Power Rankine

In apphcation to electric utihty power generation, MHD is combined with steam (qv) power generation, as shown in Figure 2. The MHD generator is used as a topping unit to the steam bottoming plant. From a thermodynamic point of view, the system is a combined cycle. The MHD generator operates in a Brayton cycle, similar to a gas turbine the steam plant operates in a conventional Rankine cycle (11). 

Because of the simplicity and reUabiUty of the Rankine cycle, faciUties employing this method have dominated the power industry in the twentieth century and typically play an important role in most modem combined-cycle faciUties. Water is the working fluid of choice in nearly all Rankine cycle power plants because water is nontoxic, abundant, and low cost. 

Power plants based on the Rankine thermodynamic cycle have served the majority of the world s electric power generation needs in the twentieth century. The most common heat sources employed by Rankine cycle power plants are either fossil fuel-fired or nuclear steam generators. The former are the most widely used. 

Fossil Fuel-Fired Plants. In modem, fossil fuel-fired power plants, the Rankine cycle typically operates as a closed loop. In describing the steam—water cycle of a modem Rankine cycle plant, it is easiest to start with the condensate system (see Fig. 1). Condensate is the water that remains after the steam employed by the plant s steam turbines exhausts into the plant s condenser, where it is collected for reuse in the cycle. Many modem power plants employ a series of heat exchangers to boost efficiency. As a first step, the condensate is heated in a series of heat exchangers, usually sheU-and-tube heat exchangers, by steam extracted from strategic locations on the plant s steam turbines (see HeaT-EXCHANGETECHNOLOGy). 

The Combined (Brayton-Rankine) Cycle The 1990s has seen the rebirth of the combined cycle, the combination of gas turbine technologies with the steam turbine. This has been a major shift for the utility industry, which was heavily steam-tnrbine-oriented with the use of the gas turbine for peaking power. In this combined cycle, the hot gases from the turbine exhaust are used in a heat recoveiy steam generator or in some cases in a snpplementaiy fired boiler to produce superheated steam. 

Another applieation for turboexpanders is in power reeovery from various heat sourees utilizing the Rankine eyele. The heat sourees presently being eonsidered for large seale power plants inelude geothermal and oeean-thermal energy, while small systems are direeted at solar heat, waste heat from reaetor proeesses, gas turbine exhaust and many other industrial waste heat sourees. Some of these systems are diseussed below in greater detail. 

One form of solar heat does offer interesting possibilities and is refeiTcd to as OTEC (Ocean-Thermal Energy Conversion). The OTEC power plant principle uses the solar heat of ocean surface water to vaporize ammonia as a working fluid in a Rankine cycle. After the fluid is expanded in the turbine, it is condensed by the 22°C colder... 

An external combustion engine that has been widely supported as a low-emission power source is the Rankine cycle steam engine. Many different types of expanders can be used to convert the energy in the working fluid... 

The most effective cycle is the Brayton-Rankine cycle. This cycle has tremendous potential in power plants and in the process industries where steam turbines are in use in many areas. The initial cost of this system is high however, in most cases where steam turbines are being used this initial cost can be greatly reduced. 

Plots of the properties of various substances as well as tables and charts are extremely useful in solving engineering thermodynamic problems. Two-dimensional representations of processes on P-V, T-S, or H-S diagrams are especially useful in analyzing cyclical processes. The use of the P-V diagram was illustrated earlier. A typical T-S diagram for a Rankine vapor power cycle is depicted in Figure 2-36. 

The efficiency of power cycles such as the Rankine cycle is given by the ratio of the net work out to the heat added. Thus from Figure 2-36 the efficiency is... 

Figure 9-14. Combined Brayton-Rankine Cycle Fuel Cell Power Generation System... 

Another advantage is that the IGCC system generates electricity by both combustion (Brayton cycle) and steam (Rankine cycle) turbines. The inclusion of the Brayton topping cycle improves efficiency compared to a conventional power plant s Rankine cycle-only generating wstem. Typically about two-thirds of the power generated comes from the Brayton cycle and one-third from the Rankine cycle. 

The rate of radiant thermal energy transfer between two bodies is described by the Stefan-Boltzmann law. Originally proposed in 1879 by Joseph Stefan and verified in 1884 by Ludwig Boltzmann, the Stefan-Boltzmann law states thatthe emission of thermal radiative energy is proportional to the fourth power of the absolute temperature (Kelvin or Rankine) ... 

An example of this is a commercial central power station using a heat engine called a Rankine steam power plant. The Rankine heat engine... 

Determine the efficiency and power output of a basic Rankine cycle using steam as the working fluid in which the condenser pressure is 80 kPa. The boiler pressure is 3 MPa. The steam leaves the boiler as saturated vapor. The mass rate of steam flow is Ikg/sec. Show the cycle on a T-s diagram. Plot the sensitivity diagram of cycle efficiency versus boiler pressure. 

A simple Rankine cycle using water as the working fluid operates between a boiler pressure of 500 psia and a condenser pressure of 20 psia. The mass flow rate of the water is 31bm/sec. Determine (1) the quality of the steam at the exit of the turbine, (2) the net power of the cycle, and (3) the cycle efficiency. Then change the boiler pressure to 600 psia, and determine (4) the quality of the steam at the exit of the turbine and (5) the net power of the cycle. 

COMMENTS The effect of increasing the boiler pressure on the quality of the steam at the exit of the turbine can be seen by comparing the two cases. The higher the boiler pressure, the higher the moisture content (or the lower the quality) at the exit of the turbine. Steam with qualities less than 90% at the exit of the turbine, cannot be tolerated in the operation of actual Rankine steam power plants. To increase steam quality at the exit of the turbine, superheating and reheating are used. 

An ideal Rankine cycle uses water as a working fluid, which circulates at a rate of 80kg/sec. The boiler pressure is 6 MPa, and the condenser pressure is 10 kPa. Determine (1) the power required to operate the pump, (2) the heat transfer added to the water in the boiler, (3) the power developed by the turbine, (4) the heat transfer removed from the condenser, (5) the quality of steam at the exit of the turbine, and (6) the thermal efficiency of the cycle. 

In a Rankine power plant, the steam temperature and pressure at the turbine inlet are 1000°F and 2000 psia. The temperature of the condensing steam in the condenser is maintained at 60° F. The power generated by the turbine is 30,000 hp. Assuming all processes to be ideal, determine (1) the pump power required (hp), (2) the mass flow rate, (3) the heat transfer added in the boiler (Btu/hr), (4) the heat transfer removed from the condenser (Btu/hr), and (5) the cycle thermal efficiency (%). 

For an ideal Rankine cycle, steam enters the turbine at 5 MPa and 400°C, and exhausts to the condenser at lOkPa. The turbine produces 20 MW of power. 

Steam is generated in the boiler of a steam power plant operating on an ideal Rankine cycle at 10 MPa and 500° C at a steady rate of 80 kg/sec. The steam expands in the turbine to a pressure of 7.5 kPa. Determine (1) the quality of the steam at the turbine exit, (2) rate of heat rejection in the condenser, (3) the power delivered by the turbine, and (4) the cycle thermal efficiency (%). 

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