Large Temperature Jump Method Research Articles

Evaluation of ad/desorption dynamics of S-PEEK/Zeolite composite coatings by T-LTJ method

In this work, the adsorption/desorption kinetics of a consolidated bed, for adsorption heat pumps application, coated with a new composite adsorbent material based on SAPO-34 zeolite in a matrix of sulfonated poly(ether-ether-ketone) was investigated. The heat exchanger was coated by drop casting with a composite zeolite/polymer mixture containing 90 wt% of SAPO-34 powder. The adsorption/desorption kinetic was investigated using the Thermal Large Temperature Jump method and compared with the same heat exchanger filled with loose grains of SAPO-34. The coated heat exchanger exhibits very high specific cooling capacity with a very fast adsorption kinetic. The characteristic times were approximately 39% lower compared to zeolite grains filled adsorber. Analogously, the obtained maximum specific power value was 85% higher than that of the uncoated one.

Adsorber heat exchanger using Al-fumarate beads for heat-pump applications - a transport study.

Metal-Organic Frameworks (MOFs), thanks to their type V water adsorption isotherms ("S-Shape") and large water capacities, are considered as potential breakthrough adsorbents for heat-pump applications. In particular, Al(OH)-fumarate could enable efficient regeneration at a lower temperature than silica-gel which would allow us to address the conversion of waste heat at low temperature such as found in data centers. Despite its greater adsorption capacity features, heat and mass transport limitations could jeopardize the potential performance of Al(OH)-fumarate. Heat and mass transport depend on the size of the bodies (mm range), their packing and on the pore structures, i.e. macro-mesopore volumes and sizes. This paper describes the cost-efficient and scalable synthesis and shaping processes of Al(OH)-fumarate beads of various sizes appropriate for use in water Adsorption Heat-Pumps (AHPs). The objective was to study transport limitations (i.e. mass and heat) in practical e beads which meet mechanical stability requirements. Dynamic data at the grain scale was obtained by the Large Temperature Jump method while dynamic data at the adsorber scale was obtained on a heat exchanger filled with more than 1 kg of Al(OH)-fumarate beads. Whereas the binder content had little impact on mass and heat transfer in this study, we found that Knudsen diffusion in mesopores of the grain may be the main limiting factor at the grain scale. At the adsorber scale, heat-transfer within the bed packing as well as to the heat exchanger is likely responsible for the slow adsorption and desorption kinetics which have been observed for very low desorption temperature. Finally, the dynamic aspects of the observed water adsorption isotherm shift with temperature are discussed in light of reported reversible structure modification upon temperature triggered water adsorption-desorption.

Composite “LiCl/MWCNT/PVA” for adsorption thermal battery: Dynamics of methanol sorption

Adsorption thermal storage and transformation (ATST) of low-temperature heat is an energy saving technology towards the efficient use of renewable and waste heat. A solid sorption thermal battery (SSTB) is a promising concept for low-grade heat storage, combined cooling and heating, integrated energy storage and energy upgrade. Current progress in SSTB is related to the selection of advanced adsorbents and cycles which are properly adapted to ATST in various climatic zones. This paper mainly addresses such adaptation for China, Russia, and India which are among the top CO2 emitters and partially for Italy and Portugal. First, climatic data for selected cities of these countries were analyzed to specify adsorbents optimal from the thermodynamic point of view. It was found that an innovative sorbent “LiCl inside Multi-Wall Carbon NanoTubes (MWCNT)” is one of the most promising and universal for SSTB operating in several selected climatic conditions.To further elucidate the composite usability, especially, in cooling/(air conditioning) cycles, the experimental dynamic study of methanol sorption on this sorbent was performed. The study included shaping the LiCl/MWCNT composite as grains using polyvinyl alcohol as a binder, and the measurements of methanol sorption/desorption dynamics under conditions of the selected ATST cycle. The dynamics, studied by a Large Temperature Jump method, revealed the fast ad/desorption that led to high specific power and smaller SSTBs. Hence, the selected composite is a promising candidate for SSTB applications in the climatic zones involved.

NH2-MIL-125 as promising adsorbent for adsorptive cooling: Water adsorption dynamics

Adsorption heat transformation (AHT) has attracted an increasing research interest as energy saving and environmentally benign alternative to vapor compression systems. Novel adsorbent NH2-MIL-125 could be promising for AHT owing to its high water adsorption capacity and good hydrothermal stability, although its dynamic properties have not been tested yet. In this paper the results of dynamic study of water adsorption on loose grains of NH2-MIL-125 are presented. The adsorption dynamics is studied by a Large Temperature Jump method under typical operating conditions of isobaric stages of adsorptive cooling cycle. The effects of the adsorption/ desorption temperature, adsorbent grain size and number of the grain layers are explored. The water adsorption on the grains of 0.2–1.8mm size are shown to occur under at “grain size insensitive” mode as the adsorption rate is determined by the ratio S/m of the heat transfer area to the adsorbent mass regardless the grain size. Indeed, the ad/desorpion rate and the Specific Cooling Power (SCP) are proportional to the (S/m)-ratio. Quite high SCP-values of 0.4–2.8kW/kg can be obtained in adsorption chillers having a large (S/m)-ratio of 1.6–6.9m2/kg, which is of high practical interest.

Dynamics of water vapour adsorption by a monolayer of loose AQSOA™-FAM-Z02 grains: Indication of inseparably coupled heat and mass transfer

Dynamics of water adsorption in an adsorption chiller (AC) determines the specific cooling power and the AC size. The dynamics, in its turn, strongly depends on the configuration of an “adsorbent – heat exchanger” (Ad-HEx). Here we considered the simplest, monolayer, configuration of the Ad-HEx and studied the water ad-/desorption dynamics as a function of the grain size (0.2–2.1 mm), temperature drop (16–36 °C), and pressure (7–47 mbar) for a commercial adsorbent AQSOA™-FAM-Z02. The dynamics was measured by a Volumetric Large Temperature Jump method under typical conditions of isobaric stages of real AC cycle.It is found that the initial part of all kinetic curves is exponential and can be described by a single characteristic time τ. This time depends on the grain size R as τ ∼ Rα, where the index α is constant over the whole range of the grain sizes, and its value is 1.33 and 1.20 for adsorption and desorption runs, respectively. The α-constancy may indicate an inseparable coupling of heat and mass transfer in the monolayer configuration of AQSOA™-FAM-Z02 adsorbent bed. For grains of about 2 mm size, quite acceptable initial power (>2 kW/kg) is obtained. Thus, the monolayer configurations may result in fast adsorption dynamics, reasonable specific power and compact AC units.

Dynamic optimization of adsorptive chillers: The “AQSOA™-FAM-Z02 – Water” working pair

Despite significant progress in commercial ACs (adsorption chillers) achieved for the last thirty years, still there is a much room for their further improvement. This paper addresses the optimization of dynamic performance of ACs utilizing loose grains of a novel adsorbent AQSOA™-FAM-Z02. The dynamic data were measured by a Volumetric Large Temperature Jump method under typical conditions of isobaric ad/desorption stages of real AC cycle. For the first time, the effects of number of the adsorbent layers (N = 2, 4, and 8), grain size (0.2–0.9 mm) and cycle boundary conditions were comprehensively studied. The most notable findings revealed for the water ad/desorption dynamics are: 1) the initial part of all kinetic curves is exponential and can be described by a single characteristic time τ; 2) at equal values of the ratio (S/m)=(heat transfer surface)/(adsorbent mass), the dynamic curves are very close regardless the adsorbent grain size R (the “grain size insensitive” regime is observed at 0.44 m2/kg ≤ (S/m) ≤ 1.75 m2/kg); 3) the maximal specific cooling power is a linear function of the (S/m)-ratio. Appropriate recommendations on improving the AC cycle dynamics which concern the optimal conversion degree, grain size and (S/m)-ratio are made.

Prediction of SCP and COP for adsorption heat pumps and chillers by combining the large-temperature-jump method and dynamic modeling

Adsorption heat pumps and chillers provide sustainable heating and cooling by transforming waste or solar heat. To increase the efficiency of adsorption heat pumps and chillers, new adsorber designs are constantly proposed. However, evaluating the efficiency of new adsorber designs is a time- and money-consuming task. To reduce time and costs, we combine two methods in this work: First, the kinetic performance is evaluated experimentally using the Large Temperature Jump method for a representative adsorber part. Second, a dynamic model for the adsorber is developed and validated. The validated adsorber model is used in a dynamic model of a complete adsorption heat pump cycle. Our approach allows for a sound analysis of novel adsorber designs by determination of the efficiency indicators specific cooling power (SCP) and coefficient of performance (COP) based on a representative adsorber part. We studied a representative adsorber part made from an extruded aluminum tube heat exchanger filled with silica gel 123. We determined SCP and COP as function of adsorption and desorption times and found a maximum SCP 268 W kg−1 with a corresponding COP of 0.51 for a desorption time of 125 s and an adsorption time of 200 s.

Adsorption Dynamics in Adsorptive Heat Transformers: Review of New Trends

This review addresses recent advances in understanding and trends in studying dynamics in adsorptive heat transformers (AHTs), which are considered as an alternative to common compression and absorption machines. Experimental data have been obtained mainly by a new large temperature jump method (LTJM) specifically developed to imitate the conditions of isobaric stages of AHTs. Here we consider merely a simple but quite realistic configuration of an “adsorbent–heat exchanger” (Ad-HEx) with n-layers of loose adsorbent grains (n = 1–8). Water, methanol, and ammonia are used as adsorbates. Several recommendations following from this experimental study have been checked with prototypes of adsorptive chiller in ITAE-CNR (Italy) and the University of Warwick (UK). These tests clearly demonstrated that the dynamic performance of AHT can be significantly improved by a proper management of AHT cycle and Ad-HEx geometry. The most important findings and general regularities that have been revealed by systematic studying of the adsorption dynamics of water, methanol, and ammonia in AHT systems are summarized, illustrated, and discussed in this review.

Dynamic study of adsorbers by a new gravimetric version of the Large Temperature Jump method

This paper presents a new experimental setup devoted to measure the ad-/desorption kinetics of an Ad-HEX (adsorbent+heat exchanger) under typical boundary conditions of an Adsorption Heat Transformer (AHT) as well as the results of the first test campaign carried out. The experimental apparatus can be considered as a gravimetric version of the known Large Temperature Jump method. In fact, the dynamic evolution of the uptake during the isobaric ad-/desorption stages is directly measured by a weighing system suitable to work in the range of 5–600g of sample mass (adsorbent+HEX) with the accuracy ±0.1g and the time response shorter than 0.1s The experimental campaign was conducted on an Ad-HEX composed of granules of a commercial SAPO-34 adsorbent placed on a flat type aluminum HEX, under operating conditions reproducing two different thermodynamic cycles (Th=90°C, Te=10°C, Tc=30 and 35°C), typical for adsorption air conditioning.The influence of the grain size (ranging from 0.350 to 2.5mm) on the adsorption dynamics both in monolayer and multilayer configurations at variable and constant “heat transfer surface/adsorbent mass” ratios (S/m) was studied. The results showed that, for the Ad-HEX configurations tested, the adsorption dynamics can be properly described by a modified Linear Driving Force approach by the use of a single temperature-invariant characteristic time τ. The invariance of the specific cooling power was revealed when the S/m ratio was kept constant (S/m=1.23m2/kg). This ratio is found to be a useful parameter for both assessment of the dynamic perfection and optimization of various Ad-HEX configurations.

Novel ammonia sorbents “porous matrix modified by active salt” for adsorptive heat transformation: 5. Designing the composite adsorbent for ice makers

The novel adsorbent of ammonia based on binary salt system BaCl 2 + BaBr 2 inside vermiculite pores was intently designed for adsorption cooling cycle specialized for ice-making. On the base of analysis of the working conditions of the cycle the requirement for the optimal adsorbent was formulated in terms of the equilibrium temperature of reaction between the salt and ammonia and the real composite adsorbent (BaCl 2 + BaBr 2 )/vermiculite with required properties was prepared. The dynamics of ammonia adsorption on the composite was studied by a Large Temperature Jump method under working conditions of the ice-making cycle. The maximum cooling power W 0 realized at t → 0 was estimated as 1.2 kW/kg. The composite allows the production of 2 kg/(kg h) of ice. Such a good performance demonstrates an advantage of the target-oriented design of the composite adsorbent with predetermined properties matching the particular conditions of the cooling cycle. ► Composite adsorbent (BaCl 2 + BaBr 2 )/vermiculite was intently prepared for ice makers. ► Dynamics of ammonia sorption was studied under conditions of adsorption cooling cycle. ► The maximum cooling power W 0 was estimated as 1.2 kW/kg. ► The composite allows production of 2 kg of ice per 1 kg of the adsorbent per hour. ► Such a good performance was a result of the target-oriented design of the adsorbent.

Composite sorbent of methanol “LiCl in mesoporous silica gel” for adsorption cooling: Dynamic optimization

Abstract A novel composite sorbent of methanol “LiCl in mesoporous silica gel” has recently been proposed for AC (adsorption cooling). Its testing in a lab-scale adsorption chiller resulted in the specific cooling power of 210–290 W/kg and the cooling COP of 0.32–0.4. Although these values are rather encouraging, a room for their enhancement still exists. The aim of this paper was a dynamic optimization of the composite performance in AC cycles. Dynamics of methanol sorption on loose grains of the LiCl/silica composites was studied by a Large Temperature Jump method under typical conditions of AC cycle. Effects of number of the sorbent layers, salt content, grain size and cycle boundary temperatures were studied. Physico-chemical processes in the three-phase system (salt, solution, vapor) were shown to be quite complex and can strongly affect the dynamics of methanol ad-/desorption. Several obstacles which can retard the sorption were analyzed. Appropriate recommendations on improving the cycle dynamics, which concern optimal conversion degree, salt content and relative durations of ad- and desorption phases, were made.

Novel ammonia sorbents “porous matrix modified by active salt” for adsorptive heat transformation: 4. Dynamics of quasi-isobaric ammonia sorption and desorption on BaCl2/vermiculite

A novel composite sorbent of ammonia BaCl 2 /vermiculite has recently been proposed and tested for adsorption cooling. The aim of this paper was a detailed study of its dynamic operation under conditions of isobaric stages of an adsorption chiller cycle. Experiments were performed by a Large Temperature Jump method at the ammonia pressures 5.8, 6.9, 12.2 and 15.8 bar. The ad- and desorption temperatures were fixed, respectively, at 15, 20, 25, 30, 35 °C and 65, 70, 80, 90 °C. All kinetic curves were described by an exponential function with a single characteristic time τ . The rate constant k = 1/ τ was found to be a linear function of the temperature difference which drives the process, hence, heat transfer between the sorbent layer and the plate heat exchanger determines the process dynamics. The appropriate heat transfer coefficient does not depend on the ammonia pressure and process temperatures and equals 90 W/(m 2 ·K). The data obtained were used for analyzing the dynamic performance of adsorption chiller with this composite as a solid sorbent. Appropriate recommendations on optimization of working cycle are made. The analysis showed that the specific cooling power could reach 690–860 W/kg at the cycle duration of 100–300 s.