Approaching theoretical maximum energy performance for desiccant dehumidification using staged and optimized metal-organic frameworks

Dehumidification systems including desiccant wheels are widely used in commercial and a few residential air conditioning systems to remove latent loads. These wheels require high-temperature heat to regenerate and are often quite inefficient, with a typical single-pass efficiency of around 20%. In this work we propose a novel desiccant wheel system that takes advantage of the unique behavior of metal-organic frameworks (MOF) sorbents. We show that for any given entering air condition, there exists an optimal MOF that if included in the dehumidification systems will result in maximum efficiency. Based on this idea, we develop a multi-stage dehumidification system in which each stage uses the optimal MOF. To do so, we first mathematically model and validate a MOF-based desiccant wheel system and use it to systematically identify the optimal MOF isotherm shape as a function of inlet air conditions. Then we extend the model to different multi-stage MOF-based desiccant wheel systems and optimize them to achieve energy performance far exceeding a single-desiccant wheel. We use our validated discretized dynamic model to compare the energy needed for operation of these systems with other possible configurations. Finally, we show that an optimized staged MOF system can approach the theoretical maximum energy performance for desiccant dehumidification, using nearly 100% of regeneration energy for desorption of water and wasting very little. The results also show that a multi-stage MOF-based dehumidification system can have a regeneration efficiency 2 to 5 times greater than a single-stage system. Adding adsorption heat removal stages between the desiccant wheel stages can increase the regeneration efficiency by 5–20%, and the dehumidification effectiveness of the system 20–40%.