**3. Solar adsorption systems**

where *Qs*

where *Pc*

efficiency, and *η<sup>d</sup>*

For a collector area of 2.4 m2

and *Qt*

8 Sustainable Air Conditioning Systems

air-conditioning process), and *m*̇

*dE*

is the battery charging power, *Pd*

is the discharge efficiency.

ization of water, *Tb*

are, respectively, the sensible and total cooling power, *hfg* is the heat of vapor-

is the supply air flow rate (0.275 kg/s) [23].

*η<sup>d</sup> Pd*

is the battery discharging power, *η<sup>c</sup>*

/kW of cooling capacity and a storage tank volume of 40 L/m2

(5)

, the

is the charge

Thanks to the presence of the battery, the system can be used during peak times to provide the energy required. Indeed, the energy stored in the battery *Ebattery* is determined using Eq. (5) [23].

*dt* <sup>=</sup> *<sup>η</sup><sup>c</sup> Pc* <sup>−</sup> \_\_\_\_ <sup>1</sup>

The simulation results of the internal temperature and humidity were carried out for different types of buildings and climates using TRNSYS software. The system increased the solar fraction of 30% [23]. Moreover, medium-temperature, concentrated solar thermal collectors are used in an air-conditioning system with an auxiliary heater (used to compensate for a lack of energy) and a double-effect absorption chiller [24] to cool a building. The main components

simulation results using TRNSYS software show that the system is able to cover 50% of the load needs of the building [24]. In addition, the COP system is 1.4, which reveals the system efficiency. On the other hand, the investigation [25] couples the solar energy to a traditional vapor compression air conditioner to perform a new hybrid solar-driven AC system. The proposed system was modeled and controlled using TRNSYS software in order to improve its energy efficiency. It is constituted of three main parts (a vapor compression system, a solar vacuum collector, and a solar storage tank). At steady-state conditions, the compressor power consumption was decreased from 1.45 to 1.24 kW, which is traduced by a global energy saving of about 14 and 7.1% for only the compressor. Likewise, an energy saving achieved by the condenser fan is about 2.6% [25], which allows increasing the COP. Hence, the authors reported

\_\_\_\_\_\_ *battery*

of the proposed AC system are shown in **Figure 4**, according to Ref. [24].

that the system is able to satisfy efficiently the cooling requirements.

**Figure 4.** General scheme of the components constituting the solar AC system.

is the building air temperature (it must be higher than 25°C to activate the

These systems have long-term environmental benefits and significant energy efficiency like the absorption AC systems [26]. In fact, they use natural refrigerants such as the water [27] and can be driven by a low-temperature heat source [28].

Several studies have been focused on the design of solar adsorption AC systems. Nonetheless, their design is complex and some parameters, like the heat rejection, are not easy to be determined using classical tools [27]. In this investigation, the authors developed a dynamic model to simulate a solar cooling system equipped with a backup unit, a heat rejection unit (having a thermal capacity of 35 kW), and adsorption chillers, which are driven by solar collectors distributed over an area of 27.52 m2 to cool a flat building area of 130 m2 in Italy.

The authors expressed the thermal performance of the solar collectors as [27]:

$$\frac{Q}{A} = G\left(\eta\_0 - 1.485\frac{(T\_n - T\_s)}{G} - 0.002\frac{(T\_n - T\_s)^2}{G}\right) \tag{6}$$

where *Q* is the power of solar collectors, *A* is their area, *G* is the intensity of the solar radiation, *η*<sup>0</sup> is the ratio of the efficiency measured at actual admitted irradiance to vertical admitted irradiance, *Tm* is the collector average temperature, and *Ta* is the ambient temperature.

The system also cooled about 1000 l of water that can be used in numerous activities. However, the COP of the chiller is much low compared with the electric one: 0.35 and 2.5, respectively. They are computed using Eq. (7) [27].

$$\begin{aligned} \text{(\\_} \text{)}\\ \text{COP}\_{\text{chiller}} &= \frac{\text{Q}\_{\text{v}}}{\text{Q} + \text{Q}\_{\text{batter}}}\\ \text{COP}\_{\text{dcrtic}} &= \frac{\text{Q}\_{\text{v}}}{E\_{\text{d,tot}}} \end{aligned} \tag{7}$$

where *Qev* is the evaporation energy representing the useful effect of the chiller, *Qs* is the energy supplied by the solar collectors, *Qheater* is the energy supplied by the backup unit, and *Eel,tot* is the total electric consumption of all the system components.

The ratio between the energy supplied by the thermal collectors and the total energy required by the complete system, called solar fraction, is given by Eq. (8) [27].

$$SF = \frac{Q\_\*}{Q\_\* \star Q\_{\text{bart}}} \tag{8}$$

In addition, the installation costs are very high, about \$ 29.022. They can be paid back after about 13 years. In fact, about \$ 1085 and 3942.45 kWh of electric energy are saved per year. Another adsorption cooling system using a tubular solar 1-m2 double-glazed collector/ adsorber was designed, as shown in **Figure 5**, according to Ref. [28]. The main objective is to decrease the energy consumption of cooling systems in the sub-Sahara regions in Algeria. Indeed, an energy saving of about 28.3 MWh could be reached during August [28]. However, the solar COP is too low (about 0.21).

**4. Solar desiccant systems**

addition, this kind of system (having 1 m2

solar flat-plate collectors (having an area of 2 m2

and CO<sup>2</sup>

of 48 m<sup>3</sup>

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

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,

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

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

emissions of 50.2% and 49.8%, respectively. Moreover, the system costs can be paid

a solar desiccant cooling unit equipped with evacuated solar collectors (having 16 m2

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

absorber, tubes fixed and set under the absorber plate, and insulation on the back side of them.

. The authors used a fixed solid desiccant bed in place of a rotary desiccant wheel. The

) consist mainly of a transparent cover, a plate

of dehumidifier area and 80 m2

).

11

of solar collector

Solar Air-Conditioning Systems

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

of area),

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

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 adapted to the fixed solar cells to air-condition a housing volume of 330 m<sup>3</sup> using 20 m2 flatplate collector and 2 m<sup>3</sup> hot water tank [30], which can be employed in household activities. 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 adsorption chiller, a solar chimney (having 12 m2 of area), and a cooling channel (having 24 m2 of area), through which the hot air is cooled and distributed in the test room (having 200 m<sup>3</sup> of 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 a split inverter air conditioner having the same cooling power [31].

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 and cooling of the air-conditioned spaces.
