*4.2. Primary Energy Consumption*

Figure 14 shows the outdoor air temperature and the accumulated energy throughout five days in summer. WFG absorbed solar energy and prevented it from entering the building. The amount of energy absorbed by the water could be connected to the district heating network, geothermal boreholes, or to domestic hot water devices. In the final energy balance, the accumulated energy was subtracted from the cooling loads and added to the renewable energy production.

**Figure 14.** Energy absorption on eastern, southern, and western WFG facades. Sample summer days from 14 July 2020 to 18 July 2020.

Article 41 of the Energy Performance of Buildings Directive (EPDB 2018) recommended the use of primary energy factors to calculate the energy parameters of building envelopes [2]. Table 5 shows these energy factors, such as cooling energy demand (CED in kWh/m2), final energy consumption (FEC in kWh/m2), non-renewable primary energy consumption (NRPEC in kWh/m2). The energy absorbed by the water was considered as renewable primary energy production (RPE in kWh/m2) and CO2 emissions (kg CO2/m2). The electricity consumption of the circulation water pumps was not considered because they were connected to photovoltaic panels. The circulation pump was working 24 h per day. The primary energy factor (PEF) is the inverse of electricity production efficiency from fuel source to electricity at the building, taken from official European Union documents [42,43], if the energy was produced using heat pumps, considering a Coefficient of Performance (COP) of 2.5 and a conversion factor between final energy and non-renewable primary energy (KWh NRPE/KWh FE) of 1.954. The factor of emitted CO2 for electricity was 0.331.


**Table 5.** Primary energy balance of WFG in summer.

Table 6 shows the estimated winter heating loads over the working hours. Indoor (*T\_int*) and outdoor (*T\_ext*) temperatures were taken from Figure 10 with a surface area of 3.9 m2. The same procedure was repeated to calculate the values on five sample days. A high-performance triple glass made of three glass panes with an argon chamber and Low-E coating was compared with the selected WFG cases. The triple-glass *U* value was taken from a glazing catalog [41], whereas the *U* value of the WFG was defined in Table 1. The total heat losses through the passive triple glazing and the WFG were 288.38 Wh/m<sup>2</sup> and 15.31 Wh/m2. The energy savings per day was 273.07 Wh/m2. In addition, the WFG was able to produce renewable energy.


**Table 6.** Winter heating loads on 14 January 2020.

<sup>1</sup> Values are taken from Figure 10; <sup>2</sup> values are taken from [41].

Table 7 illustrates the primary energy savings, the reduction of CO2 emissions, and renewable energy production of WFG in five winter days.

**Table 7.** Primary energy balance of WFG in winter.

