Systems, controllers, and methods for controlling a CO<sub>2 </sub>refrigeration system
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Solution Overview
Problem
Existing CO2 refrigeration systems experience inefficiencies due to frequent mode switching between subcritical and transcritical operations, instability in subcooling control, and inefficient pressure regulation, leading to degraded performance and reduced efficiency, particularly at low ambient conditions.
Innovation Solution
A CO2 refrigeration system with integrated control algorithms that utilize inlet air temperature to manage both transcritical and subcritical modes, adjusting the high-pressure valve and fan speed to maintain optimal exit pressure and temperature targets without mode-specific switching, thereby stabilizing operation and enhancing efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If two-mode control structure is used to switch between subcritical and transcritical modes based on critical temperature, then the system can adapt to different operating conditions, but the system experiences frequent mode switching and instability particularly near the critical temperature
Solution Approach 1:
The control system is divided into separate subcritical mode controller and transcritical mode controller, each optimized for their specific operating range. The controller determines the operating mode based on ambient temperature thresholds and activates the appropriate control algorithm, avoiding the instability of frequent mode switching near critical temperature by clearly defining operational boundaries for each mode.
Solution Approach 2:
The control system dynamically adapts its parameters based on the determined operating mode. In subcritical mode, the system optimizes for condensing efficiency while in transcritical mode, it optimizes for heat rejection. This dynamic adaptation allows the system to maintain stability within each mode while preserving versatility across different operating conditions.
2Measurement precision
If pressure control target is determined from refrigerant temperature at gas cooler exit, then the system can respond to temperature changes, but the system exhibits tail-chasing behavior with oscillating pressure and temperature
Solution Approach 1:
The control system determines the operating mode in advance based on ambient temperature before attempting pressure control. By pre-determining whether the system should operate in subcritical or transcritical mode based on ambient conditions, the controller establishes a stable pressure target range before control actions are taken, preventing oscillatory tail-chasing behavior.
Solution Approach 2:
The system uses feedback from ambient temperature sensors to continuously monitor operating conditions and adjust the control mode accordingly. This feedback mechanism allows the system to maintain stable pressure and temperature by switching between control algorithms based on actual environmental conditions rather than reacting to transient fluctuations.
3Stability of the object's composition
If minimal condensing pressure limit is imposed to maintain stability in subcooling control, then the system can avoid instability, but the system operates at lower efficiency
Solution Approach 1:
The control system changes its operating parameters based on the determined mode. In subcritical mode, it operates with higher condensing pressures optimized for efficiency while in transcritical mode, it operates with lower pressures optimized for heat rejection. This parameter adaptation allows the system to maintain stability in each mode without being constrained by a universal minimal pressure limit that would reduce 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 achieves stable and efficient operation across varying conditions by dynamically controlling the high-pressure valve and fan based on inlet air temperature, reducing unnecessary mode transitions and improving system performance and energy efficiency.
Implementation Method 1
a gas cooler/condenser that receives refrigerant discharged from the compressor and outputs a flow of refrigerant
Implementation Method 2
a fan positioned to supply a flow of air into the gas cooler/condenser
Data Source
AI summary
A controller for a CO2 refrigeration system includes at least one input configured to receive an inlet air temperature from a first temperature sensor, an exit temperature from a second temperature sensor and an exit pressure from a pressure sensor, at least one output for controlling the valve and the fan, a memory, and a processor. The processor is configured by instructions stored in the memory to receive, through the at least one input, the inlet air temperature from the first temperature sensor, and control, through the at least one output, the valve and the fan based on the inlet air temperature both when the CO2 refrigeration system is operating in a transcritical mode and when the CO2 refrigeration system is operating in a subcritical mode.