Performance analysis of a solar-driven integrated direct-contact membrane distillation and humidification–dehumidification system

•Integration of membrane distillation with membrane humidification-dehumidification units.•Accurate multi-step model for modelling integrated system validated by experiments.•Effective energy recovery by averaged freshwater production increase of 33.5%•Averaged GOR and COP improvement of 37% and 27% by using the integrated system. Despite significant advantages of solar-driven membrane-based distillation systems, their large-scale application is limited due to high energy losses, and low technical and economic feasibility. To overcome the mentioned issues, a Direct-Contact Membrane Distillation (DCMD) module (as the primary distillation stage) was coupled with a hollow-fibre membrane-based air humidification–dehumidification (HD) unit (as the secondary distillation stage) to increase the water productivity of the system by recovering some of the lost energy in the first stage. The membrane-based humidifier was designed and implemented instead of the conventional packed columns to avert their drawbacks of being bulky, complex, less durable, and less reliable. An accurate and effective multi-step model was developed for modelling the integrated system considering the changing nature of thermo-physical properties. The suggested system was then built and tested in Perth, Western Australia. Experiments were conducted to determine the technical efficacy of incorporating the distillation from the second stage and its effects on freshwater production rate, gained output ratio, and overall efficiency of the integrated system. The results clearly indicated that the proposed two-stage system outperformed the conventional standalone solar DCMD and HD systems. The energy recovery unit was effective by increasing the average freshwater production by 34 % and 33 % for the replicated temperatures representing typical summer and winter climate conditions of Perth, respectively. The improvement in gained output ratio and coefficient of performance were observed to be 36 % and 38 %, and 26 % and 28 % for summer and winter temperatures, respectively. Moreover, the optimised air velocity of HD systems was found to be 1 m/s and the results indicated that the water production of the HD system at the considered flow rates (2–5 Lpm) was not sensitive to feed temperature at low values (