Phase change thermal storage technology analysis

有机相变材料 ,另一类是 无机相变材料 Global research on phase change materials has a history of decades. At present, phase change materials are basically divided into two categories, one is organic phase change material , and the other is inorganic phase change material .

The biggest advantage of organic materials is good stability, but low energy storage density, poor thermal conductivity, and high cost; the advantages of inorganic materials are high energy storage density, good thermal conductivity, low cost, but poor cycle stability, and some even fail after a few cycles It cannot be commercialized. A new energy material company in Jiangsu has made a major breakthrough in the stability of inorganic phase change materials, and no attenuation has been seen for more than 5,000 cycles of the material, so it can be well implemented.

1.1 Phase change thermal energy storage

If phase change materials are applied in the field of energy storage, direct use of materials is difficult to achieve in engineering projects. Often, a specific container needs to be filled with phase change materials, and a heat exchange system is required for phase change materials and heat transfer media. Heat exchange between. In addition, sufficient insulation is required around the container. Phase change thermal storage equipment is called thermal storage. Inorganic phase change nanocomposites have a phase transition temperature of about 78 ° C and a rated heat storage of 650MJ.

The external shape of the thermal storage is a cubic structure, the outer contour size is 942mm × 942mm × 1835mm, and the weight is 2.1t. There is a special steel liner inside. The liner is filled with phase change material. Copper heat exchangers are evenly arranged in the material. The outside of the liner is a polyurethane insulation layer with a thickness of 50mm. The appearance of the thermal storage and its structural principle are shown in Figure 1.
Figure 1: Appearance and structural principle of thermal storage

For thermal storage equipment, the quality of the thermal storage capacity, the performance of charging and releasing heat, the efficiency of charging and discharging, and the performance of heat loss directly affect the availability of the device. The thermal storage product was entrusted to the National Air Conditioning Quality Inspection Center of the Chinese Academy of Building Sciences to test the above thermal performance, and conducted constant heating power and constant water inlet temperature heat release tests, constant water inlet temperature heat and constant power discharge. The thermal test and the thermal loss test have a total of three tests. The experimental results are analyzed as follows.
Table 1 shows the results of constant heating power and constant water temperature exothermic test. It can be seen from the data that the entire heating time is 377min, less than 7h, the average heating power is 28.38kW, and the heating capacity is 641.93MJ; when the heat is released, the temperature is 20 ° C and the maximum power is 258kW The heat release is 639.49MJ, which is less than 2% from the rated heat storage of 650MJ, the charge and heat release efficiency is as high as 99.6%, and the electrical energy conversion rate is 95%.

Table 1: Constant heating power and constant water temperature exothermic test results

Table 2 shows the results of constant water temperature charging and constant power exothermic test. It can be seen from the data that under the condition of constant temperature inlet water heating at 90 ° C, the heating time is only 224min, and it can be filled in less than 4h. With a constant exothermic power of 9.9kW, the exothermic time is as long as 957min (about 16h).

Table 2: Test results of constant water inlet temperature heating and constant power exotherm

In the heat loss performance test experiment, under the ambient temperature of 20 ° C, the heat loss of the thermal storage in 24 hours is 4.85%.

1.2 Valley electricity heat storage heating system
Figure 2 shows a schematic diagram of a typical valley electricity heat storage heating system. The working principle is that the electric boiler works to heat the circulating water during the valley electricity period, and the heat in the hot water is brought into the thermal storage by the circulation pump to heat the thermal storage. If heating is required at the end of the night, the secondary side is directly operated, and the required heat is directly provided by the electric boiler. During the non-valley period, the circulation pump works to bring out the heat in the thermal storage. The heat is transferred to the secondary side through plate replacement to provide heating for the end.

Figure 2: Schematic diagram of Gudian heat storage heating system

Comparison of Valley Power Phase Change Thermal Storage Heating with Other Heating Technologies
At present, winter heating in the cold regions of northern China generally adopts the following methods: municipal heating, gas boilers, direct electric heating, ground source heat pumps, air source heat pumps, etc. Most of the municipal heating is centralized coal-fired hot water boilers, or heat is provided by thermal power plants around the city. Due to the limitation of the heating pipe and heat source heating capacity, the municipal heating has been under the pressure of rapid urban expansion in recent years. The construction of the municipal heating pipe network and the heating capacity are far from meeting the actual heating needs.
     At the same time, under the pressure of environmental governance, some small and medium-sized coal-fired boilers are being converted into gas-fired boilers, which has significantly increased the cost of use. Gas heating has developed rapidly in recent years, but like municipal heating, gas heating relies heavily on the city's infrastructure, imbalances in the development of gas pipeline networks, gas sources, and other factors have great constraints on gas heating, and the cost of gas is high. , And there are great security risks.
     There are many types of direct electricity heating, such as heating cables, carbon crystal heating, electric heaters, etc. The disadvantage is that the operating cost is high. The advantage of ground source heat pump is that the operating cost is lower, but the initial investment cost is very high. And, because of the large amount of construction projects and the inability to transform existing buildings, this type of heating is generally used less often. Air-source heat pumps have emerged in recent years, but due to the power and usage effects of their units, they are currently only applicable to areas with low heating requirements in rural areas such as northern China.
The valley electricity phase change heat storage heating scheme is adopted to take advantage of the lower electricity price in the valley period, which saves users on operating costs. At the same time, the system has a low dependence on infrastructure. In commercial and office buildings, there is basically no need to increase power capacity. The entire system is safe and reliable, runs automatically, and requires no maintenance.

Valley electricity phase change heat storage heating application
(1) Project overview
Tianjin Shuiyoucheng Commercial Center used municipal hot water for heating. The height of the building was charged according to the building area of 130,000 m², the heating fee was 40 yuan / m², and the total heating season cost 5.21 million yuan. At present, the project has implemented an electric boiler + thermal storage heating system. During the night Gudian (23:00 to 7:00 the next day), the thermal storage is heated while the space is frozen and insulated, and the thermal storage is used at other times. Heating, individual extreme weather is supplemented by flat electricity.
(2) System design
According to the analysis of the actual daily heat consumption data of Tianjin Shuiyoucheng in the two heating seasons from 2012 to 2014, the average daily heat demand of the commercial body during the day and night is shown in Table 3. From the data, it can be seen that the heat consumption in 2012-2013 is larger than that in 2013-2014, which is due to the lower winter temperature in that year. A query of the local temperature records in Tianjin for the past 10 years shows that the winter temperature in 2013-2014 is at a historical average level, so the design of the project selected the heat data from 2013-2014 for reference. According to Tianjin Shuiyoucheng Valley electricity time from 23:00 to 7:00 the next day, it is determined that the amount of heat to be stored during the Valley electricity time period is 105863MJ.

Table 3: Statistics of measured historical data
According to the required storage heat of 105863MJ, the number of required phase change thermal storages can be calculated to be 163 (single heat storage is 650MJ). Considering the 5% margin, the design number is 170. Because the building needs a small amount of anti-freezing heating at night at the same time during the Gudian heat storage period, the theoretical heat supply required during the Gudian time period is calculated to be 132365MJ. The required electric heating power is calculated to be 4936.6kW, and the calculation is shown in Table 4. In this calculation, the heat loss of the system piping is considered 5%, and the efficiency of the electric boiler is 98%.

Table 4: Calculation of required electric power

(3) Equipment selection The main equipment quantity and technical parameters are shown in Table 5.

Table 5: Main equipment parameter table
(4) Project construction
The project officially entered construction in late September 2014, and commissioning was completed by early November 2014. The actual effective project construction time is about one month. Figure 3 shows the actual scene after the project is completed, (a) is the electric boiler and the central control room, and (b) is the entire row of the thermal storage.

Figure 3: Gudian heat storage heating system site

(5) Project operation effect
During the heating season from November 14, 2014 to March 15, 2015, Tianjin Shuiyou Chenggu Electric Heating Depot System operated steadily for 122 days, the system operating cost was 1.9 million yuan, and the valley electricity price was 0.4123 yuan / kWh in the plan. Based on the adjustment of 0.4683 yuan / kWh, compared with the original municipal heating system, it saved 3.31 million yuan, a saving rate of up to 64%, and the unit energy consumption was 14.6 yuan / m² / heating season. The actual daily average heating cost per square meter was only 0.12 yuan.
    During the heating season from November 14, 2014 to March 15, 2015, Tianjin Shuiyou Chenggu Electric Heating Depot System used a total of 1.58 million yuan in low electricity costs, accounting for 83% of the total cost, and 250,000 yuan in flat electricity costs, accounting for all The cost is 13%, and the peak electricity cost is 70,000 yuan, accounting for 4% of the total cost. After the optimization of the late-stage control system, it is not necessary to add heat to the flat electricity, and it is expected that the valley electricity cost will account for much more than 90%. After the heating season ends, read out the power of each part of the system from the metering energy meter as shown in Table 6. The calculated efficiency of the electricity boiler's power consumption is converted to heating panels for heat exchange efficiency up to 97.5%.

Table 6: Electricity statistics of each part of the system
     After using the thermal storage heating system, Tianjin Shuiyoucheng averages 30639 kWh of electricity per day for heating, of which 27882.4 kWh is valley power, 2322.5 kWh is flat power, and 434.1 kWh is peak power. The power consumption during the peak time period can be calculated to be 54.3 kW. . According to the 2013-2014 winter data, if the direct power heating method is used, the power consumption during peak periods is about 1418.4kW. Therefore, after using the Gudian thermal storage heating method, the Shuiyoucheng project can transfer the 1364.1kW peak power load every day. If there are 100 similar heating projects, after using the valley heating system heating system, about 136.41MW peak load can be transferred during the peak power period, which can reduce the equivalent installed power plant load.

Concluding remarks
Phase change energy storage is an advanced heat storage technology. The use of phase change technology to perform electric heat storage during valley time for building heating is of great value for power grid peak shaving and user heating operation costs. This article compares the phase-change heat storage heating technology with several common heating methods. From the perspective of initial investment and operating costs, the phase-change heat storage technology has advantages.
This article specifically tests and analyzes the thermal performance parameters of 650MJ phase-change thermal storage products. The heating time is less than 7h, the heating capacity is 641.93MJ, the heat generation is 639.49MJ, the charging and discharging efficiency is as high as 99.6%, and the electrical energy conversion rate is 95%. , 24h heat loss is 4.85%.
The valley electricity heat storage heating solution using phase change technology reduces the operating costs for users, provides a good solution for the replacement of coal-fired boilers, and can also contribute to the peak shaving of electric power. The heating project of this project can save 136.4MW generating set power during peak power period after using the thermal storage heating system, which can greatly reduce the power load during peak hours.

Note: The author of the article is Zhang Jihuang, Sun Li, Li Wen of Jiangsu Qineng New Energy Materials Co., Ltd .; Yang Qiang, Research Institute of Environment and Energy, Chinese Academy of Building Sciences.