Cities nowadays form a global urban system, deeply interconnected, and play an increasing role in the economic, environmental, and social level. As urban development is unfolding and developing, the cities as a global phenomenon, i.e. global cities, represent at the same time a formidable sustainability challenge and a priceless source of opportunities. As of today, cities represent more than the 70% of the global CO₂emissions, and the urban system is the most important subject that must be involved to accelerate the energy transition leaving no one behind, through a progressive electrification of energy consumption while making power sector carbon-neutral.

Indeed, it is widely accepted that in order to reduce the impact of mankind on the planet cities play a key role: they are living laboratories where to first tackle Planet criticalities.

In order to design the cities of tomorrow a paradigm shift is needed, cities are complex systems and need to be treated as living organisms, where flows of energy, material, and information are essential for their development. In recent years, a new model based on circular flows (therefore resembling the basic laws of thermodynamics) has emerged as an opportunity for sustainable development: the circular economy.

The circular economy is a set of new visions aimed at eliminating waste from the system by closing loops of energy and material flows, realizing a fully developed vision of the city as a living ecosystem, where waste of subsystems are used as input for other subsystems (e.g. district heating systems based on industrial heat recovery). The circular economy is based on five pillars: renewable material and energy; increased use of assets through sharing, product as a service and life extension; closing loops by flow reusing.

Although it may seem just a semantic difference, there is a conceptual distinction between a smart and circular city. The concept of smart city is mainly focused on the role of information technology, whereas circular cities focus on a more holistic approach including all dimensions, with objectives in terms of competitiveness, environmental sustainability and social inclusion. In this vision governance plays a central role too, and in order to allow for a transition towards the circular paradigm, it has to be two-fold: both top-down and bottom-up. This means that all elements of the urban ecosystem (citizens, companies, start-ups, organizations, policy/decision makers, etc.) need to be strongly involved and able to participate and contribute in terms of ideas, proposals, projects, and initiatives.

Technology is also at the core of the circular city vision, as a tool to support the achievement of economic, environmental and social targets.

According to a circular view we could consider five urban subsystems:

• Built environment
• Energy
• Mobility
• Bio-economy
• Material flows (e.g. waste, water, food, etc.)

Each one of these subsystems has a key role in supporting the transition.

Infrastructures are pervasive in this vision, they represent the backbone on which circular cities operate: smart grids, smart lighting, ports, optical fiber. All these infrastructures need to be (re)designed in order to support the proper operation of the circular city. New technologies have a key role, and we can define the most relevant of them as renewable technologies, electrical appliances and digital and Artificial Intelligence solutions.

Renewable energy sources, distributed and integrated with electrical appliances (such as electric vehicles and heat pumps), represent a new paradigm that can both eliminate global and local pollution emitted from current technologies and improve life quality. Big data coming from digital technologies, such as sensors, the Internet of things, etc., feed Artificial Intelligence application that can maximize the benefit not only of new physical technologies but also of the old ones, as they allow new business models and uses. Solutions such as car sharing, waste tracking, pay per–use, peer-to-peer exchanges enabled by blockchain technologies have been allowed from strong digital evolutions of the last years.

The circular vision of the city is strongly interconnected with Sustainable Development Goals (SDGs), impacting all of them. Most of the relationships are straightforward, and we just mention a few that could be less immediate to spot:

• SDG 4 (quality education): the circular transition is related to increased environmental and civic education of all the citizens, who must actively contribute to its implementation
• SDG 8 (decent work and economic growth): shifting the economic model from a linear one to a new one based on the low cost of materials, lack of externalities and costs  and strong automation of processes toward a circular one based on maintenance, reuse, regeneration and recycle would create knowledge based jobs not replaceable from robots.
• SDG 9 (industry, innovation and infrastructure): the circular economy represents an overall vision that shapes all specific initiatives (industry 4.0, bio-economy, etc.)

Enel has placed the circular economy at the center of its strategy and strongly focused on the perspective of circular cities. Enel contribution is not just in terms of business solutions and services, but also in terms of theoretical elaboration, recently publishing a Position Paper on Circular cities.

Starting from 2015 Enel Foundation is publishing in the scientific literature a number of studies on the ‘Urban metabolism of megacities’, focusing on energy flows, electric cities, and net-negative electric cities presenting the analysis of energy and material flows of megacities around the world.

Enel Foundation is now looking at updating and extending such results, assessing and analyzing the flow of material and energy of a set of representative cities around the world in a circular metabolism perspective with the aim to comprehend current and ‘to be’ circularity according to three main dimensions:

• Untapped economical potential: waste of resources, unclosed loops, missing infrastructure, low penetration of electrical technologies, are all a loss of economic potential that could be recovered
• Environment: waste generation, water and air pollution, CO2 emissions, are some of the negative effects related to current linear system that could be avoided
• Social: the circular economy, through recycle, reuse and maintenance, or smaller size renewable based production, would shift the labor structure. The impact of the green transition and of the circular vision needs to be assessed according to principles of social equality (i.e. social cost needs to be carefully assessed).

A real transition towards circularity cannot be achieved if a further fundamental element is not considered: metrics. Circularity is a wide concept, that could be even misleading. Any solution has to be assessed in terms of energy and material deltas vs. alternatives solutions. In this respect Enel has developed its own circular KPI (Circulability@ model) in order to systematically review and asses real impact of new solutions, services and approaches.

It is also worth to underline that circularity is not about being a little more efficient, but about adopting a completely new and different way of thinking and behaving. To provide an example, a new gasoline engine that increases efficiency of 2%, or that uses 10% of biofuel, is not a circular evolution but just an improvement of a linear model. Integrating renewable energy and electric vehicles, using digital infrastructure to monitor flows, reusing deteriorated energy storage systems for cars, recovering heat from industrial processes, represent instead a paradigm shift and a strong contribution towards circularity.

Circular cities represents therefore the only chance to drive cities into a sustainable development path, and we consider it the perspective for the cities of tomorrow literally because all necessary technologies are already available; what still needs to be overcome is the linear model are the old and unfit infrastructures, the current ‘silos’ laws that does not support circular solutions and a culture that is still embedded with linear concepts.