Improving the Thermal Efficiency and Performance of Refrigeration Systems:Numerical-Experimental Analysis of Minimization of Frost Formation

The frost growth on cold surfaces in evaporators is an undesirable phenomenon which becomes a problem for the thermal efficiency of the refrigeration systems because the ice layer acts as a thermal insulation,drastically reducing the rate of heat transfer in the system.Its accumulation implies an increase in energy demand and a decrease in the performance of various components involved in the refrigeration process,reducing its efficiency and making it necessary to periodically remove the frost,resulting in expenses for the defrost process.In the present work,a numerical-experimental analysis was performed in order to understand the formation process of porous ice in flat plates with different surface treatments and parameters.This understanding is of utmost importance to minimize the formation of porous ice on cold surfaces and improve equipment efficiency and performance.In this context,a low-cost experimental apparatus was developed,enabling an experimental analysis of the phenomenon under study.The environmental conditions evaluated are the temperature of the cold surface,roomtemperature,humidity,and air velocity.The material of the surfaces under study are aluminum,copper,and brass with different surface finishes,designated as smooth,grooved(hydrophilic),and varnished(hydrophobic).The numerical-experimental analysis demonstrates measurements and simulations of the thickness,surface temperature,and growth rate of the porous ice layer as a function of the elapsed time.The numerical results were in good agreement with the experimental results,indicating that the varnished surface,with hydrophobic characteristics,presents greater difficulty in providing the phenomenon.Therefore,the results showed that application of a coating allowed a significant reduction on the frost formation process contributing to the improvement of thermal efficiency and performance of refrigeration systems.

Grey box modeling of supermarket refrigeration cabinets

Aiming to enable robust large-scale fault diagnostics and optimized control for supermarket refrigerationsystems, a data-driven grey box model for an evaporator and its surrounding cooling cabinet (or room) ispresented. It is a non-linear model with two states: the cabinet temperature and the refrigerant mass in theevaporator. To demonstrate its applicability, data with one-minute sampling resolution from ten evaporators ina supermarket in Otterup (Denmark) was used. The model parameters were estimated using a Kalman filter andthe maximum likelihood method. Since the dynamical properties of the cabinets constantly change as goodsare added and removed, the parameters were re-estimated for each night, over a period of approximately 2.5years. The model is validated through a statistical analysis of the residuals and the importance of the ongoingre-estimation of parameters is highlighted. Furthermore, the physical meaning of the estimated parameters isdiscussed and potential applications for characterization and classification of cabinets are demonstrated, byshowing how they can be differentiated as either open- or closed cabinets or rooms, using only the estimatedheat transfer coefficients and heat capacities. For a selected case it is shown that the estimated parametervalues are close to physics derived values, and that the accuracy measured by the standard errors of theestimates is approximately ±10% relative to the estimated values. The analysis demonstrates that the modelis robust, accurate and reliable in terms of estimating physically meaningful parameters and it is thereforeappropriate for large-scale implementation.

Review on Applications of Zeotropic Mixtures

Compared with the pure fluids,the zeotropic mixtures can balance the requirements of environmental protection,heat source matching and system safety,and exhibit excellent thermodynamic performance.However,compared to the widespread applications of pure fluids,zeotropic mixtures are rarely exploited in thermodynamic cycles,and there is a lack of targeted summary on refrigeration systems,organic Rankine cycle systems and combined power and refrigeration systems.In the recent years,zeotropic mixtures are developing at an unprecedented pace,while the working fluids components are inevitably explored in the process.In this paper,the research progress of zeotropic mixtures in the field of refrigeration systems,organic Rankine cycle systems and combined power and refrigeration systems are reviewed.Based on the review of zeotropic working mixtures,the reasonable predictions can be proposed.In the future,environmental problems will still be one of the most important concerned issues.Therefore,the zeotropic mixtures consisting of natural hydrocarbons and carbon dioxide,which are environmentally friendly,have great potential for development.Furthermore,zeotropic mixtures of natural working fluids can improve comprehensive energy efficiency of combined systems and will play an important role in future carbon emission reduction technologies.