High temperature latent heat thermal energy storage using heat pipes

A thermal network model is developed and used to analyze heat transfer in a high temperature latent heat thermal energy storage unit for solar thermal electricity generation. Specifically, the benefits of inserting multiple heat pipes between a heat transfer fluid and a phase change material (PCM) are of interest. Two storage configurations are considered; one with PCM surrounding a tube that conveys the heat transfer fluid, and the second with the PCM contained within a tube over which the heat transfer fluid flows. Both melting and solidification are simulated. It is demonstrated that adding heat pipes enhances thermal performance, which is quantified in terms of dimensionless heat pipe effectiveness.     

Thermal performance enhancement of melting and solidification process of phase‐change material in triplex tube heat exchanger using longitudinal fins

Efficient application of intermittent renewable energy sources, like solar, waste heat recovery, and so forth, depends on a large extent on the thermal energy storage methods. Latent heat energy storage with the use of phase‐change material (PCM) is the most promising one because it stores large energy in the form of latent heat at a constant temperature. The current study investigates melting and solidification of PCM in the triplex tube heat exchanger (TTHX) numerically. The two‐dimensional numerical model has been developed using Ansys Fluent 16.2, which considers the effects of conduction as well as natural convection. To overcome the limitation imposed by the poor thermal conductivity of PCM, use of fins is the better solution. In the current study, longitudinal fins are used for better performance of TTHX, which increases heat‐transfer area between PCM and heat‐transfer fluid. The effects of location of fins, that is, internal, external, and combined internal‐external fins, are observed. All three configurations improve melting as well as solidification process. During the melting process, internal and combined internal‐external fins are equally efficient, in which maximum 59% to 60% reduction in melting time is achieved. For solidification, internal‐external fins combination gives maximum 58% reduction in solidification time.     

Heat pipe-assisted melting of a phase change material

Heat pipe-assisted melting of a phase change material (PCM) housed within a vertical cylindrical enclosure is simulated and is compared to melting induced by heating from an isothermal surface, or with a solid rod or a hollow tube. A parametric study reveals that the heat pipe-assisted melting rates are significantly higher than those associated with the rod or tube, and approach the maximum attainable rates associated with the isothermal surface. Melting rates are enhanced as either the condenser length or the diameter of the heat pipe is increased. The heat pipe is particularly effective in augmenting melting in configurations involving PCM heating from above.     

Research on melting and solidification processes for enhanced double tubes with constant wall temperature/wall heat flux

Latent heat thermal energy storage (LHTES) improves the energy utilization efficiency between energy supply and energy demand of heating storage in buildings and liquid desiccant air conditioning systems. The present work is focused on validated numerical investigation of the thermal performances of LHTES inside enhanced double tubes. The effects of the number of fins ranging from 2 to 10 and boundary conditions of the inner tube wall on the melting and solidification processes are examined. The results indicate that number of fins and wall boundary conditions play an important role in the thermal performances of LHTES. It is noted that recirculation flow in the liquid phase change material region is formed remarkably. The enhancement ratio for constant wall temperature is more significant than that of constant wall heat flux during the melting process. However, the discrepancy of the enhancement ratio for different inner wall temperatures is limited during the solidification process.     

三套管式相变蓄热器强化传热研究

基于三套管式相变蓄热器的特点,提出应用T字形翅片来强化相变蓄热器的传热性能。研究结果表明:添加翅片可有效地降低蓄热器中相变材料的凝固和融化时间,直翅片和T字形翅片的混合强化结构能使凝固过程比未强化结构节省74%的时间,使融化过程节省60%的时间。因此直翅片和T字形翅片的混合使用可以达到进一步强化传热的目的。     

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.     

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.     

Heat transfer and exergy analysis of cascaded latent heat storage with gravity-assisted heat pipes for concentrating solar power applications

A thermal network model is developed to predict the performance of latent heat thermal energy storage (LHTES) systems including cascaded phase change materials (PCMs) and embedded heat pipes/thermosyphons. Because the design of LHTES systems involves a compromise between the amount of energy stored, the heat transfer rate, and the quality of the released thermal energy, an exergy analysis is also carried out to identify the preferred LHTES design. It is found that the LHTES with the lowest melting temperature PCM yields the highest exergy efficiency. However, a cascaded LHTES recovers the largest amount of exergy during a 24 h charging–discharging cycle. Quantitatively, the cascaded LHTES recovers about 10% more exergy during a 24 h charging–discharging cycle compared to the best non-cascaded LHTES considered in this work.     

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.     

Thermal Performance Enhancement of Triplex Tube Latent Thermal Storage Using Fins-Nano-Phase Change Material Technique

Latent heat thermal energy storage (LHTES) systems using a phase change material (PCM) can reduce the heat-transfer rates during charging/discharging processes because of their inherently low thermal conductivity. In this study, heat-transfer enhancement using various configurations of longitudinal fins employing both a PCM and a nano-PCM in a large triplex-tube heat exchanger (TTHX) was numerically investigated via the Fluent 15 software. The results showed that the thermal conductivity of the pure PCM (0.2 W/m K) can be observably enhanced by dispersing 10% alumina (Al₂O₃) to 25%. Therefore, the melting time is reduced to 12%, 11%, and 17% for the internal, internal-external, and external fins, respectively, compared with the case of the PCM without nanoparticle. It is concluded that the model of external fins-nano-PCM embedded in a large TTHX is the most efficient model for achieving complete PCM melting in a short time (188 min), where improving the thermal performance to 14% and 11% compared with the TTHX with internal and internal-external fins-nano-PCM, respectively. The simulation results are validated and agree well with experimental results for the PCM and nano-PCM.     

A comparison of heat transfer enhancement in a medium temperature thermal energy storage heat exchanger using fins

An experimental energy storage system has been designed using a horizontal concentric tube heat exchanger incorporating a medium temperature phase change material (PCM) Erythritol, with a melting point of 117.7 °C. Three experimental configurations, a control system with no heat transfer enhancement and systems augmented with circular and longitudinal fins have been studied. The results presented compare the system heat transfer characteristics using isotherm plots and temperature-time curves. The system with longitudinal fins gave the best performance with increased thermal response during charging and reduced subcooling in the melt during discharging. The experimentally measured data for the control, circular finned and longitudinal finned systems have been shown to vindicate the assumption of axissymmetry (direction parallel to the heat transfer fluid flow) using temperature gradients in the axial, radial and angular directions in the double pipe PCM system.     

Analysis of solidification in a finite PCM storage with internal fins by employing heat balance integral method

Here, a simplified analytical model has been proposed to predict solid fraction, solid–liquid interface, solidification time, and temperature distribution during solidification of phase change material (PCM) in a two‐dimensional latent heat thermal energy storage system (LHTES) with horizontal internal plate fins. Host of boundary conditions such as imposed constant heat flux, end‐wall temperature, and convective air environment on the vertical walls are considered for the analysis. Heat balance integral method was used to obtain the solution. Present model yields closed‐form solution for temperature variation and solid fraction as a function of various modeling parameters. Also, solidification time of PCM, which is useful in optimum design of PCM‐based thermal energy storages, has been evaluated during the analysis. The solidification time was found to be reduced by 93% by reducing the aspect ratio from 8 to 0.125 for constant heat flux boundary condition. While, for constant wall temperature boundary condition, the solidification time reduces by 99% by changing the aspect ratio from 5 to 0.05. In case of convective air boundary surrounding, the solidification time is found to reduce by 88% by reducing the aspect ratio from 8 to 0.125. Based on the analytical solution, correlations have been proposed to predict solidification time in terms of aspect ratio and end‐wall boundary condition.     

Thermal and heat transfer characteristics in a latent heat storage system using lauric acid

The thermal and heat transfer characteristics of lauric acid during the melting and solidification processes were determined experimentally in a vertical double pipe energy storage system. In this study, three important subjects were addressed. The first one is temperature distributions and temporal temperature variations in the radial and axial distances in the phase change material (PCM) during phase change processes. The second one is the thermal characteristics of the lauric acid, which include total melting and total solidification times, the nature of heat transfer in melted and solidified PCM and the effect of Reynolds and Stefan numbers as inlet heat transfer fluid (HTF) conditions on the phase transition parameters. The final one is to calculate the heat transfer coefficient and the heat flow rate and also discuss the role of Reynolds and Stefan numbers on the heat transfer parameters. The experimental results proved that the PCM melts and solidifies congruently, and the melting and solidification front moved from the outer wall of the HTF pipe (HTFP) to the inner wall of the PCM container in radial distances as the melting front moved from the top to the bottom of the PCM container in axial distances. However, it was difficult to establish the solidification proceeding at the axial distances in the PCM. Though natural convection in the liquid phase played a dominant role during the melting process due to buoyancy effects, the solidification process was controlled by conduction heat transfer, and it was slowed by the conduction thermal resistance through the solidified layer. The results also indicated that the average heat transfer coefficient and the heat flow rate were affected by varying the Reynolds and Stefan numbers more during the melting process than during the solidification process due to the natural convection effect during the melting process.     

Enrichment of heat transfer in a latent heat storage unit using longitudinal fins

The charging and discharging rates of a phase change material (PCM) in a horizontal latent heat storage unit (LHSU) is largely influenced by the lower thermal conductivity of the PCM. In the present research, four different configurations of longitudinal fins are proposed to augment the heat transfer in horizontal shell and tube type LHSUs. Numerical investigations are reported to establish the thermal performance augmentation with rectangular, triangular, and Y‐shaped (bifurcated) fins. From the results, it has been inferred that all fin configurations provide a faster charging and discharging rate. In the present set of geometric dimensions of LHSU considered, a reduction in charging time of 68.71% is evaluated for case III (three rectangular fins with one fin positioned in the area of the heat transfer fluid [HTF] surface) and case V (two bifurcated fins with one fin positioned in the area of the HTF surface). Moreover, overall cycle (charging + discharging) time is reduced by 58.3% for case III. Employment of fins results in a faster rate of absorption and extraction of energy from the PCM.     

Numerical investigation on latent heat thermal energy storage in a phase change material using a heat exchanger

The use of a heat exchanger using phase change material (PCM) is an example of latent heat thermal energy storage (LHTES). In this study, the charging of PCM (RT50) is studied in a double pipe heat exchanger. The designing of the heat exchanger needs to be optimized for operating and boundary conditions to store latent heat efficiently. The size of the equipment and the amount of PCM are also important to calculate the latent heat storage capacity of the LHTES device. In this study, the amount of PCM taken is quite high to avoid sensible heat transfer and to maximize the heat content of PCM. The charging process of PCM is numerically simulated using an enthalpy-porosity model. The study includes the effect of inlet temperature and flow rate of high-temperature-fluid (HTF) and concludes that both play an important role in determining the charging time. The continuous increase in inlet temperature of HTF can decrease the charging time of PCM in the heat exchanger. However, the continuous increase in the HTF flow rate cannot show the same effect. The charging time can only be minimized with a specified flow rate regime for a specific inlet temperature of HTF. These factors consequently affect the efficiency of the heat exchanger.     

Economic evaluation of latent heat thermal energy storage using embedded thermosyphons for concentrating solar power applications

An economic evaluation of a latent heat thermal energy storage (LHTES) system for large scale concentrating solar power (CSP) applications is conducted. The concept of embedding gravity-assisted wickless heat pipes (thermosyphons) within a commercial-scale LHTES system is explored through use of a thermal network model. A new design is proposed for charging and discharging a large-scale LHTES system. The size and cost of the LHTES system is estimated and compared with a two-tank sensible heat energy storage (SHTES) system. The results suggest that LHTES with embedded thermosyphons is economically competitive with current SHTES technology, with the potential to reduce capital costs by at least 15%. Further investigation of different phase change materials (PCMs), thermosyphon working fluids, and system configurations has the potential to lead to designs that can further reduce capital costs beyond those reported in this study.     

The optimum fin length distribution of tabular PCM heat exchanger

The study aims to find the optimal fin length distribution for improved heat transfer during melting and solidification in a tubular phase change material (PCM) heat exchanger (HE) designed for heat storage. Three types of horizontal PCM tabular HEs, all with five longitudinal fins, were studied numerically. While maintaining a constant heat-transfer area, each model depicts a unique fin length distribution design. The first model, which serves as the reference design, has a uniform fin length distribution and each fi\n is 30 mm long. The second model has shorter upper and side fins and longer lower fins. The third model has long lower fins but shorter than that of the second model, with short side fins and no change in upper fin length with reference design. The findings indicate that the second model exhibits the best heat-transfer performance for the melting process, while the first model is most effective for solidification. Interestingly, the third design emerges as the optimum choice for both melting and solidification processes, where for 1 h of melting operation, results obtained 87%, 92%, and 90% for three models, respectively, from the first uniform model to the third model. While for 2 h of solidification the result obtained 11%, 17%, and 13% liquid fraction for the three models, respectively.     

基于火积理论的多级套管式相变蓄热系统优化

本文基于最小火积耗散热阻原理,在考虑相变材料导热热阻以及非稳态传热过程的基础上,对多级套管式相变蓄热系统的融化温度进行了数值优化,获得了最优融化温度分布。在此基础上,研究了相变材料导热系数和传热管长度对最优融化温度、火积耗散热阻和平均蓄热速率的影响。研究结果表明,与现有理论优化方法相比,本文提出的数值优化方法具有更好的适用性;优化后多级套管式相变蓄热系统可有效提高相变蓄热系统的平均蓄热速率,降低火积耗散热阻;随着相变材料导热系数增大和传热管长度增加,多级套管式相变蓄热系统最优融化温度的温差愈加明显,其强化传热性能呈上升趋势。     

Numerical analysis and parameters optimization of shell-and-tube heat storage unit using three phase change materials

In this paper, a mathematical model of shell-and-tube latent heat thermal energy storage (LHTES) unit of two-dimension of three phase change materials (PCMs) named PCM1, PCM2 and PCM3 with different high melting temperatures (983 K, 823 K and 670 K, respectively) and heat transfer fluid (HTF: air) with flowing resistance and viscous dissipation based on the enthalpy method has been developed. Instantaneous solid–liquid interface positions and liquid fractions of PCMs as well as the effects of inlet temperatures of the air and lengths of the shell-and-tube LHTES unit on melting times of PCMs were numerically analyzed. The results show that melting rates of PCM3 are the fastest and that of PCM1 are the slowest both x, r directions. It is also found that the melting times of PCM1, PCM2 and PCM3 decrease with increase in inlet temperatures of the air. Moreover, with increase in inlet temperatures of the air, decreasing degree of their melting times are different, decreasing degree of the melting time of PCM1 is the biggest and that of PCM3 is the smallest. Considering actual application of solar thermal power, we suggest that the optimum lengths are L₁ = 250 mm, L₂ = 400 mm, L₃ = 550 mm (L = 1200 mm) which corresponds to the same melting times of PCM1, PCM2 and PCM3 are about 3230 s and inlet temperature of the air is about 1200 K. The present analysis provides theoretical guidance for designing optimization of the shell-and-tube LHTES unit with three PCMs for solar thermal power.     

Experimental research on thermal characteristics of PCM thermal energy storage units

To explore thermal management integration in electric vehicles (EVs), a phase change materials (PCMs) thermal energy storage unit using flat tubes and corrugated fins is designed. The investigation focuses on the thermal characteristics of the PCM unit, such as the temperature variation, heat capacity, and heat transfer time, etc. Meanwhile, the heat storage and release process will be influenced by different inlet temperature, liquid flow rate, melting point of the PCM, and the combination order of the units. Under the same inlet temperature and flow rate condition, the PCM unit with higher melting point enters the latent heat storage stage slowly and enters the phase change melting release stage quickly. Furthermore, the heat storage and release rates increase with increasing liquid flow rates, but the effects are diminishing in the middle and later periods. The multiple PCM units with different melting temperatures are cascaded to help recycle low-grade heat energy with different temperature classes and exhibit well heat storage and release rates.