Heat Transfer Fluid Inlet Research Articles

Thermodynamic analysis and optimization of multistage latent heat storage unit under unsteady inlet temperature based on entransy theory

An optimization model for a multistage latent heat storage (LHS) unit with unsteady heat transfer fluid (HTF) inlet temperature was proposed. Thermodynamic analysis and optimization were performed based on the entransy theory. The expressions of the optimum phase change material (PCM) melting temperatures (Tm,opt) were derived. The effects of geometric parameters and unsteady HTF inlet temperature on the optimum phase change temperatures were investigated. The results indicate that with the increase of stage number (n), Tm1,opt increases and Tmn,opt decreases, which is beneficial to extend the selection range of PCM. For fixed entransy dissipation condition, increasing n will not change the fluctuation of the HTF outlet temperature; however a nearly uniform HTF outlet temperature can be obtained by increasing unit length (L). The unsteady HTF inlet temperature has great effects on the optimum phase change temperature. For a 3-stage LHS unit, the optimum phase change temperature of each stage increases by 14.9 K, 26.4 K and 38.0 K respectively with respect to the values obtained by steady method, which causes the heat storage capacity decreases by 6.1% and entransy dissipation decreases by 10.6%. The present work can provide guidance for the design of the multistage LHS unit with unsteady HTF inlet temperature.

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

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

Thermal behavior of latent thermal energy storage unit using two phase change materials: Effects of HTF inlet temperature

This work presents a numerical study of the thermal behavior of shell-and-tube latent thermal energy storage (LTES) unit using two phase change materials (PCMs). The heat transfer fluid (HTF) flow through the inner tube and transfer the heat to PCMs. First, a mathematical model is developed based on the enthalpy formulation and solved through the governing equations. Second, the effects of HTF inlet temperature on the unsteady temperature evolution of PCMs, the total energy stored evolution as well as the total melting time is studied. Numerical results show that for all HTF inlet temperature, melting rate of PCM1 is the fastest and that of PCM2 is the slowest; increasing the HTF inlet temperature considerably increases the temperature evolution of PCMs. The maximum energy stored is observed in PCM2 with high melting temperature and high specific heat; heat storage capacity is large for high HTF inlet temperature. When the HTF inlet temperature increases from 338K to 353K, decreasing degree of melting time of PCM2 is the biggest from 1870s to 490s, which reduces about 73.8%; decreasing degree of melting time of PCM1 is the smallest from 530s to 270s, which reduces about 49.1%.

Thermal analysis of nanoparticle-enhanced phase change material solidification in a rectangular latent heat storage unit including natural convection

The present study aims to investigate the solidification process of nanoparticle-enhanced phase change material (NEPCM) filled in a rectangular latent heat storage unit (LHSU). The storage unit consists of several slabs filled with n-octadecane as phase change material (PCM) dispersed with high conductivity nanoparticles (copper). The NEPCM slabs are vertically oriented and separated by rectangular channels in which circulates water as heat transfer fluid (HTF). A two-dimensional model has been developed and validated by experimental, theoretical and numerical data available in literature. The enthalpy porosity method was adopted for modeling the phase change process and the finite volume approach was used to discretize the resulting equations. The numerical simulations were conducted to investigate the effect of the volumetric fraction of nanoparticles, aspect ratio of NEPCM slabs and dimensionless HTF inlet temperature on the heat transfer enhancement and storage performance of the LHSU during solidification process. A correlation encompassing all investigated control parameters has been developed to estimate the solidification time of NEPCM slabs.

Investigation of the Dynamic Melting Process in a Thermal Energy Storage Unit Using a Helical Coil Heat Exchanger

In this study, the dynamic melting process of the phase change material (PCM) in a vertical cylindrical tube-in-tank thermal energy storage (TES) unit was investigated through numerical simulations and experimental measurements. To ensure good heat exchange performance, a concentric helical coil was inserted into the TES unit to pipe the heat transfer fluid (HTF). A numerical model using the computational fluid dynamics (CFD) approach was developed based on the enthalpy-porosity method to simulate the unsteady melting process including temperature and liquid fraction variations. Temperature measurements using evenly spaced thermocouples were conducted, and the temperature variation at three locations inside the TES unit was recorded. The effects of the HTF inlet parameters were investigated by parametric studies with different temperatures and flow rate values. Reasonably good agreement was achieved between the numerical prediction and the temperature measurement, which confirmed the numerical simulation accuracy. The numerical results showed the significance of buoyancy effect for the dynamic melting process. The system TES performance was very sensitive to the HTF inlet temperature. By contrast, no apparent influences can be found when changing the HTF flow rates. This study provides a comprehensive solution to investigate the heat exchange process of the TES system using PCM.

Performance investigation of a lab-scale latent heat storage prototype – Experimental results

Abstract This paper presents the performance tests carried out on a lab-scale latent heat storage (LHS) prototype during charging and discharging processes. The storage unit is a shell-and-tube type heat exchanger with embedded finned tubes, designed for an LHS capacity of 10 MJ. A ternary mixture comprising of potassium nitrate, sodium nitrate and sodium nitrite in the weight proportion of 53:7:40 is used as the phase change material (PCM). Hi-Tech Therm 60 is used as the heat transfer fluid (HTF). Performance parameters viz., melt fraction, charging/discharging time and energy storage/discharge rate were evaluated at different operating conditions. The effects of HTF inlet temperature and flow rate on the storage characteristics of LHS prototype were analyzed. It is observed that the temperature variation in the angular direction of the prototype during charging process is significant. This is due to the natural convection heat transfer that occurred around the molten layer of PCM while melting. During the discharging process, the angular temperature variation is negligible as the solidification phenomenon is controlled mainly by the conduction heat transfer. It took about 124 min/131 min for charging/discharging of the LHS prototype.

Experimental investigation on the heat transfer performance of a latent thermal energy storage device based on flat miniature heat pipe arrays

An experimental latent-thermal energy storage device (LTESD) with a flat miniature heat pipe array (FMHPA) as a core heat transfer element is designed. Multihole flat pipes are utilized as a heat supply and heat removal loop for the passage of heat transfer fluid (HTF). Experiments are performed at different HTF volume flow rates and inlet temperatures to investigate the performance of the thermal storage unit consisting of FMHPA and vertical fins and to observe the change in the temperature of phase-change materials, such as lauric acid. The effects of heating/cooling section length and thermal resistance are also examined. Results indicate that LTESD works stably and efficiently and the respective storing and releasing power are 1299 and 1120 W under standard operating conditions of 2 L/min at 70 °C and 2 L/min at 15 °C.

Experimental thermal storage research of organic binary phase change materials in building environment

ABSTRACTThis paper aims to reveal the heat transfer mechanism of low-temperature phase change material (PCM) and design PCM heat storage device in building heating environment. Firstly, low-temperature binary PCMs of lauric acid and stearic acid are prepared, and their thermal properties are investigated by DSC. Then, shell and tube latent heat thermal energy storage units are conducted, and heat transfer experiments are carried out to analyze the heat transfer mechanism of PCM. The results demonstrate that natural convection plays an important role in heat transfer process, and the heat storage efficiency of PCMs can be significantly enhanced by increasing the fin width and improving the inlet heat transfer fluid (HTF) temperature. Furthermore, some proposals are put forward to guide the design of PCM storage device in building heating environment.

Comparative study of melting and solidification processes in different configurations of shell and tube high temperature latent heat storage system

The performance of parallel and counter flow configurations of a shell and tube heat exchanger used as a latent heat thermal storage system was investigated. The comparative analysis of the melting and solidification processes (charging and discharging) in the vertical and horizontal orientations of counter and parallel flow was included in this study to make the results conclusive for the purpose of design of the storage system to be used in a concentrated solar power (CSP) plant. In this analyses, taking into account natural convection, numerical calculations were performed using ANSYS Fluent. The vertical parallel flow configuration shows 12% higher effectiveness compared to the counter flow for the charging and discharging processes. A higher rate of phase change occurs for both the charging and discharging processes in a parallel flow configuration, as a higher fraction of phase change material (PCM) is exposed to the heat transfer fluid inlet temperature compared to the counter flow. However, a lower temperature gradient and nearly constant effectiveness for a longer period of time are observed in the counter flow arrangement due to the higher rate of natural convection. An interesting result is the higher rate of natural convection in the horizontal orientation compared to the vertical one due to the Benard convection phenomenon, despite the fact that Ra is higher in the vertical orientations (1010>107). The horizontal counter flow and parallel flow configurations show on average 10% higher effectiveness for the charging process which is constant for a longer period during the process compared to the vertical configurations whereas the horizontal arrangement during the discharging process improves the effectiveness by about 2% due to the minor role of natural convection. The horizontal configurations provide a more uniform phase change process with the lowest peak temperatures (in the melting processes) and temperature gradient which are correlated with the highest second law efficiency and exergy recovery.

Optimization of Multistage Latent Heat Storage Unit Under Unsteady Inlet Temperature Based on Entransy Theory

Thermodynamic analysis and optimization of a multistage latent heat storage (LHS) unit with unsteady heat transfer fluid (HTF) inlet temperature were performed based on the entransy theory. The expressions of the optimum phase change material (PCM) melting temperatures (Tm,opt) were derived. The results indicate that with the increase of stage number (n), Tm1,opt increases and Tmn,opt decreases, which extends the selection range of PCM. For fixed entransy dissipation condition, increasing n will not change the fluctuation of the HTF outlet temperature; however, a nearly uniform HTF outlet temperature can be obtained by increasing unit length (L). The unsteady HTF inlet temperature has great effects on the optimum phase change temperature. For a 3-stage LHS unit, the optimum phase change temperature of each stage increases by 14.9 K, 26.4 K and 38.0 K respectively with respect to the values obtained by steady method, which causes the heat storage capacity decreases by 6.1% and entransy dissipation decreases by 10.6%. The greater the temperature fluctuation is, the greater the effects will be. The present work can provide guidance for the design of the multistage LHS unit with unsteady HTF inlet temperature.

Experimental study using different heat transfer fluid of a packed bed thermal energy storage system during charging process

Heat transfer analysis of a packed bed-PCM latent heat thermal energy storage system is presented in this article. The cylindrical packed bed is filled with spherical capsules of PCM (water) that is used for an air conditioning commercial application. In the study, the experimental model is tested to analyse the thermal performance of packed bed-PCM latent. The results obtained are used for the thermal behaviour of charging process. The main objective is to compare the loading time using as heat transfer fluid (HTF) ethylene glycol and ethanol. The effects of mass flow rate, freeze rate and inlet HTF temperature are investigated. The results of the study indicate that changing the HTF type and inlet temperature have different effects on total time in charging process.

Impact of periodic flow reversal of heat transfer fluid on the melting and solidification processes in a latent heat shell and tube storage system

A numerical study has been conducted on a shell and tube latent heat storage system whereby the inlet heat transfer fluid direction is periodically reversed during charging and discharging. The impact of varying the boundary condition at the interface of the tubes carrying the heat transfer fluid and phase change material (PCM) on the evolution of the phase change front, heat transfer area and heat transfer rate have been evaluated during the charging and discharging processes. Results for the charging processes show a higher heat transfer area develops during the early stages and amplification of natural convection after 40% melt fraction, leading to a higher heat transfer rate. In comparison to the fixed flow condition, periodic flow reversal for the discharge cases results in an increased heat transfer area for a longer period of time, leading to a higher heat transfer rate particularly after 75% solidification. This effect is more important for discharging cases in the absence of convection heat transfer. Periodically reversing the direction of heat transfer fluid, which produced a periodic boundary condition at the tube-PCM interface, also resulted in a lower temperature gradient in space and time and consequently higher exergy recovery, and about a 6% increase in the time-average heat transfer rate in the charging and discharging cases. The novel reversal flow method provides a means to implement a periodic boundary condition without changing the heat source/sink, enhancing the thermal performance and cost effectiveness of latent heat storage systems. Phase change storage systems incorporating periodic flow reversal provide higher energy delivery rates, greater power density and more exergy recovery. This method can support fast heat release to respond to a peak load in a CSP plant or fast heat storage to protect a tubular receiver from high thermal stresses.

Effect of the phase change material in a solar receiver on thermal performance of parabolic dish collector

In this work, the use of phase change material in the circular tank solar receiver is proposed for a 16 m2 Scheffler parabolic dish solar concentrator to improve the heat transfer in the receiver. Magnesium chloride hexahydrate with melting temperature of 117?C is selected as the phase change material in the annular space of the receiver with rectangular fins inside the phase change material. Experimental work is carried out to analyze heat transfer from the receiver to heat transfer fluid with and without phase change material in the inner periphery. Energy and exergy efficiency are determined from the measurements of solar radiation intensity, receiver temperature, surroundings temperature, heat transfer fluid inlet and outlet temperatures, storage tank temperature, and wind speed. The experiments were conducted in SRM University, Chennai, India (latitude: 13? 5? N, longitude: 80?16? E) in April 2014. Use of phase change material in receiver periphery increased energy efficiency by 5.62%, exergy efficiency by 12.8% and decreased time to reach the boiling point of water by 20% when compared with the receiver without phase change material.

Experimental and numerical investigation of discharging process of direct contact thermal energy storage for use in conventional air-conditioning systems

Direct contact thermal energy storage (TES) for use in conventional air-conditioning systems is proposed to reduce the operational energy demand. Thermal performance of a novel kind of phase change material (PCM) prepared for use in conventional air-conditioning systems with the proposed direct contact TES tank, is evaluated. A 3-dimensional (3D) numerical model is built using ANSYS FLUENT to investigate dynamic characteristics of the discharging process of TES system. The model is validated by comparing the numerical with the experimental results. The effects of heat transfer fluid (HTF) flow rate, HTF inlet temperature, liquid PCM volume fraction, complete discharging time, discharging capacity of the tank, and temperature distribution in direct contact TES tank are investigated. The results indicate that increasing the HTF flow rate speeds up the discharging capacity and the discharging time of the direct contact TES system reduces. When the flow rate increases from 0.503m3/h to 0.936m3/h, the melting PCM increases from 63.33vol.% to 70.74vol.% within 600s. The discharging capacity increases with HTF inlet temperature; however, the whole discharging capacity does not change obviously by changing HTF inlet temperature.

Role of PCM addition on stratification behaviour in a thermal storage tank – An experimental study

The present paper aims to analyse the effect of addition of phase change materials (PCM) encapsulated in spherical capsules at the top of a hot water storage tank for stratification enhancements during the charging process. The experiments were performed in a cylindrical storage tank of capacity 115 L at various flow rates and inlet temperatures of heat transfer fluid (HTF). The temperature profile along the height of the storage tank was analysed for both the thermal energy storage (TES) systems with and without the addition of PCM capsules. The experimental results showed that the overall charging time required was lesser in the sensible thermal energy storage system compared to the system with PCM capsules. However, this difference in charging time was reduced with increase in HTF inlet temperature. The stratification behaviour was analysed using the various non-dimensional numbers such as stratification number, Richardson number, charging efficiency and Cumulative charge fraction for both the TES systems. Better stratification was attained in sensible TES system at lower HTF inlet temperature for all flow rates considered. However, the effect of addition of PCM capsules increased the stratification capability as the temperature difference between the inlet HTF and PCM melting temperature was increased.

Numerical and statistical study on melting of nanoparticle enhanced phase change material in a shell-and-tube thermal energy storage system

This paper presents a numerical and statistical study on heat transfer and energy storage performance of a vertical shell and tube thermal energy storage unit, where CuO – water nanofluid is used as heat transfer fluid (HTF) in the tube and CuO – paraffin nanoparticle enhanced phase change material (NePCM) is used on the shell side for latent thermal energy storage. A computational fluid dynamics (CFD) model is developed to simulate the laminar nanofluid flow, melting of NePCM and the overall heat transfer between the HTF and NePCM, where an enthalpy porosity technique is used in modeling the melting process. With this numerical model, the response surface methodology (RSM) is applied to statistically investigate the effects of three main operational parameters on the NePCM melting process: (1) concentration of CuO nanoparticles in the HTF, with a range of 0–4vol.%; (2) concentration of CuO nanoparticles in the NePCM, with a range of 0–7vol.%; and (3) inlet temperature of the HTF. Thermophysical properties of the NePCM are considered to be temperature dependent in the analysis. Face-centered central composite design (CCF) is employed to find the significant factors and their interactions in affecting liquid fraction rate in the melting process. It is found that changes in all these three factors are responsible for 99.8% liquid fraction variation, with HTF inlet temperature as the most significant parameter. Moreover, all three factors show a parabolic behavior on the liquid fraction variation. Finally, a regression model is developed for prediction of liquid fraction in the NePCM.

Evaluation of Key Design Parameters of an Encapsulated Latent Heat Thermal Storage Unit

The present work numerically investigates for a latent heat thermal storage (LHTS) unit, the effect of key design parameters such as the inlet temperature of the heat transfer fluid (HTF), initial and melting temperatures of the PCM, thermophysical parameters such as specific heat, thermal conductivity etc., on its performance. A numerical model has been developed considering the discharging mode of operation and solved employing finite difference technique. The parametric study reveals that the effect of HTF inlet temperature on the unit's thermal response is more compared to initial temperature of the PCM and the influence of thermal conductivity of the PCM is very strong compared to specific heat capacity of the solid PCM

Exergy analysis of single effect absorption refrigeration systems: The heat exchange aspect

The main limitation of conventional energy analysis for the thermal performance of energy systems is that this approach does not consider the quality of energy. On the other hand, exergy analysis not only provides information about the systems performance, but also it can specify the locations and magnitudes of losses. A number of studies investigated the effect of parameters such as the component temperature, and heat transfer fluid (HTF) temperature and mass flow rate on the exergetic performance of the same absorption refrigeration system; thus, reported different coefficient of performance (COP) values. However, in this study, the system COP was considered to remain constant during the investigation. This means comparing systems with different heat exchanger designs (based on HTF mass flow rate and temperature) having the same COP value. The effect of HTF mass flow rate and inlet temperature of the cooling water, chilled water and heat source on the outlet specific exergy and exergy destruction rate of each component was investigated. It was found that the lower HTF mass flow rate decreased exergy destruction of the corresponding component. Moreover, the lower temperature of heat source and chilled water inlet increased the system exergetic efficiency. That was also the case for the higher cooling water inlet temperature. Based on the analysis, since the absorber and condenser accounted for a large portion of the total exergy destruction, cooling tower modification with lower cooling water mass flow rate is recommended. Furthermore, increasing the cooling water temperature is also recommended as long as the cooling tower can meet the cooling load.

Melting of graphene based phase change nanocomposites in vertical latent heat thermal energy storage unit

The present study deals with the investigation of melting phenomena in single vertical shell-and-tube latent heat thermal energy storage unit. A two dimensional axi-symmetric computational fluid dynamics model based on the enthalpy-porosity method was developed to investigate the melting behaviour. Organic alkane n-eicosane and n-eicosane/graphene nanosheets with different volume fractions were considered as the phase change materials (PCMs). Water was considered as the heat transfer fluid (HTF) flowing inside the tube and the PCM is filled in the shell side of thermal energy storage unit. A variety of numerical simulations were performed for different heat transfer fluid inlet temperatures and varying loadings of graphene nanosheets. Numerical calculations show that higher inlet temperature of the heat transfer fluid decreases the melting time due to accelerated natural convection. We also show that the inclusion of graphene nanosheets significantly decreases the melting time due to the enhanced thermal conductivity of PCM. At 2vol% graphene laoding, melting time reduces significantly by ∼41% when the HTF temperature is 60°C and ∼37% when the HTF temperature is 70°C.

Thermal Performance of a Single-Pass All-Glass Parabolic Trough Receiver

AbstractA novel single-pass all-glass parabolic trough receiver (SPAG-PTR) was developed and the parabolic trough solar collector with concentration ratio of 7.5 was built to investigate the thermal performance of the SPAG-PTR. The indoor heat loss tests were settled as well as the outdoor experiments in quasi-steady-state and transient-state test conditions. The heat loss per unit absorber area of the SPAG-PTR in the average absorber temperature from 100 to 287°C was within the range of 85–674 W/m2, which represents a better heat loss performance (−15%) upon the Sanle-3 receiver (Nanjing Sanle Electronic Information Industry Group, Nanjing, China). The experimental efficiency in transient state was within the range of 0.53–0.73 with the experimental relative error of approximately 3.6–6.5%. In addition, the experimental efficiency curve in quasi-steady-state was obtained. In quasi-steady-state, the experimental efficiency was approximately 0.65–0.71 when the inlet heat transfer fluid temperature was in ...