Thomas E. Murphy

Flux balancing of light and nutrients in a biofilm photobioreactor for maximizing photosynthetic productivity

This article reports a combined experimental and numerical study on the efficient operation of Porous Substrate Bioreactors. A comprehensive model integrating light transport, mass transport, and algal growth kinetics was used to understand the productivity of photosynthetic biofilms in response to delivery rates of photons and nutrients. The reactor under consideration was an evaporation driven Porous Substrate Bioreactor (PSBR) cultivating the cyanobacteria Anabaena variabilis as a biofilm on a porous substrate which delivers water and nutrients to the cells. In an unoptimized experimental case, this reactor was operated with a photosynthetic efficiency of 2.3%, competitive with conventional photobioreactors. Moreover, through a scaling analysis, the location at which the phosphate delivery rate decreased the growth rate to half of its uninhibited value was predicted as a function of microorganism and bioreactor properties. The numerical model along with the flux balancing techniques presented herein can serve as tools for designing and selecting operating parameters of biofilm based cultivation systems for maximum productivity.

Temperature Fluctuation and Evaporative Loss Rate in an Algae Biofilm Photobioreactor

This study describes the thermal modeling of a novel algal biofilm photobioreactor aimed at cultivating algae for biofuel production. The thermal model is developed to assess the photobioreactor’s thermal profile and evaporative water loss rate for a range of environmental parameters, including ambient air temperature, solar irradiation, relative humidity, and wind speed. First, a week-long simulation of the system has been performed using environmental data for Memphis, TN, on a typical week during the spring, summer, fall, and winter. Then, a sensitivity analysis was performed to assess the effect of each weather parameter on the temperature and evaporative loss rate of the photobioreactor. The range of the daily algae temperature variation was observed to be 12.2 C, 13.2 C, 11.7 C, and 8.2 C in the spring, summer, fall, and winter, respectively. Furthermore, without active cooling, the characteristic evaporative water loss from the system is approximately 6.0 L/m day, 7.3 L/m day, 3.4 L/m day, and 1.0 L/m day in the spring, summer, fall, and winter, respectively. [DOI: 10.1115/1.4005088]

Multispectral image analysis for algal biomass quantification

This article reports a novel multispectral image processing technique for rapid, noninvasive quantification of biomass concentration in attached and suspended algae cultures. Monitoring the biomass concentration is critical for efficient production of biofuel feedstocks, food supplements, and bioactive chemicals. Particularly, noninvasive and rapid detection techniques can significantly aid in providing delay‐free process control feedback in large‐scale cultivation platforms. In this technique, three‐band spectral images of Anabaena variabilis cultures were acquired and separated into their red, green, and blue components. A correlation between the magnitude of the green component and the areal biomass concentration was generated. The correlation predicted the biomass concentrations of independently prepared attached and suspended cultures with errors of 7 and 15%, respectively, and the effect of varying lighting conditions and background color were investigated. This method can provide necessary feedback for dilution and harvesting strategies to maximize photosynthetic conversion efficiency in large‐scale operation.

Rapid Algal Culture Diagnostics for Open Ponds Using Multispectral Image Analysis

This article presents a multispectral image analysis approach for probing the spectral backscattered irradiance from algal cultures. It was demonstrated how this spectral information can be used to measure algal biomass concentration, detect invasive species, and monitor culture health in real time. To accomplish this, a conventional RGB camera was used as a three band photodetector for imaging cultures of the green alga Chlorella sp. and the cyanobacterium Anabaena variabilis. A novel floating reference platform was placed in the culture, which enhanced the sensitivity of image color intensity to biomass concentration. Correlations were generated between the RGB color vector of culture images and the biomass concentrations for monocultures of each strain. These correlations predicted the biomass concentrations of independently prepared cultures with average errors of 22 and 14%, respectively. Moreover, the difference in spectral signatures between the two strains was exploited to detect the invasion of Chlorella sp. cultures by A. variabilis. Invasion was successfully detected for A. variabilis to Chlorella sp. mass ratios as small as 0.08. Finally, a method was presented for using multispectral imaging to detect thermal stress in A. variabilis. These methods can be extended to field applications to provide delay free process control feedback for efficient operation of large scale algae cultivation systems.

Near infrared laser penetration and absorption in human skin

For understanding the mechanisms of low level laser/light therapy (LLLT), accurate knowledge of light interaction with tissue is necessary. In this paper, we present a three dimensional, multi-layer Monte Carlo simulation tool for studying light penetration and absorption in human skin. The skin is modeled as a three-layer participating medium, namely epidermis, dermis, and subcutaneous, where its geometrical and optical properties are obtained from the literature. Both refraction and reflection are taken into account at the boundaries according to Snell’s law and Fresnel relations. A forward Monte Carlo method was implemented and validated for accurately simulating light penetration and absorption in absorbing and anisotropically scattering media. Local profiles of light penetration and volumetric absorption densities were simulated for uniform as well as Gaussian profile beams with different spreads at 155 mW average power over the spectral range from 1000 nm to 1900 nm. The results show the effects of beam profiles and wavelength on the local fluence within each skin layer. Particularly, the results identify different wavelength bands for targeted deposition of power in different skin layers. Finally, we show that light penetration scales well with the transport optical thickness of skin. We expect that this tool along with the results presented will aid researchers resolve issues related to dose and targeted delivery of energy in tissues for LLLT.

Simulation of laser propagation through a three-layer human skin model in the spectral range from 1000 to 1900 nm

Abstract. For understanding the mechanisms of low-level laser/light therapy (LLLT), accurate knowledge of light interaction with tissue is necessary. We present a three-dimensional, multilayer reduced-variance Monte Carlo simulation tool for studying light penetration and absorption in human skin. Local profiles of light penetration and volumetric absorption were calculated for uniform as well as Gaussian profile beams with different spreads over the spectral range from 1000 to 1900 nm. The results showed that lasers within this wavelength range could be used to effectively and safely deliver energy to specific skin layers as well as achieve large penetration depths for treating deep tissues, without causing skin damage. In addition, by changing the beam profile from uniform to Gaussian, the local volumetric dosage could increase as much as three times for otherwise similar lasers. We expect that this tool along with the results presented will aid researchers in selecting wavelength and laser power in LLLT.

An Image Processing Technique to Recover the Biomass Concentration in Algae Biofilm Photobioreactors

This paper reports a simple technique based on multispectral image processing that can be used to recover the biomass concentration of an algal biofilm photobioreactor. Monitoring the biomass concentration of a culture is critical in achieving successful algae cultivation for biofuel or food supplement production. In particular, non-invasive and rapid detection techniques that can provide estimates of the biomass concentration can significantly aid in providing delay-free process control feedback in large scale cultivation platforms. In this technique, the digital images of the biofilms of the cyanobacteria Anabaena variabilis were obtained under consistent lighting conditions and analyzed as a function of their biomass content. The image analysis was carried out using a custom Matlab code where the red, green and blue content of the images were correlated with the biomass concentration. The obtained correlation was consistent across biofilms generated from different stock cultures of varying culture age. Challenges facing application of the image processing technique for scaled up outdoor photobioreactors under various lighting conditions and color backgrounds were discussed.Copyright

Transient Analysis of Microorganism Temperature and Evaporative Losses in an Algae Biofilm Photobioreactor

This study describes the thermal modeling of a novel algal biofilm photobioreactor aimed at cultivating algae for biofuel production. The thermal model is developed to assess the photo-bioreactor’s thermal profile and evaporative water loss rate for a range of environmental parameters, including relative humidity, ambient air temperature, solar irradiation, and wind speed. First, a 24 hour simulation of the system has been performed using environmental data for Memphis, TN, USA on a typical spring day to assess the diurnal variations in system performance. Then, a sensitivity analysis is performed to assess the effect of each environmental parameter on the temperature and evaporative losses of the photobioreactor. It is observed that because of the high surface area-to-volume ratio of the system, the temperature of the system exceeds that of the maximum ambient temperature during daylight hours by approximately 0.5 °C and is lower than the minimum ambient temperature at night by approximately 1.4 °C because of evaporative and radiative cooling. Furthermore, without active cooling, the characteristic evaporative water loss from the system is approximately 4.8 L/m2 -day.Copyright

Increased Photobioreactor Productivity Using Algae With Low Pigmentation: A Light Transfer Perspective

This paper reports a numerical study on the photosynthetic productivity of algae photobioreactors using wild strains and genetically engineered strains with low chlorophyll pigment content. First, the specific photosynthetic rates of the wild strain algae Chlammydomonas reinhardtii CC125 and the truncated chlorophyll transformant tla1 were obtained from experiments as a function of the total available irradiation for each cell. This rate was modeled with a modified Monod function accounting for light saturation and light inhibition. The models were coupled with the radiative transport equation (RTE) to predict both the local and total photosynthetic rate of an open pond photobioreactor. The benefits and limitations of using truncated antenna transformants for increasing productivity of mass algae culturing are discussed.© 2010 ASME