Summary The optimization of the total annual cost in heat exchanger networks has been one of the overarching goals when synthesizing these networks. Several methodologies and techniques have been developed to achieve optimal costs in mixed material heat exchanger networks. This paper demonstrates the application of two decomposition methodologies (total decomposition and partial decomposition) for typical cost rules. The objective function was defined as the optimization and minimization of the total annual cost in mixed materials heat exchanger network. Three optimization algorithms, hybrid genetic‐particle swarm optimization (GA‐PSO), shuffled frog leaping algorithm (SFLA) techniques, and ant colony optimization (ACO), were used to further optimize the total cost in mixed materials heat exchanger network. The results indicate that the total annual cost in partial decomposition method was smaller than that in full integration method and total decomposition method. The reduction of the total annual cost was about 27% for GA‐PSO algorithm, 24% for SFLA and 10% for ACO relative to the results reported in this work. In partial decomposition method, at least one mixed material of heat exchanger was used to reduce the hot and cold utility for decreasing the total annual cost. Partial decomposition method resulted in the highest reduction of the total annual cost compared with other methods. Percentage of difference of the total annual cost were 0.36%, 1.92%, and 5.05% for full integration, total decomposition, and partial decomposition methods, respectively, in comparison with the previous studies. Results have been compared with the results of other studies to demonstrate the accuracy of the applied algorithms.

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Summary In the present experimental research, a five‐stage solar still was investigated using phase change material paraffin wax. Owing to Iran's climatic conditions and its location in a dry area and nonexistence of fresh water sources, producing this water from saline water sources is essential. In this research, thus, a multistage solar still was planned and examined as a commonly used freshwater source via (phase change material) paraffin wax. On the back of the collector absorber plate, paraffin wax (phase change material) was used as an additional energy source for the sun. The volume of water produced in different thicknesses of 2.5 cm, 5 cm, and without PCM was investigated. The study results specified that the application of phase change material paraffin wax in the thickness of 2.5 cm agent led to a 15% increase in freshwater production. The quantity of water generated in the flow rates of 0.7, 1.3, and 1.8 L/min was also studied. The highest quantity of water produced was obtained in the 1.3 L/min flow rate. Moreover, the quantity of water generated in the five stages of the desalination process was separately measured that the highest amount of production was associated with the first stage.

In the present study, a five‐stage solar still is investigated in which paraffin wax was considered as the phase change material. Considering the climatic conditions of Iran and its locating in an arid area and shortage of freshwater resources, it is essential to produce freshwater from saline water resources. So, herein, a multi‐stage solar still as a commonly used water desalination apparatus with paraffin wax phase change materials was designed, fabricated, and tested. On the back of the collector absorber, the paraffin wax was applied for storing the additional solar energy. The results showed that the application of paraffin‐wax phase‐change agent leads to a 15% increase in freshwater production. The quantity of water generated was also studied in the flow rates of 0.7, 1.3, and 1.8 L/min with and without phase change materials. The results showed that the highest quantity of water produced is obtained in the 1.3 L/min flow rate. Finally, the quantity of water generated in the five stages of the desalination process was separately measured and found that the water produced in the first stage was 70% of the total water.