sign in sign up
<<
Home , Coolant

10th IHPS, Taipei, Taiwan, Nov. 6-9, 2011

Thermal Performance of a for Medium-Temperaturethermal Storage System

Min Kyu Park a, Joon Hong Boo b a Graduate School, Korea Aerospace University b School of Aerospace and , Korea Aerospace University 200-1 Hwajeon, Goyang, Gyeonggi-do, 412-791 Korea Tel : +82-2-300-0107, Fax: +82-2-3158-2191, E-mail: [email protected]

ABSTRACT An experimental study was conducted to investigate the thermal performance of a heat pipe having two dissimilar condenser sections which were subject to different boundary conditions. The first condenser dissipates heat to the surrounding air through annular fins by natural , while the second one is cooled by a by at different temperatures. The container and the wick were made of stainless steel and the was Dowtherm-A for medium-temperature applications. The diameter and length of the heat pipe were 25.4 mm and 1 m, respectively. The maximum thermal load was 1 kW and the operating temperature of the heat pipe was around 250 ℃. The liquid fill charge was adjusted so that the first condenser section may work as an when the original evaporator was inactive in a vertical configuration. The experimental results were analyzed in terms of thermal resistance and effective thermal conductance against input heat flux and operating temperature. Keywords: heat pipe, medium-high temperature, dissimilar condensers, experiment

1. INTRODUCTION heat pipe for various configurations and operating conditions. Recent experimental study on the The medium-temperature range for a heat pipe fundamental aspects of the Dowtherm-A heat is normally between 550 and 750 K, based on the pipes can be found in Park et al (2006) for a operating temperature (Faghri, 1995). Heat pipes thermosyphon type with single evaporator and have been considered as promising means for condenser. effective in energy transport and storage systems in a medium-high temperature This study was focused on a heat pipe for a range, such as concentrated system charging and systems. Although is the best working fluid discharging heat at 400 oC and 220 oC, respectively, for heat pipes operating below 500 K unless the in a typical solar thermal power plant. The electric insulation is required, it is not suitable to proposed thermal energy storage system consisted be used in the medium temperature range in of a solid-liquid phase-change material (PCM, general. This renders considerable difficulties in hereafter), typically nitrates of alkali metals, and a fabricating medium temperature heat pipes. penetrating heat pipe. The advantage of using a PCM may contribute to a volume reduction of the being excluded due to its toxicity, thermal storage system utilizing the . there are only few options, to date, for heat pipe The heat pipe interfaced with heat source and sink working fluids in the medium temperature range: at heat exchanging sections located below and synthetic fluids such as Flutec PP2 and PP9, and above the PCM. During a heat charging mode, the Dowtherm-A. Among these, Flutec fluids are heat pipe part interfacing with heat source usually employed for dielectric applications, and functioned as an evaporator and the portion the maximum temperatures for use of PP2 and embedded in the PCM worked as a condenser PP9 are known to be 160 oC and 225 oC. On the section. During a heat discharging mode however, other hand, Dowtherm-A, which is known also the heat pipe part embedded in the PCM with commercial name Thermex, is a eutectic functioned as an evaporator and the part mixture of diphenyl and diphenyl ether and can be o interfacing with the heat sink worked as a used in the range of 150 to 400 C (420 to 670 K). condenser. A similar configuration was From the viewpoint of usable temperature range, investigated experimentally by Liu et al.(2006) Dowtherm-A is the most useful working fluid for using a thermosyhon made of 950-mm long medium-temperature heat pipes (Reay and Kew, pipe filled with acetone. Unfortunately 2006). Therefore, it is meaningful to investigate however, the result of their study is not applicable the performance characteristics of a Dowtherm-A to medium-temperature devices since the heat

- 13 -

source was below 100 oC and the melting evaporator internal volume. These values temperature of the PCM was 52 oC (that of corresponded to 23 to 32% of evaporator internal industrial paraffin wax). volume during a heat discharging mode, and 372 to 420% based on the total void volume of the In an experimental study by Lee et al (2009) on wick. The specification of the heat pipe is a Dowtherm-A heat pipe with internal screen wick, summarized in Table 1. the working fluid charge was greater than the whole evaporator internal volume to ensure a startup during the heat discharging mode. The operating temperature of the heat pipe was nearly 320 oC and the effective thermal conductance reached 6,000 W/m-K. However, their study was limited to a thermal performance of the heat pipe with single condenser simulating a heat charging mode. This study is aimed at conducting experimental study on the Dowtherm-A heat pipe with two dissimilar condenser sections subject to different boundary conditions that may be encountered in medium-temperature application. 2. HEAT PIPE AND EXPERIMENTAL SETUP 2.1. Fabrication of a Dowtherm-A Heat Pipe The Dowtherm-A heat pipe was fabricated with two different condenser sections. The first condenser section in actual thermal storage system would be embedded in a PCM. For convenience of the experimental setup however, it was cooled by natural convection in ambient air. It is considered that typical PCM’s in the medium temperature range normally exhibit very low thermal Figure 1. Schematic of the heat pipe with conductivity values (typically, 0.1 to 0.5 W/m-K), thermocouple locations. which can be approximated conservatively by that of air (typically, 0.05 W/m-K). The second condenser part was cooled by forced convection with liquid through block. Annular fins Table 1. Specification of the heat pipe were attached only to the first condenser to STS 316L enhance heat transfer. The heat pipe container and Container (mm) fins were made of stainless steel. The diameter and O.D.: 25.4, Thickness : 2 length of the heat pipe were 25.4 mm and 1 m, STS 304 Wick respectively. The lengths of the evaporator, the Screen mesh : #40, 2 layers first condenser, and the second condenser were Dowtherm-A 200, 470, and 200 mm, respectively, with two Working fluid Fill charge : 372 ~ 420% (based adiabatic sections of 65 mm in between the three on void volume of the wick) sections (See Figure 1). Evaporator : 200 Heat was supplied to the evaporator by a adiabatic section : Length (mm) ceramic mold type electric heater simulating the 65 (lower) + 65 (higher) high-temperature in an actual solar thermal Condenser : 470 + 200 system. To simulate a heat discharging mode, heat Thickness : 1, Height : 32.5, should be supplied to the first condenser section Fin (mm) Pitch : 10 while the evaporator was kept insulated. To ensure a heat pipe startup during a heat discharging mode, the working fluid charge was 133.3 to 150.5 ml, which corresponded to 155 to 175% of the original

- 14 -

2.2. Experimental Setup and Experimental Methods The temperatures of the heat pipe wall were measured by K-type thermocouples at the locations depicted in Figure 1. A cooling block was attached to the heat pipe wall in the second condenser section (condenser 2) at the top. The flow rate, the coolant temperatures at the inlet and outlet of the cooling block were used to estimate the amount of heat actually transported through the heat pipe, .

(1)

where and represent mass flow rate and Figure 2. Axial temperature variation of the specific heat of coolant. A series of experiments heat pipe as a function of coolant temperature was conducted during this study against variations (for a 900-W heat input, condenser1 natural in the working fluid charge, heat input, and cooling) coolant inlet temperature. The conbination of the last two was utilized to acquire a desired operating temperature of the heat pipe so that three different coolant inlet temperature of 40, 60 and 80 oC were imposed for every heat input, from the electric heater. Based on the comparison between and , the heat loss of the setup was estimated to be less than 15% in most cases except when a dry-out occurred for a single-condenser (condenser 2 only) operation, where the calculated heat loss exceeded 20%. The amount of heat discharged through condenser 1 was estimated combining two equations for a vertical cylindrical wall and for annular fin surfaces by natural convection. The heat loss was estimated by substracting the heats discharged through condenser 1 and condenser 2 from . Figure 3. Axial temperature variation of the heat pipe as a function of coolant temperature 3. RESULTS AND DISCUSSION (for a 900-W heat input with condenser1 insulated) Figures 2 and 3 compares steady-state axial temperature distributions of the heat pipe against It is observed that the operating temperature of fill charge and coolant inlet temperature variations, the heat pipe decreased with fill charge amount for for the cases the condenser 1 was cooled by a same coolant inlet temperature. The heat pipe natural convection and was insulated, when the wall temperatures in the condenser 1 as well as those in the evaporator increased as much as 25 to heat inputs were same with 900 W. The operating o temperature of the heat pipe was assumed to be 50 C when the condenser 1 was inactive by insulation. In addition, the operating temperature the same as the wall temperature in the adiabatic o section, as normally adopted in heat pipe studies remained constant, with maximum of 5 C fluctuation at 275 oC, when condenser 1 was insulated, while the coolant temperature and fill charge ratio varied.

- 15 -

Figures 4 and 5 represent the variation of thermal resistance as functions of the heat flux and the operating temperature, respectively. The results are shown only for a coolant inlet temperature of 80 oC since the highest operating temperatures were achieved among three different coolant inlet conditions. The thermal resistance of a heat pipe in this study was calculated by the following equation, considering that two different condensers should be combined properly to represent the average condenser surface temperature:

(2) where and represent the average wall temperatures of the evaporator and condenser, respectively. was determined by the Figure 4. Thermal resistance of the heat pipe as following equation. a function of heat flux and fill charge ratio (for a coolant inlet of 80 oC) (3) where the subscripts cond1 and cond2 denote the values for condenser 1 and condenser 2, respectively. The thermal resistance reduced monotonically with increase of heat flux in every case. The minimum thermal resistance of 0.12 oC/W was achieved for a heat pipe with dissimilar condensers with fill charge of 420% and operating temperature of 268.1 oC, which corresponded to an input heat flux of 69 kW/m 2. When condenser 1 was insulated, the minimum thermal resistance for the same heat flux was about 0.24 oC/W. In general, the thermal resistance value of the case with insulated condenser 1 was less than a half of the value with insulation. The effective thermal conductance, , of a heat pipe can be calculated by the following equation. Figure 5. Thermal resistance of the heat pipe as (4) a function of the operating temperature and fill charge ratio (for a coolant inlet of 80 oC) where is the cross-sectional area of the heat Figures 6 and 7 summarize the effective pipe, and is an effective transport length thermal conductance as functions of heat flux and calculated by operating temperature, respectively. In general, 0.5( ), which was the effective thermal conductance of the heat pipe 0.9 m and 0.533 m for the heat pipes with single increased with heat flux and operating temperature. condenser and the one with dissimilar condensers, While it was obvious that the effective thermal respectively. conductance increased with fluid fill charge for a heat pipe with inactive condenser 1, no definite trends can be stated for a heat pipe with active condenser 1. However, the absolute value of the effective thermal conductance of the heat pipe with active condenser 1 exhibited higher

- 16 -

Figure 8. Thermal resistance of the heat pipe as a function of input heat flux (for coolant inlet Figure 6. Effective of the o heat pipe as a function of heat flux (for coolant 80 C, heat discharging mode) inlet of 80 oC )

Figure 91. Effective thermal conductivity of the heat pipe as a function of input heat flux (for coolant inlet 80 oC, heat discharging mode) Figure 7. Effective thermal conductivity of the The effective thermal conductance for a heat pipe as a function of the operating discharging mode exhibited linear increase with temperature (for coolant inlet of 80 oC) input heat flux, and those for fill charge ratio of than twice the values. The maximum value 408% showed higher values than other fill charges. was about 8,900 W/m-K, which corresponded to The maximum input heat flux before a dry-out about 22 times of the thermal conductivity of a occurred was 32.5 kW/m 2, and the corresponding commercial copper. effective thermal conductivity was 3,400 W/m-K, which was less than a half of the maximum value The same liquid fill charge ratios and coolant achieved for a charging mode simulation. The inlet temperatures were imposed for the same heat corresponding thermal resistance was almost twice pipe for discharging mode, where the original of the minimum value for charging mode. evaporator was kept inactive, the heat was input through the condenser 1 in Figure 1, and the heat 4. CONCLUSIONS was removed only from condenser 2. Figures 8 and 9 summarize the experimental results in terms The following conclusions can be stated based of thermal resistance and effective thermal on the observation through the experiments conductance as a function of input heat flux. Both conducted in this study. figures represent the results for a typical coolant (1) The Dowtherm-A heat pipe having two inlet temperature of 80 oC. dissimilar condensers during a charging mode

- 17 -

exhibited an effective thermal conductance as Renewable Energy Technology Development much as twenty two times that of commercial Program of the Korea Institute of Energy copper for the operating temperature near Technology Evaluation and Planning (KETEP) 270 oC. The heat pipe with inactive condenser grant funded by the Korea Government Ministry 1 was able to reach a highest operating of Knowledge and Economy under the Project temperature of 322 oC, where the maximum Number 2005-N- SO17-P-01. effective thermal conductance exhibit a half of the above value. During a discharging mode it REFERENCES reduced to a lower value, which was still of tenth order of copper. [1] Faghri, A., Heat Pipe Science and (2) For the heat pipe with inactive condenser 1, Technology, Taylor and Francis, pp. 19-24, the thermal resistance reduced distinctly with 1995 increase of working fluid fill charge. However, the thermal resistance variation was [2] Reay, D. and Kew, P., Heat Pipes, Elsevier, insensitive with liquid fill charge for the heat pp.108-114, 2006. pipe with dissimilar condensers. [3] Park, K.H., Lee, Y.S., Na, S.H., and Chang, (3) Despite of a narrower operating range of input K.C., "An Experimental Study on the heat flux and lower temperature, the heat pipe Operating Characteristics of the Naphthalene with two dissimilar condensers had a superior and Dowtherm Heat Pipe," Proc. KSME effective thermal conductance to the heat pipe Annual Meeting, pp.1966-1971, 2006. with single condenser

(4) During a charging mode, dry-out occurred [4] Lee, S.K., Kwak, H.H., Boo, J.H., Kim, J.K. only for a heat pipe having inactive condenser and Kang, Y.H., "Thermal Performance of a 1 with liquid fill charge of 372% at a heat flux Dowtherm-A Heat Pipe for the Thermal of 66 kW/m 2. Dry-out was not observed for Storage System at Medium-high heat pipes with higher liquid fill charges up to Temperature," Proc. KSME Fall Annual heat flux of 75 kW/m 2, which was the Conference, pp. 1645 ∼1650, 2009. maximum heater capacity. For a heat pipe with [5] Liu, Z., Wang, Z., and Ma, C., "An dissimilar condensers, dry-out occurred for Experimental Study on Heat Transfer heat fluxes between 60 to 70 kW/m 2. Characteristics of Heat Pipe (5) For a discharging mode, a liquid fill charge of with Latent Heat Storage. Part Ι: Charging 408% exhibited optimum performance. only and Discharging only Modes," Energy Conversion and Management, Vol. 47, pp. NOMENCLATURE 944 ∼966, 2006. average temperature ( oC ) Q thermal load (W) L length (m) k thermal conductivity (W/m-K) A area (m 2) Cp specific heat (kJ/kg-K) thermal resistance (K/W) mass flow rate (kg/s)

Subscripts evp evaporator cond condenser cond1 condenser 1 (see Figure1) cond2 condenser 2 (see Figure1) eff effective rec recovered

ACKNOWLEDGEMENT This work was supported by the New &

- 18 -