###### Abstract  
In this paper, dynamic simulations of the seasonal coefficient of performance (SCOP) of Air-Source Heat Pumps will be presented by considering three different heat pump systems coupled with the same building located in three different Italian municipalities: S. Benedetto del Tronto (42\({}^{\circ}\)58\({}^{\prime}\) North, 13\({}^{\circ}\)53\({}^{\prime}\) East), Milan (45\({}^{\circ}\)28\({}^{\prime}\) North, 9\({}^{\circ}\)10\({}^{\prime}\) East), and Livigno (46\({}^{\circ}\)28\({}^{\prime}\) North, 10\({}^{\circ}\)8\({}^{\prime}\) East). Dynamic simulations were conducted by employing the software package TRNSYS and by considering real weather data (i.e., outdoor air temperature and humidity as well as solar radiation) referring to the three abovementioned cities for a period of 8 years (2013-2020) and collected from on-site weather stations. Attention has been paid to the modeling of the heat pump defrost cycles in order to evaluate their influence on the unit's seasonal performance. Results show that, when referring to different years, the thermal energy demand displays huge variations (in some cases it can even double its value), while the effective SCOP is characterized by scarce variability. Sensible variations in SCOP values are achieved for Livigno. air-source heat pumps; SCOP; heat pump seasonal performance; defrosting; real weather data 2021 11 8003 11.18003 11.  
hourly meteorological data taken from series lasting at least 10 years, according to the procedure reported in the standard ISO 15927-4 [9]. However, effective ambient temperature varies every year, and recently, climate change seems to lead to significant yearly variations. Indeed, the use of TRY might underestimate, or overestimate the effective energy performance of ASHP systems [10]. A well-known issue that characterizes ASHPs is the frost build-up on the external heat exchanger during winter operation and the need to remove it. In fact, in certain weather conditions such as temperature and relative humidity, if the surface temperature of the evaporator coil is below the air dew point temperature and the water freezing point, ice deposits on it, causing a general degradation of the performance of the air-source heat pump. The reduction of HP performance is due to the reduction of the evaporator heat transfer coefficient, which is mainly for two reasons: the ice layer reduces the air flow through the heat exchanger, and the ice layer also acts as an insulant on the coil surface [11]. Ice build-up on a heat exchanger is a transient phenomenon, characterized by three different stages, as proposed by [PERSON] et al. in [12]: the frost layer growth rate in the first stage increases with time; in the second stage, depending on the relative humidity value, the frost thickness growth rate decreases with time or remains invariable; and finally, in the third stage, the frost layer thickness rapidly increases with time. Some papers investigate the importance of temperature and relative humidity for frost build-up on the evaporator of the heat pump. In [13], [PERSON] et al. developed a frosting map (temperature-relative humidity) to guide defrosting control for air-source heat pumps, obtaining three different zones in the frosting region representing different frosting levels from severe to mild. The proposed map can be used to avoid mal-defrosting and to improve defrosting efficiency. In some papers, frosting/defrosting is taken into account, such as, for instance, in [13; 14; 15; 16], from a numerical or an experimental point of view. Reported papers mainly investigate the ASHP, and papers dealing with ground coupled exchangers do not consider the possible effect of natural convection in the soil [17]. Moreover, as far as the authors are concerned, papers that numerically investigate the defrosting inverse cycle do not take into account the effect of real climate data. Recently, the authors have analysed the effect of real climate data on the seasonal coefficient of performance of ASHPs [10].