Algae offer a renewable source of hydrogen
Scientists have discovered how to use green algae to produce hydrogen from sunlight and water. They say the technique could one day enable a small commercial pond to produce enough hydrogen gas to meet the weekly fuel needs of a dozen cars.
The scientists from the University of California, Berkeley and the US National Renewable Energy Laboratory (NREL) have used the fact that the usual photosynthetic apparatus of certain types of green algae can be turned off and redirected to create hydrogen.
The ‘metabolic switch’ is very simple: preventing the algae from coming into contact with sulphur stops photosynthesis and prevents the cell’s production of oxygen. Without oxygen, the anaerobic cells are not able to burn stored fuel in the usual way, through metabolic respiration, and are forced to activate an alternative metabolic pathway, which generates the hydrogen and may be present in many
types of algae.
“The cells utilise stored compounds and bleed hydrogen just to survive,” UC Berkeley Plant and Microbial Biology Professor Tasios Melis said. “It’s probably an ancient strategy that the organism developed to live in sulphur-poor anaerobic conditions.”
The algae culture cannot live forever when it is switched over to hydrogen production, but it can manage for a considerable period without negative effects.
Until now, hydrogen fuel has been extracted from natural gas. Only small-scale cultures of the microscopic green alga, Chlamydomonas reinhardtii have been examined in the laboratory for their hydrogen production capabilities, Melis said. “In the future, both small-scale industrial and commercial operations and larger utility photobioreactor complexes can be envisioned using this process,” he said.
While current production rates are not yet high enough to make the process commercially viable, the researchers believe that yields could rise at least ten-fold, making the technique a fuel-producing option.
Preliminary rough estimates, for instance, suggest it is conceivable that a single, small commercial pond could produce enough hydrogen gas to meet the weekly fuel needs of a dozen or so automobiles.
The scientific team has begun to test ways to maximise hydrogen production, including varying the type of microalga used and its growth conditions.
“What has been lacking is a renewable source of hydrogen,” another of the researchers, Dr. Michael Seibert, said. For nearly 60 years, scientists have known that certain types of algae can produce the gas, but only in trace amounts. Despite tinkering with the process, no one has been able to make the yield rise significantly without elaborate and costly procedures.
The researchers first grow the alga in the same way as any other plant. This allows the algae to collect sunlight and accumulate a supply of carbohydrates.
When enough energy has been stored, the researchers transfer the liquid alga culture to stoppered one-litre glass bottles with no sulphur present. Then, the culture is allowed to consume all oxygen.
After about 24 hours, photosynthesis and normal metabolic respiration stop, and hydrogen begins to bubble to the top of the bottles and bleed off into hydrogen-collection tubes.
“It was actually a surprise when we detected significant amounts of hydrogen coming out of the culture,” Melis said. “We thought we would get trace amounts, but we got bulk amounts.”
After up to four days of generating an hourly average of about 3ml of hydrogen per litre of culture, the culture is depleted of stored fuel and must be allowed to return to photosynthesis. Then, two or three days later, it again can be tapped for hydrogen. The cell culture can go back and forth like this many times.