Zoran R. Jovanovic

The effect of the gas–solid contacting pattern in a high-temperature thermochemical energy storage on the performance of a concentrated solar power plant

This work investigates how the gas–solid contacting pattern in a thermochemical energy storage system charged and discharged by air as the heat-transfer fluid influences (1) the integration of the storage into a concentrated solar power plant and (2) the performance of the power plant. The investigation uses 6Mn2O3 ↔ 4Mn3O4 + O2 as the model reaction taking place in packed- and fluidized-bed reactors simulated based on idealized contacting patterns and empirical reduction/oxidation rate laws. Considering computed heat-transfer fluid outflow temperatures and the operating requirements by the power block and the solar field, the preferred integration into the power plant is identified to be parallel for the packed bed and serial for the fluidized bed. A more detailed comparison of these two plant configurations shows that a parallel integration of a well-designed storage system is advantageous due to the increased power block inlet temperatures and the absence of limitations on the attainable duration of discharging. Furthermore, combining thermochemical and sensible energy storage systems in one storage unit is beneficial for the parallel integration of a batch-type storage system because it merges increased volumetric and gravimetric storage densities provided by the thermochemical section with reduced outflow temperatures during charging provided by a low-cost sensible section.

Mechanism of Zn Particle Oxidation by H2O and CO2 in the Presence of ZnO.

In this work we investigate the mechanism of Zn oxidation with CO2 and/or H2O to produce solar derived fuels (CO and/or H2) as part of the Zn/ZnO thermochemical redox cycle. It has been observed that the ZnO contamination of Zn produced by solar thermal reduction of ZnO (solar Zn) facilitates oxidation of the metallic Zn by CO2 and H2O, allowing for nearly complete conversion at temperatures as low as 350 °C. Reaching the same reaction extent starting with pure Zn requires considerably higher temperatures which imposes use of unconventional hard-to-operate reaction configurations utilizing Zn as vapor. The mechanism of this enhancement is investigated by studying the oxidation of solid Zn diluted with ZnO or Al2O3 at 350–400 °C utilizing thermogravimetry. It is found that ZnO acts as the site for the oxidation of Zn originating from the vapor phase, thereby serving as a sink for Zn vapor and maintaining the driving force for sustainable Zn sublimation. As this Zn sublimation competes with the growth of an impervious ZnO scale over the surface of the remaining solid Zn, the presence of the ZnO increases the reaction extent according to the magnitude of its surface area. This mechanism is supported by energy-dispersive X-ray (EDX) spectroscopy, revealing a substantial deposition of produced ZnO over the surface of the ZnO-seeded Al2O3 diluent.

One-Dimensional Heat and Mass Transfer and Discrete Granule Model of a Tubular Packed-Bed Reactor for Thermochemical Storage of Solar Energy

Thermochemical storage of high-temperature (450°C – 1000°C) thermal energy can be applied to concentrated solar power systems for round-the-clock electricity dispatchability. Reversible, non-catalytic gas-solid reactions are used to convert thermal into chemical energy during endothermic charging, and vice versa during exothermic discharging. To assist the experimental investigation of such chemical reactors, a numerical model of the heat and mass transfer in a tubular packed bed reactor has been developed. The fluid serves as both heat transfer medium and gas reactant supplier and is modeled as a homogeneous phase using unsteady one-dimensional mass and energy balances. Solid reactants are modeled as spherical porous granules. The unsteady radial energy and mass balances are solved in the granules. This allows for the accurate treatment of larger granules with a diameter in the range of mm to cm in which radial gradients may occur. The temperature profile within a granule is calculated from an energy balance, whereas mass balances track the local concentration of the gases within the pores, affected by diffusion and an eventual mass source/sink due to the gas-solid reaction. The fluid and granule phases are coupled through local mass and energy exchange terms. The numerical implementation of the model is tested through a thorough code-verification study and a general assessment on mass and energy conservation within the coupled fluid and granule phases.Copyright

A 1 kWel thermoelectric stack for geothermal power generation - Modeling and geometrical optimization

A thermoelectric stack composed of arrays of Bi-Te alloy thermoelectric converter (TEC) modules is considered for geothermal heat conversion. The TEC modules consist of Al2O3 plates with surface 30×30 mm2 and 127 p-type (Bi0.2Sb0.8)2Te3 and n-type Bi2(Te0.96Se0.04)3 thermoelement pairs, each having a cross-section of 1.05×1.05 mm2, and with a figure-of-merit of 1 and a heat-to-electricity conversion efficiency of ∼5%. A heat transfer model is formulated to couple conduction in the thermoelements with convection between the Al2O3 plates and the water flow in counter-flow channel configuration. The calculated open-circuit voltages are compared to those resulting from the mean temperature differences across the TEC modules computed by CFD. The investigated parameters are: hot water inlet and outlet temperatures (373 - 413 K and 323 - 363 K, respectively), stack length (300 - 1500 mm), thermoelement length (1 - 4 mm) and hot channel heights (0.2 - 2 mm). The heat transfer model is then applied to optimize a 1...