The cost analyses for large-scale microalgae production for fuels reviewed earlier evolved from the rather superficial analysis of the 1970s to the much more detailed and sophisticated studies during the 1980s, with some updates and advances during the present decade. The basic process did not change significantly from the conceptual designs first suggested by Oswald and Golueke (1960): very large open,
Ttarowr%nfneursiíxidI,l spc^lwaygpp?iicisr e rHowve wyiihethalSlgliT^tglF5ínhgvtudVppl•vl^r shnfengnfly md rgMwyenpnefe ^gigfe: dose gnguyseo fasc akiaRmn negator s ome type of pond lining. Current commercial microalgae production ponds are typically
0.25-0.5-ha in size, and are lined with plastics to prevent percolation and silt suspension and to allow pond cleaning. However, there are also examples of much larger and unlined raceway ponds in a commercial production facility, specifically the
Earthrise Farms Spirulina plant in southern California, where two large
(approximately 5-ha) unlined ponds are currently operating. Similar systems are also used in wastewater treatment. The City of Hollister wastewater treatment plant includes a single 7-ha raceway unlined pond mixed with an Archimedes screw. Even larger (>50 ha) unlined, unmixed ponds are also used for microalgae production in For the harvesting, fuel processing, and media/nutrient recycling subsystem designs Australia for commercial production of Dunaliella, and in several countries in the cost analyses are perhaps less robust, based on often untested assumptions. wastewater treatment. Thus, although some uncertainties remain (such as allowable However, overall, none of these appear to provide a likely major show stopper. Still, channel width and wind fetch effects), in general the basic engineering designs and most of these issues require more R&D. One area where little work has been done is assumptions for the microalgae cultivation ponds appear well established. in the extraction of the algal oils. Although in the most recent studies the use of hgsugnsoveffihesmudgy . effect on costs, the cost projections are optimistic; therefore, there is relatively little scope for any further cost reductions. In most cases, hgsugnsoveffihesmudgy . effect on costs, the cost projections are optimistic; therefore, there is relatively little scope for any further cost reductions. In most cases, engineering designs and specifications were based on the cheapest possible design and likely lowest costs. Also, the engineering design and system construction approaches were based on agricultural engineering practices, rather than those of , A major conclusion from the cost analyses is that there is little prospect for any chemical engineering, as agricultural materials and construction methods are more alternative designs for microalgae production systems that would be able to meet the applicable, in addition to being of lower cost.
requirements of microalgae production for fuels. This is particularly true of closed photobioreactors, in which the culture is entirely enclosed, in greenhouses, plastic tubes or bags, or other transparent enclosures. The costs of even the simplest such system would likely be well above what is affordable for fuel production processes. Even the simplest plastic sheeting cover over the ponds would much more than double total systems capital and operating costs. The simplest tubular photobioreactors are projected to have capital costs some ten times higher
(e.g., $50/m) than open pond designs (Benemann 1998). And, despite many proponents of such closed photobioreactors, current commercial microalgae
Bfo(diicís8nclose13lmho^6i0fecliisovel^oos^fial,e)]6^ne^t1sirlísarev^lsucof avebytthrgknarue emiCFo(sSvíeorí?fioSiiCltscc olí(lPi€ti oSsWip!Hf1tetlematSeratufLi:^g€lTiC abeologlc at/y^tproinafitso nanodf
^o^ges ^ncsfitoons^eMci ^ ^md handling and harvesting costs. Thus, it would be theoretically possible to grow algal strains not able to dominate in open ponds, at higher productivities and reduced harvesting costs, thereby making up for the higher costs of closed photobioreactors (which proponents assume to be only marginally higher than open pond systems). Closed systems of various types may find important applications in the production of the "starter culture" or inoculum that wt lthe^hqu i?n§ ^rniade sJni emtreM n rij r§e- scry ^pen op<0r Jta0 rislrmixefS pound5 bus lpa]irlctlíiarPy0imiiciir(t1nnt fwDunageinj tInaAysímpfoVJlc(co Ug€ir€€fcesyye rgs8e,€S'€ii
Jog aS ^trriainnsaaperoiSiiati on in Mexico). Such production processes are of even lower cost than the mixed raceway designs. However, due to hydraulic and CO2 supply
limitations (among others), productivities are maximally only a few g/m /d, a small fraction of those required for microalgae fuels production. Thus, there seem to be few practical choices in the basic engineering design of a raceway pond system. Even the m(PXIsge0pttt0rs mei remF«^!^ whsed are ovsaifmmic onnmiarigifr€llfbr?
andissMe than tthe aeí€?irJftIVf^g(e(|:siAgnshimredeV5encrthWs, cosc r€utJtlot€;puím[pts,t0
biflliotSiccal assumptions on which such designs are based. These have changed dramatically during the past 2 decades in one major aspect: productivity. Productivity projections have escalated from less than 50 mt/ha/y in the initial studies (e.g., Benemann et al. 1977), to almost 300 mt/ha/y (on an equivalent heat of combustion basis) in the most recent extrapolations (Benemann et al. 1993). In terms of photosynthetic efficiency, these improvements are from about 2% total solar energy conversion to a near-theoretical 10% efficiency. This dramatic increase in projected productivities was based on two main factors: first the significant advances in the state-of-the-art during these 2 decades, with significantly higher productivities than originally anticipated being measured in outdoor systems. And second, the clear necessity to achieve very high efficiencies for any sunlight-to-fuels process. Although there are theoretical, and practical, approaches to achieving such high efficiencies, they will without a doubt require relatively long-term R&D efforts (see Section IV.A.2.).
Productivity, in terms of solar conversion efficiency, is only one of the objectives of future R&D in this field. A related issue is that much of this productivity must be in the form of algal lipids, suitable for utilization and upgrading to fuels. Although some progress was made in this area in the laboratory, through physiological and genetic means (see Section III.B.5.d.; also Section II), this still will require considerable research. Another area that will require significant research is the development of a
Tcw-cma1 íi?ucvesiln?l0Pf0caliSie^aihulIheeerlgiDfigcinalcr°dlaeKnPrni0Jetl^:ie0rcfíD?lifici Oil]rrenolio]ncií0iltlltgl1owaaDai]Deeírl?iiPle?warl?hbwypuyl thPPndD3t0n^oUl?iinpCulSvUt1iDD PlfiftT?ss;frQtlowsp1-bfetiM®eenaIUfiU] ^ucaoctf^tgmd p^esaivsa ting and lipid accumulation to overall productivity. Future R&D must focus on these biological issues as a primary research objective, in the quest for low-cost production processes.
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