Keywords

1 Introduction

With the advancement of manned space missions in China, there is a growing demand for energy systems with high output power and long working time to meet the operational requirements of spacecraft, especially in environments with extended periods of no sunlight, such as lunar nights. The integrated support system of spacecraft needs to have high specific energy and volumetric energy to reduce the spacecraft mass and increase payload capacity. Fuel cell technology has emerged as one of the key directions for the development of space power technology, both domestically and internationally [1,2,3]. Fuel cell power systems involve the conversion and utilization of multiple forms of energy, including water, gas, heat, and electricity. They can be used as independent power generation systems on spacecraft and can also integrate with spacecraft propulsion and thermal control systems to form a comprehensive energy system [4,5,6].

In this project, an analysis of energy transfer and resource sharing modes among subsystems, such as energy, propulsion, thermal control, and environmental control, is conducted from the perspectives of energy storage at the source end and energy consumption at the terminal end. Key technologies, including low-temperature propellant utilization, fuel cell power generation, product water purification, hydrogen and oxygen electrolysis regeneration, and heat recovery utilization, are studied through integrated analysis of energy utilization in subsystems such as power, propulsion, environmental control, and life support. A modular and replaceable prototype of a regenerative fuel cell integrated energy system is constructed, and ground demonstration and verification tests are conducted to provide technical reserves and design references for the future energy system of new manned spacecraft in China.

2 Energy Utilization Modes

The comprehensive utilization of energy encompasses the conversion of solar energy, chemical energy, electrical energy, and thermal energy, as well as the efficient utilization of hydrogen and oxygen, and the purification and utilization of product water. During the launch phase, the gaseous hydrogen and oxygen evaporated from the propulsion subsystem are supplied to the regenerative fuel cell system for power generation, which is used to supply the load while generating water for storage as a circulating substance in the regenerative fuel cell system [7].

During the lunar day period, solar energy is used as the primary energy source to power the bus load, and water is electrolyzed in the electrolysis cell, converting electrical energy into chemical energy for energy storage. This process enables the production of hydrogen and oxygen to meet the fuel cell power generation needs during the lunar night period. The excess hydrogen and oxygen can also be utilized to provide breathable oxygen for astronauts and hydrogen for reducing carbon dioxide in the environmental control and life support system. During the lunar shadow period without sunlight, the fuel cell system utilizes the stored hydrogen and oxygen for power generation, supplying power to the bus load as the main energy source, while generating water for storage after purification [8, 9]. The excess water can also be used as drinking water for the environmental control and life support system. The heat generated during the operation of the fuel cell can be used as a heat source for the thermal control subsystem during the lunar night period [10]. The comprehensive utilization of energy based on regenerative fuel cells is depicted in Fig. 1 [1].

Fig. 1.
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Comprehensive energy utilization mode [1]

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3 Integrated Energy System Solution

The integrated system of combined heat, power, and environmental control is designed to integrate multiple subsystems including energy storage and generation, environmental control and life support, and thermal control. It consists of eight functional units, including power generation subsystem, gas supply and pressure regulation, gas regeneration, water purification, temperature and humidity control, ventilation and purification, environmental monitoring, and information processing and control. The system includes the following aspect (Fig. 2).

Fig. 2.
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Integrated energy system solution

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3.1 Power Generation Function

The integrated energy system adopts a combined power generation method using fuel cells and lithium batteries. A 1000 W fuel cell is used as the main power source for external power supply, while lithium batteries serve as auxiliary power source for internal system power supply. When the system is started, the battery supplies power to the system components through a controller. Once the fuel cell stack reaches the operating state, it switches to supplying power to the controller and loads through voltage transformation/thermal backup, and enters the power generation state. When the battery is low on power, the fuel cell stack charges the battery through constant current and constant voltage DC-DC conversion.

3.2 Gas Supply and Pressure Regulation Function

Gas supply and pressure regulation mainly achieve gas supply for fuel cell power generation and environmental gas supply pressure regulation.

  1. (1)

    The power generation process of the system requires providing hydrogen and oxygen gases with a stable pressure of about 100 ± 2 kPa for the internal reaction. The hydrogen and oxygen gases from the gas bottles are regulated by pressure reducing valves to stable pressure for fuel cell power generation.

  2. (2)

    The integrated energy system provides oxygen for the sealed compartment through electrolysis of water. To meet the requirements of the living environment, the pressure inside the compartment is controlled at 94 kPa with an oxygen partial pressure of 20–24 kPa [11].

3.3 Gas Regeneration and Control

Gas regeneration and control mainly include pressure control and gas purification processes. During the electrolysis of water to produce hydrogen and oxygen gases, the pressure of the gases needs to be controlled to maintain a balanced pressure on both sides of the hydrogen and oxygen gases, which rises steadily and is input into the gas bottles. In addition, the hydrogen and oxygen gases produced by the electrolysis of water contain a large amount of water vapor and impurities. The membrane filter and dryer can recover water vapor from the gases, and palladium catalyst can remove impurities from the gases, achieving gas purification with hydrogen and oxygen purity of over 99.99%.

3.4 Water Regeneration and Purification

To improve water resource recovery efficiency, simplify management processes, and reduce system coupling to increase reliability, a unified water source, unified water quality, unified management, and unified distribution scheme are adopted for the system’s water source. Water regeneration and purification mainly target the water from four processes, including product water from the power generation process, circulating water from the electrolysis process, condensate water from the temperature and humidity control process, and wastewater from daily use.

3.5 Temperature and Humidity Control Function

The temperature and humidity control function for the sealed compartment’s atmosphere mainly includes temperature control, humidity control, condensate water collection, and cold source supply functions. According to the system’s functional units, temperature and humidity are controlled through sensors and actuators, ensuring a comfortable living environment for the occupants. Condensate water, which is generated during the temperature and humidity control process, is collected and recycled for water regeneration and purification.

3.6 Ventilation and Purification Function

The ventilation and purification function is responsible for maintaining a clean and healthy air environment inside the sealed compartment. Air is circulated through filters to remove particulate matter and harmful gases. Carbon dioxide produced by human respiration is removed through a carbon dioxide removal system, and fresh oxygen is supplied through the oxygen generation process. The ventilation and purification function ensures a continuous supply of fresh air and maintains a safe and healthy living environment.

3.7 Environmental Monitoring Function

The environmental monitoring function continuously monitors various parameters, such as temperature, humidity, air quality, and gas concentration, to ensure the system operates within the designated range. Data is collected and processed in real-time, providing feedback for system control and optimization. Any abnormal conditions are detected and addressed promptly to maintain the system’s stability and reliability [12].

3.8 Information Processing and Control Function

The information processing and control function integrates all the subsystems and coordinates their operation. It includes data collection, processing, and control algorithms to ensure optimal performance of the integrated energy system. Real-time monitoring and control are performed to adjust system parameters and respond to changing conditions. The information processing and control function also includes human–machine interface (HMI) for operators to monitor system status and make manual adjustments when needed.

4 Regenerative Fuel Cell Energy System

In an integrated energy system, the regenerative fuel cell energy subsystem consists of the gas regeneration subsystem, gas supply and pressure regulation subsystem, and power generation subsystem. Currently, domestic regenerative hydrogen–oxygen fuel cell energy systems mainly face challenges such as insufficient specific energy, durability, and reliability that need further demonstration and verification. Based on the requirements of the application scenarios for the overall energy system and the preliminary analysis of the renewable energy system, the overall design adopts a split-type regenerative fuel cell system, where the electrolysis and power generation are carried out by separate fuel cell stacks, while the pipelines and controllers can be shared. The schematic diagram of the overall design principle is shown below. The regenerative fuel cell energy system mainly consists of five parts: the PEM water electrolysis subsystem, the hydrogen–oxygen fuel cell subsystem, the reactant storage subsystem, the environmental control subsystem, and the power regulation and control subsystem (Fig. 3).

Fig. 3.
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Prototype of regenerative fuel cell principle

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4.1 Power Generation Function

The hydrogen–oxygen fuel cell subsystem directly converts the chemical energy of hydrogen and oxygen into electrical energy. To improve energy conversion efficiency, the generating power of fuel cell is 1000 W, and the generating efficiency is greater than 65%. The polarization curve of the fuel cell stack is shown in Fig. 4.

Fig. 4.
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Power generation performance of fuel cell

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During the startup process of a hydrogen–oxygen fuel cell stack, liquid circulating water needs to be supplied. Therefore, the startup temperature of the stack needs to be above 0 °C. Additionally, the proton exchange membrane exhibits noticeable swelling phenomena at temperatures above 70 °C. To study the temperature sensitivity of a fuel cell stack within the temperature range of 10–65 °C, a preliminary design using MATLAB was conducted. Specific parameter conditions were set to evaluate the electrochemical performance of the stack at different temperatures. The obtained results were compared and analyzed against experimental measurements, the stack exhibits power generation capability within the temperature range of 10–65°C, with a difference of approximately 15% in power output and efficiency (Table 1).

Table 1. Comparison of electrochemical performance of a 1000W fuel cell stack with temperature variations
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The pressure of a single cell in the fuel cell stack is influenced by the partial pressures of hydrogen and oxygen gases. To analyze the electrochemical performance of the fuel cell stack under different gas pressures and evaluate its temperature sensitivity, simulations were conducted, and the results were compared with experimental measurements. According to the experimental results, the voltage of a single cell in the fuel cell stack is observed to be 786 mV at a current density of 200 mA/cm2 when the absolute pressure of hydrogen gas is 150 kPa. With an increase in the hydrogen gas absolute pressure to 250 kPa, the voltage of a single cell under the same current density reaches 815 mV. Therefore, within the range of 150–250 kPa, the fuel cell stack exhibits a difference of approximately 10% in terms of power generation efficiency and output (Table 2).

Table 2. Comparison of electrochemical performance of a 1000 W fuel cell stack with pressure variations
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4.2 Efficient Utilization of Hydrogen and Oxygen

With a fuel cell power generation of 1000 W and an average single cell voltage of 0.8 V, the theoretical consumption of hydrogen and oxygen can be calculated using Faraday’s law. Assuming intermittent exhaust, the collected exhaust gas can be used to calculate the utilization rate of hydrogen and oxygen. The calculation results are shown in Table 3.

Table 3. Gas consumption volume and collection volume
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4.3 Product Water Recovery

After adjusting the reaction gas pressure to the set value, the fuel cell stack is started and loaded to the rated power. After the fuel cell stack stabilizes, the working current of the stack and the number of single cells in the stack are recorded, as well as the weight of the recovered product water. By conducting a comparison between the theoretically calculated weight of water produced in a fuel cell and the actual weight of water collected, it is determined that the water recovery rate of the fuel cell is greater than 95% (Table 4).

Table 4. Quality of product water generation and collection
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4.4 Hydrogen and Oxygen Production by Water Electrolysis

The water electrolysis subsystem re-electrolyzes water into hydrogen and oxygen using external electrical energy, and the gas production rate should meet the requirements of the fuel cell during eclipse periods. The electrolytic power of water cell is 1000 W, and the voltage efficiency is more than 90% (Fig. 5).

Fig. 5.
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Electrolytic performance of water electrolyzer

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4.5 Hydrogen and Oxygen Purification

With a power of 1000 W for the water electrolysis cell, the gas produced at atmospheric pressure is purified through a purification device. The oxygen content in hydrogen gas and the hydrogen content in oxygen gas before and after purification are measured. The purification results are shown in Table 5.

Table 5. Purity before and after gas purification
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5 Summary

Regenerative fuel cell systems offer high specific power and specific energy, making them well-suited for future manned space missions. These systems can achieve comprehensive material utilization through integration with propulsion, environmental control, and life support subsystems. Additionally, they can also integrate energy utilization with thermal control subsystems, resulting in an organic fusion of energy systems with other subsystems. By implementing multi-system integrated design, spacecraft launch weight can be reduced, and overall energy utilization efficiency can be improved throughout the spacecraft’s lifecycle.

Regenerative fuel cell energy systems are a promising sustainable energy technology for future energy supply. However, they still face technical and engineering challenges in practical applications. To overcome these challenges, careful consideration should be given to system design, efficient power generation and hydrogen–oxygen utilization, advanced thermal control technology, reliable power regulation and control, durability, reliability, as well as regular maintenance and monitoring. By addressing these aspects, the performance and reliability of regenerative fuel cell energy systems can be enhanced, promoting their widespread adoption in various fields.