The Experts below are selected from a list of 195 Experts worldwide ranked by ideXlab platform

Ziwen Xing - One of the best experts on this subject based on the ideXlab platform.

  • extremum seeking control of cop optimization for air source transcritical co2 heat pump water heater system
    Applied Energy, 2015
    Co-Authors: Bin Hu, Yaoyu Li, Ziwen Xing
    Abstract:

    Air-source transcritical CO2 heat pump systems have found attractive applications in automobile air conditioning, regional heating and commercial water heating systems. The coefficient of performance (COP) of such systems is affected by several operational variables, such as ambient temperatures, water outlet temperature, and Discharge Pressure. For practical operation, it is desirable to maintain the maximum achievable efficiency or COP of such systems in real time. However, performance of model based control/optimization strategies can be quite limited by the difficulty in acquiring accurate system models due to system nonlinearity, large variations in ambient conditions and thermal load, as well as equipment variation and degradation. In this study, a self-optimizing control scheme is proposed to maximize the COP in real time by using the extremum seeking control (ESC) strategy. ESC is a class of self-optimizing control strategy that can search for the unknown or slowly varying optimum input with respect to certain performance index, which is effectively a dynamic realization of the gradient search based on a dither-demodulation scheme. For the air-source transcritical CO2 heat-pump water heater, the Discharge Pressure setpoint is taken as the input to the ESC controller, while the system COP is taken as the performance index, i.e. the feedback signal for the extremum seeking process. To evaluate the proposed ESC strategy, a Modelica based dynamic simulation model is developed for the plant to perform the simulation study. Simulations are conducted for several scenarios: a fixed operation condition, change of the water outlet temperature setpoint, change of ambient condition, and changes of both the ambient temperature and water outlet temperature setpoint. For all simulation cases, the water inlet temperature is fixed at 12°C, the ambient temperature is assumed to range from −15°C to −35°C, while the water outlet temperature ranges from 55°C to 80°C. Simulation results show that ESC is able to search and even track both fixed and slowly varying optimum COP without need for system model. Significant recovery of energy efficiency can thus be obtained for practical operation of such systems in a nearly model-free fashion.

  • effect of heat transfer area and refrigerant mass flux in a gas cooler on heating performance of air source transcritical co2 heat pump water heater system
    Energy and Buildings, 2013
    Co-Authors: Shouguo Wang, Yongning He, Ziwen Xing
    Abstract:

    Abstract This study investigated the effect of geometrical parameters of gas coolers on system performance and the optimal Discharge Pressure with a developed and experimentally validated numerical model of an air-source transcritical CO2 heat pump water heater system. This model fully considered actual heat transfer process in the gas cooler and characteristics of major components adopted in the prototype system. Simulation results showed a good agreement with experimental results in terms of heat yields, COP and the optimal Discharge Pressure. Parametric analysis reveals that a larger heat transfer area of the gas cooler increases heat yields but decreases the optimal Discharge Pressure while other components in the system remain unchanged. In addition, at the same operating condition with the same heat transfer area, heat yields increase with CO2 mass fluxes by reducing tube diameters and flow passages of gas coolers, and beyond certain range of mass fluxes, the optimal Discharge Pressure increases sharply. As one result, the optimal range of CO2 mass flux in heat exchange tubes should be the range of 155–325 kg m−2 s−1 at given condition in this paper.

  • experimental investigation on air source transcritical co2 heat pump water heater system at a fixed water inlet temperature
    International Journal of Refrigeration-revue Internationale Du Froid, 2013
    Co-Authors: Shouguo Wang, Ziwen Xing
    Abstract:

    Abstract For clarify main affecting factors on air-source transcritical CO2 heat pump water heaters, and the influence extent of these main affecting factors, at fixed water inlet temperature of 12 °C, an experimental research was conducted on a prototype in a range of ambient temperatures (−15 °C–35 °C) and water outlet temperatures (55 °C–80 °C). Based on the experimental results, effect of water outlet temperatures and ambient temperatures on the optimal Discharge Pressure and system COP was illustrated, and with change of water outlet temperatures and ambient temperatures, variation of CO2 gas cooler outlet temperatures, evaporating temperatures and other cycle variables was presented and interpreted. According established simulation model, we conducted simulation for the prototype and the cycle respectively adopted another two gas coolers with difference heat transfer surface. As one conclusion, for an established plant, correlation for the optimal Discharge Pressure as the function of ambient temperatures and water outlet temperatures can be achieved by fitting numerous simulation or experimental results.

  • experimental investigation of the Discharge valve dynamics in a reciprocating compressor for trans critical co2 refrigeration cycle
    Applied Thermal Engineering, 2012
    Co-Authors: Zhilong He, Xueyuan Peng, Ziwen Xing
    Abstract:

    The self-acting valve has a significant influence on the efficiency and reliability of the reciprocating compressor. In the trans-critical CO2 cycle, the large density and high Pressure difference across the valve cause serious bending and impact stresses in the valve, offering great challenges for successful valve design. Experimental investigation of the valve dynamics is required in order to design a self-acting valve with a high efficiency and long life span for the trans-critical CO2 compressor. A semi-hermetic reciprocating compressor was developed for application in CO2 refrigeration, and a test system was incorporated into the compressor performance test rig, with a focus on investigating the dynamics of the Discharge valves. With the experimental results, the movement of the valve was discussed in detail for the trans-critical CO2 compressor, allowing for the study of the thermodynamic performance of the compressor. While varying design parameters such as Pressure ratio, valve lift, spring stiffness and compressor speed, the movement of the Discharge valve in the reciprocating CO2 compressor was measured in order to investigate the major factors that influence the valve dynamics. The average valve speed increased from 0.71 m/s to 0.81 m/s as the Discharge Pressure changed from 7.8 MPa to 12 MPa. The experimental methods and results discussed in this paper could provide useful information for both valve testing and the optimization of their reliability in trans-critical CO2 compressors.

Shouguo Wang - One of the best experts on this subject based on the ideXlab platform.

  • effect of heat transfer area and refrigerant mass flux in a gas cooler on heating performance of air source transcritical co2 heat pump water heater system
    Energy and Buildings, 2013
    Co-Authors: Shouguo Wang, Yongning He, Ziwen Xing
    Abstract:

    Abstract This study investigated the effect of geometrical parameters of gas coolers on system performance and the optimal Discharge Pressure with a developed and experimentally validated numerical model of an air-source transcritical CO2 heat pump water heater system. This model fully considered actual heat transfer process in the gas cooler and characteristics of major components adopted in the prototype system. Simulation results showed a good agreement with experimental results in terms of heat yields, COP and the optimal Discharge Pressure. Parametric analysis reveals that a larger heat transfer area of the gas cooler increases heat yields but decreases the optimal Discharge Pressure while other components in the system remain unchanged. In addition, at the same operating condition with the same heat transfer area, heat yields increase with CO2 mass fluxes by reducing tube diameters and flow passages of gas coolers, and beyond certain range of mass fluxes, the optimal Discharge Pressure increases sharply. As one result, the optimal range of CO2 mass flux in heat exchange tubes should be the range of 155–325 kg m−2 s−1 at given condition in this paper.

  • experimental investigation on air source transcritical co2 heat pump water heater system at a fixed water inlet temperature
    International Journal of Refrigeration-revue Internationale Du Froid, 2013
    Co-Authors: Shouguo Wang, Ziwen Xing
    Abstract:

    Abstract For clarify main affecting factors on air-source transcritical CO2 heat pump water heaters, and the influence extent of these main affecting factors, at fixed water inlet temperature of 12 °C, an experimental research was conducted on a prototype in a range of ambient temperatures (−15 °C–35 °C) and water outlet temperatures (55 °C–80 °C). Based on the experimental results, effect of water outlet temperatures and ambient temperatures on the optimal Discharge Pressure and system COP was illustrated, and with change of water outlet temperatures and ambient temperatures, variation of CO2 gas cooler outlet temperatures, evaporating temperatures and other cycle variables was presented and interpreted. According established simulation model, we conducted simulation for the prototype and the cycle respectively adopted another two gas coolers with difference heat transfer surface. As one conclusion, for an established plant, correlation for the optimal Discharge Pressure as the function of ambient temperatures and water outlet temperatures can be achieved by fitting numerous simulation or experimental results.

Jahar Sarkar - One of the best experts on this subject based on the ideXlab platform.

  • cycle parameter optimization of vortex tube expansion transcritical co2 system
    International Journal of Thermal Sciences, 2009
    Co-Authors: Jahar Sarkar
    Abstract:

    Abstract Optimization studies along with optimum parameter correlations are presented in this article for a vortex tube expansion transcritical CO2 refrigeration cycle with two cycle layouts based on the Maurer model (1999) and the Keller model (1997). A simple thermodynamic model is proposed and used for vortex tube analysis. Finally, the COP improvement and effect on optimum Discharge Pressure by using vortex tube in transcritical CO2 cycle instead of expansion valve are presented. The results show that the effect of cold mass fraction and inlet water temperature to desuperheater (used to cool hot gas from vortex tube) on the cycle optimization is negligible. The Maurer model is better than the Keller model in terms of moderately more COP improvement and lower cost due to less components. The use of a vortex tube is more effective for higher gas cooler exit temperature for both models. Results show that the vortex tube expansion transcritical CO2 cycle for the Maurer model can give higher COP improvement for lower cooling temperature applications; however the trend is reverse for the Keller model.

  • simulation of a transcritical co2 heat pump cycle for simultaneous cooling and heating applicationselimination des cristaux de givre sur une plaque froide effets des frequences stationnaires et de balayage des champs electriques
    International Journal of Refrigeration-revue Internationale Du Froid, 2006
    Co-Authors: Jahar Sarkar, Souvik Bhattacharyya, Ram M Gopal
    Abstract:

    A steady state simulation model has been developed to evaluate the system performance of a transcritical carbon dioxide heat pump for simultaneous heating and cooling. The simulated results are found to be in reasonable agreement with experimental results reported in the literature. Such a system is suitable, for example, in dairy plants where simultaneous cooling at 4 °C and heating at 73 °C are required. The optimal COP was found to be a function of the compressor speed, the coolant inlet temperature to the evaporator and inlet temperature of the fluid to be heated in the gas cooler and compressor Discharge Pressure. An optimizing study for the best allocation of the fixed total heat exchanger inventory between the evaporator and the gas cooler based on the heat exchanger area has been carried out. Effect of heat transfer in the heat exchangers on system performance has been presented as well. Finally, a novel nomogram has been developed and it is expected to offer useful guidelines for system design and its optimisation.

Zhilong He - One of the best experts on this subject based on the ideXlab platform.

  • experimental investigation of the Discharge valve dynamics in a reciprocating compressor for trans critical co2 refrigeration cycle
    Applied Thermal Engineering, 2012
    Co-Authors: Zhilong He, Xueyuan Peng, Ziwen Xing
    Abstract:

    The self-acting valve has a significant influence on the efficiency and reliability of the reciprocating compressor. In the trans-critical CO2 cycle, the large density and high Pressure difference across the valve cause serious bending and impact stresses in the valve, offering great challenges for successful valve design. Experimental investigation of the valve dynamics is required in order to design a self-acting valve with a high efficiency and long life span for the trans-critical CO2 compressor. A semi-hermetic reciprocating compressor was developed for application in CO2 refrigeration, and a test system was incorporated into the compressor performance test rig, with a focus on investigating the dynamics of the Discharge valves. With the experimental results, the movement of the valve was discussed in detail for the trans-critical CO2 compressor, allowing for the study of the thermodynamic performance of the compressor. While varying design parameters such as Pressure ratio, valve lift, spring stiffness and compressor speed, the movement of the Discharge valve in the reciprocating CO2 compressor was measured in order to investigate the major factors that influence the valve dynamics. The average valve speed increased from 0.71 m/s to 0.81 m/s as the Discharge Pressure changed from 7.8 MPa to 12 MPa. The experimental methods and results discussed in this paper could provide useful information for both valve testing and the optimization of their reliability in trans-critical CO2 compressors.

A A M Redhwan - One of the best experts on this subject based on the ideXlab platform.

  • potential of nanorefrigerant and nanolubricant on energy saving in refrigeration system a review
    Renewable & Sustainable Energy Reviews, 2017
    Co-Authors: W H Azmi, M Z Sharif, Taib Mohd Yusof, Rizalman Mamat, A A M Redhwan
    Abstract:

    Refrigeration system is one of the biggest reason of the expanding pattern of energy consumption, thus, energy saving is one of the best approach to overcome this issue. Nanofluids show extraordinary potential in upgrading the thermodynamic and mechanical performance of the refrigeration system. In the refrigeration system, the effort to improve the efficiency of the system is by introducing nanoparticles in refrigerant (nanorefrigerant) and in lubricant (nanolubricant). In this paper, a comprehensive review is carried out to investigate the impact of nanorefrigerant and nanolubricant on energy saving. The overview consists of properties enhancement of nanorefrigerant and nanolubricant, tribological performance, heat transfer enhancement, performance in heat exchanger, improvement in refrigeration system and Pressure drop characteristic. The previous results showed that the best energy saving with 21% less energy used was with the use of 0.5% volume ZnO-R152a refrigerant nanolubricant. Both the suction Pressure and Discharge Pressure were brought down by 10.5% when nanorefrigerant was utilized. The evaporator temperature was lessened by 6% with the utilization of nanorefrigerant. The replacement of R134a with R152a gives a green and clean environment, with zero ozone depleting potential (ODP) and less global warming potential (GWP). The performance of refrigeration system was significantly enhanced.