Energy input, carbon intensity and cost for ethanol produced from farmed seaweed

Macroalgae, commonly known as seaweed, has received significant interest as a potential source of ethanol because of its fast growth, significant sugar content and successful lab-scale conversion to ethanol. Issues such as energy input in seaweed conversion, lifecycle emissions, global production potential and cost have received limited attention. To address this gap, a well-to-tank model of ethanol production from brown seaweed is developed and applied to the case of ethanol production from Saccharina latissima in British Columbia, Canada. Animal feed is proposed as a co-product and co-product credits are estimated. In the case considered, seaweed ethanol is found to have an energy return on invested (EROI) of 1.7 and a carbon intensity (CI) of 10.8 gCO2e MJ−1. Ethanol production from conventionally farmed seaweed could cost less than conventional ethanol and be produced on a scale comparable to 1% of global gasoline production. A drying system is required in regions such as British Columbia that require seasonal seaweed storage due to a limited harvest season. The results are significantly influenced by variations in animal feed processing energy, co-product credit value, seaweed composition, the value of seaweed animal feed and the cost of seaweed farming. We find EROI ranges from 0.64 to 26.7, CI from 33 to −41 gCO2e MJ−1 and ethanol production is not financially viable without animal feed production in some scenarios.