*4.2. Estimation of Final Energy Consumption*

Tables 7 and 8 show the estimated heating (positive) and cooling (negative) loads. Ventilation loads (*Vent*) were calculated with the number of occupants (*n*) at each hour. Internal loads (*IH*) are calculated with the number of occupants, the metabolic rate of typical office activity, and 20 W/m<sup>2</sup> for lighting and equipment. Solar radiation (*SR*) was taken from Figure 6a with a surface area of 7.8 m<sup>2</sup> and a solar heat gain coefficient of 0.5. The same procedure was repeated to calculate the values on five sample days.

Tables 9 and 10 compare the thermal energy consumption of the air-to-water heat pump with the calculated cooling and heating loads. The values are taken from Figures 11 and 12 (*kWh\_heatpump*) and Tables 7 and 8 by adding the heating and cooling loads over the working hours.


**Table 7.** Summer cooling loads on 10 July 2019.

<sup>1</sup> Values are taken from Figure 6.

**Table 8.** Winter heating loads on 14 January 2020.


<sup>1</sup> Values are taken from Figure 8.

**Table 9.** Sample summer week energy consumption (kWh).


<sup>1</sup> Values are taken from Figure 11.

**Table 10.** Sample winter week energy consumption (kWh).


<sup>1</sup> Values are taken from Figure 12.

Final energy (FE) consumption, non-renewable final energy (NRFE) consumption, and the CO2 emissions in kg are primary energy factors in calculating the energy performance of buildings, according to the Energy Performance of Buildings Directive (EPDB 2018) [39]. The Spanish regulation of building thermal systems (RITE) recommends a conversion factor between final energy (FE) and non-renewable final energy (NRFE) of 1.954 [40]. The factor of emitted CO2 for electricity is 0.331. The final energy consumption and CO2 emissions were calculated with two different heat pumps. Table 11 illustrates the performance of the air-to-water heat pump in cooling and heating mode. The performance depends on the outlet temperature of the WFG (*To* = 15 ◦C in summer, *To* = 30 ◦C in winter) and the source inlet temperature in the heat pump (*Ts,i* = 20–35 ◦C in summer, *Ts,i* = 15–20 ◦C in winter). The outdoor temperature, *T\_ext*, is shown in Figures 5 and 7, respectively. *Ts,i* values were taken from the top tank temperatures (*T\_tank\_top*) shown in Figures 11 and 12. The air-to-water heat pump shows a better coefficient of performance (COP) when the water temperature is close to 35 ◦C and a better energy efficiency ratio (EER) when the water temperature is close to 18 ◦C. The top tank temperatures (*T\_tank\_top*) in Figures 11 and 12 confirmed the range of optimal operating temperatures. Although the actual heat pump electrical energy consumption has not been measured, the estimated COP and EER have been taken from [41].


**Table 11.** Final energy analysis. Air-to-water heat pump.

<sup>1</sup> Energy efficiency ratio (EER)/ <sup>2</sup> coefficient of performance (COP) values are taken from [41].

Air-to-air heat pumps were also analyzed using the cooling and heating loads from Tables 9 and 10. The parameters that influence air-to-air heat pump performance are the dry bulb exterior air temperature (*T\_ext\_db*) and the dry bulb interior return air temperature (*Tri\_db*). Table 12 shows the final energy (FE), non-renewable final energy (NRFE), and the emitted CO2 for electricity of the air-to-air heat pump.


**Table 12.** Final energy analysis—air-to-air heat pump.

1, 2 EER/COP values are taken from [41].

The radiant WFG panel system coupled with a buffer tank and air-to-water heat pump showed non-renewable final energy (NRFE) consumption of 72.13 kWh in cooling mode and 24.29 kWh in heating mode, whereas the expected values of an air-to-air system were 93.56 kWh and 32.05 kWh in the studied summer and winter weeks. This resulted in a final energy savings of 23% in summer and 24% in winter. The reductions of CO2 emissions were 3.63 kg/week in summer and 1.32 kg/week in winter. As stated in Section 2.1, the ventilation device was not a component of the energy management system, and its performance was not controlled. The ventilation load was estimated by multiplying the air flow by the specific enthalpy (kJ/kg) difference between indoor and outdoor conditions. In summer, the specific enthalpy of outdoor air at 31.3 ◦C with 35% relative humidity was 58.8 kJ/kg. At 26 ◦C and 36% relative humidity, the indoor air specific enthalpy was 46.7 kJ/kg. At a ventilation air flow rate of 75 L per second, the total ventilation cooling load over 12 h was 10.8 kWh. In winter, the indoor and outdoor specific enthalpy were 37.11 kJ/kg and 16.36 kJ/kg, respectively, and the ventilation load over

the working hours was 13.24 kWh. The electrical consumption of the ventilation device, including the engine and the fan, was 3.24 kWh [42].

The non-renewable energy consumption was 72 kWh in a summer week and 24 kWh in a winter week. The expected energy consumption projection throughout the year was 1700 kWh with a floor area of 40 m2. Therefore, the yearly heating and cooling energy consumption per m2 was 42.5 kWh/m2 per year. If the average energy savings compared to an air-to-air heat pump with multi-split were 23%, the total non renewable energy consumption (NREC) savings accounted for 391 kWh/year. The average price of electricity in Spain is 0.12 EUR/kWh [22], and the system overcosts compared to traditional indoor wall partitions plus the split system can be 50 EUR/m2. For 24 m2 of radiant WFG panels, the expected payback period would be 20 years. WFG technology is not competitive nowadays, so future research is needed in industrialization and standardization to bring down the initial costs.
