*2.1. Description of the Facility*

The testing facility was an office space of the Department of Applied Mathematics in the School of Aeronautics and Space Engineering in Madrid, Spain (40,44389◦ N, −3,7261972◦ E). Two faculty members occupy the room from 8:00 a.m. to 8:00 p.m., and there are meetings with students during office hours. The occupancy is limited to six people at a time. The facility validated the WFG behavior as a component of the heating and cooling system. Figure 1 illustrates the floor plan. Four transparent WFG panels (WFG1, WFG2, WFG3, and WFG4) separated the corridor from the office. The thermal and spectral properties of these transparent panels were carefully selected to absorb the maximum heat from the beam solar radiation, which entered through the main glazed facade, impinging into the WFG in the afternoon for four to five hours, depending on the season. The northeast facade was an insulated opaque wall, and the rest of the interior partitions were translucent WFG (WFG\_TP01 to WFG\_TP09). In all, there were thirteen WFG panels of 1500 mm height by 1300 mm width. The energy management system is placed outdoors, in the north-east facade. The electronic control unit (ECU) monitored the temperatures of the WFG and the indoor, corridor, and outdoor temperatures.

Table 1 shows the thermal transmittance and areas of the office envelope. The opaque internal partitions were modular walls with melamine panel finish (0.5 cm) and rock–wool acoustic insulation (3 cm). The northeast facade was an insulated opaque wall made up of a zinc plate external finish (1 mm), ventilated air chamber (3 cm), a brick wall (11 cm), rock–wool thermal insulation (6 cm), air chamber (5 cm), and a plaster board (12 mm). The roof was composed of a zinc plate external finish (1 mm), ventilated air chamber (3 cm), metal deck with concrete (10 cm), air chamber (10 cm), rock–wool thermal insulation (6 cm), and a plaster board (12 mm). The thermal transmittances met the requirements of the Spanish Building Code [38].

**Figure 1.** Plan view of the office spaces. The transparent Water-Flow Glazing (WFG) was connected directly to the primary circuit. The translucent interior partitions were connected in parallel to the circulating device.



<sup>1</sup> Values meet the Spanish Building Code (CTE DB HE1) requirements [38].

Figure 2 shows the space with transparent WFG (a) facing south-west and translucent interior partitions (b). The former was double glazed; each glass pane was composed of 8 mm planiclear, 1.54 mm saflex Rsolar SG41, 8 mm planiclear, and a 20 mm water chamber. The latter was double glazed; each glass pane was formed of 10 mm planiclear, 1 mm translucent Polyvinyl butyral (PVB) 000A CoolWhite, 3 mm planiclear, and a 16 mm water chamber. The mass flow rate through the transparent WFG was set to be 2 L/min, and through the translucent glazing, it was 1 L/min. The transparent panes were exposed to western solar radiation and had to absorb a large amount of heat. In contrast, the translucent panes were designed to deliver heat or cold in winter or summer.

**Figure 2.** Top: View from the access corridor during the construction process. Bottom: Glass configuration of the office space. (**a**) Transparent Water-Flow Glazing (corridor), (**b**) translucent Water-Flow Glazing (interior partitions).

Table 2 shows the estimated heating and cooling loads in the office space. Ventilation loads (*Vent*) and internal loads (*IL*) were calculated for an occupancy of six people and average office equipment [38,39]. The total glazed surface was 7.8 m2 of transparent WFG and 17.55 m<sup>2</sup> of translucent interior partitions. The wall-to-window ratio of the wall exposed to solar radiation was 40%. The total area of WFG radiant panels was 25.35 m2, with a floor area of 40 m2. The expected power delivered by WFG was 130 W/m2 when the difference between the circulating water and the indoor air temperature was 13 ◦C. The dew point temperature for the indoor air temperature was at 27 ◦C and relative humidity at 40% was 12 ◦C. Therefore, keeping the WFG inlet temperature above 12 ◦C, the indoor air temperature at 27 ◦C, and the average water temperature at 14 ◦C, the delivered cooling power would be 130 W/m2. The total WFG surface area was 25.35 m2 and the total cooling power was 3295 W, which was above the predicted cooling loads shown in Table 2. The cost of the system depends on a few different factors, including the dimensions of the glass, thickness, and distance between the energy management system and the panels. A typical 2 m2 double glass panel costs 900 USD (around 450 USD/m2), including the piping and individual circulating devices. Installation of WFG requires a professional team, which could run 50 to 70 USD per hour.

**Table 2.** Estimation of heating and cooling loads in the offices.


<sup>1</sup> Values are taken from CTE DB HE1 [38].

Figure 3 shows the schematics of both circuits. The energy management circuit consisted of a 370 L buffer tank, an expansion tank, an air-to-water heat pump, and an air heat exchanger. The heat pump's (Saunier Duval Genia Air 8/1 Power A7/W35 = Power A35/W18) nominal power was 7.60 kW in winter (at an outdoor air temperature of 7 ◦C and inlet water temperature of 35 ◦C) and summer (at an outdoor air temperature of 35 ◦C and inlet water temperature of 18 ◦C). The heat pump was selected for commercial reasons, regarding availability and budget constraints. Some malfunctions and operating issues related to the oversized cooling and heating power are addressed in the following sections. The air heat exchanger works when the outdoor air temperature is low enough to cool down water. This cooling mode can only be used when outdoor ambient air temperatures are below 12 ◦C. When the air heat exchanger is used for free cooling, the control system uses valves to isolate the heat pump from the rest of the loop, and the heat exchanger is used like a chiller. Once the buffer tank is heated or cooled down, the water flows to transfer heat or cold to the circulating device. Then, the secondary circuit transports the heated or cooled water to thirteen radiant WFG units. A control system with a thermostat based on the indoor temperature turned the heat pump and the flow rate ON and OFF. The secondary circuit was made up of two branches—one that transferred heat or cold to translucent partitions and another one for the transparent WFG modules. Each transparent WFG module had a circulating device (*CDi*). The mass flow rate through the transparent modules was set to *m˙* = 2 L/min m<sup>2</sup> when the system was ON. All the translucent WFG panels were connected to the same circulating device (*CD TP*), and the flow rate was *m˙* = 1 L/min m2. The influence of the mass flow rate on the ability to deliver or absorb heat and the recommended values have been studied in previous articles [37]. Transparent WFG panels are exposed to solar radiation, so the mass flow rate had to be higher to absorb heat in summer and keep the water temperature within acceptable values. The electronic control unit actuated the WFG circulating devices, the heat pump, and the air heat exchanger using the basic commands of ON and OFF, with the control logic explained in Table 3. There was a mechanical ventilation system that met the requirements of the Spanish Regulation of Thermal Installations in Buildings (RITE) for ventilation of office spaces (12.5 L per second per person) [40]. The mechanical ventilation provided conditioned air and operated over the working hours (8:00 a.m. to 8:00 p.m.) at a constant air volume. However, it was not a component of the controlled energy management system. The lack of control of the ventilation device was one of the system's uncertainties because high relative humidity can cause condensation in radiant panels when operating in cooling mode, and can affect the latent loads.

**Figure 3.** Schematics of the testing facility. The primary circuit connects the energy management devices (heat pump, air heat exchanger, and buffer tank). The secondary circuit goes from the buffer tank to the WFG.


**Table 3.** Energy management system in cooling mode.

Tables 3 and 4 show the proposed energy management strategy in the heating and cooling modes. The heat pump (*HP*) was set to operate during working hours, whereas the air heat exchanger (*AHX*) operated only in cooling mode during non-working hours. The first condition was related to the indoor air temperature (*T\_int*) and the second condition depended on the difference between the outdoor air temperature (*T\_ext*) and the tank temperatures (*T\_tank\_top, T\_tank\_bottom*).

**Device HP AHX WFG WFG\_HP** Condition 1 T\_int < 20 ◦C T\_int < 20 ◦C Condition 2 - (T\_tank\_bottom − T\_int) > 10 ◦C 8:00 p.m.–7:00 a.m. - - ON ON 7:00 a.m.–8:00 p.m. ON - ON ON

**Table 4.** Energy management system in heating mode.
