**2. Solar absorption systems**

The harmful effects of conventional AC systems (use of environmentally unfriendly refrigerants; CO<sup>2</sup> emission) and their high primary energy consumption lead scientists to invest in clean energy resources, especially the solar energy [3]. The absorption technology is the most used in air-conditioning [4–6]. It uses an absorber and a generator instead of the compressor. Therefore, no electrical power is needed to pressurize the refrigerant (water or ammonia) [7]. In fact, the refrigerant is first absorbed in an absorbing material and then pressurized in the absorbed liquid phase. The pressurized absorption mixture is then reheated in a solar-powered generator to regenerate the pressurized refrigerant vapor. After that, it is deliquesced in the condenser in order to become liquid, which is then expanded through an expansion valve. The chilled refrigerant causes the cooling effect in the evaporator. Finally, the refrigerant is transferred to the absorber and a new cycle is beginning. Thereby, absorption systems contribute to reducing the greenhouse gas emissions to the atmosphere and the energy costs. Nonetheless, they have a low coefficient of performance (COP) (between about 0.3 and 0.75 according to the cooling capacity) compared with the electrical vapor compression AC systems that their COP can reach up to 3 [7].

The operating principle of a solar air-conditioning system is illustrated in **Figure 1**.

**Figure 1.** Absorption solar air-conditioning system.

Several research studies around the world aimed to design various modern solar-powered plants with energy storage. They allow minimizing the environmental effects and satisfying the energy demand [4, 8, 9]. We find single-stage or double-stage absorption systems with and without crystallization [4]. The single-stage systems are equipped with two heat exchangers and two or three storage tanks. However, the double-stage systems are different from the preview systems by adding the two pairs of absorber/generator and evaporator/condenser. In addition, the crystallization process occurs that the refrigerant undergoes three-phase transformation (solid: usually crystallized salt, liquid, and vapor) [4]. Furthermore, these plants and their performance are closely linked to the climatic conditions (especially solar irradiance) of the regions where they are installed. For instance, Mediterranean countries are characterized by a hot climate, which encourages the use of solar air-conditioning systems [5]. In fact, Tunisia widely invests in solar energy that this country is characterized by a sunny climate over long periods of the year [10]. In this reference, an absorption solar installation is applied to a room of 150 m2 to minimize the energy consumption during the summer. It consists of a water-lithium bromide absorption chiller having a capacity of 11 kW, a flat-plate solar collector having an area of 30 m2 , and a hot water storage tank having a volume of 0.8 m<sup>3</sup> . The simulation results showed that the COP reached 0.725 for a cooling capacity of 16.5 kW as long as the heat source temperature increases, which causes the growth of the heat transfer between the system exchangers and then the quantity of heat distributed in the surroundings [11]. Moreover, another study analyzed the energy performance of a solar air-conditioning office building that maximum monthly consumes about 380 kWh [12]. It consists of insulating the walls and cooling the roof. Hence, it allows reaching an energy saving of 46 and 80% in winter and summer, respectively, as well as, reducing the cooling load from 14.09 to 8.68 kW. In the same framework, the studies [13, 14] aimed to improve the efficiency of a solar installation equipped with parabolic solar collectors (having an area of 39 m2 ), an absorption chiller associated with a cooling tower, a backup heater, two tanks for storage and drain-back storage, and a set of fan coils installed in the building to be cooled [14]. The synoptic scheme presenting the main components of the proposed cooling system is illustrated in **Figure 2**, according to Ref. [14].

heat) into conditioned air and sometimes chilling storage water. They are outstandingly used in residential and other sectors (offices, hotels, restaurants, storage warehouses, schools, hospi-

The present chapter reviews recent studies focusing on three technologies of solar AC sys-

The harmful effects of conventional AC systems (use of environmentally unfriendly refriger-

clean energy resources, especially the solar energy [3]. The absorption technology is the most used in air-conditioning [4–6]. It uses an absorber and a generator instead of the compressor. Therefore, no electrical power is needed to pressurize the refrigerant (water or ammonia) [7]. In fact, the refrigerant is first absorbed in an absorbing material and then pressurized in the absorbed liquid phase. The pressurized absorption mixture is then reheated in a solar-powered generator to regenerate the pressurized refrigerant vapor. After that, it is deliquesced in the condenser in order to become liquid, which is then expanded through an expansion valve. The chilled refrigerant causes the cooling effect in the evaporator. Finally, the refrigerant is transferred to the absorber and a new cycle is beginning. Thereby, absorption systems contribute to reducing the greenhouse gas emissions to the atmosphere and the energy costs. Nonetheless, they have a low coefficient of performance (COP) (between about 0.3 and 0.75 according to the cooling capacity) compared with the electrical vapor compression AC sys-

The operating principle of a solar air-conditioning system is illustrated in **Figure 1**.

emission) and their high primary energy consumption lead scientists to invest in

tals, etc.) [2], what makes them classified among the most energy consumers.

tems: absorption, adsorption, and desiccant systems.

**2. Solar absorption systems**

4 Sustainable Air Conditioning Systems

tems that their COP can reach up to 3 [7].

**Figure 1.** Absorption solar air-conditioning system.

ants; CO<sup>2</sup>

**Figure 2.** Synoptic schema of the solar cooling system using parabolic collectors.

The analysis of the system performance showed that the absorption chiller output could reach up to about 12 kW. Also, its COP is ranged between about 0.8 and 0.9 [15]. Furthermore, it allowed reducing the CO2 emission of about 3000 kg during hot seasons and reaching an energy saving of 1154 l of gasoil. This type of solar air-conditioning plants was reviewed in Ref. [16] and performed in the investigation [17] for cooling and heating office buildings in Greece. It is characterized by lower thermal losses, high efficiency, and a small collecting surface of about 14 m2 . An energy saving of up to 50% can be obtained [17]. Nonetheless, the installation costs are higher (about 924 €/m2 of the solar collector), especially for large areas [17]. Moreover, the maintenance is frequent and also expensive. Reference 18 reports the performance statistics of a solar AC system constituted by thermal parabolic collectors (having an area of 588 m2 ) and a double-effect absorption chiller. The system has an annual average efficiency of 40% and a peak efficiency of 58% [18]. It also allows chilling water contained in a storage tank of 23.000 l that is used as a buffer tank. The annual average COP of the absorption chiller, which is a water-cooled double-effect chiller, can reach 1.1. Nonetheless, its costs are very high and reach up to \$ 680.000 that can be paid after about 21 years [18].

Moreover, the governing equations of the mass flow rates of the weak and strong refrigerantabsorbent solutions (ws and ss, respectively) for lithium bromide-water are given by Eq. (2) [22].

*ss* = *Xws*/(*Xss* − *Xws*) . *m*̇

*ws* = *Xss*/(*Xss* − *Xws*) . *m*̇

(*h*<sup>4</sup> <sup>−</sup> *<sup>h</sup>*3)) . <sup>∆</sup>*<sup>t</sup>*

where Δ*t* is a 1-h time-step interval and the enthalpy *h*(1 to 10) is based on the thermodynamic

tion results using Engineering Equation Solver (EES) software indicated that the COP of the system is about 0.85. However, the experimental results showed that it can reach 0.9 [22]. At the level of the size of system components (collector and tanks), they become smaller in summer that the solar intensity is high. Thus, the required mass storage will be reduced and the

In the same context, many related studies were carried out in Australia. For example, in Ref.

solar photovoltaic-battery air conditioner in order to satisfy the desired comfort with the min-

*m*̇ *cp*

O absorption system.

*HRs* = *HRb* − (*Qt* − *Qs*)/*m*̇*hfg*

where *Xws* and *Xss* are the mass concentrations for weak and strong solutions. The generator and evaporator heat and pump work are written in Eq. (3) [22].

*<sup>r</sup> h*<sup>1</sup> + *m*̇

*WP* = (*mss*(*h*<sup>6</sup> − *h*5)) . ∆*t*

*r*

*QG* = (*m*̇

*QE* = (*m*̇

[23], the author modeled a building (having a volume of 60 m<sup>3</sup>

imum energy consumption. The conditioned supply air temperature *Ts*

*<sup>m</sup>*̇

*r*

(2)

7

Solar Air-Conditioning Systems

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

(3)

(4)

*r*

, a hot storage mass of 1500 kg, and a constant load, the simula-

) equipped with an autonomous

and the humidity ratio

*ws h*<sup>8</sup> − *mss h*7) . ∆*t*

*<sup>m</sup>*̇

state shown in **Figure 3**, according to Ref. [22].

are computed using Eq. (4) [23].

**Figure 3.** Thermodynamic state of the LiBr-H2

*Ts* <sup>=</sup> *Tb* <sup>−</sup> \_\_\_\_ *Qs*

For a collector area of 48 m2

COP will be enhanced.

*HRs*

In Algeria, the solar energy was also harnessed to cool houses in hot climates [19]. In this investigation, the authors developed a model of the air conditioner and the absorption cooling system of 10 kW, which is constituted by solar collectors (having a surface of 28 m2 ) and a 900-l hot storage tank, as well as a cooling tower and a thermally driven chiller. The results obtained showed that the solar system with a thermal COP equal to 0.73 can satisfy the required conditioned air of a house having a surface of 120 m2 [20]. The study [21] proposed a very efficient hybrid combined cooling, heating, and power system driven by solar energy and biomass applied to a building with a 100-kW electricity load. It consists of a biomass gasification subsystem, solar evacuated collector (having an area of 96 m2 for an 800 W/m2 solar irradiance), internal combustion engine, and dual-source powered mixed-effect absorption water chiller. In fact, the system allowed an energy saving of about 57%, a reduction of the carbon emission ratio of about 95%, and providing about 200 kW of cooling power. In addition, the COP of the system is high (1.1) [21].

Furthermore, the high ambient temperatures in Gulf countries cause a ceaseless demand for cooling, which allows achieving a significant scientific development in the solar AC field. For instance, in Saudi Arabia, the investigation [22] focused on optimizing the performance of a solar-powered LiBr-water absorption AC system. It is equipped with a flat-plate collector and storage tanks of cold and refrigerant, which ensure a continuous operation of 24 h/7 days. The chiller has a cooling capacity of 5 kW. The authors also give its complete mathematical model using unsteady time-dependent values of the solar intensity and the ambient temperature that are assumed to be constant over given small time intervals Δt. In fact, the generalized energy equation over each Δt, assuming uniform flow processes, is given by Eq. 1 [22].

$$Q - \mathcal{W} = \left(\sum m \cdot h\right)\_{\text{out}} - \left(\sum m \cdot h\right)\_{\text{in}} + m \left(u\_f - u\_i\right)\_{\text{system}} \tag{1}$$

where *Q* and *W* are the net thermal and mechanical energies, *m* is the mass inside the volume (V) of each system component, and (*uf – ui* ) is the change in internal energy per unit mass inside the volume (V) during the time Δ*t* (*uf* is the final internal energy per unit mass inside the volume (V) at the end of the time step (Δ*t*), while *ui* is the initial value).

Moreover, the governing equations of the mass flow rates of the weak and strong refrigerantabsorbent solutions (ws and ss, respectively) for lithium bromide-water are given by Eq. (2) [22].

Moreover, the governing equations of the mass flow rates of the weak and strong refrigerant-absorbed solutions (ws and ss, respectively) for lithium bromide-water are given by Eq. (2) [22].

$$
\dot{m}\_{ss} = X\_{us} / \left(X\_{us} - X\_{us}\right) \cdot \dot{m}\_{r}
$$

$$
\dot{m}\_{ss} = X\_{us} / \left(X\_{us} - X\_{us}\right) \cdot \dot{m}\_{r}
$$

where *Xws* and *Xss* are the mass concentrations for weak and strong solutions.

The analysis of the system performance showed that the absorption chiller output could reach up to about 12 kW. Also, its COP is ranged between about 0.8 and 0.9 [15]. Furthermore, it

energy saving of 1154 l of gasoil. This type of solar air-conditioning plants was reviewed in Ref. [16] and performed in the investigation [17] for cooling and heating office buildings in Greece. It is characterized by lower thermal losses, high efficiency, and a small collecting

[17]. Moreover, the maintenance is frequent and also expensive. Reference 18 reports the performance statistics of a solar AC system constituted by thermal parabolic collectors (having

efficiency of 40% and a peak efficiency of 58% [18]. It also allows chilling water contained in a storage tank of 23.000 l that is used as a buffer tank. The annual average COP of the absorption chiller, which is a water-cooled double-effect chiller, can reach 1.1. Nonetheless, its costs are

In Algeria, the solar energy was also harnessed to cool houses in hot climates [19]. In this investigation, the authors developed a model of the air conditioner and the absorption cooling system

age tank, as well as a cooling tower and a thermally driven chiller. The results obtained showed that the solar system with a thermal COP equal to 0.73 can satisfy the required conditioned air of

cooling, heating, and power system driven by solar energy and biomass applied to a building with a 100-kW electricity load. It consists of a biomass gasification subsystem, solar evacuated

and dual-source powered mixed-effect absorption water chiller. In fact, the system allowed an energy saving of about 57%, a reduction of the carbon emission ratio of about 95%, and provid-

Furthermore, the high ambient temperatures in Gulf countries cause a ceaseless demand for cooling, which allows achieving a significant scientific development in the solar AC field. For instance, in Saudi Arabia, the investigation [22] focused on optimizing the performance of a solar-powered LiBr-water absorption AC system. It is equipped with a flat-plate collector and storage tanks of cold and refrigerant, which ensure a continuous operation of 24 h/7 days. The chiller has a cooling capacity of 5 kW. The authors also give its complete mathematical model using unsteady time-dependent values of the solar intensity and the ambient temperature that are assumed to be constant over given small time intervals Δt. In fact, the generalized energy equation over each Δt, assuming uniform flow processes, is given by Eq. 1 [22].

where *Q* and *W* are the net thermal and mechanical energies, *m* is the mass inside the volume

 *– ui*

very high and reach up to \$ 680.000 that can be paid after about 21 years [18].

for an 800 W/m2

ing about 200 kW of cooling power. In addition, the COP of the system is high (1.1) [21].

of 10 kW, which is constituted by solar collectors (having a surface of 28 m2

*<sup>Q</sup>* <sup>−</sup> *<sup>W</sup>* <sup>=</sup> (∑*<sup>m</sup>* . *<sup>h</sup>*)*out* <sup>−</sup> (∑*<sup>m</sup>* . *<sup>h</sup>*)*in* <sup>+</sup> *<sup>m</sup>* (*uf* <sup>−</sup> *ui*)*system*

emission of about 3000 kg during hot seasons and reaching an

. An energy saving of up to 50% can be obtained [17]. Nonetheless, the

) and a double-effect absorption chiller. The system has an annual average

[20]. The study [21] proposed a very efficient hybrid combined

solar irradiance), internal combustion engine,

) is the change in internal energy per unit mass

is the final internal energy per unit mass inside

is the initial value).

of the solar collector), especially for large areas

) and a 900-l hot stor-

(1)

allowed reducing the CO2

6 Sustainable Air Conditioning Systems

surface of about 14 m2

an area of 588 m2

a house having a surface of 120 m2

collector (having an area of 96 m2

(V) of each system component, and (*uf*

inside the volume (V) during the time Δ*t* (*uf*

the volume (V) at the end of the time step (Δ*t*), while *ui*

installation costs are higher (about 924 €/m2

The generator and evaporator heat and pump work are written in Eq. (3) [22].

$$\begin{aligned} \text{mean and pump work are written in Eq. (3) [22].}\\ \Delta Q\_{\odot} &= \left( \dot{m}\_{\text{\text{\textquotedblleft}}{\text{\textquotedblright}}} h\_{\text{\textquotedblright}} - m\_{\text{\textquotedblleft}} h\_{\text{\textquotedblright}} \right) \cdot \Delta t \\ Q\_{\text{\textquotedblleft}} &= \left( \dot{m}\_{\text{\textquotedblleft}} (h\_{\text{\textquotedblright}} - h\_{\text{\textquotedblright}}) \right) \cdot \Delta t \\ \mathcal{W}\_{\text{P}} &= \left( m\_{\text{\textquotedblleft}} (h\_{\text{\textquotedblright}} - h\_{\text{\textquotedblright}}) \right) \cdot \Delta t \end{aligned} \tag{3}$$

where Δ*t* is a 1-h time-step interval and the enthalpy *h*(1 to 10) is based on the thermodynamic state shown in **Figure 3**, according to Ref. [22].

For a collector area of 48 m2 , a hot storage mass of 1500 kg, and a constant load, the simulation results using Engineering Equation Solver (EES) software indicated that the COP of the system is about 0.85. However, the experimental results showed that it can reach 0.9 [22]. At the level of the size of system components (collector and tanks), they become smaller in summer that the solar intensity is high. Thus, the required mass storage will be reduced and the COP will be enhanced.

In the same context, many related studies were carried out in Australia. For example, in Ref. [23], the author modeled a building (having a volume of 60 m<sup>3</sup> ) equipped with an autonomous solar photovoltaic-battery air conditioner in order to satisfy the desired comfort with the minimum energy consumption. The conditioned supply air temperature *Ts* and the humidity ratio *HRs* are computed using Eq. (4) [23].

$$\begin{aligned} T\_s &= T\_b - \frac{Q\_s}{\dot{m}c\_p} \\ HR\_s &= HR\_b - (Q\_i - Q\_s) / \dot{m} \, h\_{j\_k} \end{aligned} \tag{4}$$

**Figure 3.** Thermodynamic state of the LiBr-H2 O absorption system.

where *Qs* and *Qt* are, respectively, the sensible and total cooling power, *hfg* is the heat of vaporization of water, *Tb* is the building air temperature (it must be higher than 25°C to activate the air-conditioning process), and *m*̇ is the supply air flow rate (0.275 kg/s) [23].

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].

$$\frac{dE\_{\text{battery}}}{dt} = \eta\_c P\_c - \frac{1}{\eta\_d P\_d} \tag{5}$$

**3. Solar adsorption systems**

distributed over an area of 27.52 m2

They are computed using Eq. (7) [27].

the solar COP is too low (about 0.21).

\_\_

tion, *η*<sup>0</sup>

and can be driven by a low-temperature heat source [28].

*Q*

irradiance, *Tm* is the collector average temperature, and *Ta*

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]

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

where *Q* is the power of solar collectors, *A* is their area, *G* is the intensity of the solar radia-

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.

*COPchiller* <sup>=</sup> \_\_\_\_\_\_\_\_ *Qev*

energy supplied by the solar collectors, *Qheater* is the energy supplied by the backup unit, and

The ratio between the energy supplied by the thermal collectors and the total energy required

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.

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,

*Qs* + *Qheater*

*COPelectric* <sup>=</sup> \_\_\_\_

where *Qev* is the evaporation energy representing the useful effect of the chiller, *Qs*

*Eel,tot* is the total electric consumption of all the system components.

by the complete system, called solar fraction, is given by Eq. (8) [27].

Another adsorption cooling system using a tubular solar 1-m2

*SF* <sup>=</sup> \_\_\_\_\_\_\_\_ *Qs*

is the ratio of the efficiency measured at actual admitted irradiance to vertical admitted

*Qs* + *Qheater*

*Qev Eel*,*tot* The authors expressed the thermal performance of the solar collectors as [27]:

*<sup>A</sup>* <sup>=</sup> *<sup>G</sup>*(*η*<sup>0</sup> <sup>−</sup> 1.485 (*Tm* <sup>−</sup> *Ta*) \_\_\_\_\_\_

to cool a flat building area of 130 m2

*<sup>G</sup>* <sup>−</sup> 0.002 (*Tm* <sup>−</sup> *Ta*)<sup>2</sup>

\_\_\_\_\_\_\_

in Italy.

Solar Air-Conditioning Systems

9

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

*<sup>G</sup>* ) (6)

(7)

is the

(8)

double-glazed collector/

is the ambient temperature.

where *Pc* is the battery charging power, *Pd* is the battery discharging power, *η<sup>c</sup>* is the charge efficiency, and *η<sup>d</sup>* is the discharge efficiency.

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

For a collector area of 2.4 m2 /kW of cooling capacity and a storage tank volume of 40 L/m2 , the 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 that the system is able to satisfy efficiently the cooling requirements.

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