• Energy Research

Abstract The work discusses a problem of harvesting and upgrading of ultra-low grade heat with thermochemical energy storage technology for space and domestic water heating in residential area. The laboratory scale prototype, operating on the principle of an open packed bed sorption reactor and using moist air as a heat/mass transfer fluid, is experimented. The range of experimental air temperature was set to 17–40 °C, which corresponds to the typical range of domestic waste thermal energy. The tested sorbent was a salt-in-matrix composite material composed of a silica gel containing 43 wt.% of calcium chloride (CaCl2) salt. Hygrothermal behavior and energy performances of the prototype control volume filled with 245 g of material, representing the reactive front of a thermal wave, were analyzed at constant inlet hydration conditions (water vapor pressure of 12.5 mbar). The average temperature lift was recorded as 9–13 °C, representing the amplification of a supplied heat on 23% – 75% depending on the inlet temperature. The average specific thermal power inside the material bed was measured to be 168–267 W kg-1. The apparent energy density, based on the prototype control volume, ranged between 1.0 and 1.6 GJ m-3. Taking into account the heat of water vaporization, the coefficient of performance of the process was determined to be 0.96–1.57.

Abstract Water sorption thermochemical heat storage is a promising way to provide dwellings with renewable central heating. It requires the use of several cubic meters of materials per dwelling. Depending on the design of the heating system, specific heat and mass transfer issues occur. For instance, the heat transfer rate in reactive medium and the kinetics of sorption process determine the system thermal power. In addition, the moisture propagation during inter-seasonal storage must be understood. In this paper, the influence of the water mass uptake on the apparent thermal conductivity and apparent mass diffusivity of solid material were studied. The studied material was a composite of calcium chloride (CaCl2) encapsulated in mesoporous silica with a salt content of 40–43 wt.%. The thermal conductivity was measured by the transient hot bridge method and varied from 0.13 to 0.16 W m−1 K−1, having a threshold at 0.14 g/g of water mass uptake. The apparent water mass diffusivity was studied using a diffusion column. The water diffusivity – concentration dependency was established by using the modified Hall method. The apparent diffusion coefficient ranged from 3 × 10−10 to 2 × 10−8 m2 s−1 in experimental conditions.

Abstract Composite materials based on a silica gel loaded with CaCl 2 are of great interest for seasonal thermochemical heat storage. In order to improve the performance of these materials for this application, and to evaluate their multi-cycle stability, a new synthesis protocol is proposed, based on successive impregnation/drying steps by using a matrix with a broad pore size distribution. Through this method, a CaCl 2 content of 43 wt%, a high cycle loading lift of 0.40 g/g and an unprecedented energy storage density for this type of material of 211 kW h/m 3 of packed bed composite, in conditions of a solar heat storage system (adsorption at 30 °C, desorption at 80 °C, and water vapor pressure of 12.5 mbar) can be reached. Moreover, the distribution of the salt inside the pores and the absence of any salt crust outside the matrix prevent salt leakage, leading to an outstanding preservation of the cycle loading lift after 10 cycles. Based on Polanyi theory, it can be assumed that the energy storage density can exceed 350 kW h/m 3 for water sorption at 20 °C, desorption at 80 °C, with both steps at a water vapor pressure of 12.5 mbar.

Thermal energy used below 100 °C for space heating/cooling and hot water preparation is responsible for a big amount of greenhouse gas emissions in the residential sector. The conjecture of thermal solar and thermochemical solid/gas energy storage processes renders the heat generation to become ecologically clean technology. However, until present, few pilot scale installations were developed and tested. The present work is devoted to the experimental study of global performance of a pilot scale thermochemical energy storage prototype. Two working modes, namely fixed packed bed and moving bed, were tested using 2.2 kg and 5.5 kg of composite material (silica gel impregnated with calcium chloride) under indoor atmospheric conditions. The global experimental efficiency of a 49l water tank charging process during 75 min was found as high as 0.8–0.85. The energy storage density reached in the fixed bed mode by the material was 158 kWh/m3, while in the moving bed mode it was 2.5 times lower. The reasons for such a difference are discussed in depth in the text.

Abstract This work presents the results of the performance characterization for novel salt-in-silica composites, destined for seasonal energy storage. The materials were synthesized using the Davisil® silica gel to support the hygroscopic inorganic salts of calcium chloride ( C a C l 2 ), magnesium chloride ( M g C l 2 ) or strontium bromide ( S r B r 2 ). The experiments were carried out on an open sorption laboratory setup under constant hydration conditions. The sample mass was 245 g, which is representative of a prototype control volume of 0.5 l, and the nominal air flow rate was 215 l min−1. Energy storage densities between 70 and 145 k W h m - 3 of control volume were experimentally found at 30 °C (inlet air temperature) and for different inlet air relative humidity levels. Average specific thermal powers in the range between 93 and 311 W k g - 1 were measured. Based on the experimental results, the design and upscaling of the seasonal energy storage were demonstrated. The best feasible energy storage densities ranging from 50 to 90 kWh m-3 of up-scaled reactor were predicted to deliver 1000 W of rated thermal power.

Abstract The excellent matching between the sorption and desorption temperatures of hexahydrated SrBr2 and those required for solar heat storage for building applications, the high heat of reaction (67.5 kJ/mol of water) coupled with the gain/loss of 5 mol of water per mole of salt make this salt an appealing sorbent for solar thermal energy storage applications coupled to space heating. Due to the morphological instability of this salt, it is necessary to incorporate it in a porous matrix as a composite sorbent. A new composite material for thermochemical energy storage applications was developed. It consists of a mesoporous silica gel impregnated by strontium bromide with salt content equal to 58 wt.%. The structure and the sorption properties of the composite were characterized by SEM-EDX, temperature dependent XRD, XRF, and N2 sorption measurements. The salt is homogeneously distributed inside the pores of the silica gel. Water sorption isotherms were measured between 20 °C and 80 °C, which enabled us to understand the sorption mechanism. A mathematical model was developed and used to fit the experimental data in order to predict the sorption behavior of the composite at different conditions (influence of temperature and pressure conditions on the cycle loading lift and energy storage density). The interest of using such a composite for thermal energy storage application is then discussed (thermal energy produced by solar collector and used for space heating). A high cycle loading lift of 0.22 g/g is obtained corresponding to an energy storage capacity of 230 W h/kg and an energy storage density of 203 kW h/m3 of packed bed composite (between 30 °C and 80 °C at 12.5 mbar) is reported, with an excellent stability over 14 sorption/desorption cycles. The sorption kinetics of this composite is enhanced compared to pure salt. Test on a laboratory scale open type reactor gives a maximum specific thermal power of 200 W/kg and a mean specific thermal power of 92 W/kg at 30 °C and 12.5 mbar for an extent of reaction of 0.68.

Abstract A novel thermochemical reactor was designed, built, and tested at Besol’s and CEA-INES’ labs. Its circular shape and its vibrating bed allow to move the solid hydrate constantly, therefore increasing turbulence in the moist air heat and mass transfer region. A reactor model was developed and identified in order to calculate its performances over a wide range of operating conditions and to understand what are the key factors leading to increased performances, especially regarding the specific behavior of the composite material made of calcium chloride incorporated in a silica gel matrix. New kinetic equations were developed while combining sorption and porous medium physical phenomena. The model was further refined with a 10 layers solid bed spatial discretization. However vibrations actually mix those layers, which would require a more detailed approach. Nevertheless, outlet parameters were predicted in both modes with a deviation lower than 0.72 K equivalent. Test results in realistic conditions showed an average 356 W heating power with an air temperature elevation of +6.0 K, while desorption cooling power was 278 W with an average 4.5 K temperature decrease. The usable energy density with this 0.163 m3 uninsulated reactor was 200.4 W h per kg of 9% hydrated solid composite, which is adapted to seasonal storage since it is close to the 207.8 W h per kg theoretical target. Electrical consumption for air circulation is only about 10 W, but vibration accounts for almost 70 W, which still needs to be reduced. Detailed results showed that a continuous solid flow and a counter current configuration would lead to further increase average outlet temperature.