Thermal performance analysis for a heat receiver using multiple phase change materials

To solve the problems associated with employing the single melt point phase change material in a heat receiver for the NASA 2 kW solar dynamic power system, this paper presents a practically easy to carry-out PCM receiver model composed of three different phase change temperature materials together with the corresponding physical model. A numerical solution is also given by which the maximal temperature for heat transfer, working fluid exit temperature, and liquid PCM fraction of the total heat transfer tube in whole are calculated. Furthermore, the results are compared with those obtained from the single PCM heat receiver. The results show that it is possible to improve the receiver performance and to reduce both the fluctuation of working fluid temperature and the weight of the heat receiver. All results of the calculation can be used to guide the heat receiver design.     

Experimental investigation of aluminum oxide nanofluid on heat pipe thermal performance

In this research, the effect of using aluminum oxide nanofluid (pure water mixed with Al₂O₃ nanoparticle with 35 nm diameter) on the thermal efficiency enhancement of a heat pipe on the different operating state was investigated.     

Heat transfer performance of an external receiver pipe under unilateral concentrated solar radiation

The heat transfer and absorption characteristics of an external receiver pipe under unilateral concentrated solar radiation are theoretically investigated. Since the heat loss ratio of the infrared radiation has maximum at moderate energy flux, the heat absorption efficiency will first increase and then decrease with the incident energy flux. The local absorption efficiency will increase with the flow velocity, while the wall temperature drops quickly. Because of the unilateral concentrated solar radiation and different incident angle, the heat transfer is uneven along the circumference. Near the perpendicularly incident region, the wall temperature and absorption efficiency slowly approaches to the maximum, while the absorption efficiency sharply drops near the parallelly incident region. The calculation results show that the heat transfer parameters calculated from the average incident energy flux have a good agreement with the average values of the circumference under different boundary conditions. For the whole pipe with coating of Pyromark, the absorption efficiency of the main region is above 85%, and only the absorption efficiency near the parallelly incident region is below 80%. In general, the absorption efficiency of the whole pipe increases with flow velocity rising and pipe length decreasing, and it approaches to the maximum at optimal concentrated solar flux.     

Experimental investigation of silver nano-fluid on heat pipe thermal performance

Nano-fluid is employed as the working medium for a conventional 211 μm wide × 217 μm deep grooved circular heat pipe. The nano-fluid used in this study is an aqueous solution of 35 nm diameter silver nano-particles. The experiment was performed to measure the temperature distribution and to compare the heat pipe thermal resistance using nano-fluid and DI-water. The tested nano-particle concentrations ranged from 1 mg/l to 100 mg/l. The condenser section of the heat pipe was attached to a heat sink that was cooled by water supplied from a constant-temperature bath maintained at 40 °C.At a same charge volume, the measured nano-fluid filled heat pipe temperature distribution demonstrated that the thermal resistance decreased 10–80% compared to DI-water at an input power of 30–60 W. The measured results also show that the thermal resistances of the heat pipe decrease as the silver nano-particle size and concentration increase.     

Thermal analysis of void cavity for heat pipe receiver under microgravity

Based on theoretical analysis of PCM (Phase Change Material) solidification process,the model of improved void cavity distribution tending to high temperature region is established.Numerical results are compared with NASA (National Aeronautics and Space Administration) results.Analysis results show that the outer wall temperature,the melting ratio of PCM and the temperature gradient of PCM canister,have great difference in different void cavity distribution.The form of void distribution has a great effect on the process of phase change.Based on simulation results under the model of improved void cavity distribution,phase change heat transfer process in thermal storage container is analyzed.The main goal of the improved designing for PCM canister is to take measures in reducing the concentration distribution of void cavity by adding some foam metal into phase change material.     

Thermal performance simulation of a solar cavity receiver under windy conditions

Solar cavity receiver plays a dominant role in the light-heat conversion. Its performance can directly affect the efficiency of the whole power generation system. A combined calculation method for evaluating the thermal performance of the solar cavity receiver is raised in this paper. This method couples the Monte-Carlo method, the correlations of the flow boiling heat transfer, and the calculation of air flow field. And this method can ultimately figure out the surface heat flux inside the cavity, the wall temperature of the boiling tubes, and the heat loss of the solar receiver with an iterative solution. With this method, the thermal performance of a solar cavity receiver, a saturated steam receiver, is simulated under different wind environments. The highest wall temperature of the boiling tubes is about 150 °C higher than the water saturation temperature. And it appears in the upper middle parts of the absorbing panels. Changing the wind angle or velocity can obviously affect the air velocity inside the receiver. The air velocity reaches the maximum value when the wind comes from the side of the receiver (flow angle α = 90°). The heat loss of the solar cavity receiver also reaches a maximum for the side-on wind.     

A study on the new separate heat pipe refrigerator and heat pump

A new separate heat pipe refrigerator and heat pump is suggested based on the general three temperature thermal jet refrigerator and heat pump cycle. Sub-cooled hot water or other appropriate liquid heated by low grade heat sources forms the hot end and another heat pipe containing evaporator and condenser ends, adiabatic section of two-phase ejector and throttling tube is as the cold end of the separate heat pipe system. Performance relations for the thermal jet refrigerator and heat pump of such system is analyzed and a method of thermodynamic performance analysis is recommended. Primary prediction shows the feasibility of such heat pipe system for cold and warm water supply.     

Heat transfer analysis of parabolic trough solar receiver

Solar Parabolic Trough Collectors (PTCs) are currently used for the production of electricity and applications with relatively higher temperatures. A heat transfer fluid circulates through a metal tube (receiver) with an external selective surface that absorbs solar radiation reflected from the mirror surfaces of the PTC. In order to reduce the heat losses, the receiver is covered by an envelope and the enclosure is usually kept under vacuum pressure. The heat transfer and optical analysis of the PTC is essential to optimize and understand its performance under different operating conditions. In this paper a detailed one dimensional numerical heat transfer analysis of a PTC is performed. The receiver and envelope were divided into several segments and mass and energy balance were applied in each segment. Improvements either in the heat transfer correlations or radiative heat transfer analysis are presented as well. The partial differential equations were discretized and the nonlinear algebraic equations were solved simultaneously. Finally, to validate the numerical results, the model was compared with experimental data obtained from Sandia National Laboratory (SNL) and other one dimensional heat transfer models. Our results showed a better agreement with experimental data compared to other models.     

Thermal stress analysis of eccentric tube receiver using concentrated solar radiation

In the parabolic trough concentrator with tube receiver system, the heat transfer fluid flowing through the tube receiver can induce high thermal stress and deflection. In this study, the eccentric tube receiver is introduced with the aim to reduce the thermal stresses of tube receiver. The ray-thermal-structural sequential coupled numerical analyses are adopted to obtain the concentrated heat flux distributions, temperature distributions and thermal stress fields of both the eccentric and concentric tube receivers. During the sequential coupled numerical analyses, the concentrated heat flux distribution on the bottom half periphery of tube receiver is obtained by Monte-Carlo ray tracing method, and the fitting function method is introduced for the calculated heat flux distribution transformation from the Monte-Carlo ray tracing model to the CFD analysis model. The temperature distributions and thermal stress fields are obtained by the CFD and FEA analyses, respectively. The effects of eccentricity and oriented angle variation on the thermal stresses of eccentric tube receiver are also investigated. It is recommended to adopt the eccentric tube receiver with optimum eccentricity and 90° oriented angle as tube receiver for the parabolic trough concentrator system to reduce the thermal stresses.     

Parametrical analysis of the design and performance of a solar heat pipe thermoelectric generator unit

This paper describes a solar heat pipe thermoelectric generator (SHP-TEG) unit comprising an evacuated double-skin glass tube, a finned heat pipe and a TEG module. The system takes the advantage of heat pipe to convert the absorbed solar irradiation to a high heat flux to meet the TEG operating requirement. An analytical model of the SHP-TEG unit is presented for the condition of constant solar irradiation, which may lead to different performance characteristics and optimal design parameters compared with the condition of constant temperature difference usually dealt with in other studies. The analytical model presents the complex influence of basic parameters such as solar irradiation, cooling water temperature, thermoelement length and cross-section area and number of thermoelements, etc. on the maximum power output and conversion efficiency of the SHP-TEG. Simulation based on the analytical model has been carried out to study the performance and design optimization of the SHP-TEG.     

Experimental investigation of titanium nanofluids on the heat pipe thermal efficiency

The enhancement heat transfer of the heat transfer devices can be done by changing the fluid transport properties and flow features of working fluids. In the present study, therefore, the enhancement of heat pipe thermal efficiency with nanofluids is presented. The heat pipe is fabricated from the straight copper tube with the outer diameter and length of 15, 600 mm, respectively. The heat pipe with the de-ionic water, alcohol, and nanofluids (alcohol and nanoparticles) are tested. The titanium nanoparticles with diameter of 21 nm are used in the present study which the mixtures of alcohol and nanoparticles are prepared using an ultrasonic homogenizer. Effects of %charge amount of working fluid, heat pipe tilt angle and %nanoparticles volume concentrations on the thermal efficiency of heat pipe are considered. The nanoparticles have a significant effect on the enhancement of thermal efficiency of heat pipe. The thermal efficiency of heat pipe with the nanofluids is compared with that the based fluid.     

Heat extraction from salinity-gradient solar ponds using heat pipe heat exchangers

This paper presents the results of experimental and theoretical analysis on the heat extraction process from solar pond by using the heat pipe heat exchanger. In order to conduct research work, a small scale experimental solar pond with an area of 7.0 m² and a depth of 1.5 m was built at Khon Kaen in North-Eastern Thailand (16°27′N102°E). Heat was successfully extracted from the lower convective zone (LCZ) of the solar pond by using a heat pipe heat exchanger made from 60 copper tubes with 21 mm inside diameter and 22 mm outside diameter. The length of the evaporator and condenser section was 800 mm and 200 mm respectively. R134a was used as the heat transfer fluid in the experiment. The theoretical model was formulated for the solar pond heat extraction on the basis of the energy conservation equations and by using the solar radiation data for the above location. Numerical methods were used to solve the modeling equations. In the analysis, the performance of heat exchanger is investigated by varying the velocity of inlet air used to extract heat from the condenser end of the heat pipe heat exchanger (HPHE). Air velocity was found to have a significant influence on the effectiveness of heat pipe heat exchanger. In the present investigation, there was an increase in effectiveness by 43% as the air velocity was decreased from 5 m/s to 1 m/s. The results obtained from the theoretical model showed good agreement with the experimental data.     

Thermal simulation of a single particle in a falling-particle solar receiver

A transient simulation of the heat transfer in a particle directly heated by the sun, in a falling-particle receiver, is presented. The local temperature can have a non-uniform distribution, that depends on many factors, such as the size of the particle, its rotation, and the exposition time. This fact should be considered when studying the material resistance and aging, or direct chemical reactions in the falling particle. The single-particle simulation presented here can be useful to test the validity of the global fluid dynamics simulations, to study single-particle related aspects such as aging, thermal stresses and chemical stability, to study the distribution of temperature if the particles are not uniform in size, or to study the efficiency in chemical applications.     

Compact cooler for electronics on the basis of a pulsating heat pipe

The paper presents the results of developing and investigating a compact cooler for electronics made on the basis of a closed loop pulsating heat pipe (CLPHP). The cooler is made of a copper tube 5.6 m long with OD of 2 mm and ID of 1.2 mm in the form a 3D spiral containing 17 turns. The device is equipped with a light copper radiator with a finning area of 1670 cm², which was blown by an axial fan located inside the spiral. The thermal interface of the cooler situated in the heating zone is made of a copper plate with a thermocontact surface measuring 40 × 35 mm, which was in thermal contact with all the turns of the device. The cooler overall dimensions are 105 × 100 × 60 mm, its mass is 350 g.The operation of the cooler has been investigated with water, methanol and R141b as working fluids at a uniform and concentrated supply of a heat load in different heating modes. A reliable operation of the device has been demonstrated in the range of heat loads from 5 to 250 W. A minimum thermal resistance “heat source–ambient air” equal to 0.32 °C/W was attained with water and methanol as working fluids at a uniform heat load of 250 W. With a heat load concentrated on a section of the thermal interface limited by an area of 1 cm², a minimum value of thermal resistance equal to 0.62 °C/W was attained at a heat load of 125 W when methanol was used as a working fluid.     

Analyzing the pressure-difference phenomenon between the condensing and boiling sections in a heat pipe cooling system

The heat pipe cooling system in this study consists of a flat evaporator, a condenser, and rising and falling tubes with water as working fluid. The working fluid has different water levels inside the two components. This is due to the vapor pressure deficits of the evaporation section and condenser section. This paper utilizes condensing and boiling pressure-difference theory and measures the temperature of the condenser wall to develop a theoretic model for the water level deficit inside the thermal module. Results indicate that the working fluid infiltrates the condenser and indirectly verifies the phenomenon leading to the different water levels inside the cooling system. Moreover, the water level height difference theory presented in this study may reduce the length of the condenser by 3.14 cm.     

Model-based design and analysis of heat pipe working fluid for optimal performance in a concentric evacuated tube solar water heater

In this study, a semi-dynamic model of a concentric evacuated tube solar water heater is developed to investigate the effect of working fluid design on technical and economic performance of a typical solar water heater in a household located at Sydney, Australia. The model is validated against experimental data. The effects of using water, ammonia, acetone, methanol, and pentane as working fluids of the built-in heat pipe are discussed comparatively during a typical day of operation. Water is identified as the best working fluid amongst the others. The variation of thermal resistance and critical heat flux of the heat pipes due to change in weather condition is presented and discussed. Three hypothetical working fluids are then proposed for further analysis which led to a working fluid design superior to water in performance. It is shown that the performance of the solar water heater can be significantly enhanced up to 28% and 50% from economical and technical points of view, respectively.     

An experimental investigation on heat transfer performance of nanofluid pulsating heat pipe

The effect of SiO2 particles on heat transfer performance of a pulsating heat pipe(PHP) was investigated experimentally.DI water was used as the base fluid and contrast medium for the PHP.In order to study and measure the character,there are SiO2 /H2 O nanofluids with different concentration and applying with various heating powers during the experiment investigation.According to the experimental result,the high fraction of SiO2 /H2 O will deteriorate the performance of PHP compared with DI water,i.e.the thermal resistance and the temperature of evaporation section increases.It is in contrary in the case of low fraction of SiO2 /H2 O.Finally,the comparison of the thermal performances between the normal operation system and the static settlement system is given.It is found that both the thermal resistance of nanofluid PHP and the temperature of the evaporation section increase after standing for a period,and it is the same trend for the temperature fluctuation at the identical heating power for PHP.     

Thermal performance of a solar cooker integrated vacuum-tube collector with heat pipes containing different refrigerants

A solar cooking system using vacuum-tube collectors with heat pipes containing a refrigerant as working fluid has been fabricated, and its performance has been analysed experimentally. The experiments were conducted during clear days in July and August of 2002 in Elazı , Turkey under similar meteorological conditions for three refrigerants and water. Detailed temperature distributions and their time dependences were measured. The maximum temperature obtained in a pot containing 7 l of edible oil was 175 °C. Also, the cooker was successfully used to cook several foods. The cooking processes were performed with the cooker in 27–70 min periods.     

A numerical and experimental study on a heat pipe PV/T system

A novel heat-pipe photovoltaic/thermal system was designed and constructed by the authors. This system can simultaneously supply electrical and thermal energy. In addition, when compared with the traditional water-type photovoltaic/thermal system, this system can be used in cold regions without freezing. A dynamic model was developed to predict the performances of the heat-pipe photovoltaic/thermal system. Experiments were also conducted to validate results obtained for the simulation. A comparison between simulation values and experimental results demonstrated that the model was able to yield satisfactory predictions. Results indicated that the daily thermal and electrical efficiencies of the heat-pipe photovoltaic/thermal system were 41.9% and 9.4%, respectively, while the average heat and electrical gains were 276.9 and 62.3 W/m², respectively. In addition, second-law efficiency, based on the second law of thermodynamics, is provided to analyze the total efficiency of the heat-pipe photovoltaic/thermal system, and the average total second-law efficiency of the system is 6.8%.     

A review of heat pipe systems for heat recovery and renewable energy applications

Advancements into the computational studies have increased the development of heat pipe arrangements, displaying multiphase flow regimes and highlighting the broad scope of the respective technology for utilization in passive and active applications. The purpose of this review is to evaluate current heat pipe systems for heat recovery and renewable applications utility. Basic features and limitations are outlined and theoretical comparisons are drawn with respect to the operating temperature profiles for the reviewed industrial systems. Working fluids are compared on the basis of the figure of merit for the range of temperatures. The review established that standard tubular heat pipe systems present the largest operating temperature range in comparison to other systems and therefore offer viable potential for optimization and integration into renewable energy systems.