Impact of Cascaded and Series/Parallel Configurations on the Thermal Performance of Flat-Plate Phase-Change Thermal Energy Storage Systems

Abstract

This study investigates the thermal performance of a flat-plate phase-change thermal energy storage system, focusing on two structural innovations: a cascaded arrangement of multiple phase-change materials (PCMs) with varying melting points, and the implementation of series/parallel flow configurations. A combined numerical and experimental approach is employed to analyze dynamic charging/discharging behavior. Quantitative results indicate that the cascaded configuration (three PCMs) reduces phase-change completion time by 13% and increases cooling energy storage power from 2.00 kW to 2.43 kW during charging compared to single-PCM systems. Flow configuration significantly impacts thermal response: the parallel layout delivers more stable cooling output, while the series layout achieves faster initial cooling (reaching 6.24 °C within 1200 s, 31% faster than the parallel layout). Experimental results reveal that inlet water temperature is the most critical operating parameter, with each 2 °C increase significantly prolonging charging time. This work offers practical guidance for the design and optimization of efficient cascaded PCM thermal storage systems.

1. Introduction

Against the backdrop of continuously rising global temperatures and increasingly pressing energy demands, the effective reduction of energy consumption and the curbing of carbon emissions have become energy policies prioritized by nations worldwide. Consequently, the pursuit of energy-saving and emission-reduction technologies has emerged as a prominent research focus within the energy sector [1]. Phase-change energy storage technology, which utilizes the latent heat absorbed or released during the solid–liquid phase transition of phase-change materials (PCMs), offers a pathway to enhance the thermal storage efficiency of systems. This technology helps mitigate the temporal and spatial mismatch between energy supply and demand [2,3], and has consequently been extensively investigated in fields such as renewable energy utilization, building energy conservation, and waste heat recovery [4,5,6,7].

Among various thermal energy storage (TES) systems, latent heat storage using PCMs exhibits a distinct advantage due to its high energy storage density, as it can store and release significant amounts of heat at a nearly constant temperature during the phase-change process [8]. Integrating phase-change energy storage with air-conditioning systems not only enables efficient heat storage and release but also improves the system’s energy efficiency and reliability. Compared to traditional sensible heat storage technologies, phase-change energy storage offers higher energy density, more flexible temperature control, and a more compact design, making it suitable for diverse applications requiring stable thermal regulation [9,10].

The implementation of phase-change energy storage technology relies upon energy storage systems. Common encapsulation structures for phase-change energy storage systems can be categorized into three types: cylindrical storage units, shell-and-tube storage units, and rectangular storage units [11]. Among these, the parallel-arranged plate-type device, owing to its simple geometry, permits fluid flow between adjacent PCMs and exhibits a high surface area-to-container volume ratio. Stathopoulos et al. experimentally revealed the non-linear temperature characteristics of paraffin-based PCMs during charging and discharging processes in flat-plate thermal storage systems [12]. To enhance heat transfer efficiency during this process, research by Marín et al. demonstrated that flat-plate heat accumulators constructed using paraffin/graphite composite materials could reduce charge/discharge times by approximately half compared to pure paraffin at equivalent thicknesses [13]. The overall heat transfer efficiency was also significantly improved due to the incorporation of graphite.

In terms of modelling analysis, several scholars have laid the groundwork for understanding the performance of flat-plate thermal storage devices. Halawa et al. developed a one-dimensional model accounting for wall temperature variations to calculate the temperature fields and phase interface evolution within the PCM and heat transfer fluid [14]. Ding et al. further proposed an approximate heat transfer model for this device and derived universal dimensionless design criteria [15]. To evaluate its long-term operational characteristics, Liao et al. established a one-dimensional model that systematically revealed the dynamic process from initialization to stable cycling, quantifying the influence of key structural and operational parameters on thermal storage efficiency and capacity [16].

In single-phase-change material (single-PCM) thermal storage systems, temperature variations along the flow direction of the heat transfer fluid (HTF) reduce the temperature difference between the HTF and PCM, thereby diminishing heat transfer dynamics and limiting energy storage capacity [17,18]. However, employing multiple PCMs arranged in cascading order with decreasing phase-change temperatures along the HTF flow direction maintains a relatively stable temperature difference. This ensures efficient heat transfer and enables greater energy storage capacity [19]. Multiphase change material systems enhance overall performance through cascading integration, a benefit validated across multiple energy sectors. On the power generation side, Khandelwal et al. found that cascaded thermal storage systems coupled with solar power plants increase turbine output, with molten salts outperforming thermal oils as heat transfer fluids [20]. On the energy consumption side, Yang et al. constructed a dual-PCM system that significantly improved industrial waste heat recovery efficiency [21]. Bagherizadeh et al. further demonstrated through experiments that multi-PCM systems consistently exhibit higher energy storage capacity than single-PCM systems across multiple heating applications [22].

Single phase-change materials (PCMs) often struggle to maintain high efficiency throughout the entire charging and discharging cycle in applications such as wide-temperature-range waste heat recovery. To address this, research proposes employing cascaded structures composed of multiple PCMs with differing melting points to match the temperature variations of heat sources, thereby enhancing the overall performance of latent heat energy storage systems (LHTES) [23,24,25]. Experiments by Fang et al. validated this advantage: compared to a single-stage system of identical total volume, a three-stage PCM system effectively resolved issues of low efficiency and insufficient heating temperature through optimized thermal matching, significantly enhancing process efficiency [26]. While these studies have successfully demonstrated the benefits of material cascading, they typically do so within a single, fixed-flow-path heat exchanger. Conversely, research on series/parallel connections has largely focused on flow distribution among identical units without the added complexity of cascaded PCMs with varying melting points.

However, existing research has predominantly focused on optimizing the structural parameters of individual flat-plate units. Previous studies on cascaded PCM systems have primarily investigated material-level arrangements within a single flow channel, while investigations on series/parallel connections have mainly dealt with identical PCM units without considering melting point gradients. The coupled effect of cascaded PCMs and flow topology (series vs. parallel) on system-level thermal performance remains unexplored. To address this gap, the present study aims to investigate the combined effects of cascaded arrangements and series/parallel connections on the storage/discharge characteristics, temperature distribution, and overall energy efficiency of flat-plate phase-change energy storage systems. By systematically combining both design dimensions, this work reveals how their interaction influences thermal performance, offering a more integrated perspective compared to conventional single-PCM or single-flow-path systems.

2. Experimental and Simulation Methods

2.1. Numerical Simulation Method

2.1.1. Physical Model

Building upon the design of plate-type thermal energy storage devices, this research will focus on innovative studies concerning cascaded structural design and series/parallel connection methods. Through systematic comparison of the characteristics of different geometric structures—including shell-and-tube, cylindrical, and plate-type designs—it was discovered that the plate structure demonstrates significant advantages in heat transfer efficiency due to its high surface area-to-volume ratio. This configuration is particularly well-suited for establishing fluid channels between phase-change material (PCM) plates. Not only does it optimize the heat exchange process and reduce thermal resistance, but its compact nature also provides ideal conditions for achieving cascaded layouts and multi-path series/parallel configurations.

Compared to the partial advantages of shell-and-tube and cylindrical structures in terms of flow and heat transfer, the plate structure demonstrates greater prominence in simplicity and cost-effectiveness, and has therefore been designated as the core configuration for this study. Consequently, we shall systematically design and construct flat-plate energy storage units with cascading characteristics. This will enable exploration of their thermal performance under various series and parallel flow configurations, alongside assessment of multiple connection methods’ impact on system heat storage/release characteristics, temperature distribution, and overall energy efficiency. This approach aims to provide novel insights for designing and optimizing highly efficient, compact thermal energy storage systems.

To investigate the impact of different PCM temperatures and connection configurations on the energy storage/release performance of flat-plate phase-change storage devices, a cascaded flat-plate phase-change storage experimental apparatus was designed based on the fundamental flat-plate configuration, as illustrated in Figure 1. This apparatus employs a rectangular water tank as its outer casing, internally housing multiple horizontally stacked layers of phase-change material plates. By controlling the inter-plate gaps, a continuous heat exchange flow channel is established, forming a cascaded structural layout. Building upon this cascaded arrangement, multiple flat-plate units were interconnected through a combination of series and parallel flow configurations by rationally arranging inlet/outlet ports and internal flow channel connections. This design creates a versatile experimental platform capable of systematically investigating the coupled heat transfer characteristics arising from both the PCM material cascade (melting point gradient) and the system-level flow topology (series/parallel connections). Diffusers were installed at all inlet and outlet ports to ensure uniform water distribution across each heat exchange stage within the interconnected series/parallel flow channels.

The three-dimensional physical model was simplified to a two-dimensional geometry, justified by the uniform flow distribution perpendicular to the flow direction. Key assumptions adopted in this model include the following:

(1)

Uniform mass flow rate and identical inlet temperature for all heat transfer fluid (HTF) channels;

(2)

Negligible natural convection within the liquid PCM due to the confinement by thin, horizontal plate geometry;

(3)

Adiabatic boundary conditions, neglecting any heat loss to the ambient environment;

(4)

Homogeneous and isotropic PCM with constant thermophysical properties;

(5)

Uniform initial temperature for all PCM, HTF, and the container walls.

Negligible natural convection within the liquid PCM. This assumption is justified by the confinement of the PCM within thin, horizontally oriented plates, where the small height-to-length ratio and the dominance of conduction in such confined spaces significantly suppress the development of buoyancy-driven convection cells. The primary heat transfer mechanism is therefore considered to be conduction.

Research on cascaded plate structures has neglected natural convection within the liquid phase of phase-change materials, thereby limiting the influence of gravitational effects on heat transfer kinetics. Utilizing this periodic arrangement, the computational domain is simplified into representative segments of alternating pathways for the high-temperature heat transfer fluid and phase-change material. Subsequently, a parametric study was conducted on this fundamental unit to evaluate cooling storage and release performance under cascaded and series/parallel configurations. This established a foundation for systematically analyzing how PCM temperature and device connection arrangements influence the operational behavior of plate-type phase-change energy storage devices.

2.1.2. Mathematical Model

In CFD simulations, the solidification and melting processes of phase-change materials (PCMs) are modelled using the ‘solidification/melting’ approach, which is based on the enthalpy-porosity theory. This method tracks the phase-change interface by defining the liquid phase fraction and treats the molten region as a porous medium exhibiting a gradual porosity gradient. Consequently, it accurately describes the heat transfer and flow behavior during solid–liquid phase transitions.

The liquid phase fraction β is defined as the volume fraction of the liquid phase-change material (PCM) within each control volume. It serves as a key parameter for tracking the position of the phase interface. This parameter enables effective handling of solid–liquid boundary movement, thereby simplifying computational processes involving phase-change heat transfer. The mathematical definition of the liquid phase fraction is as follows [27]:

$$\beta =\left\{\begin{array}{c}0,if T_{pcm}<T_s\\ {\displaystyle \frac{T_{pcm}-T_s}{T_l-T_s}}\\ 1,if T_{pcm}>T_l\end{array},{if T}_s<T_{pcm}<T_l\right.$$

(1)

where $T_s$ and $T_l$ denote the solidification temperature and melting temperature (°C) of the phase-change material, respectively, while $T_{pcm}$ represents the reference temperature for the PCM.

The enthalpy method adopts a unified set of conservation equations (mass, momentum, and energy) that apply seamlessly to the solid, mushy, and liquid regions of the PCM. This framework solves for the enthalpy and temperature fields concurrently [28,29,30,31]. Based on the above assumptions, the governing equations are defined as follows:

$${\displaystyle \frac{\partial \rho }{\partial t}}+{\displaystyle \frac{\partial \left(\rho u\right)}{\partial x}}+{\displaystyle \frac{\partial \left(\rho v\right)}{\partial y}}+{\displaystyle \frac{\partial \left(\rho \omega \right)}{\partial y}}=0$$

(2)

$$\rho [{\displaystyle \frac{\partial u}{\partial t}}+u{\displaystyle \frac{\partial u}{\partial x}}+v{\displaystyle \frac{\partial u}{\partial y}}+\omega {\displaystyle \frac{\partial u}{\partial y}}]=\mu [{\displaystyle \frac{{\partial }^{2}u}{\partial t^2}}+u{\displaystyle \frac{{\partial }^{2}u}{\partial x^2}}+v{\displaystyle \frac{{\partial }^{2}u}{\partial y^2}}+\omega {\displaystyle \frac{{\partial }^{2}u}{\partial z^2}}]-{\displaystyle \frac{\partial p}{\partial x}}$$

(3)

$$\rho [{\displaystyle \frac{\partial v}{\partial t}}+u{\displaystyle \frac{\partial v}{\partial x}}+v{\displaystyle \frac{\partial v}{\partial y}}+\omega {\displaystyle \frac{\partial v}{\partial y}}]=\mu [{\displaystyle \frac{{\partial }^{2}v}{\partial t^2}}+u{\displaystyle \frac{{\partial }^{2}v}{\partial x^2}}+v{\displaystyle \frac{{\partial }^{2}v}{\partial y^2}}+\omega {\displaystyle \frac{{\partial }^{2}v}{\partial z^2}}]-{\displaystyle \frac{\partial p}{\partial y}}$$

(4)

$$\rho [{\displaystyle \frac{\partial \omega }{\partial t}}+u{\displaystyle \frac{\partial \omega }{\partial x}}+v{\displaystyle \frac{\partial \omega }{\partial y}}+\omega {\displaystyle \frac{\partial \omega }{\partial y}}]=\mu [{\displaystyle \frac{{\partial }^2\omega }{\partial t^2}}+u{\displaystyle \frac{{\partial }^2\omega }{\partial x^2}}+v{\displaystyle \frac{{\partial }^2\omega }{\partial y^2}}+\omega {\displaystyle \frac{{\partial }^2\omega }{\partial z^2}}]-{\displaystyle \frac{\partial p}{\partial z}}$$

(5)

$$\rho [{\displaystyle \frac{\partial \left(C_f{T}_f\right)}{\partial t}}+{\displaystyle \frac{\partial \left(u{C}_f{T}_f\right)}{\partial x}}+{\displaystyle \frac{\partial \left(v{C}_f{T}_f\right)}{\partial y}}+{\displaystyle \frac{\partial \left(\omega C_f{T}_f\right)}{\partial y}}]={\displaystyle \frac{\partial }{\partial x}}\left(k_f{\displaystyle \frac{\partial T_f}{\partial x}}\right)+{\displaystyle \frac{\partial }{\partial y}}\left(k_f{\displaystyle \frac{\partial T_f}{\partial y}}\right)+{\displaystyle \frac{\partial }{\partial z}}\left(k_f{\displaystyle \frac{\partial T_f}{\partial z}}\right)$$

(6)

$${\displaystyle \frac{\partial \left({\rho }_{pcm}H\right)}{\partial t}}={\displaystyle \frac{\partial }{\partial x}}\left(k_{pcm}{\displaystyle \frac{\partial T_{pcm}}{\partial x}}\right)+{\displaystyle \frac{\partial }{\partial y}}\left(k_{pcm}{\displaystyle \frac{\partial T_{pcm}}{\partial y}}\right)+{\displaystyle \frac{\partial }{\partial z}}\left(k_{pcm}{\displaystyle \frac{\partial T_{pcm}}{\partial z}}\right)$$

(7)

$$H=h_{ref}+{\int }_{T_{ref}}^T{c}_{pcm}dT+\beta L$$

(8)

$${\displaystyle \frac{\partial \left(\rho \overrightarrow{v}\right)}{\partial t}}+\nabla \left(\rho \cdot \overrightarrow{v}\cdot \overrightarrow{v}\right)=-\nabla P+\nabla \left(\mu \overrightarrow{V}\right)+S$$

(9)

$$S={\displaystyle \frac{{\left(1-\beta \right)}^2}{\left({\beta }^2+\epsilon \right)}}{A}_{mush}(\overrightarrow{v}-\overrightarrow{v_p})$$

(10)

The physical significance of each parameter is as follows: ρ—density (kg/m³); $\overrightarrow{u},\overrightarrow{v},\overrightarrow{\omega }$—velocity (m/s) in different directions; ${\rho }_{pcm}$, $c_{pcm}$ and $T_{pcm}$ denote the effective density, effective thermal conductivity and temperature of the composite phase-change material, respectively; H represents the total enthalpy of the composite phase-change material.; $T_{ref}$—reference temperature (K); $h_{ref}$—reference enthalpy value (kJ/kg); $C_{ref}$—reference specific heat capacity (J/(kg·K)); P—pressure (Pa); μ—kinematic viscosity (Pa·s). The source term S in the momentum equation describes momentum loss, its calculation being related to the $A_{mush}$ paste zone constant, the zero-crossing correction term ε (set to less than 10⁻⁴), and the entrainment velocity $\overrightarrow{v_p}$.

2.1.3. Boundary Conditions and Parameter Settings

For the designed cascaded flat-plate energy storage unit, its numerical model involves five core materials: phase-change material (PCM1–3), high-density polyethylene (HDPE) as the structural material [14], and water (heat transfer fluid). PCM1 is a composite phase-change energy storage material comprising n-octanoic acid and lauric acid; PCM2 is a composite phase-change energy storage material comprising pentadecane and undecacosane; PCM3 is a composite phase-change energy storage material comprising dodecylalcohol and octanoic acid. In the material settings, the physical properties of water were directly adopted from the software’s built-in data, while the key physical properties of PCM and HDPE were introduced via customized methods, as detailed in

Table 1. Material parameter settings.

Physical Properties	PCM1	PCM2	PCM3	HDPE
Density (kg·m−3)	945	910	1350	940
Specific heat capacity (J·kg−1·K−1)	1935	1905	1930	1900
Thermal conductivity (W·m−1·K−1)	2.23	0.9975	1.876	0.48
Phase-change latent heat (kJ·kg−1)	143	136.3	117.4	--
Phase-change temperature (°C)	5.2	6.5	7.3	--

.

In the numerical calculations, the upper and lower surfaces of the computational domain are defined as symmetrical planes. The interface between the PCM and HTF is designated as a coupled heat transfer surface with a thickness of 2 mm, while all other surfaces are modelled as thermally insulated walls. The HTF inlet is configured as a velocity inlet, and the outlet as an outflow.

Based on the unsteady characteristics and coupling mechanisms of the physical processes, the solver is configured with the following key settings:

(a)

Solver and Model: Employ a transient (non-steady) solver, enabling the energy equation, laminar flow model, and solidification/melting model.

(b)

Algorithm and Discretization Scheme: Pressure–velocity coupling utilizes the SIMPLE algorithm, with the pressure term discretized using the PRESTO scheme.

(c)

Convergence and Initialization: Set the convergence tolerance criteria for the continuity equation, momentum equation, and energy equation to 10⁻⁵, 10⁻⁵, and 10⁻⁷, respectively. During computational initialization, the initial temperature of the phase-change material (PCM) is determined by the operating mode: 12 °C for the cooling storage process and 2 °C for the heat release process.

2.1.4. Grid Independence Verification and Model Verification

To ensure the accuracy of the numerical model for the cascaded plate unit, rigorous quality control and independence verification were conducted on the mesh of its basic unit. Building upon the integrated use of structured and unstructured meshes, a mesh independence analysis was performed for a representative unit (H_HTF = 10 mm, H_PCM = 30 mm). By comparing the PCM liquid phase fraction during a 4000-second heat release process across five distinct mesh densities (14,012–86,250 elements) (Figure 2a), it was determined that 55,000 elements suffice to achieve a converged solution. This mesh strategy for the base unit provides a reliable foundation for subsequent system-level cascade and series/parallel performance studies.

To ensure the reliability of subsequent cascade and series/parallel performance studies, the experimental values of the fundamental flat-plate phase-change cold storage unit were first validated through simulation. The model was constructed strictly according to the experimental setup in Reference [14]. Under identical cold storage operating conditions, the simulated and experimental outlet temperature curves exhibited high agreement (Figure 2b), with an average relative error below 5%. This validation confirmed that the employed model accurately captures the unit’s transient phase-change heat transfer behavior, laying the foundation for subsequent system-level investigations.

2.1.5. Evaluation Index

In the field of phase-change cooling energy storage systems, the performance evaluation of flat-plate phase-change cooling storage units is primarily based on their cooling energy storage/release capacity, heat transfer power, and energy utilization efficiency. To systematically investigate the impact of different combinations on these key performance indicators, the following parameters are defined, with their specific calculation methods as follows:

$$Q=m_{pcm}(\int {\left(c_{p,s}\right)}_{pcm}dT+\Delta H+{\left(c_{p,l}\right)}_{pcm})DT$$

(11)

where Q denotes the total stored or released energy (kJ) of the flat-panel phase-change unit, encompassing both latent and sensible heat; $m_{pcm}$ is the mass of the phase-change material (kg); and $c_{p,s}$ and $c_{p,l}$ represent the specific heat capacity at constant pressure (kJ·kg⁻¹·K⁻¹) for the solid and liquid phases of the PCM, respectively.

$$\mathrm{W}={\displaystyle \frac{Q}{t}}$$

(12)

where t is the duration of the phase transition process.

$$\mathrm{\eta }={\displaystyle \frac{Q}{\int c·m·\Delta Tdt}}$$

(13)

where η is the energy utilization efficiency of the device, defined as the ratio of useful thermal energy to the input energy over a complete cycle; m is the mass flow rate of the HTF (kg·s⁻¹); and ΔT denotes the temperature difference (°C) between the HTF inlet and outlet.

2.2. Phase-Change Cold Storage Experiment

2.2.1. Experimental Bench System Principle

To investigate the impact of different connection configurations on the energy storage performance of flat-plate phase-change thermal storage units, a high-performance experimental system was designed and constructed for studying the performance characteristics of such units.

The experimental setup, depicted schematically and photographically in Figure 3, is designed to evaluate the performance of the cascaded flat-panel storage unit. It integrates three key subsystems: the phase-change storage device under test, a closed-loop water circulation system for thermal charging/discharging (featuring a thermostatic bath and insulated DN25 piping), and a data acquisition system for parameter monitoring (including thermocouples, flow sensors, and an Agilent logger). In operation, the circulated water undergoes forced convection heat transfer with the stacked panels, while the acquisition system records the dynamic thermal response.

2.2.2. Experimental Content

(1)

Experimental Preparation

Prior to commencing the experiment, a comprehensive pre-operation inspection was conducted on the entire phase-change energy storage system. This process focused on verifying the functional status of critical instruments, including the cryogenic constant-temperature reaction bath, flow meters, and temperature sensors. Additionally, the sealing integrity and thermal insulation performance of the circulation piping system were confirmed to ensure all components were operating correctly.

(2)

Cold storage experiment

Prior to commencing the experiment, the temperature of the constant-temperature reaction bath tank was first set to 12 °C. Subsequently, the pipeline circulation pump was activated, and the connecting valves to the piping system opened, allowing the 12 °C cold water to flow fully through the plate-type phase-change energy storage device for heat exchange. Throughout this process, the temperatures at all internal measurement points and the inlet/outlet water temperatures were monitored in real time. Once all measurement points reached and stabilized at their preset values, the initial operating conditions for the cold storage experimental system were deemed established.

Subsequently, the system pump and corresponding valves were shut off, and the temperature of the constant-temperature reaction bath tank was reset to 2 °C. Once the tank temperature stabilized, the pump was restarted, and the valves reopened, allowing the low-temperature chilled water to flow into the phase-change energy storage device for heat release and exchange. Concurrently, the data acquisition system was activated to record the entire process. When all measurement points reached a stable state once more, this signified the conclusion of the cold storage experiment.

During the cold storage experiments, variable-condition storage tests were conducted under different operating conditions by adjusting the inlet temperature and flow rate of the fluid. The specific experimental conditions are detailed in

Table 2. Cold storage experimental condition settings.

Serial Number	Fluid Inlet Temperature (°C)	Fluid Inlet Flow (L·h−1)
A1	2	1500
A2	2	1200
A3	2	900
A4	3	1500
A5	4	1500

.

(3)

Cold release experiment

At the commencement of the heat release experiment, the temperature of the constant-temperature reaction bath tank was first set to 2 °C. The circulation pump was activated, and the pipeline valves opened, allowing the 2 °C cold water to flow through the plate-type phase-change energy storage device, facilitating thorough heat exchange with the internal units. Throughout this process, the temperatures at all measurement points within the device and the inlet/outlet water temperatures were monitored in real time. Once all measurement point temperatures had stabilized, the system was deemed to have completed its initialization.

Subsequently, the pump and pipeline valves were shut off, and the tank temperature was adjusted to 12 °C. Once the temperature stabilized, the pump was restarted, and the valves reopened, allowing 12 °C hot water to flow through the energy storage device for heat release exchange. Concurrently, the data acquisition system was activated to record real-time data. When all measurement points reached a stable state once more, this signified the conclusion of the heat release experiment.

To investigate the impact of varying operating conditions on performance, the experiment further conducted variable-condition studies by altering inlet temperature and flow rate. Specific parameter settings are detailed in

Table 3. Cold release experimental condition settings.

Serial Number	Fluid Inlet Temperature (°C)	Fluid Inlet Flow (L·h−1)
B1	12	1500
B2	12	1200
B3	12	900
B4	11	1500
B5	13	1500

.

3. Results and Discussions

3.1. The Impact of Cascade Configuration on the Comprehensive Performance of the Device’s Cold Storage Process

The cascaded phase-change cold storage device comprises multiple PCMs arranged sequentially within the heat exchanger, featuring progressively higher phase-change temperatures. Compared to single-PCM configurations, this design achieves superior thermodynamic matching, thereby significantly enhancing overall heat transfer performance. Concurrently, the cascaded structure effectively broadens the compatible temperature window between air conditioning operating conditions and the cold storage medium, expanding both the range of PCM options and the system’s application potential. Within the planar stacked configuration investigated herein, three PCM units (PCM1, PCM2, PCM3) were arranged in ascending order of phase-change temperature to establish the cascading effect.

To investigate the cold storage characteristics of a cascaded phase-change storage device employing phase-change materials with differing phase-change temperatures, simulated operating conditions were established with an inlet velocity of 0.1 m/s and an inlet temperature of 2 °C. Figure 4 illustrates the variation in liquid phase fraction for different PCMs. It was observed that phase-change cooling completion times for PCM1, PCM2, and PCM3 units were 5240 s, 5959 s, and 5419 s, respectively. In contrast, the phase-change cooling completion time for a single PCM1 unit was 6879 s, representing a 13% reduction in completion time. Although the heat transfer fluid temperature continuously increased during the flow heat exchange process, the cascade distribution of the phase-change material caused the PCM phase-change temperature to rise accordingly. Consequently, the heat exchange temperature difference within the PCM units remained relatively stable, leading to substantially identical cold storage completion times across different PCM units.

As evident from the variation in heat transfer per unit area across different PCM units in Figure 5, during the second phase of phase-change heat transfer, PCM3 exhibited the highest heat transfer per unit area, followed by PCM1 and PCM2. This was primarily due to the heat transfer fluid maintaining a substantial state with the phase-change units throughout. However, PCM2 exhibited a lower thermal conductivity, resulting in reduced heat transfer per unit area. This caused a slower temperature rise in the heat transfer fluid and an increased temperature difference, consequently leading to PCM3 achieving the highest heat transfer per unit area and the fastest completion of phase change. Therefore, during the cooling storage phase, the cascaded phase-change cooling storage device can effectively accelerate the cooling storage process, addressing the challenge of insufficient night-time storage duration.

To better analyze the performance of the cascaded phase-change cold storage system, a comparative analysis was conducted between the cold storage capacity of the PCM cascaded phase-change energy storage system and that of the single PCM1 phase-change cold storage unit, as illustrated in Figure 6. Owing to PCM3’s greater volumetric energy storage density, the cascaded PCM system exhibited a higher overall volumetric energy storage capacity than that of the single PCM1 unit, with cooling capacities of 14.5 MJ and 13.8 MJ, respectively. Concurrently, the cascaded system achieved faster phase-change cooling rates, resulting in higher cooling power outputs of 2.43 kW and 2.00 kW, respectively. As the structural design of the flat-plate phase-change cooling unit directly influenced the energy utilization efficiency of the storage system, and given that the cooling efficiency of both the cascaded and single PCM1 phase-change cooling systems was nearly identical, the selection of phase-change material exerted a negligible effect on the overall energy utilization efficiency of the storage system.

3.2. The Effect of Cascade Configuration on the Comprehensive Performance of the Device’s Heat Dissipation Process

To ensure the cascaded phase-change material (PCM) thermal storage unit fully exploits temperature gradient utilization during the heat release process, the original inlet and outlet positions must be reversed. The inlet velocity was set to 0.1 m/s, with an inlet temperature of 12 °C. As shown in Figure 7, the liquid phase fraction variation during the heat release process indicated that, unlike the heat storage phase, PCM1 units exhibited the shortest heat release completion time during discharge, followed by PCM3 and PCM2 units, with respective completion times of 3040 s, 4931 s, and 4336 s. The single-PCM1 phase-change cold storage unit completed desorption in 3220 s. The phase-change desorption time increased by 53%, primarily because during desorption, the high-temperature heat transfer fluid exchanged heat with PCM3 and PCM2, which had higher phase-change temperatures, before the phase-change process. Consequently, the temperature difference during the second stage of phase-change heat exchange was significantly lower than that observed in the single-PCM1 phase-change cold storage unit.

As demonstrated by the heat transfer coefficient variations per unit area across different PCM units in Figure 8, PCM1 exhibited the highest heat transfer coefficient, followed by PCM3. This is similarly attributable to PCM2’s lower thermal conductivity, which resulted in a slower rate of temperature reduction during heat exchange between the high-temperature heat transfer fluid and the PCM2 unit. Consequently, this increased the temperature differential between the heat transfer fluid and PCM3.

Figure 9 illustrates the variations in heat release performance across different PCM compositions within phase-change thermal storage structures. Due to PCM3’s high volumetric energy storage density, the cascaded system exhibited a greater overall heat release capacity. However, the extended heat release duration of the cascaded storage system resulted in a heat release power of 2.94 kW. While comparable to the heat exchange power during the storage process, this figure remained significantly lower than the 4.28 kW achieved by the single PCM1 storage unit. During the heat release process, both configurations exhibited high heat release energy utilization rates. Figure 10 illustrates the outlet temperature variations for different PCM compositions. The outlet temperature of the cascaded storage unit was higher than that of the single PCM1 unit, primarily due to its lower phase-change heat transfer power. Nevertheless, it also maintained a relatively stable outlet temperature.

3.3. The Effect of Series/Parallel Configuration on the Comprehensive Performance of the Device’s Cold Storage Process

To investigate the cold storage characteristics of series/parallel phase-change storage devices, a simulated operating condition was established with an inlet velocity of 0.05 m/s and an inlet temperature of 2 °C. A single module comprised 48 flat-plate phase-change cold storage units stacked in a configuration of H_PCM = 30 mm and H_HTF = 10 mm. Temperature distributions and outlet temperature variations were analyzed for single modules, series-connected, and parallel-connected phase-change storage devices across different operating durations to elucidate differences in their cold storage performance.

Figure 11 illustrates the temporal evolution of internal temperature distribution within a single phase-change module. Simulation results indicate that water within the apparatus rapidly dissipated heat from the phase-change thermal storage units. By 3000 s of operation, the temperature of plate-type phase-change thermal storage units near the inlet side had stabilized below 6 °C, whilst units near the top and bottom of the outlet side maintained temperatures around 7 °C. This temperature gradient primarily stemmed from the higher fluid velocity in the central region, enabling more efficient heat removal from that area. Furthermore, the simulation revealed that phase change began occurring in the inlet-side units at 1800 s, indicating the gradual progression of the phase-change process throughout the device.

Figure 12 and Figure 13 compare the internal temperature distributions of series and parallel phase-change cold storage units at 600 s and 3000 s. In the series unit, as water sequentially passed through each phase-change module, heat transfer was relatively uniform; however, the overall temperature distribution exhibited a gradient change from inlet to outlet.

Figure 14 illustrates the temporal variation in outlet temperature for single-module, series-connected, and parallel-connected phase-change cold storage units. Results indicate that the outlet temperatures of all three configurations exhibited a trend of initial rise, subsequent plateauing, and eventual decline. This phenomenon arose because the units initially stored substantial thermal energy. As water flow persisted, this heat was progressively dissipated, causing the outlet temperature to decrease after reaching its peak.

The outlet temperature of the single-module configuration stabilized at 5.48 °C after 3000 s of operation, whereas the series configuration stabilized at 7.91 °C over the same period. The parallel configuration stabilized at 6.38 °C. Within the series configuration, the temperature rise rate exceeded that of the single-module mode, peaking at 7.97 °C—significantly higher than the single-module peak of 6.13 °C. This disparity primarily stemmed from the cumulative thermal resistance inherent in series configurations, compounded by uneven coolant distribution or temperature measurement positional bias caused by low flow rates. Furthermore, the parallel configuration demonstrated superior cooling efficiency at the outlet compared to the series mode. It achieved 92 kilowatts of cooling capacity within 9600 s, extending operational duration by 2400 s relative to the single-module setup while enhancing cooling efficacy by 50%.

3.4. The Effect of Series/Parallel Configuration on the Comprehensive Performance of the Device’s Heat Dissipation Process

To investigate the heat release characteristics of series/parallel phase-change storage devices, a simulated operating condition was established with an inlet velocity of 0.05 m/s and an inlet temperature of 12 °C. A single module comprised 48 flat-plate phase-change cold storage units stacked in a configuration of H_PCM = 30 mm and H_HTF = 10 mm.

Figure 15 illustrates the temporal evolution of internal temperature distribution within a single phase-change module. Simulation results indicate that by 600 s of operation, the temperature of the plate-type phase-change cold storage unit near the inlet side had risen to exceed the melting point of the phase-change material (5.2 °C), initiating phase transition.

Figure 16 and Figure 17 compare the internal temperature distributions of series and parallel phase-change cold storage units at 600 s and 3000 s. In the series unit, as the water flow sequentially passed through each phase-change module, heat transfer occurred more uniformly, with the phase-change process progressing gradually from the inlet side towards the outlet side.

Figure 18 illustrates the temporal variation in outlet temperature for single-module, series-connected, and parallel-connected phase-change cold storage units. The results reveal that all three configurations exhibited three distinct phases: rapid cooling, phase transition, and temperature recovery. The series configuration demonstrated a faster cooling rate during the initial phase (reaching 6.24 °C within 1200 s, 31% faster than the parallel configuration) due to its cascading heat transfer effect, making it suitable for emergency scenarios requiring rapid cooling. In contrast, while the parallel configuration exhibited slower initial cooling (reaching phase transition temperature 1800 s later than the series design), it achieved more sustained stable cooling and superior hydraulic performance through strictly synchronized phase transitions across all branches. This makes it an ideal choice for applications requiring long-term stable cooling, such as data centers.

This is because the series flow path forces the HTF to interact sequentially with the thermal mass of all modules. When combined with a material cascade (higher temperature PCMs near the inlet), this effect could be further amplified or modulated. The current study, which used identical PCMs in all modules for the series/parallel experiments, isolated the pure effect of flow topology, providing a baseline for future studies on their coupled influence.

3.5. Experimental Study of Flat-Panel Phase-Change Energy Storage Device

3.5.1. Analysis of Heat Transfer Performance

The phase-change energy storage material used in the phase-change energy storage device was PCM1. Figure 19a illustrates the temperature curves over time for the inlet and outlet of the phase-change energy storage device, along with temperatures at various monitoring points within the device, under operating conditions of an inlet temperature of 2 °C and an inlet flow rate of 1500 L/h. Based on the data presented, three distinct phases of temperature variation were clearly observable: During the initial cold storage phase (approximately 0–500 s), as the phase-change process had not yet commenced, convective heat transfer occurred between the phase-change units (approximately 12 °C) and the 2 °C cold water. This caused the PCM temperature to decrease rapidly, leading to a swift drop in temperature at the monitoring points. Upon entering the stable cold storage phase (500–5000 s), the phase-change units gradually solidified, stabilizing the PCM temperature. Consequently, the rates of temperature change at all monitoring points slowed, exhibiting a gradual decline. By the final cold storage phase (5000–6300 s), the phase-change process was largely complete, causing monitoring point temperatures to drop rapidly and ultimately approach the inlet temperature. Furthermore, it was observed that monitoring points at the same height but further from the inlet exhibited a greater temperature differential compared to the inlet fluid. Simultaneously, significant temperature variations were noted between monitoring points at different heights, with the lowest temperatures recorded at the mid-height position, followed by the bottom and top. This indicates more effective heat exchange between the phase-change units and the inlet fluid at the mid-height, potentially attributable to non-uniform water velocity distribution within the apparatus, resulting in differing heat transfer efficiencies at various heights.

Figure 19b illustrates the temperature curves over time for the inlet and outlet of the phase-change energy storage device, along with temperatures at various monitoring points within the device, under operating conditions of an inlet temperature of 12 °C and an inlet flow rate of 1500 L/h. The figure indicates that the temperature trends at each monitoring point during the heat release process resembled those during the heat storage process. However, owing to the greater heat transfer temperature difference during heat release, the duration required for heat release was significantly shorter than that for heat storage. Concurrently, the figure also reveals that during the actual cycle, the inlet temperature did not remain constant but exhibited fluctuations of approximately 1 °C. This fluctuation may be related to fluid flow within the system or the response characteristics of the temperature control equipment.

3.5.2. Effect of Inlet Temperature on the Cold Storage and Release Performance of the Unit

As shown in Figure 20a, which illustrates the inlet and outlet temperature variations of the unit under different inlet temperature operating conditions during the cold storage process, the completion time for cold storage gradually increased with rising inlet temperatures. This effect was particularly pronounced at an inlet temperature of 4 °C, where the cold storage completion time increased significantly, rendering it highly unsuitable for night-time cold storage. Concurrently, the temperature difference between inlet and outlet diminished as the cold storage inlet temperature rose, indicating a gradual reduction in heat exchange capacity. Furthermore, as illustrated in Figure 20b, which depicts the influence of varying inlet temperatures on inlet and outlet temperatures during the heat release process, the completion time for heat release shortened as the inlet temperature increased. Given the substantial heat exchange temperature difference between the heat release inlet temperature and the material’s phase transition temperature, the inlet temperature exerted a relatively greater impact on heat exchange during the heat release phase.

Figure 21 illustrates the impact of varying inlet temperatures on the energy storage performance of the unit. Results indicate that the heat exchange power during cold storage decreased as the inlet temperature increased, whereas the heat release power increased with rising inlet temperature. The variation observed during the cold storage process was more pronounced. This was primarily due to the relatively small phase-change temperature difference during cold storage, where even minor temperature alterations significantly affected the heat transfer rate. Furthermore, the energy utilization efficiency during the cold release process diminished as the inlet temperature rose. From an inlet temperature of 11 °C to 13 °C, the energy utilization efficiency decreased by approximately 2.5%.

3.5.3. Effect of Import Flow Rate on the Cold Storage and Release Performance of the Unit

Figure 22 illustrates the inlet and outlet temperature variations during the charging and discharging processes of the phase-change thermal storage unit under three inlet flow conditions: 900 L/h, 1200 L/h, and 1500 L/h. As the inlet flow rate increased, the unit exhibited a trend of progressively decreasing inlet temperature differential during phase-change heat transfer. Furthermore, the time required to complete the phase change gradually diminished with increasing flow rate. This indicates that higher flow rates enhanced heat exchange efficiency between the fluid and phase-change material, thereby accelerating the phase-change process and optimizing the system’s energy transfer and storage performance.

Figure 23 illustrates the effect of inlet flow rate on the energy storage performance of the phase-change thermal storage unit. The figure demonstrates that increasing the inlet flow rate effectively enhanced the unit’s phase-change heat transfer capacity. Furthermore, the slope of the curve indicates that the increase in heat release capacity was more pronounced. This suggests that a higher inlet flow rate contributed to improved heat exchange efficiency, particularly during the heat release phase. Furthermore, although energy utilization efficiency did improve with increasing inlet flow rate, the enhancement was relatively limited and may essentially be regarded as a minor variation. Consequently, its impact on the overall system performance was not particularly significant.

4. Conclusions

This study systematically investigated the thermal performance of a flat-plate phase-change thermal energy storage system, focusing on the coupled effects of two system-level design dimensions: cascaded PCM arrangement and series/parallel flow configurations. The main conclusions are summarized as follows:

(1)

The cascaded PCM configuration significantly enhanced charging performance. Compared to a single PCM1 unit, the three-PCM cascade reduced phase-change completion time by 13%, increased cooling storage power from 2.00 kW to 2.43 kW, and boosted total storage capacity due to PCM3’s higher volumetric energy density. This charging advantage was maximized when the flow path aligned with the cascade order, demonstrating the coupled benefit of material and flow design.

(2)

During discharging, the same cascade structure reduced discharge power from 4.28 kW (single PCM1) to 2.94 kW and prolonged discharge time by 53%. The impact of material cascade on discharge performance is modulated by flow topology: parallel configurations can distribute the discharge load more evenly, mitigating the power drop and improving temperature uniformity—a critical trade-off for application-specific design.

(3)

Series and parallel configurations exhibited distinct thermal responses that interacted with the cascaded PCM arrangement. Series flow achieved faster initial cooling (reaching 6.24 °C within 1200 s, 31% faster than parallel), suitable for rapid cooling scenarios, while parallel flow provided more sustained and stable cooling output through synchronized phase change. The choice between series and parallel must therefore consider both the cascaded PCM layout and the target application requirements.

(4)

Operational parameters also play critical roles within this system-level framework. Inlet temperature was the most influential factor: a 2 °C increase significantly prolonged charging time and reduced charging power, but shortened discharging time and increased discharging power. Flow rate accelerated the phase-change process but had minimal impact on energy utilization efficiency. Additionally, vertical position affected heat exchange due to non-uniform velocity distribution, with mid-height units exhibiting the most effective thermal interaction.