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A judicious use of energy is a fundamental factor to achieve sustainable development. The limited character of energetic fossil resources and the pollution generated by burning fossil fuels for electricity production generate the need to replace them with other sources of energy. Despite the fact that fossil fuels would continue to play a prevailing role in the energy supply for decades to come, renewable energy resources have the potential of contributing to the increasing global energy demands, while simultaneously emerge the most efficient solutions for clean and sustainable energy development in the world. In this framework, the main scope of the present study is to provide an analysis of the current state of world natural resources used to produce energy and energy consumption degree across different regions of the world. At the same time, this paper aims to compare the environmental impacts in water, air, soil and ecosystem produced by a range of conventional and renewable energy sources, which is necessary to be reduced for building a genuine low-carbon society.

The European Directive 2010/31 claims that from 2020 on, only (nearly-) zeroenergy buildings may be built. To reach this, buildings have to be optimised energetically. The remaining energy demand has to be covered by renewable energy sources (PV, geothermal etc.) gained nearby, e.g. on site. Comparable predefined plot and office building sizes are investigated for all main climate zones around the globe. Each case study leads to a different energy demand, even though, the same energy generation systems have been applied being PV for power and geothermal energy for heating and cooling. The analysis investigates first, whether the limiting factor to reach a nearly-zero-energy building is in the PV or the geothermal system. Furthermore, in all case studies, the balance between the usable floor area and the surface area for renewable energy sources (roof surface area for PV, plot size for geothermal) is investigated. From this, the maximum numbers of storeys (which can be heated/ cooled) are given per location. Furthermore, consequences for the urban pattern of nearly-zero-energy buildings (like street width and building heights) are highlighted. The main outcome is a correlation between simulated data for the energy demand and climate data. If the architectural optimization follows the local climatic needs, linear equations appear between the simulated energy demand and the climate data for a scenario of optimised buildings. The equations for annual heating or cooling demand and heating or cooling degree hours are given.