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RESEARCH PAPERS

J. Energy Resour. Technol. 2006;129(1):1-9. doi:10.1115/1.2424960.

Diagnosis procedures primarily aim at locating the control volumes where anomalies occurred. This is not a simple task since the effects of anomalies generally propagate through the whole system and affect the behavior of several components. Some components may therefore present a reduced efficiency, although they are not sources of operation anomalies, due to nonflat efficiency curves. These induced effects are a big obstacle in the use of thermoeconomic techniques for the search of the origin of the anomalies. On the other hand, the real cause of the alteration of component behavior is the modification of its characteristic curve, due to degradation or failures. According to this concept, a new approach, based on an indicator measuring the alteration of the characteristic curve of the component affected by the operation anomaly, is proposed and applied to a test case power plant.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(1):10-17. doi:10.1115/1.2424965.

Worldwide power resources that could be extracted from Ocean Thermal Energy Conversion (OTEC) plants are estimated with a simple one-dimensional time-domain model of the thermal structure of the ocean. Recently published steady-state results are extended by partitioning the potential OTEC production region in one-degree-by-one-degree “squares” and by allowing the operational adjustment of OTEC operations. This raises the estimated maximum steady-state OTEC electrical power from about 3TW(109kW) to 5TW. The time-domain code allows a more realistic assessment of scenarios that could reflect the gradual implementation of large-scale OTEC operations. Results confirm that OTEC could supply power of the order of a few terawatts. They also reveal the scale of the perturbation that could be caused by massive OTEC seawater flow rates: a small transient cooling of the tropical mixed layer would temporarily allow heat flow into the oceanic water column. This would generate a long-term steady-state warming of deep tropical waters, and the corresponding degradation of OTEC resources at deep cold seawater flow rates per unit area of the order of the average abyssal upwelling. More importantly, such profound effects point to the need for a fully three-dimensional modeling evaluation to better understand potential modifications of the oceanic thermohaline circulation.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(1):18-28. doi:10.1115/1.2424962.

The energy supply method by distributed power supply is expected from the transportation loss of the power and a decrease in heat. The system using especially renewable energy and a fuel cell is excellent from the viewpoint of the environment. However, the efficiency of the whole system depends on the amount of utilization of exhaust heat. The system assumed in this paper installs a small fuel cell and a solar module in 12 buildings, connects each building with hot-water piping, and supplies fuel cell exhaust heat. The hot-water piping path that results in minimum heat release loss was investigated in the optimization analysis using a genetic algorithm. As a result, the optimal path of the hot-water piping considering the production of electricity of a solar module and ambient temperature was clarified.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(1):29-32. doi:10.1115/1.2424961.

As an alternative to conventional methods of conveying and delivering energy in mobile applications or to remote locations, we have examined the combustion of nanostructured metal particles assembled into metal clusters. Clusters containing iron nanoparticles (50nm in diameter) were found to combust entirely in the solid state due to the high surface-to-volume ratio typical of nanoparticles. Optical temperature measurements indicated that combustion was rapid (500ms), and occurred at relatively low peak combustion temperatures (10001200K). Combustion produces a mixture of Fe(III) oxides. X-ray diffraction and gravimetric analysis indicated that combustion was nearly complete (93–95% oxidation). Oxide nanoparticles could be readily reduced at temperatures between 673K and 773K using hydrogen at 1atm pressure, and then passivated by the growth of a thin oxide layer. The nanostructuring of the particles is retained throughout the combustion–regeneration cycle. Modeling of the combustion process is in good agreement with observed combustion characteristics.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(1):33-41. doi:10.1115/1.2424957.

This paper introduces a model for the description of the homogeneous combustion of various fuels in fluidized bed combustors (FBC) at temperatures lower than the classical value for solid fuels, i.e., 850°C. The model construction is based on a key bubbling fluidized bed feature: A fuel-rich (endogenous) bubble is generated at the fuel injection point, travels inside the bed at constant pressure, and undergoes chemical conversion in the presence of mass transfer with the emulsion phase and of coalescence with air (exogenous) bubbles formed at the distributor and, possibly, with other endogenous bubbles. The model couples a fluid-dynamic submodel based on two-phase fluidization theory with a submodel of gas phase oxidation. To this end, the model development takes full advantage of a detailed chemical kinetic scheme, which includes both the low and high temperature mechanisms of hydrocarbon oxidation, and accounts for about 200 molecular and radical species involved in more than 5000 reactions. Simple hypotheses are made to set up and close mass balances for the various species as well as enthalpy balances in the bed. First, the conversion and oxidation of gaseous fuels (e.g., methane) were calculated as a test case for the model; then, n-dodecane was taken into consideration to give a simple representation of diesel fuel using a pure hydrocarbon. The model predictions qualitatively agree with some of the evidence from the experimental data reported in the literature. The fate of hydrocarbon species is extremely sensitive to temperature change and oxygen availability in the rising bubble. A preliminary model validation was attempted with results of experiments carried out on a prepilot, bubbling combustor fired by underbed injection of a diesel fuel. Specifically, the model results confirm that heat release both in the bed and in the freeboard is a function of bed temperature. At lower emulsion phase temperatures many combustible species leave the bed unburned, while post-combustion occurs after the bed and freeboard temperature considerably increases. This is a well-recognized undesirable feature from the viewpoint of practical application and emission control.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(1):42-49. doi:10.1115/1.2424958.

Salt-laden hog fuel (wood waste) is burnt in a fluidized bed boiler converted from a traveling grate boiler to generate steam for a specialty paper mill. The converted boiler has a design capacity of 156th of steam from hog and actual generation has varied from 76% to 107% of the design capacity. The conversion has resulted in more stable operation, more complete combustion, less ash production, reduced boiler maintenance, and lower fossil fuel consumption. Tire derived fuel (TDF) is used as a supplementary fuel. With an energy content of 31GJt for TDF, as compared to 8GJt for wet hog, addition of 2%–5% TDF by weight increased the bed temperature by an average of 55°C, stabilized and improved the combustion of low quality hog and high moisture content sludge. The impact of TDF addition was studied in detail. Stack emissions were tested and bottom and flyash samples were analyzed. Although TDF contains an average of 1.6% zinc and 9.2% steel wire by weight, addition of TDF did not affect total particulate emissions from the boiler. SO2 emissions were increased due to the high sulfur content of TDF (1.4%), while NOx emissions were reduced. A good correlation was obtained from the test results, showing that the addition of TDF resulted in a reduction in both the total formation and the stack emissions of dioxins and furans.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(1):50-53. doi:10.1115/1.2424959.

A numerical three-dimensional (3D) simulation was performed on the flow of a particle-laden steam jet impinging on the tubes of the superheating section of a waste-burning industrial boiler: the study was conducted to investigate the causes of excessive erosion systematically detected under otherwise normal operation. Prior experimental evidence had led to the identification of the slag particles as a possible agent for such an erosion: these particles are removed from the surface of the first row of the tubes by the steam jet ejected from the sootblower, and may cause erosion if they impinge on the tubes of the following (upstream, in the direction of the gas path) rows. Three sootblower configurations were considered to analyze this presumed phenomenology of tube erosion. The (significant) effects of turbulence on the motion of the solid particles are taken into account by means of a stochastic flow model. The simulation generates a prediction of the particles trajectories, both in a geometrical sense (to locate the portions of the tube surfaces on which the individual particles may impinge) and in a statistical one (to identify the relative frequency of the impacts on each portion of surface): the erosion rate for each sootblower configuration has been calculated on the basis of the impact frequency indicated by the numerical calculations.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(1):54-65. doi:10.1115/1.2424964.

The applicability and usefulness of combustion in porous media is of much interest due to its competitive combustion efficiency and lower pollutants formation. In the previous works, the focus has been on the effects of combustion and heat transfer parameters such as excess air ratio, thermal power, solid conductivity, convective heat transfer coefficient, and radiation properties on centerline temperature and pollutant formations. A premixed combustion scheme and a fixed porous medium with constant geometrical parameters have been used in these works; therefore, the effects of porous material parameters have been less considered. In this research, the effects of geometrical parameters of porous medium, namely porosity and permeability, on centerline temperature distributions, peak flame temperature, flame structure, and gas mixture preheating have been investigated by numerical methods. To this, a two-dimensional axis-symmetric physical model of porous burner is considered. As the most typical porous burners, a two stage one which has preheating porous zone (PPZ) and combustion porous zone (CPZ) is studied. The continuity, momentum, energy, turbulence, and species transport equations are solved employing a one-step chemical reaction mechanism with an eddy-dissipation model for rate of reactions. The turbulence is modeled with two transport equations which are not considered in similar works. The combustion regime is assumed to be diffusion and combustion parameters are fixed in all cases. Porosity effects on the structure and temperature characteristic of the flame are probed in a wide range for PPZ and CPZ. Critical permeability is defined and permeability effects on flame characters in both of the preheating and combustion regions are studied thoroughly.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(1):66-70. doi:10.1115/1.2424963.

Biomass is the largest renewable energy source used in the world and its importance grows larger in the future energy market. Since most biomass sources are low in energy density and are widespread in space, a small scale biomass conversion system is therefore more competitive than a large stand-alone conversion plant. The current study proposes a small scale solid biomass powering system to explore the viability of direct coupling of an updraft fixed bed gasifier with a Stirling engine. The modified updraft fixed bed gasifier employs an embedded combustor inside the gasifier to fully combust the syngas generated by the gasifier. The flue gas produced by the syngas combustion inside the combustion tube is piped directly to the heater head of the Stirling engine. The engine will then extract and convert the heat contained in the flue gas into electricity automatically. Output depends on heat input and the heat input is proportional to the flow rate and temperature of the flue gas. The preliminary study of the proposed direct coupling of an updraft gasifier with a 25kW Stirling engine demonstrates that full power output could be produced by the current system. It could be found from the current investigation that very little attention and no assisting fuel are required to operate the current system. The proposed system could be considered as a feasible solid biomass powering technology.

Commentary by Dr. Valentin Fuster

DESIGN INNOVATION PAPER

J. Energy Resour. Technol. 2006;129(1):71-78. doi:10.1115/1.2424967.

Cooling, heating, and power (CHP) energy systems provide higher fuel efficiency than conventional systems, resulting in reduced fuel consumption, reduced emissions, and other environmental benefits. Until recently the focus of CHP system development has been primarily on medium-scale commercial applications in a limited number of market segments where clear value propositions lead to short term payback. Small-scale integrated CHP systems that show promise of achieving economic viability through significant improvements in fuel utilization have received increased attention lately. In this paper the economic potential is quantified for small-scale (microgrid) integrated CHP systems suitable for groups of buildings with aggregate electric loads in the 15120kW range. Technologies are evaluated for community building groups (CBGs) consisting of aggregation of pure residential entities and combined residential and light commercial entities. Emphasis is on determination of the minimum load size (i.e., the smallest electric and thermal load for a given CBG that is supplied with electric, heating, cooling power from a CHP) for which a microgrid CHP system is both technically and economically viable. In this paper, the operation of the CHP system is parallel with the public utility grid at all times, i.e., the grid is interconnected. Evaluations of CHP technology options using simulation studies in a “three-dimensional” space (CHP technology option, CBG load aggregation, and geographical location in the USA) were evaluated based on comparisons of net present value (NPV). The simulations indicated that as electric load increases, the viability of the CHP system (independent of the system’s size) becomes more favorable. Exceeding a system runtime (utilization) of 70% was shown to pass the break-even line in the NPV analysis. Finally, geographic location was found to have a relatively weak effect on the reported trends. These results suggest that microgrid CHP systems have the potential to be economically viable with relative independence of geographic location if adequately sized to match the specific load requirements.

Commentary by Dr. Valentin Fuster

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