CO2 High-Side Pressure Control for Wide-Range Refrigeration
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Solution Overview
Problem
Conventional control schemes for CO2 vapor compression systems in transport refrigeration units, which operate over a large range of heat source temperatures, fail to accurately determine the optimum high-side pressure, leading to suboptimal energy efficiency and cooling capacity.
Innovation Solution
The system controls high-side pressure based on conditions on both the high-pressure side (cooler) and low-pressure side (evaporator), using various combinations of sensed temperature and pressure conditions to determine the optimum pressure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If conventional control schemes use only heat sink temperature to control heat rejection pressure, then the control system is simple, but the energy efficiency and cooling capacity are suboptimal in systems with large heat source temperature ranges
Solution Approach 1:
The control system incorporates feedback from multiple sensors monitoring heat sink temperature, heat source temperature, compressor discharge pressure, and refrigerant flow rate. This multi-parameter feedback enables dynamic adjustment of the expansion valve to maintain optimal heat rejection pressure, resolving the contradiction between control simplicity and cooling capacity optimization
Solution Approach 2:
The system dynamically changes operating parameters (heat rejection pressure, expansion valve opening) based on real-time measurements of heat source temperature and other system conditions. This parameter adaptation allows the system to maintain optimal performance across varying operating conditions, improving cooling capacity while keeping the control logic manageable
2Device complexity
If conventional control schemes use only heat sink temperature to control heat rejection pressure, then the control algorithm is simple, but the energy efficiency is suboptimal in systems with large heat source temperature ranges
Solution Approach 1:
Multiple sensors provide continuous feedback on system state (heat source temperature, heat sink temperature, compressor discharge pressure, refrigerant flow rate), enabling the control algorithm to calculate and maintain optimal heat rejection pressure. This feedback mechanism improves energy efficiency by ensuring operation at or near optimal pressure points across the full operating range
Solution Approach 2:
The control algorithm dynamically adjusts the optimal heat rejection pressure setpoint based on real-time heat source temperature and system conditions, rather than using a fixed pressure target. This dynamic adaptation allows the system to maintain optimal energy efficiency across varying operating conditions without requiring excessively complex control logic
3Adaptability or versatility
If the system operates over a large range of heat source temperatures, then the system versatility is improved, but the single-parameter control based on heat sink temperature becomes insufficient
Solution Approach 1:
The control system serves multiple functions: it monitors heat sink temperature, heat source temperature, compressor discharge pressure, and refrigerant flow rate; calculates optimal heat rejection pressure; and controls the expansion valve. This multi-functional approach allows a single control system to handle the full operating range effectively
Solution Approach 2:
The system adapts its control strategy by changing the optimal heat rejection pressure setpoint based on heat source temperature and other operating conditions. This parameter adaptation enables the system to maintain optimal performance across the full operating range from -20°F to 57°F without requiring fundamentally different control systems for different conditions
Data Source
AI summary
To accommodate a transcritical vapor compression system with an operating envelope which covers a large range of heat source temperatures, a high side pressure is maintained at a level determined not only by operating conditions at the condenser but also at the evaporator. A control is provided to vary the expansion device in response to various combinations of refrigerant conditions sensed at both the condenser and the evaporator in order to maintain a desired high side pressure.