Simultaneous Charging/Discharging of Phase Change Materials: Characterization of Natural Convection Process

In recent decades, latent heat storage in phase change materials (PCMs) received considerable attention. This is due to their high latent heat capacity, which is essentially required for managing and overcoming the temporal mismatch between energy supply and demand. Thus, at the time of energy availability at supply side, it is stored in PCMs so as to be extracted later on when it is needed. In order to provide continuous operation, there are some periods when a thermal storage has to be simultaneously charged and discharged. Most studies focused either on charging, discharging, or consecutive charging and discharging process, while limited work has been conducted for the case of simultaneous charging and discharging (SCD). The first objective of this dissertation is to develop a numerical model to analyze the heat transfer mechanism within a horizontal PCM storage under SCD. Since the possible heat transfer mechanisms within PCMs are conduction, convection or a combination of both, two models are used to identify the mechanism under SCD; i.e. the pure conduction (PC) model and combined conduction and natural convection (CCNC) model. The PC model is a hypothetical model, which neglects the natural convection during phase change process; however, the CCNC model is the real case one. Validation of the model results by comparison with experimental data shows an acceptable agreement both under melting and solidification. Therefore, the developed model can be used to numerically study the phase change process in PCMs. Natural convection is the result of density changes, which create buoyancy forces within melted PCM and plays a significant role during melting. Currently, the most widely used method to account for natural convection is the effective thermal conductivity method. The method considers an artificial increase in thermal conductivity values to take into consideration the effect of natural convection by comparing the results with experimental data. Two major shortcomings of this method are that first, it is tedious to obtain the proper value and second, the method does not provide information about the melting front location. In this dissertation, a novel simplified front tracking method is presented to replace the thermal conductivity method. The novel method is based on considering two separate melting fronts for the upper and lower halves of a horizontal thermal storage system. Therefore, two dimensionless correlations are developed to map the results of the simple PC model to that of the complicated CCNC model based on the presented logic. The method essentially creates a link between CCNC and PC models, which is also missing in the literature. Based on verification, the correlations can provide results within ±15% discrepancy

Cold thermal energy storage in solid-liquid transition of carbon dioxide: Investigating the possibility

Industrial freezing is an energy-intensive process which is growing due to the increasing demand. This is exerting stress on electrical grids, especially at peak hours. To tackle this issue, thermal energy storage has received attention; however, there is a gap in terms of suitable materials for thermal energy storage with temperatures below -40 °C commonly needed in these applications. In this paper, the solid-liquid phase change of carbon dioxide has been conceptually considered for thermal energy storage in a special type of heat exchangers known as pillow plate heat exchangers. Characteristically, these heat exchangers can withstand very high pressures which is a technical requirement for carbon dioxide thermal energy storage. This paper discusses the potential system layout and challenges ahead of this technology, along with the proposal for further investigation to verify the concept.Cold thermal energy storage in solid-liquid transition of carbon dioxide: Investigating the possibilityacceptedVersio

Investigation of the effect of geometric and operating parameters on thermal behavior of vertical shell-and-tube latent heat energy storage systems

In this study, the effect of the geometrical and operational parameters on vertical cylindrical shell-and-tube LHTES systems is investigated. Four different ratios of the shell-to-tube radius are considered with the phase change material (PCM) on the shell side and the heat transfer fluid (HTF) flowing through the tube. The PCM temperature distributions are measured and compared experimentally among the studied storage units. A weighting method is utilized to calculate the average PCM temperature, liquid fraction, and stored energy fraction to evaluate the performance of the storage units. The results show that a shell to tube radius ratio of 5.4 offers better system performance in terms of the charging time and stored energy in the studied LHTES systems. Furthermore, the effects of HTF flow rate and temperature on the storage performance are studied. The HTF flow rate does not show a significant effect on the storage performance; however, the HTF temperature shows large impacts on the charging time. As the HTF temperature increases from 70 to 80 °C, the charging time reduces by up to 68% depending on the radius ratio

Cold thermal energy storage in solid-liquid transition of carbon dioxide: Investigating the possibility

Industrial freezing is an energy-intensive process which is growing due to the increasing demand. This is exerting stress on electrical grids, especially at peak hours. To tackle this issue, thermal energy storage has received attention; however, there is a gap in terms of suitable materials for thermal energy storage with temperatures below -40 °C commonly needed in these applications. In this paper, the solid-liquid phase change of carbon dioxide has been conceptually considered for thermal energy storage in a special type of heat exchangers known as pillow plate heat exchangers. Characteristically, these heat exchangers can withstand very high pressures which is a technical requirement for carbon dioxide thermal energy storage. This paper discusses the potential system layout and challenges ahead of this technology, along with the proposal for further investigation to verify the concept

Potentials and challenges for pillow-plate heat exchangers: State-of-the-art review

For a long time, shell-and-tube heat exchangers have been the most common choice in the industry. Despite some limitations, their main advantage is that they have been extensively investigated and reliable design tools are readily available. In other words, any alternative design should not only outperform shell-and-tube heat exchangers, but also be equipped with a comparably reliable and robust design platform for engineers. In recent years, a growing market share can be observed for plate heat exchangers, particularly for a special class known as pillow-plate heat exchangers. Recently, the research on pillow-plate heat exchangers has gained momentum primarily due to their relative low cost, compact design and structural integrity. However, a comprehensive literature review on the advancements so far and challenges ahead of their widespread application is missing. This study aims to fill this gap in the literature by systematically, critically and comprehensively surveying earlier studies on pillow-plate heat exchangers and providing an in-depth discussion regarding the achievements so far. The findings of this study are expected to help direct the forthcoming research on pillow-plate heat exchangers, especially towards more robust design tools. Overall, further investigation is needed on cost, plate material, flow configuration, heat transfer enhancement, correlation development, two-phase flow, etc. © 2022 The Author(s

Potentials and challenges for pillow-plate heat exchangers: State-of-the-art review

For a long time, shell-and-tube heat exchangers have been the most common choice in the industry. Despite some limitations, their main advantage is that they have been extensively investigated and reliable design tools are readily available. In other words, any alternative design should not only outperform shell-and-tube heat exchangers, but also be equipped with a comparably reliable and robust design platform for engineers. In recent years, a growing market share can be observed for plate heat exchangers, particularly for a special class known as pillow-plate heat exchangers. Recently, the research on pillow-plate heat exchangers has gained momentum primarily due to their relative low cost, compact design and structural integrity. However, a comprehensive literature review on the advancements so far and challenges ahead of their widespread application is missing. This study aims to fill this gap in the literature by systematically, critically and comprehensively surveying earlier studies on pillow-plate heat exchangers and providing an in-depth discussion regarding the achievements so far. The findings of this study are expected to help direct the forthcoming research on pillow-plate heat exchangers, especially towards more robust design tools. Overall, further investigation is needed on cost, plate material, flow configuration, heat transfer enhancement, correlation development, two-phase flow, etc. © 2022 The Author(s)Potentials and challenges for pillow-plate heat exchangers: State-of-the-art reviewpublishedVersio

Parametric study of low-temperature thermal energy storage using carbon dioxide as the phase change material in pillow plate heat exchangers

Industrial low-temperature freezing applications are often batch processes, requiring a lot of energy, exerting stress on the electrical grid. To relieve this stress, thermal energy storage can be used. However, there is a lack of suitable storage material for low temperature applications (around −50 °C). Under high pressures, carbon dioxide can be used as the phase change material for storage temperatures around −55 °C. In this study, a parametric study was conducted on the design and operational parameters of an industrial-scale pillow plate heat exchanger with carbon dioxide. Two responses were selected for the analysis where R1 considered the storage size over the phase change time (kWh/h), while R2 indicated the cost over the storage size (USD/kWh). Using design of experiments, a total of 52 simulations were carried out to investigate the parameters under constant heat transfer surface area. Analysis of variance was then carried out followed by correlation development and optimization. It was found that regardless of the process (charging or discharging), for R1 and R2, the difference between refrigerant and carbon dioxide phase change temperatures followed by plate material had the highest significance. In contrast, the refrigerant flow rate had the lowest significance in almost all cases. Moreover, considering an equal weight for the responses, overall optimal conditions were determined for the processes. The recommended values for plate pitch, plate material, difference between refrigerant and carbon dioxide phase change temperatures and refrigerant flow rate were 25 mm, aluminum, 15 °C and 4 kg/s, respectively.publishedVersio

Systèmes frigorifiques à ultra-basse température : configurations et frigorigènes permettant de réduire l’impact environnemental

Several environmental protection policies have been enforced restricting working fluids with high global warming potential (GWP) values used in many types of refrigeration and heat pump systems. However, ultralow-temperature (ULT) refrigeration has not been included, which commonly uses refrigerants with very high GWP values (such as R23 and R508B). Therefore, publicly available research programs seeking low GWP alternative refrigerants do not cover this application and the transition to more environmentally friendly fluids is slowed down. This work presents a comprehensive review that summarizes and discusses the available studies about ULT refrigeration systems. The current status of the technology, system architectures and refrigerants are analyzed. Moreover, the transition towards low GWP refrigerants is proposed, presenting the most promising low GWP alternatives. The most commonly used architectures for ULT refrigeration are the two-stage cascade and auto-cascade, in which the use of ejector has recently been considered in research papers. R170 and R1150 are the available natural refrigerants suitable for ULT, but they have not yet been included in many flammability and risk assessment studies. The A2 hydrofluoroolefin R1132a has been recently proposed as a blend component to avoid problems of stability. However, more information is still necessary to start with simulation and experimental studies. R41 could be an alternative due to its low GWP and suitable normal boiling point, but it has not been thoroughly investigated yet. Overall, there is a gap in the literature in terms of developing alternative refrigerants for ULT refrigeration. This study aims at shedding light on this gap to direct future research in this field towards reliable, environmentally friendly and marketable alternative refrigerants

Computer modelling and experimental investigation of phase change hysteresis of PCMs: The state-of-the-art review

International audienc