Mass Flow Rate Of Heat Transfer Fluid Research Articles

Tracking of the melting front in tube-in-tube latent thermal energy storage heat exchangers

In this study, the shrinking solid regime is investigated in a high aspect ratio tube-in-tube latent thermal energy storage (LTES) heat exchanger by tracking the phase change front evolution. In the setup, water flows through the inner tube as heat transfer fluid (HTF) and the shell contains a paraffin RT35HC as phase change material (PCM). A series of melting experiments have been performed in which the HTF mass flow rate, HTF inlet temperature and initial PCM temperature were varied. During each experiment, the movement of the phase change front was tracked using a camera placed next to the PCM tube. The front position as a function of time follows an S-shape curve. The effect of the operating conditions on the evolution of the phase change front is analysed. It is found that the temperature difference between the HTF and PCM exerts a more pronounced influence on the front position compared to the HTF Reynolds number. Finally, a correlation was developed to determine the temporal evolution of the front position. This research contributes to a deeper understanding of phase change phenomena in LTES systems and the found correlation can be a valuable tool for predicting and optimizing the performance of tube-in-tube LTES heat exchangers.

Effect of steam flow parameters on the partial load behaviour of a PCM storage

To estimate the achievable thermal power in passive latent heat thermal energy storage (LH-TES) systems and the corresponding charging/discharging times for customised storage design and operation, relevant influencing parameters and their impact must be sufficiently known and understood. These parameters include not only the well-researched parameters on the phase change material (PCM) side, but also the parameters on the heat transfer fluid (HTF) side, such as the inlet temperature, the mass flow rate and the resulting heat transfer coefficient (HTC) between the HTF flow and the tube wall. For heat exchangers, these influences have been studied well, however for highly transient LH-TES studies are scarce. This article presents the influence of relevant parameters focusing on the ones related to the HTF side, using analytical and numerical parameter studies in Matlab®. The effect of different HTCs for varying flow conditions of the HTF in different zones of the tube is utilised to introduce the dominant heat transfer surface between the storage and the HTF (DHTS) as an indirect operating parameter. It corresponds to the area where the HTF phase change occurs and where most of the total energy is transferred. The numerical study indicates that the size of the dominant heat transfer area and thus the DHTS can be adapted by the HTF mass flow rate and the temperature difference between the HTF inlet temperature and the PCM phase change temperature. Based on this knowledge, new conjugate design and operating strategies for a test rig with a nominal thermal power of 1 kW using the commercial PCM PLUSICE A133 as storage material are proposed. They lead to a peak shaving behaviour of the storage with defined charging and discharging times.

Energy performance analysis of solar assisted gas-fired boiler heating system for floating roof oil tank

An innovative solar assisted gas-fired boiler heating system for floating roof oil tank is proposed to achieve the low carbonization of the conventional crude oil heating method. However, the dynamic energy transport performance of the proposed heating system affected by the key engineering parameters (crude oil capacity load, mass flow rate of heat transfer fluid (HTF), crude oil initial temperature) is unclear, which hinders the engineering application of this technology. In this regard, a simulation platform for solar assisted gas-fired boiler heating system for floating roof oil tank is established, and the above mentioned engineering parameters are investigated. The results show that under the condition of combined heating of evacuated tube solar collector (ETSC), phase change heat storage tank (PCHT), and auxiliary heat source (AHS), the system can achieve stable operation by means of controllers. With the increase of crude oil capacity load, the time for the crude oil to reach its design temperature is delayed, and up to 84.21% time lag is recorded. Apart from the contribution of AHS, the maximum annual heat supply of 43.37% is provided by PCHT and ETSC, presenting good energy-saving potential. Increasing HTF mass flow rate and crude oil initial temperature is a good strategy to reduce the output of AHS, in which the annual heat supply proportion of AHS decreases by 2.25% and 2.05%, respectively. The operating stability of proposed system is enhanced under the condition of higher HTF mass flow rate, compared to the 3.5 kg/s HTF mass flow rate, the safe reserve time increases by 31.44% for 7 kg/s and 34.24% for 14 kg/s HTF mass flow rate, respectively.

Investigating heat transfer enhancement for PCM melting in a novel multi-tube heat exchanger with external fins

The low thermal conductivity coefficient is the limiting parameter of phase change materials (PCMs) for using in thermal energy storage systems. Various heat transfer enhancement methods such as utilizing extended surfaces techniques which include finned tubes and multi-tubes are studied. In this paper the melting of a PCM is investigated in a novel multi-tube heat exchanger (MTHX). RT35 and water are used in the outer and inner tube as the PCM and the heat transfer fluid (HTF), in turn. The main purpose of this study is to peruse the effect of installing external longitudinal fins on the inner tube (tubes) during the charging process. Two, three, and four tubes are engaged to check the effect of inner tubes number on the melting process with a constant amount of PCM and the HTF mass flow rate. A two-dimensional finite volume method based on the enthalpy–porosity technique has been developed using FLUENT software. A comparison between MTHX without fins and with external fins shows that adding of fins improves the heat transfer rate, more uniform temperature distribution, and reduced melting time. The PCM melting time was reduced by 27.84 %, 33.8 %, 29.84 %, and 29.55 % in cases A, B, C, and D, respectively, by installing external fins on the MTHX.

Unsteady-state thermal performance analysis of cascaded packed-bed latent thermal storage in solar heating system

Thermal storage system is crucial to economically and stably utilize solar energy to address its volatility and fluctuation. A cascaded packed-bed latent heat storage system combined with solar evacuated tube collector is constructed in this paper, and the dynamic thermal performances are numerically investigated. The solar evacuated tube collector model is established to simulate the heat transfer fluid (HTF) charging process, which is experimentally verified by comparing the inlet/outlet temperatures. A latent heat storage system in three-stage packed-bed using phase-change materials with different melting temperatures ranging from 311 K to 335 K is modeled by enthalpy-porosity method, which is validated by comparing with the experimental data of literature. Then, the solar thermal storage system is integrated, and the impacts of the mass flow rate of HTF, solar collector area and initial temperature of packed-bed on its dynamic energetic and exergetic performances are analyzed. The results and discussions are analyzed to obtain optimal operation conditions. The increase of mass flow rate by 0.04 kg/s declines the maximum temperature by 9.53%. The initial temperature of packed-bed ranging from 293 K to 300 K achieves higher HTF temperature, heat storage efficiency and charging capacity. The solar collector area is necessarily matched to the phase-change materials to obtain high efficiency and also avoid HTF vaporization.

Numerical investigation of a latent cold storage system using shell-and-tube unit

The use of cold thermal storage systems in low-temperature industrial applications is considered one of the most promising ways of improving energy efficiency and reducing the use of power during peak periods. In this study, the thermal performance of a shell-and-tube cold storage system under realistic operating conditions is investigated numerically. The proposed model is developed based on energy balances and then validated using existing experimental data from the literature. Glycol/water is used as the heat transfer fluid (HTF) and the phase transition phenomena in the phase change material (PCM) is simulated using the enthalpy–porosity approach. The influence of several design and operating parameters, including the HTF mass flow rate, HTF temperature, PCM type, and volume, on the cold storage performance during the crystallization process is presented and analyzed. The numerical results show that increasing the HTF mass flow rate accelerates the PCM crystallization process. However, the delivery periods of constant thermal power and constant HTF outlet temperature are reduced. The HTF inlet temperature has a significant effect on the cold storage performance, and the complete charging period is reduced by approximately 37% when the HTF inlet temperature is reduced from −4 °C to −7 °C. Increasing the number of tubes in the cold storage unit is concluded to significantly improve the thermal performance of the system, and using water/ice as a cold storage medium is more suitable than using the commercial PCMs RT2-HC and RT4-HC. Finally, the proposed numerical model for cold storage systems can be successfully used to design and simulate their realistic operation under different conditions.

Application of Artificial Intelligence to Improve the Thermal Energy and Exergy of Nanofluid-Based PV Thermal/Nano-Enhanced Phase Change Material

Photovoltaic-thermal (PVT) technologies have demonstrated several attractive features, such as higher power and comparative efficiencies. Improving the thermal recovery from the PVT system would further improve the power output and the efficiency of the PVT system. This paper identifies the best operating factors of nanofluid-based PV thermal/nano-enhanced phase change material using artificial intelligence. The target is the maximization of thermal energy and exergy outputs. The suggested approach combines ANFIS modelling and particle swarm optimization (PSO). Four operating factors are taken into consideration: PCM (phase change material) layer thickness, HTF (heat transfer fluid) mass flow rate, MFNPCM (“mass fraction of nanoparticles in PCM”) and MFNfluid (“mass fraction of nanoparticles in nanofluid”). Using a dataset, an “adaptive neuro-fuzzy inference system” (ANFIS) model has been established for simulating the thermal energy and exergy outputs in terms of the mentioned operating factors. Then, using PSO, the best values of PCM thickness, mass flow rate, MFNPCM and MFNfluid are estimated. The proposed model’s accuracy was examined by comparing the results with those obtained by response surface methodology and the experimental dataset.

Dynamic thermal analysis and creep-fatigue lifetime assessment of solar tower external receivers

This paper proposes a methodology for the dynamic thermal analysis of solar tower external receivers and for the assessment of their lifetime. A dynamic thermal model is implemented in Modelica to assess the temperature distributions of receiver tubes and heat transfer fluid (HTF), considering real weather data with a 10-minute time resolution and PI controllers based mass flow rate control. Three representative days are simulated, and the resulting tubes temperature distributions are used to assess the elastic stresses distributions during the investigated days. Subsequently, the creep-fatigue lifetime of each receiver panel is evaluated accounting for material plasticity and creep-induced stress relaxation. The developed methodology is applied to compare three materials for solar receivers: Inconel 740H, Haynes 230, and Incoloy 800H.Results show that fatigue can be negligible with respect to creep for all investigated materials and the shortest lifetime is obtained for 800H, followed by H230, and 740H. The approximation of a receiver operation for 365 clear-sky days per year leads to errors around 30 % on the lifetime. For 740H and 800H, the damage accumulation during cloudy days with high DNI peaks is greater than during clear-sky days due to the remarkable creep damage originated during temperature spikes which follow clouds passages. H230 instead, accumulates more creep damage during clear-sky operation. This emphasizes the potential of the developed methodology to compare different HTF mass flow rate control strategies as well as receiver geometries and materials, HTFs, and heliostats aiming strategies, accounting for the trade-off between energy yield and receiver lifetime.

Numerical analysis and optimization of the charging process on a shell‐and‐tube latent heat thermal energy storage unit for a solar power plant with direct steam generation

AbstractA two‐dimensional model of the charging process on a heat storage unit in a shell‐and‐tube type latent heat subsystem of a solar power plant with direct steam generation was constructed in this study. The effects of the outer diameter to inner diameter ratio, aspect ratio, phase change material (PCM) thermal conductivity, and heat transfer fluid (HTF) mass flow rate were investigated. Results show that increasing the PCM thermal conductivity, HTF mass flow rate, and aspect ratio of the heat storage unit can shorten heat storage time, but the ratio of the outer diameter to the inner diameter of the heat storage unit has an optimal value of 6 in this problem. Using response surface methodology analysis, the influence of the aspect ratio, outer‐to‐inner‐diameter ratio, PCM thermal conductivity, and HTF mass flow rate on the storage time of the phase change heat storage unit is in descending order. After a genetic algorithm optimization, the storage rate of the heat storage unit increased by 35%. The results of this study can guide the heat storage unit to achieve a better practical application performance.

Numerical Investigation of Concentrated Solar Central Billboard with Hexagonal Tubes.

The solar receiver is the most important component of any central solar tower power plant (CSP) system. A numerical analysis of four billboard geometry designs of the central tower receiver was undertaken in this study. On the receiver, the surface area, heat transfer fluid (HTF), and intensity of beam radiation were all constant. The mass flow rate of HTF was modified, and the temperature gain was used as a metric to determine the most efficient design based on the analytical results. Four different models of vertical tube receivers were designed and analyzed. In the analysis, the surface area of the receivers was the same. When compared to the vertical variable circular tube receiver design, the vertical variable hexagonal tube receiver design achieved a marginally higher temperature for the HTF fluid at lower mass flow rates.

Numerical modelling and optimization of packed-bed thermal-energy storage system

In this work, numerical modelling (including convection and radiation heat transfer) of Packed-bed storage systems (PBSS) is carried out and the effect of various system parameters affecting the efficiency and temperature is studied. One dimensional forms of the time-dependent, coupled partial differential equations for energy conservation of the heat transfer fluid (HTF) and the packing materials are solved using the finite difference method to design a PBSS for a given mass flow rate for 0.125 MWh (0.45 GJ). The packed-bed material is assumed to be spherical with an average diameter, and the mass flow rate, void fraction, packed bed material diameter, heat transfer fluid (HTF), and the packed-bed materials are varied for optimal efficiency during charging (i.e. when the temperature of HTF is greater than that of the packed bed). The results are compared with the analytical solutions for validation. Present solutions (considering only convection) are found to be matching reasonably well with the analytical data. With the validated model, the radiation heat transfer effect on the packed-bed is modelled (along with the convection) and void fraction, mass flow rate of HTF, a diameter of the packed-bed material, different HTF, various packed-bed materials are compared for maximum efficiency. From the present work, a PBSS design with void fraction =0.45, mass flow rate =1.8 kg/s, the average diameter of the packed-bed material = 0.02 m, water as HTF, and brick as packed-bed material is found to give the maximum efficiency along the length of the packed-bed.

Analysis and optimisation of packed bed thermocline energy storage tank for CSP power plant

A 17 kWhth packed bed thermal energy storage (PBTES) system for concentrated solar power application (CSP) has been investigated by using a two-phase energy model. The PBTES system used in this study consists of quartzite as storage material and caloria HT-43 as a heat transfer fluid (HTF). Simulation of 17 kWhth packed bed system has been utilised to study the effect of void fraction, storage material on charging efficiency and pressure drop at varying mass flow rate (0.01∼0.04 kg/s) of HTF. In this study, parametric optimisation using the Taguchi method has also been done to get the optimum parameter for maximum efficiency and minimum pressure drop. Results show that void fraction has more effect on charging efficiency as compared to pebbles diameter. The percentage decrease in charging efficiency is 8.1% for pebbles diameter varying from 0.03 to 0.05 and 13.5% for void fraction varying from 0.3 to 0.5.

Effect of HTF flow direction, mass flow rate and fins on melting and solidification in a latent-heat-based thermal energy storage device

Latent-heat-based thermal energy systems (LHTES) have commonly been used as a potential energy storage mode over any other mode of thermal energy storage. Many heat transfer enhancement techniques have been proposed over the past years. These techniques reduce the melting and solidification times. Most of these techniques focus on the phase change material (PCM). However, the flow direction of the heat transfer fluid (HTF) can affect the heat transfer performance and pumping power requirement of the system. In this paper, the effect of HTF-flow direction, HTF mass flow rate and addition of the fins on the melting and solidification of the PCM in a shell-and-tube type of energy storage is numerically studied. Two-dimensional transient simulations are performed with ANSYS-Fluent where the phase-change process is modelled using the enthalpy-porosity formulation. The model is verified and validated by comparing with the available experimental data. A reasonable match is observed. The validated model, is used to study the effects of various parameters, such as, mass flow rate of the HTF, and triangular fin (at a fixed fin pitch) for both charging and discharging of the PCM. Finally, an influence of flow direction on the melting and solidification time has been studied. It is found that the contribution of HTF mass flow rate, the addition of the fin and HTF flow directions respectively is 1.3-3.01%, 16.97-17.62%, and 1.3-1.77% of overall heat transfer performance. A major contribution to the enhancement of overall heat transfer of the system is from the addition of fins.

Melting and solidification characteristics of cylindrical encapsulated phase change materials

Thermal energy storage using encapsulated phase change materials (EPCM) has been attracting the attention of researchers in solar energy applications due to their high energy storage capabilities. However, the melting characteristics of EPCMs are affected by many parameters. In this study, experimental and CFD model were used to investigate the parameters influencing the melting and solidification characteristics of a cylindrical EPCM. The developed CFD model was validated with the experimental model's results and with experimental data from the literature. The CFD model is used to investigate the effect of changing the PCM type, heat transfer fluid (HTF) temperatures, HTF flowrates, number of fins and solidification temperatures. Lauric acid and paraffin wax at HTF temperature ranging from 55 to 73 °C and flowrates from 0.064 to 0.323 kg/s are compared. Then the number of fins from 4 to 12 were inserted inside the capsule and compared with the case without fins at the same amount of PCM. The solidification is simulated at HTF temperature from 20-35 °C. Results revealed that increasing the HTF flow rate by five times decreases the melting times of lauric acid and paraffin wax by 30% and 22%, respectively. The total energy stored in paraffin wax is much higher compared with that in lauric acid under the same HTF conditions. Furthermore, increasing the number of internal fins in the capsules to 12 enhances the melting time by 41% of paraffin wax at an HTF mass flow rate of 0.3238 kg/s and temperature of 73 °C. The presence of fins effectively enhances the solidification and melting time of lauric acid and paraffin wax.

Effect of parameters on the melting performance of triplex tube heat exchanger incorporating phase change material

In this study, a two-dimensional mathematical model of a phase change material (PCM) melting in a triplex tube heat exchanger (TTHX) unit was developed. Based on two parameters, exergy efficiency and heat storage rate, the effects of the operating conditions and unit configurations on the thermal storage performance of the investigated TTHX unit were numerically studied. The results indicate that the increase in heat transfer fluid (HTF) inlet temperature improves the heat storage rate but reduces the exergy efficiency. The HTF mass flow rate in the inner and outer tubes should be minimized under the premise that the flow states in both tubes are turbulent. If the flow states in both tubes are turbulent and the HTF mass flow rate remains unchanged, then the HTF mass flow rate in the inner tube should be decreased for higher exergy efficiency. When the PCM radius varies from 30 mm to 70 mm under the same operating conditions, the exergy efficiency increased by 15.6%, while the heat storage rate decreased by 36.8%. If the HTF mass flow rate is large, the variation in TTHX length has a slight influence on the exergy efficiency, but it significantly affects the heat storage rate.

A systematic assessment on a solar collector integrated packed-bed single/multi-layered latent heat thermal energy storage system

Thermal energy storage (TES) methods stand out as a key technology that eliminates the supply-demand imbalance in cooling or heating systems. This paper focuses on investigating the charging performance of the solar-aided packed-bed latent heat TES system. An in-house 1D transient mathematical model is developed to simulate the latent heat TES system's heat transfer phenomena. The TES tank is integrated with a flat plate solar collector to include the variable boundary condition effects of solar energy and ambient air. The enhanced thermal conductivity approach is implemented into the mathematical model to consider the natural convection effects of the PCM's liquid phase, and the method is validated. The mathematical model of the packed bed latent heat TES tank is also validated with the experimental and numerical results from the literature. As design and working parameters, the mass flow rate of heat transfer fluid (HTF) and spherical storage material diameter are varied. Single-, double- and triple-phase change material (PCM) arrangements are also studied by varying the melting temperature of PCM along the flow direction. The time-wise variation of energetic and exergetic efficiencies of the integrated system, PCM mean temperature inside the storage tank, and the liquid fraction in flow direction are investigated. Further extensive parametric analyses are conducted for heating season months, i.e., November, December, January, and February. As a result, the best design and working conditions for the integrated latent heat TES system within the range investigated are proposed. Also, it is concluded that decreasing the capsule diameter and increasing the HTF's mass flow rate has a positive effect on system indicators.

Performance analysis of a waste heat recovery solar chimney for nocturnal use

Solar chimney is one of the best ways to produce artificial wind to drive wind turbines. But the main problem associated with is that it’s dependence on solar energy and hence can’t be used during night times. The objective of the present work is to modify the existing model; so that it can be used for the generation of power even in nocturnal and cloudy conditions. There are many ways to recover waste heat from different industries, but solar chimney can give one of the leading possible ways to recover it. Waste heat can be used to heat the base or absorber of the solar chimney’s collector; or it can be used to heat the air flowing inside the chimney. Ansys 2019 R1 has been used for the Computational Fluid Dynamics (CFD) validation and analysis of the solar chimney with heat recovery heat exchanger. A heat exchanger (HE) plate of aluminium alloy has been used with syltherm-800 as heat transfer fluid (HTF), and exchanging heat with the air inside the chimney near its base. Effect of different parameters like HTF inlet temperature, mass flow rate of HTF, ambient temperature of air, and diameter of heat exchanger plate on the performance of solar collector of chimney has been analysed. Also, the maximum conversion efficiency of about 5% is achieved, which is more in comparison to solar chimney operating at normal day conditions. Limitation of the present work is related to the storage of heat during day time or there should be continuous supply of waste heat during chimney operations. In this paper, authors have proposed a CFD method for the modification and improvement of solar chimney in Manzanares (Spain) with a waste heat recovery heat exchanger for nocturnal use.

Steady-state off-design modeling of the supercritical carbon dioxide recompression cycle for concentrating solar power applications with two-tank sensible-heat storage

Concentrating solar power researchers are evaluating the potential of the supercritical carbon dioxide recompression cycle to improve the thermal efficiency and decrease the capital costs of next-generation systems. This analysis investigates the steady-state off-design performance of a recompression cycle integrated with two-tank sensible-heat thermal energy storage as the ambient temperature and heat-transfer fluid (HTF) inlet conditions to the cycle change. This paper presents off-design component models and then cycle convergence and control models to maximize net cycle power output while constraining the high-side pressure and air-cooler fan power to their respective design values and fixing the cycle HTF outlet temperature to its design value. Results show that inventory control and air-cooler fan power are important control parameters that can be optimized to maximize off-design power output as the ambient temperature and HTF mass flow rate diverge from design. The high-side pressure and fan power constraints cause the cycle net power to degrade when the ambient temperature is warmer than the design value, and this study calculates a maximum mass flow rate for hot days above which the cycle cannot achieve the design HTF outlet temperature. Finally, the analysis shows that optimizing compressor shaft speeds can improve cold-day performance by around 1.5 percentage points and part-load performance by up to 2.5 percentage points versus a baseline case with no active compressor control.

Experimental investigation on heat-transfer characteristics of a cylindrical cavity receiver with pressurized air in helical pipe

A concentrating solar system is established for the experimental investigation on a cylindrical cavity receiver. With a built-in helical pipe, the receiver is loaded at the focus point of a parabolic dish, driven by a double-axis tracking system. The helical pipe contains pressurized air as the heat transfer fluid (HTF), heated by concentrated solar irradiation. The experimental investigation analyzed operation parameters effects on both heat losses and the thermal efficiencies, which are direct normal irradiance (DNI), the inlet temperature of HTF and the mass flow rate of HTF. This study also proposed an innovative mathematical model for experiment data processing. The results indicated the increase of DNI from 201.7 W/m2 to 504.3 W/m2 led to a decrease of the thermal efficiency, the increase of the inlet temperature from 150 °C to 200 °C emerged a same tendency. The increase of the mass flow rate from 0.05 kg/s to 0.11 kg/s was on the contrary, since it decreased total heat losses. Good agreements were achieved by compared numerical simulation results of previous studies to the experiment results. Subsequently, a method for thermal performance enhancement was summarized through experimental investigation results, which could be a mentionable improvement for cavity receiver research.

Novel dimension scaling for optimal mass flow rate estimation in low temperature flat plate solar collector based on thermal performance parameters

Uncomplicated configuration, efficient collection of incident solar radiation and less frequent maintenance issues compared to the other prevalent alternatives enable the flat plate solar collectors to remain relevant in the present age of continuously evolving cutting-edge technologies. However, the quest for energy-efficient and sustainable solutions necessitates resizing of the flat plate collectors to achieve enhanced efficiency. Detailed performance analysis of the collector requires a prohibitively higher number of experimental trials and computationally expensive numerical models. Hence, in this study, a simplified energy equations-based dynamic model is used to develop working relationships for calculating the collector dimensions and the optimum heat transfer fluid (HTF) mass flow rate for achieving a predefined thermal efficiency. The developed formulae can predict the geometrical and operating parameters very accurately, with maximum deviation in estimating the collector channel dimensions being 12.5%. Hence, the formulae can be used as an initial guideline for the design of the collector. The computational results for the geometrical configuration corresponding to an average DNI, calculated from actual solar radiation data, indicate that to achieve a fixed temperature and a collector efficiency, the required HTF mass flow rate increases with increasing level of solar radiation.