Buoyancy Driven Mass and Heat Transfer in a Novel Calcium Chloride Absorption Storage Tank

A major challenge for the use of solar thermal systems for space heating is the lack of compact, long-term storage. The exothermic sorption of water into a host material has been identified as a promising mechanism for long-term storage due to a relatively high energy density and low losses. A novel sorption storage concept for seasonal storage is presented and investigated in this study.;This new concept combines sorption and sensible storage using aqueous calcium chloride. The theoretical material energy storage density of this combination is 106 kWh/m3, more than double that of sensible water storage. A single-vessel design increases system energy storage density compared to multi-vessel systems. The proposed design relies on the density interface between solutions of different mass fraction to prevent them from mixing. Two immersed devices are used to preserve the density interfaces and minimize convective mass transfer. A stratification manifold returns solutions to the storage tank into a region of uniform density and an immersed heat exchanger heats the tank via natural convection.;This study focuses on the heat and mass transfer due to natural convection from the immersed heat exchanger during the sensible charging operating mode. The ability to heat or cool the tank via natural convection while maintaining stable density interfaces and minimizing mass transfer between regions is the central goal of the design. The dimensionless parameters governing heat and mass transfer are the Rayleigh number, Ra, buoyancy ratio, N, Lewis number, Le, Prandlt number, Pr, and aspect ratio, a. Eleven experiments and twenty-one numerical simulations were conducted over a wide range of the governing dimensionless parameters expected in a practical storage tank.;This study demonstrates a stable density interface between water and aqueous calcium chloride and a very low rate of mass transfer over the operating conditions, 7x103 < Ra0 < 3.9x10 10, 0.8 < N0 < 103, 15 < Le < 1.5x10 4, and 0.125 < a < 1.43. Planer laser induced fluorescence (PLIF) and particle image velocity (PIV) measurements of calcium chloride mass fraction and velocity confirm the presence of rotating flow fields within each region of nearly uniform CaCl2 mass fraction and sharp a density interface between regions as predicted by the numerical simulations. No instabilities are observed in the density interface and the stability of the density interface increases in time due to increasing N. Experimentally measured mass transfer rates from the density interface into the upper region, represented by the Sherwood number, are 3 < Sh < 62. Very slow shearing velocities (less than 2.1 mm/s) near the density interface minimize mass transfer because the kinetic energy of the flow does not overcome the stable potential energy gradient in the interface. Low mass transfer rates from the density interface into the lower region are due to different physics; a mixed density region quickly forms beneath the density interface, isolating it from the higher velocity boundary layer. Correlations are developed from numerical data for Sh from the interface into each region during the quasi-steady heat transfer regime in terms of initial Ra, N, and Le. These correlations are Sh1 = 0.001Ra 0.28N-0.69Le1.17 and Sh2 = 0.002Ra0.18N-0.68Le1.17. The correlation for Sh2 predicts the average mass transfer rate within the uncertainty of eight out of ten experiments within the same parameter range.;This study has shown heat transfer into each region of uniform mass fraction is unaffected by mass transfer and each region can be treated as its own enclosure. Thermal stratification is developed and sustained during sensible charging. Quasi-steady and transient Nussult number correlations are obtained from the numerical data for use over the operating range of 7x103 < Ra < 3.3x109, 1 < N0 < 26.4, 15 < Le < 1.5x104, and 0.125 < a < 1. The quasi-steady correlation is Nu = 0.45a0.5Ra0.28.;A storage timescale analysis suggests the long-term storage of absorption energy under the continuous operation of an immersed vertical tube heat exchanger is possible. Storage timescales of 160 days or longer are at least double that of sensible energy storage. This study has demonstrated the potential of a single-vessel absorption storage system based on calcium chloride to provide the seasonal storage of solar thermal energy for space heating.