermodynamics all energy transformation processes result in exergy losses. The size of these losses depends on the way in which they are used. Exergy analysis has been used to assess the supply, demand and technology characteristics of regional and national economies but the majority of these studies focussed on one single year. Examples include, for the US (Reistad, 1975), Sweden, Japan and Italy (Wall, 1987, Wall, 1990 and Wall et al., 1994), Canada (Rosen, 1992) and Turkey (Ertesvag and Mielnik, 2000). Fewer studies have examined the historical evolution of resource exergy supply and utilization. Examples include studies for China covering all major sectors of productive activity over the period 1980 to 2002 (Chen and Chen, 2007a, Chen and Chen, 2007b, Chen and Chen, 2007c, Chen and Chen, 2007d and Chen and Chen, 2007e) and long-term studies that cover the entire 20th century, for the US (Ayres et al., 2003), Japan (Williams et al., 2008) and the UK (Schandl and Schulz, 2002 and Warr et al., 2008). In previous work some of the authors have argued that exergy analysis provides an approach for the better integration of ‘productive energy use’ in economic growth theory through inclusion of useful work in the production function having shown that useful work supplied to an economy is ‘Granger’ causal to output growth (Warr and Ayres, 2010). While other studies have used energy as a factor of production, much of the total consumed available energy (exergy) is actually wasted, and therefore does not contribute to growth. Ayres and Warr (2005) concluded that “useful work” delivered to the economy is a more appropriate factor of production to use in representing physical resource flows, than total primary energy (exergy) inputs.2 The inclusion of useful work as a factor of production representing the productive component of exergy inputs (productive potential) eliminates much of the unexplained Solow residual by effectively accounting for technological progress in energy related processes. Using this work augmented production function, Warr and Ayres (2006) developed a simple yet robust3 economic forecasting model taking useful work as a factor of production (named REXS). This model has been shown to be able to reproduce observed economic growth in the US economy for the entire of the 20th century and eliminates the assumption of exogenously driven exponential growth along a so-called “optimal trajectory”. Instead, the growth trajectory is dependent on endogenous technological change described in terms of the decreasing exergy intensity of output and increasing efficiency of conversion of fuel inputs (exergy) to primary exergy services (“useful work”). In this paper, we present exergy and useful work data for additional countries. The first national data set for useful work used here was published for the US in 2003 (Ayres et al., 2003). Since then, the approach has been standardised and applied to the United Kingdom (Warr et al., 2008), Japan (Williams et al., 2008 and Ayres, 2008) and Austria (Eisenmenger et al., 2009). Despite significant variability in the availability and detail of source data we attempt to analyse each country using a standardised methodology to provide comparable data for the last century (1900–2000). Calibrated studies of this length are rare (and by necessity less detailed than static single year analyses), but necessary to test the long-term stability of identified parameters needed for forecasting. The time period studied covers a critical period of the late industrialization process these now mature industrialized economies underwent. The four national case studies provide a unique and novel database enabling us to investigate the trends and dynamics of energy transition. By including useful work we enhance understanding of the relations between technological progress, energy supply and use, and economic growth. The cross-country comparison of the historical energy transition presented here concentrates on the development of a number of key characteristics of the socioeconomic energy system. In the remainder of the paper we describe the concepts and the methods used to obtain estimates of exergy inputs, the breakdown of exergy inputs to different types of useful work, the efficiency of exergy to useful work conversion, required to obtain estimate of useful work outputs. We highlight similarities and differences in the trends in relation to the development of population, economic growth and carbon dioxide emissions. The paper ends with a comparative summary of the observed characteristics of the energy transition and draws some conclusions on the decoupling of energy use, carbon emissions and economic growth in consideration of the intensity measures generated.
نتیجه گیری انگلیسی
We have presented a methodology for exergy analysis of national economies suitable for the estimation of long run trends in exergy and useful work consumption, and energy efficiency. The methodology is theoretically based on the principles of thermodynamics and specifically consideration of the 2nd law (the ‘entropy law’) and as such bears many similarities to those used by others for single year, single country assessments cited previously. Our analysis is arguably less exhaustive. This is a necessary compromise to ensure that a consistent approach is applied to source data that differs in detail and quality over time and between countries. Where historical statistics are consistent with our approach the analysis is relatively straightforward. Such is the case for electricity. However, more commonly the essential information (exergy input, useful work allocation or efficiency) was not available and needed to be estimated.23 The greatest uncertainty involves industrial uses of energy for heat which are multiple and complex. We present a means of assessing the energy efficiency of industrial use with a simple three category division of exergy use into high, medium and low temperature heat. The division is based on reported flows to industry, residential and commercial uses. The efficiency coefficients, required to estimate useful work output were obtained by standardised methods; in the case of high temperature heat, steel manufacture was used as a proxy; for mid and low temperature heat 2nd law efficiencies appropriate to the energy conversion device considered were approximated using the Carnot equation for the relevant temperature differentials. This last point highlights perhaps not only the major strength but also the limitation of the approach. The exergy efficiencies we estimate are specifically task related. Perhaps the clearest example of this is with regard mobility. A ‘complete’ assessment of the efficiency of the service provided must consider the distance and the speed of the voyage and the load of the vehicle. In practice as we discuss this is not feasible. We are restricted to providing estimates of the task efficiency specific to a given device or technology. By so doing we avoid issues of non-technical tradeoffs. For example, we do not consider the relative efficiency of wearing warm clothing over resistance domestic space heating of a room to a comfortable temperature. We are limited to consideration of the technical aspects of service delivery and do not consider qualitative preferences. The power of the exergy approach is that it enables us to compare general physical performance, by considering the actual device used in relation to the task, “such analysis delineates the limitations and inefficiencies of the devices we now have, and indicates where they should either be improved or replaced or integrated to form new systems which perform joint tasks more efficiently than either could separately accomplish”, (AIP, 1975). The concept of exergy allows us to define a theoretical maximum efficiency (or a minimum exergy requirement) to complete any given task. It follows from the definition of exergy that the actual amount of useful work delivered to all economic activities is less than the theoretical maximum or alternatively that the exergy input exceeds the minimum requirement. The ratio of the actual to the theoretical maximum can be described as the technical efficiency (as opposed to economic efficiency) with which the economy converts raw materials into finished materials. This, in turn, can be regarded as a reliable measure of the state of technology of energy conversion devices and systems. Given the prevalence and importance of such systems in industrialized economies, and the rigorous theoretical foundations of the energy-to-work framework 24 we propose in this paper that the change in efficiency, over time is a reasonable proxy measure of technical progress. The data presented has enabled us to compare the impacts of a century of unparalleled change on energy consumption. The energy transition experienced in each country has dramatically altered the structure of the energy system in each country. Common characteristics of the transition process include a rapid growth in exergy consumption accompanied by a shift from a biomass to a fossil fuel powered system. The former was constrained in size by our ability to capture energy from the sun and convert this into useful forms of energy, notably muscle work. The latter is limited only by our supplies of fossil fuels and the capacity for assimilation of wastes without catastrophic change. Useful work output shows a characteristic shift from muscle work and low temperature heat in the early phases of the energy transition, to a period of high and medium temperature heat dominating the energy system (coal-iron/steel-railroad technology regime), to a dominance of electricity-consuming services (by businesses and households) and petroleum-based transportation services. The drivers of change have been many and include industrialization, urbanisation and electrification, but specifically growth itself. The data we provide will permit examination of the relationship between these drivers of change and efficiency improvements in the way that energy is used and most importantly economic growth. We have qualitatively described a process whereby efficiency improvements provide more useful work per unit of energy purchased and hence drive down the costs of products and services (ceteris paribus). Subsequent research will seek to quantitatively assess the importance of energy efficiency improvements as a source of growth and the potential for decoupling of energy use from growth in the future.