# A Review of Small-Scale Vapor Compression Refrigeration Technologies
## 3 Small-Scale Vapor Compression Refrigeration (VCR) Systems
### Components
#### 3.4.1 Compressors  
The compressor is considered the heart of the refrigeration system; its function is to pressurize the refrigerant that leaves the evaporator as a vapor at low temperature and pressure, taking it to a thermodynamic state of superheated vapor at high temperature and pressure. There are different types of compressors; however, the most common in small-scale systems, based on what has been reported in the literature, are rotary, reciprocating, and linear. Scroll, centrifugal, diaphragm, and acoustic compressors have also been evaluated. The compressor deserves the most attention when it comes to miniaturization, mainly because its moving parts make it the component with the greatest irreversibility. In addition, there is a strong correlation between reduction in size, operating speed, and volumetric efficiency. This correlation, as well as the coupling between the drive mechanism and the compression system demand considerable attention [51]. Technological progress and improved manufacturing methods have resulted in very sophisticated compressors that a few decades ago could not even be imagined. Undoubtedly, this scientific and technological progress used in the development of these new compact compressors is impressive. This opens a wide field of research to characterize the operation and performance of these devices and find new applications for them. Some recent works where compressors for small-scale systems have been studied are described next. [PERSON] et al. [61] developed a small-scale rotary variable speed Wankel-type compressor by using micro-electro-mechanical system technology. It consists of a triangular motor rotating within an epitrochoidal chamber without a gas outlet valve. It was manufactured using iron and aluminum alloy, weighing less than 400 g, measuring 5 cm diameter and 7 cm height, and designed to provide 300 W of cooling capacity. At this working condition, the speed of the compressor was 1650 rpm with a shaft power consumption of 95 W. The system ran with R22, and mineral oil lubricant. More information on this compressor was provided in a subsequent work [62], where a second Wankel compressor of 700 g was manufactured but not tested. In another work [82], a Wankel compressor with a flat design was used for a microscale vapor compression cycle that can theoretically provide 45 W of cooling power at 1000 rpm at a temperature 40 K above ambient, and a COP of 4.6 while using R134a. Device specifications include 367 mm\({}^{3}\) displacement, 25 mm \(\times\) 30 mm footprint, and 6.25 mm thickness. The inherent advantage of the Wankel rotary compressor is its flat shape facilitating packaging in tight spaces. Figure 18a shows a similar scheme of this flat-shaped Wankel compressor. [PERSON] et al. [83] recently presented the development and performance evaluation of a lightweight miniature Wankel compressor for aerospace applications (see Figure 18b). The main objective was demonstrating a compact and portable device, investigating its sealing, housing, and materials. Two approaches were adopted for lightweighting: (1) optimize and redesign the structure; and (2) use lightweight materials instead of steel. The final product was a 340.2 g compressor with 65 mm diameter and 85 mm length made of aluminum 7075 alloy that reached 1.49 kg and 155 mm in length once the coupling of a DC brushless motor was included. For the performance tests, a vapor compression cycle was constructed using R134a to evaluate the optimal operating conditions by varying the refrigerant charge, compressor speed, and condenser cooling water temperature. Under a 100 W heat load, the optimum refrigerant charge is 220 g with 40 W minimum power consumption and a COP of 2.5. When evaluating the rotational speed, a maximum COP of 2.8 was obtained for 4800 rpm, while the increase in rotational speed led to a decrease in COP due to an increase in compressor power consumption. Finally, the increase in inlet temperature of cooling water led to a decrease in COP. [PERSON] et al.