## 1 Introduction  
The European Green Deal [1] includes new ambitious targets in Europe related to greenhouse gas (GHG) emissions to move towards a climate-neutral economy and to achieve the commitments under the Paris Agreement [2]. These key targets for 2030 include cuts of at least 40% GHG emissions compared to that of 1990 levels to achieve at least 32% share for renewable energy and to improve the energy efficiency at least by 32.5%. With these targets, Europe aims at becoming the first climate-neutral continent. Key actions highlighted within this new strategy are the decarbonization of the energy sector, which clearly needs more renewable energy use and more energy storage implementation and to ensure that buildings are more energy efficient [3]. This can be obtained by merging green and smart technologies into a green smart building (GSB), as suggested and discussed in detail by [PERSON] et al. [4]. However, a building automation control system is needed, especially in complex systems, such as the one reported by [PERSON] et al. [5]. In that study, an economic model predictive control approach was used to handle the problem of managing both electric and heating resources in a smart building to achieve nearly zero energy consumption and automated participation to demand response programs. An intelligent supervisory predictive control (ISPC) was proposed by [PERSON] et al. [6] to minimize energy consumption without sacrificing the thermal comfort of building occupants. The proposed methodology proved to be capable of assisting supervisory predictive control in commercial buildings for real-time applications. A comprehensive review of the importance of sensors in the built environment and their influence on energy saving, thermal and visual comfort, as well as indoor air quality was reported by [PERSON] et al. [7]. With these targets in mind, a new concept was developed, where heating and domestic hot water (DHW) for buildings are produced with a high share of renewable energy (solarphotovoltaic (PV)) and integration of both thermal and electrical storage [8]. Moreover, the concept is based on the use of a heat pump with a refrigerant that has low global warming potential (GWP) and is connected to the PV panels with direct current (DC). This approach is also a contribution towards the development of net-zero energy buildings (NZEB) [9]. Heat pumps have been integrated with energy storage for different purposes. For example, [PERSON] et al. [10] showed that integrating thermal energy storage (TES) with an air source heat pump can reduce the number of on-off operations. [PERSON] et al. [11] evaluated the heating energy and economic viability of an air source heat pump with latent TES for heating. [PERSON] and [PERSON] [12] analyzed the use of a PV system driving a heat pump with water storage and with batteries storage. According to the literature, the choice of refrigerant plays a key role in vapor compression heat pumps since not only is the performance affected by this selection but also the environmental impact [13,14]. Refrigerants to be used should have low GWP and ozone depletion potential (ODP); GWP should be lower than 150 (EU Regulation 517/2014). Refrigerants complying with these requirements are natural refrigerants, hydrocarbons (HCs), low-GWP hydrofluorocarbons (HFCs), novel hydrofluoroolefins (HFOs), and hydrochlororofluorolefins (HCFOs) [15]. In this paper, an innovative system to produce heating and domestic hot water for heating demand-dominated climates is assessed from an environmental point of view. This system includes sensible, latent, and electrical storage to increase the use of renewable energy decreasing the use of fossil fuels, both as final energy in heating production in the building and as primary energy in the production of electricity for the grid. Moreover, this system integrates a three-media refrigerant/phase change material (PCM)/water heat exchanger in a heat pump using a low GWP refrigerant, R32.