Ultrahigh freshwater production achieved by unidirectional heat transfer interfacial evaporation solar still integrated with waste heat recovery

•Propose a unidirectional heat transfer solar still (UHTSS) with multistage condensation structure.•Analyze the heat transfer advantages of UHTSS and optimize the parameters of the solar still based on this.•The evaporation rate can reach 6.0 kg·m−2·h−1 under sunlight, and the collection rate of fresh water can reach 4.5 kg·m−2·h−1. Solar-driven interfacial evaporation technology exhibits notable performance in evaporation, with evaporation efficiency typically maintained at over 80 %. However, utilizing interfacial evaporation for efficient seawater desalination remains a challenge due to low production of freshwater. Therefore, we propose a desalination system based on unidirectional heat transfer solar still, which is capable of combining efficient evaporation with waste heat recovery to achieve high energy utilization. The design of the solar still decomposes the light transmission and condensation functions of the glass cover plate in traditional solar still for independent optimization and reserves space for integrating high-performance interfacial evaporation materials, while minimizing heat loss by ensuring unidirectional heat transfer inside the still. In particular, the proposed unidirectional heat transfer solar still provides a free interface to condense the vapor, where a multi-stage condensation is integrated to recycle latent heat. When using a self-made hydrogel sponge as the interfacial evaporation material, the solar still achieve efficient and stable freshwater production. Outdoor experiments showcase a evaporation rate of 6.0 kg·m−2·h−1, coupled with an freshwater production rate of 4.5 kg·m−2·h−1 under an average sunlight intensity of 1.07 kW·m−2. Our results demonstrate that the freshwater production of the solar still can be further improved. This work is beneficial for further enhancing freshwater collection of solar desalination.