Large Temperature Jump Research Articles

Design and manufacture of a proof-of-concept resorption heat pump using ammonia-salt chemisorption reactions

Using the Large Temperature Jump (LTJ) experimental technique, alongside a review of the literature, sodium bromide (NaBr) and manganese chloride (MnCl2) have been identified as a suitable working pair with ammonia refrigerant for a proof-of-concept resorption heat pump system. LTJ tests using a tube-side and shell-side unit cell reactor (sorption heat exchanger), show that the experimentally obtained equilibrium lines for adsorption and desorption of sodium bromide are: ΔHADS = 30,102.5 J/mol; ΔSADS = 207.7 J/(mol·K); ΔHDES = 30,216.4 J/mol; and ΔSDES = 206.8 J/(mol·K). Using a semi-empirical model, the NaBr composite salt (salt impregnated in expanded natural graphite (ENG)) has been characterised for use as a low temperature salt in a resorption heat pump, with manganese chloride as the high-temperature salt. The model constants, A and n, for adsorption are 1 and 3, and for desorption are 5 and 4 respectively for NaBr. Manganese chloride data has been previously reported (Hinmers et al., 2022). With an appreciation of the reaction dynamics and behaviour of the NaBr and MnCl2 composite salts, a proof-of-concept resorption system has been designed and manufactured. The reactor design, alongside the overall experimental rig design (including data acquisition system) is reported. Initial filling and flushing tests show the success of the data acquisition and control system, and thus the overall suitability of the proof of-concept system for investigations into the coupled nature of ammonia salt reactions for a resorption heat pump application.

Scaling analysis of large pressure jump driven adsorption of water vapour in columnar porous silica gel bed

Adsorption process can be initiated either by Large Temperature Jump (LTJ) or Large Pressure Jump (LPJ). In this study, a theoretical analysis of coupled heat and mass transfer process during LPJ driven adsorption has been carried out using scaling principles of the governing conservation equations. A columnar domain consisting of silica gel (adsorbent) and water vapour (adsorbate) pair has been considered with heat and mass transfer directions orthogonal to each other. This domain is subjected to a sudden LPJ with isothermal boundary condition. Adsorption process has been divided into three phases for scaling analysis viz. i) pressure equalization ii) adsorption accompanied by heat generation and iii) heat removal phase. From order of magnitude arguments, various important physical scales for each phase have been derived and validated with a 2-dimensional computational fluid dynamics (CFD) study. The temperature rise in the adsorber bed is found to be 14.5 K, and the time scale to attain peak temperature is ~100 s from the initiation of adsorption. This justifies the short cycle time deployed in practical PSA systems to operate near isothermal condition. The outcomes of this investigation serve as a fundamental insight into LPJ driven adsorption process and help identify the various factors which practically effect this mode of adsorption.

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.

Modelling and Analysis of Ammonia Sorption Reactions in Halide Salts

This work has focussed on the development of an accurate method for testing and modelling the reaction kinetics involved in ammonia-salt adsorption reactions, something not achieved consistently to date. A Large Temperature Jump (LTJ) test cell has been developed for testing ammonia-salt reactions in real machine conditions. A Large Temperature Jump (LTJ) test cell has been developed for testing ammonia-salt reactions in real machine conditions. This was used to validate a new approach to modelling the behaviour and simulate the performance of chemisorption machines. A derivation of the heat transfer and thermodynamic equations are presented, and a finite difference model described which has been validated for the adsorption and desorption reactions of ammonia into halide salts within a porous matrix. The model is implemented in a MATLAB® program. Large Temperature Jump (LTJ) tests have been conducted on manganese chloride and barium chloride to validate the model and to identify the physical parameters which characterise the dynamic performance of the sorbent. The manganese chloride and barium chloride were impregnated in expanded natural graphite (ENG) (SGL SIGRATHERM®) board. The ENG board gave rise to practicable samples (31.5 mm OD ø over ½” tube) undergoing a desorption reaction in under 250 seconds with the fluid temperature 15°C above the equilibrium temperature, an order of magnitude faster than observed elsewhere. A new test method has been developed enabling an accurate single heat of reaction to be identified due to reduced hysteresis, which is reported for barium and manganese chloride. The model has been validated using experimental data from LTJ tests of two geometric configurations in radial heat transfer: discs heated/cooled from the outside radius (‘tube-side’) and annuli heated from the inner radius (‘shell-side’). The empirical data obtained is a milestone towards designed and optimised salt generators.

Ammonium Chloride (NH4Cl)—Ammonia (NH3): Sorption Characteristics for Heat Pump Applications

In a resorption heat pump, the adsorption and desorption reaction of ammonium chloride (NH4Cl) with ammonia (NH3) is of interest as a Low Temperature Salt (LTS). Reviewing previously published NH4Cl-NH3 equilibrium lines, ammonium chloride appears to offer useable working temperatures (50–70 °C) in the 10–15 bar pressure range during the adsorption reaction, and provides beneficial working conditions for the desorption reaction, when compared with alternative LTS candidates at atmospheric pressure. The NH4Cl-NH3 adsorption and desorption reactions, using a NH4Cl composite salt, have been evaluated under dynamic ‘real-world’ conditions in a Large Temperature Jump (LTJ) experimental testing rig; although there are concerns with mass transfer characteristics, the salt exhibits no hysteresis between the adsorption and desorption reactions, contrary to previous literature. The experimentally obtained equilibrium line values for the reaction enthalpy and entropy are 29,835 J/mol and 207 J/(mol∙K), respectively. Using a semi-empirical model, the NH4Cl composite salt has been successfully characterised, enabling the prediction of salt reaction behaviour. The model constants, A and n, identified are 4.5 and 5 for adsorption and 5 and 4 for desorption, with an overall salt active fraction (applicable to both reactions) of 0.98. Overall, the working equilibrium line and the dynamic performance of ammonium chloride has been investigated and the applicability of NH4Cl as a LTS for a resorption heat pump determined.

Design and Large Temperature Jump Testing of a Modular Finned-Tube Carbon–Ammonia Adsorption Generator for Gas-Fired Heat Pumps

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.

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.

Evaluation of in-situ coated porous structures for hybrid heat pumps

One of the main limitations for the wide diffusion of sorption systems, either as stand-alone and in hybrid configurations, is the low heat transfer inside the adsorber, as well as the low volumetric cooling power. In this context, the present paper reports the experimental activity on four different advanced configurations for the adsorber, based on microchannel heat exchangers where the gap between the channels is filled with porous structures where zeotypes of SAPO-34 family were synthetized. The porous structures evaluated are high-density fins, two different aluminium foams and compressed chips from the waste of aluminium machining. The sorption dynamic and cooling power density of each structure were measured through a Gravimetric Large Temperature Jump testing apparatus. The results obtained showed that the best-performing configuration is the one with high-density fins, that, for a 90/30/20 °C cycle showed a Specific Cooling Power up to 1.1 kW/kg. The other structures exhibit a much slower adsorption process, corresponding to power densities of about 0.3 kW/kg. The results were used for sizing a full-scale adsorber, whose expected Volumetric Cooling Power is 500 kW/m3.

Dynamics of adsorptive heat conversion systems: Review of basics and recent advances

The dynamics of adsorption heat conversion (AHC) systems is of crucial importance because it determines the AHC specific power, size, and cost. This paper addresses a survey of the basics and recent advances in the adsorption dynamics in various AHC beds to outline the current state of the art and future progress in this field. A survey of closed-form and numerical solutions reported in the literature for isothermal and non-isothermal adsorption systems is presented. New experimental methods based on reproducing the real driving force for AHC cycles are shown to be adequate to AHC dynamic study. A Large Temperature Jump (LTJ) method is now the most informative and fruitful approach for analyzing the AHC dynamics. Four LTJ-versions have been proposed and installed in many research labs all over the world. Old qualitative and new quantitative approaches to distinguish between various resistances in AHC systems are surveyed and analyzed. Further development of these methods could make them powerful tools for the practical optimization of adsorption dynamics in real AHC units.

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.

Modelling the Ammoniation of Barium Chloride for Chemical Heat Transformations

The coupling of reversible ammoniation reactions between two salts presents a method for the exploitation of low grade waste heat. This resorption configuration can be used for thermal transformation or heat pumping, to recover waste heat to primary producers, or for integration in heat networks. To understand the solid/gas reaction behaviour and to model its kinetics, Large Temperature Jump (LTJ) experiments were performed on a composite of barium chloride in an expanded natural graphite (ENG) matrix. A model has been built using a semi-empirical equation from the literature, which has been validated with the LTJ results. The results suggest the semi-empirical model provides a reasonable prediction for solid/gas reactions once the constants have been identified. Enhancing the model to handle sequential phase change reactions will enable a wide number of salts to be modelled, making the design of a resorption system practicable.

A new version of the Large Temperature Jump method: The thermal response (T–LTJ)

In this communication, we propose a new version of the Large Temperature Jump (LTJ) method for studying the ad/desorption dynamics on representative pieces of heat exchangers (HEx) used in real adsorption chillers. This method is based on direct measurement of the temperature difference ΔT of a heat carrier at the inlet and outlet of the tested HEx fragment after a fast drop/jump of the inlet temperature. This tightly repeats the procedure used in real HExs for transformation and storage of low temperature heat. For the sake of validation, the measurements were carried out with the same adsorbent (AQSOA FAM-Z02) and HEx as well as under the same conditions already comprehensively studied in [1]. It is demonstrated that the measured ΔT-response allows studying ad/desorption dynamics, extracting the characteristic process time and heat with sufficient accuracy. The new Thermal Large Temperature Jump (T-LTJ) method gives similar information as the G-LTJ version being more simple in realization and close to the common procedure for evaluating dynamic performance of real adsorptive chillers. Moreover, the T-LTJ provides valuable information about the heat flux directly transferred to a heat carrier fluid that is not available from other LTJ versions.

Dynamic optimization of adsorptive chillers: Compact layer vs. bed of loose grains

This paper addresses the dynamic study on methanol ad-/desorption in compact adsorbent layers, performed by a Volumetric Large Temperature Jump (V-LTJ) method, appropriate for dynamic examination of adsorptive chillers. The commercial active carbon ACM-35.4 was used as an adsorbent, and polyvinyl alcohol as a binder. The influence of the carbon grain size and the layer thickness on the V-LTJ dynamics was studied at a fixed binder content of 12wt%. To single out the effect of consolidation, a detailed comparison was made with reference beds of the loose carbon grains, having the same configuration, however, containing no binder.The following findings are reported for the compact layers: (i) near-exponential shape of the dynamic curves of methanol adsorption; (ii) invariance of the dynamics with respect to the ratio (S/m)=<heat transfer surface>/<adsorbent mass>; (iii) linear relationship between the maximal specific power and this ratio. For the compact layers, the process becomes faster by a factor of 1.5–3.5 as compared to the reference beds.

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.

Performance of a finned activated carbon cloth-ethanol adsorption chiller

Measurements of cooling power and heat demand are presented for an adsorption heat pump (AHP) that integrated a finned adsorbent heat exchanger and a solar collector. For this study the adsorbent heat exchanger was heated/cooled with fluid at near constant temperature. Results from a bench scale, large temperature jump (LTJ) test were scaled to predict the outcome of a larger experiment (adjusting for heat losses and additional heat capacities). The AHP’s measured coefficients of performance were COP∈[0.119, 0.236] versus COP∈[0.233, 0.337] expected. The factor of discrepancy in specific cooling power (predicted cooling power divided by measured cooling power) is 1.1–2.0 versus a range of 2–6 suggested elsewhere. Although the scale-up procedure accounted for additional heat capacities, unwanted air ingress (even for mole fractions<0.1%) might have substantially reduced adsorption/evaporation rates.

Dynamics study of ethanol adsorption on microporous activated carbon for adsorptive cooling applications

The present paper reports the experimental characterization of dynamic performance of three Ad-HEx configurations based on the activated carbon/ethanol working pair. The kinetic behavior of ethanol adsorption onto activated carbon was studied by the LTJ (Large Temperature Jump) method that reproduces the real conditions of isobaric stages of adsorptive cooling cycles. The study consists of two main parts: the first one is related to the adsorption kinetics of the Flat Adsorbent Bed configuration, composed of loose adsorbent grains placed on the metal support. While the second one describes the performance evaluation of finned flat – tube HEx configurations loaded with adsorbent grains. The effects of the grain size, the ratio S/m of heat transfer surface to the adsorbent mass and the Ad-HEx geometry are explored in detail in order to find the main factors affecting the adsorption dynamics. On the base of the main findings obtained, the practical recommendations on optimization of the Ad-HEx unit are formulated in order to enhance the specific cooling power of an adsorptive chiller. Furthermore, a comparison of the adsorption characteristic times between the activated carbon and a properly synthesized porous composite, composed of LiBr inside a silica gel host matrix is shown.

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.

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.