Prime Mover Heat Balance

Heat recovery from each of the main types of prime movers (i.e., reciprocating engines, gas turbines, and steam turbines) presents a different set of conditions. A heat balance is an accounting of the energy flows involved in the process, including input fuel energy, power output, and rejected heat. The amount of rejected heat that can be used will depend on the form and condition in which it is rejected and the form and condition in which it can be recovered and applied for a given application.

For example, reciprocating engine-generated heat can be recovered in substantial quantity from both the coolant system and the exhaust gas. Most of the heat of friction can be recovered in reciprocating engine operations. But some of the heat is lost to the ambient surroundings as radiation or convection. Some energy is also lost due to inefficiency in the heat exchange process. Exhaust heat recovery is limited by the allowable final exhaust temperature and the required temperature of the available heat sink. Coolant temperatures are often too low to be useful for heat recovery.

The heat balance for a reciprocating engine can be summarized as:

Total heat input = work ouput + exhaust heat + radiation + jacket water + oil cooler + inter/after cooler

Gas turbines reject a high proportion of input energy to exhaust gases, most of which can be recovered. Exhaust gas heat recovery is limited mainly by process temperature requirement and the practical limitations of reducing final exhaust gas exit temperatures below acid dewpoint levels.

Steam turbine heat recovery requires power output to be purposefully reduced in order to improve overall system efficiency. Steam is extracted or discharged from the turbine at a higher than optimal pressure for secondary uses.

In a large condensing turbine, approximately one third of input energy can be used for generating shaft power; the rest is dissipated at relatively low temperature to the environment. By reducing power output to one quarter of input energy, output steam can be provided at a high enough temperature to be effectively used for process heating.

Natural gas offers significant advantages as an engine fuel for exhaust gas heat recovery applications. In particular, gas use allows increased effectiveness due to reduced fouling of heat exchange surfaces. Heat recovery capacity is also increased because the products of gas combustion can be cooled to lower temperatures than those of sulfur-bearing fuels.

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