Heat pumps can drastically reduce energy requirements in industry. Operating a compression resorption heat pump with an NH₃-CO₂-H₂O mixture has been identified as a promising option that can have an increased performance compared to only NH₃-H₂O. In this paper an important process of the heat pump cycle is investigated: The absorption process. A mini-channel heat exchanger with 116 tubes of inside diameter of 0.5 mm is used for this purpose. For the NH₃-H₂O experiments overall heat transfer coefficients of 2.7–6 kW/(m²K) were reached for mixture mass flows of 0.71–2.5 kg/h. For the NH₃-CO₂-H₂O mixture pumping instabilities limited the operating range which resulted in higher pressures and higher mixture mass flows compared to NH₃-H₂O. The overall heat transfer coefficients were lower in the case of the added CO₂, with the maximum of 3 kW/(m²K) corresponding to a mixture mass flow of 4.2 kg/h. However, an increase in heat transfer of approximately 5% was reached with the added CO₂ which is beneficial for heat pump applications. Additionally, limited research has been conducted on absorption in upward versus downward flow and, therefore, these two configurations have also been tested in the mini-channel heat exchanger. Even though the pumping instabilities vanished with absorption in upward flow it was confirmed that absorption in downward flow with the mixture on the tube side is the most beneficial configuration for absorption of ammonia in NH₃-CO₂-H₂O or NH₃-H₂O in a mini-channel heat exchanger. The performance increased by approximately 10% with absorption in downward flow.

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Heat pumps can efficiently upgrade waste heat from the industry and in that way reduce emissions. One of the main reasons why heat pumps are not applied to a greater extent in industry are large payback periods. Compression–resorption heat pumps (CRHP) enhanced by wet compression are considered a very promising option that can have higher coefficient of performance compared to traditional technologies when the heat source and/or sink have a large temperature glide. In this study the thermodynamic and economic performance of two potential industrial cases are examined for CRHP operating with NH₃–H₂O and NH₃–CO₂–H₂O. A detailed thermodynamic model of the compressor is used to evaluate the isentropic efficiency for each case. The results are used to calculate the simple payback period, when a boiler is replaced by a CRHP, as a function of the predicted gas and electricity prices in the Netherlands from 2020 to 2035. The results indicate that adding CO₂ to the NH₃–H₂O mixture increases the cycle COP when the temperature glide of the heat sink is 40 K while the opposite occurs when the glide is 80 K. The highest COPs and lowest payback times are obtained when the outlet vapor quality is around 0.50 for both the binary and ternary mixtures. Larger glides require higher outlet qualities. However, it is clear that even for high temperature glides the payback period can be within acceptable limits, especially if the cost of CO₂ emissions is taken into account.

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One of the most important steps we can take to minimize global warming is to become more energy efficient. An important example with potential for improvement is the large amount of energy wasted through waste heat streams in industry. For a long time there has already existed a solution that can efficiently upgrade waste heat streams to useful levels – heat pumps. However, heat pumps are rarely applied in industry. There are various reasons for this, including lack of knowledge and increased complexity. Another major reason is the long payback periods. This work focuses on a promising option that, for certain applications, can outperform traditional technologies by having a higher Coefficient of Performance (COP) and reaching higher temperatures: compression-resorption heat pumps (CRHP) using wet compression.@en

Compression-resorption heat pumps (CRHP) utilizing wet compression are a very promising option to upgrade waste heat from industry. CRHPs have the potential to have higher coefficient of performance (COP) than the traditionally used vapour-compression heat pumps (VCHP). However, commercial solutions utilizing wet compression are not available yet. Also, wet compression is a feasible option only if the efficiency of the compressor is sufficiently high, 0.7 or higher, as identified by several authors. In this study, we develop and validate a model of a twin screw compressor that is suitable for wet compression. The model is adapted to calculate the entropy production generation in order to identify where the major irreversibilities are located in the compressor. The effects of clearance size, rotational speed, ammonia concentrations, compressor inlet vapor quality as well as under- and over compression are analysed. The results show that the clearance size and the rotational speed have the largest effects on the entropy production. Additionally, increased ammonia concentration and decreased vapor quality lead to decreased losses. The results indicate that it should be feasible to reach the targeted performance if the clearances size is limited to 50 μm, the rotational speed maintained above 10,000 rpm, the ammonia concentration kept in the range of 30–40 wt.%, and the inlet vapor quality in the range 0.5–0.7.

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Compression-resorption heat pumps (CRHP) enhanced by wet compression are considered a very promising option to upgrade low grade waste heat from the industry. This is especially true for applications where the heat source and/or sink have a large temperature glide. But commercial solutions using wet compression are not yet available on the market. Compared to the classically used vapor compression heat pumps (VCHP), CRHP can provide much better performance but only if the efficiency of the compressor exceeds 0.7. In this respect, a twin screw compressor has been identified as a potential solution to reach this efficiency goal. This study makes use of the entropy production minimization to identify where the major irreversibilites are located in a CRHP system operated with an ammonia-water mixture. A detailed model of the wet compressor and the resorber are coupled with simplified models of other components of the HP cycle to analyze where potential improvements can be made. The results show that significant benefits can be obtained using CRHP.@en

Compression resorption heat pumps (CRHP) are a promising option to upgrade waste heat from industry. One way to optimize a CRHP is by minimizing the entropy production rate of the heat pump; that is to minimize the lost work of the system. In this paper a CRHP operating with an ammonia-water mixture is analysed based on an application case from the chemical industry. First, a global approach is used to analyse the entropy production rate of each of the components in the heat pump cycle. Based on the result from the global approach the component with the maximum energy dissipation in the heat pump is identified, in this case the resorber. The resorber is therefore further analysed with a detailed thermodynamic state model and entropy production is calculated using the theory of non-equilibrium thermodynamics. Measures to reduce the non-uniformity of the entropy production are then proposed based on the theory of equipartition. @en

Compression resorption heat pumps (CRHP) are a promising option to upgrade waste heat from industry. Alternative working fluids can further improve the efficiency of CRHP. The ternary mixture NH3-CO2-H2O has been identified as a promising working fluid for CRHP and has the potential to further enhance the coefficient of performance (COP) of the cycle compared to the traditionally used ammonia water mixture. So far the studies on the NH3-CO2-H2O mixture have focused mainly on carbon capture applications. But the desired operating conditions are different than for CRHP applications, e.g. the NH3 concentration. Additionally the absorption process with the mixture in tubular absorbers has not yet been reported. The focus of this study is therefore to investigate experimentally the absorption process of a CRHP with this ternary mixture. To reach this goal a model is developed for ammonia-water that takes into account the kinetics and mass transfer during the absorption process. To validate the model, experiments were performed for an absorption process in a mini channel heat exchanger with NH3 concentration of 35 wt%. The results show a good match between the model and the experiments. Additionally CO2 has been added to the solution and the experimental performance was compared with the experimental performance of the NH3-H2O mixture. A concentration of 2 wt% CO2 resulted in a performance increase of up to 5% however the working fluid flow became limited by pumping instabilities.@en

This paper identifies the advantages of compression resorption heat pumps for the recovery of waste heat and its upgrading to temperatures significantly above 100 oC. Experimental work illustrates the operation of the heat rejection side of the heat pump which indicates the feasibility of operation under such conditions. The experiments are mainly executed with ammonia-water as the working fluid but additionally experiments are reported with ammonia-water-carbon dioxide as the working fluid. These last experiments indicate that the ternary mixture leads to increased performance of the heat pump. It is concluded that the relative simplicity of compression resorption heat pumps and higher performance in comparison to alternative heat pump concepts makes this heat pump type very attractive for heat recovery purposes.@en

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.

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Couple of models have been developed to calculate thermodynamic properties of NH3-CO2-H2O systems. These models are typically an equation of state for the vapor phase and an activity coefficient model for the liquid phase (Que & Chen, 2011). The activity coefficient models can be divided into three groups based on previous studies, Pitzer model, electrolyte Non Random Two Liquid (e-NRTL) model and extended UNIQUAC model. Que & Chen (2011) deem the e-NRTL model the model the most suitable for process modelling and simulations since it requires only binary interaction parameters and makes use of mole fraction concentration scale consistently for both the short range local compositions interactions and the long range Debey-Huckel expression. Darde (2011) compared the built in e-NRTL model from Aspen Plus to an upgraded version of the extended UNIQUAC model developed by Thomsen et al. (1996). His findings were that the extended UNIQUAC model is significantly more accurate than the e-NRTL model from Aspen. He does mention that if the binary interaction parameters were better fitted to experimental data for NH3-CO2-H2O mixture, the e-NRTL model might become more competitive with the extended UNIQUAC model. Since then the e-NRTL model has been modified in this way by couple of authors, included Que & Chen (2011) and Niu et al. (2013). Both of their adjusted models have then been used by other authors for process modelling, for example Zhang & Guo (2014) used the model with adjusted parameters from Niu et al. (2013) and Liu et al. (2015) used the modified model from Que & Chen (2011). In this paper the extended UNIQUAC model is compared with the e-NRTL thermodynamic model that is built into the most recent version of Aspen Plus, and two modified e-NRTL models; the one developed by Que and Chen (2011) and a new fit. This is done to confirm if the modified models can reach similar accuracy as the extended UNIQUAC model and how much more accurate they are compared to the built in model in Aspen Plus.@en