Renewable Energy

Michael Grätzel

Laboratory of Photonics and Interfaces

Ecole Polytechnique Fédérale, CH-1015-Lausanne, Switzerland
The development of renewable energy resources is encouraged by increasing public awareness that the earth’s oil reserves will run out during this century. As the energy needs of the planet will at least double within the next 50 years, the stage is set for a major energy supply shortage, unless alternative sources can cover the substantial deficit arising from the exhaustion of fossil fuels.
This lecture will review the current status of renewable energy options, and analyze in detail the production of electricity and hydrogen from sunlight. The sun provides about 100,000 terawatts of energy to the Earth − which is about 10,000 times more than the world’s present energy consumption.

Photovoltaic cells are being increasingly used to tap into this huge resource and will play a key role in sustainable energy systems of the future. So far, photovoltaic solar energy converters have been dominated by solid-state junction devices, usually in crystalline or amorphous silicon, made available − almost as a by-product – by the semiconductor industry. The technology of these devices is now mature, and is able to supply a rapidly-growing market. More than 1 Gigawatt of peak power production capability had been installed in 2004. By 2030 the output capability is expected to reach 300 Gigawatt <1>.

However, the cost of photovoltaic electricity is still too high to be competitive with nuclear or fossil energy. For the best systems, installed at well-chosen sites, the price per kWh is presently 0.25-0.65 Euro. To be competitive, the price would have to fall to below 0.05 Euro/kWh. Clearly, a change in paradigm is required to meet this cost goal − and the enormous future energy demands.

Recently-discovered devices based on mesoscopic inorganic or organic semiconductors commonly referred to as bulk junctions (due to their three-dimensional structure) offer great promise in this regard. They can be formed from nanocrystalline inorganic oxides, ionic liquids and organic hole conductors, or conducting polymer devices. They offer the prospect of very low fabrication costs since they do not require expensive and energy-intensive high temperature and high vacuum processes; they are also compatible with flexible substrates and can be made in various presentations and appearances to facilitate entry both to the domestic devices market and to architectural or decorative applications.

The prospect therefore is that it will soon be possible to depart completely from the classic solid-state cells, which will be replaced by devices based on interpenetrating network junctions. Contrary to expectation, these devices have shown strikingly high conversion efficiencies, which compete with those of conventional systems. The prototype is the dye-sensitzed solar cell (DSC), invented at the École Polytechnique Fédérale de Lausanne <2,3>, which accomplishes the optical absorption and the charge separation processes by the association of a sensitizer (as light-absorbing material) with a wide band gap semiconductor such as titanium dioxide.

Literature

1) “A Vision for Photovoltaic Energy Production” Report by the European Photovoltaic Technology Research Advisory Council ((PV-TRAC)” EUR 21242 (2005)

2) B. O’Regan and M. Grätzel “A Low Cost, High Efficiency Solar Cell " Nature, London 353, 1991.

3) M Grätzel , “Photoelectrochemical Cells” Nature 414, 338−344 (2001).