Networks: Energy, Cities and the Control of Complex Systems

Networks: Energy, Cities and the Control of Complex Systems

Adilson E. Motter[a]

Northwestern University

and

Robert N. Schock[b]

Center for Global Security Research

I. INTRODUCTION

Energy and Cities is an on-going activity of the World Federation of Scientists’ Energy Permanent Monitoring Panel. An integral part of this effortis to considerpotential applications of the physics and mathematicsof complex systems. This paper is a report froma 3-day workshop held 12-14May, 2014 in Erice, Sicily, to examine applications of complex systems theory to energy systems, and how this can be applied to rapidly growing urban populations.

The most acute energy emergency today is the very rapid migration to, and resulting growth of, cities.Trends indicate that at least two-thirds of the global population will live in cities by 2050.While the populations of mostcities are growing rapidly, the most affected are cities between 100,000 and 1 million in populationthat have little or no energy infrastructure to handle this influx. Building stable and efficient power and transportation networksoffers the opportunity to substantially contribute to solutions for all cities going forward, and by extrapolation to the sustainability of the entire planet. The science of complex networks offers the promise of enhancing stability and maximizingefficiency, and therefore of increasing resilience and facilitating a transition to sustainable cities. Leaders in thearea of complex networks, together with global leaders in the design and operation of electricity and transportation systems,brainstormedand met to identify effective solutions to building and operatingthe networks ofthe cities of the future.

Network dynamics are typically non-linear and comprise a large number of dynamic variables. Systemic properties of a network can depend as much on the interactions between components asthey do on the properties of the individual components (e.g., generators, distribution stations, households, etc.).The same connections that provide functionality to a network can also serveas conduits for the spread of failures and instabilities that wouldotherwise be limited to small areas.Optimization and control of network behavior in general requires ongoing systematic assessment of the state of the system, and in many cases forecasts. Determining the state of a network depends on the number and placement of measurements made in one or more variables across the network. Likewise, control depends on number, placement, and form ofthe control inputs.Recent research shows that network response to perturbations can be largely controlled byinterventions using a relatively smallfraction of the total number of nodes.These conceptsare applicable to the control of power-grid networks, as well as efficient, clean, and cost effective transportation systems and other interconnected networks (e.g., waste disposition networks).

The goal of thisworkshop was to explore the potential application of complex network theory to energy systems, especially to power grids and integrated transport networks, as applied to cities, and to identify limitations and priorities for research and demonstrations.In addition there are applications to many natural systems that affect human beings such as health, climate, pollution, and governance. This goal is part of the longer-term effort to develop a management tool useful for mayors and city managers in concert with large providers of energy services to cities.

Physicists, engineers, transportation researchers, network scientists, and computer scientists participated, as well as energy researchers, an atmospheric scientist, a statistician, several policy makers, an operations researcher and a network economist. Their common interest is in the control of complex systems, understanding specific systems (e.g., electric grid, transport), and charting a sustainable path forward for urban areas.

The following summaries serve as an introduction to the workshop presentations (posted here).No presentation is anisolated entity, andcollectively they bring a wide range of perspectives to the problems. While grouped into five general categories, they complement in developingsuggestions and solutions applicable to more than one, and in some cases all, areas:

1. Introduction to the subject of urban status and solutions;

2. Urban Studiesfocused on city structure, organization, and dynamics;

3. Intelligent ElectricitySystems and their potential in the development of urban centers;

4. Transportation and the key role its systems playin the functioning of cities;

5. Complex Networks and Control, the science and applicationsto urban areas.

II. SUMMARIES

1. Introductions

In an overview of the three themes (energy, cities, and complex systems) and of the goals of the workshop, Robert Schock suggested key questions for discussionthat led tofurther debate in the Round Table that closed the meeting.

Aristides Patrinosin a keynote address showed that the world is urbanizing and cities are the loci of consumption, economic activity, and innovation. Cities must be efficient, reliable and sustainable, and address issues of quality of life and equity. Urban informatics, the handling of “big data”, taking advantage of the ability to have a proliferation of sensors (IR, radar, lidar, magnetic, etc.)is where much of his team’sdata activitiesare focused.Feeding all of this information into the operations of cities is a major research areabeing addressed.

ArnulfGrüblerdrew,from the Assessment of Urban Energy Systems as part of the 2012 Global Energy Assessment (Cambridge University Press, 2012), the first-ever energy assessment of urbanization. Of the major factors in urban energy use, three stand out as amenable to urban policymaking and therefore have highest priority: the quality of the built environment, urban form and density (including transport), and the integration of urban energy systems. More decentralized but more integrated urban infrastructures are necessary but achievement is complicated by weak institutional capacities. Yet, optimism comes from urban areas continuing to lead in innovation.

Peter Droege pointed out that the existence, expansion and prosperity of modern cities - and urbanization itself - is to a largeextent driven by and based on fossil fuels. Associated carbon risks and supply uncertainties make urban growth the energy emergency it is today. Cities and communities can transition to a renewable energy base drawing on supplies from within their boundaries and elsewhere. Distributed infrastructure innovations encompass intelligent grids and diverse storage systems, serving to stimulate community prosperity and buildresilience. Droege sketched out the tools, measures and means, as well as the processes that have begun to transform urban energy systems.

2. Urban Studies

Kimberly Gray examined energy, water, food, and land-use cycles collectivelyto ask what makes a city sustainable and how to design such a city. Arguably, sustainability cannot be achieved by focusing on single systems. A sustainable city requires a complete departure from the current state of affairs and there are three major obstacles to charting a path to a desirable state:

How to couple the systems that constitutea city;

How to notviolate current codes, laws, and regulations;

How to addresssocial equity, which tremendously complicates the solution.

HyejinYounasked“whatare the fundamental scaling laws and underlying principles governing human activities in cities?”By creating models to describe existing large-scale data, she identified universal properties. For example, bigger cities have larger GDP, more intellectual property, and higher crime incidents, while they need less infrastructure, and consume less energy in a systematic way, all of which are manifested as a universal scaling law.It is argued that, together with the Zipf’s law of population and fractal growth of cities, future models for cities must satisfy these universal properties.

Francesco Calabrese showed that by assimilating sensor data (“big data”) and then dealing with the diversity, heterogeneity, accuracy, sparseness and total volume of the data, planning and operations can be implemented to help cities become more efficient. Use is made of social media (e.g., mobile phones) to understand demand and recommend changes to optimize use of a resource.

MateiGeorgescu looked at the built environment in cities and its significant hydro-climatic, energy and health impacts using a numerical weather prediction and atmospheric simulation model. The model is then used to predict the effects of various technological innovations (e.g., cool and/or green roofs) over a wide range of climate, geographic, and political regions, relating to energy demand and effects on health. Adaptation strategies are then prioritized.

Steve Doreylooked at the challenges and opportunities in a major urban area (Toronto) and outlineda growth plan with targets whose major elements involve pricing carbon and congestion, coordination among governments, changing lifestyle choices, combined heat and power, climate change adaptation, and involvement of the private sector.

Henrik Madsen modeled Denmark to attain an energy system with 100% renewables based on intelligent integration. Part of the plan involves harnessing the substantial thermal mass of the urban building stock to store thermal energy and consequently increase the flexibility of energy demand. Flexibility in the energy supply is facilitated by allowing end-users and energy suppliers (e.g., district heating plants) to substitute between different forms of energy supply for the provision of a given energy service. Connecting combined heat and power plants to the storage network can facilitate seasonal energy storage.

3. Intelligent Electricity Systems

Kurt Yeager presented an argument for a microgrid revolution in community electricity infrastructure. The goal is a power system to ensure fail-proof and universal availability of electricity in the quantity and quality necessary to meet every person’s needs. Integrated microgrids, diverse generation and storage resources, assembled into a smart self-healing grid system, are key. This transformation in electrical service quality to 21stcentury standards is critical to resolving serious economic and environmental global threats.

Praveen K. Agarwaldescribed the evolution of the Indian electric power grid and the introduction of synchrophasors for rapid (sub-second) detection of disturbances and automatic control, as well as early warning of possible transition to emergency states. A compendium of natural events and responses in the form of case studies is published and available to the public (

Yoshiaki Shibata focusedon the role of demand in a distributed energy system. It was shown that effective operation of a distributed energy system is a crucial issue to offset the disadvantages of the variable power output of renewables and more expensive investments than centralized energy systems. Leveling the superposed load curve within an urban block by accommodating a wider variety of end-use customers(e.g., households and commercial buildings) can benefit efficient operation. Urban planning should be based on a concept to create diversity in end-uses.

HemendraAgrawal described the issues and challenges regarding development of smart grid/cities on the Indian subcontinent. Issues related to the business model, technology maturity, regulation & policies, standardization and cyber security werehighlighted and ways to tackle these issues weresuggested.

4. Transportation

Moshe Ben-Akiva developed tools and solutions for future urban mobility. Urban mobility options enabled by networked computing and control technologies offer infrastructure-free traffic management, real-time traffic prediction and personalized public transportation. Behavior models were introduced where travel demand is derived from activities and schedules. The result is a personalized, demand-responsive, public transportation system that gives travelers a choice among levels of service at different prices. It optimizes the assortment of products (ride options) available in order to maximize expected profit and personal welfare.

EnnioCascettaintroduced the main mathematical models that have been adopted to simulate urban mobility based on the modeling of travelers’ behavior, as part of more complex activity participation patterns. The complexity of decision-making in urban mobility systems wasintroduced. Challenges were addressed in reconciling the research progress in urban mobility systems analysis, mobility and ICT technologieswith effective decision-making.

Marc Barthelemydiscussed how revisiting spatial economics with a physicist’s eye can enhance models that simply consider cities as being in equilibrium, and provide some clues for solving puzzlesabout urban systems. Urban morphology and morphogenesis, activity and residencelocation choice, mobility, urban sprawling and the evolution of urban networks are just afew of the important processes that can be discussed now from a quantitative point ofview.

5. Complex Networks and Control

Dirk Witthaut examined energy systems’ security in the context of statistical physicsand network science. Models evaluated the impact of increased decentralizationof power production on the stability of synchronous states in power-grid networks. Careful analysis showed that new connections in complex networks, such as power grids,can in fact destabilizethe network. These observations have implications for design, operation and control of power grids, particularly when new connections are called for to reduce risks of instabilities and cascading failure.

Anna Nagurneymodeled supply chains that provide the critical infrastructure for the production and distribution ofgoods and services in the economy. These models offer a powerful tool to identify more sustainable paths of operation. They compare sustainable and unsustainable operations of the supply chain in terms of costs (operational, environmental, and waste disposition) to a business and to society.Electricity supply and food supply chains were illustrated.

EckehardSchöll lookedat controllingthe dynamics of complex networks of coupled oscillatory systems. Apart fromcomplete in-phase synchronization, spontaneous symmetry-breaking in coupled-oscillator systems is applicable in a variety of physical, chemical, biological, and technological contexts. These models allude to practices that can increase the stability of complex networks in general, with potential applications to infrastructure and social systems.

Santo Fortunatodiscussedstriking structural features of complex networks, such as their tendency to have few nodes with a high number of neighbors and many others with a few neighbors. , It was shown that, because most networks also exhibit community structures, formed by groups of densely connected nodes,local optimization techniques can be more useful in addressing network problems than global optimization algorithms.

Guido Caldarellishowed that real-world systems have a large number of interdependencies(physical, cyber, geographic, and logical), and that they all need financial and political coordination. The need to build robust complex networks requires going beyond minimal systems and exploiting instead network redundancies, which can allow recovery in the presence of failures.It is argued that it is possible to design systems to optimize resilience without compromising the achievement of goals.

Jie Sun investigated the impact of scheduling of interventions in the control of crises and emergencies in complex networks. Identification of control/rescue interventions can be formulated as an optimization problem. With amathematical solution of the optimization problem, one can identifystate interventions (bring the system back into its optimal state), parameter interventions (to transform the optimal state), and the optimal scheduling ofsuch interventions (time for best implementation). In this way optimal response to unforeseen events can be designed.

Sean Cornelius showed that one can profit from nonlinearity—a hallmark of both natural and engineered complex networks—to control systems that would otherwise not be controllable. Nonlinearity permits the coexistence of multiple stable states (some desirable, others not), which correspond to different possible modes of operation of a network. A general computational framework is proposed to identify feasible control interventions that can orient the system toward desired states. This framework can prove useful for the control of interdependent urban networks in the presence of perturbations.

Adilson E. Motter presented a nonlinear dynamics approach to increase stability and allow control of network behavior. This approach isformulated in the context of power transmission networks. A condition for the functioning of such networks is that the power generators remain synchronized. By modeling power-grid synchronization in terms of an effective network representation, a methodologywas derived to specify parameter assignments that enhance synchronization and that can be used to devise new control schemes to facilitate recovery from failures in real time.

III. SUMMARY OF DISCUSSIONS

Based on the discussions and genuine interestof the participants, urban areas are a very reasonable focus for working on several important problems, including energy,with a common goal of improved sustainability. To address urban energy issues, integrating water, mobility, land use, information,governance, human behavior, and policy — which are systems in and of themselves —mustalso be addressed, at least eventually. This leaves the question of how muchprogress can be made by lookingat a subset, or subsets, of the overall system. Ultimately, any one system we are dealing with is a very complex system in itself, but which is also part of a set of coupled complex systems.

A great deal of discussion centered on the need for more and better datato test models for their applicability. This raises the question of whether model developers should forecastusing the best available data (or even speculative hypotheses of data) rather than wait for more and better data.Forecasts, even if speculative,have the power to stimulate more discussion and better data collection, and hence improve both analyses and further forecasts. Better models and better data often go hand-in-hand. In this area it may also be useful to physically mockup experiments in the laboratory to test how well the results agree with predictions for smaller scale systems.

A key initiative is to involve operational managers from industry. In this way the need for products and near-term profits can be included in the models. But even more important, this could foster “real world” experience and ascertain needs and goals. This complements necessary government-industrial partnerships that bring needed financial and political leadership to the basic objective.

Several basic questions arise: Is network control a key element of urban (and global) sustainability? Is partial control better than no control at all? What are the key nodes and/or interactions in the systems we are dealing with? The major obstacles to charting a path forward are how to couple the systems of interdependent networks and flows. Can policies relating to governance and human behavior be identified and, most importantly,be implemented(many sustainable practices currently violate codes, laws, or regulations)? Can social equity be addressed (jobs, housing, education)?