4.3. M-Cycle evaporative cooling (MEC)

The Maisotsenko cycle (M-Cycle) evaporative cooling (MEC) is an advance thermodynamic conception of IEC by which the product air can be cooled to the ambient air dew-point temperature [10]. The fundamental scheme of MEC operation is presented by Figure 9(a), whereas details can be found from reference [10]. The MEC apparatus (dry and wet channels in Figure 9(a)) exploits the psychrometric renewable energy (available from the latent heat of water vaporization) in such a way that product air can be cooled to the ambient air dew-point.

ðToutÞdb ≥ ðTinÞwb ð7Þ

RHout > RHin ð8Þ

wout ¼ win ð9Þ

hout < hin ð10Þ

ð11Þ

<sup>ð</sup>εwbÞIEC <sup>¼</sup> <sup>ð</sup>TinÞdb � ðToutÞdb

It can be observed from Eqs. (2)–(11) that the numerical value of outlet temperature by DEC and IEC units will be identical for same effectiveness; however, humidity makes the difference in cooling performance. In this regard, IEC system performance is investigated for summer season of Multan and Fukuoka cities for nonhuman AC applications as shown in Figure 8(b) for εwb = 0.95. According to the results, "tobacco softening" and "storage of fruits & vegetables" cannot be entertained by IEC system for both cities. On the other hand, unlike DEC, the IEC can provide optimum conditions for "animals' AC" for Fukuoka city. In addition, IEC can support/assist conventional AC system for the industrial AC process of "optical lens grinding" for Multan. Similarly, rest of the nonhuman AC applications can be examined by Figure 8(b).

Figure 8. Indirect evaporative cooling (IEC) system: (a) schematic representing the fundamentals and thermodynamic principle; and (b) system performance for nonhuman AC applications for Multan and Fukuoka, where each legend point

The Maisotsenko cycle (M-Cycle) evaporative cooling (MEC) is an advance thermodynamic conception of IEC by which the product air can be cooled to the ambient air dew-point temperature [10]. The fundamental scheme of MEC operation is presented by Figure 9(a), whereas details can be found from reference [10]. The MEC apparatus (dry and wet channels in Figure 9(a)) exploits the psychrometric renewable energy (available from the latent heat of water vaporization) in such a way that product air can be cooled to the ambient air dew-point.

4.3. M-Cycle evaporative cooling (MEC)

represents hourly value (average) of a day.

108 Refrigeration

ðTinÞdb � ðTinÞwb

Figure 9. Maisotsenko cycle evaporative cooling (MEC) system: (a) schematic representing the fundamentals and thermodynamic principle; and (b) comparison of outlet air temperatures with DEC/IEC system for Multan and Fukuoka, where each point represents hourly value (average) of a day.

According to experimental results available in the literature, MEC systems have resulted the wet-bulb effectiveness substantially more than unity, which means that MEC can sensibly cool the product air below the ambient air wet-bulb temperature. For insights into MEC, inlet and outlet air conditions are presented by Eqs. (12)–(15).

$$(\left(T\_{\dot{m}}\right)\_{dp} \leq \left(T\_{out}\right)\_{db} \leq \left(T\_{\dot{m}}\right)\_{wb} \tag{12}$$

$$\text{RH}\_{\text{out}} > \text{RH}\_{\text{in}} \tag{13}$$

$$
\varpi\_{\rm out} = \varpi\_{\rm in} \tag{14}
$$

$$h\_{\rm out} < \ h\_{\rm in} \tag{15}$$

Unlike DEC and IEC systems, the thermodynamic limit of MEC cooling is dew-point temperature; therefore, cooling potential of MEC is function of ðTinÞdb � ðTinÞdp. Accordingly, dew-bulb effectiveness of MEC εdp [–] can be expressed as follows:

$$(\varepsilon\_{d\mathfrak{p}})\_{\rm MEC} = \frac{(T\_{\dot{m}})\_{\rm d\mathfrak{b}} - (T\_{\rm out})\_{\rm d\mathfrak{b}}}{(T\_{\dot{m}})\_{\rm d\mathfrak{b}} - (T\_{\dot{m}})\_{\rm d\mathfrak{p}}} \tag{16}$$

For general overview, outlet air temperature of MEC unit for Fukuoka and Multan climates is calculated for εdp = 0.95, and the results are compared with IEC as shown in Figure 9(b). It can be seen that the MEC can provide much better conditions as compared to conventional IEC throughout the day for both cities. Moreover, the performance of MEC can be further improved as compared to IEC at drier ambient air conditions. For brief understanding, the applicability of standalone MEC can be considered limited when w ≥ 11.2 g/kgDA [10]; however, it may not be limited if utilized intelligently, for example, pre-dehumidification of ambient air (by desiccant dehumidifier) before it passes through MEC channels. As the results presented in Figure 9(b) are based on constant εdp, they may not be exactly similar to real experiments. In this regard, lots of studies have reported experimental results along with numerical models for MEC which can be found from references [50–52]. In addition, a recent study [6] provides basic correlation for performance evaluation of MEC, which can be represented by Eq. (17). The correction is valid for the range of Tin = 20–45�C and win = 10–25 g/kgDA.

$$(T\_{out})\_{db} = A\_1 + B\_1(T\_{in})\_{db} + C\_1(w\_{in}) \tag{17}$$

where win, Tin, and Tout represent inlet humidity ratio [g/kgDA], inlet, and outlet air temperatures [�C], respectively. The values of constant A1, B1, and C<sup>1</sup> are 6.70, 0.26, and 0.53, respectively.

### 4.4. Desiccant AC (DAC)

It can be noticed from Sections 4.1–4.3 that cooling potential of evaporative cooling techniques is function of ðTinÞdb � ðTinÞwb or ðTinÞdb � ðTinÞdp; therefore, it can be only applied in dry regions/climates. In contrary, desiccant AC (DAC) could be an energy-efficient and viable solution for humid climates [1]. The DAC possesses ability to deal sensible and latent load of AC distinctly and can be operated on low-grade waste heat, biogas, and/or solar energy. The ability of desiccant material to adsorb water vapors from ambient air makes DAC system a suitable choice for AC in high humidity regions [3]. A typical DAC system based on solid desiccant rotor is shown in Figure 10(a). First, ambient air (1) is passed through the desiccant dehumidifier where it is dehumidified due to water vapor pressure difference between air and desiccant (process 1–2). This process will be isenthalpic in case of ideal situation, that is, neglecting sorption heat. Thus, the temperature of dehumidified air (2) is increased due to heat of water vapor condensation. Second, the dehumidified air (2) is sensibly cooled initially by heat exchanger (process 2–3) followed by low-cost cooling processes (process 3–4), for example, IEC. On the other hand, desiccant will be saturated with water vapor adsorption after some time. Therefore, regeneration/hot air (6) is passed through the desiccant (process 6–7) which removes the adsorbed water vapors for cyclic usage of desiccant. Referring to Figure 10(a), inlet and outlet air conditions of DAC can be simply expressed by Eqs. (18)–(25), while detailed DAC models can be found from references [4, 53].

$$(T\_2)\_{wb} = (T\_1)\_{wb} \tag{18}$$

$$(T\_3)\_{db} = \ (T\_2)\_{db} - \varepsilon\_{HX} \left( (T\_2)\_{db} - (T\_1)\_{db} \right) \tag{19}$$

$$\varepsilon\_1(T\_4)\_{db} = (T\_3)\_{db} - \varepsilon\_{IEC} \left( (T\_3)\_{db} - (T\_1)\_{wb} \right) \tag{20}$$

$$\varepsilon\_{\!\!\!-1}(T\_5)\_{db} = \begin{pmatrix} T\_1 \end{pmatrix}\_{db} + \varepsilon\_{\!\!\!-1} \left( (T\_2)\_{db} - (T\_1)\_{db} \right) \tag{21}$$

$$(T\_6)\_{db} = f(w\_1, RH\_2) \tag{22}$$

$$RH\_6 \le RH\_2 \tag{23}$$

$$
\varpi\_4 = \varpi\_3 = \varpi\_2 \tag{24}
$$

$$w\_6 = w\_5 = w\_1\tag{25}$$

where subscript numbers are associated with Figure 10(a). Effectiveness of heat exchanger (HX) and cooling source (IEC), that is, εHX and εIEC, is considered 0.95 for analysis. For the analysis of DAC system, ambient air conditions of both cities (Figure 6) are considered on average basis, that is, T = 36.2�C, RH = 62% for Multan and T = 27.6�C, RH = 69% for Fukuoka. Consequently, effect of regeneration temperature (T6) on supply air condition (4) is investigated as shown in Figure 10(b). It can be seen that supply air relative humidity is reduced linearly with the increase in regeneration temperature for both cities. Similarly, supply air enthalpy is also reduced at higher regeneration temperatures by which the conditioning is improved. While the supply air temperature at all regeneration temperatures is found 30 � 0.1�C and 23.5 � 0.10�C for Multan and Fukuoka, respectively. It can be observed that DAC system can provide variety of supply air conditions by manipulating regeneration temperature; therefore, it can be considered a viable AC system for various nonhuman applications expressed in Section 2.

Finally, it can be summarized that evaporative cooling systems (Sections 4.1–4.3) could provide low-cost AC for dry regions, whereas DAC system can be used efficiently for humid climates. Additionally hybrid systems based on evaporative cooling and/or DAC can also be established for efficient and sustainable AC performance. Hence, it can be concluded that one or other AC system (presented in Section 4) can provide optimum AC for presented nonhuman applications.

Figure 10. Desiccant air-conditioning (DAC) system: (a) schematic representing the fundamentals and thermodynamic principle; and (b) effect of regeneration temperature on supply air enthalpy and relative humidity.
