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The EROI of algae biofuels (part II): just add wastewater and…get a little better

Earlier this year I discussed (The EROI of Algae) some research at The University of Texas on an experimental algae to fuels experiment. A couple of new papers have now been published on the energy return on investment (EROI) of algae-based bioenergy when coupled to a wastewater treatment plant (Energy Return on Investment for Algal Biofuel Production Coupled with Wastewater Treatment: http://www.ingentaconnect.com/content/wef/wer/2012/00000084/00000009/art00002). The reason why it is energetically beneficial to couple a wastewater treatment plant to an algal growth facility is because algae, like terrestrial crops such as corn and soybeans, require nutrients as inputs. Wastewater has a high quantity of nutrients that actually must be removed before discharging the water into the environment…in some cases to prevent algae blooms! Oh the irony…

In another recently published work, we established that a best case scenario using known technology (not possible future ideas of new strains of algae), the EROI of algae-based fuels was 0.36 when adjusting the relative energy quality of the energy inputs and outputs (for a free accessible copy of the paper see Comprehensive Evaluation of Algal Biofuel Production: Experimental and Target Results

 http://www.mdpi.com/1996-1073/5/6/1943).  Note that this value of EROI = 0.36 is what we called a 2^nd Order EROI that considers only the energy inputs from direct energy consumption (e.g. electricity, fuels) and the energy embodied in materials (mostly nutrients such as nitrogen and phosphorous). If you want to see work that documents the methodology and assumptions of input values, this work is a good place to start to then lead to other literature.

When you add the wastewater to the algae-to-fuels process…

…it could be possible to obtain a 2^nd order EROI of 1.4 (as compared to approximately 0.4 without that wastewater). Note that there is energetic value in extracting energy directly from the wastewater, such as via anaerobic digestion, and this can lead to a second-order EROI of 0.4 from a process that otherwise would have EROI = 0 (e.g. generate no useful energy).  This 2^nd order EROI = 1.4 sounds promising, indicating that more energy is produced than is needed for production (EROI = energy output / energy input). However, it does not include every investment needed to produce algae and convert it to a fuel. A rough rule of thumb (as if there really is a rule of thumb for producing fuels from algae) is that the capital and labor costs would be near half of the costs of other operating and maintenance costs. By doubling the energy inputs to account for capital and labor, the EROI will be < 1. 

Further, just because a fuel or electricity resource has EROI > 1 even after accounting for all system inputs, that criteria does not relate to societal viability. For example, there is some evidence to support the idea that an EROI > 5 might need to hold for transportation services or liquid fuels (at least in the modern economy). But this is an ongoing area for study.

What does all of this mean? Well, in my opinion, it means we need to treat our liquid fuels (with high-energy density) as very precious resources as we have yet to prove that we can create a renewable liquid fuel even nearly equivalent to gasoline (petrol) or diesel. There is some evidence to show that the United States goes into recession if the EROI of gasoline at the pump drops to near 5 (http://environmentalresearchweb.org/cws/article/news/44403). Let’s keep trying for alternative fuels, but also keep perspective to think about alternative modes of organization and transportation in general. The new targets for US mileage standards to increase to 54.5 miles per gallon are a good step toward focusing people on the constraints on the supply of oil and oil/transportation alternatives.

Posted by Carey King on September 6, 2012 4:52 AM | Permalink