Simone Ferrari

Energy Performance Analysis of Typical Buildings

The study reported in this chapter addresses the assessment of the thermal behaviour of typical buildings within the Italian context, taken as representative of the countries in southern Europe. Five representative case-studies are reported for three principal construction ages (newly built, 1960/80, very old), the two more recent periods having seen the use of more glazed and lighter walls as an alternative to more conventional solutions. The performances of the buildings are predicted, through detailed dynamic simulations, with reference to the resulting indoor operative temperatures in order to consider the actual overall thermal comfort sensation provided by the different envelope solutions. Considering that southern Europe is mostly characterized by warm climates, the study also analyses the effectiveness of two basic passive cooling strategies (i.e. shading and night ventilation) in relation to the different building characteristics. Moreover, the implications in the seasonal cooling need assessment when considering the adoption of climate-related set-point temperatures (i.e. adaptive comfort approach), beyond the assumed common standard, are also evaluated.

Climate-Related Assessment of Building Energy Needs

This chapter reports the analysis of the correlation between the seasonal thermal energy needs of different building solutions and their climatic locations, assuming the variety of the Italian context taken as representative of southern Europe. The case-studies, simulated through a detailed dynamic tool, are representative of common practices from three main construction ages (newly built, 1960/80 and very old), and therefore include both different envelope characteristics and window percentages. Consideration is also given to the implementation of passive cooling strategies, such as shading and night ventilation, and to the adoption of variable set-point temperatures, directed towards climate-related user expectations, i.e. adaptive comfort. The graphic representation, according to the heating and cooling degree days adopted for supporting the climate-related analysis, can also be useful for extending the assessment to other locations within the same climatic area through simple interpolation. The data sheets reporting the detailed results of the case-studies, on the other hand, can be used directly for assessing the building energy needs of similar locations.

Defining Representative Building Energy Models

The energy need of buildings is strictly related to the local climate and to the construction characteristics (in particular the elements of the building envelope, the ratio between glazed and opaque facade, and orientation). In order to assess the energy performance of different building solutions characterising a large building stock, it can be useful to perform detailed simulation on reference building models. The results obtained can highlight critical issues that are useful for defining adequate policies for improving the built environment. The methodology for characterizing the building energy models, which could constitute a reference for other studies, is described in this chapter as applied in Italy. The set of detailed parameters defining the reference buildings could also be adopted for similar contexts in Southern Europe.

Implications of the Assumptions in Assessing Building Thermal Balance

The assessment of building energy performance in Europe is widely based on standards that usually refer to the simplified method provided for by EN ISO 13790 (2008). In this simplified method, the estimation of heat transfer is steady-state based, and the building energy balance takes into account the behaviour of the building when interacting with the unsteady parameters by introducing a corrector factor related to the thermal mass effect (i.e. utilization factor). Nevertheless, this method, called “quasi-steady-state”, is often unreliable compared to the results provided by a detailed dynamic simulation, in particular concerning the assessment of the thermal need in warmer climates. This chapter highlights this critical point through case studies applications. Additionally, since the common goal of the building energy assessment is to depict a building with an average usage pattern, the implications concerning the assumptions of the main input parameters to be adopted in the building heat balance assessment are also investigated, showing their very high importance in influencing the final climatization need.

Building Envelope and Thermal Balance

From the thermal balance point of view, in addition to air mass transfer due to infiltration and ventilation, building energy performance strictly depends on the characteristics of the envelope, in that it constitutes the boundary between the indoor and outdoor environments. Most of the currently available building performance assessment methods evaluate the heat exchange through the envelope by the means of a steady-state analysis, leading to the diffusion of strict regulations regarding the heat transmittance of the envelope elements. Although this approach is simple to use, it does not take into account the dynamic behaviour of the construction materials, which tend to store heat and release it after a certain length of time (Van Geem 1987). This phenomenon, usually referred to as thermal inertia or thermal mass, strongly affects the heat transfer process, influencing the building thermal energy need. This chapter summarises the theoretical basis of building thermal balance and heat transfer through the envelope. Furthermore, the different implications when considering the dynamic and the steady-state assessment methods are presented with the help of a practical example.

Thermal Comfort Approaches and Building Performance

Currently, the suitable indoor temperature is commonly defined according to the thermal comfort theory formulated by Fanger (1970). This approach bases the definition of thermal comfort on mere physics and completely neglects the social and psychological aspects of thermal perception. Moreover, its formulation is completely steady-state, determining a very narrow range of allowable temperatures throughout the year regardless of the outdoor conditions. An alternative approach to defining comfortable temperatures is the adaptive approach, which stems from the results of a wide range of field studies. It assumes that the thermal expectations of the users are linked to the outside climatic conditions on a variable basis. This chapter describes these two approaches and summarises the most relevant adaptive comfort indices. Furthermore, based on the different adaptive formulations, the comfortable temperature trends are derived and compared for two locations in Italy characterized by opposite climatic conditions. Finally, the possible implications on building thermal performance are analysed by means of a case study application.

Buildings Performance Comparison: From Energy Need to Energy Consumption

Building heating and cooling energy needs result from the thermal balance calculation, and can be determined with a high degree of accuracy through the use of detailed dynamic energy simulation tools. On this basis, starting from a reference building shape, assuming the same time-related input parameters (such as the internal sources of heat, the ventilation and the indoor temperatures) and varying the construction elements, the energy performance of a set of alternative building solutions can be evaluated and compared. However, the related building energy consumption, which is also based on the characteristics of the active air conditioning systems, does not necessarily depend linearly on the calculated heating and cooling needs. A further overall energy performance comparison between the different building solutions should therefore be adjusted accordingly. In this chapter, reference climatization systems are defined and assigned to a set of case-study buildings. Final and primary energy consumption values are then calculated and the differences in the overall energy performance trends, compared to those based on the calculated energy needs, are assessed.

Approximating Dynamic Thermal Behaviour of the Building Envelope

Assessing the building energy performance through the simplified assumptions of the steady-state conditions can prove restrictive. The estimation of heat transfer through the envelope is based on the U-value of the building construction, neglecting the thermal mass effect. Several approaches have, therefore, been developed to approximate the dynamic behaviour of buildings when performing a standard steady-state analysis. According to Barnaby (1982), these methods can be generally divided into those analysing an isolated building element and those considering the context of whole building. The first category introduces some correction values that affect the main parameters of the steady-state heat transmission calculation (i.e. the U-value of the envelope elements and the temperature difference between the indoor and outdoor environments). This chapter investigates a set of correction values that try to represent the dynamic thermal envelope performance in a simplified manner, applying some of them to a set of case-study wall types.