J. Energy Resour. Technol. 2006;129(2):81-88. doi:10.1115/1.2718576.

A temperature response factors model of vertical thermal energy extraction boreholes is presented to evaluate electricity generation from underground coal fires and waste banks. Sensitivity and life-cycle cost analyses are conducted to assess the impact of system parameters on the production of 1 MW of electrical power using a theoretical binary-cycle power plant. Sensitivity analyses indicate that the average underground temperature has the greatest impact on the exiting fluid temperatures from the ground followed by fluid flow rate and ground thermal conductivity. System simulations show that a binary-cycle power plant may be economically feasible at ground temperatures as low as 190°C.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(2):89-95. doi:10.1115/1.2718577.

The popular Joshi model slightly overestimated the flow resistance of a horizontal well. As a result of this, the Joshi model underpredicts the productivity index (PI) of a horizontal well by a few percent. In the extreme case in which vertical permeability goes to zero, the Joshi model predicts a 0.0 stb∕day-psi PI, which is wrong. In this paper, the flow for a horizontal well is divided into three flows: the flow in the reservoir above the horizontal wellbore, the flow in the reservoir with a thickness of 2rw containing the horizontal wellbore, and a flow in the reservoir below the horizontal well bore. The second flow is assumed to be pure horizontal flow. The first and third flows can be further divided into a horizontal flow and a vertical flow. In this paper, the equation for each flow is provided, and then combining these flows we give the equation to calculate the effective PI of horizontal wells. In addition, when the horizontal wellbore is not located at the h∕2 midpoint of a reservoir, the Joshi model predicts an increasing PI, which is intuitively and mathematically an incorrect trend. This paper derives a new equation to compute the PI of horizontal wells when the wellbore is eccentric relative to the reservoir midpoint. The new equation generates the correct trend.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(2):96-101. doi:10.1115/1.2718578.

In this paper, an advanced energy-saving petroleum machinery, the hydraulic energy-recovering workover rig, is researched. The equipped power of this rig is only one third of an ordinary rig, and this rig can also recover and reuse the potential energy which is released by the pipestring when lowered. The special working theory of this rig is introduced. An energy-saving analysis is conducted. Analysis shows that when lowering the pipestring which weighs 260kN, the energy recovered by this rig is about 240×106J. The mathematical model of lifting the pipestring is established and a simulation analysis is conducted. Through simulation, some conclusions are obtained: (1) the lighter the pipestring the shorter the pipestring lifting time; (2) the smaller the throttle valve path area the longer the pipestring lifting time; (3) the smaller the air vessel volume the shorter the pipestring lifting time. The actual measurement results prove that the simulation results are right.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(2):102-106. doi:10.1115/1.2718579.

The development of methane hydrate (MH), which exists under the ocean floor, has recently been brought to public attention. However, the production technology has not yet been established. It is important to understand the decomposition phenomenon of MH for an investigation of the safety and the profitability of production systems. In this research, the gas hydrate decomposition rate in flowing water was measured using HCFC141b hydrate as a substitute for MH. When the water temperature was higher than the boiling point of the decomposition gas, it was observed that the decomposition gas increased the decomposition rate. Moreover, the decomposition phenomenon was simulated by the lattice gas automaton method in order to establish the technique which analytically estimates the decomposition rate. The validity of the simulation method was shown by comparing the experiments. Furthermore, the formula between Reynolds number and Nusselt number was obtained, which express the heat transfer around the gas hydrate lump.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(2):107-116. doi:10.1115/1.2718581.

The paper describes the logic of the vehicle’s power management unit (VMU) for the prototype configuration of the LETHE™ (low emissions turbo hybrid electric) vehicle designed by the University of Roma 1. The theoretical and practical feasibility of the concept (a series hybrid in which the thermal engine is a small turbo-gas and the traction is fully electric) was demonstrated in a series of previous works by the same authors, and some experimental tests were conducted at the ENEA-Casaccia Laboratories on a small 45 kW gas turbine set, to investigate the performance of the propulsive unit (turbine plus batteries and electrical motor) under the European vehicular emission (ECE) tests. After successful completion of these tests, a further analysis was carried out to identify an optimal hybridization ratio with respect both to driveability and fuel consumption: the results led to the conclusion that such an absolute optimal configuration does not exist, because not only the system performance, but also the absolute and relative sizes (i.e., nameplate power) of turbines and battery pack depend largely on the type of the proposed driving mission of the car. In the final configuration discussed in this paper, the vehicle is equipped with an additional energy storage device, a compact ultra-fast flywheel, to partially compensate for the low recharge capability of the Pb-acid batteries and to exploit better brake recovery for futher reduction of the fuel consumption. The present status report describes the VMU control logic, the individual components of the propulsive system and the proposed chassis configuration for the prototype.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(2):117-124. doi:10.1115/1.2719203.

The power output of gas turbines (GT) reduces greatly with the increase of the inlet air temperature. This is a serious problem because gas turbines have been used traditionally to provide electricity during the peak power demands, and the peak power demands in many areas occur on summer afternoons. An aquifer thermal energy storage (ATES) was employed for cooling of the inlet air of the GT. Water from a confined aquifer was cooled in winter and was injected back into the aquifer. The stored chilled water was withdrawn in summer to cool the GT inlet air. The heated water was then injected back into the aquifer. A 20MW GT power plant with 6 and 12h of operation per day, along with a two-well aquifer, was considered for analysis. The purpose of this investigation was to estimate the GT performance improvement. The conventional inlet air cooling methods such as evaporative cooling, fogging and absorption refrigeration were studied and compared with the ATES system. It was shown that for 6h of operation per day, the power output and efficiency of the GT on the warmest day of the year could be increased from 16.5 to 19.7MW and from 31.8% to 34.2%, respectively. The performance of the ATES system was the best among the cooling methods considered on the warmest day of the year. The use of ATES is a viable option for the increase of gas turbines power output and efficiency, provided that suitable confined aquifers are available at their sites. Air cooling in ATES is not dependent on the wet-bulb temperature and therefore can be used in humid areas. This system can also be used in combined cycle power plants.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(2):125-133. doi:10.1115/1.2719204.

One of the most important cycles for electricity generation from geothermal energy is the double-flash cycle. Approximately 25% of the total geothermal based electricity generation all over the world comes from double-flash geothermal power plants. In this paper, performance analysis of a hypothetical double-flash geothermal power plant is performed and variations of fundamental characteristics of the plant are examined. In the performance analysis, initially, optimum flashing pressures are determined, and energy and exergy values of the base points of the plant are calculated. In addition, first and second law efficiencies of the power plant are calculated. Main exergy destruction locations are determined and these losses are illustrated in an exergy flow diagram. For these purposes, it is assumed that a hypothetical double-flash geothermal power plant is constructed in the conditions of western Turkey. The geothermal field where the power plant will be built produces geofluid at a temperature of 210°C and a mass flow rate of 200kgs. According to simulation results, it is possible to produce 11,488kWe electrical power output in this field. Optimum first and second flashing pressures are determined to be 530kPa and 95kPa, respectively. Based on the exergy of the geothermal fluid at reservoir, overall first and second law efficiencies of the power plant are also calculated to be 6.88% and 28.55%, respectively.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2005;129(2):134-143. doi:10.1115/1.2141636.

Preliminary study has shown that the flue gas recirculation (FGR) is one of the effective ways to reduce the nitric oxides (NOx) emission in industrial furnaces. The sensitivity of the NOx emission from a FGR industrial furnace to the change in three major furnace input variables—inlet combustion air mass flow rate, inlet combustion air temperature, and pressure head of the FGR fan—is investigated numerically in this study. The investigation is carried out in frequency domain by superimposing sinusoidal signals of different frequencies on to the furnace control inputs around the design operating condition, and observing the frequency responses. The results obtained in this study can be used in the design of active combustion control systems to reduce NOx emission. The numerical simulation of the turbulent non-premixed combustion process in the furnace is conducted using a moment closure method with the assumed β probability density function for the mixture fraction. The combustion model is derived based on the assumption of instantaneous full chemical equilibrium. The discrete transfer radiation model is chosen as the radiation heat transfer model, and the weighted-sum-of-gray-gases model is used to calculate the absorption coefficient.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(2):144-151. doi:10.1115/1.2719205.

The fluidized bed combustion (FBC) process, used in power generation, can handle a variety of fuels. However, the range of fuels that can be accommodated by an FBC boiler system is affected by the ability of the fuel, sorbent, and ash-handling equipment to move the required solids through the boiler. Of specific interest is the bottom ash handling equipment, which must have sufficient capacity to remove ash from the system in order to maintain a constant bed inventory level, and must have sufficient capability to cool the ash well below the bed temperature. Quantification of a fuel’s bottom ash removal requirements can be useful for plant design. The effect of fuel properties, on the rate of bottom ash production in a laboratory FBC test system was examined. The work used coal products ranging in ash content from 20to40+wt.%. The system’s classification of solids by particle size into flyash and bottom ash was characterized using a partition curve. Fuel sizing was compared to the partition curve, and fuels were fractionated by particle size. Fuel fractions in the size range characteristic of bottom ash were further analyzed for distributions of ash content with respect to specific gravity, using float sink tests. The fuel fractions were then ashed in a fixed bed. In each case, the highest ash content fraction produced ash with the coarsest size consist (characteristic of bottom ash). The lower ash content fractions were found to produce ash in the size range characteristic of flyash, suggesting that the high ash content fractions were largely responsible for the production of bottom ash. The contributions of the specific gravity fractions to the composite ash in the fuels were quantified. The fuels were fired in the laboratory test system. Fuels with higher amounts of high specific gravity particles, in the size ranges characteristic of bottom ash, were found to produce more bottom ash, indicating the potential utility of float sink methods in the prediction of bottom ash removal requirements.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(2):152-158. doi:10.1115/1.2719208.

Coal resource is abundant in China, while the reserves of natural gas and petroleum are limited. Due to the rapid increase in the number of automobiles, a competitive way to produce liquid fuels from coal is urgently needed in China. A so-called “coal topping process” is under development at the Institute of Process Engineering, Chinese Academy of Sciences, from which liquid products can be obtained by flash pyrolysis in an integrated circulating fluidized bed system. In order to achieve a high yield of liquid products from high volatile coal, controlling the residence time of coal particles and produced gas may be of importance for minimizing the degree of the secondary reactions; i.e., polymerization and cracking of the liquid products. Experiments of the flash pyrolysis of coal have been conducted in an entrained bed reactor, which is especially designed to study the influence of the coal particle residence time on the product distribution. The results show that the gaseous, liquid, and solid product distribution, the gas compositions as well as the liquid compositions depend strongly on the gas and particle residence time.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(2):159-167. doi:10.1115/1.2719209.

Modeling of a natural circulation boiler for a coal-fired thermal power station is presented here. The boiler system is divided into seven subcomponents, and for each section, models based on conservation of mass, momentum, and energy are formulated. The pressure drop at various sections and the heat transfer coefficients are computed using empirical correlations. Solutions are obtained by using SIMULINK . The model is validated by comparing its steady state and dynamic responses with the actual plant data. Open loop responses of the model to the step changes in the operating parameters, such as pressure, temperature, steam flow, feed water flow, are also analyzed. The present model can be used for the development and design of effective boiler control systems.

Commentary by Dr. Valentin Fuster

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