CO2 Refrigeration Cycle With Expander-Subcompressor Pressure Matching
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Solution Overview
Problem
Refrigeration cycle devices using carbon dioxide as a refrigerant face efficiency issues due to the constraint of constant density ratio, leading to difficulties in adjusting high-pressure-side pressure to optimal levels, which affects the recovery of expansion power and overall operating efficiency.
Innovation Solution
A refrigeration cycle device configuration that includes a main compressor, an expander, a sub-compressor, and a sub-compression passage, where the design volume ratio is set to efficiently recover power across a wide operating range by adjusting the high-pressure-side pressure through the use of an intermediate-pressure bypass valve and pre-expansion valve, allowing for optimal power recovery without bypassing the expander.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a bypass passage is provided to adjust high-pressure-side pressure, then pressure adjustment flexibility is improved, but refrigeration efficiency deteriorates due to isenthalpic expansion loss
Solution Approach 1:
The compression process is segmented into two stages: a sub-compressor handles the first stage compression from suction pressure to intermediate pressure, while the main compressor handles the second stage from intermediate pressure to discharge pressure. This segmentation allows the expander to operate independently at optimal conditions without requiring bypass passages for pressure adjustment, eliminating the isenthalpic expansion loss while maintaining pressure adaptability through the sub-compressor's variable displacement control.
2Ease of manufacture
If the design volume ratio is fixed, then manufacturing simplicity is improved, but operating efficiency deteriorates under varying conditions
Solution Approach 1:
The sub-compressor is designed with variable displacement capability, allowing its compression ratio and flow rate to be dynamically adjusted based on operating conditions. This dynamic adjustment enables the system to maintain optimal pressure ratios across varying loads and temperatures, significantly improving operating efficiency while the fixed geometry of the main compressor and expander preserves manufacturing simplicity.
3Ease of operation
If the expander is bypassed to adjust pressure, then pressure control capability is improved, but power recovery efficiency deteriorates
Solution Approach 1:
The sub-compressor acts as an intermediary device between the expander and the main compressor. It receives the intermediate-pressure refrigerant from the expander and compresses it to the final discharge pressure, allowing the expander to operate at optimal pressure ratio for maximum power recovery while the sub-compressor handles the additional pressure rise needed for system operation.
4Power
If a sub-compressor is added to the system, then power recovery efficiency is improved, but device complexity increases
Solution Approach 1:
The sub-compressor and expander are mechanically coupled through a common shaft, merging their functions into a single integrated unit. The expander's output shaft is directly connected to the sub-compressor's input shaft, allowing the expander to drive the sub-compressor without requiring additional motors or control systems. This merging reduces device complexity while maintaining the power recovery efficiency benefits of the two-stage compression system.
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 solution enables highly efficient operation by effectively recovering power across a wide range, maintaining high efficiency even when optimal high-pressure-side pressure adjustment is challenging, while preventing degradation in lubrication and reliability issues.
Implementation Method 1
an expander that reduces a pressure of a refrigerant
Implementation Method 2
a main compressor that compresses a refrigerant from a low pressure to a high pressure
Implementation Method 3
a sub-compressor that is provided in the sub-compression passage, compresses part of the refrigerant with the low pressure, which has flowed out from the evaporator, to an intermediate pressure
Implementation Method 4
a radiator that dissipates heat of the refrigerant, which has been discharged from the main compressor
Implementation Method 5
an evaporator that causes the refrigerant, which has flowed out from the expander, to evaporate
Data Source
AI summary
In a refrigeration cycle device, a design volume ratio, obtained by dividing a stroke volume of a sub-compressor by a stroke volume of an expander, is set to be smaller than (DE/DC)×(hE−hF)/(hB−hA). With an operating efficiency being the maximum in an operating range allowed to be set of the refrigeration cycle device, DE is a density of a refrigerant, which has flowed out from a radiator, DC is a density of the refrigerant, which has flowed out from an evaporator, hE is a specific enthalpy of the refrigerant flowing into the expander, hF is a specific enthalpy of the refrigerant, which has flowed out from the expander, hA is a specific enthalpy of the refrigerant sucked by a main compressor, and hB is a specific enthalpy of the refrigerant at an intermediate position of a compression process of the main compressor.