Comparison Of Energy Consumption Of Ventilated And Natural Convection Evaporators Of Refrigerators And Freezers

Forced convection on heat exchangers yields to higher heat exchange coefficient and so permits to limit the temperature difference between air and the evaporator. Higher energy performances of the refrigerating cycle is affordable compared to natural convection evaporators. In Europe, many refrigerators and freezers integrate natural convection heat exchangers. Making a review of the actual energy consumption of European appliances, it is obvious that a number of natural convection refrigerators and freezers show higher energy performances compared to ventilated and no frost appliances. The actual inefficiency of usual small electrical motors of fans spoils the energy gains possibly reached by forced convection. Based on experimental data and dynamic simulation, the paper presents comparisons between heat exchange coefficients, evaporating temperature levels, and overall energy consumption of both ventilated and natural convection refrigerators. Conclusions are drawn on the required energy efficiency of electrical motors in order to reach better energy performances for ventilated refrigerators and freezers. NOMENCLATURE C: daily energy consumption (Wh/day) Cp: heat capacity (J/kg.K) COP: coefficient of performance D: tube diameter (m) Fp : fin pitch (m) Gc: mass velocity (kg/m.s) HL: heat losses (W) h: heat transfer coefficient (W/m.K) LTD: logarithmic temperature difference (K) P: power consumption (W) Pt: transversal tube pitch (m) Q: cooling capacity (W) S: surface area (m) s: spacing between adjacent fins (m) T: temperature (K) UA: overall heat exchange coefficient (W/K) Greek letters σ : Stephan-Boltzman constant ε : total emissivity of the freezer wall Subscripts air_i: air temperature at the inlet of the evaporator air_o: air temperature at the outlet of the evaporator c: Fin collar outside diameter cond: condensing def: average daily defrosting e: external evap: evaporating fan: fan i: internal rad: equivalent radiative Dimensionless numbers J: Colburn factor NuL: Nusselt number based on the height L Pr: Prandtl number RaL: Rayleigh number based on the height L ReDc: Reynolds number based on the tube collar diameter INTRODUCTION The performance of an ideal refrigerating cycle is calculated by the Carnot Coefficient Of Performance (COP), which is the ratio of the cooling capacity to the mechanical power. The ideal COP is expressed as a function of the cycle operating temperatures as shown in the equation 1. evap cond evap Carnot T T T COP − = (1) The real refrigerating cycle presents irreversibilities that lead to lower performance compared to the ideal one, however, the performance variation of both ideal and real cycles is similar. Equation (1) shows clearly that the performance is non linearly dependant of the evaporating temperature and that the COP decreases very rapidly when the evaporating temperature decreases. For refrigerators, the evaporating temperature is fixed by the air side heat exchange coefficient, which is very low compared to the refrigerant side one. The no-frost appliances use ventilated fin and tube heat exchanger while the natural convection ones use a static heat exchanger with an increased heat exchange area. Experimental measurements show an average of 5K difference in evaporating temperature between those 2 technologies [ZOU00]. This temperature difference implies that higher energy performances can be affordable with the no-frost technology. However, no-frost appliances require a fan that blows air over the evaporator and a defrosting system to melt the ice that clogs up the evaporator. These 2 accessories yield to extra energy consumption which can spoil the energy gain, and in many cases the energy consumption of a no-frost appliance is higher than an equivalent static one. In addition, because of the higher evaporating temperature, the compressor to be used in a no-frost appliance is smaller than the one used in the equivalent static appliance. For actual hermetic compressors, efficiency decreases when the swept volume decreases, which leads to additional energy consumption. 1. DESCRIPTION OF APPLIANCES For comparison the chosen appliances present the same geometry and insulation thickness. Figure 1 shows the geometry description in the ENEREF software [CLO01] and the calculated net volume. Figure 1 – The geometry description in the ENEREF software. The no-frost appliance has a fin and tube heat exchanger and the natural convection appliance uses the vertical walls as exchange area. The evaporator tube length is assumed to be equal for both appliances. The heat losses, calculated by ENEREF, are of 60 W. The running time ratio, defined by the ratio of the compressor running time to the overall cycle time, is considered to be 40% for a 25°C test-room temperature. Hence the needed average cooling capacity is 150 W. 2. EVAPORATING TEMPERATURE CALCULATION The evaporating temperature is calculated by analyzing both evaporators using the logarithmic temperature difference method (LTD). The superheating section of the evaporator is neglected for both evaporators and hence the LTD is defined by equation (2).