The study of latent heat transport characteristics by solid particles and saccharide solution mixtures

The purpose of this study is the development of latent heat transport system by using the mixture of the minute latent heat storage materials and the saccharine solution as medium. The experimental studies are carried out by the evaluation of viscosity and pressure loss in a pipe. Polyethylene (P.E.) is selected as the dispersed minute material that has closeness density (920kg/m3) of ice (917kg/m3). D-sorbitol and D-xylose solutions are picked as continuum phase of the test mixture. The concentration of D-sorbitol solution is set 48mass% from measured results of saturation solubility and the melting point. 40mass% solution of D-xylose is selected as the other test continuum phase. The non-ion surfactant, EA157 Dai-ichiseiyaku CO. Ltd, is used in order to prevent of dispersed P.E. powder cohere. The pressure loss of test mixture is measured by the straight circular pipe that has smooth inner surface. The measuring length for pressure loss is 1000 mm, and the inner diameter of pipe is 15mm. The accuracy of experiment apparatus for measuring pressure loss is within ±5%. The pressure loss data is estimated by the relationship between the heat transport ratio and the required pump power. It is clarified that the optimum range of mixing ratio exists over 10mass% of latent heat storage material.     

Heat transferential study on a heat storage container with phase change material

This paper presents heat transfer characteristics of a heat storage container utilizing latent heat storage material, calcium chloride hexahydrate (melting point 28°C). The heat storage container is consisted of a vertical cylindrical pipe with a vertical heat transfer tube at the center and the latent heat storage material containing into the cylindrical pipe. The heat release and heat transfer characteristics during the solidification of the latent heat storage material are experimentally studied. The following results are obtained. The relation between the time and total heat flow rate, that is, the heat release characteristics have the following three stages: (1) the stage of sensible heat release from liquid heat storage material, (2) the latent heat release stage, (3) the stage of sensible heat release from solid heat storage material. The actually measured values of thermal resistance show a tendency similar to that of the theoretical values, but their absolute values are smaller than those of theoretical values by about 40%. This is because the solidified crystals of heat storage material do not grow in circular shape concentric with the heat transfer pipe but grow in dendritical shape, and dendritical crystallized portions function as fins.     

Performance enhancement of high temperature latent heat thermal storage systems using heat pipes with and without fins for concentrating solar thermal power plants

A key drawback of using latent heat thermal storage systems for concentrating solar thermal power plants is the low thermal conductivity of the phase change material during the melting and solidification processes. This paper investigates an approach for reducing the thermal resistance by utilising axially finned heat pipes. A numerical model simulating the phase change material melting and solidification processes has been developed. This paper also includes the models of the evaporation and condensation of the heat pipe working fluid. The results show that by adding four axial fins and including the evaporation and condensation, the overall thermal performance of the storage system is enhanced significantly compared to having bare heat pipes. After 3 h a total of 106% increase in energy storage is obtained during the charging process. The results also show that the combined effect of incorporating the evaporation/condensation process and adding the fins leads to a threefold increase in the heat storage during the first 3 h. During the discharge process, there was a 79% increase in energy discharged and also the combined effect of incorporating the evaporation/condensation as well as adding the fins results in an almost four fold increase in the heat extracted within the first 3 h. A parametric analysis has also been carried out to analyse the effect of the finned heat pipe parameters after incorporating evaporation and condensation of the heat pipe working fluid.     

Fundamental study of the melting process of crushed ice in a heat storage container

This report deals with heat transfer in the melting process of crushed ice filling an ice/water heat storage container. The volumetric heat transfer rate and melting end-times are measured for the cases when rectangular-type, small-stone-type, and particle-type ice in the container are melted using circulating warm water. The melting end-time is shortest for small-stone-type ice and longest for particle-type ice. The volumetric heat transfer rate is greater for small-stone-type ice and rectangular-type ice than for particle-type ice. The flow rate of the circulating warm water fed into the tank from an inlet pipe has a major effect on the heat transfer rate. © 1999 Scripta Technica, Heat Trans Asian Res, 28(7): 583–596, 1999     

Novel heat dissipation design incorporating heat pipes for DC combiner boxes of a PV system

In a Photovoltaic (PV) system, heat is generated by an operating diode. Because DC combiner boxes are waterproof, dustproof, air tight and made of heat-insulating material, thermal energy is easily accumulated, affecting the performance and safety of power cables and other electronic components near the diodes in the DC combiner box. This study utilizes a heat pipe as a channel for heat dissipation to conduct the heat out of a DC combiner box without destroying the air-tightness of the box. An existing DC combiner box was improved upon using this method of heat dissipation. The measured heat flow and temperature demonstrate that the proposed method is feasible. The influence of the condensation section temperature on the maximum heat transfer of the heat pipe was also investigated by experiment. The maximum heat transfer rate of the heat pipe was found to increase with the condensation section temperature of the heat pipe. When the condensation temperature was 20 °C, 30 °C and 40 °C, the maximum heat transfer rate of the heat pipe was 21.6 W, 29.6 W and 39.7 W, respectively.     

Performance enhancement of finned heat pipe assisted latent heat thermal energy storage system in the presence of nano-enhanced H2O as phase change material

In this research, Latent Heat Thermal Energy Storage Systems (LHTESS) containing Nano-Enhanced Phase Change Material (NEPCM) in the presence of novel shape finned heat pipe is numerically investigated from the viewpoint of discharging process. In recent years, LHTESS have been used to establish a balance between energy supply and demand. Since conventional PCMs are characterized with high latent heat and low thermal conductivity, these systems are capable of storing large amount of energy, but storage and retrieval processes cannot be achieved in the desired time duration. In this paper, CFD simulation and multi-objective Response Surface Method (RSM) optimization is used simultaneously to find the optimum configuration of novel shaped fin, which is then attached to a heat pipe and immersed into the LHTESS. The performance of finned heat pipe assisted LHTESS is compared to the LHTESS containing NEPCM, and LHTESS with other common fin structures. Since the immersion of finned heat pipe into the system decreases the amount of employed PCM, the maximum energy storage capacity of the LHTESS drops subsequently. Thus, energy storage capacity, as one of the objectives of optimization procedure of this research is studied quantitatively, which is proposed as the novelty here. Results indicate that employing maximum energy storage capacity as an evaluation parameter, leads to efficient design of LHTESS. Also it is inferred that immersing finned heat pipe into LHTESS as a heat transfer enhancement technique is superior to nanoparticles dispersion.     

Heat pipe based cold energy storage systems for datacenter energy conservation

In the present paper, design and economics of the novel type of thermal control system for datacenter using heat pipe based cold energy storage has been proposed and discussed. Two types of cold energy storage system namely: ice storage system and cold water storage system are explained and sized for datacenter with heat output capacity of 8800 kW. Basically, the cold energy storage will help to reduce the chiller running time that will save electricity related cost and decrease greenhouse gas emissions resulting from the electricity generation from non-renewable sources. The proposed cold energy storage system can be retrofit or connected in the existing datacenter facilities without major design changes. Out of the two proposed systems, ice based cold energy storage system is mainly recommended for datacenters which are located in very cold locations and therefore can offer long term seasonal storage of cold energy within reasonable cost. One of the potential application domains for ice based cold energy storage system using heat pipes is the emergency backup system for datacenter. Water based cold energy storage system provides more compact size with short term storage (hours to days) and is potential for datacenters located in areas with yearly average temperature below the permissible cooling water temperature (∼25 °C). The aforesaid cold energy storage systems were sized on the basis of metrological conditions in Poughkeepsie, New York. As an outcome of the thermal and cost analysis, water based cold energy storage system with cooling capability to handle 60% of datacenter yearly heat load will provide an optimum system size with minimum payback period of 3.5 years. Water based cold energy storage system using heat pipes can be essentially used as precooler for chiller. Preliminary results obtained from the experimental system to test the capability of heat pipe based cold energy storage system have provided satisfactory outcomes and validated the proposed system concept.     

Feasibility of trichlorofluoromethane (CCl3F,R11) heptadecahydrate as a heat storage material

Trichlorofluoromethane (CCl₃F, R11) heptadecahydrate, which melts at 8.5°C with a heat of fusion of 274 kJ/kg (65.4 cal/g), was investigated as a latent heat storage material. Addition of nonionic surfactant, 2 wt% to water in R11 hydrate, increased the formation rate of the hydrate about 4 times which is sufficient for practical use. Phase separation of R11 was prevented effectively in a gel matrix formed by polyacrylamide (PAA) or gelatin. The PAA gel containing the hydrate-surfactant had an apparent latent heat of fusion of 167–209 kJ/kg (40–50 cal/g), repeating phase change successfully over 100 cycles without deterioration in a heat cycle test (0–20°C, 5 cycles/day). Tetrabutyl ammonium fluoride hydrate [(C₄H₉)₄NF 34.4 H₂O] was found effective as a nucleating agent.     

A comparative study on the performances of different shell-and-tube type latent heat thermal energy storage units including the effects of natural convection

Numerical modeling was performed to simulate the melting process of a fixed volume/mass phase-change material (PCM) in different shell-and-tube type latent thermal energy storage units with identical heat transfer area. The effect of liquid PCM natural convection (NC) on the latent heat storage performance of the pipe and cylinder models was investigated using a 3D numerical model with FLUENT software. Result shows that NC can cause a non-uniform distribution of the solid–liquid interface, which accelerates PCM melting rate. The PCM melting rate and heat storage rate in the horizontal cylinder model are higher than those in the horizontal pipe model because of the combined effects of heat conduction and NC. A comparative study was conducted to determine the effects of horizontal and vertical shell-and-tube models with different heat transfer fluid (HTF) inlets including the effects of NC. The results indicate that the vertical model with an HTF inlet at the bottom exhibits the highest PCM melting rate and heat storage rate for the pipe models. For the cylinder models, the horizontal model and the vertical model with an HTF inlet at the bottom can achieve nearly the same completed melting time. In addition, NC has minimal effect on any model with an HTF inlet at the top.     

Emulsification of thermal energy storage materials in an immiscible fluid

The problems of thermal energy storage are of major importance in the development of intermittent energy sources and the efficient usage of conventional energy supplies. Utilization of latent heat materials for thermal energy storage has been plagued by the build-up of solids on cooling surfaces and the resulting low heat transfer rates. A novel system has been investigated in order to alleviate these difficulties. Small droplets of latent heat material were suspended in an immiscible heat transfer fluid to form an emulsion. The generation of stable emulsions is an empirical art, for which the selection of surface-active agents and the method of mixing play the key roles. A total of 42 latent heat storage emulsion samples were prepared using a diphenyl compound as the organic phase. Most of the samples were prepared using a high speed mixing apparatus. Several emulsified blends exhibited favourable prolonged storage and cycling behaviour. Estimates based on apparent viscosity measurements indicated that high rates of heat transfer could be obtained with this system. Assuming turbulent flow conditions and 60 per cent salt loading, a value for the mean film coefficient of heat transfer was calculated to be about 1045 J/m² s °C. The concept offers potentially large heat exchanger cost reductions, while retaining 60 per cent of the volume savings attainable in latent heat systems.     

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.     

Phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as PCM in a latent heat storage system

The phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as phase change material (PCM) during the melting and solidification processes were determined experimentally in a vertical two concentric pipes energy storage system. This study deals with three important subjects. First is determination of the eutectic composition ratio of the palmitic acid (PA) and stearic acid (SA) binary system and measurement of its thermophysical properties by differential scanning calorimetry (DSC). Second is establishment of the phase transition characteristics of the mixture, such as the total melting and solidification temperatures and times, the heat transfer modes in the melted and solidified PCM and the effect of Reynolds and Stefan numbers as initial heat transfer fluid (HTF) conditions on the phase transition behaviors. Third is calculation of the heat transfer coefficients between the outside wall of the HTF pipe and the PCM, the heat recovery rates and heat fractions during the phase change processes of the mixture and also discussion of the effect of the inlet HTF parameters on these characteristics. The DSC results showed that the PA–SA binary system in the mixture ratio of 64.2:35.8 wt% forms a eutectic, which melts at 52.3 °C and has a latent heat of 181.7 J g⁻¹, and thus, these properties make it a suitable PCM for passive solar space heating and domestic water heating applications with respect to climate conditions. The experimental results also indicated that the eutectic mixture of PA–SA encapsulated in the annulus of concentric double pipes has good phase change and heat transfer characteristics during the melting and solidification processes, and it is an attractive candidate as a potential PCM for heat storage in latent heat thermal energy storage systems.     

Investigation of nitrate salts for solar latent heat storage

Solar thermal applications require some means of thermal energy storage. Amongst several storage concepts, latent heat storage is quite suitable because of its high storage density and almost constant temperature during charging and discharging. The temperature range between 200 and 300°C is considered to be important for solar total energy systems. In this temperature range, sodium nitrate and its mixed salts with other nitrates including eutectic and off-eutectic salts are candidates.

The present paper deals with heat transfer in a latent heat storage unit utilizing these salts. A method of rough estimation of the thermal conductivity of the storage materials is described, and the temperature history of the storage material experimentally obtained is compared with numerical solutions and found to be in reasonably good agreement.

It is seen that the temperature of the heat transfer surface quickly drops soon after the appearance of a solid phase due to low thermal conductivity of these salts. Ways to avoid this temperature drop are discussed.     

Evaluation of paraffin/water emulsion as a phase change slurry for cooling applications

A Phase Change Slurry (PCS) is a latent heat storage and transfer medium consisting of a Phase Change Material (PCM) as a dispersed phase and a carrier fluid as a continuous phase. PCSs have a high energy density because they use not only the sensible heat capacity of the carrier fluid, but also the latent heat capacity of the PCM during the phase transition. In this paper, a paraffin/water emulsion has been studied as a PCS for comfort cooling applications in a temperature range of 0–20 °C. A paraffin blend having a melting temperature range of 2–12 °C was used for preparing the emulsion. The properties of the emulsion were studied in view of the application requirements on PCSs. The dependence between the paraffin fraction and emulsion properties was investigated. The stability of the emulsion was examined both during the storage period and under mechanical–thermal loads in a test rig by determining the change in the properties. The results indicate that the paraffin/water emulsion containing a paraffin weight fraction of 30–50 wt.% is an attractive candidate for cold storage and distribution applications.     

The correlation between the number of fins and the discharge time for a finned heat pipe latent heat storage system

The concept of a solar energy heat pipe latent heat storage system is presented. In order to assure large charging and discharging rates, finned heat pipes are used to transfer heat to and from the phase-change material (paraffin in this case). The evolution of the solid - liquid interface is studied by considering the radial heat transfer (due to the heat pipe wall) and the angular one (due to the fin). Two mathematical models, corresponding to exponential, respectively polynomial functions describing the fin temperature profile are presented and the results are compared. The two models allow the evaluation of the discharge time of the storage unit for a certain number of fins for a single heat pipe. When the discharge time has a fixed value, the methods presented in the paper allow to conclude whether the number of fins is sufficiently large to assure the complete solidification of the phase-change material.     

Enhancement of hydrogen charging in metal hydride-based storage systems using heat pipe

Heat transfer in metal hydride bed significantly affects the performance of metal hydride reactors (MHRs). Enhancing heat transfer within the reaction bed improves the hydriding rate. This study presents performance analysis in terms of storage capacity and time of three different cylindrical MHR configurations using storage media LaNi₅: a) reactor cooled with natural convection, b) reactor with a heat pipe on the central axis, c) reactor with finned heat pipe. This study shows the impact of using heat pipes and fins for enhancing heat transfer in MHRs at varying hydrogen supply pressures (2–15 bar). At any absorption temperature, hydrogen absorption rate and hydrogen storage capacity increase with the supply pressure. Results show that using a heat pipe improves hydrogen absorption rate. It was found that finned heat pipe has a significant effect on the hydrogen charge time, which reduced by approximately 75% at 10 bar hydrogen supply pressure.     

结构参数对全玻璃真空管太阳热水器夜间热损失的影响研究

文章建立了三维非稳态的全玻璃真空管太阳热水器的数值模型,分析了夜间散热时,该热水器内的流动和传热特征,并且在夜间同一工况下,模拟研究了贮热水箱保温材料的导热系数、保温厚度,以及真空管涂层的发射率对贮热水箱温度、真空管温度和该热水器夜间热损失的影响。分析结果表明:随着散热过程的持续进行,全玻璃真空管太阳热水器内温度分层情况越来越明显,内部流体的流速越来越小,真空管内静滞区域自下往上逐渐扩大;当贮热水箱保温材料的导热系数由0.035 W/(m·℃)减小至0.020 W/(m·℃)时,该热水器的夜间热损失减少了8.5%;当贮热水箱保温厚度由50 mm增加至60 mm时,该热水器的夜间热损失减少了5.0%;当真空管涂层的发射率由0.06减小至0.05时,该热水器的夜间热损失减少了4.0%。     

Novel ventilation cooling system for reducing air conditioning in buildings.: Part I: testing and theoretical modelling

A latent heat storage unit incorporating heat pipes embedded in phase change material (PCM) is developed and tested for a novel application in low energy cooling of buildings. A one-dimensional mathematical model of the heat transfer from air to PCM is presented to allow sizing of a test unit. Details of the construction and testing of one heat pipe/PCM unit in a controlled environment are described, and measurements of heat transfer rate and melting times are presented. When the difference between air and PCM temperature was 5°C, the heat transfer rate was approximately 40 W over a melt period of 19 h. The heat transfer rate could be improved, and the phase change time shortened, with an alternative design for finning of the heat pipe inside the PCM.     

Effects of heater size and working fluids on nucleate boiling heat transfer

The present study is an experimental investigation of nucleate boiling heat transfer mechanism in pool boiling from wire heaters immersed in saturated FC-72 coolant and water. The vapor volume flow rate departing from a wire during nucleate boiling was determined by measuring the volume of bubbles from the wire utilizing the consecutive-photo method. The effects of the wire size on heat transfer mechanism during a nucleate boiling were investigated, varying 25 μm, 75 μm, and 390 μm, by measuring vapor volume flow rate and the frequency of bubbles departing from a wire immersed in saturated FC-72. One wire diameter of 390 μm was selected and tested in saturated water to investigate the fluid effect on the nucleate boiling heat transfer mechanism. Results of the study showed that an increase in nucleate boiling heat transfer coefficients with reductions in wire diameter was related to the decreased latent heat contribution. The latent heat contribution of boiling heat transfer for the water test was found to be higher than that of FC-72. The frequency of departing bubbles was correlated as a function of bubble diameters.     

Thermal property measurement and heat transfer analysis of acetamide and acetamide/expanded graphite composite phase change material for solar heat storage

As a phase change material (PCM), acetamide (AC) can be a potential candidate for energy storage application in the active solar systems. Its utilization is however hampered by poor thermal conductivity. In this work, AC/expanded graphite (EG) composite PCM with 10 wt% (mass fraction) EG as the effective heat transfer promoter was prepared; its thermal properties were studied and compared with those of pure AC. Transient hot-wire tests showed that the addition of 10 wt% EG led to about five-fold increase in thermal conductivity. Investigations using a differential scanning calorimeter revealed that the melting/freezing points shifted from 66.95/42.46 °C for pure AC to 65.91/65.52 °C for AC/EG composite, and the latent heat decreased from 194.92 to 163.71 kJ kg⁻¹. In addition, heat storage and retrieval tests in a latent thermal energy storage unit showed that the heat storage and retrieval durations were reduced by 45% and 78%, respectively. Further numerical investigations demonstrated that the less improvement in heat transfer rate during the storage process could be attributed to the weakened natural convection in liquid (melted) AC because of the presence of EG.