In order to measure the thermophysical properties of ammoniated salt (CaCl 2 .mNH 3: m = 4, 8) as an energy storage system utilizing natural resources, the measurement unit was developed, and the thermophysical properties (effective thermal conductivity and thermal diffusivity) of CaCl 2 .mNH 3 and CaCl 2 .mNH 3 with heat transfer media (Ti: titanium) were measured by the any heating method. The effective thermal conductivities of CaCl 2 .4NH 3 + Ti and CaCl 2 .8NH 3 + Ti were 0.14 - 0.17 and 0.18 - 0.20 W/(m .K) in the measuring temperature range of 290 - 350 K, respectively, and these values were approximately 1.5 - 2.2 times larger than those of CaCl 2 .4NH 3 and CaCl 2 .8NH 3. The effective thermal diffusivities were 0.22 - 0.24 × 10 -6 and 0.18 - 0.19 × 10 -6 m 2/sin the measuring temperature range of 290 - 350 K, respectively, and these values were approximately 1.3 - 1.5 times larger than those of CaCl 2 .4NH 3 and CaCl 2 .8NH 3. The obtained results show that the thermophysical properties have a dependence on the bulk densities and specific heats of CaCl 2 .mNH 3 and CaCl 2 .mNH 3 + Ti. It reveals that the thermophysical properties in this measurement would be the valuable design factors to develop energy and H 2 storage systems utilizing natural resources such as solar energy.
These days, the possibility of significant global warming resulting from emissions of greenhouse gases by fossil fuel combustion has become an important concern within the international community. In order to save energy and utilize the renewable energy as natural resources, The thermal energy storage systems utilizing the low temperature heat sources such as solar energy (approx. 353 - 373 K) have been proposed and developed, the processes using the chemical reaction of an anhydrous salt with NH3 have been proposed and discussed for their practicability [
In order to develop the energy storage system and H2 storage system utilizing the above chemical reaction, the measurement unit was developed, and the thermophysical properties (effective thermal conductivity and effective thermal diffusivity) of CaCl2・mNH3 (m = 4, 8) and CaCl2・mNH3with heat transfer media (Ti: titanium) as the important design factors were measured in this study.
Regarding the measurement principle and method, the “any heating method” developed by Iida et al. [
In this study, the thermophysical properties of CaCl2・mNH3 system were measured by the any heating method developed by Iida et al. [
where
The fundamental heat conduction equation can be expressed as
where
Taking Laplace transform of Equation (2) and substituting
where
ro order modified Bessel functions of the first and the second kinds and
On the other hand, the heat flux
where
Taking Laplace transform of Equation (4) and substituting Equation (4) into Equation (4), then Equation (5) is given as
In this study, the hollow cylindrical sample [I] is the reference specimen and the cylindrical sample [II] is the measured specimen, and measurement point 4 is unnecessary in this case. In the measured specimen [II],
where
By measuring
Therefore,
The Laplace integration of heat flux at
where
On the other hand, in the reference specimen [I], by measuring
where
Therefore, the Laplace integration of heat flux at
Since it is clear that
where
Hence, in this case,
Regarding thermal conductivity of the measured specimen
CaCl2 used in this experiment is produced by Wako Pure Chemicals Industries, Ltd. It is guaranteed reagent
grade, and it is specified as the pure grade having minimum purity of 95.0% and used without further purification. The powdered crystal of CaCl2 is dried at 773 K and is stored over silica gel in a desiccator. NH3 gas of 99.99% purity is provided from Sumitomo Seika Co. Ltd. Titanium sponge (Ti) of 10 - 28 JIS mesh 90% up is provided from Wako Pure Chemical Industries, Ltd., and it is used as the heat transfer media and has minimum purity of 99.0%.
In order to insulate this measurement cell from the surroundings, the apparatus is wrapped by the foamed polystyrol. The each temperature of this apparatus is measured by using C-A (Chromel-Alumel) thermocouples corrected by the digital thermometer, and the temperature data as the digital signal (change of mV) is transferred to the microcomputer and stored. The amount of liquid NH3 transferred to the measurement cell from NH3 vessel can be measured by the microscope. The temperatures of this cell and NH3 vessel are controlled by using the constant temperature water bath throughout the reaction, and the accuracy of temperature control is minimum accuracy within ±0.1 K. The each pressure in these vessels is measured by Bourdon gauge, whose accuracy is ±0.1% of full scale (up to 2.0 MPa). The pressure control in this cell is carried out using the pressure regulator valve.
The temperature of this measurement cell is increased and controlled by Ni-Cr wire heater and thermistor type temperature controller, and the accuracy of temperature control is minimum accuracy within ±0.1 K. In order to escape non-uniform temperature field and to decrease the thermal resistance, Al2O3 powder is packed between the stainless steel pipe and the reference specimen.
CaCl2 of 1.31 mole (approx. 145 g ) is crushed below size of 200 JIS mesh and was dried at 773 K for 3 hours by an oven. A dried CaCl2 as measured specimen is placed in this measurement cell. It is sealed, and the thermophysical properties (effective thermal conductivity and effective thermal diffusivity) are measured at atmospheric pressure (0.1 MPa) by the same measurement method for ammoniated salts (see 3.3.2 ).
Similarly, CaCl2 of 0.218 mole (approx. 24.2 g ) is crushed below size of 200 JIS mesh and was dried at 773 K for 3 hours by an oven. A dried CaCl2 as measured specimen (or a specimen mixed with weighed Ti: weight ratio; Ti/CaCl2 = n, where n = 3) is placed in this cell. It is sealed, worked by the vacuum pump in order to remove an air and any water from this system. NH3 vessel is also evacuated for 2 hours and NH3 gas is introduced from the NH3 gas bomb into NH3 vessel, which is kept at a constant temperature (273 K) by the cooling liquid. After liquid NH3 is charged in it, its volume is measured by the microscope rapidly and recorded. Then this cell is connected with NH3 vessel shown in
When the temperatures of the cell and NH3 vessel are stabilized, a needle valve is opened to keep the constant pressure using the pressure regulator valve in this cell. Operating temperature and pressure in this cell are controlled to 303 K and 0.5 MPa, respectively. The amount of liquid NH3 transferred to the cell from NH3 vessel is measured by reading the scale of NH3 vessel using the microscope. The NH3 mole number absorbed to CaCl2 is calculated from the volume change of liquid NH 3 in NH3 vessel. When 8 moles of NH3 is absorbed to the pureCaCl2, the experiment of ammoniation is just finished.
The deammoniation from an ammoniated salt (CaCl2・8NH3(+Ti)) is carried out by using the same experimental apparatus. In this case, the NH3 vessel is kept at constant temperature of 293 K by the circulating water from the constant temperature water bath, and the temperatures on horizontal axis in the cell are heated to 353 K by the heating water. The NH3 mole number desorbed from ammoniated salt is calculated by the same method of ammoniation. When 4 moles of NH3 is desorbed from CaCl2・8NH3(+Ti), this deammoniation process is finished. In order to measure the thermophysical properties on repeated runs (ammoniation and deammoniation), the thermophysical properties are measured after the repeated runs (≥10 times each).
When the measurement temperature and the temperature of measuring points are stabilized in each ammoniated salt (CaCl2・4NH3(+Ti) and CaCl2・8NH3(+Ti)) under the equilibrium pressure, the heating of the measurement cell by charging electricity to the heater is started, and the heating rate and maximum heating temperature are 5 K/min and 10 K/min, respectively. The temperature response as the change of mV by thermocouple of each measuring point is measured, and the scan rate of temperature response is every 9 seconds and the measurement time is 30 minutes. The data of temperature response is corrected by the digital thermometer and the temperature data is transferred to the microcomputer and stored. The thermophysical properties (effective thermal conductivity and effective thermal diffusivity) are calculated from the stored data based on the preceding measurement principle.
ical properties (
authors’ previous work [
In order to develop the energy storage unit and H2 storage unit using CaCl2・mNH3 (m = 4, 8) + Ti (weight ratio; Ti/CaCl2 = n, where n = 3) system, the thermophysical properties (effective thermal conductivity
Regarding
It reveals that the thermophysical properties in this measurement would be the valuable design factors to develop energy and H2 storage systems utilizing natural resources such as solar energy.
Greek letters
ΔH Enthalpy change
i Measurement point (i = 0, 1, 2, 3, 4)
[I] Hollow cylindrical sample and Reference specimen
[II] Cylindrical sample and Measured specimen