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Aqueous ammonia‐based post‐combustion carbon capture (PCC) is a well recognized and leading technology for the reduction of CO2 emissions from coal‐fired power stations. Despite its many techno‐economic advantages over its counterparts, there are still a few major challenges that could prevent its large‐scale adoption. This model‐based study addresses the most common problems of solid precipitation at the stripper overhead and ammonia slipping with the product CO2. We propose using a direct contact condenser (DCC) to replace the conventional gas / liquid heat exchanger (HX) at the stripper overhead. Three scenarios were proposed and assessed for DCC configuration: open, closed, and combined DCC circuits. It was found that combined‐circuit DCC could better enhance the CO2 product's purity, bringing its temperature to the nominal target, and producing less condensate, which is diluted and easier for downstream integration. Most important, this configuration can effectively eliminate ammonia slipping in the product line and solid precipitation at the stripper overhead. Validating these improvements at our current pilot plant would bring tremendous benefits for the commercialization of this technology. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Compression-thermochemical cooling system with electricity & low-grade heat input. • 35 K heat source temperature reduction through compression-assisted desorption. • Quasi-steady simulation model validated accounting for heat and mass transfers. • Preliminary case study with bi-stage compression and cold production at −25 °C. • 2 times higher COP than vapor compression during ammonia de-storage phase. Abstract This paper studies a hybrid system for cold production consisting of a compression cycle combined with a thermochemical process by sharing the same condenser, evaporator and refrigerant fluid. The aim of this hybridization is to solve mismatch issues between the demand of cold and the source of energy (availability and/or price) with a system as compact as possible. One important side benefit is that the interaction between the compressor and the thermochemical reactor reduces the activation temperature for ammonia desorption in the thermochemical reactor. To study this interaction a quasi-steady simulation model for both storage and de-storage phases has been developed and experimentally validated by means of a small scale (approx. 300 Wh of cold storage) experimental bench with ammonia as refrigerant and barium chloride (BaCl 2) as reactant salt. Experiments proved a 35 K reduction in the activation temperature of the desorption reaction with respect to desorption without compressor. Model validation by adjusting permeability and thermal conductivity of the reactive composite showed an acceptable agreement between predicted and experimental reaction advancement-time curves. The validated model was used for simulation of the system in a preliminary case study, representative in power (40 kW) and temperature (−25 °C) of an industrial cold demand. It is shown that during ammonia de-storage, the hybrid achieves a higher COP than a conventional mechanical vapor compression system. It increases exponentially with the relative share of thermochemical storage in the cold production. [ABSTRACT FROM AUTHOR]

Two stage ammonia-water absorption refrigeration system is applicable for low driving temperature heat source and large temperature lift applications. However, low system performance weakens its advantages because the large heat dissipation brings unacceptable power consumption. Better internal heat recovery is significantly effective for system performance improvement. This paper presents an analysis on maximum internal heat recovery of a mass-coupled two stage ammonia-water absorption refrigeration system by pinch technology. Two sets of freezing conditions are assumed to carry out the analysis. The minimum system heat input and the relevant heat matching are determined by problem table method and grid method. Moreover, the feasible system configurations with optimal energy target are presented under the set conditions according to the grid diagram. The system performance is calculated directly from the problem table. The key point of the maximum internal heat recovery is the heat matching of the streams in the temperature intervals which are adjacent to the pinch point. Compared to a conventional system, the thermal COP of the derived system can be improved by 14.5% and 34.1% under the studied freezing conditions. The improvement is more effective when there is a temperature overlap between the generation and absorption processes. [ABSTRACT FROM AUTHOR]

The present study deals with a compression-absorption machine. The proposed hybrid system operates with water-ammonia and functions at three pressure levels. The absorber operates at an intermediate pressure Pint taken between the evaporator pressure (PEV) and the condenser pressure (PCD), unlike the one stage machine which functions at two level pressures. The system object of this work is examined and compared to the conventional cycle. The analyses of the numerical results highlight that the coefficient of performance of the novel proposed configuration is better than that relative to the simple effect absorption machine and reveals the great importance of the intermediate pressure not only in the performance improvement but also on reducing the generator temperature allowing the system to operate at low enthalpy sources. We showed for an evaporator temperature and a condenser temperature fixed respectively to -10 °C and 40 °C, the proposed hybrid refrigeration cycle, operates at a generator temperature TGE = 75 °C with an installation's COP about 0.56. [ABSTRACT FROM AUTHOR]

Abstract: A one-dimensional, steady state model for absorption of ammonia vapor into a constrained microscale film is presented. A weak solution of ammonia–water flows in a microchannel into which ammonia vapor bubbles are injected in cross flow from a porous wall. A counter flowing coolant solution removes the heat generated due to absorption from the opposite wall. The 1-D, steady state species and energy transport equations are solved to yield, along the length of the channel, concentration and temperature profiles of the solution stream and the temperature profile of the coolant fluid stream. The model is validated from experimental measurements of global parameters. A parametric study of fluid and geometrical parameters based on the validated model is presented. Results show that a balance between the residence time within the absorber and the absorption time scales, by way of adjusting the mass flow rates of the vapor and weak solution, is needed to ensure complete absorption. A lower coolant inlet temperature significantly enhances absorption rate by increasing the local concentration difference between the saturation and bulk values. The absorption rate is more sensitive to the liquid–vapor interfacial area than to the heat transfer area between the solution and the coolant. [Copyright &y& Elsevier]

Abstract: This paper reports on the experimental research conducted to study the condensation of ammonia on smooth and integral-fin (32fpi) titanium tubes of 19.05mm outer diameter. Experiments were carried out at saturation temperatures of 30, 35, 40 and 45°C and wall subcoolings from 1 to 8°C. The results show that the condensation coefficients on the smooth tubes are well predicted by the Nusselt theory with an average error of +0.66% and within a deviation between −6.6% and +8.3%. The enhancement factors provided by the integral-fin tubes range from 0.77 to 1.22. The low enhancement factors are due to the high condensate retention between fins, which brings about flooded fractions of the tube perimeter from 62.9% to 73.2%, and the low thermal conductivity of titanium. The Briggs and Rose [1994. Effect of fin efficiency on a model for condensation heat transfer on a horizontal, integral-fin tube. Int. J. Heat Mass Transfer 37, 457–463.] model, which accounts for the conduction in the fins, predicts the experimental data with a mean overestimation of 20%. The analysis of the partial thermal resistances in the overall heat transfer process points out the convenience of enhancing the outside ammonia condensation when high water velocities are considered inside the tubes. [Copyright &y& Elsevier]

Abstract: Attention is drawn, to the fact that, while four different correlations for condensation in microchannels are in fair agreement for the case of R134a (on which the empirical constants in the correlations are predominately based) they differ markedly when applied to other fluids such as ammonia. A wholly theoretical model is compared with the correlations for both R134a and ammonia. [Copyright &y& Elsevier]

Studies of diffusion-absorption (DAR) refrigerators have intensified, as well as the market's interest in increasing energy efficiency through cogeneration systems. Several studies have been published with the objective of improving the coefficient of performance, through the most different procedures. This article evaluates a non-intrusive method for the calculation of the ammonia mass flow rate at the inlet of the expansion device. The required parameters are geometric data and condenser temperature. The air-side convection heat transfer coefficient was calculated considering six empirical correlations. Then, throughout an energy balance in the condenser, the mass flow rate was computed. Four correlations predicted similar heat transfer coefficient. Thus, it is possible to estimate the ammonia mass flow using only the geometric data and the condenser temperatures. The strategy adopted to achieve this goal it was to use only the geometric data and the temperatures of the condenser and to calculate the energy balance in the superheated and saturation phases, since the absence of the subcooling in the condenser is a characteristic of the DAR cycle. • A non-intrusive method for the calculation of the ammonia mass flow rate of the DAR cycles to replace flow meters. • The ammonia mass flow is calculated based only in the temperatures and geometric parameters of the condenser • The air-side convection heat transfer coefficient is calculated considering six empirical correlations. • Throughout an energy balance in the condenser the mass flow rate is computed • The applied methodology proved to be satisfactory due to the equivalence of the results with the studied models. [ABSTRACT FROM AUTHOR]