Heat Transfer Fluid Temperature Research Articles

Role of aloe vera based nanofluids for cool thermal energy storage system: A comparative study

The present experimental investigation incorporates the preparation of aloe vera gel-based phase change nanofluid (NFPCM) for a cool thermal energy storage (CTES) system. Two sets of NFPCMs were produced by adding graphene nanoplatelets (GNP) in deionized water and aloe vera gel (0.20 wt%, 0.40 wt%, and 0.60 wt%). The zeta potential analysis shows that the NFPCM with aloe vera gel is more stable even after 60 days from the preparation (−44 mV). The thermal conductivity of NFPCMs was analyzed through the transient hot wire method and the enhancement of 42 %, and 37.5 % were measured in the ice phase for NFPCMs with aloe vera gel and DI water (0.60 wt%) respectively at the heat transfer fluid temperature of −15 °C. The NFPCMs with aloe vera gel show that the addition of GNP prepended the peak melting temperature from −16 °C to −10.5 °C, which is due to the quick formation of nuclei as compared to NFPCMs with DI water. Additionally, a slight reduction in the latent heat of only 7 % was observed. The charging characteristics of NFPCMs were studied in a single spherical capsule at −8 °C. The maximum reduction in total charging time of 25 % was recorded for the aloe vera gel-based NFPCM with a maximum concentration of GNP (0.60 wt%). The green synthesis of aloe vera gel-based NFPCM exhibited noteworthy enhancements in thermophysical properties. Additionally, it was observed that the long-term stability of the NFPCM was achieved by excluding surfactants during the preparation process. The incorporation of graphene nanoplatelets (GNP) into aloe vera gel as the foundational phase change material (PCM) facilitates the partial charging of the CTES system with a reduced energy input.

An improved dynamic analytical model and the parameter sensitivity analysis of spherical stacking latent thermal energy storage devices

Thermal energy storage technology plays an important role on improving energy usage flexibility for the end users. The spherical stacking latent thermal energy storage devices inherits the advantages of direct thermal energy usage efficiency and simplified practical applications that received widely attentions. However, due to dispersed and multi-layer structural arrangement, the non-ignorable dynamic heat transfer process poses difficulties in the device design and operation control. In this study, an improved analytical model for predicting the transient heat transfer process of spherical stacking devices is developed. After verification, the parameter sensitivity analysis of the device is conducted by using the model. The results show that the dynamic change trend of thermal resistance is distinct for the melting and the solidification process. The developed model can dynamically predict the outlet temperature of heat transfer fluid and the heat transfer efficiency in a good agreement, and the prediction error of outlet temperature during solidification can reach −2.49 %–4.70 %. The correlation between sphere radius and heat transfer rate is negative and reaches a kind of high correlation above 0.8 for solidification process. The inlet temperature and the mass flow rate of heat transfer fluid are the most influential parameters for melting process.

Evaluation of a concentrated solar power-driven system designed for combined production of cooling and hydrogen

The current technique of hydrogen generation and cooling production raises climatic problems, which calls for the investigation of cleaner alternatives. No research has been conducted to propose and analyze a solar thermal power cycle-based water electrolysis for on-site production of hydrogen and the detailed exergy analysis of a series flow double-effect absorption chilling machine employed for large solar cooling. In this regard, a helically coiled tube embedded central receiver is used in the heliostat field for the simultaneous production of solar cooling and green hydrogen to meet the increasing needs of energy and fuel sustainably. The combined system is composed of four sub-systems including a heliostat field, LiBr-H2O operated series-flow double effect absorption refrigeration cycle (DEARC), organic Rankine cycle (ORC), and proton exchange membrane (PEM) electrolyzer. The system was examined from energy and exergy perspectives in the Engineering Equation Solver (EES) using library data of REFPROP for obtaining the thermodynamic properties of working fluids. The results of the thermodynamic model for the combined system are validated. Furthermore, the effect of ambient temperature, solar irradiation, and type of ORC working fluid on the hydrogen and cooling production, and the overall energy and exergy efficiency of the combined system are examined. A computational fluid dynamics simulation applying ANSYS-FLUENT was used to examine how coil curvature ratio and coil torsion affect solar heat transfer fluid (SHTF) pressure and temperature leaving the receiver. The coil curvature ratio affects temperature increase more than coil torsion. Solar heat transfer fluid outlet temperature increases by 36.9% when the coil curvature ratio increases from 0.063 to 0.095 at direct normal irradiations (DNI) 1200 W/m2, input temperature of 92oC, and coil torsion of 0.032. The response of the cogeneration cycle to varying operating parameters is examined. Application of R141b as the ORC fluid results in a large increase of cooling exergy efficiency from 14.98% to 63.48% when the ambient temperature is increased from 15°C to 45°C whereas the exergy efficiency of the combined system is marginally improved. The exergy distribution presents for ORC operating on R141b, out of 100% input solar exergy, 16.42% is generated as hydrogen exergy and 13.67% cooling exergy, 66.79% is the exergy destroyed, and the rest 3.12% is exergy loss. Exergy analysis is utilized to identify the sources of losses in useful energy within the components of the system considered and provides a more realistic and meaningful assessment than the traditional energy analysis. The results justify a need for the promotion of exergy analysis through policy decisions in the context of the global energy and environment crisis.

Influence of fin shape design on thermal efficiency of phase change material-based thermal energy storage unit

The present numerical study treats the impact of fin shape design on the thermal efficiency of phase change material (PCM)-based thermal energy storage (TES) unit, focusing on the same surface area occupied by fins. Comparing two different finned TES units equipped with rectangular and triangular fin shapes, respectively, showed significant enhancements in PCM melting activity. Comparative analysis demonstrated that triangular fin shape lowers PCM melting time by 12.64% for equivalent fin numbers, and by 15.38% for equal fin lengths due to the enlargement of the heat transfer area provided by the triangular shape. Further examination of fins with triangular shape in terms of spacing and length, under fixed thickness and size parameters, revealed significant reduction in melting time with increasing fins length. Notably, 50.75% decrease in melting time was achieved by decreasing the number of fins to 20 while increasing fin length to 10 mm. Moreover, maintaining a heat transfer fluid (HTF) temperature 20 K higher than the melting PCM temperature maximizes TES thermal efficiency. These outcomes emphasize the importance of optimizing fin shape design for enhancing heat transfer without affecting the energy storage capacity of TES systems, with potential applications in building thermal management.

Experimental investigation on thermodynamic performance of a copper foam-based cascaded latent heat storage tubes

Cascaded latent heat storage (CLHS) systems offer an effective means for energy storage from moderate to medium and low temperature heat sources (100-200℃). In this study, a three-stage shell-and-tube cascaded latent heat storage system was designed and fabricated. Three types of paraffin wax with distinct melting points were employed as phase change materials (PCMs), and foam copper was utilized to enhance heat transfer. By adjusting the inlet temperature and flow rate of heat transfer fluid (HTF), the thermodynamic performance of each LHS stage and the entire system under different operating conditions were investigated. Exergy analysis, entransy analysis, and sensitivity analysis were employed to study the primary influential factors on system performance and the impact weights of diverse operating conditions. Experimental results highlighted the crucial significance of maintaining consistency in the physical parameters of PCMs across different stages in the CLHS system for its thermodynamic performance. The high thermal conductivity and low latent heat of PCM#2 in the second stage result in substantial energy losses during the heat storage process, reducing the efficiency of energy cascaded utilization within the system. With an increase in the HTF inlet temperature and flow rate, the energy storage capacity and rate increase, while the energy storage efficiency exhibits a decreasing trend. Moreover, the effect of varying inlet temperature on the energy storage efficiency is much smaller compared to that of inlet flow rate. This study emphasizes that heat dissipation is a critical factor affecting the thermodynamic performance of the cascaded latent heat storage system. For CLHS systems targeting high-temperature and high-flow rate heat sources, enhancing insulation is crucial for ensuring the sustained high-efficiency operation.

CFD Analysis of a Latent Thermal Storage System (PCM) for Integration with an Air-Water Heat Pump

Heat pumps driven by sustainable electricity sources have been identified as a technology that can contribute to reduce carbon dioxide emissions. Furthermore, a heat pump can also provide energy savings when combined with a thermal energy storage system. Heat pumps can optimize their efficiency by accumulating thermal energy during periods of lower electricity demand, resulting in shorter operational durations and decreased overall energy consumption. In this work, the combination of a latent heat storage system with an air-water heat pump has been numerically analysed and experimentally tested. A phase change material (PCM) heat exchanger with an immersed plate was designed using a 3D CFD model (COMSOL Multiphysics®). The heat exchanger configuration with six steel plates immersed in the phase change material tank was proposed to enhance heat transfer in the storage system. The developed model is validated against experimental data from a real case study, demonstrating a maximum error of approximately 3% during the discharging phase. Additionally, the study explores the effects of different inlet heat transfer fluid temperatures and flow rates on the PCM solidification time.

Innovative arrangement of circular angled fins for maximizing the discharge performance of triple-tube heat storage systems

Abstract This study introduces a novel triple-tube latent heat storage system enhanced with circular angled fins to improve solidification and heat recovery performance. The fins are arranged in staggered pattern with alternating upward and downward orientations on both sides of the PCM shell. A validated numerical model was developed using the enthalpy method to simulate the intricate heat transfer and phase change physics. Effects of circular fins geometry and operating conditions were systematically quantified on discharge rates and temperature uniformity. Four fin dimension cases (thickness × length): (2 × 5), (1 × 10), (0.66 × 15), and (0.57 × 17.5) mm2 were analyzed. The results demonstrate that fins with greater length and reduced thickness exhibit superior performance due to enhanced heat transfer capabilities, resulting in quicker solidification and faster heat retrieval. The longest 17.5-mm fins, achieving full solidification in 1973 s with 44%, 19%, and 1.9% quicker than cases with 5-mm, 10-mm, and 15-mm long fins, respectively. Incorporating an additional fin upward further reduces the solidification time by 4.5% while improving heat recovery by 3.6%. The 17.5-mm long fins increase heat discharge by 48% and outlet heat-transfer fluid temperatures by 39% versus straight fin baselines. Lower inlet heat-transfer fluid temperatures (10°C vs 20°C) reduce PCM solidification times by 31% (1755s vs 2554s) while increasing heat recovery rates by 57% (56.3 W vs 35.8 W). Overall, the integrated angled fins create a customizable latent heat storage system with greatly intensified heat transfer and thermal performance compared to conventional shell-and-tube arrangements.

Experimental investigation of thermal performance of vertical multitube cylindrical latent heat thermal energy storage systems.

The multitube design in the shell-and-tube type latent heat thermal energy storage (LHTES) system has received intensive attention due to its promising benefits in enhancing heat storage efficiency. In this paper, single and multi-tube shell LHTES systems were experimentally investigated. First, this study experimentally compared the thermal characteristics between a multiple-tube heat exchanger (MTHX) and a single-tube heat exchanger (STHX). The STHX's geometrical parameters coincided with a virtual cylindrical domain in the MTHX, being similar to the single-tube model formulated by simplifying the numerical solution to investigate the MTHX. The experimental data was then used to validate the simplified numerical model commonly used in the literature that converted the multi-tube problem to a single-tube model by formulating a virtual cylindrical domain for each tube in the MTHX system. The results showed that there was a noticeable difference in the thermal characteristics between the actual STHX and the virtual cylindrical STHX domain in the MTHX system. The comparison indicated that the simplified numerical model could not accurately reflect the thermal performance of the MTHX system. An experimental study or three-dimensional numerical modelling was required for the thermal analysis of the multi-tube problems. Second, the effect of tube number in the MTHX was experimentally investigated. It was found that an increase in tube number boosted both charging and discharging rates without inhibiting the natural convection. The five-tube configuration decreased the total charging and discharging duration by 50% compared to the two-tube one. Finally, the effect of heat transfer fluid (HTF) operating parameters on the system performance was evaluated on the five-tube MTHX system. The results revealed that the adoption of higher HTF temperature considerably improved the charging performance. The charging time decreased by up to 41% with the HTF temperature increasing from 70 to 80°C. Meanwhile, a variation in the HTF flow rate from 5 to 20 L/min showed a more pronounced influence on charging than on discharging due to the different dominant heat transfer mechanisms.

Mapping the waste heat recovery potential of CO2 intercooling compression via ORC

The goal of the present work is the systematic mapping of the electricity savings potential of waste heat recovery (WHR) from CO2 compression intercoolers via Organic Rankine Cycle (ORC) technology. Α generalized approach covering a broad range of target CO2 discharge pressures from 50 to 500 bar for 3 to up to 8 compression stages is followed. The maximum, theoretical electricity savings based on the exergy of the heat transfer fluid (HTF) used for intercooling range between 10 % and 24 %. Considering the exergy efficiency of optimized air-cooled ORCs operating with alkanes and hydrofluoroolefins for HTF temperatures between 70 °C and 130 °C, the maximum electricity savings ratio (i.e., the electricity savings compared to the total electricity consumption of the compression unit) is about 5 %, while the installed power of the WHR-ORC ranges from about 3 to 23 kWe per kg/s of compressed CO2. Because of the intense variability of CO2 specific heat capacity over a wide range of pressures, there is a very strong dependence of the waste heat potential on the final discharge pressure and number of stages. Through optimized mixing of the HTF streams, the performance of the WHR-ORC can be improved for particular discharge pressure ranges. Proper sizing of the ORC air-cooled condenser for minimizing air-side pressure drop is critical to ensure the feasibility of WHR-ORCs. According to a techno-economic analysis, WHR-ORCs can result in discounted payback periods from 6 to 12 years and 2.5 to 4 years for 4000 and 8000 operating hours, respectively.

Development and experimental investigation of full-scale phase change material thermal energy storage prototype for domestic hot water applications

The paper presents an experimental analysis of the full-scale phase change material (PCM) thermal energy storage (TES) prototype that is designed for use in domestic hot water preparation systems. The PCM-TES prototype is based on the external arrangement of organic PCM and a custom-made compact fin-and-tube type of heat exchanger. The prototype has been designed, constructed, and tested according to the Ecodesign Directive requirements for water heaters and hot water storage tanks. Several parameters have been analysed, including the heat transfer fluid (HTF) temperature and mass flow rate, heat transfer rate, accumulated and released energy, and quantity of domestic hot water (DHW) delivered. Additional attention was paid to the reliability and repeatability of the results. Experimental research allowed to determine the quantity and temperature of DHW at a constant mass flow rate of 0.1 kg/s. The prototype unit showed that it can deliver 99.1 l of hot water at 40 °C for up to 15 min and 39 s, while the use of lower temperature DHW (up to 37 °C) allows to increase the quantity of hot water delivered up to 156.2 l. Experimental results showed the viability of the prototype, areas for improvement and further possibilities for its optimization. Overall, the study results are relevant for developing latent heat thermal energy storage systems using fin-and-tube heat exchangers in tanks and the feasibility of integrating such systems into DHW systems.

Multi-factor analysis and optimization design of a cascaded packed-bed thermal storage system coupled with adiabatic compressed air energy storage

In an adiabatic compressed air energy storage (A-CAES), one of the key components is the heat storage system, in which the packed bed filled with encapsulated phase-change capsules has been widely investigated because of its excellent thermal performance. In this paper, a packed-bed thermal energy storage model with three layers of phase change materials (PCM) is proposed in the context of an A-CAES. Four factors primarily affecting the thermal performance of the packed bed thermal storage system, namely the mass flow rate of heat transfer fluid (HTF), the inlet temperature of HTF, the cascade situation of PCM, and the arrangement of particle sizes, are critically analyzed by conducing the orthogonal experiments. On this basis, the machine learning method is applied to predict and optimize the thermal performance of the packed bed aiming to find the optimal results. The comparison of results between the optimized and original models expressively shows a reduction of 8.46% in PCM mass, an increase of 92.18%/116.82% in the overall heat storage/release quantity, and an increase of 19.23% in the overall efficiency. This work outlooks a guideline for the potential application of packed bed in A-CAES.

Heat transfer and thermal resistance analysis under various heat transfer fluid flow rates based on a medium-temperature pilot-scale latent heat storage system

Assessments often rely on experimentally observed factors when examining heat transfer variations due to different heat transfer fluid (HTF) flow rates. To better comprehend limiting factors and guide operational parameter design for shell-and-tube latent heat system (LHS) systems, a quantitative analysis of thermal resistance hindering heat transfer is crucial. Hence, this article conducted experimental evaluations at various flow rates to critically analyze its thermal performance based on a large-capacity (1000 MJ) medium-temperature LHS system loaded with 4130 kg commercial nitrate mixture (peak melting temperature: 219.90 °C). The heat transfer coefficient and thermal resistance during charging and discharging were quantitatively investigated to identify the main factors influencing heat transfer and dominant mechanisms. The results suggested that augmenting the charging flow rate from 5 to 15 L/min resulted in 2.65 times increase in average power (from 24.01 to 63.64 kW). Besides, during charging, the overall thermal resistance primarily came from the phase change material (PCM) side initially. As PCM melted, the thermal resistance on the PCM side became equivalent to that on the HTF side. Increasing the charging flow rate enhanced heat transfer through two mechanisms: reducing the thermal resistance on the HTF side and increasing the driving force of heat transfer by elevating the average HTF temperature. During discharging, the overall thermal resistance was predominantly derived from the PCM side after solidification. Increasing the discharging flow rate caused a limited influence on decreasing thermal resistance. Enhancing the discharging flow rate only bolstered the driving force for heat transfer by elevating the average HTF temperature. This study provides valuable insights into the factors influencing heat transfer in LHS systems and contributes to optimizing LHS technology.

Thermal investigation and parametric analysis of cascaded latent heat storage system enhanced by porous media

Cascaded latent heat storage (CLHS) has become a promising solution to the recovery and utilization of thermal energy. Many studies optimized the inlet temperature and velocity of heat transfer fluid to improve the heat transfer rate for the charging process of CLHS systems with pure phase change materials (PCMs), but the effect is not significant, mainly because the thermal resistance of the system comes from the PCMs side due to the low thermal conductivity of pure PCMs. Porous media composite phase change materials (CPCMs) have been shown to have promising prospects in a single PCM system, but no research precedent has been found for the application of porous media CPCM in the CLHS system. Besides, when CPCMs are encapsulated in the cavities of the CLHS system instead of pure PCMs, the difference in the degree of impact between the porosity of a porous media and boundary conditions on the thermal performance of a CLHS system is still unclear. The purpose of this article is to significantly improve the heat transfer rate by applying the CPCM to the CLHS system and to comprehensively compare the impact of porosity and HTF inlet boundary conditions on the charging/discharging time and energy/exergy charged/discharged rates of a CLHS system with CPCMs through two sensitivity analysis methods. When the porosity reduces from 0.97 to 0.92, the charging time and the discharging time decrease by 74.50 % and 73.39 % respectively; the average energy charge rate and average exergy charge rate increase by about 4.11 times; the average energy and exergy discharge rates increase by 5.76 times and 5.8 times respectively. Sensitivity analysis shows that the porosity of a porous media is the most important factor affecting the charging and discharging rates of a CLHS system, but its impact on energy and exergy charge/discharge rates is not monotonic. The effect of the inlet temperature and velocity of the HTF on a CLHS system is complex, and simply changing a single variable may not be effective in optimizing thermal performance parameters. This study provides a comprehensive performance enhancement strategy for the thermal energy storage and utilization processes of CLHS systems, which have great potential in fields such as solar energy collection, waste heat recovery, and building heating.

Performance Prediction of a Dual-axis Tracking Solar Trough Collector Based on Artificial Neural Network

A dual-axis tracking parabolic trough solar collector, using a certain straight-trough tube, was set up to experimentally investigate the heat collecting performance. An artificial neural network(ANN) model was developed. Experimental data were used to train and predict the mean temperature of Heat transfer fluid in the solar trough collector based on the developed model. The Levenberg-Marquardt (LM) method was also applied to optimize the weights and thresholds for the classic BP Newton algorithm, providing an ANN model with 9 hidden nodes and 30,000 training times. The predicted values are all in good agreement with the experimental data, with a mean relative error of 0.21% and a maximum error of 1.2%. In comparison, the mean relative error of the one-dimensional steady-state model reaches 1.07%. It indicates that the ANN exhibits excellent performance in predicting the export temperature of the solar collector with a specific flow rate of Heat transfer fluid. This ANN model is promising to predict the performance of solar trough collectors under different operating and environmental conditions.

Influence of upstream and downstream flow on different packed bed configurations in phase change process

One approach to storing latent heat in large quantities is through PBSS (Packed bed storage system) with spheres filled with PCM (Phase Change Materials). In these storage systems, an HTF (heat transfer fluid) flows over the spheres. So, both the sphere wall temperature and the heat flux can vary depending on the geometric conditions of the bed of spheres, the PCM thermodynamic conditions, and the HTF thermal conditions. Thus, this work aims to verify the influence of the thermal conditions of the external flow on the PBSS filled with PCM in the melting process. Cases with variable heat flux as well as uniform temperature and uniform heat flux conditions were tested. Three sphere positions, 4 sphere spacing ratios (blockage ratio), and three temperature differences between HTF and PCM phase change temperature were also analyzed, for two external flow directions (upstream and downstream), totaling 74 different configurations. The study found that the uniform temperature condition resulted in a 33 % faster total melting time than the uniform heat flow condition. Additionally, the upstream flow showed a total melting time that was approximately 24 % shorter than the flows in the opposite direction. The relationship between the total melting time and the temperature differences was found to be non-linear. Moreover, the blockage ratio showed a non-linear dependence with total melting time. The configuration with a blockage ratio of 1.125 demonstrated the shortest total melting time, with values on average 22.4 % shorter than the blocking ratio of 0.550, which had the longest melting times. These results demonstrate that optimizing external flow thermal conditions is crucial for improving the performance of PBSS with PCM-filled spheres in large-scale latent heat storage systems.

Influence of carbon graphene flakes enriched PCM on partial charging conditions for cool thermal energy storage system applications

The present research work is aimed at studying the influence of carbon graphene flakes nanoparticle concentrations in DI water at various heat transfer fluid temperatures of −6, −9, and −12 °C. The carbon graphene flakes (CGF) with higher concentrations of 0.4, 0.8, and 1.2% with the base PCM is investigated experimentally in a stainless steel spherical capsule of 100 mm diameter, and the thermophysical properties of the CGF PCMs were interpreted with the charging behaviours of the base PCM. The CGF PCMs help to reduce the solidification time by 47 to 56% for the higher bath temperature of −6 °C in partial charging conditions. For the complete solidification, the maximum reduction in solidification time was attained between 14 and 21% for the CGF PCM with a concentration of 1.2%. The partial charging conditions favour the increase in heat flux two-fold when the bath temperature is operated at −6 °C, this reduces the demand for energy during the peak hours and shifts the peak load conditions. It is concluded that the CGF PCMs with a concentration of 1.2% help in enhancing the heat transfer rate and significantly compress the solidification duration compared to the bath temperatures of −9 °C and −12 °C.

Modelling a latent heat thermal storage demonstrator and identification of the model key-parameters

Latent Heat Thermal Energy Storage (LHTES) has a great potential to reduce CO2 emissions in urban areas, when associated to a district heating network. It facilitates the flexibility of the network by decoupling the production and consumption. Thus, consumption peaks that are often managed with fossil fuels will be managed by unloading the storages of the network. LHTES based on tube and shell is the most widely studied technology, as it combines low cost, high maturity, and high compactness factor. This paper presents a numerical model to simulate the thermohydraulic behaviour of a shell-and-tube LHTES, based on a 1D approach for the heat transfer fluid (HTF) and a 1.5D approach for the Phase Change Material (PCM). The objective is to develop a model representing a whole system, faster than a Computational Fluid Dynamic approach while keeping a sufficient degree of accuracy. The paper focuses on the presentation of the model and its validation against experimental data from a real-scale 180 kWh demonstrator, currently in operation in the substation of the urban heating network of Grenoble, France. The validation against three different charges is presented, two charges at a constant flow rate (0.6 and 1.6 kg/s-1) and one at constant power (60 kW). The model gives results with an accuracy superior to 95 % and >80 % of the points are between the experimental uncertainty curves. It has also been found that the model loses its accuracy when the inlet HTF temperature abruptly decreases and the top collector does not behave as a stratified tank anymore (the relative error on total energy accumulated reaches 35 %). In the second part of the paper, a sensitivity study to the main parameters of the model shows that the model is sensible to the discretization of the collectors, the precision of the Nusselt correlation for convective transfers in the tubes, and the fusion temperature of the PCM. These parameters can induce up to respectively 22 % (0D model for the collectors' volume), 4.5 % to 9 % (considering the annular space as a flat plane or a cylindrical tube) and 3 % to 6 % (error of 0.5 to 1 °C in the PCM fusion temperature) error.

A solar spectral splitting-based PVT collector with packed transparent tube receiver for co-generation of heat and electricity

Combined heating and power (CHP) systems co-generating electrical energy as well as thermal energy is indispensable for modern-day distributed applications. Photovoltaic thermal (PVT) systems are such hybrid co-generation systems. However, in a conventional PVT system, the PV module/cell temperature restricts the thermal output, making these systems unsuitable for various applications. This study offers a novel and resilient design for a spectral beam splitting-powered compound parabolic collector-based PVT system that employs a nanofluid optical filter within borosilicate glass tubes for volumetric absorption. By maximizing the “splitting effect”, the goal of this work is to disrupt the long-standing relationship between the thermal and electrical yield from a PVT collector, i.e., obtaining a higher heat transfer fluid temperature than the PV cell temperature. Several configurations with varying array width-to-receiver tube diameter ratios, i.e. bdo ratios (ranging from 3 to 8) and one, two, and three rows/layers of receiver tubes were evaluated and compared. The optical and thermal performance of the collector designs are compared using metrics such as heat flux distribution, optical efficiency, temperature distribution, receiver efficiency, and splitting ratio. This work determines how the splitting effect dictates the performances of the collector. Through the ray-tracing routine and thermal analysis, it is demonstrated that for certain bdo ratios, the developed collector attained a maximum cumulative temperature rise of >100∘ C using ZnO–water–ethylene glycol nanofluid as the optical filter. Also, the receiver and electrical efficiency of 51.81% and 18.77%, respectively, are achieved. Moreover, the packed borosilicate glass receiver tubes with a bdo ratio of 8 and two receiver rows/layers achieved the so-called “splitting ratio” (ratio of filter fluid temperature to cell temperature; THTFTcell) of >1.6. The studied configurations find their application depending on the extent of the achieved splitting ratio. Based on these findings, it is concluded that the analyzed packed glass tube receiver designs provide a viable new route towards cost-effective and dependable robust PVT collectors for distributed and other applications.

A predictive method for pipe in pipe, cylindrical modules and spherical packed bed latent thermal energy storage systems

Due to the transient operation of latent thermal energy storage (LTES) heat exchangers, methods from classical heat exchanger theory such as the effectiveness number of transfer unit method are not applicable. As a result, the study of these systems lacks a general theoretical framework. Recently, an approximate analytical solution method was developed which integrates phase fraction under a constant boundary condition to an LTES heat exchanger with a varying heat transfer fluid temperature. In this paper, the method is applied to three geometries: pipe in pipe, cylindrical modules and spherical packed bed geometries. The resulting model predicts the phase change fraction and effectiveness as a function of time, based on one-dimensional solutions for the movement of the phase change front. The predictions of the analytical model are compared to results of a simplified numerical method of an LTES heat exchanger. The average deviation of the predicted phase change fraction is 3.3 10−2 %, 4.6 10−1 % and 3.7 10−1 % for the pipe in pipe, cylindrical and spherical module respectively. The difference in model performance stems from the description of the fundamental heat transfer behavior. In a second validation step, the outlet temperature predicted by the model is compared to an experimental dataset for a LTES shell and tube heat exchanger. The predictions of the model can be improved further by including sensible heat, heat losses and allowing time-varying inlet conditions.

Influence of heat transfer fluid on thermal performance improvement of latent heat storage unit with helical fins

The helical fins can improve the hampering effect of classical fins on the convection flows of latent heat storage units (LHSU) as an innovative design, but their thermal performance improvement was largely affected by the operational conditions of heat transfer fluid (HTF), especially for fluid temperature and velocity. Based on this, an experimental system was established to measure thermal performance of latent heat storage units with perforated and solid helical fins (PHF-LHSU and SHF-LHSU). Two inlet temperatures were considered as 65 °C and 75 °C in the charge process and 20 °C and 30 °C in the discharge process, while three velocities were employed as 0.5 L/min, 1.0 L/min and 1.5 L/min. Experimental results showed PHF-LHSU exhibited the higher thermal performance than SHF-LHSU, especially in temperature response and average Nusselt number. Increasing HTF temperature from 65 °C to 75 °C increased melting rate, the average heat flux, Nusselt number and thermal efficiency by 25.4%–31.6%, 65.9%–70.6%, 15.9–25.9% and 9.7%–11.6% in the charge process, while reducing HTF temperature from 30 °C to 20 °C lowered the solidifying rate, the average heat flux and thermal efficiency by 39.3%–47.4%, 52.5%–56.6%, and 30.7%–35.3% in the discharge process. The HTF velocity had the certain effects on thermal properties and with the HTF velocity increasing from 0.5 L/min to 1.5 L/min, the thermal efficiency was increased by 3.9%–4.7%.