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All-electric ships are now a standard offering for energy/propulsion systems in boats. In this context, integrating fuel cells (FCs) as power sources in hybrid energy systems can be an interesting solution because of their high efficiency and low emission. The energy management strategy for different power sources has a great influence on the fuel consumption, dynamic performance and service life of these power sources. This paper presents a hybrid FC/battery power system for a low power boat. The hybrid system consists of the association of a proton exchange membrane fuel cell (PEMFC) and battery bank. The mathematical models for the components of the hybrid system are presented. These models are implemented in Matlab/Simulink environment. Simulations allow analyzing the dynamic performance and power allocation according to a typical driving cycle. In this system, an efficient energy management system (EMS) based on operation states is proposed. This EMS strategy determines the operating point of each component of the system in order to maximize the system efficiency. Simulation results validate the adequacy of the hybrid power system and the proposed EMS for real ship driving cycles.

Emissions related to ship propulsion (CO_{2}, SO_{x}_{x}_{2} emissions from ships accounts for about 3%–5% of total CO_{2} emissions in the world and corresponded to about 1 billion tons in 2010. It has been estimated by international maritime organization that, without any countermeasures, the carbon dioxide emission from ships will increase to 2.5 billion tons in 2050. Moreover, SO_{2} emissions of ship are also a problem. It is estimated that it will increase by 10%–20% in 2012, which corresponds to 5.2% of the total sulphate burden in the world [

Although shipping emissions are not covered by the Kyoto Agreement, stricter regulations have been developed in different countries. As an example, European Union (EU) has implemented stricter regulation to control the SO_{2} emissions in particularly sensitive sea areas, including the Western European Waters and the Baltic Sea areas. In those areas, sulphur content in the fuel used in ship should not exceed 0.1%. However, this content is more than 1% in most marine fuels [

One of the most promising technologies which can be used in a ship is FC system (FCS). There are several types of FCs [

Some successful demonstrations have been realized in the past few years for marine applications with FCs [

Although FCSs have good capability to follow the power demand during steady state operating conditions, the dynamic response to transient power demands is relatively poor. The dynamic variations of power demand will impose a serious stress on the FC membrane, thus reducing the lifetime of FCSs [

In this paper, we will focus on the use of hybrid power configurations for a low power boat. The aim of the paper is to investigate the performance of such a hybrid system which associates a FC with a battery for the specification of low-power pleasure boat propulsion. For this case study, an efficient optimal EMS based on operation states is proposed. This EMS strategy determines the operating point of each component of the system in order to maximize the system efficiency.

The paper is organized as follows: Section 2 introduces the configuration of the hybrid power system structure for a FC boat; Section 3 presents the modeling of the hybrid power system; Section 4 proposes the EMS-based operating states; the simulations and discussion are presented in Section 5; and final conclusions are summarized in Section 6.

In this paper, we used a common specification which corresponds to a real pleasure boat. These common specifications correspond to the FC boat of “Alsterwasser” and can be found in [

The propulsion peak power is around 110 kW;

The acceleration time is around 32 s (163–195 s);

The cruising power is around 40 kW.

The average power is around 41 kW, without considering auxiliary power during the power cycle.

The FCS is an electrochemical device that converts chemical energy directly into electrical energy, which involves a set of physical and chemical reactions. ^{+} ions thanks to its excellent selective permeability.

The hydrogen fuel is fed continuously to anode electrode. Oxygen is simultaneously fed to cathode. Protons and ions are produced with an oxidation reaction shown in

Oxidation half reaction:

Reduction half reaction:

Cell reaction:

In this paper, the following assumptions are considered to obtain a simplified FC model:

The stack is fed with hydrogen and air and the gases are considered to be ideal;

The pressure drops across flow channels are considered to be negligible;

The stack operates at a constant temperature of 65 °C;

The humidity inside the stack is kept constant;

The stack is assumed not to work in the mass transport operating conditions.

The ideal open circuit voltage for a FC can be obtained by the Nernst equation, which is influenced by the partial pressures of reactants and products, temperature, reactant concentrations. Then, the output voltage of an FC is function of the Nernst's voltage, activation voltage, ohmic voltage drop, and concentration voltage [_{fc} is output voltage (V); _{OC} is open circuit voltage (V); _{fc} is output current (A); _{0} is exchange current (A); and _{fcin}_{C} is voltage constant, which can be determined by internal current and the Tafel slope under nominal operation condition and the value is less than or equals to one; R is gas constant (8.3145 J/mol K); _{H2} is partial pressure of H_{2} (atm); and _{O2} is partial pressure of O_{2} (atm). The partial pressures of gases can be calculated as follows:
_{f}_{H2} is hydrogen conversion rate; _{f}_{O2} is oxygen conversion rate; _{fuel} and _{air} are supply pressure of hydrogen and air, respectively. The rates of conversion of hydrogen and oxygen are determined as follows:
_{lpmf} and _{lpma} are the flow rates of hydrogen and air in L/min.

The studied FC is a PEMFC with a rated power of 80 kW. This FCS's parameters are summarized in

The characteristics of this studied device (

In a FCS, auxiliary power is often necessary because of the auxiliary components, such as compressor and cooling system. The auxiliary power requirement is estimated to be up to 30% of the FC stack power. The compressor consumes the main part (as much as 93.5%) of this auxiliary system power [

The FC voltage will drop when the output current arises. Therefore it should not to be connected to load directly, and a DC/DC converter is needed to control the output voltage.

In our study, a lithium-ion battery has been chosen. This kind of battery is an attractive candidate for electric vehicle (EV) applications (bus, truck and boat) due to its high energy density, long cycle life and relatively low self-discharge rate, compared to traditional lead acid batteries. Therefore, in our case, a high energy polymer Li-ion battery bank is used as energy storage device. The battery is modeled classically as a controlled voltage source in series with a resistance as shown in

The battery output voltage (_{Bat}) is derived as follows [_{Batt} is no load voltage (V); _{Batt} is battery current (A); and _{Bat} is the internal resistance (Ω). The internal resistance is assumed constant for simplicity reasons, although it normally changes according charging and discharging conditions or value of battery current.

The open circuit battery voltage which depends on the battery current, state of charge (

The value of battery voltage in the discharge mode (

The voltage value in charge mode (

In _{Batdis} and _{Batcha} are the open circuit voltage (V); _{0} is constant voltage (V); K is polarization constant (V/A h); _{Batt} is battery current (A); _{t} is actual battery charge (A h);

The

C rate of battery is used to describe its discharging or charging. The C rate is defined as:

The studied 576 V/180 A·h battery bank is composed of three packs in parallel, and every pack is made up of 12 × 48 V/60 A·h modules in series. The parameters are listed in

One DC/DC converter is used in this hybrid power system. A boost DC/DC converter is associated with FC and allows control the energy exchange between FC and DC bus. This converter shown in

In this study, this converter is modeled by a classical average state model with ideal components shown in _{i}_{2} can be simply identified by _{1}.

In this paper, the auxiliary load (light and pump,

There are two power sources in the hybrid power system. The energy management strategy for different power sources has great effect in influencing the fuel economy, dynamic performance and service life of power sources [

The proposed EMS is used to control the energy flow between the FC and the battery, which depends on the operating states considering the optimum power of FC and battery. The operating condition determination is based on the load power, the battery, and the FC states and operating points.

_{FCmin} is selected as 10 kW, with an efficiency of about 60%. Considering the average power of boat, the optimum power _{FCopt} is determined as 46 kW, where the net efficiency is 61%. The maximum net power _{FCmax} which can be delivered by the FCS is 80 kW.

Because of the battery's current and voltage limits, the following battery limits have been considered: (1) charging, discharging, and optimum power values for battery (_{optchar}, _{optdis}, and _{BATopt}), where charging power equals to discharging power with 30% capacity of battery and optimum power value is 20% capacity of the battery; and (2) battery _{max} and _{min}). Three levels for the battery

The power control strategy can be described by 11 possible cases which correspond to several combinations between P_{load} and battery

_{Load} ≤ _{FCmin}. In this case, the FC operates at its minimum power, then _{FCref} = _{FCmin}. The battery power will be _{BAT} = _{FCmin} − _{Load}, therefore the battery _{max}.

_{FCmin} < _{Load} ≤ _{FCmin} + _{optdis}. In this case, the FC operates also at its minimum power. The battery works at _{BAT} = _{Load} − _{FCmin}. Therefore the battery

_{FCmin} + _{optdis} < _{Load} ≤ _{FCmax} + _{optdis}. In this case, the FC power will be fluctuant according the load power at a power of _{FC} = _{Load} − _{optdis}. The battery works at its optimum discharging power _{optdis}, so that the battery

_{FCmax} + _{optdis} < _{Load}. In this case, the FC works at its maximum power P_{FCmax}. And the battery power will be _{BAT} = _{Load} − _{FCmax}, which means that the discharging current rate will be more than 0.3C. The battery will not keep working in this mode for a long time, because the capacity of FC and battery has been designed appropriately.

_{Load} ≤ _{FC_min}. This is similar to the Case 1. The FC operates at its minimum power, and the battery power will be _{BAT} = _{FCmin} − _{Load}. Therefore the battery

_{FCmin} < _{Load} ≤ _{FCopt} − _{BATopt}. In this case, the FC will work in strategy of load following. The FC reference power will be equal to the commanded load power. As the low response time of FC, the battery will compensate the transient power.

_{FCopt} − _{BATopt} < _{Load} ≤ _{FCopt} + _{BATopt}. In this case, the FC will work in optimum power P_{FCopt}. The FCS power will be constant and the battery will compensate the transient power.

_{FCopt} + _{BATopt} < _{Load} ≤ _{FCmax}. This case is similar to the State 6, the FC power is regulated to follow the load power.

_{FCmax} < _{Load}. This state is similar to State 4, where the FC operates at maximum power _{FC_max}, and the battery power will be _{BAT} = _{Load} − _{FCmax}, therefore the battery

_{Load} ≤ _{FCmax} − _{optchar}. In this state, the FC will provide the power of load and will charge the battery. Therefore the battery

_{FCmax} − _{optchar} < _{Load}. In this state, the FC power will be limited to the maximum power. And the battery charging power is _{FCmax} − _{Load} if the FC maximum power is more than the load power. Otherwise the battery will discharge to keep the balance of the power. It should be noted that the battery will have to stop working if the battery _{min}.

The hybrid system model is shown in ^{®} environment. Simulation results are obtained based on the typical driving cycle of the studied passenger boat presented in

In this case, the initial battery

As shown in the _{optchar} during the cruising and stop conditions. Then the battery

In this second case, the initial battery

In this case, the initial battery

There is a focus on all-electric ships powered by FCs because of their high efficiency and low emissions, and there are some R&D projects aiming to develop this technology. This paper presents a hybrid FC/battery power system for a low power boat. The hybrid system consists of PEMFC and a Li-ion battery bank. The FCS is connected to a DC bus by a boost converter, whereas the battery is connected directly to the DC bus. Mathematical models for the components of the hybrid systems are presented, which are implemented in Matlab/Simulink environment. An optimal EMS based on operation states is proposed to determine the operating point of each component of the system and optimize the system efficiency. The hybrid system behavior is then evaluated based on the real driving cycle of a low power FC boat, and the FCS works with optimized efficiency. Simulation results show that the proposed EMS can achieve higher efficiency compared with load commands tracking control, and the discharging and charging rate can be mostly controlled at less than 0.3C.

This work was partly supported by the National Natural Science Foundation of China under Grant No. 51007056 and by Brittany Region (France) funds for Post-Doctoral Foreign research stay.

The authors declare no conflict of interest.

_{2}Zero Canal Cruise

(

Schematic of an individual PEMFC.

Typical characteristics of voltage, power

(

Model of the lithium-ion battery.

Comparison between experiment and simulation discharge characteristics.

(

Relationship between FCS efficiency and power.

Power map of FC.

Model of the hybrid system. PWM: pulse width modulation; LPF: low-pass filter; PI: proportional-integral controller; and EMS: energy management system.

Power of load, FC and battery for Case 1.

Battery characteristics for Case 1.

C rate of battery for Case 1.

FC characteristics for Case 1.

Power of load, FC and battery for Case 2.

Battery characteristics for Case 2.

C rate of battery for Case 2.

FC characteristics for Case 2.

Power of load, FC and battery for Case 3.

Battery characteristics for Case 3.

C rate of battery for Case 3.

FC characteristics for Case 3.

Hydrogen mass consumption.

Parameter of the fuel cell (FC) boat (data extracted from [

Motor peak power | 120 | kW |

PEMFC rated power | 80 | kW |

PEMFC voltage | 140–260 | V |

PEMFC current | 280–520 | A |

Battery (lead gel) | 560 V/360 | A·h |

Displacement of water | 72 | t |

Length | 25.56 | m |

Width | 5.20 | m |

Passenger capacity | 100 | - |

Maximum speed | 15 | km/h |

Parameters of PEMFC.

Nominal stack power | 80 | kW |

Nominal stack efficiency | 54.5% | - |

FC resistance | 0.070 | Ω |

Nernst voltage of a cell | 1.1125 | V |

Exchange current | 0.50886 | A |

Exchange coefficient | 0.77139 | - |

Fuel composition | 99.99% | - |

Oxidant composition | 21% | - |

Operating temperature | 65 | °C |

Parameters of battery (data extracted from [

Capacity | 60 A h | 180 A h |

Output voltage | 36–55 V | 430–660 V |

Continuous current (0.5C) | 30 A | 90 A |

Energy | 2.6 kW h | 94.5 kW h |

Life cycle (0.3C, 80% DOD) | 1500 | 1500 |

Maximum discharging current (2C in 10 s) | 60 A | 360 A |

Summay of the operating states.

1 | _{load} ≤ _{FCmin} |
_{FCmin}/braking | |

2 | _{load} ≤ _{FCmin} + _{optdis} |
_{FCmin} | |

3 | _{Load} ≤ _{FCmax} + _{optdis} |
_{FC} = _{Load} − _{optdis} | |

4 | _{FCmax} + _{optdis} < _{Load} |
_{FCmax} | |

| |||

50% ≤ |
5 | _{load} ≤ _{FCmin} |
_{FCmin} |

6 | _{load} ≤ _{FCopt} − _{BATopt} |
_{load} | |

7 | _{load} ≤ _{FCopt} + _{BATopt} |
_{FCopt} | |

8 | _{load} ≤ _{FCmax} |
_{load} | |

| |||

9 | _{load} > _{FCmax} |
_{FCmax} | |

10 | _{Load} ≤ _{FCmax} − _{optchar} |
_{load} + _{optchar} | |

11 | _{Load} > _{FCmax} − _{optchar} |
_{FCmax} |