Showing papers on "Thermodynamic cycle published in 1999"

Analysis of a New Thermodynamic Cycle for Combined Power and Cooling Using Low and Mid Temperature Solar Collectors

Abstract: A combined thermal power and cooling cycle is proposed which combines the Rankine and absorption refrigeration cycles. It can provide power output as well as refrigeration with power generation as a primary goal. Ammonia-water mixture is used as a working fluid. The boiling temperature of the ammonia-water mixture increases as the boiling process proceeds until all liquid is vaporized, so that a better thermal match is obtained in the boiler. The proposed cycle takes advantage of the low boiling temperature of ammonia vapor so that it can be expanded to a low temperature while it is still in a vapor state or a high quality two phase state. This cycle is ideally suited for solar thermal power using low cost concentrating collectors, with the potential to reduce the capital cost of a solar thermal power plant. The cycle can also be used as a bottoming cycle for any thermal power plant. This paper presents a parametric analysis of the proposed cycle.

Ecological optimization of an irreversible Brayton heat engine

Abstract: Finite-time thermodynamics is used to determine the maximum ecological function, its corresponding thermal efficiency and power output of an irreversible Brayton heat engine. The ecological function of a heat engine is defined as the power output minus the loss power, which is equal to the product of the environmental temperature and the entropy production rate. The ecological function is optimized with respect to the thermal conductance ratio and the adiabatic temperature ratio. The optimum values of adiabatic temperature ratios and thermal conductance ratios of irreversible Brayton heat engines are presented. To obtain a higher ecological function, the thermal conductance of the cold-side heat exchanger should be larger than that of the hot-side heat exchanger. The effects of the total number of transfer units of heat exchangers, turbine and compressor isentropic efficiencies, thermal reservoir temperature ratios and heat leaks on the maximum ecological function and its corresponding parameters are studied and discussed. Results can be used as important criteria in the design of Brayton heat engines.

On the power cycles working with ideal quantum gases : I. The Ericsson cycle

Abstract: The Ericsson power cycles working with ideal Bose and Fermi monoatomic gases are examined. They are conveniently called the Bose and Fermi cycles. Efficiencies of Bose and Fermi cycles are derived ( and respectively). Variations of them with the temperature ratio and pressure ratio of the cycle are examined. A comparison of the efficiencies with each other and that of the classical Ericsson cycle is made. In the degenerate gas state it is seen that , although in the classical gas state. In a Bose cycle, it is shown that there is an optimum value for the lowest temperature at which the efficiency reaches its maximum value for a given pressure ratio. Furthermore, Bose-Einstein condensation restricts the value of of a Bose cycle for a given value of . In a Fermi cycle, there is no an optimum value for . However, goes to a finite value of less than unity when goes to zero.

Forced convection adsorption cycle with packed bed heat regeneration

Abstract: The convective thermal wave is part of a patented cycle which uses heat transfer intensification to achieve both high efficiency and small size from a solid adsorption cycle. Such cycles normally suffer from low power density because of poor heat transfer through the adsorbent bed. Rather than attempting to heat the bed directly, it is possible to heat the refrigerant gas outside the bed and to circulate it through the bed in order to heat the sorbent. The high surface area of the grains leads to very effective heat transfer with only low levels of parasitic power needed for pumping. The new cycle presented here also utilises a packed bed of inert material to store heat between the adsorption and desorption phases of the cycle. The high degree of regeneration possible leads to good coefficients of performance (COPs). Thermodynamic modelling, based on measured heat transfer data, predicts a COP (for a specific carbon) of 0.90 when evaporating at 5°C and condensing at 40°C, with a generating temperature of 200°C and a modest system regenerator effectiveness of 0.8. Further improvement is possible. Experimental heat transfer measurements and cycle simulations are presented which show the potential of the concept to provide the basis of a gas-fired air conditioner in the range 10–100 kW cooling. A research project to build a 10-kW water chiller is underway. The laboratory system, which should be operational by June 1997, is described.

Performance analysis for a real closed regenerated Brayton cycle via methods of finite-time thermodynamics

Abstract: SYNOPSIS Performance analysis of a real power cycle has been performed using finite-time thermodynamics. The analytical formulae about the relations between power output and pressure ratio, and between efficiency and pressure ratio of a real closed regenerated Brayton cycle coupled to variable-temperature heat reservoirs are derived. In the analysis, the irreversibilities involve the heat resistance losses in the hot- and cold-side heat exchangers and the regenerator, the irreversible (non-isentropic) expansion and compression losses in the turbine and compressor, and the pressure drop loss in the pipe and system. The optimal performance characteristics of the cycle may be obtained by optimising the distribution of heat conductance or heat-transfer surface areas among two heat exchangers and regenerator, and the matching between working fluid and heat reservoirs. For the specified heat reservoir conditions, the power output is dependent on the effectiveness of the regenerator, and there exists an optimal ...

Dual expansion energy recovery (DEER) air cycle system with mid pressure water separation

Abstract: A liquid-to-air cycle system for conditioning water vapor bearing air and cooling a liquid load comprises an air cycle subsystem and a liquid cycle subsystem. The air cycle subsystem includes a first air-to-air heat exchanger, a reheater downstream of the first air-to-air heat exchanger, a first turbine downstream of the reheater, a first water extractor downstream of the first turbine, a first liquid-to-air heat exchanger downstream of the water extractor, and a second turbine downstream of the first liquid-to-air heat exchanger. Thereby, the second turbine can recover rejected heat from the first liquid-to-air heat exchanger. The liquid cycle subsystem is in heat exchange relationship the air cycle subsystem at the first liquid-to-air heat exchanger such that the first liquid-to-air heat exchanger absorbs the rejected heat from the liquid cycle subsystem.

Maximum profit performance of a three-heat-reservoir heat pump

Abstract: The operation of a three-heat-reservoir heat pump is viewed as a production process with exergy as its output. The relations between the optimal profit and COP (coefficient of performance), and the COP bound at the maximum profit of the heat pump are derived based on a general heat transfer law. The results provide a theoretical basis for developing and utilizing a variety of heat pumps. The focus of this paper is to search the compromised optimization between economics (profit) and the utilization factor (COP) for finite-time endoreversible thermodynamic cycles. Copyright © 1999 John Wiley & Sons, Ltd.

Analysis of a combined law power-optimized open Joule-Brayton heat-engine cycle with a finite interactive heat source

Abstract: Through concurrent employment of the first and second laws of thermodynamics, a set of optimum power expressions for the open irreversible Brayton and open Joule-Brayton heat-engine cycles has been obtained. These expressions are applicable to configurations with a finite thermal reservoir in which the values of the source outlet temperatures are forced to interact with the overall cycle during the power optimization of the cycle's working substance temperatures. Use of the concurrent law procedure simultaneously allows both the minimization of internal cycle entropy generation and the maximization of specific cycle work in order to provide the internal cycle operating temperatures necessary for power optimization. The study is carried out for open versions of these cycles in which the energy input is provided from an external source through a heat exchanger, the conductance of which is optimally allocated with respect to the cycle flow rate. The work includes a novel comparative study of the optimized power output of these cycles both with non-interactive and with interactive sources. The results of this study conclusively indicate the importance of considering variable (interactive) sink outlet temperatures in obtaining the power optimum for these cycles.

Optimization of the irreversible Stirling heat engine

Abstract: The effects of inefficiencies in the compression, expansion and regeneration processes on engine performance have been evaluated theoretically for a Stirling heat engine operating in a closed regenerative thermodynamic cycle. The irreversible cycle has been optimized by using the maximum power density technique. Maximized power and maximized power density are obtained for different n ex , τ, α c , α h , η c , η ex and η reg values. The maximum efficiencies have been found very close to the values corresponding to the maximum power density conditions but far from the values at maximum power. It has been found that the engines designed by considering the maximum power density have high efficiencies and small sizes under the same prescribed conditions.

Heat dissipation structure for semiconductor component and semiconductor device provided with that

Abstract: PROBLEM TO BE SOLVED: To provide a highly reliable heat dissipation structure for a semiconductor component which can effectively dissipate heat generated from semiconductor device chips in compliance with an increase in the density of heat generation of the chips and never breaks insulating boards even though a heat cycle is applied between a high temperature and a low temperature, and a semiconductor device provided with the structure. SOLUTION: A heat dissipation structure for a semiconductor component is provided with insulating boards 103 having the surfaces, which are respectively mounted with each semiconductor device chip 101, on one side of the surfaces and the other surfaces to come into contact directly with a refrigerant 10 and a radiator ceiling plate 104a, which has the surface bonded to the sidewalls of the boards 103. A semiconductor power module 1 is provided with the heat dissipation structure.

Thermodynamic cycle using hydrostatic head for compression

Abstract: A thermodynamic cycle is disclosed that uses compression and expansion to generate refrigeration or power in which at least some of the compression is effected by hydrostatic head of the heat-exchange medium used in the cycle. In a refrigeration cycle, the head of a heat-exchange medium in the refrigeration cycle is used to compress the heat-exchange medium. A vaporous heat-exchange medium is introduced into the upper end of a down riser that extends downwardly through a heat sink. The vaporous heat-exchange medium descends through the down riser and the head of the heat-exchange medium compresses the heat-exchange medium. The heat generated by the compression is transferred to the heat sink. The heat-exchange medium is then pumped up through a return riser and passed through a pressure expansion means and evaporator. From the evaporator the heat-exchange medium is returned to the upper end of the down riser for recycling.

Second law study of the einstein refrigeration cycle

Abstract: After formulating the theory of relativity, Albert Einstein spent several years developing absorption refrigeration cycles. In 1930, he obtained a U.S. patent for a unique single pressure absorption cycle. The single pressure throughout the cycle eliminates the need for the solution pump found in conventional absorption cycles. The Einstein cycle utilizes butane as a refrigerant, ammonia as a pressure equalizing fluid, and water as an absorbing fluid. This cycle is dramatically different in both concept and detail than the better known ammonia-water-hydrogen cycle. Recent studies have shown that the cycle’s COP is 0.17, which is relatively low compared to two-pressure cycles. This limits the cycle to refrigeration applications where simplicity, compactness, silent operation, and low cost are the important characteristics. Improved efficiency would open up other potential applications. In this study, a comprehensive second law analysis of the cycle was carried out on each component and process to determine the thermodynamic source of the low efficiency. The results show that the reversible COP for the cycle is 0.58, and that the component with the largest irreversibility is the generator. The entropic average temperatures for the heat flows into and out of the cycle are 353 K for the generator, 266 K for the evaporator, and 315 K for the absorber/condenser. The COP degradations from the ideal due to irreversibilities are 0.12 for the evaporator, 0.11 for the absorber/condenser, and 0.17 for the generator. The generator irreversibility is due to the inherent temperature difference in the internal heat exchange. The results show that there is a large potential for increasing the cycle’s efficiency through design changes to raise the low generator temperature and to reduce the large generator irreversibilities.

Control of incipience hysteresis effects in liquid cooled electronics heat sinks

Abstract: Although air cooling continues to be the primary cooling technique for electronics, increases in chip density and power dissipation drive the need to study techniques such as liquid immersion cooling. This paper describes a saturated pool boiling study of interactions between different heat sources, located in close proximity so as to simulate neighboring ICs on a vertically oriented silicon multichip module, immersed in FC-72. The heat sinks tested consisted of re-entrant cavities etched in silicon to enhance thermal performance, while the heat sources were in the form of serpentine thin film heaters. The benchmark case to which all multiple heater tests were compared was the isolated central heater case with no heat dissipating neighbors. With just this heat source activated, the usual boiling incipience temperature hysteresis was experienced during the first increasing heat cycle. During the second cycle of increasing heat flux, this hysteresis all but disappeared, proving the efficacy of re-entrant cavities in trapping vapor, as long as the pool remained saturated. The presence of a neighboring heat source located below the test heater, dramatically improves the thermal performance, virtually eliminating hysteresis effects altogether.

Air-conditioning system and operation control method thereof

Abstract: A hybrid air-conditioner comprises a vapor adsorption cycle air-conditioner, a vapor compression refrigerating cycle air-conditioner, an external heat source circuit and the like. In the case where the external heat source temperature is in a high temperature range, the cooling operation is performed using only the vapor adsorption cycle air-conditioner. In the case where the external heat source temperature is in a low temperature range, the cooling operation is performed using only the vapor compression refrigerating cycle air-conditioner. In the case where the external heat source temperature is in an intermediate temperature range, the cooling operation is performed using both the vapor adsorption cycle air-conditioner and the vapor compression refrigerating cycle air-conditioner. As a result, the temperature restrictions on the vapor adsorption cycle air-conditioner can be removed and the range of operation allowance can be widened.

Thermodynamic power and cryogenic refrigeration system using low temperature heat source

Abstract: A thermodynamic power and cryogenic refrigeration system using a first and second (binary) working fluid has a low-temperature closed bottoming cycle and an open or closed topping cycle. In the bottoming cycle a mixture of a first gas such as helium or hydrogen and a low temperature liquid such as liquefied nitrogen is compressed in a polytropic process and then the liquid content is separated. The separated first gas is heated using rejected heat from a second gas expanded in the topping cycle or ambient air and then the heated first gas is adiabatically expanded and supercooled while performing useful work and thereafter is fed to the compressor and mixed with the separated liquid to serve as a coolant and facilitate rejection of polytropic heat and to supplement the cool gas/liquid mixture providing polytropic compression of the first gas and thus completes the bottoming cycle. The bottoming cycle functions to cool the second gas and liquefy it in the topping cycle. The topping cycle is an open or closed modified Rankine cycle. The topping cycle uses heat of seawater and/or ambient air or other low-temperature heat source to simultaneously produce a refrigerant and power. The apparatus of the closed bottoming cycle may also function without the apparatus of the topping cycle using a low temperature heat source to produce a refrigerant and power.

Component fabrication and testing for a meso-scale refrigerator

Abstract: Preliminary design of a meso-scale refrigerator is the focus of this paper. Two design variations for two different applications are presented here. The two intended applications are (1) an integrated heat removal system for electronics or photonic chips or modules, and (2) an actively cooled jacket for personnel. The proposed device is based on a vapor compression refrigeration cycle. The paper starts with a general system description and preliminary design along with a functional decomposition where the overall system function is decomposed into nine major sub-functions. Different alternatives for compression and actuation are considered. Design specifications and some design details for each application are presented next. The specifics of the design requirements were found to suggest the same thermodynamic cycle for both applications, with the same temperature lift of 48

Energy conversion using thermoacoustic devices

Abstract: Thermoacoustic engines offer the possibility for simple and efficient energy conversion devices. They can be prime movers where heat produces sound or heat pumps and refrigerators where sound pumps heat. An important element in such engines is the secondary thermodynamic medium, the stack which provide the phasing between heat transfer and pressure changes at acoustical frequencies. Other elements are a resonator, working fluid (usually a gas), heat exchangers and a driver or source of heat. Similar to thermoelectric devices, the engines require a temperature difference for their operation and when run in a conjugate mode they produce a temperature difference. They are interesting because they are simple and they have essentially no moving parts.