*3.2. Analysis in Winter Conditions*

Figure 7 shows the temperatures and the irradiance curve of a sample winter week from 08 January 2020 to 14 January 2020. On sunny working days (from 08 January 2020 to 10 January 2020), the outdoor temperature showed typical values of winter in Madrid, with a minimum temperature slightly below 0 ◦C and a maximum temperature between 10 and 15 ◦C. The solar radiation impinged on the south-west facade as of 11:00 a.m. with a peak value of 300 W/m2. The indoor air temperature (*T\_int*) was below comfort until 7:00 p.m. because the heating system was off. In the morning, the heat pump started working, and the radiant WFG panels were delivering heat. In the afternoon, the temperature in the corridor (*T\_ext\_C*) rose to 30 ◦C, which helped to heat the office space air temperature (*T\_int*) to 22 ◦C. The solar radiation in the afternoon made the heat delivered by the WFG unnecessary. Over the weekend (11 January 2020 and 12 January 2020), the heat pump was not operating, and the indoor air temperature declined and reached its lowest value (14 ◦C) on Monday 13 July 2020 at 7:00 a.m. Due to the solar radiation, the temperature in the corridor rose to 28 ◦C. On weekend days, the heat pump did not operate in the morning, so the indoor temperature continued to drop until the afternoon, when the

solar radiation and the corridor overheating contributed to raising the indoor air temperature from 17 to 19 ◦C on 11 January 2020 and from 15 to 17 ◦C on 12 January 2020. Nevertheless, the indoor temperature on 11 January 2020 at 7:00 a.m. was 18 ◦C, and on 13 January 2020, it was 14 ◦C after two days without operating the heat pump. On working days, the indoor air temperature was above 18 ◦C at the beginning of the working hours. On 13 July 2020 and 14 July 2020, the solar irradiance was low, and the outdoor temperature variation over the day was only 5 ◦C. The heat pump operated most of the working hours, unlike on sunny days, when it operated only in the morning.

**Figure 6.** Solar irradiance and indoor and outdoor temperatures—summer sample days (**a**) 10 July 2019 and (**b**) 11 July 2019.

**Figure 7.** Solar irradiance and indoor and outdoor temperatures—sample winter week of 08 January 2020 to 14 January 2020.

Figure 8 details the parameters on two winter days with different outdoor conditions. Figure 8a illustrates a sunny winter day when the solar irradiance reached a peak value of 300 W/m2, and the outdoor temperature ranged from −1 to 11 ◦C. The WFG started working in heating mode from 7:00 a.m., when the indoor air temperature was 18 ◦C, to 9:30 a.m., when the indoor air temperature reached 20 ◦C. The indoor air temperature continued to rise to 22 ◦C because the corridor air temperature reached a peak of 30 ◦C. Figure 8b shows a winter day with little solar radiation and an outdoor temperature that ranged from 4 to 10 ◦C. The WFG started working in heating mode at 7:00 a.m., when the indoor air temperature was 16 ◦C. It took the system four hours to increase the indoor air temperature to 20 ◦C. The heat pump was connected to the buffer tank, so the heating time seemed too long due to the thermal inertia. Starting the heat pump four hours before the working hours would be an excellent strategy to improve comfort conditions on winter days after the holidays.

**Figure 8.** Solar irradiance and indoor and outdoor temperatures—sample winter days (**a**) 09 January 2020 and (**b**) 14 January 2020.

Figure 9 presents a sample week of February, from 19 February 2020 to 25 February 2020. The minimum outdoor air temperature was 0 ◦C on 20 February 2020, and the maximum temperature was 21 ◦C on 24 February 2020. The indoor air temperature (*T\_int*) in the office space maintained comfortable conditions operating in a free-floating temperature regime with zero energy consumption. The WFG circuit was never empty. During the free-floating regime, the mass flow rate was 0 and the heat pump was not in operation. Temperature in the corridor (*T\_ext\_C*) showed peak values above 32 ◦C in the afternoon. The solar irradiance on the west facade (*Sun\_rad*) reached a peak of 480 W/m2.

Figure 10 illustrates the performance on two consecutive February days. Although the minimum outdoor air temperature was 0 ◦C on 19 February 2020 and 20 February 2020, the peak solar radiation (440 W/m2) increased the temperature inside the studied office in the afternoon. When the indoor air temperature reached 25 ◦C, the water inlet temperature dropped, and the outlet temperature was above the inlet. As stated in Table 3, the heat pump was set to operate in cooling mode when indoor temperature was above 25 ◦C. The heat pump cooled down water three times between 5:00 p.m. and 7:00 p.m. on 19 February 2020, and only once at 6:30 p.m. on 20 February 2020.

**Figure 9.** Solar irradiance and indoor and outdoor temperatures—sample week in February from 19 February 2020 to 25 February 2020.

**Figure 10.** Solar irradiance and indoor and outdoor temperatures—sample February days (**a**) 19 February 2020 and (**b**) 20 February 2020.

Table 6 shows a summary of the energy performance on four days in different seasons. On 10 July 2019, the system was working in cooling mode. The heat removed from the office space by the transparent WFG (*kWh\_WFG*) and by the translucent partitions (*kWh\_WFG\_TP*) was 4.9 KWh. The transparent WFG absorbed the most significant amount of heat during the working hours because of the high mass flow rate (*m˙* = 2 L/min m2), whereas the translucent interior partitions performed better during the night. The contribution of the air heat exchanger (*kWh\_AXH*) during the night was negligible compared with the heat pump, which operated from 7:00 a.m. to 8:00 p.m.


**Table 6.** Thermal energy summary on four sample days.

<sup>1</sup> Energy values in kWh.

On 09 January 2020, the heat delivered by the translucent WFG (*KWh\_WFG\_TP*) was 18.7 kWh, whereas the total amount of energy delivered by the transparent WFG (*KWh\_WFG*) was 3.4 kWh. In the afternoon, the transparent WFG circuit was stopped to allow solar radiation to enter the office space. The translucent WFG supplied most of the heat during the working hours. The thermal energy delivered by the heat pump (*kWh\_HP*) was 7.12 kWh from 7:00 a.m. to 11:00 a.m. In the afternoon, the thermal inertia of the tank and the solar radiation made it unnecessary to operate the heat pump again. On 14 January 2020, the contribution of the transparent WFG was higher because there was little solar radiation in the afternoon. The heat pump operated over the working hours and released twice as much thermal energy as on 09 January 2020.

On 20 February 2020, the system was working in cooling mode. The air heat exchanger (*kW\_AXH*) was cooling down the buffer tank during the night, and the heat pump operated during the working hours. The heat removed by the translucent WFG (*kWh\_WFG\_TP*) was 4.8 kWh. The energy delivered by the heat pump was 0.6 kWh, and the thermal inertia of the buffer tank was enough to keep indoor temperature between 20 and 26 ◦C. In Section 4.4, these conditions are assessed to evaluate the occupants' comfort.
