Development of a thermoelectric refrigerator with two-phase thermosyphons and capillary lift

A thermoelectric domestic refrigerator has been developed, with a single compartment of 0.225 m³, for food preservation at 5 °C. The cooling system is made up of two equal thermoelectric devices, each composed of a Peltier module (50 W) with its hot side in contact with a two-phase and natural convection thermosyphon (TSV) and a two-phase and capillary lift thermosyphon (TPM), in contact with the cold side.The entire refrigerator has been simulated and designed using a computational model, based on the finite difference method. Subsequently an experimental optimization phase of the thermosyphons was carried out, until thermal resistance values of R_TSV = 0.256 K/W and R_TPM = 0.323 K/W were obtained. These values were lower than those obtained with finned heat sinks.Finally, a functional prototype of a thermoelectric refrigerator was built, and the results which were obtained demonstrate that it is able to maintain a thermal drop (Ambient Temperature–Inside Temperature) of 19 °C. The electric power consumption at nominal conditions was 45 W, reaching a COP value of 0.45. The study demonstrated that by incorporating these two-phase devices into thermoelectric refrigeration increases the COP by 66%, compared with those which use finned heat sinks.     

Development of a heat exchanger for the cold side of a thermoelectric module

A heat exchanger for the cold side of Peltier pellets in thermoelectric refrigeration, based on the principle of a thermosyphon with phase change and capillary action has been developed. This device improves the thermal resistance between the cold side of a Peltier pellet and the refrigerated ambient by 37% (from 0.513 of the finned heat sink, to 0.323 K/W). Analytic calculations and experimental optimisation of the TPM have been carried out by building and testing several prototypes. It also has been experimentally proved that the COP of thermoelectric refrigerators can be improved up to 32% (from 0.297 to 0.393) by incorporating the developed device.     

Influence of carbon nanotube suspension on the thermal performance of a miniature thermosyphon

An experimental study was carried out to understand the heat transfer performance of a miniature thermosyphon using water-based carbon nanotube (CNT) suspensions as the working fluid. The suspensions consisted of deionized water and multi-wall carbon nanotubes with an average diameter of 15 nm and a length range of 5–15 μm. Experiments were performed under three steady operation pressures of 7.4 kPa, 13.2 kPa and 20 kPa, respectively. Effects of the CNT mass concentration and the operation pressure on the average evaporation and condensation heat transfer coefficients, the critical heat flux and the total heat resistance of the thermosyphon were investigated and discussed. Experimental results show that CNT suspensions can apparently improve the thermal performance of the thermosyphon and there is an optimal CNT mass concentration (about 2.0%) to achieve the maximum heat transfer enhancement. The operation pressure has a significant influence on the enhancement of the evaporation heat transfer coefficient, and slight influences on the enhancement of the critical heat flux. The enhanced heat transfer effect is weak at low heat fluxes while it is increased gradually with increasing the heat flux. The present experiment confirms that the thermal performance of a miniature thermosyphon can be strengthened evidently by using CNT suspensions.     

Assessment of thermoelectric module with nanofluid heat exchanger

For applications such as cooling of electronic devices, it is a common practice to sandwich the thermoelectric module between an integrated chip and a heat exchanger, with the cold-side of the module attached to the chip. This configuration results thermal contact resistances in series between the chip, module, and heat exchanger. In this paper, an appraisal of thermal augmentation of thermoelectric module using nanofluid-based heat exchanger is presented. The system under consideration uses commercially available thermoelectric module, 27 nm Al₂O₃–H₂O nanofluid, and a heat source to replicate the chip. The volume fraction of nanofluid is varied between 0% and 2%. At optimum input current conditions, experimental simulations were performed to measure the transient and steady-state thermal response of the module to imposed isoflux conditions. Data collected from the nanofluid-based exchanger is compared with that of deionized water.Results show that there exist a lag-time in thermal response between the module and the heat exchanger. This is attributed to thermal contact resistance between the two components. A comparison of nanofluid and deionized water data reveals that the temperature difference between the hot- and cold-side, ΔT = T_h ? T_c ≈ 0, is almost zero for nanofluid whereas ΔT > 0 for water. When ΔT ≈ 0, the contribution of Fourier effect to the overall heating is approximately zero hence enhancing the module cooling capacity. Experimental evidence further shows that temperature gradient across the thermal paste that bonds the chip and heat exchanger is much lower for the nanofluid than for deionized water. Low temperature gradient results in low resistance to the flow of heat across the thermal paste. The average thermal contact resistance, R = ΔT/Q, is 0.18 and 0.12 °C/W, respectively for the deionized water and nanofluid. For the range of optimum current, 1.2 ? current ? 4.1 A, considered in this study, the COP ranges between 1.96 and 0.68.     

Experimental investigation of small diameter two-phase closed thermosyphons charged with water,FC-84, FC-77 and FC-3283

An experimental investigation of the performance of thermosyphons charged with water as well as the dielectric heat transfer liquids FC-84, FC-77 and FC-3283 has been carried out. The copper thermosyphon was 200 mm long with an inner diameter of 6 mm, which can be considered quite small compared with the vast majority of thermosyphons reported in the open literature. The evaporator length was 40 mm and the condenser length was 60 mm which corresponds with what might be expected in compact heat exchangers. With water as the working fluid two fluid loadings were investigated, that being 0.6 ml and 1.8 ml, corresponding to approximately half filled and overfilled evaporator section in order to ensure combined pool boiling and thin film evaporation/boiling and pool boiling only conditions, respectively. For the Fluorinert? liquids, only the higher fill volume was tested as the aim was to investigate pool boiling opposed to thin film evaporation. Generally, the water-charged thermosyphon evaporator and condenser heat transfer characteristics compared well with available predictive correlations and theories. The thermal performance of the water-charged thermosyphon also outperformed the other three working fluids in both the effective thermal resistance as well as maximum heat transport capabilities. Even so, FC-84, the lowest saturation temperature fluid tested, shows marginal improvement in the heat transfer at low operating temperatures. All of the tested Fluorinert? liquids offer the advantage of being dielectric fluids, which may be better suited for sensitive electronics cooling applications and were all found to provide adequate thermal performance up to approximately 30–50 W after which liquid entrainment compromised their performance.     

Development and experimental validation of a computational model in order to simulate ice cube production in a thermoelectric ice-maker

We have developed a computational model which allows the simulation of a thermoelectric device to make ice cubes in a vapour compression domestic fridge. This model solves both the thermoelectric and heat transfer equations, including the phase change equations in the ice cube production.The inputs of the model are: the thermoelectric parameters as a function of the temperature; dimensions; material properties (thermal resistances and capacities) and the boundary conditions (room temperature and voltage supplied to the Peltier module). The outputs are the values of the temperature for all the elements of the thermoelectric ice-maker and the ice production.In the experimental phase a prototype of a thermoelectric ice-maker incorporated in a vapour compression domestic fridge was constructed in order to adjust and validate the computational model, and to optimise the experimental application. This ice-maker has two Peltier modules, some aluminum cylinders, called fingers, where the ice is made, and a component that acts as heat extender and dissipater which connects the hot side of Peltier module with the freezer compartment. The ice formation on the fingers is obtained by the cooling on the Peltier modules. When the ice cubes are formed, the voltage polarity of the thermoelectric modules is switched so the fingers warm up until the ice around the fingers melts. Then the ice cubes are dropped by gravity.This paper studies the production of ice cubes using the computational model and the experiment results and analyses the most important parameters for the optimisation of the ice-maker (voltage supplied to the Peltier module, thermal resistance of the hot side dissipater and initial water temperature).     

Carbon nanotube thermal interfaces on gadolinium foil

We report the thermal behavior of gadolinium foils to be used in magneto thermoelectric generator cells. Magneto thermoelectric generator cell technology exploits the ferromagnetic phase transition of gadolinium to drive the movement of a diaphragm ‘shuttle’ whose mechanical energy is converted to electrical form and which enhances heat transfer through both conduction and convection. Efficient heat transfer at mechanical interfaces is critical to increase shuttle speed and the commensurate rate of heat transfer. The synthesis and characterization of carbon nanotube thermal interfaces for the gadolinium foils are described. The total thermal interface resistance of the carbon nanotube coated gadolinium was measured using a one-dimensional reference calorimeter technique. Improvement of carbon nanotube growth based on parametric process variations is described, and the effect of hydrogen embrittlement on the magnetic properties of the gadolinium foils during carbon nanotube growth is quantified. The samples generated in this study were consistently measured with total thermal interface resistances in the range of 65–105 mm² K/W, a reduction of 55–70% compared to bare gadolinium (R_int ≈ 230 mm² K/W). The addition of carbon nanotube arrays did not alter the magnetic properties of the gadolinium foils and only a slight decrease in the magnetic moment of the gadolinium samples (8–13%) was measured after growth.     

Thermoelectric air-cooling module for electronic devices

This article investigates the thermoelectric air-cooling module for electronic devices. The effects of heat load of heater and input current to thermoelectric cooler are experimentally determined. A theoretical model of thermal analogy network is developed to predict the thermal performance of the thermoelectric air-cooling module. The result shows that the prediction by the model agrees with the experimental data. At a specific heat load, the thermoelectric air-cooling module reaches the best cooling performance at an optimum input current. In this study, the optimum input currents are from 6 A to 7 A at the heat loads from 20 W to 100 W. The result also demonstrates that the thermoelectric air-cooling module performs better performance at a lower heat load. The lowest total temperature difference-heat load ratio is experimentally estimated as ?0.54 W K^?1 at the low heat load of 20 W, while it is 0.664 W K^?1 at the high heat load of 100 W. In some conditions, the thermoelectric air-cooling module performs worse than the air-cooling heat sink only. This article shows the effective operating range in which the cooling performance of the thermoelectric air-cooling module excels that of the air-cooling heat sink only.     

Sintered porous heat sink for cooling of high-powered microprocessors for server applications

This paper experimentally investigates the sintered porous heat sink for the cooling of the high-powered compact microprocessors for server applications. Heat sink cold plate consisted of rectangular channel with sintered porous copper insert of 40% porosity and 1.44 × 10^?11 m² permeability. Forced convection heat transfer and pressure drop through the porous structure were studied at Re ? 408 with water as the coolant medium. In the study, heat fluxes of up to 2.9 MW/m² were successfully removed at the source with the coolant pressure drop of 34 kPa across the porous sample while maintaining the heater junction temperature below the permissible limit of 100 ± 5 °C for chipsets. The minimum value of 0.48 °C/W for cold plate thermal resistance (R_cp) was achieved at maximum flow rate of 4.2 cm³/s in the experiment. For the designed heat sink, different components of the cold plate thermal resistance (R_cp) from the thermal footprint of source to the coolant were identified and it was found that contact resistance at the interface of source and cold plate makes up 44% of R_cp and proved to be the main component. Convection resistance from heated channel wall with porous insert to coolant accounts for 37% of the R_cp. With forced convection of water at Re = 408 through porous copper media, maximum values of 20 kW/m² K for heat transfer coefficient and 126 for Nusselt number were recorded. The measured effective thermal conductivity of the water saturated porous copper was as high as 32 W/m K that supported the superior heat augmentation characteristics of the copper–water based sintered porous heat sink. The present investigation helps to classify the sintered porous heat sink as a potential thermal management device for high-end microprocessors.     

Modeling and testing of an advanced compact two-phase cooler for electronics cooling

This paper describes the modeling, design, and testing of a high flux and yet compact two-phase CPU cooler, with excellent attributes of low thermal resistance that are derived from the intrinsic design features of phase change phenomena and minimal vapor pressure drop of the device. For the same footprint of a conventional cooler, the prototype rejects more than twice the capacity of CPUs of today. The unique design minimizes its overall size and yet provides adequate area for forced convection cooling. Testing was conducted over an assorted heat loads and air flow rates flowing through the fins, achieving a best performance of 0.206 K/W of device thermal resistance at a rating of 203 W under an air flow rate of 0.98 m³/min. The prototype device is orientation free where a 90° tilt could perform at the same rating conditions.     

Experimental investigations on phase change material based finned heat sinks for electronic equipment cooling

This paper reports the results of an experimental investigation of the performance of finned heat sinks filled with phase change materials for thermal management of portable electronic devices. The phase change material (PCM) used in this study is n-eicosane and is placed inside a heat sink made of aluminium. Aluminium acts as thermal conductivity enhancer (TCE), as the thermal conductivity of the PCM is very low. The heat sink acts as an energy storage and a heat-spreading module. Studies are conducted for heat sinks on which a uniform heat load is applied for the unfinned and finned cases. The test section considered in all cases in the present work is a 80 × 62 mm² base with TCE height of 25 mm. A 60 × 42 mm² plate heater with 2 mm thickness is used to mimic the heat generation in electronic chips. Heat sinks with pin fin and plate fin geometries having the same volume fraction of the TCE are used. The effect of different types of fins for different power level (ranging from 2 to 7 W) in enhancing the operating time for different set point temperatures and on the duration of latent heating phase were explored in this study. The results indicate that the operational performance of portable electronic device can be significantly improved by the use of fins in heat sinks filled with PCM.     

Design and experimental investigation of portable solar thermoelectric refrigerator

The main objective of this study is to design and build an affordable solar thermoelectric refrigerator for the Bedouin people (e.g. deserts) living in remote parts of Oman where electricity is still not available. The refrigerator could be used to store perishable items and facilitate the transportation of medications as well as biological material that must be stored at low temperatures to maintain effectiveness. The design of the solar-powered refrigerator is based on the principles of a thermoelectric module (i.e., Peltier effect) to create a hot side and a cold side. The cold side of the thermoelectric module is utilized for refrigeration purposes; provide cooling to the refrigerator space. On the other hand, the heat from the hot side of the module is rejected to ambient surroundings by using heat sinks and fans. The designed solar thermoelectric refrigerator was experimentally tested for the cooling purpose. The results indicated that the temperature of the refrigeration was reduced from 27 °C to 5 °C in approximately 44 min. The coefficient of performance of the refrigerator (COP_R) was calculated and found to be about 0.16.     

Air heat exchangers with long heat pipes: Experiments and predictions

This paper presents measurements and predictions of a heat pipe-equipped heat exchanger with two filling ratios of R134a, 19% and 59%. The length of the heat pipe, or rather thermosyphon, is long (1.5 m) as compared to its diameter (16 mm). The airflow rate varied from 0.4 to 2.0 kg/s. The temperatures at the evaporator side of the heat pipe varied from 40 to 70 °C and at the condenser part from 20 to 50 °C. The measured performance of the heat pipe has been compared with predictions of two pool boiling models and two filmwise condensation models. A good agreement is found. This study demonstrates that a heat pipe equipped heat exchanger is a good alternative for air–air exchangers in process conditions when air–water cooling is impossible, typically in warmer countries.     

Experimental investigations on thermal performance enhancement and effect of orientation on porous matrix filled PCM based heat sink

Thermal performance in terms of enhancement ratios and the effect of orientation of a copper porous matrix filled phase change material (PCM) based heat sink are experimentally studied in this paper. N-eicosane is used as the phase change material. A copper open cell metal foam, press fitted into an aluminium casing is the thermal conductivity enhancer. In PCM based heat sinks, low thermal conductivity associated with PCMs makes the use of enhancement techniques inevitable for better thermal performance. A plate heater with an overall dimension of 60 × 42 mm² with 2 mm thickness is used to mimic the heat generation in electronic chips. The effect of orientation of the heat sink on thermal performance is studied by developing a tracking system, capable of placing the heat sink at any specified orientation.     

Reducing cold-start emission from internal combustion engines by means of thermal energy storage system

Increasing environmental pollutions is an important problem appearing at cold start of internal combustion engines. Developments of new devices that solve this problem are an extremely urgent need especially for cold regions. In this study, a developed experimental sample of thermal energy storage system (TESS) for pre-heating of internal combustion engines has been designed and tested. The development thermal energy storage device (TESD) works on the effect of absorption and rejection of heat during the solid–liquid phase change of heat storage material (Na₂SO₄ · 10H₂O). The TESS has been applied to a gasoline engine at 2 °C temperature and 1 atm pressure. Charging and discharging time of the TESD are about 500 and 600 s, respectively and temperature of engine is increased 17.4 °C averagely with pre-heating. Maximum thermal efficiency of the TESS system is 57.5 % after 12 h waiting duration. CO and HC emissions decrease about 64% and 15%, respectively, with effect of pre-heating engine at cold start and warming-up period.     

Experimental analysis of flow and heat transfer in a miniature porous heat sink for high heat flux application

A novel miniature porous heat sink system was presented for dissipating high heat fluxes of electronic device, and its operational principle and characteristics were analyzed. The flow and heat transfer of miniature porous heat sink was experimentally investigated at high heat fluxes. It was observed that the heat load of up to 280 W (heat flux of 140 W/cm²) was removed by the heat sink with the coolant pressure drop of about 34 kPa across the heat sink system and the heater junction temperature of 62.9 °C at the coolant flow rate of 6.2 cm³/s. Nu number of heat sink increased with the increase of Re number, and maximum value of 323 for Nu was achieved at highest Re of 518. The overall heat transfer coefficient of heat sink increased with the increase of coolant flow rate and heat load, and the maximal heat transfer coefficient was 36.8 kW(m² °C)^?1 in the experiment. The minimum value of 0.16 °C/W for the whole thermal resistance of heat sink was achieved at flow rate of 6.2 cm³/s, and increasing coolant flow rate and heat fluxes could lead to the decrease in thermal resistance. The micro heat sink has good performance for electronics cooling at high heat fluxes, and it can improve the reliability and lifetime of electronic device.     

Effect of functionalized MWCNTs/water nanofluids on thermal resistance and pressure fluctuation characteristics in oscillating heat pipe

An influence of multi-walled carbon nanotube (MWCNT) based aqueous nanofluids with different concentrations on the heat transport and the relevant pressure distribution in oscillating heat pipe (OHP) has been investigated. The present paper describes the heat transfer phenomena in terms of thermal resistance, pressure and frequency of pressure fluctuation in multi-loop oscillating heat pipe (OHP) charged by aqueous nanofluids with MWCNT loadings of 0.05 wt.%, 0.1 wt.%, 0.2 wt.% and 0.3 wt.%. The multi-loop OHP with 3 mm inner diameter has been conducted in the experiment at 60% filling ratio. Experimental results show that thermal characteristics are significantly inter-related with pressure distribution and strongly depend upon the number of pressure fluctuations with time. The investigation shows that the 0.2 wt.% MWCNTs based aqueous nanofluids obtain maximum number of the fluctuation frequency and low thermal resistance at any evaporator power input. Based on the experimental results, we discuss the reasons for enhancement and decrement of thermal characteristics of the nanofluids.     

Experimental investigation on performance of a rotating closed loop pulsating heat pipe

Pulsating heat pipes (PHPs) are interesting heat transfer devices. Their simple, high maintaining, and cheap arrangement has made PHPs very efficient compared to conventional heat pipes. Rotating closed loop PHP (RCLPHP) is a novel kind of them, in which the thermodynamic principles of PHP are combined with rotation. In this paper, effect of rotational speed on thermal performance of a RCLPHP is investigated experimentally. The research was carried out by changing input power (from 25 W to 100 W, with 15 W steps) and filling ratio (25%, 50%, and 75%) for different rotational speeds (from 50 rpm to 800 rpm with an increment of 125 rpm). The results presented that at a fixed filling ratio, thermal resistance of RCLPHP decreased with increasing heat input applied to evaporator. Above a certain range of heat input, probability of partial dry-out of evaporator existed, which led into thermal performance deterioration of RCLPHP. Moreover, thermal resistance of RCLPHP decreased with increasing rotational speed and probability of partial dry-out in the evaporating section reached to its least amount.     

Simulation and experiment of thermal energy management with phase change material for ageing LiFePO4 power battery

Thermal energy management performance of ageing commercial rectangular LiFePO₄ power batteries using phase change material (PCM) and thermal behavior related to thermal conductivity between the PCM and the cell are discussed in this paper. The heat sources are simplified according to the experimental results of the cells discharged at 35 A (≈5 C). 3-D modules of a single cell and battery pack are formulated, respectively. The results show that the thermal resistance in the cell leads to an inevitable temperature difference. It is necessary to improve the thermal conductivity and to lower the melting point of the PCM for heat transfer enhancement. The PCM with a melting point lower than 45 °C will be more effective for heat dissipation, with a desired maximum temperature below 50 °C. The temperature difference in the whole unit before PCM melting will be decreased significantly. In addition, a proper k_PCM:k_c is necessary for a well designed battery thermal energy management system.     

Operating limit of heat transport in two-phase thermosyphon with connecting pipe (heated surface temperature fluctuation and flow pattern)

An experiment on heat transport phenomena has been carried out in a two-phase thermosyphon with an adiabatic connecting pipe using water as the working fluid at atmospheric pressure. The thermosyphon has an upper liquid chamber and a lower vapor chamber, which are connected with an adiabatic pipe. A horizontal upward-facing heated surface is installed in the bottom of the lower vapor chamber.The size of the connecting pipe is an inner diameter D_p = 2, 3, 4, 5, 6 and 8 mm and a length L = 250, 500 and 1000 mm. As the heat is supplied into the thermosyphon, the temperature of heated surface starts fluctuating at a heat flux at which unstable vapor–liquid counter current flow is generated in the connecting pipe. Bubbles at the upper end of the connecting pipe were photographed when the temperature fluctuation started. It was found that the heat flux at the onset of the temperature fluctuation increases with an increase in D_p and then can be predicted well by Eq. (1), which was derived based on the flooding velocity presented by Wallis [G.B. Wallis, One dimensional two-phase flow, McGraw Hill, New York, 1969], with C_w = 0.7 for D_p = 5, 6 and 8 mm. Furthermore, we clarified that the cause of this fluctuation comes from the inlet effect of the connecting pipe and we demonstrated this finding using a bell mouth, which was installed at either the bottom end or both ends of the connecting pipe.