Energy Energy (RE) into the power grid

Energy Storage to Enhance Renewable Energy Share


Walid Nassar

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Olimpo Anaya-Lara

Department of Electrical and Electronic Engineering

Department of Electrical and Electronic Engineering

University of Strathclyde

University of Strathclyde

Glasgow, Charitable body SC015263, UK

Glasgow, Charitable body SC015263, UK

[email protected]

[email protected]


        Abstract – The study aims to explore different kinds of
storage energy techniques as a contributor to increase the share of renewable
energy into the existing utility grid. The study focuses on wind energy as it
is the most promising resource of renewable energy. To do so, the study
examines the recent and future trends of different kinds of energy storage. The
methodology for selecting the proper energy technology has been provided.
Classification of energy storage based on energy form is explored. The findings
of this study came up with compressed air energy storage as the most convenient
storage for offshore energy and under construction in Canada is a big project
for this energy storage. The LCOE is projected to be in favor of energy storage
such as compressed air energy storage and pumped hydro storage with respect to
that of natural gas combined cycle power plants. This study is prepared for conference
proceeding to be published by IEEE.


                Index Terms – Energy storage
selection, Renewable energy, power quality, system stability.


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 2. So, it is
projected that the wind energy will play a vital role in the power market in
the future and for these reasons wind energy is considered under this study.



II.  The 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 not be possible to depend on renewable
energy alone and still the fossil fuel power plants play a vital role in the
grid stability 3.

Distributed Generation (DG’s) within 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. In
addition, wind turbines have a limited control on the grid frequency which is a
crucial issue in terms of grid stability 4.
The aforementioned constraints could be rectified either by developing in wind
turbine itself or using small energy storage system (ESS) 4.

III.  Energy Storage Background

The idea of use ESS is to store the surplus energy
from RE in different forms of energy for example chemical, then this energy will be
retrieved to electrical form again when required at peak loads. To store the electrical
energy produced by RE, there are various forms of energy such as thermal,
electromagnetic, chemical, mechanical and electrochemical. 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. 

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. 


Figure 2 Forms of Energy storage and applications 2


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 used. For example, ESS used for few seconds to ensure
delivering good quality power. Other application use ESS for minutes as an
emergency backup such as uninterruptible power supply (UPS) 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).
et al. 6 concluded his study with 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.


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 see figure 4 7.


Figure 3. Maturity of energy storage technologies 7



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. The study 8
depended 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. Three different storage systems are considered
under this study: pumped hydro storage, batteries and hydrogen storage. The
study concluded that the combination of renewable energy sources and storage
systems would be able to supply energy at lower costs. They added that the
energy storages considered are short or medium-term because considering long-term
storage would result in electricity costs increase.

mentioned earlier ESS could be used in power applications to improve system
reliability and hence improving system performance. Mohamed et al. 9 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, 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 the study 9 used
FESS for stability improvement, Wang et al. 10 proposed superconducting
magnetic energy storage with superconducting fault current limiter (SFCL) for the
same reason with a grid-connected large-scale offshore wind farm. Wang et al. 10
claimed that the combination between SMES and SFCL improved the stability of
the system.  

recent study has been held by Machteld et al. 11 to compare the costs of
intermittent renewable sources 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 favour 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

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 12.



        Classification of energy storage techniques has been
presented by this study as trial towards increasing the RE share in the existing
utility grid. The main problems under this study are examined in section 2 with
highlighting the problem of intermittency which is shared by most of renewable
energy resources. ESS proposed by different studies as an alternative solution
for most of power quality problems such as harmonics, flicker, unstable voltage
and power fluctuations.

        The ESS is proposed to be used in a large-scale within RE
field to store the surplus energy into different energy forms such as chemical
or mechanical, then retrieve it to electrical energy at peak load. Five energy
forms are stated under this study which could store the electrical energy. Selecting
the convenient ESS technology should be performed based on the ESS’s
characteristics and the application which used for. RE system with combination
of PHS, batteries and hydrogen as storage facilities could supply energy at
lower costs. CAES consider the most convenient ESS for offshore energy farms.

        For future work, there is need to address the following ESS
topics which could help in increasing the share of RE:

 The optimal placement of ESS in the power

Virtual energy storage which
combine distributed energy storage system and controlled centrally,

Coordinated control of wind farms
and on-site ESS,

 Modularization of energy storage technologies to be
flexible such as batteries for grid connection.



authors would thank University of Strathclyde Glasgow, for
its financial support to my PhD study.




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