Integration of Vapor Compression and Thermoelectric Cooling Systems for Enhanced Refrigeration Performance

Abstract

Refrigeration is vital in daily life and industries, traditionally relying on single-system cooling. The two predominant kinds of single-system cooling are vapor compression refrigeration (VCR) and thermoelectric cooling (TEC). Each of these two single systems has its own disadvantages, such as higher input energy requirements and lower efficiency. However, the effect of the integration of VCR and TEC for achieving higher cooling performance with lower energy input has not been well studied and reported in the existing literature. Therefore, the aim of this study is to conduct a thorough investigation into an integrated refrigeration system that combines VCR and TEC. This integration allows switching between systems based on specific requirements, leveraging the high coefficient of performance (COP) of VCR and the benefits of TEC. Three configurations have been studied, and each of them has three operating conditions: VCR alone, TEC alone, and TEC hybrid with VCR. Configuration I corresponds to the results from the individual refrigeration test. In Configuration II, the hot end of the thermoelectric cooling module is installed at the insulation layer between the TEC layer and the VCR compartment. In Configuration III, the cold end of the thermoelectric cooling module is positioned at the insulation layer between the TEC layer and the VCR compartment. Configuration III of the integrated system demonstrated good performance by reducing the time required to reach the target temperature. It took 40 min for TEC alone to reach a temperature of 11.1 °C, 13 min for VCR alone, and only 9.6 min for a hybrid system. The hybrid system shows increased versatility and potential for future applications, providing valuable insight into optimizing advanced cooling technologies. Furthermore, from an economic and sustainability standpoint, the proposed hybrid refrigeration system is advantageous and ambitious as it offers superior cooling capacity and greater efficiency than current refrigeration systems.

1. Introduction

Global energy consumption is steadily increasing, and excessive reliance on non-renewable energy sources has resulted in serious environmental problems [1,2]. In response to growing energy usage and a global commitment to energy conservation and environmental protection, countries around the world are actively exploring strategies to transition to cleaner energy sources, reduce dependence on fossil fuels, and enhance energy utilization efficiency [3,4]. Initiatives such as the Montreal Protocol and the Kigali Amendment have been introduced to address environmental concerns. These agreements focus on restricting the use of refrigerants, particularly hydrofluorocarbons, which have been shown to harm both the environment and the ozone layer [5,6]. Additionally, greenhouse gases are released into the atmosphere due to widespread use of refrigerators, heating, and air conditioning systems. However, by adopting clean or renewable energy technologies such as ocean energy [7], photovoltaics [8], solar thermal energy [9], wind power [10], and energy storage solutions [11], energy consumption can be reduced. This shift is critical in addressing both energy and environmental challenges. Air conditioning and refrigeration are integral parts of modern life. Various forms of refrigeration exist today, with vapor compression refrigeration (VCR) technology standing out due to its superior refrigeration coefficients. It is worth noting that higher refrigeration coefficients correspond to reduced energy consumption and a smaller carbon footprint [12]. Despite its advantages, this refrigeration technology has its drawbacks. A significant issue lies in the environmental impact of the refrigerants, which serve as the working medium in the refrigeration cycle. Currently, many refrigerants pose environmental risks. However, there is a positive trend toward replacing environmentally harmful refrigerants with cleaner, non-polluting alternatives [13]. Additionally, a drawback of VCR systems arises when employing a fixed-speed compressor. The challenge lies in the difficulty of precisely controlling the temperature within the target compartment. This difficulty stems from the oscillation of temperature caused by the start–stop cycle of the compressor, resulting in significant temperature fluctuations within the refrigeration compartment, affecting the preservation of food or perishable goods. To address the mentioned drawbacks of vapor compression, numerous researchers and scholars globally have undertaken extensive research on alternative cooling technologies [14,15]. Among these alternatives, thermoelectric cooling (TEC) has emerged as one of the promising methods for tackling environmental pollution [16].

In 1821, German physicist Thomas Johann Seebeck discovered that when two different metals are joined together to form a closed loop and the temperature at the connection points differs, an electric potential is generated. This phenomenon, known as the Seebeck effect, was the first recorded thermoelectric effect. In 1834, Jean Peltier expanded on this concept by showing that a closed loop of two different metals could not only generate electricity through heat but could also absorb or release heat depending on the direction of the current passing through it [17,18]. In addition to the Seebeck and Peltier effects, thermoelectric cooling modules operate based on five thermoelectric effects, including the Thomson effect, the Joule effect, and the Fourier effect [19].

Thermoelectric cooling modules are widely used for applications such as cooling PC processors, portable food and beverage storage, automotive seats with temperature control, and thermoelectric air conditioning. Significant efforts have been invested by the scientific community to develop TEC systems [16]. However, despite extensive research, the low efficiency and high cost of thermoelectric cooling modules have hindered their widespread adoption and commercialization. At present, thermoelectric cooling modules cannot compete with widely commercialized technologies such as the organic Rankine cycle and VCR systems [20,21]. Improving the energy conversion efficiency and thermoelectric performance of thermoelectric cooling modules is a key focus of ongoing research. Current efforts to enhance and optimize thermoelectric cooling modules are generally categorized into three main strategies: (1) improving semiconductor materials, (2) optimizing the structure and geometry of thermoelectric elements, and (3) enhancing thermal management techniques [22,23,24,25,26,27,28]. TEC boasts several advantages, including a compact structure, the absence of moving parts, and a small size [29]. The technical characteristics of VCR and TEC are shown in

Table 1. Characteristics of VCR and TEC technologies.

Projects	TEC	VCR
Main components	Thermoelectric modules, hot and cold end heat sinks	Compressor, Evaporator, Condenser, Throttling device
Refrigerant	Absence	Presence
COP	0.1~1	0.5~4
Capacity control methods	Linear and continuous adjustment of cooling capacity from milliwatts to kilowatts is achieved by adjusting the input voltage and current	Fixed-speed compressors experience periodic start–stop cycles, but the control mode is complex, and the cost is high
Temperature control accuracy	0.01 °C	0.1 °C
Startup power	10 W is enough	Usually more than 30 W
Service life	10–15 years	15–20 years
Payback period	5–7 years	3–5
Cost of the received cold (USD/kW)	300–600	50–150

. Many researchers have studied individual VCR systems or TEC systems. Salilih et al. [30] proposed a method for analyzing and simulating a solar-driven variable-speed compression VCR system. The research revealed that, when the compressor operates at low speed, the COP of the refrigeration cycle is approximately 2.25, and it decreases to 1.85 at the highest speed. Kishore et al. [31] explored the impact of key performance parameters of thermoelectric materials on the efficiency of a thermoelectric cooling module. Additionally, they optimized a high-efficiency thermoelectric cooling module capable of cooling human skin to 8.2 °C below ambient temperature. The optimized thermoelectric cooling module achieved enhanced cooling performance and a smaller and more economical design. Belarbi et al. [32] conducted experiments involving a microchannel heat sink-coupled TEC module for the airflow cooling of a desktop central processor. The results demonstrated a sizable impact of the thermoelectric modules on central processor cooling, showing a 15% improvement over conventional forced-air cooling. Reviewing the existing research, it is evident that both the VCR and TEC systems have their respective advantages and disadvantages. The VCR system offers a high COP and cooling capacity, while the TEC system operates with fewer temperature fluctuations.

Hybrid refrigeration systems have been recognized by some researchers as a way to achieve lower temperatures and enhance cooling capacity. These hybrid refrigeration systems combine Stirling cycle refrigeration systems with magnetic refrigeration [33], VCR with absorption refrigeration [34], VCR with vapor injection refrigeration units [35], and TEC with VCR systems [14]. At present, numerous researchers are concentrating on the VCR–TEC hybrid system, given that both VCR and TEC are appropriate for use in household refrigerators. Additionally, TEC offers greater flexibility and stability in refrigeration, making the VCR–TEC hybrid system a promising solution with strong commercial potential. This approach aims not only to capitalize on the strengths of both systems but also to broaden the scope of applicable scenarios. A hybrid refrigeration system featuring TEC and VCR may offer the flexibility to switch between different refrigeration systems based on specific scenarios. Additionally, these systems can operate simultaneously, either individually or in combination.

In recent years, some studies attempted to explore the possible integration of TEC systems with VCR systems, leading to the proposal and optimization of various hybrid refrigeration architectures. Fu et al. [36] introduced a hybrid refrigeration system in which the hot end of the TEC wafer is in direct contact with the VCR evaporator, achieving lower refrigeration temperatures than single systems. Experiments indicated a refrigeration temperature of −38.1 °C in a 0.018 m³ compartment, highlighting its potential for small, low-capacity systems. Vián et al. [37] developed a three-compartment TEC-hybrid VCR system, optimizing the structure and validating it with a computational model that showed a 1.2 °C error. The system maintained constant refrigeration temperatures, enhancing food preservation. Astrain et al. [38] employed a cascade refrigeration system, improving the thermoelectric cooling module capacity by configuring it so that the TEC discharges heat into the VCR freezer. This configuration enabled the TEC compartment to operate between 0 °C and −4 °C, with temperature oscillations kept below 0.4 °C. Sayyad et al. [39] integrated TEC with VCR, preventing wet vapor from entering the compressor and comparing the performance to VCR-only systems.

Söylemez et al. [40] designed a hybrid refrigeration system integrating both TEC and VCR technologies. While this hybrid system exhibits higher power consumption and noise levels compared to conventional VCR refrigerators, it provides advantages in precise temperature control and rapid cooling capacity. Additionally, Söylemez et al. [41] extended their research to the refrigerator’s freshness chamber in the aforementioned model. They examined how the thermoelectric cooling module’s mounting position affected temperature, temperature uniformity, and cooling time in the freshness chamber. Their research showed that mounting the thermoelectric cooling assembly in the middle of the rear wall matched the cooling time of the original design, which had the cooler positioned in the middle of the front wall. However, energy efficiency decreased by 10% and 27% when cooling from ambient temperatures of 16 °C and 32 °C, respectively. Jamali et al. [42] optimized and enhanced a hybrid thermoelectric and transcritical CO₂ refrigeration system by incorporating a thermoelectric generator to convert heat energy from an air cooler into electrical energy. Their findings showed that the COP of this hybrid system increased by approximately 19% compared to a simple CO₂ refrigeration cycle. Additionally, another hybrid refrigeration system combines VCR and TEC specifically for refrigeration applications. By connecting the two systems in series and using the VCR system’s refrigeration capacity to manage the heat load of the TEC system, this approach aims to achieve lower refrigeration temperatures and improve the stability of temperature control.

Hybrid refrigeration systems integrate the strengths of both TEC and VCR systems while compensating their respective drawbacks. These systems can provide lower cooling temperatures, faster cooling rates, and more consistent temperature control compared to VCR systems. Additionally, they can offer superior cooling capacity and greater efficiency than TEC systems. Therefore, from an economic and sustainability standpoint, adopting the hybrid refrigeration system is advantageous and ambitious.

However, current research on TEC and VCR systems has primarily focused on optimization of individual systems or improving refrigeration temperatures and COP in hybrid systems without addressing the impact of varying input power on the refrigeration system. Therefore, understanding how temperature changes in a hybrid refrigeration system under different power inputs is crucial. Furthermore, the temperature range suitable for domestic refrigerators has been the focus of most research on hybrid refrigeration systems. Developing a refrigeration system capable of producing multiple temperature ranges with the same equipment is essential for storing specific items, such as food, pharmaceuticals, chemical reagents, and other goods. This prevents resource waste and contributes to sustainable development.

Therefore, the aim of this study is to design a hybrid refrigeration unit with three configurations that combines the advantages of TEC and VCR. By leveraging the high COP of VCR and the stability and precise temperature control of TEC, the equipment enables the transportation of goods and/or in-house appliances at various storage temperatures. The study specifically involves the development of the TEC system, the VCR system, and the hybrid cooling system in these three configurations. The refrigeration effect of each configuration is analyzed experimentally, summarizing the cooling characteristics of each setup. The goal of this study is to provide insights into optimizing the cooling system and explore the potential for a sustainable version.

2. Materials and Methods

2.1. Experimental Equipment

2.1.1. Heat Sinks

When using natural convection for heat dissipation, the surface heat transfer coefficient is relatively low, leading to reduced heat dissipation efficiency. To enhance cooling, small fans were employed at the upper end for forced convection. The specifications for the fans at both the cold and hot ends are provided in

Table 2. Hot and cold end fan parameters.

Equipment	Model Number	Type	Type of Current	Rated Voltage	Rated Current	Rating
Hot end fan	CJY9040	Brushless fan	Direct current	12 V	0.2 A	2.4 W
Cold end fan	CJY4020	Brushless fan	Direct current	12 V	0.12 A	1.44 W

.

2.1.2. Refrigerants and Refrigeration Box

R290 (Propane, C₃H₈) was procured from Sinopharm Chemical Reagent Co., Ltd. in Shanghai, China and selected as the experimental refrigerant due to its low global warming potential and ozone depletion potential, making it an environmentally friendly option that reduces pollution and mitigates the greenhouse effect.

The refrigeration box was constructed from rigid polyurethane foam, with thermal conductivity ranging from 0.018 to 0.022 W/(m·K) and a density of 0.06 g/cm³. The inner lining was made from ABS engineering plastic, with a density of 1.05 g/cm³ and thermal conductivity of 0.22 W/(m·K). The outer shell was fabricated from steel plate, with a density of 7.85 g/cm³ and a specific heat capacity of 0.46 × 10³ J/(kg·°C).

2.1.3. Compressor, Evaporator, Condenser, and Throttling Device

This study used the fully enclosed Donper L58CU1 compressor model manufactured by Hubei Dongbei Electromechanical Group Co. (Huangshi, China), which is compatible with R290 refrigerant, has a cooling capacity of up to 275 W, and operates at a speed of 4234 rpm. The experimental setup included a serpentine copper tube evaporator with a diameter of 6 mm and an air-cooled condenser (model FNF-0.8/3.4) manufactured by Covili (Pistoia, Italy). The high-pressure liquid discharged from the condenser expands into a low-pressure, low-temperature gas–liquid two-phase mixture through the throttling device. Due to its simple structure and low cost, the capillary tube was used as the throttling device in this experiment. For this purpose, a copper capillary tube with a diameter of 2.2 mm was selected.

2.1.4. Thermoelectric Cooling Module

Based on market research, two commonly used thermoelectric cooling modules—TEC1-12706 and TEC1-12705—were identified. The parameters for these are listed in

Table 3. Parameters of TEC1-12706 and TEC1-12705 thermoelectric cooling modules.

Model Number	Size	Maximum Voltage Vmax	Maximum Current Imax	Maximum Cooling Capacity Qcmax	Logarithm of PN Section N
TEC1-12706	40 mm × 40 mm × 3.8 mm	15 V	5.8 A	58–65 W	127
TEC1-12705	40 mm × 40 mm × 3.4 mm	15.4 A	5 A	42.5 W	127

. A review of the parameter table reveals that the two thermoelectric cooling modules have similar dimensions, with only a 0.4 mm difference in thickness. The TEC1-12706 exhibits higher maximum refrigeration power, which is observed at a thickness of 0.4 mm. However, TEC performance varies due to differences in material composition, manufacturing processes, the number of PN pairs, and variations in length and shape, all of which influence cooling capacity. These differences are reflected in the distinct refrigeration curves obtained during experiments with both types of thermoelectric cooling modules. The optimal thermoelectric cooling module will be selected based on experimental outcomes.

2.1.5. Test Equipment

The experiment needed to test the temperature and energy consumption of the refrigeration system, and after comprehensive consideration of factors such as accuracy and cost, the YOGOKAWA-type multi-point data collector manufactured by Yokogawa Measurement Technology Corporation (Shanghai, China) with K-type thermocouples was selected. The experimental facilities described are shown in Figure 1.

2.2. Experimental Setup

To investigate the cooling performance of a hybrid refrigeration system, structural designs were implemented for both TEC and VCR systems. This experiment involved three different configurations each comprising three cooling conditions: VCR alone, TEC alone, and TEC–VCR hybrid.

2.2.1. Configuration I

Configuration I, as shown in Figure 2a, features a hybrid cooling system with two refrigeration compartments: one for the TEC system and another for the VCR system. These compartments are arranged according to their respective cooling systems. The volume of the TEC compartment is 18 L. Figure 3 illustrates the structure of the TEC system along with a physical diagram of the cooling box. The evaporator of the VCR system is located inside the VCR compartment to cool the VCR layer, while the TEC system is positioned within the rear wall of the TEC compartment. When the system operates, the TEC system absorbs heat from the TEC compartment and directly releases it into the environment. Because the heat from the hot side of the TEC system is expelled directly into the surroundings, a significant temperature differential occurs across the module, resulting in a very low energy efficiency ratio for the thermoelectric cooling module. There is minimal heat exchange between the TEC and VCR systems, and the structure can be regarded as a stack of VCR and TEC refrigeration compartments. The tests of the single-system TEC and VCR cooling systems yielded results identical to those of Configuration I. As a result, the subsequent experimental tests focused on the characteristics of the TEC-only and VCR-only systems.

2.2.2. Configuration II

Configuration II, widely studied in recent years, can achieve an energy efficiency rating of Grade A. In Configuration II, the thermoelectric cooling module is installed between the TEC compartment and the insulation layer of the freezer, as shown in Figure 2b. The heat from the hot side of the TEC system is discharged into the VCR compartment, absorbed by the evaporator, and released into the environment through the condenser. This significantly improves the COP of the TEC system and expands the cooling range. This forms a hybrid refrigeration system combining TEC and VCR technologies. The system also operates under three different refrigeration conditions. Compared to Configuration I, Configuration II only changes the installation position of the TEC system. When the TEC system operates alone, the lower compartment door needs to be opened to improve the heat dissipation of the hot end of the thermoelectric cooling module. For the VCR-only mode, insulation must be installed at the TEC and VCR installation site to prevent heat loss due to thermal conduction. Under combined cooling conditions in Configuration II, the power requirement for the VCR system increases, but the energy efficiency ratio of the TEC system significantly improves. This configuration reduces energy consumption compared to Configuration I. Since in Configuration II, compared to Configuration I, only the installation position of the TEC system is changed, with its structure and dimensions remaining the same, the results for both alone cooling conditions are the same as those for Configuration I, and no repeat tests were conducted in subsequent experiments.

The interaction between the TEC system and the VCR system in Configuration II was further investigated to enhance the overall energy efficiency of the system while maintaining the cooling benefits of each subsystem. The experimental setup for Configuration II is shown in Figure 4a, and the internal structure of the system is depicted in Figure 4b.

2.2.3. Configuration III

In Configuration III, as shown in Figure 2c, TEC and VCR are used for hybrid cooling of the compartment. The TEC system is placed in the rear wall of the TEC compartment’s casing. When the system operates, it absorbs heat from the compartment and directly discharges it into the environment. The VCR system absorbs heat from the compartment through the evaporator and releases it through the condenser. The TEC–VCR system provides hybrid cooling to the compartment and additive cooling effects. The configuration also operates under three different refrigeration conditions.

Based on the above content, it is evident that all three configurations feature three cooling modes, allowing for the selection of a cooling mode according to varying application requirements, significantly enhancing the system’s universality. Additionally, in Configurations II and III, the TEC system, as an independent system, can switch between cooling and heating by changing the direction of the input current. After reversing the current, the hot end of the thermoelectric cooling module can perform defrosting functions within the enclosure.

2.3. Cooling Capacity and COP

Cooling capacity is the amount of heat that a refrigeration unit can remove per unit of time from an area that needs to be cooled. For a TEC system, ignoring the contact resistance and thermal resistance between its internal components, the cooling capacity $Q_{\mathrm{c},\mathrm{T}\mathrm{E}\mathrm{C}}$ (W) can be defined as shown in Equation (1) [43].

$$Q_{\mathrm{c},\mathrm{T}\mathrm{E}\mathrm{C}}=\overline{\alpha }{T}_{\mathrm{c}}I-\overline{K}\left(T_{\mathrm{h}}-T_{\mathrm{c}}\right)-0.5\overline{R}{I}^2$$

(1)

where I is the current (A) of the thermoelectric cooling module, and T_h and T_c are the hot-side and cold-side temperatures of the thermoelectric cooling module, respectively. $\overline{\alpha }$ (V/K), $\overline{K}$ (W/K), and $\overline{R}$ (Ω m) denote the Seebeck coefficient, thermal conductivity, and the resistance of the thermoelectric cooling module, respectively. In a TEC system, the lower the temperature of the cold end, the greater the amount of cold that can be produced. However, the efficiency of the system decreases at low temperatures due to the thermal and electrical conductivity limitations of the thermoelectric material. As the cold-end temperature decreases, the cooling effect diminishes. Due to the nonlinear variation of the Seebeck coefficient, thermal conductivity, and resistance of the thermoelectric cooling module with temperature, it is assumed for convenience that these parameters are temperature independent.

The $\overline{\alpha }$, $\overline{K}$, and $\overline{R}$ of the thermoelectric cooling module can be calculated by Equations (2)–(4) [44].

$$\overline{\alpha }={\displaystyle \frac{Q_{max}\left(T_{\mathrm{h}}-\Delta T_{max}\right)}{T_{\mathrm{h}}^2{I}_{max}}}$$

(2)

$$\overline{K}={\displaystyle \frac{{\left(T_{\mathrm{h}}-\Delta T_{max}\right)}^2}{T_{\mathrm{h}}^2}}{\displaystyle \frac{Q_{max}}{\Delta T_{max}}}$$

(3)

$$\overline{R}={\displaystyle \frac{{\left(T_{\mathrm{h}}-\Delta T_{max}\right)}^2}{2T_{\mathrm{h}}^2}}{\displaystyle \frac{Q_{max}}{I_{max}^2}}$$

(4)

The VCR system transfers energy through a phase change in the refrigerant. The basic cycle of the system is shown in Figure 5. For the purpose of theoretical calculations, the following assumptions were made:

(1)

There are negligible pressure drops and heat loss within the evaporator, condenser, and connecting piping;

(2)

The system operates under steady-state flow conditions, ensuring a steady flow of working fluid;

(3)

The expansion process at the throttle valve is considered adiabatic, and there is no heat exchange with the surroundings.

The cooling capacity $Q_{\mathrm{c}, \mathrm{VCR}}$(W) of a VCR system can be defined as shown in Equation (5).

$$Q_{\mathrm{c}, \mathrm{VCR}}=m\left(h_1-h_4\right)$$

(5)

where m (kg s⁻¹) is the mass flow rate of the refrigerant, and h₁ (kJ kg⁻¹) and h₄ (kJ kg⁻¹) represent the refrigerant-specific enthalpies at the inlet and outlet of the evaporator. The lower the temperature of the evaporator, the higher the required compressor operating pressure, and, therefore, the efficiency of the system is affected. At low temperatures, the evaporator absorbs more heat and the cooling capacity of the system usually increases, but more compression power is required.

COP is a measure of refrigeration equipment, which indicates the ratio of the cooling capacity that the equipment can produce while consuming a certain amount of power. The specific formula is shown in Equation (6).

$$\mathrm{COP}={\displaystyle \frac{Q_{\mathrm{c}}}{P}}$$

(6)

where $Q_{\mathrm{c}}$ is the cooling capacity (W) of the system, which can be a TEC system, a VCR system, or a hybrid system; P is the input power of the system (W).

Figure 5. Pressure–enthalpy diagram of the VCR system.

Figure 5. Pressure–enthalpy diagram of the VCR system.

2.4. Uncertainty

In the experiment, several parameters were measured, including current, voltage, temperature, and electric power, which are stabilized values. The absolute uncertainties of the measured values were as follows: current uncertainty (0.25%), voltage uncertainty (0.5%), temperature uncertainty (1 K), and electric power uncertainty (5 mW). The combined uncertainties of the calculated values were determined using 95% confidence intervals. The absolute uncertainty of the COP was 0.02.

3. Results and Discussion

Since there is no heat exchange between the two refrigeration systems in the configuration I structure, which is a superposition of two refrigeration compartments, the results of the individual refrigeration experiments of the two forms of refrigeration are the same as the configuration, and the following are analyzed separately for VCR and TEC systems.

3.1. Experimental Study of VCR Systems

The experiment studied the constructed VCR system, which has a starting power of 58 W for the compressor and a cooling compartment size of 30 L. The internal temperature fluctuations of the VCR system can be adjusted using a temperature control chart, which regulates the range of internal temperature variations. Figure 6 illustrates the relationship between temperature changes over time at various positions during its operation. In the first 0–22 min, the VCR compartment temperature gradually decreases from the ambient temperature of 21.5 °C to −18 °C. Between 22 min and 29 min, the internal temperature rises from −18 °C to 0 °C, and between 29 min and 40 min, it decreases again from 0 °C to −18 °C. Subsequently, a cyclical start–stop pattern is maintained for the temperature changes in the 29–40 min range, with each cycle lasting 11 min and a temperature fluctuation of 18 °C per cycle. The maximum and minimum temperatures inside the box and the extent of temperature fluctuations can be adjusted using the temperature controller. The evaporator temperature rapidly decreases in the first 0–2 min, the rate of cooling gradually slows down from 2 min to 22 min, stabilizing, with the lowest temperature reaching −27 °C. From 22 min to 29 min, the compressor stops, and the surface temperature of the evaporator gradually increases. After reaching 0 °C, the temperature decreases again between 29 min and 40 min, stopping after reaching −27 °C, followed by a regular cyclical start–stop pattern from 22 min to 40 min. The inlet and outlet temperatures of the condenser exhibit a cyclical pattern. The inlet temperature can reach a maximum of 38.7 °C, resulting in a maximum temperature difference of 17.2 °C relative to the environment. The outlet temperature can reach a maximum of 30.9 °C, with a corresponding temperature difference of 9.4 °C from the environment. The VCR system has a cooling capacity of 133 W and a COP of 2.3.

After the compressor begins operating, the reduction in the evaporator and refrigeration compartment temperatures is primarily due to the drop in refrigerant temperature within the evaporator. The phase change of the liquid refrigerant, which absorbs heat, leads to the temperature decrease in the evaporator. Additionally, heat from the insulated compartment air is transferred to the evaporator, further lowering its temperature. Initially, the condenser temperature shows an upward trend after a slight decrease. This is because, when the VCR system starts, the refrigerant temperature is high, causing an increase in heat within the condenser. As the refrigeration cycle continues, the refrigerant temperature gradually decreases, which in turn causes the condenser temperature to drop. The condenser inlet temperature is higher than the outlet temperature because the refrigerant is compressed by the compressor, raising its temperature. As the refrigerant dissipates heat in the condenser, the outlet temperature is lower than the inlet temperature. The rate of temperature rise in the evaporator is greater than in the air in the compartment because the evaporator is made of copper tubing, which has a lower specific heat capacity than air. This results in a faster temperature change in the evaporator compared to the compartment air.

By adjusting the temperature control system to eliminate cyclical starts and stops, the system continuously cools at its rated refrigeration power. The compressor operates consistently at a constant power and speed, maintaining an input power of 58 W. As shown in Figure 7, which represents an experimental setup of the VCR system, the temperature in the refrigeration compartment continues to decrease and then stabilizes. The rate of temperature decrease gradually slows, reaching a minimum of −24.6 °C. The temperature of the evaporator initially drops rapidly in the first 2 min, then the rate of decrease gradually slows, reaching a minimum of −27.9 °C. The inlet and outlet temperatures of the condenser initially rise and then decrease, stabilizing at 30.2 °C and 24.5 °C, respectively. The temperature differences generated compared to the environment within the insulated compartment, at the condenser inlet and outlet, and at the evaporator are 46.1 °C, 8.7 °C, 3 °C, and 49.4 °C, respectively.

3.2. Experimental Study of TEC Systems

3.2.1. Testing of Thermoelectric Cooling Modules

The selection of thermoelectric cooling modules was tested and analyzed, focusing on TEC1-12705 and TEC1-12706, which are common thermoelectric cooling modules in the market. The testing involved placing these thermoelectric cooling modules in an ambient temperature of 21.5 °C with forced air cooling for heat dissipation. As illustrated in Figure 8, the relationship between input voltage and load current for the two types of thermoelectric cooling modules indicates a proportional linear relationship. Both TEC1-12705 and TEC1-12706 exhibit a linear correlation between current and voltage, with an increase in input voltage leading to an increase in input current. A notable observation is that, at the same voltage, the input current flowing through TEC1-12705 is smaller compared to TEC1-12706. The internal resistance of TEC1-12706 is stated as 3.3 Ω, while the internal resistance of TEC1-12705 is 3.6 Ω, both measured at the input rated power of 12 V. This suggests that, under the same conditions, the TEC1-12705 has a higher internal resistance than TEC1-12706. Moreover, at the same input voltage, the input current of TEC1-12705 is less than that of TEC1-12706. This implies that the internal resistance of TEC1-12705 is greater than that of TEC1-12706 when operating at the maximum allowable voltage.

Under the same conditions of heat dissipation, Figure 9 depicts the relationship between the hot- and cold-end temperatures of the thermoelectric cooling module concerning current and voltage. It is observed that the cold-end temperature of TEC1-12706 initially decreases and then increases with the rise in input current and voltage. Notably, the lowest temperature of the cold end of TEC1-12706 is achieved at −16.6 °C when the current is 3.17 A and the voltage is 10 V. Similarly, TEC1-12705 exhibits a similar trend, reaching a minimum cold-end temperature of −14.7 °C at a current of 3.35 A and a voltage of 12 V. Comparing TEC1-12706 and TEC1-12705 under the same input conditions, the cold-end temperature of TEC1-12706 consistently remains lower than that of TEC1-12705. This discrepancy is attributed to factors such as the material’s Seebeck coefficient, temperatures of the hot and cold ends, semiconductor resistance, and input current. Notably, effective heat dissipation at the hot end significantly influences the cooling capacity at the cold end. Improved heat dissipation results in lower temperatures at the cold end, enhancing overall cooling capacity. As input power increases, the hot-end temperature gradually rises, impacting its heat dissipation efficiency. In the early stages, the Boll patch effect dominates, leading to a decrease in the cold-end temperature with increasing input power. However, as input power further increases, the Joule effect heat production surpasses the cooling capacity of the Boll patch effect. Consequently, in the later stages of the test, the cold-end temperature rises with increased input power. In contrast to the cold-end temperature trend, the hot-end temperature increases with rising input current and voltage. For instance, TEC1-12706 exhibits a hot-end temperature of 40.9 °C at 4.7 A and 16 V, while TEC1-12705 registers a higher temperature of 45.3 °C at 4.46 A and 16 V. This consistent pattern indicates that under identical current and voltage conditions, the hot-end temperature of TEC1-12705 remains higher than that of TEC1-12706. Notably, further increasing the input voltage may lead to overheating at the hot end, exceeding the thermoelectric cooling module ’s maximum allowable current and resulting in chip failure. This overheating phenomenon is closely tied to factors such as the material’s Seebeck coefficient, temperatures at the hot and cold ends, semiconductor resistance, and input current. The increase in input current exacerbates the Joule effect, leading to a significant rise in hot-end temperature.

In this paper, the thermoelectric cooling module is used for the volume size of 18 L compartment refrigeration; two kinds of thermoelectric cooling module power are similar under the premise of selecting the cold-end temperature, which can reach a lower temperature of the thermoelectric cooling module. According to the results of the experimental test, it can be seen that the TEC1-12706 thermoelectric cooling module is suitable for the system’s cold source.

3.2.2. Analysis of TEC Characteristics and Influencing Factors

Based on the refrigeration sheet test results, it is evident that the thermoelectric cooling module’s cold-end temperature initially decreases and then increases with the augmentation of input power. Consequently, this section delves into an experimental study examining the relationship between the temperatures of the hot- and cold-end fins in the TEC compartments and the refrigeration compartments over time at three distinct power levels: 10 W, 30 W, and 50 W. In Figure 10, the behavior of TEC1-12706 at an input power of 10 W under ambient conditions of 21.5 °C is depicted. The temperature at each position over time reveals a gradual smoothing trend. The order of temperatures from high to low is as follows: thermoelectric cooling module hot-end-fin temperature, ambient temperature, box temperature, and cold-end-fin temperature. The hot-end-fin temperature experiences a higher warming rate in the first 5 min, stabilizing at approximately 24.1 °C. The cold-end-fin temperature, on the other hand, exhibits a greater initial temperature drop in the first 5 min, followed by a gradual reduction in the rate of decline and eventual stabilization at around 9.7 °C. The temperature inside the box steadily decreases, with the cooling rate gradually diminishing. The lowest temperature reaches 13.3 °C, resulting in a temperature difference of 8.2 °C from the ambient temperature. The test results indicate that the box temperature tends to stabilize with minor fluctuations after initial cooling. In Figure 11, post-stabilization temperature fluctuations within the box are depicted. The temperature fluctuations, as expressed in Equation (1), are observed to be ±0.2 °C or less. Notably, even with a low input power of 10 W, the TEC demonstrates effective refrigeration. This is significant, as typical VCR refrigeration systems usually have a starting power greater than 10 W. The results imply that at lower power inputs, the TEC can achieve noticeable refrigeration effects. Specifically, at 10 W power, it successfully produces an 8.2 °C temperature difference in the 18 L refrigeration compartment compared to the ambient temperature (Equation (7)).

$$\Delta Tf=\pm (Tfmax-Tfmin)/2$$

(7)

where $\Delta Tf$ is the temperature fluctuation, °C. $Tfmax$ is the maximum temperature value of the equipment during the measured time of the experiment, °C. $Tfmin$ is the minimum temperature value of the equipment during the experimental measurement time, °C.

As illustrated in Figure 12, showcasing the temperature-versus-time curves for the thermoelectric cooling module at 30 W and 50 W power, the hot- and cold-end-fin temperatures, as well as the refrigeration compartment temperatures, exhibit a consistent pattern as observed at 10 W power. Specifically, the hot-end-fin temperatures stabilize at 26.4 °C and 28.1 °C, the cold-end-fin temperatures reach equilibrium at 5.5 °C and 6.7 °C, and the refrigeration compartment temperatures maintain stability at 11.1 °C and 11.8 °C. Additionally, the temperature difference between the cooling compartment and ambient temperature is noted as 10.4 °C and 9.7 °C. The experimental results indicate that with an increase in power, the internal temperature of the box does not consistently decrease. A comparison between the 50 W and 30 W thermoelectric cooling module reveals a temperature difference of 0.7 °C in the refrigeration compartment compared to ambient temperature, indicating a smaller refrigeration capacity for the 50 W thermoelectric cooling module.

The analysis presented in Figure 13, comparing the equilibrium temperatures of the TEC compartment thermoelectric cooling module at different power levels, reveals important trends in the TEC system. Notably, the hot-end-fin temperature increases with the rise in input power, while the cold-end-fin temperature initially decreases and then increases with the increase in input power. This behavior is attributed to the thermoelectric cooling module, where the lower temperature of the cold end influences the refrigeration compartment temperature. Consequently, the refrigeration compartment follows a similar pattern of change as the thermoelectric cooling module cold-end fins. Figure 14 illustrates the COP in relation to the change in input power. It is evident that as the power of the thermoelectric cooling module increases, the COP gradually decreases. At an input power of 10 W, the COP reaches 0.53, the cooling capacity is 3.6 W, and the COP is 0.12 when the input power is 30 W. But when the input power surpasses 50 W, the COP drops to less than 0.1. The observed decrease in COP with increasing input power signifies a reduction in refrigeration energy efficiency. This diminishing efficiency is a significant challenge and a primary obstacle to the widespread commercialization of TEC technology. The findings emphasize the need for further research and development to address efficiency concerns and enhance the feasibility of TEC systems for broader applications.

Based on the conducted test, it is observed that for a single thermoelectric cooling module, the optimum power results in a maximum temperature difference of 10.4 °C between the refrigeration compartment and ambient temperature. To achieve further temperature reduction, improvements in hot-end heat dissipation methods, such as water-cooled heat or heat pipe dissipation, can be employed. However, these methods may lead to an increase in system volume, and there is a limit to the cooling capacity of a single refrigeration chip. For applications requiring lower refrigeration temperatures, a common approach involves increasing the number of thermoelectric cooling modules, as well as enhancing the hot- and cold-end heat sinks and cooling fans. While this strategy leads to a further reduction in refrigeration temperature, it comes at the cost of increased system power consumption. In Figure 15, when using a single thermoelectric cooling module at 10 W, 30 W, and 50 W power, temperature differences of 7.5 °C, 10.4 °C, and 10 °C can be achieved, respectively. Conversely, with five thermoelectric cooling modules in the system at 50 W, 150 W, and 250 W power, temperature differences of 17.4 °C, 24.6 °C, and 22 °C are attainable. Importantly, under the same power input conditions, increasing the number of thermoelectric cooling modules enhances the refrigeration effect. Notably, using three 10 W thermoelectric cooling modules concurrently yields a better refrigeration effect than employing a single 30 W thermoelectric cooling module, emphasizing the impact of power on the energy efficiency ratio of the thermoelectric cooling module.

3.3. Experimental Study of Configuration II

Experimental tests were conducted on Configuration II with both systems operating in cooling mode simultaneously. The material dimensions of the cooling box remained unchanged, with the TEC system input power at 2 W and the VCR system operating at its rated power. Figure 16 illustrates the temperature variation over time at different locations within the Configuration II TEC system. It is evident that under the condition of a 2 W input power, a temperature difference of 18.8 °C can be achieved within the TEC compartment. When the TEC system operates in cooling mode independently, even with an input power of 30 W, the temperature difference within the thermal insulation compartment is only 10.4 °C. Therefore, Configuration II significantly enhances TEC system performance under simultaneous cooling conditions. The cooling capacity and COP of the TEC are significantly higher compared to when the TEC system operates in cooling mode independently. However, the temperature fluctuation in the TEC system increases from ±0.2 °C in Configuration I to ±0.5 °C in Configuration II. The experimental results indicate that in Configuration II, compared to Configuration I, both the cooling capacity and COP of the TEC system significantly increase, with a slight rise in temperature fluctuations. Additionally, under Configuration II, the VCR system experiences a more gradual cooling process due to the absorption of the TEC system’s hot-end temperature by its evaporator and subsequent release by the condenser. This results in an extension of the time taken for the VCR system to cool from ambient temperature to its lowest temperature, from 22 min to 26 min, compared to when the VCR system operates independently. The start–stop cycle time extends from 11 min per cycle to 15 min per cycle. In addition, with a TEC system input power of 30 W, the cooling capacity of the VCR system is 104 W, and the COP of the hybrid system is 1.17.

The input power of the TEC system under Configuration II is increased from 2 W to 10 W, as shown in Figure 17. As the input power to the TEC system increases, the temperature within the TEC compartment decreases further, reaching a temperature difference of 25.2 °C. Consequently, the time required for the VCR system to cool from ambient temperature to its minimum value extends to 37 min, with a corresponding increase in the cycle time to 28 min. This increase in cooling time can be attributed to several factors. In the input power range of 0–10 W, the TEC cooling capacity improves as the input power increases. Specifically, the poloidal effect of the thermoelectric cooling module intensifies with increasing input power, which results in a further reduction in the compartment temperature. However, the increased input power also amplifies the Joule heating effect within the thermoelectric cooling module, raising the temperature at the hot end. This increased heat at the hot end is absorbed by the evaporator, leading to a longer time for the VCR system to reach its minimum temperature.

As shown in Figure 18, the relationship between the temperature at each position and time under the TEC system input powers of 30 W and 50 W in Configuration II reveals that, when the input power is in the range of 30–50 W, the temperatures at each position within the TEC and VCR system no longer exhibit periodic start–stop behavior. This phenomenon occurs because according to the temperature control table, the VCR regulates its compressor start and stop to maintain the compartment temperature within the preset range. However, when the TEC system in Configuration II releases heat to the lower compartment, the temperature in the compartment rises, preventing the VCR system from achieving the desired maximum temperature difference. As a result, the VCR system ceases to operate in a cyclic manner. The results also highlight the importance of considering the heat release from the hot end of the thermoelectric cooling module at high input power levels when designing the system. This heat release can affect the VCR system’s ability to maintain periodic start–stop cycles and should be accounted for in future designs.

3.3.1. The Impact of VCR System Temperature Fluctuations on TEC System in Configuration II

Currently, commercial refrigerators equipped with fixed-speed compressors in VCR systems exhibit internal temperature fluctuations of about 6 °C. Therefore, this section investigates the impact of VCR temperature fluctuations on the TEC system under Configuration II. Gradually reducing the temperature fluctuations inside the VCR compartment, the temperature was maintained at ranges from −18 to 0 °C, from −18 to −6 °C, from −18 to −12 °C, and under no fluctuation conditions. As shown in Figure 19, the test results indicate that with the reduction in temperature fluctuations of the VCR system, the temperature fluctuations in the TEC cooling compartment gradually decrease. Even without any control algorithm involved, when paired with the current commercial VCR systems having 6 °C temperature fluctuations, the temperature fluctuation in the cooling compartment remains relatively small, maintained within ±0.3 °C.

3.3.2. Optimal Operating Condition Analysis

Increasing the input power of the TEC system has two effects. First, an increase in TEC system input power initially increases and then decreases the cooling capacity of the TEC system. Second, the temperature in the VCR compartment rises due to the absorption of heat generated by the TEC compartment. Therefore, it is necessary to determine the optimal input power range for TEC system, where the TEC compartment should achieve significant cooling while minimally affecting the VCR compartment. In the experiment, the VCR system was set to operate continuously at a constant power, with varying input power to the TEC system. Figure 20a shows the effect of different TEC system input powers on the VCR compartment’s cooling, with thermoelectric cooling module input power gradually increasing from 2 W to 30 W. Test results indicate that as the input power of the thermoelectric cooling module increases, the lowest achievable temperature in the VCR compartment rises, and the cooling rate gradually slows down. As shown in Figure 20b, the lowest temperature in the VCR compartment with the thermoelectric cooling module at 0 W–8 W input power increases from −24.4 °C to −17.8 °C.

Figure 21 shows the temperature of the TEC compartment over time for different input power levels of the TEC system in Configuration II. As the input power of the thermoelectric cooling system increases within the 0–8 W range, the minimum temperature reached by the TEC compartment decreases. Specifically, the cooling capacity of the thermoelectric cooling system improves as the input power increases within this range. The lowest temperature achieved in the compartment is −1.5 °C at an input power of 2 W, −5 °C at 8 W, −2.4 °C at 30 W, and 0 °C at 50 W. This trend can be explained by the relationship between the cooling capacity of the thermoelectric cooling module and the input power, as well as the heat dissipation from the hot end of the thermoelectric cooling module. In the 8–50 W range, as input power increases, the heat dissipation condition at the hot end deteriorates, leading to a decrease in the overall cooling capacity. Given that the TEC system provides a significant cooling effect, while the impact on the VCR system is minimized, the optimal input power range for cooling in Configuration II is determined to be 0–8 W. Additionally, there is a linear relationship between the temperature difference between the cold and hot ends of the thermoelectric cooling module and its cooling capacity. For a given temperature difference, the cooling capacity increases with higher input current.

As shown in Figure 22, under the condition of the same input current, with the decrease of the temperature of the hot and cold ends, its cooling capacity gradually increases, and the maximum cooling capacity can reach 45 W at the input current of 5 A and the temperature difference between the hot and cold ends of 5 °C.

3.4. Configuration III Experimental Study

To investigate the interplay between TEC systems and VCR systems under Configuration III, an experimental setup was constructed. Part of the evaporator was installed in the TEC compartment, with the TEC system operating at its maximum cooling power of 30 W.

The container had a capacity of 18 L. Figure 23 illustrates the temperature changes over time within the cooling compartment under three different conditions: when the TEC system is operating alone, when the VCR system is functioning independently, and when both systems are cooling simultaneously. The temperatures for these respective conditions reached 11.1 °C, 5.9 °C, and −0.2 °C. It was also observed that the TEC system alone took 40 min to stabilize at its lowest temperature of 11.1 °C, while the VCR system alone reached 11.1 °C in 13 min, and the hybrid systems took 9.6 min to reach the same temperature. In addition, for the hybrid system in configuration III, the VCR system has a cooling capacity of 139 W, the TEC system has a cooling capacity of 3.4 W, and the COP value is 1.55. This demonstrates that under Configuration III, simultaneous cooling allows for the combined cooling capacity of both systems, significantly enhancing the cooling effect, reducing the time taken to reach the same temperature, and further increasing the temperature difference inside the container. Under fixed cooling capabilities of the VCR system, the greater the cooling capacity of the thermoelectric cooling module, the larger the overall cooling capacity of the hybrid system.

3.5. Summary of Three Different Configurations

Based on the conducted experiments, it is evident that three configurations demonstrate the benefits of VCR and TEC. In all three configurations, the compartment temperature can be adjusted by using either or both, creating a temperature gradient to meet the temperature required for different goods or things. Particularly, the refrigerator equipped with a hybrid refrigeration system proves to be versatile and applicable to a broader spectrum of scenarios, including solar power supply, vehicle usage, outdoor settings, and more. The hybrid refrigeration system is capable of flexibly switching or adjusting the working mode of the system according to different load demands. This structure allows the system to make diverse configuration choices, and this hybrid system provides different cooling capacities and required temperature through different configurations (configurations I, II, and III). In this hybrid refrigeration system, users can set the system configuration according to the optimal temperature at which they need to preserve their goods. The hybrid refrigeration system has good commercial potential to decarbonize and reduce power costs in the future using solar energy.

The findings from all three configurations serve as valuable references for optimizing refrigeration systems.

Configuration I is a baseline configuration that represents the cooling effectiveness of the TEC or VCR when used alone. Although it is reliable, it does not represent the optimal solution in terms of cooling capacity. Configuration I features two systems with no significant energy exchange. Using a 10 W power single TEC, it achieves an 8.2 °C temperature difference compared to the ambient temperature. And the temperature in the TEC system, which creates a different temperature range from the VCR system, allows for the storage of different goods in the same equipment. The temperature in the compartment can be adjusted by varying the TEC input power, resulting in minimal fluctuations (±0.2 °C). By adding a TEC compartment, it provides precise temperature control, low thermal inertia, and the ability to both refrigerate and heat in the VCR refrigeration system with a fixed-speed compressor. The disadvantage of this configuration is that the heat from the semiconductor cooling system is discharged directly into the environment, resulting in a large temperature difference between the two ends of the thermoelectric cooling module, leading to a low energy efficiency ratio of the thermoelectric cooling module.

Configuration II combines two systems for simultaneous cooling and energy exchange, thus enhancing the cooling range and energy efficiency ratio of TEC. Configuration II improves the overall stability and temperature control accuracy of the system. However, its cooling time is slightly longer than that of a VCR alone, which may affect some occasions where rapid cooling is required. Configuration II supports three cooling conditions: TEC alone, VCR alone, and hybrid TEC and VCR. In the hybrid condition, the COP of the TEC system is significantly increased, while the COP of the VCR system is decreased. In Configuration II, the COP is 1.17 for the hybrid cooling system consisting of TEC and VCR. In Configuration II, a temperature difference of 18.8 °C can be achieved with a TEC input power of 2 watts, whereas in Configuration I, a single thermoelectric cooling module is unable to achieve this temperature difference even under optimal conditions, requiring an increase in the number of cooling modules and a significant increase in the power requirement (three thermoelectric cooling modules require 90 watts of power). Although Configuration II may increase the energy consumption of the VCR system, this effect is smaller at lower power levels, and the refrigeration demand can still be met. Configuration II, by contrast, is better suited for applications requiring a constant low temperature, such as refrigerated transport and precision instrument cooling. Additionally, Configuration II has the potential to be integrated with renewable energy sources, such as solar energy, to create a sustainable cooling system that supports energy sustainability. As a result, Configuration II offers significant advantages over Configuration I in terms of energy efficiency, reducing electricity demand, and enabling more effective refrigeration. However, this configuration also has some limitations. Compared to Configuration I, Configuration II has a higher commercialization potential and is more scalable. One concern is that more careful planning and testing are needed due to the increased likelihood of compatibility issues arising from integrating the two systems. Despite these challenges, the ability to stabilize refrigeration temperature fluctuations highlights its huge potential for widespread commercial application.

Configuration III involves both systems cooling the same compartment, offering more sensitive temperature control with the TEC. The most significant advantage of Configuration III is the ability to notably reduce the time taken to reach the target temperature. Experimental results show that the integrated system requires substantially less time than when the TEC and VCR are used separately. This configuration excels in temperature regulation response speed and is particularly suitable for applications with stringent cooling time requirements. The hybrid refrigerator excels in refrigeration capacity, significantly reducing the time needed to lower the temperature compared to a single system. Specifically, the TEC alone takes 40 min to reach 11.1 °C, the VCR alone takes 13 min, and the hybrid refrigeration of both systems takes 9.6 min, and the COP value is 1.55. Configuration III allows the coupling of the cooling capacities of the two refrigeration systems compared to a single refrigeration system, reducing the time required to achieve the target temperature.

4. Conclusions

TEC systems offer precise temperature control with low power consumption and do not rely on refrigerants, making them environmentally friendly. In contrast, VCR systems deliver higher cooling capacity at high loads but depend on refrigerants, which can harm the environment. By combining these two technologies into a hybrid system, cooling capacity is enhanced, temperature fluctuation is minimized, and the environmental impact is reduced. The development of hybrid refrigeration systems contributes to sustainability.

This study introduced the development and evaluation of a hybrid refrigeration system that combines TEC and VCR to meet diverse refrigeration requirements across different scenarios. Three TEC–VCR hybrid configurations were proposed, and their refrigeration characteristics were systematically evaluated through experimental testing of both TEC and VCR systems. The findings demonstrate the system’s versatility and potential for applications in solar-powered, vehicle-mounted, and outdoor refrigeration.

Furthermore, the proposed hybrid refrigeration system can be integrated with renewable energy sources, such as solar energy, wind energy, etc., to promote sustainability, reduce dependence on fossil fuel, and drive the advancement of refrigeration technologies toward greater environmental responsibility and efficiency.

This study yielded the following conclusions:

• Testing the TEC system at 10 W, 30 W, and 50 W showed temperatures of 8.2 °C, 10.4 °C, and 9.7 °C, respectively, with minimal fluctuations (±0.2 °C). Increasing the number of thermoelectric cooling modules enhanced cooling performance; however, efficiency declined as input power exceeded 50 W. The VCR system exhibited periodic temperature fluctuations, with a cycle time of 11 min and variations of 0.5 °C. Under stable conditions, the minimum temperature reached −24.6 °C. Additionally, at an input power of 30 W for the TEC system, the COP is 0.12. In comparison, the VCR system has a COP of 2.3.
• The study demonstrated a significant improvement in TEC performance, achieving an 18.8 °C temperature difference at only 2 W input power compared to 10.4 °C with TEC only at 30 W; COP increased from 0.1 to 1.17, although temperature fluctuations rose slightly to ±0.5 °C. The input power for Configuration II ranged from 2 W to 8 W, achieving temperatures from −1.5 °C to −5 °C in the TEC compartment with minimal impact on the VCR performance.
• Configuration III demonstrated good performance, reducing the time required to reach the target temperature. Reaching a temperature of 11.1 °C took 40 min with TEC only, 13 min with VCR only, and 9.6 min with a hybrid system. Hybrid systems in Configuration III have a cooling factor of up to 1.55.
• Future research should concentrate on advancing sustainable development in the refrigeration sector. This will include scaling up thermoelectric cooling modules, optimizing hybrid systems to ensure commercial viability, and integrating renewable energy sources for comprehensive testing. In addition, the temperature inside the compartments will be further reduced, refrigeration at different temperatures will be tested, and a wider range of storage temperatures will be designed. An intelligent temperature control system is also expected to be developed to automatically adjust system operation under different loads and conditions.

In summary, the proposed hybrid refrigeration system demonstrates significant advantages in energy efficiency, cooling capacity, and adaptability, making it a promising solution for renewable energy-based refrigeration applications.