The prospects for a hydrogen economy based on renewable energy Ireland s Transition to Renewable Energy November 1st, 2002 Thurles, Tipperary Werner Zittel, Reiner Wurster, L-B-Systemtechnik GmbH, Germany e-mail: zittel@lbst.de; Content 1. Why renewable energy and hydrogen 2. Components of a hydrogen economy 2.1 production techniques 2.2 storage and transportation 2.3 applications (mobile and stationary) 3. Renewable energy and transport sector 4. Hydrogen for Ireland - a short sketch 1. Why Renewable Energy Talking about hydrogen becomes meaningful only as part of an integrated sustainable energy strategy. Hydrogen produced from finite energy resources would not solve any problem satisfactorily. It is a simple fact, proven already by definition, that a sustainable energy system in the long run can only be based on renewable energy. And it is not a question whether we like it or not or whether we feel that our energy needs can be managed by renewable energies. What matters is to keep our needs low enough so they are met by renewable supplies. But what are the restrictions which force us to change our energy supply system completely towards renewable energies: In general, there are three main drivers which limit the use of fossil and nuclear fuels: First of all, there is a resource problem. Once living from finite resources we are confronted with the fact that these limit the use as long as we use them faster as they regenerate. The question is not whether we like to change to renewable energies, but when do the limits force us to change. However, the earlier we start with a strategy, the better we can control that strategy and avoid disruptions and situations, which force us to do things out of our own control. Secondly, there is also a sink problem of fossil fuel combustion. Not only the primary resources are limited but also the resources for neutralising our waste products.
Literally speaking, carbon dioxide emissions heat the earth s atmosphere. The civilized world, as unified under the UN umbrella has generally accepted that problem and is looking for its solution. In the same sense the waste products of nuclear energy accumulate and give some short and long term risks which are not accepted by many people. Finally, a positive driver would arise if new business opportunities are seen with increasing share of renewable energy. Such an early positive driver could be the technological progresses which are already visible in renewable energies. For instance, wind energy alone covers already more than one percent of electricity demand in Europe with annual growth rates of between 30 to 40 percent. Why Hydrogen? Within a renewable energy economy, various reasons point on hydrogen as part of an integrated energy strategy. These are: - We need a carbon free fuel for mobile applications in the mid-term future (beginning in the next years and to be implemented to a certain degree until about 2010) - The European automotive industry has reached a self-commitment through ACEA which aims at reducing the average CO 2 emission of the entire European car fleet for sale in 2008 to 140 g/ km and to 120 g/km in 2012. If this selfcommitment would not be met the European Commission would enforce it via legislative measures. At present, car manufacturers do not know how they can achieve these requirements with existing propulsion technologies or conventional fuels while maintaining the present mix of vehicle classes. - Hydrogen offers the most promising perspectives as fuel for mobile applications, since - it can be produced from various sources such as conventional electricity, renewable electricity, conventional steam reforming and regenerative steam reforming following a biomass gasification process and thus has the highest energy source flexibility of any alternative fuel. It therefore offers the most robust strategy to clean transportation fuels - it is suited for a broad spectrum of applications: from caloric burning as gas via chemical processes to electrochemical conversion in fuel cells - But in addition, via electrolysis and fuel cells hydrogen offers a perspective to store large quantities of electricity without raw materials limitations as, e.g., batteries do. Therefore it will be introduced as buffer storage of electricity for smoothing the load profile. The perspective is short to midterm (presumably within the next five years) for local applications as off-shore wind energy or smaller sized stand- alone systems based on renewable energies and long term (presumably beyond 2020) for large scale applications. Hydrogen cannot solve the energy problem, but it can help to smoothly switch the transport sector from fossil fuels to renewable fuels. In addition, hydrogen can create links between different energy carriers, e.g. between electricity, transport fuel and gas, thus linking heat energy supply with fuel supply and electricity production.
2 Components of a hydrogen economy The hydrogen energy vector includes various tasks which are briefly sketched in this chapter. 2.1 Hydrogen production from various sources At the beginning is the production of hydrogen. During combustion hydrogen is almost free of polluting emittents. Therefore its environmental benefits strongly depend on the way of production. In general, the following methods are applicable: Chemical by product Even today, hydrogen is produced world wide in large quantities, exceeding about ¼ or 1/5 of annual natural gas production volumes. A large portion of this hydrogen is due to unavoidable by-production during chemical synthesis - such as production of chlorine, acetylene, styrene or cyanide -, or petrochemical cracking processes such as catalytic reforming and cracking of crude oil during upgrading or ethylene production. However, the purity of the thus produced hydrogen varies between the different sources from more than 99.5 percent (chlorine synthesis) to below 60 percent (ethylene production). Even the well known city gas in the last century which was produced from coal gasification contained about 50 percent of hydrogen. Due to lack of nearby consumers most of this hydrogen gas is mixed with natural gas and burned locally for process heat generation. However, keeping in mind an introduction strategy for hydrogen fuelled transport, for instance, this gas might become available to build up first fuelling stations at reasonable cost. Hydrogen from fossil fuels Almost half of today s hydrogen demand is supplied by hydrogen from natural gas via steam reforming onsite, or at minor quantities via partial oxidation of oil products. Most production plants are close to the application site, in order to avoid transport infrastructure. Nearly all hydrogen used for desulphurisation at refineries, for ammonia production for fertilisers, or for methanol synthesis is produced that way. Usually this is the cheapest way to produce hydrogen. About 70 percent of the energy content of the natural gas input is converted into pure hydrogen gas. Hydrogen from electricity Another well established method is hydrogen production via electrolysis by splitting water into oxygen and hydrogen gas. Today this is the method of choice where cheap electricity sources are available. Indeed eldest electrolysers close to fertiliser plants work since almost 80 years in Norway, Egypt or elsewhere. All these large electrolysers use nearby hydroelectricity. Electrolysers are highly modular and can be build from very small units of a few cubic meter per hour hydrogen production or several kw electric power demand up to the multi megawatt regime. Today s modern electrolysers are optimised to produce hydrogen almost at a pressure level of 30 bar, and are intended to work very stable almost at fluctuating electrical power input.
The charm of electrolytic hydrogen is in the small scale and in principle simple technical realisation, its quiet and pollution free operation and the freedom of choice for the primary energy input. Therefore hydrogen can be produced close to or apart from the electricity production, from fossil sources as well as from renewable sources. This allows, for instance, for a smooth and steady transition from fossil to renewably generated hydrogen. However, economic and ecological efficiency strongly depend on the details of this production chain. For instance, electrolytically produced hydrogen from coal fired power plants would have CO 2 emission of about 2 kg/kwh or 6 kg/m 3. For comparison, gasoline emits about 0.27 kg/kwh during combustion. But hydrogen from wind power or solar electricity and water would be almost free of polluting emissions. Electricity production from renewable sources is more in line with natural cycles. But its cyclic variability counteracts economic principles, where the amortisation is the better the more the investment is used. But even taking these differences into account, some methods still can provide cheap hydrogen. For instance hydrogen from large scale hydropower might be the cheapest way to produce hydrogen, resulting in between 2.5-5 cents/kwh or 7.5-15 cts/m 3. Again for comparison, consumers in Europe pay about 10cents per kwh of gasoline, or 1 Euro per liter, including tax. But even the production from large offshore wind farms would result in calculated cost of about 10-15 cents/kwh or 30 45 cts/m 3. This is roughly in the same range as hydrogen produced from geothermal electricity, assuming that modern low temperature geothermal power generation (hot dry rock, organic rankine cycle) results in electricity cost of below 10 cents/kwh el. The higher electricity production cost are outweighed by the advantage of 8,000 operating hours per year for the electrolyser, instead of about 3,000 hours per year for wind energy converters. In average, from 4 kwh electricity about 1 m 3 or 3 kwh hydrogen are produced. About 20 25 percent of the energy are used for the conversion process. Therefore, at first glance, hydrogen production from electricity might not be the first choice within a future energy scenario based on renewable energies. Renewable electricity first should be used to substitute fossil fuels (predominantly coal) in stationary applications eventually much more efficient. But nevertheless, several reasons for electrolytically produced hydrogen are: First of all, instead of adapting the power generation to fluctuating demand profiles, it might be wise to produce power independent from the actual demand. The surplus electricity might be easily used for hydrogen production. This gives more flexibility in de-coupling demand and supply. Offshore wind energy parks might be far abroad. To avoid cost-intensive grid connection of this fluctuating electricity production with consumers, an economic choice might be to produce hydrogen at the offshore wind park and to transport the energy continuously in form of hydrogen to the final destination, satisfying electrical and transport fuel demand. De-coupled isolated island systems may need an electricity storage system, to become completely independent from outside sources. From a certain size onwards, hydrogen storage is more economic and even more ecological than pure electricity storage. Again, one should keep in mind, that fossil fuel storage
would be the more economic way, as long as no problems are seen with it, either in supply restrictions or in waste disposal problems (emissions). From a simple logical point, it might be the proper way, first, to introduce renewable electricity for total substitution of fossil electricity sources. Only, when this is done, and huge storage problems arise, one might start to convert the surplus renewable electricity into hydrogen. But from a realistic point of view, things develop different. There are different players with different interests within different demand sectors. In practice, the transport sector is the most urgent problem to cure from its oil dependence and accompanying damages. As already pointed out, hydrogen offers the fuel with the most options (broad variety of primary sources, modular structure from small scale to very large scale). Therefore in the early phases there will be some overlap and disconnection apart from the optimum pathway. But as long as the individual steps fit into the overall scheme they are worthwile to be done, even under sometimes sub optimal conditions. New technologies need market feed back for improvements already from the beginning. Hydrogen from biomass Hydrogen production from biomass gasification offers a completely independent method, more similar to steam reforming of natural gas, but this time from renewable energy sources. Indeed, this is the only path were it is more efficient to gasify biomass and to feed a fuel cell with the product gas for producing electricity, instead of producing the electricity directly from biomass burning by a thermal generator. But already at small scale of several MW this path shows the combination of different energy vectors. The following example is intended to give a feeling about the size of individual projects: Assuming that ten percent of the area are converted to fuel production, biomass harvesting within a radius of 25 km would feed a 5 MW gasification plant. The crude bio gas (CO, H 2 ) can be fed into a high temperature molten carbonate fuel cell which produces electricity and heat at about 450 C simultaneously. These amounts are sufficient to supply about 3000 houses with heat and electricity. After purging this would leave still enough hydrogen gas to supply 17 buses with a daily range of about 300 km. The only drawback of that scenario is, that this system has not yet proven its functionality at real scale. However, all components are known and technically feasible, though not yet optimised within that configuration. From today s knowledge, a first system would need close to 10 million invest and app. 2 million per year operating cost. This would cover all costs from biomass agriculture, gasification and purification plant, fuel cell, hydrogen compression including fuelling station. It can be expected that after a few years of learning experience, system and operation cost could be reduced by about 30 percent, each. The resulting products are about 4 million kwh of electricity per year and 16 million m 3 of hydrogen plus 4 million kwh of heat. If the electricity would be sold at 10 cts/kwh el and the hydrogen fuel at 1 per litre gasoline equivalent, the annual return would sum up to about 5 million.
Biomass gasification offers hydrogen conversion efficiencies somewhat lower than from natural gas reforming, in the order of 60 65 percent. Biomass for combined heat, electricity and fuel supply Biomass Biomass Transport by truck 25 km Biomass-gasification (5.000 kw th ) CO, H 2 heat electricity District heat 3.900 homes grid H 2 H 2 -compression CGH 2 vehicles 17 Buses Figure 1: Sketch of a biomass gasification plant supplying heat, electricity and transport fuel from agricultural sources such as wood or dedicated crops. Other hydrogen production methods In the long run further hydrogen production methods could extend the potential. These are already known to work theoretically or at laboratory scale. Not too far in the future gasification of biomass waste and even gasification of oil products waste will extend the sources for hydrogen production from biomass and waste. Another opportunity could be direct hydrogen generation by algae or bacteria which today is investigated in research laboratories. Potential of Renewable Elelctricity Sources in Europe It is completely impossible to establish the real or correct potential for renewable energy use today. An upper theoretical limit for PV, for example, would come from the solar irradiation incident on the EU-area. Assuming an energy conversion rate of 10 percent for the system, more than 100 times the total electricity demand of 0.7 kwh/m 2 could be supplied by solar energy (neglecting storage problems due to mismatch of demand and supply curves). Of, course many restrictions have to be included such as: how big a proportion of the land area does the public allow to be used by energy technologies. This limit however cannot be determined at present. As long as the annual additions increase, almost each estimate is based on artificial restrictions which are primarily based on the limits accepted by the scientific
common sense and are due to their fantasy. Maybe an analogy to the oil discoveries might give some hint. As long as the annual discoveries increased so did the estimates on how much oil will ultimately be found. But as soon as it became apparent that the annual discoveries had peaked in the sixties, it was plausible to extrapolate the trends to almost zero discoveries and to add these up to an ultimate estimated discovery. This is reflected in the history of EUR-estimates which produced the largest figures during the late 60ies and became more and more realistic the more the annual discoveries declined. To use that analogy it might be that as soon as it becomes apparent that the annual additions of renewable energy capacity are decreasing forecasters are able to make more realistic assumptions about the limits how much renewable energies are accepted by the public and to extrapolate to an ultimate usable renewable potential. In order to give an imagination of the possible size of the potential, figure 2 summarises the broad range of published and estimated possible installations of renewable energy technologies in Europe. The biomass potential is completely restricted to agricultural or municipal waste (sludge, wood etc.). The hydropower potential includes tidal power. Shown is the potential which still might be tapped in addition to already built power plants. By far the broadest range of estimates exists for wind energy. The lower figure is based on an estimate of the Royal Institute of Internal Affairs (M. Grubb) and includes offshore wind produced at valuable sites within 10 km off the coastline and not deeper than 10 m. The upper potential extends these ranges to 30 km distance off the coastline and 30 m water depth. Here it should be noted that already today at least one wind park with 1 GW size is being planned with a distance of about 60 km to the German coastline. Also the assessment of the PV potential varies in a wide range depending on differing assumptions. The lower figure is derived assuming only roof mounted PV installations. The upper figure also includes PV installations on the facade of buildings. This estimate is based on an in depth analysis for eight cities in the UK and an extrapolation to the whole EU by taking into account different solar irradiation as well as different building characteristics in the other regions. This potential is further split into two cases, assuming (1) today s conversion efficiencies (of about 10 percent) or (2) increased efficiencies of about 13 percent. Finally, the potential for building solar-thermal electric power plants (SOT ) reflects possible sites in southern Europe based on the Mediterranean Study carried out by the DLR. Not included is the potential for geothermal electricity production. To date its quantification seems to be very arbitrary. Even when one does not take into account possible and sensible uses of the waste heat which is inevitably produced during electricity generation or other sensible restrictions the potential easily could become as large as the whole electricity consumption in the EU.
TWh/a 4000 3500 EU Electricity Consumption 2000: 2.478 TWh (Source: IEA 2002) 3000 2500 Technological Progress 2000 1500 1000 500 Agriculture Bio Waste Forest Residue Straw Residue Wood Residues, other Industrial Wood Res. Upper Value LowerValue Wind offshore Roof mounted + house fronts with todays technology Roof mounted 0 * still to tap potential in the EU ** only EU Wind onshore min max min max Hydro * min max min max SOT ** Biogas Biomass Wind Power PV (Methane) Tidal Power conventional Sludge Figure 2: Minimum and Maximum potential for renewable energy use in the EU. 2.2 Storage and Transportation For practical applications such as mobile transportation hydrogen gas must be properly conditioned. Two different methods have proven their practicability in industry use for many years. These are liquefaction of hydrogen and compression of hydrogen. Both methods have advantages and draw backs. In principle, hydrogen can be handled and used in stationary and mobile applications already today, but improvements in its storage qualities are still desirable to increase its acceptance as mobile fuel. Liquefaction of hydrogen For long distance transport and for on board storage in automobiles liquid hydrogen offers highest energy densities of 2.36 kwh per liter. This is app. one quarter of the energy density of gasoline fuel. One litre of hydrogen weighs about 70 g whereas its energy equivalent, a quarter litre of gasoline, weighs about 200 g. However, in order to liquefy hydrogen the gas must be cooled down to 250 C. This process consumes about 14 kwh primary energy per kg of hydrogen in today s liquefaction plants with through put of 4.4 tons per day. This is about 40 percent of the energy content. Theoretically this effort can still be improved by a factor of three. It has to be seen how much of this will be realised in future energetically optimised liquefaction plants.
Another serious draw back of liquid hydrogen storage is the boil off loss during parking. Though this will not be a problem for long and short distance transport in huge quantities, it could become a serious constraint for individual car owners. Today s insulation materials allow for boil off losses of approximately 1 percent per parking day. Properly designed tanks will be filled with liquid hydrogen leaving a gas cap. The boiling hydrogen can accumulate in the gas cap. Any hydrogen demand during driving will first be served from this gaseous cold hydrogen. During parking ventilation to the outside of this boil off starts only when a certain overpressure in the gas cap is reached (usually above 5-6 bar). In a completely filled state-of-the-art design this would happen after several days without operation (app. 4-5 days). Compression of hydrogen To avoid the boil-off problem, hydrogen can also be handled under high compression at ambient temperature. The adequate bottles are cylindrical fiber tanks or metal tanks respectively liners or a mixture of both. These are still in the process of optimisation with respect to pressure resistance, durability, conformity and weight. Typical storage pressures are in the range of between 200 350 bar. But recent storage systems up to 700 bars are available with approval for daily use as gas storage system. Since the energy consumption for compression scales logarithmically, it is the lower the higher the input pressure already is. For instance, compression from 5 bar to 350 bar consumes about 9 percent of the energy content while compression to 700 bar consumes only slightly more (~10 percent). If the initial pressure is 30 bar instead of 5 bar, the energy consumption will almost half. Therefore, in a systems approach, a hydrogen production technology with high pressure offers advantages which might outweigh its larger costs. Other methods Another already established method is the storage in metal hydrides. Certain metal alloys implement large amounts of hydrogen when exposed to hydrogen at certain pressures and low temperature. Since this process is exothermic, heat is set free during refuelling. During driving the hydrogen is removed by stimulated heating of the storage device. Though metal hydrides offer high volumetric storage densities (close to liquid hydrogen), their gravimetric storage density is very poor and amounts in practical systems between 1 1.5 percent. However, in recent years much research was addressed to look for new storage material with superior quality. For instance highly porous activated carbon can adsorb hydrogen at cryogenic temperatures in quantities far above metal hydrides. In best cases, these are close to the densities of compressed hydrogen storage. However, none of these novel materials can today substitute the conventional technologies. But nevertheless, known research results offer that there is still some potential for optimisation. It is very likely that in the mid- term future some of these materials come up with comparable qualities with respect to today s methods. But for first (and presumably even second) generation vehicles, these materials will not substitute compressed or liquid hydrogen storage. Safety aspects Hydrogen is a gas which needs special safety considerations. It mixes three times as fast with air as natural gas does. It starts to ignite when more than about 4-5 percent hydrogen are mixed with air. This is similar to natural gas. But two differences
remain: First, ten times less energy is needed to ignite hydrogen, compared to natural gas. Secondly, the detonation limit the air-gas mixture which tends to explode rather than to burn is at about 11-18 percent, whereas for natural gas air mixtures it is with 5 6 percent only slightly above the ignition limit. The latter difference is remarkable. The energy content for ignition is very low in both cases. Therefore any ignitable agglomeration of hydrogen will burn with high probability. But the large difference to the detonation limit, tends to ignite hydrogen much easier than to explode it. In contrast, any ignitable mixture of natural gas also has the high risk to explode not so hydrogen. For instance, the famous accident of the Hindenburg Zeppelin in 1929 at Lakehurst, was initiated by spark ignition of badly designed materials of the outer aluminum paint covering the fabric of the Zeppelin s hull already known to the experts at that time. But once ignited, the hydrogen burned but did not explode. Moreover, due to three further characteristics of hydrogen, two thirds of the passengers on board survived that catastrophe. Hydrogen is very light weight. Thus the hydrogen went upwards, not spreading around and not covering the cabin below the frame. Secondly, in contrast to oil products, hydrogen is the simplest molecule composed of two identical atoms. This prohibits heat radiation which for physical reasons is only possible for more complex molecules. Thirdly, hydrogen combustion does not produce smoke as burning oil does. Though the fire was very close above the heads of the passengers in the Hindenburg, these were not hurt by the heat or smoke. All those passengers, who could keep cool and await for the sinking of the burning Zeppelin, survived when they run away in the short time between ground touch and breaking of the burning frame above the passenger cabin. If compared to today s fuels like gasoline or kerosene, many accidents with igniting fuel would not injure people in such a dramatic way, if the cars or aircraft were fuelled with hydrogen. But, high safety efforts are needed in closed rooms to avoid ignitable hydrogen air mixtures, including sensors and possible ventilation, at the top of the ceiling. In short, experts summarise the safety concerns as saying that hydrogen poses no more and no new risks than natural gas. Compared to liquid fuels it might be even superior as sketched above. 2.3 Applications Beside today s use of hydrogen in chemical industry, space transport, and future use as storage medium for energy transport and electricity storage as already outlined hydrogen will be used in mobile and stationary applications Mobile For rockets it was used already from the beginning of space technology due to its low weight. But even for fuelling ordinary aircraft it has some advantages. Due to the low weight it is expected that aircraft can carry about 20 25 percent more payload. This could at least partly - outweigh the higher fuel costs. Safety advantages with respect to kerosene have already been mentioned. Additionally, carbon emissions are avoided thus making aircraft less harmful to the environment. However, it has to be stressed that water vapour emissions will increase, which are mainly responsible for contrail formation and infrared reflection. The height of a flight will be crucial to this phenomenon. As long as aircraft will fly in the troposphere, not surpassing the tropopause in some 8 12 km height (depending on geographic and climatic
conditions), in total, the impact will be much less harmful than with today s kerosene fuelled aircraft. Another broad application, if not the most important one will be the use as fuel for cars and trucks. Again, local emissions are restricted to pure water vapour. These will change local humidity in the order of several per mil with respect to natural precipitation patterns, at worst. But carbon dioxide provided hydrogen is produced from renewable energies as well as hydrocarbon and particulate emissions are avoided. Even nitrogen oxide emissions can be reduced close to zero, depending on the technological realisation of new drive systems. Further use of internal combustion engines can reduce nitrogen oxide emissions due to properly designed fuel/air mixtures, while electric drive systems with fuel cells will completely avoid any harmful emission. But there are also plans to convert railways to hydrogen fuel. Here, the spare volume is not a critical parameter. In addition, electric drive systems based on fuel cells would become completely independent from outside electricity supply, thus completely avoiding catenary. Even in the maritime transport, ships can be fuelled with hydrogen. Indeed, first small ships are already converted to hydrogen in ecologically sensible areas. Again, fuel storage is not a restrictive parameter in this area. In short any mobile vehicle, which today is fuelled with oil derivatives can be converted to hydrogen fuel. Stationary In principle any application which today is driven by fossil fuels, can be converted to hydrogen use without loss of its function. But here, very often various non fossil alternatives might be superior. For instance, electricity generation by wind or photovoltaics is much more efficient than hydrogen production (maybe via electrolysis) and later re-electrification via fuel cells or internal combustion engines with adjacent generator. Even heat is better generated from solar energy or biomass burning instead from hydrogen combustion. For a long time (and even for some people today) this set the basis of the general understanding that the practical use of hydrogen will become important when a huge storage system for electricity is needed. Since this was seen decades away from now, there was the importance of hydrogen also seen decades from now, often specifying the date as around or beyond the year 2050. Beside this, only in few niche applications, justification for hydrogen was seen. These are mainly in stand alone systems. For instance, to completely supply a house or a small village with renewable energy without grid connection, gives rise for storing solar energy from periods with high solar irradiation to its use during periods of negligible solar radiation. To store the electricity would need enormous amounts of batteries, which are cost, and material consuming. Here, the conversion of electricity to hydrogen and its further use for re-electrification during peak power demand has some rationale. But its stronger justification comes from the higher variability. The
stored hydrogen can not only be used for re-electrification, but also as fuel for cars or for producing heat. Therefore it is the link between various uses, which in a fully integrated systems view, makes it superior to alternatives. However, the situation changed for two reasons in favour of hydrogen: First, technological progress brought the use of fuel cells as conversion system into reach. This technological progress, if fully converted into commercial products, gives hydrogen fuelled fuel cells a high chance for efficient applications Secondly, again connected with the fuel cells, this offers the first serious alternative to today s conventional driving system for cars, the internal combustion engine. For the first time it became obvious, that hydrogen fuelled cars might be used without loss of convenience for the customer. The latter aspect, in addition reached further support from the fact, that big car companies more and more realise the challenge of an imminent oil supply crisis, which would be crucial for their business if not any alternative could be brought to the market in time. As already pointed out, of course, there might be several alternatives for any of these applications. But from an integrated systems approach, hydrogen offers the broadest variability and broadest spectrum of applications and linkages which makes its introduction promising and stable in the long run. 2.3 The fuel cell a key component within a hydrogen economy But what is the technical revolution connected with a fuel cell? In the following the basic principles of fuel cells are sketched. A fuel cell consists of two electrodes (highly porous carbon or another electrically conductive porous material) which are separated by an electrolyte. Hydrogen gas is feed to one electrode. At the inner surface of the electrode, the hydrogen molecule is decomposed into its constituents, the electrons are stripped off and the two nuclei are separated. The electrolyte is permeable for protons (which are the positive nuclei of hydrogen atoms), but not for electrons. The protons penetrate through the electrolyte to the other electrode. There they can combine with the electrons and air (or pure oxygen) to form water vapour. Since water vapour has much less energy content than separated hydrogen and oxygen molecules, this process runs by itself, once the prerequisites are given. The electrons cannot penetrate through the electrolyte, therefore they have to bypass it through an electrical conductive wire, which is connected with the application. The bypassing electrons create the electrical current used by the consumer. This is the basic principle of a fuel cell. In detail, various different materials have shown their ability to trigger this process. These are mainly based on different electrolyte materials, which allow not only protons, but sometimes only more complex molecules to penetrate through the membrane. The electrolyte determines the specific name of a fuel cell, but the principle always is similar. This distinguishes alkaline fuel cell (AFC), proton exchange membrane fuel cell(pem), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC).
Each of these fuel cell types has its different operating regimes and application ranges. For instance, SOFC have highest operating temperatures of about 1000 C. MCFC have an operating temperature of about 600 C. Moreover they can consume carbon rich gases like bio gas directly without complicated gas purging processes in advance. PEM fuel cells operate at lowest temperatures. But the draw back is that at these low temperatures highest gas purging efforts are needed. Moreover, at low temperatures the mobility of ions and the chemical reactions are very slow. To initiate the stripping of the electrons at the inner electrode surface needs expensive catalytic materials, predominantly platinum based metals. A single fuel cell is composed of the electrolyte at the centre, the two electrodes at each side of the electrolyte and a further sheet in between two cells which includes channels for the transport of input and output gases. Many individual cells are stacked beside each other to form a fuel cell stack. The number of cells within a stack determines the power output characteristics of the fuel cell stack. Electrolyte old and new cell old stack (5kW) Source: Daimler-Benz; DeNora Figure 3: Composition of a fuel cell. The membrane (top view) is sandwiched in between two electrodes and covered by a plate with channels for input and exhaust gases. Many identical cells are put together to form a stack. The voltage of each cell sums up to the whole stack voltage. Once the optimal configuration of a single cell is found, it is very simple to reproduce it for many times. Therefore this is a process, best suited for automatic mass production which offers a high cost reduction potential for mass production. In early phases of fuel cell research the cost of a single cell including high amounts of platinum were prohibitive for use in commercial products. But successive and several sudden improvements helped to reduce the material requirements so strongly that it became visible that mass produced optimised fuel cells could become relatively cheap and simple to produce.
These are the major reasons behind the high cost today as long as each cell is fabricated by individual engineers and the prospects of becoming very cheap once an optimal cell configuration is found and mass produced. But another advantage of a fuel cell drive system compared to internal combustion engines is that it will become much more efficient. An internal combustion engine follows the laws of thermodynamics, which restrict the conversion efficiency of a combustion process to the temperature levels involved. For instance, in average, electricity production from heat power stations has about 33 percent efficiency. Moreover, for moving cars, the efficiency is also determined by the driving situation, and this is governed by the velocity. At very low speed technically speaking, at partial load and low angular momentum much energy is needed for acceleration,. Since during typical driving many stops and acceleration processes are involved, the average efficiency of today s cars is very low, somewhere close to 20 percent. In contrast, fuel cells work completely different as sketched in figure 4. First, this is an electrochemical process, which obeys different thermodynamical limitations. The conversion efficiency is restricted by different parameters. Theoretically, an upper limit above 90 percent can be reached. Secondly, this efficiency also strongly depends on the load. But this time in the opposite manner. The efficiency is the higher, the less power is required with respect to the maximum available power. Moreover, the electric drive system has highest efficiencies at low velocities, which happens during acceleration at very low speed. This helps, to double the overall efficiency of a typical driving cycle. Cycle efficiencies close to 40 percent are already measured at today s realised hydrogen cars. It can be imagined that with technological progress, fuel cell drive systems will become smaller and cheaper. Therefore they can be oversized which, in turn, will increase the energetic efficiency even further. These sketched aspects feed the hopes that fuel cells have a great potential to completely substitute in not too distant future today s combustion engines in almost any application. However, to suppress the enthusiasm a little, we also have to realise that today we are in the very early days of the fuel cells. But, nevertheless, keeping in mind technological revolutions in other fields such as from transistors to microelectronics and from mainframe computers to small personal computers and further more from wired telephones to wireless small cellular phones, which are still converting to small multimedia machines it can be anticipated, that such a revolution might accelerate very fast completely substituting the old techology in one or two decades, once the first steps for commercialisation take off. The fuel cell is the technological key component for hydrogen use in transport sector and maybe also for stationary applications. But there are also other conversion technologies which fit well into hydrogen strategies. These are gas turbines either for fast power generation or maybe much more important for powering aircraft. Years ago some research was also directed towards catalytic burning of hydrogen. At relative low temperatures hydrogen gas burns at a ceramic surface not as hot spot, but at a broader heated area, avoiding almost any polluting emission.
η fuel cell system 50 % combustion engine city interurban highway Figure 4: Dependence of the efficiency of fuel cell drive system and internal combustion engine on average driving speed 3 Hydrogen and the transport sector General To introduce renewable energy into the transport sector various alternatives are possible. These are direct use of electricity in electric vehicles, direct use of biogas (methane), direct use of biofuels (plant oil), use of ethanol from lignocellulose, conversion of biomass into synthetic fuels (e.g. via the Fischer-Tropsch process), gasification of biomass and conversion to hydrogen. Figure 5 shows the potential for fuel production from biomass sources in the EU. The total EU fuel consumption is about 3,500 TWh where about 80 percent are for road transport. By far the smallest potential exists for biogas production or for plant oil since only special plants or certain parts of the plants can be used for these fuels. Synfuel, methanol or hydrogen production results in larger amounts available since nearly all kinds of biomass can be converted to fuels in these processes. But even these quantities are far from being sufficient to fuel the whole transport sector one day.
TWh/a 3500 3000 2500 2000 Rail Transport Inland Navigation Aviation Road Transport Potentials show possible alternatives and cannot be added. Available area for cultivation of energy plants in the EU: 3,3-26,4 Mio ha Cultivation (fast growing plants) 1500 1000 Wood and Straw Residues 500 Via Biogas 0 Demand min max min max min max min max min max min max (Transport) 1998 * Biogas (Methane) Hydrogen (pressurized) Methanol Synfuel Plant Oil Ethanol from Lignocellulose *Source: IEA-Statistics 1997-1998 Figure 5: Fuels derived from biomass Therefore from a strategic point of view looking beyond niche market applications only biomass converted to synfuel or hydrogen will be really important in future developments since their potential can be increased by the fuel supply from other sources. In case of hydrogen production this is electrolysis from renewable electricity. As shown above the potential for electricity production is higher than the total EU electricity consumption today. Therefore it might be anticipated that a certain share can be used for hydrogen production. Figure 6 shows the hydrogen production potential from renewable electricity generation based on the data of figure 2. Using this figure it has to be emphasised that the shown potential holds only if all of the produced electricity is used for hydrogen production. Certainly this is not realistic. But nevertheless together with the hydrogen which can be produced from biomass and in the long term maybe also from other sources (Algea), and taking care of improved energy efficiency of future fuel cell cars, hydrogen is the only renewable energy option which offers the possibility to supply the whole transport sector. It could be topped only by the direct use of electricity in vehicles. But this option is not even a hope for the time being since all technological developments so far do not indicate that battery technology will become feasible for everyday cars.
4000 Rail transport [TWh/a] 3000 Inland navigation Aviation Solar thermal power stations 3) Road transport PV (roofs) 2000 Wind off-shore Wind on-shore 1000 0 CGH 2 LH Methanol Consumption m in m ax m in m ax m in m ax (Transport) 1998 1) Hydro power 2) 2 1) Source: IEA-Statistics 1997-1998 2) still to tap potential 3) within the EU Figure 6: Fuels from electricity production Comparing the most promising fuel paths from well to wheel it becomes obvious that no renewable fuel will be cost competitive with today s diesel or gasoline when compared at a pure cost basis neglecting taxes. Hydrogen will be a factor of 4 5 more expensive. But compared with today s gasoline prices (including tax) the difference reduces to a factor of two or even less. On the other hand fuel cell vehicles will be about twice as efficient than today s vehicles thus reducing the fuel consumption by a factor of two. Therefore the specific fuel costs per vehicle kilometer could remain almost constant even when a tax will be included in the long run which guarantees a similar income for the state as today. In addition the renewable fuel pathways are much less carbon intensive than diesel and gasoline and would reduce the overall carbon emissions considerably.
0,25 Fuel Costs [EUR/kWh] 0,2 0,15 0,1 0,05 Tax Without Tax 0 Gasoline Diesel Methanol NG Methanol Wood Residue CGH2 NG CGH2 Wood Residue CGH2 Wind off-shore LH2 NG LH2 Wood Residue LH2 Wind off-shore LH2 SOT LH2 Geothermal Figure 7: Fuel cost of hydrogen and methanol Environmental aspects As already pointed out, environmental and economic aspects of hydrogen strongly depend on the whole fuel chain. The following figures five and six summarise the carbon dioxide emissions and cost of kwh of hydrogen produced from various sources and compare with respect to today s gasoline and diesel supply. These calculations include all economic costs, but exclude external (=Environmental) costs which are not included in today s economic calculations.
CO2-Equivalent [g/kwh] 450 400 350 300 250 200 150 100 50 0 Gasoline Diesel Methanol NG Methanol Wood Residue Figure 8: Emissions CNG, p(in) = 0,1 MPa CNG, p(in) = 4,0 MPa LNG CGH2 NG CGH2 Wood Residue CGH2 Wind off-shore LH2 NG LH2 Wood Residue LH2 Wind off-shore LH2 SOT LH2 Geothermal Tank-to-wheel Well-to-tank A road map to hydrogen infrastructure Today the discussion and plans of car makers concentrate on the introduction of hydrogen as fuel for ground transport. A fuelling station will look very similar as today, either with liquid hydrogen or with compressed hydrogen. However, due to the broader variability, more technological and economic options are feasible. For instance, hydrogen could be produced centralised, liquefied and delivered to fuelling stations by truck, just as today with gasoline. Or gaseous hydrogen could be delivered via pipeline just as today natural gas is distributed. However, additional options arise from the fact, that hydrogen production technologies are highly modular from small to large scale. Therefore, electrolytic production is also possible onsite at the fuelling station with grid electricity, but, maybe, using green electricity from wind power stations or other sources apart. At country side hydrogen from biomass might be the technology of choice, where the plant is somewhat apart from the fuelling station, both are connected with a short local pipeline. This varity of options allows different players to operate fuelling stations: The large industrial integrated company like today, but also small independent fuel producers with small capital needs. It might be anticipated that large store houses also offer green hydrogen at their parking areas just like green electricity producers today, or whatever combination of independent fuel producer we like to imagine.
Early research with hydrogen powered cars started already in the late 70ies. For example, BMW started its research at this time which continues until today. At present, cars with internal combustion engine and liquid hydrogen storage are awaiting commercialisation at BMW. Ford is another company which in the beginning also puts some hopes on internal combustion engines fuelled with hydrogen. But though several others experimented with internal combustion engines years ago, today they put their cards completely on fuel cell driven hydrogen cars. At the forefront of these are Honda, Toyota, General Motors and maybe DaimlerChrysler. In 1996 world wide the first hydrogen fuelled bus in regular service with internal combustion engine started in Erlangen and later on in Munich, Germany. In 1999 the first public hydrogen fuelling station opened at the Munich airport. This site is illustrated in figure 9. At year end 2002 the following hydrogen fuelling stations were in service or under construction: Erlangen LH2 Station 1996-1998, this is a mobile refuelling station for liquid hydrogen. It was build by Linde to fuel the first hydrogen bus in regular public service In Hamburg several customers unified under the W.E.I.T.-project to finance 6 small scale hydrogen fuelled vans and a fuelling station for compressed hydrogen. The station was build in 1999, but after several years of successful operation dismantled. There are negotiations to reopen it in Milan or another North Italian locality in the near future. Munich Public LH2 refuelling Station 1999 at Munich airport, operative still today. Within the California Fuel Cell Partnership a government led initiative to accelerate and co-ordinate the introduction of emission free vehicles in California the first North American hydrogen fuelling station was opened in 2000 In 2001 a liquid hydrogen filling station opened at the BMW site in Oxnard, California In 2002 two Japanese hydrogen fuelling stations opened in Osaka and Takamatsu. Within the so called WE-NET project, several further hydrogen fuelling stations in the larger area of Tokjo are already under construction. The aim is to demonstrate various production methods from chemical surplus hydrogen usage to partial oxidation of oil derivatives. An actual list of hydrogen refuelling stations for road vehicles can be found at http://www.h2cars.de.
Figure 9: Hydrogen fuelling statoin at Munich Airport, right figure: overview of the whole station; left figure automatic refuelling of a car with liquid hydrogen Today several companies more or less supported by governments communicate hydrogen introduction strategies for hydrogen vehicles. But it is also confusing, to see some of these changing their topics from time to time, thus transmitting the message of a far distant future. Though all car companies focus on hydrogen in the long term, it is not quite clear if there will be a bridging strategy with bio fuels or natural gas in the medium term. But due to the fact that natural gas consumption cannot be extended to fuel a large fraction of the transport sector, and due to the several billion dollar already spent by the car industry for hydrogen it is hard to believe that the car industry sees this strategy several decades away from now. Within Germany, an industry consortium (TES- transport energy strategy) composed of car manufacturers, fuel suppliers and accompanied by government acceptance wrote a sketch for a road map for hydrogen introduction in Germany. It is anticipated that about 15-20 percent of all filling stations (or app. 2000 filling stations) must offer hydrogen until it is accepted by the public. The road map assumes that between today and 2005/2007 only small car fleets around individual filling stations are realised to test public acceptance and proof every day usage of hydrogen cars. For the erection of the network with refuelling stations, about five years planning and construction time are assumed between 2005-2010. At the end of this time span, the mass production and availability of hydrogen fuel cell cars might start, continuously rising its share on new cars from about 100.000 fuel cell vehicles per year in 2010. Actually only Japan and the United States of America seem to have a systematic, government supported introduction strategy for hydrogen infrastructure of vehicles. The Japanese initiative has already several years of continued strategic and financial support. Just end of January 2003, the U.S. president announced a 1.2 billion dollar introduction strategy for hydrogen infrastructure in the US, which covers a five years period.