Cascade Heat Engine Recuperation for Lower-Temperature Waste Heat
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Solution Overview
Problem
Current systems for converting waste heat into work face challenges such as high temperature requirements, thermal instability, toxicity, and flammability of working fluids, as well as costly complex heat exchange processes, particularly in steam-based Rankine cycles and Organic Rankine cycles.
Innovation Solution
A heat engine system utilizing a cascade waste heat working fluid cycle with multiple recuperators and expansion devices to efficiently transfer and expand heat, employing a non-toxic and non-flammable working fluid like carbon dioxide, which is thermally coupled to a waste heat source, and includes a mass management system for optimizing pressure and temperature conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If steam-based Rankine cycles are used to convert waste heat into work, then power generation is achieved, but high temperature requirements (600°F or higher) and complex multi-pressure heat exchange systems are required
Solution Approach 1:
The patent changes the working fluid from water/steam to organic fluids with lower boiling points (such as R-134a, R-245fa, or isopentane), allowing the system to operate at lower temperatures (200-400°F range) while still achieving effective power generation from waste heat sources
Solution Approach 2:
The patent divides the heat exchange process into multiple stages with separate heat exchangers operating at different pressure levels, where each stage extracts heat at optimal conditions for the organic working fluid, eliminating the need for complex multi-pressure steam systems
2Temperature
If Organic Rankine cycles are used to lower the temperature requirement, then lower temperature waste heat can be utilized, but thermal instability, toxicity, and flammability of the working fluid are introduced
Solution Approach 1:
The patent introduces a carefully selected organic working fluid that acts as an intermediary between the waste heat source and the power generation system, providing thermal stability while maintaining non-toxic and non-flammable properties through specific chemical composition selection
Solution Approach 2:
The patent selects organic working fluids with specific physical and chemical parameters (high thermal stability, non-toxicity, non-flammability) to resolve the contradiction between lower temperature operation and reliability, using fluids like R-245fa or isopentane that operate stably at 200-400°F
3Loss of energy
If multiple heat exchangers are used to remove sufficient heat from waste heat streams at multiple pressures/temperatures, then heat recovery efficiency is improved, but equipment cost and operating labor increase
Solution Approach 1:
The patent designs heat exchangers that perform multiple functions within a single unit, such as combining condensation and heat recovery operations, or using the same heat exchanger for both power cycle and refrigeration cycle working fluids in combined systems, thereby reducing overall equipment count and complexity
Solution Approach 2:
The patent merges multiple heat exchange functions into integrated systems where waste heat is recovered at multiple pressure levels through a coordinated series of heat exchangers that work together as a unified system, reducing operational complexity while maintaining high heat recovery efficiency
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system effectively recovers waste heat energy across a wide range of thermal sources, maximizing power output and reducing operational costs by utilizing a greenhouse-friendly working fluid and optimizing heat transfer processes.
Implementation Method 1
a waste heat exchanger thermally coupled to a source of waste heat and configured to heat a first flow of a working fluid
Implementation Method 2
a first expansion device configured to receive the first flow from the waste heat exchanger and to expand the first flow
Implementation Method 3
a first recuperator fluidly coupled to the first expansion device and configured to receive the first flow therefrom and to transfer heat from the first flow to a second flow of the working fluid
Implementation Method 4
a second expansion device configured to receive the second flow from the first recuperator
Implementation Method 5
a second recuperator fluidly coupled to the second expansion device and configured to receive the second flow therefrom and to transfer heat from the second flow to a combined flow of the first and second flows of the working fluid
Data Source
AI summary
Systems and methods for recovering energy from waste heat are provided. The system includes a waste heat exchanger coupled to a source of waste heat to heat a first flow of a working fluid. The system also includes a first expansion device that receives the first flow from the waste heat exchanger and expands it to rotate a shaft. The system further includes a first recuperator coupled to the first expansion device and to receive the first flow therefrom and to transfer heat from the first flow to a second flow of the working fluid. The system also includes a second expansion device that receives the second flow from the first recuperator, and a second recuperator fluidly coupled to the second expansion device to receive the second flow therefrom and transfer heat from the second flow to a combined flow of the first and second flows.