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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.         

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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 that the wind
energy will play a vital role in the power market in the future. Onshore wind
energy faced human oppositions due to its noise and view, 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.

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 such as 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.

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 wind forecasting 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

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


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 appropriate ESS 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. 

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.

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 and for 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

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


Figure 4. Global grid-connected electricity storage capacity (MW) 7


IV.  Energy Storage Discussion

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.

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.

specifically, Ole et al. 9 investigated the size of an offshore wind energy
storage to be connected 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 study recommended for future studies
to use different forecast error models to improve the sizing of the storage

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.  

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 on superconducting
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.

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.

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

recent study 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. 

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 sector faces
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 the different 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.

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
energy storage technologies to be flexible such as batteries for grid

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. 

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

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