Bioresource Technology 101 (2010) 1406–1413

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Bioresource Technology

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Case Study

Comparative energy life-cycle analyses of microalgal biomass productionin open ponds and photobioreactors

Orlando Jorquera a, Asher Kiperstok a, Emerson A. Sales b, Marcelo Embiruçu c, Maria L. Ghirardi d,*

a Department of Environmental Engineering, Bahia Center for Clean Technologies (TECLIM), Federal University of Bahia, Rua Aristides Novis No. 2, 4� andar,Polytechnique Institute, Salvador 40.210-630, BA, Brazilb Department of Physical Chemistry, Chemistry Institute, Federal University of Bahia, Brazilc Polytechnique Institute, Federal University of Bahia, Salvador 40.210-630, BA, Brazild National Renewable Energy Laboratory (NREL), 1617 Cole Blvd., Golden, CO 80401, USA

a r t i c l e i n f o

Article history:Received 3 August 2009Received in revised form 9 September 2009Accepted 10 September 2009Available online 2 October 2009

Keywords:PhotobioreactorsOil productionMicroalgaeLife-cycle analysisEnergy

0960-8524/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.biortech.2009.09.038

* Corresponding author. Tel.: +1 303 384 6312; faxE-mail address: maria.ghirardi@nrel.gov (M.L. Ghi

a b s t r a c t

An analysis of the energy life-cycle for production of biomass using the oil-rich microalgae Nannochlor-opsis sp. was performed, which included both raceway ponds, tubular and flat-plate photobioreactors foralgal cultivation. The net energy ratio (NER) for each process was calculated. The results showed that theuse of horizontal tubular photobioreactors (PBRs) is not economically feasible ([NER] < 1) and that theestimated NERs for flat-plate PBRs and raceway ponds is >1. The NER for ponds and flat-plate PBRs couldbe raised to significantly higher values if the lipid content of the biomass were increased to 60% dw/cwd.Although neither system is currently competitive with petroleum, the threshold oil cost at which thiswould occur was also estimated.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The continuous increase in atmospheric CO2 coupled to globalwarming and to an increase in energy consumption has becomea major problem worldwide, whose solution must include a searchfor new sources of alternative and sustainable fuels. Microalgalbiomass is one of the most promising sources of energy since itis renewable and neutral with respect to CO2 emissions. Microal-gae use solar energy to convert CO2 into carbohydrates, lipidsand proteins, with higher areal efficiency than land plants.

A comparative analysis of microalgal oil production with thatfrom different oilseed plants indicates that the former can achievea 9–300 times higher oil production per hectare if their oil contentis around 30% dw/cdw, and 21–800 times higher production if 70%of their dry cell weight is comprised of oil (Chisti, 2007). Theeustigmatophycea Nannochloropsis sp. has been reported to beone of the best candidates for mass production of biodiesel (Rodolfiet al., 2009) Recently, Chiu et al. (2009) reported that a semi-con-tinuous cultivation process for the microalgae Nannochloropsis ocu-lata under 2% CO2 yielded 0.48 g/l � d of total biomass, with an oilyield of 0.142 g/l � d. This value corresponds to 29.7% dw/cdw lipidcontent. It has also been reported that Nannochloropsis sp. mayaccumulate up to 60% lipids under nitrogen-limited conditions,

ll rights reserved.

: +1 303 384 6150.rardi).

with the potential for an annual production of 20 tons per hectareof lipids, when cultivated in a Mediterranean climate, and for morethan 30 tons per hectare, when cultivated in tropical areas (Rodolfiet al., 2009).

Different cultivation systems for microalgal biomass productionhave been tested and used in laboratories and in industrial scale(Carvalho et al., 2006; Ugwu et al., 2008). Raceway ponds (seeFig. 1A) are shallow ponds (between 10 and 50 cm deep, to allowappropriate illumination) and they incorporate low-energy-con-suming paddlewheels for gas/liquid mixing and circulation. Theculture medium is directly exposed to the atmosphere, allowing li-quid evaporation and thus helping to regulate the temperature ofthe process. These systems are typically used in commercial scalefor the cultivation of microalgae and cyanobacteria, such as Arthro-spira platensis, Dunaliella salina, Anabaena sp., Phaeodactylum tri-cornotum, Pleurochrysis carterae, Chlorella sp. and Nannochloropsisamong others, (Moheimani and Borowitzka, 2006; Pushparajet al., 1997; Moreno et al., 2003; Jiménez et al., 2003; Garcia-Gonz-alez et al., 2003; Richmond, 1992; Richmond and Cheng-Wu, 2001;Laws et al., 1983; Radmann et al., 2007). Photobioreactors areclosed systems that can be designed in a variety of configurations,e.g. tubular (Fig. 1B) and flat-plate (Fig. 1C) reactors. They havebeen used in the cultivation of Porphyridium cruentum, Phaeodacty-lum tricornotum, Arthrospira platensis, Nannochloropsis sp., Chlorellasarokiniana, Haematococcus pluvialis, Tetraselmis suecica and Chlo-rella vulgaris, among others (Fuentes et al., 1999; Acién et al.,

Fig. 1. Schematic drawing of the three different cultivation systems for massproduction of microalgal biomass addressed in this work: (a) raceway ponds(Molina Grima, 1999); (b) tubular horizontal photobioreactors (Chisti, 2007); (c)flat-plate photobioreactors (Cheng-Wu et al., 2001; Tredici and Rodolfi, 2004, andSierra et al., 2008).

O. Jorquera et al. / Bioresource Technology 101 (2010) 1406–1413 1407

2001; Tredici and Zitelli, 1988; Chini Zittelli et al., 1999, 2006;Ugwu et al., 2002; García-Malea et al., 2006; Cheng-Wu et al.,2001; Degen et al., 2001; Rodolfi et al., 2009). Both ponds and pho-tobioreactor systems are used in commercial scale to obtain bio-mass for the production of protein, pigments, animal feed, fattyacids and anti-oxidants (Huesemann and Benemann, 2009).

Typically, photobioreactors show higher volumetric productiv-ity than open ponds, with better capture of radiant energy, moreoptimal use of the cultivation area and variable energy consump-tion values for mixing and gas/liquid mass transfer (dependingon the type of photobioreactor). Values of 55 W/m3 for flat platesand up to 2000–3000 W/m3 for horizontal tubular reactors havebeen reported (Carvalho et al., 2006; Sierra et al., 2008). Racewayponds are made of less expensive materials, their construction in-

volves lower costs, and they require less energy for mixing, on theorder of only 4 W/m3 (Ami Ben-Amotz, personal communication).However, open ponds are limited as to the type of microalgae thatcan be used for cultivation, the relatively larger area required, thelower efficiency of light utilization, the poor gas/liquid mass trans-fer, the lack of temperature control, the high risk of culture con-tamination, and the low final density of microalgae (Pulz, 2001;Carvalho et al., 2006; Richmond, 2004).

The net energy ratio (NER) of a system is defined as the ratio ofthe total energy produced (energy content of the oil and residualbiomass) over the energy content of photobioreactor constructionand materials, plus the energy required for all plant operations.

NER ¼ Net energy ratio

¼P

Energy producedðlipid or biomassÞP

Energy requirementsð1Þ

An estimate of the NER for microalgal hydrogen production intubular reactors (Burgess and Fernandez-Velasco, 2007) showedthat the costs associated with reactor construction and materialsdominate the energy consumption term. The process, however,had an estimated NER > 1. Huesemann and Benemann (2009), onthe other hand, report that the use of raceway ponds for microalgalbiomass production is the only process with an NER > 1. Rodolfiet al. (2009) found that second-generation flat-panel photobioreac-tors could achieve NER > 1, but that the costs for mixing and har-vesting kept this value just barely above 1.

The objective of this work is to perform comparative analyses ofthe energy life-cycle for production of biomass from the oil-richmicroalgae Nannochloropsis sp., using both raceway ponds, tubularand flat-plate photobioreactors, and to calculate their correspond-ing NERs in order to evaluate their feasibility, thus revisiting Rodol-fi et al.’s 2009 and Huesemann and Benemann’s 2009 analyses.

2. Methods

For this study, a review of the literature data related to the cul-tivation of the microalga Nannochloropsis sp. was done. An average29.6% dw/cdw oil content was assumed, based on Rodolfi et al.(2009), who showed that this alga accumulates between 20% and40% oil per cdw. A production level of 100,000 kilograms of bio-mass dry weight was set as the basis for comparison of all systems(raceway ponds, tubular and flat-plate photobioreactors). The en-ergy consumption term included only the energy required for airpumping, used to maintain appropriate culture mixing and li-quid/gas mass transfer.

For raceway ponds, the values for volume, area, volumetric pro-ductivity and biomass were based on the literature. For tubularphotobioreactors, these data included volumetric productivity, to-tal area and total reactor volume. Finally, for flat-plate photobior-eactors, literature data was used for total biomass produced,areal and volumetric productivity, rate of dilution, illuminatedand occupied area, and reactor volume. Due to the lack of pub-lished information concerning the cultivation of Nannochloropsissp. in horizontal tubular photobioreactors (the only referenceavailable, Rodolfi et al., 2007 is from a conference abstract), the en-ergy consumption for air pumping was assumed to be 2500 W/m3

(Sierra et al., 2008). Finally, the geometrical and operating param-eters for each system in the life-cycle analyses were calculatedusing the following equations:

PA ¼ PVVA

ð2Þ

PV ¼ lX ð3Þ

where PA, areal productivity (kg/m2 d); PV , volumetric productivity(kg/m3 d); V , volume (m3); A, area (m2); l, specific growth rate

1408 O. Jorquera et al. / Bioresource Technology 101 (2010) 1406–1413

(under stationary state, l, D, d1); D, dilution rate (D ¼ FV, (d�1); F,

Flow rate (m3/h); X, biomass (kg/m3).

2.1. Life-cycle analysis using the GaBi program

The GaBi software (http://www.gabi-software.com/) was usedto perform a comparative energy consumption analysis of the dif-ferent algal cultivation processes. GaBi is a tool that allows one toestimate the output of a particular process (in terms of biomassproduced and/or energy generated, in this particular application)following input of energy costs associated with each process,including the price of the raw material, pumps and pipes, andthe cost of transportation. The limits of the system assume thatthe greatest contributions to the energy consumption term are(a) air pumping, used to facilitate mixing, gas/liquid mass transfer,and liquid cooling, and (b) cost of materials and construction of theprocess units (Fig. 2). The output was calculated on a per systemsunit.

The energy required for preparation of the culture medium,inoculation, oil extraction and subsequent production of biodieselor other biofuel was not included on our analyses. The cost of thematerial for the flat-plate photobioreactors was estimated basedon Cheng-Wu et al. (2001), Tredici and Rodolfi (2004), and Sierraet al. (2008). For tubular photobioreactors, the values were basedon Chini Zittelli et al. (1999) and Molina Grima (1999). In orderto estimate the cost for manufacturing a photobioreactor (Fig. 3)it was assumed that each unit would be made of transparentpolyethylene (Acién et al., 2001). The pumping and agitation en-ergy estimates were based on Sierra et al. (2008) and personalcommunications with Professor Ami Ben-Amotz.

The parameters used in the life-cycle analyses for flat-plate photobioreactors and raceway ponds are summarized inTable 1.

Unit of photobioreactors (flat

plate) or Unit of raceway ponds

Gas pumping and liquid mix

Liquid pumping

CO2,Air

Water

Electric energy

Electric energy

Electric energy

Pumping for systems cooling

Fig. 2. Diagram illustrating process inputs and outputs plus the energy consumed by eacthe references to color in this figure legend, the reader is referred to the web version of

3. Results and discussion

3.1. Analysis of energy consumption

Table 2 shows the results of the comparative energy analysis forthe production of Nannochloropsis sp. biomass, using the three dif-ferent cultivation systems: raceway open ponds, flat-plate photo-bioreactors and tubular horizontal photobioreactors. All systemswere compared at the 100,000 kg/year biomass production level,assuming a lipid content of 29.6%, yielding 32.9 m3 or 206.97 bar-rels per year. Higher volumetric and areal productivity were ob-served for both photobioreactor systems when compared toraceway ponds, which is due to their higher ratio of illuminatedarea to cultivation volume when compared to open ponds. The fi-nal biomass concentration was also higher in photobioreactorsthan in ponds. Indeed, in order to generate the same amount ofbiomass, the ponds would have to be more than twice as large inarea as the photobioreactors. The water consumption required tomaintain the rate of dilution (D) at 0.1 d�1 in the pond was morethan 16 times higher than that required by tubular photobioreac-tors, and more than 7 times the amount required by the flat-platephotobioreactors, which makes open ponds less efficient in this re-spect than the other systems.

Pumping is required to generate turbulent flow for optimizedgas/liquid mixing and mass transfer in the three systems. Table 2shows that the energy consumption for pumping in the horizontaltubular-type photobioreactors is too high (2500 W/m3), which ren-ders this system economically unfeasible at present (see projectedNER < 1.0). However, if the pumping energy could be reduced to180 W/m3, it would be possible to raise the NER for oil productionto 1 in these photobioreactors. Alternatively, if the pumping energywere reduced to 495 W/m3, the NER for biomass production couldbe raised to >1. The analysis done in this study does not considercosts for downstream processing of the biomass.

Physico-chemical

separation (centrifugation, extraction by solvent, etc)

Residual biomass (proteins,pigments

, others)

oil

Electric energy

Water

Electric energy

Process of biomass

separation

Biodiesel process

h system. The red line indicates the boundaries of the system. (For interpretation ofthis paper.)

Transport

Transport

Diesel oil energy

Diesel oil energy

Raw material (plastic, glass, PET) depending on

the type of photobioreactors (flat

plate, tubular,etcl), PVC cover pond

Other implements (pumps, pipes valves, seals, wires, etc.).

Fig. 3. Diagram illustrating process inputs and outputs required for manufacturing each unit of the raceway ponds or photobioreactors.

Table 1Parameters used in the energy life-cycle analysis of microalgal cultivation using flat-plate photobioreactors and raceway ponds.

Parameter Value flat-plate photobioreactors Value raceway ponds Unit

Energy consumed for air pumping 53a 3.72b W/m3

Energy consumed for water pumping (cooling) 3.26 � 10�5c Not cooled MJ/kgWater consumed for cooling 2d 0 m3

Flow rate of sea water needed to maintain the specified dilution rate 37,037e 285,714e m3/yearCaloric content equipment (lifetime = 10 years) 4.73f 1.78g MJ/kg

a Sierra et al. (2008).b Personal communication, A. Ben-Amotz.c Estimated by the authors.d Estimated by the authors.e Calculated using the equations described in Section 2.f GaBi data base (polyethylene-low density compound (PE-LD) for photobioreactors).g GaBi data base (plastic cover).

O. Jorquera et al. / Bioresource Technology 101 (2010) 1406–1413 1409

Based on Chisti’s data (2008) and assuming a lipid content of29.6% (dw lipid/dw biomass), the algal biomass has to be producedat a cost of US $ 152.00 per ton in order to be competitive withpetroleum at US $ 60.00 per barrel. Using a value of US $ 0.22per kWh for the cost of pumping, the production of 1 tons of bio-mass using horizontal tubular reactors was estimated to be US $9540.00. The use of flat-plate reactors for biomass cultivationyielded a cost of US $ 419.00, and the production cost using race-way ponds was US $ 227.00. These estimates indicate that neithersystem would be currently competitive with petroleum, if only thecost of pumping is considered. A more detailed analysis wouldhave to incorporate many other cost-inputs. The cost of the biodie-sel produced in flat-plate bioreactors and raceway ponds will becompetitive if the cost of petroleum rises to, respectively, US $165.00 and US $ 89.00 per barrel. Moreover, if the lipid contentof biomass could reach 60%, as is reported to occur upon nitrogendeficiency (Rodolfi et al., 2009), the total oil production could reach65 m3/ha year in a flat-plate bioreactor and 25.7 m3/ha year in araceway pond, with NERs of, respectively, 6.21 and 11.47. The price

of a ton of biomass would then be competitive with oil at US $117.00 and US $ 63.00 per barrel of petroleum, respectively. Allof these systems are based on continuous culture of the algae.

The results presented here are different from other work foundin the literature, in which the NER for flat-plate bioreactors wasestimated to be <1 (Huesemann and Benemann, 2009), and slightlymore optimistic than those of Rodolfi et al. (2009), where the NERfor flat-panel photobioreactors was barely above 1.

3.2. Life-cycle analysis using the GaBi energy program

Based on the data in Table 2, the following geometric configura-tions were chosen for subsequent life-cycle analyses (as their NREenergy ratio was very unfavorable, the horizontal tubular photobi-oreactors were not further considered in this study):

(a) Flat-plate photobioreactors: comprised of 112 units consist-ing of 20 flat plates, each 4.51 m in length, 1 m in heightand 10 cm in thickness for appropriate light absorption.

Table 2Comparative analysis of microalgal biomass and bio-oil production using three different cultivation systems: raceway ponds, tubular photobioreactors and flat-platephotobioreactors. The only energy cost included in the calculation of the net energy ration (NET) is for air pumping, which is required for gas/liquid mixing and mass transfer inthe photobioreactors.

Variable Raceway ponds Flat-platephotobioreactors

Tubularphotobioreactors

Annual biomass production (kg/year) 100,000 100,000 100,000Volumetric productivity (g/l � d) or (kg/m3 � d) 0.035 0.27 0.56 1Illuminated areal productivity (kg/m2 � d) 0.011 0.0142 0.0081 2Occupied areal productivity (kg/m2 � d) 0.011 0.027 0.025 3Occupied areal productivity (t/ha x year) 38.5 98.6 92.9 4Illuminated areal volume (m�2) 301 50 14.46 5Illuminated Area/volume ratio (m�1) 3.32 19.01 69.15 6Occupied Area/volume ratio (m�1) 2.3 10 22 7Biomass concentration (g/l) or (kg/m3) 0.35 2.7 1.02 8Dilution rate, D (d�1) 0.1 0.1 0.1 9Space required for a biomass annual production of 100,000 kg/year (m2) 25,988.25 10,147.00a 10,763.20 10Reactor volume required to support a biomass annual production

of 100,000 kg/year (m3)7827.79 1014.71 489.24 11

Flow rate required to maintain a 0.1 d�1 dilution rate (m3/d) 782.79 101.47 48.9 12Hydraulic retention time (volume/ flow rate) 10 10 10 13Relative oil content (%) 29.6 29.6 29.6 14Net oil yield (m3/year) 32.9 32.9 32.9 15Oil yield per area (m3/ha x year) 12.65 31.6 30.56 16Energy consumption (W/m3) 3.72 53 2500 17Energy consumption required for accumulation of 100,000 kg/year biomass (W) 29,119.37 53,779.80 1,223,091.98 18Total energy consumption (KWh/months)b 8735.81 16,133.94 366,927.6 19Total energy consumption (GJ/year) 378.45 698.94 15,895.8 20Energy produced as oil (GJ/year)c 1155.49 1155.49 1155.49 21Total energy content in 100,000.00 kg biomass (GJ/year) 3155.30 3155.30 3155.30 22NER for oil production 3.05 1.65 0.07 23NER for biomass production 8.34 4.51 0.20 24

1: Richmond and Cheng-Wu (2001), Cheng-Wu et al. (2001), Chini Zittelli et al. (1999), respectively.2: Determined from Eq. (2) (see Section 2).3 and 4: Determined from Eq. (2) (see Section 2).5: Determined by dividing the actual production the volume of each unit by illuminated area.6: Determined by dividing the illuminated area by the production volume of each unit.7: Determined by dividing the occupied area by the volume of each unit.8: Richmond and Cheng-Wu (2001), Cheng-Wu et al. (2001), Chini Zittelli et al. (1999), respectively.9: Determined from Eq. (2); Cheng-Wu et al. (2001), determined from Eq. (3).10: Determined by dividing the annual biomass production by the annual areal productivity.11: Determined by dividing the annual biomass production by the annual volumetric productivity.12: Determined by multiplying the dilution rate by the reactor volume.13: Determined by dividing the reactor volume by the flow rate.14: Rodolfi et al. (2009).15: Determined by dividing the product of the annual biomass and relative oil content by the density of oil (assumed to be 0.9 kg/l).16: Determined by dividing the net oil yield by the space required, in hectares.17: Personal communication A. Ben-Amotz; Sierra et al. (2008), respectively.18: Determined by multiplying the energy consumption by the reactor volume required to support the assumed annual biomass production.19 and 20: Determined by multiplying the energy consumption by the number of hours of daily pumping.21: Energy content of net oil yield (assumed value of 35,133.33 kJ/L).22: Assumed energy content of the protein, carbohydrate and lipid components in the annual biomass produced.23: Determined from Eq. (1), considering only oil production.24: Determined from Eq. (1), considering total biomass production.

a Includes a 1-m spacing between plates to prevent shading.b Includes 10 h of daily pumping.c Assumes an average oil content of 29.6% in Nannochloropsis sp.

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The plates are located 1 m apart from each other, occupyinga total area of 10,147.00 m2. The system, which is similar tothat described in by Tredici and Rodolfi (2004), consists of ametal frame covered with 0.3-mm thick low density poly-ethylene film. The cooling system is made of stainless steeltubing, with three tubes per unit, using flowing cold wateras the coolant. The life of the system was estimated at10 years.

(b) Raceway ponds: comprised of 22.7 units of open ponds, each100 m in length by 10 m wide and 35 cm in depth, occupy-ing a total area of 22,700.00 m2. The material for the con-crete cavity is covered with 2-mm thick polyvinylchlorideplastic (PVC). The process does not include any temperaturecontrol, except what occurs through evaporation. The life ofthe system was estimated at 10 years.

Figs. 4 and 5 represent the energy fluxogram for production ofmicroalgal biomass using flat-plate photobioreactors and racewayponds, respectively. The assumptions used for the calculations areshown in Table 1. In both cases, the estimated NREs were >1 (4.33to for flat-plates and 7.01 for open ponds), indicating favorable en-ergy processes (Table 3). The higher energy consumption in bothcases is related mostly to pumping and not so much to materialscosts, since the latter are offset by the long lifetime of the systems.The process is more favorable using raceway ponds, which is inagreement with Huesemann and Benemann (2009). However, thecurrent disadvantage of raceway ponds is that they need largeareas and this becomes a problem is they are built in areas cur-rently dedicated to food crops. Raceway ponds, as expected, alsohave lower materials and construction costs. Photobioreactors, onthe other hand, are easier to control and yield larger amounts of

Fig. 4. Flowchart for the biomass production process using flat-plate photobioreactors, generated by the GaBi program. The arrows indicate the flow of energy in the system.The thickness of the arrows represents the amount of energy required or produced by the system. The abbreviations are: US, United State; GLO, global; PE-LD, polyethylene-low density; PE, PE International software (http://www.pe-international.com/); RER, conglomerate of European plastic manufacturers.

O. Jorquera et al. / Bioresource Technology 101 (2010) 1406–1413 1411

biomass than raceway ponds, but they are associated with higherconstruction and operating costs.

In comparison with Burgess and Fernandez-Velasco (2007), thiswork estimates higher energy costs for pumping, compared toreactor material costs. This is due to the fact that Burgess esti-mated pumping needs by using standard expressions for pressuredrop due to turbulent flow in a pipe, in contrast to this work, wherethe calculation was based on experimental data described in liter-ature (Sierra et al., 2008).

It is clear that the use of photobioreactors for biomass produc-tion becomes unfavorable when additional energy costs such asthose required for concentrating the microalgae from the culturemedium and extracting oil are considered. Microalgae such as Nan-nochloropsis sp., are 2–5 lm in size, which adds additional chal-lenges to their harvesting. A new, solvent extraction processes

that does not cause cellular destruction (‘‘milking” the algae foroil) appears promising (R. Sayre, personal communication).

An alternative approach to optimize the economics of the pro-cess, based on multiple-stage, coupled systems has been proposedby Rodolfi et al. (2009). The production of biomass would occur un-der optimal nutrient conditions in closed systems, such as flat-plate photobioreactors. The biomass would then be moved to openraceway ponds in which nitrogen limitation would be imposed forinduction of lipid accumulation. The closed system (photobioreac-tors) would allow greater control and maintenance of the culturefor long periods. The open system (raceway ponds) would allowthe induction of lipid production and its accumulation in shorterperiods of time, without increasing the production costs. The re-sults from this work support this idea and demonstrate its poten-tial feasibility in terms of estimated NERs.

Fig. 5. Flowchart for the biomass production process using raceway ponds, generated by GaBi program. The arrows indicate the flow of energy in the system. The thickness ofthe arrows represents the amount of energy required or produced by the system. Abbreviations as in Fig. 4.

Table 3Analysis of the net energy ratio (NER) for microalgal cultivation in flat-plate photobioreactors and in raceway ponds, using the GaBi program.

Parameter analyzed by the GaBi program Flat-plate photobioreactors Raceway ponds

Total energy consumption (GJ/year) 729 450 1Total energy content in 100,000 kg biomass (GJ/year) 3155 3155 2Total energy content of oil produced (MJ/year) 1155 1155 3NER for oil production 1.58 2.56 4NER for total biomass 4.33 7.01 5

1: Determined from the sum all inputs (see Fig. 4).2: Assumed energy content of the protein, carbohydrate and lipid components in the annual biomass produced.3: Energy content of net oil yield (assumed 35,133.33 kJ/L).4: Determined by Eq. (1), considering only oil production.5: Determined by equation1, considering total biomass production.

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4. Conclusions

Both flat-plate photobioreactors and raceway pond showedNER > 1 in this study, and are thus considered economically feasiblefor mass cultivation of Nannochloropsis for the purpose of biofuelgeneration. However, this study did not consider the costs requiredfor microalgal harvesting and oil extraction, which could signifi-cantly add to the energy consumption parameter. Alternative non-destructive approaches such as ‘‘milking” the algae for oil and multi-

ple-stage, coupled systems may circumvent these additional costs.As their NRE energy ratio was very unfavorable, the horizontal tubu-lar photobioreactors were not further considered in this study.

Acknowledgements

We would like to acknowledge the Research Support Founda-tion for the State of Bahia (FAPESB) (OJ). We are also grateful toProf. Ben-Amotz for useful information.

O. Jorquera et al. / Bioresource Technology 101 (2010) 1406–1413 1413

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