Heat Transfer Fluid Flow Rate Research Articles

Thermodynamic performance of cascaded latent heat storage systems for building heating

Decarbonization of building space heating is essential for China to meet its carbon neutrality goal by 2060. Cascaded latent heat storage (CLHS) coupled with electric heating is a promising technology to promote renewable energy consumption, reduce carbon emissions, and save on heating bills. However, few studies have focused on the thorough investigation of the superiority of the CLHS system over a non-cascaded system. This study investigated the effects of heat transfer fluid (HTF) flow rates, HTF inlet temperatures, and number of stages on the thermodynamic performance of non-cascaded and CLHS systems based on energy and entransy analysis. Results showed that, compared with the non-cascaded system, the charged thermal energy and discharged thermal energy of the two-stage CLHS system increased by 26.5%–44.6% and 19.8%–74.5%, respectively, and the entransy increase of cold HTF increased by 20.0%–75.7%; while, in heating systems, it is not guaranteed that thermodynamic performance improves by using more stages in the CLHS system.

Heat transfer enhancement and performance study on latent heat thermal energy storage system using different configurations of spherical PCM balls

Thermal energy storage (TES) systems utilizing latent heat storage substances have gained significant attention recently due to their large heat storage capacity and isothermal behavior during charging and discharging processes. However, many phase change materials (PCM) used for storage exhibit relatively low thermal conductivity. Prolonged surfaces like fins are incorporated in PCM balls to improve heat transfer performance. These fins significantly improve the overall performance of the TES system. This study conducted experiments to evaluate the performance of a latent heat thermal energy storage (LHTES) system using spherical balls filled with PCM, both with and without solid internal fins. Water was used as the heat transfer fluid (HTF) at a constant temperature of 70 °C, while paraffin served as the PCM with a melting temperature of 61 °C. Time-based temperature variation of HTF and PCM has been investigated for various HTF flow rates. Moreover, the effect of non-dimensional numbers, such as Reynolds, Rayleigh, Nusselt, and Stanton numbers, has also been investigated. Performance parameters such as thermodynamic efficiencies and entropy generation are analyzed. The heat storage process aims to get a highly efficient configuration for accumulating the maximum energy possible and higher exergy efficiencies during the charging process. The charging duration is reduced by 47.37 % when comparing the solid internal fin configuration at the higher flow rate (6 L/min) with the no fin configuration at the lower flow rate (2 L/min). At 100 min, the cumulative heat stored is higher in the solid internal fin configurations than in the configurations without fins. Furthermore, the internal fin configurations have less entropy generation than without fin configurations, justifying that providing solid internal fins in PCM balls enhanced the LHTES system performance.

Clustering-based model predictive control of solar parabolic trough plants

This paper presents a clustering-based model predictive controller for optimizing the heat transfer fluid (HTF) flow rates circulating through every loop in solar parabolic trough plants. In particular, we present a hierarchical approach consisting of two layers: a bottom layer, composed of a set of model predictive control (MPC) agents; and a top layer, which dynamically partitions the set of loops into clusters. Likewise, the top layer allocates a certain share of the total available HTF to each cluster, which is then distributed among the loops by the bottom layer in response to the varying conditions of the solar field, e.g., to deal with passing clouds. The dynamic clustering of the system reduces the number of variables to be coordinated in comparison with centralized MPC, thereby speeding up the computations. Moreover, the loops efficiencies and the heat losses coefficients, which influence the loops control model, are also estimated at the bottom layer. Numerical results on a 10-loop and an 80-loop plant are provided.

Thermo-fluid performance enhancement in a long double-tube latent heat thermal energy storage system

Thermodynamic analysis and flow rate optimization for the long double-tube latent heat thermal energy storage systems (LHTESS) are performed. Computer modeling is carried out using created software and is based on a developed 3D non-steady non-linear coupled thermo-fluid mathematical model that combines the apparent heat capacity method and finite volume method for the sodium nitrate (NaNO3) phase change materials (PCM) and for the Syltherm800 heat transfer fluid (HTF). The mathematical model and numerical algorithm are verified through comparison with experimental data. It is found that the laminar flow of HTF ensures the uniform temperature in PCM and in HTF and this temperature is equal to the PCM melting temperature. It is proved that the PCM and HTF average temperatures along the LHTESS can be equalized using spatial and temporal velocity variations of the HTF. Mutual impact of the heat transfer and fluid mechanics parameters in the long double-tube LHTESS was studied by correlation between Nusselt numbers and Reynolds numbers. The optimal HTF flow rate parameters were analyzed based on the non-equilibrium thermodynamics method. It is discovered that the laminar regime 0 < Re < 2000 and the turbulent regime 12000<Re<15000 are the most optimal for long double-tube LHTESS of the described geometry.

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.

An absorber of parabolic trough collector for hydrogen production in a solid oxide fuel cell

Sustainable energy development is a globally attractive and indispensable research area. An effective design process and good sustainable sources are crucial for meeting the objective. This investigation utilizes solar energy by utilizing an effectively designed the integration of two-parabolic trough collector (PTC) with an area of 2.8 m2 to operate Rankine cycle-based fuel cell for hydrogen production. The heat transfer rate is optimized for maximum net energy production to operate the solid oxide fuel cell (SOFC) to maximize the hydrogen production by experimenting with the solar heating system with 3 different heat transfer fluid (HTF) flow rates such as 0.12 kg/s, 0.27 kg/s, and 0.38 kg/s. The experimental outcomes at each flow rate setting were analyzed in such a way that solar radiation received, inlet and outlet temperatures of HTF with respect to clock time, Useful heat gain and Thermal efficiency of solar heating system with respect to solar radiation received and rate of Hydrogen production (kg/s) and Net power output (kW) by sustainable solar power generation arrangement were analyzed with respect to solar radiation received. The results recommended that the flow rate of 0.38 kg/s recorded the highest thermal efficiency 82%, for the heat gain of 27589 kW. Increasing the flow velocity of HTF improves both the solar collector's efficiency and the electrical energy for the fuel cell. As a result, more solar energy is falling on the absorber and enhances the rate of hydrogen production. A preliminary cost analysis was conducted for the PTC system integrated with the hydrogen production system to determine the electric cost based on the solar advisor model (SAM). A black coating material was used to absorb more solar radiation from the PTC.

Improving the melting performance of phase change material (PCM) in a latent heat thermal energy storage unit via a non-uniform arrangement of longitudinal fin

The application of latent heat thermal energy storage (LHTES) technology in solar energy utilization is greatly restricted by the low thermal conductivity of phase change material (PCM). This paper proposed a non-uniform arrangement of a longitudinal fin in a typical LHTES unit to improve the melting performance of PCM. Water is used as the heat transfer fluid (HTF) and paraffin (RT35) is selected as the PCM. A three-dimensional computational model is built to simulate the melting process of PCM, and its result is verified by experimental data. Firstly, the melting performance of PCM in the LHTES unit with four kinds of fin arrangements is comparatively studied, and its heat transfer mechanism is characterized via the maximum velocity of liquid PCM. In addition, the influences of HTF inlet temperature and flow rate, as well as heat transfer tube material on the complete melting time of three typical fin arrangements are also investigated. The results implied that the complete melting time of the non-uniform arrangement of the longitudinal fin reduces by 30.91% relative to the conventional longitudinal fin (Case 2). The results also revealed that the melting rate of PCM is related not only to the intensity of natural convection but also to the distribution of natural convection in the whole melting process.

Discharge performance of a high temperature phase change material with low-cost wire mesh

Thermal energy storage is increasingly needed in a sustainable world because of its potential of capturing waste heat and being incorporated in solar power plants. For power generation, in particular, as turbine technology advances, a demand for higher temperature thermal energy storage materials also grows. For this purpose, latent thermal energy storage fits in well since it uses phase change materials (PCMs) which generally have a higher energy density compared to their sensible heat counterparts. In the present study, a eutectic Na2CO3(41.69%)-(33.1%)KCl-(25.21%)NaCl phase change material (PCM) with a melting temperature of 569 ° C was chosen as the storage material to experimentally assess the performance benefit of using a readily available stainless steel (ss304) wire mesh (as the periodic structure) to enhance heat transfer within the domain. In addition, for discharging, a numerical model was developed and compared with the experimental results. Furthermore, for discharging, a numerical investigation of the influence of the heat transfer fluid (HTF) flow-rate to the rate of heat transfer was performed. Overall, it was experimentally observed that the charging time for the composite case was shortened by about 35%, compared to the pure PCM case. For discharging, in the axial direction, the composite solidification time when compared to the pure PCM case was on average 10% shorter. Regarding the radial discharging performance of the composite, there was only about 5% improvement compared to the pure PCM case, which was expected due to the thermal contact resistance in the radial direction. Discharging experimental results were used to validate a discharging numerical model. Discharging results from the model showed that increasing the flow rate of the heat transfer fluid (HTF) reduced the time for solidification. It was observed that for the HTF flow rate of 5 L/min, 10 L/min, 20 L/min and 30 L/min, the discharge time was shortened by 23%, 30%, 33% and 35%, respectively.

Effect of cooktop cone angle on the performance of thermosyphon heat transport device for indoor solar cooking – A numerical study

Solar cookers are less attractive as their outdoor operation is laborious. A recent innovation encouraged the thermosyphon heat transport device (THTD) based indoor solar cooking. However, the cooktop THTD design is still in the conceptual stage. Hence, in the present study, the cooktop THTD cone angle is optimized through the steady-state and transient analysis using an analytically validated CFD model. Six cooktop cone angles (0° to 75° in steps of 15°) were evaluated and found a drop in heat transfer fluid flow rate with an increase in the cooktop cone angle, due to the existence of recirculating loops, which is confirmed by velocity vector plots. Under concentrated solar flux boundary conditions, the transient analysis showcased the advantage of 15° model over 0° and 45° variants. The efficacy of cooktop THTD is proved as all three lower cone angle designs took a mere 180 s to boil 1 L of water.

Experimental Investigation on Performance Enhancement of Parabolic Trough Concentrator with Helical Rotating Shaft Insert

The parabolic trough collector provides an extensive range of solar heating and electricity production applications in solar power plants. The receiver tube of the parabolic trough collector has a vital role in enhancing its performance by using different inserts inside it. In the present work, outdoor experimental tests were conducted to study the performance of a small-scale parabolic trough collector equipped with a centrally placed rotating helical shaft. Three cases were studied: a parabolic trough collector without helical shaft insert, a parabolic trough collector with stationary helical shaft insert, and a parabolic trough collector with a rotating helical shaft insert. The experiments are performed for different shaft rotational speeds (4, 11, and 21 RPM) and various flow rates (0.5, 1, 1.5, 2, and 2.5 LPM) of water as a heat transfer fluid. The fluid flow and heat transfer parameters (friction factor, Reynolds number, Nusselt number, and thermal enhancement factor) and performance parameters (thermal, overall, and exergetic efficiencies) are studied. The results indicated that the helical shaft insert had increased the required pumping power for the same flow rate. However, the parabolic trough collector thermal performance has enhanced with the shaft rotational speed. For all cases, the parabolic trough collector efficiency increases with the flow rate of the heat transfer fluid, but the percentage enhancement in efficiency decreases. Using a shaft rotational speed of 21 RPM and heat transfer fluid flow rates of 0.5 LPM leads to maximum thermal efficiency enhancement and a maximum friction factor ratio of 46.47% and 7.7 times, respectively, compared to plain tube. A comparison based on the same pumping power (thermal enhancement factor) shows that the maximum enhancement occurs at a flow rate of 1 LPM, and the efficiency enhancement is about 37% at a shaft rotational speed of 21 RPM. From an economic point of view, using a rotating helical shaft produces the lower annual cost of useful heat per kWh.

Effect of shell shape on the charging and discharging performance of a vertical latent heat thermal energy storage unit: An experimental study

In this paper, the effect of shell shape on vertical latent heat thermal energy storage (LHTES) unit is experimentally investigated to alleviate the heat transfer lagging region and enhance its charging and discharging performance. A cylindrical (CYL) and two conical (CON-I and CON-II) LHTES units are designed and fabricated, and they have the same heat transfer area and internal volume. Paraffin with a narrow range of phase change temperature and high enthalpy was selected as phase change material (PCM), and water is used as heat transfer fluid (HTF). The thermal behaviors of CYL, CON-I and CON-II such as evaluation of melting and solidification fronts, temperature response characteristics, heat transfer characteristics and energy and exergy conversion are comparatively studied. A correlation is developed to predict the entire melting and solidification time of different LHTES units under different HTF flow rates and temperatures. The results demonstrate that CON-I and CON-II can provide a better temperature field uniformity and enhance the heat transfer rate between HTF and PCM. When compared with CYL, the entire melting/solidification time is reduced by 16.95 %/8.07 % and 29.94 %/23.22 % for CON-I and CON-II, and the time-averaged heat transfer rate is increased by 20.69 %/23.61 % and 35.87 %/41.24 % for CON-I and CON-II during the melting/solidification processes. In addition, the developed correlation is able to well predict the entire melting and solidification time of PCM in the LHTES unit with a relative error of ±5 %.

Investigation on the Melting Performance of a Phase Change Material Based on a Shell-and-Tube Thermal Energy Storage Unit with a Rectangular Fin Configuration

A case study on the melting performance of a shell-and-tube phase change material (PCM) thermal energy storage unit with a novel rectangular fin configuration is conducted in this paper. Paraffin wax and circulated water are employed as the PCM and heat transfer fluid (HTF), respectively. It can be observed that the melting performance could be significantly improved by using rectangular fins. Melting photographs demonstrate that the melting of the PCM is firstly dominated by heat conduction; then, the melting rate is improved further due to natural convection. Moreover, the results illustrate that the influence of the inlet HTF temperature on the melting performance is significantly greater than that of the inlet HTF flow rate. The liquid fraction of paraffin wax in the PCM unit with a higher inlet HTF temperature is always higher than that with a lower inlet HTF temperature at the same time. The total charging time is reduced by 62.38% and the average charging rate is increased by 165.51% when the inlet HTF temperature is increased from 57 °C to 68 °C. As a result, a higher value of the inlet HTF temperature and a lower value of the HTF flow rate are able to improve the energy efficiency of the PCM unit with a rectangular fin configuration.

Thermal energy storage based on cold phase change materials: Charge phase assessment

• Experimental study of water–ice energy storage behavior at density inversion temperature . • Experimental analysis of a finned shell & tube latent heat thermal energy storage . • Effect of temperature, mass flow rate and charge direction on storage performance. • Presence of metastable phase is confirmed. Integration of thermal energy storage in energy systems provides flexibility in demand–supply management and in supporting novel operational schemes. In a combined heat and power cycle, it has been shown that integration of cold thermal energy storage is beneficial to fine-tune electric power and heating/cooling production profiles to better match the load demand. Latent heat storage systems have the advantage of compactness and low temperature swing, however storage performance analysis on large scale setup operating around the density inversion temperature is still limited. In this work, a shell & tube, latent heat based cold thermal energy storage was studied around the density inversion temperature of ice-water at 4 °C and the performance was characterized. Sensitivity analyses on heat transfer fluid flow rate, flow direction and inlet temperature were performed. The results show 27% power increase with doubled mass flow and 18% shorter charge time with 2 °C lower charge temperature. Contrary to general expectations during solidification, the cold thermal energy storage actually shows between 5% and 6% better thermal performance and reducing instant icing power jump of 36% due to supercooling with downwards cold heat transfer fluid flow in cooling charge cycle due to buoyancy change around density inversion temperature. This fact highlights the importance of accounting for the buoyancy effect due to density inversion when designing the operational schemes of large size cold thermal energy storage.

Charging and discharging processes of low capacity nano-PCM based cool thermal energy storage system: An experimental study

The present study aims to investigate the performance of the low-capacity energy storage tank in different heat transfer fluid (HTF) conditions (at various flow rates) filled with spherical capsules containing nano-enhanced phase change material (nano-PCM). The nano-PCM is prepared by dispersing functionalized graphene nanoplatelets (f-GNP) with deionized (DI) water. The influence of HTF inlet temperature and volumetric flow rates on the total charging and discharging time of an energy storage tank filled with 35 spherical capsules are analyzed. The maximum reduction in total charging and discharging time of 18.26% and 22.81% is recorded for different HTF conditions. The amount of latent heat energy stored is nearly 5.5 times higher than the sensible heat stored at the HTF temperature of −4 °C. The cumulative energy recovery of 2637 kJ is recorded during the discharging process, which is 85.89% of the actual energy stored (3070 kJ) in the storage tank. In addition, the dispersion of f-GNP reduces the specific energy consumption (SEC) by around 28% for the nano-PCM at −4 °C HTF temperature. It is found that the required output temperature and average melting rate can be achieved in practical applications through the control of the flow rate of HTF and its inlet temperature.

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.

Experimental analysis to improve charge/discharge of thermal energy storage in phase change materials using helical coil and porous metal foam

The use of phase change materials (PCM) for latent heat thermal energy storage (LHTES) is a common method of storing thermal energy in buildings. Because the thermal conductivity of the PCMs is low, so the rate of their energy charge/discharge would be confined. To overcome this issue, a new design is proposed using helical coil to maximize the contact surface between the heat transfer fluid (HTF) and the PCM. Both sides of the helical coil are filled with PCM, while in the inner region, copper metal foams (porosity of 0.9 and density of 12 PPI) are utilized for heat transfer rate enhancement. Effects of various parameters, including the inlet HTF flow rate, its temperature, and use of porous media on thermal energy storage performance in charge and discharge processes are analyzed experimentally. Six thermocouples are placed inside the PCM in different regions and melting and solidification processes are recorded using a thermal imaging camera. Results are presented in terms of temperature variation in different regions, temperature contours, mean temperature response, Stefan number, the charge/discharge time, and the extracted power of discharge. The results reveal that using porous media can remarkably reduce the charge/discharge time up to 57 %, which is depended on the inlet working fluid temperature. The charging time reduction is higher when the inlet HTF temperature increases. When the flow rate of HTF is increased, the charging time can be slightly shortened, while this effect becomes dominant when using porous media. Use of porous media could also accelerate the discharge time, while a faster discharge rate was observed for lower HTF flow rates during discharging the stored energy. Also, a lower initial temperature of the inlet HTF provides faster depletion rate.

Investigative Study of Solidification and Melting of Stearic Acid in Triplex Pipe with Perforated Fin Surface

The energy storage capacity of latent heat thermal energy storage systems is high therefore these systems are efficient among all other thermal heat storage. This study experimentally investigates the charging and solidification of stearic acid (C18H36O2) in the finned triplex pipe heat exchanger with holes on the fin surface. The melting and discharging of stearic acid are carried out at steady inlet heat transfer fluid (HTF) i.e., water temperature. This study also investigates the result of the change in HTF flow rate on solidification and melting of phase change material. Finally, the study focuses on the comparison of average effectiveness for finned triplex tube heat exchanger and finned triplex tube with cylindrical holes on fin surface with rates of 0.33 and 0.43 Kg/s. The results showed that the average effectiveness with a finned triplex tube with a perforated fin surface is 4% more than that of a finned triplex tube at 0.33 Kg/s whereas it is 5 % more at 0.43 Kg/s during the charging process. On the other side during discharging process average effectiveness is 14 % more at 0.33 Kg/s and 11 % at 0.43 Kg/s.

Experimentally-validated numerical studies of thermal performance enhancement of latent thermal energy storage systems with periodically reciprocating flow

The effects of periodic reversal of flow direction of a heat transfer fluid (HTF) on the heat transfer rate in a latent thermal energy storage system (LTESS) are numerically investigated for consecutive melting and freezing cycles of phase change material (PCM). Numerical results are validated with experimental measurements. Parametric studies are performed, where different parameters that affect the thermal performance of LTESS are varied independently. The effects of these variations on the thermal performance of LTESS with periodically reciprocating and unidirectional HTF flow are studied. Periodically reciprocating HTF flow results in increased uniformity of temperature and temperature gradients throughout the PCM for both melting and freezing cycles, compared to unidirectional flow. Increasing the flow rate of HTF, increases the relative heat transfer rate enhancement with reciprocating flow for both melting and freezing cycles. An optimal reciprocation period of LTESS which is dependent on other parameters, is noticed to produce the maximum relative thermal performance enhancement with the periodically reciprocating flow. Increasing the latent heat of fusion or thermal conductivity of PCM increases the relative effects of reciprocation nonlinearly.

Experimental and numerical investigation of longitudinal and annular finned latent heat thermal energy storage unit

The melting process of phase change material (PCM) in horizontal latent heat thermal energy storage (LHTES) units with longitudinal and annular fins was experimentally studied under the same fin volume condition. The PCM melting time of the annular fin unit is reduced by 13.7% compared to that of the longitudinal fin unit. The annular fins are more responsive to the increase in heat transfer fluid (HTF) inlet temperature and the longitudinal fins are more responsive to the increase in HTF flow rate. After that, numerical studies were also performed for the two LHTES units to reveal in-depth the differences in heat storage processes. The results show that the longitudinal fins perform better in the early stage due to their larger contact area with the inner tube and a denser distribution near the inner tube. But it also limits the natural convection in the latter stage. The annular fins have better natural convection performance in the latter stage and therefore have better heat storage performance. Based on the analysis, a composite fin was designed to combine the advantages of longitudinal and annular fins. Numerical results show that the newly designed fin results in a 21.6% melting time reduction compared to the longitudinal fin model and a 9.5% melting time reduction compared to the annular fin model. Finally, two principles to guide the fin design in horizontal shell-and-tube LHTES units were summarized.

Hydrogen charging and discharging studies on embedded cooling tube metal hydride reactor designed for fuel cell applications

In the present study, an embedded cooling tube (ECT) type metal hydride (MH) reactor has been designed, manufactured, and tested for possible integration with a low-temperature Polymer Electrolyte Membrane Fuel Cell (LT-PEMFC) at 40–60 °C. The MH reactor filled with 21.5 kg La0.9Ce0.1Ni5 has been subjected to a series of parametric tests. At 20 bar, 25 °C, the MH reactor absorbed 276.08 g hydrogen in 1129 s, accounting for 1.28 wt.% charged capacity. At 40 °C, the reactor desorbed 282.8 g hydrogen within 4860 s, which, if discharged through a mass flow controller (MFC) at a steady rate, could feed a 3.5 kW LT-PEMFC for the same duration maintaining the reactor pressure above 1 bar. An increase in discharging temperature to 60 °C showed about 54% increase in average hydrogen discharge rate. Influence of variation in heat transfer fluid (HTF) flow rate was discovered to be insignificant for both charging and discharging.