M. M. Tokarev

“Chemical Heat Accumulators”: A new approach to accumulating low potential heat

The first presentation of new composite Chemical Heat Accumulation materials based on granulated open-porous matrix filled with a hygroscopic substance is given. At storing heat, the materials operate in a reversible hydration/dehydration mode. When crystalline hydrates of simple salts are used as the hygroscopic substance, the new materials allow to reach the heat storing capacity up to 2000 kJ kg−1 even for accumulation of low temperature heat (of circa 20–40°C). The materials also possess improved properties for mass and heat transfer. All these make a serious advantage of these materials in comparison with those using a latent melting-solidification heat as well as with zeolites capable of reversible hydration/dehydration. The new materials can be widely used in energy efficient and freons-less air conditioning devices, for cooling the electronic units, fire-extinguishing and some other applications.

NEW COMPOSITE SORBENTS FOR SOLAR-DRIVEN TECHNOLOGY OF FRESH WATER PRODUCTION FROM THE ATMOSPHERE

Abstract In this communication we present new selective water sorbents developed at the Boreskov Institute of Catalysis (Novosibirsk, Russia) and discuss their application for fresh water production from the atmosphere. We present a general scheme of the water production and suggest the particular solar-driven unit which may be recommended for desert areas with a hot and dry climate. The results of our lab- scale tests have demonstrated a feasibility of the fresh water production with the output of 3–5 tonnes of water per 10 tonnes of the dry sorbent per day.

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.

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.

Selective water sorbents for multiple applications, 3. CaCl2 solution confined in micro- and mesoporous silica gels: Pore size effect on the “solidification-melting” diagram

In this communication we present a low-temperature “solidification-melting” phase diagram for CaCl2/H2O solutions confined in KSK and KSM silica gels. At salt concentrations of 0–48 wt. %, the diagram has been found to lie below the diagram reported for the bulk system by 15–30°C. It shows a depression of the solution melting point due to its confinment to the pores. Several other peculiarities of melting and solidification in this system are also reported and discussed. Beside fundamental interest, the data obtained could be of importance in many commercial areas such as refrigeration, accumulation of low temperature heat, frost prevention in building materials,etc.

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, 4. CaCl2 confined in silica gel pores: Sorption/desorption kinetics

This paper presents experimental data on water sorption/desorption kinetics on composite SWS-1L and SWS-1S materials, bulk aqueous CaCl2 solutions and pure KSKG silica gel. Desorption kinetic curves are measured in the temperature range of 328–363 K at different vapor pressures. First order kinetics is found for both SWS materials and the bulk aqueous solution with the apparent activation energies of the water desorption equal to 23.5 kJ/mol and 48.6 kJ/mol, respectively. The difference in the kinetic behavior of the bulk and disperse systems is discussed.

Selective water sorbents for multiple application, 6. Freshwater production from the atmosphere

In this communication we consider an application of new selective water sorbents for freshwater production from the atmosphere. We present a general scheme of water production and discuss its efficiency in different climatic areas. The results of our lab-scale tests have demonstrated the feasibility of this method with the output of 3–5 kg of water per 10 kg of the dry sorbent per day.

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.

Selective water sorbents for multiple applications. 12. Water sorption equilibrium at elevated temperature

In this communication we present the experimental data on water sorption equilibrium for SWS-1L (CaCl2 in mesoporous silica) measured at an elevated temperature of 150o to 260oC and the corresponding vapor pressure PH2O of 1.1 to 9.2 bar. A comparison of these data with those obtained at lower T (30-150oC) and PH2O (8-130 mbar) is made. All experimental data appear to fall on the same temperature-independent curve «water sorption vs. relative vapor pressure» which indicates a universal mechanism of water sorption over the whole T-P range.