Rankine cycle efficiency

Figure 2.9c Rankine cycle efficiency versus pump efficiency sensitivity analysis.

Is Rankine cycle efficiency sensitive to the pump inefficiency Why ... 

The portion of the potential energy that can be used to produce power is called the available energy and is represented by the isentropic enthalpy difference between the initial steam condition hi and the final condition corresponding to the exhaust pressure h. If the condensate enthalpy is hu, the ideal Rankine cycle efficiency, or the thermal efficiency, is... 

The efficiency of the Rankine cycle itself can be increased by higher motive steam pressures and superheat temperatures, and lower surface condenser pressures in addition to rotating equipment selection. These parameters are generally optimized on the basis of materials of constmction as well as equipment sizes. Typical high pressure steam system conditions are in excess of 10,350 kPa (1500 psi) and 510 °C. 

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). 

Rankine Cycle Thermodynamics. Carnot cycles provide the highest theoretical efficiency possible, but these are entirely gas phase. A drawback to a Carnot cycle is the need for gas compression. Producing efficient, large-volume compressors has been such a problem that combustion turbines and jet engines were not practical until the late 1940s. 

Figure 3-19 shows the thermal efficiency of the gas turbine and the Brayton-Rankin cycle (gas turbine exhaust being used in the boiler) based on the LHV of the gas. This figure shows that below 50% of the rated load, the combination cycle is not effective. At full load, it is obvious the benefits one can reap from a combination cycle. Figure 3-20 shows the fuel consumption as a function of the load, and Figure 3-21 shows the amount of steam generated by the recovery boiler. 

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... 

The results of the performance calculations for the fuel cell, Rankine cycle heat recovery system, summarized in Table 9-24, indicate that the efficiency of the overall system is increased from 57% for the fuel cell alone to 72% for the overall system. This Rankine cycle heat-fuel recovery arrangement is less complex but less efficient than the combined Brayton-Rankine cycle approach, and more complex and less efficient than the regenerative Brayton approach. It does, however, eliminate the requirement for a large, high temperature gas to gas heat exchanger. And in applications where cogeneration and the supply of heat is desired, it provides a source of steam. 

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. 

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. 

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. 

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 (%). 

A steam power plant operates on the Rankine cycle. The steam enters the turbine at 7 MPa and 550°C. It discharges to the condenser at 20 kPa. Determine the quality of the steam at the exit of the turbine, pump work, turbine work, heat added to the boiler, and thermal cycle efficiency. 

For actual Rankine cycles, many irreversibilities are present in various components. Fluid friction causes pressure drops in the boiler and condenser. These drops in the boiler and condenser are usually small. The major irreversibilities occur within the turbine and pump. To account for these irreversibility effects, turbine efficiency and pump efficiency must be used in computing the actual work produced or consumed. The T-s diagram of the actual Rankine cycle is shown in Fig. 2.9. The effect of irreversibilities on the thermal efficiency of a Rankine cycle is illustrated in the following example. 

The power output of the Rankine cycle can be controlled by a throttling valve. The inlet steam pressure and temperature may be throttled down to a lower pressure and temperature if desired. Adding a throttling valve to the Rankine cycle decreases the cycle efficiency. The throttling Rankine cycle is shown in Fig. 2.10. An example illustrating the throttling Rankine cycle is given in Example 2.6. 

What is the purpose of the throttling valve in the Rankine throttling cycle Does it improve the cycle efficiency ... 

---