Problem 4.5 - A Cogeneration Steam Power Plant

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 ststement: In an effort to decentralise the power grid and utilize the waste heat which accompanies power generation, the Athenai Power Consulting Corp. has proposed a Cogeneration system for O'Bleness Hospital to provide both 500kW electric power and hot water at 60°C. The basic approach to this unique design is shown in the following schematic diagram:

A unique aspect of the power plant is that the turbine output is at 100kPa close to 100°C. This high pressure output both eliminates the need for a deaerator and the condenser will be able to directly heat the water to the required 60°C. During the lull period when no hot water is required, water is drawn from the Hocking River to condense and subcool the steam in the hotwell to 60°C. As a young engineer at Athenai your purpose is to evaluate the basic design and discuss its effectiveness.

Justify all values used and derive all equations used starting from the basic energy equation for a flow system, the basic definition of thermal efficiency ηth, and the enthalpy change of incompressible liquid water Δh.


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