We review the literature on analytical models of advanced adiabatic compressed air energy storage plants with isochoric reservoirs, with a focus on the insights that can be extracted from the models.The review indicates that models for plants with adiabatic reservoirs, adiabatic turbomachinery, and without throttling is missing from the literature.We proceed to derive such models, assuming that the plant is operating at the quasi-steady state, that air can be treated as a calorically and thermally perfect gas, and that thermal-energy storage units are free of thermal and pressure losses.The models result in closed-form expressions for key performance indicators like the plant efficiency and volumetric energy density in terms of component efficiencies and pressure ratios.The derivation of these expressions rests on approximating integrals involving simultaneous temporal variations of temperature and pressure.The approximation leads to relative errors with magnitudes smaller than 1%.The models show that the compression and expansion work, the plant efficiency, and the maximum process temperature exhibit minima.The models also show that for a given non-dimensional storage capacity and maximum reservoir pressure, the maximum efficiency of plants that minimize the maximum process temperature is approximately equal to the minimum efficiency of plants that maximize the efficiency.For a two-stage plant with a diabatic cavern and diabatic thermal-energy storage units, our analytical model predicts the volumetric energy density to within 4.76%, indicating that it is accurate enough to be used for initial plant design.

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.

Thermal-energy storage systems consisting of multiple tanks allow the implementation of thermocline-control methods, which can reduce the drop in the outflow temperature during discharging and increase the volumetric storage density and utilization factor. Multi-tank systems based on the extraction and mixing thermocline-control methods were assessed using simulations assuming fluvial rocks as storage material and compressed air as heat-transfer fluid. For adiabatic conditions, the simulations showed improved performance for all multi-tank systems, with diminishing improvements as the number of tanks increases. The mixing method performed better than the extraction method. The mixing method delivered an outflow temperature drop of 5.1% using two tanks whose total volume was 2.15 times smaller than that of the single-tank system. For diabatic conditions, more than three tanks were not beneficial. With two tanks, the mixing method attained a temperature drop of 5.8% with a volume that is 2.5 times smaller than that of the single-tank system. The exergy efficiency of the two-tank system was 91.3% compared to 98.1% of the single-tank system. The specific material costs of the two-tank system were 1.5 times lower than those of the single-tank system.

La fermeture des centrales nucléaires et le développement de l’énergie solaire et éolienne rendent la production d’électricité plus volatile. De nouveaux systèmes de stockage sont nécessaires pour s’assurer que l’électricité est disponible au moment où elle est nécessaire. Le stockage adiabatique d’air comprimé représente une technologie prometteuse. Il utilise l’excédent de production des installations solaires et éoliennes pour comprimer l’air ambiant et le stocker dans une cavité souterraine. Au besoin, l’air comprimé est à nouveau détendu et entraîne alors une turbine qui produit de l’électricité. En tirant profit de la chaleur générée lors de la compression, cette technologie atteint un rendement de 65 à 75 %, ce qui est semblable à celui obtenu avec l’accumulation par pompage. En termes de potentiel d’émission de gaz à effet de serre et de dommages aux écosystèmes, la compatibilité environnementale des réservoirs d’air comprimé est également comparable à celle des systèmes à accumulation par pompage. Les réservoirs d’air comprimé sont techniquement réalisables. Les composants importants, comme les turbomachines et les accumulateurs thermiques, sont déjà disponibles sur le marché ou ont été testés dans une installation pilote. La construction de cavités bénéficie de l’expérience acquise lors de la réalisation de tunnels et de cavernes. Les réservoirs adiabatiques d’air comprimé constituent par conséquent une solution de stockage efficace, écologique et techniquement réalisable. En raison de leurs coûts d’investissement élevés et du manque de clarté qui entoure leur cadre économique et juridique, leur rentabilité demeure toutefois incertaine. Cela complique également le financement d’une installation de démonstration.

Der Verzicht auf Kernkraftwerke und der Ausbau von Solar- und Windenergie führen dazu, dass die Stromproduktion volatiler wird. Damit Strom dann zur Verfügung steht, wenn er gebraucht wird, braucht es neue Speichersysteme. Eine vielversprechende Technologie ist die adiabatische Druckluftspeicherung. Sie nutzt überschüssigen Strom aus Solar- und Windanlagen, um Umgebungsluft zu komprimieren und diese in einem unterirdischen Hohlraum zu speichern. Bei Bedarf wird die komprimierte Luft wieder expandiert; sie treibt dabei eine Turbine an und erzeugt wieder Strom. Da die bei der Komprimierung entstandene Wärme genutzt wird, beträgt die Effizienz 65 bis 75 Prozent; das ist ein ähnlicher Wert wie jener, den Pumpspeicher erreichen. Auch die Umweltverträglichkeit von Druckluftspeichern ist, gemessen am Treibhausgaspotenzial und an Schäden an Ökosystemen, vergleichbar mit jener von Pumpspeichern. Druckluftspeicher sind technisch machbar. Wichtige Komponenten wie Turbomaschinen und Wärmespeicher sind entweder bereits auf dem Markt erhältlich oder wurden in einer Pilotanlage erprobt. Der Bau von Hohlräumen ist zudem durch die Erfahrungen im Tunnel- und Kavernenbau ausgereift. Adiabatische Druckluftspeicher sind also eine effiziente, umweltverträgliche und technisch machbare Speicherlösung. Wegen der hohen Kapitalkosten sowie der unklaren wirtschaftlichen und rechtlichen Rahmenbedingungen ist allerdings ungewiss, ob sie wirtschaftlich sein können. Dies erschwert auch die Finanzierung einer Demonstrationsanlage.

La fermeture des centrales nucléaires et le développement de l’énergie solaire et éolienne rendent la production d’électricité plus volatile. De nouveaux systèmes de stockage sont nécessaires pour s’assurer que l’électricité est disponible au moment où elle est nécessaire. Le stockage adiabatique d’air comprimé représente une technologie prometteuse. Il utilise l’excédent de production des installations solaires et éoliennes pour comprimer l’air ambiant et le stocker dans une cavité souterraine. Au besoin, l’air comprimé est à nouveau détendu et entraîne alors une turbine qui produit de l’électricité. En tirant profit de la chaleur générée lors de la compression, cette technologie atteint un rendement de 65 à 75 %, ce qui est semblable à celui obtenu avec l’accumulation par pompage. En termes de potentiel d’émission de gaz à effet de serre et de dommages aux écosystèmes, la compatibilité environnementale des réservoirs d’air comprimé est également comparable à celle des systèmes à accumulation par pompage. Les réservoirs d’air comprimé sont techniquement réalisables. Les composants importants, comme les turbomachines et les accumulateurs thermiques, sont déjà disponibles sur le marché ou ont été testés dans une installation pilote. La construction de cavités bénéficie de l’expérience acquise lors de la réalisation de tunnels et de cavernes. Les réservoirs adiabatiques d’air comprimé constituent par conséquent une solution de stockage efficace, écologique et techniquement réalisable. En raison de leurs coûts d’investissement élevés et du manque de clarté qui entoure leur cadre économique et juridique, leur rentabilité demeure toutefois incertaine. Cela complique également le financement d’une installation de démonstration.

In the present study, a computational fluid dynamics approach has been developed with the aim of replicating the thermo-fluid dynamics behavior of the high-temperature thermal energy storage (TES) system integrated into the world's first underground advanced adiabatic compressed-air energy storage (AA-CAES) pilot plant. A 120 m long section of an unused tunnel (4.9 m diameter) under the Swiss Alps (Canton of Ticino) was exploited as air reservoir. A 12 MWhth packed rock bed TES system, directly integrated into the pressure chamber, was used to store and release the thermal energy produced during air compression. Several experimental tests were performed operating the pilot plant under different charge/discharge cycles with air temperature up to 550 degrees C and a maximum pressure of 7 bars gauge. The numerical model developed was satisfactorily validated against experimental data demonstrating its accuracy in replicating the thermo-fluid dynamics behavior of the experimental TES unit. The performance of the TES unit were also evaluated in terms of energy and exergy efficiencies resulting to be in the range of 0.77-0.91 and 0.72-0.89 respectively.

Thermocline thermal-energy storage (TES) suffers from so-called thermocline degradation, which refers to the flattening of temperature gradients in the TES with successive charging-discharging cycles. Thermocline degradation increases the variations of the heat-transfer fluid (HTF) outflow temperatures, decreases storage utilization factors, and increases specific TES material costs. Methods that prevent or reduce thermocline degradation by changing the operation of the storage are called thermocline-control (TCC) methods. The assessment of TCC methods is the main objective of this work. Three TCC methods that were chosen for this assessment are described in this paper. Two methods, based on either extracting or injecting HTF through ports, were derived from previously published methods while the third method, based on mixing multiple HTF streams, one of which is extracted through a port, is novel. In a companion paper (Geissbühler et al., Solar Energy, submitted 2018), the three TCC methods are assessed for air and molten salt as HTF using simulations of stand-alone storages as well as storages integrated into a concentrated solar power plant.

Placing an encapsulated phase-change material (PCM) on top of a packed bed of sensible filler material is an effective way of reducing the drop in the heat-transfer fluid (HTF) outflow temperature during discharging associated with a sensible thermal-energy storage (TES). So far, the literature lacks guidelines for the design of a combined sensible/latent TES. This study aims at developing a new method for the design of combined TES based on non-dimensional analysis. The method will provide a designer with non-dimensional plots, produced from quasi-steady-state results of simulations with a one-dimensional model, that relate performance parameters to geometrical, thermophysical, and operational parameters of the combined TES. In this paper, a simplified version of the method is demonstrated that allows the selection of a metallic PCM and its amount such that a specified drop in the HTF outflow temperature is attained during discharging, assuming a fixed sensible section of natural rocks and air as HTF. The plots show that the drop in the outflow temperature during discharging is minimized by selecting a PCM with a melting temperature equal to 98% of the HTF inflow temperature during charging. The plots also show that the heat of fusion, provided it exceeds a threshold, has a subordinate effect on the drop in the outflow temperature. Finally, the plots show that a smaller heat of fusion can be compensated with a larger height of the latent section. The method is illustrated with a specific example.

Three thermocline-control (TCC) methods are assessed through numerical simulations for a thermal-energy storage (TES) filled with a packed bed of rocks. Two previously suggested methods are based on extracting or injecting heat-transfer fluid (HTF) through ports, while the third is a novel method based on mixing HTF streams. The assessment was carried out using simulations with a model that resolves the packed bed in one dimension. Simulations of stand-alone TES with maximum allowed outflow temperature differences of 10% at quasi-steady conditions showed that the mixing method with three ports led to the largest utilization factors - the fraction of the maximum storage capacity that is actually utilized of - 90.8% and 85.1% for molten salt (MS) and compressed air (CA) as HTF, respectively. These represent relative improvements of 38.8% and 73.4% compared to the baseline configurations without TCC. The increased utilization factors come at the expense of small decreases in the cycle exergy efficiency. For the mixing method with three ports, the exergy efficiencies were 97.3% and 95.6% for MS and CA, respectively. Simulations of a TES with MS as HTF integrated into a CSP plant operating on a Rankine steam cycle showed that TCC increases the annually averaged plant efficiency and the annual net electricity generated solely from thermal energy supplied by the TES. These results suggest that the small decreases in the exergy efficiency of the TES are outweighed by the large increases in the utilization factor.

We investigate the mechanical behavior of a confined granular packing of irregular polyhedral particles under repeated heating and cooling cycles by means of numerical simulations with the non-smooth contact dynamics method. Assuming a homogeneous temperature distribution as well as constant temperature rate, we study the effect of the container shape, and coefficients of thermal expansions on the pressure buildup at the confining walls and the density evolution. We observe that small changes in the opening angle of the confinement can lead to a drastic peak pressure reduction. Furthermore, the displacement fields over several thermal cycles are obtained and we discover the formation of toroidal convection cells inside the granular material. The root mean square of the vorticity is then calculated from the displacement fields and a quadratic dependency on the ratio of thermal expansion coefficients is established.

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 MWh(th) was placed inside the cavern. The pilot plant was operated with charging/discharging cycles of various durations, air temperatures of up to 550 degrees 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. (C) 2018 The Authors. Published by Elsevier Ltd.

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 MWh(th) and 171.5 kWh(th), 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 degrees 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. (C) 2018 The Authors. Published by Elsevier Ltd.

A constrained multi-objective optimization approach is used to optimize the exergy efficiency and material costs of thermocline packed-bed thermal energy storage systems. The storage height, top and bottom radii, insulation-layer thickness, and particle diameter were chosen as design variables. The competing objectives of maximizing the exergy efficiency and minimizing the material costs are dealt with by forming a Pareto front. The Pareto front allows the identification of the most efficient design for a given cost and is an important tool in the design of thermal energy storage systems. Constraints are imposed to obtain storage systems with a specified capacity. The optimization approach is applied to identify the influence of various design variables on the exergy efficiency and the material costs. The results show that a storage shaped like a truncated cone with the smallest cross-section on top has a higher exergy efficiency than common designs with a cylindrical shape or a truncated cone with the largest cross-section on top. The basic thermodynamic mechanisms leading to this superior performance are identified with detailed information about the axial temperature distribution in the packed bed and thermal losses through the structure and insulation materials.

Solar-driven hydrothermal gasification is an efficient process to exploit the energetic potential of biomass containing high amounts of water. This process requires effective removal of the type-2 salts from the feedstock as their precipitates can plug the equipment and poison the gasification catalyst. This work investigates the effect of the axial temperature profile on type-2 salt deposition in a dip-tube salt separator operated with an aqueous Na2SO4 solution as model feed and presents a method for online detection and monitoring of the growth of the deposits. Our results suggest that preventing the salt deposition at the wall of the separator by controlling the axial temperature profile is not possible. The online monitoring of the salt deposition is thus indispensable for scheduling the timely removal of the salt deposits.

Using concentrated solar energy to power a hydrothermal gasification (HTG) of biomass requires thermal energy storage (TES) to compensate for the inherent intermittence of solar irradiation. The energy transfer from the TES to the HTG process is accomplished through a heat-transfer fluid (HTF) passing through a heat exchanger (HX) incorporated into the salt-separation step of the HTG process. The HX performance determines the temperature profile inside the salt separator, thereby influencing the removal of the salts from the feedstock. In this work, we compare the performan-ces of three HX types based on exploiting fluidized beds, porous media, and axially finned tubes. The effect of the HX configuration on the temperature profile inside the salt separator is assessed through CFD simulations considering pure water as the model feed to the separator. We find that all considered HX types could provide the desired temperature profile within the separator. However, the estimate for the power required to pump the HTF through the fluidized-bed HX is roughly two orders of magnitude higher than those for the axially finned tubular and porous-media HXs.

The concept of combined sensible/latent heat thermal energy storage (TES) has been exploited to mitigate an intrinsic thermocline TES systems drawback of heat transfer fluid outflow temperature reduction during discharging. In this study, the combined sensible/latent TES prototype under investigation is constituted by a packed bed of rocks and a small amount of encapsulated phase change material (AlSi12) as sensible heat and latent heat sections respectively. The thermo-fluid dynamics behavior of the combined TES prototype was analyzed by means of a computational fluid dynamics approach. Due to the small value of the characteristic vessel-to-particles diameter ratio, the effect of radial void-fraction variation, also known as channeling, was accounted for. Both the sensible and the latent heat sections of the storage were modeled as porous media under the assumption of local thermal non-equilibrium (LTNE). The commercial code ANSYS Fluent 15.0 was used to solve the model's constitutive conservation and transport equations obtaining a fairly good agreement with reference experimental measurements.