Showing papers on "Thermal energy published in 2005"

Latent Heat Storage Materials and Systems: A Review

Abstract: The use of a latent heat storage system using Phase Change Materials (PCM) is an effective way of storing thermal energy (solar energy, off-peak electricity, industrial waste heat) and has the advantages of high storage density and the isothermal nature of the storage process. It has been demonstrated that, for the development of a latent heat storage system, choice of the PCM plays an important role in addition to heat transfer mechanism. The information on the latent heat storage materials and systems is enormous and published widely in the literatures. In this paper, we make an effort to gather the information from the previous works on PCMs and latent heat storage systems. This review will help to find a suitable PCM for various purposes a suitable heat exchanger with ways to enhance the heat transfer, and it will also help to provide a variety of designs to store the heat using PCMs for different applications, i.e. space heating & cooling, solar cooking, greenhouses, solar water heating and waste heat recovery systems. Measurement techniques of thermophysical properties, studies on thermal cycles for long term stability, corrosion of the PCMs and enhancement of heat transfer in PCM are discussed. New PCM innovations are also included for the awareness of new applications. This paper contains a list of about 250 PCMs and more than 250 references.

A thermally self-sustained micro solid-oxide fuel-cell stack with high power density

Abstract: High energy efficiency and energy density, together with rapid refuelling capability, render fuel cells highly attractive for portable power generation. Accordingly, polymer-electrolyte direct-methanol fuel cells are of increasing interest as possible alternatives to Li ion batteries. However, such fuel cells face several design challenges and cannot operate with hydrocarbon fuels of higher energy density. Solid-oxide fuel cells (SOFCs) enable direct use of higher hydrocarbons, but have not been seriously considered for portable applications because of thermal management difficulties at small scales, slow start-up and poor thermal cyclability. Here we demonstrate a thermally self-sustaining micro-SOFC stack with high power output and rapid start-up by using single chamber operation on propane fuel. The catalytic oxidation reactions supply sufficient thermal energy to maintain the fuel cells at 500–600 °C. A power output of ~350 mW (at 1.0 V) was obtained from a device with a total cathode area of only 1.42 cm^2.

Thermal and non-thermal energies of solar flares

Abstract: The energy of the thermal flare plasma and the kinetic energy of the non-thermal electrons in 14 hard X-ray peaks from 9 medium-sized solar flares have been determined from RHESSI observations. The emissions have been carefully separated in the spectrum. The turnover or cutoff in the low-energy distribution of electrons has been studied by simulation and fitting, yielding a reliable lower limit to the non-thermal energy. It remains the largest contribution to the error budget. Other effects, such as albedo, non-uniform target ionization, hot target, and cross-sections on the spectrum have been studied. The errors of the thermal energy are about equally as large. They are due to the estimate of the flare volume, the assumption of the filling factor, and energy losses. Within a flare, the non-thermal/thermal ratio increases with accumulation time, as expected from loss of thermal energy due to radiative cooling or heat conduction. Our analysis suggests that the thermal and non-thermal energies are of the same magnitude. This surprising result may be interpreted by an efficient conversion of non-thermal energy to hot flare plasma.

Energy release and conversion by reconnection in the magnetotail

Abstract: . Magnetic reconnection is the crucial process in the release of magnetic energy previously stored in the magnetotail in association with substorms. However, energy transfer and dissipation in the vicinity of the reconnection site is only a minor part of the energy conversion. We discuss the energy release, transport, and conversion based on large-scale resistive MHD simulations of magnetotail dynamics and more localized full particle simulations of reconnection. We address in particular, where the energy is released, how it propagates and where and how it is converted from one form into another. We find that Joule (or ohmic) dissipation plays only a minor role in the overall energy transfer. Bulk kinetic energy, although locally significant in the outflow from the reconnection site, plays a more important role as mediator or catalyst in the transfer between magnetic and thermal energy. Generator regions with potential auroral consequences are located primarily off the equatorial plane in the boundary regions of the plasma sheet.

Meaning of ionospheric Joule heating

Abstract: [1] The possibility of relating the electric current in the ionosphere to the electric field in the frame of reference either of the neutral atmosphere (as commonly done) or of the plasma raises the question: which should be used in calculating Joule heating? The energy equations for the plasma and for the neutral medium, including collision effects, can be combined with momentum equations to separate energy transfer into work done and heating (this is not the same as separation of energy content into bulk-flow kinetic energy and thermal energy). Heating/dissipation processes can be unambiguously identified, showing that true electromagnetic (Joule) heating rate is given by J · E in the frame of reference of the plasma. The major part of energy dissipation associated with J and E in the ionosphere is heating by collisions between plasma and neutrals, proportional to the square of the difference in their bulk flow velocities and distributed approximately equally between the two. The conventionally calculated ionospheric Joule heating as J · E in the frame of reference of the neutral atmosphere equals the sum of total heating rate of plasma (only a small part of which is true Joule heating) plus total heating rate of neutrals, plus a term equal to work done on plasma in the neutral-atmosphere frame of reference; the latter is assumed negligible, with J × B/c balanced entirely by plasma-neutral collisions and with no net work done on the plasma. This assumption implies that all the magnetic stresses exerted on the plasma at ionospheric heights are taken up by the local neutral medium; hence the dynamics of the neutral atmosphere play an important role in magnetosphere/ionosphere interactions.

Direct observation of enhanced residual thermal energy coupling to solids in femtosecond laser ablation

Abstract: We perform direct measurement of the thermal energy remaining in the bulk of Cu, Mg, Au, and Si samples following multi-pulse femtosecond laser ablation. In contrast to the previous belief that the thermal energy remaining in the ablated sample is negligible using femtosecond pulses, we show a significant amount of residual thermal energy deposited in various materials. In fact, with a sufficiently large number of pulses at high fluence, virtually all the incident laser energy can be retained in the sample. Several possible mechanisms are investigated for their role in residual heating, including laser-induced surface modification, exothermic chemical processes, and pressure effects.

Thermal and non-thermal energies in solar flares

Abstract: The energy of the thermal flare plasma and the kinetic energy of the non-thermal electrons in 14 hard X-ray peaks from 9 medium-sized solar flares have been determined from RHESSI observations. The emissions have been carefully separated in the spectrum. The turnover or cutoff in the low-energy distribution of electrons has been studied by simulation and fitting, yielding a reliable lower limit to the non-thermal energy. It remains the largest contribution to the error budget. Other effects, such as albedo, non-uniform target ionization, hot target, and cross-sections on the spectrum have been studied. The errors of the thermal energy are about equally as large. They are due to the estimate of the flare volume, the assumption of the filling factor, and energy losses. Within a flare, the non-thermal/thermal ratio increases with accumulation time, as expected from loss of thermal energy due to radiative cooling or heat conduction. Our analysis suggests that the thermal and non-thermal energies are of the same magnitude. This surprising result may be interpreted by an efficient conversion of non-thermal energy to hot flare plasma.

Thermoelectric based heating and cooling system for a hybrid-electric vehicle

Abstract: A heating and cooling system including a first and second thermoelectric module each has a first surface and a second surface. In thermal communication with the first and second surfaces of the thermoelectric modules are separate heat exchangers. A thermal storage device is in thermal communication with the first surface of the second thermoelectric module. The thermal storage device is configured to store thermal energy generated by the second surface of the second thermoelectric module.

Apparatus for producing single crystal and quasi-single crystal, and associated method

Abstract: An apparatus including a crucible, an energy source, and a controller is provided. The crucible may be sealed to a nitrogen-containing gas, and may be chemically inert to at least ammonia at a temperature in a range of about 400 degrees Celsius to about 2500 degrees Celsius. The energy source may supply thermal energy to the crucible. The controller may control the energy source to selectively direct sufficient thermal energy to a predefined first volume within the crucible to attain and maintain a temperature in the first volume to be in a range of from about 400 degrees Celsius to about 2500 degrees Celsius. The thermal energy may be sufficient to initiate, sustain, or both initiate and sustain growth of a crystal in the first volume. The first temperature in the first volume may be controllable separately from a second temperature in another volume within the crucible. The first temperature and the second temperature differ from each other. Associated methods are provided.

A new principle of synthetic cascade utilization of chemical energy and physical energy

Abstract: We propose a new principle of the cascade utilization of both chemical energy and physical energy in energy systems with the integration of chemical processes and thermal cycles. Particularly, a general equation of energy levels of substance, Gibbs free energy of chemical reaction and physical energy is explicitly founded. On the basis of this equation, a chemical-looping combustion and an indirect combustion are investigated. Furthermore, a mechanism of energy release, with the combination of decreasing the energy level of Gibbs free energy and upgrading the energy level of low or middle- temperature thermal energy, is clarified. The promising results obtained here establish a theoretical basis for the further investigation of multi-function systems in which energy and the environment are compatible, and create a new approach to improve the performance of traditional thermal cycles.

Energy and Cost Minimal Control of Active and Passive Building Thermal Storage Inventory

Abstract: In contrast to building energy conversion equipment, less improvement has been achieved in thermal energy distribution, storage and control systems in terms of energy efficiency and peak load reduction potential. Cooling of commercial buildings contributes significantly to the peak demand placed on an electrical utility grid and time-of-use electricity rates are designed to encourage shifting of electrical loads to off-peak periods at night and on weekends. Buildings can respond to these pricing signals by shifting cooling-related thermal loads either by precooling the building s massive structure (passive storage) or by using active thermal energy storage systems such as ice storage. Recent theoretical and experimental work showed that the simultaneous utilization of active and passive building thermal storage inventory can save significant amounts of utility costs to the building operator, yet increased electrical energy consumption may result. The article investigates the relationship between cost savings and energy consumption associated with conventional control, minimal cost and minimal energy control, while accounting for variations in fan power consumption, chiller capacity, chiller coefftcient-of-performance, and part-load performance. The model-based predictive building controller is employed to either minimize electricity cost including a target demand charge or electrical energy consumption. This work shows that buildings can be operated in a demand-responsive fashion to substantially reduce utility costs with marginal increases in overall energy consumption. In the case of energy optimal control, the reference control was replicated, i.e., if only energy consumption is of concern, neither active nor passive building thermal storage should be utilized. On the other hand, cost optimal control suggests strongly utilizing both thermal storage inventories.

An Order-of-Magnitude Estimate of Ocean Thermal Energy Conversion Resources

Abstract: Worldwide power resources that could be extracted from the steady-state operation of ocean thermal energy conversion (OTEC) plants are estimated using a simple model. This order-of-magnitude analysis indicates that about 3 × 10 9 kW (3 TW) may be available, at most. This value is much smaller than estimates currently suggested in the technical literature. It reflects the scale of the perturbation caused by massive OTEC seawater flow rates on the thermal structure of the ocean. Not surprisingly, maximum OTEC power nearly corresponds to deep cold seawater flow rates of the order of the average abyssal upwelling representative of the global thermohaline circulation.

Method and apparatus for converting thermal energy to mechanical energy

Abstract: A method and apparatus for converting thermal energy to mechanical energy which can use a wide range of fuels and perform with a high efficiency. Operating on a little utilized thermodynamic cycle of isentropic compression, isothermal expansion, isentropic expansion and finally constant pressure cooling and contraction. The external heat engine utilizes a heat exchanger carrying heat from the external energy source to the working parts of the engine. Pistons and cylinders are activated by appropriate means to adiabatically compress the working fluid, for example ambient air, to transfer the entire mass of the air through the heat exchanger to accomplish isothermal expansion followed by adiabatic expansion and, finally, exhaust the air to ambient to allow for constant pressure cooling and contraction. Valve pistons in conjunction with the cylinders form valves that allow for the exchange of working fluid with ambient. Energy is added to the engine during isothermal expansion, whereby the energy of compression is added by a flywheel or other appropriate energy storage means, said flywheel stores energy recovered during adiabatic expansion. The thermodynamic cycle described and the engine embodiments disclosed, when run in reverse, perform as a heat pump or refrigeration device.

Electronics assembly and heat pipe device

Abstract: An electronics assembly is provided having a heat pipe device for cooling electronics. The assembly includes a substrate and an electronics package supported on the substrate. The assembly also includes a heat pipe device in thermal communication with an exposed surface of the electronics package. The heat pipe device includes a thermal conductive pipe having an internal volume and an open end. The heat pipe device also includes a thermal conductive end cap positioned to close the open end of the pipe. The end cap has an outer surface for receiving in thermal communication an electronics package. The heat pipe device further includes a cooling fluid disposed in the internal volume of the pipe for transferring thermal energy from the end cap to the outside environment.

Timescale and magnitude of plasma thermal energy loss before and during disruptions in JET

Abstract: In this paper we analyse and discuss the thermal energy loss dynamics before and during JET disruptions that occurred between 2002 and 2004 in discharges which reached >4.5 MJ of thermal energy. We observe the slow thermal energy transients with diamagnetic loops and the fast ones with electron cyclotron emission and soft x-ray diagnostics. For most disruption types in JET, the plasma thermal energy at the time of the thermal quench is substantially less than that of the full performance plasma, typically in the range of 10–50% depending on plasma conditions and disruption type. The exceptions to this observation are disruptions in plasmas with a strong internal transport barrier (ITB) and in discharges terminating in a pure vertical displacement event, in which the plasma conserves a very high energy content up to the thermal quench. These disruption types are very sudden, leaving little scope for the combined action of soft plasma landing strategies and intrinsic performance degradation, both requiring >500 ms to be effective, to decrease the available thermal energy. The characteristic time for the loss of energy from the main plasma towards the PFCs in the thermal quench of JET disruptions is in the range 0.05–3.0 ms. The shortest timescales are typical of disruptions caused by excessive pressure peaking in ITB discharges. The available thermal energy fraction and thermal quench duration observed in JET can be processed (with due caution) into estimates for the projected PFC lifetime of the ITER target.