Problem 8.2 - A Cogeneration Steam Power Plant with an Open Feedwater Heater

What is Cogeneration? - We like the definition presented by the Midwest Cogeneration Association as follows: Cogneration is the utilization of 2 forms of energy from 1 source i.e.: hot water/heat and electricity from one gen-set.

According to Cogeneration Technologies, the world's first commercial power plant - Thomas Edison's Pearl Street Station built in 1882 - was a cogeneration plant as it made and distributed both electricity and thermal energy, thus the concept has been around for many years, With the recent interest in greener energy technologies it is currently becoming more popular.

This brings us to the current problem statement:- In an effort to decentralise the power grid and utilize the waste heat which accompanies power generation, Athenai Power Consulting have proposed a cogeneration system for Ohio University to provide both 1MW electric power and hot water at around 60°C. They realise that the power consumption of OU is much higher, however they believe that a significant portion of this power is required for hot water heating.

The basic approach to this unique design is shown in the following schematic diagram:

A unique aspect of the power plant is that the hot water storage tank is located in the hotwell of the condenser allowing direct heating and storage of the water at the required temperature of 60°C with no heat loss. The continuous supply of hot water is pumped from the storage tank through the hot water distribution system in the various buildings, and makeup water is added as required. Notice that in order to ensure continuous condensation of the steam there is a parallel cooling system using water pumped from the Hocking River which can be controlled as needed based on the hot water heating requirement Notice also that Athenai have proposed an Open Feedwater Heater / de-aerator in an attempt to improve the efficiency of the basic design.

As a young engineer at Athenai your purpose is to complete the design and evaluate and discuss its effectiveness.

Justify all values used and derive all equations used starting from the basic energy equation for a flow system, the enthalpy difference [Δh] for an incompressible liquid, the basic definition of thermal efficiency [ηth] of a heat engine, and the definitions of adiabatic efficiency [ηT], irreversibility, and Second Law efficiency [ηII] of a turbine.


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