Impacts of architectural design choices on building energy performance. Applications of uncertainty and sensitivity techniques

In this paper, a sensitivity analysis has been carried out on a set of variables identified during the building conceptual design stage. The sensitivity analysis is performed on a simple intermediate floor of a typical multi-storey office representative of the office building sector for different Italian climatic zones. The conclusions are drawn in terms of sensitivity indexes of energy performance indicators for a set of different design variables and for different climatic zones. A multi-linear regression analysis is carried out to develop the polynomial approximation of the energy performance indicators. INTRODUCTION During the last decades, a large research activity has been dedicated to the architectural design choices during the preliminary conceptual design stage for code compliance, energy saving, environmental impacts purposes. Traditionally, the used approach is based on a discrete number of alternative solutions – orientation, building envelope transmittance, window size and transmittance ... – with a base case which plays the role of a reference against which the different solutions are compared (Shaviv E., 1978, Almeida P.F., 2007, Pushkar S. et al. 2005. Wildea P. et al., 2002). The results from the analysis are the best solutions in terms of energy or environmental impacts. In this paper, we suggest a different approach based on continuous alternative solutions of design choices, applying uncertainty and sensitivity techniques, to analyze some indicators of building energy performance. The first part of the work concerns the identification and description of the design variables. Variables will be synthetic, simple and related to architectural significance. The second part is dedicated to the description of the case study: a simple intermediate floor of a typical multi-story office is used as a generic case study for office buildings sector. Uniform density probability functions (with upper and lower bounds) are assigned for the different design variables. In the third part, the Latin hypercube method is used for generating random values of design variables. Then, simulations are performed, repetitively with Monte Carlo technique, using the quasi-steady simplified monthly method, specified in ISO 13790:2008 (ISO, 2008), to calculate the energy needs for heating and cooling for the generates sample. After that, the mean and the variance of the heating and cooling energy needs for the case study considering five sites representing five different climatic zones are calculated. The analysis of variance Fourier Amplitude Sensitivity Test (FAST) method is used for the sensitivity analysis to distribute the variance to the different design variables. A multilinear-regression analysis is used to find a polynomial approximation for the energy needs for heating and cooling. DESIGN VARIABLES RELATIONSHIPS The design variables in the conceptual design stage in building construction are defined as the variables that describe the building in the conceptual phase. Variables have to be synthetic, simple and meaningful either from thermal engineering or from architectural point of views: therefore architects can take rational conclusions easily. The following aspects, related to the building, were considered in the analysis: shape, orientation, glazed facade area, external shading devices, outer color and internal thermal heat capacity (uncontrollable variables such as internal heat sources or air change rate are not considered as desing varaibles because they depend on inhabitant’s behavior). Building shape At the preliminary design stage, the shape of buildings is often settled accordingly to aesthetics and landscape urbanism constraints. Energy performance criteria are less involved in this work step even though the building shape represents an important factor for building performance purposes. The building shape determinates how large is the surface exposed to the external environment and then provides information about the heat gain and loss through the envelope. CEN standard EN 15217 (CEN, 2007) presents two parameters to identify the shape of a building: Eleventh International IBPSA Conference Glasgow, Scotland July 27-30, 2009