Showing papers in "Journal of Energy Resources Technology-transactions of The Asme in 1996"

JP-8+100: The Development of High-Thermal-Stability Jet Fuel

Abstract: Jet fuel requirements have evolved over the years as a balance of the demands placed by advanced aircraft performance (technological need), fuel cost (economic factors), and fuel availability (strategic factors). In a modern aircraft, the jet fuel not only provides the propulsive energy for flight, but also is the primary coolant for aircraft and engine subsystems. To meet the evolving challenge of improving the cooling potential of jet fuel while maintaining the current availability at a minimal price increase, the US Air Force, industry, and academia have teamed to develop an additive package for JP-8 fuels. This paper describes the development of an additive package for JP-8, to produce JP-8+100. This new fuel offers a 55 C increase in the bulk maximum temperature (from 325 F to 425 F) and improves the heat sink capability by 50%. Major advances made during the development JP-8 + 100 fuel include the development of several new quantitative fuel analysis tests, a free radical theory of autooxidation, adaptation of new chemistry models to computational fluid dynamics programs, and a nonparametric statistical analysis to evaluate thermal stability. Hundreds of additives were tested for effectiveness, and a package of additives was then formulated for JP-8 fuel.more » This package has been tested for fuel system materials compatibility and general fuel applicability. To date, the flight testing ha shown an improvement in thermal stability of JP-8 fuel. This improvement has resulted in a significant reduction in fuel-related maintenance costs and a threefold increase in mean time between fuel-related failures. In this manner, a novel high-thermal-stability jet fuel for the 21st century has been successfully developed.« less

Application of a Genetic Algorithm to Wind Turbine Design

Abstract: This paper presents an optimization procedure for stall-regulated horizontal-axis wind-turbines. A hybrid approach is used that combines the advantages of a genetic algorithm and an inverse design method. This method is used to determine the optimum blade pitch and blade chord and twist distributions that maximize the annual energy production. To illustrate the method, a family of 25 wind turbines was designed to examine the sensitivity of annual energy production to changes in the rotor blade length and peak rotor power. Trends are revealed that should aid in the design of new rotors for existing turbines. In the second application, a series of five wind turbines was designed to determine the benefits of specifically tailoring wind turbine blades for the average wind speed at a particular site. The results have important practical implications related to rotors designed for the Midwest versus those where the average wind speed may be greater.

Performance of a High-Solidity Wells Turbine for an OWC Wave Power Plant

Abstract: The paper describes an experimental investigation, and presents the results of the aerodynamic performance of a high-solidity Wells turbine for a wave power plant. A monoplane turbine of 0.6 m rotor diameter with guide vanes was built and tested. The tests were conducted in unidirectional steady airflow. Measurements taken include flow rate, pressure drop, torque, and rotational speed, as well as velocity and pressure distributions. Experimental results show that the presence of guide vanes can provide a remarkable increase in turbine efficiency.

A Revised Decomposition Method for MILP Problems and Its Application to Operational Planning of Thermal Storage Systems

Abstract: A revised decomposition method for solving large-scale mixed-integer linear programming (MILP) problems with block angular structure is presented to efficiently conduct the operational planning of thermal storage systems. The fundamental algorithm adopted here is composed of solving large-scale linear programming (LP) master problems by the Dantzig-Wolfe decomposition method and small-scale MILP subproblems by the branch and bound method, and these problems are solved repeatedly until an optimality or suboptimality criterion is satisfied. As one of the revision strategies to improve computation efficiency, a two-phase approach is introduced, by which a next LP master problem can be solved efficiently by utilizing the results of a previous one. An illustrative example on a heat supply system for district heating and cooling is given to show the effectiveness of the above revision strategy. A practical example on a heat supply system with multiple thermal storage tanks for brewing is also presented.

Heat Transfer in a High-Temperature Packed Bed Thermal Energy Storage System—Roles of Radiation and Intraparticle Conduction

Abstract: A model is developed and experimentally verified to study the heat transfer in a high-temperature packed bed thermal energy storage system utilizing zirconium oxide pellets. The packed bed receives flue gas at elevated temperatures varying with time during the storage process and utilizes air for the recovery process. Both convection and radiation are included in the model of the total heat transfer between the gas and the pellets. It is found that thermal radiation and intraparticle conduction do not play a major role in the overall heat transfer in the packed bed under the specified operating conditions. An uncertainty analysis is performed to investigate the propagation of the uncertainties in the variables to the overall uncertainty in the model predictions and the experimental results.

Combined Cycle Gas Turbine Power Plant With Coal Gasification and Solid Oxide Fuel Cell

Abstract: The United States has extensive coal resources; thus, it is important to consider coal as a fuel for electric power production. This work presents a theoretical study of a novel high-efficiency coal-fired power plant. In the proposed combined cycle power plant, the Conoco coal gasification process is linked with solid oxide fuel cells (SOFC) and state-of-the-art gas turbines. The overall efficiency of such a plant can be around 60 percent, considering realistic heat, pressure, and other losses in the different components of the plant. If an additional steam turbine is incorporated, the overall efficiency can be increased to about 62 percent.

An Investigation of Lean Combustion in a Natural Gas-Fueled Spark-Ignited Engine

Abstract: Natural gas has been used extensively as an engine fuel in gas pipeline transmission applications and, more recently, as a fuel for transportation applications including both light-duty and heavy-duty vehicles. The objective of this work was to investigate the performance and emission characteristics of natural gas in an original equipment manufacturer (OEM), light-duty, spark-ignited engine being operated in the lean fueling regime and compare the operation with gasoline fueling cases. Data were acquired for several operating conditions of speed, throttle position, air-fuel equivalence ratio, and spark timing for both fuels. Results showed that for stoichiometric fueling, with a naturally aspirated engine, a power loss of 10 to 15 percent can be expected for natural gas over gasoline fueling. For lean operation, however, power increases can be expected for equivalence ratios below about Φ = 0.80 with natural gas fueling as compared to gasoline. Higher brake thermal efficiencies can also be expected with natural gas fueling with maximum brake torque (MBT) timings over the range of equivalence ratios investigated in this work. Coefficient of variation (COV) data based on the indicated mean effective pressure (IMEP) demonstrated that the engine is much less sensitive to equivalence ratio leaning for natural gas fueling as compared to gasoline cases. The lean limit for a COV of 10 percent was about Φ = 0.72 for gasoline and Φ = 0.63 for natural gas. Lean fueling resulted in significantly reduced NO x levels where a lower plateau for NO x concentrations was reached at Φ near or below 0. 70, which corresponded to about 220 ppm. For natural gas fueling, this corresponded to about 1.21 gm/(kW-h). Finally, with MBT timings, relatively short heat release durations were obtained for lean fueling with natural gas compared to gasoline.

Computer Simulation of a High-Temperature Thermal Energy Storage System Employing Multiple Families of Phase-Change Storage Materials

Abstract: Previous work by one of the authors entailed modeling of a packed bed thermal energy storage system utilizing phase-change materials (PCM). A principal conclusion reached is that the use of a single family of phase-change storage material may not in fact produce a thermodynamically superior system relative to one utilizing sensible heat storage material. This paper describes the model constructed for the high-temperature thermal energy storage system utilizing multiple families of phase-change materials and presents results obtained in the exercise of the model. Other factors investigated include the effect on system performance due to the thermal mass of the containment vessel wall and variable temperature of the flue gas entering the packed bed during the storage process. The results obtained indicate efficiencies for the system utilizing the five PCM families exceeding those for the single PCM family by as much as 13 to 26 percent. It was also found that the heat transfer to the containment vessel wall could have a significant detrimental effect on system performance.

Experiments in thermal spallation of various rocks

Abstract: The greatest limitation of the spallation process is its inability to spall (or to consistently spall) many rocks encountered in petroleum drilling and mining operations. The New Mexico Institute of Mining and Technology has conducted a series of experiments to investigate the possibility of expanding the use of the spallation process to the penetration of rocks generally considered not to be spallable. The methods used during this work were 1) spalling at temperatures below that produced by the stoichiometric burning of fuel oil and air, and 2) spalling by alternately heating and quenching the rock surfaces. No success was experienced in spalling at the lower temperatures, but initial tests showed the alternate heating and chilling system to be successful, particularly in penetrating travertine limestone. However, continued testing indicated that, unless the rocks are extremely uniform in composition, spalling will result in highly irregular holes or holes that cannot be directionally controlled.

Second Law Analysis of Adsorption Cycles With Thermal Regeneration

Abstract: Adsorption processes can be used for operating environment-friendly refrigeration cycles. When combined with the thermal regeneration process, these cycles can have quite high performance. The second law analysis of the adsorption cycles with thermal regeneration is fully developed. The different heat transports between heat transfer fluid and adsorbent, between adsorbate and condenser/evaporator heat sources, and between heat transfer fluid and heat sources are analyzed. The entropy balance is then completely established. Consistency between the first law and second law analysis is verified by the numerical values of the entropy productions. The optimal operation of an adsorber is then described, and the study of those optimal conditions lead to some correlation between the different internal entropy productions.

Reliability-Based Maintenance Strategies for Heat Exchangers Subject to Fouling

Abstract: Fouling in heat exchangers is an unavoidable by-product of the heat transfer process. The decision regarding periodic maintenance (cleaning) of the exchangers subject to fouling is generally based on thermal and economic behavior of the process. In this paper, a reliability-based maintenance strategy is discussed by incorporating the risk and scatter parameters of the linear random fouling growth model. In addition, the dimensionless cost-objective function is formulated by considering various cost elements for a heat exchanger used in a crude oil preheat train. The variation in the dimensionless cost Γ with reduced time t p /(t p + t down) is presented for different values of unit cost parameters γ 1 , γ 2 , and γ 3 representing additional fuel cost, antifoulant cost, and miscellaneous costs, respectively. Furthermore, the optimal cleaning cycle of the heat exchanger recently investigated by Casado (1990) is demonstrated to be a special case of the results presented in the paper.

A numerical method for power plant simulations

Abstract: This paper describes a highly flexible computerized method of calculating operating data in a power cycle. The computerized method presented here permits the study of steam, gas and combined plants. Its flexibility is not restricted by any defined cycle scheme. A power plant consists of simple elements (turbine, compressor, combustor chamber, pump, etc.). Each power plant component is represented by its typical equations relating to fundamental mechanical and thermodynamic laws, so a power plant system is represented by algebraic equations, which are the typical equations of components, continuity equations, and data concerning plant conditions. This equation system is not linear, but can be reduced to a linear equation system with variable coefficients. The solution is simultaneous for each component and it is determined by an iterative process. An example of a simple gas turbine cycle demonstrates the applied technique. This paper also presents the user interface based on MS-Windows. The input data, the results, and any characteristic parameters of a complex cycle scheme are also shown.

Storage of Fuel in Hydrates for Natural Gas Vehicles (NGVs)

Abstract: The need for alternative fuels to replace liquid petroleum-based fuels has been accelerated in recent years by environmental concerns, concerns of shortage of imported liquid hydrocarbon, and congressional prompting. The fact is accepted that natural gas is the cheapest, most domestically abundant, and cleanest burning of fossil fuels. However, socio-economical and technical handicaps associated with the safety and efficiency of on-board fuel storage inhibit its practical use in vehicles as an alternative fuel. A concept is presented for safely storing fuel at low pressures in the form of hydrates in natural gas vehicles. Experimental results lead to gas storage capacities of 143 to 159 volumes/volume. Vehicle travel range could be up to 204 mi. Controlled decomposition rate of hydrates is possible for feeding an automotive vehicle. Upon sudden pressure decrease in the event of a vehicle accident, the rate of release of hydrocarbons from the hydrates at constant temperature is 2.63 to 12.50 percent per min, slow enough to prevent an explosion or a fireball. A model is given for predicting the rates of gas release from hydrates in a vehicle wreck. A storage tank design is proposed and a process is suggested for forming and decomposing hydrates on-board vehicles. A consistent fuel composition is obtained with hydrates.

Incorporation of Phase-Change Materials Into a Ground Thermal Energy Storage System: Theoretical Study

Abstract: An investigation of a ground thermal energy storage system, which includes storage units containing phase-change materials (PCM), is presented. This study is related to a large-diameter helical heat exchanger, which is placed vertically in the ground. The PCM storage units under consideration have a cylindrical shell shape and are located inside and/or outside the helix. A modified numerical scheme for the solution of heat transfer in the ground, in the PCM units, and within the heat exchanger pipe, is presented. The theoretical results show that the thermal diffusivity of the PCM dominates the thermal performance of the system. Incorporation of PCM storage units containing paraffin wax results in a reduction of the thermal efficiency in comparison with a system not containing these units. However, incorporation of PCM having the same thermal diffusivity as of the soil results in a significant improvement of the thermal performance.

The Equivalence of Maximum Power and Minimum Entropy Generation Rate in the Optimization of Power Plants

Abstract: It is shown that to maximize the power output of a power plant is equivalent to minimizing the total entropy generation rate associated with the power plant. This equivalence is illustrated by using two of the oldest and simplest models of power plants with heat transfer irreversibilities. To calculate the total entropy generation rate correctly, one must recognize that the optimization process (e.g., the variability of the heat input) requires ``room to move,`` i.e., an additional, usually overlooked, contribution to the total entropy generation rate.

Thermal Destruction of Solid Wastes

Abstract: The United States generates the largest amount of solid waste per person in the world The old practice of direct landfilling and storage is receiving greater public resistance and is attributing to the search for alternative disposal methods The evergrowing problem of solid wastes requires environmentally benign and good public acceptance for the safe and ultimate disposal of the various kinds of solid wastes Incineration and various kinds of mass burn-type systems have been used to reduce the volume and mass of the wastes, which can be characterized by their operational temperature In all types of incineration systems, different kinds of gas clean-up devices are used to meet the local, state, and federal regulations for the gases before being released into the environment A major concern over these systems have been in the by-products produced from these systems during their normal design and off-design point of operation Indeed, the by-products generated from some incineration systems, under certain operational conditions, can be a health hazard and the solid residue may be leachable Recent trends in advanced thermal destruction systems are described which can destroy the solid waste to the molecular level Advanced systems can be designed to meet almost any emission standards The use of oxygen-enriched air in place of air for the combustion of gases released from the solid waste reduces the amount of effluent gas, and, hence, the reduced size and cost of the gas clean-up system The use of an excess enthalpy system offers attractive benefits in which the energy released from the waste is recycled back into the system under controlled conditions with the final desired objectives of reduced emissions, higher efficiency, and lower costs Thermal destruction of solid wastes using advanced techniques makes good technical, environmental, economical, and human health and safety The issues concerning recyclability, life cycle integration, and health effects from incineration are only expected to grow in the future

Energetic Fuels for Combustion Applications

Abstract: New fuels with high-energy-density are desirable for many combustion applications. Two types are reviewed in this paper, namely, mixtures resulting from addition of certain metallic or nonmetallic elements to conventional hydrocarbon fuels, and newly synthesized hydrocarbon fuels with strained molecular conformations or more densely packed molecular structures. Despite the favorable effects of high-energy content, these materials often exhibit low reactivity and their ability to improve the performance of practical combustion systems relies strongly on their interaction with the dynamics of the surrounding fluid flow. The intensity of the combustion processes of these materials is dictated, in general, by the melting, evaporation, pyrolysis, mixing, and exothermic reactions processes. Unlike other conventional hydrocarbon fuels, all these processes time scales are often comparable with each other, causing difficulties to devise simpler theoretical models to predict the combustion characteristics. Both the advances made in recent years and the needs for future research and development in the field of energetic fuels are discussed.

Effect of burner geometry on the blowout limits of jet diffusion flames in a co-flowing oxidizing stream

Abstract: In many industrial thermal applications, such as gas turbines and furnaces, a fuel jet is burned in the presence of a co-flowing oxidizing stream. This not only enhances the efficiency of the process by maintaining an uninterrupted supply of thermal energy, but it reduces the flame length and NO{sub x} emissions. The effects of changes in the jet nozzle geometry, i.e., nozzle shape and lip thickness, on the blowout limits of jet diffusion flames in a co-flowing air stream were experimentally investigated for a range of co-flow air stream velocities. Circular and elongated nozzles of different axes ratios were employed. Preliminary results showed that nozzles with low major-to-minor axes ratios improved, while high ratios reduced the blowout limit of attached flames compared with that for an equivalent circular nozzle. The nozzle shape had no apparent influence on the blowout limits lifted flames and the limiting stream velocity. The experimental blowout limits of lifted flames were found to be a function of the co-flowing stream velocity and jet discharge area. On the other hand, the stability of attached flames was a function of the co-flowing stream velocity, jet discharge area as well as the nozzle shape. The effect of premixing amore » fuel with the surrounding air was also studied. Generally, the introduction of auxiliary fuel into the surrounding stream either increased or decreased the blowout limit depending on the type of flame stabilization mechanism prior to blowout. The stability mechanism of the flame was found to be a function of the co-flow stream velocity and the auxiliary fuel employed. Methane was employed as the jet fuel, while methane, propane, ethylene, and hydrogen were used as auxiliary fuels in the co-flowing stream.« less

A New Approach for Determining In-Situ Rock Strength While Drilling

Abstract: A new method is developed for determining rock strength while drilling with a tri-cone roller bit. The technique employs the well-known drill-off test, using commonly measured drilling data, and does not require knowledge of drill bit parameters. The method was evaluated with drill-off test data collected from the Gas Research Institute's SFE (staged field experiment) well no. 4 and compared with calculated in-situ compressive rock strengths provided by Hareland (1992). The results indicate that the proposed approach produces reasonable results and is easily implemented.

An Analytical Comparison of Adsorption and Vapor Compression Air Conditioners for Electric Vehicle Applications

Abstract: This paper shows an analysis of the applicability of an adsorption system for electric vehicle (EV) air conditioning. Adsorption systems are designed and optimized to provide the required cooling for four combinations of vehicle characteristics and driving cycles. The resulting adsorption systems are compared with vapor compression air conditioners that can satisfy the cooling load. The objective function is the overall system weight, which includes the cooling system weight and the weight of the battery necessary to provide energy for air conditioner operation. The system with the minimum overall weight is considered to be the best. The results show the optimum values of all the variables, as well as temperatures and amounts adsorbed, for the adsorption and desorption processes. The results indicate that, for the conditions analyzed in this paper, vapor compression air conditioners are superior to adsorption systems, not only because they are lighter, but also because they have a higher COP and are more compact.

Development of an experimentally flexible facility for mixing-combustion interactions in supersonic flow

Abstract: A new experimental facility has been designed, built and calibrated to support research topics involving supersonic mixing and combustion and will be used to investigate: (1) molecular level mixing; (2) mixing-combustion coupling effects for gaseous and liquid fuels; and (3) atomization, droplet break-up, mixing, and combustion of high-energy/high-density slurry fuels in a supersonic airstream. The facility simulates flight enthalpies corresponding to altitudes between 26 and 36 km and flight Mach numbers up to 4.75. The major features of this facility are the geometric flexibility of the test section configuration, of the fuel injection, and of the experimental conditions, enabling the investigation of a broad range of topics. Parallel and transverse injection, independently controlled in four regions, is available. The construction of the test section is modular and the entrance Mach numbers can be varied between 1.6 and 3.6. An experimental program to define the test flow at the entrance of the facility`s test section is presented with emphasis on the test section entrance temperature distribution and calibration. Selected research topics currently being investigated are summarized to highlight the usefulness of the present facility.

An Investigation of the Lean Operational Limits of Gas-Fueled Spark Ignition Engines

Abstract: Examination is made of the operational limits in two variable compression-ratio single-cylinder engines when operating on the gaseous fuels methane, propane, LPG, and hydrogen under a wide range of conditions. Two definitions for the limits were employed. The first was associated with the first detectable misfire on leaning the mixture, while the second was the first detectable firing under motoring condition in the presence of a spark when the mixture was being enriched slowly. Attempts were also made to relate these limits to the corresponding values for quiescent conditions reckoned on the basis of the flammability limits evaluated at the mean temperature and pressure prevailing within the cylinder charge at the time of the spark. The measured limits in the engine were always higher than the corresponding flammability. limit values for the three fuels. Both of these limits appear to correlate reasonably well with the calculated mean temperature of the mixture at the time of passing the spark.

Simultaneous Measuring Method for Both Volumetric Flow Rates of Air-Water Mixture Using a Turbine Flowmeter

Abstract: A turbine flowmeter is employed in this study in connection with offshore oil field development, in order to measure simultaneously both the volumetric flow rates of air-water two-phase mixture. Though a conventional turbine flowmeter is generally used to measure the single-phase volumetric flow rate by obtaining the rotational rotor speed, the method proposed additionally reads the pressure drop across the meter. After the pressure drop and rotor speed measured are correlated as functions of the volumetric flow ratio of the air to the whole fluid and the total volumetric flow rate, both the flow rates are iteratively evaluated with the functions on the premise that the liquid density is known. The evaluated flow rates are confirmed to have adequate accuracy, and thus the applicability of the method to oil fields.

Exergetic analysis of energy storage using multiple phase-change materials

Abstract: This paper presents an exergetic analysis of the charge process in energy storage using multiple phase-change materials (PCMs). Thermal storage using two, three, five, as well as an infinite number of PCMs is analyzed with a distributed model for the heat transfer fluid. Analytical results show the relative merits of using multiple PCMs compared with a single PCM for thermal energy storage. Sample results are presented and discussed.

An Appraisal of One-Dimensional Analytical Models for the Packed Bed Thermal Storage Systems Utilizing Sensible Heat Storage Materials

Abstract: This article gives an appraisal of existing analytical one-dimensional models for the packed bed thermal energy storage (TES) systems utilizing sensible heat storage (SHS) materials. The models include that of Schumann, which is for separate phases, but does not include axial conductivity (or dispersion) in the bed, nd the single-phase model of Riaz which includes axial dispersion. An alternative axial conductivity model is proposed which compares well with the Schumann model when axial dispersion is negligible, but otherwise caters adequately for axial dispersion at the low Peclet number condition.

Adiabatic Losses in Stirling Refrigerators

Abstract: The Stirling cycle has been used very effectively in cryocoolers; but efficiencies relative to the Carnot limit are typically observed to peak for absolute temperature ratios of about two, which makes it less suitable for low-life refrigeration. The adiabatic loss appears to be responsible for poor performance at small temperature differences. In this paper, adiabatic losses are evaluated, for a temperature ratio of 2/3, taking into account the effect of phase angle between pistons, of volume ratio, of the distribution of the dead volume necessary to reduce the volume ratio, and of the distribution of displacement between expansion and compression spaces. The study is carried out numerically, using an adiabatic Stirling engine model in which cylinder flow is assumed to be stratified. Results show that the best location for the cylinder dead volume is on the compression side. Otherwise, all strategies used to trade off refrigeration for coefficient of performance are found to be roughly equivalent.

Thermal Destruction Behavior of Plastic and Nonplastic Wastes in a Laboratory-Scale Facility

Abstract: Experimental and theoretical studies are presented from a laboratory-scale thermal destruction facility on the destruction behavior of surrogate plastic and nonplastic solid wastes. The nonplastic waste was cellulosic, while the plastic waste contained compounds, such as polyethylene, polyvinyl chloride, polystyrene, polypropylene, nylon, rubber, and polyurethane, or any of their desired mixtures. A series of combustion tests was performed with samples containing varying composition of plastic and nonplastic. Experimental results are presented on combustion parameters ( CO, excess air, residence time) and toxic emissions (dioxin, furan, metals). Equilibrium thermochemical calculations are presented on the thermal destruction behavior of samples under conditions of pyrolysis, combustion, and pyrolysis followed by combustion. Special interest is on the effect of waste properties and input operational parameters on chemistry and product composition. STANJAN and SOLGASMIX computer codes were used in the chemical equilibrium study. Analysis and interpretation of the data reveal the effect of waste feed composition on combustion parameters and dioxin, furan, and metals emission. Equilibrium calculation results are used to describe the experimentally observed trends for the thermal destruction behavior of these wastes. The results show significant influence of plastic on combustion characteristics, and dioxin, furan, and metals emission.

Combustion Technology for Low-Emissions Gas-Turbines: Some Recent Modeling Results

Abstract: Computer modeling of low-emissions gas-turbine combustors requires inclusion of finite-rate chemistry and its interactions with turbulence. The purpose of this review is to outline some recent developments in and applications of the physical models of combusting flows. The models reviewed included the sophisticated and computationally intensive velocity-composition pdf transport method, with applications shown for both a laboratory flame and for a practical gas-turbine combustor, as well as a new and computationally fast PSR-microstructure-based method, with applications shown for both premixed and nonpremixed flames. Calculations are compared with laser-based spectroscopic data where available. The review concentrates on natural-gas-fueled machines, and liquid-fueled machines operating at high power, such that spray vaporization effects can be neglected. Radiation and heat transfer is also outside the scope of this review.

Operational Strategy of a Cogeneration System Under a Complex Utility Rate Structure

Abstract: An operational planning problem for a cogeneration system is discussed under a complex utility rate structure which imposes demand charges due to total utility consumption over a specified period as well as demand and energy charges due to hourly utility consumption. Operational strategy of constituent equipment and contract demands for total utility consumption are assessed so as to minimize the operational cost over the period subject to energy demand requirement. This problem is formulated as a large-scale mixed-integer linear programming (MILP) one, and it is solved efficiently by a revised decomposition method for MILP problems with block angular structure. Through a numerical study on a gas engine-driven cogeneration system installed in a hotel or an office building, the effect of rate structure on operational strategy is clarified.

Fundamental Factors in the Design of a Fast-Responding Methanol-to-Hydrogen Steam Reformer for Transportation Applications

Abstract: Improving the dynamic response of the steam reformer in a fuel cell power plant designed for transportation applications will enable the power plant to operate in a transient manner with a reduced need for supplementary batteries and their associated cost, weight, and life cycle limitations. As a method of seeking improvements to the dynamic response, a sixth-order dynamic model of a steam reformer is used with a design optimization process to determine the values of the steam reformer design parameters which will yield the fastest response time to a step input in hydrogen demand under a variety of initial conditions. Results of this analysis suggest that a steam reformer designed to have a maximum output of approximately 12,600 mol/h of hydrogen and optimized for fast response could have response times on the order of 15-20 s. A sensitivity analysis suggests that this response can be achieved primarily by reducing the thermal capacity of the reformer and improving the rate of heat transfer to the gaseous constituents within the reformer. With a steam reformer response time on the order of 15-20 s, supplementary energy storage devices, such as the ultracapacitor and flywheel, become more feasible. These devices are attractive because they have superior life cycle and power density characteristics when compared with traditional chemical batteries.