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Analysis of indoor environment and performance of net-zero energy building with vacuum glazed windows

The total energy and indoor thermal environment of an office building, which aims at the net-zero energy building, were measured and analysed. The annual total primary energy consumption of ‘Measurement’ was smaller than the value of ‘Calculation’ at design phase and achieved net-zero. The result of analysis of the thermal environment shows that the comfortable thermal environment was maintained. Also, the insulation performance and heat balance of the vacuum glazed windows in winter was evaluated. The overall heat transfer coefficients calculated by using the monitoring data were almost equal to the rated overall heat transfer coefficient and the high insulation performance of vacuum glazed windows was maintained even in the second year’s operation. In addition, the amount of heat gain due to solar radiation on the window surface was much larger than the amount of heat loss due to transmission. The vacuum glazed windows with high thermal insulation performance on the south side can reduce the heating load and contribute to the achievement of net-zero in the buildings.


Introduction
In order to prevent global warming, which is one of the global environmental problem, reducing the energy consumption of building becomes an important issue [1,2]. As such there are a number of progressive technologies are under investigation for mass deployment, such as translucent vacuum insulation panel [3,4], triple vacuum glazed windows [5][6][7][8], advanced vacuum based photovoltaic solar thermal collectors [9] and utilising the waste heat into electricity using thermoelectric [10][11][12]. The reality of carbon capture and energy storage in buildings has also been investigated [13]. In Japan, with the aim of reducing energy consumption of the building sector, the compliance of energy conservation has begun to be mandatory for newly built non-residential buildings with a total floor area of 2,000 m 2 or more since April 2017. In addition, Net-Zero Energy Building (ZEB) is attracting attention as a building that is more energy efficient than a building that meets the energy conservation standards. ZEB is listed as one of the measures to reduce energy consumption in the building sector. In this paper, the authors measured and analysed an office building completed in Sapporo, Japan in January 2018. This target building is the first building certificated as (ZEB-Net-Zero Energy Building) in the cold climate region in Japan, and several energysaving technologies have been installed. As the most attractive point, the vacuum glazed windows were applied in all windows facing the outside air.
Vacuum glazed windows has a layer of vacuum insulation which is a space sandwiched between two glass plates [14] and is categorized as one of the vacuum insulation technologies [15]. The first idea of vacuum glazed window was proposed by Zoller in 1913 [16] and the first successful vacuum glazing sample was fabricated by Collins et al in 1989 [17]. In 2000s, Nippon Sheet Glass Group attempted the commercialization of vacuum glazed windows and vacuum glazing products called SPACIA were fabricated [18]. Currently, the market of vacuum glazing windows is dominated by three products supplied form Nippon Sheet Glass Group [19]. With regard to the experimental thermal performance investigation with vacuum glazed windows, Papaefthimiou et al. conducted an experiment with the guarded hot box calorimeter to evaluate the thermal performance of vacuum glazed window and reported the lowest U-value (Overall heat transfer coefficient) of 1.17 W m -2 K -1 [20]. Additionally, Fang et al. carried out a similar experiment [21]. However, the research works related to the evaluation of vacuum glazed windows installed in the actual building were hardly observed [22]. So far solder glass edge seal for vacuum glazed windows has been successfully at mass manufacturing by SPACIA, however other progressive methods such as laser sealing and bonding glasses mechanisms are being explored [23][24][25].
In this paper, the authors present the experimentally measured thermal transmission of vacuum glazed windows installed in the office building by using the heat flux meter. In addition, the indoor temperature and the ambient air temperature were measured, and the overall heat transfer coefficient was estimated by using the measurement data. Furthermore, the total energy and indoor thermal environment of this office building were analysed. Then, the contribution of vacuum glazed windows to ZEB was evaluated. Fig. 1 shows an appearance of ZEB (Office building) with vacuum glazed windows and Table 1 indicated the outlines of ZEB. This office building was completed in Sapporo, Japan in January 2018 and firstly authenticated as (ZEB Net-Zero Energy Building) in the cold climate region in Japan.  The energy saving technologies installed in the office building are indicated in Fig. 2 and Table. 2. The appearance of energy technologies heat recovery ventilation, GCHP, PV system and LED energy efficient lighting system installed is shown in Fig. 3. The envelope performance was improved by the insulation materials, the vacuum glazed windows, and the blind systems. The heat recovery ventilation system was installed and additionally reduces the heating and cooling load [26]. The ground source heat pump system was applied as the heat source for heating and cooling, and it can operate with high efficiency. In addition, the energy for lighting can be saved by the LED lightings and the illuminance control systems. The reduction of energy consumption could be calculated as approx.56% at design phase by combining these energy saving technologies.  Table 2 Energy saving components installed characteristics. Furthermore, the PV and battery storage system were installed in this building. The electric power generation by PV would be more than the total electric power consumption of building.

Vacuum glazed windows
The vacuum glazed windows were applied in all windows facing the outside air. The outlines of vacuum glazed window are shown in Fig. 4. The vacuum glazed window consists of three-layer glasses with the thickness of 3 mm. Two of the three glasses have the low-emissivity coating. One of the two layers sandwiched between glasses is the vacuum layer and the other was filled with the Krypton gas. These reduces the heat loss from windows and the rated overall heat transfer coefficient decreases to 0.68 W m -2 K -1 . The shading coefficient is 0.5.

Total energy
The monitoring points by BEMS are indicated in Table 3. The electric power consumption of each item was monitored. Also, the indoor temperature and humidity were monitored. Therefore, the electric power consumption of building was tried to minimize by checking the monitoring data. Table 3 Monitoring points by BEMS in the office building

Indoor thermal environment
The sensors independent of BEMS were equipped as shown in Fig. 5 and the indoor thermal environment of this building was evaluated. The specifications of sensors are indicated in Table 4. The temperature difference due to the difference in place was analysed by the temperature sensors 1~5. The vertical temperature distributions at point B and point C were measured by the thermo couples placed at height of 0, 0.1, 1.0, 1.2, 1.8 m. In addition, temperature, globe temperature, humidity and air velocity at point A were measured. By using these measurement data, the mean radiant temperature (MRT) and predicted mean vote (PMV) at point A were calculated. Measurement was carried out from February 7 th and 11 th in 2019.  Table 4 Sensors to evaluate thermal environment and their measurement accuracy.

Heat loss and gain via vacuum glazed windows
A heat flux meter was placed on the south-facing window inside surface as shown in Fig. 6, and the heat loss and heat gain via the window surface were measured. Also, the indoor air temperature and ambient air temperature were monitored. The sensors to evaluate vacuum glazed windows and their accuracy are listed in Table. 5. In the night, there is no heat gain due to the solar radiation and only the heat loss due to heat transmission is generated. Therefore, the overall heat transfer coefficient was evaluated by using the monitoring data of heat flux, indoor air temperature, and ambient air temperature in the night. In the daytime, the value of heat flux changes from negative to positive due to the solar radiation. The heat balance of the vacuum glazed windows in winter was analysed by applying the measured value of the heat flux in the daytime. Measurement was carried out from February 17 th to March 5 th (2 nd floor) and from March 5 th to March 20 th (3 rd floor) in 2019. In order to verify the long-term performance, additional measurement was carried out from December 10 th to December 27 th (2 nd floor) in 2019.   Fig. 7 shows the monthly primary energy consumption. It can be seen that the primary energy consumption for air-conditioning (AC) increases and the amount of power generation from PV decreases in winter season. The amount of power generation from PV is smaller than the total energy consumption from November to February. On the other hand, the amount of power generation is lager from March to October. Table 6 compares the annual primary energy consumptions of 'Baseline', 'Calculation', and 'Measurement'. Here, 'Baseline' and 'Calculation' indicate the annual primary energy consumption of standard building and this building, calculated at the time of design. The annual total primary energy consumption of 'Measurement' is smaller than the value of 'Calculation'. Especially for air-conditioning (AC) and Lighting (Light), the values of 'Measurement' are significantly smaller. This is due to the users' consideration for energy saving. Also, the annual primary energy consumption of 'measurement' was reduced by more than 60% compared to the value of 'Baseline' in all items except for others and the total achieved net-zero. In Japan, when the net energy consumption becomes less than zero except for others as shown by 'Calculation', the building is certified as (ZEB). However, the net energy consumption achieved zero even if 'Others' was included in 'Measurement'.  Table 6 Annual primary energy consumptions of 'Baseline', 'Calculation', and 'Measurement' in FY2018 and reduction rate of primary energy consumption

Indoor thermal environment
As measurements result on the representative day, Fig. 8 shows the variations of indoor air temperature at point 1~5 indicated in Fig. 5 and ambient air temperature on February 8 th and 9 th in 2019. February 8 th was the coldest day of 2019 in Sapporo. The room temperature became the lowest around 8:00, when heating started, and reached 15~17 o C. After the start of heating, the temperature rose and reached 20 o C around 12:00. The temperature difference at each measurement point was 2 o C or less, and it was confirmed that the temperature difference was small in the interior. As shown in Fig. 9 is the variations of indoor air temperature, MRT, and PMV at point A indicated in Fig. 5 on February 8 th and 9 th in 2019. The MRT was almost equal to the indoor air temperature. PMV increased with temperature rising and became larger than -0.5 in the time zone above 20 o C. Fig. 10 are the vertical temperature distributions at point B (Perimeter zone) and point C (Interior zone) indicated in Fig. 5 on February 8 th in 2019. Except for 12:00, the temperature at point C is approximately 2 o C higher than that at point B. In addition, the temperature at height of 0 and 0.1 m is almost the same as the others at point B and the temperature at height of 0 and 0.1 m is approximately 1 o C higher at point C. From this, it was confirmed that the underfloor air-conditioning could prevent the temperature decrease at the feet. It can be said that the comfortable thermal environment was maintained even though the temperature in the morning was low because of the users' highly conscious for energy saving.

Heat loss and gain via vacuum glazed windows
As measurement results at night on the representative day, Fig. 11 show the variations of heat flux, indoor air temperature, and ambient air temperature. It can be seen that there is no large variation of heat flux. By using the measurement result, the average overall heat transfer coefficient was calculated. The average overall heat transfer coefficients of centre of vacuum glazed windows were estimated as 0.64 W m -2 K -1 (Fig. 11(a), 0.62 W m -2 K -1 ), (Fig. 11(b), 0.62 W m -2 K -1 ) and Fig. 11(c), respectively. These overall heat transfer coefficients were almost equal to the rated overall heat transfer coefficient 0.68 W m -2 K -1 . In addition, there was no decrease of average overall heat transfer coefficient between Fig. 11(a) and Fig. 11(c). This result indicates that the high insulation performance of vacuum glazed windows was maintained even in the second year's operation. Next, Figure 12 introduce the variations of heat flux, surface temperature on window. The calculated values of heat gain are also indicated in Fig. 12. Here, the heat gain Qgain is calculated by the following equation.

= × ×
The vertical solar radiation on the south side is calculated by using the measurement value of horizontal solar radiation. The absorption rate of solar radiation and the shading coefficient of window are the constants ( = 0.9, = 0.5). Comparing the measured values with the calculated values, it was confirmed that the errors between the measured values and the calculated values were within 20% on both the 2 nd and 3 rd floors. Therefore, it can be said that the heat gain due to solar radiation can be estimated. The amount of heat gain due to solar radiation on the window surface in winter was calculated by using the vertical solar radiation, and the amount of heat gain was compared to the heat loss due to the transmission on the window. Fig. 13 is the monthly average ambient air temperature and vertical solar radiation on the south side and Fig. 14 shows the amount of heat gain due to solar radiation and heat loss due to transmission from inside to outside. In every month, the amount of heat gain due to solar radiation was larger than the amount of heat loss due to transmission, and the total amount of heat gain in winter was 218.5 kWh m -2 , and the total amount of heat loss was 51.6 kWh m -2 . From the above, it was found that even in cold regions, the vacuum glazed windows with high thermal insulation performance on the south side can reduce the heating load and contribute to the achievement of net-zero in the buildings.

Conclusions
The total energy and indoor thermal environment of an office building, which aims at the netzero energy building, were measured and analysed. Also, the insulation performance and heat balance of the vacuum glazed windows in winter was evaluated. As the results, the following findings were obtained.
1) The annual total primary energy consumption of 'Measurement' was smaller than the value of 'Calculation' at design phase and achieved net-zero even when the annual primary energy consumption of 'Others' was included. This is due to the users' consideration for energy saving.
2) In the coldest day in Sapporo, the room temperature became the lowest around 8:00, when heating started, and reached 15~17 o C. However, the temperature rose and reached 20 o C around 12:00 after the start of heating. Then, PMV increased with temperature rising and became larger than -0.5 in the time zone above 20 o C. The comfortable thermal environment was maintained even though the temperature in the morning was low because of the users' highly conscious for energy saving.
3) These overall heat transfer coefficients calculated by using the monitoring data were almost equal to the rated overall heat transfer coefficient and the high insulation performance of vacuum glazed windows was maintained even in the second year's operation. In addition, the amount of heat gain due to solar radiation on the window surface was much larger than the amount of heat loss due to transmission. Therefore, the vacuum glazed windows with high thermal insulation performance on the south side can reduce the heating load and contribute to the achievement of net-zero in the buildings.