Performance of an air-cooled looped thermoacoustic engine capable of recovering low-grade thermal energy

An air-cooled looped thermoacoustic engine is designed and constructed, where an air-cooled cold heat exchanger (consisting of copper heat transfer block, aluminum flange, and aluminum fin plate) is adopted to extract heat and the resonant tube is spiraled and shaped to fit to the available space. Experiments have been conducted to observe how onset temperature difference and resonant frequency are affected by mean pressure, working fluid, and diameter of compliance tube. Besides, the influences of temperature difference, mean pressure, working fluid and diameter of compliance tube on pressure amplitude, output acoustic power, and thermal efficiency of the system have been investigated. The air-cooled looped thermoacoustic engine can start to oscillate at a lowest temperature difference of 46°C, with the working fluid of carbon dioxide at 2.34 MPa. A highest output acoustic power obtained is 6.65 W at a temperature difference of 199°C, with the working gas of helium at 2.58 MPa, and the thermal efficiency is 2.21%. This work verifies the feasibility of utilizing low-grade thermal energy to drive an air-cooled looped thermoacoustic engine and extends its application in the water deficient areas.     

Performance comparison of looped thermoacoustic electric generators with various thermoacoustic stages

Thermoacoustic engine is a kind of novel heat engine based on thermoacoustic effect, with the merits of environmental benignity, simplicity, and reliability. In this work, looped travelling-wave thermoacoustic electric generators (LTTEGs) with one to four thermoacoustic stages have been developed and experimentally studied. It is observed that adding thermoacoustic stages can improve the thermal-electric efficiency of LTTEGs, while whether the extra stages lead to efficiency gain depends on the number of existing stages and other operating parameters (hot temperature, for instance). One main reason is that the Gedeon streaming, which might cause severe heat loss, can be enhanced by adding thermoacoustic stages and increasing hot temperature. The results suggest that the suppression of streaming in the looped thermoacoustic engine with multiple stages is even more urgent than in the traditional travelling-wave engine with only one stage.     

Power generation by a thermomagnetic engine by hybrid operation of an electromagnetic generator and a triboelectric nanogenerator

This paper reports a thermomagnetic generator based on the magneto‐caloric effect of Gadolinium to scavenge low‐grade thermal energy and its eventual conversion into electrical energy. Gadolinium is one of the distinct materials whose Curie temperature is nearly room temperature. For this reason, a Gadolinium‐based thermomagnetic engine offers enormous possibilities to harness available waste heat from the industries. In this work, the performance of an electromagnetic generator (EMG) coupled with a triboelectric nanogenerator (TENG) was experimentally investigated as it is driven by a thermomagnetic engine exploiting low temperature waste heat. At the temperature difference of 46.5°C between the hot and cold water jets, the thermomagnetic engine produced an average rotational speed of 263 ± 1.5% rpm and a mechanical output power of 76.38 ± 0.5% mW. At the aforementioned rotational speed, the hybrid generator delivered a rectified peak voltage of 15.34 V on an open‐circuit, a short‐circuit current of 20.1 mA, and the largest power output, 12.1 mW, at a 120 Ω load. It was demonstrated that the proposed energy harvester could light dozens of light‐emitting diodes or power a digital thermometer. It is feasible that the proposed thermomagnetic generator can be utilized as an effective power source for various applications.     

The performance improvement of a cascade thermoacoustic engine by adjusting the acoustic impedance in the regenerator

A novel cascade configuration consisting of one standing wave unit and one travelling wave unit arranged in series is studied in this paper. Theoretically, a straight‐line cascade engine provides an efficient energy conversion, reduces the difficulties of fabrication and allows no Gedeon streaming. In order to achieve such a powerful cascade thermoacoustic engine, the regenerator of the travelling wave unit must be operated in high impedance and travelling wave phasing region. Various techniques of phase adjustment by modifying the configurations and geometrical dimensions of the system are investigated both numerically and experimentally in order to adjust the position of the sweet spot as well as to promote the acoustic impedance in the regenerator. It is found that the effective tuning methods with less modification here are accomplished by changing the volume of down‐cavity and reducing the flow area of down‐resonator by inserting the pencil. The exploration also shows that the acoustic field in the system is quite sensitive to the effect of down‐resonator length. The performance of the proposed system is clearly improved after the phase‐adjustment schemes are completely implemented, in which the regenerator works within the sweep spot zone with high acoustic impedance. Copyright © 2016 John Wiley & Sons, Ltd.     

Experimental study on a small‐scale pumpless organic Rankine cycle with R1233zd(E) as working fluid at low temperature heat source

This paper focuses on the novelty pumpless organic Rankine cycle (ORC) and its choice of working fluids. Based on the selection criteria, the refrigerant of R1233zd(E) is firstly chosen and investigated in the pumpless ORC system. In the system, the feed pump is removed, and the refrigerant flows back and forth between two heat exchangers, which act as the evaporator or condenser, respectively. The impacts of the heating water temperature and loads on the system performance are studied to find out the best operating conditions. The low‐grade heat source is simulated by an electric boiler. The temperature of the heat resource ranges from 80°C to 100°C with the interval of 5°C. The temperature of the cooling water inlet is 10°C and is kept constant. The largest average power output is 127 W under the condition of 100°C heating water with nine loads. Because the cycle efficiency with heating steam temperature of 100°C cannot be determined, the highest energy and exergy efficiencies are 3.5% and 17.1%, respectively, for heating water of 95°C with seven loads. The experimental results show that the energy and exergy efficiencies increase with the increase of the heating temperature. The power and current outputs increase when the loads increase under the condition of the constant heating water temperature, whereas the voltage output decreases meanwhile. The generating time increases when the loads increase. This phenomenon is mainly caused by the increasing evaporating pressure and decreasing condensing pressure when the loads increases.     

Performance analysis and validation on transportation of heat energy over long distance by ammonia–water absorption cycle

This paper presents the thermodynamic and hydrodynamic feasibility of the application of the ammonia–water absorption system for heat or cold transportation over long distance. A model of a long‐distance heat energy transportation system is built and analyzed, and it shows satisfactory and attractive results. When a steam heat source at the temperature of 120°C is available, the user site can get hot water output at about 55°C with the thermal COP of about 0.6 and the electric COP of about 100 in winter, and cold water output at about 8°C with the thermal COP of about 0.5 and the electric COP of 50 in summer. A small‐size prototype is built to verify the performance analysis. Basically the experimental data show good accordance with the analysis results. The ammonia–water absorption system is a potential prospective solution for the heat or cold transportation over long distance. Copyright © 2009 John Wiley & Sons, Ltd.     

Thermodynamic analysis and performance optimization of organic rankine cycles for the conversion of low‐to‐moderate grade geothermal heat

The present study considers a thermodynamic analysis and performance optimization of geothermal power cycles. The proposed binary‐cycles operate with moderately low temperature and liquid‐dominated geothermal resources in the range of 110°C to 160°C, and cooling air at ambient conditions of 25°C and 101.3 kPa reference temperature and atmospheric pressure, respectively. A thermodynamic optimization process and an irreversibility analysis were performed to maximize the power output while minimizing the overall exergy destruction and improving the First‐law and Second‐law efficiencies of the cycle. Maximum net power output was observed to increase exponentially with the geothermal resource temperature to yield 16–49 kW per unit mass flow rate of the geothermal fluid for the non‐regenerative organic Rankine cycles (ORCs), as compared with 8–34 kW for the regenerative cycles. The cycle First‐law efficiency was determined in the range of 8–15% for the investigated geothermal binary power cycles. Maximum Second‐law efficiency of approximately 56% was achieved by the ORC with an internal heat exchanger. In addition, a performance analysis of selected pure organic fluids such as R123, R152a, isobutane and n‐pentane, with boiling points in the range of ?24°C to 36°C, was conducted under saturation temperature and subcritical pressure operating conditions of the turbine. Organic fluids with higher boiling point temperature, such as n‐pentane, were recommended for non‐regenerative cycles. The regenerative ORCs, however, require organic fluids with lower vapour specific heat capacity (i.e. isobutane) for an optimal operation of the binary‐cycle. Copyright © 2015 John Wiley & Sons, Ltd.     

Oscillatory Flow in Thermoacoustic Sound Wave Generator

Oscillatory flow in a thermoacoustic sound wave generator is described. The thermoacoustic sound wave generator plays an important role in thermoacoustic equipment. The heat exchange between the working fluid and the stack, the acceleration and deceleration of the working fluid and viscous friction loss both in the stack and in the resonance tube influence the performance of the thermoacoustic sound wave generator. Particularly, oscillatory flow significantly influences the heat exchange mechanism between the working fluid and the stack. Temporal changes in pressure and velocity are sinusoidal inside the resonance tube. Flow forms an oscillatory jet just behind the tube outlet, and becomes intermittent far downstream outside the resonance tube. The open-end corrections of 0.63R, that is, the region where oscillatory flow characteristics are maintained downstream in spite of being outside the tube outlet, are confirmed by velocity measurements and flow visualization. Also, they are almost equal to acoustical theoretical results.     

A solar-powered traveling-wave thermoacoustic electricity generator

Traveling-wave thermoacoustic electricity generator is a new external-combustion type device capable of converting heat such as solar energy into electric power. In this paper, a 1 kW solar-powered traveling-wave thermoacoustic electricity generation system is designed and fabricated. The system consists of a traveling-wave thermoacoustic electricity generator, a solar dish collector and a heat receiver. In the preliminary tests, using electric cartridge heaters to simulate the solar energy, a maximum electric power of 481 W and a maximum thermal-to-electric efficiency of 15.0% were achieved with 3.5 MPa pressurized helium and 74 Hz working frequency. Then, after integrating the traveling-wave thermoacoustic electricity generator with the solar dish collector and the heat receiver, the solar-powered experiments were performed. In the experiments, a maximum electric power of about 200 W was obtained. However, due to the solar dish collector problems, the heating temperature of the receiver was much lower than expected. Optimizations of the collector and the heat receiver are under way.     

Techno‐economic comparison of boiler cold‐end flue gas heat recovery processes for efficient hard‐coal‐fired power generation

An important method to increase the efficiency of thermal power plants is to recover the exhaust gas heat at the boiler cold‐end with the stepwise integration of a steam turbine heat regenerative system. To this end, there are currently three typical heat recovery processes, that is, a low‐temperature economizer (LTE), segmented air heating (SAH) and bypass flue (BPF). To provide useful guidance to thermal power plants for optimal and efficient processes, the thermal economy and techno‐economic performance of the three aforementioned processes were calculated and compared using an in‐service 600‐MW hard‐coal‐fired ultra‐supercritical power unit as a reference. The results demonstrate that with the use of the LTE, SAH and BPF, respectively, to recover the exhaust heat, reducing the exhaust temperature from 122 °C to 90 °C, the net standard coal consumption rate of the 600‐MW unit can be reduced by 1.51, 1.71 and 2.81 g/(kW h). The initial costs of the three heat recovery projects are 1.69, 2.91 and 2.53 million USD. If the 600‐MW unit runs 5500 h per year at the rated load, the three processes can increase the earnings of the unit by 0.49, 0.52 and 0.94 million USD from coal savings annually, meaning that their dynamic payback periods are 4.42, 8.66 and 3.29 years, respectively. The results indicate that for a hard‐coal‐fired power unit, the coal savings achieved by exhaust heat recovery are notable. Among the three processes, SAH shows the worst techno‐economic performance because it induces a significant increase in initial costs while obtaining a limited increase in thermal economy, while BPF exhibits the best techno‐economic performance owing to the significant increase in thermal economy. Copyright © 2016 John Wiley & Sons, Ltd.     

Optimum packing factor of the stack in a standing‐wave thermoacoustic prime mover

A bench consisting of a pulse tube refrigerator driven by a standing‐wave thermoacoustic prime mover has been set up to study the relationship among stack, regenerator and working fluids. The stack of the thermoacoustic prime mover is packed with dense‐mesh wire screens because of their low cost and easy manufacture. The effect of the packing factor in the stack on onset temperature, refrigeration temperature and input power is explored. The optimum packing factor of 1.15 pieces per millimeter has been found experimentally, which supplies an empirical value to satisfy a compromise for enhancing thermoacoustic effect, decreasing heat conduction and fluid‐friction losses along the stack. The pulse tube cooler driven by the thermoacoustic prime mover is able to obtain refrigeration temperatures as low as 138 and 196K with helium and nitrogen, respectively. Copyright © 2002 John Wiley & Sons, Ltd.     

Comparative analysis of natural and conventional working fluids for use in transcritical Rankine cycle using low‐temperature geothermal source

Theoretical analyses of natural and conventional working fluids‐based transcritical Rankine power cycles driven by low‐temperature geothermal sources have been carried out with the methodology of pinch point analysis using computer models. The regenerator has been introduced and analyzed with a modified methodology considering the considerable variation of specific heat with temperature near the critical state. The evaluations of transcritical Rankine cycles have been performed based on equal thermodynamic mean heat rejection temperature and optimized gas heater pressures at various geothermal source temperature levels ranging from 80 to 120°C. The performances of CO₂, a natural working fluid most commonly used in a transcritical power cycle, have been indicated as baselines. The results obtained show: optimum thermodynamic mean heat injection temperatures of transcritical Rankine cycles are distributed in the range of 60 to 70% of given geothermal source temperature level; optimum gas heater pressures of working fluids considered are lower than baselines; thermal efficiencies and expansion ratios (Exp_r) are higher than baselines while net power output, volume flow rate at turbine inlet (V₁) and heat transfer capacity curves are distributed at both sides of baselines. From thermodynamic and techno‐economic point of view, R125 presents the best performances. It shows 10% higher net power output, 3% lower V₁, 1.0 time higher Exp_r, and 22% reduction of total heat transfer areas compared with baselines given geothermal source temperature of 90°C. With the geothermal source temperature above 100°C, R32 and R143a also show better performances. R170 shows nearly the same performances with baselines except for the higher V₁ value. It also shows that better temperature gliding match between fluids in the gas heater can lead to more net power output. Copyright © 2010 John Wiley & Sons, Ltd.     

Visualization investigation of the flow and heat transfer in thermoacoustic engine driven by loudspeaker

Heat transfer process in thermoacoustic engine is affected by acoustic oscillation which makes it different from the heat transfer in steady flow. This study pays attention to the flow and heat transfer characteristics of thermoacoustic engine driven by loudspeaker. Thermal infrared imager and particle image velocimetry (PIV) were used to investigate the temperature and flow fields under two heat levels (150 °C and 200 °C). The radial and axial temperature distribution was analyzed through dimensionless temperature. To explore the appropriate working frequency, resonance characteristic was discussed. The experimental results illustrated that the first resonance frequency is the most effective driving frequency where thermoacoustic system shows the best performance. Heat transfer mode changed from natural convection to forced convection with the addition of acoustic oscillation. Original temperature field induced by heat convection was destroyed and temperature gradient redistributed as parabolic after sound addition.     

Effects of ambient conditions on the thermodynamic performance of hybrid nuclear‐combined cycle power plant

This paper provides a theoretical study of the effects of ambient conditions on the thermodynamic performance of a hybrid combined‐nuclear cycle power plant. The operational parameters investigated are based on the first and second laws of thermodynamics, which include the ambient air temperature and ambient relative humidity (Φ). The results obtained for the gas turbine model are shown to agree very well with operational data from the Al‐Zour Emergency power plant in Kuwait. The ambient temperature was studied within the range of 0–55 °C. The analysis shows that the ambient air temperature has strong effects on plant performance and that operating the system at a high temperature will degrade the performance. Power output is reduced when the temperature is above the standard ambient temperature of 15 °C, and this loss rate is about 17% at 55 °C. The effect of ambient relative humidity (Φ) becomes significant only at higher temperatures. The ambient temperature has a large effect on the exergy destruction of the heat recovery steam generator exhaust, but it has little effect on other components of the plant. The analysis also indicates that reducing the temperature from 55 to 15 °C could help decrease the total exergy destruction of the plant by only 2%. Copyright © 2011 John Wiley & Sons, Ltd.     

Study of a longitudinal flux permanent magnet linear generator for wave energy converters

A directly coupled linear permanent magnet generator of longitudinal flux‐type is investigated. The generator will be used for power take‐off in a wave energy converter. A combined field‐ and circuit model, solved by a time stepping finite element technique, is used to model and analyse the electromagnetic behaviour of the machine. A large number of simulations form the basis of a design study where the influence of armature current level, number of cables per slot, and pole width is investigated with respect to efficiency, generator size, and the load angle. A case study is performed for a chosen generator design. The electromagnetic behaviour is examined both for nominal load and for overloads. The generator has a nominal output power of 10 kW for a constant piston speed of 0.7 ms^?1. The electromagnetic efficiency at nominal load is 86.0%, the load angle 6.6°, and the power fluctuation 1.3%. At 300% overload the load angle barely exceeds 12° and the cable temperature is below 25°C provided that the stator back is thermally connected to the sea water. The numerical calculations have been verified for small speeds by experiments. Copyright © 2006 John Wiley & Sons, Ltd.     

Power generation characteristics of a sandwich‐type self‐heating thermoelectric generator with spatially varying embedded heat source

Power generation characteristics of a sandwich‐type thermoelectric generator in which the heat source is embedded into thermoelectric elements are investigated. Our previous work on a similar concept only considered a uniform heat source distribution inside thermoelectric elements. In this work, the effect of the spatial distribution of a heat source is examined. In particular, the effect of the concentration of heat source near the one end, that is, the hot end, is intensively studied as a potential means of improving the efficiency of the device. Although the effects of heat source concentration in impractical cases without heat transfer limitations on the cold side remain ambiguous, it become clear that heat source concentration indeed has positive effects in more realistic cases with finite heat transfer coefficients imposed on the cold side. Because of the relatively low efficiency of typical thermoelectric generation, a significant amount of heat must be dissipated from the cold end of the thermoelectric element. Greater heat source concentration near the hot end leads to more effective utilization of available heat source, reduces the amount of heat rejected at the cold end, and lowers the hot end temperature of the thermoelectric element. Overall, it is suggested that heat source concentration can be used as a method to achieve more efficient operation and better structural integrity of the system. Copyright © 2015 John Wiley & Sons, Ltd.     

Influence of acoustic pressure amplifier dimensions on the performance of a standing-wave thermoacoustic system

A standing-wave thermoacoustic engine, employing an acoustic pressure amplifier (APA), is simulated with linear thermoacoustics to study the influence of APA’s dimensions on performance of the thermoacoustic system. Variations of operating parameters, including pressure ratio, acoustic power, hot end temperature of stack etc., versus length and diameter of APA are presented and discussed based on an analysis of pressure and velocity distribution in APA. Simulation results indicate that a largest amplification effect of both pressure ratio and acoustic power output is achieved at a critical length for the occurrence of pressure node and velocity antinode in APA, close to but less than one fourth of the wavelength. The distribution characteristics of pressure and velocity in APA are similar to a standing-wave acoustic field, which is the reason for the amplification effect. From the viewpoint of energy, the amplification effect results from the changed distribution of acoustic energy and acoustic power loss in the thermoacoustic system by APA. Experiments have been carried out to validate the simulation, and experimental data are presented.     

An electret-based thermoacoustic-electrostatic power generator

This study reports a new concept for power generation from thermal energy, which integrates a thermoacoustic engine (TAE) with a contact-free electret-based electrostatic transducer. The TAE converts thermal energy into high-intensity acoustic energy, while the electret-based electrostatic transducer converts the generated acoustic energy into electricity. The experiments demonstrate the feasibility and potential of the proposed electret-based thermoacoustic-electrostatic power generator (TAEPG). The dynamic response of the electrostatic transducer and energy conversion inside the TAE are further investigated using a lumped element model and a frequency-domain reduced-order network model. Good agreement is achieved between experimental measurements and theoretical predictions. Furthermore, a parametric study is performed to study the effect of key parameters including the external heating power, air gap, and resistive load on the performance of the TAEPG. Results show that an open-circuit voltage amplitude of 4.7 V is produced at a closed-end pressure amplitude of 480 Pa in the experiment, and it is estimated that 25.2% of the acoustic power generated by the TAE has been extracted by the electret-based electrostatic transducer. In this case, the maximum electric power output is measured to be 2.91 μW at a resistive load of around 2.2 MΩ. By increasing the external heating power, the TAEPG can produce a maximum voltage amplitude of 8 V. This work shows that the proposed concept has great potential for developing miniature heat-driven power generators.     

Performance analysis of a waste‐heat‐powered thermodynamic cycle for multieffect refrigeration

A multieffect refrigeration system that is based on a waste‐heat‐driven organic Rankine cycle that could produce refrigeration output of different magnitudes at different levels of temperature is presented. The proposed system is integration of combined ejector–absorption refrigeration cycle and ejector expansion Joule–Thomson (EJT) cooling cycle that can meet the requirements of air‐conditioning, refrigeration, and cryogenic cooling simultaneously at the expense of industrial waste heat. The variation of the parameters that affect the system performance such as industrial waste heat temperature, refrigerant turbine inlet pressure, and the evaporator temperature of ejector refrigeration cycle (ERC) and EJT cycles was examined, respectively. It was found that refrigeration output and thermal efficiency of the multieffect cycle decrease considerably with the increase in industrial waste heat temperature, while its exergy efficiency varies marginally. A thermal efficiency value of 22.5% and exergy efficiency value of 8.6% were obtained at an industrial waste heat temperature of 210°C, a turbine inlet pressure of 1.3 MPa, and ejector evaporator temperature of 268 K. Both refrigeration output and thermal efficiency increase with the increase in turbine inlet pressure and ERC evaporator temperature. Change in EJT cycle evaporator temperature shows a little impact on both thermal and exergy efficiency values of the multieffect cycle. Analysis of the results clearly shows that the proposed cycle has an effective potential for cooling production through exploitation of lost energy from the industry. Copyright © 2014 John Wiley & Sons, Ltd.     

Numerical oscillatory on the simulation of thermoacoustic waves

The SIMPLE algorithm for compressible flows is introduced to predict the thermoacoustic wave in a one‐dimensional closed region. The thermoacoustic waves are generated by an impulsive rise of the temperature on the left wall, while the other walls are kept at the initial temperature. Four different schemes are employed to deal with the convection‐diffusion terms, i.e., CD, FUD, QUICK, and MUSCL. The calculations and analysis show that numerical oscillation occurs under all four schemes, affected by the intensity of the waves, type of schemes, and other factors. The results are beneficial for the further investigation of the thermoacoustic waves and high efficiency scheme development. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(5): 265– 275, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20162