Dossier Océan et énergie - Énergie Thermique des Mers
Sommaire IOA News Letters
The Abstract of Hygen project of producing large quantities of hydrogen and possible fresh water if open cycle OTEC is used to produce the electricity used in the electrolysis process
C. S. Hopman
Dipl. Ing. ETH, New Caledonia
A method is presented for producing large quantities of hydrogen (H2), and possibly drinking water, cheaply. It involves high temperature electrolysis performed with heat obtained from combustion, solar, nuclear fusion or other 'hot' sources and electricity acquired from 'colder' processes such as OTEC (ocean thermal energy conversion), or hydro-electric power generation.
The advantages of combining hydrocarbon combustion and OTEC with this process, for instance, would be:
- when hydrocarbon fuels are burnt in air, some 20 to 40% of their energy is lost to heat the inert gases in the air. These stack losses, which include the heat in the spent combustion gases, are eliminated by burning with co-produced oxygen (O2). In this case, the combustion gases are mainly carbon dioxide (CO2) and water vapor. They are initially so hot that they are ionized to an electrically conducting plasma. The electrolytic splitting of water vapor in this plasma is called 'plasmolysis'.
- About half the energy required for plasmolysis may be obtained from heat. This direct conversion of heat to H2 is not subject to the Carnot constraint. Thus most of chemical energy in the fuel is converted to chemical energy in H2 with the efficiency of the plasmolysis process - about 90%.
- Water splits into its component gases and ions more readily at high temperatures and low pressures. It is thus advantageous to release spent plasmolysis gases at low pressures - if possible below one atmosphere. The hot, thin, spent plasmolysis gases can be used to preheat fuels and O2, and to increase OTEC yields before being released to condensation. If closed condensers are used, the condensate will be impure water containing dissolved acid rain gases, ash etc. It can be neutralized by adding limestone and used for irrigation. The remaining CO2 can be compressed and released to the ocean where it remains liquid or semisolid (clathrate or hydrate). The ocean contains some 50 to 80 times more CO2 than does the atmosphere so that releasing it there is relatively benign.
- The electricity required for plasmolysis can be obtained from an OTEC system. This is also converted to H2 with the plasmolysis efficiency. Very large quantities of pure distilled water suitable for drinking can be obtained as a byproduct.
Though not all the above advantages would be available on land based Hygen devices for from the sea, benefits would still be substantial.
This 'Hygen' method of producing H2 is essentially an innovative combination of well-proven hydrogen production technologies: electrolysis and the steam reforming of hydrocarbons. A process similar to MHD (magneto hydrodynamic) electricity production may be used to charge the plasma electrically, and 'magnetic bottle' techniques, similar to those which have been tested in thermonuclear fusion processes, to contain the charged plasma and to prevent the recombination of the O2 and H2 gases produced.
Latent heat recovery in the proposed OTEC process makes it possible to produce about six times more electric energy than is possible on conventional open cycle OTEC devices. OTEC energy yields are increased further by running cool and warm ocean water flows through syphons, using the energy of focused waves for pumping these flows, and by adding the residual heat of spent plasmolysis gases to that used for OTEC power generation. Wave energy is stored from stormier to calmer periods in simple, cheap reservoirs.
Costs are reduced substantially with new construction techniques - e.g. osmotically stiffened plastic cells are used to take up compressive forces in underwater pipes, wave lenses, buoyancy compartments and the like. Coupling OTEC turbines directly to homopolar generators which provide the plasmolyzers the high amperage, low voltage current they require, eliminates the alternators, transformers, rectifiers and busbars used in conventional electrolysis. Since this implies a short path between the electricity generators and plasmolyzers, it is envisaged that both processes will ultimately be combined on platforms floating out in tropical oceans. They can, however, be tested and used separately and on land. The Hygen process could be used, for example, to make cheap H2 where electric power and fossil fuels are abundant.
The proposed technology facilitates the transition to the 'post petroleum age':
-contemporary automobiles have efficiencies of about 15 to 30%; efficiencies of steam power generation plants are about 30 to 40%. It is expected that heat will be converted to H2 in plasmolyzers with an efficiency of about 90%. The OTEC process will probably contribute about as much electric energy as hydrocarbons or other heat sources provide heat energy. Fuel cells and aphodid engines running on H2 have two to three times the efficiency of air breathing combustion engines. If these are used for transport and electric power production, some 2 to 6 times more useful energy (kWh electricity, km traveled) will be obtained from each unit of fuel. This is roughly doubled if the electric energy component is included.
- Hygen H2 will be inexpensive. Little or no refining will be required of the fuels used. Electricity and heat are converted to H2 directly. The process can start small and cheap with small devices which are used mainly to produce fresh water. Latent heat recovery, cold, then hotter electrolysis can be tested. Experience and profits obtained at each stage can help prepare for the next. Hygen H2 will thus probably be very competitive with fossil fuels burnt in air - even if the enormous damage caused by their combustion and the benefits of co-produced fresh water and O2, and other uses of Hygen - OTEC devices - food production, ocean bed mining, etc., are not taken into account.
- arid fossil fuel exporting countries would benefit from the large quantities of cheap fresh water co-produced with H2. H2 importers will benefit from less acid rain and oil spills, and fewer of the health problems associated with fossil fuels.
- existing fuel reserves can be used several times longer due to the higher production and utilization efficiencies of Hygen H2. H2 can also be used to fluidize and purify heavy and dirty petroleum, shale oils, tar sands and the like.
- existing energy producing and distribution organizations and environmentalists will both benefit from an orderly, rapid transition to the H2 economy. There can be no more efficient ways of converting renewable energies - OTEC, wave, solar, wind, biomass - to H2 than by using them to boost Hygen yields. In most cases the equipment required to do this will also be substantially cheaper than when these renewable energy sources are exploited by themselves.
Hygen thus has the potential to be a very profitable technology which can be introduced in co-operation between existing energy interests and the generally perceived needs of the public to have cleaner, healthier, more environmentally friendly energy infrastructures.
Over the last century several proposals were made for producing large quantities of energy with little pollution:
- open cycle OTEC which would also produce much drinking water and help access the vast resources of the ocean,
- the MHD conversion of hydrocarbons to electric energy and,
- nuclear fusion which produces practically no wastes and can result in no meltdowns.
It is very probable that the promise of the first two will be realized with Hygen, and that this technology will advance the study of the last.