Enhanced performance of wet compression-resorption heat pumps by using NH₃-CO₂-H₂O as working fluid

Abstract

Upgrading waste heat by compression resorption heat pumps (CRHP) has the potential to make a strong impact in industry. The efficiency of CRHP can be further improved by using alternative working fluids. In this work, the addition of carbon dioxide to aqueous ammonia solutions for application in CRHP is investigated. The previously published thermodynamic models for the ternary mixture are evaluated by comparing their results with experimental thermodynamic data, and checking their advantages and disadvantages. Then the models are used to investigate the impact of adding CO₂ to NH₃-H₂O in wet compression resorption heat pump applications. For an application where a waste stream is heated from 60 to 105 °C, a COP increase of up to 5% can be attained by adding CO₂ to the ammonia-water mixture, without any risk of salt formation. Additional advantages of adding CO₂ to the ammonia-water mixture in that case are decreased pressure ratio, as well as an increase in the lower pressure level. When practical pressure restrictions are considered the benefits of the added CO₂ become even larger or around 25% increase in the COP. Nonetheless, when the waste stream was considered to be additionally cooled down, no significant benefits were observed.