**1. Introduction**

There is growing demand for air conditioning in hot climate countries (due to increase in internal loads in buildings), and greater demand for thermal comfort by its users; thus, it is becoming one of the most important types of energy consumption [1]. Accordingly, the consumption of electrical power by refrigeration equipment begins to cause problems in the supply network on the hottest summer days.

Most buildings are provided with electrically driven vapor compression chillers. Currently, the energy for air conditioning is expected to increase tenfold by 2050 [2]. In Iraq, the demand for cooling and air conditioning is more than 50%−60% of total electricity demand (48% in the residential sector) [3]; thus, it contributes to increased CO2 emissions, which could increase by 60% by 2030, compared to the beginning of the century (even though we urgently need to reduce) [4]. On the other hand, mechanical compression chillers utilize various types of halogenated organic refrigerants, such as HCFCs (hydrochlorofluorocarbons), which still contribute to the depletion of the ozone layer; this is why many of these refrigerants have been banned or are in the process of being banned.

To enhance a building's energy e fficiency, solar-driven cooling systems seem to be an attractive alternative to conventional electrical driven compression units, as they achieve primary energy savings and reduce greenhouse gas emissions for solar fractions higher than about 50% [5]. They use refrigerants that do not harm the ozone layer and demand little external electric power supply.

The simulations of lithium bromide (LiBr)/water (H2O) absorption cooling systems have a long history, but a general model for all circumstances is still elusive. Bani Younes et al. [6] presented a

simulation of a LiBr–H2O absorption chiller of 10 kW capacity for a small area of 100 m<sup>2</sup> under three different zones in Australia. They concluded that the best system configuration consists of a 50 m<sup>2</sup> flat plate collector and a hot water storage tank of 1.8 m3. In Tunisia, a feasibility and sensitivity analysis of the solar absorption cooling system was conducted by Barghouti et al. [7] using TRNSYS (University of Wisconsin-Madison, Madison, WI, USA, 1994) and EES software. They concluded that a house of 150 m<sup>2</sup> required 11 kW of absorption chiller, with 30 m<sup>2</sup> of flat plate solar collectors and a 0.8 m<sup>3</sup> storage tank to cover the cooling load.

For their part, Martínez et al. [8] compared the simulation of a solar cooling system using TRNSYS software, with real data from a system installed in Alicante, Spain. The air-conditioning system was composed of a LiBr–H2O absorption chiller with 17.6 kW capacity and 1 m<sup>3</sup> hot storage tank. The results show an approximation between the measured and simulated data, where the coefficient of performance (COP) of the absorption chiller from the experimental data was 0.691 while the COP of the simulated system reached a value of 0.73.

Burckhartyotros [9] described a 250 m<sup>2</sup> field of vacuum tube solar thermal collectors, which provided hot water at temperatures of about 90 ◦C, to drive lithium bromide/water absorption chiller with a capacity of 95 kW, utilized to cover the thermal loads for a building of 4000 m2, which included offices, laboratories, and a public area.

Ketjoy et al. [10] evaluated the performance of a LiBr–H2O absorption chiller with 35 kW cooling capacity, integrated with 72 m<sup>2</sup> of evacuated tube collectors (ETC) and an auxiliary boiler. They found that the solar absorption system had high performance with a ratio of 2.63 m<sup>2</sup> of collector area for each kW of air-conditioning.

A solar parabolic trough collector has been used beside a single effect LiBr–H2O absorption chiller [11]. Peter Jenkins [12] studied the principles of the operation of the solar absorption cooling system. The total solar area was 1450 m2. Wang [13] investigated the effect of large temperature gradients and serious nanoparticles, photothermal conversion efficiency on direct absorption solar collectors.

Hamza [14] studied the development of a dynamic model of a 3TR (Ton of Refrigeration) single-effect absorption cooling cycle that employs LiBr–water as an absorbent/refrigerant pair, coupled with an evacuated tube solar collector and a hot storage unit.

Rasool Elahi [15] studied the effect of using solar plasma for the enhancement operation of solar assisted absorption cycles. Behi [16] presented an applied experimental and numerical evaluation of a triple-state sorption solar cooling module. The performance of a LiCl–H2O based sorption module for cooling/heating systems with the integration of external energy storage has been evaluated. Special design for solar collectors was investigated by Behi [16]. Related to thermo-economics, Salehi [17] studied the feasibility of solar assisted absorption heat pumps for space heating. In this study, single-effect LiBr–H2O and NH3-H2O absorption, and absorption compression-assisted heat pumps were analyzed for heating loads of 2MW (Mega Watt). Using the geothermal hot springs as heat sources for refrigerant evaporation, the problem of freezing was prevented. The COP ranged between 1.4 and 1.6. Buonomano et al. [18] studied the feasibility of a solar assisted absorption cooling system based on a new generation flat plate ETC integrated with a double-effect LiBr–H2O absorption chiller. The results of the experiment show that maximum collector efficiency is above 60% and average daily efficiency is about 40%, and they show that systems coupled with flat-plate ETC achieve a higher solar fraction (77%), in comparison with 66.3% for PTC (Parabolic Through Collector) collectors.

Mateus and Oliveira [19] performed energy and detailed economic analysis of the application of solar air conditioning for different buildings and weather conditions. According to their analysis, they consider that the use of vacuum tube collectors reduces the solar collector surface area of about 15% and 50% in comparison with flat solar collectors. According to the final report of the European Solar Combi+ project, the use of evacuated tube collectors allows for greater energy savings (between 15% and 30%) but the investment increases significantly [20].

Shirazi et al. [21] simulated four configurations of solar-driven LiBr–H2O air-conditioning systems for heating and cooling purpose. Their simulation results revealed that the solar fraction of 71.8% and primary energy conservation of 54.51% could be achieved by the configuration that includes an absorption chiller with a vapor compression cycle as an assistance cooling system.

Vasta et al. [22] analyzed the performance of an adsorption cycle under different climate zones in Italy. It was concluded that the performance parameters were influenced significantly by the design variables. They found that with the dry and wet cooler, the solar fraction could archive values of 81% and 50% at lower solar collector areas. In addition, it was found that the COP could reach 57% and 35% in the same collector arrangement.

In recent years, research projects on solar refrigeration have been carried out to develop new equipment, reducing costs and stimulating integration into the building air conditioning market.

Calise et al. [23] carried out a transitional simulation model using the TRNSYS software. The building was 1600 m<sup>2</sup> building; the system included the vacuum tube collectors of 300 m<sup>2</sup> and a LiBr–H2O absorption chiller. It was found that a higher coefficient of performance (COP) was 0.80; optimum storage volume of 75 <sup>L</sup>/m<sup>2</sup> was determined when the chiller cooling capacity was 157.5 kW.

Djelloul et al. [24] simulated a solar air conditioning system for a domestic house using TRNSYS software. They indicate that to cover the cooling load of a house of 120 m<sup>2</sup> the best air-conditioning system configuration consisted of a single-effect Yazaki absorption chiller of 10 kW, 28 m<sup>2</sup> flat plate collectors with 35◦ inclination, and a hot storage tank of 0.8 m3. They concluded that the ratio of the collector area per kW cooling was 2.80 m<sup>2</sup>/kW.

In Iraq, the conventional electricity grid is not working well as the country struggles to recuperate from years of war [25]. However, Iraq is blessed with an abundance of solar energy, which is evident from the average daily solar irradiance, ranging from 6.5–7 kWh/m<sup>2</sup> (which is one of the highest in the world). This corresponds to total annual sunshine duration ranging between 2800–3000 h [26]. Accordingly, solar cooling technology promotion in Iraq appears to be of high importance, concerning development, and is part of the government's new strategy for promoting renewable energy projects.

It is clear from the literature that solar energy has a grea<sup>t</sup> influence on refrigeration and/or air conditioning processes. Different types and configurations of solar collectors have been applied for such purposes. The most used type was the evacuated tube collector (ETC). Moreover, it was noticed that LiBr–H2O have been used for most of the research activities in this regard [27,28].

The aim of this work is to provide (1) a valuable roadmap related to solar-driven cooling systems operating under the Iraq climate to allow for sustained greenhouse gas emission reductions in the residential air conditioning sector, and (2) energetic performance analysis of solar driven cooling systems to investigate the best system design parameters.
