Sunday, August 07, 2011

Looking at Algal Energy

A new study from the University of Virginia compares 4 different energy-from-algae approaches by VKT -- vehicle kilometres traveled.
In this new study, reported in the ACS journal Environmental Science & Technology, Clarens et al. assess four algae conversion pathways resulting in combinations of bioelectricity and biodiesel:
  1. Anaerobic digestion of bulk algae biomass to produce methane-derived bioelectricity;
  2. Production of biodiesel from algae lipids with anaerobic digestion of residual algae biomass to produce methane-derived bioelectricity;
  3. Production of biodiesel from algae lipids with direct combustion of residual algae biomass to produce bioelectricity;
  4. Direct combustion of bulk algae biomass to produce bioelectricity.
Images from GCC

...their results suggested that conversion pathways involving direct combustion for bioelectricity production generally outperformed systems involving anaerobic digestion and biodiesel production, and they were found to generate four and fifteen times as many vehicle kilometers traveled (VKT) per hectare as switchgrass or canola, respectively. _GCC

Among the many findings:

Algae EROI values computed in the study ranged from 0.65 to 4.10. Previously reported EROI for corn ethanol has been on the order of 1.25. It has been suggested, the authors noted, that the minimum sustainable EROI is roughly 3 but that values from 5 to 10 will be required to maintain quality of life in the absence of readily abundant fossil energy.

Direct combustion of algae to produce bioelectricity is seemingly more efficient than anaerobic digestion regardless of whether or not algae lipids are extracted to make biodiesel.

Selected algae systems dramatically outperform the terrestrial crop systems in terms of VKT production per hectare. Algae generates, on average, 4.2 times and 15.7 times more VKT than the switchgrass and canola systems, respectively.

Misalignment of system boundaries precludes direct comparison with corn ethanol, the authors note, but they estimate that the average algae VKT is roughly nineteen times greater than could be derived from corn ethanol (27,000 km/ha-yr) even when accounting for ethanol coproducts. 29

In terms of VKT, algae bioelectricity systems outperform algae combined biodiesel/bioelectricity systems.

Algae biodiesel and bioelectricity systems exhibit higher net energy use but lower water use and GHG emissions per km than their respective terrestrial benchmarks.

...the tremendous demand for transportation energy, increasing fuel prices, and a lack of mechanisms for monetizing environmental performance in the US make it reasonable to expect that algaeā€™s excellent land use efficiency could render it financially attractive over the next several decades. For this reason, environmental and economic LCA studies will be key tools for improving the overall sustainability of algae-derived transportation energy systems. _GCC

Once again we see the benefits of cellulosic electricity -- or biomass to electric power -- when compared to most current methods of creating biofuels from biomass.

The authors of the study missed a prime opportunity to compare efficiencies from pyrolysis of algal biomass, and gasification of algal biomass via IGCC and CHP, with the 4 approaches analysed.

Anaerobic digestion of algal biomass to produce methane is unlikely to be economical for at least the next 50 years, as the global shale gas bonanza works its way through the markets.

One of the biggest problems with public perception of biomass energy and biofuels, is the expectation that if biofuels cannot replace all other forms of energy, then there is no use pursuing their production. That type of magical thinking, with its hair-trigger relapse to utter futility, is profoundly destructive.

There are no magic bullets. A wide range of approaches will have to be taken before humans can emerge from their fossil fuels Earth-bound economies to more sustainably abundant and widespread economies of the future.

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