DC Environment for a Refrigerator With Variable Speed Compressor; Power Consumption Profile and Performance Comparison

DC power distribution in residential sector has regained interest among researchers and industrial players as new electronics-based appliances became locally available. However, the compatibility of appliances with DC distribution systems still requires much research effort. This work mainly explores on the power consumption profile of an inverter-driven Variable Speed Controller (VSC-based) refrigerator that has not yet been analyzed as one of the most important household loads. This paper compares the power consumption in two scenarios; 1) using three supply configurations for a VSC-based refrigerator, a Battery-Inverter-Load, a Battery-Load and Grid-Load, and 2) using a same AC power source to supply a VSC-based refrigerator and a same-size conventional refrigerator. This analysis helps toward modeling and energy estimation of PV system applications involving storage batteries. A wireless monitoring circuit has been employed to handle temperature, current and voltage measurements with a high sampling rate to cover the potential surge power. The experimental measurements show a better performance on using DC over AC power source and the power rate consumed has a smooth pattern at the starting-on time until approach a rated power. The measured efficiency of the Battery-Load topology approaches 99% compared to that of the Battery-inverter-load topology, which is approximately 78.5%. It is also found that the tested refrigerator with Battery-Load topology consumes an energy amounting to 1.850 kWh daily, while with Battery-inverter-load topology consumes 2.466 kWh daily under the same operating conditions. These results can serve as a model for modeling refrigerators and other appliances that adopt speed controller technology to drive their motors.


I. INTRODUCTION
The information on power consumptions and load patterns of household appliances is necessary to achieve a load management in the dwelling correctly. For the refrigeration industries, improvements are crucial to reduce the compressor power consumption of refrigerators. Household refrigeration system is possible to be powered by renewable technologies such as photovoltaic (PV) power and DC sources. Renewable knowledge is usually considering techniques of producing environmentally friendly methods, or reducing energy usage worldwide. Experimental and numerical investigations for The associate editor coordinating the review of this manuscript and approving it for publication was Jenny Mahoney. thermal performances of natural wet cooling refrigerant is presented in [1]. Two different alternatives for powering refrigeration systems, they are direct current (DC) or alternative current (AC) [2]. In recent years, appliances with power electronics technology have widely spread which not only reduces the variable frequency, drive size, and cost but also improves consumption performance. Advances in semiconductor switching devices, simulation and control techniques, power driving topologies, control software and hardware all contribute to better appliance performance.
A compressor combines mechanical compression parts and an electrical motor. The electrical motor drives mechanical parts by converting an electrical energy into mechanical type in a refrigerator's compressor. Variable Speed Compressor  based (VSC-based) refrigerator includes permanent magnet brushless DC motor (PMBLDC). This motor is a type of self-synchronous rotary motor controlled by electronic commutations, where the rotor is a permanent magnet with position sensors with its rotor [3]. The main diagram of a BLDC motor is shown in Figure 1.
The inverter of a VSC-based refrigerator receives DC power via a rectifier from an AC power source to drive the motor and control its speed by taking a feedback from the rotor. Therefore, it is possible to supply such type of motor/compressor by a normal AC or its equivalent DC power source.
Recently, refrigerator manufacturers have produced compressors that can operate at smoothly variable speeds depending on a difference value between setting and actual temperatures inside the refrigerator. Traditional refrigerators have compressors with a fixed speed, where the set temperature is maintained by operating the compressor at that speed. The compressor continues running until the temperature inside the refrigerator approaches equal or lower than the set temperature or when the compressor at defrosting heating. At this moment, the compressor turns off. When the ambient temperature becomes one or two degrees higher than the setting, the compressor starts running again. Although this operation has a stable steady state rating power, it experiences a high surge power on starting that inversely affects its life cycles.
A basic refrigeration system can be modeled as shown in Figure 2.
The compressor supplies the refrigerant gas throughout a condenser, a capillary pipe, and an evaporator. The accumulator ensures only vapor feeds the compressor and  two fans supply moist air-flow through the evaporator and the condenser. The air-flow of the evaporator is separated between the regular compartment and the freezer compartment. The function of the controller is to turn the compressor ON and OFF to remain the air temperature in the compartment at a specified temperature deg. C [4].
Unlike shiftable appliances, that have a fixable delay with a certain consumption-cycle profile, refrigerators are classified as continuous interruptible load, which is either ON with fixed energy consumption or OFF. However, their ON cycle duration depends upon user preference setting [5]. Refrigerators can be categorized as appliances that are ON 24 hours a day. To understand the power consumption of a conventional refrigeration system, Figure 3 shows the temperature of food and cold air in the freezer and compartment. The compartment door has opened at the time 11000 sec for 60 sec, which results in a spike in the temperature. The plot in Figure 4 demonstrates the power consumption of the cooling load and the compressor of this system that interprets the rate change of the heat transfer of the evaporator.
The compressor of VSC-based refrigerators uses electronic power inverter to drive its motor by controlling the speed according to cooling or heating the compartment load. It starts running at slow speed and gradually growing up to a maximum power at a high-speed to achieve the setting temperature. Unlike a traditional AC compressor that runs at a fixed speed that consuming all the power directly, the inverter circuit of VSC drives the compressor with the appropriate voltage that saving energy. The compressor can then regulate its running speed according to the ambient temperature. The VSC-based refrigerators work in silent mode since they run at slow-speed and do not export loud noise when the compressor starts. Furthermore, its potential failure is less and has longer lifetime.
The energy consumption of a particular refrigerator/freezers is dependent on several parameters such as; thermostat setting position, ambient temperature, door opening [6]and relative humidity [7], [8]. The effects of these parameters on energy consumption are investigated in this paper. Several studies address the analysis of refrigerator energy consumptions [9], [10]. Modeling refrigerator systems have presented modeling of individual components that include the compressor [11]- [13] and refrigerator evaporator [14]. Another type of modeling multivariable systems such as refrigeration is a measurements-based mathematical model that relies totally on experimental data [15]- [17]. For energy saving considerations, modeling the consumption patterns of household appliances is a great challenge [18]. Therefore, a model to simplify the behavior of the system energy consumption is crucial with estimating the variables that affect the accuracy of an appropriate degree. This will develop a highly accurate and detailed model that involves all variables and factors, which in turn requires substantial measurements of experimental data. The whole energy efficiency standard foundation labels are procedures of energy experiments, which denote agreed-upon procedures for measuring the performance of appliances energy [19]. Such procedures present methods for engineers, consumers and manufacturers to evaluate and compare the energy performance consistently for different household appliances [20], [21]. Experimental measurements for recently produced refrigerators help to model the power consumption profile since this type of load has not yet been analyzed in details after employing variable speed controller. Several research related fields that discussed DC environment applications can also benefit from the outcomes of this study such as [22].
Studies presented frameworks that analyze the power consumption and load profile of the home appliances individually with a high frequency sampling such as [23], while the authors in [24] modeling of building end-use power profile using wireless sensor network. The switch-ON times, switch-OFF times for three appliances including fridge freezer is identified by [25]. Modeling of such energy profile for detecting and estimating individual home appliance loads is performed by [26]. Although these related studies address the modeling and energy estimation for home appliances as an overall load or individually, they didn't discuss the power consumption profile of inverter-driven refrigerator and its principle of operation. They didn't address the compatibility of such appliances with DC power supply. A very related study also used a DC power to supply a refrigerator [27] and [28], where authors used the DC power of a solar PV to supply a small size (92L) refrigerator that its compressor has been replaced by a variable-speed direct current (VSDC) (BD35F Danfoss compressor). However, such studies have the following limitations; 1) the new VSDC compressor might not fit the refrigerator size because the temperature was not identical when comparing the performance with the same refrigerator with its original AC compressor, 2) Authors didn't use the variable speed feature and fixed on 3000 rpm instead, and 3) Authors have forced to use a DC voltage-level that a VSDC compressor requires (12V/24V) though these levels become useless with high power loads due to its losses.
The proposed power topology overcomes the above gaps by; 1) using a VSC-based refrigerator that recently available in most brands that can accept DC and AC power, 2) employing the normal operation with a feature of variable speed control to accurately controlling temperature with minimal consumption, and 3) proposing 276-311 V DC power that is equivalent to the 220V AC, this level can be connected directly to the refrigerator without any modification. This work mainly discusses on the power consumption profile of an inverter-driven refrigerator as one of the most important household loads, in which electronic speed-controller has been employed to drive its compressor. The paper discusses and compares the power consumption patterns of this refrigerator towards the modeling and energy estimation for PV systems involving storage batteries. A wireless monitoring circuit with a high sampling rate has been employed to access current and voltage measurements and to cover the potential surge power [29]. The work considers the overall system efficiency as a factor to evaluate the system performance by comparing two DC system topologies, Battery-Load and Battery-inverter-load.
The main contribution of this work is to exploit the recent advances of power electronics in the commercial VSC-based refrigerators to present the appropriate voltage level of a DC power source equivalent to the normal grid AC power to operate these refrigerators. This work will contribute to provide ability for solar PV systems with battery energy storage to operate without AC power inverters with DC environment, which will increase the overall system efficiency and reduce the cost. The study contributes to understand how a VSC-based refrigerator consume power and also analyses the consumption patterns of such refrigerators and compare it with a traditional one, which contribute to modeling its power profile.

II. METHODOLOGY
Since this work proposes using batteries with a voltage level equivalent to the 220V AC of the grid to feed a VSC-refrigerator, it is essential to select the standard nominal voltage of the system's battery. In order to ensure the same performance in terms of power consumption when connecting DC source to such a refrigerator, the concept that we are based on is the voltage matching between the source and the normal operation voltage of the load [18], [30]. Therefore, for lead-acid batteries, the voltage level is constrained by the following formula: where K bat ∈ N + . The rectified maximum voltage (V max ) that can be measured of the AC input, which is equivalent to VOLUME 8, 2020 the single phase, 220 V rms grid voltage, is given by: This voltage is supposed to be the maximum battery voltage, which is the fully-charged voltage. Since the lead-acid battery of 12V nominal voltage has about (13.5-13.6) V fullycharged voltage, the number of serially-connected batteries (K bat ) is 311/13.5 ≈ 23. Thus, the nominal voltage is 276V.
With such a voltage, the circuit of charging batteries by the grid is more efficient since the voltage drop between the grid (as a source) and the battery are close together [31]. The experimental set-up in this work uses two configuration scenarios; 1) The first configuration is to supply the VSC-based refrigerator by two source types. The proposed DC level, which is between 276-311 V DC, and an AC power (220V), by using an inverter and directly from a grid. The objective of this experiment is to verify the suitability of the proposed DC level and to get the power pattern for future modeling and energy estimation purposes. 2) The second configuration is to use an AC power source for refrigerators, a VSC-based and a traditional. The objective here is to check the effectiveness of the VSC-based technology as compared with the old traditional experimentally. Figure 5 demonstrates these scenarios respectively, where serially connected batteries provide an energy with a voltage DC to a DC-AC inverter then to a VSC-based refrigerator, and to compare with the directly connection from the battery pack to the same refrigerator as shown in Figure 5(a). The performance of the VSC-based refrigerator is evaluated by comparing its cabinet temperature and rate of change of power consumption with that of a traditional refrigerator in same size and brand as shown in Figure 5(b).
An appropriate application for the proposed configuration of using DC with VSC-based refrigerators is the topology of solar PV systems with battery energy storage that shown in Figure 6.

A. MONITORING CIRCUIT
A wireless power monitoring circuit has been developed to record voltage and current measurements with a high sampling rate of 11Hz to cover the potential surge power, this monitoring system presented in our previous effort [18], [32]. The circuit represents a wireless communication transceiver node located at the household of the refrigerator, which has the necessary elements used for measuring the energy parameters (the current and the voltage along with the time) as well as receiving control signals required for managing the potential consumption of energy. The measurements have been calibrated with respect to the Fluke 434-II Energy Analyzer. Three pieces of thermocouple amplifier and K-Type thermocouple interface were used to measure the ambient temperature and temperatures inside refrigerator cabinet of both VSC-based and traditional refrigerators.
Mainly, the instantaneous electrical power consumed from a power source (P s ) is divided into two parts, power conversion losses (P c ) and the refrigerator power consumption (P r ), which in turn can be subdivided into ON-state power (P r−ON ) and OFF-state power (P r−OFF ) consumptions as given in the following equations: where I s−ON and I s−ON are the ON-state and the OFF-state measured currents. µ c is the converter efficiency, it is one when the source is directly linked to a load (refrigerator). The electrical energy consumption over the interval (t start − t end ) is E r and is given by: The calculation of efficiency (µ) is based on the following formula: where E delivered denotes the energy delivered from the supply, while, E consumed denotes that energy consumed by the load. The current and voltage signals are delivered to a conditioning circuit which synthesizes the inputs to be suitable for XBee node limits. The circuit diagram and its physical photo are shown in Figure 7(a) and 7(b), respectively.

B. VSC-BASED REFRIGERATORS
The speed of the VSC is controlled by varying the frequency and the magnitude of the electric input voltage. Both AC-DC and DC-DC converters are used in VSC-based refrigerators, where the input power is converted from single-phase 220V/50 Hz AC of home electricity outlet to DC, and then the later is converted into a controlled voltage level and frequency to deliver to the compressor motor. Therefore, the controller provides a variable frequency to the compressor motor with an ability to control its speed. This is done by passing the input AC voltage of utility through a full wave rectifier bridge and the DC output of the rectifier is smoothed by a capacitor bank. The smoothed DC is converted into an output of a required voltage and frequency via inverter bridge circuit controlled by pulse width modulation (PWM) techniques. The block diagram describing the main system components for such a refrigerator is shown in Figure 8. The objective of these measurements is to understand the power consumed of such type of refrigerator over one cycle, and to recognize the difference in the power pattern as compared with traditional refrigerators. The interesting specifications of the tested refrigerator are with a 476L capacity, a digital inverter compressor, 50Hz, 425W, and a (220-240) V supply voltage. The conducted experiments include supplying the refrigerator with both AC and DC power. The experimental measurements of the power consumption profile have been conducted on a VSC-based refrigerator shown in Figure 9.

III. RESULTS
Experiments have been carried out with the developed logging and monitoring prototype of around 11 samples per second to capture a potential surge power that might happen at a starting on-time of the load operation. To understand how this type of refrigerator consumes energy, one power cycle of the VOLUME 8, 2020  VSC-based refrigerator is performed for analyzing with some details. About more than one hour of logging measurements has been recorded started from the first second of the turning ON of the refrigerator. The refrigerator consumption-rate profile of one-cycle, with RMS voltage equal to 220V, over one-cycle operation is shown in Figure 10.
After about 15 minutes, the consumption has risen due to start running of the compressor to recover the difference between the setting and the ambient temperature. It starts with about 50% (210W) of its maximum consumption (420W) and gradually increases, which agrees with the model in [33]. When the cabinet temperature inside the refrigerator becomes lower than the setting, the power will decline back to its lower value.
In order to verify the suitability of the proposed DC level for PV systems and to get the power pattern for future modeling and energy estimation purposes, we divide the remaining results into two scenarios according to the adopted configuration in the methodology section as follows: A. SCENARIO 1: AC AND DC SUPPLYING THE VSC-BASED REFRIGERATOR In this scenario, the average values of each one-hour measurements of the power consumption have been calculated and plotted to compare the consumption of the VSC-based refrigerator under the traditional configuration (Battery-Inverter-Load), the proposed one with the appropriate voltage-level of DC (Battery-Load), and a normal Grid-Load configuration. Figure 11 demonstrates the difference between these supplying environments on the refrigerator' consumption over a full day. To quantify the experiment conditions and its outcome, Table 1 lists these values in detail.
In order to determine the daily energy consumed by a refrigerator (E r−day ) in (Watt.h), the area under the power consumption at an 11Hz sampling frequency (f smp ) is accumulated over a full day time by using: where t end = f smp * 24 * 60 * 60 (6) It is found that this refrigerator consumes electricity amounting to 2.466 kWh, including the DC-AC inverter (red color), while amounting to 1.850 kWh per day when connected directly to the proposed DC voltage-level (blue color). As compared with AC Grid-Load configuration, the full day energy (kWh) is slightly higher than the direct Battery-Load  energy since the experiment of the Battery-Load topology excludes the rectifier circuit of the VSC-based refrigerator.
The experimental measurements show a better performance on using DC over AC power source and the power rate consumed has a smooth pattern at the starting-on time until approach a rated power, which agree with results obtained in [28]. The measured efficiency for (Battery-Load) topology approaches 99% over that for (Battery-inverter-load) which is around 78.5% due to inverter losses.
For PV system applications, the harvested daily energy of the solar PV can also be obtained from (6) by substituting the instantaneous power (P s ) with the PV power at maximum power (P mp ), while µ c represents the efficiency of the solar charge controller including MPPT, µ c < 1. The PV daily energy (E PV −day ) must satisfy the following: k=t end k=0 P mp (k) = E r−day + E losses−day (7) VOLUME 8, 2020  where E losses−day represents the daily losses energy such as that in system batteries and wires.
For example, if we assume E losses−day = 0.2E r−day . For the 1.850 kWh, E PV −day = 2.22kWh.
According to the practical measurements of one-day weather conditions in Malaysia (Selangor), it is possible to size the required solar PV panels [34]. Furthermore, the total power of the required PV panels over a third of a day must be greater than 2.22 kWh/8 = 277.5 W, if we assume there is an eight hours a day time. Therefore, one standard (280-300) W panel is sufficient to cover the demand of the refrigerator energy for such size in Malaysia weather.

B. SCENARIO 2: AC POWER SOURCE SUPPLYING BOTH A VSC-BASED AND A TRADITIONAL REFRIGERATORS
In order to check the effectiveness of using a refrigerator of VSC-based technology with respect to the old traditional one experimentally, both refrigerators were monitored.
The experimental measurements were carried out under the following conditions: • All measurements were performed with empty cabinet and after making sure that the temperature inside and outside the refrigerator cabinet are identical.
• Making sure that the refrigerator door remains closed during the experiment time.
• The thermostat was adjusted to a maximum cooling level in this refrigerator (-6C o ), which may make the compressor to operate at a high running conditions to maintain the lower temperature.
• Maintaining same ambient conditions for both refrigerators during the test.
• Switching ON both refrigerators in a same time, and all the measurements have recorded with 11 sample/sec by a base station computer. The measurements of the temperatures inside the cabinet and the ambient were recorded over about three and a half hours. These profiles are shown in Figure 12. On a same context, measurements of the power consumptions over three and a half hours were recorded for the two refrigerators simultaneously as shown in Figure 13.
It can observe that the data recording starts at 27 o C in both refrigerators, which is the same as the ambient temperature. From temperature recordings, it is observed that the profiles for the traditional and VSC-based refrigerators are almost same in the cycling but VSC-based has a less band of variation. Although both refrigerators approach the setting temperature on same time during the initial pull-down period, the power consumption of the VSC-based refrigerator has a soft rate with lower energy as compared to the traditional one  that has a high surge power in the starting ON time. For the traditional refrigerator, each OFF-ON transient has a surge power of a value that may double the rated power, which requires a hard stress done by the compressor. In contrast, for the VSC-based refrigerator, the frequency of the OFF-ON transient is much smaller and with no surge power.
Based on several experimental attempts and referring to most references in the literature motioned in the introduction section, the comparison in ideal operations between a traditional compressor and VSC can be described in Figure 14. The figure demonstrates the variation of the compressor rotational speed and the energy consumed over an operation time.
It can be observed that the traditional refrigerator cycles ON and OFF at a fixed speed whereas the VSC-based refrigerator operates with various speed by controlling its power electronically. VSC-based refrigerator works at a low speed and consumes a lower amount of energy efficiently, and its power rate declines along with ON time. The figure also shows that VSC-based starts its cycles at low speed and grows gradually. Therefore releases less noise.
Unlike the soft rate of power consumption in the VSC-based refrigerator, the high surge transients in power experienced by the traditional refrigerator have negative effects on its operation since it is required to be connecting via a higher capacity inverter to the battery pack of a PV system. A higher capacity of a charge controller and an inverter lead to high cost system as well as higher conversion losses.

IV. CONCLUSION
In this paper, experimental tests have been conducted to evaluate the performance of a VSC-based refrigerator and the effectiveness of using such refrigerator with the proposed DC level of voltage towards the solar PV applications. Based on the obtained results, the followings can be concluded: 1) Since the power circuit of the new commerciallyavailable inverter-driven refrigerators is already rectifying an AC input, it is possible to efficiently use such refrigerators with the DC voltage level that equivalent to the rectified grid AC without any modification. 2) Using VSC-based refrigerators with the proposed (Battery-Load) has better performance than that with the traditional (Battery-Inverter-Load) configuration of PV systems, which increases system efficiency, prolongs system storage and decreases cost through dispensing the use of inverter. 3) As compared with the traditional, the power consumption pattern of the VSC-based refrigerator does not have any surge power, in which it is possible avoiding the using a very high capacity inverter and batteries even with the traditional configuration Battery-Inverter-Load. 4) The outcome of the conducted experiments for power consumption patterns can serve for further analysis such as; refrigerator consumption modeling, forecasting, and control purposes. 5) It is recommended to use a VSC-based refrigerator for systems that have battery pack within the solar PV scheme as the system would be more efficient and economical.