**Preface to "Evaluation of Energy Efficiency and Flexibility in Smart Buildings"**

Energy use in the residential sector accounts for about 40% of the total energy use both in Europe and the US. In particular, heating and cooling in buildings represent a high share of the overall energy use. Nevertheless, buildings are considered a resource in power systems thanks to the high energy flexibility they provide. Indeed, in buildings, there are several deferrable loads (e.g., laundry machines and dishwashers) and thermostatically controlled loads (TCL), such as heat pumps, refrigerators, and air conditioners. The latter technologies, together with the properties of the building envelope, contain various forms of storage, which can be used to alter the electric load without affecting the quality of the energy service. The energy flexibility provided by buildings is paramount to mitigating the upcoming challenges of future power systems, and its exact definition and quantification have a central role. Furthermore, the EU's Energy Performance of Buildings Directive pushes towards new and better-performing buildings—nearly zero energy buildings (nZEB)—where energy efficiency and energy flexibility are essential to achieve the required performance targets. Given this premise, the sector of heating and cooling in buildings is promising for the application of demand-side managemen<sup>t</sup> (DSM) strategies aimed at modifying the final user's electricity demand on the basis of electricity grid needs. The relevance of DSM is related to the growing share of renewable energy sources (RES) in the generation mix and consequently to the necessity of integrating them and of adapting the energy demand to their intermittent and unpredictable production. DSM technologies can be used to activate the energy flexibility of buildings. They can be divided into three main categories: (i) energy-efficient end-use devices; (ii) additional equipment, systems, and controls to enable load shaping (e.g., energy storage); and (iii) communication systems between end-users and external parties, for example, demand response (DR) programs. The identification of the different technologies within the aforementioned categories, their technical details, and the effects of their application at the system or even country level is paramount. Different possibilities and analysis are collected in this book to shed light on this very interesting topic.

> **Alessia Arteconi** *Editor*

*Article*
