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Advances in the development of energy storage technologies are making them attractive for grid integration together with wind power plants. Thus, the new system, the virtual power plant, is able to emulate the characteristics of today's conventional power plants. However, at present, energy storage devices are expensive and proper selection of the energy storage technology that is to be grid integrated with wind power plants is necessary. In this paper, a methodology for selection of the most suitable energy storage technology for grid integration with wind power plants is proposed. The selection process is service-oriented and thus the energy storage technology is selected based on certain requirement criteria. The primary frequency regulation service was chosen, for example, due to its potential economic benefits. For this service, low cost per cycle at partial charge/discharge was found as the requirement criterion while Li-ion batteries where found as the devices which could best fulfil this requirement. Since accurate and fast battery performance models are indispensable for studying the virtual power plant behavior under different operating conditions, impedance-based performance-degradation models were developed for the two most suitable Li-ion chemistries for the primary frequency regulation service: LiMO /Li Ti O and LiFePO /C. Index Terms-Electrochemical impedance spectroscopy, energy storage, Li-ion batteries, primary frequency regulation, wind power.

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Lithium-ion batteries have developed into a popular energy storage choice for a wide range of applications because of their superior characteristics in comparison to other energy storage technologies. Besides modelling the performance behavior of Lithium-ion batteries, it has become of huge interest to accurately diagnose their state-of-health (SOH). At present, Lithium-ion batteries are diagnosed by performing capacity or resistance (current pulse) measurements; however, in the majority of the cases, these measurements are time consuming and result in changing the state of the battery as well. This paper investigates the use of the electrochemical impedance spectroscopy (EIS) technique for SOH diagnosis of Lithium-ion battery cells, instead of using the aforementioned techniques, since this new method allows for online and direct measurement of the battery cell response in any working point. For the investigation, a 2.5 Ah Lithium-ion battery cell based on lithium iron phosphate and graphite (LiFePO 4 /C), as active material, was used. The obtained results at different battery SOH levels have been analyzed in detail and are suggesting that the EIS technique represents a promising tool for diagnosis the lithium-ion battery cells' SOH; meaningful results regarding the pulse power capability decrease of the tested Li-ion battery cells were obtained by applying the EIS technique. I.978-1-4799-5776-7/14/$31.00 ©2014 IEEE

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Uninterruptible power supply (UPS) systems have incorporated in their structure an electrochemical battery which allows for smooth power supply when a power failure occurs. In general, UPS systems are based on lead acid batteries; mainly a valve regulated lead acid (VRLA) battery. Recently, lithium ion batteries are getting more and more attention for their use in the back-up power systems and UPSs, because of their superior characteristics, which include increased safety and higher gravimetric and volumetric energy densities. This fact allows them to be smaller in size and weight less than VRLA batteries, which are currently used in UPS applications.The main purpose of this paper is to analyze how Li-ion batteries can become a useful alternative to present VRLA. In this study, three different electrochemical battery technologies were investigated; two of the most appealing Li-ion chemistries, lithium iron phosphate (LFP) and lithium titanate oxide (LTO) were compared with lead acid batteries, in terms of their basics characteristics (e.g. capacity, internal resistance) and their dependence on the operating conditions.