Cycle efficiency

The next generation of gas turbine-based, combined-cycle power plants, under constmction in many parts of the world, is to feature net plant efficiencies in the 60% range based on LHV of fuel input. These faciUties, scheduled for start-up in the latter 1990s, are anchored by large gas turbines capable of simple-cycle efficiencies >40% LHV in some cases. To develop these machines, manufacturers have scaled up and improved upon designs that have already proved to be highly rehable. 

A 165-MW-class gas turbine/generator has been introduced by another manufacturer. This machine, also developed by scaling up a proven design, features a simple-cycle efficiency of 37.5% a turbine inlet temperature of 1235°C a pressure ratio of 30 1, up from 16 1 on the previous generation and an output of 165 MW for gas fuel firing under International Standards Organization (ISO) conditions (101 kPa, 15°C (14.7 psia, 59°F)). A combined-cycle facihty based around this machine could achieve efficiencies up to 58% or a heat rate of about 6209 kj/kWh (5885 Btu/kWh). 

At least two manufacturers have developed and installed machines rated to produce more than 210 MW of electricity in the simple-cycle mode. In both cases, the machines were designed and manufactured through cooperative ventures between two or more international gas turbine developers. One 50-Hz unit, first installed as a peaking power faciUty in France, is rated for a gross output of 212 MW and a net simple-cycle efficiency of 34.2% for natural-gas firing. When integrated into an enhanced three-pressure, combined-cycle with reheat, net plant efficiencies in excess of 54% reportedly can be achieved. 

As of the mid-1990s, many older conventional steam plants have been converted to combined cycle. The old boiler is removed and replaced by a combustion turbine and heat recovery steam generator. Although the cycle efficiency is not as high as completely new plants, substantial capital cost is avoided by the modification and reuse of existing steam turbine and auxiHary equipment. In many combined cycle power plants, steam is injected into the combustors of the combustion turbine to lower peak flame temperatures and consequendy lower NO. ... 

Environmental Profile Facts About the Economics, Efe Cycle Efficiencies, and Peformance Benefits, The Vinyl Institute, Morristown, N.J., July 1993. 

There are many ways to increase cycle efficiency (COP). Some of them are better suited to one, but not for the other refrigerant. Sometimes, for the same refrigerant, the impact on COP could be different for various temperatures. One typical example is the use of aliqmd-to-suctiou heat exchanger (Fig. H-75). 

In the area of performance, the steam turbine power plants have an efficiency of about 35%, as compared to combined cycle power plants, which have an efficiency of about 55%. Newer Gas Turbine technology will make combined cycle efficiencies range between 60-65%. As a rule of thumb a 1% increase in efficiency could mean that 3.3% more capital can be invested. However one must be careful that the increase in efficiency does not lead to a decrease in availability. From 1996-2000 we have seen a growth in efficiency of about 10% and a loss in availability of about 10%. This trend must be turned around since many analysis show that a 1% drop in the availability needs about 2-3% increase in efficiency to offset that loss. 

The increase in pressure ratio increases the gas turbine thermal efficiency when accompanied with the increase in turbine firing temperature. Figure 1 -5 shows the effect on the overall cycle efficiency of the increasing pressure ratio and the firing temperature. The increase in the pressure ratio increases the overall efficiency at a given temperature, however increasing the pressure... 

Inereasing the pressure ratio and the turbine firing temperature inereases the Brayton cycle efficiency. This relationship of overall cycle efficiency is based on certain simplification assumptions such as (1) liia > nif, (2) the gas is caloricaly and thermally perfect, which means that... 

In the case of the actual cycle the effect of the turbine compressor (rjc), and expander (rjt) efficiencies must also be taken into account, to obtain the overall cycle efficiency between the firing temperature Tf and the ambient temperature Tamb of the turbine. This relationship is given in the following equation ... 

To obtain a more accurate relationship between the overall thermal efficiency and the inlet turbine temperatures, overall pressure ratios, and output work, consider the following relationships. For maximum overall thermal cycle efficiency, the following equation gives the optimum pressure ratio for fixed inlet temperatures and efficiencies to the compressor and turbine ... 

Figure 2-5 shows the improvement in eyele effieieney beeause of heat recovery with respect to a simple open-cycle gas turbine of 4.33.T ratio pressure and 1,200°F inlet temperature. Cycle efficiency drops with an increasing pressure drop in the regenerator. 

Thus, the overall adiabatic thermal cycle efficiency can be calculated from the following equation ... 

Analysis of this cycle indicates that an increase in inlet temperature to the turbine causes an increase in the cycle efficiency. The optimum pressure ratio for maximum efficiency varies with the turbine inlet temperature from an optimum of about 15.5 1 at a temperature of 1500°F (816°C) to about 43 1 at a temperature of about 2400 °F (1316 °C). The pressure ratio for maximum work, however, varies from about 11.5 1 to about 35 1 for the same respective temperatures. 

The split-shaft regenerative turbine is very similar to the split-shaft cycle. The advantage of this turbine is the same as that mentioned before namely, high torque at low rpm. The cycle efficiencies are also about the same. Figure 2-14 indicates the performance that may be expected from such a cycle. 

Figure 2-21 show the effect of 5% by weight of steam injection at a turbine inlet temperature of 2400 °F (1316 °C) on the system. With about 5% injection at 2400°F (1316 °C) and a pressure ratio of 17 1, an 8.3% increase in work output is noted with an increase of about 19% in cycle efficiency over that experienced in the simple cycle. The assumption here is that steam is injected at a pressure of about 60 psi (4 Bar) above the air from the compressor discharge and that all the steam is created by heat from the turbine exhaust. Calculations indicate that there is more than enough waste heat to achieve these goals. 

The injection of coolant air in the turbine rotor or stator causes a slight decrease in turbine efficiency however, the higher turbine inlet temperature usually makes up for the loss of the turbine component efficiency, giving an overall increase in cycle efficiency. Tests by NASA on three different types of cooled stator blades were conducted on a specially built 30-inch turbine cold-air test facility. The outer shell profile of all three blade types was the same, as seen in Figure 9-24. 

OVERALL CYCLE EFFICIENCY Tamb=15C EFF COMP = 87% EFFTURB, =92%... 

Another reason for the increased use of gas turbines as prime movers in the process industry is the high thermodynamic cycle efficiencies and subsequent low operating cost. 

Simple-cycle efficiency does not usually mean as much to process users as total-cycle efficiency, because the gas turbine is not usually economic in process applications without some type of heat recovery. Total-cycle efficiency is most important in any economic evaluation. In a cycle with heat recovery, the only major loss that is charged to the cycle is the heat exhausting from the boiler stack. With the good comes the bad. Gas turbine maintenance is generally somewhat higher in cost and should be included in the total evaluation. 

As an alternative to the thermal or cycle efficiency of Eq. (1.1), the cyclic heat rate (the ratio of heat supply rate to power output) is sometimes used ... 

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