**4. Desiccant cooling cycles**

**3. System description**

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The cooling needs for thermal comfort and sunshine timings follow the same pattern. The demand of air conditioning is higher in summer when the sun shines with higher intensity. What if this intensity of sun can be used as an input energy source for the cooling devices?

The issues related to conventional air conditioning technology can be addressed using a new technology called desiccant-based evaporative cooling. This technology is a combination of a desiccant dehumidifier and an evaporative cooler. The schematic representation of desiccant cooling system is shown in **Figure 6** [1]. In such systems, the energy is required to drive the fans, operate the water pump, and to regenerate the desiccant dehumidifier. The required energy can be provided using solar thermal collector for desiccant dehumidifier and photovoltaic modules to drive the fans and the water pump according to the load requirements. The desiccant dehumidifier controls the latent load whereas; evaporative cooler controls the sensible load. Heat recovery medium is used to make the system more energy efficient. For the continuous operation of the system, the regeneration air is heated up to the required regeneration temperature using a solar thermal collector to regenerate the desiccant dehumidifier. The load demand and sunshine follows the same profile which makes this system an effective alternative to conventional air conditioning system. The energy demands for continuous operation of this system can be fulfilled using solar heat according to the load profile. The use of desiccant cooling technology reduces the energy consumption substantially because of no overcooling and reheating of supply air for moisture removal. More research

**Figure 6.** Principle of desiccant-based evaporative cooling technology [1].

Desiccant cooling is used as an alternative to conventional cooling system. These systems operate without the use of any refrigerant and control the latent as well as sensible load independently which helps in better control of moisture and improve air quality. Thermal energy required for regeneration of these units can be supplied from different heat sources such as solar, biomass, waste heat, etc.

Desiccant technology is a cooling technology which removes moisture from air by a process known as sorption (adsorption or absorption). Different desiccant materials are used for this process. Due to vapor pressure difference, adsorption material absorbs the water vapors from the air. To repeat the cycle continuously, moisture from the desiccant wheel is removed using thermal energy. Basic operating cycles of solid and liquid desiccant cooling are illustrated in **Figure 7** [12] and **Figure 8** [13], respectively. In both cooling systems, desiccant dehumidifier is the major component which controls the latent load followed by an addition cooing system i.e. evaporative cooler. Input heat is provided through some thermal energy medium

**Figure 7.** (a) Systematic solid desiccant cooling system with evaporative cooler (b) psychometric processes [12].

**Figure 8.** Basic configuration of liquid desiccant system [13].

for desorption of desiccant dehumidifier and continuous operation of the cycle. There are different modifications to the basic desiccant cooling cycles. A summary of different desiccant cooling cycles is:

• **Ventilation cycle**: In this cycle outdoor air is cooled and 100% return air from the conditioned room is utilized for regeneration process. The air leaving at point E is cooled in an evaporative cooler and is used as cold-sink for room return air as shown in **Figure 9**. The room return air is heated in a heat exchanger and then further heated using a heating medium up to a desired regeneration temperature.

**Figure 10.** Desiccant evaporative cooling system operating on recirculation cycle.

**Figure 9.** Desiccant evaporative cooling system operating on ventilation cycle.

Renewable and Sustainable Air Conditioning http://dx.doi.org/10.5772/intechopen.73166 129

**Figure 11.** Desiccant evaporative cooling system operating on Dunkle cycle.


**Figure 9.** Desiccant evaporative cooling system operating on ventilation cycle.

for desorption of desiccant dehumidifier and continuous operation of the cycle. There are different modifications to the basic desiccant cooling cycles. A summary of different desiccant

• **Ventilation cycle**: In this cycle outdoor air is cooled and 100% return air from the conditioned room is utilized for regeneration process. The air leaving at point E is cooled in an evaporative cooler and is used as cold-sink for room return air as shown in **Figure 9**. The room return air is heated in a heat exchanger and then further heated using a heating me-

• **Recirculation cycle:** this cycle compromises of the same components as ventilation cycle except 100% return air from the conditioned room is mixed with the process air stream at the inlet of desiccant wheel as illustrated in **Figure 10**. This cycle has a Thermodynamic advantage that it can process the air with greater availability for cooling. But this cycle has

• **Dunkle cycle**: This cycle is an effort to combine the thermodynamic advantages of both the ventilation and recirculation cycles. It is a recirculation cycle with an additional heat exchanger to improve the performance of the system. The cycle is shown in **Figure 11**. • **Recirculation ventilated cycle**: Recirculation ventilated cycle is mix of ventilation and re-

• **Wet-surface heat exchanger (WSHE)**: The other desiccant cooling cycle make use of a wetsurface heat exchanger (WSHE), wherein the incoming air can be cooled to its dew point temperature. In WSHE, water indirectly cools the process air and then this air is used to

a higher cold-sink temperature as compared to the ventilation cycle.

circulation cycle in which 10% ventilation air mixed with the return air.

cooling cycles is:

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dium up to a desired regeneration temperature.

**Figure 8.** Basic configuration of liquid desiccant system [13].

cool the return air from the room.

**Figure 10.** Desiccant evaporative cooling system operating on recirculation cycle.

**Figure 11.** Desiccant evaporative cooling system operating on Dunkle cycle.

