The Future Role of Hydrogen in Petrochemistry and Energy Supply DGMK Conference October 4-6, 2010, Berlin, Germany

Hydrogen for Automotive Applications and Beyond U. Eberle Adam Opel GmbH, Rüsselsheim, Germany

Abstract The energy storage system is of decisive importance for all types of electric vehicles, in contrast to the case of vehicles powered by a conventional fossil fuel or bio-fuel based internal combustion engine. Two major alternatives exist and need to be discussed: on the one hand, there is the possibility of electrical energy storage using batteries, whilst on the other hand there is the storage of energy in chemical form as hydrogen and the application of a fuel cell as energy converter. [1]

Considering the latter concept, hydrogen is a promising energy carrier in future energy systems. However, storage of hydrogen is a substantial challenge, especially for applications in vehicles with fuel cells that use proton-exchange membranes (PEMs). Different methods for hydrogen storage are discussed, including high-pressure and cryogenic-liquid storage, adsorptive storage on high-surface-area adsorbents, chemical storage in metal hydrides and complex hydrides, and storage in boranes. For the latter chemical solutions, reversible options and hydrolytic release of hydrogen with off-board regeneration are both possible. Reforming of liquid hydrogen-containing compounds is also a possible means of hydrogen generation. The advantages and disadvantages of the different systems are compared. [2]

Introduction Approximately 900 million vehicles worldwide are on the roads today. About 96% of the fuel used for transportation purposes is thereby produced from fossil sources of energy. There are estimates for the year 2020 that the number mentioned above will increase to approximately 1.1 billion vehicles worldwide, in particular due to the economic expansion and industrial development of India and China. This will inevitably have consequences for global crude oil demand and also consequently for worldwide CO2 emissions. Since an increase in demand for oil and CO2 production proportional to the projected number of vehicles is unsustainable for financial, ecological and political reasons, every implementation strategy must aim at the replacement of fossil fuels as a source of energy for automotive applications. [1]

Therefore, a key element of the advanced propulsion strategy of many global car manufacturers is the electrification of the automobile and the displacement of gasoline by alternative energy carriers. This would lead to reduced fuel consumption, reduced emissions and also to increased energy security via geographic diversification of the available energy sources. At General Motors, this strategy has its roots with the introduction of the first modern electric vehicle, the 1996 GM EV1. The EV1 was a pure battery electric vehicle (BEV), developed as a mass production car for the average driver. Unfortunately, the market experience with the EV1 and its initial lessees indicated that further significant improvements in BEVs were needed. Some EV1 drivers coined the term ‗‗range anxiety‘‘, describing their omnipresent concern, or even fear, of becoming stranded with a discharged battery in a limited-range vehicle, away from the electric infrastructure. Hence, improvements in on-board energy storage are needed (directly proportional to vehicle range) and shorter charging times were assessed to be essential for a more widespread deployment of BEVs. [1]

DGMK-Tagungsbericht 2010-3, ISBN 978-3-941721-07-4 43 The Future Role of Hydrogen in Petrochemistry and Energy Supply

Zero-emission vehicles based on hydrogen fuel cells (FCEV) or pure battery-electric systems (BEV) that are fully competitive to conventional vehicles regarding performance and ease-of- use represent the ultimate target of the GM strategy. [1]

Fuel cell electric vehicles and battery electric vehicles - a comparison

These days, within the general public and also within the automotive and energy R&D community, very often the impression is created that a decision has to be made between hydrogen fuel cell vehicles (FCEV) and pure electric vehicles (BEV), which is seen as a question of either/or. However, this is definitely not the case since both technologies address different areas of the vehicle market. This is caused by the extremely different energy densities of the applied energy carriers (see Fig. 1). [1]

Figure 1: Energy densities of various energy carriers (based on a vehicle range of 500 km)

The pure battery vehicle is the technology of choice for small urban vehicles with ranges up to 150 km. Besides GM activities (e.g., the EN-V two-wheeler concept for urban mobility) other car manufacturers, mainly Mitsubishi and the Renault-Nissan alliance, are currently working on the development and market introduction of large volume battery electric vehicles for such an application. By contrast, a so-called E-REV vehicle (extended-range EV), such as the Chevrolet Volt or the Opel Ampera, is perfectly suited for those customers who sometimes - but not too often - need longer ranges of up to 500 km; and especially for those willing to accept a small internal combustion engine in order to ensure the range beyond the initial 60 km of pure EV operation (see reference [1]). Also, further car manufacturers, such as Daimler and Toyota, pursue similar or related technology paths for concept vehicles.

On the other hand, fuel cell electric vehicles (FCEV) offer a different set of advantages: they always operate as zero-emission vehicles, can be refueled within three to five minutes, and offer ZEV ranges of about 500 km at full performance for family-sized cars. Due to its comparatively high energy density of 1600 W h per kilogram of tank system weight (70 MPa Compressed Gaseous Hydrogen vessels based on carbon-fiber composite technology, CGH2), hydrogen is the ideal energy carrier to serve as intermediate store of fluctuating renewable energy such as solar and wind power, and to enable the usage of this green energy as transportation fuel. [1]

44 DGMK-Tagungsbericht 2010-3 The Future Role of Hydrogen in Petrochemistry and Energy Supply

a)

b)

Figure 2: a) GM HydroGen4, b) tank-to-wheel efficiencies of the GM HydroGen4 (upper line) and the gasoline-powered base model Chevrolet Equinox (lower line).

The fuel cell (FC) running on hydrogen is the most attractive long-term powertrain option for passenger cars. It eliminates CO2 emissions on the tank-to-wheel path, the fuel can be produced from many sources, and it provides very high average efficiencies. The latter is

DGMK-Tagungsbericht 2010-3 45 The Future Role of Hydrogen in Petrochemistry and Energy Supply

especially based on the fact that the fuel cell reaches its highest efficiency at part load. By contrast, at full load, there exists virtually no advantage over internal combustion engines anymore. However, particularly passenger vehicles are mostly operated at loads significantly below their rated power: For such operating conditions, the gain in efficiency offered by fuel cells is maximal. At very low power output, even the fuel cell system efficiency sharply drops. This is attributed to many balance-of-plant components such as the air compressor, as those have to be operated even at idle power (see Figure 2b). [3]

GM HydroGen4 Vehicle Type 5-door , cross over vehicle, front- wheel drive, based on the Chevrolet Equinox Dimensions Length 4796 mm Width 1814 mm Height 1760 mm Wheelbase 2858 mm Trunk space 906 Liter Weight 2010 kg Payload 340 kg Hydrogen storage system Type 3 Type IV CGH2 vessels Operating pressure 70 MPa Capacity 4.2 kg Fuel cell system Type Proton Exchange Membrane (PEM) Cells 440 Power 93 kW Battery system Type Nickel-metal hydride (NiMH) Power 35 kW Energy content 1.8 kWh Electric propulsion system Type 3-phase, synchronous motor Continous power 73 kW Maximal power 94 kW Maximal torque 320 Nm Performance Top speed 160 km/h Acceleration 12 s (0-100 km/h) Range 320 km Operating temperature -25°C to +45°C, vehicle can be parked at ambient temperature < 0°C (without external heating)

Table 1: Technical specifications of the GM HydroGen4

The scalability of fuel cell systems also facilitates the adaptation to different vehicle sizes. One example is the fuel cell system that was originally developed for the GM HydroGen3 (a van based on the Opel Zafira) and afterwards was adapted to a small vehicle, the Suzuki MR Wagon FCV, using a shorter fuel cell stack with reduced cell count. Eventually, it was adapted to a GMT800 truck by doubling the stack and some other components. [1]

46 DGMK-Tagungsbericht 2010-3 The Future Role of Hydrogen in Petrochemistry and Energy Supply

Typically, 4 - 7 kg of hydrogen have to be stored on-board. This remains to be a serious issue for the vehicle integration. Furthermore, cylindrical vessels are required for CGH2 and most other types of fuel storage. In existing vehicles, without modifications, there is no space for hydrogen storage devices that could provide a sufficient range. Hence, rear body modifications are necessary to integrate the hydrogen storage vessel(s). In an extreme case, one could imagine concepts where the car is built around the hydrogen storage such as the Chevrolet Sequel (total fuel capacity: 8 kg of hydrogen). [3] By doing so, for the very first time, an FCEV operating range of significantly more than 300 miles could be achieved and demonstrated on public roads between suburban Rochester and New York City in May 2007. [1]

Since autumn 2007, within the framework of „Project Driveway―, more than 100 cars of the current generation HydroGen4, were deployed to demonstration projects all over the world (e.g. in the U.S. and in Germany). These vehicles offer an improved everyday-capability and a higher performance than their predecessors (see Table 1). For instance, the cars can be both operated and started at very low temperatures down to -25°C. The electrical propulsion system provides a maximum torque of 320 Nm at the motor and accelerates the HydroGen4 in less than 12 seconds from 0 to 100 km/h. The continuous power output of the electric motor of 73 kW is sufficient for a maximum speed of 160 km/h; the maximum performance is 93 kW. Three carbon-fiber tanks on board of the vehicle store 4.2 kg of hydrogen and enabled a range of 320 km. The empty hydrogen storage system can be completely re-filled again within 3 minutes. To further improve the agility of the vehicle and to increase the efficiency by enabling recuperation, a nickel metal-hydride battery with an energy content of 1.8 kWh is installed on board of the vehicle. The vehicles proved to be more efficient than the comparable conventional Chevrolet Equinox vehicle with gasoline engine by a factor of 2 (EPA Composite cycle 4.6 liters/100 km of gasoline equivalent in comparison with 9.6 liters/100 km of gasoline, see Figure 2 for corresponding efficiency values). [1] Since the initial HydroGen4 vehicle deployment, these efficiency, respectively, fuel consumption values have been significantly improved, in particular at part load, by implementing a further optimized controls software. [4]

Durability is a major issue for the fuel cell stack, especially when membranes are becoming thinner and catalyst loadings lower, which is necessary for performance and cost reasons. Among the factors limiting the lifetime of PEM fuel cells, chemical degradation has been identified as one of the major challenges, although the detailed mechanisms and influencing factors are still under investigation. But even though not yet all degradation phenomena are fully understood, there has been remarkable progress in system reliability (see Figure 3). A fuel cell system durability of about 30,000 miles has been demonstrated within Project Driveway, and an updated HydroGen4 system is projected to reach 80,000 miles. Further improvements will be achieved for the 2015 – 2020 early commercialization timeframe. The planned next-generation fuel-cell propulsion system for 2015 is half the size, 220 pounds lighter than the current 4th generation and uses about a third of the platinum of the system in the Chevrolet Equinox Fuel Cell electric vehicles (see Figure 3). [1] Both could be achieved by continuous improvements in the engineering and operation of complete fuel cell systems and vehicles. Besides the fuel cell stack, another subsystem has turned out to have major influence on vehicle cost and performance: the fuel storage. [3]

DGMK-Tagungsbericht 2010-3 47 The Future Role of Hydrogen in Petrochemistry and Energy Supply

a)

b)

Figure 3: a) Fuel cell system durability, b) projected system cost over production volume [4]

On-board hydrogen storage options

There are four major options for on-board hydrogen storage:

(1) CGH2 compressed gaseous hydrogen, 35–70 MPa and room temperature. (2) LH2 liquid hydrogen, 20–30 K, 0.5–1 MPa. (3) Solid state absorbers (such as hydrides or high-surface materials). (4) Hybrid solutions, utilizing at least two of the mentioned above technologies.

These technologies are described in detail in several other publications (inter alia, see

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reference [2]) and shall therefore be discussed here only shortly. It also has to be stated that values for gravimetric and volumetric energy densities may correspond either to just a materials approach (a) or a systems approach (b) including all required components and mountings. From an engineering perspective, the second approach is preferable; also the target values provided by the US Department of Energy are defined on this basis. The options (1) and (2) have been implemented by the automotive industry in recent years. For example, the GM HydroGen3 (a multi-purpose vehicle based on the Opel Zafira mass production architecture) was adaptable to both storage types. The HydroGen3 vehicles were capable of storing either 3.1 kg H2 (70 MPa CGH2 variant) or 4.6 kg H2 (LH2 variant). These values correspond to ranges of 270 km, respectively, 400 km in the New European Driving Cycle (NEDC). [3] The GM HydroGen4, GM‘s fourth generation fuel cell vehicle, incorporates a 4.2 kg 70 MPa CGH2 storage system (see previous section).

As mentioned before, a completely different approach is pursued for GM‘s Chevrolet Sequel. This concept car is not based on an existing vehicle platform, but was designed around the hydrogen propulsion and fuel storage systems. Doing so, the integration of an 8 kg 70 MPa CGH2 tank (corresponding to a range of more than 480 km) was enabled. Due to the comparatively high operating pressures of these storage systems, a cylindrical vessel design is both essential and obvious. Since the volumetric storage density of CGH2 tanks is rather low (already on a materials basis, see Figure 1 and 4), the integration of large fuel systems into an existing mass production vehicle architecture remains to be a challenge. Thus, in all of the cases described above, the storage system comprises for vehicle packaging reasons not only a single but two, respectively, three pressure vessels. Despite the described limitations of the CGH2 approach, this option yields today the best overall technical performance and shows the highest maturity for automotive applications. [3]

Figure 4: Compressed gaseous hydrogen vessels, comparison type III and type IV [4]

Until very recently, liquid hydrogen has also been considered to be a technology viable for automotive implementation. But its drawbacks concerning an efficient thermal insulation could not be tackled in a satisfying manner. Due to the low LH2 operating temperature of in- between 20 and 30 K, an unavoidable heat flow takes place (2–3 W for the complete tank system).

This heat input consists of three components:

(1) Thermal conduction

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(2) Convection (3) Thermal radiation

Among these, the thermal conduction through mountings, pipes and cables to the inner storage vessel and the heat radiation from the environment to the cryogenic liquid are dominant. To achieve those low overall heat transfer numbers mentioned above, it is very important to also work with cylindrical tank structures (compare to CGH2), because this geometry is very close to the optimal surface-to-volume ratio (only beaten by completely spherical structures). Additionally, it is required to implement a very efficient multi-layer vacuum super insulation, consisting of approximately 40 layers of metal foil. Wrapping these foils around the storage vessel in general, and around the dome areas (further complicated by the in- and outlets for H2, respectively, the mountings) in particular, is time-consuming and highly demanding. [3]

The remaining significant heat input leads to an enhanced evaporation of the liquid hydrogen stored inside which eventually causes a pressure rise. Typically, when a system pressure of approximately 1 MPa is reached, a valve has to be opened to vent some of the hydrogen gas. The time period between putting the vehicle into an idle or parking mode and the venting process is usually called ―dormancy‖. Typical values for the length of this period are several days. After that point in time, hydrogen is continuously lost to the environment. This amount of hydrogen (heat flow multiplied by time period and divided by the H2 heat of evaporation of 0.45 MJ/kg) is known as boil-off gas. A related problem are the cooling-down losses during refueling (due to the evaporation of hydrogen), since all the pipes, dispensers, nozzles and valves have to be cooled down to cryogenic temperatures before a considerable amount of LH2 can be filled into the tank system. Both effects lead to unacceptable hydrogen losses in the eyes of the customer, respectively, the infrastructure operators. The complexity of the LH2 storage system combined with the challenge to reduce the boil-off as much as possible leads to overall LH2 system costs that are – at large scale – not favourable over CGH2 systems. Despite the fact that the volumetric storage density of LH2 tanks is slightly higher compared to CGH2 systems, we do not see strong advantages in vehicle packaging that might outweigh the mentioned above disadvantages. Additionally, the conformability of LH2 tank systems is actually regarded to be not better than that of pressure vessel systems. Since also the energy required to liquefy hydrogen consumes 30% of the stored chemical energy compared to just 15% for 70 MPa CGH2 (12% for 35 MPa CGH2), based on the H2 net calorific value of 120 MJ kg-1, CGH2 was evaluated to be the superior technology. For the time being, thus, a decision at GM was made to concentrate on the development of 70 MPa CGH2 systems for current vehicle applications such as the GM HydroGen4 and the Chevrolet Sequel or, respectively, other next generation vehicles (anticipated for 2015). And to establish this technology as benchmark for any emerging breakthrough technology (such as tank systems based on hydrides, high-surface materials or other advanced solid state absorbers). The corresponding gravimetric and volumetric hydrogen densities of current CGH2 tanks (single vessel, 6 kg H2 capacity) are 0.023 kg (H2) per liter (system) and 0.048 kg (H2) per kg (system). A maximum extraction rate of 2 g H2 per second and a re-filling time of about 3 minutes can be realized. [3]

All relevant advanced concepts (see Figure 5) have to beat those figures in most of the categories. Below, these alternative options and their respective challenges shall be discussed. What are now the boundary conditions for any novel storage concept? Basically, there are: (1) Limitations of available space (caused by vehicle packaging needs, especially when utilizing existing architectures), (2) Technical operating parameters defined by the requirements of the fuel cell propulsion system (e.g. hydrogen extraction rate, supply pressure, and temperature), and (3) Customer demands (such as cost, overall capacity, refueling time and efficiency). Solid state absorbers of hydrogen offer an impressive volumetric hydrogen density on a materials basis. But unfortunately, this is not the complete story. In particular, the very short refueling times of three minutes, demanded by the customer, result in a significant increase in system complexity. [3]

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Figure 5: Operating temperatures of various hydrogen storage technologies

When we consider a 6 kg H2 tank system comprising a storage material M with a heat of formation deltaH of about 25 MJ kg-1 H2 (typical for many hydrides, see Fig. 5), a thermal load of 150 MJ would have to be compensated during refueling:

M + H2  MH2 + Heat

That leads to an average heat exchanger power of at least 800 kW. Such a high- performance device is not imaginable to be installed on-board a vehicle due to cost, volume and weight reasons. Typically, values significantly less than 100 kW would be reasonable for an automotive application. Merely enabling a H2 supply rate of 2 g s-1 to the fuel cell system, under full throttle conditions, causes a heat management challenge of about 50 kW. A further constraint is that many solid state absorber systems (in particular many hydride systems) require operating pressures of about or above 10 MPa (at least during refueling). Hence, a pressure tank consisting of advanced components and materials is required additionally. Also, the operating temperature of a solid state absorber has to be limited to about 70°C for practical reasons. The necessary energy to maintain that temperature level could be provided by using the waste heat of the fuel cell system. To obtain even higher temperatures, hydrogen would have to be either converted into electricity or be burned directly. Since this amount of hydrogen could not be used for propulsion purposes, both options would lower the effective H2 capacity and therefore also the range of the vehicle. [3] Last but not least, hydrogen absorbers often consist of powder materials. Thereby, it has to be evaluated whether the apparent density of the absorber is comparable to the crystal density of a compact block of the base compound. Thus, it has to be stated that an empirical rule of thumb exists that roughly describes the correlation between the materials value of the storage density and the respective systems value: The absorber relates to about 50% of the total weight, whereas the remaining 50% relate to the additional system components (such as pressure vessel, heat exchanger, valves, pipes, etc.). [3]

What are the lessons to be learned from that behaviour and what are therefore the objectives for future research:

(1) Heat of formation has to be reduced as low as thermodynamically possible. (2) Operating temperature should be less than 70°C. (3) Operating pressure should be limited to values less than 5 MPa for cryogenic or elevated temperature (up to 70°C) operation modes.

DGMK-Tagungsbericht 2010-3 51 The Future Role of Hydrogen in Petrochemistry and Energy Supply

(4) Operating pressure should be less than 35 MPa for room-temperature applications using low-deltaH hydrides.

These points should be defined as target values for any breakthrough storage compound. Currently, three major paths for material scientists exist to achieve those design parameters:

(1) The destabilization of the hydride state through alloy formation or more complex reaction schemes (e.g. LiBH4+(1/2)MgH2  LiH+(1/2)MgB2+2H2). (2) Cryo-adsorption of hydrogen on high-surface materials (such as high-performance activated carbon or metal-organic frameworks). (3) A hybrid solution combining low-deltaH hydride approaches with a 35 MPa compressed hydrogen design (e.g. using TiCrMn-alloys or more optimized, novel materials).

Using the third path and conventional hydrides, the volumetric storage density of a 70 MPa CGH2 system could already be achieved at an operating pressure of about 35 MPa. This could simplify the vehicle packaging and infrastructure challenges significantly. Unfortunately, the CGH2 system gets more complex by integrating the storage compound and a heat exchanger into the pressure vessel. That higher complexity leads to considerably greater system cost. [3]

A competing technology to the points above, the decomposition of hydrogen-rich, but non- reversible, compounds (such as sodium borohydride or in particular ammonia borane), is at the moment not considered to be a viable alternative for the automotive industry. The assessment for sodium borohydride was caused by the inherent complex on-board system design (i.a. concerning the treatment of waste materials), the infrastructure implications (i.a. related to the need for exchangeable fuel cartridges) and the off-board recycling, respectively, energy issues. [3] Nevertheless, the same reservations hold for amine–boranes as for the hydrolytic routes for hydrogen storage discussed above for metals or metal hydrides. The system is comparable to the NaBH4 system introduced by Millennium Cell, with the advantage of higher storage capacities, but the disadvantage of the possible breakthrough of ammonia to the fuel cell, which requires additional safety precautions and gas clean-up. For practical applications, new methods for the regeneration of the borates from aqueous solutions are also required. This process is highly energy-intensive, because the reduction of borate compounds into the amine–boranes in an aqueous solution appears to be impossible, which then requires further isolation of the boron oxide. Upon infusion of ammonia–borane in a mesoporous silica material SBA-15, the kinetics (not the thermodynamics) of the hydrogen release can significantly be enhanced at low temperatures.The incorporation of hydrogen storage materials in porous systems may alter the kinetic of desorption and absorption of H2 of the solid-state material, but it reduces the storage capacity of such composites drastically. [2]

For all systems in which hydrogen storage and release is not nearly thermoneutral, heat management is a crucial issue. If short refuelling times are desired, the heat exchanger must have a capacity of some hundred kW if the enthalpy of hydrogenation is in the order of 40 kJ per mol of H2. This poses severe engineering problems and limits the use of some of the technology options. For large volume storage, liquid hydrogen is probably the most promising option, as evaporation losses are not a severe problem under such conditions. For smaller- scale storage applications, such as in cars, pressure storage as used today appears to be the best solution, but this may change with possible advances in the other technologies. Reforming solutions do not seem to be able to reach the performance targets set for transportation applications, although they may play a role in hydrogen supply for APUs and the leisure sector. Adsorptive storage on high-surface-area materials has already reached a state which comes close to the limits expected from physical considerations, and thus it is improbable that much more progress can be achieved in this field. Advances of solid hydrogen storage materials based on hydrides are difficult to predict. Chemically, it appears

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that higher storage capacities can be achieved. The thermodynamics of these systems are not known at present, and the same holds for the kinetics. This is certainly a field where the exploration of possible storage materials still is worthwhile and promising materials may be discovered. Solid-state materials do have the potential to outperform physical methods of storage, such as cryo-storage or high-pressure technologies. However, on a shorter time scale, these latter technologies seem to be the ones closest to practical large-scale applicability. Although this section focuses mostly on experimental studies into hydrogen storage, theoretical investigations have made great progress over the last years and may develop into a highly valuable tool to guide the search for new hydrogen storage options and new materials. [2]

At this point it may be concluded, that a 70 MPa CGH2 system is currently the best-in-class option available for automotive on-board hydrogen storage. [3] For a detailed review on hydrogen storage materials and on hydrocarbon-reforming technologies, reference [2] is recommended for further reading.

Infrastructure issues

To set up a sufficiently dense (and sufficiently consumer-friendly) hydrogen filling station infrastructure, for example in the United States, approx. 12 000 gas stations need to be built. The underlying model assumes that in the 100 biggest metropolitan areas of the USA (comprising about 70% of total population) the maximum distance between two filling stations would not exceed two miles, resulting in 6500 intra-urban stations. On the freeways connecting these large conurbations, a filling station would have to be installed every 25 miles. This highway network would correspond to a number of 5500 additional stations. [1] Such a comprehensive filling station network (see Fig. 6) could serve about one million hydrogen vehicles and would cost approx. US$ 10 to 15 billion over a time period of 10 years. Considering Germany as another example, a network of 1000 to 2000 hydrogen filling stations would be required to provide a very convenient network. Similar to today‘s gasoline infrastructure, a hydrogen gas station would be able to serve hundreds of vehicles per day, since just three to five minutes are needed to re-fill a hydrogen car. This is not the case for pure battery electric vehicles. For battery technology and electric grid stability reasons, charging times of at least one to several hours or even longer periods are required; a standard public charging point becomes blocked for hours by just one customer. Hence, such a station could only serve a few vehicles per day. For EVs, there exists a strong inter- dependency between two normally distinct activities, namely ‗‗parking‘‘ and ‗‗refueling‘‘. Furthermore, the typical customer does not want to wait near the vehicle for extended time periods until it is recharged. On the other hand, the charging points are comparatively cheap even at low volumes (US$ 5000 to 10,000 including installation and excavation) leading to low initial costs for early fleet demonstrations. This is in particular valid when the conventional 230 V/16 A technology (e.g. chargers, connectors and wall outlets) could be used. Considering scenarios with an increasing penetration of the vehicle fleet with electric vehicles (i.e. >1 million zero-emission vehicles in Germany), the cost for the implementation of a local battery re-charging infrastructure approaches the initially much higher cost of a more centralized hydrogen infrastructure. This is caused by the high number of required charging pole installations. In fact, the ratio of public charging points to vehicles needs to be close to 1 or even higher. However, for small to mid-sized fleets of zero-emission vehicles, the infrastructure for pure battery or extended-range electric vehicles can be set up more easily due to the better scalability and the lower initial cost for a sufficiently dense network. But hydrogen offers a different and very important advantage: due to its high energy density, hydrogen as an energy carrier is the ideal partner for the intermediate storage of fluctuating, renewable energies. In doing so, excess amounts of sustainable energy sources such as solar and wind power can be made available not only for stationary but also for automotive applications. Let us consider, for example, the North German electric power grid, the so- called ‗‗E.ON Regelzone‘‘ (in the meantime acquired by the Dutch company TenneT). In October 2008, the power fed into the grid by wind turbines fluctuated—sometimes within a

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few hours, sometimes within days—between a maximum of approx. 8000 MW and virtually zero. An excess amount of available wind power, for example, caused at several points in time in late 2009 and early 2010 dramatic effects on the energy markets, such as significantly negative prices for electric energy at the European Energy Exchange (EEX). To solve similar challenges will become even more important and urgent when gradually the already approved or planned off-shore wind farms (e.g., in the North Sea) will come online later this decade. Hence it is self-evident that it would be extremely helpful to ‗‗buffer‘‘ excess energy in intermediate stores to handle these fluctuations, i.e. to absorb energy during a certain time period from the grid or, vice versa, to provide energy back to the grid in case of a high market demand. [1] Today, this ‗‗buffer‘‘ is realized as pumped hydro stores (the largest facility in Germany, Goldisthal, offers a maximum storage capacity of 8000 MWh or in compressed air reservoirs (typical salt cavern, volume two million m3 of volume, max. storage capacity 4000 MWh). If hydrogen is used as a medium instead of compressed air, up to 600,000 MWh energy could be stored in an identical salt cavern. Unlike conventional technology, hydrogen therefore offers not only a buffer store for short time periods ranging from a few minutes to hours, but also such a large-scale hydrogen store could absorb the excess wind energy of several days The stored gas eventually could be either converted back into electrical energy or could simply be used as a fuel for hydrogen vehicles. By contrast, even large fleets of pure battery EVs are not able to provide a competitive energy storage dimension: if 5 kWh of the usable energy content of an EV battery (for operating lifetime and customer ease-of-use considerations, 10% of the total nominal energy content should not be exceeded) could be subscribed to and utilized by the electric utility, just to replace the pumped hydro store of Goldisthal, 1.6 million EVs would be needed. Considering a smaller battery pack typical for urban vehicle applications, the required fleet size even needs to be significantly larger: if only a value of 2 kWh could be used, that number would increase to about 4 million EVs. Also, other large-scale stationary battery systems (based on Na–S or even advanced Li-ion technology) are by far not able to provide energy storage dimensions comparable to a hydrogen-based system. [1]

Conclusions

The long-term advanced propulsion strategy of virtually all car manufacturers consists in displacing gasoline and diesel as energy sources for automotive applications. This will be achieved by a continuously increasing electrification of the powertrain. However, the energy density of current and future automotive batteries unfortunately provides limitations for the development of pure battery electrical vehicles as soon as vehicle ranges significantly longer than 100 miles are required. Therefore, GM and Opel pursue the concept of the extended- range electric vehicle (E-REV) and the hydrogen fuel cell vehicle (fuel cell electric vehicle, FCEV) for this application field. Ranges of 500 km can be achieved with hydrogen fuel cell vehicles and 70 MPa CGH2 tanks; moreover, hydrogen can be produced on a large scale at competitive cost.

In the area of longer-range sustainable mobility, the future of automotive propulsion thus belongs to a smart combination of E-REV and FCEV vehicles and, for some urban applications, also to small-sized BEVs. During the early commercialization phases, all of these vehicles will be more expensive than comparable conventional vehicles. Therefore, the support and cooperation of all involved stakeholders are indispensable during this initial phase. The most important players here are primarily car manufacturers, energy companies and the governments. But also the end customer has to be willing to accept and purchase these innovative vehicles despite the remaining incremental cost and initial technology limitations compared to conventional vehicles. [1]

References [1] U. Eberle, R. von Helmolt, Energy Environ. Sci., 2010, 3, 689–699 [2] U. Eberle, M. Felderhoff, F. Schüth, Angew. Chem. Int. Ed. 2009, 48, 6608 – 6630 [3] R. von Helmolt, U. Eberle, Journal of Power Sources 165 (2007) 833-843 [4] R. von Helmolt, U. Eberle, ―Fuel Cell Vehicles 2010 – A Progress Report‖, 12th Ulm ElectroChemical Talks, 2010

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