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Hydrogen solar concept home
The authors believe that small-scale, distributed hydrogen at the residential level like this concept home illustration makes the most economic sense in a 'Hydrogen Economy.'

The Future of the Hydrogen Economy - Part 3

Conclusion of white paper on 'Hydrogen Economy' feasibility.

By Baldur Eliasson and Ulf Bossel

To Part Two

Delivery of Hydrogen

Energy Needed to Deliver Hydrogen by Road Transport
A hydrogen economy would certainly involve some hydrogen transport by trucks. There are other options for a hydrogen infrastructure, but road transport will always play a role, be it to serve remote locations or to provide back-up fuel to filling stations at times of peak demand.

The comparative analysis is based on information obtained from the fuel and gas transport companies Messer-Griesheim [6a], Esso (Schweiz) [6b], Jani GmbH [6c] and Hover [6d] some of the leading providers of industrial gases in Germany and Switzerland. The following assumptions are made: Hydrogen (at 200 bar), methane (at 200 bar), methanol, propane and octane (representing gasoline) are trucked from the refinery or hydrogen plant to the consumer. The delivery of liquid hydrogen is not considered at this time.

In all cases, trucks with a gross weight of 40 tons are fitted with suitable tanks or pressure vessels. Also, at full load the trucks consume 40 kg of Diesel per 100 km. This is equivalent of 1 kg per ton per 100 km. The fuel consumption is reduced accordingly for the return run with emptied tanks. We assume the same engine efficiency for all transport vehicles.

Furthermore, the hydrogen and methane pressure tanks can be emptied only from 200 bar to about 42 bar to accommodate for the 40 bar pressure systems of the receiver. Such pressure cascades are standard praxis today. Otherwise compressors must be used to completely empty the content of the delivery tank into a higher-pressure storage vessel. This would not only make the gas transfer more difficult, but also require additional compression energy.

Therefore, pressurized gas carriers deliver only 80% of their freight, while 20% of the load remains in the tanks and is returned to the gas plant.

Each 40-ton truck is designed to carry a maximum of fuel. For methanol and octane the tare load it is about 25 tons, for propane about 20 tons because of some degree of pressurization. At 200-bar pressure a 40-ton truck can deliver about 3.2 tons of methane, but only 320 kg of Hydrogen. This is a direct consequence of the low density of hydrogen, as well as the weight of the pressure vessels and safety armatures. In anticipation of technical developments, the analysis was performed for 500 kg of hydrogen, of which 80% or 400 kg are delivered to the consumer. With this assumption, 39.6 tons of dead weight have to be moved on the road to deliver 400 kg of hydrogen.

The results of this analysis are presented in Figure 6. The energy needed to transport any of the three liquid fuels is reasonably small. It remains below 5% of the HHV energy content of the delivered commodity for a delivery distance of

500 km. More energy is needed to truck methane. But the relative energy consumption becomes unacceptable for hydrogen at almost any distance.

The following note may serve to illustrate the consequences of the scenario. A mid-size filling station on any frequented freeway easily sells 25 tons of fuel each day. This fuel can be delivered by one 40-ton gasoline truck. But it would need 21 hydrogen trucks to deliver the same amount of energy to the station, i.e. to provide fuel for the same number of cars per day. Efficient fuel cell vehicles would change this number somewhat, but not considerably. The transfer of pressurized hydrogen from the truck to the filling station takes much more time than draining gasoline form the tanker into an underground storage tank. The filling station may have to close operations during some hours per day for safety reasons.

Today about one in 100 trucks is a gasoline or diesel tanker. For hydrogen distributed by road one may see 120 trucks on the road, 21 or 17% of them transport hydrogen. One out of six accidents involving trucks would involve a hydrogen truck. This scenario is unacceptable for political and social reasons.

Energy Needed to Deliver Hydrogen through Pipelines
Hydrogen pipelines exist, but they are used to transport a chemical commodity from one to another production site. The energy required to move the gas is irrelevant in this context, because energy costs are part of the production costs.

This is not so for energy transport through pipelines. Normally, pumps are installed at regular intervals to keep the natural gas moving. These pumps are energized by energy taken from the delivery stream. About 0.3% of the natural gas is used every 150 km to energize a compressor to keep the gas moving [7].

The assessment of the energy consumed to move hydrogen through pipelines must be based on a rough comparison with natural gas pipeline operating experience. The comparison is done for equal energy flows, i.e. the same amount of energy is delivered to the customer through the same pipeline either in the form of natural gas or hydrogen. But it is well established, though, that existing pipelines cannot be used for hydrogen, because of diffusion losses, brittleness of materials and seals, incompatibility of pump lubrication with hydrogen and other technical issue. The comparison further considers the different viscosities of hydrogen and methane.

The theoretical pumping power N [W] requirement is given by

The symbols have the following meaning:

Furthermore, the flow of energy through the pipeline, Q [J/s] is given by
with HHV being the higher heating value of the transported gas. Combining equations (2), (3), (4) and (5) one can asses the theoretical pumping power NH2 for hydrogen and NCH4 for methane and relate both to each other. One obtains

Since the pumps run continuously, the power ratio also represents the ratio of energy consumption.

Because of the low volumetric energy density of hydrogen, the flow velocity must be increased by over three times. Consequently, the flow resistance is increased, but the effect is partially compensated for by the viscosity difference. Still, about 4.6 times more energy is required to move hydrogen through the pipeline than is needed for the same natural gas energy transport.

Figure 7 shows the results of this approximate analysis. While the energy consumption for methane (representing natural gas) appears reasonable, the energy needed to move hydrogen through pipelines makes this type of hydrogen distributions difficult. Not 0.3% but almost 1.4% of the hydrogen flow is consumed every 150 km to energize the compressors. Only 60 to 70% of the hydrogen fed into a pipeline in Northern Africa would actually arrive in Europe.

Energy Needed to Generate Hydrogen at Filling Stations
One option for providing clean hydrogen at filling stations and dispersed depots would be to generate the gas on-site by electrolysis. Again, the energy needed to generate and compress hydrogen by this scheme is compared to the HHV energy content of the hydrogen delivered to local customers. Natural gas reforming is not considered for reasons stated earlier.

The analysis is done for hydrogen energy equivalent of conventional fuel necessary to serve 100 to 2,000 conventional road vehicles per day at a single gas station. On the average, each car or truck is assumed to accept 60 liters of gasoline or diesel. The hydrogen energy equivalent would be about 1,700 to 34,000 kg H2 per day for 100 and 2000 vehicles per day, respectively.

The efficiency of the electrolysers varies from 70 to 85% for 100 and 2,000 vehicles per day, respectively. Also, losses occur in the AC-DC power conversion. Between 4 and 73 MW of power are needed for making hydrogen by electrolysis.

Additional power is needed for the water make-up (0.1 to 2.2 MW) and for the compression of the hydrogen to 200 bar (0.4 to 6.MW). In all, between 5 and 81 MW of electric power must be supplied to the station to generate hydrogen for 100 to 2,000 vehicles per day.

It may be of interest that between 15 and 305 m 3 of water are consumed daily. The higher number corresponds to about 3.5 liters per second.

The results of this analysis are presented in Figure 8. The total energy needed to generate and compress hydrogen at filling stations exceeds the HHV energy of the delivered hydrogen by at least factor 1.5. The availability of electricity may certainly be questioned. Today, about one sixth of the energy for end-use is supplied by copper wires. The generation of hydrogen at filling stations would make a threefold increase of the electric power generating capacity necessary.

The Limits of a Hydrogen Economy
The results of this analysis indicate the weakness of a "Pure-Hydrogen-Only- Economy". All problems are directly related to the nature of hydrogen. Most of the problems cannot be solved by additional research and development. We have to accept that hydrogen is the lightest of all gases and, as a consequence, that its physical properties are incompatible with the requirements of the energy market.

Production, packaging, storage, transfer and delivery of the gas, in essence all key component of an economy, are so energy consuming that alternatives should be considered. Mankind cannot afford to waste energy for idealistic goals, but it will look for practical solutions and select the most energy-saving solutions. The Pure-Hydrogen-Only solution may never be acceptable.

But the degree of energy waste depends on the chosen path. Hydrogen generated from rooftop solar electricity and stored at low pressure in stationary tanks may be a viable solution for private buildings. On the other hand, hydrogen generated in the Sahara desert, transported to the Mediterranean Sea through pipelines, then liquefied for sea transport, docked in London and locally distributed by trucks may not provide an acceptable energy solution at all. Too much energy is lost in the process to justify the scheme.

But there are solutions between these two extremes, niche applications, special cases or luxury installations. For instance, combusting the hydrogen at the same site where it is produced, as the Norwegian company Norsk Hydro suggested some years ago, is probably a workable solution. Simply because there is no transport and storage involved. Norsk Hydro proposed to separate natural gas on shore into hydrogen and carbon dioxide, sequester the carbon dioxide under the North Sea and burn the hydrogen in a power plant to make clean electricity.

As stated in the beginning, hydrogen may be the only link between physical energy from renewable sources and chemical energy. It is also the ideal fuel for modern clean energy conversion devices like fuel cells or even hydrogen engines. But hydrogen is not the ideal energy carrier between primary sources and distant end users. New solutions must be considered for the commercial bridge between the hydrogen electrolyser and the hydrogen consumer.

Methanol Energy Economy
The ideal energy carrier would be a liquid with a boiling point of at least 60°C and a point of solidification below -40°C. Such energy carrier would stay liquid under normal weather conditions and at high altitudes. Gasoline, diesel and methanol are good examples of such fuels. They are in common use not only because oil companies distill them from crude oil or natural gas, but mainly, because they qualify for widespread use because of their physical properties. Even if oil had never been discovered, the world would not use synthetic hydrogen, but a synthetic hydrocarbon fuel. Gasoline, diesel, heating oil etc. have emerged as the best solutions with respect to handling, storage, transport and energetic use. With high certainty, such liquids will be synthesized from hydrogen and carbon in a distant energy future.

Methanol is certainly a serious candidate. It carries four hydrogen atoms per carbon atom. It is liquid under normal conditions. The infrastructure for liquid fuels exists. Also, methanol can either be directly converted to electricity by Direct Methanol Fuel Cells (DMFC), Molten Carbonate Fuel Cells (MCFC) and Solid Oxide Fuel Cells (SOFC). It can also be reformed easily to hydrogen for use in Polymer Electrolyte Fuel Cells (PEFC or PEM). Methanol could become a universal fuel for fuel cells and many other applications.

But the synthesis of methanol requires hydrogen and carbon atoms. In a future sustainable energy world carbon could come from plant biomass, from organic waste and from captured CO2. Typically, biomass has a hydrogen-to-carbon ratio of two. In the methanol synthesis two additional hydrogen atoms could be attached to every biomass carbon. Carbon from the biosphere may become the key element for in a sustainable energy future. Instead of converting biomass into hydrogen, hydrogen from renewable sources should be added to biomass to form methanol. Carbon atoms should stay bound in the energy chain as long as possible. They are returned to the atmosphere (or recycled) after the final use of energy. But synthetic methanol is one of a number of options to be seriously considered for the planning of a clean and sustainable energy future.

Time has come to shift the attention for a ioHydrogen Economyle to a "Methanol (or else) Economy" and to direct manpower and resources to find technical solutions for a sustainable energy future characterized by two closed natural cycles of water and CO2 or hydrogen and carbon.

Energy and the Hydrogen Economy (Updated 08Jan2003) 1.2Mb

References
[1] Ulf Bossel and Baldur Eliasson, Energy Efficiency of a Hydrogen Economy. To be published
[2] Handbook of Chemistry and Physics, recent editions
[3] G. H. Aylward, T. J. V. Findlay, Datensammlung Chemie in SI-Einheiten, 3. Auflage (German Edition), WILEY-VCH, 1999
[4] E. Schmidt, Technische Thermodynamik. 11th Edition, Vol.1, p287 (1975)
[5a] Burckhardt Compression AG, Winterthur / Switzerland (private communication)
[5b] Linde Kryotechnik AG, Pfungen / Switzerland (private communication)
[6a] Messer-Griesheim AG, Krefeld / Germany (hydrogen gas, private communication)
[6b] Esso (Schweiz) AG, Zurich / Switzerland (gasoline and diesel, private communication)
[6c] Jani GmbH & Co. KG, Seevetal / Germany (propane, private communication)
[6d] Hoyer GmbH, Köln / Germany (liguid natural gas, private communication)
[7] Swissgas Schweiz AG, Zurich, Switzerland (private Communication)
[8] VDI Wärmeatlas, VDI Düsseldorf, Germany 1977
[9] Synthetic Fuels, R.F.Probstein and R.E.Hicks, Mc-Graw Hill,1982
[10] H. Audus, Olav Kaarstad and Mark Kowal, Decarbonisation of Fossil Fuels:
Hydrogen as an Energy Carrier, CO2 Conference, Boston/Cambridge 1997, published in Energy Conversion Management, Vol. 38, Suppl., pp. 431-436.
[11] Hydrogen as an Energy Carrier, C. J. Winter and J. Nitsch, Editors, Springer Verlag, 1988

Times Article Viewed: 6964
Published: 11-Jan-2003

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