Energy/Exergy Analysis

Primary Source: Suomalainen, K. (2006) Energy / Exergy Analysis in Report on the SWOT analysis of concepts, methods, and models potentially supporting LCA. Eds. Schepelmann, Ritthoff & Santman (Wuppertal Institute for Climate and Energy) & Jeswani and Azapagic (University of Manchester), pp 141-147

Level of analysis: All, can be used to assess specific processes from an energy efficiency point of view, as well as entire socio-economic systems to assess sustainability from a global perspective (sustainability indicators).

Assessed aspects of sustainability: as energy efficiency – environmental, as sustainability indicators – socio-economical.

Main purpose of the assessment:

To assess sustainability/efficiency of a system through evaluation of its energy flows and energy efficiency.

Description of the methodology:

Energy analysis has traditionally focused on production processes at the micro-level, often with the aim to lower production costs by increasing energy efficiency. It has also commonly been used to analyse resource use, in a manner similar to material flow analysis (Ahlroth et al, 2003). There are several different ways to measure energy. One example is exergy, which can be defined as a measure of available energy. This has been applied on e.g. process engineering systems, nations and products. Similar to cumulative energy requirements analysis, in emergy analysis the total inputs of energy, materials, information and labour are added using “emergy”-equivalents which describe the accumulated energy associated with the different types of inputs. Recently, however, energy analysis has been developed as a method to evaluate sustainability of complex systems, such as socio-economic systems (Giampietro, 1999).

Detailed description

Exergy as an indicator for resource depletion

The energetic argument claims that useful energy (exergy) is the ultimate limiting resource. Each material resource has an associated energy cost and thus every potentially limited resource is limited in part because of it’s energy costs are to high (Finnveden, 1997). Another argument for the use of exergy asks “what is being depleted or consumed?”. Material or energy may only be transformed, but not destroyed (excluding nuclear reactions). It is the usable material and energy that is being consumed. Societies feed on low entropy matter (useful matter) converting it to high entropy matter and energy. In this sense entropy could be a useful indicator of resource consumption.

The chemical exergy of natural resources may be calculated for system boundaries compatible with LCA (Finnveden, 1997). Thus exergy consumption could be used as a characterisation parameter for abiotic fund and flow resources, and possibly also to describe competition of biotic resources. However, exergy is not fit to describe all relevant aspects of resource depletion, as biological diversity for one example.

Cumulative energy requirements analysis (CERA)

CERA is used to quantify the primary energy requirement for products and services in a lifecycle perspective. The method was developed with the purpose of taking in to account the upstream energy flows of production processes. The cumulative energy requirement indicates a basic environmental pressure associated with the use of energy. Similar to material intensity the energy intensity can not be used to quantify specific environmental pressures (e.g. ozone depletion) rather than a generic pressure.

The primary energy requirements are measured in Joules and aggregated into one number. A lower value may be interpreted as being associated with less environmental burden only if the relative share of the energy carriers will not be changed towards more hazardous ones.

CERA can be used to:

–          quantify the energy intensity of products, services and national economies;

–          analyse options for energy savings in industry;

–          provide energy input coefficients for base materials to support engineering and design of products (Leiden University Institute of Environmental Sciences website).

Energy analysis of socio-economic systems

In the following review energy analysis is used to assess sustainability of socio-economic systems. According to Giampietro et al. socio-economic systems can be described as nested, dissipative, adaptive systems, where stability can be studied by assessing the exergy budget of elements in the system. The two main goals of dissipative systems are guaranteeing short-term stability of current dissipative structures that maintain the system’s metabolism (efficiency) and guaranteeing long-term stability through maintaining high compatibility of patterns of selforganization within a changing environment (adaptability).

In this approach the structure of a socio-economic system consists of a purely dissipative part and a hypercyclic part (Vlanowicz, x), as shown in figure 1. The purely dissipative part includes households and services whereas the hypercyclic part contains the sectors of productive activities. Energy investment in households (HH in figure 1) is purely dissipative, consuming net energy in the short term. HH includes sleeping, personal care, leisure time and such activities that are performed by the economically inactive population. Energy investment in service sector activities (SS in figure 1) are also dissipative, but are defined by social roles e.g. job positions, police, army, health care, education and insurance, i.e. they are included in economic activities. Energy investments in activities in the productive economic sectors (CI in figure 1) are characterized by positive returns in energy flows. They are also defined by social roles within the energy and mining sector, manufacturing sector, food security, environmental security. ET in figure 1 stands for the flow of exosomatic energy consumed by society.

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The dynamic exergy budget is defined by the socio-economic characteristics of society generating demand and the characteristics of the exosomatic autocatalytic loop generating supply. Stabilizing society’s exergy budget thus means equalling demand with supply.

Giampietro et al. study the dynamic equilibrium between demand and supply by using existing relationships between parameters that they are determined by. Demand is given by:

–          ET = flow of exosomatic energy consumed by society [J/yr]

–          MF = metabolic flow, energy per kg of human body mass [J/kg hr]

–          ABM = average body mass [kg]

–          Exo/Endo = ratio between exosomatic and endosomatic energy flows

–          THT = total human time = population × hours in a year [hr], this can also be given as

–          THT = A+WS, where WS = B+C;

–          A = non-working time (sleeping, leisure etc)

–          WS = work supply, amount of time that the economically active population allocates to work annually

–          B = hours of work delivered in the service sector

–          C = hours of work delivered in productive sectors

A sustainability indicator, the bioeconomic pressure (BEP), which measures exosomatic energy throughput consumed by a society per hour of labor time in the productive sector of the economy, is defined by;

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From the supply side the same indicator is defined as the strength of the exosomatic hypercycle (SEH).

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where strength of the exosomatoc hypercycle is defined as the exosomatic energy throughput (societal power) generated in the society per unit of work delivered in productive sectors. SHE depends on how much exosomatic energy is “eaten” by the hypercycle (ET/CI), relating to the output-to-input energy ratio of processes that make resources available to the economy, and the power consumption per worker in the productive sector (CI/C), which is affected by the existing technology and the quality of the available natural resources and defined by technical input-output coefficients. Setting demand equal to supply gives the dynamic exergy budget.

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This links physiological and socio-economic variables with technological variables.

In assessing sustainability of a society, this model can deal with the question whether current material standard of living is technically feasible and culturally acceptable. According to Giampietro et al. BEP is a good indicator of development showing reasonable correlation with several economic, physiological and social indicators of development. The data used in this model consists of official data of the UN, FAO, and the World Bank.

Energy analysis is a flexible method that can be used with various types of energy measures.  However, there are distinct differences in approach between a bottom-up and top-down assessments; one is used more for micro-level analysis and the other for the macro-level.

EA has also been applied for example for analysis of impacts of waste emissions (Rosen and Dincer 1999), material life cycle assessments (Michaelis et al. 1998), exergy flows in societies (Ertesvåg, 2001) and emergy accounting (Brown and Herendeen 1996, Ulgiati et al. 1994).

Strengths

Scope of assessment (what is being assessed)

The main strengths of energy/exergy analysis are:

–          advantage of working with one unit only (energy-unit),

–          applicable to most situations/problems/case studies and at all levels (micro, meso, macro),

–          flexible in scope (defined by the user of the method)

–          scientific potential/strength in thermodynamic basis, entropi, exergi,

–          often acceptable as a first estimative of sustainability analysis.

Methodology (robustness, validity & reliability)

–          Energy analysis is robust when the study is well-defined and documented. It is a flexible method at all levels (micro, meso, macro) and allows for a wide range of application from identifying opportunities to save costs to assessing societal sustainability. If the scope of the study is well-documented, EA results are easily comparable (energy units) and due to this transparency their reliability can quite easily be assessed.

–          Due to its flexibility EA also has potential to be integrated with LC approach tools. Methodologically CERA is closest to LCA at micro/meso scale. Thus EA presents opportunities to deepen LCA.

Weaknesses/Limitations

Scope of assessment

EA does not take in to account all aspects of sustainability; economic and social on one hand, or discrete aspects such as biodiversity or ozone depletion on the other. It is limited to the energy aspects of the studied system.

Methodology

–          the variety among EA methods presents the lack of definition of a stepwise, easy to follow methodological approach to EA,

–          may be practically difficult to include all relevant energy aspects -> lack of documentation guidelines raise the question of validity and reliability of results,

–          not robust due to lack of definition and common rules.

Opportunities for broadening and deepening LCA

Due to the importance of energy efficiency, EA has many opportunities to be included in EU research at various levels; Eco-design of Energy Using Product Directive (EuP). There is the opportunity to use EA as a tool to study and interpret eco-efficiency performance and targets of processes, products, services and even more complex systems at meso- or macro-level.

Threats for broadening and deepening LCA

EA alone does not seem to be sufficient for comprehensive sustainability analysis. EA is focused on a narrow, though relevant, part of sustainability, but there is the risk of focusing too much on energy aspects and leaving out other important aspects. Such development may lead to a narrower use of EA as a tool.

Literature/Internet Links

Ahlroth, S., Ekvall, T., Wadeskog, A., Finnveden, G., Hochschorner, E., Palm, V., 2003, Ekonomi, energi energy och miljo – metoder att analysera samband, Totalförsvarets forskningsinstitut, forskningsgruppen for niljöstrategiska studier, Stockholm.

Brown, M. T., Herendeen, R. A., 1996, Embodied energy analysis and EMERGY analysis: a comparative view, Ecological Economics 19, pp. 219-235.

Erstesvåg, I. S., 2001, Society exergy analysis: a comparison of different societies, Energy 26, pp. 253-270.

Finnveden, G., Moberg, Å., 2005, Environmental systems analysis tools – an overview, Journal of Cleaner Production 13, 1165-1173.

Finneden, G., Östlund, P., 1997, Exergies of natural resources in life-cycle assessment and other applications, Energy Vol. 22, No. 9, pp. 923-931.

Giampietro, M., Mayumi, K., Pastore, G., 1999, Energy Analyses as a Tool for Sustainability: Lessons from Complex System Theory, Annals of the New York Academy of SciencesVolume 879 Tempos in Science and Nature: Structures, Relations, and Complexity, pp344-367.

Michaelis, P., Jackson, T., Clift, R., 1998, Exergy analysis of the life cycle of steel, Energy, Volume 23, Vol. 3, pp. 213-220.

Rosen, M. A., Dincer, I., 1999, Exergy analysis of waste emissions, International Journal of Energy Research, Vol. 23, no, 13, pp. 1153-1163.

Ugliati, S, Odum, H. T., Bastianoni, S., 1994, Emergy use, environmental loading and sustainability. An emergy analysis of Italy, Ecological Modelling, Vol. 73, no. 3-4, pp. 215 268.

Leiden University Institute of Environmental Sciences website; http://www.leidenuniv.nl/cml/ssp/projects/chainet/tools-cera.html, last consulted in July 2008.

 

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