Hybrid Fuel Cell / Battery Power Systems for Underwater ... - CiteSeerX
Hybrid Fuel Cell / Battery Power Systems for Underwater Vehicles Q. Cai,a D. J. Browning,b D. J. Brett,a N. P. Brandona Department of Earth Science and Engineering, Imperial College London, SW7 2BP b Physical Sciences Department, DSTL Porton Down, Salisbury, Wiltshire, SP4 0JQ
a
Abstract A system-level design and analysis of the power system for a lightweight unmanned underwater vehicle (UUV) is presented with recommendations of viable technologies that can meet the UUV mission requirements. A hybrid fuel cell / battery system is designed to power the UUV as it has advantages over a pure fuel cell or battery system. The power system is designed to use a lithium-ion battery hybridised with a polymer electrolyte fuel cell. The analysis is focused on the mass, size, and the energy balance of the system components. It is shown that hydrogen and oxygen storage systems dominate the mass and volume of the energy system compared to the fuel cell and battery. Liquid oxygen is recommended for oxidant storage based on the mission length requirement. Keywords: Fuel Cell, Battery, Underwater Vehicle, System Design and Analysis Introduction Unmanned underwater vehicles (UUVs) are ideally suited to provide surveillance, remote sensing and communication relay capabilities for both military and civilian applications. Practical examples include oceanographic data gathering, environmental monitoring, mine detecting and coastal defence. The power system of a UUV has long been a major consideration in designing and manufacturing these vehicles for particular missions. This is because the power system usually determines the ultimate performance (e.g. endurance, cruising speed and distance) of a UUV. The work reported here aims to investigate viable power system architectures that meet the requirement of UUVs. Stealth is the highest design priority of a UUV as it enables the UUV to operate anywhere, at any time, without being detected. Besides helping to avoid detection, stealth enhances a submarine’s ability (by eliminating / reducing selfnoise) to detect targets. To meet the stealth requirement, an air independent power
(AIP) system is beneficial to UUVs. The ideal AIP source for a submarine will be quiet, have a low thermal signature, will not need to discharge anything from the submarine system, and will of course be capable of operating without atmospheric air. In its simplest form, the AIP power source is a battery. However, batteries alone encounter technology difficulties for use as the power source of UUVs, as current battery technologies cannot provide sufficient endurance to allow for large area coverage and short turnaround time between missions. Hybrid fuel cell / battery systems have a number of advantages over either stand-alone fuel cells or batteries. For example, the battery would enable instant cold-start operation whilst the fuel cell was initiating. The battery, as the dynamic energy storage device, would supply peak and pulse power and power for start-up of the hybrid system. The fuel cell, as the device that converts the energy from the fuel, supplies base-load power and recharges the battery. A hybrid system would allow both components to be of smaller dimensions and operate with higher efficiency, since neither would have to provide the full load power.
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The hybridisation of fuel cell and battery depends on the system power requirement. To characterise this, it is useful to define a variable called ‘the degree of hybridisation’ as follows [1] : DOH =
Battery power Battery power + Fuel cell power
(1)
For the generic power profile as shown in Figure 1, which is composed of cyclic periods of high and low power requirement both of variable time, the hybridisation of the fuel cell and battery intends for the fuel cell to supply the constant average power, and then has the battery to supply the peak power that is above the average power. The battery will then be recharged during the period when the propulsive power is below the average power. If we define T as the ratio of the battery discharge time t1 to the battery recharge time t2, and F as the ratio of the peak power P1 to the base power P2, the degree of hybridisation can be represented as a function of T and F, as in equation (5).
Figure 2 shows a map that relates the power profiles to the degree of hybridisation. The map shows that as F increases, DOH increases. The increase of DOH with F becomes more prominent in the region of very small T, i.e. DOH has a dramatic increase with decreasing T. Graphical representations have been added to the map at various regions to show how the characteristics of the load profile vary across the range of possible missions. It is clear that the critical factor that determines the DOH is the difference between the peak power and the average power. If a power profile gives an average power which is significantly different from the peak power, then the DOH is high (which means the battery needs to supply a higher power than that fuel cell does), and vice versa. 1 0.9
0.8
0.8
0.6 DOH
The Degree of Hybridisation
0.7 0.4 0.6 0.2
0.5
0
0.4
3
10
0.3 2
10
0.2
F
0.1
1
10
t1
0
10
-3
10
-2
10
0
-1
10
10
1
10
2
10
3
10
T
P1 PFC
Figure 2: A map showing the degree of hybridisation (DOH) and the power profiles
t2 P2
A Hybrid Fuel Cell / Battery Power System for a Lightweight Underwater Vehicle
t
Figure 1: A generic power profile
P1t 1+ P2t 2 t1 + t2 t T= 1 t2
PFC =
F=
P1 P2
DOH =
P1 ≥ P2
(2) (3) F ≥1
(4)
P1 − PFC F −1 = P1 F (T + 1)
(5)
UUV Description and its Power Profile In order to examine issues relating to fuel and oxidant storage as well as system design issues, representative parameters for a lightweight UUV have been chosen for our analysis and based on a particular mission power cycle. The analysis performed here is solely based on the mass and energy balance of the system. UUV parameters are given in Table 1. The UUV
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has a limited space for the energy section. The neutral buoyancy requirement for the UUV determines the limited weight of the energy section. This means that the components of the power system need to be compact enough, in order to achieve a long mission duration. Table 1: Parameters for a light weight UUV Lightweight vehicle (LWV) External diameter
12.75 inch (32.4 cm)
showing that a high power demand is required from the battery. It is worth noting that, as the pulse power lasts for ten minutes at 6700 W, which corresponds to a discharge energy of 1117 Wh from the battery, a super-capacitor is not suitable to provide the peak power, since present technology is not able to store that amount of energy in a device that would fit within a UUV. The maximum energy density of current super-capacitor technology is only of the order of 5 Whkg-1. 8000
Internal diameter
11.26inch (28.6 cm)
Length
148 inch (3.76m)
7000 6000
Weight
~500 lbs (227 kg)
Payload
92.6 kg
Power (W)
5000 4000 3000 2000
Operation depth Energy
200 m
Volume
1000
24% Vtotal = 57.94L
0 0
10
20
30
40
50
60
70
80
90
100
Time (h)
Section
Weight
24%Vtotal*1025kg/m3 =59.39kg
The power profile for the UUV is given in Figure 3, which shows periodic high power demand. The number of activity cycles determines the mission length of the UUV. The degree of hybridisation is 0.928,
Figure 3: The power profile for a light weight UUV. Red lines indicate the propulsive power, green lines indicate the payload power, and the black dotted line indicates the average power (i.e. fuel cell power)
H2 Tank Circulation Pump
Coolant Pump
Fan
Coolant Pump
Humidifier Constant Voltage Regulation system
PEMFC
Circulation Pump
Humidifier
Smart Battery Charger
Battery
Electricity Supply Regulation System
O2 Tank
Power Supply
Figure 4: System architecture of the hybrid fuel cell/battery power system for UUVs
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System Architecture Based on the power profile in Figure 3, we proposed the power system architecture of the hybrid fuel cell / battery power system for the UUV, as shown in Figure 4. A polymer electrolyte fuel cell (PEFC) is chosen to be hybridised with a battery to supply the power for the UUV. Hydrogen and oxygen are supplied directly from a H2 tank and O2 tank respectively, which simplify the system. Note that a H2 tank in this context represents hydrogen in compressed cylinders, in cryogenic cylinders, or in the form of a metal hydride or complex hydride, which can release hydrogen directly. The rationale for choosing the type battery and fuel cell, as well as the method of hydrogen and oxygen storage, is discussed in more detail later. To cool the PEFC, water is driven through the stack using pumps to transfer the excess heat from the fuel cell to the hull and subsequently to the submarine environment. The hybrid system also includes a constant voltage regulation system, a smart battery charger, and an electricity supply regulation system, which all require careful design, but are beyond the scope of this study. It is worth noting that the possibility of using hydrocarbon fuels to supply hydrogen to the fuel cell has also been considered. However, using these fuels adds complexity, weight and volume to the system. A reformer is needed to convert hydrocarbon fuels into hydrogen rich fuels. As the UUV is a closed system (i.e. no gases are allowed to be released outside of the UUV), a CO2 capture stage would also be required. An analysis was performed on the mass and energy balance of such a system operating on hydrocarbon fuel, which predicted a reduced mission length compared to using direct hydrogen. This is mainly because the reformer and the CO2 capture stage add to the weight of the
UUV. We do not report the results here due to the limited space available. The 2007 report on ‘Cost analysis of PEM
fuel cell systems for transportation’ by the National Renewable Energy Laboratory (USA) [2] also showed that, compared to a reformate system, the use of direct hydrogen increased the power density of the stack and the fuel utilization, leading to a reduction in stack size and an increase in system efficiency. Battery Design Based on the power profile in Figure 3, the energy required from the battery is calculated taking into account the efficiency of the battery and the discharge limit. The efficiency of the battery is a function of the charge / discharge rate and the state of charge [3]. Here we have taken an average efficiency of 70%. It is not conducive to a battery’s performance to be fully discharged. Hence, in practice, a lower limit is set below which a battery should not be discharged, in this case it is taken to be 80% of full discharge. Given the constraint of battery efficiency and discharge limit, the size of the battery is calculated as follows: Stored energy = energy consumed × 1 × 1 0.7 0.8
For the duty cycle shown in Figure 3, the energy needed to be stored in the battery is 2875 Wh. The weight and volume of the battery system capable of storing this energy is calculated based on available values for the specific density (Whkg-1) and energy density (Wh l-1). Five commercially available batteries are compared. The technologies all have a track record of submarine operation, for example, the Norwegian Navy have used Ni-Cd, NiMH and lithium-ion batteries for their HUGIN submarines [4]. Table 2 lists the characteristics of these batteries and Figure 5 compares the weight and volume
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of each technology. On a weight and volume basis, it is clear that lithium-ion batteries are best. Since these batteries can be packed from relatively small cells, they are easily configured to fit within most UUV hull shapes. From a practical perspective, lithium-ion batteries need careful maintenance and should never be depleted to below their minimum voltage. To ensure this does not occur, a battery management unit (BMU) is employed which measures all cell voltages, the battery temperature and the battery current. It also keeps track of the charge balance of the battery, cycle count, production number, etc. Examples of BMUs for submarines have been reported [4].
VRLA
Ni-Cd
NiMH
Li-ion
ZEBRA
Specific energy (Wh kg-1)
34
45
65
120
120
Energy density (Wh L-1)
120
110
135
230
140
Specific power (W kg-1)
75
120
90
220
180
Selfdischarge per month (%)
8
10
30
5
None
Nameplate cycle life
500
1000
500
>1000
2000
Efficiency (%)
70
80
80
85
90
-15~45
-40~70
-30~70
-20 ~60
275~350
105175
200300
250350
2501000
70-270
Cost (£ kW h-1)
100 90 80 70
Weight (kg) Volume (L)
60 50 40 30 20 10 0
VRLA
Ni-Cd
NiMH
ZEBRA
Lithium-ion
Figure 5: Estimation of the weight and volume of the battery required for the example system in Figure 4
Table 2: Comparison of different battery technologies
Operation temperature (oC)
PNU100-V56-C09 from Modular Energy Devices would require a battery package of 24.6 kg and 7 l, whereas SAFT claim to be able to provide the same performance using their VL34P module with a weight of 16 kg and 9 l volume. Our continued analysis is based on the SAFT figures.
The performance of current lithium-ion battery technologies varies significantly between manufactures. Based on the energy requirement of the system in Figure 3, the
Fuel Cell System A polymer electrolyte fuel cell (PEFC) is chosen for our system design model for a number of reasons. The PEFC operates at low temperature, up to 80°C. This low temperature operation offers almost instantaneous power output, resulting in rapid start-up, and making the PEFC ideally suited for transportation applications. Compared with other low temperature fuel cell technologies such as the alkaline fuel cell, the PEFC is better developed and is commercially available from manufacturers such as Ballard, who can provide systems ranging in power from ~1 kW to over 100 kW. Several papers have addressed the use of PEFCs in UUVs [5,6]. The advantage of using a PEFC in our design is that a water cooling system is enabled, so avoiding the need for air; furthermore, the cooling effect of water is much greater, and a low-power pump would be sufficient for driving the coolant flow. In contrast, high temperature fuel cells such as the solid oxide fuel cell (SOFC) operate at temperatures of ~500-
3rd SEAS DTC Technical Conference - Edinburgh 2007
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1000°C, and require large amounts of air for cooling and are therefore less suitable for use in an air independent system.
Table 3: Hydrogen storage methods. The gravimetric density ρm, the volumetric density ρv, the working temperature T and pressure P. RT refers to room temperature.
The system efficiency ηsystem of a fuel cell operating on hydrogen is given by: [7]
Storage method
ρm (kg H2/kg)
ρv (kg H2 m-3)
High pressure gas cylinders [8]
0.012
ηsystem =
Pnet ⋅ t HHVH2 ⋅ n H2
(6)
where Pnet is the usable power, t is the operation time, HHVfuel is the higher heating value of hydrogen, and nH2 is the moles of hydrogen consumed. Pnet is the average power (500 W) derived from the power cycle in Figure 3, plus the power consumed by the auxiliaries such as pumps and fans (estimated to be 100 W), giving a total of 600 W. In this work, we assume that the PEFC operates at its rated power with an efficiency of ηsystem = 50%, which is in accord with the US DOE target for PEFCs in transportation applications [2]. Hydrogen Storage Hydrogen storage is a key enabling technology for PEFCs – the ability to supply hydrogen as required and the storage capacity of a hydrogen storage unit is the main factor in determining the range of the vehicle. In this work, we compare several commercially available and potentially available hydrogen storage methods, as summarised in Table 3. In addition to those listed, other technologies exist, including sodium boronhydride, lithium amide and magnesium hydride. We do not consider these methods here since reliable estimates of storage capacity are not available. It is difficult to say which hydrogen storage technique is the best, as each of them has its advantages and disadvantages. In this study, energy storage capacity is of primary concern. Based on equation (6) and the parameters given in Table 3, the weight and volume of the various hydrogen storage technologies are compared later, in the context of the analysis of the whole system.
o
T C
P bar
Remarks
16
RT
200
0.032
14
RT
350
Compressed gas, in lightweight composite cylinders
0.055
25
RT
700
0.13
a
Abstract A system-level design and analysis of the power system for a lightweight unmanned underwater vehicle (UUV) is presented with recommendations of viable technologies that can meet the UUV mission requirements. A hybrid fuel cell / battery system is designed to power the UUV as it has advantages over a pure fuel cell or battery system. The power system is designed to use a lithium-ion battery hybridised with a polymer electrolyte fuel cell. The analysis is focused on the mass, size, and the energy balance of the system components. It is shown that hydrogen and oxygen storage systems dominate the mass and volume of the energy system compared to the fuel cell and battery. Liquid oxygen is recommended for oxidant storage based on the mission length requirement. Keywords: Fuel Cell, Battery, Underwater Vehicle, System Design and Analysis Introduction Unmanned underwater vehicles (UUVs) are ideally suited to provide surveillance, remote sensing and communication relay capabilities for both military and civilian applications. Practical examples include oceanographic data gathering, environmental monitoring, mine detecting and coastal defence. The power system of a UUV has long been a major consideration in designing and manufacturing these vehicles for particular missions. This is because the power system usually determines the ultimate performance (e.g. endurance, cruising speed and distance) of a UUV. The work reported here aims to investigate viable power system architectures that meet the requirement of UUVs. Stealth is the highest design priority of a UUV as it enables the UUV to operate anywhere, at any time, without being detected. Besides helping to avoid detection, stealth enhances a submarine’s ability (by eliminating / reducing selfnoise) to detect targets. To meet the stealth requirement, an air independent power
(AIP) system is beneficial to UUVs. The ideal AIP source for a submarine will be quiet, have a low thermal signature, will not need to discharge anything from the submarine system, and will of course be capable of operating without atmospheric air. In its simplest form, the AIP power source is a battery. However, batteries alone encounter technology difficulties for use as the power source of UUVs, as current battery technologies cannot provide sufficient endurance to allow for large area coverage and short turnaround time between missions. Hybrid fuel cell / battery systems have a number of advantages over either stand-alone fuel cells or batteries. For example, the battery would enable instant cold-start operation whilst the fuel cell was initiating. The battery, as the dynamic energy storage device, would supply peak and pulse power and power for start-up of the hybrid system. The fuel cell, as the device that converts the energy from the fuel, supplies base-load power and recharges the battery. A hybrid system would allow both components to be of smaller dimensions and operate with higher efficiency, since neither would have to provide the full load power.
3rd SEAS DTC Technical Conference - Edinburgh 2007
C14
The hybridisation of fuel cell and battery depends on the system power requirement. To characterise this, it is useful to define a variable called ‘the degree of hybridisation’ as follows [1] : DOH =
Battery power Battery power + Fuel cell power
(1)
For the generic power profile as shown in Figure 1, which is composed of cyclic periods of high and low power requirement both of variable time, the hybridisation of the fuel cell and battery intends for the fuel cell to supply the constant average power, and then has the battery to supply the peak power that is above the average power. The battery will then be recharged during the period when the propulsive power is below the average power. If we define T as the ratio of the battery discharge time t1 to the battery recharge time t2, and F as the ratio of the peak power P1 to the base power P2, the degree of hybridisation can be represented as a function of T and F, as in equation (5).
Figure 2 shows a map that relates the power profiles to the degree of hybridisation. The map shows that as F increases, DOH increases. The increase of DOH with F becomes more prominent in the region of very small T, i.e. DOH has a dramatic increase with decreasing T. Graphical representations have been added to the map at various regions to show how the characteristics of the load profile vary across the range of possible missions. It is clear that the critical factor that determines the DOH is the difference between the peak power and the average power. If a power profile gives an average power which is significantly different from the peak power, then the DOH is high (which means the battery needs to supply a higher power than that fuel cell does), and vice versa. 1 0.9
0.8
0.8
0.6 DOH
The Degree of Hybridisation
0.7 0.4 0.6 0.2
0.5
0
0.4
3
10
0.3 2
10
0.2
F
0.1
1
10
t1
0
10
-3
10
-2
10
0
-1
10
10
1
10
2
10
3
10
T
P1 PFC
Figure 2: A map showing the degree of hybridisation (DOH) and the power profiles
t2 P2
A Hybrid Fuel Cell / Battery Power System for a Lightweight Underwater Vehicle
t
Figure 1: A generic power profile
P1t 1+ P2t 2 t1 + t2 t T= 1 t2
PFC =
F=
P1 P2
DOH =
P1 ≥ P2
(2) (3) F ≥1
(4)
P1 − PFC F −1 = P1 F (T + 1)
(5)
UUV Description and its Power Profile In order to examine issues relating to fuel and oxidant storage as well as system design issues, representative parameters for a lightweight UUV have been chosen for our analysis and based on a particular mission power cycle. The analysis performed here is solely based on the mass and energy balance of the system. UUV parameters are given in Table 1. The UUV
3rd SEAS DTC Technical Conference - Edinburgh 2007
C14
has a limited space for the energy section. The neutral buoyancy requirement for the UUV determines the limited weight of the energy section. This means that the components of the power system need to be compact enough, in order to achieve a long mission duration. Table 1: Parameters for a light weight UUV Lightweight vehicle (LWV) External diameter
12.75 inch (32.4 cm)
showing that a high power demand is required from the battery. It is worth noting that, as the pulse power lasts for ten minutes at 6700 W, which corresponds to a discharge energy of 1117 Wh from the battery, a super-capacitor is not suitable to provide the peak power, since present technology is not able to store that amount of energy in a device that would fit within a UUV. The maximum energy density of current super-capacitor technology is only of the order of 5 Whkg-1. 8000
Internal diameter
11.26inch (28.6 cm)
Length
148 inch (3.76m)
7000 6000
Weight
~500 lbs (227 kg)
Payload
92.6 kg
Power (W)
5000 4000 3000 2000
Operation depth Energy
200 m
Volume
1000
24% Vtotal = 57.94L
0 0
10
20
30
40
50
60
70
80
90
100
Time (h)
Section
Weight
24%Vtotal*1025kg/m3 =59.39kg
The power profile for the UUV is given in Figure 3, which shows periodic high power demand. The number of activity cycles determines the mission length of the UUV. The degree of hybridisation is 0.928,
Figure 3: The power profile for a light weight UUV. Red lines indicate the propulsive power, green lines indicate the payload power, and the black dotted line indicates the average power (i.e. fuel cell power)
H2 Tank Circulation Pump
Coolant Pump
Fan
Coolant Pump
Humidifier Constant Voltage Regulation system
PEMFC
Circulation Pump
Humidifier
Smart Battery Charger
Battery
Electricity Supply Regulation System
O2 Tank
Power Supply
Figure 4: System architecture of the hybrid fuel cell/battery power system for UUVs
3rd SEAS DTC Technical Conference - Edinburgh 2007
C14
System Architecture Based on the power profile in Figure 3, we proposed the power system architecture of the hybrid fuel cell / battery power system for the UUV, as shown in Figure 4. A polymer electrolyte fuel cell (PEFC) is chosen to be hybridised with a battery to supply the power for the UUV. Hydrogen and oxygen are supplied directly from a H2 tank and O2 tank respectively, which simplify the system. Note that a H2 tank in this context represents hydrogen in compressed cylinders, in cryogenic cylinders, or in the form of a metal hydride or complex hydride, which can release hydrogen directly. The rationale for choosing the type battery and fuel cell, as well as the method of hydrogen and oxygen storage, is discussed in more detail later. To cool the PEFC, water is driven through the stack using pumps to transfer the excess heat from the fuel cell to the hull and subsequently to the submarine environment. The hybrid system also includes a constant voltage regulation system, a smart battery charger, and an electricity supply regulation system, which all require careful design, but are beyond the scope of this study. It is worth noting that the possibility of using hydrocarbon fuels to supply hydrogen to the fuel cell has also been considered. However, using these fuels adds complexity, weight and volume to the system. A reformer is needed to convert hydrocarbon fuels into hydrogen rich fuels. As the UUV is a closed system (i.e. no gases are allowed to be released outside of the UUV), a CO2 capture stage would also be required. An analysis was performed on the mass and energy balance of such a system operating on hydrocarbon fuel, which predicted a reduced mission length compared to using direct hydrogen. This is mainly because the reformer and the CO2 capture stage add to the weight of the
UUV. We do not report the results here due to the limited space available. The 2007 report on ‘Cost analysis of PEM
fuel cell systems for transportation’ by the National Renewable Energy Laboratory (USA) [2] also showed that, compared to a reformate system, the use of direct hydrogen increased the power density of the stack and the fuel utilization, leading to a reduction in stack size and an increase in system efficiency. Battery Design Based on the power profile in Figure 3, the energy required from the battery is calculated taking into account the efficiency of the battery and the discharge limit. The efficiency of the battery is a function of the charge / discharge rate and the state of charge [3]. Here we have taken an average efficiency of 70%. It is not conducive to a battery’s performance to be fully discharged. Hence, in practice, a lower limit is set below which a battery should not be discharged, in this case it is taken to be 80% of full discharge. Given the constraint of battery efficiency and discharge limit, the size of the battery is calculated as follows: Stored energy = energy consumed × 1 × 1 0.7 0.8
For the duty cycle shown in Figure 3, the energy needed to be stored in the battery is 2875 Wh. The weight and volume of the battery system capable of storing this energy is calculated based on available values for the specific density (Whkg-1) and energy density (Wh l-1). Five commercially available batteries are compared. The technologies all have a track record of submarine operation, for example, the Norwegian Navy have used Ni-Cd, NiMH and lithium-ion batteries for their HUGIN submarines [4]. Table 2 lists the characteristics of these batteries and Figure 5 compares the weight and volume
3rd SEAS DTC Technical Conference - Edinburgh 2007
C14
of each technology. On a weight and volume basis, it is clear that lithium-ion batteries are best. Since these batteries can be packed from relatively small cells, they are easily configured to fit within most UUV hull shapes. From a practical perspective, lithium-ion batteries need careful maintenance and should never be depleted to below their minimum voltage. To ensure this does not occur, a battery management unit (BMU) is employed which measures all cell voltages, the battery temperature and the battery current. It also keeps track of the charge balance of the battery, cycle count, production number, etc. Examples of BMUs for submarines have been reported [4].
VRLA
Ni-Cd
NiMH
Li-ion
ZEBRA
Specific energy (Wh kg-1)
34
45
65
120
120
Energy density (Wh L-1)
120
110
135
230
140
Specific power (W kg-1)
75
120
90
220
180
Selfdischarge per month (%)
8
10
30
5
None
Nameplate cycle life
500
1000
500
>1000
2000
Efficiency (%)
70
80
80
85
90
-15~45
-40~70
-30~70
-20 ~60
275~350
105175
200300
250350
2501000
70-270
Cost (£ kW h-1)
100 90 80 70
Weight (kg) Volume (L)
60 50 40 30 20 10 0
VRLA
Ni-Cd
NiMH
ZEBRA
Lithium-ion
Figure 5: Estimation of the weight and volume of the battery required for the example system in Figure 4
Table 2: Comparison of different battery technologies
Operation temperature (oC)
PNU100-V56-C09 from Modular Energy Devices would require a battery package of 24.6 kg and 7 l, whereas SAFT claim to be able to provide the same performance using their VL34P module with a weight of 16 kg and 9 l volume. Our continued analysis is based on the SAFT figures.
The performance of current lithium-ion battery technologies varies significantly between manufactures. Based on the energy requirement of the system in Figure 3, the
Fuel Cell System A polymer electrolyte fuel cell (PEFC) is chosen for our system design model for a number of reasons. The PEFC operates at low temperature, up to 80°C. This low temperature operation offers almost instantaneous power output, resulting in rapid start-up, and making the PEFC ideally suited for transportation applications. Compared with other low temperature fuel cell technologies such as the alkaline fuel cell, the PEFC is better developed and is commercially available from manufacturers such as Ballard, who can provide systems ranging in power from ~1 kW to over 100 kW. Several papers have addressed the use of PEFCs in UUVs [5,6]. The advantage of using a PEFC in our design is that a water cooling system is enabled, so avoiding the need for air; furthermore, the cooling effect of water is much greater, and a low-power pump would be sufficient for driving the coolant flow. In contrast, high temperature fuel cells such as the solid oxide fuel cell (SOFC) operate at temperatures of ~500-
3rd SEAS DTC Technical Conference - Edinburgh 2007
C14
1000°C, and require large amounts of air for cooling and are therefore less suitable for use in an air independent system.
Table 3: Hydrogen storage methods. The gravimetric density ρm, the volumetric density ρv, the working temperature T and pressure P. RT refers to room temperature.
The system efficiency ηsystem of a fuel cell operating on hydrogen is given by: [7]
Storage method
ρm (kg H2/kg)
ρv (kg H2 m-3)
High pressure gas cylinders [8]
0.012
ηsystem =
Pnet ⋅ t HHVH2 ⋅ n H2
(6)
where Pnet is the usable power, t is the operation time, HHVfuel is the higher heating value of hydrogen, and nH2 is the moles of hydrogen consumed. Pnet is the average power (500 W) derived from the power cycle in Figure 3, plus the power consumed by the auxiliaries such as pumps and fans (estimated to be 100 W), giving a total of 600 W. In this work, we assume that the PEFC operates at its rated power with an efficiency of ηsystem = 50%, which is in accord with the US DOE target for PEFCs in transportation applications [2]. Hydrogen Storage Hydrogen storage is a key enabling technology for PEFCs – the ability to supply hydrogen as required and the storage capacity of a hydrogen storage unit is the main factor in determining the range of the vehicle. In this work, we compare several commercially available and potentially available hydrogen storage methods, as summarised in Table 3. In addition to those listed, other technologies exist, including sodium boronhydride, lithium amide and magnesium hydride. We do not consider these methods here since reliable estimates of storage capacity are not available. It is difficult to say which hydrogen storage technique is the best, as each of them has its advantages and disadvantages. In this study, energy storage capacity is of primary concern. Based on equation (6) and the parameters given in Table 3, the weight and volume of the various hydrogen storage technologies are compared later, in the context of the analysis of the whole system.
o
T C
P bar
Remarks
16
RT
200
0.032
14
RT
350
Compressed gas, in lightweight composite cylinders
0.055
25
RT
700
0.13
