PCM Unit Research Articles

Influence of variable gravity on the phase change material for enclosed vertical channels with helical fins

This study numerically investigates the influence of variable gravity on the melting behaviour of a phase change material (PCM- paraffin wax RT27) contained within an enclosed vertical channel under different gravity conditions, specifically those found on the Earth, the Moon, and under the microgravity environments. Furthermore, the impacts of incorporating various number of helical fins within the PCM unit under different gravitational conditions are assessed for the first time. The findings indicate that the melting efficiency of PCM is considerably affected by the reduced gravity conditions, due to changes in the natural convection, velocity, and temperature; the melting time is twice as long under the moon-gravity conditions and sixfold longer under the microgravity conditions compared to the earth-gravity. It is also observed that reduced efficiency can be remedied by the insertion of fins into the PCM unit. The breakdown of the natural convection vortices with different number and orientation of fins expedites the melting process for all gravity conditions thus leading to improved performance in melting and energy storage.

Improving the performance of a PCM integrated solar air collector by adding porous fins over the bottom side of the absorber: A transient CFD study

In this study, it is aimed to improve the utilization time period of a solar air collector by integrating a PCM unit. In this context, a PCM unit was integrated to a single-pass solar air collector. Additionally, porous fins were added into the PCM unit in order to shorten PCM melting time. Thus, six different solar collectors were designed and numerically analyzed by adding a storage unit and fins to the collector. A collector without storage and five PCM added collectors with different number of porous fins were studied. In other words, the effect of integrating PCM container and adding porous fins inside the PCM container on various parameters such as utilization time period of the collector, outlet temperature, thermal efficiency and exergy efficiency were investigated. The ANSYS Fluent program was used in numerical analyses. As a result, integrating the PCM unit to the analyzed air collector caused to extend utilization time period of the collector. Adding porous fins into the PCM unit has resulted in a significant increase in the overall performance of the system. In addition, increasing the number of porous fins accelerated the melting process of paraffin. Increasing the number of porous fins was significantly improved average energy efficiency. Average thermal efficiency of the tested collectors was obtained between 38.66 and 41.73 %. In general, as a result of numerical analysis of six different systems, the utilization time was increased by adding the PCM unit to the solar air collector. Moreover, placing porous fins inside the PCM unit increased the performance of the system by accelerating the melting of the paraffin.

High power and energy density of a wavy PCM unit embedded in a solar air heater utilizing fractional porous media

This experimental and numerical investigation presents the optimization of a wavy PCM unit to obtain high power and energy density with and without porous media for solar air heater applications at low ambient temperature (12oC≤Ta≤24oC). Hence, paraffin wax as a thermal energy storage material and wire mesh as a porous media are selected in this investigation. The effect of the fractional porous region (25%≤ψ≤100%) and porosity (94%≤ϕ≤100%) of the PCM unit is investigated to obtain desired power and energy density. The obtained numerical results revealed that the porous fraction region (ψ) of 50% and porosity (ϕ) of 96% are the optimum value in wavy PCM unit to obtain high power and energy density. As an extension of this work, the optimized thermal energy storage unit is further utilized in an impinging jet solar air heater design to obtain instant and long thermal backups and, consequently, outlet air at high temperature. The wavy PCM unit embedded impinging jet solar air heater is experimentally investigated under outdoor conditions for two cases; viz, to obtain instant and long thermal backups. To obtain instant and long thermal backups, a full day and alternative operations are conducted. Whereas, in case of alternative operation, charging of the PCM unit is only done up to the maximum sunshine hour of the day and solar air heater is operated during the night to utilize stored thermal energy at high power and energy density to deliver outlet air at high temperature for the long duration. Moreover, the experimental results revealed a maximum outlet air temperature of 59°C at 0.01 kg/s and a significant thermal energy backup up to 5 h at an outlet air temperature gain of ≥ 5°C. The economic analysis is also conducted for the solar air heater design and the operating cost is obtained as 0.02 INR/kWh at the optimum condition.

Design of a recyclic solar air heater for low temperature regions utilizing integrated wavy corrugated thermal energy storage systems

This work presents thermal performance prediction of a design of solar air heater for low temperature regions utilizing integrated thermal energy storage systems (PCM unit). Recyclic double air pass is considered for a range of recycle ratio, G = 0–1 at a mass flow rate of 0.01 kg/s. Paraffin wax as a PCM for thermal energy storage is used in two different designs of wavy corrugated PCM unit. Geometrical parameters related to PCM units are considered such as wavelength 0.05≤λ≤0.1m, amplitude 0≤α≤0.015m, length 1≤L≤2m, width 0.5≤W≤1.414m, and height 0.02≤h≤0.04m. Energy-enthalpy method is used to predict the temperatures and phase change of PCM employing finite volume method. Explicit time marching scheme is applied to present transient thermal performance of solar air heater designs. In addition, employed non-orthogonal structured mesh in the complex wavy corrugated PCM units, ensured the numerical accuracy in the prediction. Ambient temperature, Ta and heat flux of the range 5o≤Ta≤20oC and 1000 W/m2 is considered to present realistic situation of low temperature regions, respectively. Thermal backup of 24 h at a significant high outlet air temperature than ambient is aimed to provide in order to support wide thermal applications of solar air heater. The obtained results reported the significantly higher outlet air temperature for the heater design i.e. 9 °C than ambient corresponding to 24 h. Optimum value of different geometrical and flow parameters is obtained as ṁ=0.01kg/s,G=1,λ=0.1m,α=0.005m,L=1.414m,W=1.414mandh=0.04. In addition, presented contours results revealed the associated physics and important guidelines upon the use and size of the thermal energy storage systems in solar air heaters.

On the effects of an optimized inlet acoustically flow on a heat storage system's performance: A numerical study

BackgroundAcoustic flows are the rotational flows that are generated near solid surfaces and vibrating components, as well as near boundary walls in a high-intensity sound field. By making the flow oscillate, this type of flow can be a good way to improve HT. Besides, phase change materials provide high cooling in a short time because they absorb the latent heat of their melting during melting. MethodThe use of the phase change material and adding some pulsing function of inlet flow have been considered to investigate the battery's performance. To this end, the finite volume method, along with interface mesh and grid adaptation techniques, has been used to achieve more accurate results. FindingThe comparisons of the obtained results were made to the existing data, and after ensuring the numerical methodology, four functions were used to investigate the effects of variations in inlet velocity. The results showed that different kinds of pulsation flows have different qualities and ways to improve HT in different situations. For instance, the sine profile helps the average temperature of the PCM unit reach the maximum value in approximately 50 percent less time.

Performance improvement of adsorptive hydrogen storage on activated carbon: Effects of phase change material and inconstant mass flow rate

Due to the multiple advantages of activated carbon (e.g. high specific surface area, safety and low cost) toward hydrogen storage, the subject of heat management in H2-storage tanks has become a research focus in recent years. In this work, using a computational fluid dynamics (CFD) model (implemented in Fluent software), the performance of a hydrogen storage tank (packed with activated carbon) was analyzed in the presence of PCM/oil-based thermal recovery/release systems. The charging, dormancy, and discharging cycles of the storage tank were investigated with respect to the variation of the injected mass flow rate of hydrogen (from 2 × 10−4 to 8 × 10−4 kg/s). There was good agreement between our findings and those obtained from the experiment. It was observed that the adsorption capacity of the solid bed is proportionally enhanced with the rise in the mass flow rate of hydrogen. In addition, independently of the used hydrogen mass flow rate, it has been observed that the charging periods were ended at the same instant (i.e. t = 400 s). According to the enthalpy contours and liquid fraction of PCM (~18 and 30 % at 400 and 4000 s, respectively), it has been concluded that the thermal conductivities of the adopted PCM and that of solid bed (activated carbon) should be enhanced for better performance of the hydrogen stocking tank. Finally, from a viewpoint of storage performance and process cost, it is therefore possible to realize rapid charging/discharging of the activated carbon tank with adopting a PCM and oil-based thermal unit for heat management.

Investigation on effect of connection angle of “L” shaped fin on charging and discharging process of PCM in vertical enclosure

This study probes the effect of connection angle of “L” shaped fin on the melting and discharge process of a vertical PCM unit. The PCM enclosure had dimensions of 50 ×50 mm and the L shaped fin was located at middle of enclosure. The enclosure was heated and cooled by considering constant temperature at the vertical wall. The Lauric acid was considered as the phase change material. Seven different coincidence angles of 0°, 30 °, 60 °, 90 °, −30 °, −60 ° and −90 ° were considered in present investigation. Results indicated that L shaped fin with Teta = −30 ° could provide 14% reduction in the melting process. Furthermore, the case with Teta = 0° provided 45% enhancement in solidification time. The outcomes revealed that maximum charging rate was associated with case with Teta = −30 ° which had the value of 103.41 J/S and provided a 32% enhancement in comparison with the base case. Also, the best discharge rate for the solidification process was about 34.184 J/s which was related to case with Teta = 0° and presented a 2.23 times improvement in the average solidification rate.

Numerical investigation on transient thermal performance predictions of phase change material embedded solar air heater

In this work, thermal performance of PCM embedded parallel flow solar air heater employing differential air flow arrangements is predicted using transient computational iterative solution procedure. The present iterative approach is capable of predicting transient temperatures of the complete collector design for 24 h working cycle including sunshine and nocturnal hours. Finite volume method is used employing explicit time marching scheme in a MATLAB code to solve the discretized transient governing differential equations. First Order Upwind advection scheme is applied to predict the air temperatures. Obtained results revealed all possibilities of obtaining significant thermal energy storage in PCM, instant and long thermal backups, and heat gain of air as a function of time of the day and variable air flow rate arrangements. The maximum thermal efficiency of about 63% is obtained at the total mass flow rate of 0.05 kg/s. In addition, air flow rate conditions for the present design are revealed that can provide the outlet air temperature ≥303 K during nocturnal hours. An off sunshine situation is presented that depicts the scope of obtaining the possible instant thermal energy backup. The effect of PCM unit thickness on thermal performance of the collector is also considered in the present work.

Experimental study on a binary mixture ratio of fatty acid-based PCM integrated to PV panel for thermal regulation on a hot and cold month

ABSTRACT This paper presents the thermal regulation for photovoltaic panel by integrating PCM unit of 30 mm and 50 mm thickness. Eutectic mixture of Lauric and Palmitic Acids in the ratio of 69:31 and 85:15 was used to achieve the melting temperature of 35°C and 40°C, derived from average ambient and cell temperature for hot and cold months. This experimental study shows that average increase inperformance ratio from reference panel to 30 mm and 50 mm PCM panels were 1.54% and 2.36%, for March and 1.28% and 1.66%, for December. Further, the average efficiency of the panel was increased from 9.28% to 9.43% and 9.51% in March and 10.19% to 10.31% and 10.35% in December.

Compared Analysis of Calculation and Simulation on Heat Transfer Characteristics in PCM Heat Exchange Unit, Part II: Thermal Release Process

In this part, the thermal release processes of PCM phase change in the same energy storage unit are analyzed, and the PCM unit model is the same as in Part I: channel number is 39, the plate spacing is 0.01 m, the air channel is 40 with its plate spacing of 0.003 m, and the unit length is total of 1.62 m. MATLAB solution about the above PCM energy storage unit was performed according to the model and physical property data, and the numerical simulation was carried out by using Fluent software as well. The theoretical calculation and numerical simulation were compared and analyzed. For the release process, it also can be seen that only in the case of smaller or larger flow rate working conditions, the solution results of MATLAB for the changing rate of the average temperature of the phase change material inside the system is quite different from the FLUENT simulation result; the total results are very similar to thermal storage process. When the flow rate is higher, the difference between the two results is even smaller than the release process; the comparison result of the PCM liquid rate is different under the small flow rate condition, and this is obvious for the release process, especially under the condition of small flow rate. However, also the maximum error does not exceed 10%. Conclusion can be drown that the flow rate conditions in the model and working conditions are the important factors for affecting the compared results, but the calculation and simulation results can more accurately describe the thermal release characteristics of the PCM phase change energy storage unit, which was calculated in nearly 90% ranges of working conditions, which furtherly illustrates the effectiveness of analytical method used in this paper.

Demonstration of a solar combi-system utilizing stable supercooling of sodium acetate trihydrate for heat storage

Achieving a high fraction of solar heat in heat supply for domestic buildings would reduce the use of fossil fuels for heat generation and has been a goal for a long time. Combined short and long-term heat storage has been identified as one way of achieving solar fractions higher than 50 percent in heat supply for domestic buildings. To this end, a laboratory solar heating system was built with heat-pipe tubular collectors 22.4 m2 in aperture and a heat-storage prototype consisting of a 735 L water tank and four PCM units each containing 200 kg sodium acetate trihydrate (SAT) composite. The SAT composite was utilized as sensible heat storage with the additional ability to release heat of fusion on demand. Operation was demonstrated with the space heating and hot water demand patterns of a standard-size Passive House in the Danish climate. A strategy was developed to control the system. Seven operation modes enabled combined charging of water tank and PCM units, heat transfer from PCM units to the water tank when heat was in demand, and the right timing of auxiliary heating. We present the controller settings identified and the heat transfer fluid flow rates applied. Sequences of water tank charging, and single and parallel PCM unit charging were used to match the collector power available and the heat transfer limitations of the stores. During the charging of PCM units, the flow temperature was kept between 70 and 95 °C to allow continuous heat transfer rates of up to 16 kW. Peaks of up to 36 kW occurred when PCM units were added to the charging circuit. During heat transfer from PCM units to the water tank, flow temperatures were close to the SAT composite temperature and thermal power of up to 6 kW was measured. The heat stores were efficiently utilized in spring and autumn. The developed control strategy and measurement data from system demonstration will form the basis for numerical performance investigations.

Validation of a melting fraction-based effective thermal conductivity correlation for prediction of melting phase change inside a sphere

It is complicated and costly to directly simulate the natural convection influenced melting processes involving multiple PCM units in packed bed phase change system. An effective method to simulate such melting process is using an equivalent conduction model, in which the effective thermal conductivity (ETC) of PCM considering the natural convection effects is an important thermal parameter needing to be determined. In this paper, the ETC of PCM is investigated and determined by comparing numerical results based on models considering natural convection and pure conduction, respectively. Parameters of interest include melting fraction (α), Rayleigh number (Ra, 1.5 × 104 ≤ Ra ≤ 6.3 × 107), Prandtl number (Pr, 17 ≤ Pr ≤ 550) and Stefan number (Ste, 0.011 ≤ Ste ≤ 0.276.). Results show that the ETC of PCM has a strong relation to α and Ra, while it has little relation to Pr and Ste. Hence, an ETC correlation is developed as a function of Ra and α. Validation and application of the ETC correlation are further carried out, and it is found that the melting fraction predicted based on the present ETC correlation can better reflect the effect of liquid zone geometry on the melting heat transfer with natural convection than that predicted based on the previously reported correlations using a liquid gap parameter.

An experimental investigation on solar powered solid desiccant air conditioning (SPSDAC) based on regenerative evaporative cooling system with PCM unit

ABSTRACT A solar powered solid desiccant air conditioning (SPSDAC) with PCM unit was explored for cooling purposes for the duration of sunlight and sunset hours. Two unique modes of the indirect evaporative cooler (IEC) was adopted during the operation of the SPSDAC. During mode-1, the water circulated through the copper coil and annular space of the hollow circular pipe was vacant. Whereas in mode-2, the water circulates through copper coil and annular space. It was concluded that COP of the system is more than the exergy efficiency and exergy efficiency varied from 1.83% to 1.99%. The desiccant wheel thermal effectiveness (28.81% and 46.06%) is lower than the indirect and direct evaporative cooler effectiveness. The setup under mode-2 is advantageous as compared to mode-1. The pay back time of the setup is 23.2 months. The mean overall thermal storage capacity of the system in mode-1 is 1796.1 W whereas in mode-2 is 1817.7 W.

Numerical simulation on thermal characteristics of supercooled salt hydrate PCM for energy storage: Multiphase model

Multiphase model is set up and applied to perform a 3D transient CFD simulation on the discharging characteristics of supercooled PCM. Three typical processes are identified and analyzed, i.e. stable supercooling, triggering crystallization and regular solidification periods. The simulation is achieved by loading the source term in energy equation and phase transition term in volume fraction equation. PCM temperature, solid fraction (SF) and heat transfer rate (HTR) to the surroundings are monitored and analyzed during the three stages. It is found that there is little difference among the temperature and SF curves of different z-direction planes during the first two periods but they are distinct during the regular solidification period. For the triggering crystallization period, PCM temperature rises much rapidly up to the melting point within 30s and SF increases up to 0.33 correspondingly. The heat loss is small in this short period and could be simplified as adiabatic. Variations of temperature and SF extend from up and side walls to the interior of the PCM unit. HTR curve fluctuates greatly throughout the three processes with highest value of 1420W occurring in regular solidification period. The developed method can be used for parameter analysis of supercooled PCM thermal storage devices.

Coupling waste heat extraction by phase change materials with superheated steam generation in the steel industry

To allow a better exergy exploitation than the current state-of-the-art waste heat to power solutions in the steel industry, a new type of energy recovery system based on Phase Change Materials is proposed. In particular, the use of high temperature PCMs evolves from simply smoothing off gas temperature, as in the most recent studies for energy recovery from electric arc furnaces, to generating constant superheated steam able to feed the downstream turbine nearly at nominal load. This result is achieved by introducing an auxiliary section between the PCM Section and the steam generation one, which provides the auxiliary heat needed to level the thermal content of off gas. The auxiliary heat is extracted from the PCM units by a heat transfer fluid flowing across the inner tube of each PCM container.Different models to properly size and simulate the operations of the proposed energy recovery system have been developed and integrated. Results show how the size of the steam generator and the turbine can be reduced of about 41% with respect to traditional solutions, while increasing electric power production by 22% thanks to the reduced fluctuation in steam parameters at the turbine inlet, which leads to a greater overall efficiency.

Coupled cooling method and application of latent heat thermal energy storage combined with pre-cooling of envelope: Sensitivity analysis and optimization

Abstract Cooling system for mine refuge chamber provides comfortable environment for miners to avoid heat damage. The existing cooling systems have their own application scopes and limitations. The coupled cooling method of Latent Heat Thermal Energy Storage (LHTES) combined with pre-cooling of envelope (PE) is a new free cooling method which is suitable for high-temperature, passive, impact and other harsh environment. Then, to improve the thermal comfort and reduce energy consumption, the effect of the pre-cooling temperature, melting temperature of PCM, aspect ratio and amounts of PCM unit on the indoor temperature are investigated in a systematic manner. Furthermore, the system is optimized and the generalized results for the evaluation parameter are given. Analysis of the results may lead to following main conclusions: (i) the method really controls the indoor temperature and the saving amount of PCM is more than 50% compared to the traditional LHTES systems; (ii) the temperature control (TC) performance of PCM would drop significantly if it melts more than 80%; (iii) under current operating conditions, the optimal melting temperature is about 29 °C and the aspect ratio of PCM unit is 60:500; (iv) per 1 °C the pre-cooling temperature dropped, 19% the actual amount of PCM decreased for the case studied.

Effective delayed neutron fraction by Rossi-α method in accelerator-driven system experiments with 100 MeV protons at kyoto university critical assembly

ABSTRACTIn the accelerator-driven system (ADS), the effective delayed neutron fraction (βeff) is a requisite for converting the subcriticality from dollar units to pcm units. To evaluate the accurate βeff value in ADS, measurement of βeff complements its calculation methodology and the nuclear data on delayed neutrons. Subcriticality measurements are carried out by the pulsed neutron source method in the Kyoto University Critical Assembly, and the neutron noise analyses are conducted by the Rossi-α method with the pulsed shape of the spallation neutron source. The value of βeff is deduced with the combined use of measured subcriticality in dollar units and correction parameters by MCNP6.1 together with JENDL-4.0 and JENDL/HE-2007. A comparison between the calculated and the measured βeff represents the acceptable accuracy within the subcriticality range of around keff = 0.93 in the ADS operations. Here, the applicability of the measurement methodology based on the Rossi-α method is demonstrated by varying the subcriticality.

Monte Carlo Approach of Effective Delayed Neutron Fraction by k-Ratio Method with External Neutron Source

In an accelerator-driven system (ADS), the effective delayed neutron fraction βeff is required for subcriticality measurements in order to convert measured reactivity in dollar units into pcm units. Here, an alternative calculation methodology with source-dependent reaction rates is proposed to estimate the subcriticality (pcm units) with an external neutron source for the newly defined parameter. In the proposed methodology, the parameter is obtained by the k-ratio method with reaction rates considering the external neutron source (fixed-source calculation) and is compared with that obtained by the eigenvalue calculations (MCNP6.1). To investigate its applicability to the estimation of subcriticality, subcriticality measurements by the extrapolated area ratio method are carried out in the experiments in ADS at the Kyoto University Critical Assembly, with the variation of subcriticality (pcm units) and the spectrum of the external neutron source. Then, is estimated by the proposed methodology with MCNPX-2.5.0. The measured subcriticality with the use of obtained by the fixed-source calculations shows good agreement with that obtained by the different measurement methodology and MCNPX-2.5.0. Through the subcriticality estimation, the applicability of the proposed methodology is confirmed for different subcriticality levels and the spectrum of the external neutron source.

Solar air heater with phase change material: An energy analysis and a comparative study

In the present work an experimental study is made to enhance the efficiency of a simple fabricated Solar Air Heater (SAH) by using the thermal heat storage. A rectangular cavity filled with a Paraffin wax is used as a latent storage unit. An experimental study of two similar designed solar air collector (with and without Phase Change Material), is carried out to evaluate the PCM unit importance. The results show that during the night the outlet air temperature of the SAH was enhanced with using PCM. The daily energy efficiency of the SAH without PCM reached 17%, while the daily energy efficiency of the SAH with PCM reached 33%.

Experimental study on the thermal performance of air-PCM unit

In the present research paper thermal performance of an air-phase change material (PCM) unit has been experimentally studied. The influences of the air inlet temperatures and velocities have been investigated on the charging/discharging time of the PCM panels, heating/cooling load and effectiveness over the phase transition. These parameters play a vital on determining thermal performance of an air-PCM unit. Air inlet temperatures of 34 °C, 36 °C and 38 °C and air inlet velocities of 0.6 m/s, 1.6 m/s and 2.5 m/s. have been studied. The increase of the air inlet velocity reduced the charging and discharging time, however not linearly. The time for the complete melting and solidification is substantially reduced when the velocity has been increased from 0.6 m/s to 1.6 m/s. The air inlet temperature has been proved to be most important factor affecting the discharging time. For lower air inlet velocity the cooling and heating loads achieve lower values but remain nearly constant over a longer period of time. The effectiveness reaches its highest values for the air inlet velocity of 0.6 m/s for the charging and discharging processes. It has been concluded that discharging time can be extended by reducing the air inlet velocity and making the technology more suitable for heating and cooling applications for the buildings. A simplified methodology is proposed for the analysis of the PCM charging and discharging process through the identification of the critical points as result of the heat transfer behaviour in the air- PCM unit.