Showing papers on "Thermal energy published in 2002"

Cosmological smoothed particle hydrodynamics simulations: the entropy equation

Abstract: We discuss differences in simulation results that arise between the use of either the thermal energy or the entropy as an independent variable in smoothed particle hydrodynamics (SPH). In this context, we derive a new version of SPH that, when appropriate, manifestly conserves both energy and entropy if smoothing lengths are allowed to adapt freely to the local mass resolution. To test various formulations of SPH, we consider point-like energy injection, as in certain models of supernova feedback, and find that powerful explosions are well represented by SPH even when the energy is deposited into a single particle, provided that the entropy equation is integrated. If the thermal energy is instead used as an independent variable, unphysical solutions can be obtained for this problem. We also examine the radiative cooling of gas spheres that collapse and virialize in isolation, and of haloes that form in cosmological simulations of structure formation. When applied to these problems, the thermal energy version of SPH leads to substantial overcooling in haloes that are resolved with up to a few thousand particles, while the entropy formulation is biased only moderately low for these haloes under the same circumstances. For objects resolved with much larger particle numbers, the two approaches yield consistent results. We trace the origin of the differences to systematic resolution effects in the outer parts of cooling flows. When the thermal energy equation is integrated and the resolution is low, the compressional heating of the gas in the inflow region is underestimated, violating entropy conservation and improperly accelerating cooling. The cumulative effect of this overcooling can be significant. In cosmological simulations of moderate size, we find that the fraction of baryons which cool and condense can be reduced by up to a factor ∼2 if the entropy equation is employed rather than the thermal energy equation, partly explaining discrepancies with semi-analytic treatments of galaxy formation. We also demonstrate that the entropy method leads to a greatly reduced scatter in the density–temperature relation of the low-density Lyα forest relative to the thermal energy approach, in accord with theoretical expectations.

Disruption heat loads on the JET MkIIGB divertor

Abstract: Combined analysis of divertor thermocouple and IR camera measurements during JET disruptions can provide valuable information on the distribution of the energy loads, even if the stored energy of the JET plasmas is small compared to that foreseen for the next-generation tokamaks. Typically the energy collected at the divertor represents a small fraction of the pre-disruption plasma energy; this is consistent with the high level of radiation observed and with part of the magnetic energy being transferred to the plasma-coupled conductors. The data for this paper are taken from the whole set of disruptive plasmas of JET operation in the years 2000 and 2001. In most of the MkIIGB disruptions, the plasma displaces upwards (away from the divertor); therefore, only a small number of downward events are available for analysis. However, divertor heat loads seem to be more strongly correlated to the delay of the loss of the X-point with respect to the thermal quench than the direction of the plasma displacement. When the plasma thermal energy is lost with the plasma still in X-point configuration, the septum and the tiles wetted by the strike-points, often more than one tile per strike-point, experience a sharp increase in temperature, equivalent to up to 1 MJ m-2. When the thermal quench occurs at the same time as, or after, the loss of plasma vertical control, no significant divertor tile temperature obreak increase can be observed for both upwards and downwards events. Most of the disruptions purposely made to produce runaway electrons went towards the divertor and, although not systematically, lead to local (mostly at the septum) temperature increase equivalent to a load up to 2 MJ m-2, often toroidally asymmetric.

Molecular dynamics simulation of brittle fracture in silicon

Abstract: The fracture process involves converting potential energy from a strained body into surface energy, thermal energy, and the energy needed to create lattice defects. In dynamic fracture, energy is also initially converted into kinetic energy. This paper uses molecular dynamics (MD) to simulate brittle frcture in silicon and determine how energy is converted from potential energy (strain energy) into other forms.

Modeling and Performance of Microscale Thermophotovoltaic Energy Conversion Devices

Abstract: We analyze the feasibility of energy conversion devices that exploit microscale radiative transfer of thermal energy in thermophotovoltaic devices. By bringing a hot source of thermal energy very close to a receiver fashioned as a pn-junction, the near-field effect of radiation tunneling can enhance the net power flux. We use the fluctuational electrodynamic approach to microscale radiative transfer to account for the spacing effect, which provides the net transfer of photons to the receiver as a function of the separation between the emitter and receiver. We calculate the power output from the microscale device using standard thermophotovoltaic device relations. The results for the performance of a device based on indium gallium arsenide indicate that a ten-fold increase in power throughput may be realized with little loss in efficiency. Furthermore, we develop a model of the microscale device itself that indicates the influence of semiconductor band-gap energy, carrier lifetime, and doping.

Ocean thermal energy conversion primer

Abstract: The vertical temperature distribution in the open ocean can be simplistically described as consisting of two layers separated by an interface. The upper layer is warmed by the sun and mixed to depths of about 100 m by wave motion. The bottom layer consists of colder water formed at high latitudes. The interface or thermocline is sometimes marked by an abrupt change in temperature but more often the change is gradual. The temperature difference between the upper (warm) and bottom (cold) layers ranges from 10°C to 25 °C, with the higher values found in equatorial waters. This implies that there are two enormous reservoirs providing the heat source and the heat sink required for a heat engine. A practical application is found in a system (heat engine) designed to transform the thermal energy into electricity. This is referred to as OTECfor Ocean Thermal Energy Conversion. Several techniques have been proposed to use this ocean thermal resource; however, at present it appears that only the closed cycle (CC-OTEC) and the open cycle (OC-OTEC) schemes have a solid foundation of theoretical as well as experimental work. In the CC-OTEC system, warm surface seawater and cold seawater are used to vaporize and condense a working fluid, such as anhydrous ammonia, which drives a turbine-generator in a closed loop producing electricity. In the OC-OTEC system, seawater is flash-evaporated in a vacuum chamber. The resulting low-pressure steam is used to drive a turbine-generator. Cold seawater is used to condense the steam after it has passed through the turbine. The open-cycle care, therefore, be configured to produce desalinated water as well as electricity.

Evaporation of water droplets placed on a heated horizontal surface

Abstract: A numerical analysis of the evaporation process of small water droplets with diameters of I mm or less that are gently deposited on a hot isothermal solid surface has been performed. This study considers the internal fluid motion that occurs as a result of the thermocapillary convection in the droplet and it determines the effect of fluid motion on the heat transfer between the drop and the solid surface. This study is particularly relevant because the internal fluid motion has not been considered in previous numerical and analytical models presented in the literature. To assess the effects of internal fluid motion, the model results are compared to numerical results provided by a heat conduction model that neglects the fluid motion. The Navier-Stokes and Thermal Energy equations are solved using the Artificial Compressibility Method with Dual Time Stepping. Boundary-fitted grids are used to track the changes in the droplet surface shape during the evaporation process

Thermal imaging system

Abstract: A multicolor imaging system is described wherein at least two, and preferably three, different image-forming layers of a thermal imaging member are addressed at least partially independently by a thermal printhead or printheads from the same surface of the imaging member by controlling the temperature of the thermal printhead(s) and the time thermal energy is applied to the image-forming layers Each color of the thermal imaging member can be printed alone or in selectable proportion to the other color(s) Novel thermal imaging members are also described

Combined solar electric power and liquid heat transfer collector panel

Abstract: An apparatus for converting solar energy to thermal and electrical energy including a photovoltaic grid for converting the concentrated solar energy into electrical energy mounted on a copper plate that provides even temperature dispersion across the plate and acts as a thermal radiator when the apparatus is used in the radiant cooling mode; and a plurality of interconnected heat transfer tubes located within the enclosure and disposed on the plane below the copper plate but conductively coupled to the copper plate for converting the solar energy to thermal energy in a fluid disposed within the heat transfer tubes. Fresnel lenses are affixed to the apparatus on mountings for concentrating the solar energy on to the photovoltiac grid and functioning as a passive solar tracker.

Fluid heat exchanger assembly

Abstract: A vehicle system for transferring thermal energy in relation to a vehicle fluid includes at least one thermoelectric device, having at least two surfaces, concurrently dissipating thermal energy on a first surface and absorbing thermal energy on a second surface, mounted in proximity to a contained vehicle fluid so as to provide thermal communication between the contained vehicle fluid and either the cooler or the warmer surface of the thermoelectric device. Additionally, a method of cooling a vehicle fluid includes the steps of: (a) providing at least one thermoelectric device, having at least a first surface that changes temperature in a first direction upon activation of the thermoelectric device and a second surface opposing the first surface that changes temperature in an opposite direction upon activation of the thermoelectric device; (b) positioning the thermoelectric device such that the first surface is in thermal communication with a contained vehicle fluid; and (c) activating the thermoelectric device to develop a thermal gradient between the contained vehicle fluid and the first surface.

Effect of Extrusion Cooking on Molecular Parameters of Corn Starch

Abstract: By using a systems analytical model (SAM) and a fuzzy logic control software (fuzzy CIM) extrusion experiments were designed, that enabled a differentiation of the influence of the thermal energy input, expressed by the product temperature (PT), and the influence of the specific mechanical energy input (SME) on the molecular structure of extruded starch. The chromatographic examination of the molecular changes in the starch clearly revealed the influence of the extrusion cooking conditions on molecular degradation. The molecular size of extruded starch, expressed as the weight average of the molecular weight (Mw), decreased exponentially when SME increased. In the range of 110—180 °C, PT had no significant influence on Mw so that the observed reduction of Mw was primarily dependent on the increase in SME. By contrast, the polydispersity depended both on PT and SME. The influence of PT on the polydispersity was of minor significance up to 160 °C, increasing more steeply at higher temperatures. PT increase above 180 °C resulted in increasing reducing power of the extruded starch, whereas SME had almost no effect on reducing power. Only at a PT of more than 180 °C small amounts of short chain molecules with a degree of polymerisation (DP) smaller than 6 could be determined.

Double closed loop thermoelectric heat exchanger

Abstract: A double pass heat exchanger comprising: a first bank of thermoelectric devices that includes at least one thermoelectric device, the thermoelectric devices having cooling surfaces capable of absorbing thermal energy and opposed heating surfaces capable of dissipating thermal energy; a second bank of thermoelectric devices that includes at least one thermoelectric device, the thermoelectric devices having cooling surfaces capable of absorbing thermal energy and opposed heating surfaces capable of dissipating thermal energy; a first block of heat transfer material in concurrent thermal communication with a first fluid conduit and the heating surfaces of the first bank of thermoelectric devices and the heating surfaces of the second bank of thermoelectric devices; and, a second block of heat transfer material in concurrent thermal communication with a second fluid conduit and the cooling surfaces of the first bank of thermoelectric devices and the second bank of thermoelectric devices.

Thermal challenges facing new-generation light-emitting diodes (LEDs) for lighting applications

Abstract: Light Emitting Diodes (LEDs) have progressed in recent years from emitting indicator level lighting to emitting enough light for illumination applications. This has opened a new field for LED applications, resulting in significant advantages over conventional light sources and creating some application challenges unique to LEDs. Methods used in the past for packaging LEDs, such as the T 1 3/4 or 5mm through-hole package, were suitable for the indicating lamp industry but have proved poor for high brightness LEDs driven at relatively high power due to temperature rises. Conventional lighting methods provide little guidance for LED thermal problems since these usually involve a very high temperature source, such as a filament or an arc, and radiant heat transfer dissipates the thermal energy. A typical incandescent bulb, for example, radiates 85-95% of the thermal energy away; the remainder is dissipated by conduction to the socket or naturally convected to the atmosphere. LED junction temperatures are limited to much lower values and hence the heat transfer system cannot depend upon radiant energy transfer. This means the cooling methods for lighting now shift from primarily radiation to conduction and natural convection, and this is paradigm shift that lighting designers must recognize when moving to LEDs. In this presentation, the thermal challenges facing LED lighting applications are discussed. Developments in packaging of die into Level 1 products are shown, and the thermal challenges of high brightness LED applications caused by the paradigm shift of heat transfer methods for lighting are discussed.

Hot dry rock (hdr) geothermal energy research and development at fenton hill, new mexico

Abstract: INTRODUCTION Conventional geothermal technology entails the production of useful energy from natural sources of steam or, much more commonly, hot water. These hydrothermal resources are found in a number of locations around the world, but they are the exception rather than the rule. In most places, the earth grows hotter with increasing depth, but mobile water is absent. The vast majority of the world’s accessible geothermal energy is found in rock that is hot but essentially dry -the so-called hot dry rock (HDR) resource. The total amount of heat contained in HDR at accessible depths has been estimated to be on the order of 10 billion quads (a quad is the energy equivalent of about 180 million barrels of oil and 90 quads represents the total US energy consumption in 2001). This is about 800 times greater than the estimated energy content of all hydrothermal resources and 300 times greater than the fossil fuel resource base that includes all petroleum, natural gas, and coal. (Tester, et al. 1989). Like hydrothermal energy resources already being commercially extracted, HDR holds the promise for being an environmentally clean energy resource, particularly with regard to carbon dioxide emissions, which can be expected to be practically zero. The total HDR resource base noted above was calculated by summing the thermal energy content of rock beneath the landmasses of the world at temperatures above 25C (77F), from the surface to a depth of 30,000 ft (9,150 m). Obviously, much of this HDR resource resides in rock that is only marginally warmer than 25C and is thus of such low-grade that it is not practical to recover it. In addition, a large part of the resource may be located in parts of the world where its exploitation may not be economically worthwhile. Nevertheless, with such a large resource base, the potential for HDR to be a major contributor to the world’s energy supply makes its development well worth pursuing, especially when considered in light of its environmental advantages. One method of evaluating the potential for HDR development in a region is to examine its geothermal gradient -the rate at which the earth gets hotter with depth. The geothermal gradient varies widely from place to place, being much higher in tectonically active regions and in areas of volcanic activity. Figure 1 shows a geothermal gradient map of the United States. It is apparent from this map that HDR resources at useful temperatures (above 100C) are abundant in many parts of the west.

Quantitative internal thermal energy mapping of semiconductor devices under short current stress using backside laser interferometry

Abstract: In the backside interferometric thermal mapping technique, an infrared (IR) laser beam probes the temperature-induced changes in the semiconductor refractive index inside a semiconductor device, which results in a change in the measured optical phase shift. In this paper, a theoretical analysis of the phase shift is reported. The focus is on nanosecond-to-microsecond time-scale thermal mapping during high current stress, as occurring e.g., during an electrostatic discharge (ESD) event or in some power applications. An analytical expression for phase shift is obtained from the analysis of the thermal diffusion equation. The phase shift is directly proportional to the two-dimensional (2-D) heat energy density in the semiconductor active region of the device. The phase shift is also expressed in terms of the local dissipated heat energy and the heat transferred to the device top and lateral sides. In addition, the space integral of the phase shift is expressed in terms of a total energy dissipated in the device and the total heat transferred from the semiconductor to the top device layers. The theory shows an excellent agreement with experimental data obtained for a p-n diode ESD protection structure working in the avalanche regime.

Thermal-diffusive ignition and flame initiation by a local energy source

Abstract: The ignition and flame initiation in a gaseous reacting mixture subject to a local source of thermal energy is analysed by means of large activation energy asymptotics. The ignition transient is assumed to be long enough for heat conduction to be the dominant cooling mechanism. We show the existence of a critical value of the Damkohler number, defined as the ratio of appropriate characteristic times of conduction and chemical reaction, such that ignition only occurs for supercritical values. Additional conditions are required to ensure self-propagation of a flame after ignition. These are obtained, with the thermal-diffusive model, for a source of energy represented by an instantaneous point, line or planar source. The analysis, involving an unsteady free-boundary problem, shows that the initial flame kernel evolves to a self-propagating flame only if the energy released by the source is greater than a critical value.

Gas temperature gradients in a CF4 inductive discharge

Abstract: The neutral gas temperature in a CF4 planar inductive discharge was measured with space and time resolution using laser-induced fluorescence of the CF radical with analysis of the rotationally resolved excitation spectra. Strong temperature gradients are observed and temperatures as high as 900 K are reached at the reactor center at 50 mTorr with a power density of 0.15 W/cm3. The temperature at the reactor center increases with both gas pressure and power, but is independent of the gas flow rate. A simple model based on the global thermal energy balance is proposed to explain these results. The fraction of the injected rf power consumed in gas heating varies from 4.4% to 42% under our conditions (5–50 mTorr, 250 W rf power). Axial temperature profiles were measured in the steady state and in the time afterglow. The typical temperature relaxation times are several hundreds of microseconds. A numerical two-dimensional, time-dependent thermal model is in good agreement with the results.

Cascading closed loop cycle (CCLC)

Abstract: A Cascading Closed Loop Cycle (CCLC) system is described for recovering power in the form of mechanical or electrical energy from any thermal energy source whose temperature is sufficiently high to vaporize a pressurized light hydrocarbon medium such as propane or propylene. A light hydrocarbon medium is vaporized in multiple indirect heat exchangers; expanded in multiple cascading expansion turbines to generate useful power; and condensed to a liquid using a cooling system. The light hydrocarbon liquid medium is then pressurized with a pump and returned to the indirect heat exchangers to repeat the vaporization, expansion, liquefaction and pressurization cycle in a closed, hermetic process.

Energy budget and imaging spectroscopy of a compact flare

Abstract: We present the analysis of a compact flare that occurred on 26 February 2002 at 10:26 UT, seen by both RHESSI and TRACE. The size of the nearly circular hard X-ray source is determined to be 5.6 (±0.8)″, using different methods. The power-law distribution of non-thermal photons is observed to extend down to 10 keV without flattening, and to soften with increasing distance from the flare kernel. The former indicates that the energy of the precipitating flare electron population is larger than previously estimated: it amounts to 2.6 (±0.8) × 1030 erg above 10 keV, assuming thick-target emission. The thermal energy content of the soft X-ray source (isothermal temperature of 20.8 (±0.9) MK) and its radiated power were derived from the thermal emission at low energies. TRACE has observed a low-temperature ejection in the form of a constricted bubble, which is interpreted as a reconnection jet. Its initial energy of motion is estimated. Using data from both satellites, an energy budget for this flare is derived. The kinetic energy of the jet bulk motion and the thermal and radiated energies of the flare kernel were more than an order of magnitude smaller than the derived electron beam energy. A movie is available on the CD-ROM accompanying this volume.

Thermoelectrically controlled blower

Abstract: A compact, energy efficient blower that is controlled by a Peltier thermoelectric device to supply either hot or cold air. The Peltier device is sandwiched between a pair of heat exchangers that are surrounded by a plastic enclosure. Each heat exchanger includes a plurality of parallel aligned thermal energy conducting fins that are folded to maximize the surface area thereof. A fan is mounted atop the enclosure to pump a first supply of intake air through a first air flow path in a first heat exchanger at one side of the Peltier device to exhaust the waste energy emitted by the Peltier device and collected by the first heat exchanger. The fan also pumps a second supply of intake air through a second air flow path in the second heat exchanger at the opposite side of the Peltier device to blow the useful energy emitted by the Peltier device and collected by the second heat exchanger to an application device or a space within which the blower is located. The first and second heat exchangers are turned upside down relative to one another at opposite sides of the Peltier device so that the waste energy and useful energy are pumped from the blower in directions which are aligned perpendicular to one another.

Chemical kinetics with electrical and gas dynamics modelization for NOx removal in an air corona discharge

Abstract: A non-stationary reactive gas dynamics model in a mono-dimensional geometry, including radial mass diffusion, gas temperature variation and chemical kinetics, is developed in this paper. The aim is to analyse the spatio-temporal evolution of the main neutral species involved in a corona discharge used for NO pollution control in polluted air at atmospheric pressure and ambient temperature. The present reactive gas dynamics model takes into account 16 neutral chemical species (including certain metastable species) reacting following 110 selected chemical reactions. The initial concentration of each neutral species is obtained from a 1.5D electrical discharge model. The gas temperature variations are due to direct Joule heating during the discharge phase, and also result from the delayed heating due to the relaxation of the vibrational energy into a random thermal energy during the post-discharge phase. The simulation conditions are those of an existing experimental setup (anode voltage of 10 kV in the case of a point to plane geometry with an interelectrode distance of 10 mm). The obtained results show that the diffusion phenomena and the gas temperature rise affect quite well the gas reactivity and the neutral species evolution. This allows us to better understand the different reaction processes and transport phenomena affecting the NO concentration magnitude inside the discharge channel.

Solar panel for simultaneous generation of electric and thermal energy

Abstract: A solar panel for simultaneous generation of electric and thermal energy with efficiency improvements is disclosed. A combined panel provided with a photovoltaic panel thermally contacting a fluid-containing panel by means of a heat exchanger, has reflective means mounted thereon for directing solar radiation to the photosensitive surface of the photovoltaic panel. The increased light concentration together with the cooling action of the water circulating in the fluid-containing panel, permits to highly increase the electric energy generated by the photovoltaic panel and the thermal power carried outside the fluid-containing panel by means of the water.

Improvement of hydrogen storage by adsorption using 2-D modeling of heat effects

Abstract: The use of hydrogen storage based on an adsorption process for cars operating on a fuel cell will be seriously considered when the amount of stored gas is high enough, the tank safe and economically viable, and the filling process fast enough. To maximize the storage capacity for rapid filling, it is necessary to minimize heating of the adsorbent bed and to imagine means to extract energy toward the outside. Numerical simulation of such a hydrogen storage tank, based on a 2-D model where mass and thermal energy balances are coupled, is presented. Results from simulations agreed well with experiments, when various modified parameters were calculated. Solutions for improving adsorbent beds for hydrogen storage tanks are then proposed.

Solar Thermal Reduction of ZnO Using CH4:ZnO and C:ZnO Molar Ratios Less Than 1

Abstract: The solar thermal reduction of ZnO, using solar process heat and CH 4 or C as reducing agent, is investigated for CH 4 :ZnO or C:ZnO molar ratios ranging from 0 (thermal decomposition at above about 2000°C) to 1 (stoichiometric reduction at above about 1000°C). At 1400°C, in thermodynamic equilibrium ZnO can be completely reduced using a CH 4 :ZnO molar ratio of 0.3 and produces one fuel (Zn-metal) rather than two for the stoichiometric case (Zn and syngas). The maximal reactor thermal efficiency without heat recovery from the offgas, defined as the ratio of the heating-value of the zinc produced to the total thermal energy input, is 55%. CO 2 -emissions are reduced by a factor of 10-15 compared to fossil-fuel-based zinc-production technologies. For a closed materials cycle, in which power is extracted from the solar zinc using a fuel cell and the ZnO formed is recycled to the solar reactor, the total exergy efficiency, defined as the work output of the fuel cell to the thermal energy input, varies between 30 to 40% when based on the absorbed solar power in the reactor. These efficiency values are very encouraging, especially since the solar ZnO/Zn cycle allows-in contrast to other regenerative power plants-to store and transport solar energy.