Experimental study on heat storing and release properties of fluidized bed regenerative device

Abstract

By using the fluidization technique and electromagnetic heating technique, this paper designed a new type solid electric regenerative device using quartz sand particles to store heat, completed the setting of experimental bench of fluidized bed regenerative device, and analyzed the heat storing and release properties of this regenerative device. The experimental results show that the fluidized bed regenerative device has a good heat storing and release performance. When the metal wall surface of regenerative device was heated by electromagnetic heating to 600 ${{}^{\circ}}$ C and later kept at this temperature, the heat storing temperature of quartz sand particle material can reach above 500 ${{}^{\circ}}$ C after storing heat for 12 hours. After the 12-hour heat storing was finished, the total power of regenerative device’s heat storage obtained according to the temperatures of measuring points in fluidized bed and the physical properties of quartz sand can satisfy the design requirement. The 12-hour total effective heat release of this device is 29.5 kW $\cdot$ h and its heat release per unit time is kept at 2–3 kW, indicating that this device has a stable and controllable heat release process and is of actual application value.

Keywords

Regenerative device fluidized bed electromagnetic heating

1. Introduction

At present, the energy consumed in heat supply is rising year by year, but renewable energy couldn’t meet heat users’ need of stable heat supply. Meanwhile, a problem going with it is the difference between peak and valley power consumption is gradually widening, resulting in increasing waste of electric energy. In order to realize peak load shifting, it is necessary to develop new energy storage techniques. Recently, the phase-change material has low heat conductivity coefficient that it is difficult to extract the stored thermal energy [1, 2, 3], the fused salt-based regenerative system has such defects as poor thermal stability, high fused salt solidification temperature, corroded storage tank and high cost, while the solid heat storage has such problems as nonuniform heat storage, low heat transfer coefficient and system hard to control, which severely limit the application of phase-change materials and sensible heat and heat storing techniques. The solid granular heat storage materials with high temperature resistance and good economy are feasible approach for solving above problems.

Based on this, a fluidized bed regenerative device was proposed. Through literature consultation, it is discovered that at present, most studies on fluidized bed concentrate in the fields of efficient use of fossil energy, coal combustion and power generation, etc. [4, 5, 6, 7]. However, few scholars have made research on the design of regenerative device as well as the properties of heat storing and release.

This paper selected quartz sand solid particles as the heat storing material, compared the features and differences of various regenerative devices and materials in heat storing and release, heat transfer characteristics, economy and safety [8, 9], and adopted the structure of fluidized bed [10, 11]. A fluidized bed regenerative device using valley power for heat storage was designed, and the heat storing and release properties of this device was studied by experiment, laying a foundation for its engineering application.

2. Deign of regenerative device

The fluidized bed regenerative device mainly adopts fluidization technique and electromagnetic heating technique and uses solid particles to store heat. During the valley electricity period, the fluidized bed regenerative device utilizes valley electricity and combines with the internal quartz sand solid particles to store thermal energy, and the electrical energy is converted into heat energy through electromagnetic induction and transferred to the heat storage solid particles. The heating is converted into heat energy, so that the temperature of the heated wall surface rises to a high temperature of more than 900K. At the same time, after heat storage at night, the temperature of the solid particles of the heat-storing quartz sand can be evenly raised to 900K, and the stored heat can meet the needs of heat users. In the off-valley power period, when heat output is required, the fan is turned on, and the power is reasonably adjusted in different periods, so that the low-temperature air flows through the pores of the quartz sand particles and drives some of the quartz sand particles to fluidize. At this time, the heat in the particles will transfer to the air and heat the air to a high temperature. After the air exchanges heat with the quartz sand particles, it flows into the heat exchanger from the air outlet pipe and transfers the heat to the heat user.

2.1 Selection of heat storing material

A key of the design of regenerative device is the reasonable selection of heat storing material. Heat storing material is a carrier through which a regenerative device stores heat, and a good such material should have large thermal capacity and high cost performance. The properties of some frequently-used solid heat storing particle materials are compared, see their properties in Table 1.

Table 1
Comparison of properties of commonly used solid heat storing particle materials

Material name (main ingredient)	Density (kg/m ${}^{3}$ )	Specific heat capacity (J/kg $\cdot$ K)	Properties
Quartz sand (SiO ${}_{2}$ )	2600	1000	Exist abundantly in the nature, with the lowest cost, high abrasion resistance, stable property
Bauxite (Al ${}_{2}$ O ${}_{3}$ )	3950	1200	Stable property, high cost, slightly higher specific heat capacity
Coal ash (Al ${}_{2}$ O ${}_{3}$ , SiO ${}_{2}$ )	2200	720	Low cost, slightly lower specific heat capacity, low abrasion resistance
Ceramic proppant (75% Al ${}_{2}$ O ${}_{3}$ )	3300	1200	Highest cost

Through comparative analysis, the 4–6 mm quartz sand solid particles were selected as the heat storing material of this regenerative device, because this material has low cost, higher specific heat capacity, and stable property, At the same time, it is found by means of numerical simulation that the quartz sand with a particle size of about 6 mm has a low lifting height, which is beneficial to the stable operation of the device.

2.2 Structural design of regenerative device

Figure 1.

The main structure of the regenerative device and the internal schematic diagram of the device.

The structure of regenerative device plays a vital role in its heat storing and release property. In order to enhance heat transfer, the fluidization technique and electromagnetic heating technique were adopted to design the structure of fluidized bed regenerative device. The main structure of the regenerative device is shown in Fig. 1. The external surface of the device is carbon steel, with quartz sand solid particles closely packed inside; outside the device, electromagnetic coils are twined to heat solid particles in valley power period. When the device is operated, the particles are fluidized by adjusting the air volume at air inlet, exchanging heat with the air, and then the air enters the rear exchanger via air outlet. To reduce the operating power consumption of the fan, air inlet pipes are set at the side and bottom of the device, and controlling the air valves of different pipes can realize a stable heat storing and release process, controlling the power of the heating device can adjust the rate at which the fluidized bed regenerative device stores heat. A gas-solid separation device is arranged inside the device, which can block the raised solid particles and prevent them from escaping.

By combining the theoretical calculation and numerical simulation, the structure size of regenerative device is optimized. The design power of the device is set as 2 kW, and the total height is set as 1600 mm to prevent elutriation during fluidization, with a gas-solid separator set at the top; the piling height of quartz sand particles is 0.8 m, and the mass is about 250 kg; in order to control the heat release velocity, the diameters of side air inlet and bottom air inlet are set as 200 mm, and the pipe diameter of upper hot air outlet is 100 mm.

The innovation point of this regenerative device’s structure is mainly the application of fluidization technique and electromagnetic heating technique. The fluidization technique applied in the regenerative device can greatly enhance the heat transfer property between air and solid particles, and solid particles may have intense heat exchange with other particles and air in the process of fluidization, making the bed temperature to keep an even state during heating. For the regenerative device, it is easy to control the bed temperature, which enhances the controllability and safety of heating process. Meanwhile, the fluidized bed regenerative device uses the electromagnetic heating technique, making it easier to control the wall surface temperature of the device during heating, and realizing the uniform heating in the process of heating the heat storing materials. Besides, the electromagnetic inductive heating has high safety and thermoelectric conversion efficiency, because in the process of heating, the electromagnetic heating coil itself doesn’t produce heat, which also results in relatively higher service life than that of traditional resistance wire as well as higher economy.

Figure 2.

Experimental system flow chart.

3. Heat storing and release experiment

3.1 Heat storing and release experimental system

In order to master the heat storing and release properties of fluidized bed regenerative device, an experimental system was designed as shown in Fig. 2. This experimental system mainly includes five parts: automatic control device, power input system, fluidized bed regenerative device, electromagnetic inductive heating device, gas-water heat exchanger and radiator. The control unit is mainly used to control the electric power of electromagnetic inductive heating device, realizing the stepless regulation of heating power during inductive heating, and to prevent the device to overheat and thereby damage the equipment. The power input system mainly supplies power to the electromagnetic heating device, fan and water pump. The regenerative device is equipped with air inlet duct at the side surface and bottom and with exhausting valve and air outlet duct on the top, in which the air outlet duct is connected with gas-water heat exchanger, the return air duct at heat exchanger side is connected with the air inlet of regenerative device to form a loop, and the heat exchanger is connected with the radiator via supply and return water pipe. The return air duct is equipped with fan, the return water pipe is equipped with water pump, and the outside of regenerative device body, air supply and return air ducts, and supply water and return water pipes are laid with thermal insulation material. The experimental bench is shown in Fig. 3.

Figure 3.

Overall diagram of the experimental bench.

Figure 4.

Layout of experimental temperature measuring points inside the regenerative device.

3.2 Arrangement of measuring points

The arrangement of measurement points of experimental system is shown in Figs 3 and 4. Inside the regenerative device body, seven temperature sensors are evenly arranged, numbered from 1 to 7, for measuring the gas-solid phase temperature inside the device. The section of the quartz sand thermal storing particles bed is round, and the temperature sensor measures the temperature at the center of this bed section. At the same time, the wind temperature and flow at inlet and outlet, the air flow at air inlet, the pressure at air duct, and the supply and return water flow of this system are also monitored. The data collection system uses YOKOGAWA GP-20 paperless recorder, and the data collected by all instruments are recorded and displayed via paperless recorded. The main experiment of experimental system as well as the relevant properties of test instruments are shown in Table 2.

Table 2
Type and parameters of main experimental instruments

Equipment name	Type	Parameters
Circulating water pump	YE2-71M1-2	Voltage: 380 V, Power: 0.37 kW, Head: 19 m, Flow: 7.5 m ${}^{3}$ /h.
Fan	YW5-47	Voltage: 380 V, Blowing rate: 1200 m ${}^{3}$ /h, Air pressure: 1210 Pa, Rotate speed: 2800 r/min.
Air velocity transducer	AF121B-10M/S	Output: 0–10 V, duct diameter 20 cm, resistant to high temperature of 600 degree; Installation: movable thread mounting, Range: 0–10 m/s, Precision level: 3%, Display resolution: 0.01, Power supply voltage: 24VDC, load resistance: $\leqslant$ 500 $\Omega$ , long-term stability: $\pm$ 1% F $\cdot$ S/year, ambient relative humidity: 0 $\sim$ 95%, compensation temperature: 0 $\sim$ 70 ${{}^{\circ}}$ C, working temperature: $-$ 40 $\sim$ 400 ${{}^{\circ}}$ C, Protection class: IP65
Capsule pressure gauge		Range: 0–2.5 kPa, Precision level: level 1.6
Temperature sensor		Range: 0–1200 ${{}^{\circ}}$ C/Range: 0–100C ${{}^{\circ}}$ , Precision level: $\pm$ 0.05%
Electromagnetic flowmeter		Medium: water, Voltage: 220 V, Pipe diameter: DN25, Output: 4–20 mA, Connection: flange connected, Field display, Precision: level 0.5.
Gas-water heat exchanger		Dimension: 600600800 mm, Material: full carbon material Dimension of finned tube: $\varphi$ 322.557 mm High-frequency welded finned tubes: 7 tubes/row*5 rows $=$ 35 tubes.
Electromagnetic heating host		Max. power: 60 kW
Intelligent temperature &		Temperature range: $-$ 200 $\sim$ 200 ${{}^{\circ}}$ C
humidity recorder
Air disk radiator	FP-51	Max. power: 2.5 kW (low/medium/high power available)
Electromagnetic coil		Material: Copper, Wire diameter: 25 mm ${}^{2}$ , Wire length: 50 m

3.3 Experimental scheme

In view of the practical usage of regenerative device, an experimental scheme of heat storing and release was designed for the designed fluidized bed regenerative system.

(1)
Experimental scheme of heat storing: the heat storing process is that the low-cost electric energy converts to heat energy by heating the heat storing material through electromagnetic induction, so as to store the heat required by heat consumers. To reduce the power consumption of fan, the heat storing experiment was performed on stationary bed, with experiment heat storing time of 12 hours and the heating device power of 50% (the heat storing time can be determined by regulating heating device power according to the peak & valley electricity policy of application district). The temperature of heating wall surface of regenerative device is set as 600 ${{}^{\circ}}$ C.
(2)
Experimental scheme of heat release: in the process of heat release, the fan and air valve are turned on, the electromagnetic heating device is shut down, then the heat energy stored in the regenerative device is supplied to heat consumers by heat transfer between hot air and heat exchanger. The experimental heat release time is 12 hours. During heat release, the heat can be released stably by adjusting air valve and fan frequency.

3.4 Experimental uncertainty analysis

The experimental errors mainly include air density factors, differential pressure signal transmission errors and errors caused by sensor failures. Since the measurement of air density usually requires compensation of temperature and pressure, in order to reduce the experimental error, a look-up table method was used to compensate the air density. High temperature-resistant sensors are selected for the measurement of particle temperature, and at the same time, the correct installation and maintenance of various measuring instruments are ensured, and the failure of sensors and experimental errors caused by abnormal installation are eliminated.

4. Analysis of heat storing and release experimental results

4.1 Heat storing experimental result and analysis

Figure 5.

The internal temperature change curve of the regenerative device during the heat storing process.

Figure 5 shows the change curves of the temperatures of seven measuring points 1–7 in regenerative device with the time during heat storing process. It is obvious that the temperatures of these measuring points display three groups of different variation trends: measuring points 1–4, measuring points 6–7, and measuring point 5. Therein, the temperatures of measuring points 1–4 are lower, showing no great difference, uniform change law, and a gradual ascending trend in the whole heat storing process, and all of them rise above 150 ${{}^{\circ}}$ C when heat storing is finished. This is because the measuring point 1–4 are inside the regenerative device to measure air temperature but not inside the heat storing material, and its change law indicates that in the stationary bed heat storing mode, the heat of high-temperature heat storing particle material at the bottom is gradually transferred to the upper air, making the air temperature to increase continuously. As the thermal capacity of air is low, so the air temperature rises gradually more slowly.

Similarly, it can be seen from Fig. 5 that the measuring points 5–7 inside the solid particles in regenerative device obviously have higher temperature rise rate than the measuring points 1–4, in which, the measuring points 6 and 7 has an evidently higher temperature rise rate than measuring point 5. This demonstrates that though these three measuring points show the bed temperature, the measuring points 6 and 7 are located inside the electromagnetic heating coil, the temperature of wall surface at the coil quickly rises to 600 ${{}^{\circ}}$ C after the heating is started, and the great temperature difference between high-temperature wall surface and solid particles accelerates the heat transfer to regenerative bed. While, the temperature of metal wall surface without electromagnetic heating coil is lower, so the temperature rise of regenerative bed where the measuring point 5 is located mainly comes from the heat conduction of higher temperature solid particles at the bottom, and in the stationary bed heat storing mode, the process that the heat transfers from bottom particles to upper solid particles through heat conduction is slower.

By observing the slope of temperature rise curves, it is known that after the temperature of a measuring point rises above 300 ${{}^{\circ}}$ C, the temperature rise velocity of each measuring point in regenerative bed changes to some extent. After 300 ${{}^{\circ}}$ C, the temperature rise rates of measuring points 6 and 7 start to decrease, which is due to that the bed temperature is already high at this time and has a less difference with the temperature of metal wall surface, leading to the decline of heat transfer efficiency. Meanwhile, the temperature rise rate of measuring point 5 start to be faster, indicating that the overall temperature of regenerative bed has risen at this time, and the increase of the temperature difference between upper part and bottom part of regenerative bed makes the heat transfer efficiency to increase.

At 720 min, the heat storing process finishes, the temperature of measuring points 6 and 7 is around 500 ${{}^{\circ}}$ C, measuring point 5 rises to around 300 ${{}^{\circ}}$ C, and measuring points 1–4 is stable at around 150 ${{}^{\circ}}$ C. But it can be discovered from the curve slope that if the heat storing time is longer, the temperature of the central point of regenerative bed still goes higher. Under stationary bed heat storing mode, the radial temperature distribution law of regenerative bed is that the closer it is to heat wall surface, the higher the temperature it will be, and at this time, the heat is still transferred from outside the bed to central part of bed, so temperature distribution does not yet reach to a steady state. The experiment shows that if the fan is turned on during experiment to bring a fluidized operation, the temperature distribution of regenerative bed will be much uniform,but meanwhile, the electricity power consumption will also be increased, so it is suggested to adopt heat storing under stationary bed mode in case of not affecting heat storing volume.

The heat storing volume is measured according to the temperatures of measuring points in regenerative bed and the physical properties of quartz sand. After the 12-hour heat storing is finished, the total heat storing power of regenerative device is 25.17 kW $\cdot$ h. Since the measuring points in regenerative bed are arranged in the central position of regenerative cylinder, the temperature of central point should be the lowest in stationary bed heat storing mode. Besides, it is discovered from the heat release curves that, after stopping the heating, the temperature of central point still shows an ascending trend in a certain period of time, so this power shall be the least heat storing power of regenerative device and the practical device power will be larger, which can meet the design requirement.

4.2 Heat release experimental result and analysis

To ensure the outlet air temperature, the heat release process is divided into two stages: bottom blowing-in method is used before 420 min, and side blowing-in method is used after 420 min. Figure 6 shows the change curves of measuring points 1–7 in regenerative device with the time during heat release process. Comparing Figs 5 and 6, it is found out that the temperatures of three central measuring points in regenerative bed in initial heat release moment are increased compared to those when heat storing is finished, increased from 369 ${{}^{\circ}}$ C, 477 ${{}^{\circ}}$ C, 465 ${{}^{\circ}}$ C to 372 ${{}^{\circ}}$ C, 480 ${{}^{\circ}}$ C, 469 ${{}^{\circ}}$ C. As the heat release experiment is started 12 min after the heat storing experiment is finished, so this demonstrates that the temperature of central part of regenerative bed is lower than that of peripheral temperature in this period of time, and the heat is still transferred inward.

Figure 6.

The internal temperature change curve of the regenerative device during the heat release process.

In Fig. 6, the temperature change law of each measuring point in heat release process is similar to heat storing process, showing three different groups of variation trends. Therein, the temperatures of measuring points 1–4 are always stable at around 200 ${{}^{\circ}}$ C in the whole heat release process and doesn’t change much in the earlier and later periods of heat release. The temperatures of measuring points 5–7 decline after a rise before 420 min, because the heat release experiment is started immediately when the heat storing is finished, the internal temperature of bed is not stable yet at that time, and the temperature of central part still rises under the effect of external high-temperature bed body. Considering the entire heat release velocity of bed, the heat release velocity of measuring point 7 is faster than measuring point 6, and the velocity of measuring point is the slowest. The reason is that within 420 min, only the bottom air valve is turned on with an openness of 100%, the cold air enters the device from the bed bottom center, making the sand particles in central part of device to fluidize, greatly increasing the heat exchange efficiency with air. Then the heat energy of quartz sand at bed bottom is quickly absorbed by the air and taken away, and the heat is transferred to the middle and upper part of regenerative bed body. Therefore, we can see from Fig. 6 that when the temperature of measuring point 7 starts to fall, the temperatures of measuring points 5 and 6 still rise; when the temperature of measuring point 6 start to fall, the temperature of measuring point 5 still rises. This suggests that in case of only bottom valve is turned on, the heat release law of fluidized bed is the closer the quartz sand particles are to the bottom air inlet, the quicker their heat is released. Meanwhile, as the temperature of quartz sand particles at the bottom decreases, it can be observed from the curve slope that its heat release efficiency will also greatly decline, and the heat release velocity of quartz sand in middle and upper part obviously increases. In this stage, to guarantee the overall heat release velocity of regenerative device, the fan frequency should be controlled around 25 Hz.

At 420Smin, the temperatures of measuring points 6 and 7 both fall down below 150 ${{}^{\circ}}$ C, indicating the heat of central fluidized region in middle and lower part of bed has been fully released. While the temperature of measuring point 5 is still high, so it’s the time to shut down the bottom air valve and turn on the side air valve with openness of 100%, and fan frequency is set as 30 Hz. Figure 6 shows that after the side air valve is turned on, the temperature decline velocity of measuring point 5 is obviously promoted, and the change trend of temperature curve of measuring points 6 and 7 displays a reversal: their temperature changes show an ascending trend. This supports that the temperature of particles in non-fluidized region at both sides of bottom air inlet is still high in the previous heat release stage, the particles show a stationary bed state after the air valve is shut down, and the heat energy is transferred from outer part to central region under the effect of temperature difference. At 720 min, the heat release experiment is finished, but at this time the temperature of measuring point 5 is still above 300 ${{}^{\circ}}$ C, and the outlet air temperature is 195 ${{}^{\circ}}$ C, manifesting that the regenerative device could continue to release heat at this time.

Figure 7.

The temperature change diagram of the outlet and return air of the regenerative device during the heat release process.

Figure 7 shows the experimental data of outlet and inlet air temperatures of regenerative device in the process of heat release working condition. Before 420 min, the average flow rate of air is 0.36 m/s. Between 420–720 min, the average flow rate of air is 0.52 m/s. According to the design code [12], the effective heat release of regenerative device can be worked out by the following formula:

$\displaystyle Q_{\textit{mdlo}}=\mathop{\sum}\limits_{j=1}^{N_{1}}c_{p}m_{{hd}% }({t_{o}-t_{i}})\cdot{\Delta}\tau_{j}\cdot 10^{-6}/3.6$

in which, $Q_{\textit{mdlo}}$ – denotes to the maximum effective heat release of regenerative device, in kW $\cdot$ h;

$N_{1}$ – denotes to total record number from the beginning to the end of the experiment;

$c_{p}$ – denotes to the average specific heat of air, in J/(kg $\cdot$ K);

$m_{{hd}}$ – denotes to the mass flow rate of air, in kg/s;

$t_{o}$ – denotes to outlet air temperature of regenerative device, in ${{}^{\circ}}$ C;

$t_{i}$ – denotes to inlet air temperature of regenerative device, in ${{}^{\circ}}$ C;

${\Delta}\tau_{j}$ – denotes to the time interval of the jth record, in second.

According to the above formula combining with experimental data, we can figure out the total 12 hour effective heat release of this regenerative device is 29.5 kW $\cdot$ h, and the hourly heat release of the device under such heat release working condition is stable at 2–3 kW. This indicates that if the air valve and fan frequency are reasonably regulated, this regenerative device can stably supply heat to heat consumers. From the results of heat storing and release experiment, this regenerative device has a practical heat release power greater than design power, and the time of heat storing experiment under current experimental working condition can be shortened.

5. Conclusions

(1)

This paper adopts the fluidization technique and electromagnetic heating technique to design a new type generative device using quartz sand particle to store heat. This improves gas-solid heat transfer efficiency, enhances heat transfer safety and controllability. The experimental study indicate this regenerative deice has good heat storing performance and the heat storing volume can meet the design requirement.

(2)

This regenerative device can realize a stable heat storing and release process, and its heat storing time can be controlled by setting the temperature of heating wall surface and the coil power. The heat release experiment shows that by adjusting the bottom air valve, side air valve and fan frequency, this regenerative device can realize a stable heat supply, and the hourly heat release of the device can be kept at 2–3 kW.

(3)

The fluidized bed regenerative device studied in this paper has high practical application value. The research of this topic expands the application scope of solid particulate materials, improves the technical economy of solid heat storage, and is of great significance to energy conservation and emission reduction.

Footnotes

Acknowledgments

This work is supported by the Central Government Guides Local Funds for Scientific and Technological Development Project (216Z5201G) and Hebei innovation capability improvement project (19244503D).

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Experimental study on heat storing and release properties of fluidized bed regenerative device

Abstract

Keywords

1. Introduction

2. Deign of regenerative device

2.1 Selection of heat storing material

Table 1 Comparison of properties of commonly used solid heat storing particle materials

3.1 Heat storing and release experimental system

Table 2 Type and parameters of main experimental instruments

4. Analysis of heat storing and release experimental results

4.1 Heat storing experimental result and analysis

Footnotes

Acknowledgments

References

Table 1
Comparison of properties of commonly used solid heat storing particle materials

Table 2
Type and parameters of main experimental instruments