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Energy-efficient photobioreactor configuration for algal biomass production Ambica Koushik Pegallapati, Yalini Arudchelvam, Nagamany Nirmalakhandan ⇑ Civil Engineering Department, New Mexico State University, MSC 3CE, Las Cruces, NM 88003, USA
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Developed an internally illuminated
photobioreactor (IIPBR) for algal cultivation. " Theoretical reactor analysis of IIPBR showed improvement over bubble columns. " Experimental results on two algal strains validated theoretical advantages of IIPBR. " Biomass production per energy input of IIPBR is twice that of current PBRs.
a r t i c l e
i n f o
Article history: Received 11 July 2012 Received in revised form 19 August 2012 Accepted 22 August 2012 Available online 5 September 2012 Keywords: Bioreactors Bubble columns Energy Internal illumination Biomass production
a b s t r a c t An internally illuminated photobioreactor (IIPBR) design is proposed for energy-efficient biomass production. Theoretical rationale of the IIPBR design and its advantages over the traditional bubble column photobioreactors (PBRs) are presented, followed by experimental results from prototype scale cultivation of freshwater and marine algal strains in an 18 L IIPBR. Based on theoretical considerations, the proposed IIPBR design has the potential to support 160% higher biomass density and higher biomass productivity per unit energy input, B/E, than a bubble column PBR of equal incident area per unit culture volume. Experimental B/E values recorded in this study with fresh water algae and marine algae (1.42 and 0.37 g W1 d1, respectively) are at least twice as those reported in the literature for comparable species cultivated in bubble column and airlift PBRs. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In the past, photobioreactors, such as bubble columns and airlift reactors, have been utilized to cultivate algae as a feedstock to manufacture high-value products in the food, pharmaceutical, and chemical industries (Chisti, 2007; Mata et al., 2010). Recently, algae have been identified as one of the preferred feedstocks to produce fuels because of their fast growth rate, carbon-neutrality, and sustainability. Nevertheless, for algal fuels to be cost-effective against fossil fuels, traditional algal cultivation systems used in the
⇑ Corresponding author. Tel.: +1 575 646 5378; fax: +1 575 646 6049. E-mail address:
[email protected] (N. Nirmalakhandan). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.08.090
production of low-volume, high-value products have to be reevaluated and reengineered to produce fuels in large volumes, in an energy-efficient and economic manner (Wijffels and Barbosa, 2010). For instance, the current practice of evaluating PBRs on the basis of biomass productivity per unit culture volume without considering the energy input to the process may not result in an energyefficient and sustainable solution. In assessing PBRs for producing algal biomass as a feedstock for fuel production, it would be preferable to maximize net energy gain rather than maximizing biomass productivity. The first approach to maximizing net energy gain is to minimize the energy input to the process. Energy input to indoor bubble column and airlift PBRs includes the light energy input for illuminating the cultures and the sparging energy input for providing the
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Nomenclature a b B/E B/V Cf Ci D d Ei/V Es/V G h Io Iave k
empirical constant, unitless empirical constant, unitless biomass productivity per unit energy input, g W1 d1 biomass productivity per unit culture volume, g L1 d1 final biomass concentration, g L1 initial biomass concentration, g L1 diameter of the external column in IIPBR, m diameter of the internal column in IIPBR, m illumination energy per unit culture volume, W m3 sparging energy per unit culture volume, W m3 gas flow rate, m3 min1 culture depth, m incident illumination intensity, l Einsteins m2 s1 average light available to the cultures, l Einsteins m2 s1 constant, unitless
CO2 needs, and mixing the cultures and keeping them suspended. Since various combinations of culture depth, culture volume, incident area, sparging rate, biomass density, and light supply can result in different energy inputs, biomass productivities, and energy outputs, biomass production per unit energy input is proposed here as a comprehensive metric to compare the performance of different PBRs designed for biofuel production. Another metric could be the energy equivalent of the produced biomass per unit energy input. Hulatt and Thomas (2011), for example, have proposed the lower heating value of biomass as the energy equivalent of the produced biomass. When evaluating algal PBRs for liquid biofuel production, however, a more appropriate metric could be the energy equivalent of the lipid content of the biomass produced per unit energy input. The second approach to maximize net energy gain is to maximize the lipid content of the biomass. Lipid content of the biomass is a function of the growth phase and the cultivation conditions. The most common approach for maximizing lipid content of the algal biomass is stressing the cultures (Chiu et al., 2009) by nitrogen depletion (Li et al., 2008). Other approaches include phosphate limitation (Reitan et al., 1994), high salinity (Rao et al., 2007), high iron concentration (Liu et al., 2008), or light control. Previous studies (Li et al., 2008) have pointed out that, though, stressing increases the lipid content, it results in low biomass productivity and hence low lipid productivity. To circumvent this dilemma, a two-stage cultivation strategy has been suggested (Ho et al., 2010). In this strategy, optimal operating conditions and nutrient rich medium are provided in the first stage to promote the growth of the biomass; outflow from the first stage are then fed to a second stage, where appropriate stressing conditions are imposed to promote lipid accumulation.
KI Kx Kw S t V X z
half saturation constant of light, l Einsteins m2 s1 light extinction coefficient due to biomass, cm2 mg1 biomass light extinction coefficient due to water, cm1 incident surface area, m2 time, d culture volume, m3 algal biomass concentration, g L1 light path length, m
Greek symbols l specific growth rate of algal cultures, d1 lmax maximum specific growth rate of algal cultures, d1 c specific weight of the algal culture, N m3
Even though the internally illuminated design could be adapted for outdoor use under sunlight, this proof-of-concept study was conducted indoors, under artificial lights. Based on the projected high energy-efficiency of the IIPBR design, artificially lit IIPBRs may find application in the following areas: (1) as a mother reactor for providing seed to mass cultivation systems such as open raceway reactors, where it is essential to maintain a high-output, energy-efficient reactor to feed high quality cultures for scheduled re-seeding of the raceways (Belay, 1997; Kawaguchi, 1980) or to revive contaminated reactors; (2) as a hybrid reactor integrating artificial light and solar light for round-the-clock cultivation (Matsunaga et al., 1991; Mori, 1985; Ogbonna et al., 1999); and (3) as a production reactor for high-value end products with optimal supply of CO2, nutrients, and mixing, keeping the light input as the only growth-limiting factor. The proposed IIPBR design consists of two concentric columns, where the cultures are grown in the annular space between the two columns under radial illumination provided by the lights housed inside the inner column. The circular cross-section was chosen here because it has been shown to have better light penetration and higher share of illuminated zone in the reactor (Fernandes et al., 2010). The theoretical rationale behind the IIPBR configuration is presented first, followed by experimental results with two model algal strains – fresh water algae and marine algae, cultivated in an 18 L prototype version of this design to validate the theoretical rationale. Then, the performance of this IIPBR is compared with those of traditional PBRs reported in the literature to demonstrate its energetic advantage. Since the literature reports covered a variety of algal species with differing growth characteristics, results from a parallel bubble column test conducted as part of this study with one of the selected test species – Nannochloropsis salina, were used to make a more direct comparison.
1.1. Goals of this study 2. Theoretical background This study pursued the first approach towards improving net energy gain, wherein, an internally illuminated photobioreactor (IIPBR) design is proposed to maximize biomass productivity per unit energy input. The goals of this paper are to: (i) present the theoretical rationale behind the internally illuminated design and its advantage over a traditional bubble column PBR; (ii) conduct a proof-of-concept study of the IIPBR design on two algal species to validate its theoretical advantages; and (iii) to compare the performance of the IIPBR design with those of other PBR designs reported in the literature.
The proposed IIPBR design can be best rationalized by comparing it with a traditional bubble column photobioreactor (BCPBR) of the same external column diameter D (m); culture depth, h (m); incident light input, Io (l Einsteins m2 s1); and illuminated surface area per culture volume, S/V (m1). The diameter of the internal column of the IIPBR is d (m) (=kD (m, where, k < 1). Since the lights are external in the case of the BCPBR, its footprint (>pD2/4) will obviously be larger than that of the IIPBR (=pD2/4). Additional advantages of the IIPBR over the BCPBR in terms of light path
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length, maximum biomass density that they can support, and biomass productivity per unit energy input are derived in the following sections.
The illuminated surface area per unit culture volume S/V in the case of the BCPBR can be expressed as
S pDh 4 ¼ 2 ¼ pD h V BCPBR D
ð1Þ
(a) PAR (µ Einsteins/sq m-s) as a function of X Average PAR in broth
2.1. Shorter light path length of IIPBR
100
and, in the case of the IIPBR can be expressed as
S pkDh 4k ¼ 2 ¼ 2 pD pk2 D2 h V IIPBR Dð1 k Þ 4
ð2Þ
4
If the illuminated surface area per unit culture volume is to be the same for the two designs, an appropriate value for k can be found by equating the above two expressions to one another, and solving for k, yielding k = 0.62. With this value of k, the light path length, z (m), for the IIPBR can be found as
Z¼
1 1 ðD kDÞ ¼ ðD 0:62DÞ ¼ 0:19D 2 2
ð3Þ
which is 62% shorter than that of the BCPBR under comparison (=0.5 D).
Productivity/energy input, B/E [g/W-d]
4
80 60
IIPBR 40
BCPBR
20 12.00 0 10.0
1
Iave ¼
Io ½1 expð100ðK x X þ K w ÞzÞ 100ðK x X þ K w Þz
ð4Þ 2
6
IIPBR 1.0
BCPBR 0.1 0
1
2 3 4 Biomass density, X [g/L]
5
6
Fig. 1. BCPBR vs IIPBR, both of the same light input and incident area per unit volume. (a) Simulated photosynthetic active radiation (PAR) as a function of biomass density (X); Kx = 0.16 cm2 mg1; Kw = 0.0018 cm1. (b) Simulated biomass productivity per unit energy input (B/E) as a function of biomass density (X); lmax = 0.2 d1; KI = 0.4 l Einstein m2 s1.
Es Gch ¼ 60V V
ð6Þ
1
where, I0 is the incident light level (l Einsteins m s ), Kx is the light extinction coefficient due to biomass (cm2 mg1 biomass); and Kw is the light extinction coefficient of water (cm1). Feeding the minimum light level required to sustain the growth of a given species in Eq. (4), the maximum biomass density of that species that can be supported by the two PBR designs can be estimated by substituting for z, the respective light path length values (z = 0.5 D for BCPBR; and z = 0.19 D for IIPBR). The average light levels in the two designs as a function of biomass density given by Eq. (4) are illustrated in Fig. 1(a) for typical values of Kx and Kw. It can be deduced from Fig. 1(a) that the IIPBR can support a higher biomass density than a BCPBR for a desired light level. 2.3. Higher biomass production per unit energy input in IIPBR The minimum sparging energy required per unit culture volume, Es/V (W m3), to keep a biomass density of X well mixed can be estimated from the empirical relationship
Es ¼ aX þ b V
5
(b) B/E (g/W-d) as a function of X
2.2. Higher biomass density in IIPBR The average light level within the broth, Iave (l Einsteins m2 s1), in the two designs as a function of biomass density, X (g L1), can be described by (Sciandra, 1985)
Biomass density, X [g/L] 2 3 4
ð5Þ
where, a and b are empirical constants (Rich, 1982). Even though this equation has not been explicitly validated for algal species, it was selected in this study as no other relationships could be found in the literature. However, in a study by Arudchelvam and Nirmalakhandan (2012), several conclusions derived from this equation have been found to agree well with the experimental results obtained on the same algal species as that being tested here. Alternatively, in the case of gas-sparged columns, the energy input to the reactor can be related to the gas sparging rate, G (m3 min1), and the culture depth, h (m), as
where, c is the specific weight of the broth (N m3). The volumetric biomass production rate, B/V (g L1 d1), can be related to the specific growth rate l (d1) and the biomass density:
B ¼ lX V
ð7Þ
Hence, the biomass production per unit sparging energy input, B/Es, (g W1 d1) can be found from Eqs. (5) and (7):
B B=V 1000lX ¼ ¼ Es Es =V aX þ b
ð8Þ
Considering the energy for illumination per unit culture volume, Ei/V (W m3) (= 0.217 I0S/V) and that for sparging, Es/V (= aX + b) as the total energy input, biomass production per unit total energy input, B/E in terms of biomass concentration can now be estimated as
B B=V 1000lX ¼ ¼ E ðEi =V þ Es =VÞ ð0:217Io S=V þ ðaX þ bÞÞ
ð9Þ
When X is expressed in g/L and Es/V in W/m3, a = 4 and b = 5 (Rich, 1982). Alternately, the biomass production per unit total energy input, B/E in terms of the gas-sparging rate can be estimated as
B B=V 1000lX ¼ ¼ E ðEi =V þ Es =VÞ ð0:217Io S=V þ ðGch=VÞÞ
ð10Þ
Assuming that all growth requirements other than light are available in excess, and that the growth is not inhibited, the specific growth rate, l can be expressed as a function of the average available light, Iave, as follows:
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l ¼ lmax
Iave K I þ Iave
269
ð11Þ
Combining Eqs. (9) and (11),
ave 1000lmax K IIþI X B ave ¼ E ð0:217Io S=V þ ðaX þ bÞÞ
ð12Þ
The energetic efficiencies of the BCPBR and IIPBR configurations in producing biomass can now be compared in terms of B/E using Eqs. (4) and (12). Fig. 1(b), developed from Eq. (12), shows the variation of B/E as a function of biomass density in the two designs. Since the light input and the incident area per unit volume are the same for the two reactors, the B/E plots overlap in the two cases, but cover different ranges depending on X. Fig. 1(a) and (b) together illustrate the relationship between average light intensity, biomass density, and biomass productivity per unit energy input. For example, for a given surface photosynthetic active radiation (PAR) of 100, and assuming the minimum light level required for biomass growth to be PAR of 50, the BCPBR can support a biomass density of 1.0 g/L while the IIPBR can support 160% more biomass density of 2.6 g/L as shown in Fig. 1(a), if all the other nutrient requirements are satisfied. The biomass productivities per unit energy input can be read from Fig. 1(b) as 0.7 g W1 d1 for the BCPPBR and 1.8 g W1 d1 for the IIPBR. In summary, for a given incident illumination area per unit culture volume, the IIPBR design can be seen to have a smaller footprint and smaller light path length; it can support a higher biomass density for a desired average illumination (1.0 vs. 2.65 g L1 at 50 PAR) to yield higher biomass productivity per unit energy input (0.7 vs. 1.8 g W1 d1). While the above advantages are based on simplified assumptions and ideal conditions, they provided the impetus to experimentally validate the premise of the IIPBR design. 3. Experimental method An 18 L prototype version of the IIPBR design was fabricated and tested under laboratory conditions to validate the theoretical rationale and evaluate its energy-efficiency. Fig. 2 shows the schematic of the prototype IIPBR, details of which have been presented elsewhere (Pegallapati and Nirmalakhandan, 2011). 3.1. Algal strains and growth media Growth characteristics of two algal species – Scenedesmus sp. a representative fresh water algae, and N. salina, a representative marine algae, were evaluated in this study. The media composition and preparation methods adopted for the two cultures have been detailed elsewhere (Pegallapati and Nirmalakhandan, 2011). 3.2. Experimental scheme The two algal strains were cultivated in the IIPBR sparged with CO2-enriched air (CEA) at various CO2-air ratios to determine optimal ratio for maximum growth. The CO2-air mixture was sparged via four porous silica air diffusers placed at the bottom of the annular space of the IIPBR. In all the tests, the culture depth in the reactor was kept constant at 81 cm and the working volume, at 18 L. All tests were conducted under laboratory conditions, where the temperature ranged between 26–27 °C. Growth of Scenedesmus sp. was evaluated under sparging with ambient air and with CEA at CO2-air ratios of 1–5% (v/v). Tests with ambient air and with CO2-air ratio of 1% CEA were done in batch mode, while the tests with CO2-air ratios of 2–5% were done in
Fig. 2. Schematic of internally illuminated photobioreactor. Arrow A indicates the orientation of the PAR meter.
semi-continuous mode. Growth of N. salina was evaluated under sparging with ambient air in batch mode, and with CEA at CO2-air ratios of 0.5, 1, and 2% in semi-continuous mode. All these tests were conducted with the same light source of 30 W (florescent day light tube lights) and gas-to-culture volume (G/V) ratio of 0.044 min1. Under semi-continuous cultivation of both the strains, 10% of the culture volume was harvested and the harvested volume was replaced with fresh medium. The fact that the literature reports on other PBR designs had utilized a variety of algal species rendered the energetic comparison among different designs debatable. As such, additional tests were conducted with one of the test species used in the above IIPBR experiments- N. salina, in an externally illuminated bubble column to generate our own data for direct comparison with the IIPBR. These tests were run in 900 mL bubble columns, in triplicate, with the same culture conditions and analytical procedures and at CO2-air ratio of 0.5%, over a range of gas-to-culture volume ratios from 0.02 to 1.0 min1 to determine the optimal G/V for maximizing biomass production. 3.3. Methods Algal growth was monitored in terms of optical density at 750 nm, which was then converted to biomass density based on dry weight (g L1), based on a correlation developed between optical density at 750 nm and dry biomass concentration. Dry weight
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of biomass was determined by centrifuging followed by oven drying. Incident light intensity was measured (at A as shown in the Fig. 2) at eight equally spaced radial locations and at four points along the height of the IIPBR and averaged (Pegallapati and Nirmalakhandan, 2011). 3.4. Algal productivity Productivity of algal biomass B (g DW L1 d1) was estimated using the following equation under batch and semi-continuous modes (Goksan et al., 2003; Ryu et al., 2009):
B¼
ðC f C i Þ t
ð13Þ
where, under batch operation, Cf is final biomass concentration, (g L1), Ci is initial dry biomass concentration (g L1), and t is duration of the run (day); and under semi-continuous operation, Ci is the dry biomass concentration after harvesting and replenishment with media; Cf is the dry biomass concentration prior to harvesting after time interval (t); and t is the time interval between harvests. 4. Results and discussion The maximum biomass productivities recorded in the IIPBR with the two test species at the corresponding CO2-air ratios are summarized in Table 1. These maximum productivities were used in evaluating the B/E of the IIPBR. The incident light measured at point A (Fig. 2) averaged 91 l Einsteins m2 s1. Data reported in the literature for various photobioreactor designs were compiled to estimate their energetic efficiencies in terms of B/E as shown in Table 2. Recognizing that this database covers various types of PBRs tested with diverse algal species of different growth characteristics, the comparison on the basis of B/E is provided here has to be considered a token of their energetic efficiencies. However, a more convincing comparison between a traditional externally illuminated bubble column (BCPBR) and the IIPBR is done utilizing the data from the bubble column tests conducted in this study using the same test species. The maximum productivity recorded in the tests on the BCPBR run at the CO2-air ratio of 0.5% and over a range of G/V ratios of 0.02–1.0 min1 was used in calculating its B/E, which is included in the last row of Table 2. 4.1. Effect of sparging energy input The energy expended for sparging per unit culture volume depends on the reactor height and the G/V ratio according to Eq. (6). Even though sparging energy is approximately 10–100 times lower than the light energy input in the artificially lit PBRs listed in Table 2, mixing is critical to maintain optimal productivity and higher concentrations. For example, in the bubble column tests conducted as part of this study with N. salina, when the G/V ratio was increased from 0.02 to 1.0 min1, biomass productivity increased to a certain point and remained constant thereafter while,
Table 1 Summary of growth results in IIPBR for Scenedesmus sp. and N. salina. Test strain
G/V ratio (min1) Optimum CO2-air ratio (%) Biomass productivity (g L1 d1)a Biomass density (g L1)a Biomass prod./energy input (g W1 d1) a
Scenedesmus sp.
N. salina
0.044 4 0.401 (±0.04) 1.40 (±0.04) 1.42
0.044 1 0.104 (±0.007) 0.52 (±0.01) 0.37
Averaged during harvesting cycles with standard deviations in brackets.
the sparging energy continued to increase directly with G/V according to Eq. (6). While the light input was the same in all these tests, the B/E reached a maximum value of 0.14 g W1 d1 at G/V of 0.18 min1 with no apparent increase in B/E thereafter, due to no further increase in productivity with G/V (Fig. 3). Estimates from the results reported by Ryu et al. (2009) for a bubble column PBR also showed that B/E increased with increase of G/V, from 0.28 to 0.40 g W1 d1 (Table 2) up to G/V of 0.2 min1, with no significant increase in B/E thereafter. Even though the energy input for sparging is low compared to light energy input in the case of artificially lit PBRs, it becomes critical in the case of outdoor PBRs used in large-scale cultivation (Sierra et al., 2008). The energetic approach proposed here could be readily extended to such outdoor PBRs as well, to assess their performance on the basis of B/E rather than on B alone. 4.2. Effect of light energy input Of the two forms of energy input to artificially lit PBRs, light energy is the dominant form that can significantly impact their productivity and energetic efficiency. Comparing the PBRs listed in Table 2 on the basis of light energy input per unit culture volume, Ei/V, the three designs with the lowest light input are the flat panel airlift (FPA) PBR (= 127.1 W m3), the IIPBR (= 276.4 W m3), and the transparent rectangular chamber (TRC) PBR (= 399.1 W m3). However, on comparing these three PBRs on the basis of volumetric productivity, B/V, the IIPBR ranked highest with a productivity of 0.401 g L1 d1 of Scenedesmus sp. (a fresh water algae) followed by the TRC PBR with 0.297 g L1 d1 of Chlorella sp. (a fresh water algae) and the FPA PBR with 0.12 g L1 d1 of Spirulina sp. (a fresh water algae). As propositioned in the theoretical section, this higher performance of the IIPBR is attributed to the internal illumination and the shorter light path length of 0.048 m; in contrast, the TRC and FPA PBRs were both externally illuminated, with longer light path lengths of 0.2 and 0.15 m, respectively (Hsieh and Wu, 2009; Reyna-Velarde et al., 2010). Even though several studies included in Table 2 had utilized shorter light path lengths than the IIPBR, they had applied higher light levels but without any justifiable increase in biomass production in terms of B/E. Data in Table 2 are plotted in Fig. 4 to show the variation of B/E as a function of light energy input and light path length, supporting the advantage of the IIPBR design in efficient utilization of light as theoretically postulated. The above advantage stems from one of the premises of the IIPBR design, that it is geometrically more efficient than other PBR designs. This is justified in Fig. 5, where various PBRs are compared on the basis of B/E, as a function of incident area per unit culture volume and the light path length. It is evident from this comparison that different combinations of PBR designs, algal species, incident area per unit culture volume, and light path length can result in different B/E values. In general, B/E is seen to decrease with increasing incident area per unit culture volume, because as the incident area increases the light input increases while the productivity does not continue to increase in proportion. 4.3. Effect of internal illumination and external illumination Comparing the three internally illuminated PBR studies listed in Table 2, the B/E found from Zittelli et al. (2003) with a marine species, Nannochloropsis sp., (0.07–0.15 g W1 d1) is lower than that recorded in the current study with a similar marine species, N. salina (0.37 g W1 d1); likewise, the B/E found from Loubiere et al. (2010) with a fresh water algae Chlamydomonas (0.19 g W1 d1), is lower than that recorded in this study with a similar fresh water algae, Scenedesmus sp (1.42 g W1 d1). Since the light path lengths of all these internally illuminated designs are of the same order of
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A.K. Pegallapati et al. / Bioresource Technology 126 (2012) 266–273 Table 2 Comparison of biomass productivity per unit energy input (B/E) in several PBRs. Type of PBRa,b FPA (E) MFPP (E) MFPP (E) TRC (E) BC BC BC BC
(E) (E) (E) (E)
Lightpath (m)
Modec
CO2-aird (%)
7.30
0.15
B
b
165.85 165.85 15.29
0.01 0.01 0.20
SC SC B
82.47 82.47 51.05 58.90
0.04 0.04 0.05 0.04
S/V (m1)
Light input (W m3)
B/Ej (g W1 d1)
Straine,f
Lightg,h
References
12.02
0.86
S (F)
80(N)
0.61 0.85 0.30
129.30 129.30 9.73
0.142 0.101 0.73
N (M) N (M) C (F)
115(F) 230(F) 660(H)
Reyna-velarde et al. (2010) Zittelli et al. (2000) Zittelli et al. (2000) Hsieh and Wu, 2009
0.25 0.25 0.25 1.00
0.50 0.60 0.61 0.77
8.49 8.49 20.82 122.63
0.092 0.11 0.18 0.38
NO (M) C (F) C (F) A (F)
300(F) 300(F) 300(F) 150(F)
G/V (min1)
B (g L1 d1)
127.1
0.30
0.12
3 3 7.20
4152.2 8304.5 399.1
0.50 0.50 0.30
SC B SC B
2 2 5 15
5385.9 5385.9 3334.1 1923.5
Sparging energy (W m3)i
HAL (E)
127
0.01
B
a
2491.8
2.38
2.76
249.14
1.01
P (M)
171(F)
Chiu et al. (2009) Chiu et al. (2008) Chiu et al. (2009) Jacob-Lopes et al. (2009) Edmund et al. (1990)
FP (E)
123.33
0.01
SC
2
5101.4
0.50
0.22
24.53
0.04
CM (M)
190(F)
Goksan et al. (2003)
FP (E)
41.11
0.03
SC
2
1700.5
0.50
0.10
24.53
0.06
CM (M)
190(F)
Goksan et al. (2003)
AL (E)
21.75
0.11
SC
3.33
1988.7
0.16
0.66
7.70
0.33
CR (F)
420(F)
Fischer et al. (1994)
BC (E)
124.95
0.16
B
4
9520.2
0.03
0.43
9.81
0.05
DT (M)
350(F)
BC (E)
124.95
0.16
B
12
9520.2
0.03
0.31
9.81
0.03
CV (F)
350(F)
BC (E)
124.95
0.16
B
12
9520.2
0.07
0.34
19.61
0.04
CV (F)
350(F)
BC (E)
124.95
0.16
B
12
9520.2
0.17
0.38
49.03
0.04
CV (F)
350(F)
39.79 48.17 48.17 48.17 48.17 50.00 45.30 67.88 45.30 67.88 13.89 13.89 75.78
0.05 0.04 0.04 0.04 0.04 0.02 0.045 0.030 0.045 0.030 0.048 0.048 0.030
B B B B B B SC SC SC SC SC SC B
20 5 5 5 5 a 2 2 2 2 4 1 0.5
1299.5 1048.7 1048.7 1048.7 1048.7 1534.8 1725.8 1315.3 2554.2 1965.50 276.4 276.4 1157.32
0.20 0.06 0.10 0.20 0.40 0.13 0.10 0.10 0.10 0.10 0.04 0.04 0.22
0.09 0.30 0.30 0.43 0.45 0.30 0.17 0.2 0.19 0.25 0.40 0.10 0.16
12.43 3.54 5.91 11.81 23.63 21.80 30.07 27.14 30.07 27.14 5.83 5.83 11.81
0.07 0.28 0.28 0.40 0.42 0.19 0.10 0.15 0.07 0.13 1.42 0.37 0.14
BB (F) C (F) C (F) C (F) C (F) Ch (F) N (M) N (M) N (M) N (M) Sc (F) NS (M) NS (M)
150(F) 100(F) 100(F) 100(F) 100(F) 141(F) 175(F) 89(F) 259(M) 133(M) 91(F) 91(F) 79(F)
Hulatt and Thomas, 2011 Hulatt and Thomas, 2011 Hulatt and Thomas, 2011 Hulatt and Thomas, 2011 Ge et al. (2011) Ryu et al. (2009) Ryu et al. (2009) Ryu et al. (2009) Ryu et al. (2009) Loubiere et al., 2010 Zittelli et al. (2003) Zittelli et al. (2003) Zittelli et al. (2003) Zittelli et al. (2003) This study This study This study
BC (E) BC (E) BC (E) BC (E) BC (E) II E (I) A (I) A (I) A (I) A (I) IIPBR (I) IIPBR (I) BC (E)
a TRC, Transparent rectangular chamber; IIE, internally illuminated external swirl flow reactor; FPA, flat panel airlift reactor; MFPP, modular flat panel photobioreactor; BC, bubble column; HAL, helical airlift reactor; FP, flat plate reactor; AL, airlift reactor; A, annular columns; IIPBR, internally illuminated photobioreactor. b E, Externally illuminated; I, internally illuminated. c B, Batch; SC, semi-continuous. d a, pH regulated; b, ambient air. e C, Chlorella sp.; Ch, Chlamydomonas; S, Spirulina sp.; N, Nannochloropsis sp; A, Aphanothece microscopica Nägeli; P, Porphyridium cruentum; CM, Chaetoceros muelleri; CR, Chenopodium rubrum; DT, Dunaliella tertiolecta; CV, C. vulgaris; BB, B. braunii; Sc, Scendesmus sp.; NS, N. salina. f F, fresh water algae; M, marine algae. g Light in l Einsteins m2 s1. h H, Halogen lamp; F, florescent; N, neon lamp; M, metal halide lamp. i Estimated using Eq. (6) except for airlift reactors for which power equation from Chisti (1998) was adopted. j Estimated using Eq. (11).
magnitude (0.02–0.048 m), the higher B/E of the IIPBR in the current study is attributed primarily to the low light energy input per unit volume, which is an order of magnitude less than that in the internally illuminated PBR studies by Zittelli et al. (2003) and Loubiere et al. (2010). From this comparison, it is concluded that, for any given species, the IIPBR design has to be optimized further with respect to the light energy input per unit culture volume. Another premise of the IIPBR was that, it is energetically more efficient than other PBR designs. This is justified in Fig. 6, where the different PBRs are compared on the basis of B/E, as a function of light energy input per unit culture volume and the G/V ratio (which is proportional to the sparging energy, Eq. (6)). The highest B/E of 1.42 g W1 d1 achieved with Scenedesmus sp. in the current study on the IIPBR design is due to a combination of efficient utilization of the low level of light input from the internal light source; the low G/V ratio that was adequate to maintain high growth rate;
and the high growth characteristics of the species. The B/E for N. salina in the current study on the IIPBR was not as high due to its low growth characteristics. While the comparisons presented above disregarded the growth characteristics of the different algal species, the low B/E values estimated from most studies that had used high light input per unit volume are attributed partly to poor utilization of the external light sources and longer light path lengths. Among the externally illuminated reactors included in Table 2, the B/E value of the bubble column (BC) of this study of 0.14 g W1 d1 is comparable to those of the BCs by Ryu et al. (2009) (= 0.28 and 0.4 g W1 d1); Chiu et al. (2008) and Chiu et al. (2009) (= 0.11 and 0.18 g W1 d1); and, is at least twice higher than those of the BCs by Hulatt and Thomas, 2011 (= 0.03– 0.05 g W1 d1); and Ge et al. (2011) (= 0.07 g W1 d1). Though the light path lengths of the BCs in this study and in Ryu et al.
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0.20
a) Biomass productivity, B [g/L-d]
0.15 0.10 0.05
b) Sparging energy, Es [W/cu m]
45 30 15 0.200
Fig. 5. Comparison of B/E estimated for PBRs using literature results vs. B/E found in this study for IIPBR, as a function of incident area per unit volume and light path length.
c) B/Es and B/E [g/W-d]
B/E
0.15 0.10 0.05
B/Es
0.00 0.01
0.1
1
Gas:culture volume ratio [1/min] Fig. 3. Effect of gas-to-culture volume ratio on: (a) Biomass production (B); (b) sparging energy (Es); and (c) B/Es and B/E, at fixed CO2-air ratio of 0.5% in bubble column with N. salina. Error bars in (a) indicate ⁄std. dev. from triplicates.
Fig. 6. Comparison of B/E estimated for PBRs using literature results vs. B/E found in this study for IIPBR, as a function of light energy input per unit culture volume and G/V ratio.
Fig. 4. Comparison of B/E estimated for PBRs using literature results vs. B/E found in this study for IIPBR, as a function of light energy input per unit volume and light path length.
(2009) are of the same order, the higher B/E achieved in the latter study could be due to difference in algal species. The lower B/E value recorded by Hulatt and Thomas (2011) compared to the BC in this study could be due to sub-optimal S/V resulting in higher light energy input, reducing the B/E value. 4.4. Effect of algal species In spite of the differences of the algal species in the literature studies listed in Table 2, the performance data from the different PBRs could be compared on a broad sense by classifying the species as either fresh water algae or marine algae. In the subgroup of fresh water algae, B/E found in this study with the IIPBR (= 1.42 g W1 d1)
for Scenedesmus sp. is at least twice higher than those reported by Hsieh and Wu (2009) for Chlorella sp. (= 0.73 g W1 d1); by Chiu et al. (2008) for Chlorella sp. (= 0.11 g W1 d1); by Ryu et al. (2009) for Chlorella sp. (= 0.28 g W1 d1); by Louibere et al. (2010) for Chlamydomonas (= 0.19 g W1 d1); by Ge et al. (2011) for Botryococcus braunii (= 0.07 g W1 d1); and by ⁄Hulatt and Thomas (2011) for Chlorella vulgaris (= 0.03–0.04 g W1 d1). In the subgroup of marine algae too, B/E found in this study with N. salina in the IIPBR (= 0.37 g W1 d1 at G/V of 0.044 min1) is 2– 3 times higher than the B/E for Nannochloropsis sp. (= 0.14 and 0.101 g W1 d1 at G/V of 0.5 min1) estimated from Zittelli et al. (2000); and 3–4 times higher than the B/E for Nannochloropsis oculata (= 0.092 g W1 d1) estimated from Chiu et al. (2009). A more direct comparison can be made between the IIPBR and the BCPBR using the growth data on the same test species – N. salina cultivated as part of the current study. The incident light was nearly the same in the two PBRs: 77–104 l Einsteins m2 s1 in the IIPBR and 70–105 l Einsteins m2 s1 in the BCPBR. The maximum biomass productivity achieved in each case was selected in the calculation of B/E, yielding B/E of 0.37 g W1 d1 in the case of the IIPBR and 0.139 g W1 d1 in the case of the BCPBR. This improvement of nearly 165% in B/E in the IIPBR over the BCPBR for the same test species confirms the advantage of the internal illumination design as hypothesized.
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