G. Restuccia

A family of new working materials for solid sorption air conditioning systems

Abstract In this communication a family of new working materials, so called selective water sorbents, is presented for sorption air conditioning. These materials are composites “hygroscopic salt inside porous matrix with open pores” and possess intermediate behaviour between solid adsorbents, salt hydrates and liquid absorbents, so that their water sorption properties can be controllably modified by varying (a) porous structure of the host matrix, (b) chemical nature of the impregnated salt and (c) the amount of the salt inside the pores. For these materials the water sorption equilibrium and specific heat have been measured in a wide temperature and uptake range. This study shows that composites based on CaCl2 and LiBr as impregnated salts and different micro- and mesoporous silica gels as host matrices are able to absorb up to 0.75 g H2O per 1 g of the dry sorbent. After presentation of sorption equilibrium curves, the thermodynamic performance for their utilisation in heat pump and refrigeration systems, is calculated. The results show that these new materials can be operated with cycles whose maximum temperature is about 95 °C (for cooling) and 140 °C (for heating) that are lower than those for other pairs known so far. The values of coefficient of performance are considerably higher than those reported for silica gel/water system at the same temperature of the heat source.

SELECTIVE WATER SORBENTS FOR MULTIPLE APPLICATIONS, 2. CACL2 CONFINED IN MICROPORES OF SILICA GEL : SORPTION PROPERTIES

This paper presents sorption properties of a selective water sorbent based on mesoporous KSKG silica gel as a host matrix and calcium chloride as a hygroscopic salt. Sorption isobars, isochores and isotherms at T=20–150°C and vapor partial pressures of 8–133 mbar clearly showed two types of water sorption: 1) the formation of solid crystal hydrates at low amounts N of sorbed water, and 2) vapor absorption mainly by the salt solution at higher N. Sorption properties of CaCl2 crystal hydrates were found to change strongly due to their impregnation into mesoporous silica gel, whereas the solution confinement to the mesopores did not change its water sorption properties with respect to the bulk solution. Isosteric sorption heat was measured to depend on water sorption and to change from 62.5 kJ/mol for solid hydrates to 42.2–45.6 kJ/mol for solution.

A zeolite-coated bed for air conditioning adsorption systems: parametric study of heat and mass transfer by dynamic simulation

In this paper the heat and mass transfer properties of a new zeolite-coated adsorbent bed to be employed in sorption air conditioning systems are investigated by a modelling approach. It consists of a dynamic model which allows to calculate the exchanged energies, the cycle time and, thus, the specific power of the bed. The analysis of the model results, has shown that the proposed configuration (in which the heat transfer enhancement is mainly related to the good adhesion between metal and adsorbent) is very interesting if compared with the traditional beds. Furthermore, to determine the conditions which allow to obtain the most effective heat and mass transfer in the new adsorbent bed, an optimisation study has been carried out.

Composite Materials Based on Zeolite 4A for Adsorption Heat Pumps

Some additives and binders were chosen for the preparation of 4A-zeolite-based composites with high equivalent thermal conductivity for heat pumps application. Additives (SiC, Si3N4, graphite) and binders (PTFE, Al(OH)3) were tested for their effectiveness in terms of equivalent thermal conductivity and maximum water adsorption capacity of the composites. The influence of the equivalent thermal conductivity of the composite adsorbents on the specific power of the heat pump was also calculated. Results show a significant improvement in the equivalent thermal conductivity of the composite samples which are prepared using aluminum hydroxide as binder, over that of zeolite pellet beds. Such composite materials could be used to build adsorption heat pumps with higher specific power and, consequently, with lower investment cost.

Thermal conductivity of selective water sorbents under the working conditions of a sorption chiller

Abstract This paper presents the experimental data on the thermal conductivity of the composite sorbents, CaCl2/SiO2 and LiBr/SiO2, measured by the “hot wire method” under various conditions of vapour pressure PH2O, temperature T and water uptake w. The measurement ranges were chosen according to the operating conditions of a typical sorption cooling cycle ( 10 H 2 O mbar , 40 ° C ). The obtained results show that the sorbent thermal conductivity λ(T,PH2O,w) increases considerably as the uptake of sorbate rises, while its dependence on pressure and temperature is, in the studied range, almost negligible. Finally, the influence of thermal conductivity on the specific power of the sorption chiller is discussed.

Water sorption on composites LiBr in a porous carbon

Water sorption equilibrium of LiBr confined to pores of a mesoporous synthetic carbon Sibunit and a macroporous expanded graphite (samples SWS-2C and SWS-2EG, respectively) was studied. Isobars of water sorption on these composites are measured at vapor pressure 6–81 mbar and temperature 30–145 jC. The type of sorption equilibrium for the two composites appears to be quite different. The isobars for SWS-2EG have a plateau corresponding to one molecule of H2O adsorbed by one molecule of LiBr, which indicates the formation of crystalline hydrate LiBrH2O inside pores with a monovariant type of equilibrium. At lower temperatures, the equilibrium becomes divariant that is typical for LiBr–water solutions. On the contrary, the water sorption equilibrium for SWS-2C is divariant over the whole temperature and pressure range which means that no crystalline hydrates are formed inside Sibunit pores. In our opinion, this distinction results from differences in a pore structure of the host carbons. The composite sorption capacity can reach 0.6–1.1 g H2O per 1 g of the dry sorbent at relative humidity 70%. The advanced sorption capacity makes the sorbents promising for gas drying, thermal storage of energy and other applications. D 2002 Elsevier Science B.V. All rights reserved.

Selective Water Sorbents for Multiple Applications. 11. CaCl2 Confined to Expanded Vermiculite

This paper presents the water sorption properties of a new selective water sorbent based on expanded vermiculite as a host matrix and calcium chloride as a hygroscopic salt. Sorption isobars, isosters and isotherms at T = 30–150°C and vapor partial pressure 8.2–42.0 mbar clearly show that at low water contents crystalline hydrates with 0.33, 1 and 2 molecules of water per 1 molecule of CaCl2 are formed in the pores. These hydrates are stable over a temperature change of 20–30°C and exhibit kinetically slow transformations. At higher water uptake, the vapor absorption leads to the formation of a CaCl2 aqueous solution inside the pores, which properties are close to those in the bulk. Isosteric sorption heat was found to depend on water sorption and change from 76.3 kJ/mol for solid hydrates to 39.1–46.6 kJ/mol.

Dynamic simulation of a recuperative adsorption heat pump

We present a method for dynamic simulation of a two-reactor adsorption heat pump. The operating principle of the system is described and the governing equations are derived, taking into account the efficiencies of the various components. A non-dimensional form of the model shows the important parameters which influence system performance. The resulting set of nonlinear differential equations is solved numerically using an iterative method. The model is tested using the zeolite-water adsorption pair and design data from the literature.

Selective Water Sorbents for Multiple Applications, 10. Energy Storage Ability

The application of new selective water sorbents for storage of low temperature heat is analyzed. Values of energy storage ability E are measured by a DSC technique for more than fifteen samples of selective water sorbents based on silica gels, aluminas, carbon Sibunit and aerogels as host matrices and CaCl2, LiBr, MgCl2 as impregnated salts. E-values up to 8.4 kJ/g and 4.0 kJ/g are found for forced and naturally saturated sorbents, which are much higher than for common sorbents, like zeolites and unimpregnated silica gels. The temperature dependence of E gives an estimation of sensible and latent heat contributions as well as the average heat of water desorption and average specific heat of dry sorbents. A comparison between the SWSs and others materials proposed in literature is done.

Selective water sorbents for multiple applications, 7. Heat conductivity of CaCl2−SiO2 composites

In this communication we present experimental data on the low temperature heat conductivity of a consolidated bed made of the CaCl2/SiO2 composite material measured by the “hot wire method”. The conductivity appears to increase strongly with a raise of the sorbed water amount and reaches 0.53 W/m K at a high water content when the bed is completely saturated with the salt solution. λ values obtained appear to be much higher than those for zeolite 4A, which is a competitor solid adsorbent proposed for sorption cooling and heating machines. Finally, the influence of the thermal conductivity on the specific power of sorption heat pump based on the “CaCl2/SiO2-water” pair is briefly discussed.