As we walk through city streets, we walk through time, encountering the city-building legacy of each past generation.

Cliff Ellis, History of Cities and City Planning

In the 1980s, American city planner Kevin Lynch identified in the book Good City Form five basic dimensions by which to evaluate a city: vitality, sense, fit, access, and control. It is our impression that if city performance is ever to be measurable, something similar to Lynchean vitality is very likely to appear among its defining parameters.

Vitality is defined in this context as the ability to fulfill the biological needs of citizens, while providing a safe and healthy environment. In contrast, cities are nowadays responsible for approximately three-quarters of the world's energy consumption, and face the consequent problems related to overpopulation, exhausted energy resources, and pollution. Abandoning the city model and hence scattering in small villages is not an option. People and ideas flowing in cities are at the root of creativeness, innovation, and transformation that spread through all of the civilized world. City planners (and also current residents, through a rational use of energy resources) are the ones endowed with the task of assuring the vitality of citizens for generations to come.

Securing enough resources, especially clean and renewable energy, is one of the challenges for city planners. The job is not trivial, since most cities are already built. Founded by crossroads, rivers, or natural harbours to guarantee their energy supply, cities have grown along with the availability of new sources of energy which contribute to their nurturing. New energy is transported into the city from wind farms, solar plants, nuclear plants, oil wells, or gas reservoirs, installed not always around the corner, with a growing environmental, economic, and energetic cost.

Out of all of the above sources of energy, there is one that could be directly integrated into the city with no need for transportation: solar power. The Sun provides the Earth with as much energy every hour as the worldwide energy consumption over a year (data from 2002).