Environmental Engineering Reference
In-Depth Information
revealed that the viability of a commercial system depends heavily on obtaining algal mutants
that can produce H2 under near-atmospheric concentrations of O2 [64]. This level of oxygen
tolerance represents an approximate 70-fold improvement over wild-type Fe-hydrogenases in
the concentration of oxygen tolerated (from 0.3-2 1 percent), or a 720-fold increase in the
half-life of Fe-hydrogenase activity (from ~1 minute to ~1 2 hours) in the presence of
atmospheric oxygen levels. In either case, it is clear that such an improvement is well within
the range of improvements that have been reported for a variety of other enzymes [65-72].
2.3.5. H2 recycling
. A third minimizer of photon-to-H
2
conversion efficiency is the
presence in many green algae, including those most well-studied for H
2
production,
Chlamydomonas reinhardtii, Scenedesmus obliquus, and Chlorella fusca, of H
2
uptake
activity [73, 44]. However, the enzymes responsible for this activity have not yet been
positively identified; in fact, the possibility remains that some of the same Fe-hydrogenases
may be responsible for both H
2
uptake and H
2
production activity. The investigation of
mechanisms regulating hydrogen production, involving mutants deficient in individual
functions of the pathway, remains an important area of research in the development of H2
production by direct biophotolysis [11].
2.3.6. Rate and cost estimate
. To date, the two-stage bioreactors making use of wild-type
green algae are the only systems that have been developed for sustained production of H2 by
means of direct photolysis. Rates of H
2
production achieved in this way are reported as 0.07-
0.08 millimole per liter culture per hour [5] as seen in Table 5, the lowest rate reported for
existing biohydrogen production systems. However, the substrate costs for direct photolysis
are also extremely low, improving its relative efficiency in comparison with higher-rate,
higher- input requiring systems.
Table 5. Comparison of rates of H2 biosynthesis [5]
BioH2 System
H2 synthesis rate
(reported units)
H2 synthesis rate
(converted units)
Direct photolysis
4.67 mmol H2/l/80 h
0.07 mmol H2/(l x h)
Indirect photolysis
12.6 nmol H2/μg protein/h
0.355 mmol H2/(l x h)
Photo-fermentation
4.0 ml H2/ml/h
0.16 mmol H2/(l x h)
CO-Oxidation by R. gelatinosus
0.8 mmol H2/g cdw/min
96.0 mmol H2/(l x h)
Dark-fermentations
Mesophilic, pure straina Mesophilic,
undefinedb
21.0 mmol H2/l 1/h 1,600.0
l H2/m3/h
21.0 mmol H2/(l x h) 64.5
mmol H2/(l x h)
Mesophilic, undefined
3.0 l H2/l/h
121.0 mmol H2/(l x h)
Thermophilic, undefined
198.0 mmol H2/l/24 h
8.2 mmol H2/(l x h)
Extreme thermophilic, pure strainc
8.4 mmol H2/l/h
8.4 mmol H2/(l x h)
a
Clostridium species #2.
b
A consortium of unknown microorganisms cultured from a natural substrate and selected by the
bioreactor culture conditions.
c
Caldicellulosiruptor saccharolyticus
In addition, the potential benefit of realistic research accomplishments must be taken into
account. In the Thorough-cost analysis by modelers at the NREL, the cost of H
2
generated by
direct photolysis was estimated under current laboratory conditions, as well as with
improvements available in the near-term (incorporating measures definitely possible), in the
