Superconductive magnetic energy storage

Technical and economic aspects of large scale superconductive magnetic energy storage are discussed. This paper is a review of a program which has been under way at the University of Wisconsin since 1970. Early work between 1970 and 1976 was primarily involved in proving economic and technical feasibility of the concept The present program deals with component development and detailed design ultimately leading to construction of a large superconducting magnet capable of storing 1000–10,000 MWh. The magnet is a single-layered segmented solenoid of approx. 100 m radius. Energy containment is achieved economically by burying the magnet underground in bedrock tunnels. Magnetic loads are transmitted from the conductor to bedrock through glass fiber reinforced composite struts. The conductor consists of a composite of aluminum and NbTi and is designed for full cryogenic stability in 1.8 K superfluid helium. The dewar-conductor assembly will be rippled in a l m radius of curvature to reduce the hoop stress tension. A Graetz bridge is required to convert the d.c. superconducting current into a.c. current in the three-phase power system. Economic analysis indicates that superconductive magnetic energy storage is competitive with alternative large scale storage schemes for units greater than 1000 MWh size.     

Application of magnetic energy storage unit as load-frequencystabilizer

Fast-acting energy storage devices can effectively damp electromechanical oscillations in a power system because they provide storage capacity in addition to the kinetic energy of the generator rotor, which can share the sudden changes in power requirement. The effectiveness of small-sized magnetic energy storage (MES) units (both superconducting and normal loss types) for this application is shown, and means of best utilizing the small energy storage capacity of such units to improve the load-frequency dynamics of large power areas are suggested. The proposed method of improving the load frequency control of power systems has the advantage that it does not require the governor or any other part of the power system to perform any sophisticated control action. The control logic suggested for this purpose takes the area control error as its input and uses inductor current deviation feedback. In a power system with a SMES (superconducting MES) unit, the optimal setting of the integrator gain is altered to a higher value. With the suggested control measure, SMES units of 4-6 MJ capacity would suffice in reducing the maximum deviations of frequency and tie-line power flow by about 40% in power areas of 1000-2000 MW capacity     

Design and performance of hot-oil storage tanks

Because the ratio of surface area to capacity decreases with increasing volume for a particular shaped hot-oil storage tank, there is a trend towards larger tanks in order to incur only relatively small heat losses per unit capacity. The present investigation suggests, for conditions encountered in the UK, that the aspect ratio (i.e. height-to-radius) for least heat losses from bare or fully insulated hot-oil tanks should be approximately 0·4. If the cylindrical walls of the tank are to be insulated, the roof remaining bare, then the optimal aspect ratio should be appropriately greater. However, when designing a tank, the running cost is only one of several considerations—although an increasingly important one as fuel costs inflate—the capital investment in the tank and site also radically affect the choice. The economically most favourable aspect ratio, i.e. that leading to the least total financial expenditure over the lifetime of the tank, is considerably in excess of the optimal aspect ratio corresponding to minimum rate of energy loss. This maximum energy thrift or minimum financial expenditure dichotomy is now serious when neither sufficient, cheap fuel nor adequate industrial investment is available.     

Operation principle and applications of multiterminalsuperconductive magnetic energy storage systems

The basic operation principle of a multiterminal superconductive magnetic energy storage (MSMES) system is introduced. The motivation for developing the MSMES systems is to combine and maximize the flexibility benefits provided by energy storage and the controllability benefits provided by power electronic systems. A MSMES system can be used simultaneously as an energy storage device and a power flow control device. This attribute enables MSMES systems to perform some unique functions in electric power systems. Potential applications of MSMES systems and their impact on solving the problems faced by power systems today are discussed     

Application of magnetic energy storage unit as continuous VArcontroller

It is shown that magnetic energy storage units can simultaneously operate as continuous VAr (volt-ampere reactive) controllers while performing the role of load-frequency stabilizers in electrical power systems. This is achieved by operating the converter in the buck-boost mode with a switched capacitor bank placed across its terminals. The P versus Q modulation ranges of the 12-pulse converter depend on the source inductance, secondary voltage of the input transformers, and output current. Once the input transformer is chosen, the Q modulation range depends on the active power transfer and the current through the inductor at any instant of time. The actual reactive power consumption of the converter is varied continuously, depending on the requirements of the power system, while keeping within the Q-modulation range. Switching of the capacitor bank keeps the required Q consumption of the converter within the available range. It is shown that this mode of control improves the overall performance of the power system in P-f and Q-V loops and obviates the use of any additional VAr compensator in the power area where the SMES (superconducting magnetic energy storage) unit is located     

Impact of superconductive magnetic energy storage on electric powertransmission

The authors demonstrate that a superconductive magnetic energy storage (SMES) system can provide a significant positive impact on electric power transmission. By using SMES, transmission-line loadings during heavy load hours can be reduced if the SMES system is located near the major load. Transmission losses as well as the fuel cost for the losses over a 24 hr period can also be decreased. An SMES scheme, the SMES-DC link, is introduced for energy storage and control of power flow. The operation of this scheme and the benefits it provides are described     

Thermal performance of a two-phase thermosyphon energy storage system

This article presents an energy storage system, which can be readily integrated with the building structure. It stores heat supplied by solar energy via the two-phase closed loop thermosyphon to storage tank and releases stored heat in energy storage material via two-phase closed thermosyphon to the heat exchanger through the flow of transport fluid. The functions of such energy storage system have three operating modes, i.e., heat charge, heat discharge, and simultaneous charge and discharge. The thermal performance of the system with alcohol and water as working fluid is experimentally investigated. The results show that the storage system employing alcohol as working fluid in the loop thermosyphon provides better performance; the system gives optimum heat charge and discharge performance under 35–40% fill ratio, regardless whether the working fluid is water or alcohol. The system displays optimum charge efficiency of 73% and optimum discharge efficiency of 85% with alcohol as working fluid.     

Variable geometry wind turbine for performance enhancement,improved survivability and reduced cost of energy

The conceptual design and proof‐of‐concept testing of a furling vertical axis wind turbine, suited to large‐scale offshore deployment, is described. Through the implementation of variable geometry capabilities, extreme storm loads can be reduced, and unsteady flow‐related fatigue loads can be minimized thereby reducing capital (structural) and maintenance costs. Moreover, annual power generation can be optimized in real‐time to account for unsteady wind effects related to weather and siting thus improving efficiency and annual power generation. Copyright © 2014 John Wiley & Sons, Ltd.     

Improvement of synchronous generator damping throughsuperconducting magnetic energy storage systems

Stabilization of a synchronous generator through control of firing angle of the power converters in superconducting magnetic energy storage (SMES) systems is considered. An optimum strategy of the firing angle control is designed so as to eliminate the transients in minimum time. A nonlinear model of a synchronous generator, its governor and exciter systems, and an SMES system connected to the generator terminal is considered. The optimum firing angle control is derived retaining the nonlinearities of the system dynamics. Digital simulation results indicate that the proposed strategy controls the slowly growing as well as first swing instabilities very effectively     

Minimizing the energy cost for microgrids integrated with renewable energy resources and conventional generation using controlled battery energy storage

This paper presents a methodology to minimize the total cost of buying power from different energy producers including renewable energy generations particularly within the context of a microgrid. The proposed idea is primarily based on the controlled operation of a battery energy storage system (BESS) in the presence of practical system constraints coupled with our proposed cost optimization algorithm. The complex optimization problem with constraints has been solved using the well-known concept of dynamic programming. The methodology has been assessed using actual power and price data from six different power generation sites and cost reduction has been calculated for a number of BESSs by varying their energy and power capacities. Twofold benefits of the proposed methodology lie in minimizing the total cost along with the constraint-based efficient operation of the BESS. Simulation results depict that the given power demand at a particular region can be fulfilled properly at all times using a BESS and multiple power generation.     

Dynamic performance of wind-diesel power system with capacitive energy storage

This paper presents an analysis of the dynamic performance of a wind-Diesel power system which operates in isolation from the grid. The simulation studies of the dynamic response are conducted in two different configurations of the power system, firstly, without storage and, secondly, with capacitive energy storage. The frequency and power deviations resulting from a step load disturbance of 1% are presented. It is shown that improvement in the transient responses of the stand alone wind and the hybrid wind-Diesel power system is achieved when capacitive energy storage is included in the systems.     

Key performance indicators in thermal energy storage: Survey and assessment

Thermal energy storage (TES) is recognised as a key technology for further deployment of renewable energy and to increase energy efficiency in our systems. Several technology roadmaps include this technology in their portfolio to achieve such objectives. In this paper, a first attempt to collect, organise and classify key performance indicators (KPI) used for TES is presented. Up to now, only KPI for TES in solar power plants (CSP) and in buildings can be found. The listed KPI are quantified in the literature and compared in this paper. This paper shows that TES can only be implemented by policy makers if more KPI are identified for more applications. Moreover, close monitoring of the achievements of the already identified KPI needs to be carried out to demonstrate the potential of TES.     

Enhancing the utilization of photovoltaic power generation bysuperconductive magnetic energy storage

The authors demonstrate that a superconductive magnetic energy storage (SMES) system can enhance large-scale utilization of photovoltaic (PV) generation. Results show that power output from a SMES system can be used to smooth out PV power fluctuations so that the combined PV/SMES output is dispatchable and free from fluctuations. Power generated from PV arrays is shown to be fully utilized under different weather conditions, and PV penetration is increased to significant levels without adversely affecting the power system. Coupled with PV generation, a SMES system is even more effective in performing diurnal load leveling. A coordinated PV/SMES operation scheme is proposed, and its demonstration under different weather conditions is discussed     

高温相变蓄热的研究进展

从如下两个方面总结了高温相变蓄热的研究现状：①在高温相变材料(PCM)方面,重点介绍了高温相变材料的一些重要性能及其测量,高温相变材料的封装,高温复合相变材料及高温相变材料的应用;②在传热分析方面,主要介绍了相变过程的数值模拟和相变蓄热系统(LTES)的热力学优化。     

Self-discharge characteristics and performance degradation of Ni-MH batteries for storage applications

The needs for onboard energy storage are practically dependent on the Ni-MH and Li-ion battery packs, because these two power-assisting systems have features of proper energy density, longer cycle lifetime, quick charge acceptance, and proper operating windows for both voltage and temperature. In particular, the Ni-MH power system has a proper tolerance mechanism for overcharge and overdischarge, a lower cost for battery pack maintenance, and a slightly longer cycle lifetime profile. We studied the self-discharge characteristics, state-of-health, state-of-charge, and energy efficiencies at various charge input levels. The end-of-voltages during charge and discharge were evaluated for the Ni-MH storage batteries. The impedance measurements and data analysis have also been conducted for equivalent circuit simulations. The performance deterioration and capacity decay are fundamentally analyzed and discussed in details, including electrode side-reactions, structure degradations, separator weakening, and level changes of electrolyte saturation in the battery. Further battery quality enhancement through cycle duration improvement for onboard energy storage potentially provides more suitable power and energy delivery in order to obtain higher efficiency, save more fuels, and reduce CO₂, SO₂, and NO_x emissions.     

System-level power-to-gas energy storage for high penetrations of variable renewables

According to outlooks by the IEA and the U.S. EIA, renewables will become the largest source of electricity by 2050 if global temperature rise is to be limited to 2 °C. However, at penetrations greater than 30%, curtailment of wind and solar can be significant in even the most flexible systems. Energy storage can reduce curtailment and increase utilisation of variable renewables. Power-to-gas is a form of long-term storage based on electrolytic production of hydrogen. This research models the co-sizing of wind and solar PV capacity and electrolyser capacity in a jurisdiction targeting 80% penetration of variable renewable electricity. Results indicate that power-to-gas can reduce required wind and solar capacity by as much as 23% and curtailment by as much as 87%. While the majority of charging events last less than 12 h, the majority of the total annual stored energy comes from longer-term events. Additional scenarios reveal that geographic diversity of wind farms reduces capacity requirements, but the same benefit is not found for distributing solar PV.     

Design, construction, performance evaluation and economic analysis of an integrated collector storage system

The design and construction of an Integrated Collector Storage (ICS) system is presented in this paper. The main advantage that such a collector system presents, with respect to conventional flat-plate collectors, is the fact that it is of a very low profile. The main disadvantage of these collectors comes from the design of the system, i.e. with the receiver of the collector being also the storage vessel, it is not possible to insulate it properly and there are significant heat losses during the night. System modelling and optimisation is carried out by the use of a computer code written for the purpose. Performance results presented are in good agreement with the predicted results, especially for the end-of-day storage temperature which is predicted to within 5.1%. The initial cost of the system presented here is 13% cheaper than the corresponding flat-plate (FP) collector of the same aperture area and storage volume. Additionally, the economic analysis of the two systems, performed with the F-Chart program, showed a yearly F-value of 0.85 for the ICS system compared to 0.83 for the FP system, a pay-back period of nine years for the ICS system, compared to 11 years for the FP system and a life cycle saving of C£330 for the ICS system compared to C£201 for the FP system.     

Thermal performance of a high conductive shape-stabilized thermal storage material

A composite based on paraffin, styrene–butadiene–styrene (SBS) triblock copolymer and exfoliated graphite (EG) is prepared. In this composite, paraffin undergoes solid–liquid phase change in the SBS network, and there is no leakage of it even in the state of melting. The composite exhibits high thermal conductivity and nearly 80% of the latent heat of fusion per unit mass of the paraffin.     

脂肪醇相变蓄能特性的数值模拟

利用FLUENT凝固/融化模型对多种脂肪醇在蓄能球内的凝固过程进行数值模拟,得到了蓄能球液相随时间的变化规律。针对适用于常规空调工况的十醇,分析了凝固时间与壁面温度、球径和导热系数的关系,对脂肪醇在蓄能领域的应用具有一定的参考价值。     

Development of the high performance magnesium based hydrogen storage alloy

Series of MgNi type alloys with Ti, Al, Zr, Pd and Co additive elements were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. Systematical alloy designing indicated that Mg_0.80Ti_0.15Al_0.05Zr_0.05Ni_0.95 alloy has the best electrode performance. The atomic fractions in this alloy were believed to be optimum to get the reasonable amount of hydrogen storage with the improved cyclic stability. Titanium was estimated to enter into Mg(OH)₂ layer during the discharging process and make this barrier layer more penetrable by elemental hydrogen. Al and/or Al-oxides were predicted to dissolve selectively throughout the barrier hydroxide layer and thus reduce the stability of this layer. The main contribution of Zr was estimated to arise from its large atomic size that Zr atoms can create extra sites for the elemental hydrogen in the alloy structure. As the alloy charge transfer resistances decreased, the alloy retention rates increased. Improvement in the alloy capacity retaining rate was also closely related with the hydrogen diffusion coefficients in the alloy.