Membrane Absorber for Absorption Refrigeration Technology

Abstract

This work addresses efficiency improvements in vapor-compression refrigeration and heat pumps by advancing absorption technology, which can run on low-cost heat sources such as waste heat or solar energy but is currently limited by large component size and high investment cost. It focuses on membrane contactors for ammonia–water absorption, aiming to shrink absorber volume while improving mass transfer through high interfacial area and higher liquid velocities, with the membrane keeping gas and liquid bulk phases separated for robust operation and potential mobile use. Several polypropylene hollow-fiber membrane modules with varying fiber geometries, pore sizes, and fiber counts were designed and built, using ammonia-resistant epoxy potting. Lab-scale absorption experiments were performed and compared with simulations, showing that the dominant mass-transfer resistance is on the liquid side and that the membrane adds little additional resistance. The model requires fitting only one module-specific parameter that is independent of temperature, pressure, and ammonia concentration, enabling reliable predictions from limited data. Operationally, preventing liquid breakthrough is critical; breakthrough pressure depends on pore size, liquid surface tension, and contact angle, and breakthroughs are most likely under high liquid pressure drop or during start-up/shut-down, while gas breakthrough can slightly enhance absorption by creating extra interfacial area. The technology is validated in a small (≤10 kW) ammonia–water absorption refrigeration plant where a membrane absorber with metal housings and plate heat exchangers was installed in a bypass and compared against a conventional plate absorber. Plant experiments match lab findings and are well predicted by the model, demonstrating that membrane absorbers can reduce absorber volume by roughly 50–90% relative to plate absorbers (with shell-and-tube absorbers being much larger still). The results indicate a pathway to small-capacity absorption refrigeration (about 5–20 kW) with reduced size and cost. Future work highlights integrating cooling directly into membrane modules, designing for economical manufacturing, and exploring alternative working pairs, potentially including ionic-liquid absorbents. The project was conducted through a multi-institution collaboration and funding, and the outcomes contributed to founding Makatec GmbH to commercialize membrane absorption refrigeration technology.