Elsevier

Applied Energy

Volume 136, 31 December 2014, Pages 1035-1042
Applied Energy

Sankey diagram framework for energy and exergy flows

https://doi.org/10.1016/j.apenergy.2014.08.070Get rights and content

Highlights

  • A framework for designing Sankey diagrams for national level analysis is presented.

  • Provided insights of how Sankey diagrams can be designed for various objectives.

  • Identified areas for energy savings by overlaying energy and exergy flow diagrams.

Abstract

Energy flow diagrams in the form of Sankey diagrams have been identified as a useful tool in energy management and performance improvement. However there is a lack of understanding on how such diagrams should be designed and developed for different applications and objectives. At the national level, a framework matching features of Sankey diagrams with various objectives of energy performance management provides better understanding how Sankey diagrams can be designed. As part of the framework, boundaries outlined around a group of facilities provide a refined representation of sub-systems that trace energy use in various conversion devices, products and services, which identifies potential areas for energy savings. There are some differences between energy flows and exergy flows represented in Sankey diagrams. Furthermore, exergy flow Sankey diagrams offer advantages in identifying potential areas for energy savings. This is illustrated at a national level, using UK’s industrial heating processes that overlays both energy and exergy flow diagrams.

Introduction

Sankey diagrams have been used as an effective tool to focus on energy flow and its distribution across various energy systems. It is represented by arrows, whose width represents the magnitude of the flow. Schmidt [1] presented a comprehensive review of the historical uses of Sankey diagrams stressing its rising importance in decision making and public policy. For example, the use of Sankey diagrams for identifying energy efficiency improvements for a society was addressed in a 1971 paper [2]. Many countries and international agencies have represented energy flows using Sankey diagrams from supply to end use sectors. International organizations such as ISO have developed energy management systems standards such as ISO 50001 (Energy Management Systems) to improve organizational energy performance. Such standards recognize the Sankey diagram as one of the tools that could be used in the energy review process that analyzes energy consumption, identifying significant consumption and identifying areas for improvements for energy performance planning [3, p. 14].

However, there is a lack of understanding on how such Sankey diagrams should be designed and developed for different applications and objectives. Issues such as the diagram’s structure (e.g. should flows be centered on processes, physical equipment, final energy services or a combination of these) and the appropriate level of detail and granularity are not fully addressed. Furthermore inconsistencies can be observed as to how energy losses are represented for various end-use products and services, showing a lack of consensus in how energy losses should be quantified for end-use products and services.

Representing energy losses for end-use products and services can be particularly challenging given that energy analysis does not support assessment of the quality and usefulness of energy. On the other hand, exergy analysis of energy systems and processes serves as a quantitative measure of quality and usefulness of energy [4]. Therefore, exergy flow Sankey diagrams can be very useful in identifying thermodynamic losses and potential areas for energy savings, and providing a rational basis for energy performance benchmarking, and for improving energy management systems and activities.

The purpose of this paper is to provide a framework to use energy and exergy flow Sankey diagrams for national level analysis. The remaining sections are organized as follows: Section 2 conducts a literature review of recent energy flow Sankey diagrams to identify and understand their features and differences. In Section 3, a framework matching key features of Sankey diagrams with different objectives of energy performance is presented. Section 4 explains the difference between energy balance and exergy balance, and provides a case study of UK’s industrial heating to show how and why exergy flow Sankey diagrams can be used in identifying potential energy savings. Section 5 concludes the paper.

Section snippets

Literature review of Sankey diagrams at national level

Although Sankey diagrams were originally developed to trace energy flows for steam engines, its application to energy flows in a society (e.g. at national and global level) has become increasingly important in recent years. This section conducts a review of the use of Sankey diagrams at the national level to identify and understand the features and differences of these Sankey diagrams, including system boundaries (both spatial and temporal), level of granularity, and representation of energy

Sankey framework for energy flow analysis at national level

National level analysis of energy systems are often performed under three categories of interests: energy economics, environmental impacts of energy, and energy security. Alternative classification of the objectives of analysis, such as increasing the use of alternative energy, and identifying areas for energy efficiency improvements and energy conservation have ramifications in all three categories mentioned above. These inter-linkages and inter-dependence of objectives make it challenging to

Exergy as a metric for loss representation

Sankey diagrams are often used to trace energy flows based on energy balance/conservation concepts and the First law of Thermodynamics. Under these principles, energy input and outputs at each stage of the energy system is balanced by considering the energy content of various sources and flows. Differences between magnitude of energy input and output are represented as losses only at relatively low levels of details and granularity (e.g. power generation losses, oil refining losses,

Conclusion

The matching of various key features with different objectives provides a framework for designing Sankey diagrams for national level and facility level analysis. The analysis makes three unique contributions to our understanding of how Sankey diagrams are to be designed for various objectives. Firstly, energy loss representations at low levels of granularity do not contribute significantly to energy performance objectives. Secondly, features based on refining sub-systems to trace energy use in

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