High Energy Storage Systems Research Articles

Invited Presentation: Materials and Electrode Design for Long Life Lithium-Ion Batteries

Long life, efficient and safe high energy storage systems are the key components for new car concepts (hybrid and full electric cars) as well as for intermediate storage of renewable energy sources. However, present lithium ion technology does not fulfil completely the requirements in terms of energy density, wide operation temperature, long life behaviour and safety even under abuse conditions. In the last twenty years the improvement of energy density of portable lithium ion batteries has mainly been achieved by optimization of electrode microstructure and cell design and not by introducing new electrode materials. The life time and safety of the cells depend strongly on both cell chemistry and proper cell design. Most life time and safety studies focus either on the stability of single components like positive or negative materials from half cell measurements or on performance or safety tests of complete batteries. However, for a better understanding of aging mechanisms and safety issues it is important to analyse the full range from components up to battery level and especially the contribution of negative and positive electrode and their interactions in the complete cell under various operation conditions. We studied the aging behaviour of different complete cells (pouch or 18650) with various cell chemistries by monitoring the voltage profile of the individual electrodes with a reference electrode. The results clearly demonstrate that cycling life and safety are strongly influenced by a proper adjustment of the electrode microstructure and kinetics of the negative and positive electrode. Abuse tests, accelerating rate calorimetry (ARC) and differential scanning calorimetry (DSC) techniques were combined to study potential safety hazards. Gas chromatography (GC) provides powerful information about the chemical decomposition of electrolyte and of interactions within the battery. This combination of methods leads to a detailed description of life-time and safety-relevant processes at different temperatures and rates. The results clearly demonstrate that the safety behaviour of the cells changes with prolonged cycling and capacity fading. Based on the analytical results the microstructure for both electrodes anode and cathode has been optimized in order to minimize aging effects. On the cathode side blend electrodes of different sorts of materials and particle sizes can be used to optimize reversible capacity, rate capability, long term behaviour. For example LFP/NCM blend electrodes combine high mass loading (higher energy density) with excellent cycling life and improved safety. It has been demonstrated that a pouch cell containing synthetic graphite as negative and a NMC/LFP (3:1) blend as positive gives > 96% of initial capacity after 5760 cycles charged and discharged with 1 C rate between 3 – 4.2 V.The presentation will summarize various strategies to improve the cycling stability for high energy and high voltage lithium ion batteries with various materials combinations.

Monitoring the Location of Cathode Reactions in Li-O2systems

Lithium-oxygen (Li-O2) batteries have been considered as a promising alternative for high energy storage systems due to their exceptional theoretical energy density. However, many obstacles need to be overcome before developing a safe, green and efficient commercial dispositive [1,2].One of the key issues of these batteries is the employed electrolyte. Carbonates have been inherited from Li-ion technology, becoming their use popular in the early steps of development of Li-O2 systems. However, due to their decomposition at high potentials [3,4], they are slowly being replaced by other type of electrolytes such as glymes or ionic liquids. In order to study these processes, in addition to common electrochemical characterisation techniques, X-ray photoelectron spectroscopy (XPS), impedance spectroscopy and transmission electron microscopy (TEM) have proved to be fundamental techniques for in situ monitoring of reaction products that take place during charge and discharge [5-7]In this work, it is proved that the use of organic solvents with low oxygen solubility reduces the active area for the oxygen reduction reaction (ORR) to the oxygen cathode electrolyte triple interface (OCETI). For this aim electrodes loaded with different carbon masses have been cycled in different conditions and post-mortem analyzed by means of X-ray diffraction (XRD), scanning electrode microscope (SEM) and x-ray photoelectron spectroscopy (XPS). The obtained data evidence that the most of the cathode is not active in each stage of the cycling process (Figure 1.a and 1.b). Consequently, expressing the obtained specific capacity values in mA h g-1 units or the current densities in mA g-1 becomes undesirable as it leads to non reasonable comparisons. Moreover, the analysis of the cyclability has evidenced the competition between the OCETI (Figure 1.c) and the applied intensity (Figure 1.d) as the main limiting factors for the performance of these Li-O2 systems.

Advanced Lithium-Sulfur Batteries with Carbon Nanotubes As Conductive Networks

Lithium-sulfur batteries, with the high theoretical capacity (1675 mAh g-1) and energy density (2600 Wh kg-1) as well as the easy access and non-toxic of raw materials, demonstrate a promising prospect among the high energy storage systems for electric vehicles (EVs) and portable electronic devices. However, there are also several issues to be handled before commercial application.Firstly, different from traditional lithium-ion batteries, intermediate polysulfides generate and dissolve in the organic electrolyte during the discharge process of lithium-sulfur batteries. The dissolved polysulfides, shuttle between cathode and anode sides, lowering the utilization of sulfur and also react with the lithium anode without any electron transfer. However, the dissolution of polysulfide in a lithium-sulfur battery is highly dependent on the sulfur/electrolyte loading. Thus an appropriate dosage of electrolyte will build a concentration gradient, which will limit the dissolution of polysfulfides. In our work, A lithium-sulfur cell with a high initial discharge capacity of 1053 mAh g-1 at a high rate of 1 C and an ultralow decay rate of 0.049 %/per cycle during 1000 cycles was obtained by using CNT-sulfur cathode and suppressing polysulfide shuttle to a shuttle factor of 0.02 by matching the sulfur/electrolyte loading. Secondly, sulfur is with low electrical conductivity (5×10-30 S cm-1 at 25 oC) and extra conductive network is highly required. Generally, the addition of conductive materials will neutralize the advantage in high energy density of lithium-sulfur battery. Thus, the more sulfur content in the composite cathode, the larger energy density. Herein, we employed the carbon nanotubes (CNTs) to build the unblocked conductive skeleton and a room-temperature, one-step ball-milling treatment of aligned CNTs and sulfur was applied to obtain high sulfur content of 90 %. The 90 % sulfur loading cathode exhibited a superior density of 1.98 g cm-3 (2.07 g cm-3for sulfur), which will greatly increase the volumetric energy density of lithium-sulfur battery. Reference: [1] XB Cheng, et al. J. Power Sources, 2014, DOI: 10.1016/j.jpowsour.2013.12.031[2] XB Cheng, et al. Nano Energy, 2014, accepted

Encapsulation of copper-based phase change materials for high temperature thermal energy storage

Worldwide attention has been paid to high temperature phase change materials (PCMs) utilized in latent heat storage systems such as solar thermal power generation or industrial waste heat recovery. Current high temperature PCMs on basis of molten salts are suffering from inherent low thermal conductivity, which is detrimental to heat release rate and systematically thermal efficiency. Metal materials, always possessing ultrahigh thermal conductivity and satisfied heat fusion, are highly suitable as PCMs. However, the development of metal-based PCMs must overcome the package problem, namely, packing active, high temperature liquid metal into durable container. In this paper, copper capsules coated with refractory metallic shells were proposed as a novel metal PCM, which could work at temperature up to 1000°C. Copper spheres with diameter of millimeters were encapsulated with a thick chromium–nickel bilayer by a novel chromium periodic-barrel electroplating method and nickel barrel-plating method. The latent heat density of as-prepared capsules is up to 75% of the theoretical value (about 71J/g) at the melting temperature of 1077°C and the thermal resistance of chromium–nickel layer is 8.27×10−6m2k/w. Particularly, copper capsules could endure 1000 charge–discharge thermal cycles from 1050°C to 1150°C without any leakage. The structure investigations reveal the excellent oxidation resistance of capsules and good stability between copper and chromium–nickel layer, even after long-term charge–discharge cycles. The results demonstrate that as-prepared copper capsules are applicable as high temperature PCMs which can facilitate high temperature thermal energy storage systems.

Resilient aligned carbon nanotube/graphene sandwiches for robust mechanical energy storage

The use of microscale flexible mechanical energy storage devices, instead of traditional electrochemical energy storage devices based on supercapacitors and Li-ion batteries, is highly considered for portable electronics, actuators, and meso-micro scale systems. The selection of resilient and robust building blocks is the first step for high energy-density mechanical energy storage system. Herein, alternative aligned carbon nanotubes (CNTs) and graphene were effectively sandwiched into freestanding sp2 all-carbon hybrids, rendering the excellent loading transfer in the three-dimensional framework. The millimeter-scale aligned CNT/graphene sandwiches could be repeatedly compressed at high strains (ε>90%), with a highest energy absorption density of 237.1kJkg−1, an ultrahigh power density of 10.4kWkg−1, and a remarkable efficiency of 83% during steady operation, providing novel nanocomposites with outstanding mechanical energy storage performance comparable to electrochemical batteries and bridging nanoscopic structures to micro- and mesoscale applications.

Stabilizing the surface of lithium metal

Abstract

Battery Management System—Balancing Modularization Based on a Single Switched Capacitor and Bi-Directional DC/DC Converter with the Auxiliary Battery

Lithium-based batteries are considered as the most advanced batteries technology, which can be designed for high energy or high power storage systems. However, the battery cells are never fully identical due to the fabrication process, surrounding environment factors and differences between the cells tend to grow if no measures are taken. In order to have a high performance battery system, the battery cells should be continuously balanced for maintain the variation between the cells as small as possible. Without an appropriate balancing system, the individual cell voltages will differ over time and battery system capacity will decrease quickly. These issues will limit the electric range of the electric vehicle (EV) and some cells will undergo higher stress, whereby the cycle life of these cells will be shorter. Quite a lot of cell balancing/equalization topologies have been previously proposed. These balancing topologies can be categorized into passive and active balancing. Active topologies are categorized according to the active element used for storing the energy such as capacitor and/or inductive component as well as controlling switches or converters. This paper proposes an intelligent battery management system (BMS) including a battery pack charging and discharging control with a battery pack thermal management system. The BMS user input/output interfacing. The battery balancing system is based on battery pack modularization architecture. The proposed modularized balancing system has different equalization systems that operate inside and outside the modules. Innovative single switched capacitor (SSC) control strategy is proposed to balance between the battery cells in the module (inside module balancing, IMB). Novel utilization of isolated bidirectional DC/DC converter (IBC) is proposed to balance between the modules with the aid of the EV auxiliary battery (AB). Finally an experimental step-up has been implemented for the validation of the proposed balancing system.

Development of Non-Aqueous Redox Flow Batteries at Pacific Northwest National Laboratory

Redox flow battery (RFB) is considered as one of the most promising large-scale stationary energy storage technologies and has attracted much attention both academically and industrially. The unique cell architecture renders RFB with attractive advantages including separation of energy and power, exceptional design flexibility, excellent scalability and modularity, long service life, high efficiency, etc.Therefore, RFB is a well-suitable option to stabilize the power grid and tackle the intermittency of the renewable energies. Conventional aqueous RFBs have achieved significant progress such as vanadium-vanadium, iron-vanadium, zinc-bromine, polysulfide-bromine systems. However, aqueous RFBs also are confined by two major intrinsic limitations: narrow voltage window to avoid water electrolysis and relatively low concentrations of the electroactive materials, making them generally low energy density systems.Non-aqueous electrolytes provide wider electrochemically stable voltage window and examples having cell potential of >2V have been demonstrated recently. The expansion of the operational cell potential has a direct impact on the energy density of the non-aqueous RFB system. Meanwhile, the wider voltage windows make more redox couples available as electroactive material choices and increase the possibility of using multiple electron transfer redox reactions. Therefore, a number of non-aqueous RFB systems have been proposed and investigated [1]. Most of these systems are based on organic active materials that have the advantages of convenient structural tunability and availability of different redox mechanisms.Here we report the development of non-aqueous RFBs at PNNL with the focus on several lithium-organic RFB systems, each adopting a different organic cathode material [2]. Chemical functionalization to the organic active materials was carried out to increase its solubility targeting high energy density energy storage systems. The flow cells of these systems demonstrated excellent electrochemical performance with impressive cell efficiencies, high operational current density, and remarkable cycling stability that exceeded those of other reported non-aqueous RFBs. Effective protection of the lithium anode employed in these lithium-organic RFB systems will be discussed.

Mechanism Study of Li/Se Rechargeable Batteries in Different Electrolytes

Lithium-seleniume battery has been reported as one of the promising new high energy density energy storage systems considering its high theoretical gravimetric capacity of 678 mAh/g and volumetric capacity density of 3268 mAh/cm3, which is comparable to that of S (3467 mAh/cm3). It has been reported that Se cathode could be cycled versus both Li and Na at room temperature. Moreover, a long cycle life up to 1000 cycles with high efficiency have been reported by Wang et al recently. We have proved that Se is reduced into Li2Se through the formation of intermediate polyselenide in the ether-based electrolyte by in-situ synchrotron techniques. However, a different mechanism is expected in the carbonate-based electrolyte based on the different voltage profiles. Then we experimentally demonstrated that the Se is directly transformed into Li2Se without any intermediate phase formation. These results further proved that the electrolytes have a significant effect on this battery system. This mechanism study provides essential information on strategy selection for furhter performance improvement of Li-Se baterry.

Development of New Anode Composite Materials for Fluoride Ion Batteries

The increasing demand for high energy storage systems has prompted the interest to alternative chemistries for new battery systems, with better performances than currently used Li-ion batteries. Recently, the reversible charge and discharge of a battery based on a fluoride shuttle has been demonstrated [1]. Such a fluoride ion battery (FIB) can theoretically achieve a high energy density, i.e. above 5000 Wh.L-1 for some electrode combinations. This type of batteries is at an early stage of development and large improvements need to be done concerning the performance of the electrodes or of the electrolyte. In the first study, different types of metal fluorides were already tested as cathode (BiF3, CuF2, SnF2 or KBiF4) and Ce metal was used as anode. However, this material, as well as other potential anode materials like La, Ca, Li or Mg, is very sensitive to oxidation, even when stored under argon. We therefore decided to prepare electrode materials in the discharged state to investigate new anode materials, using Bi metal as cathode. We found that it was possible to obtain several cycles of charge/discharge using CeF3 or doped-CaF2 as anode but not when using MgF2. Moreover, the discharge capacities obtained after the initial charge were low. To ensure both electronic and ionic conductivity in the electrodes it is usually necessary to prepare a composite material. Here, only activated carbon was added to CeF3 and doped-CaF2 as these compounds are also good ionic conductors. In the case of MgF2, it was necessary to add also a certain amount of the electrolyte as MgF2 itself shows very poor ionic conductivity. The low performances obtained for this anode composite are thus most likely linked to interfaces issues between the different compounds of the electrode composite. In order to improve the reactivity of the interfaces, a new composite was prepared by adding Mg and MgF2 (the charge and discharge products) to the electrolyte and carbon in the anode composite. Using this new anode composite, it was possible to obtain much better cycling performances with higher charge/discharge capacities and lower polarization. Different characterization techniques were used to understand the reactions occurring when using the different anode material and understand the improvement observed when mixing Mg and MgF2.[1] M. Anji Reddy & M. Fichtner – J. Mater. Chem. 43 (2011) 17059

Status of the Development of a New High Temperature Thermal Energy Storage System (HTTESS) for CSP-power Plants

Thermal Energy Storage-Systems (TES-System or TESS) are already acknowledged to be highly needed to optimize the electricity yield of CSP-power plants and to improve the economics of CSP-projects and to reduce electricity prices. Furthermore for many electricity grid operators it is important to have spinning reserves in the grid and dispatchable power available.It has been the task of enolcon to develop a new TES-System realizing a step change which is by its principle simple, robust with regard to execution and operation, and which is reducing the electricity costs of CSP-power plants. Furthermore such system shall be open to future developments of CSP-systems with regard to increasing steam temperatures and steam pressure. Such TES-system shall be commercially available for large scale application already in year 2014/2015.As a result enolcon has invented in year 2010/2011 a TES-System using ambient air as heat transfer fluid and stones/rock as storage medium. The key elements of the enolcon-TES are the open cycle using always ambient air with an air-air-heat exchanger and the arrangement of the storage material in such way to minimize the pressure losses and the own electricity consumption. The development is progressing in a structured way by studies, engineering works, TES-pilot plants, isothermal air flow test plant for the verification of the CFD-calculations, and since end of 2012 by the operation of a high temperature TES- module with all features of the large scale modules. The actual status of the TESS-development and the key results of the charging and discharging of the high temperature TES-module TESS002 is presented following.

Progress on high-energy density lithium-sulfur batteries

Recently, lithium-sulfur batteries as high energy-density energy storage system have attracted more and more attention. In this article, we first introduce lithium-sulfur batteries and then review recent progress in carbon-sulfur cathodes and electrolytes for lithium-sulfur batteries. Furthermore, we analyze and summarize the reported strategies to inhibit lithium polysulfide dissolution. Particularly, we propose a new class of bi-functional Solvent-in-Salt (SIS) electrolyte with high lithium salt concentration for solving the problems resulting from both sulfur and lithium metal electrodes. Finally, we present the outlook for future lithium-sulfur batteries.

대용량 에너지 저장시스템을 위한 나트륨 유황전지

나트륨 유황전지(NAS)는 대용량 에너지 저장시스템(energy storage system, ESS) 중 하나로서, 최근 풍력에너지, 태양에너지, 해양에너지 등 그린재생에너지의 사용증가로 ESS에 대한 수요가 급증함에 따라 NAS 전지에 대한 관심이 고조되고 있다. NAS 전지는 에너지 밀도가 높고(납 축전지밀도의 3배), 사이클 수명이 길고, 자가방전이 없어 대용량 전력저장 시스템에 적합하다. NAS 전지는 양극으로 황(Sulfur), 음극으로 나트륨(Na), 고체전해질 및 분리막으로 ${\beta}$ -알루미나( $Al_2O_3$ )로 구성되어 있고, 양극 활물질인 황은 부도체이기 때문에 도전재인 탄소섬유(carbon felt)에 함침시켜 양극으로 사용해야 함으로, 양극재 구성 및 특성은 전지성능에 상당한 영향을 미치게 된다. 따라서 본 논문에서는 NAS 전지의 구성, 다황화나트륨( $Na_2S_x$ , 방전생성물) 및 양극재의 특성, 전지 성능에 미치는 영향인자들에 대해서 알아보고자 한다. 【Sodium sulfur (NAS) battery is a high energy storage system (ESS). These days, as the use of renewable green energy like wind energy, solar energy and ocean energy is rapidly increasing, the demand of ESS is increasing and NAS battery is considered to be one of the most promising ESS. Since NAS battery has a high energy density(3 times of lead acid battery), long cycle life and no self-charge and discharge, it is a good candidate for ESS. A NAS battery consists of sulfur as the positive electrode, sodium as the negative electrode and ${\beta}$ -alumina as the electrolyte and a separator simultaneously. Since sulfur is an insulator, carbon felt should be used as conductor with sulfur and so the composition and property of the cathode could largely influence the cell performance and life cycle. Therefore, in this paper, the composition of NAS battery, the property of carbon felt and sodium polysulfides ( $Na_2S_x$ , intermediates of discharge), and the effects of these factors on cycle performance of cells are described in detail.】

(De)Lithiation Mechanism of Li/SeSx (x = 0–7) Batteries Determined by in Situ Synchrotron X-ray Diffraction and X-ray Absorption Spectroscopy

Electrical energy storage for transportation has gone beyond the limit of converntional lithium ion batteries currently. New material or new battery system development is an alternative approach to achieve the goal of new high-energy storage system with energy densities 5 times or more greater. A series of SeSx-carbon (x = 0-7) composite materials has been prepared and evaluated as the positive electrodes in secondary lithium cells with ether-based electrolyte. In situ synchrotron high-energy X-ray diffraction was utilized to investigate the crystalline phase transition during cell cycling. Complementary, in situ Se K-edge X-ray absorption near edge structure analysis was used to track the evolution of the Se valence state for both crystalline and noncrystalline phases, including amorphous and electrolyte-dissolved phases in the (de)lithiation process. On the basis of these results, a mechanism for the (de)lithiation process is proposed, where Se is reduced to the polyselenides, Li2Sen (n ≥ 4), Li2Se2, and Li2Se sequentially during the lithiation and Li2Se is oxidized to Se through Li2Sen (n ≥ 4) during the delithiation. In addition, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy demonstrated the reversibility of the Li/Se system in ether-based electrolyte and the presence of side products in the carbonate-based electrolytes. For Li/SeS2 and Li/SeS7 cells, Li2Se and Li2S are the discharged products with the presence of Se only as the crystalline phase in the end of charge.

Effective microwave-assisted synthesis of graphenenanosheets/NiO composite for high-performance supercapacitors

A facile and fast strategy was used for synthesis of a petal-like graphene nanosheets (GNS)/NiO composite. Using this strategy, graphene oxide was reduced to graphene and formed the GNS/NiO composite via a microwave-assisted method without a complicated procedure and any controlling-agent. The product was characterized by X-ray diffraction (XRD), Raman spectra, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The analysis results have confirmed that petal-like NiO sheets are well dispersed on the surfaces of graphene nanosheets. The as-prepared composite was electrochemically tested by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy as an electrode material for supercapacitors. The GNS/NiO composite exhibits a specific supercapacitance of 799 F g−1 at a current density of 0.3 A g−1 in 6 M KOH electrolyte and a long cycle life, along with 90% specific capacitance remaining after 1000 cycles. The electrochemical performance of the composite was significantly improved compared to bare graphene and NiO. This could be attributed to their architecture. The results presented here suggest that the GNS/NiO composite could have potential applications in high energy storage systems.

Fabrication and electrochemical performance of 3D hierarchical β-Ni(OH)2 hollow microspheres wrapped in reduced graphene oxide

A novel structured reduced graphene oxide/Ni(OH)2 (rGO/Ni(OH)2) hybrid composite with enhanced electrochemical performance was prepared by wrapping β-Ni(OH)2 hollow microspheres in rGO sheets via a facile solvothermal route, using poly(L-lysine) (PLL) as reductant and ethylene glycol (EG) as coupling agent. The structural, morphological and electrochemical properties of the composite were well examined. The results show that single-crystalline β-Ni(OH)2 hollow microspheres are enveloped in rGO sheets after thermal treatment in the hybrid composite, which exhibits a high specific capacitance of 1551.8 F g−1 at a current density of 2.67 A g−1 and a capacity retention of 102% after 2000 cycles. Notably, in comparison with pure β-Ni(OH)2 hollow microspheres and the simple mixture (mixture of rGO and Ni(OH)2 spheres), the rGO/β-Ni(OH)2 composite exhibited superior electrochemical properties, which may be due to the wrapped electrically conducting graphene sheets and the unique three-dimensional (3D) structure of the composite. The rational design, interesting structure and the ideal electrochemical performance of this graphene-based composite suggest its potential applications in high energy storage systems.

An Optimal Rapid Energy-Storing Design for the Stackable Piezoelectric Power Generation Devices

The optimal design for stackable piezoelectric power generation device and its micropower energy storage method is presented in this work. In this configuration, three different arrangement designs of piezoelectric power generation devices were put and compared for obtaining a greater produced output power. Evidence shows that the Sample 2 among the three different arrangement stackable piezoelectric power generation devices could avoid damage to the PZT-5H and achieve a highest instantaneous output power (5.175mW). Moreover, to establish the high efficiency energy storage system, using low power buck converter IC (LTC3588-1) was proposed. The obtained results show that a 15mAh Ni-MH battery could be fully-charged within two hours by utilizing the optimal arrangement design stackable piezoelectric power generation device combining with micropower energy storage system.

Applications of PZT Dielectric Ceramics in High-Energy Storage Systems

The barium and strontium titanate (BST) ceramics have been used with great success as excellent dielectrics in the construction of high voltage (HV) commercial ceramic capacitors with reduced dimensions because of their high dielectric constant. However, the main point of this paper is to investigate other type of ceramic known as PZT (Lead Zirconate Titanate) normally used as piezoelectric sensors in industrial applications. The idea herein is to use the PZT ceramics as HV dielectrics for applications in high-energy storage systems by de-poling their piezoelectric properties in order to avoid dielectric damage and losses at high frequencies. For this, de-poled PZT-4 ceramic samples (30 mm × 2 mm) were submitted to HV tests, in which their dielectric breakdown strength and dielectric constant variation with the applied voltage were assessed. These results obtained confirmed the use of PZT in applications that require reasonable dielectric constant stability (< 15 %) with voltage and HV dielectric breakdown (40 kV/cm) for compact high-energy storage devices.

Polynomial Control Method of DC/DC Converters for DC-Bus Voltage and Currents Management—Battery and Supercapacitors

This paper presents an embedded energy share method between the high energy storage system (battery) and the auxiliary energy storage system such as supercapacitors (SC). Using the SC and battery with a good strategy for energy management improves the performance of hybrid electric vehicles (HEVs). The SC modules are dimensioned for peak power requirement, and the battery's module ensures the average power of HEVs. The battery module is connected to dc-bus through a dc/dc converter for the first topology and without a converter for the second configuration. Buck-boost converters are used between the SC and the dc-bus to manage the available energy for all topologies. The originality of this paper stems from its focus on the control methods of the dc-bus voltage and currents, which use adjustable polynomial controllers (correctors based on polynomial approach). These methods are implemented in the PIC18F4431 microcontroller which ensures analog to digital conversion, and the pulsewidth modulation signals generation for dc/dc converters. Due to cost and available components, such as the power semiconductors (IGBT) and the battery, the experimental tests benches are carried out in reduced scale. Through some simulations and experimental results, the performance of the proposed control is shown and analyzed.

Concentrating Solar Thermal Heat Storage Using Metal Hydrides

Increased reliance on solar energy conversion technologies will necessarily constitute a major plank of any forward global energy supply strategy. It is possible that solar photovoltaic (PV) technology and concentrating solar thermal (CST) power technology will play roughly equal, but complementary roles by 2050. The ability to increase reliance on CST power technology during this period, however, will be constrained by a number of factors: the large plant sizes dictated by economies of scale, the high associated transmission infrastructure investment cost, and the limitations of current thermal energy storage technologies. Thus, solar technology's main midterm role is seen to be as hybrid solar thermal power plant. The development of low-cost, high-temperature, high-energy density thermal energy storage systems is needed to enable CST plants to be dispatchable and accelerate the deployment of this technology. Thermochemical storage has the best potential to achieve these energy storage requirements and a brief overview of thermochemical energy storage options for CST plants points to high-temperature metal-hydride thermochemical heat energy storage systems. Hydrogen storage systems offer the highest energy storage capacity per volume and are therefore the most likely candidates for achieving the goal of fully dispatchable CST plants. A number of high-temperature metal-hydride thermochemical solar energy storage systems have been proposed and a small number of these systems are currently being investigated and developed. A key component of this work is matching the thermochemical metal-hydride system with a suitable “low-temperature” hydrogen storage material to produce systems that are self-regulating. A summary of the development status of these systems suggests that, despite the technical challenges associated with high-temperature thermochemical energy storage systems, their potential advantages are now seeing development occurring. Although in the early stages, their commercialisation could be fast tracked.