**4. Air-conditioning absorption system**

The use of solar energy to power an air-conditioning system is a convenient practice to replace conventional electricity [7]. This can be achieved by two methods: photovoltaic solar cooling and thermal driven sorption system [8]. However, thermal cooling technology is preferred because it can use more incident sunlight directly compared with the PV system [9]. Thermal cooling technologies include absorption, adsorption, desiccant systems, and ejectorcompression systems; nevertheless, absorption cooling represents the most common globally technology due to the commercial availability [7, 10]. The process absorption is based on the absorption and desorption of a working fluid named refrigerant in an absorbent. Basic absorption cycle consists of four main components: generator, condenser, evaporator, and

**Figure 5.** Schematic diagram of the absorption cooling cycle.

absorber; additionally, the system requires a solution pump and two valves, as shown in **Figure 5**. A quantity of heat (QGE) is added to the generator at a relatively high temperature (TGE) to vaporize the working fluid from the solution. The vaporized working fluid (1) goes to the condenser, where it is condensed into a saturated liquid, and the heat released from this condensation process (QCO) is discharged to the atmosphere at an intermediate temperature (TCO). The liquid leaving the condenser (2) passes through an expansion valve to reduce its pressure (3) and goes to the evaporator; as the saturation temperature of the refrigerant at lower pressure is much lower than room temperature (TEV), the refrigerant absorbs the heat of the room (QEV), and it vaporizes, producing the cooling effect. Then, the vapor generated (4) moves to the absorber where it is absorbed by the strong solution of absorbent coming from generator (7, 8), delivering heat (QAB), which is dissipated to the ambient to keep the absorption process at a desirable temperature (TAB). Finally, the mixture refrigerant/absorbent is pumped (5, 6) to the generator to restart the cycle.

The cycle can be mathematically described by the following equations derived from mass and energy balances:

$$\mathbf{Q}\_{\rm GC} = \boldsymbol{m}\_{\rm 6} \boldsymbol{H}\_{\rm 6} - \boldsymbol{m}\_{\rm 1} \boldsymbol{H}\_{\rm 1} - \boldsymbol{m}\_{\rm 7} \boldsymbol{H}\_{\rm 7} \tag{1}$$

$$Q\_{\rm CO} = \,\,\,m\_{\rm 1}\,\,H\_{\rm 1} \,\, \,=\,\,m\_{\rm 2}\,\,H\_{\rm 2}\,\,\,\tag{2}$$

$$Q\_{\rm EV} = m\_4 H\_4 - m\_3 H\_3 \tag{3}$$

$$Q\_{\rm AB} = m\_4 H\_4 + m\_8 H\_8 - m\_5 H\_5 \tag{4}$$

$$\text{COP} = \frac{Q\_{\text{EV}}}{Q\_{\text{d2}} + W\_p} \tag{5}$$

The coefficient of performance (COP) is a parameter that is defined as the ratio of available useful energy to the total power supplied to the system. As the work from the pump solution, *Wp* is relatively small (about 1%) with respect to the heat supplied in the generator, and it is usually negligible for analysis purposes.

These equations allow us to compute the system in order to simulate various operating conditions and determine which those that meet the requirements.

#### **4.1. Working pair**

**3.3. Metallic structures**

48 Sustainable Air Conditioning Systems

parison with the structures of wood.

**4. Air-conditioning absorption system**

**Figure 5.** Schematic diagram of the absorption cooling cycle.

These structures are metallic and are characterized by their low cost in comparison with structures of reinforced concrete, high resistance of the metallic structures due to their properties of the steel such as useful long life, ductility, tenacity, and high electrical conductivity. Their advantages are as follows: rapidity of assembly, great capacity of laminated, resistance to the fatigue, armor with diverse types of shaped and possible structural reutilization after dismounting. The disadvantages are as follows: corrosion, elastic bulge, and high cost in com-

The use of solar energy to power an air-conditioning system is a convenient practice to replace conventional electricity [7]. This can be achieved by two methods: photovoltaic solar cooling and thermal driven sorption system [8]. However, thermal cooling technology is preferred because it can use more incident sunlight directly compared with the PV system [9]. Thermal cooling technologies include absorption, adsorption, desiccant systems, and ejectorcompression systems; nevertheless, absorption cooling represents the most common globally technology due to the commercial availability [7, 10]. The process absorption is based on the absorption and desorption of a working fluid named refrigerant in an absorbent. Basic absorption cycle consists of four main components: generator, condenser, evaporator, and

**Table 2** shows the advantages and disadvantages of the structural materials.

The mixture refrigerant/absorbent is better known as working pair, and the performance of the cycle depends critically on it [11]. Generally, a suitable working pair should satisfy some requirements such as a high boiling point difference and a good miscibility between the components, chemically stable, nontoxic, environmental-friendly to mention a few [12, 13]. A wide variety of refrigerant/absorbent combinations have been suggested for absorption cooling systems [13], being the mixtures of water/LiBr and ammonia/water as the two most


increasing the system performance during the equipment life, thereby reducing the possible direct and indirect effects of the systems on the global and local environments. **2.** Identify, where necessary propose, and document reasonably accurate methods of predicting heat transfer, pressure drop and void fractions in these types of heat exchangers, thereby promoting or simplifying their commercial use by heat pump manufacturers. Integral with these activities was an examination of manifolding/flow distribution in compact/microheat

Design and Construction for Hydroxides Based Air Conditioning System with Solar Collectors…

http://dx.doi.org/10.5772/intechopen.72188

51

**3.** Present listings of operating limits for different types of compact heat exchangers, for example, maximum pressures, maximum temperatures, material compatibility, minimum diameters, and so on and of estimated manufacturing costs or possible market prices in a large-scale production. It is intended within this context that opportunities for technology transfer from sectors where mass-produced CHEs are used (e.g., automotive) will be

Plate heat exchanger (PHE) is a compact heat exchanger and has been used for absorption system applications [27–30]. Design, sizing, and selection of a PHE for absorption systems are restricted by the thermodynamic properties of the working mixture because they limit the heat transfer rate; consequently, the heat transfer area is in function of this. Other parameter to consider in heat exchanger selection is the operating conditions (temperature and pressure). Aqueous solutions such as LiBr, NaOH, CaCl, LiCl operate at vacuum pressure conditions (from 0.8 to 7 kPa) [31–35], but there are configurations that include a high-pressure

To the performance, a thermal or heat transfer analysis to heat exchanger is suitable to apply some of the methods such as LMTD, ε-Ntu, and P-Ntu. The methodologies have subtle variations but in essential are the same. P-Ntu method is often used for the calculation of the correlation factor F for the first method. LMDT and ε-Ntu methods have been widely applied in industrial practice [38]. A calculation procedure for plate heat exchanger and useful charts was developed as functions of the number of transfer units (Ntu) and the heat capacity ratio (R) for different heat exchanger configurations. Number of channels, number of passes of each fluids, and flow arrangement were the terms to classify the heat exchangers [39]. The ε-Ntu method avoids a rather cumbersome iteration through logarithmic terms, necessary in the LMTD method, and provides a very elegant method using dimensionless parameters that can be applied in easy way to new design and performance

> *Q*̇ *max*

Heat capacity rates are obtained by multiplying the specific heat and mass flow rate of the fluid. The fluid with the higher heat capacity is designated **Cmax** and the lower one **Cmin**. If the

cold fluid has the minimum heat capacity, then the effectiveness is defined as:

), to the maximum heat transfer poten-

(6)

exchangers, in particular in evaporators.

examined, and recommendations are made.

generator (from 150 to 300 kPa) [30, 36, 37].

rating problems of heat exchangers.

tial rate (Q̇

The effectiveness (ε) is the ratio of heat transfer rate (Q̇

*<sup>ε</sup>* <sup>=</sup> *<sup>Q</sup>*̇ \_\_\_\_\_

max),when the heat exchanger area is infinite:

**Table 3.** Properties for sodium hydroxide and potassium hydroxide.

common working fluids [12]. However, both systems have their limitations, which make necessary to research different working pairs [14].
