• Energy Research

The thermal masses of components influence the performance of many adsorption heat pump systems. However, typically when experimental adsorption systems are reported, data on thermal mass are missing or incomplete. This work provides original measurements of the thermal masses for experimental sorption heat exchanger hardware. Much of this hardware was previously reported in the literature, but without detailed thermal mass data. The data reported in this work are the first values reported in the literature to thoroughly account for all thermal masses, including heat transfer fluid. The impact of thermal mass on system performance is also discussed, with detailed calculation left for future work. The degree to which heat transfer fluid contributes to overall effective thermal mass is also discussed, with detailed calculation left for future work. This work provides a framework for future reporting of experimental thermal masses. The utilization of this framework will enrich the data available for model validation and provide a more thorough accounting of adsorption heat pumps.

Abstract In this study, a new comprehensive model is developed that can predict the effective thermal conductivity and thermal contact resistance of the packed bed adsorbers, as a function of water uptake, number of adsorbent layers, particle size, bed porosity, temperature, contact pressure, and gas pressure. The proposed model is successfully validated against experimental data for AQSOA FAM-Z02, measured by a heat flow meter. The relative differences of the experimental data and predicted values for the packed bed effective thermal conductivity are 2% and 3% at 25 and 80 °C, respectively. By increasing the water uptake from 0 to 0.3 kg kgads−1, effective thermal conductivity of a 2 mm FAM-Z02 randomly packed bed is predicted to increase by 17% for temperature of 25 °C and 18% for temperature of 80 °C.

Abstract This study investigates the performance of a low-operating pressure evaporator using capillary-assisted tubes for adsorption cooling systems (ACS). When using water as a refrigerant in an ACS, the operating pressure is low (

The performance of the adsorption cooling system using the zeolite 13X/CaCl2 composite adsorbent was studied using a numerical simulation. The novel zeolite 13X/CaCl2 composite adsorbent with superior adsorption properties was developed in previous studies [11]. It has high equilibrium water uptake of 0.404 g/g between 25°C and 100°C under 870Pa. The system specific cooling power (SCP) and coefficient of performance (COP) were successfully predicted for different operation parameters. The simulated COP with the composite adsorbent is 0.76, which is 81% higher than a system using pure zeolite 13X under desorption temperature of 75°C. The SCP is also increased by 34% to 18.4 W/kg. The actual COP can be up to 0.56 compared to 0.2 for zeolite 13X-water systems, an increase of 180%. It is predicted that an adsorption cooling system using the composite adsorbent could be powered by a low grade thermal energy source, like solar energy or waste heat, using the temperature range of 75°C to 100°C. The performance of the adsorber with different design parameters was also studied in the present numerical simulation. Adsorbents with smaller porosity can have higher thermal conductivity and may result in better system performance. The zeolite bed thickness should be limited to 10mm to reduce the thermal response time of the adsorber. Addition of high thermal conductivity materials, for example carbon nanotube, can also improve the performance of the adsorber. Multi-adsorber tube connected in parallel can be employed to provide large heat transfer surface and maintain a large SCP and COP. The desorption temperature also showed a large effect on the system performance.

A new analytical model is developed for predicting thermal contact resistance (TCR) of non-conforming rough contacts of bare solids in a vacuum. Instead of using probability relationships to model the size and number of microcontacts of Gaussian surfaces, a novel approach by employing the “scale analysis methods” is taken. It is shown that the mean size of the microcontacts is proportional to the surface roughness and inversely proportional to the surface asperity slope. A general relationship for determining TCR is derived by superposition of the macro and the effective micro thermal resistances. The present model allows TCR to be predicted over the entire range of non-conforming rough contacts from conforming rough to smooth Hertzian contacts. It is demonstrated that the geometry of heat sources on a half-space for microcontacts is justifiable and that effective micro thermal resistance is not a function of surface curvature. A comparison of the present model with 604 experimental data points, collected by many researchers during the last forty years, shows good agreement for the entire range of TCR. The data covers a wide range of materials, mechanical and thermophysical properties, micro and macro contact geometries, and similar and dissimilar metal contacts.

Abstract Adding natural graphite flakes to sorbents of sorption cooling systems can significantly enhance the overall thermal diffusivity, while reducing the active material and increasing mass transfer resistance. To find an optimum compromise between these counteracting trends, the sorption performance of CaCl2-silica gel composite sorbents with 0–20 wt% graphite flakes content are tested using a custom-built gravimetric large pressure jump (G-LPJ) test bed. It is observed that in the early stages of sorption, i.e. sorption time less than 20 min, graphite flakes additives increase the specific cooling power (SCP) (from 365 to 604 W/kg for 5 min sorption time, a 65% increase) and coefficient of performance (COP) (from 0.46 to 0.54 for 5 min sorption time, a 17% increase) due to the enhanced sorbent thermal diffusivity (from 0.23 to 1.38 mm2/s, a 500% increase). Over time, as the sorbent approaches equilibrium, the performance enhancement deteriorates by increasing graphite flakes content since there is less active material in the composite. Furthermore, adding 20 wt% graphite flakes to the composite sorbent has led to a 67% increase in SCP0.8 (SCP when the sorbent reaches 80% of equilibrium).

Abstract Low thermal conductivity in packed bed adsorbers is a crucial challenge facing widespread adoption of low-grade heat adsorption thermal energy storage systems. In this work, thermal conductivities of 2-mm diameter AQSOA FAM-Z02 packed bed adsorbers with different numbers of adsorbent layers are measured, using a NETZSCH HFM 436/3/1E Lambda, in the temperature range of 10–80 °C and under atmospheric pressure. Effects of thermal contact resistance (TCR) between the adsorbent particles and the bed metal surfaces are deconvoluted from the total thermal resistance. Effective thermal conductivities of the adsorber packed bed are 0.188 and 0.204 W m−1 K−1 at temperatures of 10 and 80 °C, respectively. It is observed that the relative importance of TCR compared to the total thermal resistance of a monolayer FAM-Z02 packed bed, is 67% at 25 °C and under contact pressure of 0.7 kPa, which is significant and should be considered in the design of adsorption systems.