In recent years, the widespread adoption of energy storage cells in various applications, particularly in renewable energy systems, has highlighted the critical need for efficient thermal management solutions. Energy storage cells, such as lithium-ion batteries, are prone to thermal issues during operation, which can compromise their safety, performance, and longevity. Effective thermal management is essential to maintain optimal operating temperatures, typically between 20°C and 40°C, and to prevent thermal runaway events. Among the various cooling techniques, direct cooling systems utilizing refrigerant-based two-phase flow have emerged as a promising approach due to their superior temperature uniformity and high energy efficiency. This study focuses on the experimental investigation of a direct cooling thermal management system designed for energy storage cells, emphasizing temperature control, pressure drop characteristics, and system performance under varying operational parameters. By leveraging the phase change properties of refrigerants, this system aims to enhance the thermal stability of energy storage cells, thereby supporting their reliable integration into large-scale energy storage applications.
The increasing demand for high-capacity energy storage systems has driven research into advanced thermal management strategies. Traditional methods like air cooling and indirect liquid cooling face limitations in terms of heat transfer efficiency and temperature uniformity. In contrast, direct cooling systems exploit the isothermal evaporation of refrigerants to achieve efficient heat dissipation with minimal temperature gradients. This paper presents a comprehensive analysis of a direct cooling system tailored for energy storage cells, including experimental setup, data processing, and performance evaluation. Key aspects such as compressor frequency optimization, pressure loss, and coefficient of performance (COP) are examined to provide insights for system design and operation. The findings demonstrate that direct cooling can effectively manage thermal loads while maintaining excellent temperature homogeneity, making it a viable solution for modern energy storage cells.
To quantify the performance of the direct cooling system, several parameters are defined and calculated. The average temperature of a cold plate is given by:
$$T_{\text{ave}} = \frac{1}{8} \sum_{i=1}^{8} T_i$$
where \( T_i \) represents the temperature at each measurement point. The maximum temperature difference on a single cold plate is expressed as:
$$\Delta T_{\text{max}} = \max(T_i) – \min(T_i)$$
Pressure loss across the cold plates is calculated using:
$$\Delta p = p_{\text{in}} – p_{\text{out}}$$
and the pressure loss ratio is:
$$\phi_{\text{loss}} = \frac{\Delta p}{p_{\text{in}}} \times 100\%$$
The system’s coefficient of performance (COP) for cooling is defined as:
$$\text{COP} = \frac{Q_T}{W_{\text{com}} – W_{\text{fan}}}$$
where \( Q_T \) is the total cooling capacity, \( W_{\text{com}} \) is the compressor power, and \( W_{\text{fan}} \) is the fan power. These formulas are essential for evaluating the efficiency and effectiveness of the thermal management system for energy storage cells.
The experimental system comprises key components such as a variable-speed compressor, cold plates, heat exchangers, and control valves, all integrated into a refrigerant circuit. The system operates in two modes: cooling and heating, achieved by switching a four-way valve. In cooling mode, the cold plates act as evaporators, where the refrigerant undergoes phase change to absorb heat from the energy storage cells. The system uses R134a as the refrigerant, and silicone heating pads simulate the heat generation of actual energy storage cells. Each cold plate has dimensions of 1030 mm × 560 mm, with a heating area of 0.47 m². Temperature sensors are strategically placed on the cold plates to monitor surface temperatures, and pressure sensors are installed at inlets and outlets to measure pressure drops. Data acquisition is performed using Agilent 34972A, and system control is managed via NI LabView software. The experimental conditions include an ambient temperature of 25°C, with heat loads set to 500 W per cold plate, representing a typical 0.5C charging scenario for energy storage cells. Tests are conducted at compressor frequencies of 35 Hz, 40 Hz, 45 Hz, and 50 Hz to analyze performance variations.
The design of the cold plates is critical for ensuring uniform temperature distribution across the energy storage cells. The flow channels within the cold plates are optimized to minimize pressure losses while facilitating efficient heat transfer. The refrigerant enters the cold plates in a two-phase state and evaporates, absorbing heat from the simulated energy storage cells. The use of thermal interface materials, such as 1 mm thick conductive pads, enhances heat conduction between the heating elements and cold plates. Additionally, insulation is applied to the system components to reduce heat exchange with the environment, ensuring accurate measurements. The system’s performance is evaluated based on temperature uniformity, pressure drop, and energy efficiency, with a focus on applications for energy storage cells in stationary storage systems.
Table 1 summarizes the key parameters of the experimental system components, providing a detailed overview of the setup used for testing the thermal management of energy storage cells.
| Component | Type | Specifications |
|---|---|---|
| Compressor | Variable-speed scroll compressor | Displacement: 14.1 ml/rev, Speed range: 1000–7200 rpm |
| Cold Plates | Aluminum with flow channels | Dimensions: 1030 mm × 560 mm, Heating area: 0.47 m² per plate |
| Refrigerant | R134a | Commonly used in direct cooling systems for energy storage cells |
| Heat Exchanger | Plate-type | 22 plates, each with 0.01 m² area, for refrigerant-refrigerant heat exchange |
| Expansion Valve | Electronic | Orifice diameter: 1.65 mm, Adjustable opening: 0–500 steps |
| Heating Source | Silicone pads | Maximum power: 1000 W per pad, Total: 4 pads for simulation |
| Data Acquisition | Agilent 34972A | For temperature and pressure monitoring |
The temperature uniformity of the cold plates is a key indicator of the system’s effectiveness in managing the thermal behavior of energy storage cells. Under standard conditions (500 W heat load per cold plate, 40 Hz compressor frequency, 25°C ambient temperature), the surface temperatures of the cold plates remain highly uniform. The maximum temperature difference on any single cold plate is less than 0.4°C, and the average temperature range across all cold plates is only 0.28°C. This level of uniformity is crucial for preventing hot spots and ensuring the stable operation of energy storage cells. The temperature data, collected from 32 measurement points across four cold plates, show that all points fall within a narrow range of 14.67°C to 16.85°C during steady-state operation. This demonstrates the system’s ability to maintain consistent cooling performance for energy storage cells under typical low heat load conditions.
Pressure drop analysis reveals important insights into the flow characteristics within the cold plates. The pressure loss between the inlet and outlet of the cold plates ranges from 4.99 kPa to 21.52 kPa, corresponding to pressure loss ratios of 1.26% to 5.30%. Although these losses are relatively small, they can lead to slight reductions in evaporation temperature, potentially affecting temperature uniformity. For instance, the maximum evaporation temperature drop observed is 1.62°C, which is acceptable for most applications involving energy storage cells. The low pressure losses are attributed to the optimized flow channel design, which ensures even refrigerant distribution and minimizes flow resistance. This design is essential for maintaining efficient heat transfer and stable operation in direct cooling systems for energy storage cells.
Table 2 presents the temperature and pressure drop data for each cold plate under the standard operating condition, highlighting the performance consistency across the system for energy storage cells.
| Cold Plate | Average Temperature (°C) | Max Temperature Difference (°C) | Pressure Loss (kPa) | Pressure Loss Ratio (%) |
|---|---|---|---|---|
| 1 | 15.34 | 0.79 | 21.52 | 5.30 |
| 2 | 15.48 | 1.31 | 14.04 | 3.47 |
| 3 | 15.62 | 1.56 | 10.67 | 2.66 |
| 4 | 15.35 | 0.85 | 4.99 | 1.26 |
To further evaluate the system, experiments are conducted at different compressor frequencies while maintaining a constant heat load of 500 W per cold plate. The results show that increasing the compressor frequency from 35 Hz to 50 Hz leads to a decrease in the average cold plate temperature from 21.03°C to 14.87°C. However, this improvement in cooling capacity comes at the cost of reduced temperature uniformity, as the maximum temperature difference across the cold plates increases from 1.13°C to 1.39°C. Additionally, the compressor’s compression ratio rises from 1.93 to 2.88, resulting in a decline in the system’s COP from 8.16 to 5.24. This trade-off between cooling performance and energy efficiency is a critical consideration for optimizing direct cooling systems for energy storage cells. The highest COP of 8.16 is achieved at 35 Hz, where the system maintains an average temperature of 21.03°C and a maximum temperature difference of 1.13°C, meeting the typical requirements for energy storage cells.
The system’s energy efficiency is a vital aspect, especially for large-scale applications of energy storage cells. The COP values obtained in this study are generally higher than those of conventional liquid cooling systems, underscoring the advantages of direct cooling. The relationship between compressor frequency and COP can be modeled using the following equation, which accounts for the compressor power and heat absorption:
$$\text{COP} = \frac{\sum_{j=1}^{4} Q_{s,j}}{W_{\text{com}} – W_{\text{fan}}}$$
where \( Q_{s,j} \) is the cooling capacity of the j-th cold plate. This formula highlights the inverse relationship between compressor power and COP, emphasizing the need for frequency optimization in systems designed for energy storage cells.
Table 3 summarizes the system performance at different compressor frequencies, providing a comparative analysis for energy storage cell applications.
| Compressor Frequency (Hz) | Average Cold Plate Temperature (°C) | Max System Temperature Difference (°C) | Compression Ratio | COP |
|---|---|---|---|---|
| 35 | 21.03 | 1.13 | 1.93 | 8.16 |
| 40 | 17.58 | 1.14 | 2.28 | 6.45 |
| 45 | 15.45 | 1.21 | 2.35 | 5.82 |
| 50 | 14.87 | 1.39 | 2.88 | 5.24 |
The discussion of these results emphasizes the importance of balancing cooling performance with energy consumption in thermal management systems for energy storage cells. The direct cooling system demonstrates robust temperature control and uniformity, making it suitable for applications where energy storage cells are subjected to low to moderate heat loads. The pressure drop characteristics indicate that the cold plate design effectively minimizes flow resistance, contributing to stable operation. However, the decrease in COP at higher compressor frequencies suggests that operational parameters should be carefully tuned to achieve optimal efficiency. For instance, in scenarios where energy storage cells operate at lower power levels, such as in grid storage, a lower compressor frequency may be preferable to maximize COP while maintaining adequate cooling.
Future work could explore the integration of advanced control strategies, such as adaptive frequency modulation based on real-time thermal loads of energy storage cells. Additionally, the use of alternative refrigerants with higher latent heats could further enhance the system’s performance. The findings from this study provide a foundation for the design and optimization of direct cooling systems in various applications involving energy storage cells, including electric vehicles and renewable energy storage.
In conclusion, the direct cooling thermal management system evaluated in this study offers significant benefits for energy storage cells, including excellent temperature uniformity, low pressure drops, and high energy efficiency. The experimental results confirm that the system can effectively manage thermal loads under standard conditions, with compressor frequency playing a key role in performance optimization. By maintaining temperatures within the desired range and minimizing energy consumption, this approach supports the safe and reliable operation of energy storage cells. As the demand for efficient energy storage solutions grows, direct cooling systems are poised to become a cornerstone of advanced thermal management strategies, ensuring the longevity and performance of energy storage cells in diverse applications.