COMPARISON OF ENERGY EFFICIENCY BETWEEN VARIABLE AND FIXED SPEED SCROLL COMPRESSORS IN REFRIGERATING SYSTEMS

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1 COMPARISON OF ENERGY EFFICIENCY BETWEEN VARIABLE AND FIXED SPEED SCROLL COMPRESSORS IN REFRIGERATING SYSTEMS A. BENAMER, D. CLODIC Ecole des Mines de Paris, Centre d Energétique, 6, bd Saint-Michel Paris Cedex 6, France benamer@cenerg.ensmp.fr Abstract This paper presents a method for the comparison of energy consumption of variable and fixed speed compressors. In the present tests both are Scroll compressors. A test bench has been developed by the laboratory and high efficiency compressors have been chosen for both technologies. Results show the lower the heat load, the higher the energy savings associated with variable speed. Variable speed generates up to 4% of energy savings. A protocol for the comparison of energy consumption of refrigerating compressors has been established based on the characterisation of their efficiency. This protocol permits the simulation of compressor energy consumption according to various heat load scenarios. Tests have shown also system effects that generate refrigerating capacity losses. Solutions are proposed to compensate these losses and take advantage of energy savings linked to variable speed. This link had to be considered in the regulation. Key words: variable speed - Scroll compressors - control - energy consumption - compressor efficiency lubrication - energy efficiency volumetric efficiency mechanical efficiency global efficiency. 1. VARIABLE SPEED COMPRESSOR Variable speed system appears to be most adequate for reducing cooling capacity, because there is a proportional ratio between the compressor speed and the mass flow rate, then a proportional ratio between the energy consumption and the cooling capacity. Scroll compressors were initially developed for maximum speed of 3 rpm. Indeed, usual compressors are designed with a lubricant mass flow rate only defined for the nominal point of operation. The energy efficiency requires the choice of a compressor technology compatible with variable speed. Variable speed can be associated with any technology of compressor but the extent of profits or losses will be different. Scroll and screw compressors differ from other families of compressors. For a 3 rpm nominal speed compressor, when the speed is reduced, the volumetric efficiency decreases because the gas leakage between the high and the low pressures increases. On the other hand the isentropic efficiency grows because the gas flow is reduced. The mechanical efficiency will be increased because the speed is reduced. Whatever the compressor technology, both lubrication pump and compressor speeds are coupled. At low rotation speed, because the lubricant flow rate can become insufficient the rotation speed of the lubrication pump shall be independent of compressor shaft. 1

2 The extra cost generated by the dissociation of the lubrication system and the compressor shaft is high. The alternative solution consist in the variation of the compressor speed on a wide range so as to push back the limit fixed by technological constraint of lubrication and efficiencies. The extension of the compressor speed range requires the development of some high speed compressors. Mechanical frictions generated by high speed can be reduced by the reinforcement of some mechanical parts. 2. 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. Electronic expansion valve Electric resistance Receiver Multiorifice expansion valve Refrigerant mass flow meter Oil separator Deshydrator Water flow meter Condenser Evaporator Thermostatic expansion valve Water tank Water flow meter Pump Compressors Pump Figure 1: The test bench. Characteristics of both compressors are shown in table 1. Refrigerating and condensing capacities are given for R-22 at evaporating temperature of C, and condensing temperature of 35 C. For the variable speed compressor, power consumption corresponds to the maximum rotation speed. Variable speed compressor Fixed speed compressor Swept volume (m3/h) Power (kw) Cooling capacity (kw) Condensing capacity (kw) Rotation speed (rpm) Table 1: Compressor characteristics. 2

3 3. CHARACTERIZATION OF PERFORMANCES The method is based on the compressor characterisation by the definition of volumetric, isentropic and global efficiencies. For both compressors, efficiencies are determined by tests. Once the efficiency curves are obtained, values can be expressed by the following equations η is =.3235 τ N η =.2974 τ +.4 N v η g =.592 τ N Isentropic, volumetric and global efficiencies for the variable speed compressor. η is η v η g 2 = τ τ =.418 τ τ =.216 τ τ Isentropic, volumetric and global efficiencies for the fixed speed compressor. Theses equations permit the calculation of the compressor power variation. Figures 2 and 3 show evolutions of calculated and measured compressor power. Figure 2 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%. Additional studies should permit to find a solution for systematic errors. Mesured and calculated power compressor (W) P mesured P calculated Time (s) Figure 2: Fixed speed compressor. Calculated and measured powers. Mesured and calculated power compresso (W) P mesured Time (s) P calculated Figure 3: Variable speed compressor. Calculated and measured powers 4. EFFICIENCY OF THE VARIABLE SPEED COMPRESSOR Usual systems for the reduction of compressor cooling capacity do not permit the proportionality between the cooling capacity and the electric power. Conversely, variable speed permits proportionality between reduction of power and of the cooling capacity. Curves of Figure 4 present the variations of the electric power and of the cooling capacity. 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 should be 3

4 optimised. Maximum rotation speeds are only used for short periods of time, usually between 3 and 5%. Qo/Qomax and W/Wmax ratio 1% 9% Qo/Qmax W/Wmax 8% 7% 6% 5% 4% 3% 2% 1% % % 2% 4% 6% 8% 1% Rotation speed ratio COP 3, 2,5 2, 1,5 1,,5, Rotation speed (rpm) Figure 4: Variations of power and refrigerating capacities as a function of the heat load Figure 5: COP evolution as a function of the rotation speed. The optimum COP is reached when the rotation speed is 4.5 rpm. The COP decreases significantly when the rotation speed is below 2.7 rpm. 5. ENERGY COMSUMPTION COMPARISONS Fixed and variable speed compressors were compared for heat loads varying by discrete step from 2 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. When the cooling capacity load is low the power consumption difference between the two compressors is higher. It can be concluded that low cooling capacity yields larger energy savings associated with the variable speed. Scenarios where low loads are most frequent were performed and confirmed measurements taken by step of power. Figure 6 indicates that the lower the heats load the higher the consumption difference. This difference reaches more than 4% when the heat load is 2% of the maximum heat load. 2 Power (W) Variable speed Fixed speed Heat load variation (% ) Figure 6: power variation of fixed and variable speed compressors as a function of the heat load. 4

5 6. VARIATION OF THE EVAPORATOR INTERNAL FLOW RATE When the compressor adjusts its speed to the refrigerating needs, the refrigerant flow rate varies. The variation of the internal flow rate is not without consequences on the thermodynamics parameters of the refrigerating system. For an increasing cooling capacity, conditions of exchange on the external fluid are modified. For a constant glycol flow rate, the differences of glycol temperature between the evaporator inlet and outlet increase with the cooling capacity. Conversely, when the cooling capacity decreases, the temperature difference of glycol at the evaporator decreases too. When the variation of the cooling capacity needs is performed with constant external fluid flow rate, and variable internal fluid flow rate, the temperature difference of the external flow rate varies. The disadvantage of these exchanges with the variation of glycol temperatures is that the evaporating temperature is fixed by the external exchange conditions. Consequently when the glycol temperature is modified, the evaporating temperature is also modified. The evaporating temperature is fixed both by the external flow rate and temperatures. Temperatures ( C) Time (s) Twater inlet Tevaporation T water outlet refrigerant flow rate (g/s) and heat transfert fluid flow rates (1*l/mn) Time (s ) mff mev (1*l/mn) Figure 7: Temperatures in the evaporator. Figure 8: Refrigerant and heat transfer fluid flow rates. Figure 7 presents variations of the glycol temperatures and of the evaporating temperature. When the evaporating pressure decreases, the mass volume at the inlet compressor increases, therefore the refrigerant mass flow decreases. Figure 8: presents the variation of the refrigerant mass flow rate and the glycol mass flow rate. Figure 9 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 21 K. The evolution of the internal heat exchange coefficient is shown on figure 1. The average value of this coefficient varies from 3 4 W/m 2.K to 5 2 W/m 2.K when the refrigerant flow rate increases. 5

6 Heat exchange coefficient (W/m2.K) Time (s) K global h glycool Internal heat exchange coefficient (W/m2.K) Time (s ) Figure 9: External and global heat exchange coefficients Figure 1: Internal heat exchange coefficient The external heat exchange coefficient varies from 62 to 7 W/m 2.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. 7. CONCLUSIONS The lubrication system is a significant parameter for the HVAC system. The lubricant flow rate is related to the compressor rotation speed. Variable speed compressor can thus be penalised by a deceleration of the lubricant flow. The sealing between the lobes for screw compressors or the spirals of Scroll compressors is less at low speeds and induces a drop of the volumetric efficiency. It is possible to maintain an adequate lubricant flow by uncoupling the rotation of lubrication pump and the compressor shaft. The energy savings generated by the electronic variation speed are a function of the thermal loads. The protocol of comparison implies the knowledge of the characteristics of the compressors: the volumetric, isentropic and total efficiencies. Theses efficiencies must be characterised according to compressor ratio for the fixed speed compressor and the speed shall be taken into account for the variable speed compressor. The energy savings generated by a variable speed Scroll compressor compared to a fixed speed Scroll compressor can reach 3% when the thermal loads range is between 2 and 3 % of the maximum thermal load. Comparisons can be made with other technologies of compressors with lower efficiency, and the relative savings can be higher. The regulations of the internal and external flows shall be coupled to benefit fully from the savings associated with the electronic variation speed. The external flow shall be controlled to maintain the glycol temperature variation. The internal flow is controlled to maintain the set temperature. The objective of the external flow variation is to modify the total heat transfer to maintain the level of the evaporating temperature exchange. The energy savings generated by the external flow rate control does not reduce only the compressor consumption but also the pump consumption. 6

7 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. 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 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, 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. 4. Erbs, D.G., Bullock, C.E., and Voorhis, R.J. «New testing and rating procedures for seasonal performance of heat pumps with variable speed compressors». ASHRAE Transactions, Vol 92, Part 2B, p696-75, Liu, Z. «Simulation of a variable speed compressor with special attention to supercharging effects». Ph.D. Thesis, Purdue University, Riegger, O.K. «Variable speed compressor performance». ASHRAE Transactions, no.1, Ruohoniemi, T.J. «Measured efficiency of variable speed drives in heat pumps». ASHRAE Transactions, Vol 94, Part 2, Shimma, Y. and al., «Inverter Control Systems in the residential heat pump air conditioner». ASHRAE Transactions, N 2, 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

8 Comparaisons de l efficacité énergétique entre un compresseur un compresseur à vitesse variable et d un compresseur à vitesse à vitesse fixe. Régulation globale du système frigorifique pour un fonctionnement à vitesse variable. RESUME : Cet article présent une méthode de comparaison sur l énergie consommée de deux compresseurs, un à vitesse variable et le second à vitesse fixe. Les deux compresseurs sont de technologies scroll. L étude expérimentale a nécessité le développement d un banc d essai. Les compresseurs utilisés ont été choisis à haute efficacité, l un muni de la vitesse variable et le second limité à une vitesse fixe. Les résultats montrent que c est aux faibles charges thermiques que les gains associés à la vitesse variable sont les plus importants. Les économies d énergie engendrée peuvent atteindre des valeurs de 4% d écart de consommation en faveur du compresseur à vitesse variable par rapport au compresseur à vitesse fixe. Un protocole de comparaison sur la consommation énergétique des compresseurs a été établi, elle est basée sur la caractérisation des rendements. Ce protocole permet de simuler la consommation des compresseurs avec la variation des charges thermiques. Les essais ont pût mettre en évident des «effets systèmes» engendrer par les baisses de puissance frigorifiques. Une solution est présentée pour compenser les pertes énergétiques et ainsi bénéficier pleinement des gains associés à la vitesse variable. Ce système doit être associé aux compresseurs à vitesse variable. 8