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Microgrid Scale Liquid Air Energy Storage: Plant Optimization and Thermal Characterization of Phase Change Materials for High Grade Cold Storage
No full textL’incremento del consumo energetico legato ad un forte aumento della domanda energetica mondiale, rappresenta la principale causa delle attuali problematiche ambientali legate al riscaldamento globale, come l’effetto serra e il cambiamento climatico. Le nuove politiche del settore energetico mirano ad una radicale decarbonizzazione della rete che ha notevolmente ridotto la produzione energia prodotta da fonti fossili, incrementando la penetrazione delle fonti rinnovabili. Il maggiore svantaggio di queste ultime è legato alla loro intermittenza e alla loro imprevedibilità. I nuovi obiettivi del settore energetico, sono quindi focalizzati nel risolvere le problematiche legate alle fonti rinnovabili e l’ottimizzazione del bilancio tra potenza prodotta e la domanda energetica. L’integrazione di sistemi di accumulo (energy storage) risulta quindi fondamentale per garantire la flessibilità del sistema energetico. Infatti questi ultimi hanno la capacità di accumulare la potenza elettrica per essere utilizzata in un tempo successivo.
Le grandi quantità di energia termica a temperatura criogenica dissipate nell’ambiente durante la rigassificazione del GNL (gas naturale liquefatto) e i grandi volumi di azoto liquido prodotto non utilizzato, hanno suggerito lo studio dei liquidi criogenici come vettori energetici alternativi che possono utilizzati sia essere mezzi di stoccaggio, ma anche applicati nel settore dei trasporti definendo una "liquid air economy". In questo contesto i sistemi LAES (Liquid Air Energy Storage) rappresentano un sistema di accumulo basato sul concetto che l’aria ambiente può essere liquefatta a -196°C riducendo il suo volume specifico di circa 700 volte, ed essere stoccata in serbatoi a bassa pressione. L’aria liquida può essere quindi utilizzata nuovamente, per produrre potenza elettrica mediante un espansore.
Alcune idee per utilizzare l’aria liquida come mezzo di stoccaggio sono state proposte nel 1977 e nei primi anni 2000, ma senza ulteriori sviluppi. Nel 2005 University of Leeds e Highview Power hanno brevettato un sistema LAES con un ciclo di liquefazione basato su un ciclo Linde-Hampson. Il progetto di ricerca ha portato allo sviluppo e alla realizzazione di un impianto pilota LAES con una potenza di 300 kW e una capacità di stoccaggio di 2.5 MWh. L’impianto, commissionato ad Aprile 2010, è stato inizialmente integrato con un impianto a biomasse di 80MW locato allo Slough Heat and Power di Greater London e successivamente rilocato a University of Birmingham. Negli ultimi anni, grazie alla realizzazione del primo impianto pilota e alla dimostrazione potenziale dei liquidi criogenici come vettori energetici, il LAES è diventato oggetto di interesse di molti ricercatori grazie ai diversi vantaggi che la tecnologia offre, come la durata dell’impianto e l’assenza di vincoli ambientali. Inoltre il LAES può essere facilmente scalabile, grazie alla consolidata tecnologia basata su componenti già utilizzati in ambito criogenico. Il maggiore svantaggio, che compromette la fattibilità dei sistemi LAES, è la bassa efficienza che può essere stimata del 50-60 % per sistemi di larga scala. La maggior parte dell’interesse in letteratura è dedicato a sistemi di scala industriale con un ciclo di liquefazione basato su una produzione giornaliera di aria liquida maggiore di 300 tonnellate. Questi sistemi sono studiati per competere con i sistemi di accumulo energetico di larga scala di medio o lungo periodo come gli storage ad aria compressa (CAES) o i sistemi di accumulo con pompaggio di acqua (PHE).
Sebbene ancora una grande porzione di potenza elettrica viene prodotta da grandi sistemi centralizzati, negli ultimi anni il sistema energetico mondiale si sta muovendo verso sistemi decentralizzati con le sorgenti di generazione distribuita (DER) e le microgrids. Il potenziale della tecnologia LAES e i suoi vantaggi, rendono interessante lo studio di un LAES di piccola scala (microgrid scale LAES) che può essere dedicato come sistema di accumulo di medio o lungo periodo per applicazioni di generazione distribuita e microgrid.
In questa tesi il microgrid scale LAES è stato proposto definendo un impianto che integra un ciclo di liquefazione con una produzione giornaliera di aria liquida da poche tonnellate a poche decine di tonnellate al giorno. L’obiettivo di questo lavoro è quello di intraprendere la sfida nel ridurre la scala di un sistema LAES contribuendo alla comunità scientifica a definirne le potenzialità e ispirare la ricerca a possibili lavori futuri.
La prima parte di questa tesi analizza diversi cicli di liquefazione, adatti per essere applicati ad un LAES di piccola scala, con l’obiettivo di trovare una configurazione ottimale che minimizza il consumo specifico. Il confronto dei diversi cicli di liquefazione è stato basato su una analisi parametrica eseguita tramite il software di simulazione Aspen HYSYS. Un ciclo Kapitza supercritico (oltre 40 bar) con un doppio stadio di compressione, risulta la migliore configurazione e, se integrato ad un serbatoio di aria liquida pressurizzato, il consumo specifico stimato può essere inferiore ai 500 kWh/t. Inoltre, l’analisi exergetica, evidenzia una grande porzione di exergia persa durante l’interrefrigerazione tra i due stadi di compressione che suggerisce l’integrazione di un sistema di recupero dell’energia termica ad alta temperatura. Combinando il ciclo di liquefazione con un ciclo ad assorbimento a singolo effetto basato su un ciclo con acqua e bromuro di litio (LiBr), il consumo specifico del microgrid scale LAES può essere ridotto del 10%.
Nella seconda parte del lavoro, il ciclo di liquefazione Kapitza è stato integrato in una configurazione completa del microgrid scale LAES che include sia la sezione di scarica, che i sistemi di recupero dell’energia termica ad alta e bassa temperatura. Una analisi parametrica del sistema è stata eseguita assumendo un’ampia variazione delle condizioni operative del sistema valutandone l’effetto nelle prestazioni e nell’efficienza finale. I risultati, sono stati usati per elaborare quattro mappe di performance. Queste possono essere usate come un semplice strumento che può essere usato per stimare l’efficienza del sistema LAES e trovare le configurazioni ottimali che migliorano il consumo specifico, la potenza prodotta e l’efficienza globale dell’impianto.
Lo storage termico a bassa temperatura o "High Grade Cold Storage" (HGCS) è il componente principale del sistema di recupero di energia termica a bassa temperatura che risulta fondamentale per migliorare le prestazioni del microgrid LAES e diminuire il consumo specifico del ciclo di liquefazione. Per ridurre le dimensioni del sistema di accumulo termico incrementando la densità energetica, una soluzione di HGCS ibrida è stata proposta in questo lavoro. Lo storage ibrido è parte di un progetto basato al TESLAB@NTU nella Nanyang Technological Uuniversity di Singapore, e prevede l’integrazione di una o più parti di materiali a cambiamento di fase (PCM) nelle attuali soluzioni di HGCS basate su sistemi di accumulo termico a calore sensibile.
Il design dei sistemi di accumulo termico che utilizzano PCM, richiedono la conoscenza del comportamento termico del mezzo di stoccaggio durante le fasi di carica e scarica. Questa tesi si focalizza nella caratterizzazione termica dei materiali a cambiamento di fase a bassa temperatura, proponendo una metodologia che può essere utilizzata per misurare e prevedere il comportamento termico dei PCM. In particolare un apparato sperimentale è stato progettato per misurare la temperatura in differenti punti di un PCM contenuto in un contenitore cilindrico. I risultati sono stati quindi usati per validare un semplice modello 1-D che può essere utilizzato per prevedere il comportamento termico di PCM a bassa temperatura. Per questo obiettivo, il container è stato progettato per ottenere dal PCM un comportamento simile ad un processo di trasferimento di calore monodimensionale. Il modello 1-D è stato calibrato e validato con i risultati sperimentali ottenuti con la carica e la scarica di acqua deionizzata. L’attendibilità dei risultati è stata verificata con due test differenti, valutando la solidificazione e lo scioglimento in differenti punti del container. La metodologia è stata quindi testata con diverso PCM basato una soluzione di glicole etilenico (EG) dissolta con una concentrazione del 30% in acqua pura. Le soluzioni alcoliche acquose risultano difficili da testare a causa del basso valore di calore latente (o entalpia di fusione) e la bassa velocità di nucleazione. I risultati ottenuti con il modello 1-D, mostrano una buona corrispondenza con i dati sperimentali. Il metodo proposto risulta quindi essere un valido e semplice strumento per approssimare e prevedere il tempo di carica e scarica di diversi PCM e confrontare i differenti comportamenti. Inoltre, la semplicità del codice numerico, permette a quest’ultimo di essere facilmente integrato in modelli numerici più complessi ed essere utilizzato per un modello completo dello storage termico a bassa temperatura.The increasing of the energy consumption related to the global energy demand is the main cause of environmental issues, such as greenhouse effect and climate change. The new policies related to the energy sector are aiming to a deep decarbonization of the grid that has led to reducing the energy produced from fossil fuels increasing the penetration of renewable energy sources in the actual energy system. The main drawback of clean energy sources is related to their unpredictable nature. The new challenge of the energy sector is then to overcome the problem related to the intermittency of renewable sources optimizing the balance between energy supply and energy demand. The use of energy storages has a fundamental role to guarantee the flexibility of the energy system capturing the wrong-time power from the renewables giving the possibility to be used in a later time or in a different place.
The large amounts of cold thermal energy wasted (from spare liquid nitrogen and LNG regasification) and the research of a new and sustainable energy vector have led to study the potential of cryogens on grid, transport and cooling applications defining a possible "liquid air economy". In the grid context, Liquid Air Energy Storage (LAES) is based on the concept that air at ambient pressure can be liquefied at -196°C, reducing the specific volume of around 700 times, to be stored in low-pressure vessels. The liquid air then can be expanded in a later time in a power producing device (e.g. turboexpander, reciprocating engine) to produce electric power.
The first concept son using the liquid air as storage medium was proposed in 1977 and again in the early 2000 but with no further developments. In 2005 University of Leeds and Highview Power patented a LAES system with a liquefaction system based on a Linde-Hampson cycle. The research starting from lab scale components has led to develop a 300kW/2.5MWh LAES pilot plant. The plant initially commissioned in April 2010 was located at Scottish and Southern Energy 80MW biomass plant at Slough Heat and Power in Greater London and now relocated at the University of Birmingham. In the recent years, the realization of the first pilot plant and the demonstration of the potential of cryogens as an energy vector, have led the LAES to attract the attention of many researchers due to their several advantages, such as long lifetime and no geological constraints. Furthermore, LAES can be easily scalable to large size due to the system made with existing mature components and sustainable materials. Although those benefits, the main disadvantage that compromises the LAES feasibility, is the low round-trip efficiency that is estimated around 50-60 % for large-scale systems. The most of the interest in the literature available is dedicated to industrial scale systems (up to 300 tons per day) to compete with large-scale technologies used for medium or long-term storage such as compressed air energy storage (CAES) and pumped hydroelectric (PHE). However, although a large portion of electric power produced still realize on large centralized systems, the energy system is moving towards to a decentralized energy production system with distributed energy resources (DER) and microgrids. The potential of the LAES technology and the related advantages make it interesting to investigate a small-scale (microgrid scale) that can be suitable for DER and microgrid applications as a medium or long-term energy storage.
In this work, a microgrid scale LAES is proposed defining a system that integrates a liquefaction plant with a daily production ranging between few tons and few tens of tons of liquid air. The aim of this work is to start the challenge of scaling down and reduce the size of the LAES contributing to the scientific community to understand the potential of this technology and inspire future research.
The first part of this thesis analyses different liquefaction cycles, suitable for small-scale LAES with the aim to find an optimal configuration that minimizes the specific consumption of the plant. Indeed, the liquefaction cycle mainly affects the roundtrip efficiency of the system. The analysis, done by software simulation Aspen HYSYS, is based on parametric analysis and a comparison between the liquefaction cycles. A Kapitza cycle configuration with a two-stage compression working at supercritical pressures (over 40 bar), is the best cycle configuration that, if integrated to a pressurized phase separator, can achieve a specific consumption below 500 kWh/t. Furthermore, the high exergy loss at the compressor aftercoolers suggests that the integration of a waste heat recycle can increase the exergy efficiency of the system. Combining the system with a 105.5 kW single-effect water-Lithium Bromide absorption chiller the specific consumption of the microgrid scale LAES liquefier can be reduced of around 10%.
In the second part of the work, the Kapitza cycle has been integrated into a complete configuration of a microgrid LAES system that includes both the discharge section and the waste heat and cold thermal energy recovery system. A wide range of operating conditions has been assumed to evaluate the effect on the system performance. The results have been used to elaborated four performance maps to be used as a simple and immediate method that can be applied by engineers and researcher to estimate the LAES performance and optimize the design in terms of term of specific consumption, specific electric power output, and round-trip efficiency.
The performance maps highlight that the reuse of the cold energy released by the liquid air before the expansion in the discharge cycle allows to improve the performance of the charge side of the LAES decreasing the specific consumption of the liquefaction cycle and increase the overall roundtrip efficiency of the system. Therefore, the last part of the thesis focuses on the High Grade Cold Storage (HGCS) that represents the main component of the waste cold recovery system. HGCS is fundamental to improve the performance the microgrid LAES and decrease the specific consumption of the liquefaction plant. To reduce the dimension of the storage increasing the energy density, a hybrid HGCS solution that integrates one or more part of phase change materials (PCM) has been proposed to improve the actual bulky solutions based on a sensible heat storage. The hybrid HGCS is part of a project based at TESLAB@NTU in the Nanyang Technological University of Singapore. The design of the storage using PCM requires a good understanding of the thermal behaviour of the storage medium during the charging and the discharging. Therefore, this work focuses on the thermal response and thermal characterization of PCM proposing a methodology to measure and predict the thermal behaviour of low temperature PCM. In particular, an experimental rig has been designed to measure the temperature in different points of a PCM placed inside a cylindrical shape container. The results are then used to validate a simple 1-D model that can be used to predict the thermal behaviour of different low temperature PCM. For this purpose, the container has been designed to obtain a similar thermal behaviour of the PCM involving a one-dimensional heat transfer process. The 1-D model has been calibrated and validated with the experimental results obtained with the melting and the solidification of pure deionized water. The reliability of the results has been verified with two different tests evaluating the solidification and the melting in different positions of the container. The methodology has been tested with a solution with a concentration of 30% of ethylene glycol (EG30) by weight in deionized water. Although aqueous alcohols are difficult to test due to their low latent heat and slow nucleation rate, the results of the 1-D model show a good agreement with the experimental data representing a valid and simple method to approximate and predict the time of charge and discharge of the PCM and to compare the thermal profiles of different materials. Furthermore, the simplicity of the code allows the numerical model to be integrated into a more complex numerical model, particularly suitable for the complete modelling of HGCS
Suitable materials to be used as phase change materials (PCM) for industrial applications
No full textThe data collection was carried out in two stages. In the first one, the scientific literature was carefully examined to find candidates and their reported properties. In the second one, commercial PCMs available on the market were identified. The considered properties were melting temperature (ºC), melting enthalpy (J/g), specific heat in solid and liquid state (J/g·ºC), density in solid and liquid state (kg/m3), thermal conductivity in solid and liquid state (W/m·ºC), degradation temperature (ºC), and hazard level
Suitable phase change materials for high-temperature applications
No full textThis dataset corresponds to a collection of suitable phase change materials (PCMs) and their main thermophysical properties, which can be applied in high-temperature applications. The data collection was carried out by consulting scientific literature to find suitable PCMs. Moreover, the reported thermophysical properties were experimentally obtained. The properties listed in the dataset are melting temperature, melting enthalpy, thermal conductivity in solid state, and maximum operational temperature
A preliminary study on the optimal configuration and operating range of a “microgrid scale” air liquefaction plant for Liquid Air Energy Storage
No full textLiquid Air Energy Storage systems represent a sustainable solution to store energy. Although a lot of interest is dedicated to large scale systems (up to 300 tons per day), a small-scale Liquid Air Energy Storage can be used as energy storage as part of a microgrid and/or an energy distribution network. However, when scaling down the size of the system, the round trip efficiency decreases due to the low performance of the liquefaction process. In this paper a preliminary study on the optimal configuration for a microgrid scale liquefaction cycle (10 tons per 12 h) for a Liquid Air Energy Storage application is proposed in order to minimize the specific consumption. The Linde, Claude and Kapitza cycles are modelled and compared by means of a parametric analysis carried out with the software Aspen HYSYS. The results show that the two stages compression Kapitza cycle operating at 40 bar represents an optimal solution in terms of performance and cycle configuration resulting in a specific consumption of about 700 kW h/t. The analysis also shows that the implementation of a pressurized phase separator leads to a reduction of the specific consumption as high as 21% (≈550 kW h/t)
Techno-economic analysis of a Liquid Air Energy Storage (LAES) for cooling application in hot climates
Full text linkThis work investigates the technical and economic feasibility of a Liquid Air Energy Storage (LAES) for building demand management applications. The thermodynamics and processes of the LAES configuration, as well as the description of the daily cooling energy demand profile, are described in details and the assumptions and constrains are pointed out. The quantitative analysis has been carried out for a daily cooling energy demand of an existing office building, located in Singapore, locality characterized by a typical hot climate. A thermodynamic analysis has been carried out for LAES configuration by means of the Aspen HYSYS® process simulation code. Under the technical assumptions formulated, LAES achieves an overall round trip efficiency of 45% with a specific consumption of 0.20 kWh/kgLA. The exergy analysis shows that LAES is characterized by an exergy efficiency of 84% and 67% for the liquefaction and the discharge processes, respectively; the compressor and the power turbines account for the highest exergy losses. Finally, the economic results show that under the actual condition of peak tariff and off-peak tariff in Singapore, the investment proposed is not convenient but in case of high values of LAES round trip efficiency and lower OPT the investment may be attractive. However, future works have to deal with the limitations introduced in the analysis, such as neglecting LAES operation costs, and the uncertainty related to capital costs figures.</p
Innovative cryogenic phase change material (PCM) based cold thermal energy storage for liquid air energy storage (LAES) – numerical dynamic modelling and experimental study of a packed bed unit
No full textElectrical energy storage represents a necessary link between sustainability goals and the enhancement of intermittent renewable energy sources penetration in electricity grids. Liquid air energy storage (LAES) is a promising large scale thermo-mechanical energy storage system whose round trip efficiency is largely affected by the performance of the sub-thermal energy storages. The high grade cold storage (HGCS) is by far the most important due to the crucial thermodynamic recovery of the waste cold stream released by the liquid air regasification process. LAES pilot plant and pre-commercial demonstrator, as well as the vast majority of the theoretical and experimental analysis found in literature studies, currently design to store that exergetically valuable cold source in sensible heat (SH) thermal energy storage, economically efficient but low energy density solution. Conversely, phase change material (PCM) has the potential to store a larger amount of energy using the same amount of storage volume. The objective of the present work is to numerically and experimentally investigate the thermal behaviour of a novel cryogenic HGCS packed bed filled by PCM and determine how the novelty introduced affects the LAES thermodynamic and economic performance compared to the SH configuration. To this end, a simplified transient one-dimensional numerical model to simulate the charging and discharging phase of the HGCS system has been developed and successfully validated against experimental results provided by literature for SH medium and an experimental campaign carried out on a novel lab scale HGCS at TESLAB@NTU for PCM, representing a unicum in literature for both PCM and LAES applications. The numerical results have shown that the introduction of a PCM in the HGCS mitigates the thermocline effect shown in SH configuration ensuring: a) longer discharge phase by means of the thermal buffer phenomena triggered by the phase change process and b) lower specific consumption compared to SH configuration (0.272 vs 0.330 kWhe/kgLA) due to a lower time average outlet temperature of the heat transfer fluid during the HGCS discharge, corresponding to LAES charge phase. From an economic perspective, the decrease of the time average specific consumptions results in a notable payback period inferior to 3 years, making the economic investment considerably attractive.This work was partially funded by the Ministerio de Ciencia, Innovacion y Universidades de Espana (RTI2018-093849-B-C31 - MCIU/AEI/FEDER, UE) and by the Ministerio de Ciencia, Innovacion y Universidades - Agencia Estatal de Investigacion (AEI) (RED2018- 102431-T). This work is partially supported by ICREA under the ICREA Academia programme
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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