**1. Introduction**

Energy consumption in buildings shows a growing trend worldwide and is of primary concern for the world population [1]. Over the last decade, nearly 60% of total net electricity consumption in Organization for Economic Co-operation and Development (OECD) economies, was in the building sector, both residential and commercial [2]. The residential building sector is responsible for more than half of the electricity consumption in developing countries [3].

In the frame of the Paris agreement in 2015, 195 countries adopted 17 sustainable development goals (SDGs) as the outcome of the UN's inclusive and comprehensive negotiations in the frame of the 2030 agenda [4]. The seventh goal states that using clean and sustainable energy sources is an opportunity to transform economies and lives, especially in developing countries.

Annual power consumption depends on the use of the building, construction year, number of floors, building structure, and building location [5]. When it comes to heating and cooling consumption, the heating, ventilation, and air conditioning (HVAC) system, exterior walls, and glazing are the

essential elements [6,7]. Building energy management systems (BEMS) and energy-saving measures are aimed at reducing buildings' energy requirements for heating and cooling [8–10].

In countries with a hot, humid climate, the excessive use of inefficient cooling systems leads to an increase in electricity consumption and causes pollution [11–13]. The energy performance of a building also depends on the solar radiation and the correlation between cooling/heating loads and the colors of surfaces [14].

In hot climate areas, the glazing solar heat gain coefficient (SHGC) must be low, and it is more relevant than the U-value because solar radiation causes the most significant part of the cooling load [15]. In cold climate areas, the goal is to reduce the need for heating energy, making the most of solar radiation [16,17]. Heating, ventilation, and air conditioning (HVAC) systems have to be efficient in providing users with a healthy environment. When fossil fuels and oil resources run out, solar energy and other renewable sources are alternatives to overcome the clean-energy demand growth [18,19]. The annual solar irradiation ranges between 100–200 W/m<sup>2</sup> as an average in Mediterranean countries, so the potential of solar energy is more than enough to provide as much energy as the building consumes [20].

Solar energy is aligned with the concept of "Regenerative Design." It implies a proactive attitude of the building beyond the traditional sustainable design practice. Regenerative buildings reduce their energy consumption to zero, and can recollect, generate, and distribute renewable resources [21]. Glass is a fundamental element in the design of regenerative buildings. Still, its extensive use has increased the heating and cooling loads. Using transparent materials requires understanding their spectral properties and developing systems to solve some of the issues regarding heat gain, heat loss, and daylight [22,23]. Accurate prediction of the performance of glazing facades has to include a thorough analysis of thermal and spectral properties that depend on the glass, spacers, coatings, and gas fillings. Solar control layers reflect and filter solar radiation, and low emissivity coatings reduce the emissivity of the glass and retain the heat charge inside [24]. Acting in the chamber can improve the insulation capacity of the double-glazed windows [25]. The chamber can also be filled with inert gas, or vacuumed, to reduce the transmittance in large glazed surfaces [26]. Thermochromic and electrochromic glazing vary in color and transparency as a reaction to light and heat excess [27]. Double-pane windows, in which the exterior photovoltaic pane produces electricity, can be designed and manufactured today [28,29]. Double-pane windows can also be developed with circulating water through the chamber, instead of inert gas, allowing the water to absorb the heat of direct and diffused solar radiation [30].

The use of the building, the orientation of the facade, and the location of the project are relevant inputs to determine the glazing composition [31]. The Fourier model does not predict variations in thermal properties as a function of time [32]. Water flow glazing (WFG) facades are considered dynamic envelopes able to react or adapt to the building's external and internal conditions. Most of the simulation engines do not include dynamic properties, so developing new tools to calculate the impact of WFG has become a goal of researchers [33,34]. Water is opaque to the near-infrared (NIR) spectrum of light, while its visible transmittance is very high [35].

Complicated simulation engines provide the designer with multiple options, and sometimes they are not useful at an early design stage because decisions have not been made yet. Architects might find better support in simple energy simulation tools than in complicated ones [35,36]. Building information modeling (BIM) has the potential to achieve performance improvements and high-quality construction, and architecture, engineering, and construction (AEC) industries have applied BIM in construction projects over the last few decades [37]. One of the features of BIM is the energy analysis of buildings. It makes the most of a friendly interface that has been tested over decades of experience by many users. However, users have identified gaps between the expected building energy consumption and the actual measured performance [38–40]. The causes of these gaps are diverse, including behavioral habits of occupants and construction flaws [41]. The evaluation of the actual thermal properties of the

building stock from monitored data is widely considered advantageous compared to tabulated data to improve the overall quality of the building process by feeding back the measured data [42].

The steady-state model is not a reliable means to analyze dynamic forms of heat transfer. Temperature, solar radiation, occupancy, and HVAC systems affect the transient state of the building envelopes. Those parameters are time-dependent and non-linear. Remote sensing systems have become indispensable in comparing the actual energy performance with simulation models and understanding the dynamic heating and cooling loads [43–45]. Cooling has represented a small share of the final energy use in buildings, but demand has been rising over the last decade [46,47]. This article considers the best available technologies (BAT) for cooling, which are innovative and economically viable [48]. The energy efficiency ratio (EER) is the parameter that measures the efficiency of cooling systems. Hydronic technologies, such as water-to-water heat pumps, are compatible with WFG and radiant floors and walls. WFG can improve the performance coefficient of cooling systems by increasing the indoor comfort temperature and the inlet temperature of the fluid through the glazing [49]. The technology of WFG has been studied in previous scientific articles. Some papers have studied the physical structure and energy performance of WFG in cooling-demand climates through numerical computation [50,51]. Recent research studied the performance of WFG compared with conventional double glazing with low-emissivity coatings. Dynamic simulation has been used to evaluate different options of glazing, and the presented simulation results concluded that improving SHGC is more efficient for thermal performance than improving the U-value [52]. Other papers have validated the numerical simulations using test prototypes. The dimensions of the tested devices varied depending on the goals of the research. Cubic boxes measuring 60 × 60 × 60 cm, with one side open, have been used to place different glass panes [53]. If the goal was to validate the performance of WFG, the prototype was designed as an adiabatic box, with high thermal insulation in the opaque walls with U values below 0.1. Other tests focused on analyzing the influence of coatings applied to the indoor surface and the heat gains by measuring the water flow rate and the inlet/outlet temperatures of WFG. These test facilities were slightly bigger (the length was 1.55 m, the width, 0.9 m, and the height, 0.9 m). In this case, the insulation of opaque walls was not relevant, and the indoor air temperature was set to 24 ◦C by a direct expansion cooling coil with an electrical heater [54]. The authors of the present paper have developed a set of equations to take into account the influence of multiple diffuse reflections, direct reflections between glazing and parallel surfaces, indirect reflections between the glazing, parallel surfaces, and perpendicular surfaces. These equations have been included in the simulation tool tested in the present article [55]. The simulation of the indoor air temperature and the water absorption in a transient state affected by changes in temperature and solar radiation was relevant when the test facility was bigger. In these cases, validating simulation tools with real data was essential in predicting thermal behavior and the fluctuations of indoor air temperature [56,57]. This paper aims to investigate the dynamic thermal parameters of WFG. The influence of WFG as a means of energy management was tested by comparing the indoor temperatures of two prototypes. The empirical tests under variable weather conditions and have been carried out over two years. There are three objectives in the analysis of the prototype. First, it allows for the comparison of the indoor temperatures of the WFG cabin and the Reference cabin. Second, the simulation tool based on the mathematical model to predict the performance of WFG was validated using real data. Finally, it aims to study the improvement of a water-to-water heat pump's performance by reducing the temperature gap between the water and the indoor air. Two cases have been tested. In the first case, the water was flowing without controlling its temperature. In the second case, there was a source of energy that provided the desired boundary conditions. The thermal performances of the WFG cabin and the Reference cabin have been recorded using a proper monitoring system. Different boundary conditions based on real data are given to the mathematical models to carry out the simulation.
