Heat Release Simulation Research Based on Solid Heat Storage and Release Materials

To promote the actual investment of solid electric heat storage heating devices, three different heat storage and heat release materials are used to simulate and analyze the heat release. The results show that “magnesia brick has the best heat storage and release capacity in 10 hours”. In terms of average temperature performance, silica brick is the best, and the standard deviation of temperature reaches the maximum value at 8 h, which is 75.13 k. It is less than the maximum value of 99.70 k for magnesia brick and 114.93 k for solid waste ceramic brick. However, the manufacturing cost of silica brick is the highest, and the cost of solid waste ceramic brick is the lowest. Solid waste ceramic bricks can be preferentially selected under the condition of meeting the requirements of heat storage and release.


Introduction
A solid electric heat storage heating device is a device that uses abandoned wind power and night power for heating.At the same time, it is an energy storage device that can heat industrial and commercial buildings, residential buildings, and other buildings at different times or all day [1][2][3].It can provide peak-shaving auxiliary services and implement the integration of source network, load, and storage [4][5][6].
At present, the commonly used heat storage and release materials include magnesia brick, silica brick, and solid waste ceramic brick [7][8].In the application scenario, the installation site of some equipment is small and requires high heat storage and release capacity.For irregular wind power absorption and large absorption capacity, low-cost heat storage and release materials are required [9].Scenarios with high frequency and long-time use require heat storage and release bricks with good uniformity, to strengthen the feasibility of putting it into practical application, reduce costs, and promote carbon peak and carbon neutralization.The performance analysis of heat storage and release materials is extremely important.

Physical model
The main structure of the solid electric heat storage heating device is composed of a heat storage and release brick stack, wind protection area, warm shell, air inlet and outlet, and internal air chamber.As shown in Figure 1, the length is 3, 153 mm, the width is 1, 900 mm, and the height is 1, 500 mm.The volume of heat storage and release brick stack is 4.42 m 3 .The porosity of the ventilation duct is 18.35%, and the area of the air inlet and outlet is 0.12 m 2 .

Mathematical model
During the heat release process of the solid electric heat storage heating device, heat convection and heat conduction are mainly used, and the heat radiation effect is ignored [10].
When the device is in an exothermic state, the heat absorbed by the heat exchanger is equal to the heat released by the thermal storage brick stack minus the heat lost by the equipment.It can be expressed by Formula (1): where s Q is the heat released by the heat storage and release brick stack; c Q is the heat absorbed by the heat exchanger;  is the thermal efficiency.
During a period of heat release, the heat ) (Q s released by the heat storage and release brick stack can be expressed as: where c is specific heat capacity and m is mass; 1

T and 2
T is the initial and intermediate temperature of heat storage and release brick stack.
The heat absorbed by the heat exchanger ) (Q s can be expressed as: T are the inlet and outlet temperature of cold fluid.After we substitute Formula (1) and Formula (2) into Formula (3), the temperature of thermal storage and release brick stack at a certain time can be obtained as:

Meshing and boundary condition setting
The solid electric heat storage heating device is divided into a fluid area, heat storage, release area, and wind protection area by the tetrahedral structured grid.
The airflow rate at the inlet boundary condition of the solid electric heat storage heating device is 3 m/s, and the air inflow temperature is 373.15 k.The outlet is a pressure outlet, the wall is insulated, the interface between the fluid and the solid surface is a thermal-fluid-solid coupling interface, and the initial temperature of the thermal storage and release brick is 1, 073.15 k.The physical characteristic parameters of different heat storage and release materials are shown in Table 1.

Analysis of heat storage and release capacity
Due to the different specific heat capacities and thermal conductivity of different heat storage and release materials, the heat stored at the same temperature is different.At the same time and under the same exothermic conditions, the heat that can be released will also be different.The 10 h transient heat release process of three different heat storage and release materials is shown in Figure 2.
It can be seen from the above figure that the average temperature of the three kinds of heat storage and release bricks decreases approximately linearly with the extension of heat release time.In the exothermic process of 10 h, the average temperature is magnesia brick > solid waste ceramic brick > silica brick.It is concluded that the thermal storage performance of magnesia brick is better than that of solid waste ceramic brick, and that of solid waste ceramic brick is better than that of silica brick.During the 10 h heat release period, the average wind temperature of the flow field is shown in Figure 3, and the temperature of the magnesium brick flow field is the highest throughout the process.In the first 6 hours, the average temperature of the flow field of silica brick is higher than that of solid waste ceramic brick, and in the last 4 hours, the opposite is true.At 6 h, the flow temperature of silica brick and solid waste ceramic brick is approximately 559 k.From this, we can conclude that magnesia brick has the best exothermic performance, and silica brick is slightly higher than solid waste ceramic brick.

Average temperature performance analysis
The temperature standard deviation and uniformity index are calculated by simulating and measuring the surface temperature of the brick stack to simulate the temperature uniformity of the actual thermal storage brick.
where R is the temperature standard deviation; n is the number of temperature measuring points of thermal storage structure; x i is the temperature of each measuring point of the thermal storage structure.
It can be seen from Figure 4 that the standard deviation of the temperature of silica brick and magnesia brick is close to the same before 2 h, and that of silica brick is smaller than magnesia brick after 2 h.The maximum value reached 75.13 k at 8 h.The maximum temperature standard deviation of magnesia brick is 99.70 k, which is higher than the maximum temperature standard deviation of silica brick 24.57k.The standard deviation of solid waste ceramic brick temperature is the largest, which is greater than the other two materials.From the uniformity index, because the thermal conductivity of solid waste ceramic brick is the lowest, the uniformity index of solid waste ceramic brick is the lowest.The uniformity index of silica brick is lower than that of magnesia brick before 4.5 h, and gradually higher than that of magnesia brick after 4.5 h.It can be seen that the temperature uniformity of silica brick is the best.

Magnesia brick
Silica brick Solid waste ceramic brick Figure 5. Instantaneous temperature distribution diagram of thermal storage and release brick stack As can be seen from Figure 5, most of the temperature of silica brick is distributed in 611-801 k, with the most uniform distribution and small temperature span; most of the temperature of magnesia brick is distributed in 831-1, 027 k, which is relatively uniform, and the temperature span is larger than that of silica brick; most of the temperature of solid waste ceramic bricks is distributed in 667-960 k, which is uneven and has the largest temperature span.It is concluded that silica brick is superior to magnesia brick and solid waste ceramic brick in terms of temperature uniformity.

Conclusion
1.In this paper, three kinds of heat storage and release bricks were analyzed for 10 hours, and the results showed that the magnesium brick had the best heat storage and release performance.
2. In terms of temperature uniformity, silica brick has the best temperature uniformity, and the standard deviation of temperature reaches the maximum value at 8 h, which is 75.13 k.It is far smaller than magnesia brick and a solid waste ceramic brick.However, the manufacturing cost of silica brick is the highest, and the cost of solid waste ceramic brick is the lowest.Solid waste ceramic bricks can be preferentially selected under the condition of meeting the requirements of heat storage and release.

Figure 1 .
Figure 1.Schematic diagram of heat storage and release structure 3) where h M is the mass flow rate of cold fluid; h p c . is the specific heat capacity of cold fluid  

Figure 2 .
Figure 2. Residual average temperature diagram of heat storage and release brick stack

Figure 3 .
Figure 3. Flow field average temperature monitoring diagram

Figure 4 .
Figure 4. Standard deviation and uniformity index change curve

Table 1 .
Physical characteristic parameters of different heat storage and release materials