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.
Highlights
One of the measures of the European Council to reduce greenhouse gas emissions is to improve energy efficiency [10]
Summarizing, this study investigates the effect of adding CO2 to the working fluid of wet compression resorption heat pumps which work with ammonia-water
The model modified by Que and Chen [36] and the new fit are especially compatible with the extended UNIQUAC model at low ammonia concentrations
Summary
One of the measures of the European Council to reduce greenhouse gas emissions is to improve energy efficiency [10]. Thermodynamic properties of the ternary mixture NH3-CO2-H2O are investigated by comparing the extended UNIQUAC model with modified and improved e-NRTL models over a large range of operating conditions. A new fit is made to further improve the e-NRTL model (extend its application range) These properties are used to predict the performance of the mixture when applied in wet CRHP making use of a model that takes into account the major irreversibility's of the cycle: driving forces for heat transfer and deviation from isentropic compression. The results indicate promising enhancement for the COP, pressure ratio, and the pressure levels for certain applications This solution has the potential to make a strong impact in the industry; increasing the energy efficiency of many processes and in that way reducing emissions. CO2ðaqÞ þ H2OðlÞ$HCOÀ3 þ Hþ HCOÀ3 $CO23À þ Hþ H2OðlÞ$Hþ þ OHÀ NH3ðaqÞ þ HCOÀ3 $NH2COOÀ þ H2OðlÞ
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