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Nuclear or Solar Hydrogen?

Chemical engineers keep coming up with clever new ideas for producing green energy from novel sources.In many, hydrogen gas plays a key role. It can be produced, as mostly at present, by high temperature steam reformation of methane (natural gas or biogas), or by electrolysis of water using electricity – which can be generated from renewable sources, or from nuclear plants. It can also be produced by very high temperature direct dissociation of water – using focused solar, or heat in, or from, nuclear plants. Or, more efficiently, by thermo-chemical processes aided by catalysis. Once produced, hydrogen can be used as a fuel for a conventional combustion engine or a gas turbine, or fed to a fuel cell, so as to generate electricity. It can also be used as a feedstock to produce synfuels.

Dr Charles Forsberg from MIT’s Nuclear Fuel Cycle Study project, has proposed using nuclear plants combined with biomass gasification to provide the energy and carbon feedstock for the production of liquid synfuel using the well known Fischer–Tropsch process. The nuclear plant provides electricity for the electrolysis of water – generating hydrogen and oxygen. The oxygen is used to run a biomass gasifier the output of which is used, along with the hydrogen, as a feed stock for the production of synfuel – diesel or gasoline. He says that the use of external heat and hydrogen can double to triple the liquid fuel output per ton of biomass compared to using just biomass as the feedstock and as the process energy source.

There are various possible variations (e.g. heat or steam from the nuclear plant can be used for high temperature biomass reforming, rather than electrolysis of water). Or you can go for direct hydrogenation of biomass with nuclear-derived hydrogen. The production of ethanol from biomass using steam from nuclear plants is another option. Forsberg’s claim is that approaches like this offer a way to make better use of high capital cost nuclear plants to produce a high-value storable fuel as well as electricity. He doesn’t see hydrogen as being a replacement fuel across the board, but as being used for producing products like this.

Even so, there may be some options for hydrogen as a new energy vector. Advocates of the hydrogen economy argue that, one attraction of hydrogen is that, like natural gas, not only can it be transported down a pipe with relatively low losses, it can also be stored, so it has advantages over electricity as a energy vector. However it’s bulky. Cryogenic storage, as a liquid, is expensive and energy inefficient. Chemi-absorption techniques exist, for trapping it in organic lattices, but are not yet widely available on any scale. Storage as a gas in pressurised tanks is the easiest option, but takes up room. Underground storage in caverns is about the cheapest bulk option.

The generation and storage of hydrogen could be one way to allow nuclear plants to be able to meet variable/ peak energy demands. Forsberg suggests using excess electricity from nuclear plants at low grid demand periods to generate hydrogen by electrolysis and then using the electrolyser in reverse to generate electricity to meet demand peaks. Evidently high-temperature electrolysis units can be operated as high-temperature fuel cells. A parallel, probably more energy efficient, approach would be to use the nuclear hydrogen and the oxygen also produced by electrolysis, to fuel an oxy-hydrogen burner unit, producing high temperature steam for a gas turbine. That could have an overall efficiency of 70% – since no boiler is required.

Solar hydrogen/synfuel

In passing, Forsberg does mention that solar thermal could be used as the heat source for some of the systems he outlines. This would make a lot of sense. The SOLASYS ‘Power Tower’ in Israel is already being used to steam ‘reform’ methane into hydrogen and carbon monoxide at around 700 °C.

Meanwhile the US Dept of Energy Energy Efficiency and Renewable Energy Web site, describes how a solar concentrator can use mirrors and a reflective or refractive lens to capture and focus sunlight to produce temperatures up to 2,000 °C. The CNRS solar heliostat/parabolic mirror system at D’Odeillo in southern France has in fact been doing that since 1970. Direct dissociation of water at temperatures like this is possible but is relatively inefficient – perhaps 1–2%. However, there are systems being developed: e.g. see Hion Solars approach: http://www.hionsolar.com/n-hion96.htm.

An alternative is to use the high-temperature focused solar to drive chemical reactions that produce hydrogen, possibly aided by catalysis. For example, the US DEn EERE web sites say that in one such system ‘zinc oxide powder is passed through a reactor heated by a solar concentrator operating at about 1,900 °C. At this temperature, the zinc oxide dissociates to zinc and oxygen gases. The zinc is cooled, separated, and reacted with water to form hydrogen gas and solid zinc oxide. The net result is hydrogen and oxygen, produced from water. The hydrogen can be separated and purified. The zinc oxide can be recycled and reused to create more hydrogen through this process. See: http://www1.eere.energy.gov/hydrogenandfuelcells/production/water_splitting.html

Clearly the nuclear fission lobby is looking at how to redeem its capital intensive technology, for example by using otherwise wasted heat. Combined Heat and Power operation is one option, if there are heat loads nearby. But supporting hydrogen to synthetic fuel production, as proposed by Forsberg, is obviously another. General Atomics has developed a Sulphur–Iodine cycle for thermo-chemical water splitting, which in principle, can, it’s claimed, achieve cycle efficiencies of 50% using heat at 850 °C. That’s achievable by some fission plants – and possibly also, at some point in future, by fusion plants. But it’s also a route that could be taken by solar, with arguably less problems- no wastes to deal with, or fuel to find.

*Forsberg spoke at the World Nuclear University in Oxford last July. For more from him, see International Journal of Hydrogen Energy 3, 4 (2009).

If you want to keep up to date on issues and developments in the renewable-energy field, then see the long-running Renew newsletter, now re-launched as a PDF-delivered file. Details can be found at http://www.natta-renew.org.

Posted by Dave Elliott on September 5, 2009 12:29 PM | Permalink