Low-grade heat-driven system utilizing thermo-responsive ionic liquid for greenhouses dehumidification and water recovery

Generating water from moist air offers a promising approach to sustainability in greenhouses. The generated water can be used for irrigation, and the resulting air can benefit crop cultivation. However, traditional methods for achieving this often rely on high-grade energy sources, such as electricity, resulting in a low energy efficiency. This study introduces a new system driven by low-grade heat at 32 °C to improve the energy efficiency. The new system replaces the conventional regeneration component of the liquid desiccant dehumidification system by a forward osmosis unit. The thermo-responsive ionic liquid, Rm-β-CD/oligo([vbim]TFSI), serves as the draw solution in the forward osmosis unit. It extracts water from a desiccant solution through osmotic pressure differences and separates the water using low-grade heat via liquid–liquid separation. The performance of the new system is assessed and compared with existing systems (vapor compression system and vacuum membrane system) based on the second law of thermodynamics. Results show the exergy efficiency, defined as the ratio of water recovery to exergy consumption, is an order of magnitude higher for the new system compared to existing systems. Following this, a theoretical study is conducted to guide the design of the new system. Findings indicate that the desiccant solution temperature and ionic liquid concentration significantly affect water recovery. Notably, lowering the inlet desiccant solution temperature of the dehumidifier from 25 °C to 15 °C can increase water recovery sixty-fold, achieving a relative humidity of 65 %, which satisfies most greenhouse requirements. Additionally, the system performs optimally in energy efficiency with CaCl₂ solution compared to LiBr solution. This work lays the groundwork for integrating a wider variety of low-grade heat sources into simultaneous dehumidification and water recovery systems designed for greenhouses. © 2024