Energy and Buildings 36 (2004) 35–40 A non-dimensional analysis of a ventilated double façade energy performance Carla Balocco ∗ Dipartimento di Energetica “S. Stecco”, Università degli Studi di Firenze, via S. Marta 3, Firenze, Italy Received 24 February 2003; received in revised form 15 May 2003; accepted 6 June 2003 Abstract A method based on dimensional analysis is proposed for a natural ventilated double façade energy performance study. The 14 non-dimensional numbers defined, with physical meaning, can be used to describe thermal and energy performance of different façade designs. A comparison between non-dimensional numbers solved by simulation, experimental data, and the obtained and validated correla- tion results is reported. A simple tool to evaluate thermal performance of a solar chimney is provided. Applying it, useful design indications can be derived also by varying simple parameters using thermal and physical data easy to get. © 2003 Elsevier B.V. All rights reserved. Keywords: Non-dimensional numbers; Ventilated double façade; Simulation 1. Introduction At present, energy consumption reduction for heating and cooling buildings is an extremely important task. Demon- stration programs, theoretical and experimental studies are supported in many countries. Passive solar heating and cool- ing of buildings has been widely studied [1,2]. In particular, different Trombe wall systems [3,4], solar chimney [5–7] and also double glass façades have been stud- ied for designing natural ventilated building façades [8,9]. Modern architecture often shows building design with dou- ble glass façades with low solar radiation transmission co- efficient. This glass façade structure is commonly built with vertical glass cover at 20–60 cm distance from the inner one. It can be also continuous in height or discontinuous at each floor level. There are some examples of Trombe wall or solar chimney application in architecture designs. Trombe wall or solar chimney façade system is a facing, preferable south facing, concrete or masonry quite blackened wall and covered on the exterior by glazing or opaque panel. Mas- sive thermal wall stores solar energy which is transferred to the inside building for winter heating and allows air natural buoyancy circulation draught effect between the lower and upper openings of the cavity during summer. Thermal per- formance of this complex system depends on its geometry, ∗ Tel.:+39-055-4796436; fax: +39-055-4796342. E-mail address: c.balocco@ing.unifi.it (C. Balocco). air circulation in the channel and local climate conditions mainly due to outdoor air temperature, solar radiation and wind velocity. Multi-dimensional and time-dependent temperature field and transient heat fluxes can be accurately computed only by numerical simulation procedures. Those results have to be compared and validated by experimental data. Thermal analysis based on steady state conditions have been re- cently developed, but complex simulation programs using finite element method, turbulence models and fluid dynamic procedures (CFD) are not so widely used to study pro- totype façade, comparing experimental results [3,10]. Un- steady simulation has also been performed with CFD code to study phase change material’s (PCM) effect as a heat storing materials [11]. Some of these simulation programs permit to control and verify surface condensation phenomena. Only analysis, simulation results and experimental data compar- isons provide understanding basis of heat transfer mecha- nism and overall energy performance of natural ventilated façades. It is a real fact the lack of empirical data on energy performance for different climatic conditions and construc- tive solutions of solar chimney, Trombe wall and in particular double glass façades. The aim of the present work is to pro- vide a useful tool to know and compare thermo-physical and energy performances of different natural ventilated façade designs, by non-dimensional analysis of a natural ventilated façade studied. The proposed method permits a simple way to assess energy potential of the system and its useful sum- mer cooling effect. 0378-7788/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0378-7788(03)00086-0