• 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.

The thermal conductivity and permeability of consolidated expanded natural graphite treated with sulphuric acid (ENG-TSA) were measured both parallel and perpendicular to the direction of compression used to produce the samples. Results showed that the thermal conductivity and permeability were highly anisotropic. The thermal conductivity perpendicular to the direction of compression was 50 times higher than that parallel to the direction of compression and the permeability was 200 times higher. The maximum thermal conductivity measured was 337 W m(-1)K(-1) at a bulk density of 831 kg m(-3). The permeability perpendicular to the direction of compression varied in the range of 10(-11) to 10(-16) m(2) as the density increased from 111 to 539 kg m(-3). The specific heat was measured, and the average value is 0.89 kJ kg(-1) K(-1) in the temperature range 30-150 degrees C. As a type of heat transfer matrix the thermal diffusivity was about five times higher than that of, for example, pure aluminium due to the combination of improved thermal conductivity with comparatively low density and reasonable specific heat. (C) 2011 Elsevier Ltd. All rights reserved.

A composite adsorbent composed of BaCl2 impregnated into expanded vermiculite has been synthesized and tested in a laboratory scale adsorption chiller. Previous work has established the promising theoretical performance of this adsorbent with ammonia as a refrigerant, in terms of equilibrium uptake, suitable equilibrium temperatures for use in air conditioning applications and good reaction dynamics. Analysis of the adsorption phase revealed a simple exponential approach to equilibrium uptake which was not previously observed in larger scale experiments. It was demonstrated that this material can provide effective operation of the chiller using a low potential heat source (80-90 degrees C) giving COP as high as 0.54 +/- 0.01 and SCP ranging from 300 to 680 W/kg,. The specific cooling power depends strongly on the driving temperature difference and the cycle duration. (C) 2010 Elsevier Ltd. All rights reserved.

Computational modelling of multiple-bed and thermal wave adsorption cycles is carried out in order to determine which method of heat recovery gives the superior trade off between coefficient of performance (COP) and power density (the power output per unit mass or volume of machine). The modelling is performed for the activated carbon-ammonia pair with a high power density plate heat exchanger type generator. It is discovered that multiple-bed cycles give a superior trade-off between COP and power density and are therefore recommended over thermal wave cycles. The principal application is considered to be a gas fired heat pump and it is found that the technology compares favourably with competing systems.

Gas-fired heat pumps are a potential replacement for condensing boilers, utilizing fossil-fuel resources more efficiently and reducing the amount of biogas or hydrogen required in sustainable gas grids. However, their adoption has been limited due to their large size and high capital cost, resulting in long payback times. For adsorption-based heat pumps, the major development challenge is to maximize the rate of heat transfer to the adsorbent, whilst minimizing the thermal mass. This work develops a modular finned-tube carbon–ammonia adsorption generator that incorporates the adsorbent in highly compacted 3-mm layers between aluminum fins. Manufacturing techniques that are amenable to low cost and high-volume production were developed. The module was tested using the large temperature jump (LTJ) method and achieved a time constant for adsorption and desorption of 50 s. The computational model predicted that if incorporated into two adsorption generators of 6 L volume each, they could be used to construct a gas-fired heat pump with a 10 kW heat output and a gas utilization efficiency (GUE, the ratio of useful heat output to higher calorific value of gas used) of 1.2.

A new type consolidated composite activated carbon (AC) was developed with a host matrix of expanded natural graphite treated with sulfuric acid (ENG-TSA). Samples with different density, different grain size of AC, and different proportion of AC were produced and thermo-physical properties were evaluated. Results show that the highest effective thermal conductivity and thermal diffusivity of consolidated composite AC were 34.2 W m−1 K−1 and 3.89 × 10−5 m2 s−1, which were 150 times and 72 times higher, respectively, than ordinary granular AC. The permeability of adsorbents ranged between 1.24 × 10−14 and 7.81 × 10−10 m2 while the density ranged between 215 and 448 kg m−3. The adsorption performance for composite AC and granular AC were evaluated by fitting experimental data with the equilibrium Dubinin–Astakhov (D–A) equations. Results show that the addition of ENG-TSA can improve the performance of adsorption refrigeration machines by increasing the concentration swing.

Abstract Composite adsorbents, comprising activated carbon and expanded natural graphite, have been developed, and their thermal conductivity, permeability and adsorption performance were tested. The thermal conductivity varied with the ratio of activated carbon to expanded natural graphite. Thermal conductivity increased as the ratio of expanded graphite increased. Considering that the density of activated carbon for the composite adsorbent should not be lower than 200 kg/m3, otherwise the volumetric cooling capacity would be unacceptably low, the highest thermal conductivity obtained from experiments was 2.47 W m−1 K−1. The permeability was also measured, and the best result obtained was 4.378 × 10−12 m2. In order to evaluate the influence of heat and mass transfer on adsorption performance, the adsorption rate was tested using a Rubotherm magnetic suspension balance, and results showed that for the freezing conditions lower than −10 °C the performance of granular activated carbon was better than that of solidified adsorbent because of the reduced mass transfer of ammonia at low saturated pressure. The adsorption performance of consolidated adsorbents increased rapidly when the evaporating temperature was higher than −10 °C. When the evaporating temperature was 8 °C, the adsorption rate of consolidated adsorbent was improved by 29% if compared with that of granular adsorbent.

The anisotropic thermal conductivities and permeabilities are investigated for discs and plates of compacted expanded natural graphite. The measuring directions of heat conductivity and permeability are both parallel and perpendicular to the pressing direction of compacted samples. An unexpected phenomenon is found in that the thermal conductivity sometimes decreases as the density of the material increases, and this phenomenon only occurs for thermal conduction parallel to the compressing direction. The results also indicate that the direction perpendicular to the compression direction shows higher thermal conductive properties and permeability values. Both anisotropic thermal conductivities and permeabilities are strongly dependent on density. Analysis shows that as a type of porous material, the ENG yields layers under the effect of pressure, and their orientation influences the values of heat conductivity and permeability of the different samples.