First of two parts:
http://biopact.com/2007/01/in-depth-look-at-biofuels-from-algae.html
Friday, January 19, 2007
An in-depth look at biofuels from algae
Over the past few years, several companies have issued press releases
about technologies they have developed to produce biofuels from
algae. The claims in these stories are that algae yield 'enormous'
amounts of biomass that can be turned into liquid fuels at low cost.
Most of the projects involve the use of closed photobioreactors, in
which the micro-organisms are grown in a controlled manner by feeding
them CO2 and nutrients. Sadly, after decades of development, none of
those projects have ever demonstrated the technology on a large
scale, let alone over long periods of time. This is why it is time to
have a look at the possible reasons as to why algae biofuels are
being talked about, but don't seem to get off the ground.
The biofuel potential of algae has been the object of considerable
research efforts in the past. Both in Europe (France and Germany),
Japan and the US, scientists have been working on algae systems since
the 1950s and especially since the oil crisis of the 1970s. One
program stood out, because it was so comprehensive. This is the
so-called "Aquatic Species Program" (ASP), which ran from 1978 to
1996 under the US National Renewable Energy Laboratory (NREL), funded
by the Office of Fuels Development, a division of the US Department
of Energy. The program's conclusions offer a handy guideline for
those of us who want to explore the challenges and opportunities of
producing biofuels from algae.
The focus of this program was to investigate high-oil algaes that
could be grown specifically for the purpose of wide scale biodiesel
production. The research began as a project looking into using
quick-growing algae to sequester carbon in CO2 emissions from coal
power plants. Algae had already been used in experiments to manage
waste water and were found to make good substrates for biogas
production, even though the sludge they fed on yielded more biogas.
Noticing however that some algae have very high oil contents, the
project shifted its focus to growing algae for another purpose -
producing biodiesel. Some species of algae were supposed to be
ideally suited to biodiesel production due to their high oil content
(between 10 and 50%, depending on many different factors), and fast
growth rates in laboratory situations. But after two decades of
fundamental research and large-scale trials, the results of the ASP
have been a mixed bag.
The following is an in-depth look at the conclusions of the different
projects carried out under the program. All quotes are taken from "A
look back at the U.S. Departmenf of Energy's Aquatic Species Program"
[*.pdf], the close-out document that was written after the program
was terminated in 1996.
For readers who want a shortcut: Table 1 offers a quick overview of
the results of all the studies under the ASP, as well as those of
some earlier research on which the program drew (click to enlarge).
Note that the results on biomass yields in the table only refer to
yields that were actually obtained in the field, not to projected,
desired or predicted yields.
Our overview includes a look at the most common problems and
challenges encountered during the program (low yields, unstable algae
cultures, harvesting difficulties, pond design, the impractability of
photobioreactors) and at the results of the separate experiments
conducted from the late 1970s to the early 1990s. We compare the
biomass yields of ordinary terrestrial tropical energy crops with
those of algae and analyze some basic risks involved in both biomass
production systems. Finally, we have a look at recent developments in
the production of gaseous fuels (biohydrogen and biogas) based on
algae.
Limited success with engineering algae
The ASP's original aim of genetically manipulating algae so that they
produce more lipids did not yield any significant results. The
researchers discovered a lot of information about the genetics and
environmental factors that play a role in the biology of different
algae species, but they failed to identify the magic 'lipid trigger'
they were looking for. The ASP concluded that:
biomass :: bioenergy :: biofuels :: energy :: sustainability ::
biogas :: biodiesel :: biohydrogen :: algae :: cyanobacteria ::
diatoms :: energy balance :: life-cycle analysis :: energy crops ::
tropical ::
"Although much remains to be done, significant progress was made in
the understanding of environmental and genetic factors that affect
lipid accumulation in microalgae, and in the ability to manipulate
these factors to produce strains with desired traits. The evidence
for a specific lipid trigger is not overwhelming." [page 142]
However, another, non-genetic way of increasing the amount of lipids
within the cells was discovered. It consisted of depriving the algae
of certain nutrients:
"Interpreting exactly what is happening in the nutrient-deprived
cells is difficult, particularly when cells are starved for N, as the
lack of an important nutrient is likely to produce multiple and
complex reactions in a cell. However, lipid accumulation in some
algal species can be induced by nutrient limitation." [142]
Even though this technique did increase the lipid content of some of
the micro-organisms, it also resulted in a decrease of the overall
biomass productivity of the algae. The net-result in terms of total
lipid production was negative:
"One of the most important findings from the studies on lipid
accumulation in the microalgae is that, although nutrient stress
causes lipid to increase in many strains as a percentage of the total
biomass, this increase is generally accompanied by a decrease in
total cell and lipid productivity." [143]
Finally, preliminary experiments were also performed within the ASP
to use the knowledge obtained on the genetic transformation system in
order to
"introduce genes into the algal cells, with the goal of manipulating
lipid biosynthesis. Additional copies of the ACCase gene were
introduced into cells of C. cryptica and N. saprophila. Although
ACCase activity was increased in these cells, there was no detectable
increase in lipid accumulation." [144]
The program was terminated before further research into this path was
undertaken. Given the rapid developments in biotechnology (a decade
has passed since the ASP was ended), we think it is very likely that
ideal algae can be engineered in the future, even though the
challenges remain high.
Such an ideal type would have to have the following properties:
1. it should have a high and constant lipid content;
2. one has to be able to grow the micro-organism continuously (the
problem of the stability of algae cultures);
3. it should have a high photosynthetic efficiency resulting in high
and constant biomass productivities;
4. it should be capable of withstanding seasonal climatic differences
and daily changes in temperatures.
5. the physical nature of the 'super' algae (especially its size)
would have to be such that it is easily harvesteable by membranes (if
the species is too small, high-strength and durable harvesting
membranes have to be designed that have to be able to withstand
fouling and water pressure drops; in the late 1970, such membranes
were deemed to be too costly) or it would have to be a type that
easily flocculates so that harvesting can occur without too much
losses and without the need for costly flocculants (see below).
All of the above are common problems associated with alga-culture
that were identified during the ASP.
Photobioreactors: dismissed as too costly and impractical
The Aquatic Species Program experimented with closed photobioreactors
for a while, but quickly dismissed them as being too impractical and
costly. It therefor concentrated on growing algae in open ponds from
the start, an effort it pursued over the decades that followed.
"The Japanese, French and German governments have invested
significant R&D dollars on novel closed bioreactor designs for algae
production. The main advantage of such closed systems is that they
are not as subject to contamination with whatever organism happens to
be carried in the wind. The Japanese have, for example, developed
optical fiber-based reactor systems that could dramatically reduce
the amount of surface area required for algae production. While
breakthroughs in these types of systems may well occur, their costs
are, for now, prohibitive-especially for production of fuels. DOE's
program focused primarily on open pond raceway systems because of
their relative low cost." [5]
At the end of the 1970s (when oil prices hit all time highs; US$80
per barrel in 2004 dollars), one last photobioreactor project
involving the use of a fibre-optic lighting system was abandoned very
soon:
"Another biophotolysis project tested an optical fiber system for
diffusing solar light into algal cultures, thereby overcoming the
light saturation limitation to photosynthetic efficiencies. This was
shown to be impractical and was abandoned after only some very
initial work." [161]
At the time, Japanese scientists started working on the same
fibre-optic lighting technology, but this effort yielded no major
breakthroughs. Fibre-optics were thought to offer a solution to the
problem of light saturation limits experienced in closed
photobioreactors. A theoretical advantage of such reactors is the
limited amount of space they need; algae move through them in a
controlled flow, so that they recieve an optimal amount of light. But
it quickly became apparent that this theory doesn't work out in
practise.
If reactors are designed in the form of large tubes or spheres, the
algae located at the center do not receive enough light. So
scientists introduced fibre-optic wires at the center of the reactor,
which continuously emit light. The kinetics of the algae would then
be finetuned so that all of them circle around the surface of the
reactor (where they receive ambient light) and near the center, where
they receive light from the fibre-optic wires. Obviously, this was
quite a costly affair, compared to simply growing algae on a
horizontal plane (in shallow ponds) where they can make use of
sunlight. Like ordinary terrestrial crops.
In the end, the ASP decided to take the latter route, and abandoned
photobioreactor research alltogether. Instead, it started designing
open ponds, to be located in the open air, in sunny deserts and other
locations that receive a lot of sunshine (like Hawaii). From then on,
the argument that algae take up "less space to grow" than ordinary
crops, no longer held.
Full life-cycle analysis required
Taking into account the ASP's experience with photobioreactors, we
should stress that current announcements surrounding this technology
are not very transparent nor complete. Advocates of algae biofuels
often look at the biomass productivities of algae in closed
photobioreactors, and compare those to the yields of energy crops or
to algae grown in open ponds. But there is more to bioenergy than
mere crop yields. Both from an economic as well as from an
environmental point of view, the entire life-cycle of the biofuels
must be analysed.
Press releases from algae-biofuel companies never disclose any
information on the actual energy balance, the greenhouse gas balance
and the costs involved in manufacturing and operating
photobioreactors. They don't because it is their obvious weak point.
As one analyst (Jonas Van Den Berg) once said: "growing algae in
reactors or in plastic ponds is like growing sugarcane in
greenhouses, it makes no sense." As the following analyses in this
essay will show, there is some truth to this observation.
There are many techniques and problems associated with life-cycle
studies of biofuels (earlier post). Depending on the chosen
parameters (system boundaries and byproduct credits), results will
differ. But in the case of closed photobioreactors, it would be
legitimate to use the technique used by an often quoted scientist
like David Pimentel (Cornell University), who made a comprehensive
energy balance analysis of ethanol, and who used a very broad "system
boundary" in his study. For example, he included the energy inputs
required to manufacture farm machinery that will be used to harvest
the corn.
It would be interesting to make a similar life-cycle analysis for
photobioreactors. The Aquatic Species Program did not do this
explicitly, but we guess that when it said reactors are "too costly",
it hinted at the overal life-cycle of the technology.
Photobioreactors are made of resources that require a lot of energy
to make. Steel, aluminum, polymers, glass, shaped in special forms
(spheres and tubes). A lot of energy goes into (building the machines
needed for) mining the raw resources (iron, aluminum ore, petroleum
for the polymers, and so on). The ores then have to be transported to
processing plants where another amount of energy is required to smelt
and cast them. The finished pieces then have to be brought together
(requires transport energy) and assembled. All this happens before
any biofuel has been produced.
When the reactor is in place, it needs to be heated and cooled in
order to operate efficiently during cold winter and hot summer
months. This too requires a considerable amount of energy. Without
heating, algae cultures die or become extremely unproductive (the ASP
showed that cultures grown in open ponds yield as low as 2g/m© of
biomass per day (on a yearly basis this equals to around 7,3 metric
tons) during winter months.
Furthermore, the ASP showed that there is a fine balance between the
optimal kinetics of the algae (the speed at which they move through a
system in order to receive enough light) and the energy inputs needed
to achieve this balance: in several experiments, the costs of keeping
the algae flowing (by pumping the medium in which they grow),
exceeded the energy the algae produced. In photobioreactors, this
same observation holds. Some reactors consist of vertically,
diagonally and horizontally stacked tubes through wich the algae are
circulated; if under the ASP's horizontal pond conditions, the energy
balance already was negative in some experiments (speeds >30cm/s), it
is not unreasonable to assume that it will be negative in such
complex reactors where the algal medium has to be pumped several
metres high through vertical tubes.
In short, comparing the biomass productivities of algae and their
resulting energy content without taking into account the entire
energy balance, is a futile exercise. Journalists and the media
should not forget this. At the Biopact, we also think that this is
one of the reasons why so many algae projects issue a press release,
but never actually implement their technology on a large scale. If
closed photobioreactors work and succeed in delivering cheap
biofuels, then all the better for us. But if they don't, we should
have the courage to say so too.
NREL's ASP research abandoned photobioreactors alltogether and
instead focused on the development of algae farms in desert regions,
using shallow saltwater pools for growing the algae. Using saltwater
eliminates the need for desalination, but could lead to problems as
far as salt build-up in ponds. Building the ponds in deserts also
leads to problems of high evaporation rates and temperature control
(at night, it can get very cold and heating ponds would be very
costly). Moreover, during winter months, biomass productivities
declined sharply, lowering the overal biomass yields per year.
Harvesting the algae posed engineering challenges. Finally, another
recurring problem was keeping the algae cultures stable. Cultures
that performed well under laboratory conditions were often lost in
the field trials, because they were invaded by stronger algae; the
experiments were often halted and new cultures had to be reintroduced
into the ponds.
All these challenges are nicely illustrated in the separate
large-scale experiments that were carried out from the late 1970s to
the early 1990s. Let us have a cursory look at them.
STUDIES PRECEDING THE ASP
Species Control in Large-Scale Algal Biomass Production (1976)
From the very beginning, in the 1950s up until today, the problem of
the stability of algae cultures in open ponds has not been resolved.
In a first series of experiments, aimed at growing algae for
waste-water treatment in open ponds, there was a consistent gap
between the stability of laboratory cultures and the instability of
cultures grown in open ponds.
This 'Species Control' project addressed this problem and at the same
time looked at potential harvesting technologies. Because the
dominant algal species found in a pond could range from small
unicellular to large colonial or filamentous species, harvesting of
the algae for biomass conversion would require a universally
applicable harvesting technology, such as centrifugation or chemical
flocculation, to enable the recovery of any algal type. However,
these processes proved to be very expensive.
If, however, algal species could be controlled in the ponds, then
filamentous microalgae species might be grown that would be easier
and cheaper to harvest using microstrainers. Microstrainers, which
are rotating screens (typically 25 to 50 µm openings) with a
backwash, are already widely used for removing filamentous algae,
mainly filamentous cyanobacteria (blue-green algae) from potable
water supplies.
Thus, the first objective of this project, initiated in 1976, was to
investigate how to selectively cultivate filamentous microalgal
species in waste treatment ponds
"Both at short and long retention times the algal cultures invariably
became unharvestable with microstrainers. Intermediate hydraulic
retention times selected for larger colonial algal species that were
more readily harvestable. However, long retention times also resulted
in low productivities. There was an optimum residence time, which
varied with depth of the culture and climatic variables that selected
for harvestable cultures. However, biomass recycling was only
marginally effective in improving biomass harvestability by
microstraining. " [148]
Problems were encountered with zooplankton grazing off the algal
cultures. Coarse (150-µm) screens did not effectively remove the
grazers. Shorter retention times reduced grazer pressures, but also
made the cultures less harvestable by microstrainers. In all the
ponds, the Scenedesmus species dominated in the winter and spring,
and then was replaced with Microactinium. Loss of dominance
correlated with the breakup of the colonies, which may have been
related to zooplankton grazing.
"The best productivity was 13.4 g/m2/d, during a 10-month period,
irrespective of harvest efficiency. For the most harvestable pond,
productivity was only 8.5 g/m2/d (of which only 7.2 g/m2/d was
harvested by the microstrainers). Clearly, optimizing for
productivity and harvestability required quite different operating
conditions. It was concluded that the use of microstrainer harvesting
and biomass recycling was unlikely to lead to both a high algal
productivity and effective harvesting process." [152]
Large-Scale Freshwater Microalgal Biomass Production for Fuel and
Fertilizer (1977-1979)
Both microstrainer harvesting and biomass recycling were seen as
unfeasible harvesting strategies, but researchers kept experimenting
with the techniques, only to abandon them relatively soon:
"Initially the approach to establish microstrainable cultures using
the 12-m2 ponds, continued to be investigated. Essentially the same
results as before were obtained: detention time was found to be the
key environmental variable determining algal colony size (but not
necessarily species composition) and a negative correlation was found
between numbers of algal grazers and the large colonial algal types
easy to harvest with microstrainers. Apparently the grazers
preferentially consumed the smaller algae. Overall, the
harvestability results with the microstrainers continued to be poor,
so this line of research was abandoned during the initial period of
this project." [156]
They then focused on another technique: harvesting after
bioflocculation. Bioflocculation refers to the tendency of normally
repulsive microalgae to aggregate in large flocs, that then exhibit a
rather high sedimentation velocity. The mechanisms of bioflocculation
involve extracellular polymers excreted by the algae. Once the algae
have flocked together, they can be harvested.
The bioflocculation research zoomed in on a "phase isolation"
process, in which the algal cells were allowed to spontaneously
settle when sewage inflow was stopped. Although generally long times
were required for this settling process (2-3 weeks), it was decided
to investigate this general phenomenon of "bioflocculation" in high
rate ponds. The process involved removing the algae from the paddle
wheel-mixed ponds and placing them in a quiescent container, where
they would spontaneously flocculate and rapidly settle.
There are several apparently distinct mechanisms by which algae
flocculate and then settle, including "autoflocculation", which is
induced by high pH in the presence of phosphate and divalent cations
(Mg2+ and Ca2+), and flocculation induced by N limitation.
Settling tests were carried out with the cultures from the 12-m2
ponds. As with microstrainer harvesting, detention time and mixing
velocity were the most important variables in promoting a
bioflocculating culture. The rather rapid settling of many of the
cultures was very encouraging. Also, the initial experiments with the
0.25-ha pond demonstrated a fairly rapid
"Bioflocculation [being] established as the method of choice for
algal harvesting, as it seemed to be achievable even with high
productivity cultures. Culture settleability was routinely determined
during all the experiments with the high rate ponds." [157]
Interesting yield data
Table 2 summarizes productivity, settleability and harvesteability of
algae grown for more than 1 year in the two 0.1-ha ponds (click to
enlarge).
These results were the only ones so far for algae grown during all
monhts of the year. They show the sensitivity of the micro-organisms
to changes in temperature, with yields in winter and spring months
declining to very low levels (lowest: 2.6g/m©/ha).
The average gross biomass productivity was maximum 14g/m©/day (51.1
tons per hectare per year), and minimum 12g/m©/day (43.8 tons per
hectare per year), of which some 90% could be harvested.
The difference in harvestable biomass yields between algae grown in
large (0.1 hectares) and small (12m©) ponds was small: both small
ponds obtained an average yield of 13g/m©/day (even though they were
only used to grow algae for a period of 10 months, excluding the two
coldest months), the two large ones 14g/m©/day and 12g/m©/day
respectively.
The numbers from these trials, showing a yield per hectare per year,
allow us to make a comparison with ordinary terrestrial energy crops
(see below).
Membrane harvesting project (1978)
Professor Harry Gregor at Columbia University was funded for 2 years
to develop membrane systems for cross-flow filtration harvesting of
microalgae. However, the membranes available at the time, the
pressure drops required, and the fouling problems encountered made
this approach impractical.
Ryther and Goldman (late 1970s)
At Woods Hole Oceanographic Institutions, Drs. John Ryther and Joel
Goldman carried out extensive research on microalgae cultivation in
outdoor ponds on mixtures of seawater-secondary sewage effluent. When
Dr. Ryther relocated to the Harbor Branch Oceanographic Foundation in
Florida in the late 1970s, he was supported by DOE and later the ASP
for the production of freshwater plants (water hyacinths, etc.) and
seaweeds, as well as for microalgae culture collection work. Dr.
Goldman also wrote a review on the theoretical and practical aspects
of microalgae cultivation under contract with the US Department of
Energy.
"One conclusion was that the productivity of microalgae systems would
be limited, because of the light saturation effect and other factors,
to below 50 tons/ha/yr." [161]
A yield of 50 tons/ha/year is considerably below average yields of
ordinary tropical energy crops (see below).
MICROALGAL MASS CULTURE: THE ASP'S OWN RESEARCH
After these previous studies and field trials, the ASP tried to
improve upon both the harvesting process as well as on keeping algae
cultures stable, and embarked on its ambitious program that consisted
of:
"extensive work on [algae] strain isolation, selection,
characterization, etc., carried out by the ASP [which] was used to a
significant extent by the field projects, through the testing of a
number of the isolates in algal mass cultures." [162]
But the gap between laboratory and field results kept appearing
throughout the program:
"Unfortunately, the laboratory-level screening protocols had, in
hindsight, relatively little predictive power for the ability of the
strains to dominate and perform in outdoor ponds. Similarly, the
laboratory work on the biochemistry, genetics and physiology of lipid
biosynthesis, was difficult to apply to the goal of increasing lipid
productivities in outdoor systems. Greater integration of laboratory
and outdoor R&D is a challenge for any future microalgae R&D
program." [162]
Despite this disconnect, the ASP went ahead an initiated two outdoor
projects in 1980, one in California using a paddle wheel-mixed
raceway pond design ("high rate pond," [HRP]), and another in Hawaii.
The Hawaii project was to demonstrate a patented algal culture
system, invented by then-ASP program manager, Dr. Larry Raymond
(1981). This "Algal Raceway Production System" (ARPS) used very
shallow flumes.
HAWAII, 1980-1987
This first major project made use of Dr Raymond's patented ARPS, a
complex 48m© raceway pond, which was expected to yield high and
consistent productivities with strains of P. tricornutum.
One difficulty noted in the laboratory experiments was the low cell
densities achieved, compared with the original reports by Raymond for
the ARPS system. Researchers tried to increase cell density by
increasing the pond depth to 0.6 m, rather than 0.1 m as proposed by
Raymond. This resulted in other problems (low cell density,
shading-see below), and the depth was again reduced to 30 cm.
Laws later reported on initial results with the 48-m2, 0.6-m deep,
airlift-mixed flume system. Cell densities were much lower than
predicted, likely because of the great depth of the culture, which
was later reduced.
The second year of this project emphasized the use of "flashing light
to enhance algal mass culture production". The basic idea was that a
"foil array" in the pond culture would generate a vortex that would
create organized mixing in the ponds, expected to result in exposure
of the cells to regular dark-light cycles.
Based on data in the literature, this effect would be predicted to
increase overall productivity. These a priori arguments were not
supported by the algal physiological literature (the flashing light
productivity enhancements are observed at much shorter time
constants), and neither were the hydraulic arguments plausible
(organized mixing would be seen only in a small fraction of the pond
volume). However, the key issue here is not the theory but the actual
experimental results.
"From November 1981 to January 1982, an average productivity of only
about 3.3 g/m2/d was recorded for the 50-m2-flume reactor, a very low
value for Hawaii, even in winter. After installation of the foils,
productivities, from February to March 1982, increased to about 11
g/m2/d." [166]
One observation was infestation of the culture by algal predators,
which could have been one reason for the rather large variability in
productivities observed during this operation. However, day-to-day
variability in productivities is a fact of outdoor pond microalgae
cultivation, even in the best of cases.
During the third year, a set of variables was tested and the
researchers concluded that "by far the most significant factor
affecting biomass production" was culture depth, arguing that the
"self-shading effects were more than offset by higher areal standing
crops." This was a rather puzzling conclusion as it is contrary to
both theory and experience, which assumes that, everything else being
equal, depth should not affect productivity. No actual productivity
data were reported.
The fourth year switched algae species, because "the fact that a
given species grows well in the laboratory is no guarantee that it
will perform well in an outdoor culture system." One reason the
project switched to different algal species was that the P.
tricornutum strain used in the experiments described above was quite
sensitive to even moderate (above 25°C) temperatures, and required
temperature control (cooling) of the reactors. A Platymonas sp. was
thus tested without temperature control in the outdoor flumes, at
several dilution rates and maximal pH levels of 7 to 8. This strain
showed a maximum productivity of about 26 g/m2/d, about the same as
observed with P. tricornutum with temperature control.
Note that, even though no energy inputs were reported, using the
tricornutum strain required continuously cooling the reactors, an
energy intensive operation.
During the fifth year, research was once again directed toward the
study of more thermotolerant species. Algal strains collected by the
ASP researchers in the southwestern United States were evaluated
using different types of water. Several species, including Platymonas
sp. (used previously), Amphora sp., C. gracilis, and Boekelovia sp.
were grown in the two water types, each at two salinities and at four
temperatures (25° to 32°C), with the data reported as the number of
doublings per day, making it difficult to compare the actual biomass
productivity with previous and later results.
One interesting, but unexplained, observation was that at higher
temperatures there was a consistent shift, among all four algae, of
maximum doubling rates to the higher salinity waters. The small
outdoor flumes were used to test this cultivation strategy. The
cultures were diluted each third day, to a concentration of 2 x 106
cells. The results were "consistent with those of earlier studies,"
with solar conversion (PAR) efficiencies close to 10% (5% of total
solar). The C. gracilis species was also tested, though at a 2-day
dilution rate (requiring a one per day doubling time), with somewhat
lower efficiencies (8%), though still rather high productivities.
Also, Tetraselmis suecica was cultivated in the ponds with good
results. Over a 78-day cycle, in spring 1984 and summer 1985,
productivity was 37+5 g/m2/d, with a corresponding PAR efficiency of
9.1%.
Research during year 6 elaborated on the two key findings mentioned
earlier: effects of a 3-day dilution interval and of the foil arrays.
The effects of foil arrays were tested over a 12-month period in the
48-m2 flume with Cyclotella sp., a diatom, which, like Chaetoceros,
is a good lipid producer. The experiment involved alternatingly
operating the pond with and without the foils for 2-week periods. The
presence of foils increased productivity by almost a third, similar
to the prior experiments.
The dilution effect was investigated with T. suecica, also in the
48-m2 flume, with similar results as before, in terms of both overall
and maximal 3rd day productivity. However, solar conversion
efficiencies were lower than observed in previous years, perhaps due
to the approximately 3°C higher temperature during this year,
compared to the previous one. The author speculated that this could
have been close to the maximal permissible temperature for growth of
T. suecica, and thus resulted in lower productivities.
However, the effect of dilution interval on production in the 48.4-m2
flume was somewhat puzzling. These findings were a subject of
considerable discussion and controversy. One possible explanation was
the measurement of actual biomass density, which varied from about
27-28 g/m2 after dilution, to 80, 140, and 160 g/m2 for the 2-, 3-,
and 4-day dilutions periods, respectively. However, this was
considered an "unlikely" explanation. Indeed, the highest
productivity was observed on day 3, with a steep decline on day 4.
However, 4-day cycle cells still had lower productivity on day 3.
Some "lingering effect of exposure to supraoptimal density
conditions" was speculated to account for this phenomenon. The
classical technique for studying such phenomena is the P versus I
curve. Such studies were carried out with T. suecica cultures grown
in the smaller 9.2-m2 flumes. However, as the author noted, the
results were "somewhat discouraging" as there was no difference as a
function of dilution intervals, and productivities were only about 24
g/m2/d, much lower than reported with the larger flumes. Thus, this
issue remained as a major focus of this project.
During the final year of the Hawaii ARPS project, the goal was to
screen for additional algal species in the smaller flumes and to
further study the effect of dilution intervals. Four species were
tested in the 9.2-m2 flumes: Navicula sp., C. cryptica, C. gracilis,
and Synechococcus sp. From prior work, photosynthetic efficiencies of
9.1% were reported with T. suecica, during a 78-day period, and 9.6%
for 122 days with C. cryptica. With the three other organisms listed
above, somewhat lower efficiencies were noted during shorter time
periods: 7.8 % for Navicula sp., 8.5% for C. gracilis, and 8.6% for
Synechococcus. Somewhat "surprisingly" (their characterization), they
observed that in a 2-day batch growth mode, initial cell
concentrations ranging from about 50 to 400 mg/L (AFDW) had no major
effect on productivity. For C. cryptica, at an initial concentration
of 40 mg/L at a depth of 12 cm, this would give an areal cell density
of about 5 g/m2. For an equal daily productivity of 30 g/m2/d,
averaged over 2 days, this would require the cells to divide 2.5
times the first day, and once the second day. Not impossible,
certainly, but somewhat problematic. There is indeed some likelihood
that some systematic measurement error influenced their productivity
measurements.
"This report also described lipid induction by Si limitation by C.
gracilis and C. cryptica. In both microalgae Si limitation greatly
reduced overall productivities, and lipid productivities, even though
lipid contents increased. Laws concluded that lipid productivities
would be maximized by maximizing total biomass production." [173]
In the final paper, Laws et al., reported on long-term (13-month)
production of C. cryptica in the large flume, with a 9.6% solar
conversion efficiency reported with the foils and 7.5% without the
foils, similar to earlier results with T. suecica. For 122 days, at
optimal dilution (2- day batch cycle) productivity of about 30 g/m2/d
was measured. This is, indeed, a high sustained productivity; on a
year's basis, it equates to roughly 15g/m2/d (54.75 ton/hectare/year).
Conclusions of the Hawaii Project
This project evolved from one that focused on a demonstration of the
ARPS concept using a single flume, to the investigation of
fundamental issues in algal mass culture, using several smaller ponds
and a simplified system design. In particular, this project reported
very high productivities achieved by two methods: organized mixing in
ponds (e.g., the foils), and optimal batch dilution (2- or 3-day
intervals, depending on species). However, the basis for these
productivity enhancements was speculative, and it proved difficult to
demonstrate the reproducibility of these effects. The effects of
foils could be better ascribed to degassing of oxygen from the ponds
with foils (e.g., higher mixing power inputs) and the results from
the 3- day dilution experiments to some uncontrolled factors, in
addition to possible methodological problems.
None of the experiments under the Hawaii Project involved growing
algae for longer than a year, which is why no final word on their
(harvesteable) biomass productivity can be said. (One experiment was
carried out for 13 months, but no yield data for it were reported.)
Laws continued his research with Electric Power Research Institute
funding for 1 year, moving the system to Kona, Hawaii. No
significantly different information was produced. However, Laws
concluded
"that lack of land area, and high costs, would make such a process
[growing algae in open ponds] impractical for fuel production in
Hawaii." [174]
CALIFORNIA, 1981-1986
The objective of this second project was to demonstrate the
functionality of a so-called High Rate Pond (HRP) system using
agricultural irrigation waters and fertilizers as nutrients. The HRP
was defined as a paddle wheel-mixed (approximately 10-20 cm/s),
moderate depth (approximately 15-30 cm), algal production system. The
R&D goal was to develop production technology for microalgae biomass
with a high content of lipids. A detailed literature review concluded
that the best option would be to use N limited (but not starved)
batch cultures of green microalgae.
The system consisted of four 200-m2 and three 100-m2 ponds, along
with three deep harvesting ponds and four water and effluent storage
ponds. This system thus provided considerable flexibility for the
testing of a large number of variables and algal species, at a scale
that would allow some confidence in the scale-up of the results. The
units were lined with 20 mil PVC, to allow complete mass balances.
Note on costs and energy balance: We wish to add an important note
here: these ponds were lined with PVC, which brings us conceptually
close to greenhouses used in terrestrial agriculture (purely speaking
from the point of view of material inputs and costs). The biomass
productivity of the algae obtained in the California ponds as well as
in previous and later projects (between 30 and 50 ton per hectare per
year) makes us conclude that the mere cost of the PVC makes such a
system uncompetitive with ordinary, rainfed, open-air (sub)tropical
agriculture. Tropical crops already yield far more biomass than algae
(see below), and if they were to be grown in greenhouses, their
productivity would be much higher still. This is why Jonas Van Den
Berg's remark - "growing algae in reactors or in plastic ponds is
like growing sugarcane in greenhouses, it makes no sense" - is not
too far-fetched.
After an initial delay and a temporary loss of funding, the actual
pond operations were initiated in August 1982.
The first inoculation of algae into one of the 100-m2 ponds consisted
of a mixed Micractinium-Scenedesmus culture, which was soon lost:
"these algae settled out due to lack of flow deflectors, and the
culture was soon dominated by a Selenastrum sp. Both biomass
concentration and productivity were quite low. Without flow
deflectors at the far end of the ponds (away from the paddle wheel)
the hydraulics were so poor that the ponds exhibited almost zero
productivity." [179]
This was due to the formation of large countercurrent eddies
resulting in "dead zones," where algal cells settled. After flow
deflectors were installed, the pond was re-inoculated with an almost
pure culture of Scenedesmus that had arisen spontaneously in one of
the 12-m2 inoculum ponds. The culture remained well suspended and
grew well.
However, a similar inoculation into a 200-m2 pond resulted in almost
complete settling of the culture, caused by poor pond hydraulics,
even with similar flow deflectors installed. This indicated that the
hydraulics of the ponds are critical to the success of the process
and further, that the hydraulics are not predictable from one scale
to another, even within a factor of two. After two flow deflectors
were installed around the bends in the 100-m2 ponds, these ponds
exhibited much improved hydraulics, with few eddies or settling of
algal cells.
In contrast, similar deflectors did not improve hydraulics
perceptibly in the larger, 200-m2 ponds. Only after two more flow
deflectors were installed at the end nearest the paddle wheels were
satisfactory hydraulics observed in these larger ponds. A
quantitative study of flow velocities was undertaken using a flow
meter. The results were counterintuitive: flow velocities are higher
on the inside than the outside of the channels. Clearly, pond
hydraulics must be customized for each pond size and design to obtain
even mixing.
"As expected, productivities were rather low in the initial
experiments carried out during October and November 1982. Maximum
productivities (measured for 2 days) were only about 9 g/m2/d and
average productivities less than 5 g/m2/d." [179]
These initial experiments included assessment of species dominance, N
limitations, and mixing velocities. Pond operations ceased by the end
of November 1982 after poor results.
In 1983-1984, a new company, Microbial Products, Inc. (EnBio was
dissolved when John Benemann left in 1983 for the Georgia Institute
of Technology), continued the project. The pond system described
earlier continued to be used for this project.
The objective was to obtain long-term productivity data with a
pilot-scale system and generally demonstrate the requirements of
large-scale algal mass cultivation
The first challenge was to obtain microalgal species that could be
grown on the fresh to slightly brackish water available at the site:
"The common experience is that either inoculated strains from culture
collections fail to grow in the outdoor ponds, or that they grow
initially but become rapidly outcompeted by indigenous strains. A
common practice is to make the best of a bad situation and cultivate
the invading organisms instead." [180]
This was also the experience and approach of this project. After
inoculation of Scenedesmus obliquus strain 1450 from the SERI Culture
Collection, a strain of Scenedesmus quadricuada invaded. This turned
out to be the most successful organism, cultivated for 13 months in
fresh water and an additional 3 months in brackish. After an Oocystis
sp. (Walker Lake isolate) was inoculated, a Chlorella sp. became
dominant and was maintained (or maintained itself) for 2 months under
semi-continuous dilution. However, some strains provided by SERI
researchers could be grown for at least a few months outdoors,
including an Ankistrodesmus falcatus and a freshwater Scenedesmus sp.
Productivity for S. quadricauda (see table 3) grown semi-continuously
which is harvested every few days (a "sequential batch" growth mode),
averaged about 15g/m2/day for the 8 month period of March through
October, with monthly averaged solar conversion efficiency ranging
from 1.2% to 2.2%. Typical biomass density just before harvest (that
is on the 3rd dilution day) ranged from 60 to 100 g/m2, except for
May, which recorded the highest standing biomass (160 g/m2) and
productivity (20 g/m2/d). The continuously diluted cultures (diluted
during the entire light period) exhibited approximately 20% higher
productivity. Over a ten month period, the average productivity stood
at 15g/m©/day (see table, click to enlarge), of which some 90% is
harvesteable.
See next message, "An in-depth look at biofuels from algae - 2"
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