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Energy Systems Analysis

J. Energy Resour. Technol. 2012;134(3):032001-032001-8. doi:10.1115/1.4006434.

Improving the efficiency of thermodynamic cycles plays a fundamental role in reducing the cost of solar power plants. These plants work normally with Rankine cycles which present some disadvantages due to the thermodynamic behavior of steam at low pressures. These disadvantages can be reduced by introducing alternatives such as combined cycles which combine the best features of each cycle. In this paper, a combined Rankine–Goswami cycle (RGC) is proposed and a thermodynamic analysis is conducted. The Goswami cycle, used as a bottoming cycle, uses ammonia–water mixture as the working fluid and produces power and refrigeration while power is the primary goal. This bottoming cycle, reduces the energy losses in the traditional condenser and eliminates the high specific volume and poor vapor quality presented in the last stages of the lower pressure turbine in the Rankine cycle. In addition, the use of absorption condensation in the Goswami cycle, for regeneration of the strong solution, allows operating the low pressure side of the cycle above atmospheric pressure which eliminates the need for maintaining a vacuum pressure in the condenser. The performance of the proposed combined Rankine–Goswami cycle, under full load, was investigated for applications in parabolic trough solar thermal plants for a range from 40 to 50 MW sizes. A sensitivity analysis to study the effect of the ammonia concentration, condenser pressure, and rectifier concentration on the cycle efficiency, network, and cooling was performed. The results indicate that the proposed RGC provide a difference in net power output between 15.7% and 42.3% for condenser pressures between 1 and 9 bars. The maximum effective first law and exergy efficiencies for an ammonia mass fraction of 0.5 are calculated as 36.7% and 24.7%, respectively, for the base case (no superheater or rectifier process).

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2012;134(3):032002-032002-8. doi:10.1115/1.4005922.

Optimization of thermodynamic cycles is important for the efficient utilization of energy sources; indeed, it is more crucial for the cycles utilizing low-grade heat sources where the cycle efficiencies are smaller compared to high temperature power cycles. This paper presents the optimization of a combined power/cooling cycle, also known as the Goswami cycle, which combines the Rankine and absorption refrigeration cycles. The cycle uses a special binary fluid mixture as the working fluid and produces a power and refrigeration. In this regard, multi-objective genetic algorithms (GAs) are used for Pareto approach optimization of the thermodynamic cycle. The optimization study includes two cases. In the first case, the performance of the cycle is evaluated as it is used as a bottoming cycle and in the second case, as it is used as a top cycle utilizing solar energy or geothermal sources. The important thermodynamic objectives that have been considered in this work are, namely, work output, cooling capacity, effective first law, and exergy efficiencies. Optimization is carried out by varying the selected design variables, such as boiler temperature and pressure, rectifier temperature, and basic solution concentration. The boiler temperature is varied between 70–150 °C and 150–250 °C for the first and the second cases, respectively.

Commentary by Dr. Valentin Fuster

Fuel Combustion

J. Energy Resour. Technol. 2012;134(3):032201-032201-7. doi:10.1115/1.4006481.

Distributed combustion is now known to provide significantly improved performance of gas turbine combustors. Key features of distributed combustion include uniform thermal field in the entire combustion chamber for significantly improved pattern factor and avoidance of hot-spot regions that promote thermal NOx emissions, negligible emissions of hydrocarbons and soot, low noise, and reduced air cooling requirements for turbine blades. Distributed combustion requires controlled mixing between the injected air, fuel, and hot reactive gasses from within the combustor prior to mixture ignition. The mixing process impacts spontaneous ignition of the mixture to result in improved distributed combustion reactions. Distributed reactions can be achieved in premixed, partially premixed, or nonpremixed modes of combustor operation with sufficient entrainment of hot and active species present in the combustion zone and their rapid turbulent mixing with the reactants. Distributed combustion with swirl is investigated here to further explore the beneficial aspects of such combustion under relevant gas turbine combustion conditions. The near term goal is to develop a high intensity combustor with ultralow emissions of NOx and CO, and a much improved pattern factor and eventual goal of near zero emission combustor. Experimental results are reported for a cylindrical geometry combustor for different modes of fuel injection with emphasis on the resulting pollutants emission. In all the cases, air was injected tangentially to impart swirl to the flow inside the combustor. Ultra low NOx emissions were found for both the premixed and nonpremixed combustion modes for the geometries investigated here. Results showed very low levels of NO (∼10 ppm) and CO (∼21 ppm) emissions under nonpremixed mode of combustion with air preheats at an equivalence ratio of 0.6 and a moderate heat release intensity of 27 MW/m3 -atm. Results are also reported on lean stability limits and OH* chemiluminescence under different fuel injection scenarios for determining the extent of distribution combustion conditions. Numerical simulations have also been performed to help develop an understanding of the mixing process for better understanding of ignition and combustion.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2012;134(3):032202-032202-6. doi:10.1115/1.4006655.

The main purpose of this research is to study the effect of various blends of an environmental friendly alternative fuel “methyl ester” on the performance of a heavy diesel engine. The biodiesel was obtained from a chemical process: the transesterification of waste oils (frying oils). Tests were conducted on an engine test bench in accordance to DIN 2020 standards. Results obtained demonstrate that the biodiesel gives very interesting ecological advantages but engine performance was reduced slightly comparatively to those obtained with a pure diesel fuel. We have noted about 5% decrease in power and torque and about 2% in Nox emission for every 10% of biodiesel blend added comparatively to pure diesel. However, the use of biodiesel has slightly increased specific fuel consumption (about 6% for every 10% of biodiesel blend added).

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2012;134(3):032203-032203-7. doi:10.1115/1.4006867.

Gas phase decarboxylation of hydrolyzed free fatty acid (FFA) from canola oil has been investigated in two fix-bed reactors by changing reaction parameters such as temperatures, FFA feed rates, and H2 -to-FFA molar ratios. FFA, which contains mostly C18 as well as a few C16 , C20 , C22 , and C24 FFA, was fed into the boiling zone, evaporated, carried by hydrogen flow at the rate of 0.5–20 ml/min, and reacted with the 5% Pd/C catalyst in the reactor. Reactions were conducted atmospherically at 380–450 °C and the products, qualified and quantified through gas chromatography-flame ionization detector (GC-FID), showed mostly n-heptadecane and a few portion of n-C15 , n-C19 , n-C21 , n-C23 as well as some cracking species. Results showed that FFA conversion increased with increasing reaction temperatures but decreased with increasing FFA feed rates and H2 -to-FFA molar ratios. The reaction rates were found to decrease with higher temperature and increase with higher H2 flow rates. Highly selective heptadecane was achieved by applying higher temperatures and higher H2 -to-FFA molar ratios. From the results, as catalyst loading and FFA feed rate were fixed, an optimal reaction temperature of 415 °C as well as H2 -to-FFA molar ratio of 4.16 were presented. These results provided good basis for studying the kinetics of decarboxylation process.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2012;134(3):032204-032204-10. doi:10.1115/1.4007216.

Combustion in direct-injection diesel engines occurs in a lifted, turbulent diffusion flame mode. Numerous studies indicate that the combustion and emissions in such engines are strongly influenced by the lifted flame characteristics, which are in turn determined by fuel and air mixing in the upstream region of the lifted flame, and consequently by the liquid breakup and spray development processes. From a numerical standpoint, these spray combustion processes depend heavily on the choice of underlying spray, combustion, and turbulence models. The present numerical study investigates the influence of different chemical kinetic mechanisms for diesel and biodiesel fuels, as well as Reynolds-averaged Navier–Stokes (RANS) and large eddy simulation (LES) turbulence models on predicting flame lift-off lengths (LOLs) and ignition delays. Specifically, two chemical kinetic mechanisms for n-heptane (NHPT) and three for biodiesel surrogates are investigated. In addition, the renormalization group (RNG) k-ε (RANS) model is compared to the Smagorinsky based LES turbulence model. Using adaptive grid resolution, minimum grid sizes of 250 μm and 125 μm were obtained for the RANS and LES cases, respectively. Validations of these models were performed against experimental data from Sandia National Laboratories in a constant volume combustion chamber. Ignition delay and flame lift-off validations were performed at different ambient temperature conditions. The LES model predicts lower ignition delays and qualitatively better flame structures compared to the RNG k-ε model. The use of realistic chemistry and a ternary surrogate mixture, which consists of methyl decanoate, methyl nine-decenoate, and NHPT, results in better predicted LOLs and ignition delays. For diesel fuel though, only marginal improvements are observed by using larger size mechanisms. However, these improved predictions come at a significant increase in computational cost.

Commentary by Dr. Valentin Fuster

Oil/Gas Reservoirs

J. Energy Resour. Technol. 2012;134(3):032801-032801-5. doi:10.1115/1.4006573.

As a highly efficient production method, multibranch horizontal well is widely used in the development of low permeability reservoirs, naturally fracture reservoirs, heavy oil reservoirs, shallow layer reservoirs, and multilayer reservoirs, because it can significantly improve the productivity of a single well, inhibit edge or bottom water coning, and enhance oil recovery. This paper presents a new productivity equation for multibranch horizontal well in 3D anisotropic reservoirs. By applying coordinate transformation, a 3D anisotropic reservoir is transformed into an equivalent isotropic reservoir with considering wellbore deformation and vertical radial flow. An analytical solution of multibranch horizontal well productivity in 3D anisotropic reservoirs is obtained by using pseudo-3D solving method and similar flow replaces theory. The results show that branch number n, branch length (l), and permeability anisotropy degree ($β3$) are the three main factors that have big effects on the production rate of multibranch horizontal well.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2012;134(3):032802-032802-7. doi:10.1115/1.4006866.

A working methodology to minimize wellbore stability problems has been established through the use of unique laboratory tests, an experimental database for fluid-rock interaction and physical properties of shales, and an integrated modeling approach utilizing different types of experimental and field data. The model simulations provide output accounting for a wide range of input parameters such as well inclination, mud chemistry, rock mechanical properties, field stresses and pressures, formation anisotropy, and shale mineralogy. The model output can subsequently be used to diagnose field drilling problems or to design drilling operations. As an example, data from a high pressure/high temperature (HP/HT) field offshore Mid-Norway as well as a field in the overpressured shales in the southern part of the Norwegian North Sea have been analyzed and compared.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2012;134(3):032803-032803-9. doi:10.1115/1.4006865.

Rate decline analysis is a significant method for predicting well performance. Previous studies on rate decline analysis of fractured wells are all based on homogeneous reservoirs rather than homogeneous ones considering fracture face damage. In this article, a well model intercepted by a finite conductivity vertical fracture with fracture face damage is established to investigate how face damage factor affects the productivity of fractured well. Calculative results show that in transient flow, dimensionless rate decreases with the increase of fracture face damage and in pseudo steady-state flow, all curves under different face damage factors coincide with each other. Then, a new pseudo steady-state analytic formula and its validation are presented. Finally, new Blasingame type curves are established. It is shown that the existence of fracture damage would decrease the rate when time is relatively small, so fracture damage is an essential factor that we should consider for type curves analysis. Compared with traditional type curves, new type curves could solve the problem of both variable rate and variable pressure drop for fractured wells with fracture face damage factor. A gas reservoir example is performed to demonstrate the methodology of new type curves analysis and its validation for calculating important formation parameters.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2012;134(3):032804-032804-11. doi:10.1115/1.4007003.

This paper presents an improved reservoir simulation approach to methane production in a longwall mining environment. The coal beds are naturally fractured systems with the gas adsorbed into the coal matrix. Fractures penetrating the coal matrix have limited storage capacity, but they play the role of a gas transportation system. The proposed simulation technique is based on the assumption that a mass of coal removed by mining transfers its gas to adjacent fractures. By using an ECLIPSE coal bed methane simulator, the pore volume of the matrix represents the coal volume of the simulation cell. Consequently, the exploitation of coal can be simulated by modifying the matrix pore volume over time. This paper presents theoretical backgrounds of this approach and investigates numerical effects. A case study of the Moszczenica coal mine in Poland, including computer simulations of methane production, is also reported to show that a long history of the methane and coal recovery can be reproduced using the proposed technique.

Commentary by Dr. Valentin Fuster

Technology Reviews

J. Energy Resour. Technol. 2012;134(3):034001-034001-6. doi:10.1115/1.4006790.

The present work is an attempt to compile and analyze the most recent literature pertaining to thermal pyrolysis of plastic waste using fluidized bed reactors. The review is short owing to the small number of work reported in the open literature in particular to the fluidized beds. Although works on pyrolysis are reported in fixed beds, autoclaves, and fluidized beds, vast majority of them address to the utilization of fluidized bed due to their advantages and large scale adaptability. The pyrolysis temperature and the residence time are reported to have major influence on the product distribution, with the increase in pyrolysis temperature favoring gas production, with significant reduction in the wax and oil. The pyrolysis gas generally contains H2 , CO, CO2 , CH4 , C2 H4 , C2 H6 while liquid product comprises benzene, toluene, xylene, styrene, light oil, heavy oil, and gasoline with the variations depending on process conditions. The effects of other process parameters, namely fuel feed rate, fuel composition, and fluidizing medium have been reviewed and presented.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2012;134(3):034002-034002-4. doi:10.1115/1.4006432.

The production and escalation of hydrogen on a large scale is a goal toward the upheaval of green and cheap energy. During the last decades, many methods have been espoused to produce more hydrogen fuel. Right now hydrogen is hauled out from different sources, such as oil, coal, natural gas, water, etc. In case of water, the hydrogen is produced by different methods such as electrolysis, photosplitting, photoionization, photocatalysis, product analysis methods, etc. This review paper thrashes out the assessment of these assorted methods of hydrogen production from water.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Energy Resour. Technol. 2012;134(3):034501-034501-13. doi:10.1115/1.4006433.

Sand management strategies become an important study to be performed as part of multiphase flow assurance assessments during oil and gas project life and especially for subsea multiphase flow network. This paper presents experimental works to investigate the sand transport characteristics and identify the sand minimum transport condition (MTC) in sand–water and sand–air–water flows in a horizontal and + 5 deg inclined pipelines. The used sand volume fraction, Cv , ranged from 1.61 × 10−5 up to 5.38 × 10−4 . The sand minimum transport velocity in single-phase water flow was obtained visually and then compared with that calculated by previous correlations for slurry transport. It was found that in sand–water flow, the pipeline inclination had negligible effect on the minimum sand transport velocity. However, the transport characteristics of sand particles were found changed significantly by changing the pipe inclination, which could result in the change of air–water flow regime. It was observed that sand particles transport more efficiently in terrain slug than stratified wavy flow in +5 deg inclined pipes. The sand transport and settling boundary for different air–water flow regimes were generated for horizontal and +5 deg inclined pipeline.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2012;134(3):034502-034502-5. doi:10.1115/1.4006960.

Coiled tubing is widely used in oil drilling and production operations. However, extreme high stress variation of coiled tubing during the processes of pulling out, rolling in the reel, and passing through the gooseneck makes coiled tubing fatigue failure easily. Thus, it is of great importance to increase coiled tubing fatigue life. This paper introduces the new technology to improve the fatigue life of coiled tubing—the prebending coiled tubing technology; proceeds mechanical analysis and strength check of the prebending coiled tubing; analyzes stress cycling characteristics of the prebending coiled tubing in field operations; establishes the fatigue life prediction model of prebending coiled tubing under arbitrary cycle, on the basis of fatigue experimental data under the symmetric cycle and the pulsating cycle, with fitting and interpolation method; makes simple comparison of the fatigue life of the prebending coiled tubing with that of the straight coiled tubing. Preliminary calculations show that the prebending coiled tubing technology may improve the fatigue life of coiled tubing multiple times.

Commentary by Dr. Valentin Fuster