Document Type

Working Paper

Date of this Version

4-2010

Abstract

The purpose of this design project is to evaluate the feasibility of power recovery from a lowgrade heat source. Specifically, the design team investigated the technical and economic issues associated with building a plant to recover power from a waste heat stream that is the byproduct of another process, such as a power or chemical plant. The project specifies that the incoming hot air stream is at 120psi and 230oF, and there are no external heat sources. The project champion suggests the design team to use a modified organic Rankine cycle with a dense liquid expander for more thermodynamically efficient heat recovery.

However, the design used in this report presents a modification on the suggested cycle by adding a flash evaporator to separate a liquid-vapor stream before the condenser. This change decreases costs associated with the size of the condenser, as well as the amount of cooling water needed. This provides for a more economical and efficient process.

Concerning the process thermodynamics and readiness, we conclude that the low-grade heat recovery cycle is feasible in its current iteration, and it provides a viable method to recover energy from a low-grade heat stream. From an economic standpoint, we conclude that this process would be economically feasible if energy costs increased by 15%-20%, to about $0.096/kWh. However, it is not currently economically competitive, because the cost of the equipment and building the plant to recover the energy are significantly high in comparison to the amount of energy recovered from the process.

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Date Posted: 18 November 2010