1、4777 Modified RC Thermal Circuit Model Applied to Cold Storage System with Multi-Loop Heat Pipes Yuan-Ching Chiang Huei-Chun Chen ABSTRACT Jen-Jie Chieh Sih-Li Chen, PhD such a type of thermal storage system. First, the need for an This paperproposes a theoretical model to investigate the thermal pe
2、rformance ofa cold storage system with multi-loop wickless heatpipes. The cold storage system utilizes the supe- rior heat transfer characteristics of heat pipe and eliminates drawbacks found in the conventional thermal storage tank. A mod$ed RC circuit model to determine the thermal charac- teristi
3、cs of the cold storage system has been developed. Exper- imental investigations are then conducted to study the cold storage thermal performance in an experimental system with the ratio of distance between heat pipes to outer diameter of heatpipe W/D = 2. DifSerent heat transfer mechanisms, includ-
4、ing nucleate boiling, geyser boiling, and natural convection, are ident$ed in diferent experimental systems with various liquidfills. This paper probes the efect ofthe fill level on cold storage rate and cumulative cold storage quantity. Compari- sons of this theory with experimental data show good
5、agree- ments in the nucleate boiling stage of cold storage process. INTRODUCTION A number of thermal energy storage systems (Yimer and Madami 1997; Hasnain 1998; Dincer and Rosen 2001; Dincer 2002) have been considered and developed in recent years. Most of the energy storage systems utilize an acti
6、ve control method to store or release thermal energy. That is, in the system design of thermal storage, a pump is included to trans- fer thermal energy from a high-temperature heat source to the thermal storage tank via flowing working fluid. To utilize the stored thermal energy, an electromagnetic
7、valve is used under control to change the flow path of the working fluid so that energy stored in the storage tank is released to and used by a low-temperature heat sink. There are two drawbacks found in _ -. operation cost and the power consumption of the pump. The thermal storage shall be unworkab
8、le in case of a system fail- ure. Second, change of the charge and discharge ability of the conventional storage systems basically relies on the system piping design; therefore, only two functions, i.e., energy stor- age and energy release, are available in its operating modes. It is impossible for
9、both the heat supply side and the heat utili- zation side of the thermal storage to operate at the same time during the energy utilization. A new cold storage system is proposed in this paper, which utilizes the superior heat transfer characteristics of heat pipe and eliminates drawbacks found in th
10、e conventional thermal storage tank. The cold storage system with multi-loop heat pipes, as shown in Figure 1, consists of an energy storage tank and two heat pipe loops. The energy storage tank is filled with phase- change medium (PCM) so that thermal energy can be stored or released via melting or
11、 freezing of the phase-change medium between solid and liquid states. Insulating material is provided to cover the outside of the energy storage tank to prevent heat loss. A top cover is provided at the top of the tank for replen- ishing the phase-change medium into the chamber, and a drain hole is
12、provided at the bottom of the tank for draining the phase-change medium. The heat pipe loops include three parts, namely, a group ofparallel wickless heat pipes vertically disposed inside the energy storage tank and vertical high- temperature heat exchange and vertical low-temperature heat exchanger
13、 separately located outside of the tank. The parallel heat pipes combine the high-temperature heat exchanger to form a two-phase closed-loop thermosyphon for cold storage. The cold release loop is constructed by connecting the parallel heat pipes and the low-temperature heat exchanger. The paral- Yu
14、an-Ching Chiang and Jen-Jie Chieh are graduate students and Sih-Li Chen is a professor in the Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan, ROC. Huei-Chun Chen is a graduate student in the Department of Air Conditioning and Refrig- eration at National Taipei Unive
15、rsity of Technology, Taipei, Taiwan, ROC. 02005 ASHRAE. 387 Ice-Storage Tank “T I ti Me T Figure I Cold storage system with multi-loop heatpipes: (a) charge mode (6) discharge mode. le1 heat pipes have external short fins densely provided around their outer surfaces to increase thermal conductive co
16、ntact areas thereof. These short fins also divide inner space of the energy storage tank into multiple energy storage cells. The phase-change medium becomes molten or frozen in these energy storage cells to store or release cold energy. The verti- cal higWlow-temperature heat exchangers outside the
17、energy storage tank are used to exchange heat with high-temperature and low-temperature flowing fluid, respectively. An adequate amount of working fluid is filled in the heat pipe loops. Figure 1 shows the manner in which the cold storage system with multi-loop heat pipes operates to store cold ener
18、gy. When an amount of low-temperature flowing fluid flows into the low-temperature heat exchanger in a direction as shown by the arrows in Figure la, it absorbs heat in the vapor working fluid inside the vertical low-temperature heat pipes and is heated to have increased enthalpy value. The vapor wo
19、rking fluid inside the heat pipes condenses into a liquid working fluid that forms a thin layer of condensate along the inner wall surface of the vertical low-temperature heat pipe, then flows downward under gravity into the vertically parallel heat pipes. At this point, the liquid working fluid in
20、the verti- cally parallel heat pipes absorbs energy stored in the liquid phase-change medium in the cells outside the heat pipes and 388 ASHRAE Transactions: Research comes to a boil to produce vapor working fluid that flows upward due to its buoyancy into the vertical low-temperature heat pipe to c
21、omplete one cycle. Energy stored in the phase- change medium is transferred to the low-temperature working fluid flowing through the vertically parallel heat to boil and evaporate the working fluid and thereby freeze the liquid phase-change medium into a solid state. Figure 1 b shows the function in
22、 which the cold storage system with multi-loop heat pipes operates to discharge cold energy. The RC thermal circuit model, which is analogous to the RC electrical circuit model, is proposed to investigate the cold storage system with multi-loop heat pipes. The analogous electric circuit is shown in
23、Figure 2. It comprises thermal resis- tance of each heat transfer device (R), thermal capacity of the energy storage tank (C), heat transfer rate in the cold storage system (q), and each temperature (T). Those parameters in the RC thermal circuit model can be analogous to the electric resistance, el
24、ectric capacity, current, and electric potential, respectively, in the RC electrical model. The thermal RC concept has been successfully applied to temperature control engineering owing to its simplicity for complex heat transfer problems. Engeler and Garfinkel (1 965) uses the thermal RC concept to
25、 analyze the temperature rise at the junction of a GaAs laser for a variety ofpulse currents and base temperatures. It is shown that the thermal behavior of the diode may be calculated analytically at room temperature because of the constancy of the thermal parameters. Min et al. (1990) studied the
26、transient thermal property of semiconduc- tor devices with the RC concept and the method of images. The software developed is useful for a variety of device struc- tures under pulsed conditions with high peak power or switch- ing conditions. Wu (2000) utilized a simple RC thermal circuit similar to
27、an electrical circuit to analyze the controlled system composed of an electrical heater and oven fast. It is shown that RC thermal circuit offers good assistance for temperature control of a system. This paper presents theoretical analysis and experimen- tal investigations of the charge characterist
28、ics of the cold storage system with multi-loop heat pipes. A modified RC thermal circuit model is proposed to study the thermal char- acteristics of the cold storage system. Experimental investi- gations are performed in an experimental system with a ratio of distance between heat pipes and outer di
29、ameter of the heat pipe W/D = 2. Flow patterns are identified in different exper- imental systems with various liquid fills. This paper also probes the effect of the fill level in multi-loop heat pipes on the cold storage rate and cumulative cold storage quantity. The parameters, including thermal r
30、esistance, refrigerant temperature, and cumulative storage quantity, are discussed and compared with experimental measurements. MODIFIED RC THERMAL CIRCUIT MODEL The modified RC thermal circuit model is shown in Figure 2. It consists of source temperature potential A: Af (8) O Figure 3 Experimental
31、systems, equipment, and measurement systems: 1 chiller, 2 resistance heater, 3 thermostatic tank, 4 brine cycle pump, 5 bypass valve, 6 purgmeter, 7 valve, 8 heat exchanger, 9 energy storage tank, 1 O drain hole, 11 upper cover, 12 liquid level indicator, 13 pressure gauge, I4 thermocouples, 15 data
32、 recorder, 16 GPIB card, 1 7 monitor 390 ASHRAE Transactions: Research ture heat exchanger employed was the high-efficiency plate type. The PCM in the storage tank was pure water and R-22 was used as the working fluid in the multi-loop heat pipes. As shown in Figure 3, the experimental equipment was
33、 set up mainly for constant production of brine through the low-temperature chiller in a large thermostatic tank. The brine tank was 71 x 130 x 66 cm with maximum capacity of 600 L. A 10 HP chiller was equipped with a 2 HP pump to circulate the brine to the low-temperature heat exchanger. Another 1/
34、2 HP pump was installed inside the thermostatic tank to recirculate the brine and achieve a uniform tempera- ture. Brine water in the thermostatic tank was 28% ethene glycol solution. The temperature range of the thermostatic tank was -20-80C with accuracy of al3“C. The measurement system consists o
35、f T-type thermocou- ples, flow meter, pressure gauge, electronic scale, liquid level indicator, digital camera, and height indicator. The physical quantities measured include the brine inlet/outlet temperature of the heat exchanger, brine cycle flow rate, temperature and pressure of refrigerant in a
36、nd out of the ice storage tank, vari- ation of water temperature in the ice storage tank, liquid refiig- erant level in the ice storage tank, change of water level inside the energy storage chamber, and the surrounding temperature. The thermal energy rate supplied to the thermal battery Qi is comput
37、ed from the temperature difference at the inlet and outlet of the brine flowing through the low-temperature heat exchanger: The cold storage rate Q, is the rate of water latent and sensible heat storage in the energy storage chamber during a period of time. The former may be calculated from the thic
38、k- ness of the ice layer by observing the variation of water level in the energy storage chamber; the latter may be obtained based on measurements of temperatures ofwater in the energy storage chamber. The water temperature at various points of the energy storing chamber is T, and the average temper
39、ature of measurement points is CTJm, where m is the number of measurement points in the energy storage chamber. Thus, the cold storage rate of the cold storage system, Q, can be expressed as Therefore, the cumulative cold storage quantity in these experiments Q, is the summation of cold storage rate
40、s for each time interval in the cold storage system with multi-loop heat pipes: t f/Ar Q, = JQ,dt = Q,At (12) O O RESULTS AND DISCUSSION To examine the effect of the fill level of liquid refrigerant in the energy storage tank on the cold storage phenomenon of the cold storage system with multi-loop
41、heat pipes, system performance was analyzed at the liquid levels of 30 cm, 40 cm, and 50 cm, respectively. The effects of fill level on liquid temperature at the lower header and vapor temperatures at the upper location are quantitatively demonstrated in Figures 4-6. It is seen that under all operat
42、ing conditions, the experimental and predicted temperature of both liquid and vapor drops steeply at the beginning, but subsequent experimental and predicted temperature profiles of liquid and vapor variation differ with different fill levels. When the fill level is at 50 cm, as shown in Figure 4, b
43、oth the experimental and predicted liquid temperatures rise from subcooling to operation temper- ature at the time of2,lOO seconds, while the vapor temperature always keeps saturated. Since the cold storage rate at the high fill level for the entire cold storage process is large enough for nucleate
44、boiling and since the heat transfer mechanism during the entire cold storage process is dominated by nucleate boil- ing, predicted temperature profiles of liquid and vapor show good agreement with experimental measurements. The vapor and liquid temperature histories for fill levels at 30 and 40 cm a
45、re shown in Figures 5 and 6. Experimental data of vapor temperature at the time of 800 and 1,300 seconds for the fill level of 30 and 40 cm rises from saturation to super- heated vapor. Oscillation of experimental vapor temperature occurs. In the case of 30 cm fill level, it can be observed that the
46、 experimental vapor temperature becomes more violent than that of 40 cm fill level. However, both experimental liquid temperatures in Figures 5 and 6 always keep saturated. Both predicted vapor and liquid temperature profiles for the fill level of 30 and 40 cm still keep constant. It is obvious that
47、 the heat transfer mechanisms at the 30 and 40 cm fill levels change from nucleate boiling to geyser boiling. Under geyser boiling conditions, liquid is periodically propelled from the evapora- tor to the condenser section with a significant velocity. This liquid with fast movement results in oscill
48、ating heat transfer and produces a strange sound in the thermosiphon. In an extreme case, it may damage the container wall. The reason for these discrepancies between the experi- mental measurements and the present model is that the appli- cation of the present theoretical model is suitable only for
49、 the nucleate boiling stage. We use the nucleate boiling correlation to determine the convective thermal resistances of refrigerant among Rtl and Rt2 in Figure 2. Nucleate boiling with an excel- lent heat transfer coefficient makes the convective thermal resistances of refrigerant far smaller than the conductive ther- mal resistance of PCM. Thus, the thermal resistances of nucle- ate boiling can be neglected. The overall thermal resistance in the cold storage system is determined only by conductive ther- mal resistance. With the lapse of time during the charge process, the heat transfer me
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