Radiant and thermal energy transport in planktonic and benthic algae systems for sustainable biofuel production
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.