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Model-free control of vapor-injection air-source heat pump based on multivariable extremum seeking.

Number: pap. F34

Author(s) : WANG W., LI Y., CAO F.

Summary

A major disadvantage of the conventional air-source heat pump (ASHP) is that the capacity and the coefficient of performance (COP) degrade significantly in extremely low ambient temperature, along with the high discharge temperature of the compressor. The refrigerant vapor-injection technique has been well received as an effective method to improve the performance of ASHP. Two common strategies of vapor injection are the so-called flash tank cycle (FTC) and the internal heat exchanger cycle (IHXC) or economizer vapor injection system. In an IHXC system, two major parameters that are critical for system performance are the refrigerant mass flow rate of injection stream into the compressor and the injection pressure. It is desirable to optimize the settings of these two parameters in real time under various ambient and load conditions. However, model-based optimization is difficult and cost prohibitive to accomplish. In this study, a multi-variable extremum seeking control (ESC) framework is proposed as a model-free real-time optimization strategy to minimize the power consumption of an IHXC ASHP system in real time, provided that the requirement of thermal regulation is satisfied. As a dynamic realization of the gradient search based on a dither demodulation scheme, ESC can find out the unknown and/or slowly-varying optimal input setting with little plant knowledge. The intermediate pressure of injured vapor and the outdoor unit fan speed are taken as the manipulated inputs of the ESC, and the total power consumption of the system is the only feedback. To evaluate the proposed ESC strategy, a Modelica-based dynamic simulation model of an ASHP water heater is developed with Dymola and TIL Library. In this model, the outlet temperature of hot water is regulated by the compressor capacity, while the opening of an expansion valve of the vapor-injection loop is used to regulate the intermediate pressure and mass flow rate of the injected vapor, and a variable speed fan model is used for the outdoor unit heat exchanger. Simulation results show that ESC is able to find the optimum values of intermediate pressure and outdoor fan speed that minimizes the total power consumption, which also maximizes the system COP, under different scenarios of ambient and thermal load conditions.

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Pages: 17

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Details

  • Original title: Model-free control of vapor-injection air-source heat pump based on multivariable extremum seeking.
  • Record ID : 30022137
  • Languages: English
  • Source: 8th international conference on compressors and refrigeration, 2017.
  • Publication date: 2017/07/20

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