Thermal energy storage improves the load stability and efficiency of solar thermal power plants by reducing fluctuations and intermittency inherent to solar radiation. This paper presents a numerical ...study on the transient response of packed bed latent heat thermal energy storage system in removing fluctuations in the heat transfer fluid (HTF) temperature during the charging and discharging period. The packed bed consisting of spherical shaped encapsulated phase change materials (PCMs) is integrated in an organic Rankine cycle-based solar thermal power plant for electricity generation. A comprehensive numerical model is developed using flow equations for HTF and two-temperature non-equilibrium energy equation for heat transfer, coupled with enthalpy method to account for phase change in PCM. Systematic parametric studies are performed to understand the effect of mass flow rate, inlet charging system, storage system dimension and encapsulation of the shell diameter on the dynamic behaviour of the storage system. The overall effectiveness and transient temperature difference in HTF temperature in a cycle are computed for different geometrical and operational parameters to evaluate the system performance. It is found that the ability of the latent heat thermal energy storage system to store and release energy is significantly improved by increasing mass flow rate and inlet charging temperature. The transient variation in the HTF temperature can be effectively reduced by decreasing porosity.
•Numerical study on packed bed LHTES consisting of spherical shaped encapsulated PCMs.•Ability of LHTES in storing and releasing energy is improved by increasing mass flow rate and inlet charging temperature.•Transient variation in HTF temperature can be reduced by decreasing porosity and encapsulation diameter.•Increasing storage diameter reduces the ability in removing fluctuations in HTF temperature.
•Developed a simplified numerical model considering axial heat diffusion in HTF and PCM.•Studied the effects of process parameters of LHTES during a thermal cycle experimentally.•Correlated heat ...transfer characteristics of a single PCM capsule and the storage system.•Energy stored and extraction are faster for lower PCM capsule diameter and higher porosity.•Maximum efficiency found is 75.69% for minimum discharging inlet HTF temperature.
The variability in solar radiation creates a gap between energy demand and supply, which necessitates the use of efficient thermal energy storage for bridging the gap to make the solar thermal power plant a viable solution for continuous power generation. In this work, a mathematical model of encapsulated phase change materials (PCMs) based latent heat thermal energy storage (LHTES) is developed considering simplified non-equilibrium two energy equations coupled with enthalpy technique to analyse the transient variation in heat transfer fluid (HTF) temperature at the outlet of LHTES and PCM temperature. Experiments on a spherical capsule reveal melting and solidification behaviour of PCM from the measured temperature field. A lab-scale LHTES is designed and fabricated to evaluate the effects of charging temperature, discharging temperature and flow rate on thermal performance of the LHTES during charging and discharging operations. Detailed parametric study on capsule diameter and porosity shows that the energy stored and extraction are faster for smaller capsule diameter and higher porosity. The maximum efficiency of the storage in this work is found to be 75.69% for charging and discharging inlet HTF temperatures of 180 and 120 °C, respectively and flow rate of 8.2 lpm.
Among the different pathways of improving the energy storage and energy utilization in a thermal energy storage system, the formation of thermal stratification in hot water tanks is a promising ...technology. In this study, we developed a novel numerical model to assess the thermal stratification performance in a hot water tank due to addition of encapsulated phase change material (PCM) by varying the bed height, bed porosity and the encapsulation diameter. The formulation of the present numerical model is devoid of complicated momentum and energy equations. A set of simplified energy balance equations is developed to account for heat transfer between the heat transfer fluid (HTF) and the PCM considering local thermal non-equilibrium. The temperature profile at the outlet of the storage tank during charging process demonstrates an enhancement in the Richardson number by 58.3%, at the end of the charging process, upon doubling the PCM bed height, and the corresponding improvement in the charging efficiency is found to be 54.6%. The melting point of PCM plays a pivotal role on the extent of stratification and storage efficiency, as it dictates the amount of heat diffusion towards the bottom section of TES. During discharge phase, if the HTF flow rate is doubled from the nominal value of 2 L/min, the extraction efficiency is enhanced by ∼16.7%.
•Developed a novel numerical model for investigating thermal stratification in TES.•The model includes the detailed heat transfer between HTF and encapsulated PCM.•Compared to steel and rock, PCM provides better thermal gradient across the storage tank.•PCM melting temperature affects the stratification number and storage efficiency.•Increasing flow rate of HTF significantly improves extraction efficiency during discharging.
•Numerically studied flow boiling in multi-parallel microchannel with inlet-outlet plenums.Three-dimensional, laminar, multi-phase, transient numerical model is developed.•Experiments are performed ...on a fourteen parallel microchannel for wide range of heat flux.•Excellent agreement between numeral and experimental results over a wide rangeVapour blocking in the channel near the outlet is primarily responsible for instabilities.•Fourteen channels demonstrate higher heat transfer coefficient compared to six channels.
A combined numerical and experimental investigation to elucidate the two-phase flow behaviour and heat transfer during subcooled boiling of water in 1 × 1 cm2 footprint area heat sinks with six, ten, and fourteen parallel microchannels is performed. A three-dimensional, laminar, multi-phase, transient numerical model is developed to simulate flow boiling in microchannels. This study is one of the first studies which reports a three-dimensional numerical simulation of flow boiling in a large area multiple parallel microchannels heat sink including the effect of inlet and outlet plenums. We show an excellent agreement in heat transfer and pressure drop data with experimental results on a heat sink with fourteen parallel microchannels over a wide range of applied heat flux spanning various boiling regimes. The results show that the vapour blocking in the channel near the outlet is primarily responsible for instabilities and oscillations in the pressure drop, the surface temperature, and the mass flux. Intensified confinement due to the decrease in the number of the channels for a constant fin width results in increased surface temperatures. Similarly, increase in the number of the channels from six to fourteen improved heat transfer significantly wherein a significant drop of 45.5 °C in surface temperature with little increase of 37% in pressure drop is observed for a mass flux of 500 kg/m2s and a heat flux of 220 W/cm2. Microchannel heat sink with fourteen channels demonstrates on an average nearly 240% higher heat transfer coefficient in comparison to the heat sink with six channels. The numerical modelling framework used in this study can be used to provide design guidelines for microchannel heat sinks.
•Studied the suppression of instability by incorporating inlet restrictor and a flexible dampener during microchannel flow boiling.•Microchannel with flexible dampener reduces fluctuations ...significantly.•Inlet restrictor microchannel with flexible dampener provides stable flow boiling at high heat fluxes.•Performance of inlet restrictor microchannel with flexible dampener is comparable with plain microchannel with lowest surface temperature.
Flow boiling suffers from the issue of instability due to the high rate of bubble generation in confined spaces within microchannel heat sinks. The cyclic changes in flow regimes lead to prolonged periods of dry out due to the backflow of vapour at high heat fluxes. Poor management of excess vapour and the resulting crowding at the outlet plenum drastically deteriorates the thermal and hydraulic performance of microchannel heat sinks. This study incorporates the simple design of a flexible membrane type pulsation dampener at the outlet port to address the aforementioned issues. The flexible dampener expands to accommodate the excess vapour, thereby minimising backflow and dryout within the channels. The heat sink design with the flexible dampener drastically reduces the fluctuations with lower surface temperature and pressure drop in compassion to the baseline case. Next, we combine the flexible dampener design with the inlet restrictor and compared the performance with and without the flexible dampener. The fluctuations/backflow are completely mitigated in this case, however, at the cost of a slight increase in pressure drop. Nonetheless, a quantitative comparison in terms of performance evaluation criteria suggests that the pulsation dampener can largely minimise the pressure drop penalty due to inlet restrictors while ensuring complete suppression of instability.
Nowadays, miniaturization of high-performance electronic devices leads to higher heat generation, which needs efficient cooling to maintain device temperature below its operating temperature. This ...work proposed an efficient cooling technique using two-phase flow boiling in a recharging microchannel (RMC) and compared its performance with the conventional simple microchannel (SMC). A three-dimensional numerical investigation is carried out using the volume-of-fluid multiphase model and evaporation-condensation phase-change model in ANSYS Fluent V15. Water with mass fluxes of 100 and 500 kg/(m
2
·s) enters through the microchannel at inlet subcooling of 5 °C, whereas the substrate bottom surface is subjected to constant heat flux in the range of 10-40 W/cm
2
. The water saturation temperature is considered as 50 °C, corresponding to the saturation pressure of 12.352 kPa. Results reveal that the RMC shows enhanced heat transfer coefficient, reduced thermal resistance, reduced wall superheat, and reduced substrate temperature compared to SMC. Besides, the RMC shows a reduced pressure drop than the SMC at lower mass flux, whereas it shows an increased pressure drop at higher mass flux. The flow boiling instabilities can mitigate during the bubbly flow regime in RMC. This study suggests the possible application of flow boiling in RMC to cool high-performance electronic devices.
•Studied the effects of scaling on the performance of cylindrical helical coil and conical spiral coil receivers.•Similarity parameters are identified and scaling law is derived to simplify the ...designing of scaled up prototypes.•Cylindrical receivers with varying aspect ratio and conical receivers with varying half-cone angle are considered.•Correlations of the convective heat losses of the cylindrical helical coil receiver and conical spiral coil receivers are derived.
A solar receiver is an integral part of any concentrated solar power based plant for energy conversion. An experimental or numerical analysis of a large-scale solar receiver is expensive due to the high infrastructure and computational costs involved. Hence, it becomes necessary to study the effects of scaling on the performance of a solar receiver so that the results of the analysis of small-scale receivers can be extended to large-scale models. Therefore, a scale analysis is carried out in this study to develop constitutional relationships amongst physical and geometrical parameters for upward-facing cylindrical helical coil and conical spiral coil cavity receivers. Similarity parameters are derived from the non-dimensional Navier Stokes equations in the curvilinear coordinate system (s,r,θ). A scaling law is derived based on the identified parameters. A validated numerical model for the cylindrical helical coil receiver and conical spiral coil receiver is used to verify the scaling analysis. Further, Nusselt number correlations for the convective heat losses from the upward-facing cylindrical and conical cavity receivers are proposed. The newly proposed correlations predict the convective losses from the upward-facing cylindrical and conical cavity receivers within the error band of ±20%.
•Design of multitube shell and tube LHTES for extending electricity generation after sunset.•Dynamic model is developed for ORC based solar thermal power plant with LHTES.•Thermal resistance based ...reduced order model with enthalpy technique is used for LHTES.•Simplified energy equations are solved for other components of the solar power plant.•Effects of geometric parameters of LHTES on the overall performance is studied.
In this paper, dynamic model for organic Rankine cycle (ORC) based solar thermal power plant integrated with latent heat thermal energy storage (LHTES) system is developed to understand its interaction with other components in the power plant for extending electricity generation after sunset. A multitube shell and tube latent heat thermal storage system is considered, in which phase change material (PCM) is stored in the shell side and heat transfer fluid (HTF) flows through the tubes. The multitube shell and tube latent heat storage is modelled considering an elemental unit obtained by encircling each HTF tube to the extent within which the surrounding PCM and HTF exchanges heat. A reduced order model coupled with the enthalpy technique is developed for analysing heat transfer in the LHTES system during charging and discharging periods. The model is validated with the experimental results reported in literature. Simplified transient energy equations are used to model collector and heat exchanger. Further, a latent heat thermal storage system is designed to generate 200kWt after sunset and the overall performance of the solar thermal power plant is evaluated for ten days of operation under Indian conditions.
•Developed generalized numerical model of volumetric shrinkage and expansion of PCM.•Evaluated effect of shrinkage on discharging performance of latent heat storage system.•Maximum difference in heat ...removal from the storage with and without shrinkage is 15%.•Studied the performance of the storage for several cycles of solidification and melting.
Prediction of shrinkage voids during solidification of phase change material (PCM) with unequal liquid and solid densities is critical to design and assess the thermal performance of latent heat thermal energy storage system. In this paper, the solidification of phase change material with shrinkage void formation is numerically investigated using the modified source-based enthalpy method. This study aims to develop a capability to predict the size and location of shrinkage voids during solidification by developing generalized governing conservation laws that account for large differences in density, specific heat, and thermal conductivity between solid and liquid phases of phase change material. The proposed numerical model incorporating volumetric shrinkage and expansion is validated with the reported experimental results. Further, the present model is used to study the effect of solidification shrinkage on the heat transfer performance of the latent heat thermal energy storage system using paraffin wax. The shrinkage during the discharging period is found to affect the thermal performance of the thermal energy storage significantly. The maximum difference in total heat extraction from the thermal energy storage is obtained as 15% between the cases of with and without shrinkage. Moreover, the proposed model is used to analyze the behaviour of latent heat thermal energy storage system under multiple cycles of solidification and melting, incorporating volumetric shrinkage and expansion. The developed methodology will provide detailed insight into the thermal performance of latent heat thermal energy storage system with shrinkage and expansion, leading to an improved material selection and design of the storage systems.
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