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The economics of producing biodiesel from algae

Brian J. Gallagher *

Ecotonics Environmental Scientists, 1801 Century Park East, Suite 2400, Los Angeles, CA 90067, USA

a r t i c l e i n f o

Article history:Received 21 October 2009Accepted 15 June 2010Available online 29 July 2010

Keywords:Renewable fuelsAlgaeBiodieselEconomicsNet present valueReturn on investment

a b s t r a c t

Biodiesel is an alternative fuel for conventional diesel that is made from natural plant oils, animal fats,and waste cooking oils. This paper discusses the economics of producing biodiesel fuel from algae grown

in open ponds. There is potential for large-scale production of biodiesel from algal farms on non-arableland; however, previous studies have failed to demonstrate an economically viable process that could bescalable to a commercialized industry. The problems include inconsistent and insuf cient algalproductivities, uncertain capital and operating costs, volatile market prices and unknown levels of government support. Although intensive work is being done on many technological issues, the economicstudies and data are incomplete and out of date. This paper presents an updated nancial analysis of theproduction and economic conditions that could have a profound effect on the success of this importantalternative fuel production process.

2010 Elsevier Ltd. All rights reserved.

1. Introduction

The US consumes over 50 billion gallons of diesel fuel per yearfor transportation purposes [1]. In 2007, the US GovernmentAccountability Of ce reported the need to develop a strategy foraddressing a peak and decline in oil production [2]. Declining oilproduction will cause oil and diesel prices to rise sharply creatinga strong market for replacement fuels. Biodiesel is an alternativeliquid fuel that can substantially replace conventional diesel andreduce exhaust pollution and engine maintenance costs [3]. Thisrenewable fuel can be produced from different feedstocks such assoybeans, waste cooking oil, and algae. Most of the biodiesel in theUS is currently made from soybeans, which will soon reacha resource limitation of arable land. Microalgae are very smallaquatic plants that produce natural vegetable oils suitable forconversion to biodiesel. Biodiesel produced from microalgae grownin circulated ponds on non-arable land is not resource limited andhas the potential for yields 50 e 100 times greater than biodieselfrom soybeans. This high production level can be achieved sus-tainably with high-energy return on investment (EROI) and withlittle impact on food production and prices. Furthermore, theproductivity of algal ponds can be enhanced by the direct additionof waste CO 2 from fossil-fueled power plants and other high carbonemitting facilities. Therefore, large commercial aquatic farms of microalgae could not only produce large amounts of biodiesel

feedstock, but also recycle waste CO 2 emissions thereby reducingtheir buildup in the atmosphere.

Starting in 1999, US biodiesel production has rapidly increasedat an annual rate of about 200% [4] . However, rising soybean pricesand falling crude oil and diesel prices have hurt the competitive-ness of the biodiesel industry during 2009. The capacity for USbiodiesel production is nowover one billion gallons peryear spreadover about 150 plants, some of which have stopped producing. Thispaper explores the near term and future economics of producinglarge quantities of biodiesel fuel from algae.

The process of making biodiesel fuel from microalgae involvesseveral steps. Fig. 1 is a simpli ed block diagram of the entiresystem operation, which includes four major sub-processes asshown:

Growing algae in engineered ponds (GROWTH) Harvesting the biomass in settling ponds (HARVEST) Extracting the algal oils from the biomass (EXTRACTION) Converting the algal oil into biodiesel (CONVERSION)

The rst three sub-processes are performed at an aquatic farmusing agricultural engineering principles. Conversion of algal oil tobiodiesel is generally accomplished in a chemical plant usinga simple process to reduce the size and viscosity of the algal oilmolecular compounds. For the purpose of this analysis, an inte-grated algae farm and biodiesel conversion plant operation will be jointly analyzed. The next section discusses the economics of thegrowth, harvest and extraction stages; and the subsequent sectionwill discuss the economics of the conversion stage; which together

* Tel.: 1 310 556 9698; fax: 1 323 874 8109.E-mail address: [email protected] .

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doi: 10.1016/j.renene.2010.06.016

Renewable Energy 36 (2011) 158 e 162
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represent an illustrative project of 1000 acres (405 ha) producing2400 to 6400 gallons of biodiesel fuel per year.

2. Growing, harvesting and extracting algal oil using openponds

The focus of this paper is the well-known method of using openponds to grow algae at high rates ( Fig. 1). Shallow water is circu-lated horizontally around some type of divided pond or circularraceway to allow controlled exposure of the algae to naturalsunlight as well as to CO 2 and nutrients dissolved in the water. TheCO2 and nutrients can come from natural sourcesor can be added tohelp produce high biomass outputs. High growth rates and biomassare desired, but the type of biomass grown is also important. Algae,like oilseed plants, produce different metabolic componentsincluding carbohydrates, proteins and biolipids. The biolipids arethe oil producing materials and therefore high biolipid concentra-tions are desirable. Most importantly, the species of algae must becarefully selected and nurtured according to its speci c needs.

The US Department of Energy (DOE) researched renewabletransportation fuels from 1978 to 1996. The Aquatic SpeciesProgram (ASP) was prompted by the 1970s middle-east oil crisis,which caused oil shortages and high prices in the US. The DOEterminated the algae program in 1996 due to budget cutbacks andthe relatively low cost of crude oil at that time (about $20/barrel).Theresearchemphasiswas shifted to bio-ethanol andhydrogen. TheDOE nal research paper on algae titled A Look Back at the UnitedStates Department of Energy s Aquatic Species Program: Biodieselfrom Algae includes the following conclusions [5] :

Algalbiodieselcould easily supply several quads of biodiesel esubstantially more than existing oilseed crops could provide.Microalgae systems use far less water than traditional oilseedcrops. Land is hardly a limitation . it is clear that resource limi-

tations are not an argument against the technology.

1 In studies conducted in California, Hawaii and New Mexico, theASP proved the concept of long term, reliable production of algae . The Roswell, New Mexico tests proved that outdoorponds could be run with extremely high ef ciency of CO 2utilization. Single day productivities reported . were as high as50 grams of algae per square meter per day, a long-term targetfor the program. A major conclusion from these analyses is that there is littleprospect for any alternatives to the open pond designs . Thefactors that most in uence cost are biological, and not

engineering-related . Even with aggressive assumptions aboutbiological productivity, we project costs for biodiesel which aretwo times higher than current petroleum diesel fuel costs.

The DOE conclusions were prepared when diesel fuel sold forabout $1.25 per gallon, including taxes. Using a conservative threetimes this amount as the projected breakeven cost for algal biodieselresults in an estimated cost of $3.75 per gallon . This gure is equiv-alent to $5.50 per gallon in today s dollars underscoring thepossibility that if modest yield improvements can be achieved anddemonstrated, biodiesel from algae could be competitive withinseveral years, if substantial subsidies or tax breaks are provided.The following production cost review is taken from the Departmentof Energy s ASP Final Report [5] .

During the ASP program, several studies on algal growthsystems designs and costs were undertaken. In 1982, Benemannand others developed costs for microalgal biomass production inopen earthen ponds [6] . Several designs were developed includingthe use of CO 2 in waste ue gas from a coal- red power plant andthe direct use of purchased CO 2 delivered on-site. The researchers

concluded that the conservative capital costs for the purchased CO 2design would be $39,850 per hectare (ha) and the operating costswouldbe $18,496 perha. The assumed productivity was 67.5 metrictons (mt) of algal biomass per ha with extractable algal oils of 40%by weight. This resulted in an estimated cost of algal oil output (asfeedstock to a biodiesel conversion plant) of about $2.75 per gallonfora conservativecase. This cost would equate to $6.10 per gallon in2009 dollars. It is important to note that these estimates are forextracted algal oil, which must compete with the cost of crude oilfor conventional diesel as well as other feedstock costs for bio-diesel. The algal oil must be converted to biodiesel in a separatechemical process. Biodiesel conversion, marketing, sales anddistribution, can add $1.00 per gallon or more to the algal oil post-production costs.

In 1987, Weissman and Goebel developed costs for an openpond system assuming productivities of 112 mt/ha/yr and 224 mt/ha/yr [7]. Only the lower productivity value will be reviewed asa conservative baseline, although many researchers are projectinghigher productivities to be achievable in the future. The researchersin this study used a 400-ha open pond system similar to theprevious one discussed. The total capital cost for their design was$72,952 per ha with operating costs (including debt service) of $30,658 per ha. This analysis resulted in a biomass production totalcost of $273/mt, which converts to about $3 per gallon of biodieselfeedstock or $5.75 in 2009 dollars. The estimated costs for thisdesign were about 65% higher than in the 1982 Benemann design,but the assumed productivity was about 65% higher also.

In 1996, Benemann and Oswald produced updated designs and

cost estimates for open ponds [8]. As in an earlier study, two

CO2

nutrientsGROWTHalgae grown

in openponds

HARVESTalgae settled

and physicallyremoved

EXTRACTIONalgal oils

chemicallyextracted

CONVERSIONalgal oil

converted tobiodiesel

return water

recycled nutrients

sunlightMICROALGAE GROWTH POND

BIODIESELCONVERSION PLANT

biodieselout

waterand algae

algalbiomass

algaloil

water

Fig. 1. Simpli ed systems block diagram of the algae to biodiesel process.

1 Quad is a unit of energy of a quadrillion BTUs or approximately 8 billion gals of

biodiesel.

B.J. Gallagher / Renewable Energy 36 (2011) 158 e 162 159


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designs ( ue gas and pure CO 2) were developed and two assump-tions on productivity were used (30 and 60 g/m 2/day). The lowerproductivity will be reviewed, which is equivalent to about 101 mt/ha/yr. 2 The extractable algal oil was assumed to be 40%. This wouldsuggest an assumed biodiesel yield of about 12,000 gallons per ha,which would be considered good. The pure CO 2 design capital costswere reported to be $74,150 per ha and the operating costs werecalculated as $26,270 per ha. These estimates were in 1996 dollarsand yet were comparable to the 1987 Weismann and Goebel studypreviously discussed [7]. If the 1987 costs are normalized to the1996 period (at 3% in ation), the Benemann and Oswald costs areabout 35% less. This is evident in the $69 per bbl of algal oil costconcluded in the study and is the best result so far reported. Inaddition, the ue gas design is 20% less costly ($56 per bbl algal oil)due to a large savings in the cost of CO 2, which was assumed to be$40 per mt at that time. When in ated to 2009 dollars, these algaloil costs would be in the range of $100 per bbl or about the averageprice of crude oil in 2008. A very rough estimate of biodiesel costsper gallon can be obtained by dividing the estimated algal oil costsper bbl by 40 and then adding $1.50 for postproduction costs andtaxes. 3 Using the $100 per bbl cost of the pure CO 2 design with thelower productivity gure results in a biodiesel retail price of about$4.00 per gallon. If one assumes a subsidy of $1 per gallon, theresulting net cost of $3.00 could be competitive in several yearsassuming that crude oil prices escalate signi cantly above thein ation rate. Table 1 summarizes the three system design costs

and productivity assumptions that were reviewed.The costs were then normalized to 2009 dollars (shown in

parentheses) and simple average values calculated. The cost pergallon of biodiesel feedstock varies from $2.41 to $6.09 per gallonwith an average value of $4.75 in today s dollars. Based on thesedata, a conservative reference case scenario was created to analyzefurther the economic viability of this process as shown below:

Productivity: 100 mt/ha/yr or 45 t/ac/yrLipid concentration: 35% by weightBiodiesel yield: 10,421 gallons per ha or 4218 gals/acCapital costs: $112,400 per ha or $45,500 per acOperating costs: $39,300 per ha or $15,900 per ac

The operating costs for this analysis do not include the debtservice cost for the capital outlays in order to use a cash ow, net-present value calculation. This method assumes all capital invest-ments as cash (non-recurring costs) and compares this amountwith discounted cash ow returns (revenues less recurring costs)projected for the future. For the analysis, the cost of land wascomputedseparately and system parameters were slightly adjustedand rounded for simplicity. The productivity and lipid concentra-tion values above are approximately equivalent in output to theaveraged data shown from the prior DOE studies.

3. Converting algal oil to biodiesel fuel using thetransesteri cation process

Biolipid algal oil consists of hydrocarbon compounds (tri-acyglycerides) that are not suitable for most diesel engines. Thecarbon chains are too viscous for good ow and combustion. Thealgal oil can be modi ed for use in diesel engines by a chemicalprocess known as transesteri cation . This process involves add-ing alcohol in a base mixture to convert the triglycerides into threesmaller hydrocarbon chains to make an alternative fuel for dieselengines. Glycerin is a by-product that is removed and sold to reducecosts. The end product of the algal oil conversion using methylalcohol is fatty acid methyl esters with the trade name of Bio-diesel . Biodiesel fuels must meet stringent chemical, physical andquality requirements imposed by the US EPA as speci ed in ASTMstandard D6751. Biodiesel has unique properties, which includealmost no sulfur or particulate matter that contribute to air pollu-tion. Sulfur and particulate matter have been responsible for mostof the black smoke and sour odor problems commonly attributed to dirty diesel fuels. High sulfur concentrations make it dif cult toprovide ef cient catalytic converters for reducing diesel exhaustemissions. Biodiesel also has greater lubricity than petroleumdiesel, which is the ability to reduce friction of moving parts. Inaddition, algal biodiesel is a carbon-neutral fuel, which means itassimilates about as much CO 2 during algal growth as it releasesupon fuel combustion. Based on life-cycle analyses, biodiesel use

can improve air quality; reduce atmospheric CO 2 concentrations;and decrease engine maintenance. Biodiesel fuel is easily blendedwith petroleum diesel to make a premium fuel with improvedperformance.

The major problem is the high cost involved in the overallproduction of biodiesel, which primarily depends on the cost of thefeedstock. According to industry data, a ve-million gallon per yearbiodiesel conversion plant (14,000 gallons perday) would have costapproximately $6,500,000 in 2005 [9] or about $7,300,000 in 2009dollars. The non-recurring capital costs required are for tanks, vats,piping, motors, pumps and various controls. This assumes thata dedicated plant is designed for microalgal oil supplied in batchesfrom an aquatic farm. No special land or power requirements arenecessary beyondthose neededfor the farm. In addition to the algal

oil feedstock, the major operating expenses for this process wouldinclude sales and administration, alcohol, utilities, maintenance,insurance, labor, bene ts and catalysts, which together representalmost 23% of the total production costs [9] . Again, cost of capital isnot included in the cash ow NPV analysis. For a reference case, wewill assume that feedstock costs are high and represent 70% of theoperating costs per gallon and all other expenses discussed aboverepresent 22.5% per gallon along with average depreciation of 7.5%.For biodiesel prices to be competitive with petroleum diesel in2009, the feedstock cost would have to be $2.30 per gallon; theselling and O&Mexpenses about $0.625 per gallon; and the averagedepreciation about $0.075 per gallon for a total of $3.00 wholesale.State (CA) and federal taxes add another $0.51 per gallon at thepump. However, biodiesel is frequently sold at premium prices

above petroleum diesel, especially to

eet customers, due to its

Table 1Summary of algae farm costs and productivity assumptions.

Source Prod.mt/ha/yr

Lipids% conc.

Cap. cost $/hectare Oper. cost $/hectare Algal oil$/bbl

Algal oil$/gal

Benemann et al. (1982) [6] 67.5 40 $39,850 ( $88,519) $18,496 ( $41,085) $115 ( $255) $2.74 ( $6.09)Weismann and Goebel (1987) [7] 112 30 $72,952 ( $139,784) $30,658 ( $58,744) $126 ( $241) $3.00 ( $5.75)Benemann and Oswald (1996) [8] 109.5 40 $74,150 ( $108,889) $26,270 ( $38,577) $69 ( $101) $1.64 ( $2.41)

Average values 96.3 36.7 $62,317 ( $112,397) $25,141 ( $46,135) $103 ( $199) $2.46 ( $4.75)

Note: bold values in parentheses represent costs in 2009 dollars.

2 Dependent on number of production days per year. An average 335 productiondays/yr is assumed for this analysis.

3

A standard barrel of oil contains 42 gallons.

B.J. Gallagher / Renewable Energy 36 (2011) 158 e 162160


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desired attributes as a clean, sustainable fuel that reduces exhaustpollutants, CO 2 emissions, and engine wear and maintenance.

4. Net present value (NPV) analyses

NPV analyses were conducted on several representative casesbased on the production and cost data summarized in Sections 2and 3 . This method computes a present worth value of future dis-counted cash ow returns (including depreciation) minus thecapital investment required. A modi ed accelerated cost recoveryschedule was used (macrs) for depreciation taken over 16 years.The discount factor was set to 10% and a 20-year period of returnswas used. An internal rate of return (IRR) was calculated which isthe discount factor that makes the NPV go to zero. The IRR isa better return on investment (ROI) measure for longer periods.Payback periods based on total discounted cash ows equal to theinvestment were also calculated.

Three different productivities of 67, 100 and 134 mt/ha withrespective lipid concentrations of 30%, 35% and 40% were selected

to represent low, moderate and high algal oil productions. Aninteractive spreadsheet model was developed to perform the NPV calculations and representative examples are included in theAppendices . This model allows various inputs, system costs,subsidies and other economic conditions to be varied withincertain limits. The size of the farm in producing acres was con-strained between 100 and 1000 acres for this analysis, althoughonly the 1000 acre farm analysis is summarized in the appendices.All variable inputs are shown in red on the attached spreadsheets.The three reference cases illustrated represent low, moderate andhigh system yields, subsidies, CO 2 costs and the estimated price of conventional diesel, which is then equated to the lowest price of biodiesel sold. These illustrative scenarios are arbitrary and manyother combinations of factors are possible. Higher lipid concen-trations are more bene cial than higher algal productivities sincethe corresponding increases in costs are considerably less. Table 2summarizes several intermediate cases to gain insight into themost sensitive parameters. There are some factors, such as the costof CO2, that have a strong in uence on pro tability. CO 2 costs could

Table 2Summary of algae to biodiesel NPV analyses.

2014 Scenarios Prodmt/ha

Lipid% conc.

Yieldgal/ac

Bio $/gal a Subs/gal Oil $inc/yr (%)

Oil$/Bbl

CO2/ton Deprec NPV Mil $

IRR % PBYrs

Low algal yield 67 30 2410 $2.28 $2.79 $0.50 3 $74 $44 ACRS (125.2) n/a n/aLow subsidiesLow oil pricesMod CO 2 pricesLow algal yield 67 30 2410 $2.28 $2.79 $1.00 3 $74 $44 ACRS (111.7) n/a n/aMod subsidiesLow oil pricesMod CO 2 pricesLow algal yield 67 30 2410 $2.60 $3.11 $1.00 6 $84 $44 ACRS (86.6) n/a n/aMod subsidiesMod oil pricesMod CO 2 pricesLow algal yield 67 30 2410 $2.60 $3.11 $1.00 6 $84 $33 ACRS (75.3) n/a n/aMod subsidies

Mod oil pricesLow CO2 pricesMod algal yield 101 35 4220 $2.60 $3.11 $1.00 6 $84 $44 ACRS (56.6) n/a n/aMod subsidiesMod oil pricesMod CO 2 pricesMod algal yield 101 35 4220 $2.60 $3.11 $1.50 6 $84 $44 ACRS (44.8) n/a n/aHigh subsidiesMod oil pricesMod CO 2 pricesMod algal yield 101 35 4220 $2.60 $3.11 $1.50 9 $96 $44 ACRS (10.1) n/a n/aHigh subsidiesHigh oil pricesMod CO 2 pricesMod algal yield 101 35 4220 $2.95 $3.46 $1.50 9 $96 $33 ACRS (2.5) n/a 19.9High subsidiesHigh oil pricesLow CO2 pricesHigh algal yield 134 40 6430 $2.95 $3.46 $1.00 9 $96 $44 ACRS 17.4 12.4 16.7Mod subsidiesHigh oil pricesMod CO 2 pricesHigh algal yield 134 40 6430 $2.95 $3.46 $1.50 9 $96 $44 ACRS 35.4 14.9 13.6High subsidiesHigh oil pricesMod CO 2 pricesHigh algal yield 134 40 6430 $2.95 $3.46 $1.50 9 $96 $33 ACRS 46.7 16.5 11.9High subsidiesHigh oil pricesLow CO2 pricesHigh algal yield 134 40 6430 $3.33 $3.84 $1.50 12 $108 $33 ACRS 126.5 23.3 8.2High subsidiesVery high oil $Low CO2 pricesa

Without taxes (1st) and with taxes (2nd).

B.J. Gallagher / Renewable Energy 36 (2011) 158 e 162 161


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decrease signi cantly if carbon taxes are imposed on power plantsand other high carbon emitting industries in the future. However,a continuing rise of the real cost of crude oil has, by far, thestrongest effect on the ultimate pro tability of the algae to bio-diesel production process.

5. Conclusions

It is apparent that an economically viable algae-to-biodieselcommercialization will initially depend on government subsidiesand the future price of oil, in addition to optimized biomass yields.The analysis shows that positive net present valueswith reasonablerates of return are only possible if moderately high yields and crudeoil prices become a reality and substantial subsidies or tax breaksare made available. This assumes that an aggressively accelerateddepreciation allowance is taken similar to an agricultural startupoperation.

The analysis also shows that more attractive returns andpayback periods arepossible if oil prices escalate signi cantly in thefuture. The cost of crude oil would have to exceed $100 per barrel inreal dollars (similar to 2008) to make a high return scenario plau-sible. Many economists feel that this is unlikely to occur in the nearterm period (3 e 5 years) due to present economic conditions andthe low global demand for oil. However, an international group of geologists and petroleum engineers are predicting that global oilproduction rates will soon peak and then begin to decline, resultingin a steady increase in oil prices [10e 15] . Average crude oil prices in2002 were $26 per barrel and increased at an annual rate of 25% to$100 per barrel by 2008 [16] , prior to the global economic down-turn. This increase was due to a combination of tightening suppliesand rising global demand, especially from China and India.

Although excessive oil speculation may have been partlyresponsible for the steep rise (and decline) in crude oil prices, thistype of increase could recur within a few years since global oildemand is unlikely to stay dormant. The resulting rise in crude oiland conventional diesel prices could make production of biodiesel

from algae very attractive if modest gains in algal productivity and/or lipid concentrationscan be reliably demonstratedin the next veto ten years. For example, the last row of Table 2 demonstrates thehuge returns possible if real crude oil prices were to rise signi -cantlyabove$100 per barrel and keeprising at a strong rate. If theseconditions are encountered, the NPV model shows an increasinginsensitivity to loss of subsidies and increases in capital and/oroperating costs. In other words, if Peak Oil and decline become

a reality, biodiesel produced from algae appears extremely attrac-tive as crude oil and conventional diesel prices sharply escalate,even if the assumed capital and operating costs herein prove to beoptimistic.

Appendix. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi: 10.1016/j.renene.2010.06.016 .

References

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[2] US Government Accountability Of ce. Crude oil: uncertainty about future oilsupply makes it important to develop a strategy for addressing a peak anddecline in oil production. US Government Accountability Of ce; 2007.

[3] US Environmental Protection Agency. Alternative fuels: biodiesel. Report#EPA420-F-06-044. Available at: http://www.epa.gov/oms/smartway/growandgo/documents/420f06044.pdf ; 2006 (accessed September 2009).

[4] US Energy Information Administration. US energy outlook, Table A11. Liquidfuels: biodiesel. Report #DOE/EIA-0383(2009). Available at: http://www.eia.doe.gov/oiaf/aeo/pdf/0383(2009).pdf ; 2009 (accessed September 2009).

[5] Benemann JR, Dunahy TG, Roessler PG, Sheehan J, Weisman JC. A look back at

the U.S. department of energy

s aquatic species program e biodiesel fromalgae. Report NREL/TP-580-24190. National Renewable Energy Laboratory, USDepartment of Energy; 1998.

[6] Benemann JR, Augenstein DC, Weissman JC. Summarized in [5], Microalgae asa source of liquid fuels. Appendix: technical feasibility analysis. U.S. Depart-ment of Energy, Final Report, Unpublished; 1982, 126 pp.

[7] Weissman JC, Goebel RP. Design and analysis of pond systems for the purposeof producing fuels. Report SERI/STR-231 e 2840. Golden, Colorado: SolarEnergy Research Institute; 1987. Summarized in [5].

[8] Benemann JR, Oswald WJ. Systems and economic analysis of microalgaeponds for conversion of CO 2 to biomass. Final Report. Pittsburgh EnergyTechnology Center; 1996. Summarized in [5]. Grant No. DE-FG22-93PC93204.

[9] Howell S. Time to take the biodiesel plunge. Render Magazine; February,2005.

[10] Bakhtiari AMS. World oil production capacity model suggests output peak by2006 e 2007. Oil and Gas Journal; 2004. Cited in [2].

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[12] Deffeyes KS. World s oil production peak reckoned in near future. Oil and Gas Journal; 2002. Cited in [2].

[13] Duncan RC. Peak oil production and the road to the Olduvai Gorge. PardeeKeynote Symposia. Geological Society of America; 2000. Summit. Cited in [2].

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