A new approach to optimize the energy efficiency of CO2 transcritical refrigeration plants

Abstract This paper proposes a model-free real-time optimization and control strategy for CO2 transcritical refrigeration plants that assures covering the cooling demand and continuous tracking of conditions for maximum efficiency. Our approach obtains the feedback with only three measurements, and controls the opening degree of a backpressure valve and the speed of the compressor. The strategy minimizes the power consumption of the compressor instead of maximizing the coefficient of performance, which avoids several sensors, and we demonstrate mathematically that both approaches are equivalent. We implemented the strategy with an algorithm that includes two independent auto tuned controllers, one devoted to regulate the high-pressure and another to regulate the outlet temperature of the secondary fluid of the evaporator. It also incorporates a real time perturb and observe procedure to locate online the optimum high-pressure that minimizes the compressor power consumption. The paper presents the experimental evaluation of the control strategy, verifying the stable operation of the algorithm and the energy optimization of the plant.

[1]  Marco Corradi,et al.  Optimisation of the throttling system in a CO2 refrigerating machine , 2003 .

[2]  Chun-Lu Zhang,et al.  A correlation-free on-line optimal control method of heat rejection pressures in CO2 transcritical systems , 2011 .

[3]  Rodrigo Llopis,et al.  Development and validation of a finite element model for water - CO2 coaxial gas-coolers , 2012 .

[4]  E. Torrella,et al.  Influence of the superheat associated to a semihermetic compressor of a transcritical CO2 refrigeration plant , 2010 .

[5]  Singiresu S. Rao.  Engineering Optimization : Theory and Practice , 2010 .

[6]  Jahar Sarkar,et al.  Optimization of a transcritical CO2 heat pump cycle for simultaneous cooling and heating applications , 2004 .

[7]  Yunting Ge,et al.  Control optimisation of CO2 cycles for medium temperature retail food refrigeration systems , 2009 .

[8]  Petter Nekså,et al.  Commercial refrigeration system using CO2 as the refrigerant , 2004 .

[9]  Graham C. Goodwin,et al.  Control System Design , 2000 .

[10]  Ciro Aprea,et al.  Heat rejection pressure optimization for a carbon dioxide split system: An experimental study , 2009 .

[11]  Souvik Bhattacharyya,et al.  Optimized transcritical CO2 heat pumps: Performance comparison of capillary tubes against expansion valves , 2008 .

[12]  Ramón Cabello,et al.  Energetic evaluation of a CO2 refrigeration plant working in supercritical and subcritical conditions , 2014 .

[13]  Marco Corradi,et al.  A critical approach to the determination of optimal heat rejection pressure in transcritical systems , 2010 .

[14]  Ian Postlethwaite,et al.  Multivariable Feedback Control: Analysis and Design , 1996 .

[15]  Ramón Cabello López,et al.  REFLAB: An Interactive Tool for Supporting Practical Learning in the Educational Field of Refrigeration , 2011 .

[16]  Shengming Liao,et al.  A correlation of optimal heat rejection pressures in transcritical carbon dioxide cycles , 2000 .

[17]  Ramón Cabello,et al.  Experimental evaluation of the energy efficiency of a CO2 refrigerating plant working in transcritical conditions , 2008 .

[18]  Samer Sawalha,et al.  Theoretical evaluation of trans-critical CO2 systems in supermarket refrigeration. Part II: System modifications and comparisons of different solutions , 2008 .

[19]  Samer Sawalha,et al.  Theoretical evaluation of trans-critical CO2 systems in supermarket refrigeration. Part I: Modeling, simulation and optimization of two system solutions , 2008 .

[20]  Ramón Cabello,et al.  New positions for an internal heat exchanger in a CO2 supercritical refrigeration plant. Experimental analysis and energetic evaluation , 2014 .

[21]  Jostein Pettersen,et al.  Fundamental process and system design issues in CO2 vapor compression systems , 2004 .

[22]  Rodrigo Llopis,et al.  Energetic evaluation of an internal heat exchanger in a CO2 transcritical refrigeration plant using experimental data , 2011 .