*4.1. Validation of Energy Management System*

The power released or absorbed by the water (*P*) is measured in watts per square meter (W/m2), and is shown in Equation (1).

$$P = \dot{m}c(T\_O - T\_i),\tag{1}$$

where *m˙* is the mass flow rate (Kg/s m2), *c* (J/Kg ◦C)is the specific heat of the water, and *To* and *Ti* are the temperatures of water leaving and entering the glazing, respectively (◦C). The mass flow rate is the mass of a fluid passing by a point over time. In summer conditions, the transparent WFG was set to operate during working hours. It had to absorb most of the solar radiation impinging on the glazing. Figure 11 illustrates the buffer tank temperatures and the thermal energy provided by the heat pump in a sample summer week. The top tank temperature (*T\_tank\_top*) showed that the heat pump was set to work when *T\_tank\_top* was between 15 and 18 ◦C. On 10 July 2019, it worked at three cycles per hour. The following days, it was fixed to operate at a minimum time between starts of forty minutes. Over the weekend, the heat pump did not operate, and the buffer tank temperature reached 35 ◦C. The maximum power delivered by the heat pump (31.13 kWh) took place on 11 July 2019, when the solar irradiance reached its maximum value without any obstructions, according to Figure 5.

**Figure 11.** Tank temperatures and heat pump thermal power—sample summer week from 1 July 2019 to 16 July 2019.

When the heat pump was working in the heating mode in winter conditions, the transparent WFG was set to operate in the morning. It did not operate in the afternoon because the solar radiation on the south-west partition helped reduce the heating load. Figure 12 shows the tank temperatures (*T\_tank\_top*, *T\_tank\_middle*, *T\_tank\_bottom*) and the thermal consumption of the heat pump (*kWh\_heatpump*) measured with the water flow rate and the difference of water temperature between the inlet and outlet in the heat pump. On sunny days, the heat pump operated mainly in the morning because the solar radiation heated up the office space in the afternoon. On 09 January 2020, when the outdoor air temperature ranged from <sup>−</sup>1 to 11 ◦C and a peak solar radiation of 300 W/m2, the heat pump heated the buffer tank from 7:00 a.m. to 9:00 a.m. The thermal inertia of the tank and the solar radiation in the afternoon made it unnecessary to operate the heat pump again. The total energy consumption per day was 7.12 kWh. The average heat pump thermal energy was 7 kWh on 08 January 2020, 09 January 2020, and 10 January 2020, whereas on Monday 13 January 2020, a cloudy winter day after non-working days, the total energy consumption was 20.05 kWh. The warm-up response was too low, and it took four hours to raise the temperature to comfort conditions. Over the weekend, the tank temperature dropped, and this made it necessary to increase the energy supplied by the heat pump. The lack of solar radiation in the afternoon was the reason to operate the heat pump until the end of the working hours.

Figure 13 shows the tank temperatures (*T\_tank\_top*, *T\_tank\_middle*, *T\_tank\_bottom*) and the thermal consumption of the heat pump (*kWh\_heatpump*) on six February days. The heat pump operated in cooling mode and cooled down the top tank temperature in the afternoon. On 21 February 2020 and 22 February 2020, the heat pump did not operate, and the buffer tank was in a free-floating regime. The average energy consumption per day was 1 kWh on working days. The difference between the heat pump consumption on 14 January 2020 (15 kWh) and on 20 February 2020 (1.1 kWh) can be explained because the peak solar radiation on 09 January 2020 was below 300 W/m2, and the outdoor temperature was above 10 ◦C for four hours. On 20 February 2020, the peak solar radiation was 450 W/m2, and the outdoor temperature was close to 18 ◦C for 7 h.

**Figure 12.** Tank temperatures and heat pump thermal power—sample winter week from 08 January 2020 to 14 January 2020.

**Figure 13.** Tank temperatures and heat pump thermal power—ample February week from 19 February 2020 to 25 February 2020.
