# IoT-Enabled Smart Sustainable Cities: Challenges and Approaches
## 3 Improving the Sustainability of a Smart City
### Energy  
The environmental impact of cities, which nowadays consume 75% of natural resources and produce 60% \(\div\) 80% of global greenhouse gas emissions, will grow as urbanization increases from 54% of the world population today to 66% by 2050 [79].  
Figure 10: Smart city areas involving soil and water, health, engagement, and culture.  
In the last years, in addition to traditional energy sources, many other terms have been associated with energy, including clean energy, green energy, sustainable energy, renewable energy, and smart energy. Therefore, the theme of energy, related to pollution and global warming, is now perceived by the people much more than in the past. In detail, clean energy (or green energy) suggests that the energy consumption has a very minimal negative impact on the environment, as shown in Figure 9.  
Another related term is \"zero-energy system\" (or zero-energy buildings), in which the consumed energy balances the generated energy, with a net consumption to be considered as zero. Increasing resource efficiency at city scale would enable more sustainable solutions and help to transition to carbon-free economy while meeting the challenges of increasing population in such areas. This also leads to the concept of Positive Energy Districts (PEDs), corresponding to city areas where the energy balance (produced-consumed) is positive. The vision for future smart energy systems is to have algorithms which can learn user demand and usage profiles from historical data [80], thus leveraging this information to optimize, coordinate, and control the distribution of different energy vectors. As embedded algorithms become more pervasive at different levels of the energy network, Artificial Intelligence (AI), including ML as a special class, would become increasingly important for the smart cities of the future.  
Finally, smart energy is much broader than any of the previous concepts, as it can be seen as related to an \"Internet of Energy\" model [81]. In detail, a smart energy system consists of the intelligent integration of decentralized sustainable energy sources, efficient distribution, and optimized power consumption. The core of a smart energy system is the information infrastructure, which is responsible for collecting the energy consumption information and sharing the provider rate information. In this case, IoT-oriented solutions can be used to control the operations with an appropriate level of energy consumption for smart appliances like dishwashers and water heaters. Another relevant aspect for smart cities of the future is the rapid diffusion of electric cars and plug-in station for charging electric vehicles; even in this case, IoT can support this carbon-to-electricity transition by providing both vehicles and recharging stations with mechanisms for contactless data exchange and billing, as well as improving data interconnection, exchange, and analysis through Vehicle-to-Vehicle (V2V) paradigms. Optimized system energy consumption is another key point of a smart energy system: the effective use of efficient energy storage, smart metering, and effective energy management can be keys for optimizing energy consumption in a smart city, possibly aided and enhanced by IoT-oriented solutions, as discussed before.