Heat Engines
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Introduction
Heat engines convert heat energy into mechanical energy. Examples include steam engines, steam and gas turbines, spark-ignition and diesel engines, and the "external combustion" engine or Stirling engine. Such engines can provide motive power for transportation, to operate machinery, or to produce electricity.
All heat engines operate in a cycle of repeated sequences of heating (or compressing) and pressurizing the working fluid, the performance of mechanical work, and rejecting unused or waste heat to a "sink." At the beginning of each cycle, energy is added to the fluid forcing it to expand under high pressure so that the fluid "performs" mechanical work. The thermal energy contained in the pressurized fluid is converted to kinetic energy. The fluid then looses pressure, and after unused energy (in the form of heat) is rejected, it must then be reheated or recompressed to restore it to high pressure.
Heat engines cannot convert all the input energy to useful mechanical energy in the same cycle; some amount, in the form of heat, is always not available for the immediate performance of mechanical work. The fraction of thermal energy that is converted to net mechanical work is called the thermal efficiency of the heat engine. The maximum possible efficiency of a heat engine is that of a hypothetical (ideal) cycle, called the Carnot Cycle. Practical heat engines operate on less efficient cycles (such as the Rankine, Brayton, or Stirling) but in general, the highest thermal efficiency is achieved when the input temperature is as high as possible and the sink temperature is as low as possible.
The "waste" or rejected heat (to the "sink") can be used for other purposes, including pressurizing a different working fluid, which operates a different heat-engine (vapor turbine) cycle, or simply for heating.
Renewable sources of heat or fuels, such as solar or geothermal energy and biomass (as well as fossil fuels) can power heat engines. The following is a brief description of four types of heat engines, the Rankine, Stirling, Brayton, and the newly developed, and highly efficient, Kalina, that can be used or are being investigated for converting renewable sources of energy to useful energy.
Rankine Cycle Engines
The Rankine cycle system uses a liquid that evaporates when heated and expands to produce work, such as turning a turbine, which when connected to a generator, produces electricity. The exhaust vapor expelled from the turbine condenses and the liquid is pumped back to the boiler to repeat the cycle. The working fluid most commonly used is water, though other liquids can also be used. Rankine cycle design is used by most commercial electric power plants. The traditional steam locomotive is also a common form of the Rankine cycle engine. The Rankine engine itself can be either a piston engine or a turbine.
Stirling Cycle Engines
The Stirling cycle engine (also called an "external" combustion engine) differs from the Rankine in that it uses a gas, such as air, helium, or hydrogen, instead of a liquid, as its working fluid. Concentrated sunlight, biomass, or fossil fuels are sources potential fuels to provide external heat to one cylinder. This causes the gas to alternately expand and contract, moving a displacer piston back and forth between a heated and an unheated cylinder.
Brayton Cycle Engines
Brayton cycle systems, which incorporate a turbine, also use a gas as the working medium. There are open-cycle and closed-cycle Brayton systems. The gas turbine is a common example of an open-cycle Brayton system. Air is drawn into a compressor, heated and expanded through a turbine, and exhausted into the atmosphere. The closed-cycle Brayton system may use air, or a more efficient gas, such as hydrogen or helium. The gas in the closed-cycle system, however, gives up some of its heat in a heat exchanger after it leaves the turbine. It then returns to the compressor to start the cycle again.
Kalina Cycle Engine
The Kalina cycle engine, which is at least 10 percent more efficient than the other heat engines, is simple in design and can use readily available, off-the-shelf components. This new technology is similar to the Rankine cycle except that it heats two fluids, such as ammonia and water, instead of one. Instead of being discarded as waste at the turbine exhaust, the dual component vapor (70% ammonia, 30% water) enters a distillation subsystem. This subsystem creates three additional mixtures. One is a 40/60 mixture, which can be completely condensed against normal cooling sources. After condensing, it is pumped to a higher pressure, where it is mixed with a rich vapor produced during the distillation process. This recreates the 70/30 working fluid. The elevated pressure completely condenses the working fluid and returns it to the boiler to complete the cycle. The mixture's composition varies throughout the cycle. The advantages of this process include variable temperature boiling and condensing, and a high level of recuperation.
The U.S. Department of Energy completed a power plant using a Kalina cycle engine in 1991, at the Energy Technology Engineering Center in Canoga Park, California. The power plant may also improve heat engine efficiency through better thermodynamic matching in the boiler and distillation subsystem, and through recuperation of the heat from the turbine exhaust. Data from the early operating trials confirmed the principle of the Kalina Cycle technology. The technology is now being used in geothermal power plants.
