## 5 Discussion  
The outdoor weather conditions of the office building are based on conditions in Guangzhou, China, which is in a hot summer and cold winter zone. The characteristic temperature method (CTM) is utilized for the research. Based on the building energy gene  
Figure 9: Indoor airflow simulation results under natural ventilation. (**a**) indoor air velocity. (**b**) indoor wind speed vector. (**c**) indoor airflow streamline. (**d**) Indoor airflow streamline (for comparison). theory [30], the dynamic load and energy consumption of buildings are simulated by CTM, and the relationship between load or energy consumption and various other factors can be revealed. According to CTM, if solar radiation gain is considered, the indoor characteristic temperature can be expressed by  
\[T_{\text{in}}=\frac{\sum K_{i}F_{i}T_{si}+\sum F_{i}I\Big{(}\eta_{i}+\frac{\alpha _{i}}{\alpha_{o}}\rho_{G}\Big{)}C_{i}\mp Q_{AC}}{\sum K_{i}F_{i}} \tag{13}\]  
where \(F_{i}\) is the building envelope area, m\({}^{2}\); \(T_{si}\) is the equivalent solar-air temperature, \({}^{\circ}\)C; \(K_{i}\) represents heat transfer coefficient, W/ \(\left(\text{m}^{2}\cdot\text{K}\right)\); \(I\) represents solar radiation, W/m\({}^{2}\); \(\eta_{i}\) is the transmittance; \(\rho_{G}\) is the absorption ratio; and \(C_{i}\) is the shading coefficient. \(Q_{AC}\) is the cooling or heating capacity provided by the air conditioner to maintain the indoor setting temperature within the thermal comfort zone [10]. On the other hand, if not considering solar radiation, \(I\) is equal to zero and \(T_{si}\) is approximately the outdoor temperature \(T_{out}\), \({}^{\circ}\)C. So, the expression is changed into  
\[T_{\text{in}}=\frac{\sum K_{i}F_{i}T_{air}\mp Q_{AC}}{\sum K_{i}F_{i}} \tag{14}\]  
This CTM approach for indoor temperature prediction and building load simulation has been validated in previous studies and verified by commercial building simulation software including DOE-2 (v 2.2, U.S. Department of Energy), EnergyPlus (v 23.2.0, U.S. Department of Energy) and DeST (v DeST-h, Tsinghua University, China) [11; 19; 31; 32]. For the other working conditions of building simulation, such as indoor air exchange rate (ACH) through a mechanical ventilation system, internal heat gains from indoor lighting, occupancy and devices, etc., key parameters are obtained based on the benchmark values according to the U.S. Department of Energy (DOE) Reference Building and ASHRAE AS (2013) Standard of thermal environmental conditions for human occupancy (building's daily office hours are 9:00-17:00) [33; 34; 35]. For the indoor thermal comfort evaluation, the integrated uncomfortable degree \(I_{\text{year}}\) is used as the index, which is defined as:  
\[I_{\text{year}}=\int \begin{matrix}\text{year}\\ T_{L}>T_{\text{in}}\end{matrix} (T_{L}-T_{\text{in}})d\tau+\int \begin{matrix} \text{year}\\ T_{\text{in}}>T_{H}\end{matrix} (T_{\text{in}}-T_{H})d\tau \tag{15}\]  
where \(T_{\text{H}}\) (26 \({}^{\circ}\)C) and \(T_{\text{L}}\) (18 \({}^{\circ}\)C) are the upper and lower indoor temperature limits of the thermal comfort zone, respectively. This evaluation index depicts the integrated degree of deviation of the indoor temperature from the thermal comfort zone [10; 11]. Based on the aforementioned building thermal performance modeling, the average indoor air temperature variations can be obtained with comparison under natural ventilation optimization (Figure 10). According to the thermal comfort demands, the upper temperature limit is set at 26 \({}^{\circ}\)C and the lower temperature limit at 18 \({}^{\circ}\)C. When indoor temperatures fall within this range, individuals experience a state of warmth and comfort, without feeling excessively cold or hot. The blue line represents the indoor temperature variations under natural ventilation conditions.