Energy balance and techno-economic assessment of algal biofuel production systems

Abstract

There has been considerable discussion in recent years about the potential of micro-algae for the production of sustainable and renewable biofuels. Unfortunately the scientific studies are accompanied by a multitude of semi-technical and commercial literature in which the claims made are difficult to substantiate or validate on the basis of theoretical considerations.

To determine whether biofuel from micro-algae is a viable source of renewable energy three questions must be answered
:
a. How much energy can be produced by the micro-algae?
b. How much energy is used in the production of micro-algae?
c. Is more energy produced than used?

A simple approach has been developed that allows calculation of maximum theoretical dry algal biomass and oil yields which can be used to counter some of the extreme yield values suggested in the 'grey' literature. No ready made platform was found that was capable of producing an energy balance model for micro-algal biofuel. A mechanistic energy balance model was successfully developed for the production of biogas from the anaerobic digestion of micro-algal biomass from raceways. Preliminary calculations had suggested this was the most promising approach. The energy balance model was used to consider the energetic viability of a number of production scenarios, and to identify the most critical parameters affecting net energy production.

These were:
a. Favourable climatic conditions. The production of micro-algal biofuel in UK would be energetically challenging at best.

b. Achievement of ‘reasonable yields’ equivalent to ~3 % photosynthetic efficiency (25 g m-2 day-1)

c. Low or no cost and embodied energy sources of CO2 and nutrients from flue gas and wastewater
d. Mesophilic rather than thermophilic digestion

e. Adequate conversion of the organic carbon to biogas (? 60 %)

f. A low dose and low embodied energy organic flocculant that is readily digested, or micro-algal communities that settle readily

g. Additional concentration after flocculation or sedimentation

h. Exploitation of the heat produced from parasitic combustion of micro-algal biogas in CHP units

i. Minimisation of pumping of dilute micro-algal suspension
It was concluded that the production of only biodiesel from micro-algae is not economically or energetically viable using current commercial technology, however, the production of micro-algal biogas is energetically viable, but is dependent on the exploitation of the heat generated by the combustion of biogas in combined heat and power units to show a positive balance.

Two novel concepts are briefly examined and proposed for further research:

a. The co-production of Dunaliella in open pan salt pans.
b. A 'Horizontal biorefinery' where micro-algae species and useful products vary with salt concentration driven by solar evaporation.

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ORCID for Sonia Heaven: orcid.org/0000-0001-7798-4683

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