Overview

A cogeneration topping cycle is based on the use of heat rejected from the power generation process for an economically useful purpose. The use of this heat results in a reduction in the amount of fuel that would otherwise be required for site burners, thus reducing total energy costs. The amount of useful heat that can be recovered per unit of electrical output will vary as a function of the type of prime mover, the particular configuration of the engine itself, and the actual operations of the engine. This review of heat recovery systems is organized by prime mover type; turbines, which generally provide the greatest flexibility in satisfying site requirements, are reviewed first followed by reciprocating engines, which provide the least flexibility.

An important factor contributing to the economic viability of cogeneration at a specific site is the ability to use that recovered heat in site processes and heating, ventilating, and air-conditioning (HVAC) systems. In general, the lower the temperature required at the site, the more useful heat that can be recovered from any prime mover and the greater the operating cost savings. The following broad guidelines are applicable regardless of prime mover type:

• Direct-drying industrial applications allow the maximum use of heat recoverable from exhaust gases, as drying is typically performed by heating ambient air. The resulting reference point for heat recovery is the ambient air temperature, which is generally less than 100 F (37.8 C). In addition, no heat recovery steam generator (HRSG) (with its associated costs) is...