Direct Conversion Of Heat Research Articles

A Novel Solar System of Electricity and Heat

Thermoelectric devices may have an essential role in the development of fuel-saving, environmentallyfriendly, and cost-effective energy sources for power generation based on the direct conversion of heat into electrical energy. A wide usage of thermoelectric energy systems already exhibits high reliability and long operation time in the space industry and gas pipe systems. The development and application of solar thermoelectric generators (TEGs) arelimited mainly by relatively low thermoelectric conversion efficiency. Forthe first time, we propose to use the direct energy conversion of solar energy by TEGs based on the high-performance multilayer thermoelectric modules with electric efficiency of ~15%. Solar energy was absorbed and converted to thermal energy, which is accumulated by a phase-change material (aluminum alloys at solidification temperature ~900 K). The heat flow from the accumulator through the thermoelectric convertor (generator) allows electrical power to be obtained and the exhaust energy to be used for household purposes (heating and hot water supply) or for the operation of a plant for thermal desalination of water.

Development of Solar Energy Systems Based on High Performance Bulk and Film Thermoelectric Modules

Abstract: Due to the increase in energy demand and depletion of natural resources, the development of energy harvesting technologies becomes very important. Thermoelectric devices, based on the direct conversion of heat into electrical energy, are being the essential part of cost-effective, environmental-friendly, and fuel-saving energy sources for power generation, temperature sensors, and thermal management. High reliability and long operation time of thermoelectric energy systems lead to their extensive use in space industry and gas pipe systems. Development and wide application of solar thermoelectric converters (generators) is mainly limited by relatively low thermoelectric conversion efficiency. In this work, we suggest for the first time to use direct conversion of solar energy by systems based on high-performance multistage thermoelectric modules operating in the temperature range of 300 - 900 K for creation of autonomic systems with electric power up to 500 W and electric efficiency up to 15 %. Furthermore, we developed film thermoelectric modules on thin flexible substrates with the figure of merit Z corresponding to that of bulk modules. Such film thermoelectric converters with output voltage of several volts and electric power of several microwatts can be used at micro-solar energy systems.

Recent trends in thermoelectrochemical cells and thermally regenerative batteries

Given the ever-growing awareness on global warming, much interest has focused on new and effective ways to manage energy, especially by harvesting and exploiting low-temperature heat sources, ubiquitous in the modern environment. Here, the holy grail is the direct conversion of heat into electricity especially using thermoelectric devices, and in this contribution, we focus on thermoelectrochemical systems. We give a brief overview of the most common thermally regenerative electrochemical cells developed nowadays with a short overview of their thermodynamic derivation, and we collect some of the most recent results in terms of their thermoelectrochemical properties, in particular, their temperature coefficients. We see that although the most used redox couples are based on Fe 3+ /Fe 2+ and their derivates, thermodiffusion effects and other entropy-related phenomena are attracting the attention of the scientific community and boosting astonishing results. On the other hand, thermally regenerative batteries are emerging, showing modest performance.

Analysis of thermoelectric generator incorporating n-magnesium silicide and p-tetrahedrite materials

Thermoelectric (TE) generators allow the direct conversion of heat into electricity without moving parts and with little maintenance needs if carefully designed. However, their high cost per unit power produced has prevented their widespread use in waste heat recovery applications. The use of earth-abundant, affordable, non-toxic and performant TE materials such as n-silicide and p-tetrahedrite for TE generators is assessed in the present analysis. A full multiphysics model was used to optimize the geometry of an n-p TE pair/module using these materials and assess its performance. The approach taken implemented the matched load condition automatically and a broad range of conditions was tested and discussed. Several pairs with the optimal geometry were preliminarily manufactured, assembled and characterized. The experimental results were compared against the predictions, allowing to validate the multiphysics model. Power densities up to 5 kW/m2 were predicted for temperatures between 290 K and 620 K, with efficiencies up to 5%. If modules with such performance could be manufactured with electric contact resistances comparable to those found in currently available commercial modules, then a door to the viable recovery of waste heat in industrial and automotive applications could be opened.

Modelling and Thermodynamic Analysis of a Hot-Cold Conversion Pipe Using R134a-DMF-He as the Working Pair

Based on the concept of a diffusion absorption system, a hot-cold conversion pipe utilizing 1,1,1,2-tetrafluoroethane (R134a)-dimethylformamide (DMF)-helium (He) as the working pair is presented with the aim of cooling output by recovering the low-grade waste heat. The model of the hot-cold conversion pipe is established, in which a heat pipe is used to transfer the waste heat as the heat input. The equations of the thermodynamic properties of the working pair are established by equation of state method (EOS). The model of the hot-cold conversion pipe is built based on the mass, species and energy balance equations of each component. The direct conversion of heat to cold is achieved by the desorption, absorption, condensation and diffusion evaporation processes of R134a. The hot-cold conversion pipe is cooled by natural convection, which can be enhanced by chimney effect. The thermodynamic analysis is carried out to analyze the effect of the boundary conditions, i.e. the heat source temperature, the refrigeration temperature, and the environmental temperature, on the system performance. This paper provides a theoretical basis for actual application of the hot-cold conversion pipe in waste heat recovery field.

Synthesis, characterization and applications of Aniline passivated bismuth selenide thermoelectric nanoparticles

Thermoelectric materials have been around since more than a hundred years with applications related to cooling and power generation. The most commonly used thermoelectric materials were typically semiconductors like tellurides and selenides of bismuth, lead, antimony and their alloys. A recent revival of interest in the study of these materials has been propelled by the global needs for alternative sources of energy and by the advances in field of novel material synthesis and characterization. In particular, bismuth selenide (Bi2Se3) has been characterized as efficient n-type thermoelectric materials. Another intriguing property of Bi2Se3 is that it behaves like a topological insulator with a conducting surface and an insulating bulk. This has also led to extensive studies on electronic properties of Bi2Se3.The thermoelectric effect or Seebeck effect is the direct con-version of heat into electric energy when two junctions made up of two dissimilar conductors are held at two different temperatures, T1and T2, with T1 > T2 lead to a voltage being developed between the two junctions. In the present investigation Bismuth selenide nanoparticles are synthesized by two different physical methods namely co-precipitation and hydrothermal method. The resultant nanostructures are characterized by X ray diffraction , SEM , FTIR , UV spectroscopy etc.

Small Autonomous kW-Level Power Generation Based on Radioisotope and Renewable Energy Sources for the Arctic Zone and the Far East

In the near future, the demand for autonomous kW-level electric power generating plants in the Arctic zone and in the Far East will number in the tens and hundreds. In the present article, the operational characteristics of radioisotope thermoelectric generators with direct conversion of heat into electricity, which are being decommissioned, are analyzed. Schemes for autonomous electric power plants based on renewable sources of energy and electrochemical cells are proposed as alternatives. The problems arising in the development of such plants are analyzed. The results achieved in meeting concrete orders from operating organizations are discussed. It is concluded that autonomous electric power plants with renewable sources of energy and fuel cells hold promise.

Enhancing Thermoelectric Properties through Control of Nickel Interstitials and Phase Separation in Heusler/Half-Heusler TiNi1.1Sn Composites.

Thermoelectric devices, which allow direct conversion of heat into electrical energy, require materials with improved figures of merit () in order to ensure widespread adoption. Several techniques have been proposed to increase the of known thermoelectric materials through the reduction of thermal conductivity, including heavy atom substitution, grain size reduction and inclusion of a semicoherent second phase. The goal in these approaches is to reduce thermal conductivity through phonon scattering without modifying the electronic properties. In this work, we demonstrate that Ni interstitials in the half-Heusler thermoelectric TiNiSn can be created and controlled in order to improve physical properties. Ni interstitials in TiNiSn are not thermodynamically stable and, instead, are kinetically trapped using appropriate heat treatments. The Ni interstitials, which act as point defect phonon scattering centers and modify the electronic states near the Fermi level, result in reduced thermal conductivity and enhance the Seebeck coefficient. The best materials tested here, created from controlled heat treatments of TiNiSn samples, display = 0.26 at 300 K, the largest value reported for compounds in the Ti–Ni–Sn family.

Thermionic Energy Conversion in the Twenty-first Century: Advances and Opportunities for Space and Terrestrial Applications

Thermionic energy conversion is the direct conversion of heat into electricity by the mechanism of thermionic emission, the spontaneous ejection of hot electrons from a surface. Although the physical mechanism has been known for over a century, it has yet to be consistently realized in a manner practical for large-scale deployment. This perspective article provides an assessment of the potential of thermionic energy conversion systems for space and terrestrial applications in the 21st century, overviewing recent advances in the field and identifying key research challenges. Recent developments as well as persisting research needs in materials, device design, fundamental understanding, and testing and validation are discussed.

Materials science: Nanomagnets boost thermoelectric output.

The direct conversion of heat into electricity — a reversible process known as the thermoelectric effect — can be greatly enhanced in some materials by embedding them with a small number of magnetic nanoparticles. See Letter p.247 Increasing the efficiency of thermal-to-electric energy conversion using thermoelectric materials typically focuses on nanostructuring—in particular, varying the chemical and physical properties at the nanoscale to modify the thermal conductivity of the whole material. Electrical conductivity is also a key parameter and now Wenyu Zhao et al. report a nanostructuring strategy that offers simultaneous control over thermal and electrical transport, with a net gain in overall thermoelectric performance of the material. They embed nanoparticles of a soft magnetic material in a thermoelectric matrix. The nanoparticles themselves provide conventional scattering sites for thermal vibrations (phonons) in the matrix while superparamagnetic fluctuations of the magnetization state of the nanoparticles interact with the matrix electrons, separately modifying their transport properties.

Prediction of Optimal Load and Performance of Thermal Batteries

Nowadays, the possibility of applying alternative energy sources is increasingly discussed. In terms of application, a Seebeck thermoelectric generator seems promising for the direct conversion of heat (preferably waste heat) into electricity. In order to increase the performance and thereby the effectiveness of different thermoelectric modules, especially advantageous thermoelectric properties of materials and also innovative design of thermoelectric devices are now solved. However, the achievement of maximum possible performance and operating efficiency is not determined solely by material efficiency and geometric dimensions of thermocouples connected in a thermal battery. It is also necessary to predict an optimum load of the specific source of energy in order to use material design improvements optimally and ensure longer life of thermal batteries. Their manufacturers, except the power input as a “heat consumption” and the maximum power output of a thermal battery (at a certain temperature difference between cold and heated walls), should also declare such load by an appliance, which is optimal for the given type of thermogenerator. The present paper, therefore, proposes a method for determining the optimum load of a thermal battery.

Top-down fabrication of silicon nanowire devices for thermoelectric applications: properties and perspectives

In this paper, the most recent achievements in the field of device fabrication, based on nanostructured silicon, will be reviewed. Top-down techniques for silicon nanowire production based on lithography, oxidation and highly anisotropic etching (wet, plasma and metal assisted) will be discussed, illustrating both advantages and drawbacks. In particular, fabrication processes for a massive production of silicon nanowires, organized and interconnected in devices with macroscopic dimensions, will be shown and discussed. These macroscopic devices offer the possibility of exploiting the nanoscale thermoelectric properties of silicon in practical applications. In particular, the reduced thermal conductivity of silicon nanowires, with respect to bulk silicon, makes possible to obtain high efficiencies in the direct conversion of heat into electrical power, with intriguing applications in the field of green energy harvesting. The main experiments elucidating the electrical and thermal properties of silicon nanowire devices will be shown and discussed, and compared with the recent theoretical works developed on the subject.

Magneto Hydrodynamic Generator

Energy provides the power to progress. The natural resources of a country may be large but they can only be turned into wealth if they are developed, used and exchanged for other goods. This cannot be achieved without energy. Availability of sufficient energy and its proper use in any country can result in its people rising from subsistence level to the highest standards of living. It is interesting to note that more than half the population lives in Asia where the energy consumption is barely 8-10 % of the world’s total. (More than half the world’s primary energy consumed in North America and Western Europe.) Apart from the conventional energy sources of production of electricity which are Hydro, Thermal, and Nuclear, technological advances in progress for generation of electricity by direct means, i.e., without rotating parts such as ‘Magneto Hydro Dynamics (MHD)’. Direct conversion of heat into electrical energy by means of MHD converters has not yet made its technological break-through though Magneto Hydro Dynamic generators are being actively examined in laboratories throughout the world. In India considerable studies have been carried out in this field by a team of scientist under the National Council of Science and Technology (NCST). It has been proposed to evolve a pilot MHD generation plant during the next few years in the country. The basic principle of MHD generation is the same as that of a conventional electrical generator, namely that the motion of a conductor through a magnetic field induces a voltage in the conductor. In MHD method gasses at high temperature are passed at high velocity through a magnetic field at right angles to the direction of flow and electricity is collected at stationary electrodes placed on opposite sides of the channel. This type of generation would greatly simplify the equipment and is likely to be developed in the next two decades.

Recent progress in thermoelectric materials

Direct conversion of heat into electricity through advanced thermoelectric (TE) materials has been one of the most attractive solutions to the severe environmental and energy issues facing humanity. In recent years, great progress has been made in improving their dimensionless figure of merit (ZT), which determines the conversion efficiency of TE devices. ZT is related to three “interlocked” factors—the Seebeck coefficient, electrical conductivity, and thermal conductivity. These three factors are interdependent in bulk TE materials, and altering one changes the other two. The difficulty in simultaneously optimizing them caused TE research to stagnate, until great reductions in thermal conductivity were both theoretically and experimentally proven in nanomaterials in 1993. In this review, we first introduce some TE fundamentals and then review the most recently improvements in ZT in different kinds of inorganic and organic TE materials, which is followed by an investigation of the outlook for new directions in TE technology.

Probing thermopower on the microscale

AbstractThermoelectric (TE) generators provide electrical energy from direct conversion of heat by means of the Seebeck effect; without moving parts, completely silent, and with negligible maintenance. As any other heat engine this conversion exploits only a fraction of the Carnot efficiency (Rowe (ed.), CRC Handbook of Thermoelectrics (CRC Press Inc., 1995), p. 19 1). The TE efficiency is linked to the thermoelectric figure of merit Z, which itself is given by basic material properties: Z = S2σ/κ. These are the electrical conductivity σ, the thermal conductivity κ and the Seebeck coefficient or thermopower S, which is known as the factor of proportionality between voltage output and applied temperature difference in a given TE sample. A distinct sensitivity to the carrier concentration and structural variations make the control and stabilisation of thermopower very challenging in complex material structures since degradation by diffusion, decomposition or evaporation can be observed in many cases during synthesis, operation and even in the process of characterisation of TE semiconductors; particularly at elevated temperatures. Investigating structural and compositional properties, stability, and performance of TE materials, and consequently aiming to understand their interaction, mainly methods like X‐ray diffraction (XRD), energy‐dispersive X‐ray spectroscopy (EDX), scanning electron microscopy (SEM) or integral temperature dependent measurements of particular transport properties are used. Although TE materials research satisfies highest requirements on accuracy, the above mentioned techniques are not perfectly qualified to investigate promising material classes thoroughly. Against the background of usually complex material structures this article aims to show, that an efficient characterisation of TE materials becomes accessible for several questions by use of a spatially resolved determination of the thermopower.

Pyroelectric energy harvesting using Olsen cycles in purified and porous poly(vinylidenefluoride-trifluoroethylene) [P(VDF-TrFE)] thin films

This paper is concerned with the direct conversion of heat into electricity using pyroelectricmaterials. The Olsen (or Ericsson) cycle was experimentally performed on three different typesof 60/40 poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer samples,namely commercial, purified, and porous films. This cycle consists of two isoelectric fieldand two isothermal processes. The commercial and purified films were about50 µm thick and produced a maximum energy density of521 J l − 1 and426 J l − 1 per cycle,respectively. This was achieved by successively dipping the films in cold and hot silicone oil baths at 25and 110 °C under low and high applied electric fields of about 200 and500 kV cm − 1, respectively.The 11 µm thick porous films achieved a maximum energy density of188 J l − 1 per cycle between 25and 100 °C and electric fieldbetween 200 and 400 kV cm − 1. The performance of the purified and porous films suffered from their lower electrical resistivityand electric breakdown compared with commercial thin films. However, the energy densities ofall 60/40 P(VDF-TrFE) films considered matched or exceeded those reported recently for0.9Pb(Mg1/3Nb2/3)O3–0.10PbTiO3 (0.9PMN–0.1PT)(186 J l − 1)and Pb(Zn1/3Nb2/3)0.955Ti0.045O3 (243 J l − 1) bulk ceramics. Furthermore, the results are discussed in light of recently proposed figuresof merit for energy harvesting applications.

From Chemical Processes to Electrochemical Processes: The Key to Minimal Entropy Production

Exothermic chemical reactions exploited in the chemical industry today represent opportunities for the development of fuel cell like devices that can perform these reactions electrochemically. Thereby, the excessive generation of waste heat is avoided and electric power is produced. Moreover, the reactions that exhibit a positive entropy change (ΔS > 0) when performed electrochemically may enable the direct conversion of heat into power to compensate for ΔS. The rapid development of fuel cell technology appears to be an ideal precursor for such new developments. On the other hand, interest form the chemical industry for the development of fuel cell technology can boost the market penetration of conventional fuel cells. In some cases, conventional fuel cell types like the MCFC and the SOFC may be used without change, in other cases maybe only the anode or cathode catalyst materials have to be modified or replaced. New, dedicated electrochemical reactors may be developed. Because of the different reactions, reactants, and products, a new research area for the design of new electrocatalyst lies ahead.

Recent developments in thermoelectric materials

The increased activity in attempts to develop improved thermoelectric semiconductors for use in the direct conversion of heat into electrical energy results mostly from research sponsorship by the US Military and NASA. Thermoelectric generators have no moving parts and are difficult to detect by visual, aural or thermal infrared means. Fossil multifuelled thermoelectric generators are the leading candidates for replacing standard US Military engine generator sets up to 1·5 kW under the SLEEP programme (Signature Suppressed Lightweight Electric Energy Plants). When coupled to an isotopic heat source, thermoelectric generators are able to operate reliably and unattended for long periods of time and have a proven performance record in supplying electrical power to the Lunar Experimental Package (Apollo Program) and in providing onboard electrical power to the Voyager spacecrafts. In both military and space applications any improvement in the thermoelectric generators' conversion efficiency would result in a saving in fuel—an important consideration. One way of improving the conversion efficiency is by increasing the so called ‘Figure of merit’ of the semiconductor material employed in the fabrication of the generators' thermocouples. In this paper an assessment is made of current thermoelectric materials; recent attempts to improve the figure of merit of existing materials are discussed and a number of new thermoelectric materials described. Significant headway has been made in reducing the lattice thermal conductivity of thermoelectric materials through the use of additives, small grain sizes or combinations of both. This development will result in substantial improvements in the thermoelectric figure of merit, provided the electrical properties can be maintained close to single crystal values. It is concluded that, because in the past the development of new thermoelectric materials has occupied long periods of time, even during periods of intense research activity, it is likely that established or ‘modified’ established materials will remain the mainstay of military and space applications at least for the forseeable future.

The temperature-entropy diagram of “solid state engines” and “solid state heat pumping systems” with shape memory alloys

The direct conversion of heat into mechanical energy using the shape memory effect which occurs in some martensitic alloys is discussed. The representation of one possible thermal mechanical energy conversion cycle is given, and some reference cycles are proposed, with the temperature-entropy diagrams giving cycles familiar for motors operating with fluids. Possible uses for solid state motors are described. (PMA)

The direct conversion of heat into electricity. Magnetohydrodynamic conversion and electrogasdynamic conversion

The direct conversion of heat into electricity. Magnetohydrodynamic conversion and electrogasdynamic conversion