Abstract:

This paper discusses the current status of energy-saving technologies for refrigerators. Based on an analysis of the main usable energy losses in the refrigeration system, it preliminarily discusses how the compression/injection mixed refrigeration cycle is an effective way to reduce the usable energy consumption losses in refrigerator cooling systems. The compression/injection mixed refrigeration cycle has advantages such as high volumetric cooling capacity, simple structure, ease of implementation, and the ability to further reduce system noise, making it a feasible method for further energy saving in refrigerator cooling systems.

Keywords: Refrigerator, Compression/Injection, Usable Energy Loss, Energy Saving

With the increasing popularity of household refrigerators, products are developing towards larger capacities, multiple compartments, and user convenience. At the same time, the energy consumption of refrigerators is accounting for an increasingly higher proportion of total household energy consumption, with refrigerators now consuming half of the total electricity used by residents. Additionally, refrigerators generate pollution during production, use, and eventual disposal. In recent years, people have recognized that the indirect impact of electricity consumption by refrigerators over long-term use is significant, as the indirect harmful substances generated from electricity consumption account for about 90% of the harmful substances produced throughout the production, use, and disposal processes. Based on current electricity consumption levels and production capacity estimates, it is projected that refrigerators produced in China over the next decade will consume 601 billion kilowatt-hours of electricity during their expected lifespan, necessitating an increase of 5,700 megawatts in power generation capacity. This will significantly exceed the current power generation load in China. Therefore, energy saving in refrigerators is of profound significance for both the economy and environmental protection. Improving refrigerator energy efficiency has received widespread attention from governments and refrigerator manufacturers worldwide. Below, we will explore the main energy-saving measures currently applied to refrigerators and further discuss methods for enhancing energy efficiency.

1. Current Analysis of Refrigerator Energy-Saving Technologies

1.1 Research and Improvement of Insulation Layers

For household refrigerators, heat loss from the cabinet and energy consumption during compressor operation play a decisive role in the overall energy consumption of the appliance. Therefore, researchers are continuously improving compressor performance and efficiency while also making unremitting efforts to enhance the thermal insulation properties of refrigerator cabinets. Designers typically increase wall thickness to enhance the insulation performance and energy-saving indicators of refrigerators. Under the same ambient temperature, better insulation results in slower heat transfer, whereas poorer insulation leads to faster heat transfer. To maintain a certain cooling capacity, the compressor’s operating frequency must increase, resulting in higher energy consumption. Thus, the quality of the insulation layer directly affects the rate of heat transfer, determining the overall energy consumption. Currently, the vast majority of refrigerators use cyclopentane as a foaming agent and appropriately increase the foaming space, significantly improving insulation performance. Additionally, utilizing microcellular foaming technology to reasonably increase insulation layer thickness is effective. The thickness of the insulation layer is determined based on the temperature differences between various positions inside and outside the refrigerator, ensuring uniform temperatures across all compartments. Strong door seals are also employed to control cold air leakage, further enhancing energy-saving effects. However, increasing wall thickness to improve insulation performance and energy-saving indicators is not without drawbacks. The increase in wall thickness raises the K value of the foaming layer and reduces its density. Therefore, the chief scientist from Bayer has advised manufacturers to carefully consider the impact of thickness on the performance indicators of polyurethane insulation materials while striving to meet new energy consumption standards at low costs.

1.2 Development of Computer Temperature Control Technology

Computer temperature control technology refers to the use of microcomputer chips and multiple sensors to monitor, detect, and analyze various states and temperatures within the refrigerator. The collected data is transmitted to the microcomputer chip. The chip synthesizes and analyzes the information to derive precise numerical values and issues operational commands, optimizing the settings for freezing and refrigeration. The primary advantage is the ability to set different temperatures for the refrigeration and freezing compartments based on needs, thereby adjusting the compressor’s operating frequency to achieve quiet operation and energy savings.

1.3 Adoption of New Throttling Devices

In conventional vapor compression refrigeration cycles, throttling devices such as expansion valves, capillary tubes, and short tube nozzles are important components. The throttling process in the cycle is irreversible, and the enthalpy remains unchanged before and after throttling, leading to inherent thermodynamic losses. It has long been recognized that replacing the expansion device with a power-generating device can reduce these losses and improve the coefficient of performance (COP) of the vapor compression refrigeration cycle. In 1999, CIodic invented a refrigeration system using a micro turbine for throttling. The mechanical work generated by the refrigerant expansion can drive one or more heat exchanger fans. Tests conducted on a 290-liter household air-cooled refrigerator using a turbine expansion device instead of a capillary tube showed that, assuming an expansion device efficiency of 80%, the COP increased by 11% due to the use of the turbine expansion device, generating 112W of mechanical work. It was also found that increasing the degree of supercooling enhances cooling capacity while reducing the mechanical work produced by the turbine expansion device. Therefore, ensuring that the mechanical work generated by the turbine expansion device can provide sufficient forced convection is crucial. To enhance the mechanical work produced by the turbine expansion device, the best approach is to reduce supercooling, thereby lowering the system’s COP. In this case, the energy savings come from replacing electrical energy with mechanical work to drive the fan. However, these devices are costly and prone to damage under harsh operating conditions of two-phase flow, making practical application challenging.

1.4 Application of Variable Frequency Technology

It is widely recognized that the most effective way to produce energy-saving refrigerators is to use variable frequency technology. Variable frequency refrigerators significantly improve cooling efficiency compared to conventional refrigerators. This is achieved by using dedicated variable frequency compressors and drivers to adjust the compressor’s speed, allowing it to vary between 2000 and 4000 RPM. When deep freezing or ice-making is required, the variable frequency compressor can reach 3600 RPM (60Hz) to quickly lower the internal temperature, ensuring rapid cooling. When the internal temperature is relatively low (for example, at night or when the door is not opened during the day), the variable frequency compressor can operate at low speeds, quickly reducing to 2700 RPM (45Hz), entering a super-quiet energy-saving state. When the food temperature reaches the normal set temperature, the compressor operates at 3000 RPM (50Hz). It is reported that using variable frequency technology improves freezing capacity by 20% compared to conventional refrigerators, with a significant reduction in daily energy consumption, achieving about a 60% increase in energy-saving efficiency. Thus, variable frequency refrigerators reduce the start-stop process, improve the energy efficiency and mechanical efficiency of the compressor, and enhance the cooling efficiency of the refrigerator. Therefore, the application of variable frequency technology greatly improves the energy-saving level of refrigerators. However, currently, variable frequency refrigerators are priced about 600 yuan higher than conventional refrigerators, and this additional cost ultimately falls on consumers. Consumers are paying a high price for a mere 0.25-degree energy-saving feature. Therefore, it is necessary to develop energy-saving technologies for refrigerators that do not rely on variable frequency technology, which can alleviate the economic burden on consumers while ensuring similar or even superior energy-saving effects compared to variable frequency technology.

1.5 Research on Multi-Cycle Systems

For refrigerators using dual-cycle refrigeration systems, the refrigeration compartments and freezing compartments are two independent refrigeration circuits. This allows for precise temperature control in each compartment, meeting the needs for food classification preservation and enhancing freshness while achieving quiet energy-saving operation. The working principle is that after the refrigerator starts, the solenoid valve first connects the capillary tube of the refrigeration compartment, causing the temperature to drop rapidly. Once the refrigeration compartment reaches the set temperature, the solenoid valve then connects the capillary tube of the freezing compartment for concentrated refrigeration. When the freezing compartment reaches the set temperature, the compressor stops. Subsequently, whenever the temperature of any compartment exceeds the set temperature, the compressor will start and connect to the corresponding capillary tube for refrigeration, ensuring reasonable distribution of cooling capacity and higher cooling efficiency, thus optimizing the refrigerator’s cooling performance using computer simulation technology.

1.6 Conversion Valve Technology

Some refrigerator manufacturers have adopted self-developed energy-saving technologies, first equipping them with a dual-valve system. This involves adding a conversion valve based on the existing variable flow valve for the refrigerant, maximizing the utilization of refrigerant in the cooling system. For example, a high-pressure shut-off valve technology at the condenser outlet has a certain energy-saving effect.

1.7 Vacuum Insulation Panel Technology

Using vacuum insulation materials (U-VACUA), this insulation material comprises aluminum alloy thin films and micro-glass fiber cores. By increasing the internal vacuum level, the insulation performance is improved by about twice compared to previous vacuum insulation materials (such as s-VIP). Its innovation lies in adding a separate vacuum layer to the foaming layer rather than relying solely on thickening the foaming layer for insulation, thereby maximizing the retention of cold within the refrigerator and ultimately achieving energy-saving goals. However, the main issue with current vacuum insulation panels is their long-term stability, specifically the maintenance of vacuum levels within the panels. If the vacuum level does not meet the required standards, the insulation effect will be poor, resulting in significantly increased energy consumption for users’ refrigerators. In such cases, most users may not be aware of this situation, which can greatly increase their refrigerator energy consumption levels. In summary, the existing major energy-saving technologies for refrigerators have certain limitations, necessitating the development of more efficient and reliable new energy-saving technologies for refrigerators.

2. Major Usable Energy Losses in Refrigerator Cooling Systems

It is well known that the factors causing usable energy losses in refrigerator cooling cycles mainly include the following four aspects: 1) irreversible heat transfer losses and flow resistance losses in the evaporator; 2) irreversible heat transfer and flow resistance losses in the condenser; 3) throttling losses in throttling devices; 4) irreversible losses in the compressor. Reducing these irreversible losses in cooling equipment has always been a focus of research for improving refrigeration cycles. For conventional household refrigerators, the temperatures of the refrigeration and freezing compartments are maintained at 5°C and -18°C, respectively. However, the evaporation temperatures of the refrigerant in the refrigeration and freezing compartment evaporators are below -25°C. The excessive temperature difference in the refrigeration compartment leads to significant cooling losses. In typical dual-temperature refrigerators, the refrigerant, after passing through a capillary tube, enters the evaporators of the freezing and refrigeration compartments sequentially,maintaining the freezing compartment temperature at -18°C and the refrigeration compartment temperature at 5°C through the same evaporation temperature (approximately -29°C). The excessive temperature difference in the refrigeration compartment results in considerable usable energy losses, and this situation is even more severe for large refrigeration compartments. Using non-azeotropic mixtures of refrigerants is one method to overcome this drawback, but there are certain difficulties due to strict composition requirements. Another effective approach is the dual-compressor dual-circuit scheme being implemented by companies like Haier in China and in Europe and the United States, which has achieved significant energy-saving effects. However, this makes the refrigerator cooling system more complex, increases costs, and does not solve the issue of usable energy loss caused by the direct mixing of two different return temperatures. At the same time, this scheme is economically reasonable for large refrigerators over 500L commonly used in Europe and the United States, but it is not suitable for the widely used 200L refrigerators in China. Therefore, developing new system cycles and researching their energy-saving methods is essential, and the vapor compression/injection mixed refrigeration cycle has advantages in this regard.

3. Methods to Reduce Usable Energy Losses in Cooling Systems

For traditional refrigeration devices, the throttling losses in cooling systems are generally small, and the phase change occurring in the expansion valve makes it generally considered not worthwhile to recover, so recovery is rarely considered. Therefore, developing and researching highly efficient energy-saving refrigeration cycles is of significant importance for energy conservation and environmental protection. In recent years, some researchers have introduced injectors into vapor compression refrigeration cycles, forming compression/injection mixed refrigeration cycles, which have been theoretically proven to have significant energy-saving effects. The compression/injection mixed refrigeration cycle is an effective method to reduce the excessive irreversible heat transfer losses in the refrigeration compartment of dual-temperature refrigerators and improve the performance of refrigerator cooling systems. By employing compression/injection mixed refrigeration cycles, it is possible to recover the throttling losses caused by the throttling in the cooling system, thereby improving the cooling efficiency of the refrigeration cycle.

3.1 Working Principle and Characteristics of Injectors

An injector is a device that mixes two fluids at different pressures, facilitating energy exchange to form a mixed fluid at an intermediate pressure. It does not directly consume mechanical work or electrical energy to increase fluid pressure, temperature, and other parameters. Its structure is simple, low-cost, and has no moving parts, making it suitable for any flow type, including two-phase flows. Injectors have long been used in cooling systems driven by low-grade heat sources, making them an excellent energy recovery method in places with waste heat. Numerous studies have shown that using injectors in cooling systems is indeed a feasible approach. An injector utilizes jet flow turbulence and dispersion to transfer energy and mass in a fluid mechanical mixing reaction device. It typically consists of a nozzle, receiving chamber, mixing chamber, and diffusion chamber. When the working vapor flows through the nozzle, the static pressure energy and thermal energy of the gas convert to kinetic energy, creating a low-pressure environment at the nozzle outlet, which generates suction for the injected fluid. Due to the turbulent dispersion effect of the jet flow boundary layer, it mixes with the surrounding drawn-in fluid to exchange energy, forming a mixed fluid at an intermediate pressure. After entering the mixing chamber, the working fluid and injected fluid equilibrate in speed, usually accompanied by an increase in pressure. Subsequently, the fluid enters the diffusion chamber, where its speed continuously decreases, and kinetic energy is continuously converted to static pressure energy. The injector can achieve two functions: one is to create a low-pressure, low-temperature environment at the inlet of the injected fluid, achieving suction functionality, which can act as a vacuum pump; the other is to provide high-pressure (high-temperature) conditions at the outlet of the diffusion chamber, achieving compression functionality, which can serve as a booster or compression device. The characteristics of injectors include: 1) simple structure, easy manufacturing, and low cost; 2) no moving parts, allowing for long-term reliable operation without maintenance; 3) no direct consumption of mechanical energy; 4) reasonable distribution of energy-carrying fluids.

3.2 Main Connection Methods of Compression/Injection Mixed Refrigeration Cycle

The compression/injection mixed refrigeration cycle is an effective method to reduce excessive irreversible heat transfer losses in the refrigeration compartment of dual-temperature refrigerators and improve the performance of refrigeration systems. In mixed refrigeration cycles, the connection methods between the refrigeration compartment evaporator and the freezing compartment evaporator can be either series or parallel. In the series connection of the freezing compartment and refrigeration compartment evaporators, the refrigerant liquid passes through two stages of throttling and evaporation to achieve different evaporation temperatures for the freezing and refrigeration compartment evaporators. The gases produced are connected by a gas-liquid separator. The different pressure gases mix in the injector and then enter the compressor. This type of cycle has various forms of heat recovery, with the most typical method being to use the steam from the injector outlet to cool the refrigerant liquid before the first stage of throttling. In the parallel mixed refrigeration cycle, the refrigerant liquid is split into two paths for throttling and evaporation. Due to the different throttling degrees, this ensures that the freezing compartment and refrigeration compartment evaporators have different evaporation temperatures. The vapor from the exits of the two evaporators mixes in the injector and enters the compressor. Generally, this type of cycle has three heat recovery options: one is to cool both refrigerant liquids simultaneously; the second is to only cool the refrigerant liquid on the freezing compartment side; the third is to cool the refrigerant liquid on the refrigeration compartment side. The compression/injection mixed refrigeration cycle achieves different evaporation temperatures for the freezing and refrigeration compartment evaporators through dual-path throttling and evaporation, reducing the irreversible heat transfer losses and throttling losses on the refrigeration compartment side compared to traditional simple compression refrigeration cycles. Additionally, the high-pressure vapor exiting the refrigeration compartment evaporator is used in the injector to draw in the low-pressure vapor from the freezing compartment evaporator, thereby increasing the inlet pressure of the compressor. Under conditions without heat recovery, compared to the parallel method, the series connection of evaporators reduces the second-stage throttling losses by separating the saturated vapor from the wet vapor that needs to undergo two-stage throttling using a separator. This not only decreases the second-stage throttling losses but also increases the flow rate of the vapor on the high-pressure side entering the injector, resulting in a higher outlet pressure for the injector than in the parallel method, thus achieving a higher coefficient of performance. The impact of heat recovery is closely related not only to the properties of the refrigerant but also to the connection method of the evaporators and the heat recovery scheme. As the heat recovery temperature increases, the COP of the series connection mode of the evaporator slightly decreases, while the COP of the heat recovery cycle under the parallel connection mode varies, with the heat recovery method on the freezing compartment side being the most advantageous. The vapor compression/injection mixed refrigeration cycle has significant differences from traditional vapor compression refrigeration cycles. The high-temperature, high-pressure liquid refrigerant directly enters the nozzle of the injector, where it accelerates and depressurizes, converting pressure energy into velocity energy. The refrigerant typically exits the nozzle at speeds exceeding the speed of sound, with pressure dropping below the evaporation pressure. The low-pressure, high-velocity refrigerant exiting the nozzle mixes with the low-pressure, low-temperature refrigerant from the evaporator, and then slows down and increases pressure in the diffusion chamber, converting velocity energy back into pressure energy, which raises the pressure of the refrigerant coming from the evaporator. After gas-liquid separation in the gas-liquid separator, the gaseous refrigerant enters the compressor, while the liquid refrigerant returns to the evaporator. This ensures that the suction pressure entering the compressor is higher than the evaporation pressure, thereby improving the cooling efficiency of the refrigeration system.

In summary, based on a systematic analysis of the existing major energy-saving technologies for refrigerators, it is indicated that the adoption of the compression/injection mixed refrigeration cycle is necessary for further energy saving in refrigerator cooling systems. The benefits of employing the compression/injection mixed refrigeration cycle include: 1) increased suction pressure for the compressor, reducing power consumption and usable energy loss; 2) reduced exit velocity from the evaporator, thereby lowering internal flow noise in the refrigeration system; 3) simple structure and ease of implementation; 4) effectively reducing the energy consumption of large-capacity refrigerators, especially for large refrigeration compartments, with more pronounced energy-saving effects. Therefore, with the development of the economy, the increasingly tight energy supply, and the growing demand for large-capacity refrigerators, the vigorous development of the compression/injection mixed refrigeration cycle becomes particularly necessary and is an effective method to enhance the performance of refrigerator cooling systems.