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RESEARCH PAPERS

J. Energy Resour. Technol. 1994;116(4):251-257. doi:10.1115/1.2906450.

This paper describes the axial, torsional, and transient buckling vibratory models developed for the selection of optimum core rod size. The axial and torsional vibratory core rod simulator (VCRS) models are coupled by way of a transient buckling wave which propagates over the length of the core rod. This paper reports the frequencies and magnitudes of the stresses in the 101 core rod now in use. In addition, four core bit vibratory forcing functions for thrust and torque were developed. The thrust and torque frequencies and magnitudes for the bit forcing functions were extracted from full-size laboratory core bit tests with fast Fourier transforms. The natural frequencies of the core rod were determined with closed-form solution models and were confirmed with a finite element model. Finally, a selection of core rod sizes were modeled to determine the best size to minimize damaging stress which stems from vibration.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1994;116(4):258-267. doi:10.1115/1.2906451.

A corehead was designed, manufactured and tested to reduce fluid invasion of the core. This is obtained by minimizing the exposure time of the core to the drilling fluid in increasing the rate of penetration (ROP). The design incorporates a medium heavyset polycrystalline diamond compact (PDC) cutting structure developed in accordance with cutting models and balancing methods used for drill bits. The highest ROP is achieved by a particular hydraulic design: flow ports shape and positioning to clean the cutting structure enhance the drilled cuttings removal while preventing drilling fluid in the throat of the corehead. Moreover, an internal lip works with a special inner barrel shoe to effectively seal off mud flow from the throat. All the design features have been subjected to laboratory tests, including measurement of pressure drop across the corehead and flow visualization studies. Flow visualization tests include high-speed filming of the flow and paint tracing to indicate the special flow pattern. In conjunction with lab tests, a numerical simulation was performed using fluid dynamics software to optimize hydraulic parameters. The low invasion core bit has been used in numerous applications. The performance achieved was significantly better than the average achieved over a period of years using various PDC coreheads. The rate of penetration was increased by a factor of 4.8 and bit life by 2.3 (often with reusable condition).

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1994;116(4):268-272. doi:10.1115/1.2906452.

Single-cutter experiments have been performed to investigate the cutting and wear of thermally stable diamond (SYNDAX3) during rock cutting. Cutting forces increase linearly with depth of cut, but are unaffected by cutting speed. The wear of the cutter per mass of rock removed is found to decrease with increasing depth of cut. Excessive cutting speed is harmful to the cutter since both the cutter temperature and the change in cutter temperature per power input increase with cutting speed. In the cutting experiments, evidence of delayed fracturing is observed. For essentially constant cutting conditions, fractures develop in the cutter only after a significant amount of cutting is done. Damage of this type is very harmful to the cutter as cutter temperature rises and efficiency drops with increasing wear.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1994;116(4):273-277. doi:10.1115/1.2906453.

Experiments have been carried out to assess the rock-cutting ability of thermally stable diamond cutters of different sizes and shapes as individual cutting elements or on various drill bits. In general, penetration is proportional to weight on bit and cutter speed, with variations being dependent on cutter shape, size, and pressure environment. However, there is a marked difference in the penetration of the cutters into the rock at equivalent weight on cutter depending upon whether they are single or are mounted on a bit. It is believed that these large differences are related to the inability of the cuttings to escape from the gap between the bit and the rock face, i.e., poor cleaning. This observation holds promise that by improving the cleaning of the bit, even higher rates of penetration may be attained than the values achieved so far (800 ft/hr in Leuders limestone).

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1994;116(4):278-286. doi:10.1115/1.2906454.

The phase composition of coal combustion ashes is important in regard to their potential uses. Here it is shown that a combination of phase separation techniques with X-ray diffraction, thermogravimetric analysis, and conventional chemical analysis, including elemental analysis and wet chemistry, can be used to obtain a fairly complete phase composition. The application of these techniques is illustrated with an FBC ash sample, indicating the procedures used to achieve the identification of calcium silicates, aluminates and ferrites, and their quantitative estimates. A new method for the quantitative determination of CaO in presence of large amounts of Ca(OH)2 has been developed which uses phase separation as a previous step to chemical titration.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1994;116(4):287-289. doi:10.1115/1.2906455.
Abstract
Topics: Water , Wave energy
Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1994;116(4):290-296. doi:10.1115/1.2906456.

A general-purpose optimal operational planning system with a user-friendly interface is developed for the purpose of determining operational strategies of energy supply plants in a simple and rational manner. The system has the following main functions: (a) data registration, (b) graphical flow sheet editing, (c) automatic programming and optimization calculation, and (d) graphical representation of results. A graphical flowsheet editor enables an easy and flexible composition or change of plant configurations and their relevant data by using fundamental data registered in a database. Plant data composed by the editor are arranged automatically to carry out an optimization calculation, in which the operational strategy is determined so as to minimize an objective function, such as operational cost subject to energy demand requirements and other operational restraints. An application example shows that the system is a useful tool for the operational planning of energy supply plants from the viewpoints, not only of rationality, but also of task and time reduction.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1994;116(4):297-304. doi:10.1115/1.2906457.

A thermally driven heat pump using a solid/vapor adsorption/desorption compression process is thermodynamically analyzed. Heat regeneration between the two adsorbent beds is accomplished through the use of a circulating heat transfer (HX) fluid. Effective heat regeneration and system performance requires that steep thermal profiles or waves be established in the beds along the path of the HX-fluid flow direction. Previous studies by Shelton, Wepfer, and Miles have used square and ramp profiles to approximate the temperature profiles in the adsorbent beds, which, in turn, enable the thermodynamic performance of the heat pump to be computed. In this study, an integrated heat transfer and thermodynamic model is described. The beds are modeled using a two-temperature approach. A partial differential equation for the lumped adsorbent bed and tube is developed to represent the bed temperature as a function of time and space (along the flow direction), while a second partial differential equation is developed for the heat transfer fluid to represent the fluid temperature as a function of time and space (along the flow direction). The resulting differential equations are nonlinear due to pressure and temperature-dependent coefficients. Energy and mass balances are made at each time step to compute the bed pressure, mass, adsorption level, and energy changes that occur during the adsorption and desorption process. Using these results, the thermodynamic performance of the heat pump is calculated. Results showing the heat pump’s performance and capacity as a function of the four major dimensionless groups, DR, Pe, Bi, and KAr , are presented.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1994;116(4):305-311. doi:10.1115/1.2906458.

In conventional energy conversion processes, the fuel combustion is usually highly irreversible, and is thus responsible for the low overall efficiency of the power generation process. The energy conversion efficiency of the combustion process can be improved if immediate contact of fuel and oxygen is prevented and an oxygen carrier is used. In a previous paper (Harvey et al., 1992), a gas turbine cycle was investigated in which part of the exhaust gases—consisting mainly of CO2 , H2 O, and N2 —are recycled and used as oxygen-carrying components. For the optimized process, a theoretical thermal efficiency of 66.3 percent was achieved, based on the lower heating value (LHV) of the methane fuel. A detailed second-law analysis of the cycle revealed that, although the exergy losses associated with the fuel oxidation were significantly less than those associated with conventional direct fuel combustion methods, these losses were still a major contributor to the overall losses of the system. One means to further improve the exergetic efficiency of a power cycle is to utilize fuel cell technology. Significant research is currently being undertaken to develop fuel cells for large-scale power production. High-efficiency fuel cells currently being investigated use high-temperature electrolytes, such as molten carbonates (~ 650°C) and solid oxides (usually doped zirconia, ~1000°C). Solid oxide fuel cells (SOFC) have many features that make them attractive for utility and industrial applications. In this paper, we will therefore consider SOFC technology. In view of their high operating temperatures and the incomplete nature of the fuel oxidation process, fuel cells must be combined with conventional power generation technology to develop power plant configurations that are both functional and efficient. In this paper, we will show how monolithic SOFC (MSOFC) technology may be integrated into the previously described gas turbine cycle using recycled exhaust gases as oxygen carriers. An optimized cycle configuration will be presented based upon a detailed cycle analysis performed using Aspen Plus™ process simulation software (Aspen Technology, 1991) and a MSOFC fuel cell simulator developed by Argonne National Labs (Ahmed et al., 1991). The optimized cycle achieves a theoretical thermal efficiency of 77.7 percent, based on the LHV of the fuel.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1994;116(4):312-318. doi:10.1115/1.2906459.

In conventional energy conversion processes, the fuel combustion is usually highly irreversible, and is thus responsible for the low overall efficiency of the power generation process. The energy conversion efficiency can be improved if immediate contact of air and fuel is prevented. One means to prevent this immediate contact is the use of fuel cell technology. Significant research is currently being undertaken to develop fuel cells for large-scale power production. High-temperature solid oxide fuel cells (SOFC) have many features that make them attractive for utility and industrial applications. However, in view of their high operating temperatures and the incomplete nature of the fuel oxidation process, such fuel cells must be combined with conventional power generation technology to develop power plant configurations that are both functional and efficient. Most fuel cell cycles proposed in the literature use a high-temperature fuel cell running at ambient pressure and a steam bottoming cycle to recover the waste heat generated by the fuel cell. With such cycles, the inherent flexibility and shorter start-up time characteristics of the fuel cell are lost. In Part I of this paper (Harvey and Richter, 1994), a pressurized cycle using a solid oxide fuel cell and an integrated gas turbine bottoming cycle was presented. The cycle is simpler than most cycles with steam bottoming cycles and more suited to flexible power generation. In this paper, we will discuss this cycle in more detail, with an in-depth discussion of all cycle component characteristics and losses. In particular, we will make use of the fuel cell’s internal fuel reforming capability. The optimal cycle parameters were obtained based on calculations performed using Aspen Technology’s ASPEN PLUS process simulation software and a fuel cell simulator developed by Argonne National Laboratory (Ahmed et al., 1991). The efficiency of the proposed cycle is 68.1 percent. A preliminary economic assessment of the cycle shows that it should compare favorably with a state-of-the-art combined cycle plant on a cost per MWe basis.

Commentary by Dr. Valentin Fuster

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