More than 40% of the total energy consumed in the EU is used to cover the needs for heating, cooling and electricity of buildings. As the major part of this energy is produced from combustion of oil and natural gas, both the EU and the EU Building Sector are highly depended on imported fossil fuels. Moreover, the Sector is also a major contributor to Green-House Gas (GHG) emissions. To address issues concerning EU security of energy supply, EU contribution to climate change and in line with the Kyoto protocol and ongoing discussions in the European and International community, the EC has set the objectives of 30% reduction of its GHG emissions by 2020 and 20% increase of the share of renewable energy. The Building Sector, as a major industrial sector, has to significantly contribute to the realisation of these objectives. Thus, the trend for the Building Sector is to move from fossil fuels based energy production to the use of renewable energy sources (RES) and clean fuels to produce the required energy to cover the building energy needs. However, in order to ensure continuous operation of energy systems based on RES it is necessary to find a proper way to balance the intermittent nature of RES. Currently, the solution is to store the excess of the produced electricity in large-scale storage batteries, which present several drawbacks. In this frame, our concept is the development of an intelligent, self-sustained and zero CO2 emission hybrid energy system to cover electric power, heating and cooling loads (tri-generation) of either residential/commercial buildings or districts of buildings. In the proposed system, the primary energy will be harvested from RES and directly used to cover contingent loads, while the excess energy will be converted to hydrogen to be used as energy storage material and to be further applied as a green fuel to cover the building heating needs through direct combustion or to produce combined heating and electricity by means of fuel cells