Found in both marine and freshwater environment, algae have also been studied as potential sources of energy. Macroalgae - more commonly known as seaweed or kelp - are fast-growing plants that can reach a considerable size (up to 60 m in length) . A few experimental seaweed farms were built along the coast of southern California in the 1970s, but due to difficult weather conditions and rough waters, the open-sea project was rapidly abandoned. More protected from the elements, near-shore farming of Macrocystis algae gave better results, and a yearly biomass production of more than 30 tonnes per hectare were reported (Fig. 12.11).
Another form of algae - microalgae - are microscopic and the most primitive form of photosynthetic organisms (Fig. 12.12). Whilst their mechanism of photo
Figure 12.12 Microalgae.
synthesis is similar to that of higher plants, the algae are generally more efficient in converting solar energy to biomass, because of their simple cellular structure and the fact that they grow in aqueous suspension surrounded by water, CO2 and other nutrients necessary for their growth. Some species of microalgae are very rich in oil, and this can account for up to 50% of their mass. It has been claimed that microalgae are capable of producing 30 times more oil per hectare than terrestrial oilseed crops. This prompted the National Renewable Energy Laboratory (NREL) from the U.S. Department of Energy (DOE) to study these organisms for the manufacture of biodiesel. Experimental production of microalgae was conducted in open, shallow ponds located in regions with high sun exposure in California, Hawaii, and New Mexico. Unlike higher, more complex organisms, microalgae reproduce very rapidly and can be harvested continuously as they are produced. To maintain high growth rates, the ponds were constantly fed with CO2, water, and other nutrients needed by the algae. For large-scale production in "algae farms", CO2 could be obtained from a number of sources, especially fossil fuel-burning power plants. The carbon contained in fossil fuel could thus be used twice, reducing global emissions of CO2. This would provide a practical approach for the recycling of CO2 into a useable fuel. Japan, which is extremely dependent on energy imports, is actively pursuing this approach to extend its domestic energy production. Ultimately it will, however, only postpone the release of CO2 into the atmosphere. To be sustainable in the long term, algae should use the CO2 contained naturally in air and dissolved in water. This will probably considerably lower the yield of biomass production, requiring vast areas of land to produce significant amounts of energy. One of simplest way to use the biomass generated from macro- or microalgae is to produce methane. Because of the high water content of algae, gasification requiring prior drying of the material is not attractive, and anaerobic digestion - which is anyway conducted in water - is therefore more adaptable. The methane generated can than be used in the same way as natural gas to produce methanol. More research is needed, however, to provide a better understanding of the biology of aquatic organisms and to identify the best species and growing conditions for optimal conversion of solar energy into biomass. Genetic engineering could also be used to improve the characteristics of such plants and algae. So far, however, the production ofbiomass from aquatic plants has proven too expensive to compete with fossil fuels, or even plants grown on land.
Was this article helpful?