3. Results and discussion
3.1. Results
3.1.1. Energy
The energy inputs and outputs for all 20 cases are shown in Fig. 2. The
energy impacts assigned to each contribution are listed in TableD-3 of the
SI. Case 1 incorporates inefficientmethods that yield high energy requirements
and a low-nitrogen growth setting with lower productivity than
the high-nitrogen cases. Case 2 improves upon Case 1 by usingmore efficient
technologies and Case 3 applies a high-nitrogen growth setting with
higher yields, further improving the EROI with respect to Case 1. Case 4
(Desmodesmus sp.) has a greater oil content than Case 3 (Staurosira sp.)
and no silicon requirement, but the EROI is less than Case 3 because
there is less animal feed produced, which receives a large displacement credit. Fermentation of the aqueous phase produced during the Valicor
process (Case 5) results in a slight reduction in the EROI as compared to
Case 4 due to energy required to distill the ethanol produced by fermentation.
Implementingwind power (Cases 6, 9, and 10) providesmajor energy
savings, reducing the energy impact for electricity fromroughly 3.85
to 1.13 MJ/MJ. HTL provides the most energetically advantageous separations/
conversion method (Cases 7 and 9), yielding an EROI above 8 in
Case 9H, which is among the highest EROI reported to-date for an endto-
end algae fuel scenario. The benefits of HTL include large biocrude
yields, on-site electricity and heat production, and low embedded energy
inputs due to high levels of nutrient recycling. The OpenAlgae process
(Cases 8 and 10) is quite similar energetically to the Valicor process, but
provides slight improvements in this assessment by reducing the heat required
for extraction. All of the Hawaii cases result in greater EROI values
than their Texas counterparts because the water supply energy is significantly
less for drilled wells in Hawaii versus a 5 kmpipeline in Texas. For
Cases 2–10 in Texas, thewater supply energy accounts for over 25% of the
total energy input,whereas all of the cases inHawaii require less than 25%
of the total energy input for water supply.