Efficiency of steam turbines

Generally, the efficiency of steam turbines decreases with decreasing load. The overall turbine efficiency can be represented by two components the isentropic efficiency and the mechanical efficiency. The mechanical efficiency reflects the efficiency with which the energy that is extracted from steam is transformed into useful power and accounts for mechanical frictional losses, heat losses, and so on. The mechanical efficiency is high (typically 0.95 to 0.99)6. However, the mechanical efficiency does not reflect the efficiency with which energy is extracted from steam. This is characterized by the isentropic efficiency introduced in Figure 2.1 and Equation 2.3, defined as ... 

Several parameters affect the efficiency of steam turbines, as shown partially on Figure 4.1. Closer examination will need to take into account specific mechanical details which usually are left to the manufacturer. Geared turbines [the dashed line of Fig. 4.1(b)] have higher efficiencies, even with reduction gear losses, because they operate with especially high bucket speeds. For example, for a service of 500 HP with 300psig steam, a geared turbine has an efficiency of 49.5% and one with a direct drive at 1800 rpm has an efficiency of 24%. 

The overall efficiency of fuel cells increases with size because problems of heat loss or of gas pumping can be dealt with properly only if the size of the fuel battery is above a certain minimum of a few kilowatts. Above this output there is little increase in efficiency as the size is increased the efficiency of steam turbine power plants increases with size up to about 500 megawatts. This consideration alone implies that fuel cells will be best used for small scale operations, such as supplying electricity in remote areas or powering vehicles, such as delivery vans or locomotives. They will also be used where the advantages of fuel cells, the quiet operation, reliability and absence of pollution, will be more important than efficiency. 

Evans provides the following graph of steam turbine efficiencies. 

A type of shell and tube heat exchanger that condenses exhaust steam and creates a vacuum, improving the efficiency of a turbine. 

A boiler plant raises 5.2 kg/s of steam at 1825 kN/m2 pressure, using coal of calorific value 27.2 MJ/kg. If the boiler efficiency is 75%, how much coal is consumed per day If the steam is used to generate electricity, what is the power generation in kilowatts, assuming a 20% conversion efficiency of the turbines and generators ... 

The overall efficiency of steam generation and distribution is 60%. Which scheme is most cost-effective, the steam turbine, the gas turbine or the heat pump ... 

Steam turbines are used to convert part of the energy of the steam into power and can be configured in different ways. Steam turbines can be divided into two basic classes back-pressure turbines and condensing turbines. The efficiency of the turbine and its power output depend on the flowrate of steam to the turbine. The performance characteristics can be modeled by a simple linear relationship over a reasonable range of operation. 

Determine the efficiency and power output of a regenerative Rankine (without superheater or reheater) cycle using steam as the working fluid and in which the condenser temperature is 50° C. The boiler temperature is 350°C. The steam leaves the boiler as saturated vapor. The mass rate of steam flow is 1 kg/sec. After expansion in the high-pressure turbine to 100°C, some of the steam is extracted from the turbine exit for the purpose of heating the feed-water in an open feed-water heater the rest of the steam is then expanded in the low-pressure turbine to the condenser. The water leaves the open feed-water heater at 120°C as a saturated liquid. The efficiency of both turbines is 85%. 

The optimization of steam turbine systems is well understood and more widely practiced than the power consumption optimization of alternatives such as electric motors and gas turbines. In addition to the initial cost, the major disadvantage of steam turbines is their low tolerance for wet or contaminated steam. Wet steam can cause rapid erosion, and contaminants can cause fouling. Both will reduce the turbine s efficiency and will shorten its life. Steam quality monitoring is therefore important to maintain the reliability and to reduce the operating cost of steam turbines. 

Today s biopower capacity is based on mature, direct-combustion boiler/steam turbine technology. The average size of biopower plants is 20 MW (the largest approach 100 MW) and the average efficiency from steam-turbine generators is 17 to 25 percent. The small plant sizes lead to higher capital cost per kilowatt-hour of power produced and the low electrical conversion efficiencies increase sensitivity to fluctuations in feedstock price.658... 

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