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Fig. 26-1 Relative Effect of Part-Load Operation on Generating Efficiency. Source: Cogen Designs, Inc.

Fig. 26-1 Relative Effect of Part-Load Operation on Generating Efficiency. Source: Cogen Designs, Inc.

Figure 26-1 shows the relative effect of part-load operation on generating efficiency for a spark-ignition reciprocating engine, a two-shaft and a single-shaft combustion gas turbine. These curves reveal the benefits achieved by maintaining prime mover systems at full load. Notice that below 80% of rated capacity, the single-shaft turbine's relative generating efficiency starts to fall significantly, as compared with the reciprocating engine and two-shaft turbine. At 60% of rated capacity, the performance of the two-shaft turbine falls off relative to the reciprocating engine. A Diesel or dual-fuel engine will show even better part-load performance than the spark-ignited gas engine depicted in this graphic.

Gas turbines will more likely show superior economic performance during preliminary screening when thermal load requirements are high relative to electric loads and when higher temperature thermal output is required. When used with heat recovery-powered absorption or steam turbine-driven chillers for cooling applications, recovered heat from a combustion gas turbine will be more effective than heat recovered from a reciprocating engine system. This is the result of both the higher quantity of recoverable heat and the ability to power double-effect absorption or steam turbine-driven vapor compression systems with all of its thermal output.

For example, a moderately thermally efficient 2,000 kW reciprocating engine-driven electric cogeneration unit directly connected to a customized heat recovery absorption chiller/heater can provide about 8.5 MMBtu/h (8,959 MJ/h) of heating, or 650 tons (2,300 kW of cooling capacity, or a combination of these heating and cooling outputs. A typical 2,000 kW gas turbine, however, can produce about 13 MMBtu/h (13,700 MJ/h) of heating, or 1,300 tons (4,600 kWr) of cooling output.

For electric generation applications with more limited thermal loads or with widely varying thermal loads, both combined-cycle and steam injection-cycle systems will commonly be considered. These options provide operating flexibility in that heat recovery steam can serve either power generation or thermal loads, depending on demand. Supplementary firing can be used with either type of system. The steam injection cycle typically produces greater capacity for a given turbine model without supple mentary firing and does so at a lower capital cost and with lower space requirements.

On the other hand, the combined-cycle system often achieves a superior heat rate and does not require abundant water use because its steam is condensed in a closed steam-cycle system. Thus, a simplified comparison shows the typical trade-off of lower capital cost for the steam injection-cycle system versus lower operating cost for the combined-cycle system. Generally, conditions of high peak electric rates, inexpensive water, and low load factor (i.e., full capacity is not required all of the time) tend to favor the steam injection cycle, while conditions of high load factor and high water and fuel costs tend to favor the combined-cycle system.

Once preliminary comparisons are made to match available alternatives to a given application, a more detailed analysis is required to determine if the project is economically feasible and to limit the driver options to a small set of candidates. At that point, a very detailed technical and financial analysis will be required before a prudent final decision can be made.

This chapter summarizes factors relating to the application of the main prime mover types in electric power generation systems. A number of detailed examples and illustrations are provided to show the effect of driver and heat recovery options in simple-, cogeneration-, combined-, and STIG-cycle applications.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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