**1. Introduction**

The residential and commercial building sector accounts for almost 40% of the European Union final energy consumption [1]. Thus, the goal of achieving a highly energy-efficient building stock by 2050 was set by The Energy Performance of Buildings Directive (EPBD 2018) [2]. In the United States, recent studies have shown that heating and cooling account for more than 30% of energy consumption in buildings [3]. Other non-OECD countries, including China and India, will be responsible for half of the global increase in energy consumption until 2040 [4]. The development of new materials, new heating and ventilation technologies, and energy-saving measures have improved the thermal performance of buildings in winter conditions [5]. However, in summer conditions, the increasing standards of life and the affordability of air-conditioning technologies have contributed to increasing the energy needs for cooling over the last decade [6]. Nowadays, air conditioning in office and commercial facilities accounts for 15% of the total electricity consumption in the world [7–9]. Occupants

and equipment are responsible for internal heat gains, and large glass areas increase solar radiation gains, especially in warm climates, thus leading to the increased total electricity consumption for the purposes of cooling [10,11].

The design of advanced glazed facades is the most promising component in building design with the highest impact on building performance [12]. In this paper, advanced facades refer to a broad spectrum of constructive solutions that allow for the dynamic response of the building envelope. They can actively manage the heat flow and energy transfer between the building and its external environment, leading to a potentially significant reduction in heating and cooling loads [13]. Advanced glazed facades include passive solutions, such as Low-E coatings, which reflect the indoor heat when the outdoor temperature is low [14], and highly selective coatings that reflect direct and diffuse solar heat radiation in summer. Scientific literature has confirmed this potential energy reduction [15,16]. However, the most promising results can be accomplished with dynamic technologies that can adapt to different outdoor conditions. Polymer dispersed liquid crystal (PDLC), Suspended Particle Devices (SPDs), and electrochromic (EC) glass switch from transparent to colored or vary transmission and reflection parameters [17,18]. The system is limited by its high initial cost and the need for an energy management system integrated with the rest of the equipment, especially the ventilation system. Controlling the relative humidity is essential in radiant panels to prevent condensation issues, especially in summer. The integrated piping does not allow movable panels, so its use is limited to buildings with mechanical ventilation [19]. Nevertheless, the measures to improve the energy performance of buildings do not focus only on the building envelope. Building designers must consider all technical and mechanical systems in a building, such as passive elements; heating, ventilation, air conditioning (HVAC); the energy use for lighting and ventilation; and other measures to improve thermal and visual comfort [20].

Water flow glazing (WFG), as an advanced facade technology, combines passive (coatings and polyvinyl butyral (PVB) layers) and active solutions (variable water mass flow rate) to absorb or reject infrared radiation and reduce the temperature of the interior glass pane [21,22]. Flowing water captures most of the solar infrared radiation, while a significant part of the visible component goes through the glazing [23,24]. WFG radiant panels can be used as components of a heating or cooling hydronic system with little difference between the water and the indoor temperature [25]. Finally, WFG can work as an integrated solar collector to provide water heating in warm seasons, and the excess of hot water can be stored in buffer tanks [26]. The use of the facade and interior partitions as radiant heating and cooling devices have advantages compared with convective cooling systems. Using radiant ceilings or walls can reduce energy consumption between 10% to 70% compared with all-air systems [27]. This article showed some of the accomplishments of the research project: "Industrialized Development of Water Flow Glazing Systems" (InDeWag), supported by program Horizon 2020–the EU.3.3.1: Reducing energy consumption and carbon footprint by smart and sustainable use. The water flow glazing unitized facade is made of three components: glazing, circulating device, and aluminum frame [28]. The glazing comprises different layers and interfaces according to determined spectral and thermal properties. The circulating device includes a water pump moving the fluid in a closed circuit, a heat exchanger, and temperature and flow sensors to monitor and control the heat. Finally, the aluminum frame provides the unitized module with structural stability.

Despite the fast pace of product innovation, a gap has been created between the new advanced facades and the available building simulation tools to model and assess building energy performance [29]. Monitoring activities are essential to support simulation models, especially in transient state [30], when changing the boundary conditions can affect the results of dynamic simulations by more than 30% [31]. Although there are commercial building energy simulation tools that include dynamic simulations [32], very few include WFG [33,34]. The authors of this paper developed a set of equations that take into account multiple direct and diffuse reflections between the glazing surfaces, the absorptance of glass and water layers, the spectral properties of coatings, and the convective heat transfer coefficient [35,36]. Then, a simulation tool was developed to allow building designers to make decisions on the glazing type. Finally, the equations and the simulation tool were validated through the

real results taken from a demonstrator placed in Sofia, Bulgaria. In addition to an accurate simulation tool, the actual challenge was to develop a monitoring system to reveal characteristic patterns of users, and to finally discover relevant design criteria that might be applicable to different orientations throughout the year. The monitoring system applied in this case study has been tested in other facilities over the last few years, and results were shown in previous articles [37,38].

This paper focused on a methodology to select and assess the performance of different WFG configurations in a test facility in Sofia, Bulgaria. Hence, to achieve this goal, it was essential to (i) simulate the steady state to select the optimum WFG in each orientation at the first stage of the design process, (ii) validate the first results by including transient boundary conditions, (iii) analyze the performance of the selected glazing in a real facility, and (iv) estimate the final energy savings, the potential renewable energy production, and CO2 emissions in summer and winter conditions.
