**5. Comparative analysis**

#### **5.1. Ventilation cycle**

The basic configuration of a ventilation cycle is shown in **Figure 9**. In ventilation cycle, hot and humid process air passes through the rotating desiccant wheel and its dry bulb temperature increases and humidity decreases. The process air is then cooled by passing through a heat recovery wheel. The further cooling of process air is achieved using an evaporative cooler according to the set-values of supply air temperature and humidity. For ventilation cycle, the exhaust air stream from the conditioned space is cooled and humidified close to saturation using an evaporator cooler. The quantity of air coming at point 5 is normally equal to quantity of air entering at point 4. The exhaust air is sensibly heated to precool the process air. Finally, the regeneration air stream is heated and is passed through desiccant wheel for its regeneration, which allows the continuous operation of the dehumidification process. For ideal desiccant wheel the air at the exit of the wheel will be completely dehumidified and specific humidity at point 2 will be zero [14].

$$
\omega\_{2,\text{ideal}} = \mathbf{0} \tag{1}
$$

Sensible effectiveness of the desiccant wheel is given by:

**5.2. Recirculation cycle**

*COP* = *m*̇

**6. Results and discussion**

between point 7 and 8 (T<sup>8</sup>


). In this analysis T<sup>8</sup>

has a maximum COP of 0.81 and 0.52, respectively for the month of September.

stant value of 120°C. The temperature at point 7 has higher value in case of ventilation cycle as compared to recirculation cycle. This larger difference makes the required regeneration heat higher for recirculation cycle as compared to ventilation cycle. Note that, average values of performance parameters are calculated when Tambient = 35°C, regeneration temperature of 120°C, process air flow rate 1.5 kg/s, and both regeneration and process mass flow rates are equal. The detailed results for monthly COP of the system operating on both cycles are presented in **Figures 12** and **13**. The system operating under ventilation and recirculation cycle

 *εDW*,*<sup>T</sup>* = (*T*<sup>2</sup> − *T*1)/(*T*<sup>8</sup> − *T*7) (8) After achieving the temperature and humidity at each state point of the cycle, the cooling capacity, regeneration load and coefficient of performance can be deduced from the following relations.

*Qcool* = (*h*<sup>5</sup> − *h*4) (9)

*Qreg* = (*h*<sup>8</sup> − *h*7) (10)

*COP* = *Qcool*/*Qreg* (11)

The basic configuration of a desiccant cooling system operating on regeneration cycle is shown in **Figure 10**. In recirculation mode all the process remains the same as that of ventilation cycle except the air leaving from the conditioned room is mixed with the process air at point 1 instead of circulating it on the regeneration side. On the regeneration side, fresh ambi-

*<sup>p</sup>*(*h*<sup>1</sup> – *h*<sup>4</sup> )/*m*̇

The conditions of air at different points of the cycle operating on both cycles are obtained using the developed mathematical model in the preceding sections. The effectiveness of both evaporative coolers and heat recovery wheel is considered to be constant for this analysis. The conditions of the supply air are one of the important parameter which plays a major role on performance and amount of latent load removed from the room by the system. The climatic conditions used for this analysis and required temperature and humidity ratio of the supply air is presented in **Table 1**. The obtained results for yearly average value of COP, required heat, and cooling capacity are presented in **Table 2**. It can be observed that, the system operating on ventilation cycle has higher value of average COP as compared to regeneration cycle. The difference in the performance of both cycles is because of required regeneration heat as illustrated in **Table 2**. The cooling load for both cycles remains almost same because of small difference in conditions of air at points 1 and 5. The difference of regeneration heat is because of temperature difference

*r*

(*h*<sup>7</sup> − *h*8) (12)

Renewable and Sustainable Air Conditioning http://dx.doi.org/10.5772/intechopen.73166 131

(regeneration temperature) is set to a con-

ent air is used. For regeneration cycle the thermal COP of the system is given as:

The heat recovery wheel is basically a counter flow heat exchanger. The desiccant wheel and heat recovery wheel latent effectiveness in term of specific humidity and energy balance for the adiabatic desiccant wheel can be represented as:

$$
\varepsilon\_{DW} = (\omega\_1 - \omega\_2) / (\omega\_1 - \omega\_{2,\text{ideal}}) \tag{2}
$$

$$(\omega\_1 - \omega\_2)h\_{\parallel \parallel} = \begin{pmatrix} h\_1 - h\_2 \end{pmatrix} \tag{3}$$

$$
\varepsilon\_{\text{HHW}} = \left\{ T\_2 - T\_3 \right\} / \left\{ T\_2 - T\_6 \right\} \tag{4}
$$

In the evaporative cooler, air undergoes a process of adiabatic dehumidification. On psychrometric chart, this process follows constant wet bulb temperature line.

$$
\varepsilon\_{\rm xCl} = \left(T\_3 - T\_4\right) / \left(T\_3 - T\_{n3}\right) \tag{5}
$$

$$
\varepsilon\_{\rm lC2} = \langle T\_5 - T\_6 \rangle / \langle T\_5 - T\_{\rm n5} \rangle \tag{6}
$$

In case of equal process and regeneration air flow rates, energy balance on the adiabatic heat recovery wheel can be written as:

$$\left(h\_{\rm 2} - h\_{\rm 3}\right) = \left(h\_{\rm 7} - h\_{\rm 8}\right) \tag{7}$$

Sensible effectiveness of the desiccant wheel is given by:

$$
\varepsilon\_{\text{DNO},T} = \left< T\_z - T\_1 \right> / \left< T\_s - T\_7 \right> \tag{8}
$$

After achieving the temperature and humidity at each state point of the cycle, the cooling capacity, regeneration load and coefficient of performance can be deduced from the following relations.

$$Q\_{out} = \left(h\_5 - h\_4\right) \tag{9}$$

$$\mathbf{Q}\_{\rm avg} = \begin{pmatrix} \mathbf{h}\_{\rm s} - \mathbf{h}\_{\rm \gamma} \end{pmatrix} \tag{10}$$

$$\text{COP} = Q\_{\text{out}} / Q\_{\text{vg}} \tag{11}$$

#### **5.2. Recirculation cycle**

• **Novel conceptual cycle**: In novel conceptual cycle, mixed air is dehumidified and then this dehumidified air is sensibly cooled in a heat exchanger. After sensible cooling it is passed

• **Three mixed-mode cycles**: In this cycle the evaporative coolers which are used as cooling

The basic configuration of a ventilation cycle is shown in **Figure 9**. In ventilation cycle, hot and humid process air passes through the rotating desiccant wheel and its dry bulb temperature increases and humidity decreases. The process air is then cooled by passing through a heat recovery wheel. The further cooling of process air is achieved using an evaporative cooler according to the set-values of supply air temperature and humidity. For ventilation cycle, the exhaust air stream from the conditioned space is cooled and humidified close to saturation using an evaporator cooler. The quantity of air coming at point 5 is normally equal to quantity of air entering at point 4. The exhaust air is sensibly heated to precool the process air. Finally, the regeneration air stream is heated and is passed through desiccant wheel for its regeneration, which allows the continuous operation of the dehumidification process. For ideal desiccant wheel the air at the exit of the wheel will be completely dehumidified and specific humidity at point 2 will be zero [14].

 *ω*2, *ideal* = 0 (1) The heat recovery wheel is basically a counter flow heat exchanger. The desiccant wheel and heat recovery wheel latent effectiveness in term of specific humidity and energy balance for

*εDW* = (*ω*<sup>1</sup> − *ω*2)/(*ω*<sup>1</sup> − *ω*2,*ideal*) (2)

(*ω*<sup>1</sup> − *ω*2) *hfg* = (*h*<sup>1</sup> − *h*2) (3)

*εHRW* = (*T*<sup>2</sup> − *T*3)/(*T*<sup>2</sup> − *T*6) (4)

In the evaporative cooler, air undergoes a process of adiabatic dehumidification. On psychro-

*εEC*<sup>1</sup> = (*T*<sup>3</sup> − *T*4)/(*T*<sup>3</sup> − *Tw*3) (5)

*εEC*<sup>2</sup> = (*T*<sup>5</sup> − *T*6)/(*T*<sup>5</sup> − *Tw*5) (6)

In case of equal process and regeneration air flow rates, energy balance on the adiabatic heat

(*h*<sup>2</sup> − *h*3) = (*h*<sup>7</sup> − *h*8) (7)

metric chart, this process follows constant wet bulb temperature line.

the adiabatic desiccant wheel can be represented as:

recovery wheel can be written as:

medium are replaced by regenerative/wet-surface heat exchangers.

through WSHE.

130 Sustainable Air Conditioning Systems

**5.1. Ventilation cycle**

**5. Comparative analysis**

The basic configuration of a desiccant cooling system operating on regeneration cycle is shown in **Figure 10**. In recirculation mode all the process remains the same as that of ventilation cycle except the air leaving from the conditioned room is mixed with the process air at point 1 instead of circulating it on the regeneration side. On the regeneration side, fresh ambient air is used. For regeneration cycle the thermal COP of the system is given as:

$$\text{COP} = \dot{m}\_p (h\_1 - h\_4) / \dot{m}\_r (h\_7 - h\_8) \tag{12}$$

#### **6. Results and discussion**

The conditions of air at different points of the cycle operating on both cycles are obtained using the developed mathematical model in the preceding sections. The effectiveness of both evaporative coolers and heat recovery wheel is considered to be constant for this analysis. The conditions of the supply air are one of the important parameter which plays a major role on performance and amount of latent load removed from the room by the system. The climatic conditions used for this analysis and required temperature and humidity ratio of the supply air is presented in **Table 1**.

The obtained results for yearly average value of COP, required heat, and cooling capacity are presented in **Table 2**. It can be observed that, the system operating on ventilation cycle has higher value of average COP as compared to regeneration cycle. The difference in the performance of both cycles is because of required regeneration heat as illustrated in **Table 2**. The cooling load for both cycles remains almost same because of small difference in conditions of air at points 1 and 5. The difference of regeneration heat is because of temperature difference between point 7 and 8 (T<sup>8</sup> - T7 ). In this analysis T<sup>8</sup> (regeneration temperature) is set to a constant value of 120°C. The temperature at point 7 has higher value in case of ventilation cycle as compared to recirculation cycle. This larger difference makes the required regeneration heat higher for recirculation cycle as compared to ventilation cycle. Note that, average values of performance parameters are calculated when Tambient = 35°C, regeneration temperature of 120°C, process air flow rate 1.5 kg/s, and both regeneration and process mass flow rates are equal. The detailed results for monthly COP of the system operating on both cycles are presented in **Figures 12** and **13**. The system operating under ventilation and recirculation cycle has a maximum COP of 0.81 and 0.52, respectively for the month of September.


**Table 1.** Climatic data and desired supply conditions.


**7. Economic evaluation**

**Figure 13.** Monthly variations of COP for recirculation cycle.

initial investment costs.

EUR/kW for large-scale systems [17].

In this section, an overview of economic aspects related to desiccant cooling technology has been presented. The economic evaluation of desiccant cooling system has been carried out by different researchers. Abdel-Salam and Simon [15] evaluated a membrane based liquid desiccant cooling system for its enviro-economic aspects. They compared primary energy consumption of four different systems. The obtained results showed that the primary energy consumption and total life cycle cost of desiccant cooling system was lower than conventional system by 19 and 12%, respectively. Addition of energy recovery ventilator improved the difference by 32% for primary energy consumption and 21% for total life cycle cost. Li et al. [16] compared vapor compression cooling system and hybrid of desiccant system for energy and economic evaluation. The results indicated that replacing the conventional system with hybrid system would reduce the size from 28 to 19 kW leading to annual effective energy savings of nearly 6760 kWh. However, the payback period would be 7 years because of the added

Renewable and Sustainable Air Conditioning http://dx.doi.org/10.5772/intechopen.73166 133

The costs of system accessories will vary depending upon the required flow rates and cooling needs [9]. The sizing charts for fans and pumps are shown in **Figures 14** and **15**, respectively. It can be observed that cost of each accessory depends upon the required output. The small desiccant cooling systems have higher specific costs as compared to large units. A comparative analysis of system specific cost with respect to its size is presented in **Figure 16**. The specific installed system costs are 7300 EUR/kW for small-scale systems and in average 1900

**Table 2.** Average performance parameters for desiccant cooling system operating on ventilation and circulation cycle.

**Figure 12.** Monthly variations of COP for ventilation cycle.

**Figure 13.** Monthly variations of COP for recirculation cycle.
