Showing papers on "Thermal efficiency published in 1987"

Thermal efficiency at maximum work output: New results for old heat engines

Abstract: What is the thermal efficiency of a heat engine producing the maximum possible work per cycle consistent with its operating‐temperature range? This question is answered here for four model reversible heat engine cycles. In each case, the work is maximized with respect to two characteristic temperatures that are intermediate between the maximum and minimum cycle temperatures T+ and T−. The maximum‐work efficiencies are found to equal or be well approximated by η*=1−(T−/T+)1/2. Because this efficiency is a function solely of the extreme cycle temperatures, it can be compared easily with the corresponding reversible Carnot cycle efficiency ηc =1−T−/T+. Here, η*, which is a much better guide to the performance of actual heat engines than ηc, is the same efficiency found by Curzon and Ahlborn [Am. J. Phys. 43, 22 (1975)] for a model irreversible heat engine operating at maximum power output. The present results show that η* is more ‘‘universal’’ than had been realized previously. If the work output per cycle i...

Process for producing power

Abstract: A process is disclosed for producing mechanical energy or electric power from chemical energy contained in a fuel, utilizing a combustion turbine. The compressed air which is used for combustion of the fuel to drive the turbine is humidified prior to combustion in a multistage countercurrent saturator to replace some or all of the thermal diluent air with water vapor. Humidification is effected with the water at a temperature below its boiling point at the operating pressure. The compressed air is cooled prior to humidification by passing in heat exchange relationship with the water used for humidification. Low level heat is rejected from the compressed air during intercooling and prior to humidification. This process provides a significant improvement in thermal efficiency, compared to combined cycle, steam injected cycle, intercooled regenerative cycle, and other air humidification based processes. Additionally, the entire steam cycle of a combined cycle process is eliminated, including the steam turbine generator, steam drums, surface condenser and cooling towers.

Compressed air energy storage turbomachinery cycle with compression heat recovery, storage, steam generation and utilization during power generation

Abstract: A thermal energy peaking/intermediate power plant is disclosed having a compression mode in which thermal energy is produced and stored in a thermal energy storage device and an expansion mode in which such stored thermal energy is utilized to produce steam from water and inject such steam into a combustion process.

A thermodynamic approach to the design and synthesis of plant utility systems

Abstract: A systematic, thermodynamically oriented method for design and synthesis of plant utility systems is proposed. Heat requirements are satisfied in preference to power requirements. Taking a combined gas-steam cycle as the most complex plant utility system, several significant new findings are obtained: the thermal efficiency of a gas turbine cycle can be a function of heat input ratio exclusively; a reheating steam cycle may not improve the overall thermal efficiency; and the improvement of overall thermal efficiency due to regenerative heating of the feedwater is effective only when the thermal efficiency of the steam generator is greater than the overall thermal efficiency.

Exergy Analysis of Combined Cycles: Part 1—Air-Cooled Brayton-Cycle Gas Turbines

Abstract: Quantitative analytical tools based on the second law of thermodynamics provide insight into the complex optimization tradeoffs encountered in the design of a combined cycle. These tools are especially valuable when considering approaches beyond the existing body of experience, whether in cycle configuration or in gas turbine cooling technology. A framework for such analysis was provided by the author [1-3] using simplified, constant-property models. In this paper, this theme is developed to include actual chemical and thermodynamic properties as well as relevant practical design details reflecting current engineering practice. The second-law model is applied to calculate and provide a detailed breakdown of the sources of inefficiency of a combined cycle. Stage-by-stage turbine cooling flow and loss analysis calculations are performed using the GASCAN program and examples of the resulting loss breakdowns presented. It is shown that the dominant interaction governing the variation of cycle efficiency with turbine inlet temperature is that between combustion irreversibility and turbine cooling losses. Compressor and pressure-drop losses are shown to be relatively small. A detailed analysis and loss breakdown of the steam bottoming cycle is presented in Part 2 of this paper.

Performance of a High-Efficiency Radial/Axial Turbine

Abstract: This paper presents the test performance of a lightly loaded, combination radial/axial turbine for a 420-hp, two-shaft gas turbine. This two-stage turbine configuration which included an interstage duct and an exhaust duct discharging vertically to ambient pressure conditions, was shown to be capable of attaining an overall isentropic efficiency of 89.7 percent. The influence of exhaust diffuser struts on the turbine performance under stalled power turbine conditions was shown to significantly affect compressor and turbine matching.

Factors limiting the improvement in thermal efficiency of S. I. engine at higher compression ratio

Abstract: An analysis of the factors that limit the improvement in thermal efficiency at higher compression ratios was performed with both thermodynamic calculation and experiment. The results showed that the major factors were cooling loss and unburned fuel. Both of these factors increase with smaller swept volume, larger S/V ratio combustion chamber, and lower engine speed and load. These effects explain the observation that thermal efficiency peaks at relatively low compression ratio.

Heat and power integration of chemical processes

Abstract: A nonlinear programming model is presented that positions heat engines above or below the pinch temperature and heat pumps across the pinch temperature. Heat engines and heat pumps compete to supply the hot deficits above the pinch temperature and to consume the cold deficits below the pinch temperature. Alternate working fluids are considered with power cycles pressures adjusted in the optimization. Arbitrary objectives include minimization of utilities and annualized cost, with the latter found necessary to give more realistic compression ratios in the power cycles. The temperature intervals are lumped to reduce the problem size, significantly increasing the efficiency of optimization without affecting the solution. Results are presented for a natural gas processing plant.

Absorption refrigeration and heat pump system

Abstract: An absorption refrigeration/heat pump system of the four chamber multiple subsystem type in which an absorption power module is employed having a desorber, condenser, and second desorber combined for more effective heat transfer and thermal efficiency.

Very high efficiency hybrid steam/gas turbine power plant wiht bottoming vapor rankine cycle

Abstract: An improved thermal efficiency power plant for converting fuel energy to shaft horsepower is described. The conventional combustor of a gas tubine power plant is replaced by a direct contact steam boiler 8, modified to produce a mixture of superheated steam and combustion gases. Combustion takes place preferably at stoichiometric conditions. The maximum thermal efficiency of the disclosed plant is achievable at much higher pressures than conventional gas turbines. Uses of multi-stage compression turbines (4, 9, 1, 10) with intercooling (2, 3) and regeneration (16, 17, 18, 19) is utilized along with a vapor bottoming cycle (11, 12, 13) to achieve a thermal efficiency greater tha 60% with a maximum drive turbine inlet temperature of 1600 degrees Fahrenheit.

Efficient heater and air conditioner

Abstract: The purpose of this invention is to cost effectively achieve more fuel efficient cooling and heating of buildings and also provide society with the resource conservation and environmental benefits that can result from the more efficient use of our fuel resources. The invention uses conventional equipment and includes an internal combustion engine and components that are now used in electric driven compression and steam driven absorption air conditioning and heat pump systems. The work from the engine provides a direct mechanical drive for the compression cycle and most of the rejected heat from the engine is recovered and utilized by the absorption cycle, which is achieved by recovering the high and medium temperature heat in the absorption cycle generator and recovering the low temperature heat from the engine exhaust by the mixture flow stream from the absorber to the generator. The result is a direct fueled engine driven combined cycle system that can be twice as fuel efficient as existing electric driven compression or steam driven absorption systems, and that can cost effectively replace most exissting systems in large building and provide substantial resource and environmental benefits.

Combustion chamber for internal combustion engines

Abstract: According to the present ivention, a combustion chamber of internal combustion engines comprises: a main combustion chamber which is created by recessing the piston top; a swirl chamber which is contiguous to said main combustion chamber via a passage; and a fuel injection nozzle to supply atomized fuel into said main combustion chamber and swirl chamber. Because of the above arrangement, a relatively quick combustion takes place in the swirl chamber and a relatively sluggish combustion takes place in the main combustion chamber. As a result, the generation of HC, NOx, or smokes are suppressed while improving an output, a fuel consumption rate, and a thermal efficiency.

Combustion-powered refrigeration with decreased fuel consumption

Abstract: In refrigeration systems wherein the refrigerant compressor is driven by a prime mover powered by combustion of a fluid fuel, a notable saving in fuel consumption is achieved by utilizing waste heat in the hot exhaust gases from the prime mover in an absorption refrigeration unit that chills a coolant stream circulated to the condenser for the compressed refrigerant. Existing combustion-powered refrigeration systems can be improved by adding a lithium halide absorption unit to utilize heat in the exhaust gases to produce refrigeration that is used to condense the compressed refrigerant. A combustion turbine coupled to a centrifugal compressor is a preferred combination of prime mover and refrigerant compressor for economically producing tonnage refrigeration.

Exergy analysis of combined cycles: Part 2 - Analysis and optimization of two-pressure steam bottoming cycles

Abstract: Results of a study for selecting the optimum parameters of a dual-pressure bottoming cycle as a function of the gas turbine exhaust temperature are presented. Realistic constraints reflecting current technological practice are assumed. Exergy analysis is applied to quantify all loss sources in each cycle. Compared to a single pressure at typical exhaust gas temperatures the optimized dual-pressure configuration is found to increase steam cycle work output on the order of 3 percent, principally through the reduction of the heat transfer irreversibility from about 15 to 8 percent of the exhaust gas energy. Measures to further reduce the heat transfer irreversibility such as three-pressure systems or use of multicomponent mixtures can therefore only result in modest additional gains. The results for the efficiency of optimized dual-pressure bottoming cycles are correlated against turbine exit temperature by simple polynomial fits. Sensitivity of the results to variations in the constraint envelope are presented.

Gas turbine cycle incorporating simultaneous, parallel, dual-mode heat recovery

Abstract: The cycle includes water-injection, steam-injection, recuperation (or regeneration) and waste-heat boiler heat recovery in an arrangement that provides high thermal efficiency, flexible operation in a cogeneration plant and favorable capital cost in relation to thermodynamic performance when compared to currently practiced cycles. In the present cycle, the sensible enthalpy of the exhaust gases between turbine exit and stack is used to simultaneously and in-parallel heat both air and water/steam. A smaller amount of water is boiled than in the known Cheng cycle, in which the exhaust heat is used only to heat water/steam. Thus, the latent heat exhausted at the stack in the present cycle is lower than that for the Cheng cycle resulting in higher efficiency.

Ethanol Fumigation of a Compression-Ignition Engine Using Advanced Injection of Diesel Fuel

Abstract: THIS paper describes the modification and testing of a compression ignition engine using diesel and fumigated ethanol as fuel. Tests on the engine fueled with diesel only were made, and the performance evaluated to form a basis for comparison for those of ethanol-diesel dual fueling. Modifications were made in the introduction of the ethanol and air and in the diesel injection timing. Both the ethanol flow rate and diesel injection timing were varied in order to observe the trend in the engine performance and to determine the preferred combinations of settings for ethanol utilization. Performance was evaluated in terms of brake thermal efficiency. Ethanol-air mixing greatly improved the performance even at the optimum injection timing for 100% diesel operation. Performance at advanced timings was better than that of diesel provided the correct fueling rates were used. Generally, for increased performance the ethanol flow rates should increase with timing advance. No study was made on the long term effects of ethanol fuel on the engine.

Parametric Analysis of Combined Gas-Steam Cycles

Abstract: Combined gas-steam cycles have been analyzed from the thermodynamic point of view. Suitable thermodynamics indices - explained in Appendix A - have been utilized. The parameters that most influence efficiency have been singled out and their ranges of variability have been specified. Calculations have been carried out taking into account the state of the art for gas turbines and the usual values for the quantities of steam cycles. The results are given. The maximal gas turbine temperature has been varied between 800/sup 0/C and 1400/sup 0/C. The gas turbine pressure ratio has been analyzed in the range of 2-24. Afterburning has also been taken into consideration. Maximal efficiency curves and the corresponding specific work curves (referred to the compressed air) related to the parameters of the analysis are given and discussed.

Process for increasing the energy input in electric arc furnaces

Abstract: To save energy in the production of steel in an electric arc furnace while increasing the total input of energy, the employment of carbonaceous fuels and oxygen-containing gases is proposed. The oxygen or the oxygen-containing gases are led into the furnace in the upper part of the furnace through stationary top blow-in devices (4) into the space between the pitch circle of the electrodes (10) and the furnace wall (3). The resulting intense gas flow repeatedly sucks in the reaction gases arising from the scrap or the melt being formed and burns them. The heat thus set free is transferred to the scrap and/or the melt with a thermal efficiency of at least 70%. By nozzles (2) arranged below the surface of the bath, and preferably in the furnace floor, preferably oxidizing gases are led into the melt and solid materials, particularly carbonaceous fuels, are supplied to the melt through one or more hollow electrodes (6) with an abrasion-resistant cladding. With this process, increased energy input into the electric arc furnace becomes possible, and thus the melting down time can be shortened and the economy of the process improved.

Device for vaporizing a cryogenic fluid

Abstract: The device operates with an internal combustion engine (1) and utilizes the combustion heat of the same. In addition to the heat of the cooling water exiting from the engine (1), the exhaust gases are transferred to a water circuit in an exhaust gas heat exchanger (4). The mechanical performance of the engine (1) is, through the direct application of a hydraulic brake (14), almost completely changed into thermal energy, and the heat thus produced is likewise conducted to the water circuit. The water heated is conducted through a vaporizing unit (7), where it brings the cryogenic fluid conducted in a pipe coil (25) or in a pipe assembly to vaporization.

Dual source external heating system for a heat pipe

Abstract: An external heating system for a heat engine such as a Stirling cycle engine which permits thermal energy to be provided by solar energy or fuel combustion sources. The system employs a complexly shaped heat pipe evaporator section having an enclosed cavity for receiving solar energy and another section forming hollow fins which is exposed to hot combustion gasses. Accordingly, either heat source may be used to evaporate working fluid within the heat pipe which is transferred to the associated heat engine.

Thermodynamic Evaluation of Gas Turbine Cogeneration Cycles: Part I—Heat Balance Method Analysis

Abstract: This paper presents a heat balance method of evaluating various open-cycle gas turbines and heat recovery systems based on the first law of thermodynamics. A useful graphic solution is presented that can be readily applied to various gas turbine cogeneration configurations. An analysis of seven commercially available gas turbines is made showing the effect of pressure ratio, exhaust temperature, intercooling, regeneration, and turbine rotor inlet temperature in regard to power output, heat recovery, and overall cycle efficiency. The method presented can be readily programmed in a computer, for any given gaseous or liquid fuel, to yield accurate evaluations. An X-Y plotter can be utilized to present the results.

Preliminary performance evaluation of a hybrid vapor-compression/liquid desiccant air conditioning system

Abstract: An efficient hybrid air-conditioning system is described that takes advantage of the high efficiency for sensible cooling inherent in vapor-compression systems and the high efficiency for latent heat removal inherent in liquid desiccant dehumidification cycles. The thermodynamic and possible economic advantages over a standard vapor-compression system are outlined. Considerable potential is shown to exist for saving energy and reducing peak electric demand over conventional vapor-compression systems that provide the same outlet air conditions.

Mechanical efficiency of kinematic heat engines

Abstract: A general theory of the mechanical efficiency of kinematic heat engines is presented. Concepts are introduced that precisely relate mechanical efficiency to general characteristics of an engine. Theorems are proven that lead to a mathematically explicit limit to the mechanical efficiency of reciprocating engines. This limit is the mechanical analogue of the Carnot limit to thermal efficiency. The ideal Stirling engine plays a key role in this theory.

A Comparison of the Predicted and Measured Thermodynamic Performance of a Gas Turbine Cogeneration System

Abstract: The thermodynamic performance of a gas turbine cogeneration system is predicted using a computer model The predicted performance is compared to the actual performance, determined by measurements, in terms of various thermodynamic performance parameters which are defined and discussed in this paper These parameters include the electric power output, fuel flow rate, steam production, electrical efficiency, steam efficiency, and total plant efficiency Other derived parameters are the net heat rate, the power-to-heat ratio, and the fuel savings rate This paper describes the cogeneration plant, the computer model, and the measurement techniques used to determine each of the necessary measurands The predicted and the measured electric power compare well The predicted fuel flow and steam production are less than measured The results demonstrate that this type of comparison is needed if computer models are to be used successfully in the design and selection of cogeneration systems

A thermal-optical analysis of a compound parabolic concentrator for single and multiphase flows, including superheat

Abstract: A thermal and optical analysis of the performance of a refrigerant charged Compound Parabolic Concentrator (CPC) for solar applications operating in non-boiling, boiling and super-heated regimes is presented. The performance of the CPC working under these single and multiphase conditions is governed by the axial fractional channel lengths of the non-boiling and the superheating regions. The overall thermal loss coefficient, the dimensionless capacitance rate and collector efficiency factors for various CPC operating regions are defined. A new “Generalized Heat Removal Factor“, ℱs for solar collectors under any operation mode is developed. The thermal efficiency of a CPC and flat-plate collector, whether under non-boiling, boiling or superheated conditions, is evaluated using ℱs which enables the selection of a suitable collector design and concentration ratio at some specified operational temperature. It is shown that, in general, a CPC has a greater thermal conversion efficiency than a flat-plate for a given operating condition.

Method of estimating the fuel/air ratio of an internal combustion engine

Abstract: A method of estimating the fuel/air ratio of the mixture supplied to an engine is described in which the in-cylinder pressure signal in a combustion cycle is processed to obtain an estimate of the fresh mixture strength supplied to the cylinder. The estimation is obtained by calculating the trapped charge mass and the total fuel mass consumed in a combustion cycle. Based on these values, the fuel/air ratio is estimated.

A comparative study of an absorber heat recovery cycle for solar refrigeration using NH3-refrigerant with liquid/solid absorbents

Abstract: This paper presents an investigation on using an ammonia refrigerant with liquid/solid absorbents in an absorber heat recovery cycle where heat released during the absorption process is used to heat up the strong solution coming out of the absorber, thereby reducing the generator heat input and hence improving the coefficient of performance. A comparative thermodynamic study is made with NH3-H2O and NH3-LiNO3 pairs as working fluids for both conventional absorption and absorber heat recovery systems. It is found that an improvement of about 10 per cent in COP for the absorber heat recovery cycle is achieved over the conventional absorption cycle and the NH3-LiNO3 system yields a higher COP than for NH3-H2O over a wide range of generator temperatures and condenser/absorber temperatures. A detailed parametric study is also presented in this paper.