# Review of Dew Point Evaporative Cooling Technology for Air Conditioning Applications
## 3 General Presentation of Technology  
The indirect evaporative cooling system provides only sensible cooling to the processed air without any moisture addition. In an indirect evaporative cooler, the product air passes the dry channels, and the working air passes through the wet channels on the opposite side of the heat exchanger plate. The working air absorbs heat from the product air, and therefore the product air is cooled without adding any moisture to the air. At the same time, latent heat (related to water evaporation) is transferred to the working air. Main types of typical indirect evaporative coolers and dew point indirect evaporative coolers are shown in the Figure 3. Due to not adding moisture to the supply air, the IEC system is more attractive than direct evaporative devices for domestic applications. IEC units are made in the form of typical heat recovery recuperators: counter-flow (Figure 3a) and cross-flow (Figure 3d). One of the first concepts of achieving higher efficiency with indirect evaporative air cooling was presented in 1935, when [PERSON] [3] published his patent of the air conditioning system where the ambient air was cooled below its inlet wet bulb temperature without mechanical refrigeration. This was achieved by the combination of direct and indirect evaporative cooling. In 1976, in the former USSR, Prof. [PERSON] developed his first ideas of the Maisotsenko-Cycle (M-Cycle) utilized in the regenerative and counter-flow exchangers. He patented his first devices (patents SU No 571,669 and 979,796 [4]). In recent years, several reviews of indirect evaporative cooling have been conducted. In 2019, [PERSON] et al. presented evaporative cooling combined with selected technologies (desicants, membranes) [5]. In 2018, [PERSON] et al. focused on application of the DPIEC in air-conditioning. They summarized and discussed different methods of M-Cycle device evaluation. In 2016, two reviews were published by [PERSON] et al. [6] and [PERSON] and [PERSON] [7]. [PERSON] mainly presented applications such as the M-Cycle cooling tower and conception of gas turbines. [PERSON] and [PERSON] focused on dew point evaporative cooling in building applications.  
Figure 2: Division of research areas of evaporative cooling included in this study.  
The key performance indicators that describe a dew point indirect evaporative cooler are wet bulb and dry bulb effectiveness, calculated from the following equations:  
\[\varepsilon^{IWB}=\frac{t_{1i}-t_{10}}{t_{1i}-t_{2i}^{WB}} \tag{1}\]  
\[\varepsilon^{IDP}=\frac{t_{1i}-t_{10}}{t_{1i}-t_{1i}^{DP}} \tag{2}\]  
\(t_{1i}\)--inlet product air temperature, \({}^{\circ}\)C  
\(t_{1o}\)--outlet product air temperature, \({}^{\circ}\)C  
\(t_{2i}^{WB}\)--wet bulb inlet working air temperature, \({}^{\circ}\)C  
\(t_{1i}^{DP}\)--dew point inlet product air temperature, \({}^{\circ}\)C  
Third performance indicator is electrical coefficient of performance (COP or EER--energy efficiency ratio):  
\[\text{COP}_{el}=\frac{Q_{CC}}{N_{el}} \tag{3}\]  
\(Q_{CC}\)--system cooling capacity, kW  
\(N_{el}\)--power consumed, kW