Buildings use 40% of the total energy in the US and European Union, [4]. Thus, any development strategy considers, in dedicated chapters, guidelines or targets for building development. Different concepts are proposed such as zero energy building (ZEB) or net zero energy building, low energy building, autonomous building, passive house that characterize the energy used/lost in building benefiting on different saving/renewable energy solutions, comparing with a traditional house using energy based on fossil fuels. The concepts are not descriptive and there are national standards mainly for the energy used for heating because in many climatic zones it represents the largest energy used. Furtherer development runs towards energy plus houses that generate an energy surplus due to a combination of very low losses (energy conservation) and efficient use of renewables. All these concepts are already implemented in demo buildings or in larger demo communities as the Sustainable Campus in Los Angeles Community College, [5] or the Plus Energy Village in Freiburg. For passing from the demo stage to the large implementation, scientific and technical progress is required along with a supporting legal and financial frame. This is why the EU COM (2008) 30 Directive promotes the idea of passive, low or zero energy building as a solution for reaching the 20/20/20 targets.

In a low energy building the energy requirement for space heating ranges from 30 to 20 kWh/m2year (German standard) or is set as 42 kWh/m2year (Swiss standard). The use of renewables is mainly addressed to for heating/cooling via solar thermal systems, alone or combined with heating pumps. Studies, [6], have proved that in temperate climatic regions, the active solar heating via solar-thermal collectors is efficient in transient seasons (autumn, spring) and the peak (in winter) is well supported due to the heating pump.

The implementation of photovoltaics (PV) is of importance in promoting zero energy buildings but has a significant contribution in the thermal energy balance of low energy buildings, too. Building integrated photovoltaics, [7], or Active Building Envelopes, [8], are investigated for calculating their efficiency and modelled in correlation with climatic data for the novel buildings design.

Modelling the (thermal) energy consumption in low energy buildings is done considering the energy flows and the consumption analysis, based on commercial software (TRANSYS, ISOFOTON, CENSOLAR) or based on other software developed according to the needs, [9-10]. The complex approach requires specific climate variables, the solar-thermal installation characteristics, the installation conditions corroborated with the consumption needs. Based on the data accuracy and the

algorithm used for data refining (average values calculated for different time steps) the results (mainly the sizing of the solar-thermal system) are closed to the real building needs or lead to over­dimensioning (and supplementary costs).

Thus, designing a low energy building uses a combination of climatology, thermodynamics (especially heat transfer), civil engineering and energy management, applied to renewables integration.