# A Review of Small-Scale Vapor Compression Refrigeration Technologies
## 1 Introduction  
Vapor compression refrigeration (VCR) systems have become a basic need in a wide range of heating, ventilation, air conditioning, and refrigeration (HVAC&R) applications in numerous areas ranging from food and air conditioning to health care, industry, and energy. According to estimates from the International Institute of Refrigeration, there were five billion units in operation worldwide in 2019, and this number is expected to double by 2050. HVAC&R systems are responsible for 20% of global electricity consumption and 7.8% of global greenhouse gas emissions [1]. Research on this topic is therefore critical for improving system efficiency and meeting the climate change mitigation targets established within the implementation of the Kigali Amendment to the Montreal Protocol [2]. Typical VCR applications include domestic/residential, commercial, industrial, and mobile refrigeration and air conditioning, as well as heat pumps, water chillers, and more [3]. In addition to these, small-scale refrigeration is receiving growing attention for low cooling capacity applications, both through active methods, such as vapor compression, as well as passive technologies. Passive cooling systems do not require an external power source [4], and at a small scale include [5] natural convection, heat pipes, vapor chambers, immersion, phase change materials, as well as thermosymbols [6], and heat sinks [7]. By contrast, active cooling systems do require a power source to perform their function: forced convection, spray cooling, jet impingement, droplet electrowetting, thermoelectric, microchannels, absorption, Stirling, Joule-Thompson, and the focus of this review: vapor compression systems [8]. While passive systems do not consume energy and meet cooling requirements for many applications, they often offer limited performance, demanding the use of active systems for satisfactory cooling. It is therefore critical to identify the right technology (or combination of technologies) for each application. The growing demand for better heat removal technologies for small-scale applications combined with the scientific and technological progress of miniaturized VCR components, mainly compressors and heat exchangers, has resulted in the development of compact systems [9; 10]. Individual components as well as complete systems are already available in the market and used for different cooling applications, such as electronics, chips, lasers, portable air conditioning, personal cooling, body cooling for athletes, medical applications, laboratory equipment, vehicles, farming, and other commercial and industrial applications [11; 12]. Due to their high energy efficiency, durability, and adequate performance on a wide range of conditions, VCR systems are considered the most promising option [13] due to the insufficient cooling capacity of conventional passive technologies for many applications [14]. Several studies have been conducted for both individual VCR components and systems to determine their behavior, evaluate their performance, improve the design, and expand their applicability through experiments and modeling. Other studies have directly compared the performance of small-scale passive and active technologies, showcasing the advantages of mini-vapor compression over other technologies. However, the importance of continued research lies in the need for increasing their applicability, since most of the current focus is on electronics and personal cooling at the expense of other potential uses. In addition, there is still a need for further basic research on important topics for small-scale VCR systems including the use of alternative refrigerants, as well as thermal and energetic improvements to the basic cycle. Considering the importance of small-scale VCR systems, we aim to describe the current state of VCR systems for small-scale applications, focusing on their advantages and disadvantages over alternative active and passive technologies.