• Energy Research

Abstract One of the main challenges of solar thermal technology is the intermittency of solar radiation which adversely affect temperature stability of the solar receiver. A promising technique to tackle this problem is the use of a variable aperture mechanism to regulate the light entry into solar receiver. Efficiency analysis confirms the advantage of this control technique over shutter adjustment method, which is also based on regulation of solar radiation entry. In order to regulate the temperature in a closed loop circuit based on aperture size adjustment, a model based control strategy was developed. To show the robustness and comprehensiveness of this control strategy, it was applied to a cavity receiver heated by two different radiative heat sources demonstrating the applicability of this control strategy consistently in most commonly practiced solar thermal systems. The first heat source studied is a solar furnace housing a parabolic dish, whereas the second one is a high flux solar simulator. For each radiative heat source, flux entering the receiver was determined using Monte Carlo ray tracing (MCRT) method. MCRT model was then coupled with energy balance equations to derive numerical model describing dynamic temperature variation in solar receiver. Comparison of simulated and experimentally measured temperatures showed appreciable accuracy of the dynamic model. Simulation results of the numerical model were then used to identify a nonlinear adaptive model for use in designing a model predictive controller (MPC). Parameters of the adaptive model were updated continuously to make the controller more robust against model mismatches and external disturbances. Simulation results for both radiative heat sources showed that the proposed controller yields faster response with less overshoot compare to proportional integral derivative (PID) controller. Results showed that this controller exhibits robust performance during sunrise and sunset times as well as passing clouds conditions where significant fluctuations in solar radiation is experienced.

In this research, a two-stage process consisting of cultivation in nutrient rich and nitrogen starvation conditions was employed to enhance lipid production in Chlorella vulgaris algal biomass. The effect of supplying different organic and inorganic carbon sources on cultivation behavior was investigated. During nutrient sufficient condition (stage I), the highest biomass productivity of 0.158±0.011g/L/d was achieved by using sodium bicarbonate followed by 0.130±0.013, 0.111±0.005 and 0.098±0.003g/L/d for sodium acetate, carbon dioxide and molasses, respectively. Cultivation under nitrogen starvation process (stage II) indicated that the lipid and fatty acid content increased continuously to a maximum value at day 2. Using carbon dioxide resulted in highest cell density, while using sodium acetate led to the highest fatty acid content. Molasses was not as effective as other carbon sources, but by taking into account its lower price, it can be considered as a suitable carbon source for algal lipid productivity.

Abstract Due to the depletion of fossil fuels and their environmental issues, it is necessary to find energy resources which are renewable. Algal biomass becomes promising feedstock for bio-fuel production. They are considered as sustainable, renewable and effective biomass and bio-fuels obtained from them are more environment-friendly than fossil fuels. The aim of this work is to provide a state of the art review on pyrolysis of microalgae for generation of bio-fuels. Initially, some general aspects of biomass such as microalgae characteristics, different thermochemical processes and advantages of microalgae pyrolysis to produce bio-fuels are discussed. Then, different pyrolysis methods are explained and parameters affecting the process are addressed. Bio-fuels including gaseous, solid and liquid products have been characterized in a separate section. Finally, the technical challenges associated with microalgal pyrolysis commercialization are discussed in the last section of this article.

In this study, a novel experimental approach was used to overcome the lack of phase equilibrium information to obtain data that is more applicable to industrial situations. Liquid–liquid equilibrium (LLE) data, tie-lines, and phase boundaries were carried out for two systems of canola oil methyl esters (containing 1 wt % KOH) + glycerol + methanol and sunflower oil methyl esters (containing 1 wt % KOH) + glycerol + methanol at three different temperatures (303.15, 313.15, and 323.15 K). The quality of data was also ascertained using Othmer–Tobias correlations. The experimental LLE data was also correlated by the nonrandom two-liquid (NRTL) and the Wilson–NRF Gibbs free energy models. The energy interaction parameters of both models were obtained for both systems. The results indicated that the Wilson–NRF provided an accurate correlation of LLE behavior with average absolute deviation (AAD%) inferior to 8.78% and 10.80% for canola and sunflower biodiesel systems, respectively. However, the NRTL model prese...

Thermal decomposition behavior and kinetics of microalgae Chlorella vulgaris, wood and polypropylene were investigated using thermogravimetric analysis (TGA). Experiments were carried out at heating rates of 10, 20 and 40°C/min from ambient temperature to 600°C. The results show that pyrolysis process of C. vulgaris and wood can be divided into three stages while pyrolysis of polypropylene occurs almost totally in one step. It is shown that wood can delay the pyrolysis of microalgae while microalgae can accelerate the pyrolysis of wood. The existence of polymer during the pyrolysis of microalgae or wood will lead to two divided groups of peaks in DTG curve of mixtures. The results showed that interaction is inhibitive rather than synergistic during the decomposition process of materials. Kinetics of process is studied by the Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO). The average E values obtained from FWO and KAS methods were 131.228 and 142.678kJ/mol, respectively.

In this research, organic solvent composed of hexane and methanol was used for lipid extraction from dry and wet biomass of Chlorella vulgaris. The results indicated that lipid and fatty acid extraction yield was decreased by increasing the moisture content of biomass. However, the maximum extraction efficiency was attained by applying equivolume mixture of hexane and methanol for both dry and wet biomass. Thermodynamic modeling was employed to estimate the effect of hexane/methanol ratio and moisture content on fatty acid extraction yield. Hansen solubility parameter was used in adjusting the interaction parameters of the model, which led to decrease the number of tuning parameters from 6 to 2. The results indicated that the model can accurately estimate the fatty acid recovery with average absolute deviation percentage (AAD%) of 13.90% and 15.00% for the two cases of using 6 and 2 adjustable parameters, respectively.

Abstract Harnessing solar energy for thermochemical processing is an exciting and fast emerging research area with significant potential for reducing CO2 emissions. However, maintenance of a uniform temperature distribution as well as avoidance of hot spots in solar cavity receivers are challenges of present technology which are adversely affecting the process efficiency. This study presents a model based methodology as a design tool for iterative creation of optimum solar receiver geometry. Several discrete solutions are demonstrated as case studies via experimental testing of a solar receiver radiated by a 7 kW solar simulator. Experimental observations are compared with the results of the numerical analysis based on two-dimensional (2D) numerical model that couples the fluid flow and heat transfer mechanisms in the solar receiver. A Monte-Carlo ray tracing method was used to model the incoming radiation from the solar simulator and radiative exchange between the inner surfaces of the cavity receiver. Comparison of the simulation results to experimentally measured steady state temperatures at different points of the solar receiver shows 6.68% average absolute error confirming appreciable accuracy of the model. The results also show that reversing the gas flow direction and increasing the insulation layer do not improve the temperature distribution in the receiver. However, reducing the front flange dimensions and decreasing the inner receiver radius do enhance the temperature distribution and increase the average receiver temperature. Numerical results show that these changes can increase the average temperature of the inner cavity cylinder walls by 27%, and increase the temperature uniformity index by 58%. These findings provide essential insight for solar reactor design to reduce hot spot problems and improve temperature uniformity.

In this research, direct conversion of wet algal biomass into biodiesel using supercritical methanol was studied. In this process, microalgal lipids simultaneously was extracted and converted to biodiesel under high pressure and temperature conditions without using any catalyst. Several experiments have been performed to optimize the methanol amount and it has been revealed that the best performance was achieved by using methanol/wet biomass ratio of 8:1. The effect of using various co-solvents in increasing the efficiency of the supercritical process was investigated. It has been shown that hexane was the most effective co-solvent and its optimal ratio respect to wet biomass was 6:1. The results indicated that compare to conventional extraction plus transesterification reaction, fatty acid methyl esters (FAMEs) yield was slightly higher in the direct conversion process. Moreover, increasing the moisture content up to 80% has no significant effect on reducing the performance of this process.

Due to the depletion of fossil fuels and their environmental issues, it is necessary to find energy resources which are renewable. Biomass becomes promising feedstock for bio-fuel production. The aim of this study is to investigate thermal decomposition behavior and the effect of third component on the binary mixture pyrolysis using thermogravimetric analysis (TGA). Experiments were carried out at heating rates of 10, 20 and 40°C/min from ambient temperature to 600°C. Two divided groups of peaks were observed in DTG curve of tertiary mixture which the first one was corresponded to microalgae and wood and the second one was belonged to polymer. It is stated that microalgae and wood can improve the degradation process while polymer can delay the decomposition process of mixture. Mentioned positive effect of microalgae and wood could be related to main decomposition temperature and component of microalgae and wood. On the other hand, polymer reduces weight loss of binary mixture and has negative effect of it. The kinetics analysis showed that activation energy (E) and pre-exponential factor (A) of tertiary mixture was slightly lower than that of microalgae-polymer mixture which had the lowest E and A.