Cascade Organic Rankine Cycle Heat Exchanger Design
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Solution Overview
Problem
Existing organic Rankine cycle (ORC) systems with direct exchange and cascade cycles face inefficiencies due to high exergetic losses and safety concerns, particularly when dealing with flammable working fluids and high temperature differences between hot and cold sources, limiting the recovery of heat from gas turbines.
Innovation Solution
An ORC system with direct exchange and cascade cycles where the high temperature fluid contacts the hot source directly, using a mixture of diphenyl/diphenyl oxide as the high temperature fluid and cyclopentane as the low temperature fluid, with specific heat exchanger configurations to optimize heat transfer and safety, allowing for efficient cooling of hot fumes and reducing the risk of fluid leakage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If direct exchange configuration is used to eliminate intermediate oil circuit, then investment costs are reduced, but safety risks increase due to flammable working fluid leakage
Solution Approach 1:
The system is divided into two separate cycles (high-temperature cycle and low-temperature cycle) with distinct working fluids. The high-temperature cycle uses a non-flammable fluid (diphenyl/diphenyl oxide mixture) that directly exchanges heat with hot gases, while the low-temperature cycle uses a flammable fluid (cyclopentane) that operates at lower temperatures. This segmentation isolates the flammable fluid from direct contact with high-temperature hot gases, reducing safety risks while maintaining the direct exchange configuration's cost advantages.
Solution Approach 2:
The high-temperature working fluid (diphenyl/diphenyl oxide mixture) acts as an intermediary between the hot gases and the low-temperature working fluid. It receives heat directly from hot gases at high temperatures, then transfers heat to the low-temperature cycle's working fluid in the condenser/evaporator. This intermediary approach allows direct exchange with hot gases without exposing the flammable low-temperature fluid to high-temperature combustion gases, thereby reducing safety risks.
2Productivity
If high temperature difference is used between hot source and ORC cycle, then heat recovery efficiency increases, but exergetic losses increase
Solution Approach 1:
The heat recovery process is segmented into two stages corresponding to two cycles operating at different temperature levels. The high-temperature cycle captures heat from the high-temperature portion of hot gases, while the low-temperature cycle captures heat from the lower-temperature portion. This segmentation allows the system to efficiently recover heat across a broader temperature range, improving overall heat recovery efficiency while reducing exergetic losses by matching temperature differences more closely at each stage.
Solution Approach 2:
The system changes the operating parameters (temperature and pressure) of two separate cycles to optimize heat recovery at different temperature levels. The high-temperature cycle operates with higher evaporation temperature and pressure to efficiently capture heat from hot gases at high temperatures, while the low-temperature cycle operates with lower parameters to efficiently capture residual heat. This parameter optimization reduces the temperature difference at each heat exchange stage, thereby reducing exergetic losses while maintaining high overall heat recovery efficiency.
3Productivity
If cascading cycles are adopted to cool fumes efficiently, then heat recovery improves, but system complexity increases
Solution Approach 1:
The system merges the functions of two cycles into a unified cascading configuration where the condenser of the high-temperature cycle serves as the evaporator of the low-temperature cycle. This merging allows efficient heat recovery by having the high-temperature cycle's working fluid condense while simultaneously evaporating the low-temperature cycle's working fluid in the same heat exchanger. The integrated design improves heat recovery efficiency while managing system complexity through functional consolidation rather than separate components.
4Device complexity
If flammable working fluid is used in direct exchange with hot gases, then system simplicity increases, but risk of fire and burst increases
Solution Approach 1:
The working fluids are segmented by temperature and flammability characteristics. The high-temperature cycle uses a non-flammable fluid (diphenyl/diphenyl oxide mixture) that safely contacts hot gases, while the low-temperature cycle uses a flammable fluid (cyclopentane) that operates at lower temperatures away from ignition risks. This segmentation maintains relative system simplicity while eliminating the fire and burst risks associated with using flammable fluids in direct contact with high-temperature hot gases.
Solution Approach 2:
The non-flammable high-temperature working fluid serves as an intermediary that contacts hot gases directly, preventing flammable low-temperature working fluid from exposure to high-temperature combustion gases. This intermediary approach maintains system simplicity by using direct exchange configuration while eliminating the safety hazards of fire and burst through the protective intermediary layer.
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
This configuration enhances the overall efficiency of the system by achieving a gross electrical efficiency of 28% with a gross output power greater than 10 MWel, while ensuring safety by minimizing the risk of fluid leakage and burst, and simplifying the heat exchanger design.
Implementation Method 1
the working fluid of which is preheated, evaporates and eventually overheats by exchanging heat directly with the fumes leaving the gas turbine
Implementation Method 2
the working fluid of which is preheated, evaporates and eventually overheats by exchanging heat directly with the fumes leaving the gas turbine
Implementation Method 3
in the high temperature working cycle, the fluid is firstly de-overheated and then is condensed
Data Source
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AI summary
An organic Rankine cycle system (100, 110, 120) with direct exchange and in cascade comprising a high temperature organic Rankine cycle (10) which carries out the direct heat exchange with a hot source (H) and a low temperature organic Rankine cycle (10') in thermal communication with the high temperature cycle (10). The organic Rankine cycle system (100, 110, 120) is configured in a way that the thermal communication between the cycles (10, 10') takes place through at least one heat exchanger (3) configured to use at least the condensation heat of the high temperature cycle to vaporize and/or preheat the working fluid of the low temperature organic Rankine cycle fluid and through a heat exchanger (4) configured to operate as working fluid sub-cooler for the high temperature organic Rankine cycle (10) and as a working fluid preheater for the low temperature organic Rankine cycle (10').