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Abstract:
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Biofuel production from microalgal biomass offers a clean and sustainable liquid fuel alternative to fossil fuels . In addition , algae cultivation is advantageous over traditional biofuel feedstocks as (i ) it does not compete with food production , (ii ) it potentially has a much greater areal productivity , (iii ) it does not require arable land , and (iv ) it can use marginal sources of water not suitable for irrigation or drinking . However , current algae cultivation technologies suffer from (i ) low solar energy conversion effiencies , (ii ) large thermal fluctuations which negatively affect the productivity , and (iii ) large evaporative losses which make the process highly water intensive . This thesis reports a numerical study that address these key issues of planktonic as well as benthic algal photobioreactor technologies .
First , radiant energy transfer in planktonic algal photobioreactors containing cells with different levels of pigmentation was studied . Chlamydomonas reinhardtii and its truncated chlorophyll antenna transformant tla1 were used as model organisms . Based on these simulations guidelines are derived for scaling the size and microorganism concentration of photobioreactors cultivating cells with different levels of pigmentation to achieve maximum photosynthetic productivity . To achieve this , the local irradiance obtained from the solution of the radiative transport equation (RTE ) was coupled with the specific photosynthetic rates of the microorganisms to predict both the local and total photosynthetic rates in a photobioreactor . For irradiances less than 50 W /m2 ,
the use of genetically modified strains with reduced pigmentation was shown to have negligible effect on increasing photobioreactor productivity . However ,
at irradiances up to 1000 W /m2 , improvements of up to 30 % were possible with cells having 63 % less pigment concentration . It was determined that the ability of tla1 to transmit light deeper into the photobioreactor was the primary mechanism by which a photobioreactor using the modified strain can achieve greater productivity . Furthermore , it was determined photobioreactors using each strain have dead zones in which the local photosynthetic rate is negligible due to nearly complete light attenuation . These dead zones occur at local optical thicknesses greater than 169 and 275 in photobioreactors using the wild strain and the genetically modified strain , respectively .
In addition , a thermal model of an algae biofilm photobioreactor was developed to assess the thermal fluctuations and evaporative loss rate of these novel photobioreactors under varying outdoor conditions . The model took into account air temperature , irradiance , relative humidity , and wind speed as inputs and computed the temperature and evaporative loss rate as a function of time and location in the photobioreactor . The model was run for a week -long period in each season using weather data from Memphis , TN . The range of the daily algae temperature variation was observed to be 13 .2C , 12 .4C , 12 .8C , and 9 .4C in the spring , summer , winter , and fall , respectively . Furthermore , without active cooling , the characteristic evaporative water loss from the system is approximately 6 .3 L /m2 -day , 7 .0 L /m2 -day , 4 .9 L /m2 -day , and 1 .5 L /m2 -day in the spring , summer , fall , and winter , respectively . |