Research Papers: Alternative Energy Sources

J. Energy Resour. Technol. 2018;140(10):101201-101201-9. doi:10.1115/1.4040060.

The desire to increase power production through renewable sources introduces a number of problems due to their inherent intermittency. One solution is to incorporate energy storage systems as a means of managing the intermittent energy and increasing the utilization of renewable sources. A novel hybrid thermal and compressed air energy storage (HT-CAES) system is presented which mitigates the shortcomings of the otherwise attractive conventional compressed air energy storage (CAES) systems and its derivatives, such as strict geological locations, low energy density, and the production of greenhouse gas emissions. The HT-CAES system is investigated, and the thermodynamic efficiency limits within which it operates have been drawn. The thermodynamic models considered assume a constant pressure cavern. It is shown that under this assumption the cavern acts just as a delay time in the operation of the plant, whereas an adiabatic constant volume cavern changes the quality of energy through the cavern. The efficiency of the HT-CAES system is compared with its Brayton cycle counterpart, in the case of pure thermal energy storage (TES). It is shown that the efficiency of the HT-CAES plant is generally not bound by the Carnot efficiency and always higher than that of the Brayton cycle, except for when the heat losses following compression rise above a critical level. The results of this paper demonstrate that the HT-CAES system has the potential of increasing the efficiency of a pure TES system executed through a Brayton cycle at the expense of an air storage medium.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(10):101202-101202-11. doi:10.1115/1.4040075.

The present paper introduces the analysis-led-design concept in attaining the thermal homogeneity at the exit section of a mixing chamber. Staggered holes (SH) chamber type is representing jet-in-crossflow (JICF) where cold air is injected radially into an axially flowing hot air with a different velocity. Streamlined body of prolate-spheroid shape is fitted in the center of the chamber, and equipped with swirl generating fins (Swirlers). Numerical simulations were first run to predict the flow and energy fields and assess the performance of seven cases representing distinct swirlers setting (shape, dimension, and number). An unsteady turbulent condition was adopted considering high Reynolds number (Re) at the boundaries and large eddy simulation (LES) model for solving the eddy motion in the domain. Afterward, experimental measurements worked on validating the numerical results through proving the effectiveness of the recommended swirler design. Graphical and tabulated results showed the difference between the mixing patterns in thermal dimensionless numbers (normalized mixture fraction and uniformity factor), and consideration of total pressure drop was taken. All swirling designs enhanced the mixing process by generating substantial central swirl besides the small eddies formed from the jet interaction. Numerically, average uniformity improvement achieved in all cases studied was 46%, while the recommended geometry (football with four short rectangular swirlers, F4SR) is 16% better than plain football (FB), but loses pressure by 17%. Upon experimentation, F4SR had almost the same positive outcomes against plain football and SH by 24% and 47%, respectively. Finally, F4SR acts well at lower Re.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(10):101203-101203-9. doi:10.1115/1.4040189.

Salt gradient solar ponds are the ponds in which due to existence of saline and salt gradient layers, lower layers are denser and avoid the natural convection phenomenon to occur so that solar radiation energy can be stored in the lowest zone. In this study, one-dimensional (1D) and two-dimensional (2D) numerical approaches have been implemented to simulate unsteady buoyancy-driven flow of solar ponds. In 1D method, the pond has been investigated in terms of the layers thicknesses so that the variation of temperature is calculated by energy conservation equation. The formulized radiation term was used as energy source term in energy equation. The results of 1D approach were validated with an experimental study and then optimization was carried out to determine the maximum thermal efficiency for an interval of layers height. Since the stability of the solar pond cannot be determined by 1D simulation, a 2D approach was considered to show the stability for different nonconvective zone (NCZ) heights and different salt gradients. In 2D study, in order to investigate hydrodynamic and thermal behavior of saltwater fluid, a numerical approach was used to simulate temperature gradients throughout the pond. The results of 2D numerical method are validated with an experimental data. The effect of linear and nonlinear salt gradient was considered.

Commentary by Dr. Valentin Fuster

Research Papers: Energy Systems Analysis

J. Energy Resour. Technol. 2018;140(10):102001-102001-8. doi:10.1115/1.4040108.

This study makes energy and exergy analysis of a sample organic Rankine cycle (ORC) with a heat exchanger which produces energy via a geothermal source with a temperature of 140 °C. R600a is preferred as refrigerant to be used in the cycle. The changes in exergy destructions (of irreversibility) and exergy efficiencies in each cycle element are calculated in the analyses made based on the effectiveness of heat exchanger used in cycle and evaporator temperature changing between 60 and 120 °C for fixed pinch point temperature differences in evaporator and condenser. Parameters showing system performance are assessed via second law approach. Effectiveness of heat exchanger and temperature of evaporator are taken into consideration within the scope of this study, and energy and exergy efficiencies of cycle are enhanced maximum 6.87% and 6.21% respectively. Similarly, exergy efficiencies of evaporator, heat exchanger, and condenser are increased 4%, 82%, and 1.57%, respectively, depending on the effectiveness of heat exchanger and temperature of evaporator.

Commentary by Dr. Valentin Fuster

Research Papers: Fuel Combustion

J. Energy Resour. Technol. 2018;140(10):102201-102201-17. doi:10.1115/1.4039745.

Fuel injection parameters such as fuel injection pressure (FIP) and start of main injection (SoMI) timings significantly affect the performance and emission characteristics of a common rail direct injection (CRDI) diesel engine. In this study, a state-of-the-art single cylinder research engine was used to investigate the effects of fuel injection parameters on combustion, performance, emission characteristics, and particulates and their morphology. The experiments were carried out at three FIPs (400, 700, and 1000 bar) and four SoMI timings (4 deg, 6 deg, 8 deg, and 10 deg bTDC) for biodiesel blends [B20 (20% v/v biodiesel and 80% v/v diesel) and B40 (40% v/v biodiesel and 60% v/v diesel)] compared to baseline mineral diesel. The experiments were performed at a constant engine speed (1500 rpm), without pilot injection and exhaust gas recirculation (EGR). The experimental results showed that FIP and SoMI timings affected the in-cylinder pressure and the heat release rate (HRR), significantly. At higher FIPs, the biodiesel blends resulted in slightly higher rate of pressure rise (RoPR) and combustion noise compared to baseline mineral diesel. All the test fuels showed relatively shorter combustion duration at higher FIPs and advanced SoMI timings. The biodiesel blends showed slightly higher NOx and smoke opacity compared to baseline mineral diesel. Lower particulate number concentration at higher FIPs was observed for all the test fuels. However, biodiesel blends showed emission of relatively higher number of particulates compared to baseline mineral diesel. Significantly lower trace metals in the particulates emitted from biodiesel blend fueled engine was an important finding of this study. The particulate morphology showed relatively smaller number of primary particles in particulate clusters from biodiesel exhaust, which resulted in relatively lower toxicity, rendering biodiesel to be more environmentally benign.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(10):102202-102202-10. doi:10.1115/1.4040010.

Gasoline compression ignition (GCI) offers the potential to reduce criteria pollutants while achieving high fuel efficiency. This study aims to investigate the fuel chemical and physical properties effects on GCI operation in a heavy-duty diesel engine through closed-cycle, three-dimensional (3D) computational fluid dynamic (CFD) combustion simulations, investigating both mixing-controlled combustion (MCC) at 18.9 compression ratio (CR) and partially premixed combustion (PPC) at 17.3 CR. For this work, fuel chemical properties were studied in terms of the primary reference fuel (PRF) number (0–91) and the octane sensitivity (0–6) while using a fixed fuel physical surrogate. For the fuel physical properties effects investigation, six physical properties were individually perturbed, varying from the gasoline to the diesel range. Combustion simulations were carried out at 1375 RPM and 10 bar brake specific mean pressure (BMEP). Reducing fuel reactivity was found to influence ignition delay time (IDT) more significantly for PPC than for MCC. 0D IDT calculations suggested that the fuel reactivity impact on IDT diminished with an increase in temperature. Moreover, higher reactivity gasolines exhibited stronger negative coefficient (NTC) behavior and their IDTs showed less sensitivity to temperature change. In addition, increasing octane sensitivity was observed to result in higher fuel reactivity and shorter IDT. Under both MCC and PPC, all six physical properties showed little impact on global combustion behavior, NOx, and fuel efficiency. Among the physical properties investigated, only density showed a notable effect on soot emissions. Increasing density led to higher soot due to deteriorated air entrainment into the spray and the slower fuel-air mixing process.

Commentary by Dr. Valentin Fuster

Research Papers: Petroleum Engineering

J. Energy Resour. Technol. 2018;140(10):102901-102901-11. doi:10.1115/1.4039981.

The phenomenon of liquefied natural gas (LNG) cargo weathering is considered in terms of the conditions influencing boil-off gas (BOG) rates during the offshore movements and handling of LNG on marine LNG carriers (LNGC), floating storage and regasification unit (FSRU), and floating storage units (FSU). The range of compositions (grades) of commercially traded LNG is significantly broader than the range of compositional changes caused by typical storage times for offshore LNG cargoes. The different nitrogen and natural gas–liquid concentrations of LNG cargoes (i.e., ethane and heavier C2+ components) significantly influence the impacts of weathering and ultimately determine whether the LNG delivered to customers is within sales specifications or not. The BOG from LNG in storage is richer in methane and nitrogen; if nitrogen is present in the LNG, otherwise just richer in methane, than the LNG from which it is derived. This leads to the LNG becoming richer in the C2+ components as ageing progresses. LNG weathering is shown not to play a significant role in the rollover phenomenon of LNG moved and stored offshore, because nitrogen contents are low (typically < 1.0%) and auto-stratification is rarely an issue. LNG stored for long periods on FSU (greater than 8 weeks, or so) experiences significant weathering effects, but most LNG processed by FSRU (and most FSU) has a residence time of less than 30 days or so, in which case weathering has only minor operational impacts. Weathering rates and LNG compositional changes on FSRU for different LNG grades are provided.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(10):102902-102902-11. doi:10.1115/1.4039870.

Heavy oil is an important hydrocarbon resource that plays a great role in petroleum supply for the world. Co-injection of steam and flue gas can be used to develop deep heavy oil reservoirs. In this paper, a series of gas dissolution experiments were implemented to analyze the properties variation of heavy oil. Then, sand-pack flooding experiments were carried out to optimize injection temperature and injection volume of this mixture. Finally, three-dimensional (3D) flooding experiments were completed to analyze the sweep efficiency and the oil recovery factor of flue gas + steam flooding. The role in enhanced oil recovery (EOR) mechanisms was summarized according to the experimental results. The results show that the dissolution of flue gas in heavy oil can largely reduce oil viscosity and its displacement efficiency is obviously higher than conventional steam injection. Flue gas gradually gathers at the top to displace remaining oil and to decrease heat loss of the reservoir top. The ultimate recovery is 49.49% that is 7.95% higher than steam flooding.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(10):102903-102903-10. doi:10.1115/1.4040059.

Optimization of oil production from petroleum reservoirs is an interesting and complex problem which can be done by optimal control of well parameters such as their flow rates and pressure. Different optimization techniques have been developed yet, and metaheuristic algorithms are commonly employed to enhance oil recovery projects. Among different metaheuristic techniques, the genetic algorithm (GA) and the particle swarm optimization (PSO) have received more attention in engineering problems. These methods require a population and many objective function calls to approach more the global optimal solution. However, for a water flooding project in a reservoir, each function call requires a long time reservoir simulation. Hence, it is necessary to reduce the number of required function evaluations to increase the rate of convergence of optimization techniques. In this study, performance of GA and PSO are compared with each other in an enhanced oil recovery (EOR) project, and Newton method is linked with PSO to improve its convergence speed. Furthermore, hybrid genetic algorithm-particle swarm optimization (GA-PSO) as the third optimization technique is introduced and all of these techniques are implemented to EOR in a water injection project with 13 decision variables. Results indicate that PSO with Newton method (NPSO) is remarkably faster than the standard PSO (SPSO). Also, the hybrid GA-PSO method is more capable of finding the optimal solution with respect to GA and PSO. In addition, GA-PSO, NPSO, and GA-NPSO methods are compared and, respectively, GA-NPSO and NPSO showed excellence over GA-PSO.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(10):102904-102904-8. doi:10.1115/1.4040200.

Oil-in-water (O/W) emulsions are expected to be formed in the process of surfactant flooding for heavy oil reservoirs in order to strengthen the fluidity of heavy oil and enhance oil recovery. However, there is still a lack of detailed understanding of mechanisms and effects involved in the flow of O/W emulsions in porous media. In this study, a pore-scale transparent model packed with glass beads was first used to investigate the transport and retention mechanisms of in situ generated O/W emulsions. Then, a double-sandpack model with different permeabilities was used to further study the effect of in situ formed O/W emulsions on the improvement of sweep efficiency and oil recovery. The pore-scale visualization experiment presented an in situ emulsification process. The in situ formed O/W emulsions could absorb to the surface of pore-throats, and plug pore-throats through mechanisms of capture-plugging (by a single emulsion droplet) and superposition-plugging or annulus-plugging (by multiple emulsion droplets). The double-sandpack experiments proved that the in situ formed O/W emulsion droplets were beneficial for the mobility control in the high permeability sandpack and the oil recovery enhancement in the low permeability sandpack. The size distribution of the produced emulsions proved that larger pressures were capable to displace larger O/W emulsion droplets out of the pore-throat and reduce their retention volumes.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(10):102905-102905-11. doi:10.1115/1.4040201.

Numerical simulation and prediction studies on horizontal well performances in gas reservoir are foundation for optimizing horizontal well completion process. To gain more understanding on this theory, a steady-state reservoir model coupling with wellbore is developed in the fractured gas reservoirs with bottom-water and different fracture intensities to predict the horizontal well performances. Based on the equivalent flow assumption, the fractured porous medium is transformed into anisotropic porous medium so that the gas reservoir flow model can be developed as a new model that incorporates formation permeability heterogeneity, reservoir anisotropy, and gas reservoir damage. The wellbore flow model which considers pressure drops in the tubing is applied. We compare this paper model solutions for inflow profile along the well to the numerical solutions obtained from a commercial simulator (ECLIPSE 2011), and the result shows a very good agreement. Moreover, sensitive analysis, in terms of various linear densities of fractures, matrix permeability, fracture width, and wellbore pressure drop, is implemented. The results show that the new model developed in this study can obtain a more practical representation to simulate the horizontal wells performance in fractured gas reservoir with different fracture intensities and bottom-water, thus can be used to optimize the parameters in horizontal well completion of fractured gas reservoirs with different fracture intensities and bottom-water.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(10):102906-102906-10. doi:10.1115/1.4039443.

Acidizing of carbonate reservoirs is a common technique used to restore and enhance production by dissolving a small fraction of the rock to create highly conductive channels. Literature review reveals that most acidizing studies are focused on acid injection at a constant volumetric rate (CVR) instead of at a constant injection pressure (CIP). Therefore, the primary objective of the present work is to investigate the benefits and recommended applications of each technique. The study analyzes dissolution patterns and wormhole propagation rate. A coreflood study was conducted using different Indiana limestone cores to assess both techniques. Additionally, a two-dimensional (2D) wormhole model was used to mathematically describe the acidizing phenomena. The algorithm is based on a 2D radial flow system that iterates time to quantify wormhole propagation and injection rate. Wormhole velocity is calculated by an empirical laboratory model that depends on two parameters measured from core flow testing. Therefore, the algorithm captures the essential physics and chemistry of the acid reaction in a carbonate porous medium. The study confirmed that conical, wormhole, and branched types of acid dissolution patterns exist for both techniques (CVR or CIP). Unlike in the CVR technique, dissolution patterns during the CIP technique can change and tend toward a branched dissolution regime. The CIP technique required a lower acid volume to achieve a breakthrough in the conical dissolution regime and a higher acid volume to achieve a breakthrough in the branched dissolution regime compared to the CVR technique. In a dominant wormhole pattern, both techniques required nearly the same acid volume for a breakthrough. A computed tomography (CT) scan confirmed that the CIP technique developed a uniform wormhole at a low initial injection rate. For the CIP technique, the acid injection rate increased exponentially with the volume of the acid injected. The CIP technique is recommended for a low-permeability reservoir where acid injection at a high rate is not possible to avoid face dissolution wormhole patterns. On the other hand, the CVR technique is recommended for a medium—to the high-permeability reservoir where high acid injection rate can be achieved.

Commentary by Dr. Valentin Fuster

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