03-12-2012, 05:16 PM
Power Cycles
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There are four power cycles that are generally used in the generation of electricity; the Rankine cycle, the Brayton cycle, the Otto cycle, and the Diesel cycle. The Rankine cycle is used in most base load power plants throughout the United States. The Brayton cycle is the power cycle used for gas turbines, often used in peaking applications. Finally, the Otto and Diesel cycles drive reciprocating engine generators, generally used in smaller power generating applications.
In order to properly discuss the cycles, some discussion of the terminology used is required. First, the term efficiency must be discussed. Every cogeneration cycle contains a number of components, each with a related efficiency. The fuel-to-electrical efficiency encompasses both the prime mover and the electrical generator (including whatever gearing is necessary between the prime mover and the generator). Generally speaking, reciprocating engines have the highest fuel-to-electrical efficiency, with gas turbines second, and steam turbines last. The fuel-to-electrical efficiency of a particular system is also referred to as the heat rate. The heat rate is the number of British Thermal Units (BTU) required to produce one kilowatt-hour of electricity. For a 100% fuel-to-electrical efficiency, this number is 3413, but the second law of thermodynamics limitations and irreversibilities in the cycle increase this number to roughly 10,000 or higher. Figure 2 shows the heat rate of some typical gas turbines. Note that in general, gas turbines become more efficient as they increase in size, becoming asymptotic at a value between 8,000 and 10,000 BTU.8
The choice of a cycle for a cogeneration unit is not as straightforward as it might first appear. Initial intuition might lead to the cycle with the highest fuel-to-electrical efficiency. It is important to remember that not just electricity is being generated from these cycles. In cogeneration applications, the thermal energy is as important as the electricity. Many industrial applications require vast amounts of steam, and a cycle with a very high electrical efficiency may be incapable of providing the required quality of thermal energy.
Capital costs must also be considered. This is most evident in a retrofit application. If some of the equipment required to operate a given cycle is already available, a given cycle may become the better economic choice, even though it is less efficient. Remember also that other issues such as fuel availability, ease of installation and maintenance, and local electric rates all play a part in the overall choice of a system.
Rankine Cycle
All large steam based power plants operate on the Rankine cycle. Basically, the Rankine cycle utilizes a steam boiler to produce high pressure, high temperature steam. The steam, leaving the boiler at temperatures and pressures as high as 1000 F and 4500 psig, is routed through a steam turbine where it is expanded to produce shaft work that drives an electric generator. In order to increase the efficiency of the overall process, the expansion of the steam is generally performed in stages. After passing through a high pressure turbine stage, the steam is returned to the steam generator to be reheated. After the final expansion stage, the steam is routed to a condenser, where it is returned fully to liquid form and pumped back to the steam generator. By using this method of power production, electric facilities are able to approach 40% efficiency.
Steam Turbine Types
The steam turbines used in the Rankine cycle can be of many types readily available. The most basic type is the backpressure turbine. The backpressure turbine produces electricity through the expansion of high pressure steam to atmospheric pressure and higher. The exiting steam can be directed to an industrial process (in a cogeneration system), or sent to a condenser.
A condensing turbine operates similarly to the backpressure turbine, but the low pressure side of the turbine is below atmospheric pressure. This results in a greater fuel-to-electrical efficiency for the cycle, but the rejected steam is of much lower thermodynamic value, thereby decreasing its value for thermal recovery. Electrical plants, whose sole purpose is the generation of electricity, use condensing turbines.
The extraction turbine allows for the removal of steam from the turbine during expansion. The turbine can be designed to provide a wide array of extraction pressures and flows. This allows for a great deal of flexibility when trying to satisfy both a thermal and electrical load. Extraction turbines are frequently used in cogeneration applications, though the efficiency of the extraction turbine is lower than other turbine types.
Cogeneration and The Rankine Cycle
The classic Rankine cycle is not generally used for heat recovery. The large base load power plants which operate on the Rankine cycle do not recover the heat from either the exhaust or the condenser. To be technically classified as a cogeneration cycle, the Rankine cycle must provide some heat recovery. This heat recovery can be taken from the combustion exhaust (if it is suitably clean), from the condenser, or, if an extraction turbine is used, directly from the steam turbine itself.
One attractive feature of a Rankine cycle system is its capability of operating on many types of fuel. Coal, chemical wastes, heavy and light oils, biomass, and even natural gas are used to fuel Rankine cycle plants. Furthermore, steam turbines are readily available in almost any size. These features combine to give the Rankine cycle unmatched flexibility to meet specific load conditions. Rankine cycle plants have a long life span, and all parts of the system are reliable and require relatively little maintenance.9
New Rankine based cogeneration systems can be quite expensive. The heat exchangers (boilers and condensers) are very expensive in terms of both equipment cost and installation cost. Usually, Rankine cycle cogeneration systems are economically attractive if the boilers and condensers are already in place, and a steam turbine is all that is required to cogenerate electricity.
Brayton Cycle
The Brayton cycle is the gas turbine power cycle. It is an open cycle where ambient air is compressed to a high temperature and pressure before it is fed to the combustion chamber. In the combustion chamber, the air-fuel mixture is ignited, dramatically increasing the temperature of the mixture. These hot gases are then expanded in a turbine, which can be coupled to an electric generator or some other load. In a turbine cogeneration system, the exhaust gases are collected and passed either to process (the relatively clean exhaust is suitable for high temperature drying), or to an HRSG. Because the exhaust gases leave the gas turbine at such high temperatures (~1000F), they have good thermal quality and are capable of producing large amounts of high pressure steam. Also, since so much excess air is used during the combustion process, the exhaust contains enough oxygen to sustain further gas firing, resulting in more usable heat. Supplementary fired HRSG units are capable of achieving total fuel utilization efficiencies of 90+%.10