• Energy Research

Abstract Experimental validation of simulated adsorber/desorber beds for sorption cooling applications is essential to obtain reliable results. We have conducted rigorous simulation of the adsorption process occurring in a finned tube adsorber utilizing 2D-axisymmetric geometry. The adsorber uses activated carbon–ethanol as adsorbent–refrigerant pair. It is cooled with water at nearly 30 °C and experiencing a sharp pressure increase of ethanol from 0.95 kPa initially to 6 kPa. The simulated temperatures at adsorbent thicknesses of 0, 1, 5 and 10 mm from tube outer diameter showed an increase in adsorbent temperature up to 20 °C from its initial temperature. They were slightly higher at start of adsorption and were consistent with experimental data at higher flow time. The validated CFD model will serve as a base for evaluating and optimizing activated carbon–ethanol adsorption cooling cycle. It can be extended also to different adsorber designs and other adsorbent–adsorbate pairs.

This paper presents adsorption isotherm data of CO2 onto two different types of highly porous activated carbons (ACs) for temperatures ranging from (−18 to 80) °C and pressures up to 10 MPa. The assorted adsorbents are activated carbon fiber (ACF) of type A-20 and activated carbon powder of type Maxsorb III. Adsorption isotherm data have been obtained using a volumetric technique and fitted to the Dubinin−Astakhov (D−A), Toth, Langmuir, and modified D−A equations. The latter considers the pseudosaturation pressure of CO2 that plays an important role for supercritical gas adsorption, and the pseudosaturation pressure was determined from the experimental data. The Toth and modified D−A isotherms correlate with the experimental data within 5 % root-mean-square deviation (rmsd) and present a better fitting than that of the Langmuir and the D−A equations. The isosteric heat of adsorption data were derived from the Toth and modified D−A isotherm equations and the correlation proposed by Chakraborty et al., and ...

Abstract Adsorption of carbon dioxide onto highly porous activated carbon based consolidated composite adsorbent has been experimentally investigated. Experiments have been conducted at temperatures of 30, 50, 70 °C and pressures up to 7 MPa using magnetic suspension adsorption measurement unit. The innovative adsorption isotherms data have been correlated using three isotherm models namely, Langmuir, Toth, and modified Dubinin-Astakhov (D-A). The studied models successfully fitted with the experimental data and Toth isotherm model shows a better fitting. Results showed that the volumetric adsorption capacity of CO2 onto the studied consolidated composite is higher than that of CO2 onto parent activated carbon powder (Maxsorb III). The isosteric heat of adsorption of the studied pairs has been calculated from isotherm data. The performance of ideal adsorption cooling cycle, employing consolidated composite adsorbent/CO2 pair, has also been simulated at three different evaporator temperatures, namely 5, 10 and 15 °C along with a coolant temperature of 25 °C and heat source temperatures ranging from 45 to 90 °C. The estimated thermodynamic parameters and isotherm data are important for further development of CO2 based adsorption cooling systems.

The adsorption rate is an important parameter for accurate performance estimation of adsorbent-refrigerant based adsorption cooling cycles. Here, we have investigated the response of two adsorption kinetics models of activated carbon–ethanol pair by means of CFD simulation. The isothermal assumption used in estimating the diffusion time constant of Fickian diffusion and linear driving force (LDF) models led to divergence and under-estimated adsorption uptakes, respectively. By including the simulated adsorbent temperature profile in fitting of LDF model to experimental data, we assessed the non-isothermal diffusion time constants which were 2.5 to 5 times higher than those evaluated previously with isothermal assumption. The goodness of fitting, evaluated with coefficient of determination (R2), improved and became higher than 0.95 from 0.73 initially. The developed non-isothermal LDF equation allows accurate heat and mass transfer simulations and performance optimization of large scale adsorption/desorption bed employing activated carbon-ethanol pair for adsorption cooling applications.

Adsorber heat exchanger design has great importance in increasing the performance of the adsorption-based cooling system. In this study, a transient two-dimensional axisymmetric Computational Fluid Dynamics (CFD) model has been developed for the performance investigation of finned tube type adsorber using activated carbon and ethanol as the working pair. The operating conditions of the cooling system were 15, 20 and 80 for evaporation, cooling and heating temperatures, respectively. The simulated temperature profiles for different adsorbent thicknesses were validated with those from experimental data measured in our laboratory. Moreover, the error in mass and energy balance were 3% and 7.88%, respectively. Besides, the performance investigation has been performed for cycle time ranging from 600 s to 1400 s. The optimum cycle time was 800 s and the corresponding evaluated specific cooling power (SCP) and coefficient of performance (COP) were found to be 488 W/kg and 0.61, respectively. The developed CFD model will be used for fin height and fin pitch optimization and can be extended to other adsorbent-adsorbate based adsorption cooling system.

Abstract In this study, the dynamic behavior of a 4-bed adsorption chiller was analyzed employing highly porous activated carbon of type Maxsorb III as the adsorbent and R1234ze(E), which global warming potential (GWP) is as low as 9, as the refrigerant. The simulated results in terms of heat transfer fluid temperatures, cycle average cooling capacity and coefficient of performance (COP) were obtained numerically. With 80 kg of Maxsorb III, the system is able to produce 2 kW of cooling power at driving heat source temperature of 85 °C which can be obtained from waste heat or solar energy. In particular, it can be powered by the waste heat from the internal combustion engine and therefore is suitable for automobile air-conditioning applications.

Abstract In this study, a transient mathematical model of a 4-bed adsorption chiller using Maxsorb III as the adsorbent and CO 2 as the refrigerant has been analyzed. The performances of the cyclic-steady-state system are presented for different heating and cooling water inlet temperatures. It is found that the desorption pressure has a big influence in the performances due to the low critical point of CO 2 ( T c = 31 °C). With 80 kg of Maxsorb III, the CO 2 based adsorption chiller produces 2 kW of cooling power and presents a COP of 0.1, at driving heat source temperature of 95 °C along with a cooling temperature of 27 °C and at optimum desorption pressure of 79 bar. The present thermal compression air-conditioning system could be driven with solar energy or waste heat from internal combustion engines and therefore is suitable for both residential and mobile air-conditioning applications.

Abstract Knowledge of adsorption characteristics of adsorbent-adsorbate pairs is essential for designing adsorption beds for adsorption cooling and adsorptive gas capturing applications. We investigated the adsorption isotherms and the adsorption kinetics of CO2 onto microporous activated carbon powder of type Maxsorb III. Measurements were performed with gravimetric apparatus for temperatures from 30 to 70 °C and pressures up to 7 MPa for adsorption isotherms and up to 4 MPa for adsorption kinetics. The gravimetric adsorption data obtained were consistent with previously measured isotherms with volumetric apparatus. Both absolute and excess adsorption data have been fitted precisely with Toth and Dubinin-Astakhov isotherm equations. The classical linear driving force (LDF) model with a constant mass transfer coefficient failed to correlate the experimental adsorption kinetics data. To overcome this problem, the authors presented a modified LDF equation with a variable mass transfer coefficient which is a function of the equilibrium and instantaneous uptakes. This modified LDF equation led to a better fitting and could be implemented easily in simulation of pressure swing adsorption (PSA), temperature swing adsorption (TSA) and adsorption chiller applications.