This chapter examines devices using hydrogen and fuel cells to provide traction power for vehicles or heat and electricity for stationary purposes. Such devices are often denoted “systems”, but the term “system” is used fairly generally, because the distinction between systems and components is not always sharp, and in consequence, systems will be mentioned in many places throughout this book. This chapter is concentrating on a systematic overview of several types of systems employing hydrogen and fuel cells, constituting comprehensive aggregates of components serving particular demands, such as providing personal or freight transportation or providing heat and power to a building. These individual systems will in Chapter 5 be combined to form nation-wide or global networks of interconnected energy supply systems, this being another conventional use of the term “system”. The reuse of the term may be defended as being less ambiguous than alternatives such as “economies” or “societies”, often found in constellations like “hydrogen economy” or “hydrogen society”.
4.1 Passenger cars 4.1.1 Overall system options for passenger cars The simplest system for using hydrogen in a passenger car, not considering straight combustion (see section 2.3.3), has a fuel storage tank, a fuel cell, and an electric motor. The electric motor is rated at the maximum power required by the vehicle, and as there is no traction battery included, the fuel cell must be able to deliver the same power input to the motor, while the hydrogen store must be large enough to give the vehicle the desired range. In case fuels other than hydrogen are used, the fuel cell must be capable of
accepting them (direct methanol fuel cell, etc.), or they must be converted to hydrogen, for example by use of a reformer (natural gas, gasoline, methanol, etc.; see Chapter 2, Fig. 2.27). The storage tank is now accommodating the fuel of choice. A control system administers the flows of fuel and timing of the components’ functions. There should in most cases also be a water management system, capable of keeping the fuel cell (if it is of PEM type) at the proper humidity. In many cases, it is not convenient to use hydrogen for cold starts, and a start-battery is used to furnish start-up power. This could be a conventional car battery (of lead-acid type) with modest storage capacity, but generally, the functionality will be improved by using a larger, high-voltage battery.
Figure 4.1. Various hybrid car concepts. (From G. Suppes, S. Lopes, C. Chiu (2004). Plug-in fuel cell hybrids as transition technology to hydrogen infrastructure. Int. J. Hydrogen Energy 29, 369-374. Used by permission from Elsevier.)
When a battery aimed at traction is added, the system is called a hybrid system. Now the power required at the wheels is delivered either by the fuel cell or by the energy already stored in the battery. In hybrid systems, the option of delivering power directly from the fuel cell to the electric engine (termed parallel operation) is not mandatory, as the fuel cell may deliver all its power output through the battery (serial operation). Whether this is the case or not, the hybrid concept allows the fuel cell rating to be smaller than that of the engine. One option is in this case to operate the fuel cell at a constant power level and have the battery charge up when the power is not needed for traction. An alternative is to allow battery charging off-road, for example, when
the car is parked or at a filling station. Such fuel cell vehicles are called plugin hybrids (Bitsche and Gutmann, 2004; Suppes et al., 2004). Figure 4.1 shows some possible hybrid layouts. For plain combustion hydrogen vehicles, the components include the engine and usually a liquid hydrogen store (to get sufficient range). The control equipment must therefore also comprise an exhaust system capable of safely handling the hydrogen boil-off from such a store (Ochmann et al., 2004). The specific problems and optimising issues arising when using hydrogen in an internal combustion engine are discussed by Verhelst and Wallner (2009).
Figure 4.2. Schematic layout of power system for a PEMFC vehicle. (From R. Ahluwalia, X. Wang, A. Rousseau, R. Kumar (2004). Fuel economy of hydrogen fuel cell vehicles. J. Power Sources 130, 192-201. Used by permission from Elsevier.)
In assessments of energy efficiency, it is important to include all components of the system. Each energy-converting device is characterised by a conversion efficiency (energy output over energy input), as well as an exergy efficiency (free energy out over free energy in), with the latter reflecting the quality of energy (Sørensen, 2010a). The fuel-to-electricity efficiency of various fuel cell types is 30-70% (Chapter 3), but to this comes the upstream efficiency of producing the fuel and the downstream efficiency of using it. Hydrogen production from fossil fuels or biofuels has an efficiency of 45-80%,
while production from electricity is at 60-90% efficiency (Chapter 2). Further, the efficiency characterising the production of electricity in the first place may add another factor of 30-45% to the efficiency, if the basis for electricity is fossil resources. For renewable energy sources, such as wind power or photovoltaic panels, the conversion efficiency is usually not included in this context because the primary source is “free”. Downstream efficiencies ranges for automobile traction are typically 35-45% (Chapter 6), while for electric lights and appliances, nearly the entire scale of efficiencies is met in various actual devices. It follows that the overall efficiency from primary energy to an energy service at the end-user, such as mobility, may be as low as 5%. The positive message contained in this fact is that there is plenty of room for improvement by proper design and combination of components.
Figure 4.3. Placement of fuel cells, hydrogen tank, and auxiliary equipment in the early DaimlerChrysler prototype car Necar 4. (Based on G. Friedlmeier, J. Friedrich, F. Panik (2001). Test experiences with the DaimlerChrysler fuel cell electric vehicle NECAR 4. Fuel Cells 1, 92-96. Used by permission from Wiley.)
4.1.2 PEM fuel cell and fuel cell hybrid cars As most current automotive fuel cell efforts use proton exchange membrane
(PEM) fuel cells, these will be described in a little more detail in this section and will be used as templates for the performance calculations presented in section 4.1.3. A typical passenger car pure (i.e., not hybrid) PEM fuel cell system is outlined in Fig. 4.2. Included are heaters for bringing the equipment from ambient temperatures to the operating temperature of around 80°C and humidifiers for ensuring the level of water in the membrane and electrode areas required for operation (see Chapter 3, Figs. 3.50, 3.51, and 3.60). The water management equipment includes a condenser, which is integrated into a conventional radiator, but operating at much lower temperatures than that of internal combustion engine cars.
Figure 4.4. The General Motors skateboard concept, having all power equipment and physical controls placed in a flat frame construction placed below the passenger cabin, and all driving control instructions transmitted electronically from the cabin to the board equipment. (From M. Herrmann and J. Meusinger (2003). Hydrogen storage systems for mobile applications. Presented at 1st European Hydrogen Energy Conf., Grenoble. Used by permission from GM.)
The fuel cell equipment adds considerably to the weight of the vehicle, implying lower efficiency but also the possibility of increased stability, achieved by placing the heavy equipment low in the vehicle construct. Figure 4.3 shows the under-floor placement of fuel cells, storage tanks, and auxiliary equipment in the Necar 4 hydrogen-fuel prototype fuel cell car of DaimlerChrysler, making it much more stable than its commercial combustion engine cousin, the commercial Mercedes-Benz A-class car. While the Necar 4 has a 75-kW fuel cell stack, the newer DaimlerChrysler 0-series f-cell car (see Chap-
ter 6, section 6.2.4) uses a 85 kW Mark-902 fuel cell stack from Ballard Power Systems. Use of these huge power systems (relative to the size of the class-A cars, normally working with a 40-50 kW diesel engine) is considered necessary because of the extra weight of the fuel cell-related equipment, and in order not to attract any criticism for lowering the “sporty” performance. A larger, B-class version of the f-cell has been built, featuring smaller volume fuel tanks by use of 70 MPa compressed hydrogen rather than the earlier 35 MPa (Orecchini and Santiangeli, 2010). An even more advanced step is the concept first proposed by General Motors (Fig. 4.4), where not only all fuel cell equipment is placed below the passenger cabin, but this “skateboard” is totally isolated from the cabin and receiving all instructions electronically (for steering, for braking, and for accelerating). Achieving optimum control is being assisted by having not just one but four electric motors, one at each wheel. The concept is used in prototype hybrid cars such as Sequel (with lithium-ion batteries) and HydroGen4 (with nickel-metal hydride batteries; Eberle and Helmolt, 2010). Other car manufacturers, such as Toyota, in their prototype fuel cell vehicles have placed the fuel cell stacks more conventionally in what traditionally constitutes the engine space in front of the passenger cabin (Takimoto, 2004). The methanol-reformer systems (e.g., DaimlerChrysler’s Necar 5) were for some time developed in parallel with the direct hydrogen fuel cell vehicles, considering that the advantages of only needing minor changes in fuelling infrastructure would outweigh the somewhat lower overall efficiency (Boettner and Moran, 2004). However, technical problems with the reformer performance have currently brought this line of development to a halt. Direct methanol fuel cell vehicles are still under development, for example, by DaimlerChrysler (Lamm and Müller, 2003). Basic performance modelling of PEM fuel cells was undertaken in Chapter 3. Further such studies, directed specifically at thermal performance in automotive applications, may be found in Nolan and Kolodziej (2010). A considerable number of investigations have been carried out in an attempt to determine the best control strategies for vehicles with PEM fuel cells with or without a traction battery or other complementary power-providing device such as a flywheel or a capacitor (fuel cell control, Al-Durra et al., 2010; hybrid system without plug-in option, Bernard et al., 2010, Ryu et al., 2010, Fadel and Zhou, 2011; hybrid with plug-in capability, Bubna et al., 2010a). Some 5% fuel saving may typically be obtained by refining the simpler control strategies. As mentioned, the hybrid system may be either serial (all fuel cell power is delivered through the energy store) or parallel (the electric motor may be powered directly by the fuel cell or by the storage). The hybrid systems studied in the references just quoted are parallel systems. An example of a serial system is given in section 4.1.3.
Hybrid systems combine hydrogen fuel cells with electrochemical energy stored, typically in a battery, and thus constitute a natural continuation of the technology choice embedded in current fossil fuel-battery hybrid vehicles such as the Toyota Prius and the similar Lexus hybrids (gasoline and nickelmetal-hydride battery; Orecchini and Santiangeli, 2010). Plug-in hybrids using advanced lithium-ion batteries include the General Motors Volt and Ampera. The Li-ion batteries are also the choice for the range of commercial purely electric vehicles marketed by Renault and by Citroën/Peugeot, because of their excellent capability for following complex and demanding driving cycles (Corbo et al., 2010). As with Li-ion batteries for small electronic products, there is a safety issue usually solved by a self-releasing emergency vent (Arora et al., 2010). Because the reaction time of a fuel cell is slower than that of most batteries, there is usually a modest-size battery even in “pure” fuel cell vehicles, aimed to provide better performance during changing operating condition (such as acceleration or deceleration). This function could conceivably be taken over by other technologies, such as capacitors. Flywheels have been considered but for safety reasons, they are more appropriate for stationary applications, where they can be placed underground. Capacitors, on the other hand, can display extremely high power densities (compared with usually adequate and sufficient batteries) and thus provide the thrills of motorcar racing and reckless driving. Control systems for capacitors and supercapacitors integrated into fuel cell drive trains with or without batteries have been considered by Ayad et al. (2010) and by Lin and Zheng (2011). Incorporating all three expensive components, fuel cells, batteries and supercapacitors, may well be overkill (Yu et al., 2011). In the opposite vein one finds the suggestion that pure battery hybrids may perform well enough to be used for road vehicles, at the extreme without use of a voltage converter (a device that allows battery terminal and motor bus voltages to be different) to maximise efficiency (Bernard et al., 2011). While early prototype passenger pure fuel cell vehicles were rated at about 100 kW, the advantage of hybrids is that the expensive fuel cells do not have to provide the peak power. The smallest vehicles may need only a few kW of rated fuel cell power (Tang et al., 2011) and a four-person car 10-20 kW (see section 4.1.3). At the same time, the presence of the fuel cell makes it possible to do with less battery capacity, affecting another expensive component. One purpose of the simulation studies below is to explore the optimum rating of the fuel cell and battery in a hybrid vehicle.
4.1.3 Performance simulation In parallel with actual tests on the road, simulations are used by both scientists and auto manufacturers as a means of getting a first orientation at low
cost, both before a new car is actually constructed and also during the sequences of testing and revision phases. Here, a brief simulation study will be made in order to illustrate the capability of simplified, but fairly realistic, model assumptions. Models are available, capable of simulating the behaviour of various vehicle types including those with conventional propulsion systems using Otto or Diesel engines, pure electric vehicles or pure fuel cell vehicles as well as hybrids among any of these, either by a detailed physical modelling or by a semi-empirical method, where different processes are simply parametrised and the parameters are adapted to measured data. The latter method is presented here, based on the software ADVISOR originally developed at the US Renewable Energy National Laboratory (Markel et al., 2002). The users can write their own subroutines, or use the existing collection of parametrised models for fuel cell stacks, electric motor, battery energy storage, power-train control in fuel cell cars with battery, exhaust control, power behaviour of the wheel/axle system under prescribed driving conditions (slope, road surface, and resistance, etc.), and also auxiliary electric energy use in the vehicle. The ADVISOR programme core uses the required driving speed at a given time to calculate torque, rotational speed, and power in each drive-train element, a procedure called a backward-facing simulation approach. However, this is combined with a forward-facing approach based on the control logic, and the simulation proceeds forward in time, but with backward consistency checks at each step. Program users can both change and add subroutines for specific processes, or change the flow between the modules. The fuel cell modelling is either a simple one where power output and efficiency are linked by an empirical curve such as the one shown in Fig. 4.5, or a combination of relationships for each component in the fuel cell system (Fig. 4.2). More detailed models−for example, treating water management−are available (Markel et al., 2002; Maxoulis et al., 2004). For the battery in hybrid modelling, additional calculations were made with the battery assumed to be rechargeable by the fuel cell. Both conventional lead-acid or nickel-metal-hydride batteries and advanced lithium ion batteries have been modelled, using internal resistance battery models. Also, for the battery segment of the simulation, much more detailed subroutines have been developed for use by technical vehicle designers. The fuel cell vehicle considered in the simulation model is an artefact loosely modelled over the Volkswagen Lupo TDI-3L (see Chapter 6, Table 6.6), so for simplicity, I shall call it Little Red Riding Hood. The 45 kW diesel engine is replaced by a PEM fuel cell engine in a hybrid configuration with battery power, ranging from pure fuel cell operation to a pure electric vehicle. Suitable component rating is initially determined on the basis of efficiency for a given driving cycle rather than (poorly known) cost. This may lead to several
systems of comparable performance.
Figure 4.5. Power curve for a 50 kW PEM fuel cell used in simulations.
The hydrogen fuel consumption for this vehicle is assumed to take the form shown in Fig. 4.6. The fuel efficiency is closer to the target value for current research than to the (lower) efficiency of the early prototypes that have been test-driven over the last decade. Figure 4.6. Hydrogen consumption of the Little Red Riding Hood vehicle assumed in simulation model, as function of power level. Note that the partload performance implied by Figs. 4.5 and 4.6 is nearly independent of power level down to around 1 kW, in clear contrast to the situation for internal combustion engines.
The total mass of the each Little Red Riding Hood configuration has the distribution shown in Table 4.1 (compare to Chapter 6, Table 6.6 for the Diesel Lupo 3L).
Component mass (kg)
Basic vehicle (incl. Li-ion start battery)
Fuel cell equipment (40, 20 and 0 kW)
Li-ion batteries (0, 15, and 250 MJ)
Electric motor (50 kW)
Transmission (manual 1-speed equivalent)
Passengers and cargo (average)
Table 4.1. Mass distribution for selected Little Red Riding Hood fuel cell-electric hybrid vehicles. For lead-acid or Ni-MeH batteries, the battery mass is 2-3 times higher.
Figure 4.7. The driving cycle used in simulations (Sørensen, 2005c, d).
The simulations are employing a mixed driving cycle (Fig. 4.7) put together from pieces of the driving cycles used in the United States and in the European Union for regulatory and taxation purposes and containing both highway driving, suburban stretches, and city driving with frequent stops at red lights. The overall speed frequency distribution of the 89 km route is shown in Fig. 4.8.
Figure 4.8. Frequency distribution of driving speeds for the driving cycle shown in Fig. 4.7.
Figure 4.9. Simulated power output from 40 kW pure fuel cell vehicle.
For a pure fuel cell vehicle rated at 40 kW, the efficiency assumptions of Figs. 4.5 and 4.6 make it possible to traverse the driving cycle of Fig. 4.7 without noticeable deviations from the prescribed speeds along the trip (maximum deviation some 2%). Figure 4.9 shows the power output from this fuel cell, as function of driving time at the specified speeds. For simplicity, no grades have been assumed present. The average energy consumption of the 40 kW fuel cell during the driving-cycle trip is 1.138 MJ/km (equivalent to about 3.5 litre of gasoline per 100 km) (Sørensen, 2010b). Figure 4.9 suggests that even with a 30 kW fuel cell rating, the particular driving cycle could be completed at the required speeds, in which case the fuel consumption would be reduced to 0.855 MJ/km (or 2.7 litre of gasoline-equivalent per 100 km) (Sørensen, 2007b, c). However, in a real car, the higher rating of at least 40 kW would be used, because the driving cycle selected for the study may not represent the actual maximum requirement met on the road (for instance, it does not contain slopes). The range of the vehicle on a 4 kg hydrogen fuel tank (equivalent to 178 litre of hydrogen at a pressure of 30 MPa) would be over 650 km and thus similar to the range of most present passenger cars. However, Fig. 4.9 also shows that the upper-level fuel cell power rating is only required during quite short parts of the trip. This suggests that an advantageous configuration would by a fuel cell-battery hybrid, where the fuel cell is rated at a lower level, such as 20 kW or even down in the range of 5-10 kW, and where the peaks are taken care of by a traction battery feeding power into the electric motor (which at little extra cost can be rated at the higher power level envisaged, here chosen as 50 kW). In the following, the performance of a range of such hybrid configurations will be simulated, using a serial coupling, i.e., that the fuel cell delivers power to the battery and the motor is driven entirely from the battery (avoiding the direct use of electric power from the fuel cell, in a parallel configuration with or without voltage conversion). As a prelude to the model simulation of various hybrid layouts, a pure electric vehicle will be treated, as it forms the end-point opposite to that of the pure fuel cell vehicle considered. Several detailed models of battery performance exist (for instance, Albertus et al., 2008), but only a simplified equivalent Kirchhoff circuit model will be employed here (Johnson, 2001; Liaw and Dubarry, 2010), judged to be sufficient for identifying the general behaviour of hybrid vehicles. For a single battery module of which about a thousand makes up the 250 MJ lithium-ion battery electric vehicle specified in Table 4.1, it predicts the voltage and resistance behaviour illustrated in Figs. 4.10 and 4.11, as function of charge state and for different temperatures. As with most batteries, low temperature (around 0°C) degrades performance, while high ambient temperatures (about 40°C) do not cause significant problems except when the battery is close to being completely discharged. The implications for power delivery is illustrated in Fig. 4.12, suggesting that the best
performance is ensured if the battery is never discharged much under 40%. Finally, Fig. 4.13 shows the round-trip efficiency of the battery at 25 °C, as function of current and state of charge. Except for the poor performance near zero charge, the efficiency is quite high at low currents and declines with increasing currents drawn, but still stays above 70% for currents up to 100 A.
Figure 4.10. Voltage diagram (volt for a given charge state) for a module of a “Saft” Liion battery (Johnson, 2001), for three ambient temperatures.
Figure 4.11a, b. Resistance diagram (ohm for a given charge state) for a module of a “Saft” Li-ion battery (Johnson, 2001), for three ambient temperatures, during charging (a) and during discharging (b).
Figure 4.12. Instantaneous power (W for given state of charge) delivered by a module of a “Saft” Li-ion battery (Johnson, 2001), for three ambient temperatures.
Figure 4.13. Round-trip efficiency as function of current and charge state for a module of a “Saft” Li-ion battery (Johnson, 2001) at ambient temperature 25°C.
Figure 4.14. Simulation of pure electric vehicle based on Little Red Riding Hood with Liion batteries: Net torque delivered by electric motor during the driving cycle.
Figure 4.15. Simulation results for Little Red Riding Hood in a pure electric vehicle version with Li-ion batteries: Power delivered by battery assembly during driving cycle. Negative values represent brake energy recovery.
Figure 4.16. Simulation results for Little Red Riding Hood in a pure electric vehicle version with 250 MJ Li-ion batteries: Battery charge state as function of cycle time. The average energy use−that is, drop in battery stored energy during the driving cycle−is 0.617 MJ/km.
The results of carrying out simulation of the performance of the pure electric Little Red Riding Hood vehicle is shown in Figs. 4.14 to 4.16, giving first the torque and power provided by the battery/motor power train and in Fig. 4.16 the charge state of the battery during the trip. From the total energy drawn from the battery during the trip, an average energy use of 0.617 MJ km-1 can be calculated, which corresponds to an equivalent 2.6 l of gasoline per 100 km. This is better than the energy performance the VW Lupo on which the vehicle is modelled (that has a fuel use of 3 l diesel fuel per 100 km or about 3.3 l gasoline per 100 km, cf. further discussion in Chapter 6). The stated electric vehicle energy use does not include the upstream losses in charging the batteries from some external source of electric power, or the losses that may have occurred in generating that electricity from some primary source. Turning now to fuel cell/battery hybrid vehicles, the results of a simulation for a specific combination of a vehicle equipped with a 20 kW fuel cell and a modest 15 MJ Li-ion battery (as specified in Table 4.1) will be presented. The battery can be recharged by the fuel cell, in the serial configuration assumed. Figures 4.17 and 4.18 show the net power output from the battery and from the fuel cell during the mixed driving cycle of Fig. 4.7. The size of the battery has been chosen so that the charge state at the end of the assumed driving cycle is approximately the same as at the beginning, as illustrated in Fig. 4.19.
For other trips made with the vehicle, this may of course not be the case.
Figure 4.17. Simulation results for Little Red Riding Hood vehicle with 20 kW fuel cells and 15 MJ Li-ion batteries: Power delivered by battery assembly during driving cycle. Negative values represent brake energy recuperation.
Figure 4.18. Simulation results for Little Red Riding Hood vehicle with 20 kW fuel cells and 15 MJ Li-ion batteries: Power delivered by fuel cells during driving cycle.
Figure 4.19. Simulation results for Little Red Riding Hood vehicle with 20 kW fuel cells and 15 MJ Li-ion batteries: Battery charge state as function of cycle time. The average energy drawn from the fuel cell during the driving cycle is 0.796 MJ/km.
It is seen from Fig. 4.19, that particularly during the high-speed highway part of the driving cycle (at 5000-6000 sec), the fuel cell input to the battery is smaller than the power required, and the battery charge state thus declines rapidly. At other times, the fuel cells deliver more power charging the batteries than drawn by the motor. The equivalent gasoline fuel use would be 2.5 l per 100 km, after subtracting a small battery charge state gain at the end of the cycle (the end-point of the curve in Fig. 4.19 is slightly above the startlevel). Lower than 20 kW fuel cell rating leads to insufficient battery charging, implying that the vehicle cannot be electrically autonomous, but has to obtain battery recharging from an external source. The latter situation describes a plug-in hybrid vehicle. The plug-in configurations allow a less costly size of fuel cells and permit recharging of batteries whenever the car is parked at a suitable power outlet (at home, at a working place, or at a special publicly located dispenser charging for a fee). The cost of recharging facilities is typically smaller than that of hydrogen filling stations, but for a plug-in hybrid, both are required. The technical performance of plug-in and self-recharging fuel cell hybrids will be considered following, and guidelines for optimising the economy of the solutions will be dealt with in Chapter 6. The electric motor used in the simulations has an efficiency of 0.92 and a manual 1-speed. In conventional vehicles the transmission often constitutes a significant loss factor, and losses in using 5-speed or automatic transmissions
(Cuddy, 1998) are usually higher than for the 1-speed gearbox. However, the Lupo 3L on which the current vehicle models are based has a computeroperated automatic transmission that actually has a higher efficiency than the corresponding manual transmission, and the 1-speed model was therefore selected in order to avoid distorting the simulation results by use of outdated gearbox technology. Figure 4.20 shows the motor characteristics during motoring and regeneration, for the 50 kW motor used in the simulations shown in Figs. 4.17 to 4.19. The limiting curves with crosses (motoring) or circles (generating) describe torque as function of rotational speed, while the free-standing crosses indicate operating points during the mixed driving cycle assumed in the simulation. Motor efficiencies are indicated inside the operating area with lines and numbers.
Figure 4.20. Torque-speed characteristics of 50 kW electric motor during motoring (× with line) or battery charging (ο with line), motor efficiencies (numbers at lines), and points of operation (crosses) during the Fig. 4.7 driving cycle for a 20 kW fuel cell and 15 MJ Li-ion battery configuration of the Little Red Riding Hood vehicle. More operational points are in the (upper) region of motoring than in the (lower) region of generating.
The time-sequence of the efficiency of the motor-controller system is shown in Fig. 4.21 (equivalent to collecting the efficiencies of the operating points in
Fig. 4.20 and ordering them along the driving cycle of the trip).
Figure 4.21. Efficiency of motor-controller assembly for the 20 kW/15 MJ Little Red Riding Hood vehicle during the mixed driving cycle of Fig. 4.7.
The efficiency of the battery charge and discharge operations stays close to 98% except for the rapid discharge during highway driving (5000-6000 s), where it goes down to about 92%. Also, the fuel cell hydrogen conversion efficiency stays between the maximum of 59% (as assumed, see Fig. 4.5) and a low of 48% during start-up, as shown in Fig. 4.22.
Figure 4.22. Little Red Riding Hood 20 kW fuel cell efficiency over Fig. 4.7 driving cycle.
One interesting feature of the power distribution between fuel cell and bat-
tery (Figs. 4.17 and 4.18) is that the battery contribution is largest during the high-speed motorway driving and occasionally even higher than the fuel cell contribution, and that during the urban sections, fuel cells are providing at least as much power as the batteries. This is a feature of the component rating, where neither power source is sufficient on its own, but it is contrary to the conventional view of (fossil fuel-electric) hybrids, where it is intended that batteries should provide most of the urban driving and the polluting fuel most of the highway driving. For the hydrogen fuel cell-battery design, the division is unimportant, since both components are pollution-free during driving. The simulation takes into account losses from rolling resistance of tyres against the road surface and from aerodynamic formation of turbulence, as well as the losses occurring along the entire drive train from fuel input or stored energy to the final transfer of power to the wheel axle. The size of individual losses during motoring in the mixed driving cycle is shown in Fig. 4.23, and Fig. 4.24 shows the losses during the segments in the driving cycle, where regeneration of charge to the batteries takes place. The motoring losses are dominated by the hydrogen conversion losses, although they are smaller than those of a vehicle with an internal combustion engine. For battery charging, the recovery of brake energy is incomplete, and the highest losses are brake energy not retrieved because it is escaping in the form of heat from braking discs or other equipment. The final step in passenger car modelling is to explore the dependence on the relative component sizes of PEM fuel cells and Li-ion batteries, going from the pure battery over plug-in hybrids to self-charging hybrids (at 20 kW fuel cell rating or higher), ending at the pure fuel cell car. Figure 4.25 shows how the charge state along the mixed driving cycle changes when passing from plug-in to grid-independent vehicles. Figures 4.16 and 4.19 already showed two situations, and in the top panels of Fig. 4.25, it is seen that a 15 kW fuel cell stabilises the (25 MJ) batteries, except during the highway part of the trip, while a 5 kW fuel cell does not change the (125 MJ) battery discharge curve much, relative to the 250 MJ battery discharge curve of Fig. 4.16. In the lower panels, it is seen that with increasing fuel cell rating and decreasing battery rating, the battery charge state excursions become more pronounced. Because a vehicle should also perform well in cases going beyond the particular driving cycle used for fair comparison between different vehicles, one would hardly reduce the battery size to values significantly below 10 MJ (for the four-passenger size of car considered). The range of hybrids considered and their calculated performance are summarised in Table 4.2 and Fig. 4.26.
Figure 4.23. Distribution of total losses during the motoring segments of the mixed driving cycle for the Little Red Riding Hood simulation (in 104 kJ).
Figure 4.24. Distribution of total losses during the battery charging segments of the mixed driving cycle for the Little Red Riding Hood simulation (in kJ).
Figure 4.25. Charge state as function of driving time in the mixed driving cycle used for simulation of selected Li-ion batteries in Little Red Riding Hood hybrid vehicles with increasing fuel cell rating and decreasing battery rating. The two hybrids in the upper panel are of plug-in type, while the two in the lower panel are gridindependent. Note that the vertical scale is different in each panel.
The hybrid simulation summary in Table 4.2 shows the total energy consumption (during driving) through the simulated trip is smallest for a pure battery vehicle, increases slightly for the plug-in hybrids and the gridindependent vehicle with the smallest fuel cell, but then increased with increased rating of the fuel cell, emphasising that electric vehicles are more efficient than even the best fuel cell vehicles. However, the table also shows the mass penalty of large batteries, making the hybrids with small fuel cells and large batteries very heavy and in consequence having shorter rage between recharging of batteries. This range more than doubles from the pure battery car to the 15 kW fuel cell plug-in hybrid car. Once into the self-recharging range, there is little advantage in increasing the fuel cell rating relative to the smallest one making the vehicle grid-independent. The battery size is modest in any case.
Table 4.2. Summary of results for hybrid fuel cell-battery vehicle simulations. Plug-in hybrids Fuel cell rating (kW) Fuel cell system mass (kg) Fuel cell energy use (MJ/km) Battery capacity (MJ) Battery system mass (kg) Battery fuel use (MJ/km) Battery recharging range (km)
0 0 0 250 1136 0.617 405
5 43 0.435 125 567 0.263 468
10 55 0.666 57.5 261 0.1 574
15 68 0.751 25 113 0.028 890
Self-recharging hybrids Fuel cell rating (kW) Fuel cell system mass (kg) Fuel cell energy use (MJ/km) Battery capacity (MJ) Battery system mass (kg)
20 80 0.796 15 68
25 93 0.809 10 45
30 105 0.818 10 45
35 118 0.842 7.5 34
40 130 1.138 0 -
Figure 4.26. Comparison of simulated energy usage for mixed driving cycle traversed by diesel Lupo 3L, plug-in, and self-recharging hybrids (Sørensen, 2010b).
In Fig. 4.26, the energy use values found in the hybrid vehicle calculations are compared to each other and to the case of the same vehicle powered by a
common rail diesel engine (the commercial VW Lupo 3L discussed in Chapter 6). The efficient diesel engine performs slightly better than the pure fuel cell configuration, but all the hybrids have better energy efficiencies, and as expected, the purely electric vehicle has the best efficiency and the plug-in hybrids with large battery components perform better than the gridindependent fuel cell-battery hybrids. The cost implications of these results are discussed further in Chapters 6 and 7.
4.2 Other road vehicles At an early stage, interest in incorporating fuel cells in larger vehicles arose, due to the easier fitting of the (then) bulky equipment as compared with installation in passenger cars. While freight companies have shown little interest, bus companies (often under public control) were among the first to volunteer to test fuel cell technology. Presently, there are about some 100 fuel cell buses driving in regular route patterns in various cities of the world. One positive consideration is that the fixed route driving and use of dedicated filling stations have made it easy to accommodate the limited range of current fuel cell buses and to establish dedicated hydrogen filling stations at suitable locations in the test cities. Typical fuel cell bus layouts have the stacks in the back and compressed hydrogen tanks on top, as illustrated in Fig. 4.27.
Figure 4.27. Hybrid fuel cell bus, showing placement of power equipment (68 kW PEM cell, 2×75 kW motor and NiMH batteries) (MAN, 2004; used by permission.)
Practical experience is accumulating through several ongoing and completed fuel cell bus programmes. Data have been obtained−for example, for a 13-t (including passengers), 52-passenger SCANIA hybrid fuel cell bus with a fuel cell rating of 50 kW and 44 standard 12-V lead-acid batteries capable of delivering power to two 50 kW wheel hub motors (Folkesson et al., 2003). Figure 4.28 shows the energy flows in this system, based on the Braunschweig driving cycle (a multiple stop-go bus cycle with maximum speeds of 35-60 km/h). The bus is equipped with a regenerative breaking system that reduces the fuel consumption with a diesel equivalent of 9.9 l/100 km to 25.8 l/100 km. The special characteristics of the driving pattern cause the modest size power supply system to be sufficient, even with an air conditioning system installed.
Figure 4.28. Average energy flows during a bus driving cycle for a SCANIA hybrid fuel cell bus (based on lower heating value of hydrogen). (From A. Folkesson, C. Andersson, P. Alvfors, M. Alaküla, L. Overgaard (2003). Real life testing of a hybrid PEM fuel cell bus. J. Power Sources 118, 349-357. Used by permission from Elsevier.)
A large fleet of fuel cell buses were put in operation with support from an EU program: about 30 DaimlerChrysler Evobus Citaro F buses (200 kW PEM fuel cells, 1629 l of H2 compressed to 35 MPa), in route application in various European cities, including a range of hydrogen production schemes delivering to the associated filling station (Mercedes-Benz, 2004). Similar demonstration projects have been implemented in other parts of the world, including the United States, Japan, Australia, and China. Operating experience from such programmes is accumulating (e.g., Bubna et al., 2010b), including indications of fuel cell system lifetimes under different control strategies. Figure 4.29 shows the power supply sharing between an 80 kW PEM fuel cell and a Ni-MeH battery for two systems manufactured in China, during operation since 2008 in a Beijing route path. Two patterns of controlling the fuel
cell use characterise the two systems: The Foton-II bus has a load-forecasting strategy, leading to periods of constant fuel cell power production with only fluctuations induced by demand, while the Foton-III bus controls fuel cell operation in an attempted instant load-following mode, causing much more strongly varying power output. It is found that the degradation of model III is larger than that of model II, and the lifetime similarly shorter.
Figure 4.29. Power distributions for hybrid city-buses operated in Beijing. Top: FotonII bus total power (upper curve) and power provided by fuel cell (middle with plateaus), plus power to or from batteries (lower curve). Bottom: Same for Foton-III bus, except that there are no fuel cell power plateaus, due to use of a different control strategy (from Li et al., 2010; used with permission).
Use of fuel cells in two-wheeled road vehicles, such as bicycles and scooters, has been demonstrated (Huang et al., 2009; SiGNa, 2011). Several initiatives have been taken to introduce fuel cells in specialised vehicles, for use not on public roads but privately on the premises of various enterprises. Typical tasks performed include hauling of merchandise within a warehouse (Wilhelm et al., 2011) or transporting disabled persons and luggage in airports (e.g., Munich project). For warehouse applications, fuel cell powered forklifts have been manufactured in small series. Figure 4.30 gives the test performance of such a device comprising a 5 kW PEM fuel cell, an ultracapacitor, and a lead-acid battery. The fuel cell is operated at a fairly constant power level and the capacitor takes care of most of the load variations, except for the period of high load, where the battery contributes.
Figure 4.30. Performance simulation of forklift system with fuel cell, ultracapacitor, and battery: Current contributions during operational cycle at top, voltage at bottom (from Keränen et al., 2011; used with permission).
4.3 Ships, trains, and airplanes The use of fuel cells on ships, as well as direct hydrogen combustion, has been suggested, but so far only tried for smaller pleasure boats and for partial provision of auxiliary power for small equipment happening to be on a boat (Tse et al., 2011). A European project looked at environmental impacts
from using hydrogen on ferries (see Chapter 6), and various proposals for use of PEM or SOFC fuel cells for propulsion have been forwarded. The first one realised (in 2008) was a European Commission-funded 48 kW fuel cell riverboat for use on the Alster at Hamburg (Anon., 2008). An early proposal considers submarines, where air-independent propulsion systems may be of particular importance (Sattler, 2000). Such a submarine would need to carry both hydrogen and oxygen in containers, presumably in liquid form in both cases. Figure 4.31 shows a possible layout of such an arrangement, suitable for retrofitting of existing submarines. Suggestions for dealing with the problems of such designs have recently been made by Nikiforov and Chigarev (2011).
Figure 4.31. Proposed submarine propulsion system based on PEM fuel cells and liquid hydrogen and oxygen. (From G. Sattler (2000). Fuel cells going on-board. J. Power Sources 86, 61-67. Used by permission from Elsevier.)
A project to explore the possibilities for using hydrogen for trains has been undertaken by Tokyo Gas Co. (2004), including assessments of arrangements for fuelling. Specific use of SOFC fuel cells in a hybrid configuration with supercapacitors and batteries has been studied for a switcher locomotive (Guo et al., 2011), taking advantage of its particular driving cycle with little displacement from a fixed location (where filling stations and battery recharging can conveniently be located). A more general railway application simulates the traversal of a 40 km stretch in the UK by a train with a PEM fuel cell (470-670 kW) and a nickel-cadmium battery, and compares various hybrid and pure diesel-power configuration (Meehagawatte et al., 2010). The driving cycle is shown in Fig. 4.32, including occasionally quite substantial grades. These grades seem to present the largest modelling problem, as the parameter settings working well for the main part of the route have difficulties in reproducing performance on the large grades near the end of the trip.
Figure 4.32a, b. Driving cycle for train route between Stratford upon Avon and Birmingham in the UK (a, top: with model simulation results), and grades along the track route (b, bottom). From Meegahawatte et al. (2010); used with permission.
Another possible tracked fuel cell application is for urban tramways, metros, or monorails. Figure 4.33 shows an example of a tramway application, aimed for Seville in Spain (Fernandez et al., 2011). The system consist of PEM fuel cells rated at 254 kW, a motor rated at 540 kW, and a 34 Ah Ni-MeH battery, and Fig. 4.33 shows the urban driving cycle (speed versus time) and the total
motor power drawn during the trip. The fuel cell contribution consists of a series of shifting plateaus of up to 200 kW power generation.
Figure 4.33. Driving cycle for Sevilla tramway, and power requirement (from Fernandez et al., 2011; used with permission).
The use of hydrogen in aircraft gas turbines has been proposed several times over the past decades (Jensen and Sørensen, 1984). Within the European Cryoplane project, it has been suggested that lower altitude cruising will make propulsion by hydrogen advantageous with respect to environmental impacts (Svensson et al., 2004). However, it is doubtful if this suggestion is practical in a world where air corridors are becoming overcrowded and new approaches to making better use of airspace are being sought. A different approach is to reconsider the airship as a means of air travel. A first approach to this is considering an airship for high-altitude cruising (or as a stratospheric platform) powered by photovoltaic panels and using a reversible fuel cell system to store surplus solar power and use it when the sun is not visible. In this way, carrying possibly heavy batteries may be avoided. The envisaged relative shares of direct use of solar power, of electrolyser operation and of fuel cell power production are shown in Fig. 4.34. So far, testing of the equipment sketched in Fig. 4.34 has been performed on a 1 kW scale in the laboratory and in simulated airship conditions. Testing fuel cell technology in a small, unmanned lightweight aircraft has been done by Bradley et al. (2007). The glider-looking plane has a 0.5 kW PEM fuel cell, capable of providing more power than the similar planes aimed for human bicycle propulsion. An example of the power disposition during maximum propulsion is given in Fig. 4.35. For cruising, the power
levels are reduced by at least a third, and during descent, idling is possible. Figure 4.34. Power modes and their timeshares for reversible fuel cell (RFC) plus solar cell system proposed as a power source for a stratospheric airship. (From K. Eguchi, T. Fujihara, N. Shinozaki, S. Okaya (2004). Current work on solar RFC technology for SPF airship. From Proc. 15th World Hydrogen Energy Conf., Yokohama. 30A-07, CD Rom by Hydrogen Energy Soc. Japan.)
Figure 4.35. Power disposition for fuel cell-propelled, light unmanned aircraft at full throttle (takeoff; from Bradley et al., 2007; used with permission).
A step up in aircraft size is the manned plane considered by Boeing (LapeñaRey et al., 2008). It has a PEM fuel cell rated slight over 20 kW and a similarly rated Li-ion battery. Both are required during take-off, while the fuel cell can provide enough power for cruising, as indicated in Fig. 4.36.
Figure 4.36. Performance of hybrid PEM fuel cell-Li-ion battery in light manned aircraft, shown as breakdown of contributions to power, according to pre-flight workshop tests (Lapeña-Rey et al., 2008; used with permission).
The prospect of saving fuel-storage mass in aircraft by replacing petroleum fuels by hydrogen has been discussed by Dollmayer et al. (2006), Barbir et al. (2005), and Verstraete et al. (2010). The influence of vibrations caused by turbulence on fuel cell stability and performance has been analysed by Rouss et al. (2008), concluding that such considerations should enter into the design of fuel cell-powered aircrafts.
4.4 Power plants and stand-alone systems Stationary power generation on a larger scale may use either low- or hightemperature fuel cell systems, and several systems rated at up to a few hundred kW have been operated (Barbir, 2003; Bischoff et al., 2003; Veyo et al., 2003). The systems comprise the basic units of PEMFC, MCFC, or SOFC as described in Chapter 3, combined with fuel preparation and exhaust cleaning equipment where necessary. Placement at conventional power plant locations allows convenient accumulation of all the processes required, or access to a hydrogen pipeline where this is preferred, without having to cope with the
limited space constraints of vehicles. Scenarios for the expansion of this type of system are dealt with in Chapter 5, including the special requirements in cases where the hydrogen plants serve as stores for primary energy sources such as wind or solar power. Also discussed in Chapter 5 is the incorporation of large hydrogen stores in a centralised set-up. Similar discussions of the requirements for large penetration of stationary fuel cell technologies have been taken up for Japan by Fukushima et al. (2004). In the transitional phase between the present and a fuel cell-based system, it has been suggested to lower the cost of hydrogen for vehicles by generating it from off-peak electricity from the present system (Oi and Wada, 2004). This idea is similar to the use of surplus wind power for the production of hydrogen, as described in Chapter 5, section 5.5. In conjunction with intermittent renewable energy sources, the fairly long up-start times for high-temperature fuel cell systems may be a problem, although the relevant time-interval for forecasting of, for example, wind power production can be handled rather accurately (Meibom et al., 1999). In order to give stationary fuel cell systems an optimum performance, shortterm storage in the form of ultra-capacitors may be incorporated (Key et al., 2003). The fast response of these storage devices allows a very precise load matching. On the other hand, it is generally assumed that a mature fuel cell technology will enable power systems to respond fast enough for most normal operations. Small-scale capacitor storage is perhaps more likely to become incorporated into electric appliances, in order to make them more resilient. Because many new appliances with growing penetration in the market (such as laptop computers and smart phones) include on-board battery storage and stabilising circuits, the demand for high power-quality (suppression of frequency and voltage excursions) has generally been declining in recent years, which is good news for many of the emerging power systems, including both primary converters for wind and solar energy and also intermediate converters such as fuel cells. One already emerging niche application of fuel cells is for emergency power system (or UPS = “uninterruptible power systems”). A different kind of stationary system application is for remote power supply. Such systems are most likely based on primary energy from renewable energy converters, because long-distance transportation of hydrogen by road will increase the current problem of transportation cost, making remote power much more expensive than the same type of power in densely inhabited areas, due to the cost of transporting fossil fuels in trucks over land. Locations that can be served by waterways may not have this problem. The fuel cell system will not be particularly different from what is used in other locations, except that low maintenance requirements will have a higher priority. Use of modest lifetime MCFCs for peak-shaving has been tested (MPS, 2004).
Power plant use of hydrogen is likely for a while to be more attractive using diesel or gas engines than fuel cells, due to the much lower cost combined with the absence of the negative effects of volume requirements by internal combustion engines using hydrogen found in automotive applications (e.g., the first BMW demonstration of hydrogen cars used 12-cylinder engines or similarly bulky ignition chambers). A study has been carried out for the North-Atlantic island of Utsira, with a couple of dozens inhabitants, based on power generated by wind, used directly or for producing hydrogen by an alkaline electrolyser, and using the hydrogen either in a PEM fuel cell or in an engine similar to those used for diesel oil (Fig. 4.37; Ulleberg et al., 2010).
Figure 4.37. Power production system on the Norwegian island Utsira in the North Sea. The components are rated as follows: Wind turbine 600 kW, electrolyser 50 kW, hydrogen store 2400 Nm3, fuel cell 10 kW, hydrogen engine 55 kW, battery 50 kWh, and flywheel 5 kWh. (From Ulleberg et al. (2010); used with permission).
Utsira is a very windy place, and the intermittency of wind power production causes need for back-up power only in relatively short periods. Figure 4.38 shows the system behaviour during such a five-hour period: When the wind power production minus electricity load drops, the electrolyser is switched off and the flywheel is employed to ensure stability of the now quite strong fluctuations in wind power, securing smooth supply by taking off surplus production and applying it during deficit, along with the engine generating power from stored hydrogen (but reacting slower than the flywheel). When wind power production again starts to rise, the engine is first shut down, and a little later, the electrolyser is restarted.
Figure 4.38. Operational data from the Utsira set-up shown in Fig. 4.37, during a 12hour period in March (see text). From Ulleberg et al. (2010); used with permission.
4.5 Building-integrated systems Smaller, building-integrated fuel cell systems, notably of the PEM type, have attracted much attention over recent years. This is partly related to the motion to more decentralised energy systems, where traditional energy supply has maintained a distinction between heat, which often has been decentralised (individual building oil or gas burners), and electric power supplied from central sources. The third important energy type, that of fuels for vehicles, has been delivered through supply chains ending at communal filling stations, and small-scale, portable power has been entirely supplied through the purchase of small batteries (where only the rechargeable ones have offered personal control). Fuel cells offer the possibility for individual buildings and thus their owners to become their own electricity providers and possibly also to provide individual filling stations for vehicles parked in building-attached garages (Sørensen, 2000). At the same time, waste heat from on-site power and hydrogen production may cover or contribute to providing heat/hot water to the building (combined power and heat, CPH). Finally, fuel cell technology may also replace small batteries for portable applications, allowing individuals to decentralised control all their energy supplies for heat, vehicle fuels and stationary or portable electricity uses.
Figure 4.39. Vision of building-integrated fuel cell system supplying heat, power, and hydrogen as a vehicle fuel. From Honda (2004), used with permission.
These possibilities are incorporated in some of the scenarios presented in Chapter 5. Figure 4.39 gives a vision for building-integrated fuel cell systems deriving their primary energy from renewable energy supply through electric power networks. An alternative would be hydrogen supply through pipelines in analogy to natural gas distribution or district heating networks. A first approach to the building-integrated use of hydrogen may be replacing the natural gas boiler unit used in many countries for home heating and hot water needs by a PEM fuel cell unit with reformer and thus capable of using the existing supply of natural gas. Such fuel cell plus reformer units have already been developed and may reach a competitive cost within the next one or two decades (Vaillant, 2004; Osaka Gas Co., 2004). Test results have been forthcoming for a number of prototype installations, such as a 4.0 plus 6.8 kW H-Power combined heat and power producing unit operated in Italy (Gigliucci et al., 2004). The natural gas reformer plus PEM fuel cell installations are often referred to as micro-CPH plants, and several discussions of design and modelling have recently emerged−for example, by BeausoleilMorrison (2010). Arsalis et al. (2011) proposes the use of high-temperature (above 100°C) PEM fuel cells. However, the possible efficiency improvement is small and heat distribution temperatures in buildings (and district heating systems) have been declining over the last several decades, making conventional PEM cells delivering heat at 50-60°C quite adequate and probably
more convenient. An interesting approach is taken by Li and Ogden (2011), adding a hydrogen compressor and making the reformer so large that it can provide enough hydrogen to cover both building electricity use (plus some heat for hot water, space heating being poorly correlated to power demand at the location considered) by the PEM fuel cell and also refill a fuel cell vehicle parked in/at the building during nights. Figure 4.40 shows the performance of the system on a typical day, under conditions prevailing in northern California, assuming a fixed-schedule, slow vehicle filling at night. Figure 4.40. Typical performance of a residential system with a 2 kW PEM fuel cell, an 8 kW natural gas reformer (including shift reactor and gas purifier) producing 0.9 kg vehicle H2 over a 10hour interval, plus electricity and heat. The building power demand is met and the associated heat produced mostly exceeds demand. Based on Li and Ogden (2011); used with permission.
Development of 1-2 kW combined cycle SOFC modules is ongoing, based, for example, on disc-shaped sub-stacks and integrated into a small unit with a fuel cleaner (for removing sulphur from natural gas), an afterburner (to reduce fuel in exhaust) and a battery storage (EnBW, 2004; CFCL, 2004; Kazempoor et al., 2010). The advantage of using SOFC technology is to be able to avoid the reformer. A recent study comparing micro-CPH systems based on PEM and SOFC technologies concludes that the PEM systems are more efficient in both energy and exergy respect (Barelli et al., 2011). This confirms the generally held view that SOFC technologies are interesting only for largescale plants, where the necessary high temperatures are easier to establish. In a next phase, assuming that resources of natural gas are so limited that alternatives should be sought and/or that greenhouse gas emission issues must be taken seriously, micro-CPH units based on an established hydrogen supply infrastructure would be introduced (Erdmann, 2003; Kato and Suzuoki, 2004). Buildings would serve to both supply fuel-cell hybrid vehicles with plug-in power and hydrogen (Syed et al., 2010). As regards micro-CPH plants, one could envisage both systems with piped hydrogen supplies (say, taking over or replacing the natural gas network) and systems where the hydrogen is produced in a decentralised fashion. Such production would be particularly attractive if the capability of fuel cells to perform in reversible
modes can be exploited at high efficiency (Sørensen, 2000; 2003a; and Chapter 5). Actually, most building-integrated systems would benefit greatly from being able to use reversible fuel cells (see Chapter 3, section 3.5.5), rather than having to install two expensive components: fuel cell and electrolyser. The problem has been that reverse operation of a fuel cell optimised for power production used to have a low efficiency (around 50%), making it less efficient than conventional alkaline electrolysis (also technically a one-way type of fuel cell). However, a breakthrough in technology, so far on laboratory scale (Ioroi et al., 2004), discussed in connection with Fig. 3.52 in Chapter 3, suggests that reversible PEM fuel cell technologies may in the future live up to the expectations made in visions of widespread in-building use, such as the decentralised scenarios presented in Chapter 5, section 5.5. An important issue for building-integrated fuel cells is the infrastructure serving the building. It might consist of an electricity grid connection and attachment to a hydrogen pipeline network. If the building is receiving hydrogen, it can use it to produce electricity and associated heat. If more electricity is produced than can be used in the building, it may be exported to the electricity grid. In this way the fuel cell capacity is used to a fuller degree, and the need for additional power stations not part of the building-based system becomes smaller. However, another mode of operation (see Chapter 5, Fig. 5.3) is to receive electricity from the grid and use it to produce hydrogen to be dispensed to vehicles parked in or at the building or to be stored for later regeneration of power and associated heat. This option is most relevant for primarily renewable energy systems with intermittent power input, because then the hydrogen can be used to produce power and heat also when no primary production is available. This points to a further possibility in case a reversible fuel cell system is used−namely that the building has a connection to an electricity grid, but not to any hydrogen network. It would then generate hydrogen from excess electricity not used immediately in the building, store it, and use some of it for regenerating power in case of low supply and the rest of it as fuel for vehicles. In such a vision, storage of hydrogen should be possible in close connection with the building. Only when there is a hydrogen network, does a centrally located hydrogen store make sense. Building-integrated hydrogen storage may be in the form of storage tanks in vehicles that happen to be occupying the garage (if present) or parked near the house. However, this option does not guarantee availability of sufficient storage space in all cases where required. It is therefore necessary to think of other hydrogen storage options dedicated to the building. These could be compressed gas containers or, as some would prefer for safety reasons, metal hydride stores. As the scenario in Chapter 5 will show, only modest volumes of such storage (about a
third m3 for a family home) are required for handling the fluctuating energy production of a wind-solar primary energy production system, including consideration of losses in all components of the system. Aki et al. (2004) have also pointed out that there are system stability advantages obtained by using a local hydrogen pipeline grid to interchange hydrogen between individual buildings.
4.6 Portable and other small-scale systems Consuming patterns have over recent years seen a dramatic increase in the use of portable equipment for entertainment and work (such as music and video players, laptop computers, and mobile phones with multi-functionality). This has increased the demand for batteries, but at the same time revealed limitations of the battery technology that seem difficult to avoid despite the steady but necessarily modest increase in conversion efficiency. Fuel cells with small-scale stores would be an obvious solution to these problems, because the technical performance in one important area already surpasses that of batteries, namely that fuel cell operation of a state-of-the-art laptop computer can offer autonomy for days rather than a few hours. The difference between these otherwise similar technologies is the external storage of chemicals for a fuel cell versus the internal storage in batteries. This is also the bottleneck, because direct storage of hydrogen would be inconvenient (high compression needed for pocket-size containers of suitable capacity), and use of portable reformers constitutes an added component presenting its own concerns. Two avenues are pursued at the moment: avoiding reformation by use of a direct methanol fuel cell, or development of a micro-reformer working on a substance with high energy density, both as regards mass and volume. Battery replacement technologies are also sought for appliances other than laptops, both with smaller energy use (e.g., smart phones) and with larger energy requirements (from garden equipment to military portable weaponry and devices for intelligence and communication). A cellular phone in use will need less than 200 mW of electric power, a video camera will need less than 6 W, and a laptop computer or a portable CD player will typically need less than 20 W. The more specialised portable equipment includes field environmental monitors, medical mobile life-support systems, and soldier communication and signalling devices used in military operations (Palo et al., 2002). Power requirements for such equipment would typically be in the range of 10-500 W. A standard size Li-ion battery has a storage capacity of 750 mAh, and the largest one convenient for a laptop computer has a capacity of 3600 mAh. For a portable video camera, the standard Li-ion battery may deliver 5 W at 7 V for an hour or two, while for the laptop at 12 V, current batteries
provide about 4 h at an average power consumption of 10 W. The current high price paid for small-scale Li-ion batteries today (some 15-25 euro or US$ per kWh for typical devices for cameras and laptops, with a 4year lifetime, as opposed to the lower but still significant prices for the larger batteries used for craftsman tools or electric vehicles) makes portable applications an inviting market for alternative technologies, and particularly those that may offer prolonged autonomy. The options that may be considered for portable fuel cell applications include PEM fuel cells with compressed hydrogen canisters, metal hydrides, or fuels such as methanol requiring use of a reformer, and as mentioned direct methanol fuel cells. Compressed hydrogen at 30 MPa and the best metal hydrides have an energy density on a volume basis (Chapter 2, Table 2.4) of 2.7 and 15 GJ m−3, while that of methanol is 17 GJ m−3. These should be compared to the energy density of Li-ion batteries, which is 1.4 GJ m−3 (Sørensen, 2010a). The implication is that by adding a 50% efficient PEM fuel cell, the performance of a device using stored compressed hydrogen at up to 30 MPa plus a fuel cell is not better than that of a Li -battery, while the other possibilities listed at least theoretically may outperform the best batteries. The consideration of size (volume) is relevant for portability, but even more is the energy density by mass, as humans have to physically carry the devices around (a tautology for “portable” equipment). In fact, lower mass would be a general advantage for many laptop computers that in their present form are more “towable” than “portable” (one reason that the market is currently splitting up into desktop replacements, laptops, notebooks, tablets, and so on, basically just signalling weight and distinguishing themselves from the smart-phone and GPS devices without a keyboard of anatomically acceptable dimensions). Without the mass of the container, the energy density of hydrogen in any form is 120 MJ kg−1, but for storage in metal hydrides, the overall density goes down to under 9 MJ kg−1 (Chapter 2, Table 2.3). For methanol the value is 21 MJ kg−1, and for Li-ion batteries it is 0.7 MJ kg−1 (Sørensen, 2010a). This comparison on a mass basis makes most fuel cell solutions preferable to Li-ion batteries (and much more to lead-acid batteries 5 times lower in energy density), as long as the volume can be tolerated. For the best metal hydride stores, the capacity (hours of autonomous operation) could be increased some 5-fold over Li-ion batteries of the same volume, and this option has attracted some attention. Güther and Otto (1999) describe a Siemens-Nixdorf experimental laptop computer with such a store, obtaining a 3-fold increase over a Li-ion battery. Recently, a 2 W smart-phone re-charger consisting of a small PEM fuel cell and a replaceable 15 Wh MeH cartridge has been marketed (Geek with laptop, 2011). Non-metal hydrides should be able to reach even better values.
The use of methanol and a PEM fuel cell in combination with a miniature reformer has been contemplated by several Japanese electronics industries and in a series of US army projects (Palo et al., 2002; Patil et al., 2004). In the early project, an energy density of 2.6 MJ kg−1 was found for a 15 W, 1 kg portable power device with a small auxiliary battery used to start the reformer system up. From the next project, Fig. 4.41 shows a 40 W micro methanol-reformer. In 2006, still with US Army funding, the company UltraCell developed another methanol cartridge-based micro-fuel cell capable of running a laptop for two or three days. Today, versions from some hours to a month of rugged, autonomous operation are offered, still primarily for military use (Fig. 4.42; UltraCell, 2011). Various theoretical models have been constructed to deal with optimisation issues (Besser, 2011), and ways of avoiding environmental emissions were already suggested at an early stage (Muradov, 2003).
Figure 4.41. Prototype of 40 W methanol-tohydrogen reformer, produced for the US Army by Pacific Northwest National Laboratory. The coin appearing in the picture has a diameter of 24 mm (Patil et al., 2004). Used with permission.
Figure 4.42. Left: Ultracell XX25 hydrogen fuel cell with methanol cartridge and reformer, for rugged military usage (2008-2011). Middle and right: NEC direct methanol prototype fuel cells with methanol cartridge (2003 and 2004 design). From UltraCell (2011); Y. Kubo, NEC Corp. (2004); NEC (2011). Used with permission.
The good energy density of methanol can be used without the extra reforming component, by using a direct methanol fuel cell, described in Chapter 3 section 3.6 (see Fig. 3.69). One drawback is the lower conversion efficiency of methanol PEM cells relative to their hydrogen cousins. This route is being pursued (Meyers and Maynard, 2002; Zenith et al., 2010), but methanol/DMFC devices are not only less efficient but also further from commercial readiness than the reformer/PEM fuel cell combination. Key factors in making the DMFC work are ambient humidity, condenser temperature, and
excess air, according to the model calculations by Zenith et al. (2010). Early prototypes of a notebook personal computer powered by a microDMFC are shown in Fig. 4.42 (middle and right). In the 2003 model, a replaceable methanol cartridge is placed behind the computer, and the 280 cm2 DMFC is placed under the laptop keyboard. The fuel cell produces 14 W at 12 V, and if the methanol volume is some 30 cm3 (judged from photo), then the storage capacity is 142 Wh. This would allow the computer to be operated for 14 h at an average consumption of 10 W. A “docking station” design was exhibited in 2004. Both Japanese and Korean computer manufacturers presented several fuel cell notebook prototypes between 2003 and 2006, and sometimes with promises of “next year” entrance into the commercial market, but none of them have by 2011 been appearing in shops. However, the German company Efoy/SFC Energy is marketing a range of DMFC units (e.g., model 2200 XT: 90 W, weight 9 kg, capable of delivering 2160 Wh of electricity over 24 hours) aimed primarily at recreational uses, such as in pleasure boats (Efoy, 2011). All the portable fuel cell units discussed use a small battery for starting the system, a process often taking 10-20 minutes. In order to take advantage of standard PEM hydrogen fuel cells without reforming, a chemical reaction concept based on sodium silicide has been employed to generate hydrogen for a small PEM fuel cell, first for bicycle use (500 g furnishing some 200 W over 50 km) and recently for off-grid recharging of small portable devices (Fig. 4.43; SiGNa, 2011; MyFC, 2011). The reaction scheme is (NSF, 2011; Lefenfeld et al., 2006) 2 NaSi + 5 H2O → Na2Si2O5 + 5 H2 + 350 kJ/mol
Figure 4.43. PowerTrekk PEM fuel cell for recharging small appliances, using stored NaSi to produce hydrogen, when water is added, and to recharge the Li-ion battery that serves to allow instant start-up (from MyFC, 2011; used by permission).
The production of NaSi from sand and kitchen salt should be straightforward (Modic, 2011), and the exogenic process producing hydrogen is quite fast but easily controllable, in contrast to some other sodium reactions. Other types of fuel cells, such as microbiological ones, have also been contemplated for small-scale, portable use (Dunn-Rankin et al., 2005), but prospects are poor due to the extremely low overall efficiency.
4.7 Problems and discussion topics 4.7.1 How much hydrogen must an airplane be able to carry for a flight from London to Tokyo? 4.7.2 Discuss the optimum rating of fuel cell and battery for a hybrid car, as a function of the relative price of batteries and fuel cell equipment. Note that the energy rating of a battery is a poorly defined parameter. The reason is that the energy released from a battery depends on the discharge rate and hence on the driving cycle (Jensen and Sørensen, 1984). Manufacturers rarely mention this when they quote energy stored E in kWh or charge, C = E/V, stored in Ah. Here V is the potential across all units connected in series (some systems use a combination of parallel and series connection of individual battery module units). 4.7.3 Write a list of those power-consuming activities in your private or work life that you believe cannot ever be expected to become powered by batteries or portable fuel cells. Based on the fraction these activities constitute of the total power usage, estimate which percentage of total electricity use it would be possible to base on renewable energy sources such as wind or photovoltaic power, used solely to charge either batteries or fuel cells (two separate estimates).