Contents
1.1 Introduction
Electricity is a form of energy. It plays a very important role in everyday human life. The demand for this electrical energy is gradually increasing with the rising population. Electrical energy has led to development of society, its economic growth and high standard of living. The generation and consumption of electrical energy has been one of the most pressing challenges for the modern societies, due to ever increasing gap between the generation and consumption. Engineering efforts and solutions can address these challenges to bridge the gap.
In India, the installed power generation capacity is 229.133 Giga-watt (GW) [3, 4]. About 57.18%
of the electricity is generated from coal based power plants, 17.62% from hydroelectric power plants, 12.45% from renewable-energy sources, 9.03% from gas based power plants, 3.15% from nuclear power plants and 0.57%from oil based power plants [3]. The base load requirement is 861,291Megau- nit (MU) against availability of 788,355MU which is a shortage of 8.5%. Due to population growth and economic development, the demand for energy has increased at a rate of 3.6% per annum over the past 30 years and the approximate energy consumption per capita is 96kWh in rural area and 288kWh in urban area [3–6]. During peak hours, the demand is 139.29GW against availability of 110.76GW which is a shortage of 20.48% [4]. The peak load energy shortage prevails in all regions of the coun- try and varies from 5.98% in the North-Eastern region to 14.51% in the Southern region [3]. Due to shortage of electricity, power cuts are common throughout India and this has adversely affected the country’s economic growth. Therefore, bridging the gap between the energy generation and its consumption becomes essential for the nation’s development. The present energy generation in India from the various energy sources is shown in Fig. 1.1.
57.18%
17.62%
12.45%
9.03%
3.15%
0.57%
Coal Hydroelectric Renewable Gas Nuclear Oil
Figure 1.1: India’s present power generation from various power plants.
The estimated total peak load demand for electricity in India is expected to cross 298GW by 2022 [3–5]. The peak load shortage would prevail in all the regions of the country and for North- Eastern region it would be 17.58GW. Assam which is a part of North-Eastern region will have peak power demand of 1.93GW [7]. The peak load demand for 132/33kV Sishugram substation in the city of Guwahati, Assam will be 0.62MW [8, 9]. From the power plant usage data it is seen that most of the power plants are underutilized during the off-peak hours. If some energy storage device can be used to store the energy during the off-peak hours, then an energy gap of 500kW during peak power demand can be met. Therefore, an energy storage system (ESS) is required to solve the issue of energy gap during peak hours. In order to handle the peak power demand, one possible solution can be to ramp-up the power generation capacity, but this option would require significant infrastructure cost of the power plants.
0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6
Load (p.u)
Time (hrs)
00:00hrs 04:00hrs 08:00hrs 12:00hrs 16:00hrs 20:00hrs 24:00hrs
0.85 0.9 0.95 1 1.05 1.1
V node (p.u) Voltage profile Load profile
Node voltage
Valley filling Peak shaving
Existing load
Valley filling
Figure 1.2: Grid load (p.u) and voltage profile of the Guwahati city, Sishugram 132/33kV Grid Substation.
The approximate overview of the grid load and voltage profile of Guwahati city, Assam is shown in Fig. 1.2 [10]. The energy demand is high during the peak hours from 07:00hrs to 10:00hrs and 18:00hrs to 22:00hrs. During these hours, power plants must ramp-up generation in order to meet up the demand. It is expensive to produce power in the peak hours because the increased generation usually comes from high cost fuel such natural gas. From Fig. 1.2, it can be seen that there is big difference between the peak power demand and off peak power demand. The gap between the peak
and off-peak power demand can be reduced if ESS is used. During the off-peak hours the ESS can be charged and during peak hours the stored energy can be injected back to the grid.
There are different types of ESS available which is given in Fig. 1.3.
Battery Supercapacitor
Energy Storage System
storage Superconducting
magnetic energy Flywheel energy storage system
Figure 1.3: Types of energy storage system.
(i) Battery is electrochemical energy storage device which convert chemical energy to an electrical energy and vice versa [11]. The battery is generally expensive, has limited charge/discharge rate, has high energy density and low power density and has limited life cycle.
(ii) Supercapacitor (SC) is electrochemical energy storage device which stores the electrical energy in the form of on electric field in the electrochemical double layer [12]. The specific energy stored in an SC is relatively low due to limitations in the accessible specific surface area of the electrode the specific power is large due to the short time constant of double layer charging but the energy density, energy stored per unit of weight is less [13].
(iii) The Superconducting magnetic energy storage (SMES) device stores the energy in the form of magnetic field created by the flow of direct current (dc) in a superconducting coil [14].
The stored energy can be drawn from the SMES unit almost instantaneously and also can be delivered over a period ranging from fraction of seconds to several hours [15]. The SMES has high power, high efficiency and four-quadrant control [16, 17].
(iv) Flywheel energy storage system stores the electrical energy in a rotating mass [18, 19]. De- pending upon the inertia and speed of rotating mass, the amount of kinetic energy is stored as a rotational energy [19]. The kinetic energy is transferred in and out of the flywheel with an electrical machine that can function either as a motor or generator depending on the load angle.
The flywheels have high power density, high energy density, ability to handle high power levels and need low recharge time [20].
The difference between daily peak and off-peak power demand is varying every year. In case the difference is small for a day, then the normal battery facilities become suitable. Since these normal battery energy storage facilities have not proven economical except in functional applications such as distributed energy storage system (DESS) upgrade a reasonable storage system is required to mitigate daily peak and off-peak power demand with less infrastructure cost.
The electric vehicles (EVs) can be used to mitigate the peak power demand because most of the time these vehicles are kept parked [21–24]. The batteries of EVs can be used as a DESS to support the electric grid when the power demand is high and store the excess amount of energy during the off-peak hours [22, 23, 25–37]. The EVs are idle most of the time and the batteries are expected to retain a significant amount of energy when the EVs are not in use [21–23, 34].
There are different types of batteries used in EVs such as lead acid, lithium-ion (Li-ion), alka- line battery etc. In the recent decades much attention has been given to the Li-ion batteries due to numerous advantages [38, 39]. The Li-ion batteries have high power (800-2000 W/kg), high specific energy (100-250Wh/kg), high working cell voltage, long life cycle, high power rate density (three time of the lead acid battery and one and half time of the alkaline battery), low self discharge rate and no memory effect [40]. Therefore, Li-ion batteries are most suitable for EVs and the higher energy density of these batteries makes them suitable as DESS. In the next section, mitigation of peak and off-peak hour power demand by using EVs is presented.