FROM HARMFUL ALGA TO CLEAN ENERGY

BIOETHANOL: Weight for weight, sea lettuce contains as much carbohydrate as wheat or maize. Danish researchers will study whether sea lettuce can cost-effectively be grown in artificial basins

When the alga species sea lettuce (Ulva lactuca), which grows everywhere along coasts worldwide, enjoys some warmth, it starts to grow. When it reaches a certain size it sinks to the seabed, decays and accelerates oxygen depletion, causing fish to die. In other words, it is a harmful organism that threatens the health of marine environments.

But for all its sins, sea lettuce has a redeeming quality that will become the subject of thorough study in Denmark. When dried, it contains up to 60% carbohydrate, a figure comparable to wheat or maize. That makes it a potential candidate for the production of bioethanol to substitute fossil-derived petrol.

Replacing food crops
Sea lettuce is interesting as a subject of research because it could replace food crops, like sugar cane and maize, which are currently used for bioethanol production. Increasing production of bioethanol is swallowing up an ever-larger proportion of the world’s agricultural land, resulting in higher food prices worldwide. Another reason for the research is that problems with algae will proliferate in the coming decades because of higher temperatures and the associated global warming.

“In Denmark alone, we estimate that we can annually harvest around 100,000 tons of sea lettuce from natural habitats,” says senior advisor Michael Bo Rasmussen of the National Environmental Research Institute at the University of Aarhus. “And if production of sea lettuce is systematised in artificial basins, we calculate a yield of 500 tons per hectare. These basins could be placed near combined heat and power stations where the hot returned water from the cooling cycle can be exploited. At the same time, CO2 rich flue gases from the stations could be piped into the basins and agricultural waste or biogas could be supplied as nutrients. Our initial calculations show that 100,000 litres of bioethanol could be produced per hectare of cultivated sea lettuce.”

Valuable source for bio-ethanol
“It looks like it could be a neat solution,” says Michael Bo Rasmussen. “From being a foul-smelling menace which people the world over are using substantial resources to remove, sea lettuce could become a valuable source material for bio-ethanol production, which we are about to study. What’s neat about it is that sea lettuce’s ability to grow is dependent on a supply of CO2 and nutrients. That can help tackle two environmental problems in one go – the CO2 can come from atmospheric emissions of the greenhouse gas while the nutrients can come from manure, which farmers have difficulty getting rid of. At the right temperature, sea lettuce will double in size in just four days. And when it reaches the right size, it can be
harvested and dried, and its carbohydrate content turned into bio-ethanol.”

World-class green energy
Michael Bo Rasmussen acknowledges that there is still a long way to go before research into the potential of sea lettuce as an energy feedstock can be commercialised.

“Besides laboratory research into maximising the carbohydrate content of sea lettuce, significant work needs to be done on economic analyses,” says Michael Bo Rasmussen. “If it turns out to be cost-effective regarding both the environmental benefit of cleaner sea water and the production of bioethanol, then we have an energy source which is almost unlimited in quantity. It could become world-class green energy.”