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

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

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

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

Design optimization of a novel receiving cavity for Concentrated Solar Power applications by means of 3D CFD simulations

The aim of this work was the design optimization, by means of accurate steady state 3D CFD simulations, of a novel receiving cavity for Concentrated Solar Power (CSP) applications. The receiving cavity has been developed by Airlight Energy Manufacturing SA in collaboration with ETH Zurich and SUPSI-DTI-ICIMSIwithin the framework of the SolAir-2 project. It is made of a helically coiled steel tube, and resulted highly effective in converting the radiative energy coming from the sun into thermal energy gathered by air which was selected as heat transfer fluid (HTF) being cheap, environmentally friendly and suitable for high temperatures applications. The main geometrical parameters considered for the optimization were: cavity height, varied by increasing or decreasing the number of coils, cavity external diameter, and the presence of a spiral coiled tube closing the cavity top. For each configuration the air flow rate, with an inlet temperature of 120 °C, was tuned in order to reach an outlet temperature close to the target value of 650 °C. Cavity performance were evaluated in terms of thermal efficiency and pressure drop under two different skew angle conditions for the incoming solar radiation, 18° and 40°. CFD simulations were performed with Ansys Fluent. Navier-Stokes and energy equations were numerically solved using the finite-volume method approach; radiation heat transfer inside the cavity was taken into account by means of the Discrete Ordinates (DO) radiation model.ISSN:1876-610

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

CFD Analysis of a Receiving Cavity Suitable for a Novel CSP Parabolic Trough Receiver

The aim of this work was to study, by means of accurate 3D steady-state CFD simulations, the thermo-fluid dynamics behavior of a helically coiled heat exchanger (HCHE) constituting the receiving cavity of the novel CSP receiver based on Airlight Energy technology. In this innovative receiver design, air is used as heat transfer fluid (HTF), which, besides being inexpensive and environmentally friendly, is optimally suited for high temperature operation well beyond the limit of conventional HTFs. According to preliminary information related to the collectors orientation of the first 3.9 MWth Airlight Energy pilot plant, under construction in Ait Baha (Morocco), two reference skew angles of the incoming solar radiation were considered and the receiving cavity performance were evaluated in terms of thermal efficiency and pressure drop. Among all, one of the main requirements was to achieve, at the outlet section of the HCHE, an air temperature of 650 °C; hence the mass flow rate was tuned accordingly. In order to minimize the pumping power requirements, the HCHE was designed to guarantee a laminar flow regime under all the operating conditions. Navier-Stokes, energy and radiation transport equations, the latter accounted for by the Discrete Ordinates (DO) model, were numerically solved, using the finite-volume method approach, with Fluent code from ANSYS. A meticulous experimental proof of concept was then carried out in Biasca (Switzerland) by the Swiss company Airlight Energy Manufacturing SA. The analysis of the experimental results, detailed in this paper, allowed to assess the reliability and effectiveness of this novel CSP receiver design in the solar energy harvesting.ISSN:1876-610