The marine lithium battery power system combines energy conservation, environmental protection, low vibration and noise The advantages of flexible layout have gradually gained attention and favor in the industry. Lithium batteries emerged in the early 2000s and are a collective term for lithium electrode materials such as lithium cobalt oxide, lithium iron phosphate, and lithium manganese oxide. Their energy density is much higher than traditional marine batteries, reaching 80-200 Wh/kg, and they also have extremely high energy efficiency (>97%) and extremely low self discharge rate (<0.3%/day) Especially, lithium iron phosphate batteries have high safety and cost-effectiveness, making them the most researched energy storage technology currently. However, with the use of lithium iron phosphate batteries on ships and the increasing capacity on board, the industry has put forward stricter requirements for the safety and reliability of lithium iron phosphate battery energy storage systems. This article analyzes and studies the key technologies of marine lithium iron phosphate battery energy storage system, and elaborates on the integrated scheme configuration of lithium iron phosphate battery energy storage system using the intelligent unmanned system mother ship as the carrier, providing reference and reference for engineering designers.
1.Characteristic Analysis of Energy Storage System for Lithium Iron Phosphate Battery
1.1 Characteristic analysis of lithium iron phosphate batteries
Energy density: Under the same mass conditions, its energy density is 3-5 times that of traditional batteries (such as lead-acid batteries).
Discharge rate: Working at different discharge rates, the discharge voltage of lithium iron phosphate batteries is basically maintained within the design threshold.
Service life: Under the same capacity, the capacity of lithium iron phosphate batteries can still be maintained within the normal range when the number of cycles is three times that of traditional lead-acid batteries.
Safety: The safety issues of most lithium batteries continue to be questioned by users, but the positive and negative electrodes of lithium iron phosphate batteries themselves have high thermal stability, and the internal structure of the battery pack can be optimized and designed to pass the current high standards of performance and safety testing in the industry. Research has shown that when there is no open flame around the environment where lithium iron phosphate batteries are used, their thermal runaway generally does not lead to active explosions.
Through the thermal runaway test of the battery cells of the lithium iron phosphate power battery on the intelligent unmanned system mother ship, the gas composition released during the test process is shown in Table 1.
| Gas composition | Under 100% State of Charge (SOC) conditions (%, mol/mol) |
| CO2 | 30 |
| H2 | 46 |
| CO | 11 |
| Alkanes | 7 |
In the case of thermal runaway, due to the stable structure of lithium iron phosphate, there is no oxygen precipitation at high temperatures, and the chemical reactions involved mainly occur between the anode and the electrolyte. When thermal runaway occurs, the internal temperature of the battery cell is about 300 ℃, which is much lower than the ignition temperature of the combustible gas released by the battery in the air (530 ℃~750 ℃), so it will not ignite the combustible gas. It can be seen that in the absence of combustion aids, the lithium iron phosphate power battery configured on this ship will not experience combustion and explosion in the event of thermal runaway, and its safety level is 2, meeting the relevant requirements of classification society standards.
1.2 Characteristic Analysis of Marine Lithium Iron Phosphate Battery Energy Storage System
High capacity power supply. Due to the inherent electrochemical characteristics of lithium iron phosphate batteries, although a single battery cell has a small capacity and low voltage, in practical applications, small battery cells can be expanded and packaged to form battery clusters. A larger battery array can be obtained by connecting multiple battery clusters in parallel, with a capacity of up to megawatts. It can not only meet the energy storage needs of ships, but also serve as an active power source for ships.
Good compatibility with DC grid connection. When used as a ship power source or grid energy storage system, the DC characteristics of lithium iron phosphate battery energy storage units do not need to consider the matching of frequency and phase compared to the AC characteristics of conventional diesel generator sets, and have the characteristics of less grid connection constraints and fast system response. Among them, as an energy storage system, there are mainly three traditional transmission modes:
1) When the load rate of the power grid is higher than the set load rate of the generator set, in order to maintain the optimal load rate of the generator set, the remaining power energy storage unit is discharged by the lithium iron phosphate battery energy storage unit;
2) When the grid load is lower than the optimal load rate of the generator set, the remaining power of the generator set is used to charge the lithium iron phosphate battery energy storage unit;
3) When the load feeds back energy to the power grid (such as the energy flow of the pod electric propulsion system during the braking process of the propulsion motor), its energy is recycled and utilized by the energy storage unit.
It can be seen that configuring lithium iron phosphate battery energy storage units in the ship’s power system can maintain the diesel generator set in ideal working conditions, improve energy efficiency and economic benefits, and avoid severe fluctuations in the power grid caused by the feedback energy of the load motor when there is energy feedback.
2.Characteristic analysis of lithium iron phosphate battery energy storage system
The China Classification Society’s “Guidelines for the Inspection of Pure Battery Powered Ships” (2019) proposes that the potential risks mainly involved in marine lithium iron phosphate power batteries include: thermal runaway, electric shock risk, fire and explosion risk, gas spread risk, and external fire risk.
2.1 Thermal runaway
In response to the risk of thermal runaway, fire detection and extinguishing systems can be configured in the cabin during design to achieve early fire perception, intelligent judgment, and suppression of battery pack fires. The cabin is equipped with corresponding gas sensors based on the gas components released during the thermal runaway test of the battery cells, along with smoke sensors and temperature sensors. The risk of thermal runaway is predicted in advance through the load judgment of multiple sensors, and sound and light alarms are carried out through professional repeaters at control stations such as the cab. At the same time, the perception system is set to interlock with the cabin exhaust fan when a certain perception threshold is reached, in order to achieve automatic emergency ventilation.
2.2 Risk of electric shock
Regarding the risk of electric shock, preventive measures are mainly relied on. In the design and selection of battery packs, priority should be given to the insulation strength between internal electrical components and the shell (meeting CCS requirements); Simultaneously, real-time insulation status tracking of the battery system and output of low insulation alarm signals are carried out through the Battery Management System (BMS).
2.3 Risk of fire and explosion
The current mainstream approach to fire risk is to combine prevention, interruption, and loss reduction [8]: 1) Design the shell structure of battery modules, enhance the strength and fire resistance level of the battery pack structure through effective protective measures, and avoid a series of chain reactions caused by single or multiple batteries losing control of heat due to collision and compression; 2) Considering the heat dissipation capacity of internal space design can to some extent alleviate the speed of internal heat accumulation; 3) When selecting the internal components of the battery, focus on their respective ignition point data and select materials with high ignition point temperature values; 4) Using BMS to track battery voltage and temperature values in real-time, outputting relevant alarm signals when exceeding the threshold, and even stopping corresponding battery charging and discharging operations; 5) Install sound and light alarm devices inside and outside the cabin, and simultaneously output extended sound and light alarm signals in public places on board, providing maximum emergency handling and escape time windows for cabin staff and onboard personnel; 6) In response to the research on the effectiveness of extinguishing fires in the event of thermal runaway of lithium iron phosphate batteries, a heptafluoropropane fire extinguishing system is installed in the battery installation location/cabin to effectively control the initial fire situation. Regarding the risk of explosion, relevant measures to prevent fire risk can be referred to, with a focus on balancing the internal and external pressure of the battery pack and the overall electrical module. The design of the battery pack also needs to consider its pressure relief ability in the event of an explosion, in order to reduce the intensity of the explosion.
2.4 Risk of gas spread
In response to the risk of gas spread, usually: 1) when designing and selecting battery packs, priority should be given to materials with excellent heat dissipation and flame retardancy; 2) At the same time, refer to measures to prevent explosion-proof risks and configure explosion-proof valves; 3) BMS is used to monitor the battery module status parameters such as cell temperature, insulation, State of Charge (SOC), and voltage in real-time. When the relevant threshold is exceeded, measures can be taken to stop the corresponding battery charging and discharging operations.
2.5 External Fire Risk
In response to external fire risks, real-time monitoring of environmental temperature is carried out. When the environmental temperature reaches the alarm value, sound and light alarm signals are output in advance inside and outside the cabin, and extended sound and light alarm signals are also output in public places on board, providing ample emergency response time for personnel on board. When the threshold is exceeded, the BMS system triggers a stop charging and discharging operation on the battery, and automatically interlocks and starts the heptafluoropropane fire extinguishing system in the cabin to extinguish and cool down.
2.6 Safety Strategy for Marine Lithium Iron Phosphate Power Batteries
In the development stage of the ship, to ensure the high safety of the marine lithium iron phosphate power battery system, multiple strategies have been adopted to prevent the risk of overcharging, discharging, overheating, and overcurrent in the battery system.
The battery management system adopts a three-level warning mechanism. Level 1 mainly provides alarm prompts for crew members, Level 2 will trigger power reduction or initiate cooling measures, etc. Level 3 will stop operating the battery system, and the working status of the lithium iron phosphate power battery system will be indicated through communication between BMS and the conventional monitoring and alarm system on board. Important data exchange between BMS and the energy management system is also carried out, To trigger the execution of actions such as controlling power reduction or initiating cooling measures.
3.Integrated configuration scheme for lithium iron phosphate battery energy storage system
Elaborate on the integrated configuration scheme of marine lithium iron phosphate energy storage system using the intelligent unmanned system mother ship as the carrier.
3.1 Battery subsystem configuration
To achieve zero emissions and meet high environmental requirements, the power battery pack is the only source of power for the ship when sailing at low speeds in the port. Considering the ship’s entry and exit conditions, in accordance with the design principles of the ship’s power grid, the continuous
Calculate the electrical load based on the classification of load and intermittent load, with an electrical propulsion load of 235.5 kW (pod propulsion load of 126 kW; slewing load (considering the load coefficient LF-0.2 and simultaneous coefficient CF-0.5 as 18 kW); The load of the bow thruster device (considering the load coefficient LF-0.16 and simultaneous coefficient CF-1 of 91.5 kW), the load of the other ship systems is approximately 287 kW, totaling approximately 525 kW. The power battery is configured for 1 hour, and considering a safety margin of 12.5%, the total capacity of the power battery is designed to be 600 kWh. This capacity configuration scheme can meet the propulsion needs (where the effective power of the electric propulsion system accounts for 40% of the grid capacity).
The DC bus voltage of the ship is 1000 V, and the total capacity of the configured battery is Pah=600 Ah, which can meet the system requirements. The specific configuration is as follows: select a 3.22 V, 271 Ah lithium iron phosphate battery cell certified by a certain manufacturer and certified by the classification society; After 5 battery cells are connected in series, a 1P5S (1 parallel 5 series) module is formed. The battery pack consists of 4 modules in a 1P20S (1 parallel 20 series) structure, and there is a module for collecting battery cell voltage and temperature inside the battery pack. According to the standard charging and discharging current of a single battery pack, it is 100 A, and the maximum continuous charging and discharging current is 200 A. A single cluster battery consists of 12 battery packs, forming a 1P240S (1 parallel 40 series) battery cluster structure with a voltage of 772.8 V and a total capacity of 209.4 kWh. Divided into 2 groups: The No.1 battery pack is composed of one battery cluster connected in parallel, with a voltage of 772.8 V and a total energy of 209.4 kWh; The No. 2 battery pack is composed of two battery clusters connected in parallel, with a voltage of 772.8 V and a total capacity of 418.8 kWh; The battery system consists of two battery packs connected in parallel, with a voltage of 772.8 V and a total capacity of 628 kWh.
3.2 Battery Management System
The ship adopts a three-level architecture mode battery management system, forming a battery domain management unit (BAMS), battery cluster management unit (BMMS), and module management unit (BCMS).
The system mainly consists of 3 battery clusters, 3 sets of BMMS, 2 sets of BAMS, and a central control room display screen. The display screen of the central control panel in the central control room is used for monitoring and some control functions, and the other components are installed in the battery compartment.
The high-voltage box and cluster management unit can achieve functions such as communication and power supply with BCMS; Collection of total voltage and total current; Insulation and room temperature testing; Charge and discharge control; SOC calculation; Status diagnosis, fault location, and implementation of protection and alarm functions.
The domain management unit box is used to achieve: 1) communication and power supply with the high-voltage box and BMMS. Receive the operation status information, fault information, and operation parameters of the battery cluster uploaded by the high-voltage box; Receive request instructions uploaded by the high-voltage box; Send commands such as standby, discharge, charging, stop discharging, and stop charging to the high-voltage box. 2) Communicate with Energy Management System (EMS) devices and ship wide monitoring alarms. Upload information about all battery clusters to the system. 3) Decision function. By interacting with EMS devices, uploading battery and system information, and receiving control commands; Implement battery cluster parallel management strategy.
3.3 Energy storage system functions
In recent years, large-capacity energy storage technology has been continuously advancing. After considering the various technical indicators of battery energy storage units, their grid connection applications on land are becoming more and more common, and the grid connection technology for energy storage units on ships is also gradually being developed. The ship’s energy storage system achieves charge and discharge control through a DC-DC converter module. Under charging conditions, the converter, in conjunction with a filter (LDC) circuit, depressurizes and chops the DC bus voltage of 1000 V DC to the voltage threshold required for battery pack charging. Under discharge conditions, the DC converter, in conjunction with a filter (LDC) circuit, boosts and chops the low voltage on the battery side, stabilizing the DC busbar voltage at 1000 V DC. Through the DC busbar, power is supplied to the thrusters and daily loads. The ship’s lithium iron phosphate battery energy storage system operates from two remote control positions: on-site and centralized control room.
During remote control in the central control room, the EMS controls the battery in real-time based on the current load status of the power grid. When the load rate of the power grid is below 30% (which can be manually set) and the battery SOC is below 85% (which can be manually set), the EMS sends a start command to start charging the battery. When the load rate of the power grid exceeds 60%, charging stops. The EMS system will monitor the changes in the current power grid load in real-time. When there are high-frequency load changes, it will allow the battery system to quickly charge and discharge, thereby avoiding fierce fluctuations in the load of the diesel generator unit.
The energy storage system also has the function of maintaining the current DC grid voltage. When the grid voltage is above 1050 V, the EMS will control the battery charging. When the DC grid voltage is below 950 V, the EMS will control the battery system to discharge, thereby maintaining the stability of the grid voltage.
4.Emergency battery configuration plan for lithium iron phosphate
Traditional ships usually use emergency generator sets as their emergency power source. This ship pioneered the use of lithium iron phosphate batteries as its emergency power source, which mainly features lightweight, clean and environmentally friendly, and high safety.
According to the regulations of China Classification Society, the capacity of the battery must meet the load requirements of the equipment used in emergency situations. According to the equipment configuration that prioritizes power supply in the emergency situation of the ship, there are two types of emergency power sources on board: 220 V AC and 24 V DC (380 V AC equipment needs to be equipped with an independent uninterrupted power supply UPS or manual starting device). Therefore, the lithium iron phosphate battery system of the ship needs to be equipped with two types of power sources. The emergency power supply system requires an additional inverter to convert the output 24V DC power of the lithium iron phosphate battery pack into 220V AC power. Special attention should be paid to the issue that the emergency power supply on board the ship is 220 V AC, and the equipment is AC single-phase or AC three-phase. Considering that some ships are in the development and design stage, in order to balance the three-phase power load of the overall ship’s power grid, 24 V DC power is usually directly converted into a three-phase 220 V AC power output scheme. However, the corresponding problem of reducing the inverter coefficient needs to be solved through appropriate power module selection. When designing the scheme, it is best to choose products with pure sine waves, low distortion rate, and high output efficiency. At the same time, the power module must also meet the protection functions of overcurrent, short circuit, over temperature, etc. Otherwise, it will greatly increase the calculation capacity of the battery pack.
The demand for power supply equipment for the two types of electricity priority protection on board is shown in Table 2.
| Electrical System | Device Name | Power/W | Using System K1 | Power consumption time/H | Demand capacity/kWh |
| 220 V AC | Navigation aids/navigation equipment | 3,184 | 1 | 18 | 57.4 |
| 220 V AC | Radio equipment | 913 | 1 | 18 | 16.4 |
| 220 V AC | Internal communication alarm equipment | 250 | 1 | 18 | 4.5 |
| 220 V AC | Program controlled telephone | 105 | 0.2 | 18 | 0.4 |
| 220 V AC | Watertight door system | 1400 | 0.25 | 18 | 6.3 |
| 220 V AC | Watertight door system | 500 | 0.2 | 18 | 1.8 |
| 220 V AC | Total | — | — | — | 86.7 |
| 24 V DC | Navigation aids/navigation equipment | 49 | 1 | 18 | 0.9 |
| 24 V DC | Internal communication alarm equipment | 1,130 | 1 | 18 | 20.4 |
| 24 V DC | Broadcast system | 400 | 0.5 | 18 | 3.6 |
| 24 V DC | Emergency battery monitoring screen | 96 | 1 | 18 | 1.7 |
| 24 V DC | Low voltage distribution board | 240 | 1 | 18 | 4.3 |
| 24 V DC | Emergency lighting | 1,069 | 1 | 18 | 19.2 |
| 24 V DC | Total | — | — | — | 50.1 |
The calculation of battery capacity is as follows:
In the formula, C represents the battery capacity demand, kWh; PAC is the power of AC electrical equipment, kW; PDC refers to the power of DC electrical equipment, kW; T is the regulated power consumption time, taken as 18 hours; K1 is the utilization coefficient of the electrical equipment; K2 is the inversion coefficient, which is taken as 0.8 according to the system design requirements. The relevant data is substituted into equation (1), and the emergency battery capacity of the ship is C=158.5 kWh.
Considering the safety margin, the configuration of the ship’s lithium iron phosphate emergency battery system mainly includes the battery system, charging and discharging board, and UPS.
1) The battery system consists of 28 lithium iron phosphate battery packs in series and parallel, using 3.22 V DC, 271Ah lithium iron phosphate cells. Four cells are connected in series to form a 1P4S module, and the battery pack consists of two 1P8S modules. A single cluster battery consists of 14 battery packs, forming a 14P8S battery cluster structure with a voltage of 25.76 V DC and a total capacity of 97.72 kWh. The battery system is composed of two battery clusters in parallel. The total designed capacity is 7588 Ah, with a rated voltage of 25.76 V DC and an output voltage of 23.2 to 28.8 V DC. The total electricity consumption is approximately 195.4 kWh.
2) The charging and discharging board consists of a rectifier, charger, and distribution board. The rectifier is powered by a 24 V DC load, with a normal output voltage of 28.8 V DC and an output power of approximately 8 kW. When the main power supply loses power, the 24 V DC load is powered by the battery pack, and the discharge depth is 90%, which can meet the demand of 8 kW discharge for 18 hours.
3) Considering the uninterrupted power supply, after the main distribution board lost power, the 220 V AC power supply of BMS was lost, resulting in the failure of the entire full board operation. Therefore, BMS is equipped with an additional set of UPS 24 V DC, 5A. According to the working time of the charging and discharging board, it is necessary to meet the discharge duration requirement of 18 hours at the same time, so the BMS of this ship is equipped with a UPS capacity of 120 Ah.
5. Conclusion
With the continuous maturity and progress of the technology of large capacity lithium iron phosphate power battery energy storage system, its application prospects in ships have received high attention. This article analyzes the characteristics of lithium iron phosphate battery systems and proposes corresponding countermeasures for the risks of lithium iron phosphate power batteries. Taking the intelligent unmanned system mothership as the carrier, this paper elaborates on key technologies such as the integrated configuration scheme of the lithium iron phosphate energy storage system and the configuration scheme of the lithium iron phosphate emergency battery, providing reference and reference for engineering designers.