Showing papers on "Thermal energy published in 2008"

Energy storage systems—Characteristics and comparisons

Abstract: Electricity generated from renewable sources, which has shown remarkable growth worldwide, can rarely provide immediate response to demand as these sources do not deliver a regular supply easily adjustable to consumption needs. Thus, the growth of this decentralized production means greater network load stability problems and requires energy storage, generally using lead batteries, as a potential solution. However, lead batteries cannot withstand high cycling rates, nor can they store large amounts of energy in a small volume. That is why other types of storage technologies are being developed and implemented. This has led to the emergence of storage as a crucial element in the management of energy from renewable sources, allowing energy to be released into the grid during peak hours when it is more valuable. The work described in this paper highlights the need to store energy in order to strengthen power networks and maintain load levels. There are various types of storage methods, some of which are already in use, while others are still in development. We have taken a look at the main characteristics of the different electricity storage techniques and their field of application (permanent or portable, long- or short-term storage, maximum power required, etc.). These characteristics will serve to make comparisons in order to determine the most appropriate technique for each type of application.

Phase change material-based building architecture for thermal management in residential and commercial establishments

Abstract: Efficient and economical technology that can be used to store large amounts of heat or cold in a definite volume is the subject of research for a long time. Thermal storage plays an important role in building energy conservation, which is greatly assisted by the incorporation of latent heat storage (LHS) in building products. LHS in a phase change material (PCM) is very attractive because of its high storage density with small temperature swing. It has been demonstrated that for the development of a latent heat storage system (LHTS) in a building fabric, the choice of the PCM plays an important role in addition to heat transfer mechanism in the PCM. Thermal energy storage in the walls, ceiling and floor of buildings may be enhanced by encapsulating or embedding suitable PCMs within these surfaces. They can either capture solar energy directly or thermal energy through natural convection. Increasing the thermal storage capacity of a building can increase human comfort by decreasing the frequency of internal air temperature swings so that the indoor air temperature is closer to the desired temperature for a longer period of time. This paper aims to gather the information from the earlier works on the developments of PCM's incorporation in building, the problems associated with the selection of PCM and the various methods used to contain them for space heating and cooling applications.

Review of mathematical modeling on latent heat thermal energy storage systems using phase-change material

Abstract: Mathematical modeling of a latent heat thermal energy storage system (LHTES) was used for the optimum material selection and to assist in the optimal designing of the systems. In this paper, two types of models are mainly discussed, on the basis of first law and second law of thermodynamics. The important characteristics of different models and their assumptions used are presented and discussed, the experimental validation of some models are also presented.

Pyroelectric energy conversion: Optimization principles

Abstract: In the framework of microgenerators, we present in this paper the key points for energy harvesting from temperature using ferroelectric materials. Thermoelectric devices profit from temperature spatial gradients, whereas ferroelectric materials require temporal fluctuation of temperature, thus leading to different applications targets. Ferroelectric materials may harvest perfectly the available thermal energy whatever the materials properties (limited by Carnot conversion efficiency) whereas thermoelectric material's efficiency is limited by materials properties (ZT figure of merit). However, it is shown that the necessary electric fields for Carnot cycles are far beyond the breakdown limit of bulk ferroelectric materials. Thin films may be an excellent solution for rising up to ultra-high electric fields and outstanding efficiency. Different thermodynamic cycles are presented in the paper: principles, advantages, and drawbacks. Using the Carnot cycle, the harvested energy would be independent of materials properties. However, using more realistic cycles, the energy conversion effectiveness remains dependent on the materials properties as discussed in the paper. A particular coupling factor is defined to quantify and check the effectiveness of pyroelectric energy harvesting. It is defined similarly to an electromechanical coupling factor as k2 = p2thetas0/(epsivthetas 33 CE), where p, thetas0, epsivthetas 33, Ce are pyroelectric coefficient, maximum working temperature, dielectric permittivity, and specific heat, respectively. The importance of the electrothermal coupling factor is shown and discussed as an energy harvesting figure of merit. It gives the effectiveness of all techniques of energy harvesting (except the Carnot cycle). It is finally shown that we could reach very high efficiency using lang111rang0.75Pb(Mg1/3Nb2/3)-0.25PbTiO3 single crystals and synchronized switch harvesting on inductor (almost 50% of Carnot efficiency). Finally, practical implementation key points of pyroelectric energy harvesting are presented showing that the different thermodynamic cycles are feasible and potentially effective, even compared to thermoelectric devices.

Combustion synthesis and characterization of nanocrystalline WO3.

Abstract: The energy payback time associated with the semiconductor active material is an important parameter in a photovoltaic solar cell device. Thus lowering the energy requirements for the semiconductor synthesis step or making it more energy-efficient is critical toward making the overall device economics more competitive relative to other nonpolluting energy options. In this communication, combustion synthesis is demonstrated to be a versatile and energy-efficient method for preparing inorganic oxide semiconductors such as tungsten trioxide (WO3) for photovoltaic or photocatalytic solar energy conversion. The energy efficiency of combustion synthesis accrues from the fact that high process temperatures are self-sustained by the exothermicity of the combustion process, and the only external thermal energy input needed is for dehydration of the fuel/oxidizer precursor mixture and bringing it to ignition. Importantly, we show that, in this approach, it is also possible to tune the optical characteristics of the oxide semiconductor (i.e., shift its response toward the visible range of the electromagnetic spectrum) in situ by doping the host semiconductor during the formative stage itself. As a bonus, the resultant material shows enhanced surface properties such as markedly improved organic dye uptake relative to benchmark samples obtained from commercial sources. Finally, this synthesis approach requires only very simple equipment, a feature that it shares with other "mild" inorganic semiconductor synthesis routes such as sol-gel chemistry, chemical bath deposition, and electrodeposition. The present study constitutes the first use of combustion synthesis for preparing WO3 powder comprising nanosized particles.

Heat exchange enhancement

Abstract: A heat exchange structure includes elongated air ducts. Each air duct has a first opening and a second opening at two ends of the air duct to allow air to enter and exit the air duct, respectively. The heat exchange structure includes an exterior heat exchange surface and interior heat exchange surfaces, in which the exterior heat exchange surface is configured to receive thermal energy from heat generators that are mounted on the exterior heat exchange surface, and the exterior heat exchange surface dissipates a portion of the thermal energy received from the heat generators and transfers another portion of the thermal energy to the interior heat exchange surfaces. The interior heat exchange surfaces are in the elongated air ducts and configured to exchange thermal energy with air flowing in the air ducts, enhancing air flow in the air ducts by buoyancy of heated air.

Power generation for implantable devices

Abstract: An implantable, rechargeable medical system comprised of an implanted device, a power storage device connected to the implantable device, and a charging device operatively connected to the electrical storage device. The charging device can be thermoelectric and have components for transferring thermal energy from an intracranial heat accumulator to an extra-cranial heat sink, for generating an electrical current from the thermal energy transfer, for charging the electrical storage device using the electrical current, for measuring power generation, usage and reserve levels, for measuring temperatures of the intracranial and extra-cranial components, for physically disrupting heat transfer and charging operations, and for generating signals relevant to the status of temperature and electricity transfer in relation to energy generation criteria. The system may also have long-range and short range wireless power harvesting capability as well as movement, and photovoltaic charging capability. Components may be dual purpose, being used for receiving wireless energy as well as for accomplishing other operations such as sensing or stimulating. Specialized accessories assist with providing enhanced wireless power charging.

Latent Heat Storage for Solar Steam Systems

Abstract: Solar thermal systems, including direct steam generation in the absorbers, require isothermal energy storage systems. One option to fulfil this requirement is the application of phase change materials (PCMs) to absorb or release energy. The implementation of cost-effective storage systems demands the compensation of the low thermal heat conductivity that is characteristic for the candidate materials for PCM. Solar steam generation for power plants requires latent heat storage systems for a saturation temperature range between 200°C and 320°C. This paper describes the basic concepts investigated and first results of research activities aiming at the demonstration of a storage system using steam provided by parabolic trough collectors.

A dynamic simulation model for transient absorption chiller performance. Part I The model

Abstract: This paper is the first of two which presents the development of a dynamic model for single-effect LiBr/water absorption chillers. The model is based on external and internal steady-state enthalpy balances for each main component. Dynamic behaviour is implemented via mass storage terms in the absorber and generator, thermal heat storage terms in all vessels and a delay time in the solution cycle. A special feature is that the thermal capacity is partly connected to external and partly to internal process temperatures. In this paper, the model is presented in detail. For verification, the model has been compared to experimental data. The dynamic agreement between experiment and simulation is very good with dynamic deviations around 10 s. General functionality of the model and a more detailed comparison with experimental data are presented in Part II of this paper.

Combined cycle power plant

Abstract: Combined cycle power plants and related methods are disclosed here. In the plants, a mediating thermal energy storage unit (18) is used to store waste or residual thermal energy recovered from a heat engine employing a top thermodynamic cycle (10) of the combined cycle power plant, so that the stored residual thermal energy may be used as an energy source in a bottom thermodynamic cycle (11) of the power plant. In the combined cycle power plants described here, the heat engine employing a top cycle may comprise a Brayton cycle heat engine and the heat engine employing the bottom thermodynamic cycle may be a Rankine cycle heat engine.

Integrated solar energy receiver-storage unit

Abstract: Receivers for use in solar energy collector systems and solar-powered electrical energy generating plants are provided. The receivers (400) comprise a solar radiation absorbing core (414) that converts absorbed solar radiation to thermal energy. The core (414) comprises a refractory material to allow the receivers to operate continuously at high temperatures reached by absorbing concentrated solar radiation. The thermal energy so generated in the core may be stored in the receiver for a transitory period, or for a more extended period. Receivers (400) may comprise one or more fluid channels (415) in and/or around the core for conveying a working fluid to facilitate extraction of stored thermal energy from the core.

Reduced-emission gasification and oxidation of hydrocarbon materials for liquid fuel production

Abstract: A system and process are disclosed for the controlled combustion of a wide variety of hydrocarbon feedstocks to produce thermal energy, liquid fuels, and other valuable products with little or no emissions. The hydrocarbon feeds, such as coal and biomass, are first gasified and then oxidized in a two-chamber system/process using pure oxygen rather than ambient air. A portion of the intermediate gases generated in the system/process are sent to a Fischer-Tropsch synthesis process for conversion into diesel fuel and other desired liquid hydrocarbons. The remaining intermediate gases are circulated and recycled through each of the gasification/oxidation chambers in order to maximize energy production. The energy produced through the system/process is used to generate steam and produce power through conventional steam turbine technology. In addition to the release of heat energy, the hydrocarbon fuels are oxidized to the pure product compounds of water and carbon dioxide, which are subsequently purified and marketed. The system/process minimizes environmental emissions.

Integrated biofuel production system

Abstract: According to an embodiment, a biomass conversion subsystem produces methane and/or alcohol and residual biomass. A pyrolysis or a gasification subsystem is used to produce thermal energy and/or process gasses. The thermal energy may be stored thermal energy in the form of a pyrolysis oil. A fuel conversion subsystem produces liquid hydrocarbon fuels from the methane and/or alcohol using thermal energy and/or process gasses produced by the gasification or pyrolysis subsystem. Because the biomass production system integrates the residual products from biomass conversion and the residual thermal energy from pyrolysis or gasification, the overall efficiency of the integrated biomass production system is greatly enhanced.