*2.2. Description of the Prototype*

A prototype has been built to compare the thermal performance of WFG with a double-glazing solution. It is located near Madrid, Spain (latitude 41◦22 6" N, longitude 3◦29 57" W, altitude 1037 MAMSL). Conceptually, this mock-up is a mobile and autonomous prototype consisting of two cabins named WFG and Reference. The prototype design allows both the primary and secondary circuits to be housed within the demonstrator structure, although in independent sectors. Besides, the mock-up has four wheels at the bottom of the structure for easy transport and orientation.

The prototype has two levels: the lower level of 500 mm high and the upper level of 1000 mm high. The lower level holds the primary circuit, which is composed of a "Peltier" unit, and a lithium battery that feeds it. The upper level includes the two cabins. Finally, a photovoltaic panel has been installed on the top roof allowing to store electric energy in the battery for the operation of the cooling device. A circulating device is made up of a 10 W solar pump. The primary circuit connects the circulating system with a "Peltier" device. The secondary circuit goes from the circulating system to the WFG, with a design flow rate of 0.5 L/min. Figure 4 illustrates a schematic of the prototype with its main components, and Figure 5 shows the layout of the prototype. It consists of two semi-detached insulated cabins, one with WFG facing South and the other with a Reference glazing in the same orientation. The dimensions of the windows are 1 m × 0.5 m.

**Figure 4.** Schematics of the prototype. 1. Solar Pump 10 W, 2. "Peltier" Thermo-Electric Liquid Cooler 184 W, 24 V, 3. Battery 12 V (2 units serial connection), 4. PV Panel Polycrystalline cells. 236 W 24 V, 5. Flow meter, 6. Control unit, 7. Temperature sensors, 8. Pyranometer.

**Figure 5.** Construction plans of the prototype: Plan, cross, and long sections.

Regarding the construction materials, the prototype is made up of a steel structure formed by welded tubular profiles. Furthermore, for the cladding of the opaque parts, a white aluminum sandwich panel with 100 mm of XPS has been selected. Hence, for the glazed facade, a high selective double glazing has been chosen for the Reference cabin and a highly selective triple glazing with a water chamber towards the inside, for the WFG cabin. Figure 6 shows the configuration of both glazing facades.

The prototype has undergone an exhaustive commissioning process from the design stage to the manufacturing, factory assembly, on-site assembly, and commissioning of all the systems involved. Hence, high reflective WFG is chosen to reject sun energy and use the water chamber of the glazing facing indoors to eliminate heat loads when required. Since the energy absorption should be minimized, a high reflective coating (Xtreme 60.28) is positioned close to the outer glass pane (face 2). This coating allows for the reduction of the U-value because it can be considered a low emissivity coating at the same time (Planitherm XN). The maximum *g* value (*gOFF*) and the minimum g value (*gON*) are almost the same and around 0.2. Table 3 shows the spectral and thermal properties of the glazing defined in Figure 6. WFG presents different values of *U* and *g,* depending on the mass flow rate. *UON* and *UOFF* have been calculated with the equation (23) using *m˙* = 1 L/min m<sup>2</sup> and *m˙* = 0 L/min m2, respectively. The steady values of the reference glazing have been placed on the columns *UOFF* and *gOFF*.

**Figure 6.** Glass configuration for the Spanish mock-up. (**a**) Water Flow Glazing (WFG), (**b**) Reference glazing.

**Table 3.** Thermal and spectral properties of the reference glazing and the WFG.


<sup>1</sup> Equation (23) with *m˙* = 1 L/min m2; <sup>2</sup> Equation (23) with *m˙* = 0 L/min m2.

The WFG was selected to eliminate internal heat loads by circulating cool water through the water chamber facing indoors. This cool isothermal envelope allows insulating the building from external boundary conditions. Furthermore, the "Peltier" device connected to a buffer tank produces cool water for the glazing. To minimize the electrical consumption of the "Peltier" device, the temperature of the buffer tank should be close to the cool water of the glazing. When the outdoor conditions made it possible, evaporative cooling and cooling by night were considered to cool down the buffer tank. Besides, the prototype allows the understanding of the real issues of glass facades, analyzing deep concepts such as overheating and thermal mass.

Figure 7 shows the prototype and the position of sensors. A short description of the monitoring devices is listed below:


**Figure 7.** Set up of the Spanish mock-up and schematic of sensors distribution of the cabins. Sensors nomenclature and position for the WFG cabin and the Reference cabin.
