Investigation of Heat Storage Coupled with Night Ventilation for Office Buildings in Summer in Cold Regions of China

Building operational energy consumption, which constitutes a significant portion of China’s total social energy consumption, is a top priority in current research on energy-saving buildings. Thermal mass can be utilized to store heat energy in buildings, resulting in energy flexibility on the demand side. Numerical simulations in this paper study the whole heat transfer process of building thermal mass and the distribution of indoor air temperature and thermal mass temperature under different ventilation conditions in Jinan, a typical city in cold regions of China. By implementing a night ventilation scheme, buildings with high thermal storage capacity can store substantial amounts of cooling energy, resulting in a lower indoor temperature on the following day, and thereby improving overall indoor comfort. During the day, the wall surface temperature drops to less than 302 K, and the indoor temperature decreases by 2∼4 K, while the wall absorbs 1535 kJ of indoor heat. The rule of heat storage and release of heat mass is studied in this paper, and the feasibility of energy use flexibility on the demand side is demonstrated.


Introduction
Buildings in China are responsible for roughly 30% of the country's total energy consumption.One way to improve energy flexibility on the demand side is by utilizing thermal mass in building design [1] .Thermal mass is capable of storing thermal energy, absorbing heat during the day, and releasing heat at night.This time lag between heat absorption and release can help shift peak heat loads, thereby increasing energy flexibility.However, the effectiveness of thermal mass in reducing cooling demand is climate-dependent.An effective measure to increase cooling energy storage capacity is night ventilation.By bringing cool outdoor air indoors, thermal mass can be flushed, creating a heat sink that absorbs space heat from the room.Consequently, cooling requirements are greatly reduced [2] .
Limited research on nighttime cooling, considering thermal mass, can be attributed to the lack of simplified yet accurate models to quantify the amount of thermal mass storage under real climate conditions [3] .Although field measurements are an effective method for detecting thermal mass storage heat, arranging sensors in thermal mass storage can be challenging [4] .Moreover, only air temperature was measured in most experiments, making it difficult to distinguish the effect of thermal mass from other influences.EnergyPlus [5] is well-suited for studying transient behavior.The simulation of internal thermal mass in EnergyPlus is achieved through simplification.While such oversimplification can lead to an overestimation of the energy storage effect of internal thermal mass.In contrast, numerical methods [6] such as FVM can obtain the temperature and velocity fields of the internal thermal mass and air inside the room for realistic temperature boundary conditions.Therefore, this method is widely used to investigate nighttime cooling of internal and external thermal mass.
In this study, the finite volume method was utilized to analyze the indoor air temperature distribution, flow field distribution, and temperature distribution in thermal mass under various ventilation conditions, based on actual climate conditions in Jinan, a typical city in cold regions of China.The law of heat storage and heat release of thermal mass was also determined.The findings of this study demonstrate the feasibility of adjusting energy use flexibility on the demand side.

Physical model
In this study, an office building in Jinan, a representative city in the cold region, is selected as the subject of research.The simulated object is an office within the building with dimensions of 4500 mm × 3500 mm × 2700 mm (length × width × height), a 390 mm thick external wall, and a single-glass plastic steel window measuring 1500 mm × 1000 mm arranged on the south wall.To simplify the air supply system, only the outlet speed is considered, and a pair of slit-type air outlets are employed.The air supply inlet is located on the lower side of the south wall, while the air exhaust outlet is situated on the higher side of the south wall.A two-dimensional transient model is established and numerically simulates the radiation process of the building, as well as the indoor heat transfer and flow process, as illustrated in Figure 1.

Figure 1. Physical model of the room
The velocity inlet must specify the air velocity, temperature, and ventilation duration.In order to investigate the indoor air temperature distribution under varying ventilation conditions, three distinct scenarios were set up.Specifically, Case-1 entailed no ventilation, Case-2 involved continuous ventilation throughout the day, and Case-3 featured ventilation solely during the night.In cold regions, winter heating energy consumption is substantial, and the building envelope typically employs energysaving external insulation.The physical parameters and layout of the north and south exterior walls, floors, and ceilings are presented in Table 1.
Table 1.Thermo-physical properties of materials.

Mathematical model
The heat transfer mechanisms between the outdoor environment and the building envelope, as well as between the indoor and outdoor environments, are complex.These mechanisms include heat conduction within the walls, convective heat transfer between the indoor and outdoor walls and the indoor and outdoor air, radiation heat transfer through the external windows, and radiation heat transfer between the walls.
To simplify the calculations, the following assumptions were made in this study.
(1) The walls are considered a multi-layered structure with a constant physical composition, and the wall material is assumed to be isotropic.(2) The contact thermal resistance between the different layers of the wall material is neglected.
The specific equation is shown below.
(1) Mass conservation equation (2) Momentum conservation equation (3) Energy conservation equation ( ) The initial conditions and boundary conditions are as follows.
(1) Initial conditions: Set the initial temperature to T0=303.2K (2) Boundary Conditions The boundary conditions of convection and radiation coupling are applied to the outer wall surface and the outer wall surface of the external windows.Both the roof and the outer surface of the floor are adiabatic boundary conditions.

Simulation methods and model validation
Unsteady state simulations are performed using a pressure-based solver.The SIMPLE algorithm is utilized for the pressure-velocity coupling method.
The numerical model utilized in this study was applied to the same physical model as that in [7] for calculation.As shown in Figure 2, the maximum relative error was within 2%, indicating the reliability of the model used in this study.

Results and Discussion
Figure 3 displays a comparison between the temperature of the inner wall surface of the south wall and the indoor air temperature under different ventilation conditions, namely Case-1, Case-2, and Case-3, which correspond to no ventilation, all-day ventilation, and night ventilation, respectively.As depicted in Figure 3(a), the average temperature of the inner wall surface of the south wall under Case-1 and Case-2 is nearly identical.However, the fluctuation degree is smaller for Case-1 compared to Case-2, because the ventilation transfers a significant amount of energy for heat exchange with the inner wall throughout the day and night.The temperature fluctuation of the inner wall is 2°C lower under Case-1.The temperature of the inner wall fluctuates less than that of the air, because ventilation and radiation entering the room from outside first change the temperature of the air, which then exchanges heat with the inner surface of the wall.Thus, the temperature amplitude of the indoor air will be larger than the temperature amplitude of the inner wall of the envelope.In Fig. 3(b), the indoor air temperature drops from 0:00 to 6:00 at night, and the temperature of the wall inside the wall drops accordingly.This decrease is mainly due to the introduction of cool outdoor air to flush the surface of the internal thermal mass and reduce the heat storage of the indoor thermal mass.After 7 h, the indoor temperature of Case-2 rises faster than that of Case-1, and after 14h in the afternoon, the temperature of Case-2 drops faster than that of Case-1.
Figure 4 shows the variation of heat flux density and internal energy of the south wall.Analyzing the time when the heat flux density of the south wall changes from releasing to absorbing heat, it can be seen that Case-2 is earlier than Case-1, and Case-3 is the latest.This is mainly because Case-2 uses allday ventilation, allowing the high-temperature outdoor air to flow into the wall gradually and absorb heat.Case-3, which uses night ventilation, is later than Case-2 because it accumulates a large amount of cooling energy in the heat mass during the night, and the indoor air temperature is correspondingly lower than that of Case-2.Therefore, the wall will only begin to absorb heat when the indoor air temperature rises above 301K at around 11 am.The amplitude of heat flux density of Case-1 is much greater than that of Case-2 and Case-3, with the maximum heat flux density of absorption occurring at 13:00, which is 11.58 W/m 2 , and the maximum heat flux density of releasing occurring at 3:00, which is 12.16 W/m 2 .The wall ends the storage of heat at 21:00, 21:30, and 22:00 for Case-1, Case-2, and Case-3, respectively.As can be obtained by integrating the heat flux density in Figure 4 (a), the wall absorbs 1535 kJ of heat from the indoor environment.From Figure 4 (b), it can be seen that the average internal energy of the wall under Case-3 is much lower than that of Case-1 and Case-2, ranging from 16,000 kJ, while it is 24,000 kJ for Case-1 and Case-2.When no ventilation or all-day ventilation is adopted, the fluctuations of the internal energy of the wall in both Case-s are at a higher level.Night ventilation can significantly allow the building wall to store a large amount of cooling energy, reducing the internal energy of the wall by 6769 kJ and 7289 kJ compared to no ventilation and all-day ventilation, respectively.Thus, it enables the wall to absorb a large amount of heat from the indoor environment on the second day.

Conclusion
In this study, the heat transfer process in a building room under three different working conditions was simulated: without ventilation, with all-day ventilation, and with night ventilation.By comparing the indoor temperature distribution, the temperature distribution of the wall surface inside the heat storage body, the heat flow density distribution, and the energy change inside the heat storage body, the following conclusions were drawn: (1) The adoption of night ventilation in the Jinan area can significantly reduce the indoor temperature of the next day and improve comfort.It can reduce the daytime room temperature by 2-4 K compared to not using ventilation or using all-day ventilation.
(2) Rooms with walls that have high heat storage capacity can absorb heat from indoor air at a heat flux of 5 to 10 W/m 2 during the day due to their high thermal mass.The cumulative daytime heat absorption can reach 1535 kJ when night ventilation is adopted.
(3) The use of night ventilation can significantly increase the storage of cooling energy in building walls, resulting in a reduction of 6769 kJ and 7289 kJ in the internal energy of the walls compared to no ventilation and all-day ventilation, respectively.

Figure
Figure 2. Model validation

Figure 3 .
Figure 3.The temperature of wall surface(a) and room air(b)