A method for controlling a vapour compression system with an ejector
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Solution Overview
Problem
Vapour compression systems face challenges in maintaining optimal pressure levels within the receiver to balance compressor work and system efficiency across varying ambient temperatures, especially when using transcritical refrigerants like CO2, as high pressures can decrease efficiency and require excessive energy consumption.
Innovation Solution
A method for controlling vapour compression systems by detecting the pressure of refrigerant leaving the evaporator and adjusting the compressor unit's operation to maintain optimal pressure within the receiver, ensuring that refrigerant is either directed to the secondary inlet of the ejector or the compressor unit based on threshold values, thereby optimizing the mass flow and reducing energy consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the pressure inside the receiver is increased to reduce compressor work, then the work required by the compressor decreases, but the liquid/gas ratio in the receiver becomes unbalanced with more liquid and less gaseous refrigerant, forcing the heat rejecting heat exchanger pressure to be even higher and decreasing system efficiency
Solution Approach 1:
The control method continuously monitors the pressure inside the receiver and dynamically adjusts the expansion device opening degree based on feedback signals. This closed-loop control ensures the receiver pressure is maintained within an optimal range that balances compressor work reduction with system efficiency, preventing the pressure from rising to levels that would cause excessive liquid refrigerant accumulation
Solution Approach 2:
The expansion device opening degree is dynamically adjusted based on real-time receiver pressure conditions rather than being fixed. This dynamic control allows the system to adapt to varying operating conditions, optimizing the balance between reducing compressor work and maintaining appropriate liquid/gas ratio in the receiver across different ambient temperatures
2Use of energy by moving object
If more refrigerant is supplied from the evaporator outlet to the ejector secondary inlet to increase mass flow and reduce compressor work, then the pressure of refrigerant supplied to compressors increases, but the pressure of refrigerant leaving the evaporator may decrease below acceptable levels
Solution Approach 1:
The control method monitors the pressure of refrigerant leaving the evaporator and uses this feedback to adjust the expansion device opening degree. When evaporator outlet pressure approaches acceptable minimum levels, the expansion device opening is reduced to limit refrigerant flow to the ejector, thereby maintaining evaporator outlet pressure above acceptable thresholds while still optimizing compressor work
Solution Approach 2:
The system changes the operating parameters of the expansion device (opening degree) based on detected pressure conditions. By adjusting this parameter, the system optimizes the refrigerant flow distribution between the ejector and compressor inlet, balancing the competing requirements of maximizing ejector mass flow for energy savings versus maintaining sufficient evaporator outlet pressure
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 method allows for efficient energy use by maximizing refrigerant flow through the ejector while preventing pressure drops below acceptable levels, thus enhancing the overall efficiency of the vapour compression system across a wide range of ambient temperatures.
Implementation Method 1
An ejector is a type of pump which uses the Venturi effect to increase the pressure energy of fluid at a suction inlet (or secondary inlet) of the ejector by means of a motive fluid supplied to a motive inlet (or primary inlet) of the ejector.
Implementation Method 2
The liquid part of the refrigerant is supplied to the evaporator, via an expansion device. The gaseous part of the refrigerant may be supplied to a compressor.
Data Source
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AI summary
A method for controlling a vapour compression system (1) is disclosed, the vapour compression system (1) comprising an ejector (5). The method comprises controlling a compressor unit (2) in order to adjust a pressure inside a receiver (6), on the basis of a detected pressure of refrigerant leaving an evaporator (8). The portion of refrigerant leaving the evaporator (8) which is supplied to a secondary inlet (15) of the ejector is maximised and the portion of refrigerant supplied directly to the compressor unit (2) is minimised, while ensuring that the pressure of refrigerant leaving the evaporator (8) does not decrease below an acceptable level.