• Energy Research

Carbon capture is one of the recently raised technologies to mitigate greenhouse gas emissions. Adsorption was introduced as an energy-efficient carbon capture process, and the literature primarily shows the utilization of circular cross-sectional adsorption beds for this purpose. In this regard, this paper investigates different shapes of adsorbent beds to determine the thermal and adsorption uptake enhancements. Three geometries are considered: circular, square, and triangular cross-sectional beds. Mg-MOF-74 is used as an adsorbent, and numerical simulation is developed using a user-defined function coupled with ANSYS-Fluent. The results show that the triangular cross-sectional bed exhibits better adsorption capacity and thermal management compared to other beds. For example, the triangular cross-sectional bed shows 6 K less than the circular one during the adsorption process. It is recommended that the triangular cross-sectional bed be used for temperature swing adsorption when pumping power is not important. The square bed comes second after the triangular one with a lower pressure drop, suggesting such beds as good candidates for pressure swing adsorption. The square bed could be an excellent choice for compact beds when CO2 uptake and pumping power are both important.

Carbon capture by physical adsorption can strongly participate in the reduction of the carbon dioxide emissions from the flue gases with minimum energy penalties. In this paper, we report the experimental data for enhancing the adsorption separation capability of zeolite 13X incorporated with carbon nanotubes (CNT). Six samples (13X, XC1, XC2, XC3, XC4, and XC5) have been characterized by XRD. Equilibrium adsorption isotherms and actual dynamic behavior of adsorption breakthrough tests were conducted. XRD analyses show that all the samples have almost identical XRD patterns to pure 13X, while the equilibrium isotherms indicate that XC3 (0.5 wt% CNT/13X) has CO2 adsorbed amounts much closer to those of pristine 13X. Interesting results of actual capturing behavior were exposed during the breakthrough tests which confirm that the optimal adsorption separation is associated with XC3 with an increased adsorption capacity and separation breakpoint by about 21.4 and 25.3%, respectively, in comparison to pure 13X.

Abstract The thermodynamic balancing is a powerful tool to minimize the entropy generation in a thermal system. This tool is investigated to thermodynamically balance a novel desiccant-based humidification-dehumidification (HDH) system by extracting the recirculated air. It is carried out for a zero, single, double, triple, quadruple, quintuple, and infinity extractions. The HDH system is incorporated with a number of air dryers to produce freshwater from the humid air. An enthalpy pinch, which is described as the minimum air-to-desiccant enthalpy difference, is used as an index to approach the system thermodynamic balancing. For small values (

Abstract Thermal-based renewable and low-grade energy sources to operate humidification-dehumidification (HDH) desalination systems are critically reviewed. The investigated renewable energy sources are solar energy and geothermal energy. The low-grade energy sources such as the waste heat of photovoltaic thermal (PV/T) panels, refrigeration and heat pump systems, and power plants are also investigated. For each hybrid HDH with another driving system, the details of HDH construction, performance, hybridization method, and general observations are summarized and compared in tabular forms. Most of the studies focused on using solar energy and refrigeration and heat pump systems to drive HDH systems. The best performance indices (i.e., gained output ratio (GOR), freshwater production, and freshwater cost) can be obtained by the integration of HDH systems with power plants and then by geothermal energy, especially when a large quantity of freshwater is needed (>200 kg/h). Refrigeration systems and solar collectors can lead to higher GOR, medium water production, and higher cost. The application of PV/T results in the lowest water production. Despite the high performance of HDH driven by power plants and vapor-compression refrigeration systems, geothermal energy, solar collectors, and PV/T panels could be the right choices for hybridization with HDH systems in off-grid regions.

Abstract In this study, we report the effect of water vapor on CO2 uptake using Mg-MOF-74 via adsorption breakthrough modeling and lab experiments. Carbon dioxide is the most influencing gas that significantly expedites global warming. Therefore, it is ultimately necessary to reduce the rapid increase of CO2 concentration in the atmosphere by means of Carbon Capture and Storage (CCS). CO2 separation by physical adsorption is an interesting technology to achieve CO2 capture with minimum energy penalties. Metal-organic framework (MOF) adsorbents forms a class of adsorbents with much higher specific surface areas than conventional porous materials such as activated carbons, and zeolites. However, most MOFs show notable hydro instability for CO2 separation from humid flue gas. Mg-MOF-74 is a superior adsorbent amongst other adsorbents owing to its high CO2 uptake at flue gas conditions. A model is developed using User-Defined-Function in an ANSYS Fluent program. Two and three-dimensional models are validated by comparing their results with experimental work carried out by the authors, at ambient temperature, and published experimental data for high temperature conditions. The effect of water vapor is studied at different temperatures and various relative humidity values for Mg-MOF-74. Results indicate that CO2 uptake has been significantly reduced with the existence of more than 5% water vapor when Mg-MOF-74 is used as an adsorbent.

Abstract One of the most immediate technologies to mitigate the emission of carbon dioxide is carbon capture and storage. This study investigates different thermal design alternatives to increase the capacity of adsorption system given a desired cycle time. This is equivalent to investigating the reduction of the cooling and heating times during the adsorption and desorption processes, respectively. The thermal design investigation involves the variation of bed aspect ratio, the option of using water as a coolant, and augmenting the bed by fins with varying length and pitch. Mg-MOF-74 is used as adsorbent since it shows high CO2 uptake at flue gas conditions. An experimentally validated CFD model is implemented by using user-defined-function (UDF) linked to the ANSYS Fluent program. Results show that rising the bed aspect ratio from 2.8 to 41.6 improves the CO2 uptake by 13% and reduces the desorption period to the third. Additional improvement in the CO2 uptake (10.9–13.6%) can be obtained by cooling and heating the bed by water as a function of the bed aspect ratio. The improvement jumps to 22.2% if fins are used to augment the heat transfer rates in the bed. The optimal CO2 uptake enhancement (35.2%) is achieved by using finned annular-pipe (fin-pitch is 12.5 mm) as the adsorbent bed with water cooling. This design methodology can be adopted to other adsorption systems, including solar-driven adsorption chillers and solar-driven adsorption desalination systems.

Abstract Humidification-dehumidification desalination systems are best suited for small-scale and off-grid applications. The primary drawback of humidification-dehumidification systems is their low thermal performance. Thermodynamic balancing of the heat and mass exchange processes between the humidifier and dehumidifier can substantially improve their performance. The optimal thermal design is still needed; thus, the present study mostly focuses on the design of a thermodynamically-balanced system both with and without extractions. This study starts with the implementation of the temperature-enthalpy diagram model to investigate the effect of system enthalpy pinch and minimum and maximum saltwater temperatures on the performance of zero-, single-, and double-extraction systems. After that, design details are investigated to determine the size of the humidifier and dehumidifier under various conditions and power requirements. It is found that the system performance increases as a function of the heat and mass transfer areas (energy effectiveness) of the humidifier and dehumidifier. The systems’ performance is represented by the gained output ratio, recovery ratio, energy effectiveness, and enthalpy pinch. The gained output ratio of the single- and double-extraction systems is better than that of the zero-extraction system by a factor of 91% and 112%, respectively. The areas of the single- and double-extraction systems are larger than that of the zero extraction system by a factor of about 51% and 80%, respectively.

Abstract This paper evaluates an optimal airside thermal-hydraulic performance of compact heat exchangers. Seventy-five airside surfaces have been chosen, i.e. standard reference of Kays and London, to represent louver-fin, strip-fin, wavy-fin, plain-fin, pin-fin, finned circular tubes and finned flat tubes. A robust evaluation method is implemented by an estimation of the heat transfer rate per unit pumping power with and without considering the heat exchanger compactness. Experimental data of Colburn j-factor and Fanning friction factor (f) are used to estimate both the heat transfer rates and the friction power, respectively. The results demonstrate that strip-fin surface 1/10-27.03 shows an optimal heat transfer values per unit friction power when the compactness is taken into consideration. Nonetheless, pin-fin surface PF-4(F) shows an optimal heat transfer rate per pumping power when neglecting the importance of the compactness. The geometries having an optimal thermal-hydraulic performance for each airside type (with or without considering the compactness) are recommended as benchmarks to assess the performance of similar airside surfaces.