Design Optimisation of CO2 Gas Cooler/Condenser in a Refrigeration System

AbstractAs a natural working fluid, CO2 has been widely applied in refrigeration and heat pump systems where heat is conventionally rereleased to ambient through external airflow. Owing to extraordinary thermophysical properties, especially its low critical temperature, the CO2 heat release through a high-pressure side heat exchanger will inevitably undergo either supercritical or subcritical processes, depending on ambient air temperatures and head pressure controls. Correspondingly, the heat exchanger will act intermittently as either a gas cooler or condenser within the system during an annual operation. Such evidence should therefore be taken into account for an optimal design of the heat exchanger and head pressure controls in order to significantly enhance the performance of both components and the associated system.To achieve these targets, two CO2 finned-tube gas coolers/condensers with different structural designs and controls have been purposely built, instrumented and connected with an existing test rig of a CO2 booster refrigeration system. Consequently, the performance of the CO2 gas coolers/condensers with different structure designs, controls and system integration at different operating conditions can be thoroughly investigated through experimentation. In the meantime, models of the finned-tube CO2 gas coolers/condensers have been developed using both the distributed (detailed model) and lumped (simple model) methods. The former is employed to give a detailed prediction of the working fluid temperature profiles, localized heat transfer rates and effects of pipe circuitry arrangements, while the latter is suitable for the simulation and optimisation of system integration with less computation time. Both models have been validated with measurements, and moreover the simple model has been integrated with other component models so as to create a system model. The effects of the CO2 gas cooler/condenser sizes and controls on the system performance can thus be compared and analysed

Thermodynamic analysis on the performance of an R717/R744 cascade refrigeration system for food retail applications

Natural refrigerants, which naturally occur in the environment and are non-synthetics substances, have been used as cooling fluids in refrigerator systems for more than hundred years. However, due to safety and performance issues the chemical refrigerant substances replaced them. Over the last two decades, due to the component availability on the market natural refrigerants are getting back all the attention to re-establish their use into applications where previously HFCs were the preferred and only options. Natural refrigerant substances include CO2 (R744,) ammonia (R717) and hydrocarbons. Due to the limitation on refrigerant charge capacity for hydrocarbon systems, ammonia and CO2 are promising alternatives for food retail centralised refrigeration systems. Both ammonia and carbon dioxide have superior thermodynamic properties. On the other hand, the high toxicity of ammonia and the very high operating pressure of carbon dioxide create barriers on the system applications. Large amount of ammonia is prohibited from the sales area of food retail shops. Moreover, carbon dioxide is less efficiency comparing to HFCs installations for warm climate applications. The combination of the two refrigerants in a cascade configuration, where the ammonia side is located far away from the sales area and the subcritical operation of the carbon dioxide without effect from external ambient conditions, may lead to high system efficiencies. In this paper, a thermodynamic analysis on the performance of a R717/R744 cascade refrigeration system is discussed. The cascade configuration consists of the R717 side, the cascade heat exchanger, a R744 liquid receiver, medium and low direct expansion load, a low temperature compressor and high temperature compressor. Modelling results show that the coefficient of performance of the analysed cascade refrigeration system decreased from 2.19 to 1.56 when the ambient temperature varied from 7 ºC to 40 ºC. In addition, the second law efficiency of the analysed cascade refrigeration system varied from 0.494 to a maximum of 0.544 between ambient temperatures of 7 and 31 ºC, respectively. Finally, the amount of exergy destruction in the condenser, MT evaporator, cascade exchanger and the R717 compressor of the analysed system accounts for large percentage in total exergy destruction

Investigation of the influence of air guiding strips on the performance of multi-deck refrigerated display cabinets using CFD modelling

To decrease the cooling capacity of open-type display cabinets, air guiding strips are proposed to be used at the front face of shelves. A refrigerated food multi-deck display cabinet with a plug-in cooling unit is selected as a prototype, and a 3-D CFD transient model is developed in ANSYS-Fluent 14.5, to simulate the performance of the cabinet. The air guiding strips were found to maintain the air velocity and the integrity of the air curtain, reducing turbulence and the entrainment of ambient air into the cabinet. For the same product temperature the cabinet with the air guiding strips was found to reduce the cooling load by up to 25% compared to the cabinet without the strips. These results need to be further validated against experimental tests at controlled conditions in the laboratory

Design considerations on a small scale supercritical CO2 power system for industrial high temperature waste heat to power recovery applications

Existing industrial processes are energy intensive environments with a multitude of waste heat streams at different temperature levels whose recovery would undoubtedly contribute to the enhancement of the sustainability of the industrial sites and their products. In particular, the geographical distribution and size of the available waste heat potential is very widespread with most sources being small to medium size, up to 1 MW and fewer of larger size above 1 MW. Among the waste heat to power conversion approaches, the usage of bottoming thermodynamic cycles based on carbon dioxide in supercritical phase (sCO2) provides significant advantages compared to more conventional technologies such as the Organic Rankine Cycle (ORC) systems that are nowadays commercially available even at small scale (~10 kWe). However, unlike the large sCO2 systems that are already on the market (~ MWe), medium and small size ones are still under development. In the current work, the challenges involved in the design of a small scale sCO2 system are considered through thermodynamic analysis (1st and 2nd law) as well as by a preliminary turbomachinery design based on the similarity approach. With reference to a simple regenerative cycle architecture, the study provides details on the design parameters and performance trends as well as on the operational and manufacturing constraints that the compressor imposes to the theoretical values

Experimental Investigation of CO2 Gas Cooler/Condenser in a Refrigeration System

Natural refrigerants including CO2 have been recognized as the most promising working fluids and have been widely applied in refrigeration systems over the last decade. Owing to its attractive thermo-physical properties and negligible environmental impact, the CO2 refrigerant can be used as a replacement for convectional HFC working fluids. Normally, CO2 refrigeration systems can be classified into three different groups as indirect, cascade and all CO2 transcritical booster structures. The CO2 booster system has some advantageous over the others in terms of functions and sizes etc. However, the performance of such system still requires further investigation and improvement. This study focused on the experimental investigation into the performance comparison of two CO2 finned-tube gas coolers/condensers with different design structures and their effects on the overall performance of a CO2 booster refrigeration system. The integrated CO2 booster refrigeration system consisted of two variable speed semi-hermetic compressors, a gas cooler/condenser, a liquid receiver, electrically operated expansion valves, a medium temperature display refrigeration cabinet and an additional water/glycol load. The refrigeration system and especially the CO2 gas cooler/condenser had been comprehensively instrumented to enable detailed monitoring of the system and the heat exchanger at different operating states. Results for the system performance were obtained and analysed for different CO2 gas coolers/condensers. The results include the effect of heat exchanger designs and fan operations on the system performance. In addition, the controls of supercritical and subcritical pressures and cooling capacity are described

MODELLING AND ANALYSIS OF CO2 GAS COOLERS FOR COMMERCIAL REFRIGERATION APPLICATIONS

CO2 is an environmentally benign refrigerant which is increasingly being used in commercial refrigeration applications. CO2 refrigeration systems can operate subcritically during periods of low ambient temperature or transcritically when the ambient temperature is above 25 oC or so, depending on the design of the heat rejection heat exchanger. Optimisation of the heat transfer performance of the heat rejection heat exchanger can extent the temperature range in which the CO2 refrigeration system can operate in the subcritical region and this will improve the seasonal efficiency of the system. This paper presents a model which has been developed to simulate the performance of CO2 heat rejection heat exchangers. The model has been validated against both sub-critical and transcritical data with the view to establishing its suitability for use as a design and selection tool for CO2 heat rejection heat exchangers over a wide range of conditions

Preliminary assessment of waste heat potential in major European industries

Abstract Industrial processes are currently responsible for almost 26% of European primary energy consumptions (275 Mtoe/yr). Furthermore, most of the energy sources that drive the industrial sector are fossil fuel based. Every industrial process is characterised by a multitude of waste heat streams at different temperature levels whose recovery would undoubtedly contribute to the enhancement of the sustainability of the industrial sites and their products. Waste heat recovery systems can offer significant energy savings and substantial greenhouse gas emission reductions. For the latter to materialise technological improvements and innovations aimed at improving the energy efficiency of heat recovery equipment and reducing installation costs should take place. This paper outlines the opportunities and the potential for industrial heat recovery in the European Union by identifying and quantifying primary energy consumption in the major industrial sectors and their related waste streams and temperature levels. Through a systematic analysis considering waste heat and Carnot’s potential estimation, detailed results are given for all industrial sectors, temperature ranges and EU countries. The ‘big picture’ is rather promising with regards to the estimated total waste heat potential.The research presented in this paper has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 680599. Aspects of the work are also funded by the Centre for Sustainable Energy Use in Food Chains (CSEF). CSEF is an End Use Energy Demand Centre funded by the Research Councils UK, Grant No: EP/K011820/1