**4. Solar desiccant systems**

In the investigation [29], a reduction of the energy consumption of about 50% was achieved, especially in hot and wet climates due to the use of solar energy for the production of cold. The authors used a traditional cooling system with a dehydrating cooling cycle that can be

However, the COP of the cooling system is low (about 0.6) and the indoor climate does not fulfill standard comfort criteria for few hours during the cooling season. A new water/airconditioning system for buildings is presented in Ref. [31]. It is constituted by a solar-driven

of area), through which the hot air is cooled and distributed in the test room (having 200 m<sup>3</sup>

volume) under hot and humid, and hot and arid climate. The cyclic cooling capacity and the COP of the chiller reached their maximum values (about 16 kW and 0.71, respectively) during the day (between 15 and 16 h). This allowed decreasing the room temperature by 26.8%. Furthermore, the electric energy consumed by the system is 37% less than that consumed by

Absorption and adsorption technologies can be combined in the same AC system in order to further improve its performance. This is the subject of [32] in which the authors proposed novel solar poly-generation systems, based on both adsorption and absorption chiller technologies fed by dish-shaped concentrating and flat photovoltaic/thermal collectors instead of conventional solar collectors. They developed a computer code to determine the optimal system configurations taking into account the operating parameters and the climatic conditions. The systems are applied to buildings (office and residential spaces) located in different climatic European regions. They provided electricity and hot water, as well as they ensured the heating

hot water tank [30], which can be employed in household activities.

of area), and a cooling channel (having 24 m2

using 20 m2

flat-

of

adapted to the fixed solar cells to air-condition a housing volume of 330 m<sup>3</sup>

a split inverter air conditioner having the same cooling power [31].

plate collector and 2 m<sup>3</sup>

10 Sustainable Air Conditioning Systems

adsorption chiller, a solar chimney (having 12 m2

**Figure 5.** Synoptic schema of the adsorption solar cooling system.

and cooling of the air-conditioned spaces.

On the environmental front, desiccant systems rank among the top efficient cooling systems [33]. In fact, they can decrease the greenhouse gas emissions and improve the energy savings given that they do not use any ozone-depleting refrigerants and consume less energy as compared with the vapor compression systems [34–36]. Their benefits are meaningful when they interact with renewable energy technologies, such as solar collectors [37, 38]. They also reduce moisture from the indoor air and enhance its quality [39–41]. For instance, liquid desiccant dehumidification solar systems are used to supply fresh air in humid climate locations using the calcium chloride liquid and a flat-plate solar collector (having an area of 86.16 m2 ). It allows reducing their latent heat load and then enhancing their efficiency [42, 43]. During the entire cooling season, the proposed system in this study provides 10 [44] and 40 kW for cooling a typical house and a small restaurant, respectively. However, the COP of the desiccant unit is too low (0.41 for the house and 0.45 for the restaurant). The costs of the installation powered by natural gas can be paid back after 11 years if the gas price is 0.5638 \$/kg [43]. In addition, this kind of system (having 1 m2 of dehumidifier area and 80 m2 of solar collector area) has been tested under hot and dry climate conditions, and a Multi-Population Genetic Algorithm (MPGA) is developed to optimize the system parameters to reach a maximum energy saving and a minimum payback period. It is shown in **Figure 6** according to Ref. [45].

In fact, 38% of electricity saving and a payback period of 14 years are achieved [45]. Furthermore, a solar desiccant cooling unit equipped with evacuated solar collectors (having 16 m2 of area), in which the regeneration thermal energy is supplied by a natural gas boiler, and with a conventional air-handling device is enough to obtain a reduction of primary energy consumption and CO<sup>2</sup> emissions of 50.2% and 49.8%, respectively. Moreover, the system costs can be paid back after 17 years [36]. A liquid desiccant solar system is combined with two evaporative coolers (a regenerative indirect evaporative cooler and a direct evaporative cooler with an adjustable bypass flow) [46]. This has the objective to improve the performance of the desiccant system by using low-grade heat for air-conditioning [46]. The liquid desiccant system is characterized by a self-cycle solution at dehumidification. Its performance was analyzed through a mathematical model that studies the impact of varying five parameters (solution self-cycle ratio, working to intake air flow ratio, regeneration temperature, ambient air temperature, and humidity ratio). The system can decrease the air temperature of the cooled space to 17.9°C. Nonetheless, the obtained thermal COP is low (0.5) for the design conditions [46]. In addition, the system has the advantage of using lower temperature heat source compared with a conventional AC system. The same technology was also invested for hot climates in Saudi Arabia [47]. The investigation shows that the desiccant evaporative AC system presents a modest performance in dry climates and does not operate in very wet conditions.

On the other hand, three models of solar solid desiccant AC system were performed in Ref. [48] under cold, humid, hot, and dry climates in Tunisia and applied to a building having a volume of 48 m<sup>3</sup> . The authors used a fixed solid desiccant bed in place of a rotary desiccant wheel. The solar flat-plate collectors (having an area of 2 m2 ) consist mainly of a transparent cover, a plate absorber, tubes fixed and set under the absorber plate, and insulation on the back side of them.

**Figure 6.** Multi-population genetic algorithm (MPGA) to optimize the desiccant AC system.

They are made of copper. Water circulates into the tubes in order to be heated. The solar collectors are coupled to the desiccant system (it consists of a desiccant dehumidifier, an air–air heat exchanger, a water-air heat exchanger, and a humidifier). This provides the heat required to regenerate it, precisely the desiccant dehumidifier. The coupling is ensured by a water storage tank and a heating coil inserted in the return air stream of the desiccant system [48].

The authors expressed the thermal balance for the absorber as follows [48]:

$$\begin{aligned} \text{Authors expressed the thermal balance for the absorber as follows [48]:}\\ \begin{aligned} \rho\_{abs} \delta\_{ds} \, \mathcal{C}\_{abs} \, \frac{dT\_{abs}}{dt} &= \mathcal{G} \, \tau\_{abs} \, \alpha\_{abs} + h\_{r\_{\alpha,\nu}} \left( T\_{\text{s}} - T\_{abs} \right) + h\_{coun} \left( T\_{\text{s}} - T\_{abs} \right) \\ &+ \left( \frac{\mathcal{S}\_{abs}}{\mathcal{S}\_{abs}} \right) h\_{coul\_{\alpha\nu\alpha}} \left( T\_{\text{tair}} - T\_{\text{abs}} \right) + \left( \frac{\mathcal{S}\_{abs\text{-}radiation}}{\mathcal{S}\_{abs}} \right) h\_{coul\_{\alpha\nu\alpha\text{-}site}} \left( T\_{\text{iscalation}} - T\_{\text{abs}} \right) \\ &+ h\_{\text{abs}} \delta\_{\text{abs}} \left( \frac{\partial^{2} T\_{\text{s}\text{s}}}{\partial x^{2}} + \frac{\partial^{2} T\_{\text{s}\text{s}}}{\partial y^{2}} \right) \end{aligned} \end{aligned} \tag{9}$$

*hcondabs*−*tube*

*hcondabs*−*insulation*

*hconva*−*abs*

= \_\_\_\_\_\_\_ <sup>1</sup> *δ* \_\_\_*abs λabs* + *δ* \_\_\_\_ *tube λtube*

**Solar air-conditioning systems**

Design values *Small storage water volume:*

Design optimization • Insulating the walls and

Effectiveness COP From 0.3 to 0.75 and

Environmental benefits

minimum 0.8 m<sup>3</sup> *Collector area:* 14–96 m2

up to 150 m2

building. • Using parabolic and concentrated thermal

collectors.

sometimes up to 1.4

Reduction of the CO<sup>2</sup> emissions up to 95% (up to

**Table 1.** Design values and effectiveness of absorption, adsorption, and desiccant solar AC systems.

summer).

3000 kg).

Reduction of the energy consumption up to 80% (especially during the

*Building volume (or surface)*:

cooling the roof of the

• Combining solar energy and biomass. • Small size of collectors and tanks • Adding batteries

Cooling capacity From 5 to 16.5 kW About 16 kW About 40 kW

= \_\_\_\_\_\_\_\_\_ <sup>1</sup> *δ* \_\_\_*abs λabs* + *δ* \_\_\_\_\_\_ *insulation λinsulation*

For the tube, the thermal balance is written as [48]:

<sup>=</sup> *Nua <sup>λ</sup>* \_\_\_\_\_*<sup>a</sup> δa*

:conductive heat transfer coefficient between absorber plate and tube

**Absorption systems [3–25] Adsorption systems** 

**[26–32]**

130 m2

*Small storage water volume:* up to 2 m<sup>3</sup> *Collector area:* 1–20 m2

*Building volume (or surface)*: minimum

, up to 330 m<sup>3</sup>

From 0.2 to 0.7 From 0.41 to 0.5

Energy saving: up to 28.3 MWh. Up to 50%.

• Using low-temperature heat source. • Using sustainable adsorption chillers. • Using tubular solar double-glazed collector/adsorber.

**Desiccant systems** 

Solar Air-Conditioning Systems

13

*Small storage water volume:* up to 2 m<sup>3</sup> *Collector area:* 2–80 m2

*Building volume (or surface)*: minimum

• Using dehumidifier to reduce moisture from the cooled air. • Using multipopulation genetic algorithm to optimize the system parameters. • Using a natural gas boiler to regenerate thermal energy.

Reduction of the energy consumption up to 52%. Reduction of the

emissions up to

CO<sup>2</sup>

49.8%.

**[33–48]**

http://dx.doi.org/10.5772/intechopen.72189

48 m<sup>3</sup>

:the convective heat transfer coefficient between air gap and the absorber

:conductive heat transfer coefficient between absorber plate and insulation

where 'abs' refers to absorber, *ρ* is the intrinsic average density, *σ* is the thickness, *C* is the specific heat, *G* is the solar global radiation, *τ* is the transmission coefficient, is the absorption coefficient, *hr.* is the radiation heat transfer coefficient, *Ttc* is the temperature of the transparent cover, *Ta* is the air temperature, *S* is the surface area, and *λ* is the conductivity.


#### **Solar air-conditioning systems**

They are made of copper. Water circulates into the tubes in order to be heated. The solar collectors are coupled to the desiccant system (it consists of a desiccant dehumidifier, an air–air heat exchanger, a water-air heat exchanger, and a humidifier). This provides the heat required to regenerate it, precisely the desiccant dehumidifier. The coupling is ensured by a water storage

tank and a heating coil inserted in the return air stream of the desiccant system [48].

(*Ttc* − *Tabs*) + *hconva*−*abs*

<sup>+</sup>( *<sup>S</sup>*

(*Ttube* − *Tabs*) + (

is the air temperature, *S* is the surface area, and *λ* is the conductivity.

where 'abs' refers to absorber, *ρ* is the intrinsic average density, *σ* is the thickness, *C* is the specific heat, *G* is the solar global radiation, *τ* is the transmission coefficient, is the absorption coefficient, *hr.* is the radiation heat transfer coefficient, *Ttc* is the temperature of the transparent

∂2 *T* \_*abs* ∂*y*<sup>2</sup> ) (*Ta* − *Tabs*)

*Sabs* )*hcondabs*−*insulation*

(*Tinsulation* <sup>−</sup> *Tabs*)

(9)

*S* \_*abs*−*insulation*

The authors expressed the thermal balance for the absorber as follows [48]:

**Figure 6.** Multi-population genetic algorithm (MPGA) to optimize the desiccant AC system.

cover, *Ta*

*ρabs δabs Cabs*

12 Sustainable Air Conditioning Systems

*dT*\_\_\_\_*abs*

*dt* <sup>=</sup> *<sup>G</sup> <sup>τ</sup>abs <sup>α</sup>abs* <sup>+</sup> *hrabs*\_*tc*

+*λabs δabs*(

\_*abs*−*tube*

*Sabs* )*hcondabs*−*tub*

∂2 *T* \_*abs* <sup>∂</sup>*x*<sup>2</sup> <sup>+</sup> **Table 1.** Design values and effectiveness of absorption, adsorption, and desiccant solar AC systems.

$$h\_{\text{cond}\_{\text{s-axis}}} = \frac{\frac{1}{\delta\_{\text{ds}}} + \frac{\delta\_{\text{ds}}}{\lambda\_{\text{ds}}}}{\frac{\delta\_{\text{ds}}} + \frac{\delta\_{\text{ds}}}{\lambda\_{\text{ds}}}}$$

$$h\_{\text{cond}\_{\text{s-axis}}} = \frac{1}{\frac{\delta\_{\text{ds}}}{\lambda\_{\text{ds}}} + \frac{\delta\_{\text{ds,wall}}}{\lambda\_{\text{usukawa}}}} \text{:conductive heat transfer coefficient between absorber plate and insulation}$$

$$h\_{\text{conv}\_{\text{s-}}} = \frac{N \underline{u}\_{s} \lambda\_{s}}{\delta\_{s}} \text{:the convective heat transfer coefficient between air gap and the absorber}$$

For the tube, the thermal balance is written as [48]:

$$\begin{aligned} \left(\rho\_{\text{tube}}\mathcal{S}\_{\text{tube}}\mathcal{C}\_{\text{tube}}\frac{dT\_{\text{tube}}}{dt} = \mathcal{S}\mathfrak{c}\_{\text{abs-tube}}h\_{\text{cond}\_{\text{solv}}}\left(T\_{\text{abs}} - T\_{\text{tube}}\right) + P\_{\text{tube}}h\_{\text{conv}\_{\text{abs-tube}}}\left(T\_{\text{lossulation}} - T\_{\text{tube}}\right) \\ + \mathsf{S}\mathfrak{c}\_{\text{tube-lossization}}h\_{\text{cond}\_{\text{solv}}}\left(T\_{\text{insulation}} - T\_{\text{tube}}\right) + \lambda\_{\text{tube}}\mathcal{S}\_{\text{tube}}\frac{\partial^2 T\_{\text{tube}}}{\partial y^2} \end{aligned} \tag{10}$$

where *P* is the perimeter.

 *hconva*−*abs* <sup>=</sup> *Nutube <sup>λ</sup>* \_\_\_\_\_\_\_*<sup>f</sup> Dh*,*tube* :the convective heat transfer coefficient between tube and circulating fluid (water) *hcondtube*−*insulation* = \_\_\_\_\_\_\_\_\_ <sup>1</sup> *δ* \_\_\_\_ *tube λtube* + *δ* \_\_\_\_\_\_ *insulation λinsulation* :conductive heat transfer coefficient between tube and insulation

The value of the Nusselt number *Nutube* depends on the Reynolds number *Re* as follows [48]:

 $\text{The value of the Nusselt number } Nu\_{\text{tube}} \text{ depends on the Keynolds } \mathbf{x}$ 
$$\begin{array}{rcl} \text{Re} \le 2300 \Rightarrow \text{Nu}\_{\text{tube}} = \text{4.364} \\ \text{Re} > 2300 \Rightarrow \text{Nu}\_{\text{tube}} = 0.023 \text{ Re}^{0.8} \text{Pr}^{0.4} \end{array}$$

The numerical values of the temperature and humidity show that the desired comfort is reached by the three proposed models under different climatic conditions.

**Table 1** summarizes the design values and the effectiveness of the three technologies of solar AC systems investigated in the present chapter.
