Sankey diagrams are powerful tools for visualizing flows in complex systems. They use arrow widths to show the magnitude of flows, making it easy to see how resources like energy or materials move and change. This type of diagram is super helpful for understanding and improving processes.

In the world of network graphs and tree diagrams, Sankey diagrams stand out for their ability to show quantities and directions simultaneously. They're great for spotting inefficiencies, identifying major pathways, and finding opportunities for optimization in various systems, from energy networks to supply chains.

Sankey Diagrams: Fundamentals and Uses

Introduction to Sankey Diagrams

• Sankey diagrams are a type of flow diagram that visualizes the magnitude and direction of flows in a system (energy, material, money)
• The width of the arrows in a Sankey diagram is proportional to the quantity of flow, allowing for a clear representation of the relative magnitudes of different flows
• Sankey diagrams adhere to the first law of thermodynamics, ensuring that the total input flow is equal to the total output flow, accounting for any losses or inefficiencies in the system
• Sankey diagrams provide a visually intuitive way to understand the distribution and transformation of resources within a complex system (industrial process, supply chain)

Applications of Sankey Diagrams

• Sankey diagrams are commonly used in energy systems analysis to visualize energy flows, conversions, and losses across different stages, from primary energy sources (fossil fuels, renewables) to end-use applications (heating, transportation)
• In material flow analysis, Sankey diagrams help track the movement and transformation of materials through a system (manufacturing process, supply chain), identifying the main sources, sinks, and intermediate stages
• Sankey diagrams can be applied to various other domains, such as financial flows (revenue, expenses), data flows in computer networks (bandwidth, data transfer), or even the flow of information or ideas in a conceptual framework
• Sankey diagrams are valuable tools for communicating the overall structure and performance of a system to both technical and non-technical audiences (policymakers, stakeholders)

Applying Sankey Diagrams for Flow Analysis

Energy Systems Analysis

• When applying Sankey diagrams to energy systems, it is essential to identify the main energy sources (coal, natural gas, solar), conversion processes (power plants, refineries), and end-use applications (industry, buildings), and to quantify the energy flows between these components
• Energy Sankey diagrams can reveal the efficiency of different energy conversion pathways (electricity generation, fuel production), highlighting the losses and identifying opportunities for improvement
• Sankey diagrams can be used to compare the energy mix and consumption patterns of different countries or regions, supporting energy policy decisions and transition strategies towards more sustainable systems

Material Flow Analysis

• In material flow analysis, Sankey diagrams help visualize the mass balance of a system, tracking the input, output, and accumulation of materials at each stage of the process
• Material flow Sankey diagrams can be used to identify the main sources of waste, recycling loops, and opportunities for resource efficiency improvements
• Sankey diagrams can illustrate the environmental impacts associated with material flows (greenhouse gas emissions, water consumption), supporting life cycle assessment and eco-design practices
• Material flow analysis using Sankey diagrams is crucial for developing circular economy strategies, where the goal is to minimize waste and maximize the reuse and recycling of materials

Process Optimization

• Sankey diagrams can be employed in process optimization to visualize the flow of intermediate products, by-products, and waste streams, helping to pinpoint inefficiencies and potential areas for process integration
• In industrial processes, Sankey diagrams can be used to map the energy and material flows, identifying opportunities for heat recovery, cogeneration, or waste-to-energy solutions
• Sankey diagrams can support the design and analysis of industrial symbiosis networks, where the waste or by-products of one process become the inputs for another, enhancing overall resource efficiency

Creating Sankey Diagrams

• When creating Sankey diagrams for different contexts, it is crucial to define clear system boundaries, ensuring that all relevant flows are accounted for and that the diagram is consistent with the mass and energy balance principles
• Sankey diagrams can be created using specialized software tools (e!Sankey, SankeyMATIC) that allow for the input of flow data, customization of the diagram layout, and the generation of interactive visualizations for exploring different scenarios or time periods
• The choice of the level of aggregation and the visual design of the Sankey diagram should be tailored to the specific purpose and audience, balancing the need for detail with the clarity and readability of the diagram

Designing Effective Sankey Diagrams

Layout and Flow Direction

• Effective Sankey diagrams should have a clear and logical layout, with a consistent flow direction (left to right, top to bottom) and minimal crossing of arrows to avoid visual clutter
• The positioning of the nodes (sources, sinks, and intermediate stages) should reflect the logical sequence of the processes or the geographical arrangement of the system components
• The use of curved or orthogonal arrows can enhance the aesthetic appeal and readability of the diagram, but should be applied consistently throughout the design

Arrow Width and Scaling

• The width of the arrows should be directly proportional to the quantity of flow, using a consistent scale throughout the diagram to allow for accurate comparisons between different flows
• The scaling factor for arrow width should be chosen carefully to ensure that the smallest flows are still visible and that the largest flows do not dominate the diagram excessively
• In some cases, logarithmic scaling or flow aggregation may be necessary to accommodate a wide range of flow magnitudes within a single diagram

Color-Coding and Labeling

• Color-coding can be used to differentiate between different types of flows (energy carriers, materials, stages in a process), enhancing the readability and interpretability of the diagram
• The choice of colors should be intuitive and consistent with common conventions (e.g., red for heat, blue for electricity), and should take into account accessibility considerations for color-blind users
• Labels and annotations should be used sparingly and strategically, providing essential information about the nature and quantity of flows without overwhelming the viewer with excessive details
• The placement and formatting of labels should ensure legibility and minimize overlap with the flow arrows

Emphasizing Key Flows and Patterns

• The overall design of the Sankey diagram should emphasize the most important flows and patterns, using techniques such as highlighting, grouping, or zooming to draw attention to key aspects of the system
• The use of transparency or dashed lines can be employed to de-emphasize minor or uncertain flows, maintaining the focus on the main insights provided by the diagram
• The inclusion of reference values, benchmarks, or targets can help contextualize the flow quantities and support the interpretation of the diagram

Interactivity and Customization

• Interactive Sankey diagrams can be designed to allow users to explore the data at different levels of aggregation, filter flows based on specific criteria, or compare different scenarios or time periods
• The integration of tooltips, hover effects, or click events can provide additional information or context to the user without cluttering the main diagram
• Customization options, such as the ability to adjust the color scheme, font sizes, or layout parameters, can enhance the usability and adaptability of the Sankey diagram to different use cases and preferences

Interpreting Sankey Diagrams for Optimization

Identifying Dominant Flows and Pathways

• When interpreting Sankey diagrams, focus on the relative magnitudes of the flows, identifying the dominant sources, sinks, and pathways in the system
• The identification of the main contributors to the overall flow can help prioritize the areas with the greatest potential for optimization or intervention
• The comparison of the flow magnitudes across different stages or branches of the system can reveal the most important transformations or losses occurring within the process

Detecting Bottlenecks and Inefficiencies

• Pay attention to the width of the arrows at different stages of the process, as sudden narrowing or widening can indicate bottlenecks or inefficiencies in the system
• Bottlenecks can be identified by locating the points where the flow is significantly restricted compared to the upstream or downstream stages, indicating a capacity limitation or a process constraint
• Inefficiencies can be spotted by comparing the magnitude of the input and output flows for a specific process, with large differences suggesting potential areas for improvement

Spotting Losses and Waste Flows

• Look for significant losses or waste flows, which may appear as arrows leaving the main flow without reaching a useful endpoint, indicating potential areas for optimization or resource recovery
• The relative size of the waste flows compared to the main product flows can indicate the scale of the opportunity for waste reduction or valorization
• The destination of the waste flows (e.g., landfill, emission to the environment) can guide the selection of appropriate waste management or recycling strategies

Identifying Opportunities for Integration and Recycling

• Identify loops or recirculation flows in the diagram, as these can represent opportunities for process integration, heat recovery, or material recycling
• The presence of multiple flows of the same type (e.g., heat at different temperature levels) can suggest the potential for heat cascading or energy integration between processes
• The existence of by-product flows that are not fully utilized can indicate the potential for developing new value streams or industrial symbiosis partnerships

Comparing Efficiency and Benchmarking

• Compare the efficiency of different pathways or subsystems by examining the ratio of output to input flows, identifying the most efficient routes or technologies
• The identification of the most efficient pathways can guide the selection of best practices or the prioritization of investment and improvement efforts
• Use Sankey diagrams to benchmark the performance of a system against industry standards or best practices, identifying areas where improvements can be made to enhance overall efficiency and sustainability
• The comparison of Sankey diagrams across different facilities, technologies, or time periods can reveal trends, gaps, and opportunities for learning and optimization