**2.7. Variable speed capacity control**

Variable speed control can be realized in different ways to regulate the speed of the compressor motor such as electronic variable frequency drives. Variable frequency drives (VFD) are also known as adjustable-frequency drives (AFD), variable speed drives (VSD), AC drives, micro-drives, or inverter drives. Compressor rotational speed can be varied to match the system's changing requirement for refrigeration capacity of a variable speed drive. Variable speed capacity control studies contain mechanical, electrical fact about compressor and other equipments. Primary studies on variable speed refrigeration systems were related with the theoretical analysis of the concept of variable speed capacity control and the investigation of the problems associated with the mechanical design of the system [35].

The following studies are related with main benefits and facts of the variable speed capacity control methods. Muir and Griffith [36] investigated different aspects of capacity modulation methods for refrigeration and domestic air-conditioning systems using the seasonal energy efficiency ratio (SEER). This method compares the seasonal efficiency of systems, taking into consideration the effects of on/off cycling and steady-state efficiency at several outdoor temperatures. The analysis showed that application of capacity modulation and significant energy savings can be possible due to decrease in on/off cycling losses and improvements in steady-state efficiency at partial loads.

Tassou et al. [4, 37–40] investigated the most fact of the variable speed capacity control methods, and they focused on the capacity modulation of domestic size of heat pumps. Energy conservation via capacity control and performance comparison with conventional systems, effects of capacity modulation, mathematical modeling of variable speed systems, part load, and dynamic performance analysis of heat pumps are the important issues investigated. The investigations showed that variable speed control could achieve a 15% improvement in energy conversion efficiency, compared to a conventional system. It was also found that superheat control with a thermostatic expansion valve was unsatisfactory during part-load operation, and it was suggested that the problem could be effectively overcome by employing a microprocessor-controlled motorized expansion valve.

can be seen as advantages of the multiple compressor capacity control. On the other hand, multiple compressor capacity provides finite number of capacity steps and limited efficiency gains. For example, in a 40 hp system requiring 25 hp of output at a given time, the system must operate at 30 hp output. Also, precise and smooth temperature and humidity control

Winandy and Cridtian [33] realized a study regarding multiple compressors in which the condensing units have tandem scroll compressors. The main drawback on this configuration

According to ASHRAE [34], the gas velocities and piping geometry are the most important matter in multiple compressor control method because of ensuring adequate oil return. When working at part load, some modifications may be needed to ensure proper oil return. In addition, it is recommended that to separate the refrigerant circuits while parallel operating of compressors, however, this configuration is not always possible and does not give the same

Variable speed control can be realized in different ways to regulate the speed of the compressor motor such as electronic variable frequency drives. Variable frequency drives (VFD) are also known as adjustable-frequency drives (AFD), variable speed drives (VSD), AC drives, micro-drives, or inverter drives. Compressor rotational speed can be varied to match the system's changing requirement for refrigeration capacity of a variable speed drive. Variable speed capacity control studies contain mechanical, electrical fact about compressor and other equipments. Primary studies on variable speed refrigeration systems were related with the theoretical analysis of the concept of variable speed capacity control and the investi-

The following studies are related with main benefits and facts of the variable speed capacity control methods. Muir and Griffith [36] investigated different aspects of capacity modulation methods for refrigeration and domestic air-conditioning systems using the seasonal energy efficiency ratio (SEER). This method compares the seasonal efficiency of systems, taking into consideration the effects of on/off cycling and steady-state efficiency at several outdoor temperatures. The analysis showed that application of capacity modulation and significant energy savings can be possible due to decrease in on/off cycling losses and improvements

Tassou et al. [4, 37–40] investigated the most fact of the variable speed capacity control methods, and they focused on the capacity modulation of domestic size of heat pumps. Energy conservation via capacity control and performance comparison with conventional systems, effects of capacity modulation, mathematical modeling of variable speed systems, part load, and dynamic performance analysis of heat pumps are the important issues investigated. The investigations showed that variable speed control could achieve a 15% improvement in energy conversion efficiency, compared to a conventional system. It was also found that superheat control with a thermostatic expansion valve was unsatisfactory during part-load

gation of the problems associated with the mechanical design of the system [35].

is oil returning to the compressor which is a serious trouble especially at part load.

may not obtainable, because of the capacity modulation step [7, 8].

operational advantage at part load.

130 Refrigeration

**2.7. Variable speed capacity control**

in steady-state efficiency at partial loads.

The study of Shimma et al. [41] is related with the evaluation of energy savings by using inverters in air-conditioners. According to the authors, the maximum energy savings and better system performance could be achieved by employing better control methods. Furthermore, improving the performance of individual components in the air-conditioning system could provide better system performance. The capacity-controlled system resulted in a reduction of the room temperature fluctuations to 50% of those for the conventional on-/off-controlled system. The authors pointed out various problems such as improvements in the refrigerant throttling mechanism, adoption of more effective noise suppression techniques (which is important to reduce radio wave interference noise and harmonic noise generated by the inverter), enhancement of the reliability and performance of inverter, and improvements in the overall system design to reduce noise at high-frequency operation and to overcome vibration problems at low-frequency operation.

According to Rasmussen [42], household refrigerators are usually thermostat (on/off)-controlled, constant speed, and mostly they have a single-phase induction motor as compressor drive. The author represents results of a prototype refrigerator using variable speed, three-phase, brushless DC motor drive. Test results of the motor and drive efficiency are presented; also, motor construction are described.

Also, different types of compressors are investigated in variable speed capacity control system by different authors. Papers dealing with rotary compressor were studied by Lida et al. [43]. On a heat pump equipped with a 4 hp (3 kW) hermetic rotary compressor, experimental studies are realized. They found that the practical limits for compressor speed variation were between 25 and 75 Hz. The results indicated improvements in EER with the inverterdriven compressor compared to a fixed-speed system. Cost and SEER analyses showed a 20% increase in the total cost for the inverter-controlled system and between 20 and 26% energy savings over the constant single capacity system. Other advantages identified for variable speed control over fixed-speed systems included accurate temperature control, system softstart capabilities, and low-noise operation at reduced loads.

In the same year, Itami et al. [44] studied the performance and reliability factor of reciprocating and rotary compressors (frequency controlled). For the different type of compressor, modifications were suggested. For example, with the reciprocating compressor, a two-stage oil pump was used over the low-frequency range to ensure proper lubrication. For the rotary compressor, a liquid injection system was used to protect from overheating. Also, a disc mechanism was adopted to prevent increased amounts of discharge oil at the higher operatingfrequency range. While the operating frequency was increased, the rotary compressor showed improvements in the volumetric and motor efficiencies; on the other hand, the reciprocating compressor shows improvement in mechanical and compression efficiencies, while the operating frequency was decreased. About 20–40% improvement in the SEER was reported with the variable speed air-conditioner compared to the conventional on/off-controlled system. In another related study, Senshu et al. [45] investigated on a small-capacity heat pump using a scroll compressor. This system showed a 30% improvement in annual performance efficiency compared with the conventional reciprocating compressor. It is important that the EER of the inverter-driven heat pump at nominal load conditions was found less than a fixed-speed system, due to the inverter losses. In an ASHRAE research, project (RP-409) analyzed a large chiller operating with a variable speed controlled centrifugal compressor [46]. The results showed that variable speed control provides a 1.5% reduction in the compressor power consumption at maximum load and about 40% reduction at minimum load.

McGovern [47] investigated the performance of a two-cylinder and open-type reciprocating compressor with the speed range between 300 and 900 rpm. Different performance parameters, such as mass flow rate, shaft power, and compressor discharge gas temperature, showed a linear increase for the tested speed range; on the other hand, the volumetric efficiency was found to remain almost constant at 66% of the given speed range. The variation of mechanical efficiency with speed was 92–94% at the speed increased from 300 to 900 rpm, respectively.

Other authors studied with compressor were Ischii et al. [48, 49]. The authors compared mechanical efficiency and dynamic performance of scroll compressors with rolling piston rotary compressors. They found that the scroll compressors exhibited better vibration than the rolling piston rotary compressor; on the other hand, they exhibited lower mechanical efficiency. They reported that the mechanical efficiency of scroll compressors could be improved via design optimization.

Another study dealing with compressors was investigated by Tassou and Qureshi [50]. This study contains application of inverter-based variable speed drives for positive displacement rotary vane-type refrigeration compressors. The effects of the inverter on a number of operating parameters such as harmonic currents and voltages, power consumption and power factor, starting current, and overall system efficiency were investigated. Results showed that inverter may cause a reduction in the power factor and in the overall efficiency of the driver. According to results, variable speed operation of a rotary vane compressor can provide better temperature control and quick response to disturbances and changes in load.

Tassou and Qureshi [51] studied on positive displacement refrigeration compressors' variable speed capacity modulation. Compressors tested include an open-type reciprocating, a semi-hermetic reciprocating, and an open-type rotary vane. The results indicate that all three compressors were designed for maximum efficiency at nominal speed and all three compressors when operated at variable speed offer energy savings compared to their fixedspeed counterparts. Also, at constant head pressure, only the open-type compressor exhibited an improvement in the COP at reduced speeds. With variable head pressure control, all three compressors showed an increase in the COP with a reduction in speed. The analysis showed the open-type reciprocating compressor to be the most efficient system offering 12% savings when operating in a temperate climate and 24% savings when operating in a warm climate.

Rarely, gas and liquid refrigerant injection to the compressor is done. Gas injection is applied to increase compressor capacity and to save energy. Since more refrigerant passes through the condenser than through the evaporator compressor, capacity may increase some. Also, liquid refrigerant is injected to the compressor to decrease high discharge refrigerant temperatures which chemically degrade oil and refrigerant and cause mechanical failure. High cost and additional component requirement are disadvantages of the gas and liquid injection system; also, liquid injection may cause slugging problem in the compressor [52, 53]. Cho et al. [54] applied the refrigerant injection to the variable speed compressor and measured performance of a liquid refrigerant injected inverterdriven scroll compressor with respect to variation of compressor frequency, injection pressure, and injection location. Furthermore, the influence of liquid injection on the performance was presented as a function of operating parameters and injection location. Results were compared with the non-injection case. For high frequency at a given injection ratio, the injection at 180°, for an injection angle at an injection port, yielded slightly better performance of the compressor as compared to that at 90°. It was found that liquid injection under high frequency was very effective, but injection under low frequency resulted some negative effects in the point-of-view compressor power, capacity, and adiabatic efficiency because of higher leakage through the scrolls.

efficiency compared with the conventional reciprocating compressor. It is important that the EER of the inverter-driven heat pump at nominal load conditions was found less than a fixed-speed system, due to the inverter losses. In an ASHRAE research, project (RP-409) analyzed a large chiller operating with a variable speed controlled centrifugal compressor [46]. The results showed that variable speed control provides a 1.5% reduction in the compressor

McGovern [47] investigated the performance of a two-cylinder and open-type reciprocating compressor with the speed range between 300 and 900 rpm. Different performance parameters, such as mass flow rate, shaft power, and compressor discharge gas temperature, showed a linear increase for the tested speed range; on the other hand, the volumetric efficiency was found to remain almost constant at 66% of the given speed range. The variation of mechanical efficiency with speed was 92–94% at the speed increased from 300 to 900 rpm, respectively.

Other authors studied with compressor were Ischii et al. [48, 49]. The authors compared mechanical efficiency and dynamic performance of scroll compressors with rolling piston rotary compressors. They found that the scroll compressors exhibited better vibration than the rolling piston rotary compressor; on the other hand, they exhibited lower mechanical efficiency. They reported that the mechanical efficiency of scroll compressors could be improved

Another study dealing with compressors was investigated by Tassou and Qureshi [50]. This study contains application of inverter-based variable speed drives for positive displacement rotary vane-type refrigeration compressors. The effects of the inverter on a number of operating parameters such as harmonic currents and voltages, power consumption and power factor, starting current, and overall system efficiency were investigated. Results showed that inverter may cause a reduction in the power factor and in the overall efficiency of the driver. According to results, variable speed operation of a rotary vane compressor can provide better

Tassou and Qureshi [51] studied on positive displacement refrigeration compressors' variable speed capacity modulation. Compressors tested include an open-type reciprocating, a semi-hermetic reciprocating, and an open-type rotary vane. The results indicate that all three compressors were designed for maximum efficiency at nominal speed and all three compressors when operated at variable speed offer energy savings compared to their fixedspeed counterparts. Also, at constant head pressure, only the open-type compressor exhibited an improvement in the COP at reduced speeds. With variable head pressure control, all three compressors showed an increase in the COP with a reduction in speed. The analysis showed the open-type reciprocating compressor to be the most efficient system offering 12% savings when operating in a temperate climate and 24% savings when operating in a warm climate. Rarely, gas and liquid refrigerant injection to the compressor is done. Gas injection is applied to increase compressor capacity and to save energy. Since more refrigerant passes through the condenser than through the evaporator compressor, capacity may increase some. Also, liquid refrigerant is injected to the compressor to decrease high discharge refrigerant temperatures which chemically degrade oil and refrigerant and cause

temperature control and quick response to disturbances and changes in load.

power consumption at maximum load and about 40% reduction at minimum load.

via design optimization.

132 Refrigeration

Aprea et al. [55] presented a research dealing with compressors too. In this study, an experimental analysis has been realized, and they compared energetic performance of variable speed compressor and on/off control controlled with a classical thermostat. They used a semi-hermetic reciprocating compressor working with the refrigerants R22, R507, and R407C. The compressor was designed for a nominal frequency of 50 Hz, but they tested it in the range 30–50 Hz. The results showed that, using the R407C, an average of an electric energy consumption about 12% smaller when an inverter was employed to control the compressor refrigeration capacity instead of the thermostatic control is possible. So the R407C confirms its superiority in comparison with the R417A and R507; only the R22 shows a better performance.

Except this, energy-saving potential of the capacity control methods is studied by some researches. A feasibility and design study of a variable capacity refrigeration system was carried out by the *Energy Efficiency Demonstration Scheme* on behalf of the Department of Energy [56]. A commercially available variable speed compressor was monitored in a supermarket refrigerator. In this study at first, a conventional system was installed and then converted to variable speed for comparison. The results showed 56% power saving with high temperature (dairy applications) and a 30% saving with low temperature (frozen food applications). The energy savings achieved were attributed mainly to variable speed control and fully floating head pressure.

Rice [57, 58] studied on a heat pump, and he reported 27% energy savings for a modulating heat pump system. In that study reduced cycling losses, heat-exchanger unloading, reduced frosting/defrosting losses, and reduced backup heating were taken into account. He found that higher motor-slip losses and distorted inverter waveform decreased the conventional three-phase induction motor efficiency up to 20%, and he suggested that these losses could be reduced by a permanent magnet and electronically commutated motor-inverter combination.

Nasutin et al. [59] studied on the potential of energy saving of a variable speed compressor. The main aim of the system is to provide thermal comfort for application in air-conditioning system and to enhance load-matching capability of the system. In this study a constant speed system was retrofitted using an inverter and a proportional-integral-derivative (PID) controller used. As a result, energy saving for the system was estimated about 25.3% at a temperature of 22°C via PID controllers.

Recently, Cuevas and Lebrun [60] introduced an experimental study dealing with drawbacks of variable speed compressors which are concerning the inverter efficiency, the effect of the inverter on the induction motor, and the effect of variable speed on the compressor isentropic and volumetric efficiencies. It was observed that the inverter efficiency varies between 95 and 98% for compressor electrical power varying between 1.5 and 6.5 kW and that compressor efficiencies are not enormously influenced by compressor supply frequency. When the compressor speed is 75 Hz, a slight degradation occurs because of the electromechanical losses. These losses increase with compressor speed. A maximal isentropic efficiency of 0.65 for a pressure ratio of the order of 2.2 was obtained. The experimental results obtained at 50 Hz were used to identify six parameters of a semi-empirical model which was then used to simulate the different tests developed at different compressor speeds. The simulated results were in very good agreement with those measured. The results showed that motor losses induced by the inverter are negligible.

Studies related with VFD are important for variable speed capacity control method. A comprehensive technology review of the application of power electronics was given by Bose [61]. According to author currently available VFD systems can be classified into three basic inverter types: the six-step voltage inverter (VSI), the six-step current inverter (CSI), and the pulsewidth-modulated voltage source inverter (PWM). A report published by the Energy Efficiency Office [62] compares typical efficiencies of six VFD types of different ratings. The PWM inverter shows a slightly better efficiency over VSI and CSI.

In addition, automotive refrigeration system, multi-evaporator, system modeling, fault diagnosis, and CO2 refrigerant were applied to variable speed compressor. These subjects are explained below, respectively.

Ryska et al. [63] presented that a new evaluation method allows the overall cooling performance improvement for truck or bus at different engine speeds and driving styles. This method was demonstrated in two refrigeration units.

Park et al. [64] investigated multi-type inverter air-conditioner with a variable speed rotary compressor and an electronic expansion valve. Performance of the system was analyzed with different operating frequencies of the compressor, different cooling loads and cooling load fraction between rooms. The optimum opening amount of the electric expansion valve (EEV) was also calculated.

Choi and Kim [65] measured performance of an inverter-driven multi-air-conditioner with two indoor units using electronic expansion valves (EEV). For the performance varying indoor loads, EEV opening and scroll compressor speed have been investigated. According to the experimental results, the author suggested around 4°C superheats for both indoor units by using EEV also as the compressor speed needs to be adjusted to provide optimum cooling capacity for the indoor units.

Saiz et al. [66] have developed a steady-state computer simulation model for refrigeration circuits of automobile air-conditioning systems. The simulation model includes a variable capacity compressor and a thermostatic expansion valve in addition to the evaporator and micro-channel parallel flow condenser. The refrigeration circuit was equipped with a variable capacity compressor run by an electric motor controlled by a frequency converter.

system was retrofitted using an inverter and a proportional-integral-derivative (PID) controller used. As a result, energy saving for the system was estimated about 25.3% at a temperature

Recently, Cuevas and Lebrun [60] introduced an experimental study dealing with drawbacks of variable speed compressors which are concerning the inverter efficiency, the effect of the inverter on the induction motor, and the effect of variable speed on the compressor isentropic and volumetric efficiencies. It was observed that the inverter efficiency varies between 95 and 98% for compressor electrical power varying between 1.5 and 6.5 kW and that compressor efficiencies are not enormously influenced by compressor supply frequency. When the compressor speed is 75 Hz, a slight degradation occurs because of the electromechanical losses. These losses increase with compressor speed. A maximal isentropic efficiency of 0.65 for a pressure ratio of the order of 2.2 was obtained. The experimental results obtained at 50 Hz were used to identify six parameters of a semi-empirical model which was then used to simulate the different tests developed at different compressor speeds. The simulated results were in very good agreement with those measured. The results showed that motor losses induced

Studies related with VFD are important for variable speed capacity control method. A comprehensive technology review of the application of power electronics was given by Bose [61]. According to author currently available VFD systems can be classified into three basic inverter types: the six-step voltage inverter (VSI), the six-step current inverter (CSI), and the pulsewidth-modulated voltage source inverter (PWM). A report published by the Energy Efficiency Office [62] compares typical efficiencies of six VFD types of different ratings. The PWM

In addition, automotive refrigeration system, multi-evaporator, system modeling, fault diag-

Ryska et al. [63] presented that a new evaluation method allows the overall cooling performance improvement for truck or bus at different engine speeds and driving styles. This

Park et al. [64] investigated multi-type inverter air-conditioner with a variable speed rotary compressor and an electronic expansion valve. Performance of the system was analyzed with different operating frequencies of the compressor, different cooling loads and cooling load fraction between rooms. The optimum opening amount of the electric expansion valve

Choi and Kim [65] measured performance of an inverter-driven multi-air-conditioner with two indoor units using electronic expansion valves (EEV). For the performance varying indoor loads, EEV opening and scroll compressor speed have been investigated. According to the experimental results, the author suggested around 4°C superheats for both indoor units by using EEV also as the compressor speed needs to be adjusted to provide optimum cooling

refrigerant were applied to variable speed compressor. These subjects are

of 22°C via PID controllers.

134 Refrigeration

by the inverter are negligible.

explained below, respectively.

(EEV) was also calculated.

capacity for the indoor units.

nosis, and CO2

inverter shows a slightly better efficiency over VSI and CSI.

method was demonstrated in two refrigeration units.

Park et al. [67] developed a thermodynamic model for a refrigerant injected variable speed scroll compressor with using continuity energy conservation and real gas equation. In this model, energy balance at the low-pressure compressor, suction gas heating, motor efficiency, and volumetric efficiency were considered. Also, gas leaking was considered as a function of compressor frequency. Results showed deviations from the measured values about 10% at the 90% of the experimental data. According to the model, mass flow rate, suction gas heating, cooling capacity, and power consumption of the compressor were estimated and analyzed as a function of frequency. Furthermore, the effects of the injection on the performance of the compressor were discussed as a function of frequency, injection geometry, and injection conditions. Another modeling study was investigated by Aprea and Renno [68]. The main aim of this study is a thermodynamic model simulating the working of a vapor compression refrigeration system. The model could evaluate the system performance, while the compressor capacity is regulated with an inverter inserted into the compressor electric motor. The author compared the outputs of the model with the experimental results. The comparison of model and experimental results is realized by varying the supply of current frequency of the compressor in the range 30–50 Hz using the R407C. The model and experimental result comparison is completely acceptable in terms of condensation temperature, compression ratio, condensation power, and coefficient of performance. Also, an exergetic analysis represented to explain the performance of the plant components at variable speed operation.

In addition to this study, Shao et al. [69] analyzed modeling of variable speed compressor for air-conditioner and heat pump. For the real operation performance of inverter-driven compressor, a map-based method was used. Since the second-order function of condensation and evaporation temperature, the model was built at the basic frequency and map conditions. The model is validated by the actual operating conditions. The author compared the data provided by the compressor manufacturers, the average relative errors are less than 2, 3, and 4% for refrigerant mass flow rate, compressor power input, and coefficient of performance (COP), respectively, and the author found out that this model of variable speed compressor is suitable for the simulation of inverter air-conditioner and heat pump systems.

Kim and Kim [70] studied on an experimental study to define the effect of four artificial faults on the performance for a variable speed refrigeration system. For the evaluation of the performance, a conventional vapor compression test system was modified to test several artificial faults by observing the variation of cooling capacity. The four major faults were compressor fault, condenser fault, evaporator fault, and refrigerant leakage. Two different rule-based modules for constant and variable speed operations were organized for an easy diagnosis of the faults. As a result, COP degradation due to the fault in a variable speed system was severer than that in a constant speed system.

Cho et al. [71] measured the cooling performance of a variable speed CO2 cycle and analyzed by varying the refrigerant charge amount, compressor frequency, EEV opening, and length of an internal heat exchanger (IHX). As a result the cooling COP decreased with the increase of compressor frequency at all normalized charges. The optimum EEV opening increased with compressor frequency. The optimal compressor discharge pressure of the modified CO<sup>2</sup> cycle with the IHX was reduced by 0.5 MPa. The IHX increased the cooling capacity and COP of the CO2 cycle by 6.2–11.9% and 7.1–9.1%, respectively, at the tested compressor frequencies from 40 to 60 Hz.

In addition to these studies, the Ekren and Kucuka study [72] has been carried out by a fuzzy logic-controlled chiller system with variable speed compressor. In this study, not only variable capacity modulation has been studied, but also fuzzy logic control effect on chiller system has been investigated. Scroll compressor, designed to study as fixed-speed compressor, was operated as variable speed with a PWM inverter. Also, electronic expansion valve, fuzzy controlled, was used. In this system 33.4% COP increase was obtained according to on-/offcontrolled system. This increase has been obtained because of the less temperature difference between condensing and evaporation temperature.
