Water shortage is one of the most crucial challenges worldwide, especially in the Middle Eastern countries, with high population and low freshwater resources. Considering this point and the increasing popularity of solar stills desalination systems, as the contribution, this study aims at finding the geographical preference for installation of those technologies in Iran, which is one of the biggest and most populated countries in the Middle East. For this purpose, from each climatic zone of Iran, one representative city is chosen, and analytical hierarchy process (AHP), as one of the most powerful tools for systematic decision-making, is applied. Annual fresh water production (AFWP) from the technical aspect, energy payback period (EPBP) from the energy perspective, and investment payback period from the economic point of view are selected as the decision criteria. Obtaining the three indicated indicators is done using artificial neural networks (ANNs) for yield and water temperature in the basin, which are developed by means of the recorded experimental data. The results indicate that hot arid cities with high received solar radiation, or the ones that have a higher water tariff compared to the others, are the preferred places for installation of solar stills. The example of the first category is Ahvaz, while Tehran is representative of the cities from the second category. AHP demonstrates that they are the first and second priorities for solar still installation, with scores of 26.9 and 22.7, respectively. Ahvaz has AWFP, EPBP, and IPP of 2706.5 L, 0.58 years, and 4.01 years; while the corresponding values for Tehran are 2115.3 L, 0.87 years, and 2.86 years. This study belongs to three classifications in the mathematical problems: 1. experimental work (code: 76–05), 2. Neural networks (code: 92B20), 3. and decision problems, (code: 20F10).

The analytical hierarchy process (AHP) was utilized to determine the optimal location on which to install flat-plate solar thermal collectors for residential buildings in a number of cities in Iran under diverse climatic conditions. The payback period of investment (IPBP) was chosen as one of the decision criteria, while payback periods of energy and greenhouse gas emissions (EPBP and GGEPBP), being two recently introduced concepts, were also taken into account to provide a broader insight from the energy, economic, and environmental (3E) benefits of the system. The novelty of this work is proposing a method to find places with the greatest potential to install flat-plate solar collectors. It was performed using AHP as a systematic decision-making tool, and based on energy, environmental, and economic criteria, which are the key aspects of an energy system. Codes developed in the MATLAB software were employed to determine the values for different investigated cities. According to the results, Yazd, located in the center of the country, was found to be the best place to install the system. This city enjoys EPBP, IPBP, and GGEPBP scores of 2.47, 3.37, and 0.71 years, respectively. The collector area for this city was also found to be 109.8 m2. Yazd gained a score of 26.5 out of 100. With scores of 24.4, 18.6, 15.9, and 14.6 out of 100, Tehran, Bandar Abbas, Rasht, and Tabriz were found to be the second, third, fourth, and fifth priorities for utilizing the system, respectively.

A hybrid air conditioning system, which is composed of a novel gravity-assisted heat pipe (GAHP)-based indirect evaporative cooler (IEC) and direct expansion (DX) cooling coil, is proposed and investigated here. After developing a mathematical model to describe the performance of the GAHP-based IEC, the hybrid system is evaluated during the cooling design day for providing thermal comfort for an office building in Poland. The results are obtained and compared with the combination of a rotary heat exchanger (RHE) and DX cooling coil as the conventional hybrid system. The comparison is performed by analyzing cooling capacity, electricity consumption, and coefficient of performance profiles, which describe the technical, energy, and efficiency aspects, respectively. The results show that the GAHP-based IEC hybrid system is able to enhance the energy performance significantly compared to the conventional one. The proposed hybrid HVAC system improves COP by 39.2% and reduces electricity consumption by 45.0%, according to the design-day of 24 August and the outdoor temperature of 30 °C. As a result, the total operating cost for the assumed cooling season is reduced by 51.7%.

A solar-driven desalination system, featuring a single-slope solar still is studied here. For this design, Al2O3 nanofluid is utilized, and the condition achieving the highest efficiency and cost-effectiveness is found using a reinforcement learning called a deep Q-value neural network (DQN). The results of optimization are implemented for the built experimental setup. Experimental data obtained under the climatic conditions of Tehran, Iran, are employed to compare the enhancement potential of the optimized solar still system with nanofluid (OSTSWNF) with the solar still system with water (STSWWA). The hourly fluid temperatures in the basin as well as the hourly and cumulative freshwater production (HFWP and CFWP) are discussed. A number of other parameters, including daily water production and efficiency in addition to the cost per liter (CPL) of the resulting desalinated water, are also taken into account. The results reveal that annual water production increases from 1326.8 L to 1652.4 L, representing ~25% growth. Moreover, the annual average efficiency improves by ~32%, rising from 41.6% to 54.7%. A great economic enhancement is seen as well, with the CPL decreasing by ~8%, i.e., from USD 0.0258/L to USD 0.0237/L.