Modeling of Interconnected Infrastructures with Unified Interface Design toward Smart Cities

In recent years, there have been tendencies to enable smart cities with interconnected infrastructures and communities. Current engineering design and operation practices are limited to handling individual systems with modeling and simulation, as well as control systems. This paper presents a holistic approach with engineering practice to design and operate interconnected systems as part of smart cities. The approach is based on modeling individual physical systems and associated processes and identifying key performance indicators to evaluate each system and interconnected systems with an understanding of the coupling among systems to increase the overall performance of interconnected systems. The multi-objective optimization technique is proposed to achieve the best performance based on system design, control, and operation parameters. Due to the multidimensional nature of the interconnected systems, a unified interface system with modular design is proposed to achieve the highest overall performance of the interconnected systems with standardized interactions among state variables and performance measures. The proposed approach can allow dynamic updates of the interconnected systems based on model libraries of each system and process. A case study is presented of interconnected energy–water–transportation–waste facilities, whereby modeling is discussed, and performance measures are evaluated for different scenarios using the unified interface design.


Introduction
The United Nations 2030 agenda for sustainable development stated the need for reliable, quality, and timely data to provide evidence-based analysis as part of Sustainable Development Goals (SDGs) [1]. The analysis showed an increase in population by more than 1% yearly, which leads to more demand on energy and related resources. Energy is essential for water, transportation, food, health, and waste management facilities and infrastructures. The focus on energy grids has led to several initiatives for moving centralized power grids to decentralized grids while integrating renewable energy. The analysis of grid-connected and standalone structures revealed techno-economic and environmental assessments to evaluate energy infrastructures [2]. To optimize the design of energy infrastructures, a survey was conducted to identify the main engineering design parameters for reaching the optimum energy grids and associated components to meet load and demand profiles [3]. The analysis of detailed cost factors of power grids while considering the population and associated factors was optimized and applied to many regions and countries worldwide [4]. The progress made in energy infrastructures requires a proper study on energy management based on a simulation that can evaluate the planning scenarios and strategies to reduce performance measures such as GHG emissions [5]. Energy system analysis includes the impacts of district heating as part of energy flow analysis optimized with computational intelligence algorithms [6]. There are a number domain. However, most of the energy-transportation-water-food-health-waste integrated systems are designed and operated on the basis of engineering design specifications that are defined in each domain and on a case-by-case basis with limited capabilities to analyze and optimize their functions and features due to the lack of integrated modeling and simulation tools and computer-aided design environments that can link these domains. Furthermore, specifying energy requirements is difficult in view of the multidimensional nature of the target integrated systems with various technologies and multiple views of these applications and infrastructures. These challenges could be resolved by conducting detailed process modeling of the integrated systems in holistic ways to include interfaces and interactions among energy, water, transportation, food, health, and waste management and the detailed model parameters in each domain. The study of the coupling among these dimensions and integrating requirements and functions can facilitate the engineering design with a lifecycle approach to consider integrated systems based on actual interfaces, coupling, and dependencies. The study of the interfaces among them can enable the engineering design of integrated systems with different channels: electricity, thermal, fuel, water, and waste, linked with transportation and city infrastructures. The development of model libraries for these interfaces can support the study of the coupling of model parameters as linked with performance measures, which can be utilized within an integrated modeling and simulation environment that can support the engineering design of these interfaces and corresponding systems. The development of control strategies for each domain and as interconnected systems could be assessed using performance measures. To ensure resilient interconnected systems, important considerations are studied within each domain and linked to the interface functions, such as risk factors, as well as safety and protection layers. The typical optimization techniques need to be revised to achieve the overall optimization of the interconnected systems with mutual negotiation and considerations of performance measures in each domain. Multi-objective optimization techniques are widely used for analysis and enhance the overall performance of complex systems and processes. Some studies are based on optimizing a given process model using multi-objective optimization techniques. Researchers have tried different approaches to achieve this. One example is based on the evaluation of KPIs associated with each process model, which achieved integrated optimization using multi-objective optimization, as shown in [27]. Examples of multi-objective optimization algorithms were applied to waste and resources management for industrial networks using mixed-integer linear programming (MILP), which considered material flow and balance equation models [28]. Although these approaches were able to achieve optimum performance, they could not handle constraints with links to domain knowledge. To address these challenges, computational intelligence algorithms can be adopted, which consider dynamic rules, constraints, learning from input-output relationships, and other domain knowledge. Intelligent multi-objective optimization techniques are required to evaluate key performance indicators in view of several operational scenarios to optimize the interfaces and related systems. Figure 1 shows the general framework of interconnected systems, including the energy network, food network, health network, water network, transportation network, waste management network, and social network, all of which are governed within the management layers, such as municipalities or regional offices. The activities within each network are linked to other networks such that the overall performance can be maximized in the overall interconnected systems.
The next section describes the proposed system modeling for each network to demonstrate possible interface systems to accurately evaluate and manage the interconnected systems. The next section describes the proposed system modeling for each network to demonstrate possible interface systems to accurately evaluate and manage the interconnected systems.

Interconnected System Modeling
Smart cities are urban areas that utilize sensor networks with technological infrastructures to manage different activities and real-time data to achieve a flexible and high-performance quality of life. Smart cities can be viewed as interconnected systems to enable real-time interactions among different systems, processes, and infrastructures. To enable the engineering and management of these interactions, it is important to understand the possible dependencies and coupling among these systems and link them to design, control, and operation parameters. Examples of the possible couplings among these systems include hybrid energy systems (i.e., energy-transportation systems, energy-water systems, energy-food systems, energy-health systems, energy-waste systems, energy-social systems) and integration among any three or more of these systems. The proposed modeling of possible interactions among these systems can be represented by defining basic building blocks of each system. Energy systems can be expanded into gas-, electricity-, and thermal-related networks, along with the interactions among them. There are conversion strategies from gas to power, power to gas, thermal to gas, and other such combinations, linked to storage and utilization components. Figure 2 shows the interactions among electricity, thermal, and gas networks, where sources are linked to processing or production systems. These networks are linked with storage and transfer or transportation, as well as utilization. Losses are also modeled as linked to all these components within each system.

Interconnected System Modeling
Smart cities are urban areas that utilize sensor networks with technological infrastructures to manage different activities and real-time data to achieve a flexible and highperformance quality of life. Smart cities can be viewed as interconnected systems to enable real-time interactions among different systems, processes, and infrastructures. To enable the engineering and management of these interactions, it is important to understand the possible dependencies and coupling among these systems and link them to design, control, and operation parameters. Examples of the possible couplings among these systems include hybrid energy systems (i.e., energy-transportation systems, energy-water systems, energy-food systems, energy-health systems, energy-waste systems, energy-social systems) and integration among any three or more of these systems. The proposed modeling of possible interactions among these systems can be represented by defining basic building blocks of each system. Energy systems can be expanded into gas-, electricity-, and thermalrelated networks, along with the interactions among them. There are conversion strategies from gas to power, power to gas, thermal to gas, and other such combinations, linked to storage and utilization components. Figure 2 shows the interactions among electricity, thermal, and gas networks, where sources are linked to processing or production systems. These networks are linked with storage and transfer or transportation, as well as utilization. Losses are also modeled as linked to all these components within each system.  The next section describes the proposed system modeling for each network to demonstrate possible interface systems to accurately evaluate and manage the interconnected systems.

Interconnected System Modeling
Smart cities are urban areas that utilize sensor networks with technological infrastructures to manage different activities and real-time data to achieve a flexible and high-performance quality of life. Smart cities can be viewed as interconnected systems to enable real-time interactions among different systems, processes, and infrastructures. To enable the engineering and management of these interactions, it is important to understand the possible dependencies and coupling among these systems and link them to design, control, and operation parameters. Examples of the possible couplings among these systems include hybrid energy systems (i.e., energy-transportation systems, energy-water systems, energy-food systems, energy-health systems, energy-waste systems, energy-social systems) and integration among any three or more of these systems. The proposed modeling of possible interactions among these systems can be represented by defining basic building blocks of each system. Energy systems can be expanded into gas-, electricity-, and thermal-related networks, along with the interactions among them. There are conversion strategies from gas to power, power to gas, thermal to gas, and other such combinations, linked to storage and utilization components. Figure 2 shows the interactions among electricity, thermal, and gas networks, where sources are linked to processing or production systems. These networks are linked with storage and transfer or transportation, as well as utilization. Losses are also modeled as linked to all these components within each system.  Similarly, the interconnections between food and water networks are modeled in Figure 3. The interconnection between waste and health networks are shown in Figure 4. The interactions among all these systems are clear by linking the different systems. In this figure, health sources could refer to medicine, natural herbal medicine, or healthcare technology. This is followed by the production and supply chain of these sources until they reach health services for treatment and medicine utilization, as well as possible recycling. Similarly, waste management systems start with waste sources, followed by collection, treatment and transfer/transportation, conversion, and possible recycle. There are interactions among these systems that require unified interfaces to enable the hybrid system modeling, engineering design, and control of the interconnected systems. Similarly, the interconnections between food and water networks are modeled in Figure 3. The interconnection between waste and health networks are shown in Figure 4. The interactions among all these systems are clear by linking the different systems. In this figure, health sources could refer to medicine, natural herbal medicine, or healthcare technology. This is followed by the production and supply chain of these sources until they reach health services for treatment and medicine utilization, as well as possible recycling. Similarly, waste management systems start with waste sources, followed by collection, treatment and transfer/transportation, conversion, and possible recycle. There are interactions among these systems that require unified interfaces to enable the hybrid system modeling, engineering design, and control of the interconnected systems.  There are interactions among different loads, whereby an increase in one load might be related to factors that impact other loads. For example, gas load might increase due to activities that are directed toward gas supply with a slight reduction in electric load. Figure 5 shows possible interactions among all possible loads, including electric, thermal, gas, transportation, water, waste, food, health, and social loads. A better understanding of the coupling and interactions among these loads can lead to better energy conservation and management. Moreover, a better understanding of the behavior of interconnected systems can improve the overall mapping between energy supply and loads with enhanced performance of the developed hybrid energy systems. Similarly, the interconnections between food and water networks are modeled in Figure 3. The interconnection between waste and health networks are shown in Figure 4. The interactions among all these systems are clear by linking the different systems. In this figure, health sources could refer to medicine, natural herbal medicine, or healthcare technology. This is followed by the production and supply chain of these sources until they reach health services for treatment and medicine utilization, as well as possible recycling. Similarly, waste management systems start with waste sources, followed by collection, treatment and transfer/transportation, conversion, and possible recycle. There are interactions among these systems that require unified interfaces to enable the hybrid system modeling, engineering design, and control of the interconnected systems.  There are interactions among different loads, whereby an increase in one load might be related to factors that impact other loads. For example, gas load might increase due to activities that are directed toward gas supply with a slight reduction in electric load. Figure 5 shows possible interactions among all possible loads, including electric, thermal, gas, transportation, water, waste, food, health, and social loads. A better understanding of the coupling and interactions among these loads can lead to better energy conservation and management. Moreover, a better understanding of the behavior of interconnected systems can improve the overall mapping between energy supply and loads with enhanced performance of the developed hybrid energy systems. There are interactions among different loads, whereby an increase in one load might be related to factors that impact other loads. For example, gas load might increase due to activities that are directed toward gas supply with a slight reduction in electric load. Figure 5 shows possible interactions among all possible loads, including electric, thermal, gas, transportation, water, waste, food, health, and social loads. A better understanding of the coupling and interactions among these loads can lead to better energy conservation and management. Moreover, a better understanding of the behavior of interconnected systems can improve the overall mapping between energy supply and loads with enhanced performance of the developed hybrid energy systems.

Case Study
To understand a possible interconnected system, a case study is illustrated in Figure  6. It shows a food cycle involving the farm, food factory, and food transfer or transportation. This is linked to the water cycle, including rivers, water wells, and water

Case Study
To understand a possible interconnected system, a case study is illustrated in Figure 6. It shows a food cycle involving the farm, food factory, and food transfer or transportation. This is linked to the water cycle, including rivers, water wells, and water pipes. The links to food utilization are represented by buildings and houses. This can be expanded to different residential areas. The waste cycle is also represented with waste collection systems and waste-to-energy conversion, which can be linked back to residential and industrial facilities and interfaces to the grid. Fuel sources start with oil and gas wells, followed by processing, production, and power plants to generate electricity and thermal byproducts, which are linked back to other infrastructures for energy utilization. This case study is only a snapshot of possible interactions among different components within interconnected systems, which illustrates the proposed solution. Transportation is represented with vehicles and stations so that it can be used for the transportation of food, social, water, and oil and gas.

Case Study
To understand a possible interconnected system, a case study is illustrated in Figure  6. It shows a food cycle involving the farm, food factory, and food transfer or transportation. This is linked to the water cycle, including rivers, water wells, and water pipes. The links to food utilization are represented by buildings and houses. This can be expanded to different residential areas. The waste cycle is also represented with waste collection systems and waste-to-energy conversion, which can be linked back to residential and industrial facilities and interfaces to the grid. Fuel sources start with oil and gas wells, followed by processing, production, and power plants to generate electricity and thermal byproducts, which are linked back to other infrastructures for energy utilization. This case study is only a snapshot of possible interactions among different components within interconnected systems, which illustrates the proposed solution. Transportation is represented with vehicles and stations so that it can be used for the transportation of food, social, water, and oil and gas. The proposed model parameters are explained in Table 1, which shows a list of key design, control, and operation parameters and performance measures used to analyze the interconnected systems. The proposed model parameters are explained in Table 1, which shows a list of key design, control, and operation parameters and performance measures used to analyze the interconnected systems.  Figure 7 shows more details regarding the interfaces among microgrid (MG), water facility, waste-to-energy facility, and EV charging station. It shows different levels of interfaces for integrated systems, connected systems, and autonomous systems. In principle, the integrated systems offer one direction of interfaces, at specific integration points, with local decision making or at a central point in the whole system. The connected systems offer two directions of interfaces with local and mutual decision making among connected systems. The autonomous systems offer two or multiple interface points with local decision making, while some decisions are assisted at the central or distributed levels. The logic behind the universal interface design is described in Figure 8a,b, which shows the mechanism followed in the unified interface design, starting from health concerns. If any concern is identified from the health interface port, a resolution is needed on the basis of other parameters and logic followed in the control design. This is followed by environmental, social, material, water, electricity, gas, thermal, transport, data, and policy. Each model library defines the local parameters and constraints that can be linked to all available interfaces for local decision making on the basis of autonomous functions. The logic behind the universal interface design is described in Figure 8a,b, which shows the mechanism followed in the unified interface design, starting from health concerns. If any concern is identified from the health interface port, a resolution is needed on the basis of other parameters and logic followed in the control design. This is followed by environmental, social, material, water, electricity, gas, thermal, transport, data, and policy. Each model library defines the local parameters and constraints that can be linked to all available interfaces for local decision making on the basis of autonomous functions.
shows the mechanism followed in the unified interface design, starting from health concerns. If any concern is identified from the health interface port, a resolution is needed on the basis of other parameters and logic followed in the control design. This is followed by environmental, social, material, water, electricity, gas, thermal, transport, data, and policy. Each model library defines the local parameters and constraints that can be linked to all available interfaces for local decision making on the basis of autonomous functions.

Unified Interface Modeling
To support the engineering design, control, and operation of such interface systems, it is essential to design a unified interface system that can enable modular interactions among interconnected systems with the systematic association among model parameters and KPIs. The proposed engineering design framework of the design of the unified interfaces of interconnected systems is shown in Figure 9. The physical system modeling can formulate the design, control, and operation parameters of each building block of each system.

Unified Interface Modeling
To support the engineering design, control, and operation of such interface systems, it is essential to design a unified interface system that can enable modular interactions among interconnected systems with the systematic association among model parameters and KPIs. The proposed engineering design framework of the design of the unified interfaces of interconnected systems is shown in Figure 9. The physical system modeling can formulate the design, control, and operation parameters of each building block of each system. This is followed by performance modeling of KPIs of each component and system. The different coupling mechanisms and parameters are defined, and equations are synthesized to identify dependencies among model parameters. According to the proper coupling among systems, control strategies can be defined for each system and evaluated with multidimensional simulations, using multiphysics capabilities to model thermal, electricity, gas, water, waste, transportation, food, health, and social. This is followed by multi-objective optimization to achieve the highest performance in each system and globally for the overall interconnected systems. The interface system design to coordinate different process variables and KPIs among model libraries can be defined for each system to enable the communication of process parameters for control and operation of the interconnected systems.
To support the engineering design, control, and operation of such interface systems, it is essential to design a unified interface system that can enable modular interactions among interconnected systems with the systematic association among model parameters and KPIs. The proposed engineering design framework of the design of the unified interfaces of interconnected systems is shown in Figure 9. The physical system modeling can formulate the design, control, and operation parameters of each building block of each system. This is followed by performance modeling of KPIs of each component and system. The different coupling mechanisms and parameters are defined, and equations are synthesized to identify dependencies among model parameters. According to the proper coupling among systems, control strategies can be defined for each system and evaluated with multidimensional simulations, using multiphysics capabilities to model thermal, electricity, gas, water, waste, transportation, food, health, and social. This is followed by multi-objective optimization to achieve the highest performance in each system and globally for the overall interconnected systems. The interface system design to coordinate different process variables and KPIs among model libraries can be defined for each system To better understand how the interface systems are designed and operated among interconnected systems, Figure 10 shows one example of interconnected systems, where the yellow box represents the unified interface system. The design of the input-output parameters and their utilizations in an engineering design example are explained in the next section. To better understand how the interface systems are designed and operated among interconnected systems, Figure 10 shows one example of interconnected systems, where the yellow box represents the unified interface system. The design of the input-output parameters and their utilizations in an engineering design example are explained in the next section. To analyze more interface examples, it is essential to expand each network. Different components within the environmental system are listed, such as air, soil, water bodies, and lands. Similarly, components are listed within physical infrastructures such as buildings, roads, bridges, and hospitals. The interactions among these systems are established via interface systems, which can support integration and management. Similarly, components within the transportation networks are highlighted, including vehicles, stations, fueling and charging stations, and marine ships.
A detailed unified interface system design is described using standardized functions, from "F1" to "F10", which are mapped to different dimensions, as shown in Figure 11. Two examples are illustrated using a vehicle, building, water pump, and machine. The understanding of these examples can facilitate the design of other components and systems. To analyze more interface examples, it is essential to expand each network. Different components within the environmental system are listed, such as air, soil, water bodies, and lands. Similarly, components are listed within physical infrastructures such as buildings, roads, bridges, and hospitals. The interactions among these systems are established via interface systems, which can support integration and management. Similarly, components within the transportation networks are highlighted, including vehicles, stations, fueling and charging stations, and marine ships.
A detailed unified interface system design is described using standardized functions, from "F1" to "F10", which are mapped to different dimensions, as shown in Figure 11. Two examples are illustrated using a vehicle, building, water pump, and machine. The understanding of these examples can facilitate the design of other components and systems.
Similarly, components within the transportation networks are highlighted, including vehicles, stations, fueling and charging stations, and marine ships.
A detailed unified interface system design is described using standardized functions, from "F1" to "F10", which are mapped to different dimensions, as shown in Figure 11. Two examples are illustrated using a vehicle, building, water pump, and machine. The understanding of these examples can facilitate the design of other components and systems. Figure 11. Interface system design and unified function mapping. Figure 11. Interface system design and unified function mapping.
The evaluation of different operation scenarios can be conducted on the basis of possible KPIs, as shown in Figure 12. The evaluation of different KPIs can be performed for design and operation scenarios as part of the simulation environment for real-time, steady-state, transient, and seasonal analysis. The list of functions is explained in Table 2, where sample parameters for each function are illustrated for the defined 11 groups of possible interfaces with any component. The unified interface systems can systematically enable the analysis and engineering design and control of interconnected systems. The evaluation of different operation scenarios can be conducted on the basis of possible KPIs, as shown in Figure 12. The evaluation of different KPIs can be performed for design and operation scenarios as part of the simulation environment for real-time, steady-state, transient, and seasonal analysis. The list of functions is explained in Table 2, where sample parameters for each function are illustrated for the defined 11 groups of possible interfaces with any component. The unified interface systems can systematically enable the analysis and engineering design and control of interconnected systems.      Table 3 provides a list can possible related standards that reflect interfaces in different equipment, which will be used to evaluate the compliance and interactions among interconnected systems.

Analysis of Unified Interface System Implementation
The proposed unified interface system was applied to interconnected systems shown in the case study used with several scenarios, as shown in Table 4.  Table 5. In this scenario, input and output material/food mass were defined for a trip and evaluated in view of the defined KPIs within selected groups. The analysis shows the importance of the defined KPIs, and how they could be used to evaluate interconnected systems.
where D is the distance of shipping in miles or kilometers, W is the weight or amount of shipment in pounds, kilograms, or tons, or volume metrics such as 20 foot equivalent unit (TFEs), and EF is the emission factor. The estimated GHG as per Equation (1) was used to estimate related performance measures, as shown in Table 6. The evaluations of the selected KPIs were estimated for the defined scenarios, which showed an air quality index of 70%, sustainability index of 75%, and lifecycle index of 60%. This is expected to improve KPIs with variations of the related scenario parameters such as gas consumption, gas production, transportation loads, and other related parameters through the interfaces defined with the 11 groups. This is useful to enable improving the performance of the target integrated system. In addition, it can enhance the overall performance of the interconnected systems. On the social side and to achieve diversity across system functions, other factors such as sustainability, economy, environment, and energy could be studied using the interfaces with the social function and gender groups. The proposed model could be used to enhance the performance of energy, transportation, traffic, water, food, waste treatment, health, and social networks. The analysis of these performance measures led to a better understanding of the impacts of different model parameters within the different domains, such as energy, transportation, and environment. The overall performance can be optimized on the basis of user preference weights of these KPIs using multi-objective optimization algorithms, which will be described in future work.

Conclusions
The world is moving toward smart cities, where systems and components are individually designed and operated as smart nodes in complex interconnected systems. There is a lack of best practices for achieving the engineering design of interconnected systems. This paper presented a holistic framework of the engineering design of interconnected systems, including energy, water, transportation, waste, environment, food, health, and social networks. The interactions among these systems require unified interface systems to analyze the multidimensional views of these interconnected systems. A generic framework was presented with the analysis of multiple scenarios applied to a selected case study. The concept of optimization was discussed in view of infrastructures and associated key performance indicators defined for each dimension and system. The presented work will be extended to provide multi-objective optimization with real-time links to the multidimensional simulation of interconnected systems. There are limitations regarding the possible ways to model interconnected systems where the interface points are not adequately understood. Existing approaches such as multidimensional modeling of integrated systems are able to map model parameters in different domains and build relationships among them. However, these approaches are not able to model the coupling information and interactions among different systems. The proposed approach is able to cover these gaps and build standardized interactions based on unified interfaces that are generalized for each system. KPIs are also mapped to these interface points to enable the evaluation of all design and operation parameters for different scenarios and control strategies, thus supporting real time performance optimization and leading to profitable implementation and deployment.
Further studies are required to investigate the detailed design of practical interfaces and multiphysics models to accurately represent interconnected systems. More case studies will be analyzed using the presented hybrid modeling approach of interconnected systems. The further development of examples will support expanding the proposed approach to several applications, which will help the transition toward smart cities.
Funding: This research is funded by NSERC Discovery Grant number 210320.