November 2004

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FRANKFURT - A German specialist in inorganic chemistry is proposing an innovative hydrogen energy system in which silicon made from common sand functions as hydrogen energy carrier - a “tailor-made” link between decentralized renewable energy production and a hydrogen infrastructure.

Alternatively, silicon can be the basis to make ammonia in a carbon-free process to serve both as an even better energy carrier and as traditional fertilizer, says Prof. Norbert Auner, of the Inorganic Chemistry Institute of Johann Wolfgang Goethe University here.

Auner argues such a system would release no carbon dioxide or other materials detrimental to our climate, would offer high energy density, would be comparatively simple to manufacture and recycle, and would require only a simple infrastructure similar to that needed for coal.

Auner, a former member of the technical advisory board of Dow Corning Corp., Midland, MI who has also consulted for Wacker Chemie, Munich, Germany, told H&FCL a team of students and co-researchers has demonstrated the process at the laboratory stage.

In this laboratory-scale process, desert sand is transformed at ambient temperature into silicon tetrafluoride, which subsequently is decomposed in a hydrogen atmosphere into silicon and hydrogen fluoride. At present, plans are being developed with a group of researchers and support from a British company, ReCycled Refuse International, Jersey (Channel Islands), to build a pilot plant in the UK and also in the United Arab Emirates.

The basic project to produce silicon from desert, mostly as a raw material for electronics components and for the production of silicones, sand was financially supported by both Dow Corning and Wacker.

Auner, who first proposed the concept four years ago in a university lecture, said at the time such a process, based on silicon and air, could be designed to work with very low or no carbon dioxide release, and the stored thermal energy could be released with 66% efficiency.

In a paper, “Silicon as an intermediary between renewable energy and hydrogen,” published May 5 as part of the “Research Notes” series of working papers by the research arm of one of Germany’s largest banks, Deutsche Bank Research, Auner lays out his case. Hydrogen generated from water, he says, takes inordinately large amounts of energy - about 286 kJ/mol compared to only 37.5 kJ/mol for the economic benchmark for lowest-cost hydrogen production, hydrogen produced from natural gas.

Even more energy efficient, however, is splitting out hydrogen from ammonia - 30.8 kJ/mol. But ammonia production via the traditional, well-established Haber-Bosch process requires high energy inputs and yields low outputs from the hydrogen and nitrogen starting materials. And since the needed hydrogen is derived today from hydrocarbons, this approach is a dead end if society’s goal is to get away from fossil fuels.

Ammonia produced via a silicon detour is very promising, Auner believes. Silicon dioxide - quartz sand, or SiO2 - makes up about 75% of the accessible earth’s crust, Auner explained. It is already converted industrially into crystalline silicon on a huge scale - about 1.2 million tons worth in 2000. Today, it requires large amounts of electrical energy - 12,000 kWh per ton of silicon - and significant amounts of carbon dioxide are released in the process: normally about 4.3 tons of CO2 per ton of silicon with the use of fossil fuel-reducing agents. If natural gas or oil is used as power source for the needed electricity, the CO2 output doubles and triples to 10.6 and 17.7 tons per ton of silicon, respectively.

An electrochemical process functioning under electrolytic conditions has been verified in pilot projects that exhibit reduced levels of carbon dioxide. It has not been implemented so far because of what Auner wrote are “commercial considerations” - it costs too much so far.

Parallel to that, Auner and his team have come up with a chemical process that permits production of silicon completely independent of carbon that makes use of sand and inexpensive energy - ideally renewable - to manufacture silicon. Thus, silicon, usually thought of as base material for electronic chips, becomes in effect an energy carrier.

Auner says his team has been able to directly produce silicon as a secondary energy carrier on a laboratory scale in a two-stage process, using energy that could be derived from solar sources and common desert sand. What’s needed now are “studies to transfer this to a large scale operation,” he says in his paper.

Silicon is an energy carrier with energy density and energy content comparable to carbon, Auner explained. Metallic silicon reacts exothermally, i.e. by giving off heat but only at high temperatures of more than 1,500 deg. C - about 9 kWh/kg. Given the energy needed to produce silicon - about 12 kWh per kg of silicon - the storage efficiency of electrical energy is almost 30%, comparable to energy stored in water and released into hydrogen via electrolysis and returned to water after combustion, Auner wrote.

Besides the very attractive route of producing hydrogen by direct reaction of silicon with water or steam at ambient temperature (T 20 – 100°C), silicon can also be combusted in a nitrogen atmosphere to make silicon nitride, a valuable and robust material for a number of chemical industry applications.

Using air as a reactant - 80 % nitrogen and 20% oxygen - silicon thus can function both as an energy carrier and as producer of various chemical materials: synthetic silica via the oxygen in the air, and silicon nitrides. Quoting published Wacker research data, Auner says that in order to get over the high-temperature barrier to get the nitride-producing process underway, a copper oxide catalyst can be used to lower the process temperature at which nitrides form to about 600 deg. C.

And silicon nitride is one of the key steps in the hydrogen storage chain: reacted with steam or bases, silicon nitride releases - along with other useful materials such as metal silicates - ammonia, widely used as fertilizer in a process that requires the least energy to release hydrogen. Produced via the silicon route instead of the conventional fossil fuel-based Haber Bosch process, ammonia is now emerging as a viable, promising hydrogen carrier, in Auner’s view.

But, Auner points out, a reaction mechanism involving ammonia as an intermediate product is not absolutely necessary in the production of hydrogen from silicon. Even more attractive is the fact, well known in the literature, that silicon already reacts with water under alkaline condition to directly form silicate and hydrogen.

Auner’s paper describes a number of details and chemical steps and alternatives that could come into play in such a system that cannot be covered here. He also compares in a table theoretical energy densities of metals and metal hydrides which release hydrogen in aqueous solution, ranging from beryllium (theoretically ideal, but toxic and extremely rare) to aluminum, zinc, magnesium as well as alkaline boranates and alanates.

The bottom line, however, is that “in a direct comparison with alternative hydrogen storage materials and hydrogen generation reactions, silicon proves to be the best compromise between production costs and handling, not to mention the abundant, worldwide availability from sand,” Auner wrote.

Furthermore, “the price of electricity in the mechanism SiO2g Sig H2gElectricity decreases proportionately with the costs for the production of hydrogen,” he added.

One assumption, unstated in Auner’s paper but crucial, is the availability of large amounts of renewable energy - solar energy in desert areas, for instance. The basic underlying problem, Auner told H&FCL in a transatlantic phone interview, is the global mismatch between renewable energy resources, abundant in distant places, but not easily transportable to where they are needed: densely populated, industrialized areas.

A silicon-hydrogen energy transport system might be the way out. An article in the German “DowJones/VWD energy weekly”, published on May 14th, 2004 (No. 19, p. 7) and describing the concept quotes a German energy expert as speculating that Auner’s energy transport chain just might be the “missing link” to a “danger-free hydrogen economy.”

Solar energy harvested in hot desert zones and transformed into silicon could be transported at “acceptable cost” to industrial energy consumption centers, says Josef Auer, energy expert at Deutsche Bank Research. Auer says Auner’s basic research could turn out to be one of the “most significant breakthrough efforts” in solving obstacles regarded so far as unsurmountable. He quotes Auner as saying that the energy losses in the entire chain might be about 30%, an amount that can be written off as negligible given the fact that solar energy would be available in these areas in large surplus quantities.

“A previously unutilized approach would be mobilized to store energy for unlimited periods of time and to transport it safely,” the story quotes Auer as saying. A key advantage of the silicon concept for a future hydrogen economy is that potentially high danger levels via transport and storage of hydrogen and the considerable energy losses during transport would be “massively” reduced.

Transport and storage of energy via silicon would require a relatively simple infrastructure similar to those built in the past for coal, Auer was quoted as saying.

Concluded Auner in his paper, “Our expectations for a solar-thermally produced carrier manufactured from conventional sand permit us to predict an unbeatable cost profile for silicon in the future.” Contact: Prof. Norbert Auner, Phone +49(69)798-29180; e-mail, auner@chemie.uni-frankfurt.de. The paper is available from Deutsche Bank Research, www.dbresearch.com, e-mail marketing.dbr@db.com.