Showing papers on "Thermal energy published in 2009"

Review on thermal energy storage with phase change materials and applications

Abstract: The use of a latent heat storage system using phase change materials (PCMs) is an effective way of storing thermal energy and has the advantages of high-energy storage density and the isothermal nature of the storage process. PCMs have been widely used in latent heat thermal-storage systems for heat pumps, solar engineering, and spacecraft thermal control applications. The uses of PCMs for heating and cooling applications for buildings have been investigated within the past decade. There are large numbers of PCMs that melt and solidify at a wide range of temperatures, making them attractive in a number of applications. This paper also summarizes the investigation and analysis of the available thermal energy storage systems incorporating PCMs for use in different applications.

The behavior of self-compacting concrete containing micro-encapsulated Phase Change Materials

Abstract: In order to come to a sustainable built environment the construction industry requires new energy saving concepts. One concept is to use Phase Change Materials (PCM), which have the ability to absorb and to release thermal energy at a specific temperature. This paper presents a set of experiments using different amounts of PCM in self-compacting concrete mixes. The study focuses on the direct mixing of micro-encapsulated PCM with concrete and its influence on the material properties. Therefore, the fresh concrete properties and the hardened properties are investigated. The hardened properties comprise strength tests and a thorough assessment of the thermal properties. It will be shown that increasing PCM amounts lead to lower thermal conductivity and increased heat capacity, which both significantly improve the thermal performance of concrete and therefore save energy. On the other hand a significant loss in strength and micro-structural analysis both indicate that a large part of the capsules is destroyed during the mixing process and releases its paraffin wax filling into the surrounding matrix. However, the compressive strength of our specimens still satisfies the demands of most structural applications.

Direct Calculation of the Radiative Efficiency of an Accretion Disk Around a Black Hole

Abstract: Numerical simulation of magnetohydrodynamic (MHD) turbulence makes it possible to study accretion dynamics in detail. However, special effort is required to connect inflow dynamics (dependent largely on angular momentum transport) to radiation (dependent largely on thermodynamics and photon diffusion). To this end, we extend the flux-conservative, general relativistic MHD (GRMHD) code HARM from axisymmetry to full three dimensions. The use of an energy conserving algorithm allows the energy dissipated in the course of relativistic accretion to be captured as heat. The inclusion of a simple optically thin cooling function permits explicit control of the simulated disk's geometric thickness as well as a direct calculation of both the amplitude and location of the radiative cooling associated with the accretion stresses. Fully relativistic ray-tracing is used to compute the luminosity received by distant observers. For a disk with aspect ratio H/r 0.1 accreting onto a black hole with spin parameter a/M = 0.9, we find that there is significant dissipation beyond that predicted by the classical Novikov-Thorne model. However, much of it occurs deep in the potential, where photon capture and gravitational redshifting can strongly limit the net photon energy escaping to infinity. In addition, with these parameters and this radiation model, significant thermal and magnetic energy remains with the gas and is accreted by the black hole. In our model, the net luminosity reaching infinity is 6% greater than the Novikov-Thorne prediction. If the accreted thermal energy were wholly radiated, the total luminosity of the accretion flow would be 20% greater than the Novikov-Thorne value.

Model Predictive Control of thermal energy storage in building cooling systems

Abstract: A preliminary study on the control of thermal energy storage in building cooling systems is presented. We focus on buildings equipped with a water tank used for actively storing cold water produced by a series of chillers. Typically the chillers are operated each night to recharge the storage tank in order to meet the buildings demand on the following day. A Model Predictive Control (MPC) for the chillers operation is designed in order to optimally store the thermal energy in the tank by using predictive knowledge of building loads and weather conditions. This paper addresses real-time implementation and feasibility issues of the MPC scheme by using a (1) simplified hybrid model of the system, (2) periodic robust invariant sets as terminal constraints and (3) a moving window blocking strategy.

Thermoelectric Power Generation Using Waste-Heat Energy as an Alternative Green Technology

Abstract: In recent years, an increasing concern of environmental issues of emissions, in particular global warming and the limitations of energy resources has resulted in extensive research into novel technologies of generating electrical power. Thermoelectric power generators have emerged as a promising alternative green technology due to their distinct advantages. Thermoelectric power generation offer a potential application in the direct conversion of waste-heat energy into electrical power where it is unnecessary to consider the cost of the thermal energy input. The application of this alternative green technology in converting waste-heat energy directly into electrical power can also improve the overall efficiencies of energy conversion systems. In this paper, a background on the basic concepts of thermoelectric power generation is presented and recent patents of thermoelectric power generation with their important and relevant applications to waste-heat energy are reviewed and discussed.

Non-Equilibrium Molecular Dynamics Study of Thermal Energy Transport in Au-SAM-Au junctions

Abstract: Non-equilibrium molecular dynamics (NEMD) simulations were performed on Au-SAM (self-assembly monolayer)-Au junctions to study the thermal energy transport across the junctions. Thermal conductance of the Au-SAM interfaces was calculated. Temperature effects, simulated external pressure effects, SAM molecule coverage effects and Au-SAM bond strength effects on the interfacial thermal conductance were studied. It was found that the interfacial thermal conductance increased with temperature increase at temperatures lower than 250K, but it did not have large changes at temperatures from 250K to 400K. Such a trend was found to be similar to experimental observations on similar junctions. The simulated external pressure did not affect the interfacial thermal conductance. SAM molecule coverage and Au-SAM bond strength were found to significantly affect on the thermal conductance. The vibration densities of state (VDOS) were calculated to explore the mechanism of thermal energy transport. Interfacial thermal resistance was found mainly due to the limited population of low-frequency vibration modes of the SAM molecule. Ballistic energy transport inside the SAM molecules was confirmed, and the anharmonicity played an important role in energy transport across the junctions. A heat pulse was imposed on the junction substrate, and heat dissipation inside the junction was studied. Analysis of the junction response to the heat pulse showed that the Au-SAM interfacial thermal resistance was much larger than the Au substrate and SAM resistances separately. This work showed that both the Au substrate and SAM molecules transported thermal energy efficiently, and it was the Au-SAM interfaces that dominated the thermal energy transport across the Au-SAM-Au junctions.

On the thermodynamic origin of the quantum potential

Abstract: In a new thermodynamic interpretation, the quantum potential is shown to result from the presence of a subtle thermal vacuum energy distributed across the whole domain of an experimental setup. Explicitly, its form is demonstrated to be exactly identical to the heat distribution derived from the defining equation for classical diffusion wave fields. For a single free particle path, this thermal energy does not significantly affect particle motion. However, in between different paths, or at interfaces, the accumulation–depletion law for diffusion waves provides an immediate new understanding of the quantum potential’s main features.

Toward zero-emission data centers through direct reuse of thermal energy

Abstract: We have tested hot water data center cooling by directly reusing the generated thermal energy in neighborhood heating systems. First, we introduce high-performance liquid cooling devices with minimal thermal resistance in order to cool a computer system board. This cooling is performed with water at a temperature as high as 608C, thereby eliminating the chillers and their electrical power consumption, and enabling direct reuse of the heat. We collect 85% of the board heat using microscale liquid coolers for CPUs (central processing units), interfaces, and dc (direct current) converters. With our concept, data centers can be cooled in all climate zones throughout the year without a pre-cooled heat carrier. Second, we analyze how the supply of heat and financial payback from customers reduce the total cost of ownership. With 5,000 district heating systems satisfying 9.7% of the thermal demand of Europe, ample opportunities exist for data centers to become heat providers, thereby reducing the associated carbon dioxide emission. Finally, we show how our concept can be developed within 5 years into a zero-emission data center and that such investments are economically viable and ecologically beneficial given increasing energy prices. With such data centers, the IT (information technology) industry can assume a key role in greatly reducing carbon dioxide emissions and global warming by replacing energy-intensive processes with more efficient, digitally assisted processes.

A review on transportation of heat energy over long distance: Exploratory development

Abstract: This paper presents a review on transportation of heat energy over long distance. For the transportation of high-temperature heat energy, the chemical catalytic reversible reaction is almost the only way available, and there are several reactions have been studied. For the relatively low-temperature heat energy, which exists widely as waste heat, there are mainly five researching aspects at present: chemical reversible reactions, phase change thermal energy storage and transportation, hydrogen-absorbing alloys, solid–gas adsorption and liquid–gas absorption. The basic principles and the characteristics of these methods are discussed.