# Power Generation from Low Grade Industrial Waste Heat

Students: Samantha Knisley Brian Kruelskie Heather Smith (Team Leader)

Date: Spring 2011

Requirements:
The designed stack system must generate 2MW of power and pay for itself in 10 years or less at a rate of \$0.05/kW-h. The available heat source is water at 200oF flowing at a rate of 3,000 gallons per minute. The emissions of the system should be zero, or if present, the CO2 and NOx emissions should be less than 5 tons/year (or 2ppm). Also, the project budget was \$500.00.

Problem:
This project focuses on utilizing a natural occurring phenomenon known as the stack effect to generate power from low-grade waste heat. The heat input to the system is low-grade waste water that is passed through a heat exchanger to transfer heat to ambient air. The heated air rises and creates a draft, or upward flow of air, which powers a turbine to generate energy.

Solution:
The team generated several versions of MATLAB code to analysis the stack system. The final version uses a discrete control volume approach and the conservation equations (energy, momentum, and mass). This code provided outputs at several heights within the system to indicate the thermodynamic state changes of the flow. Also, a wide range of stack shapes was evaluated. The team examineed the tradeoffs inherent in different stack shapes and component placements. The team performed sensitivity analyses to determine how the stack dimensions, heat exchanger dimensions, wind turbine placement, ambient environment conditions, and the temperature of the waste water affected the power output. Using the optimized values found from the sensitivity analyses, four case studies were paired with cost data (material costs only) and evaluated. None of the case studies could meet both the 2 MW power requirement and the payback period of 10 years at a rate of \$0.05/kW-h. However, the preferred solution is a system with six 200 m tall pipe shaped stacks that produce 2MW of power with a payback period of 10 years at a rate of \$0.12/kW-h. A visualization model was also built to demonstrate how the stack effect works (shown below).

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