I. Introduction Higher penetration of Renewable Energy (RE) into the power grid has become a global demand to reduce fossil fuels consumption and then reducing the greenhouse gas emissions. Recently, the wind energy has showed rapid growth and progress over the other sources of RE (the world wind energy production in 2015 was 838 TWh with respect to only 247 TWh for solar energy, see fig. 1) 1. Figure 1. 2015’s World wind and solar production 1 The world wind energy production has doubled almost 8 times over a decade period from 104 to 838 TWh during 2005 to 2015 1. The International Energy Agency (IEA) has published that the annual wind generated energy will reach 1282 TW h by 2020, and the double of 2020’s figure will be reached by 2030 2. So, it is projected thatthe wind energy will play a vital role in the power market in the future. Onshore wind energy faced human oppositions due to its noise andview, so offshore wind appeared on the horizon to avoid human objections and for harvesting higher energy due to being in higher class wind speed and the ability to build bigger turbines offshore. II. Study problem Generally speaking, wind energy like most of other renewable energies suffers from intermittency problem. In addition, problem of time variation as the peak power production is not aligned with the peak load. For these reasons, it would be possible to increase the share of renewable energy into the grid, but still the fossil fuel power plants play a vital role in the grid stability 3. Integrating Distributed Generation (DG’s) with the gird encounters some restrictions related to the grid connection, specifically, when talking about wind power integration. These constraints could be power quality issues suchas harmonics, flicker, voltage fluctuations and disturbances of remote control signal. Grid capacity would be a restriction, as well, in terms of network congestion and steady state thermal constraints, short circuit current and power and steady state voltage profile. Protection issues and dynamic behavior should be taken into account when discuss wind power integration. In addition, wind turbines have a limited control on the grid frequency which is a crucial issue in terms of grid stability. Wind farms have to contribute in voltage control and in reactive power compensation to enable power transit in the grid 4. P. Bousseau et al. 4 explored the solutions for almost all of the constraints above. The aforementioned constraints above are rectified by either developing in wind turbine itself or using small scale energy storage system (ESS). It is worth mentioning that this kind of ESS known as power application use and this is different from the energy application use which used in a large-scale and this is the core of this study. However, still the main problem which is how to increase the share of wind or RE in general to the extent that fossil fuels power plants contribute with zero power in the gird. Although the developing in the windforecasting sector, still it is not satisfying the grid operators interests in terms of the accuracy which lead to grid instability. As a result, based on the recent studies which tackled the issue of large-scale wind integration or 100% renewable energy network, ESS is crucial as load-balancing technology. III. Energy Storage Background To store the electrical energy produced by RE, there are various forms of energy. This energy will be retrieved to electrical form again when required at peak loads. Figure 2 shows the forms of energy with the applications used with each form. A. Selection of energy storage system The technical characteristics of ESS and wind power fluctuations density at different time scale are two important factors should be considered at selection of ESS. Ibrahim et al. 5 highlighted that the characteristics of ESS such as storage capacity, available power, depth of discharge, discharge time, efficiency, durability or cycling capacity, autonomy, costs, feasibility and adaptation to the generating source, mass and volume densities of energy, self-discharge rate, operational limitations, reliability and environmental aspects should be investigated when selects ESS. Figure 2 Forms of Energy storage and applications 2 Ervin et al. 6 explored in detail the main characteristics of capacitors, flywheels, pumped hydro storage (PHS), compressed air energy storage (CAES), hydrogen, batteries and superconducting magnetic energy storage (SMES). As mentioned earlier, choosing the appropriateESS depends on the application. For example, fast access time is important for power applications such as smoothing of the power output fluctuations, while this factor is not important for energy applications such as peak shaving 6. Based on the access time factor, not all ESS’s could be used for both applications power and energy. ESS such as flywheel, batteries, capacitors and SMES have the ability to supply very large amount of power for a short time that is mean they are suitable for power applications. While, on the other side, ESS such as PHS, CAES, Hydrogen and batteries can keep energy for longer time, and so they are suitable for energy application. Other studies such as 5 classified the applications of large-scale permanent energy storage into various categories or levels based on time. For example, ESS used for few seconds to ensure delivering good quality power. Other application use ESS for minutes as an emergency backup to insure service continuity when switching from electricity source to another. ESS could be used for longer period when consider network management load levelling (i.e. storing energy during off-peak hours and retrieving it during peak hours). For the purpose of this study andfor achieving higher penetration of renewable energy, ESS technologies such as CAES, PHS, hydrogen and batteries would be considered. Ervin et al. 6 concluded that NaS batteries is the most promising ESS, while hydrogen due to its high investment costs is not economical solution. In this regard, figure 3 shows the maturity of energy storage technologies 7. Figure 3. Maturity of energy storage technologies 7 B. Storage installed Capacity worldwide The recent statistics about the capacity of the global installed grid-connected ESS is 140 GW of large-scale energy storage. Roughly 99% of this capacity is based on PHS technology while batteries, flywheel, CAES and hydrogen comprise the other 1% of the global storage capacity see figure 4 7. It is shown that still PHS plays the vital role as a large-scale storage technology all over the world. Figure 4. Global grid-connected electricity storage capacity (MW) 7 IV. Energy Storage Discussion To reach the European commission’s target for 2050 (i.e. transition to a 100% renewable energy network), Bussar et al. 8 proposed energy storage systems to provide flexibility for the grid via load shifting. Bussar et al. 8 depends on “GENESYS” as a simulation tool for sizing and allocation of generation sources, storage systems and transitional grids of the European power network. The study focused on the optimal allocation of wind turbines and solar photovoltaic in Europe. Regarding the storage technologies, the study proposed three different storage systems to be used which are pumped hydro storage, batteries and hydrogen storage. The study concluded that the combination of different energy sources based on fully renewable energy sources and storage systems would be able to supply energy at lower costs. And lastly the study highlighted that the energy storages considered are short or medium term because considering long term storage would result in electricity costs increase. More specifically, Ole et al. 9 investigated the size of an offshore wind energy storage to beconnected with an offshore wind farm. The hybridization of the wind farm with the storage system has been simulated over a period of one year. The study showed that the storage sizing is highly dependent on the production forecast error and market bid length. The studyrecommended for future studies to use different forecast error models to improve the sizing of the storage system. Perry et al. 3 proposed a new compressed air energy storage system. They claimed better efficiency for their proposed CAES system than the conventional CAES as it uses near-isothermal compression/ expansion to store energy before conversion to electrical form. The study concentrated on offshore wind energy as a possible application and concluded that this new method could be used anywhere in the grid. The improvement in the efficiency of this novel system return to the liquid piston compression/ expansion, they concluded. The proposed system declines the costs of the offshore wind farms. The study proposed optimization of the overall system need to be achieved as a future work, in addition to developing the control methods related. As mentioned earlier ESS could be used in power applications. Mohamed et al. 10 utilized a low speed, large capacity flywheel energy storage system to provide reliability for VSC-HVDC transmission system which connect the offshore wind farm with the onshore grid. The system designed to absorb the surge power by FESS instead of begin dissipated as resistive losses. The study showed that the FESS could be a proper support for fault ride-through during faults. In addition to it would be used for power levelling function during normal operation. The study concluded that the proposed system with FESS provides robust performance and fast response for power levelling during normal operation and for fault ride-through during faults. The drawbacks of this system are the high initial costs and the high rating of the converter. While study 10 used FESS for stability improvement, Wang et al. 11 in a different study proposed different system for stability improvement of a grid-connected large-scale offshore wind farm based onsuperconducting magnetic energy storage with superconducting fault current limiter (SFCL). Wang et al. 11 concluded that the combination between SMES and SFCL improved the stability of the system. To enhance the dynamic-stability and achieve power fluctuations mitigation, Wang et al. 11 proposed a hybrid renewable system involves offshore wind farm and seashore wave farm with flywheel as an energy storage system. The findings of the study showed that the FESS can keep the proposed hybrid system stable under different disturbance cases. In addition, the proposed system can effectively smooth the power fluctuations supplied to the grid. Wang et al. in a similar study 12 investigated the impact of using FESS on a hybrid offshore wind / tidal energy system. The study introduced steady-state and dynamic analysis for the hybrid system connected to FESS. The study concluded, the system is stable under various circumstances when using FESS. For stability and frequency control purposes as well, Adria et al. 13 exploited the stored energy in the dc-link of voltage source converter and the kinetic energy storage from wind turbine for achieving control over the frequency. A recentstudy has been held by Machteld et al. 14 to compare the costs of intermittent renewable sources of energy (IRES) and the costs of natural gas combined cycle power plants with CO2 capture storage (NGCC-CCS) based on “experience curve”. What is interesting in this study, the LCOE for IRES is projected to be 68, 82 and 104 €2012/MWh for concentrated solar power, offshore wind energy and solar photovoltaic energy respectively. While the LCOE for NGCC-CCS is projected to be 71 €2012/MWh by 2040. The figures for energy storage such as pumped hydro, compressed air energy storage and batteries comparing with that of NGCC-CCS are projected to be in favor of energy storage, the study added. Some of the ESS witnessed advancement and credibility over long period such as PHS which has huge power capacity worldwide as mentioned earlier and batteries which have high energy and power densities, however, still storage sectorfaces big challenges. Improving the round trip efficiency is one of the biggest challenges faces ESS, specifically, the large-scale energy storage such as CAES and PHS. For example, PHS could achieve efficiency of 90%, theoretically, however the round trip efficiency for PHS, in a real life, is ranging only from 72 to 75%. Same problem for CAES which shows efficiency between 42 and 55% only. The economics of ESS is another challenge as it is hard to evaluate due to a lot of factors affected by as mentioned earlier. Lack of standard for connecting thedifferent ESS to the grid (i.e. physical connection), so modularization of the energy storage technologies to be like batteries is required in this regard. Governmental policy support very required in this regard to enhance higher penetration of ESS. To sum up, CAES consider the most convenient storage technology for storing offshore energy. The first world underwater CAES prototype is running outside the city of Toronto, Canada since summer 2104 by Canadian company which make CAES the most mature offshore storage technology at present 15. Based on the discussion above, and to enable higher penetration of renewable energy, focusing on one of the topics below would be very helpful: Optimal placement of ESS in the power system with large-scale wind integration. Combine many distributed energy storage system as a virtual storage unit and control them centrally. Coordinated control of wind farms and on-site ESS. Modularization of energystorage technologies to be flexible such as batteries for grid connection. Hybrid renewable energy power system (involves all sources) could help the problem of intermittency or still required large-scale of ESS? In addition to the topic of DC wind turbine / DC farms which make the offshore system very simple. Multi terminal HVDC scheme for offshore wind farm integration. Having said that, there are some studies such as 16 and 17 claim that the electrical power system could be supplied from 100% renewable energy sources without affecting on the reliability of the system and with prices comparable to the today’s prices. The findings of the two studies build based on feasibility study of different renewable energy sources considering the availability and the costs of investment. However, few studies have explored the configuration, the protection and the control of the future power grid tacking into account the intermittency nature of renewable sources of energy and its impact on the reliability and the stability of the grid.