Development of an automatic opening system for sliding and casement windows to control the natural ventilation rate.

. Under the influence of COVID-19, ventilation is recommended to reduce the risk of infection in the room. Window openings can increase the ventilation rate caused by indoor and outdoor temperature differences. However, there is a concern that opening the window in the air-conditioned room will increase the heating and cooling loads due to air leakage. In addition, it is difficult to maintain an adequate ventilation rate because the outdoor temperature changes from time to time. To solve this problem, we have developed an automatic opening system to control the natural ventilation rate. This system can control the adequate ventilation rate for the room by adjusting the opening area of the window automatically. In this study, the control equations for the system for sliding windows were derived, and actual measurements were taken with the system. Additionally, the flow coefficients of the casement windows were numerically analyzed. Then, the control equation for the system for casement windows was derived from the relationship between the opening angle and the flow coefficient. As a result, the ventilation rate could be controlled using an automatic opening system for the sliding windows. Also, the flow coefficient of the casement window could be approximated by a sinusoidal function of the opening angle.


Introduction
It has been reported by ASHRAE [1] and others that securing adequate ventilation is effective in reducing the risk of infection, such as COVID-19, in the room. Natural ventilation by opening windows and doors is the simplest method to increase the ventilation rate when there is an indoor and outdoor temperature difference (see Figure 1). However, some concerns about opening the window in the air-conditioned room may be affected by the outdoor temperature, resulting in poor heating and cooling efficiency and deterioration of the indoor thermal environment. Also, excessive ventilation may increase the heating and cooling load and power consumption. In addition, it is difficult to maintain an adequate ventilation rate because the outdoor temperature changes from time to time. Therefore, it is not easy to maintain the proper ventilation rate by manually opening and closing the windows.
To solve this problem, we developed an automatic window opening system for sliding and casement windows to control the natural ventilation rate. In this study, the control equations of the system for sliding windows were derived, and actual performance was measured with the system. In addition, the flow coefficients of the casement windows were numerically analyzed, and the control equations of the system for casement windows were derived.  Figure 2 shows diagrams of the developed automatic opening system. This system can control the adequate ventilation rate for the room by adjusting the opening area of the window automatically. In the case where indoor and outdoor temperature differences are large, the opening area is small. In the case where indoor and outdoor temperature differences are small, the opening area is large. In this way, the natural ventilation rate is maintained.
Using the equation from Brown et al. (Equation 1) [2], the ventilation rate can be calculated. Then, the opening width was calculated according to the ventilation rate, the opening height, and the temperature difference. In this way, the control equation for the system for sliding windows (Equation 2) was derived.

Actual measurement methods
In this study, actual measurements were conducted to understand the operating performance of the system. To confirm the indoor thermal environment, measurements were taken in the summer. The measurement was conducted on September 11, 2021. Results were compared between the automatic window opened by the system and the ordinary window opened at 95 mm in width. The width of the automatic window opened by the system was adjusted every 30 seconds. The ventilation rate controlled by the system was set at 30 m 3 /h per person [3][4][5]. Therefore, in Equation 2, Cs was set to 2/3, Q to 30 m 3 /h, and S to 1. Also, there are two types of window openings: constant opening and intermittent opening. In the case of intermittent opening, the concentration of pollutants in the room increases while the window is closed. So, the actual measurements were conducted with the window constantly open. Figure 3 shows the target experimental building in Toyama. Figure 4 shows a cross-sectional view of the temperature and concentration measurement points. Both rooms have a floor area of 6.25 m 2 , a room volume of 15 m 3 , and a window height of 2 m. In both rooms and the corridor, the air conditioner was set to 22 ºC. The cooling capacity of the air conditioner was 2.2 kW. The ventilation rate using the constant concentration method, indoor temperature distribution, and power consumption were measured. In addition, to understand the external environment, the outdoor temperature, wind direction and speed, and solar radiation were measured. Figure 5 shows the measured ventilation rate. According to the results, in the case of the automatic window opened by the system, the ventilation rate was generally maintained at 30 m 3      daily ventilation rate was measured to be about 28.9 m 3 /h. On the other hand, in the case of the ordinary window opened at 95 mm in width, the average daily ventilation rate was measured to be about 50.0 m 3 /h. As the outdoor temperature increased, the ventilation rate increased, with the maximum ventilation rate being 124.6 m 3 /h. Figure 6 shows the results of the temperature changes and the opening width of the window using the system. The maximum indoor and outdoor temperature differences were measured at 9.9 ºC in the room with the automatic window opened by the system. The system adjusted the opening width according to the indoor and outdoor temperature differences. It was confirmed that the ventilation rate could be controlled by using the system. Figure 7 shows the horizontal temperature distribution in the room at 10:00 and 13:00. Figure 8 shows the vertical temperature distribution in the cross-section of the opening at 13:00. The indoor temperature in both rooms was generally kept at the set temperature of 22 ºC. The cooling capacity of the air conditioner was high relative to the room volume, so it is considered that the effect on the indoor temperature was small. However, in the case of the ordinary window opened at 95 mm in width, the indoor temperature increased slightly due to the high outdoor temperature and increased air leakage. Figure 9 shows the power consumption of the air conditioner in each room. There was no significant difference between the two rooms, and the air conditioner operated at about 0.1 kW to 0.15 kW during the daytime when power consumption was high. However, in the case of the ordinary window, which opened at 95 mm in width, it operated at about 0.18 kW from 13:00 to 14:00. Due to increased air leakage, the cooling load temporarily increased.

Numerical analysis method
To predict ventilation rates, it is necessary to know the flow coefficient. However, the flow coefficient for each opening angle of casement windows is unclear. In this study, the flow coefficients of casement windows were analyzed. Then, the relationship between the opening angle and the flow coefficient was clarified. Figure 10 shows the types of casement windows for which analyses were conducted: single-casement windows (a) and vertical-pivoted windows (b), and the analysis model. Table 1 shows the analysis conditions. Opening angles ranging from 5 º to 90 º were changed at 5-degree intervals. The flow rate through the window was analyzed.     Figure 11 shows the results of the flow coefficient from the analysis. The flow coefficient increased with the opening angle. The flow coefficient of the casement window could be approximated by a sinusoidal function of the opening angle. In this study, the flow coefficient was calculated from the pressure difference, but the actual ventilation rate varies with the wind pressure. Casement windows open outward and catch the wind blowing along the wall, which may increase ventilation rates depending on wind direction. Therefore, the study will also be conducted about wind pressure and wind direction.

The control equation for casement windows
Based on the results of the numerical analysis, the flow coefficient of the casement window could be calculated by Equation 3. Equations 4, the control equation for the system for casement windows, was derived using Equation 1 and Equation 3. Equation 5 is a relational expression for window-dependent variables. Cc and a were inversely proportional. Figure 12 shows the estimated opening angles. For calculations, the actual measured indoor and outdoor temperature changes on September 11, 2021, were used. In Equation 3, a was set to 0.62, and in Equation 4, C c was set to 0.43, Q to 30 m 3 /h, w to 0.3 m, h to 1.22 m, and S to 1. According to the results, it was found that during the daytime, when indoor and outdoor temperature differences are large, the opening angle is adjusted to be small. At night, when indoor and outdoor temperature differences are small, the opening angle is adjusted to 90°, which is the full opening angle. In the next study, actual measurements will be taken to confirm that the ventilation rate can be controlled by using the system.

Conclusion
In this study, we developed an automatic opening system for sliding and casement windows to control the natural ventilation rate. The following findings were obtained from this study: (1) The Brown et al. equation could be used to construct the control equation for the system. (2) The developed system for sliding windows was able to control the ventilation rate. In summer, the system maintained the natural ventilation rate at around 30 m 3 /h throughout the day, and excessive ventilation during the daytime could be reduced. Although the flow coefficient was calculated based on the pressure difference, the actual ventilation rate varies with the wind pressure. Therefore, the study will be conducted about wind pressure and wind direction. Also, actual measurements will be taken to confirm that the ventilation rate can be controlled by using the system for casement windows.