Master of Science in Energy Engineering for Renewables and
Environmental Sustainability
Coordinated dispatch of Wind Power and
Electric Vehicle Charging in the electric grid
Supervisor: Prof. Morris Brenna
Master Thesis of : Cang Song
Identification Number: 851348
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Content
Abstract ... 1
Chapter 01 Electric Grid ... 3
1.1 Development of electric grid ... 3
1.2 Traditional grid ... 4
1.3 Smart grid... 7
1.4 Smart grid Technology ... 10
1.4.1 Smart Transmission grid ... 10
1.4.2 Information and Communication Technology (ICT) ... 11
1.4.3 Smart Metering Technology ... 13
1.4.3 Smart Control and Monitoring System ... 16
1.5 Distributed generation ... 19
1.6 Storage systems ... 22
1.7 Electric Vehicles as one part of the Grid ... 24
Chapter 02 Wind power ... 26
2.1 Wind power overlook ... 26
2.2 Principles of wind power generation ... 27
2.2.1 Structure and main components of wind turbines ... 27
2.2.2 Theory of wind turbines ... 30
2.3 Energy productivity ... 35
2.3.1 Weibull distribution ... 35
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2.4 Effects of wind turbines on the power system ... 39
2.4.1 Frequency variation ... 42
2.4.2 Voltage variation ... 43
2.4.3 Short-term and long-term effects ... 44
Chapter 03 : Electric Vehicles ... 47
3.1 Global EVs outlook ... 47
3.2 Electric vehicles introduction ... 50
3.2.1 Battery electric vehicle(BEV) ... 50
3.2.2 Plug-in hybrid Electric Vehicle(PHEV)... 52
3.2.3 Fuel cell vehicles ... 54
3.3 Battery technology of electricity vehicles ( BEVs and PHEVs) ... 57
3.3.1 Types of electric-vehicle battery ... 57
3.3.2 Battery performance ... 62
3.3.3 Charging Methods of EV batteries ... 64
Chapter 04 Vehicle to grid (V2G) ... 67
4.1 Overview of vehicle to grid (V2G) ... 67
4.1.1 Principle of V2G ... 67
4.1.2 Motivation towards V2G ... 70
4.2 Required elements for V2G ... 72
4.2.1 Power connection ... 72
4.2.2 Control connection ... 72
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4.3 Grid impacts by V2G ... 73
4.3.1 Bulk Power Supply ... 73
4.3.2 EV Charging Strategies ... 77
4.3.3 Charging Infrastructure impact ... 82
4.4 Coordinated dispatch of electric vehicle charging and wind power grid ... 84
4.5 Coordinated dispatch model ... 89
Chapter 05 Result and analysis ... 93
Conclusion ... 99
Tables of figures ... 101
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Abstract
Renewable energy resource is widely concerned in recent years because of the increasing pressure on climate change and fossil fuels. Wind power and electric vehicles are developed fast and getting more and more used. Unlike the traditional energy , wind energy has the characters of intermittency and volatility. The “wind curtailment” problem is also a challenge for the electric grid operator. Large scale penetration of electric vehicles and V2G(electric vehicle to grid) service is a potential way to relieve these problems. Therefore , effective coordinated dispatch of wind power and electric vehicles charging in the grid is very important.
The main contents of this thesis are as follow:
Chapter 1. “Electric grid”. The structure and components of the grid are described. Then talk about the technology of smart grid which is the essential network for large scale penetration of renewable energy resource and effective coordinated dispatch. Chapter 2. “Wind power”. This chapter mainly shows the feature of wind power and its impacts when penetrating into the grid.
Chapter 3. “Electric vehicles and battery”. The chapter describes the principle of electric vehicles and then talks about the battery technology.
Chapter4. “Vehicle to grid(V2G)”. Introduce the V2G service and talk about its impacts on grid. Then discuss the coordinated dispatch between EVs charging and wind power. At last, set up the dispatch math model.
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region grid and get the optimal dispatch results. In addition, verify the benefits of coordinated dispatch.
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Chapter 01 Electric Grid
1.1 Development of electric grid
Electrical Grid is the means through which power is generated, transmitted, and distributed to the consumers [1].The electrical grid system is mainly comprised of power plants, power transmission lines, substations, transformers, distribution lines and the consumers.
The first current electric grid system was built in Massachusetts, Unite State in 1886. This first grid was an unidirectional and simple system. Then at the beginning of the 20th century, the electric utilities were operating separately. The local power utilities operated low-voltage power plants and supplied power to the local customers by short distribution power lines. With the increased demand of power, interconnect the separated transmission systems together is more efficient and needed. The power utilities could satisfy their power demand at lower cost by building larger power plants. By the 1960s, the grid became large, mature and highly interconnected in the developed countries. A large number of power stations generated power and transmit it by high capacity electricity lines and then branched to supply the electricity to smaller plants and domestic users.
From1970s to 1990s, the number of power stations kept increasing because of the growing demand of electricity. In some countries or areas, power is difficult to meet the demand at peak times. Electricity demand patterns were established until the end of 20th century : air-conditioning and heating for home users led to the peaks were
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supplied by peak power generators. These power generators were only be turned on for some short periods. This low utilization of peaking generators and redundancy in grid lead to high costs for the power utilities and then the high cost is passed on as increased tariffs.
Since the early 21st century, the costs and limitations of the electric grid could be resolved by applying electronic communication development in to the power grid technology. Pollution and environmental influence from fossil-fired power plants got more concerned and more renewable energy were encouraged and built all over the world, especially in the developed countries, such as wind power and solar power . With large scale renewable energy connecting with the electric grid, more advanced control systems are needed. The benefits and characters of RES application in the power system, the grid need to be changed from the centralized style to highly distributed one. In the smart grid , computer intelligence and networking capabilities are applied in the traditional distribution system. Operation, maintenance and planning can be improved in the smart grid since every component in this system can both 'talk' and 'listen' and important component is also automation[2].
1.2 Traditional grid
An electrical grid is an interconnected network, including power generation plants, high voltage transmission lines which is used to transmit electricity to demand centers, and distribution lines which is connected with the end users[ 3 ].Three main components of electrical grid are as follow:
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GENERATION. Two kinds of generation are centralized generation and decentralized generation. Usually, The centralized generation are some large-scale power plants and the places are far from the users. The centralized generation could be nature gas, coal, nuclear, wind plant or large scale PV plant. While the decentralized generation is near to the users side, such as the rooftop solar of the buildings.
TRANSMISSION and DISTRIBUTION. Transmission concludes transformers, substations and power lines. The voltage is converted to very high voltages which is benefit for long-distance transmission on the power grid by the transformers at substations. Higher voltage can reduce the loss due to resistance when transmitting. Transmission is proceed by power lines overhead or underground. As soon as the power get to the consumption side, there is another substation which is used to step-down the voltage for the power users .
In addition, the transition from transmission to distribution has the following functions:
(1) The switches and circuit breakers disconnect the substation with the power grid and ensure the distribution lines be disconnected.
(2) Transformers decrease transmission voltages to distribution voltages[4].
(3) Distribution power is divided into different directions by the Bus bar . Power goes to the bus and then to distribution lines, at last to the users [6].
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and residential users. Electricity delivers important energy services for appliances even through different consumers have different needs.
Figure 1.1 The typical organization of the power grid
Nowadays, Renewable Energy Resources( RES ) is more and more used for the power generation. Which is a challenge for traditional grid to the high penetration of RES. By analyzing the daily energy demand, it is easy to find the reason why Renewable Energy Resources increase instability to the network: Figure 1.2 shows the energy load profile in a normal day. We can see : (1) In the night time, the demand is
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much lower than the day time . (2) From 9:00-1:00 AM , it is the demand Peek. (3) From 17:00 -19:00 it is the maximum peek. If Wind Power Generation is considered, in the night time the output of wind power generation is increased because there is more wind and higher density of the air in the evening than daytime in most areas . Since the energy demand is low in the evening , extra power production which is generated during the night cannot be used and may be waste. This is the problem of “wind curtailment “which happened in many countries. Therefore, with large scale RES penetrate into the grid , the power production curve and the power consumption will no longer match. A more flexible , dynamic and intelligent grid need to be developed to make electricity production to follow the energy demand.
Figure 1.2 Energy load profile in different countries
1.3 Smart grid
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Corporation conference in 2007 , Chicago US. In his definition , Smart Grid is a system which is an electric grid combined with energy, communications network, software ,hardware and control technology[5]. And it is self-healing, interactive and distributed.
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Figure 1.3 Characters of smart grid [6]
Smart grid is developed from traditional grid. While it has more elements and so its structure is more sophisticated (figure 1.4). A smart grid system has many levels of smartness , such as , diffuse generation integration, safety and reliability improvement and complementary services .
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Figure 1.4 Overview of a smart grid system
1.4 Smart grid Technology
1.4.1 Smart Transmission grid
The main function of the grid is to transmit the power which is generated by the power plants to the loads and users. Thetransmission network plays the most important role in the system.Due to the complicated topologies of the net work, the transmission has been diversified to HVAC(High Voltage Alternating Current),
HVDC(High Voltage Direct Current) transmission at different voltage levels. Increase highcapacity bundle conductor lines, high capacityHVDC system, HSIL (High Surge Impedance Loading) Line could improve the transmission network, HTLS (High
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Temperature Low Sag) Line, promotes the quality of power transmissionthrough the key of reliability and economy of the system[7]. In addition, the transmission network are also facing with challenges and issues : environmental issue, infrastructure
challenges ,marketrequirement and innovative.
These challenges can be solved by identifying the major performance features of the transmission grid with the developing of sensing, communication, control, computing andinformation (Figure 1.5).
Figure 1.5 Features of the smart transmission grid
1.4.2 Information and Communication Technology (ICT)
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side, consistent and RT information play an important role. Compared with the traditional electric grid, the smart grid not only increases the flexibility of network capacity, but also provides advance sensing and control performance because of the advance technologies applications and incorporation.
Two-direction flows of information and power is the foundation of electric smart grid. In the smart grid system, ICT(Information and Communication Technology) is a dynamic sector which realize the communication between the smart meters and data center and electrical appliance to smart meters. The network needs two-way communications between power generation, electricity transmission, distribution and utilization [8]. The system become stability and reliability because of the advanced control technology which has the advantage of high security and good robustness towards cyber attacks .Table 1.1 shows the details ,advantages and disadvantages of main communication topologies [9].
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Table 1.1 Smart Grid Network Communication Topologies
1.4.3 Smart Metering Technology
Smart metering technology is using the advanced digital meters to record electrical usage instead of analog meters. The digital meter is good at transmit information. Compare with the analog meters , energy consumption information can be sent by the digital meters to the utilities on a more precise schedule. Users can monitor their power consumption more precisely by the smart meters and consumers can make
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more informed energy choices which will be less cost for users and less electricity loss. In addition, the smart meter can also notify a power outage to the grid operator and allow switching electricity service on or off remotely.
Figure 1.6 Digital meter
Smart metering system combines power system, telecommunication and several other technologies. It can not only improve power consumption in the system but also increase the efficiency of energy and reduce total cost. The bidirectional communication of data make sure the precise and efficiency of communication infrastructure and control devices.
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Smart metering technology which is also named AMI (Advanced metering Infrastructure) . It includes communication modules, smart meters, data collectors, LAN, network management system(NMS), WAN, Outage Management System (OMS), Meter Data Management Systems (MDMS), and other subsystems[10].In figure 1.7, it shows AMI and other subsystems in a open smart metering system.
Figure 1.7 Advanced Metering Infrastructure(AMI)
In-Home Display (IHD) is an important technological device of the smart metering system. It is shown in table1.2. It is applied in the Meter Data Management
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Table 1.2 Smart Metering System using In-Home Display (IHD) units
1.4.3 Smart Control and Monitoring System
A dynamic, stochastic and computational technologies ensure a reliable, secure and efficient electric network. With the integration of distributed renewable sources and different energy storage unit , the grid become more complexity and interconnectivity. Computational Intelligence (CI) and Adaptive Critic Designs (ADCs) are promising control strategies. They make the system to be intelligent during complicated
environment. The combination of CI can form a hybrids type as neuro-fuzzy systems, neuro-swarm systems, fuzzy-PSO systems, fuzzy-GA systems, neuro-genetic systems etc., and the ADCs are based on the combination of reinforcement learning and approximate dynamic programming [11]. The outcomes of CI and ADCs based control technologies are shown in table1.3.
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Table 1.3 Innovative Control Technologies Main features of control and monitoring in smart grid are:
(i) Self-Healing
Self-healing function can maintain the stable of power grid , improve the power supply quality, avoid electricity outages and service disruption. In such independent systems , the decisions are based on the results which had been estimated and monitored before. Usually, there are two levels of self-healing: the physical layer and the logical layer.
(ii) Wide Area Monitoring and Control
Wide Area Monitoring and Control (WAMC) use the system-wide information and the communication between local information and remote location to offset the large disturbances propagation [12]. Synchrophasor Measurement Technology (SMT) has two objectives: the first is the short-term objective( increase the visualization level of the grid, post disturbance analysis, and model validations), the other is the long-term objective(develop theWAMPAC system).
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A WAMC system is mainly comprised of PMUs, application system, PDCs and a network for communication [13]. The WAMC system has three layers and the layers and relevant components is shown in Figure1.8.
Figure1. 8 Layers and Components of Wide Area Monitoring and Control
Layer 1, Data Acquisition layer . Power lines and substation bars is communicated with WAMC.
Layer 2 , Data Management layer. Phasor data concentrator collect synchrophasor measurement data and sort it.
Layer 3 , Application Layer; Synchronized PMU measurements from layer 2 is processed[14].
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1.5 Distributed generation
Distributed generation (DG) can make the electric grid more resilient and reliability. More over it also can draw the power in the emergency situation. Besides, distributed generation can prevent users from isolation of the power grid once there are faults, natural or malicious damage occur in the network. Because distributed generation can keeping on supplying electricity to the consumers until the main energy restored .
Distribution generation includes several different kinds of electric generation which are usually small electric generators. And the small generators are dispersed in the power network spatially(figure 1.9). However, managing the quality and quantity of energy of DG is a challenging task for the utilities . They must interact with the practices and standards in order to ensure the power supply. Usually, distributed generators are used to improve the efficiency of the grid and decrease the requirement of reserve generators as possible. The renewable energy sources(RES) are the suitable distribution generation. More over, considering the stability of the grid, distributed generators should track the frequency of the main power.
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Figure 1.9 Distribution generation
Main distribution generations:
(1) Wind power
Wind power is one of the promising RES for generation. It is suitable for being the distribution generation. The number of wind power plants have increased rapidly resent years and will be accounts more in the power generation to the grid in the future. Wind power can be called as distribution generation if the scale of wind plants are so small and connected to the distribution network. Compare with other forms of electricity production, the most difference of wind power is variability. The power produced by wind turbines and wind turbines work as long as the wind speed in the rang of 3m/s to 25 m/s. Besides, it is should considered that the wind power generation in night is more than that in the day time because there are larger wind
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speed in the evening in many areas. The power demand is lower and the grid load curve is at the valley in the night time. Therefore many wind turbines are suspend, “Wind curtailment” occurs. It is the most important problem of wind power in China. It is counted that the abandoned electricity generated by wind power was up to 20TWh in China in 2012.
(2) Solar power
Besides wind power , solar power is also a growing and leading renewable energy resources. Solar energy is captured by photovoltaic (PV) cells where the sunlight is transformed to electricity. Now the problems of solar energy are the efficiency of light to electricity should be improved and this kind of power generation can only work at the daytime and the output fluctuates with the weather.
(3) Small Scale Hydro Power
Small-scale hydro plants can be categorized into small hydro power plant (output is 1 – 10 MW) and mini hydropower plant( output is less than 1MW). Locally produced power better for the reliability of power supply and small scale can adapt to and have less influence on the environment it is in. While this kind of generation need water and the right geography place which can build a hydro power system.
(4) Emergency Power
Be a good emergency power , the start-up time should be short( tens of seconds) and the peak power would be longer ( several minutes). Diesel-powered plant has been proved to be the most suitable emergency power plant nowadays. Besides, the investment cost is lower. During a short peak-load-time they can even competing with
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coal power plants. The diesel plants are working at slow-speed and they can adapt to many kinds of fuels. Normally, Their efficiency is about 40% . Then the total efficiency will increase to above 60% if using some heat of the emissions for heating.
1.6 Storage systems
Wind power and solar power have the character of intermittency flexibility, so electricity demand and power supply need to be balanced. One of flexible options is using the electricity storage systems.
There are some electricity storage technologies which is shown in figure 1.10.The pumped storage hydroelectricity (PSH) is the most mature and widely used electricity storage technologies .The main idea of this technology is that water rushes through a turbine and then drive the motor to generate electricity .The dams and storage pools are needed to build. Water is stored in the storage pools and fall from the pools downward to the turbines below. Storage hydropower is used as soon as there is excessive consumption especially at the peak hours and these is no sufficient power to supply for the users. When normal electricity production is sufficient and the peak period is passed, the hydropower system will be stopped and shut down. And water which is at the low place is pumped back to the pool once the price of electricity is low.
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Figure1. 10 Energy storage systems overview
Battery storage technologies experienced a fast development and has entered in the market recent years. Although the price is high of stationary battery storage system, it is high energy and power density, high efficiency , flexible and promising.(figure 1.11) Batteries will be widely used as an energy storage unit of the grid to balance the load peaks and add capacity to the grid in the future.
Principle of using stationary battery storage technologies is : When the power price is low it is time to charge the batteries. Once the electricity consumption is at the peaks where the electricity price is high, the batteries supply power to support electricity distribution.
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Figure1. 11 The comparison between various energy storage technologies
1.7 Electric Vehicles as one part of the Grid
In section 1.6, I described the energy storage systems of the grid. One of the most important energy storage technology is the battery storage technologies. Electric vehicles has the batteries and they need to be connected with the grid for charging when they are out of power. Besides , the electric vehicles can also supply their electricity which is stored in their batteries to the grid as the energy storage system. So the electric vehicles can be part of the grid(figure 1.12)
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.
Figure1. 12 Electric vehicles as part of the grid
As it is shown in section 1.5 , wind power is one the most promising RES and will be penetrated to the electric grid in a large scale in the future. While the characters of fluctuation and Wind curtailment because of excess output in the night time are the challenge for the power system and should be solved. Energy storage systems can be used to reduce influence of the fluctuation and Wind curtailment. When the electric vehicles become part of the grid ,the batteries of electric vehicles can be the energy storage system of the grid without increasing the extra investment on adding specialized batteries.
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Chapter 02 Wind power
2.1 Wind power overlook
Renewable energy is the energy which is from renewable resources. Renewable energy can be used to generate electricity and it has quite lower emission than conventional power plants. Based on REN21's 2017 report, In 2016, for humans' global energy consumption, renewable energy contributed 19.3% of it. In his energy consumption, 9.1% is from biomass , heat energy is 4.2% (include: solar heat and geothermal), 3.6% is hydro power and 1.6% is power from wind, solar, biomass and geothermal( figure 2.1).In Global Electricity Production, the percentage of renewable energy as is shown in figure 2.2, Renewable energy accounts for 19.3% of the total electricity production which is almost one fourth of the total power. In the renewable power, besides hydropower, wind power accounts for the second largest amount of power generation.
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Figure 2. 2 Renewable Energy Share of Global Electricity Production, End 2016 In 2016, wind power capacity was increased about 55 GW and the total wind capacity in the world increase to about 487 GW. In addition, more than 90 countries had applied wind power into commercial by the end of 2016. Wind Europe’s Scenarios for 2030 predicts that capacity of wind power is 323 GW (253GW of onshore and 70GW of offshore capacity) until 2030, which is two times more than that of 2016. This accounts for about 30% of the European Union’s total power needed. From the Global Wind Energy Outlook 2016 published by Global Wind Energy Council, wind power generation will be account for about 20% of global electricity in 2030. We can see that in the following years, renewable energy will play more and more role in the power supply. More and more wind power plants will be build to supply more electricity for users.
2.2 Principles of wind power generation
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Wind power is one of the RES( renewable energy resource) . Wind power is renewable, plentiful, wide distribution, and no greenhouse gas emissions (GHG) emission. [15]. Wind power is from wind farms. A wind farm is consisted of hundreds/ thousands of wind turbines. The wind turbines generate the electricity and then transmit to the grid. The wind turbines are the most important part of the wind power plant. Wind kinetic energy can be converted into mechanical energy by a wind turbine. Then the mechanical energy is convert into electricity by the generator. As show in figure 2.3 the main components of the typical generator are:
(1) Rotor: Connects the blades to the gear box.
(2)Nacelle: Contains the gear box, generator, brake, yaw mechanism and monitors(speed monitors and direction monitors) :
(a) Gearbox: Gear box is used in the medium and large wind turbine(large than10KW). With the gearbox , the shaft rotational speed can be increased to match with the required rotation speed of the generator. Most of smaller turbines usually do not have the gearbox, they only use direct drive generators which is simple and low cost. (b) Generator: Generators of wind turbines are typically AC generators which are fixed in the nacelle. A generator can convert the mechanical energy into electrical energy.
(3)Tower: Towers are the support component which are used to support the rotors and their nacelles. Besides a tower raises height of rotor where strength/velocity of wind larger. A tower should fix sit on a reinforced concrete foundation which ensure the whole system is safe and stable.
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(4) Blades: A wind turbine has generally three blades. Length of blades are 30 meters to 50 meters. Blades need to be light and durable, so the modern rotor blades are made of some light but durable materials. Such as composite materials. Nowadays, a number of blades are made of special fiberglass and vacuum resin infusion is starting to be used by the manufacturers.
(5) Transformer: In order to deliver the electricity generated by wind turbines to the grid, the voltage needs to be stepped up by the transformer.
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Figure 2. 3 Wind turbines
2.2.2 Theory of wind turbines
Electric power generated by wind turbines is decided by the interaction between wind and rotor blades , by converting the wind kinetic energy to mechanical energy and by transforming it into electrical power.
The kinetic energy Ec = mv 〔2.1〕 m: air mass V1: air moving speed
Available specific power :
P available = = qv 〔2.2〕 Because the capacity q = = ρAv 〔2.3〕 P available = ρAv 〔2.4〕 So, the available power is increase with the cube of wind speed. It will rise about
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60%-70% if the wind speed increase 1m/s.
Power captured by the blade is represented as force by wind multiplied by the incident velocity: P = F ∙ V 〔2.5〕 Axial induction factor a= = 1 − 〔2.6〕 Power captured is P= 2∙ ρ ∙ A ∙ v ∙ a ∙ (1 − a) 〔2.7〕
So the power captured is depends on: (a) Density ρ of the incident air mass. So the power extracted will decrease when the temperature rises;(b)Rotor area A. When increasing the length of blades, area will increase. So power captured will be larger. But it should consider the strength and durable of the total structure. (c)velocity of the wind. Wind turbines should set at very wind sites to increase the power captured. Maximum power extracted from the wind :
P = ∙ ρ ∙ A ∙ v 〔2.8〕 Power/efficiency coefficient Cp: the ratio of power extracted and the wind available power.
C (a) = = ∙ ∙ ∙ ∙ ∙( )
∙ ∙ ∙ = 4 ∙ a ∙ (1 − a) 〔2.9〕
When a is 1/3 the efficiency coefficient get the maximum value :
Cp,max=0.59 ( 59% ) which is called “BetZ limit”. It is “The maximum extracted power from air flow theoretically by an ideal wind turbine cannot over 59% of the available power”. As is shown in the figure 2.4, the efficiency coefficient rises at the beginning with the axial induction a until axial induction a=1/3 where power
extracted to the maximum value. When a is 0.5, the wind velocity has slowed to 0. The air with negative velocity when a is larger than 0.5.
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Figure 2. 4 Power coefficient and axial induction
Power extracted from a wind turbine is a function of the efficiency coefficient Cp and the available wind power:
P=C ∙ ∙ ρ ∙ A ∙ V
The generated electric power is : P =ηe∙ ηm ∙ C ∙ ∙ ρ ∙ A ∙ V
ηm : the overall mechanical efficiency ηe : the efficiency of the electrical generator.
Power extracted from a wind turbine, the available wind power and electrical power can be shown in the figures 2.5 below:
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Actually, The power supplied by a wind turbine will undergo reductions because of losses. The losses may be come from:
• “altitude” because of pressure variation – the reference density is assumed as 15°C at sea level. With the altitude rising, the density decreases of about 1% each 100m of altitude;
• “altitude” because of temperature – when the temperature is different by the altitude increasing and the density decreases of approximately 3% each 10°C;
• “wake effect” – loss because the aerodynamic interference between the different turbines in the wind farm;
• freezing and dirty of blades – reducing the aerodynamic efficiency of the blades of wind turbines.
2.3 Energy productivity
2.3.1 Weibull distribution
Actually, It is not sufficient to know the mean speed of the wind at a site to determine the energy productivity of wind turbines. For a period (e.g. one year) - the histogram of percentage duration of the different wind speed shows the important data. The data are the mean value measured in the time of 10min by anemometers which is placed on the anemometric towers. Figure2.6 shows in the histogram of percentage of the time the effective speed is usually higher than the reported value. And from the histogram in Figure 2.7, we can get the speed data by the histogram of the statistical frequency of occurrence of wind speed.
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Figure 2. 6 Histogram of the velocity duration(Percentage)
The time distribution of the wind speed is showed by using the Weibull statistical distribution function usually. Because it comes nearer to the distribution frequency of the mean wind speeds .
Figure 2. 7 Histogram of the occurrence frequency of speed
The Weibull distribution gives an description of a site, which can be identified by only knowing the two parameters:
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• the shape parameter k.
The scale parameter (A), expressed in m/s, the scale parameter is relevant to the average speed. And the shape parameter (k) modifies the distribution symmetry. Value which quite near to 1 represents it is a very asymmetrical distributions. While high values (k>2-3) creates the symmetrical distributions which is similar to Gaussian functions (Figure2.8)
Figure 2. 8 Weilbull curve for different values of k
The shape parameter k represents the dispersion of the speed values which are around the mean speed. The higher the parameter k, the lower the dispersion around the mean value. The shape parameter assumes different values because the ground morphology is different. It also depends on the wind regime in a region. Typical values of the shape parameter k of different geographical sites is in Table 2.1.
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Table 2.1 Shape parameter k in different sites
Availability of scale parameter A and shape parameter B make the possibility of further assessment of the potential output energy. In addition, it has also high reliability. These two parameters have the statistical properties of the overall time series. Scale parameter A is related to average wind speed. So, to estimate the productivity ,it needs to know the mean wind speed referred to the height of the rotor hub on the ground of the installation site and the shape factor A which is showed in Figure 2.9.
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2.3.2 Assessment of energy productivity
When designing a wind power plant, the main task is to maximize the annual electrical production [kWh]. It can be expressed and assessed by Weibull distribution as regards the wind speed at the installation site and the curve of the electric power as function of instantaneous velocity of the wind[ 16 ]. Therefore, annual productivity can be expressed as below:
E = 8760 ∙ P(v) ∙ f(v) ∙ dv Where,
• 8760 : hours of a year
• P(v) : the power output of the wind turbine [kW] at a wind velocity v [m/s] deduced from the power curve.
• f(v) : Weibull statistical distribution function of occurrence frequency of wind speeds [s/m].
The total potential power output by a wind power plant is obtained by summing the productivity of the turbines and then multiplying the result by some corrective factors. It need to consider any possible aerodynamic interference between the turbines on the wind plant and all the losses happened in the different units and in the plant and the grid . The annual producibility of a turbine can be expressed as
h = E
P
It stands for “equivalent hours/year”. It is as if a turbine works for a certain number of fictitious hours heq in the rated power and stands still in the remaining (8760 – heq) hours to generate the estimated power in the period of a year.
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2.4 Effects of wind turbines on the power system
The integration of wind plants into power systems includes choosing a right place for installation and then constructing the wind turbines and the connection with the electric grid , the following management of the injected power considering the power demand of the user(network loads) and the randomness and variability of the wind power. The grid is seen as infinite short circuit power when wind turbines supply power into it. So the grid will not be affected by injection of additional loads. Actually, the deviation of the generated power would cause voltage frequency varies in the network and through the impedance of the different lines, this will lead to the variation of the voltage . So the network will be affected more once the ratio of output rated power and fault level in the power system is larger[17].
To be simplified, divide the power grid into 4 parts: power generation, electricity transmission, primary and secondary distribution (Figure 2.10).
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Generation part is carried out by the generators in large power plants. The large synchronous generators are the traditional energy sources, such as coal , nature gas. These traditional generators could meet load variations, keep network frequency constant and adjust the voltage when it is needed. Power is medium voltage when generated in these plants. Then it is transformed in high voltage and transmitted in the network. High voltage transmission can reduce the loss as much as possible.
The distribution networks are far from the power plants but near the delivery points. The fault level of them decreases sharply. They are also influenced by load fluctuation.
Wind power plants are connected to the primary distribution or to the secondary distribution part in case of small power plants. Usually, large wind power plants are connected with high voltage grids. It shows one typical connection in Figure 2.11. And shows the connection of wind power plant offshore and grid in Figure 2.12.
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Figure 2. 12 Connection of wind power offshore and grid
2.4.1 Frequency variation
The frequency of a grid is decided and influenced by active power of the electric system. For every generator connected to the grid , it has:
J ∙ dΩ
dt = C − C Where:
J -- Moment of inertia of a rotor Ω -- Angle speed of the rotor;
Cm -- Mechanical driving torque applied to the rotor; Ce -- load resistance electromagnetic torque
Therefore, from the equation, if the two torques are imbalance, the rotor will increase or decrease the speed and proportion to the difference between the torques and
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inversely proportion to its inertia. Power is coming from the torque with the angle speed. So the previous expression can be expressed by powers:
J ∙dΩ dt =
P − P Ω
In addition, the grid frequency is correlated to the angle speed . So the equation can also be written as : Ω =ω p = 2 ∙ π ∙ f p Where
ω -- Pulsation of the generated electrical quantities; p -- Number of the couples of poles of the generator
The network frequency will change if there is an imbalance between the driving power into the grid and the overall power of the connected loads. Because of the load variation, to keep the frequency as constant and within the limit value,. the output power of the power plants are modified. Usually, the higher amount of power generated by a power plant into the grid, the better this power plant is able to affect the network frequency.
2.4.2 Voltage variation
Variation of the generator excitation is one regulation method. By changing the excitation magnetic flux, the voltage and reactive power output change and the power factor also varies. In wind power plants, there will be a voltage variation between the generators and the connection points with grid. Because the connection line causes an Ohmicinductive impedance to current [18], which can be expressed as:
44 ∆V =P ∙ r + Q ∙ x V Where: P --- Active power Q--- Reactive power r--- Resistance of the line
x--- Inductive reactance of the line Vr---Grid voltage at the connection point.
Voltage variation will appear when injecting active power and reactive power into grid. Ideally speaking, if voltage of the grid could always keep constant at the rated value, by increasing the power injection, the overvoltage will appear at the generators terminals f the wind power plant, and the impedance of the connection line is higher. Actually , when the fault level of connection points to the grid gets lower, the grid voltage at the connection point will change more frequently .
The Standard EN 50160 defines the maximum levels of network voltage variation which is measured in a 10min span: （1） ±10% of the rated voltage Un during 95% of the week ，（2）for the LV grids, between -15% and +10% Un. In case of wind turbines, by varying the power factor the variation of voltage can be limited. A modest reduction in the power factor from the unit value to 0.98 inductive will lead to a the maximum voltage variation decreases 1.5% approximately.
2.4.3 Short-term and long-term effects
The impacts of the wind energy on the grid depend not only on the dimensions and flexibility of the network, but also the penetration of power generated by wind
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turbines in the electric system. it can be described as:
• short-term effects – balance of the electric system during operating time • long-term effects – supplying enough power during the period of load peaks. （1） Short-term effects
Wind power production is variable because its production is decided by the wind speed which can not control by men totally. So this causes the schedule adjust of the conventional large power plants and change of the power flow in the transmission network. Fluctuation of the wind power cannot be defined in advance and sometimes it would be predicted wrongly. So the adequate power reserve is needed. Electric system needs adequate power reserves to face the disturbances in the grid and to meet the electricity required. The wind power variation for 1 hour or less time affects the reserve of power used for the frequency control if the level of wind power injects into the grid increase the whole system variations largely. On average, if wind power penetration is 10%, the extra reserve requirement is 2%-8% of its installed capacity[19]. If the small variations in the various wind power plants which are placed on large areas are not correlated and they will cancel out each other, variations on wind power is little effect on the power reserve.
Another short-term effect of the wind power is because of transmission and distribution losses, which is depending on where the production locations are. Besides, many intermittent wind power production will lead to a lower efficiency of conventional power plants because they cannot operate in their optimum state. Therefore, the optimized unit commitment is complicated by the intermittent power
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from wind plants. When the wind power generated is over the quantity which the network can handle, keeping an dynamic control on the electrical system, some of the power produced by wind shall be limited. If the intermittent wind power is over 10% the limitation methods are very necessary.
(2) Long-term effects
The reliability of the electric system will be influenced by the intermittent wind power. The loads connected to the network which are low probability of failure should be served. To maintain the reliability of the system, wind power should replace some conventional thermal power capacity, especially, replacing some traditional capacity during the peak load period. Some variable power sources are producing power at peak load periods, such as solar energy which is following “air-conditioning” loads. It will be beneficial if wind power production coincided with the load demand. For instance, in the morning wind power is increased and in the evening wind power is decreased. So the dispersion of wind power output and the relationship between electricity demand of grid and wind power generation can determine an increase in the wind power value [20].
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Chapter 03 : Electric Vehicles
3.1 Global EVs outlook
Paris climate agreement which is announced in 2015 and enforced in 2016 set a goal that members should deal with the GHG(greenhouse gas) emission and limit the global temperature increase below 2 degrees centigrade above preindustrial levels. By looking at two IEA scenarios 2DS and B2DS, greenhouse gas emissions reduction compatible with the goal of Paris climate agreement:
• Two Degree Scenario (2DS): 1 170 Gt CO2 cumulative emissions for the 2015-2100 period, 50% chance of limiting average temperatures increases to 2°C • Beyond Two Degree Scenario (B2DS): 750 Gt CO2 of cumulative emissions for the 2015-2100 period, 50% chance of limiting average future temperatures increases to 1.75°C[21].
Figure 3. 1 GHG emission of 2DS and B2DS from 2014 to 2100 [25] After 2050, both of these two scenarios , CO2 emission will reach near 0 . The
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transfer sector accounts for 23% of greenhouse gas emissions in 2016 and it will accounts for more in the future. So countries should reduce the CO2 emission of the vehicles to achieve the goal.
Worldwide number of battery electric vehicles keeps a sharply increasing in recently years. From figure 3.2 , the number of electric vehicles is 1209 thousands. It takes the rate of growth at about two times in the following years from 2013 to 2016. In 2016 there are 1209 thousands EVS in the world.
Figure 3. 2 World wide number of electric vehicles
China is the largest EVs market in the world in 2016. There are more than 300 thousand EVs were registered .US is followed by China with about 153 thousand electric vehicles sold. While the EVs sales of European countries account for more than 200 thousand in 2016. Some European countries Norway, Netherlands and
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Sweden , the electric vehicles have taken larger market share than other countries. Especially in Norway, EVs sales accounts for 29% market share. More and more countries has published the time table for stopping to sale the diesel vehicles. Netherlands had declared that they will stop selling the diesel and petrel cars after 2025. UK and France published the timetable in July 2017, in which sales of diesel and gas vehicles would reach the end of the road by 2040. Be the largest vehicles market , China have to encourage more electric vehicles should be used. In September 2017 , Xin Guobin the Deputy Director of China's Ministry of Industry and Information said timetable about banning engine vehicles sale is in the discussion and formulation. By the forecast from International Energy Agency (Iea), the tendency from 2000 to 2050 will be show in figure 3.4.
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Figure 3. 4 Electric and Plug-in hybrid vehicle road map
3.2 Electric vehicles introduction
EVs have been used since mid-19th century. Comparing to the internal combustion engine vehicles, electricity vehicles is more comfortable, easier to operate and almost 0 emission. Even though the IC engine vehicles have been dominated the market for almost 100 years. Electric vehicles has also used in some vehicle types, such as some smaller vehicles and trains.
3.2.1 Battery electric vehicle(BEV)
BEVs are worked not by a gasoline or diesel IC engines but by their electric motors. Electricity is generated by their on-board battery packs typically. The best point of BEV is it can reach 0 emission. The electric motors of BEV is better than gasoline engine vehicles when driving. The BEV`s motor can generate near-instant torque, or turning-force. For this reason , BEVs have extremely fast acceleration compared to conventional vehicles.
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and air conditioning, etc( shown in Figure3. 5)[22].
Figure 3. 5 Main components of an Electric Vehicle
Electricity from the grid is stored in the battery pack. Under normal operation, Battery electric vehicles(BEVs) use the stored electricity to power the electric motor and then turn the vehicle `s wheels to run. The controller is determined by the accelerator pedal. By the different position of the accelerator pedal, the different amount of power to be delivered to the motor. At a given instance, the power
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electronics sends the electric signal to the control unit of how much electric power should be supplied. When the electricity is depleted, the electric vehicles can not work any more and their batteries should be recharged.
Battery electric vehicles(BEVS) will account for more than share of the vehicles market. Because it has the advantages of lowest GHG emission and good drive-ability. Although it accounts for little share in the vehicles market. But, with the development of battery technology and the decline of the cost, it will be the major kind of vehicles on the road in the future.
3.2.2 Plug-in hybrid Electric Vehicle(PHEV)
It is the vehicle has not only an electric motor but also a conventional gasoline or diesel engine. PHEVs could be plugged-in and recharged from the grid. This ensure PHEVS drive long distances with only using power. When electricity in the battery is not sufficient, the gasoline engine turns to work and at this time the vehicle operates as a traditional vehicle.
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PHEVs are more complex than the BEVs because they have both the internal combustion engine system and electricity drive system. The electricity system includes battery pack, charger, boost converter, inverter and motor. While the IC engine system mainly concludes IC engine( gasoline or diesel) ,motor and the system can be combined with the electricity system.
There are two basic operation modes can be offered by PHEVs: the charge sustaining and charge-depleting. When in the charge depleting mode, the PHEV work only on electricity until its battery get to a low level. At the same time the gasoline/diesel engine of the vehicle will be started. The charge-sustaining mode combines the two different power sources(gasoline/diesel and electricity). In this mode a PHEV work with a higher efficient and the battery pack can not be in the state of pre-determined low band. When electricity of a PHEV is exhausted in charge-depleting mode, it can turn into charge-sustaining mode automatically. Typically, when PHEVs are working, they starts in the electric mode, then working by electricity in the battery pack until the power is in a specific low state. Switching to hybrid mode once reaching high speed, usually, more than 60 miles per hour [23].
PHEVs can use less fuel and produce less emission than gasoline/diesel vehicles thanks to the electric motor and battery . Even in hybrid mode, it can also save fuel. In addition, regenerative braking component could convert part energy which is lost during braking into usable power and store it in the battery pack . The buyers of PHEVs need not to worry about the cruising ability of BEVs because they have two power system. However , PHEVs have the highest cost and their emission is higher
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than BEVS.
3.2.3 Fuel cell vehicles
Fuel cell vehicles have large differences from the BEVs and PHEVs. They use hydrogen to power the electric motor. Fuel cell vehicles combine oxygen and hydrogen to generate power and drive the motor. Fuel cell vehicles can be considered as electric vehicles because they are powered by electricity, fuel cell vehicles have different battery packs not the lithium battery which is used by BEVs and PHEVs. Besides, the range and refueling processes of fuel cell vehicles are comparable to IC engine vehicles. They can need less time to refuel like gasoline and diesel.
Electricity is generated in the fuel cell and Hydrogen as the fuel. The byproduct of this process are only water and heat. So it means that fuel cell vehicles do not produce any pollution. But producing the hydrogen can lead to some pollution, which include GHG emissions. But the total emission cut 30% compared with the IC engine vehicles. In the future they will reduce more emission when producing Hydrogen. So fuel cell vehicles are promising.
FCVs(fuel cell vehicles) look like gasoline and diesel vehicles but using cutting edge technologies. The most important part of the a fuel cell vehicle is its fuel cell. Hydrogen is stored onboard and react with oxygen to generate power that used to drive its electric motor. The major components of a typical fuel cell vehicle are showed in figure 3.7. Hydrogen is compressed in the tank at very high pressure to expand the driving distance. Hydrogen and oxygen react in the fell cell stack. Electricity is generated and power the driving motor. Thanks to the electricity , motor
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works and drives the wheels running. The electricity motor is more quite, less maintenance and more efficient than IC engine vehicles. The control strategy operate by power control unit by the electricity flow. Energy is also generated by braking system and the recovered energy can be stored and provided to motor, which can increase the total efficiency.
Figure 3. 7 Honda fuel cell vehicles
The most important component of FCV is the fuel cell stack. In order to offer sufficient power to the vehicle, fuel cell stack which has a large electricity output should be made by assembling the cells together. The potential power of a fuel cell stack is decided by the size and number of the cells. Fuel cells are comprised by the stack and the Polymer Electrolyte Membrane(PEM )as figure 3.8. Hydrogen flows to the anode where the catalyst divide hydrogen into electrons (negative) and hydrogen ions(positive). Only the ions can go through the Polymer Electrolyte Membrane to the
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cathode. While the electrons flow to the cathode in the external circuit where the current occurs. Oxygen flows on the other side of the cell where it combines with electrons and hydrogen ions into water [24].
Figure 3. 8 Fuel cell stack of FCVs
The fuel cell vehicles have some advantage: very low greenhouse gas emissions and fast to recharge the fuel than BEV and PHEV. However, there are also some challenges:
(1) Vehicles cost : cost of FCVs are more higher than and BEVs and IC engine vehicles.
(2) Hydrogen to consumers: Nowadays the infrastructure for charging is little. And the production of hydrogen can not support the worldwide use.
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(3) Reliability should be improved: Fuel cell vehicles(FCVs) are not as durable as conventional engines, and their performance will be influenced by temperature and humidity working condition. When running on the road, the fuel cell vehicles` stack durability is about only half of commercialization needed.
3.3 Battery technology of electricity vehicles ( BEVs and
PHEVs)
The electric vehicle batteries are used to supply power to drive electric vehicles. Moreover, the electric vehicles batteries are rechargeable.
3.3.1 Types of electric-vehicle battery
Electric vehicles Batteries mainly include lead–acid, NiCd, and lithium-ion batteries. Lead-acid batteries have low cost, but their weight are high and contain metals which are bad for environment. It is not suitable to use on electric vehicles. As shown in figure 3.9, two major kind of EV batteries are nickel metal hydride batteries (NiMH) and lithium ion (Li-ion) batteries. From the table, currently, nearly all HEVs are using NiMH. batteries because its technology is more mature. But Li-ion batteries gets more concern and developed. There will be more application on EVs (BEVs and PHEVs) in the future.
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Figure 3. 9 Batteries used in different electric vehicles
Nickel metal hydride
The nickel-metal hydride battery has the advantage of both the energy storage
character of metal alloys and sealed feature of nickel-cadmium battery [25]. The cutaway picture is shown in figure 3.10. A typical nickel-metal hydride battery is mainly consist of negative electrode, Electrolyte positive electrode, separator and battery construction. The active material for negative electrode is hydrogen. But the hydrogen can not use directly and should be stored in the cell as a metal hydride. The electrolyte in the NiMH cell is an solution of KOH which has a high conductivity. The positive electrode of the cell is Nickel hydroxide which exchange a proton in the charge and discharge process. The separator supplies isolation of electrodes but
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Figure 3. 10 The cutaway of a Nickel metal hydride
The electrochemistry of charging and discharging of nickel-metal battery are as follow:
Charge:
At negative electrode, water is decomposed. The product of hydrogen atoms are absorbed into the alloy.
At positive electrode , charging is depend on the oxidation of nickel hydroxide.
Discharge:
hydrogen is desorbed and combines with Hydroxyl ion into H2O and at the same time form the electron .
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At positive electrode, nickel oxyhydroxides is lower valence stare, nikel hydrxide.
Figure 3. 11 Charge and discharge of Nickel metal hydride
Lithium-ion batteries
This battery has a very good energy density and the self discharge performance of it is quite low. Besides, their weight is light and their low maintenance requirement. It is the best battery to power the electric vehicles at present.
A lithium ion battery is series of electrochemical cells which is connected in parallel or series. The electrochemical cell is composed by a cathode, an anode and an electrolytic solution.(figure 3.12). Cathode is the positive electrode. The material of cathode can be Li-Metal Oxides or Li-Metal Polyanionic such as Nickel Lithium Iron Phosphate and Manganese Cobalt Oxide. They have energy density and cell voltage and play the most important role on the batteries performance. Anode is the negative
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electrode which is mainly consisted by carbon. The electrolytic solution contains dissociated salt. It allows ions moving to the electrodes and so the current occur.
Figure 3. 12 The structure of Lithium battery
The charging and discharging principle of Li ions battery is as follow: when it is charging, a potential difference is formed because of the charging unit and it cause the Li ions intercalate into the graphite. During charging, Li+ moves to the surface then gets to the negative electrode, the increasing number of oxidation metal return electrons to circuit, Li ions flows to the electrolyte , then back to the beginning area[26](figure 3.13).
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Figure 3. 13 The charging and discharging process of Li-ion battery
3.3.2 Battery performance
Electric-vehicle batteries could supply power over sustained periods and a high ampere-hour capacity are needed. The performance of electric-vehicle batteries are as
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follow:
(1) Ampere-hour Capacity. The maximum power which could be discharged by a full charged battery is defined as Ampere-hour(Ah) Capacity [27]. The battery capacity is represent by Wh capacity which can be calculated as follow:
(2) C-rate. Define the C rate as the charge capacity of one battery in one hour. (3) Power Density. The unit volume peak power of one battery.
(4) Specific Power. Specific power is the peak power of one unit mass:
(5) Specific Energy. Specific energy is how much energy a battery can store in one unit mass. Specific energy is the key parameter to determine the total battery weight[28].
(6) Internal Resistance. The equivalent resistance of one battery. When the operating condition changes, it will be different.
(7) Peak Power. Power when the value of the terminal voltage equal to 2/3 of the open circuit voltage.
(8) Cut-off Voltage. The minimum allowable voltage.
(9) State of Charge (SOC). A important parameter for batteries which is related to healthy and safe operation of a battery. It is the remaining capacity of a battery.
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(10) Cycle Life. It is means how many discharge/charge cycles the battery has before it could not meet the criteria. The actual battery life is affected by C rate ,temperature and operation condition.
3.3.3 Charging Methods of EV batteries
For charging the EV batteries, there are mainly 3 charging methods:
(1) Constant Voltage method. It is the simplest charging method. Charging is always at a constant voltage. At the initial stage , charging current is high and drops to 0 gradually until the battery is full of charge. The shortage is that at beginning stage of the charging needs high power. So it is not suitable for some parking and residential places.
(2) Constant Current method. Constant current to the EV batteries should be maintained during charge. SOC is augment with time linearly in this method. The key point of this method is to make sure when to finish a charge process. The parameters of temperature, voltage and charging time can be used to decide the cut off time[29].
（3）Combination of constant voltage and constant current. During the charging process, constant voltage method and constant current method will be used. Show in Figure 3.14 below. At the beginning , the battery is charged at a constant low current. Then it will be supplied with high and constant current. The constant voltage mode starts once SOC reaches a threshold point[31].
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Figure 3. 14 Typical Li-ion cell charge profile
For Electric vehicles, especially PHEVs , due to regenerative braking, their batteries need to handle random charging. To maintain the safe operation of EV batteries ,safety limitation has to be set. Besides, Mechanical braking plays a role of supplementary and safe measure to aid regenerative braking. It is important to know when to stop the charging in order to avoid Over-charge. If SOC can be accurately measured, the charging can be stopped as soon as the SOC value gets to the maximum safety value. However, it is challenging to estimate SOC accurately. Even though SOC can be exactly measured, other backup methods also needed to stop charging. Typical methods currently used are as follow:
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battery., Charging process will be stopped as soon as a preset timer expires.
(2). Temperature Cut Off (TCO). Stop charging as soon as the absolute battery temperature goes up to a safe temperature.
(3). Delta Temperature Cut Off (DTCO). Charging will be stopped once delta change in temperature of battery overs a safety value.
(4). Temperature change rate dT/dt. When it exceeds a minimum value , the charging will be stopped.
(5). Minimum Current (Imin). Charging process will be stopped as soon as the current reaches the lowest limit Imin.
(6).Voltage Limit. Charging process will be stopped if the battery voltage reaches the threshold value.
(7). Voltage Change Rate(dV/dt). When there is a not change or drop tendency of the voltage, charging process will stop.
(8). Voltage Drop (-ΔV). For NiMH EV battery, when charging is over, the cell temperature starts increasing because hydrogen and hydroxide ions combines again and lead to battery voltage drops. Once voltage drops to a preset value, the charging process stops.
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Chapter 04 Vehicle to grid (V2G)
4.1 Overview of vehicle to grid (V2G)
Vehicle-to-grid is the electric vehicles (BEVs and PHEVS) can connect with the grid and provide ancillary services to operators of electric grid. It is predicted that the vehicle to grid service has much benefits for the grid when large scale penetration of EVs into the power system.
4.1.1 Principle of V2G
The framework of V2G (figure 4.1) built considering the characteristics of the EV batteries and the deployment of the system. So we can effectively harness the EVs integration with the grid as the controllable load or the storage source. In addition, we track the SOC of each battery pack of EVs. State of charge (SOC ) which was explained in Chapter 3 is an important parameter of a battery, it is the determinant parameter for each electric vehicle if connecting with the grid. The Aggregator plays the interface role with the electric vehicles and making the V2G concept realizable. The task of the system operator and ESP (Energy Service Provider)are serving BV owners. We display in Fig. 4.1 the interrelationships among these players.
