Energy Under the Sea? Seaweed as a Third-Generation Biofuel
As the search for alternative energy sources grows more urgent, we could soon turn to the sea—seaweed, specifically—to address these needs. Seaweed grows remarkably quickly and has the potential to be a sustainable source of algae-derived renewable energy, known as third-generation biofuel. This article discusses the feasibility and scalability of using seaweed as a biofuel.
Seaweed has the potential to provide up to 50 million tons of bioethanol per year, enough to supply 9% of the gasoline needs in the U.S. Labeled as a third-generation biofuel, seaweed feedstocks are renewable sources of energy that can significantly reduce our dependence on fossil fuels like gasoline. However, challenges to do with growing conditions and the high cost of production must be addressed.
Seaweed as a Renewable Feedstock
Bioethanol as a Gasoline Replacement
Just as crude oil is the feedstock for various hydrocarbons (such as naphtha and gasoline), plant matter is the feedstock for several biofuels, such as bioethanol, biodiesel and biogas. By far, the most widely produced biofuel is bioethanol, which we can obtain by fermenting plant matter with the help of yeast.
Bioethanol, chemically identical to ethanol, is considered the ideal replacement for gasoline. This is because ethanol burns cleaner and more efficiently in car engines. Many gasoline formulas contain a mix of ethanol and conventional hydrocarbons, denoted by their E rating (E10 = 10% ethanol, 90% gasoline, etc.)
Seaweed: Third-Generation Biofuel
First-generation biofuels involve converting primary food crops like corn or sugarcane into ethanol. Second-generation biofuels improve on this by only using the inedible parts of food crops (such as rice husk), reducing the competition for food supplies.
First and second-generation biofuels face challenges because of the cost of their production and the natural habitat destruction that biofuel expansion brings. Although having been used as an energy feedstock since the 1950s, seaweed (algae) is now considered the most viable and scalable source of biofuel (third-generation).
Seaweed has been associated with being the most promising feedstock for bioethanol production as it is easy to cultivate and harvest. Being photosynthetic (like plants), they can store chemical energy in their cells which we can then convert into biofuel.
However, seaweed is superior to land plants in terms of biochemical composition as they are high in carbohydrates but low in insoluble plant cellulose, making it easy to extract the sugars needed for fermentation into ethanol.
Why Seaweed is a Good Choice for Biofuel
Fast-Growing and Full of Carbs
Seaweed grows at a stupendous rate, with some kelp species able to grow 50 cm in length over the course of a single day! Other species, like spirulina, can still grow a whopping 30 cm daily. With such incredible growth rates, they can be harvested after just a few short weeks, making them an attractive source of renewable biofuel.
As mentioned above, most seaweed species are high in carbohydrate content—the feedstock for producing bioethanol. Species like the sea lettuce Ulva lactuca have an average carbohydrate content of 62%. In comparison, corn (the most widely used biomass for biofuel) only has a carbohydrate content of 21%, almost three times less!
Other families of seaweed, like Enteromorpha, are also high in fat content, making them useful for biodiesel production through lipid esterification.
Plenty of Room in the Ocean
It is well-known that 70% of the Earth’s surface is covered by water. Although most of it is unsuitable for farming (due to the water being too deep), seaweed is surprisingly adaptable and can grow in both warmer and colder climates.
Virtually any country with a coastline can farm and harvest seaweed, making it easily accessible. This is unlike terrestrial plants, where viable growth is often limited to areas with suitable soil and climate conditions. In addition, rainforests and other ecologically essential habitats are usually cleared to make way for plantations, which often leads to biodiversity loss.
Compared to first and second-generation biofuels, an area of water used to cultivate seaweed produces up to 20 times the amount of feedstock that can be harvested from a similarly sized area of land over the same period.
Clean and Efficient Bioethanol Production
Depending on the species of seaweed, the type of yeast used and the method of extraction and fermentation, the bioethanol yield can vary from 14% to 48%. It also produces a small carbon footprint since the carbon dioxide released comes from the atmosphere (instead of under the ground, like fossil fuels).
Studies estimate that 50 million tons of bioethanol can be produced from seaweed yearly, equivalent to 35 million tons (or 260 million barrels) of gasoline. This represents 9% of the annual gasoline consumption in the U.S. (based on 2020 numbers), a significant dent in the fossil fuel industry.
While highly efficient energy-wise, the cost to produce biofuels from seaweed is much higher than using traditional first and second-generation plant feedstocks. This is partly due to poor harvesting techniques (we haven’t invented underwater farming vehicles yet) compared to well-developed land farming tools and equipment.
As seaweed farming technology catches up, we’re sure to see more efficient practices combined with genetically engineered strains of seaweed that will improve its feasibility and appeal as an alternative energy source.
Challenges Facing Seaweed-Based Biofuels
Unsuccessful Large-Scale Cultivation (So Far)
The main stumbling block preventing seaweed from taking over is that large-scale cultivation has yet to be successful. Algae undergo sporulation, which stops growth and instead focuses on reproduction (creating spores). This is detrimental to feedstock production since we want the seaweed to focus on growing as much as possible.
We’re unsure what environmental triggers cause sporulation in seaweed, making preventing it difficult. Cultivating seaweed in the open ocean (instead of landlocked farms) compounds this issue, as we cannot actively control parameters like temperature and pH.
On the other hand, inland seaweed farms require more resources. Apart from the enormous amount of salt water, the tanks also require constant monitoring and adjustment of environmental conditions. This increases the energy costs of feedstock production.
Since algae mainly acquire carbon through carbon dioxide dissolved in seawater, inland farms require a constant supply to be pumped. This brings us to the next issue: carbon neutrality.
Is it Truly Carbon Neutral?
It is important to note that although most biofuels are considered renewable, producing and burning them generates emissions. Although the composition of emissions might differ from burning fossil fuels, the primary pollutants include:
- Particulate matter
- Nitrogen oxides
- Sulfur dioxide
- Carbon monoxide
- Carbon dioxide
One of the key requirements of alternative energy sources for the future is that they should be carbon neutral. That is, their use shouldn’t release trapped carbon into the atmosphere as carbon dioxide. In order for biofuel production from seaweed to be carbon-neutral, the processes involved must also not release carbon. If a seaweed farm requires carbon dioxide to be pumped through the water, a large proportion will inevitably be released as gas into the atmosphere.
Furthermore, much of the carbon dioxide on Earth is dissolved and trapped within our oceans. Seaweed can access this carbon source and transform it into plant matter. When we burn it, we emit this carbon as carbon dioxide. Thus, bioenergy from ocean-grown seaweed might still cause an overall release of carbon and be considered net carbon-positive.
About the Author
Sean is a consultant for clients in the pharmaceutical industry and is an associate lecturer at La Trobe University, where unfortunate undergrads are subject to his ramblings on chemistry and pharmacology.