Showing papers on "Thermal efficiency published in 1993"

Unglazed transpired solar collectors: Heat loss theory

Abstract: Unglazed transpired solar collectors offer a potentially low cost, high-efficiency option for once-through applications such as preheating air for ventilation, crop drying, and desiccant regeneration. This paper examines the major heat loss mechanisms associated with this concept. Radiation heat loss is determined by considering losses to both the sky and the ground. Convective heat losses are obtained by integrating the product of the temperature and velocity profiles in the boundary layer at the downwind edge of the collector. This convective heat loss is then expressed in terms of the thermal equivalent length of irradiated absorber, and analysis shows that this loss can be very low for large collectors event under windy conditions. These results are incorporated into a simple computer model which predicts collector efficiency as a function of suction velocity, wind speed, ambient temperature, and radiation. Remaining research issues are discussed.

Hybrid solar central receiver for combined cycle power plant

Abstract: A hybrid combined cycle power plant including a solar central receiver for receiving solar radiation and converting it to thermal energy. The power plant includes a molten salt heat transfer medium for transferring the thermal energy to an air heater. The air heater uses the thermal energy to preheat the air from the compressor of the gas cycle. The exhaust gases from the gas cycle are directed to a steam turbine for additional energy production.

Electric continuously variable transmission and controls for operation of a heat engine in a closed-loop power-control mode

Abstract: A heat engine operates an alternator that supplies electrical power to an electric continuously variable transmission for driving at least one ac motor through a differential to multiple wheels or to multiple independent ac wheel motors in an automobile, bus or truck. A heat engine control operates in a closed-loop power mode and decouples heat engine speed from motor speed, allowing the heat engine to produce required power at an operating speed and throttle position, or fuel injector duty cycle, that maximizes fuel economy and minimizes engine emissions. An energy storage unit (e.g., a battery, flywheel, ultracapacitor, superconducting magnetic energy storage device, or combination thereof) provides electrical energy to the electric drive system during vehicle accelerations and further reduces peak power demands on the heat engine. Advantageously, energy is recovered during vehicle deceleration and braking; and the recovered energy is used to recharge the energy storage unit. Emissions during cold starting and idling are minimized by a closed-loop temperature-controlled catalyst electric heater which is operated by the battery or the alternator.

Specific heating load of an endoreversible Carnot heat Pump

Abstract: SYNOPSIS An endoreversible Carnot cycle is presented in this paper for a heat pump, where the specific heating load is limited by the power input The endoreversible Carnot cycle is a modified Carnot cycle where the heat transferred between the heat pump and its surroundings is irreversible A specific heating load is adopted as the objective function for the performance analysis of the heat pump The relation between the maximum specific heating load and power input of the heat pump is found

Internal combustion engine with limited temperature cycle

Abstract: An expandable chamber piston type internal combustion engine operating in an open thermodynamic cycle includes a combustion process having a constant volume (isochoric) phase followed by a constant temperature (isothermal) phase.

Vapor-air steam engine

Abstract: A vapor-air steam engine is described which operates at high pressure and utilizes a working fluid consisting of a mixture of compressed uncombusted air components, fuel combustion products and steam. In the new cycle described, working fluid is provided at constant pressure and temperatures. Combustion air is supplied adiabatically by one or more stages of compression. Fuel is injected at pressure as needed. At least about 40 % to all of compressed air is burned. Inert liquid is injected at high pressure to produce steam and thus provide an inert high specific heat diluent vapor required for internal cooling of an internal combustion turbine or other type system. The use of extensive liquid injection inhibits the formation of pollutants, increases the efficiency and horsepower of an engine, and reduces specific fuel consumption. The new cycle may also be operated open or closed; in the latter case, the liquid may be recouped via condensation for regenerative reuse. When salt water is injected into the system potable water is recovered from the steam exiting the power turbine and sterile sea salt is recovered from the cumbustion chamber.

Process for the thermal conversion of biomass to liquids

Abstract: A high conversion of biomass, such as wood, sawdust, bark, or agricultural wastes, to liquids is obtained bypyrolysis at short reaction tines in a reactor capable of high heat transfer rates; the reactor being of the fluidized bed, circulating fluidized bed or transport type in which the conveying gas contains low and carefully controlled amounts of oxygen, allowing a reaction system with low concentrations of carbon monoxide or flammable gases with a resulting improvement in operating safety and potential improvement in thermal efficiency and capital costs. The oxidation steps may be carried out in one or two stages. The resulting liquid product may be used as an alternative liquid fuel or as a source of high-value chemicals.

Power and efficiency limits for internal combustion engines via methods of finite‐time thermodynamics

Abstract: Analytical expressions for the upper bounds of power and efficiency of an internal combustion engine are obtained taking into account finite rate of heat exchange with the environment and nonzero entropy production due to chemical reactions. Recommendations of theoretically possible ways of improving internal combustion engines are presented.

Availability accumulation and destruction in a DI diesel engine with special reference to the limited cooled case

Abstract: This work develops a method for the calculation of both the irreversibility produced during combustion and the working medium availability at the end of the expansion process in a high speed, direct injection (DI), naturally aspirated, four-stroke diesel engine, on which experiments were conducted at the authors' laboratory. The experimental data were processed for the determination of fuel reaction rates; the combustion irreversibility production rate was then computed from the fuel reaction rates via an analytical mathematical expression which was developed by the present research group, based on the combined resolution of the first and second laws of thermodynamics. This expression is coupled with standard first law calculations and then is integrated to give the accumulated combustion irreversibility, while the working medium availability variation is also computed throughout the engine closed cycle. These calculations are applied for a wide range of measured loads, injection timings and engine rotational speeds; they are also expanded in the direction of the intensity of the rate of heat transfer loss (to the engine cooling medium) for every combination of the experimentally determined engine variables. Therefore, apart from investigating the effect of various operating parameters on the availability balance, it is possible to evaluate the effect of the engine heat transfer reduction (limited cooled engine), from the second law analysis point of view, on the potential for efficiency improvements made by using the increased exhaust heat in recovery devices (e.g. the exhaust turbine or Rankine bottoming cycle compounding). With the present second law analysis, which forms the spearhead of this work, the exhaust gas availability offers more useful information than its enthalpy counterpart (first law analysis) for the operation of such compounding devices. The irreversibility calculation also provides useful information for the combustion loss, which cannot be isolated and evaluated at all by a first law analysis.

Jet engine performance enhancement through use of a wave-rotor topping cycle

Abstract: A simple model is used to calculate the thermal efficiency and specific power of simple jet engines and jet engines with a wave-rotor topping cycle. The performance of the wave rotor is based on measurements from a previous experiment. Applied to the case of an aircraft flying at Mach 0.8, the calculations show that an engine with a wave rotor topping cycle may have gains in thermal efficiency of approximately 1 to 2 percent and gains in specific power of approximately 10 to 16 percent over a simple jet engine with the same overall compression ratio. Even greater gains are possible if the wave rotor's performance can be improved.

Turbocharged reciprocation engine for power and refrigeration using the modified Ericsson cycle

Abstract: A Modified Ericsson Turbocharged Reciprocating Engine (METRE), is provided which exhibits a high thermal efficiency for power and refrigeration applications A Modified Ericsson cycle can include 2, 3, 4, or more stages (number of intercooling and heat/reheat cycles between stages) As stages are added, both cycle efficiency and power density (power/weight flow) increase, therefore, trade-offs between higher performance and number of stages (system complexity, cost, etc) are necessary to optimize the engine By combining a turbocompressor for the low pressures of the cycle and a multi-piston reciprocating engine for the high pressures of the cycle, a light weight, highly fuel-efficient, low-polluting, engine can be achieved The METRE is highly suited for the power range of automobiles and trucks This engine can use low technology (lower turbine temperatures, efficiencies, etc) as well as high technology components (higher turbine temperatures, efficiency etc) and remain competitive with Brayton, Stirling, gas and Diesel engines The Ericsson cycle, like the Brayton and Stirling, utilizes external combustion or heating and thus can use readily available optional fuels such as natural gas, kerosene, propane, butane and gases derived from coal Solar and nuclear energy are also useable heat source candidates

Heat recovery method and device suitable for combined cycles

Abstract: A heat recovery method is disclosed suitable for combined gas turbine/steam turbine cycles including at the exhaust of a gas turbine a waste heat boiler in which water is first treated in a degassing unit connected to a first evaporator and comprising a certain number of heat exchangers. The boiler uses an ultrasupercritical steam cycle with four pressure levels with subcritical fluid intermediate evaporator stages.

Combined water purification and power of generating plant

Abstract: A combined water purification and power generating plant is disclosed having special features designed to maximize the cycle thermal efficiency and salt recovery, with little or no concentrated brine produced therefrom. Using the plant, a volume of salt water is delivered to a plurality of indirect and direct contact feed heaters. Within the direct contact heaters, the salt water is heated and diluted by condensation therein by super-heated steam delivered thereto. Any alkaline salts having reverse solubility characteristics particulate and are filtered therefrom. From the last direct contact feed heater, the diluted salt water is delivered to a plurality of high pressure, high temperature evaporators arranged in a series which are used to further heat, evaporate and filter the salt water in multiple stages thereby improving the plant's efficiency. A steam heater is used to super-heat a steam which delivered to various areas of the plant to heat and evaporate the salt water. High and low pressure steam turbines are also provide which utilize the steam to generate electrical power. The turbines are also arranged so that the exhaust steam therefrom may be used to heat the salt water in the feed heaters and then condensed into fresh water. An optional expansion tank is also provided for additional evaporation of the concentrated brine from the last evaporator.

Endothermic fuel energy management system

Abstract: A method for managing the energy produced by a gas turbine engine, which includes utilizing an endothermic fuel to cool the combustion products in the exhaust section of the gas turbine engine prior to burning the fuel in the combustion section of the engine.

Carnot comparison of multi-temperature level absorption heat cycles

Abstract: Comparison of different absorption heat cycles is not always made on the correct manner. This also includes comparison of an ideal absorption cycle with a mechanical analogy. A new Carnot model operating with two heat engines and two mechanical heat pumps is defined to be the correct and logical way to describe the mechanical analogy for an absorption heat pump and an absorption heat transformer. General equations for the Carnot coefficient of performance, COPr, are exemplified and simulated for an absorption heat pump and an absorption heat transformer, and an entropy flow fraction diagram is introduced. The important fact that the absorption heat cycles must operate under the same conditions when they are compared is discussed.

Electric power plant for vehicles

Abstract: The drive wheels of an electric automobile are driven by one or more D.C. drive motors which derive their power from a battery. The battery is charged by two separate D.C. generators. The first D.C. generator is powered by an internal combustion engine, and the second D.C. generator is powered by a steam engine. The steam engine uses waste heat from the internal combustion engine to boil its operating liquid.

Carnot‐like processes in finite time. II. Applications to model cycles

Abstract: Physical implications of the thermodynamic inequality derived in the preceding paper are examined in the context of Carnot‐like cyclic processes. In terms of the power P and the degradation D associated with such a process, a geometrical picture is developed which vividly describes a number of important results. Our picture also leads to the concept of time efficiency as a natural complement to the concept of power efficiency. Extensions to Carnot‐like refrigerators and heat pumps are carried out. Finally, the influence of a heat leak between the two reservoirs is analyzed.

Volatile organic compound abatement system

Abstract: A volatile organic compound (VOC) abatement system is provided which incorporates a carbon or zeolite type concentrating adsorption device, with VOCs collected upon the adsorption surfaces. A heat source is required heat gases flowing to the adsorption device in order to remove the concentrated VOCs. Also, heat is required to oxidize the VOCs into a safe form prior to their release into the atmosphere. The present invention employs a regenerative thermal oxidizer for efficiently combusting the VOCs prior their release into the atmosphere. The present system also employs a bypass valve for introducing a portion of the VOCs directly into a combustion chamber without regenerative preheating. The resulting lowering of thermal efficiency of the regenerative thermal oxidizer allows the exhaust from the regenerative thermal oxidizer to carry more thermal energy. The thermal energy from the exhaust is then transferred to a heat exchanger to heat gasses flowing into the adsorption device.

Gas turbine engine operating method

Abstract: A gas turbine engine that can operate at a very high efficiency over a wide range of load conditions. In one important feature of the invention, combustion gases are discharged along two paths, one with constant energy and the other with energy that varies in accordance with the engine load. The constant energy discharge drives a turbine that, in turn, operates a compressor for the engine at a constant, high efficiency. Alternatively, the compressor is driven at a speed that varies inversely with the engine load such that the engine operates at a high efficiency for all loads. In another, independent feature of the invention, the combustion occurs at a stoichiometric ration of fuel and air, with the turbine blades being protected from the vastly increased combustion gas temperatures by an internal array of passageways through which cooling liquid, e.g., liquid fuel, and air are channeled. The fuel and air mix together to from a rich, turbulent pre-mix for ejection directly into the surrounding combustion chamber. This configuration can also be utilized to inject steam into the combustion chamber, to provide a Rankine cycle, with internal cogeneration. In yet another independent aspect of the invention, the compressor that supplied compressed air to the combustion chamber includes two counter-rotating radial impellers, with the second impeller further functioning as a diffuser for the first impeller.

Finite-Time Thermodynamics and Endoreversible Heat Engines

Abstract: An endoreversible heat engine is an internally reversible and externally irreversible cyclic device which exchanges heat and power with its surroundings. Classical engineering thermodynamics is based on the concept of equilibrium. Time is not considered in the energy interactions between the heat engine and its environment. On the other hand, although rate of energy transfer is taught in heat transfer, the course does not cover heat engines. The finite-time thermodynamics is a newly developing field to fill in the gap between thermodynamics and heat transfer. Two types of engines are modelled in this paper—a reciprocating and a steady flow—with results obtained for maximum power output and efficiency at maximum power. It is shown that the latter is the same for both types of engines but that the maximum value of power production is different.

Gas turbine cooling air control device

Abstract: PURPOSE:To improve thermal efficiency of a gas turbine by reducing the amount of discharged air and intermediate stage bleed air from an air compressor used to cool a stationary blade. CONSTITUTION:This gas turbine to supply discharge air and intermediate stage bleed air of an air compressor l to stationary blades 4-7 of various stages of a turbine 2 as cooling air is constituted to supply this cooling air to each of the stationary blades after cooling it by air cooling systems 31-34 in accordance with a driving state of the gas turbine.

Reversible Thermodynamic Cycle for AMTEC Power Conversion

Abstract: An AMTEC cell, may be described as performing two distinct energy conversion processes: (i) conversion of heat to mechanical energy via a sodium-based heat engine and (ii) conversion of mechanical energy to electrical energy by utilizing the special properties of the electrolyte material. The thermodynamic cycle appropriate to an alkali metal thermal-to-electric converter cell is discussed for both liquid- and vapor-fed modes of operation, under the assumption that all processes can be performed reversibly. In the liquid-fed mode, the reversible efficiency is greater than 89.6% of Carnot efficiency for heat input and rejection temperatures (900--1,300 and 400--800 K, respectively) typical of practical devices. Vapor-fed cells can approach the efficiency of liquid-fed cells. Quantitative estimates confirm that the efficiency is insensitive to either the work required to pressurize the sodium liquid or the details of the state changes associated with cooling the low pressure sodium gas to the heat rejection temperature.

General analysis of the mechanical efficiency of reciprocating heat engines

Abstract: This paper examines the mechanical efficiency of reciprocating heat engines in a general setting. From an abstract mathematical characterization of machines, the relation of the pressure-volume cycle of an engine and the characteristics of its mechanism to its useful work output is established. Using the ideal Stirling engine in conjunction with this relation, a functional upper bound on the mechanical efficiency of all engines is derived. This shows the existence of limits on compression ratio under conditions relating the level of performance of the mechanism to the ratio of engine operating temperatures. This has implications for the design of engines intended for operation from low grade sources, such as industrial waste heat or passive solar energy.