Solar Energy Laboratory

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Residential solar thermal energy storage using liquid desiccants

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The goal of this project is to develop a compact solar thermal storage tank using liquid calcium chloride as both the working fluid and energy storage material. Energy is stored as sensible heat and as the chemical binding energy of calcium chloride and water. Aqueous calcium chloride has a number of potential advantages as a solar storage medium compared to water. The combination of sensible and chemical binding energy yields a higher energy density, lower thermal losses, and a temperature lift during discharge.

The main challenge to successful implementation of the liquid desiccant storage is the complexity of management of heat and mass transfer during charge and discharge of the tank. It is critical to avoid mixing regions of different salt fraction and it is desirable to maintain thermal stratification. A novel concept to manage the tank is an immersed heat exchanger which isolates heat transfer and mass addition. A CFD model and experiments on a dynamically similar laboratory prototype will determine the feasibility of the concept and characterize the transient heat and mass transfer processes. Ongoing activities in the Solar Energy Laboratory include prototype development and demonstration, as well as fundamental study of the transient heat and mass transfer through a shear layer with a density interface.

Fig. 1. The material energy storage density, λ, is a metric for the energy stored in a given volume of material. The sensible heat of a water storage with a ΔT = 40 K is approximately 45 kWh/m3. For a closed system, the total material energy storage density of CaCl2-H2O is nearly twice that of H2O (85 kWh/m3 vs. 45 kWh/m3) at Δω = 0.19.