In the context of global efforts toward carbon peak and carbon neutrality, the integration of renewable energy sources like wind and solar into power grids has posed challenges to grid stability. Energy storage technologies, particularly electrochemical energy storage, have emerged as critical solutions due to their high energy density, reliability, safety, and flexibility. Among these, energy storage cells are pivotal components, but their thermal management remains a significant concern. Efficient cooling systems are essential to dissipate heat generated during operation, prevent accelerated aging, ensure longevity, and avoid thermal runaway incidents that could lead to safety hazards. This paper addresses these issues by proposing a novel indirect liquid-cooling system based on mechanical vapor recompression (MV降膜式) falling film evaporation for energy storage cells. We present a comprehensive design, performance analysis, and economic evaluation of this system, emphasizing its potential for high efficiency and low energy consumption.
Current cooling methods for energy storage cells primarily include air cooling and liquid cooling. Air cooling systems are simple and cost-effective but suffer from lower heat dissipation efficiency and cooling uniformity compared to liquid cooling. Liquid cooling, which can be indirect (e.g., cooling plates) or direct (e.g., immersion cooling), offers superior thermal performance. However, indirect liquid cooling often involves complex channel designs and high coolant consumption, while direct immersion cooling requires large volumes of expensive fluids like electronic fluorinated liquids, increasing system weight and cost. Moreover, conventional cooling systems typically rely on air conditioning units, which can lead to high energy consumption. To overcome these limitations, we leverage the high heat transfer efficiency of phase-change evaporation and MV降膜式 technology to develop a closed-loop cooling system that uses water as the coolant, reducing costs and enhancing safety.
The MV降膜式 indirect liquid-cooling system consists of two main modules: the falling film evaporation module and the MV降膜式 module. The falling film evaporation module includes energy storage cell modules, a liquid distributor, and a vacuum falling film chamber formed by falling film plates. These plates are attached to the sides of the energy storage cells with thermal grease to facilitate heat transfer. The liquid distributor ensures uniform liquid film formation on the falling film plates, where water evaporates by absorbing heat from the energy storage cells. The MV降膜式 module comprises a compressor, a liquid-vapor separator, a cooler, a pressure-reducing valve, valves, and a circulation pump. The system operates under vacuum conditions to promote evaporation at lower temperatures, while the energy storage cells remain at atmospheric pressure for safety.
The operational workflow begins with system initialization: valves are opened to evacuate air, and water is injected until the required level is reached. After closing the valves, the compressor and circulation pump are activated. In the falling film evaporation module, water from the liquid-vapor separator and condensate from the cooler are pumped to the liquid distributor, forming a thin film on the falling film plates. As the film evaporates, it absorbs heat from the energy storage cells, producing steam. Due to the pressure difference, steam is drawn from the vacuum chamber into the liquid-vapor separator, where liquid water is separated and recirculated to the distributor. The steam is then compressed by the compressor, increasing its temperature and pressure, and directed to the cooler. In the cooler, the steam condenses by exchanging heat with a cooling medium, and the condensate passes through a pressure-reducing valve to lower its pressure to the saturation pressure corresponding to the vacuum chamber temperature before returning to the falling film module. This closed-loop process ensures continuous cooling with minimal energy input.
To analyze the system’s performance, we developed a simulation model using Aspen Plus software. Water was defined as the working fluid with thermodynamic properties calculated using the IAPWS-95 standard method. The model assumes steady-state conditions, neglects pipeline energy losses, and ignores non-condensable gas effects. Key theoretical models include the compressor power consumption and evaporator heat transfer, as described by the following equations:
The compressor power consumption is given by:
$$ W = m_v \Delta H_{id,v} / \eta_c $$
where \( W \) is the compressor power consumption in kW, \( m_v \) is the steam flow rate in kg/s, \( \Delta H_{id,v} \) is the theoretical enthalpy change of steam during isentropic compression in kJ/kg, and \( \eta_c \) is the compressor efficiency.
The evaporator heat transfer is expressed as:
$$ Q = m_w \Delta H_{w,e} $$
where \( Q \) is the heat transfer rate in kW, \( m_w \) is the water flow rate in kg/s, and \( \Delta H_{w,e} \) is the enthalpy change of water during evaporation in kJ/kg. Assuming complete evaporation and negligible heat loss, \( m_w = m_v \).
We established design conditions for the simulation, as summarized in Table 1. The cooling load was set to 210 kW, with a cooling temperature of 303 K, operating pressure of 4247 Pa, falling film water flow rate of 311 kg/h, and compression temperature rise of 15 K. The simulation results, illustrated in Figure 3, show that under these conditions, the net cooling load is 204.492 kW, and the MV降膜式 compressor power consumption is only 14.227 kW. This demonstrates the system’s high efficiency, as the compressor handles the steam recompression with minimal energy input.
| Design Parameter | Value |
|---|---|
| Cooling Load (kW) | 210 |
| Cooling Temperature (K) | 303 |
| Operating Pressure (Pa) | 4247 |
| Falling Film Water Flow Rate (kg/h) | 311 |
| Compression Temperature Rise (K) | 15 |
For comparative analysis, we considered a conventional air conditioning refrigeration system with a cooling capacity of 210 kW, representing a room-level chilled water temperature control product. This system includes components like chillers, pumps, and piping, and operates with a first-level energy efficiency coefficient of performance (COP) of 3.3. The performance comparison between the MV降膜式 system and the conventional system is detailed in Table 2. At a cooling temperature of 303 K, the MV降膜式 system consumes only 14.227 kW of compressor power, whereas the conventional system requires 61.967 kW, resulting in energy savings of 77.04%. This significant reduction is attributed to the MV降膜式 system’s use of a single internal steam compression cycle, eliminating the need for additional refrigerant loops and reducing overall compression work.
| Parameter | MV降膜式 System | Conventional System | Energy Saving (%) |
|---|---|---|---|
| Cooling Temperature (K) | 303 | 303 | – |
| Latent Heat of Vaporization (kJ/kg) | 2429.67 | – | – |
| Cooling Load (kW) | 204.492 | 210 | – |
| Compressor Power (kW) | 14.227 | 61.967 | 77.04 |
We further investigated the influence of key thermodynamic parameters, such as cooling temperature and compression temperature rise, on system performance. The cooling temperature affects the steam density and compressor suction conditions, while the compression temperature rise relates to the compressor pressure ratio. Figure 5 shows the impact of cooling temperature on compressor power at different compression temperature rises. For instance, at a compression temperature rise of 12 K, increasing the cooling temperature from 283 K to 313 K reduces compressor power by 1.311 kW (10.7%). This trend aligns with previous studies and is due to higher cooling temperatures resulting in higher steam temperatures, lower pressure ratios, and reduced compressor work. Additionally, as shown in Figure 6, higher cooling temperatures increase steam density, decreasing the compressor’s volumetric flow rate and further lowering power consumption.
The compression temperature rise also significantly affects system performance. At a cooling temperature of 303 K, increasing the compression temperature rise from 10 K to 15 K raises compressor power by 4.883 kW (52.3%). This is because a higher compression temperature rise corresponds to a larger pressure ratio, increasing the compressor’s energy demand. Consequently, as illustrated in Figure 7, the system’s energy-saving potential compared to conventional systems improves with higher cooling temperatures and lower compression temperature rises. For example, at a compression temperature rise of 12 K, raising the cooling temperature from 283 K to 313 K enhances energy savings from 80.4% to 82.5%. Similarly, at 303 K, reducing the compression temperature rise from 15 K to 10 K increases energy savings from 77.0% to 83.9%. These findings highlight the importance of optimizing these parameters for efficient energy storage cell cooling.
The economic feasibility of the MV降膜式 indirect liquid-cooling system was evaluated using annualized cost (AC) and payback period (PBP) methods. The annualized cost includes capital, maintenance, and operational expenses over the system’s lifespan, while the payback period assesses the time required to recover the initial investment through energy savings. The AC is calculated as:
$$ AC = C_{acc} + C_{mc} – S_a + C_{rec} $$
where \( C_{acc} \) is the annualized capital cost, \( C_{mc} \) is the annual maintenance cost (taken as 10% of \( C_{acc} \)), \( S_a \) is the annual residual value, and \( C_{rec} \) is the annual electricity cost. The components are derived as follows:
$$ C_{acc} = C_{ccm} \times \frac{i(1+i)^n}{(1+i)^n – 1} $$
$$ S_a = S \times \frac{i}{(1+i)^n – 1} $$
$$ C_{rec} = C_{ee} \times W \times t $$
Here, \( C_{ccm} \) is the initial total investment cost, \( i \) is the interest rate (10%), \( n \) is the system lifespan (15 years), \( C_{ee} \) is the electricity price (0.8 CNY/kWh), \( S \) is the residual value (20% of \( C_{ccm} \)), \( W \) is the system power consumption in kW, and \( t \) is the annual operating hours (3000 h). The payback period is given by:
$$ PBP = \frac{\ln \left(1 – \frac{C_{ccm}}{B_j}(i – r)\right)}{\ln \left(\frac{1 + r}{1 + i}\right)} $$
where \( B_j \) is the first-year savings, and \( r \) is the discount rate (6%).
Table 3 lists the initial investment costs for the MV降膜式 system, which total 17,200 CNY, including components like the steam compressor, cooler, liquid-vapor separator, pressure-reducing valve, circulation pump, and valves. In comparison, the conventional system has an initial cost of 24,200 CNY. The economic comparison, presented in Table 4, shows that the MV降膜式 system has lower annualized capital and maintenance costs due to its simpler design. More importantly, the annual electricity cost for the MV降膜式 system is 34,145 CNY, compared to 148,721 CNY for the conventional system, resulting in annual savings of 114,576 CNY. The annualized investment cost for the MV降膜式 system is 36,525 CNY, significantly lower than the 152,069 CNY for the conventional system. The payback period for the MV降膜式 system is calculated to be 0.163 years, equivalent to approximately 2 months, indicating rapid return on investment and substantial economic benefits for energy storage cell applications.
| Component | Quantity | Cost (CNY) |
|---|---|---|
| Steam Compressor | 1 | 12,000 |
| Cooler | 1 | 3,000 |
| Liquid-Vapor Separator | 1 | 1,000 |
| Pressure-Reducing Valve | 1 | 400 |
| Circulation Pump | 1 | 500 |
| Valves | 1 | 300 |
| Total Cost | – | 17,200 |
| Parameter | MV降膜式 System | Conventional System |
|---|---|---|
| Initial Investment Cost \( C_{ccm} \) (CNY) | 17,200 | 24,200 |
| Annualized Capital \( C_{acc} \) (CNY) | 2,262 | 3,182 |
| Annual Maintenance Cost \( C_{mc} \) (CNY) | 227 | 319 |
| Annual Electricity Cost \( C_{rec} \) (CNY) | 34,145 | 148,721 |
| Annualized Investment Cost AC (CNY) | 36,525 | 152,069 |
| Payback Period PBP (years) | 0.163 | – |
In conclusion, the MV降膜式 indirect liquid-cooling system for energy storage cells demonstrates remarkable energy efficiency and economic advantages. Under a cooling load of 210 kW and cooling temperature of 303 K, the system reduces compressor power consumption by 77.04% compared to conventional first-level efficiency refrigeration systems. Optimization of cooling temperature and compression temperature rise further enhances performance, with higher cooling temperatures and lower compression rises leading to greater energy savings. Economically, the system offers a short payback period of just 2 months, making it a viable solution for large-scale energy storage applications. Future work should focus on experimental validation of the falling film evaporation module’s thermal uniformity and an in-depth analysis of the fluid dynamics and heat transfer mechanisms involved. This research underscores the potential of MV降膜式 technology to revolutionize energy storage cell cooling, contributing to more sustainable and efficient energy systems.
The design and analysis presented here highlight the critical role of advanced thermal management in enhancing the performance and safety of energy storage cells. By integrating MV降膜式 principles with falling film evaporation, we have developed a system that not only meets cooling demands but also aligns with global energy conservation goals. As the adoption of renewable energy and energy storage systems grows, such innovative cooling solutions will be essential for ensuring reliability and reducing operational costs. We encourage further exploration of this technology in various energy storage cell configurations to unlock its full potential.