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Bio-Hydrogen

Possibly the ideal solution: Using a biological process to harness solar energy and directly generate hydrogen from water.

This is a solution considered in the 1970's (like many others now coming to fruition) in response to the oil crisis.  The principle is quite simple: a plant such as algae is manipulated and grown in such conditions that it photosynthesises hydrogen from water, effectively acting as a combined biological solar panel and water electrolysis unit.

However, persuading plants to photosynthesis hydrogen en masse has not been feasible until recently.  Researchers at University of California at Berkeley (UCB) have found that using a particular type of algae (Chlamydomonas reinhardtii) they can switch off the usual photosynthesis process resulting in the the algae burning stored energy, with bulk hydrogen as a by-product.  Hence the biological 'batteries' are cycled over 2-3 day cycles of normal aerobic photosynthesis (fattening) followed by up to 4 days of atypical anaerobic metabolising of stored energy (harvesting) which produces hydrogen.  The key to switching is deprivation of sulphur, and the process thought to be "...an ancient strategy that the organism developed to live in sulfur-poor anaerobic conditions."

Current production rates are about three millilitres of hydrogen per litre of culture, but it is thought that a 10 fold increase will be possible with further research.  The technology has been patented by UCB and one assumes commercialisation will follow.

Unfortunatley, doing the sums suggests that this technology (at least in its present incarnation) cannot provide a significant proportion of our energy needs and must once again be considered only part of an integrated renewable energy infrastructure.

References:

Original University of California at Berkeley Press Release:
 -  http://www.melisenergy.com/UC_Berkeley_Press_Release.htm
UCB project description:
 -  http://plantbio.berkeley.edu/profiles/newProfiles/melis.html
Oak Ridge National Lab projects:
 -  http://www.ornl.gov/divisions/ctd/march2.htm