Experimental and Numerical Investigation of Combined Sensible/Latent Thermal Energy Storage for High-Temperature Applications

Combined sensible/latent heat storage allows the heat-transfer fluid outflow temperature during discharging to be stabilized. A lab-scale combined storage consisting of a packed bed of rocks and steel-encapsulated AlSi12 was investigated experimentally and numerically. Due to the small tank-to-particle diameter ratio of the lab-scale storage, void-fraction variations were not negligible, leading to channeling effects that cannot be resolved in 1D heat-transfer models. The void-fraction variations and channeling effects can be resolved in 2D models of the flow and heat transfer in the storage. The resulting so-called bypass fraction extracted from the 2D model was used in the 1D model and led to good agreement with experimental measurements

Pilot-scale demonstration of advanced adiabatic compressed air energy storage, Part 2: Tests with combined sensible/latent thermal-energy storage

Experimental and numerical results from the world’s first pilot-scale advanced adiabatic compressed air energy storage plant with combined sensible/latent thermal-energy storage are presented. The combined thermal-energy storage was composed of sensible and latent units with maximum capacities of 11.6 MWhth and 171.5 kWhth, respectively. The latent thermal-energy storage consisted of a steel tank with 296 stainless-steel tubes encapsulating an Al–Cu–Si alloy as phase-change material. The combined thermal-energy storage was investigated using four charging/discharging cycles with durations of about 3 h each and air inflow temperatures of up to 566 °C. The experimental results showed that the latent thermal-energy storage reduced the drop in the air outflow temperature during discharging. Minor leaks of the phase-change material were traced to the welding seams in the encapsulation as well as to holes required to insert resistance temperature detectors. Analysis of the leaked phase-change material revealed degradation and/or phase separation, which were attributed to the initial off-eutectic composition of and impurities in the phase-change material and resulted in a reduced heat of fusion. Simulations predicted the performance of the combined thermal-energy storage with good overall accuracy. Discrepancies were put down to changes in the thermophysical properties

High temperature rock-bed TES system suitable for industrial-scale CSP plant - CFD analysis under charge/discharge cyclic conditions

The present study aims at dimensioning and modeling, by means of accurate time-dependent 3D computational fluid dynamics simulations, the behavior of a high temperature rock-bed TES system. The latter is exploited to fulfill the round-the-clock energy requirements of a reference 80 MWe industrial-scale CSP plant, based upon the Airlight Energy technology, which uses air as heat transfer fluid. The TES system behavior was analyzed through 15 consecutive charge/discharge cycles to evaluate the thickness evolution of the thermocline zone, and hence the overall thermal efficiency of the system, under cyclic conditions. The numerical model was satisfactorily validated with experimental data, gathered from a 6.5 MWhth TES system prototype, located in Biasca, designed and built by the Swiss company Airlight Energy SA. The good agreement between CFD simulations results and experimental data allowed the authors to assess the relevance of radiative heat transfer, even at relatively low temperature (300 ÷ 350 °C), on the thermodynamics behavior of the TES system. Moreover, a porosity variation, with the packed bed depth, was also observed numerically and experimentally mainly due to the own weight of the packings (25m3 of natural river pebbles with 3 cm average diameter). The CFD simulations were performed with Fluent code from ANSYS.ISSN:1876-610

Experimental and Numerical Investigation of Combined Sensible/Latent Thermal Energy Storage for High-Temperature Applications

Combined sensible/latent heat storage allows the heat-transfer fluid outflow temperature during discharging to be stabilized. A lab-scale combined storage consisting of a packed bed of rocks and steel-encapsulated AlSi12 was investigated experimentally and numerically. Due to the small tank-to-particle diameter ratio of the lab-scale storage, void-fraction variations were not negligible, leading to channeling effects that cannot be resolved in 1D heat-transfer models. The void-fraction variations and channeling effects can be resolved in 2D models of the flow and heat transfer in the storage. The resulting so-called bypass fraction extracted from the 2D model was used in the 1D model and led to good agreement with experimental measurements

Towards a Commercial Parabolic Trough CSP System Using Air as Heat Transfer Fluid

We report on major design innovations related to the trough concentrator, solar receiver, and thermal storage of a parabolic trough CSP system. A 9.7 m aperture parabolic trough mirror is formed by inflated, metallized, polymeric films mounted on a rigid and durable concrete support structure. The novel receiver design, which uses air as the heat transfer fluid at ambient pressure, is based on an array of absorber cavities coupled to secondary concentrating optics for operating temperatures exceeding 600 °C. The solar-heated air is directly fed to a thermal energy storage unit based on a packed bed of rocks to guarantee round-the-clock dispatchability of high-temperature heat to the power block. A commercial CSP plant with a projected peak thermal power output of 3.9 MWth is currently under construction in Ait Baha, Morocco.ISSN:1876-610

ScienceDirect-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG Single-tank TES system -Transient evaluation of thermal stratification ac

Abstract Single-tank thermal energy storage (TES) systems represent a valuable alternative, to the most common two-tank systems with molten slat, to effectively store thermal energy in concentrating solar power (CSP) applications. From an economic standpoint, the gap between the two TES solutions is relevant. A remarkable cost reduction can be achieved if a single-tank TES system, with a low-cost filler material, is exploited. In this kind of TES system, the buoyancy driven effects of the heat transfer fluid are exploited to establish and maintain a thermocline zone which separates the hot region on top and the cold region at the bottom of the tank. The thinner the thermocline thickness, the higher the thermodynamic quality of the stored energy. As soon as the TES is charged for the first time, i.e. startup of the system, the extent of thermal stratification may vary sharply during the first cycles before achieving a stable condition. For this reason, this study aims at evaluating, by means of accurate time-dependent 3D CFD simulations, the transient evolution of thermal stratification of a single-tank TES system exploited to fulfill the round-the-clock energy requirement of a reference 80 MW e CSP plant which uses air as heat transfer fluid. A total of 30 consecutive cycles, composed by charge/discharge phases, were simulated. Since the thermal energy stored is exploited to produce electrical energy, the performances of the TES system, operating under cyclic conditions, were qualitatively characterized by means of a stratification efficiency index based upon the second-law of thermodynamics

Pilot-scale demonstration of advanced adiabatic compressed air energy storage, Part 1: Plant description and tests with sensible thermal-energy storage

Experimental and numerical results from the world's first advanced adiabatic compressed air energy storage (AA-CAES) pilot-scale plant are presented. The plant was built in an unused tunnel with a diameter of 4.9 m in which two concrete plugs delimited a mostly unlined cavern of 120 m length. The sensible thermal-energy storage (TES) with a capacity of 12 MWhth was placed inside the cavern. The pilot plant was operated with charging/discharging cycles of various durations, air temperatures of up to 550 °C, and maximum cavern gauge pressures of 7 bar. Higher pressures could not be reached because of leaks that were traced mainly to the concrete plugs. Simulations using a coupled model of the TES and cavern showed good agreement with measurements. Cycle energy efficiencies of the TES were determined to lie between 76% and 90%. The estimated round-trip efficiency of the pilot plant was based on the measured TES performance and estimated performances of the other components, yielding values of 63–74%, which compares favorably with the usually quoted values of 60–75% for prospective AA-CAES plants

Design of a 100 MWh(th) packed-bed thermal energy storage

A thermal energy storage (TES) system was designed based on a packed bed of rocks as storing material and air as heat transfer fluid. A pilot-scale 6.5 MWhth TES unit was built and tested. A dynamic numerical heat transfer and fluid flow model was developed and experimentally validated with measurements obtained from the pilot-scale TES unit. The simulation model is applied to design an industrial-scale 100 MWhth TES unit for a solar power plant currently under construction in Morocco.ISSN:1876-610

Advanced Adiabatic Compressed Air Energy Storage design and modelling accounting for turbomachinery performance

Energy storage plants are going to become a strategic asset in electric grids. This statement is confirmed looking at the increasing shares of renewables composing the energy portfolio of several nations. Therefore the power demand and production mismatches, caused by the intermittent nature of renewables, must be reconciled. Many energy storage solutions are available but Advanced Adiabatic Compressed Air Energy Storage (AA-CAES) plants have potentials similar to pumped hydro systems (PHS). A physical model was developed in Matlab-Simscape to simulate the dynamics of AA-CAES plants, implementing temperature-dependent air properties, efficiency maps for turbomachinery and realistic power ramps. Furthermore, start-up and shut-down phases and energy consumption during idle periods were accounted for. The model embeds a 1D Fortran code to model the detailed behaviour of a packed-bed TES. The grid-to-grid performance of an AA-CAES plant was determined and the assumptions implemented to take into account real turbomachinery behaviour are presented.ISSN:1742-6588ISSN:1742-659