Energy Simulation of Buildings with a Modular Object-Oriented Tool Rashmin Damle 1 , Oriol Lehmkuhl 1,2 , Guillem Colomer 1 , and Ivette Rodriguéz 2 1 TermoFluids,S.L., Magí Colet 8, 08204, Sabadell, Barcelona (Spain) 2 Centre Tecnològic de Transferència de Calor (CTTC), Universitat Politècnica De Catalunya, Colom 11, 08222 Terrassa (Barcelona), Spain Abstract The aim of this work is to develop a modular object-oriented tool “NEST” (Numerical Edifice Simulation Tool) for the energy simulation of buildings, which can be coupled with a parallel CFD software. For this purpose, a building is modelled as a collection of basic elements (walls, rooms, outdoor, openings, etc.). Different models (1D, 2D, simplified energy balances, CFD & HT, etc) are implemented for different elements which are capable of solving themselves for given boundary conditions. The elements can be linked to each other to form a specific building configuration. Thus new configurations can be quickly formed by adding or removing the required elements. Such an approach gives flexibility of choosing a model for each element and to have different levels of modelling for different elements in the system. Moreover, elements developed can be used for applications not restricted to buildings only. The object-oriented methodology, element descriptions, BESTEST cases for code validation, and transient thermal simulations of two different cases are presented in this paper. 1. Introduction This work is addressed to contribute to the progress in the numerical simulation of the thermal and fluid flow processes within and around buildings. Building energy consumption has increased from 20% to 40% in developed countries, and the HVAC (Heating Ventilating and Air-Conditioning) systems account for almost half the energy consumed in buildings (Pérez-Lombard, et al. 2008). Energy simulation of buildings is critical for optimizing the energy demands as building thermal systems are major consumers of energy as far as their construction, operation and maintenance is concerned. It can give vital information of the peak loads during the heating and cooling season, room temperatures and velocity distributions for maintaining an adequate indoor environment, and overall energy demands during an year. This information can be used at the design stage to reduce the energy costs with a good architectural and HVAC design. Buildings can be considered as thermal systems interacting with the surroundings through heat transfer and fluid flow processes. A numerical approach to handle the heat transfer and fluid flow in such systems not only helps in saving the full scale experiment time and cost, but also helps in optimizing the governing parameters for the efficient functioning of the entire system. The prediction of the physical phenomena involved in buildings is difficult due to the large and complex geometry involved, changing boundary conditions, airflows due to natural convection, stack and wind effects, infiltration of ambient air and mechanical ventilation, and the mixture of free and forced convection flows which are often are turbulent. Numerical simulations imply mathematical modeling of the different physical processes occurring in a building thermal system. The models implemented could be simple zero dimensional expressions, one dimensional models based upon experimental correlations, two or three dimensional analysis with turbulence models or direct numerical simulations (DNS) for detailed resolution of fluid flow and heat transfer, etc. The level of modelling a given process may be different depending upon the nature of the process, resources, and the accuracy desired. Numerous building energy simulation programs have been developed over the years. MODSIM (Sahlin, 1988), EKS (Clarke, et al. 1992), SPARK (Buhl, et al. 1993) are some of the earlier objected oriented initiatives for modular simulation. CONTAM (Walton and Dols, 2005) is a multizone model which has been used to calculate the indoor air quality in buildings. COMIS (Feustel, 1999) is a multizone model prepared