[peppercorn (OP)]

The following pdf describes how silicon can be used as an intermediary between solar (away from water) and hydrogen (or Nitrogen carriers, like ammonia).

Silicon as an intermediary between
renewable energy and hydrogen

Alternatively, the crude silicon could replace the carbon intensive arc process at the beginning of I.C. manufacture.

[damocles]

The "hydrogen economy" is a very doubtful direction for us to be taken, with lots of hidden "spin" and unsolved technical problems. What the general public does not understand is that it is only offering a possibility for energy storage and transportation, not a new source of energy.

Once this point is understood, the silicon economy starts to look like a potential rival. In terms of energy storage silicon is significantly superior to hydrogen on a volume and safe handling basis, even if it is behind on a mass basis. Both silicon and silica are viable as readily flowing sands which are unreactive in the normal environment (in the first case because of an adherent oxide coating).

In both cases, inexhaustible raw materials are available.
For the hydrogen economy, there are three problematic areas out of four; for the silicon economy there are two:

Hydrogen economy:  (1) Generation of hydrogen gas is problematic. At present (moderately) efficient electrolysis cells are only at the prototype stage. The cheapest hydrogen gas, and the bulk of the production, comes from reforming natural gas. Because of this oil companies are trying to fudge their way into the "hydrogen economy" via "hidden" production of carbon dioxide.
(2) Efficient fuel cells for recovering energy from hydrogen gas oxidation are an available technology.
(3) Safe storage of hydrogen gas requires very large pV per unit energy, danger from leaks, and possible high capital costs for maintenance and replacement of tanks.
(4) Safe transportation and feed lines present similar problems that are only partly solved in practice.

Silicon economy:(1) Efficient reduction of silica to silicon without the use of carbon as reductant is still a technology in early stages of development.
(2) I am unaware of any practical means of efficiently recovering energy from the silicon oxidation to silica.
(3) and (4) Storage and handling totally unproblematic, and easily encompassed within presently available technologies.

On balance, I feel that the silicon technology is well worth further exploration, mainly because I am one of a large minority who thinks that the hydrogen economy is a blind alley.

[syhprum]

" Efficient fuel cells for recovering energy from hydrogen gas oxidation are an available technology"
I would like to query this do not fuel cells require expensive and rare metals as a catalyst and very pure Hydrogen.

[damocles]

" Efficient fuel cells for recovering energy from hydrogen gas oxidation are an available technology"
I would like to query this do not fuel cells require expensive and rare metals as a catalyst and very pure Hydrogen.

Not totally sure, but I think not. Polymer membrane technology gets around this I think. Check out the CSIRO page on fuel cells:
http://www.csiro.au/Outcomes/Energy/Renewables-and-Smart-Systems/developing-fuel-cell-technologies.aspx

Heavy on promotion and light on scientific detail, I know, but I visited there a few years ago (mainly on account of my scepticism about hydrogen economy, and wanting to check out the other side of the picture so that I could give my students a fair and balanced presentation). They had working prototypes that were clearly operating very effectively and efficiently.

They also had a prototype photovoltaic electrolysis cell design that was producing hydrogen gas at a pretty good rate -- my estimate would be 2 or 3 litre/minute for a unit about 20x20x30 cm. At the time they were claiming about a 60%/80% energy efficiency for their electrolysis/fuel cell operation; their project was aiming for 80%/85%.

I did question the usefulness of consuming useful energy to produce 48% (demonstrated) or 68% (project plan) of the useful energy that you started with. They pointed out that the advantage came in the possibility of storing the energy for consumption at a place and/or time where it was more needed. If you use excess off-peak generated electricity to pump water from low storage to high storage and then use the hydro power to boost generation for peak demand, the energy efficiency is about 15%-25%, so 48%-68% represents a huge advance, but only if you have also addressed storing/safe handling issues.

The thing that most bothers me is that the general public, and even some of the scientists and engineers, do not realise that this is all you can do with the hydrogen economy -- store useful energy to get some of it back at a more useful time and/or place.

[peppercorn (OP)]

On balance, I feel that the silicon technology is well worth further exploration, mainly because I am one of a large minority who thinks that the hydrogen economy is a blind alley.

As you say, H2 future role is often so poorly explained the public that many thing it is "a new source of energy", not an energy storage medium.

Personally, when I thought to Google silicon in connection with desert solar, I was hoping to find an energy 'route' not linked to 'The hydrogen economy'; because, like you, I have major reservations about it - not least that it is a continuation for petro-firms to remain at the heart of the supply chains.

Nevertheless, I can also see that much investment has been pushed into fuel-cell tech, and (relative) low-pressure absorption techniques, etc.  And a limited number of applications are economically suited to utilising H2 fuel-cells.

The alternative route to power production from amorphous Si might be to develop a similar technology to non-rechargeable Metal-Air batteries (zinc, aluminium, etc) that might not be particularly practical for, say, electric vehicles, but would be just fine to replace Diesel-generators for a remote island community, or even a large Data-centre grid-backup.

I did question the usefulness of consuming useful energy to produce 48% (demonstrated) or 68% (project plan) of the useful energy that you started with. [CSIRO] pointed out that the advantage came in the possibility of storing the energy for consumption at a place and/or time where it was more needed. If you use excess off-peak generated electricity to pump water from low storage to high storage and then use the hydro power to boost generation for peak demand, the energy efficiency is about 15%-25%, so 48%-68% represents a huge advance, but only if you have also addressed storing/safe handling issues.

I didn't imagine that pumped-storage full-cycle efficiency was so poor!
With those sort of returns all sorts of alternative energy storage would seem much more attractive; the kind of molten-salt techniques that concentrated-solar is now employing would seem a good, low-tech alternative - the biggest issue, I suppose is the speed that electrical generation can start from cold.