A novel approach to harvesting the energy of the sun is described in a recent paper in the Journal of Applied Physics, which reports on one of the key components in the system: a solar powered laser¹.
 
 

Dancing with the sun...

They describe an energy cycle which uses magnesium as a stored source of energy which can combusted with water to generate power^{2, 3}. To make the cycle a sustainable one, it would be driven by a renewable source of energy, such as the sun, to reclaim the magnesium from the remnants of the combustion reaction. The beauty of this system is in the fact that energy can be conveniently stored in the magnesium until it is needed, so a solar power plant could provide a steady power output day and night, ramping it up to meet demands, or reducing it as required.

The cycle is sustainable in the same sense that hydro-electric power is. In that example, the sun evaporates water from the land, raising it up to form clouds, which then rain down on the hills and produce the rivers, which drive the turbines that generate the electricity. The cycle continues as the sun evaporates the river water to form more rain clouds. The clouds might be considered the energy stores, analogous to the magnesium in the proposed system.

The combustion reaction of magnesium with water is an exothermic one, which means that it gives off heat. This serves to sustain the reaction and also leaves excess heat which can be used to generate power. Hydrogen is also released during the reaction, which could either be burnt to generate more power, or stored for later. What is left over from the reaction is magnesium oxide.

To extract magnesium from magnesium oxide and continue the cycle requires that it be heated to a temperature of 4000 K. Unfortunately there is no practical way of reaching these temperatures by directly focusing sunlight onto magnesium oxide with conventional optics. These researchers have therefore proposed an alternative method which uses the sun to power lasers, which are then able to concentrate the light to generate the necessary temperatures.

The paper itself is concerned with the development of a solar pumped laser which could produce the necessary power densities for the magnesium recovery process. Their laser uses a Fresnel lens mounted on a platform which tracks the position of the sun in the sky, focusing light from the sun onto a chromium co-doped Nd:YAG laser rod. To come up with a design which most efficiently coupled sunlight into the laser cavity, they experimented with various geometries, finally demonstrating a laser with a sunlight to laser power conversion efficiency far greater than any reported elsewhere.

The laser produced a maximum power output of 80 W, still far short of the 1 kilowatt laser they say would be needed for magnesium recovery (focused down to a 1 mm diameter spot size). The researchers also successfully demonstrated the use of a Fresnel lens for solar pumping. Rather than using expensive glass lenses or parabolic mirrors, this would allow them to not only bring down the cost of a commercial system, but would also enable it to be smaller and more robust.

It will be interesting to see how this method of capturing and storing the sun's energy competes with all of the other efforts under way to develop clean and sustainable energy sources. There may be merit in the magnesium energy cycle, but it's difficult to see how the intermediate laser stage could be made into a cost-effective solution, considering the manufacture, installation and maintenance costs associated with them. However these are simply challenges along the way and one should always bear in mind that the path from research to development is rarely a direct one.

A more viable solution may yet present itself, perhaps one that uses metamaterials and that can dispense with lasers altogether, by directly concentrating the sunlight to the power densities required for the recovery of magnesium. Developments of that sort, however, may be a very long way off, while the need for clean energy solutions is already here and is urgently demanding answers.
 
 
1) T. Yabe, B. Bagheri, T. Ohkubo, S. Uchida, K. Yoshida, T. Funatsu, T. Oishi, K. Daito, M. Ishioka, N. Yasunaga, Y. Sato, C. Baasandash, Y. Okamoto, K. Yanagitani (2008). 100 W-class solar pumped laser for sustainable magnesium-hydrogen energy cycle Journal of Applied Physics, 104 (8) DOI: 10.1063/1.2998981

2) T. Yabe, S. Uchida, K. Ikuta, K. Yoshida, C. Baasandash, M. S. Mohamed, Y. Sakurai, Y. Ogata, M. Tuji, Y. Mori, Y. Satoh, T. Ohkubo, M. Murahara, A. Ikesue, M. Nakatsuka, T. Saiki, S. Motokoshi, C. Yamanaka (2006). Demonstrated fossil-fuel-free energy cycle using magnesium and laser Applied Physics Letters, 89 (26) DOI: 10.1063/1.2423320

3) T. Yabe, M. S. Mohamed, S. Uchida, C. Baasandash, Y. Sato, M. Tsuji, Y. Mori (2007). Noncatalytic dissociation of MgO by laser pulses towards sustainable energy cycle Journal of Applied Physics, 101 (12) DOI: 10.1063/1.2743730