Adsorption dynamics in heat transformers for HVAC applications : scaling up from model to real configurations of the adsorbent bed

Project leaders

Alessio Sapienza, Yuri Aristov

Agreement

RUSSIA - RFBR-suspended - Russian Foundation for Basic Research

Call

CNR/RFBR triennio 2018-2020 2018-2020

Department

Engineering, ICT and technologies for energy and transportation

Thematic area

Engineering, ICT and technologies for energy and transportation

Status of the project

New

Research proposal

Technologies and systems based on adsorption heat transformation (AHT) processes represent nowadays a fascinating option to meet the growing worldwide demand of air conditioning and space heating. Nevertheless still considerable efforts must be made in order to enhance the performance aiming at competing with commonly used electrical compression and absorption machines. For this purpose, an intelligent design of an adsorption unit should be firstly focused on a convenient choice of the adsorbent material by a comprehensive analysis that takes into account both thermodynamic and dynamic aspects. While the thermodynamic properties of AHT cycle were comprehensively studied, the dynamic optimization of AHT adsorbers is still an open issue.
The AHT dynamics is a complex process that regards all main components of a sorption machine (i.e. the adsorber, evaporator/condenser). In particular, the adsorber is the core component of an adsorption machine and it could be considered as an integrated unit "adsorbent + heat exchanger" (Ad-HEx). Sorption kinetics can be affected by the HEx design (geometry, material), adsorbent configuration (loose grains or coating), grain size, layer thickness as well as by other main variables such as heat carriers flow rates, condenser/evaporator efficiency, etc.
Consequently the dynamic optimization of an adsorption heat transformer should be based on a comprehensive study that starts from the modelling of the heat and mass transfer phenomena inside the sorbent grain and bed, goes to the experimental study of the kinetic property of the sorbent material itself and arrives to the full characterization of the integrated unit sorbent + HEX passing through an efficient design of the HEX geometry.
The CNR-ITAE and the Boreskov Institute of Catalysis SB RAS have been collaborating on this topic for several years publishing a relevant number of papers both on international scientific journals and on international conference proceedings. This collaboration was carried out thanks to several exchange visits from both sides funded by the CNR -Short Term Mobility program, Bilateral Italy-Russia agreements or by institutes own resources as visiting professor' programs. Over the years the two institution have developed complementary skills in the adsorption fields and in particular have developed experimental techniques/methods aimed at studying all aspects of the sorption dynamics for AHT systems for HVAC applications.
Accordingly, the proposed research activity aims to push forward the knowledge on adsorption dynamics. The work will be devoted to deeply analyse each aspects of the matter at small scale level (model level and small scale samples) as well as at real scale level to find out also how machine's design can affect the overall performance.
According to above, the activity will cover every aspects related with the sorption kinetics: i) modelling of the heat and mass transfer phenomena inside the sorbent grain and bed, ii) the experimental study of the kinetic properties of sorbent materials, iii) the experimental study of the kinetic properties of representative small scale adsorbers, iv) evaluation of the influence of external parameters (e.g. residual gases, efficiency of other machine components, etc), v) full characterization of real scale adsorbers under real operating conditions.
Mathematical modelling of the adsorption dynamics will be carried out by numerical solving the Fick diffusion equations for a single adsorbent particle and by Finite Elements Analysis (FEM) modelling of a bed with a few adsorbent layers using the COMSOL Multiphysics simulation environment.
The modelling results will be compared with experimental findings obtained by measurements on different sorbents carried out both in Italy and in Russia by the Large Temperature Jump method (LTJ) that is considered the most proper characterization technique in this field. In Russian labs, the volumetric version of LTJ set up will study ideal adsorber configuration based on loose sorbent grains placed of a flat plate HEx. Outcome results will be extended by test performed in Italy labs by the gravimetric version of LTJ set up. The joint action of the two version of the LTJ method will give a full definition of main parameters affecting the sorption kinetic.
As second step, the adsorber's development will be firstly carried out by the HEx's design aiming at optimizing in terms of geometry (HEx layout, fin pith, tubes dimension) as well as in terms of materials. With the aim of finding the best compromise between heat transfer properties and weight different HEx prototypes will be manufactured in metallic and plastic materials. Heat transfer behaviours will be evaluated by mathematical modelling as well as by experimental facilities.
Several adsorber prototypes will be realized to study the effect of different design parameters of the adsorber (heat exchanger design, adsorbent configuration (loose grains or coating), grain size, layer thickness) and the kinetic performance will be experimentally evaluated by lab scale kinetic set ups available at Italian and Russian laboratories.
In particular, dynamics of adsorption on adsorbers will be studied by using the large temperature (LTJ) method. Three experimental versions of this method, namely V-LTJ (Volumetric Large Temperature Jump), G-LTJ (Gravimetric Large Temperature Jump) and T-LTJ (Thermal Large Temperature Jump).
Finally, the most promising adsorbers will be tested in a laboratory scale adsorption chiller prototype available at CNR ITAE. Experimental evaluation will allow the full characterization of real performance (in terms of COP and specific cooling power) of the adsorbers under real operating conditions. This activity will allow also to identify how other components and/or operating parameters (e.g. heat carriers flow rates, condenser/evaporator efficiency) affect the prototype performance.

Research goals

The main goal of the activity is to create a deep knowledge about how every design parameter of an adsorption heat transformer affects the sorption dynamic then the overall performance of and adsorption machine. After a detailed study at small scale level, particular attention will be reserved to understand kinetic mechanism while scaling up from model/lab scale to real configurations of the adsorbers. In fact, previous studies demonstrated a wide reduction in performance going from small scale adsorbers to real scale ones especially if integrated with other components of an adsorption machine. Main goal of the activity is to cover this lack in the knowledge.
Intermediate goals can be resumed as follows: i) modelling of the heat and mass transfer phenomena inside the sorbent grain and bed, ii) the experimental study of the kinetic properties of sorbent materials, iii) the experimental study of the kinetic properties of representative small scale adsorbers, iv) evaluation of the influence of external parameters (e.g. residual gases, efficiency of other machine components, etc), v) full characterization of real scale adsorbers under real operating conditions.

Last update: 07/07/2025