Renewable Energy Storage Bui Hai Dang 2017 3723 PDF

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Hanoi University of Science and TechnologySchool of Electrical EngineeringRenewable energy storageComparison and applications in VietnamA report in Introduction to ElectricalEngineering (EE1024E)Instructor: Nguyễn Đức Tuyên,PhD, professorof SEE,HUSTReported by: Bùi Hải Đăng, student ofSEE,HUSTStuden...

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Hanoi University of Science and Technology School of Electrical Engineering

Renewable energy storage Comparison and applications in Vietnam A report in Introduction to Electrical Engineering (EE1024E) Instructor: Nguyễn Đức Tuyên,PhD, professor of SEE,HUST Reported by: Bùi Hải Đăng, student of SEE,HUST Student ID:20173723

Hanoi, June 2019

Abstract As the drastic exhaustion of the Eatrh’s fossil resources and the concerned about environmental issues of using them, the world now incresingly switch the power supply to renewable energy resources such as solar, wind, biomass, tidal, wave, etc. Solar power plants and Wind power plants are the most popular. Howerver, these types of energy is intermittent in nature and hence the energy storage systems is required to provide stable energy supply. In this report, electricty storage technologies for renewable energy power plants will be disscussed. This report will focus on the existing technologies such as pumped hydro, flywheel, compressed air, capacitors, batteries and superconducting magnetic storage. Comparison between these technologies is made regarding technical characteristics, operating requirements, applications and installation availabilities in Vietnam’s situations.

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Content List of abbreviations ……………………………………………. 1.

Introduction……………………………………………………… 1.1 Iperativeness of Renewable energy storage system 1.2 Application 1.3 Classification

2.

Theoretical Information…………………………………………. 2.1 Pumped Hydro storage system 2.2 Flywheel energy storage system 2.3 Compressed air anergy storage system 2.4 Superconducting Magnetic storage system 2.5 Capacitors/Supercapacitors storage system 2.6 Batteries

3.

Comparison and Disscusion……………………………………... 3.1 Specification differences 3.2 Installation availability in Vietnam 3.2.1 Discussion 3.2.2 Remarks

4.

Conclusion ………………………………………………………. References

PAGE 2 3 3 3 4 5 5 6 8 9 10 12 13 13 15 15 17 18 19

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LIST OF ABBREVIATIONS RE: Renewable energy ESS: Energy storage system PHES: Pumped Hydro energy storage CAES: Compressed Air energy storage SMES: Superconducting magnetic energy storage VSC: Voltage source converter AC: Alternating current DC: Direct current

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1.INTRODUCTION 1.1 Imperativeness of a energy storage system During the industrial revolution, fossil fuels (coal, oil, gas,..) become the main energy resources of human society. However,in recent decades, people have concerned about the rapid exhaustion of fossil fuels and the adverse effects of using them on global climate, thus the clean and renewable energy is being develop and utilize more and more widely. Renewable energy is energy obtained from natural repetitive and persistent flows of energy occurring in the local environment[1]. Renewable energy includes sources like wind, solar, geothermal, biomass, tidal, waves, etc. These types of energy is discontinuous in nature especially will and solar power, their meteorological parameters changes on daily, weekly, anually even hourly and depend on weather and location of the installed location[2]. Hence, the lacking of producing continuous and stable anergy capacities is recognizable. In addition, not only the power of RE is intermittent but the electricity demand also varies with time, the maximum demand only last for a few hours each day. This leads to inefficient and expensive power plant. The solution to meet renewable energy with the main grid’s demand and assure quality power supply is Electricity Storage System.

1.2 Application Generally, ESS has two main purpose: peak-shaving and loadleveling. In peak shaving mode, electricity from off-peak time is stored in ESS and then discharge to the grid during on-peak time, it means that the power plants can save fuels or prevent overloading. In load-leveling mode, ESS acts like a alternating load. When the load is low such as in off-peak period, ESS play the part of a “positive-load” (i.e it is charged by the remainder of power). On the other hand, when the consumed power is higher than average, ESS acts like a “negative-load” (i.e it

Fig.1 Load profile of a large scale ESS[3] a)Peak shaving mode;b)Load leveling mode

discharges and supports the generator).These mean that the load profile of the whole system is kept at a nearly constant rate which is good for the grid. For Renewable energy, detailed applications of ESS to enhance the integration of wind energy are reported in Ref.[4]: (i) Transmission curtailment: compensation of power delivery

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restraint caused by insufficient transmission line. (ii) Time-Shifting: firming and shaping of wind-generated energy by storing power from the grid when wind generation is inadequate and discharging during the high demand period. (iii) Grid Frequency Support: Energy storage supports grid frequency during sudden, large decreases in wind generation over a short interval. (iv) Fluctuation suppression: Wind farm generation frequency can be stabilised by suppressing fluctuations (saving and releasing energy during short duration variations in output). Eventhough the work reported in Ref. [4] only focuses on wind energy, these key applications for storing the renewable electricity should be equally relevant to solar or wave power generation as well as other intermittent renewables sources. 1.3 Classification Electricity is not easy to be stored directly but it can be converted to another form for storage then converted back to electricity when needed. This is the more convenient and costeffective method rather than store electricity directly. According to the form of energy to which eletricity is converted, ESS can be catergorized in to classes: (i)

Mechanical energy: Potential energy (Pumped hydro, Compressed air) or Kinetic energy (Flywheel)

(ii)

Electromagnetic energy: Superconducting magnetic

(iii)

Electrostatic energy: Capacitors and supercapacitors

(iv)

Chemical energy: Batteries

2. THEORETICAL INFORMATION

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2.1 Pumped Hydro Electricity Storage A PHES plant consists of three main component: (i) Lower reservoir; (ii) reversible turbine generator/turbine pump; (iii) Higher reservoir as shown in Fig.2[5]. PHES stores electricity in the form of potential energy of water that is pumped from the lower to the higher reservoir. In this kind of ESS, electricity in off-peak time is used to operate the turbine pump to raise water from the bottom reservoir to the top reservoir. When high demand occurs, water form the upper reservoir fall through the generator and generate electricity just the same as a

Fig.2 Conceptual Pumped Hydro storage system[5]

hydropower plant. The water on the high altitude has potential U =mgh , it is clear that the maximum amount of energy stored in a PHES plant proportion to the height difference between two reservoir and the volume (mass) of water. Although the energy density of PHES is relative small in comparison with other conventional fuel (calculating show that the energy density per unit volume of a W V =1.0 MJ / m

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100 m height water is

) but total energy stored in a PHES still can be very large due to the large

volume of water. PHES is a developed technology with large volume, long storage period, high efficiency (varies between 70% to 80% even up to 87%[6])and relatively low capital cost per unit of energy[3]. This technique is currently the most cost-effective way to increase the penetration level of Renewable energy into power system, particular in small autonomous island grids[5]. According to Hino and Lejeune[7], pumped hydroelectric storage plants have several advantages, such as (i) flexible start/stop and fast response speed, (ii) ability to track load changes and adapt to drastic load changes, and (iii) can modulate the frequency and maintain voltage stability. However, the installation availability of PHES is highly depend on the area geographical characteristic. PHES should be built in the area with sufficient water supply and favorable topography (i.e the height difference between two reservoir must be significant). In

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flat are are such as delta or highland, underground cavities or even open sea can be used as the lower reservoir[8]. 2.2 Flywheel energy storage sytem Ancient world had used flywheel as a mechanical energy storage device for a long time, its earliest form is the spinning table for clay pottery making. Flywheel stores energy in the form of anggular kinetic energy. The amount of energy inertia I

spinning with angular velocity

E stored in a flywheel with moment of

ω is

1 2 E= I ω . 2 Let consider the simplest case that the flywheel is a uniform disk with radius

r

which has

moment of inertia: 1 I = mr 2 2 Therfore, the energy density per unit mass of the flywheel is obtained by: E 1 2 2 W m= = r ω m 4 Hence, the faster the flywheel rotates, the larger amount of energy can be stored. However Shape K the angular velocity of a real flywheel is limitted by its material strength resisting the centrifugal force

Constant stress disk

0.931

which tends to tears the wheel apart. For a a uniform

Constant thickness disk

0.606

Thin rim

0.500

Constant stress bar

0.500

Flat pierces disk

0.305

wheel of density

ρ , the maximum tensile stress

is: σ max =ρ ω 2 r 2 (Ref.[1]) In general, the moment of inertia of a solid shape is I =Km r

2

where K is called shape factor of the

wheel (given in Table.1). So: Table.1 Flywheel shapes factor

max

E 1 1 Kσ W m = = K r 2 ω2= m 2 2 ρ

Much larger energy density can be obtained by using lighter composite material such as fiberglass in epoxy resin[1], which have higher tensile strength

σ max and smaller density

ρ .

The schematic of a Flywheel storage system is shown in Fig.3[9]

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Power source

Fig.3 Basic diagram of a Flywheel storage system

This system has three operation mode: (i)

Charge mode

(ii)

Stand-by mode

(iii)

Discharge mode

During charge mode, the Voltage Source Converter (VSC) interfacing the power source runs as a rectifier and the other as an inverter, with the transferred energy accelerating the flywheel to its rated speed. In this mode, energy is stored in the flywheel in the form of kinetic energy. Once the flywheel reaches its charge speed, the storage system is in standby mode and is ready to discharge. In this mode a little energy from the power system is used for redeem the converter and machine losses. During discharge mode, the VSC interfacing the power system runs as an inverter injecting the required power to the grid. The flywheel VSC runs as a rectifier. The flywheel slows as it discharges. The reason that the electriccity is convert ACDC-AC from source to flywheel is to simplify the control of flywheel. It easier to manipulate the speed of the wheel by manipulate the DC voltage (by pulse width modulation or other methods) rather than work with AC voltage. The advantage of Flywheel over PHES is that they take a little land area, not require any special condition thus can be installed almost everywhere. Flywheel also have long life capable, it can make thousands of fully charge-discharge cycle without requiring mantainance[10]. Although flywheel is not widely used in commercial (there is only a pilot project by Amber Kinetics in Hawaii), but it offer a promising theoretical method for electricity storage, especially for eectric vehicles since its energy can be refill more quickly than batteries.

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2.3 Compressed air electricity storage system Beside the PHES, Compressed Air Energy Storage system is the only other commercially available technology capable of providing very large energy storage deliverability (above 100 MW with a single unit).A CAES system consists of five major components as shown in Fig.4[11]: (i) A motor/generator with clutches to control engagement with the compressor or turbines. (ii) An two-stage air compressor, to achieve cost-effection and reduce the moisture in compressed air. (iii) A turbine train, containing both high- and low pressure turbines. (iv) A cavity/container for storing compressed air. (v) Equipment controls and auxiliaries such as fuel storage and heat exchanger units.

Fig.4 Compressed air storage system[11]

CAES works on the basis of conventional gas turbine generation. It separate the compression and expansion process of a conventional gas turbine into two independent processes and stores the energy in the form of elastic potential energy of compressed air. Energy is stored by compressing air into an air tight space (tank or carven) with high pressure between 4.0–8.0 MPa. Energy is extracted from CAES by two steps: (i) Compressed air is released from the

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storage tank, heated and expanded throug the high pressure turbine (ii) then the air iss mixed with gas or fuel and burned and exhauted through a low pressure turbine. Both the turbines are connected to a generator to produce electricity. The heat of the exhaust is potentially captured by a recuperator and used to heat the high pressure air in the next cycle CAES is not an independent system, it has to be combined with a conventional gas turbine plant. It cannot be used with other types of power plants such as hydropower, coal-fired, nuclear, wind turbine or solar photovoltaic plants. Moreover, the requirement of combusting fossil fuels and the contaminating emission also makes the CAES less attractive [12].

2.4 Superconducting magnetic storage Super conducting magnetic storage (SMES) is the only established technology which store electrical energy directly by electric current[13]. It store energy in the form of magnetic field energy created by a DC current passing through an inductor made from supercnducting material which have been cooled down to

4° K

in oder to maintain the coil’s

superconducting threshold. Theoretically, any coil can be considered as a ideal inductor in series with a pure resistance, let emulate the coil with a DC current flow in it as the R-L circuit in Fig.5[14]

Suppose both switch

S 1 and

S2

closed at the beginning. First,

are S 1 is

closed, after a long time, the current through the circuit is steady and equal: I 0=

ε R

Then the magnetic enargy stored in the coil is: 1 U = L I 02 2 Fig.5 R-L circuit[14]

Now closed

S2

and open

S1

at the

same time, take the EMF source out of the circuit,

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denote that moment as t=0,the current through R and but decay smoothly as shown in Fig.6[14]. The curren

( RL )t

−

i=I 0 e

Then the time rate of energy losses due to heat is equal:

Fig.6 Graph of current from t=0[14]

Ploss=i2 R Hence, if the coil is forced to reach superconducting state (i.e its resistance apprximate 0), the heat loss is approach zero and the current

i

is conserve at

i=I 0

and can circulate

indefinitely. Fig.7 shows the main components of a SMES system: (i) a superconducting unit (ii) a cryogenic refigerator and a vaccum insulated vessel (iii) a power conversion system

Fig.7 Schematic of a SMES[15]

SMES systems have been in service for some years to improve industrial power quality and to provide a fnest quality electricity for users who are the most vulnerable to voltage sag. An SMES recharges within minutes and can repeat the charge/discharge cycle thousands of times without any degradation of the coil. Although SMES also is a promissing technology to store large amount of electricity,but the high cost prohibit the widespread use of them. 2.5 Capacitor/Supercapacitor The simplest capacitor consists of two conducting plates (metal) separated by a layer of insulating material (dielectric). When a capacitor is charged, the two plates carries charges with the same magnitude

|Q| and opposite in sign. The potential V of the positive plate

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with respect to the negative one is proportional to as

Q and the capacitance

C

is defined

Q . V

Capacitors can be charged significantly faster than batteries and have long life span with a high efficiency. However, the main drawback of conventional capacitors is the low energy density. The energy stored in the capacitor is: 1 U= C V 2 2 For a conventional parallel-plate capacitor: Capacitance

C

is proportion to the area of

plates, thus if a large capacity is required, the area of the dielectric must be very large. This makes the use of large capacitors uneconomical especially in stationary EES applications [16]. Recent progress in the electrochemical capacitors/supercapacitors could lead to much greater capacitance and energy density than conventional capacitors[3]. Electrochemical capacitors (supercapacitors) consist of two electrodes separated by an ion-permeable membrane (separator), and an ionical electrolyte touching both electrodes,[17] the construction of a double layer capacitor is shown in Fig.8[17]. The electrodes are often made from porous carbon or another large surface area material. When a voltage is applied between two electrodes, the electrodes are polarized, ions in the electrolyte form electric double layers of opposite sign of charges to the electrode's charges. For example, positively polarized electrodes will have a layer of negative ions at the electrode/electrolyte interface along with a charge-balancing layer of positive ions adsorbing onto the negative layer and vice versa. Since the surface area of activated carbons is very high (about

2000 m

2

per

gram), moreover the distance between the plates is very small (less than

Fig.8 Typical construction of a double-layers capacitor (1)power source; (2)colector; (3)electrodes; (4)double layer ; (5)electrolyte with ions; (6)separator

1 nm ) thus much larger capacitances and stored energy are

obtained by using supercapacitors rather than using conventional capacitor.

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Table.2[18] present the most prospective technologies for short-term power exchange in terms of cost, time scale, and rate of efficiency. The two most promising short-term storage devices: flywheels and supercapacitors, both offer similar characteristics and are both suitable for renewable energy applications[19]. Technology

Energy cost

Power cost

Time scale

Roundtrip

Flywheel Supercapacitor SMES

($/kWh/year) 96 711 370000

($/kWh/year) 1.2 6 59

(minutes) 0.006-6 0.006-6 0.006-0.06

efficiency 89% 86% 21%

Table.2 Properties of short-term energy storage technologies[18]

The major problems with capacitors, similar to flywheels, are the short durations and high energy dissipations due to self-discharge loss. On the other hand, although the small electrochemical capacitors are well developed, large units with energy densities over 20 kWh / m3

are still in the development stage.

2.6 Batteries Rechargeable battery is the oldest form of electricity storage which stores electricity in the form...