TEST BENCH FOR COMPARATIVE MEASUREMENT OF ENERGY EFFICIENCY OF VARIABLE AND FIXED SPEED SCROLL COMPRESSOR

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1 TEST BENCH FOR COMPARATIVE MEASUREMENT OF ENERGY EFFICIENCY OF VARIABLE AND FIXED SPEED SCROLL COMPRESSOR A. BENAMER, D. CLODIC benamer@cenerg.ensmp.fr Ecole des Mines de Paris, Centre d Energétique, 6, bd Saint-Michel 757 Paris Cedex 6, France ABSTRACT This paper presents a method for the comparison of energy consumption of Scroll compressors with fixed and variable speed. Results obtained on a test bench are analyzed. High efficiency Scroll compressors have been selected. A test bench has been developed at the Center for Energy Studies of Ecole des Mines de Paris funded both by ADEME 1 and EDF. The comparison protocol is based on the characterization of compressors by their volumetric and global efficiencies. Then the energy consumption is simulated for various heat load scenarios. For low heat load, energy savings associated with variable speed are much more significant. Tests showed «system effects» that might generate losses of the cooling capacity. Solutions were implemented permitting to take advantage of benefits linked to the use of variable speed compressors in refrigerating systems. 1. VARIABLE SPEED COMPRESSOR Because a direct proportionality ratio exists between rotation speed and mass flow rate, variable speed is the power control system that seems the most adequate. Efficient variable speed compressor implies that their design integrates variable speed. On the contrary, several technological limits appear, lubrication is not appropriate at low speed and clearances for high speeds are specific [1],[]. For example, when Scroll compressors are developed for fixed speed operation, their nominal speed is 3 rpm and the lubricating system is designed only for that speed. To be able to take advantage of variable speed Scroll compressors shall have a maximum rotation speed in the range of 6 to 7 rpm. The minimum speed depends on designers. Some Scroll compressors can operate at low speed down to 5 rpm, even lower. The lubrication system is then partly independent of the compressor rotation speed [3]. Energy savings associated with variable speed are linked to the reduction of the mass flow rate, because lower internal mass flow rate permits smaller temperature differences at the heat exchangers. When the refrigerant flow is reduced at the condenser, the condensing pressure decreases and energy consumption of the compressor is lower. The control of internal and external flows at the evaporator is more complex and lower refrigerant flow does not generate automatically energy savings.. THE TEST BENCH The test bench was designed for the comparison of fixed and variable speed compressors and also for heat loads varying between 1 and 1%. The test bench is presented on figure 1. It is composed of two Scroll compressors, one with variable speed, the other with fixed speed. Each of them can operate alternatively. Both condenser and evaporator are water/refrigerant heat exchangers. Three types of expansion valves can be selected: electronic, multiorifices, and thermostatic. The evaporator is connected both to a water tank and to a heating system permitting the simulation of various heat load scenarios. Characteristics of both compressors are shown in table 1. Refrigerating and condensing capacities are given for R- at evaporating temperature of C, and condensing temperature of 35 C. For the variable speed compressor, powers correspond to the maximum rotation speed. 1 ADEME Agence pour l Environnement et la Maîtrise de l Energie (French Agency for Environment and Energy Management). EDF Electricité de France (French Electricity Company). th International Congress of Refrigeration, IIR/IIF, Sydney,

2 Electronic expansion valve Electric resistance Receiver Multiorifice expansion valve Oil Refrigerant mass flow meter separator Dehydrator Water flow meter Condenser Evaporator Thermostatic expansion valve Water tank Water flow meter Pump Pump Compressors Figure 1: The test bench Fixed speed compressor Variable speed compressor Swept volume (m3/h) Power (kw). 3.5 Cooling capacity (kw) Condensing capacity (kw) Rotation speed (rpm) Volumetric flow rate Volumetric efficiency characteristic Compressor speed Space in volumetric flow Mass flow rate Table 1: characteristics of compressors 3. CHARACTERIZATION OF PERFORMANCES Isentropic efficiency characteristic Isentropic enthalpy at the discharge port Enthalpy at the suction port Enthalpy at the suction port Enthalpy Global efficiency characteristic at the discharge port Figure : Inputs and outputs for the calculation of efficiencies. Power consumption The method is based on the compressor characterization by the definition of volumetric, isentropic and global efficiencies. Figure summarises calculations and shows the ratio between various efficiencies. The aggregation of the three efficiencies permits the calculation of the electric consumption [4],[5]. For both compressors, efficiencies are determined by tests. Once the efficiency curves are obtained, values can be expressed by the following equations taking into account the compression ratio τ and the rotation speed η is =.335 τ N η =.974 τ +.4 N v η g =.59 τ N Isentropic, volumetric and global efficiencies for the variable speed compressor. η is η v η g = τ τ =.418 τ τ =.16 τ τ Isentropic, volumetric and global efficiencies for the fixed speed compressor. th International Congress of Refrigeration, IIR/IIF, Sydney, 1999

3 These equations permit the calculation of the compressor power variation. Theses equations came from regressions from experimental data. Figures 3 and 4 show evolutions of calculated and measured compressor power [6]. Mesured and calculated power compressor (W) P measured P mesured P calculated P calculated Mesured and calculated power compressor (W) P measured P mesured P calculated P calculated Figure 3: Fixed speed compressor. Calculated and measured powers. Figure 4: Variable speed compressor. Calculated and measured powers Figure 3 indicates that the projected energy consumption of the fixed speed compressor is correct. Maximum errors between measurement and calculation are 4%. These results were obtained for significant variable heat loads. For the variable speed compressor, it appears that energy consumption is systematically under estimated. This leads to a mean error around 15% and a maximum error of 4%. Harmonic perturbations make difficult the measurement of energy consumption of the electronic speed control. Because of this difficulty the energy consumption of variable speed compressor is systematically underestimated. 4. EFFICIENCY OF THE VARIABLE SPEED COMPRESSOR Variable speed permits proportionality between reductions of the power and the cooling capacity. Qo/Qomax and W/Wmax ratio 1% 9% Qo/Qmax W/Wmax 8% 7% 6% 5% 4% 3% % 1% % % % 4% 6% 8% 1% Rotation speed ratio Figure 5: Variations of power and refrigerating capacities as a function of the heat load th International Congress of Refrigeration, IIR/IIF, Sydney,

4 3, COP,5, 1,5 1,,5, Rotation speed ( ) Figure 6: COP evolution as a function of the rotation speed. A thorough analysis of Figure 5 shows that the efficiency coefficient is improved when rotation speeds are in the range of 4 to 8% of the nominal speed. This indicates that the compressor designer knew that the more frequent rotation speeds being in this range, clearances of spirals shall be optimized. Maximum rotation speeds are only used for short periods of time, usually between 3 and 5%. Figure 6 shows more clearly what is presented on Figure 5. The optimum COP is reached when the rotation speed is 4 5 rpm. The COP decreases significantly when the rotation speed is below 7 rpm. 5. COMPARISONS BETWEEN FIXED AND VARIABLE SPEED COMPRESSORS Fixed and variable speed compressors were compared for heat loads varying by discrete step from to 1%. The fixed speed Scroll compressor is controlled by a simple system of on/off cycles when the set temperature at the evaporator outlet is reached. Figure 7 indicates that the lower the heat load, the higher the consumption difference. This difference reaches more than 4% when the heat load is % of the maximum heat load. Power (W) 18 Variable speed Fixed speed Heat load variation (%) Figure 7: power variation of fixed and variable speed compressors as a function of the heat load. Heat load Compressor consumptions (Wh) Energy consumption differences % Variable speed Fixed speed % Table : comparison of energy consumption between compressors according to heat loads This table gives value corresponding to Figure 7. th International Congress of Refrigeration, IIR/IIF, Sydney,

5 6. VARIATION OF THE EVAPORATOR INTERNAL FLOW RATE When the compressor rotation speed varies, the refrigerant flow rate varies as it is shown on figure 9. If the water flow rate remains unchanged, whereas the refrigerant capacity increases, because the temperature of water entering the evaporator is constant, the evaporating temperature decreases concomitantly, the compression ratio increases and results in a relative energy efficiency loss (see figure 8). Temperatures ( C) Twater inlet Tevaporation T water outlet refrigerant flow rate (g/s) and heat transfert fluid flow rates (1*l/mn) Refrigerant Heat transfer mff mev (1*l/mn) flow rate fluid flow rate Figure 8: Temperatures in the evaporator. Figure 9: Refrigerant and heat transfer fluid flow rates. Figure 1 presents variations of external and global heat exchange coefficients (heat transfer fluid side). The external coefficient shows little variation because the flow rate is constant. On the opposite, the mean logarithmic temperature difference increases from 17 to 1 K. The evolution of the internal heat exchange coefficient is shown on figure 11. The average value of this coefficient varies from 3 4 W/m.K to 5 W/m.K when the refrigerant flow rate increases. Heat exchange coefficient (W/m.K) K global h glycool glycol Internal heat exchange coefficient (W/m.K) Figure 1: External and global heat exchange coefficients Figure 11: Internal heat exchange coefficient The external heat exchange coefficient varies from 6 to 7 W/m.K. When the internal heat exchange coefficient increases of 35%, the global heat exchange coefficient varies of 14% only. When the internal flow rate varies, the variation of the global heat exchange coefficient is limited and the increase of the internal flow rate entails undesirable increase of the mean logarithmic temperature difference. To maintain constant the mean logarithmic temperature difference defined during the system design, it is necessary to adjust concomitantly internal and external flow rates. th International Congress of Refrigeration, IIR/IIF, Sydney,

6 7. CONCOMITANT VARIATION OF INTERNAL AND EXTERNAL FLOW RATES Control defined for the concomitant variation of internal and external flow rates will permit both constant evaporating temperature and the mean logarithmic temperature difference. Temperature at the water evaporator outlet is the control parameter of the compressor rotation speed. The temperature difference between the water inlet and outlet is maintained constant by the control of the pump rotation speed. These controls were implemented and verified for a heat load variation presented on figure 13. This heat load variation entails a variation of the compressor rotation speed and power, shown on figure 1. Figure 13 indicates that this double control permits a constant evaporating pressure. Power compressor (W) Cooling capacity (kw) and evaporation pressure (bar) Cooling capacity Evapration Evaporation pressure Figure 1: Compressor power. Figure 13: Variation of evaporation pressure and of cooling capacity. Figure 14 shows that the relative variation of the refrigerant flow rate is significant whereas the heat transfer fluid flow rate variation is lower. As indicated on figure 15, the mean logarithmic temperature difference remains constant at ± K and permits to maintain constant evaporating pressure. 1 5 Refrigerant (g/s) and heat transfert fluid (1*l/mn) flow rates Ti m e ( s) m refr m glycol Temperatures ( C) T inlet water T evaporating evaporation Toulet water D DT logaritmic logarithmic Figure 14: Refrigerant and heat transfer fluid flow rates. Figure 15: Temperatures inside evaporator. Variation of internal and external flow rates entails variation of the global heat exchange coefficient (see figure 17). The evaporating temperature is kept constant by changing the heat exchange parameters [7]. th International Congress of Refrigeration, IIR/IIF, Sydney,

7 9 1 8 Internal heat exchange coefficient (W/m.K) Ti m e (s ) Heat exchange coefficient (W/m.K) Ti m e (s) h h glycool glycol K global Figure 16: Internal heat exchange coefficient. Figure 17: Global and external heat exchange coefficients. 8. CONCLUSION To take advantage of variable speed for refrigerating compressor, it is necessary to adapt concomitantly the external flow rate. Consequently, the mean logarithmic temperature difference will remain constant, and entail constant evaporating pressure. Then when the refrigerant flow rate varies increase of irreversibilities will be avoided [8]. 9. NOMENCLATURE η is isentropic efficiency, η v volumetric efficiency, η g global efficiency, N compressor speed (rpm), τ pressure ratio (outlet compressor pressure/inlet compressor pressure). 1. REFERENCES [1] Shimma, Y. and al., «Inverter Control Systems in the residential heat pump air conditioner». ASHRAE Transactions, N, [] Tassou, S.A., and Qureshi, T.Q. «Performance of a variable speed inverter/motor drive for refrigeration applications». Computing and Control Engineering Journal, Vol 5, n 4, p , August [3] Liu, Z. «Simulation of a variable speed compressor with special attention to supercharging effects». Ph.D. Thesis, Purdue University, [4] Riegger, O.K. «Variable speed compressor performance». ASHRAE Transactions, no.1, [5] Ruohoniemi, T.J. «Measured efficiency of variable speed drives in heat pumps». ASHRAE Transactions, Vol 94, Part, [6] Domijan, A. Jr. and Embriz-Santander, E., «Measurement of electrical power inputs to variable speed motors and their solid state power converter». ASHRAE Transaction : Research. [7] Benamer, A., Clodic, D. «Calorimétrie des compresseurs à vitesse variable». Colloque : «Economie d énergie et utilisation de la vitesse variable pour les compresseurs frigorifiques», Paris, December 8, [8] Benamer, A., Clodic, D. «Analyse et simulation de systèmes frigorifiques à vitesse variable Quantification de l amélioration de l efficacité énergétique de cette technologie». Report for the French Agency for Environment and Energy Management. September 98. BANC DE MESURE POUR LA COMPARAISON DE L EFFICACITE ENERGETIQUE DE COMPRESSEURS SCROLL A VITESSE VARIABLE ET A VITESSE FIXE RESUME : Ce texte présente une méthode de comparaison des consommations d'énergie de compresseurs frigorifiques à vitesse fixe et à vitesse variable et les résultats de ces comparaisons. Des compresseurs Scroll à haute performance ont été choisis dans chacun des deux cas. Un banc d'essais a été développé au Centre d Energétique à partir de financements de l ADEME et d EDF. Le protocole de comparaison est basé sur la caractérisation des compresseurs par leur rendement volumétrique et global. Il est alors possible de simuler leur consommation selon divers scénarios de charges thermiques. Les gains énergétiques associés à la vitesse variable sont d autant plus importants que la charge thermique est faible. Les essais ont mis en évidence des "effets système" qui peuvent engendrer des pertes de puissance frigorifique. Des solutions ont été apportées pour pallier ces inconvénients et bénéficier pleinement des avantages liés à la variation électronique de vitesse. th International Congress of Refrigeration, IIR/IIF, Sydney,