Abstract
The objective of this report is to design the Electric Vehicle (EV) charging station power flow scheme with battery utilisation. This design is based on the 'time of use' pricing system for residential and business electricity. It helps to reduce the cost of charging and power grid impact. The expected outcome of this thesis is the impact of the system by comparing the recharge cost. It also compares the charging cost between work place, home and EV charging station to find out the sustainability of EV charging station. The detailed calculation such as losses and consumption estimation are based on EV sales volume estimation.
Introduction of vehicle increase the flexibility, mobility and independence of people and become an essential part of human life. For last century, the vehicle number is increased rapidly and it still shows about 3.0 % increases for last 5 years [1]. This rapid increase of vehicle provides benefit, but it leads a serious environmental problem due to the gas emission of the vehicles. Thus, nearly every major auto-maker is ready to introduce Electric Vehicle (EV), Hybrid Electric Vehicle (HEV) and Plug-in Hybrid Electric Vehicle (PHEV) to stop the pollution and to meet consumers need. The EV is predicted to be a major transport system and the EV charging station is the key infrastructure to adopt EV to the consumer.
The aim of this thesis is to
To provide an explanation of EV to help understanding.
To design EV charging station power flow scheme with utilisation of battery and photovoltaic system.
To compare the cost of charging between home, workplace and charging station based on the time.
Electric Vehicles
An electric vehicle is not a new concept. The invention of electric vehicle is in the early 20th century in the same instant with gasoline vehicle. The electric vehicle was preferred choice than the internal combustion, but the rapid development of internal combustion engine and other background reasons such as reduced oil price, driving range and invention of electric starting motor which help to start the engine without the hand cranking [2]. These made gasoline engine become an industry standard and the gasoline vehicles are increased exponentially in last century [3]. However, the environmental pollution that gasoline vehicles are creating is a serious problem in 21st century. Thus the main concern of automotive industries is the use of fuel efficient and environment friendly vehicles. There are many options such as compressed natural gas (CNG), electricity, fuel cell vehicles, hybrid vehicles (HEV) and biodiesel, but electricity is considered as the simplest and easiest option to go [4]. According to Better Place, 39% of Australian interest in buying electric vehicle as their next vehicle, this research performed not only in Australia also the other developed country such as U.S. Israel, Denmark and Canada. It shows that consumers are ready to move beyond gasoline and the electric vehicles is credited as next main stream vehicle [5].
3.1 Electric Vehicle Battery Types
3.1.1 Lead-acid Battery
The lead-acid batteries are widely used as electrical energy storage in the EV. The advantages of the lead-acid are low-cost, high power capability and good cycle. The disadvantages are low specific energy which is about 40kW/kg and poor temperature characteristics. The low energy density has significant effect on the final mass of electric vehicle and when the temperature is below 10℃, its specific power and energy are decreased, which in turn limits the operation of vehicle in cold conditions. However, recent advances in battery efficiency, capacity, materials, safety, toxicity and durability are likely car-sized EVs [6].
3.1.2 Nickel-Metal Hybrid Battery
Nickel-metal hydride batteries characteristic are now considered a relatively mature technology. The advantage of nickel-metal hybrid battery is high power, environmental friendliness, flat discharge profile and rapid recharge capabilities. It attains specific energy of 70 to 95Wh/kg and a specific power of 200 to 300W/kg. However, the initial cost of nickel-metal hybrid is relatively high and it may have memory effect and may be exothermic on charge. This battery is being used in HEV by Toyota Prius and Honda Insight [6].
3.1.3 Lithium-Ion Battery
Lithium-ion battery which is widely known through its use in laptops and consumer electronics is the most promising battery of the future. Although still at the development stage, the Li-ion battery has already gained acceptance for EV and HEV applications. There are three types of lithium-ion battery which are nickel-based, cobalt-based and manganese-based. The nickel-based Li-ion battery has 120kW/kg of energy. The cobalt-based has higher specific energy and energy density, but has a higher cost and significant increase in the self-discharge rate. The manganese-based type has the lowest cost and its specific energy and energy density lie between those of the cobalt- and nickel-based types. It is anticipated that the development of the Li-ion battery will ultimately move to the manganese-based type because of the low cost, abundance, and environmental friendliness of the manganese-based materials [6].
3.2 EV Sales Volume Estimation
The EV has not been introduced to consumer in the Australian market. Only Mitsubishi brought the i-MiEV which is used as fleet vehicle for testing and advertising purpose. Nissan is ready to launch their full electric vehicle 'LEAF' is the beginning of 2012 [7]. Thus, the estimation would only be a rough prediction of sales volume. The method of estimation will be based on sales volume of Prius, Toyota in Australia and expected price of EV also considered. The estimation analysis will provide three possible scenarios. There are the highest growth, average growth and lowest growth.
Figure - Historical sales volume of prius, worldwide
Figure - Historical sales volume of prius, australia
The above graphs show the sales volume of the Prius in worldwide and Australia from 1998 to 2008 and 2001 to 2008 respectively [8]. It provides the trends of exponential increase of hybrid vehicle sales volume. The average annual volume increase is 40% in worldwide and 72% in Australia. It is assumed that the sales volume of EV will follow this trend. Price of EV is not confirmed yet, Nissan Australia is aiming for a total cost of ownership equivalent to the Tiida and this is about $20,000 to $30,000 [9] and other manufacturers are also targeting the same price. The starting price of the Toyota Prius is $39,990 [8]. Thus, expected annual sales volume of EV would be higher than HEV. The average annual vehicle volume increase in Australia is about 2.5 to 3% [1]. With the regard of price, it shows that the Prius sales volume increase is steep and be evidence for huge success of EV in future.
Figure - EV SALES volume prediction
The projection is based on the above information and EV models which are expected to be launched within 5 years. The launching time would different, but assume it as three models and thus the total number of EV would be three times to above volume to make simple estimation.
The first projection, which is highest growth, is 80% increase annually. Thus, when it reached to 2020, the possible number of electrical vehicle will be 35,705 for single model and 107,115 in total. Vehicle sales volume in 2009 is 937,328 and regard with this factor the market share of electric vehicle is 11.43% [10].
The second projection, which is average growth, is 70% increase annually. Thus, when it reached to 2020, the possible number of electrical vehicle will be 20,160 for single model and 60,480 in total. Vehicle sales volume in 2009 is 937,328 and regard with this factor the market share of electric vehicle is 4.32% [10].
The third projection, which is minimum growth, is 60% increase annually. Thus, when it reached to 2020, the possible number of electrical vehicle will be 10,995 for single model and 32,985 in total. Vehicle sales volume in 2009 is 937,328 and regard with this factor the market share of electric vehicle is 3.52% [10].
3.3 Better Place Electric Vehicle Network
Better place is a company that is planning to set up an electric vehicle network on the east coast of Australia. The network will be a complete system providing cars, energy supply and charging infrastructure. Below are the details of Better Place's plan:
Cars - Major automaker are developing and introducing EVs to provide broad range of options to the consumers. Batter Place is working with the Renault-Nissan alliance and the plan is to provide a range of cars that are compatible with Better Place plan.
Batteries - The lithium-ion battery will be used for an EV and the charging time is about 4 to 8 hours at home or work, if they plug in the EVs. With quick charger it takes 30mins to recharge it to 80% state.
Switch Station - The battery switch stations help to minimize the time of recharge. The switch process takes less time than a stop at the gas station and the driver and passengers may remain in the car throughout.
Charging - Charging points will be set up near car park spaces around the city so that charging is convenient.
EV Driver Service - This service will provide energy monitoring, planning, service and support and charging.
EV Network Software - The banks of batteries that will be used to store electricity and be kept at the swapping stations can be used to supply electricity to the grid to meet peak demand. The network infrastructure will be used to create a smart grid.
Standards - These will be rules and regulations that will govern the use of the network. This includes costs and also the supply of energy. At the moment the plan is to have the energy sourced from renewable sources such as wind.
Better Place could have a huge influence on the introduction and adoption of electric vehicles in Sydney's transport network [11] [12].
Possible Problem with Electric Vehicles
4.1 Rare Earth Elements Problem
Rare Earth Elements (REEs) are set of materials which are represented by the minerals with atomic number 57 to 71. Particularly, when REEs are in the oxide form, it is called as Rare Earth Oxides (REO). They have unique physical, chemical and light-emitting properties that are used in many newly developed technologies. For example, Prius, Toyota use two pounds of neodymium in its permanent magnets and also it uses 20 to 30 pounds of another rare earth element, lanthanum. Thus, the consumption of REEs increased dramatically which is at 8 to 12% per annum. Global demand for REEs is already outweighing supply and sharp price increase is resulted. By 2012 global consumption of REEs is forecast to increase by 65% from current levels, driven by significant market growth for many applications that rely on REEs. Thus, the supply of REEs is important issue to develop the battery and electric motor to achieve better performance and efficiency for the HEV and EV [13].
Hybrid RRE
Figure - USe of rare earth elements in HEV [14]
4.2 Lithium Demand and Supply
A concern that has been raised with electric cars is the supply and demand for lithium to make lithium-ion batteries which are the most common battery choice for electric vehicles. Most of the known lithium deposits are located in South America, while some can also be found in Australia. Most of the lithium from batteries can be recycled and with current lithium demand predictions, supplies should easily last for the next 40 years [13]. Lithium can be used as a bridging technology until better battery technology is developed or until more lithium supplies are found. If lithium-ion battery technology is surpassed by other battery technology in electric vehicles, lithium-ion batteries could be used in lower performance applications and then recycled to be reused. There are also concerns about the supply of other rare and exotic materials used in batteries and electric motors. Electric vehicles will significantly impact the current electricity grid. By having a large number of electric vehicles relying on electrical power, more pressure will be put on the existing grid. It is expected that the load patterns will be different and that smart grid and charging technology will have to be implemented to control the load on the grid.
4.3 Power Grid Impact
During the past decade, number of HEVs are gradually increased and HEVs. Now, the automotive companies are joined in front of the market to occupy and lead the market. However, this sudden increase of EVs includes HEVs would have chance to cause a severe power grid impact and this might lead another environmental pollution due to another construction of power plant to cover a load.
The total power generated from Australia is about 266 billion kilowatt hours (TWh) per annum. Of this gross amount, power plant use 17 TWh for themselves and another 18 TWh is lost in transmission line.* The electricity consumption in Australia is about 222 TWh in 2010. It provides the extra capacity of the electricity is 10 TWh and it is not enough level to cover sudden increase of peak load.
However, most of possible EV users are planning to charge their vehicle during the night time when there is plenty of capacity on the grid. Thus, it would not be impact as much as it is predicted. Also, the new "smart grid" which charge lower during the off-peak and it is introduced by various electricity suppliers will help consumers to save their EV maintenance budget. Thus, to prevent the sudden increase of power use, government and supplier need to have adequate regulation and rate plan for the electricity consumption for EVs
4.4 Infrastructure and Battery Life Cycle
When the EVs are introduced, infrastructure and battery life cycles for the EVs have been raised as problems. In Australia, in 2008, 78% people are living in separate houses and most of them include garage***. It makes it easier for Australian to install the charger and recharge their vehicle at home. Installing the charging station can cover the limited number of public charging station in early stage of EVs era and government could enhance installation of home charging dock by tax credit or subsidy which is enforced in U.S. already.
The second problem is battery life cycle. The Nissan Leaf use 24 kW lithium-ion battery pack as an energy source of the vehicle. The lithium-ion battery is light, environmentally safe and also it has high energy density and low self-discharge rate. However, the cell capacity diminishes over a time and it is rather expensive and volatile compare to other batteries.
Nevertheless, the price of the battery become lower and those problems are being resolved with the development of battery technology. Renault-Nissan alliance announce that the battery inside the Leaf will likely maintain 70 to 80 percent of their capacity after a decade and also the warranty will offer 8 years with 100,000 miles.
EV Power Flow Design Schemes
5.1 Power Smart Time-Based Pricing
Power Smart, also known as 'Time of Use' is a new electricity pricing system which is implemented throughout the electricity network. Due to a substantial increase of electricity consumption at certain times, the generation can meet the demand. Thus, electricity pricing varies to reduce the demand at peak time. The table below is the peak, shoulder and off-peak time of energy pricing in residential and business.
Residential Energy Pricing
Peak
40.26 cents/kWh
2pm to 8pm on working weekdays
Shoulder
14.96 cents/kWh
7am to 2pm and 8pm to 10pm on working weekdays
7am to 10pm on weekends and public holidays
Off peak
8.80 cents/kWh
All other times
Table Power Smart Residential energy Rates [15]
Business Energy Pricing
Peak
40.04 cents/kWh
2pm to 8pm on working weekdays
Shoulder
14.85 cents/kWh
7am to 2pm and 8pm to 10pm on working weekdays
7am to 10pm on weekends and public holidays
Off peak
8.36 cents/kWh
All other times
Table Power Smart business energy Rates [16]
5.2 Possible Schemes
Mass adoption of EV will have impact on power grid and consumers charging cycle will be changed depends on the price of electricity. EV charging station design should satisfy those changes to minimize the problems. The possible problem of this design is life span of the battery, to ensure the quality and life span the battery will only be used for adequate situation. For the charging station power flow scheme, there are three possible scenarios with concern of electricity price and power grid impact.
5.2.1 Off-Peak
Figure Off peak Power flow
The price of electricity is cheapest during off-peak period based on 'time of use' system. Thus, recharge the vehicle during the off-peak is the best scenario for consumer and power grid system to minimize charging cost and for the efficient use of power grid. In this scheme, EV charging station recharges its battery during the off-peak period from national power grid. The vehicle will be directly charged from the power grid. The battery will not be used to recharge the vehicle during this period to maximize the life span of battery.
5.2.2 Peak and Shoulder without PV System
Figure peak power flow without PV system
The peak period have the most expensive rate and usage of electricity reach nearly maximum. During the peak and shoulder period, EV will only be recharged by battery system before the battery has discharged. Once the battery has discharged, it will use national power grid only to recharge EV and the battery will not be used to save battery life. It is good to use the battery which is large enough to last during the peak period, but the cost of battery become expensive. Thus, the battery type and capacity need to be decided with consideration of the required time, usage level and the price of battery.
5.2.3 Peak and Shoulder with PV System
To solve the problem on above scheme, the hybrid system is recommended. The PV system is the one of the best options to consider, because peak and shoulder period are the daytime. PV system is clean, semipermanent and need no maintenance [16]. Also, if the consumption increases, it is possible to add additional PV to meet the demands. The PV system will assist the battery during the peak and shoulder time to last for these period. The battery is recharged by PV system during the peak and shoulder period and it helps to increase the quantity of battery consumed to last peak and shoulder period. The problem of this system is weather dependant system and when the battery is discharged in this situation, the charging cost will be really expensive.
Figure peak power flow with PV system
Model Design
The entire system has two main power sources. This system is modification of grid tied PV system. The PV system generates the power during the daytime when the price of electricity is most expensive when 'the smart grid system' is applied. The energy generated from the PV system will be stored in the battery bank and the DC source from the battery will be converted into single phase AC source to charge the EV and when the battery is discharged, the switch will be turned on to use the electricity from the power grid. This system does not require any extra charging cost after installation when few conditions are satisfied. For the PV system and battery bank, commercial available model will be used and the charger specifications are referred from the Leaf specification. Thus, the switch and the controller will be designed for this project.
Power Grid
Switch
Controller
EV Charger
PV System
Inverter
Sealed Lead Acid Battery
6.1 Battery
6.1.1 Battery Type & Capacity
Typical requirements for a battery system to be used for long term storage are long life cycle, low self-discharge, long duty cycle, high charge storage efficiency, low cost and simple maintenance. In this project, the most important aspect to select the battery is the low cost, long life-span and availability. There are various types of battery on the market and sealed lead-acid is the most popular battery bank for the PV system. The lead-acid battery offer the best balance of capacity and cost. Lead-acid battery cells consist of two plates, positive and negative immersed in a dilute sulfuric acid solution. The positive plate, or anode, is made of lead dioxide (Pb02) and the negative plate, or cathode, is made of lead. This simulation model has two modes of operation: charge and discharge. When the current into the battery is positive, it is in charge mode and discharge mode when the current is negative.
The battery model was based on a standard 2V lead-acid battery Simulink model.
Figure Battery Model
The battery model has the following input parameters:
A. State of charge: SOC, indicating available charge.
B. Depth of charge: DOC.
(Note: battery capacity depends on charge or discharge rate.)
C. Number of 2V cells in series: ns
D. Charge/discharge battery efficiency: K
E. Battery self-discharge rate: D (h-1)
*Note: parameters D and E are empirical constants that depend on the battery characteristics.
The state of charge (in %) has a very linear relationship to the open-circuit terminal voltage of the battery. SOC1 can be estimated based on the current open-circuit terminal voltage of the battery. The relationship can be estimated using Table.
Voltage
State of Charge (%)
12.63
100
12.54
90
12.45
80
12.39
75
12.27
60
12.18
50
11.97
25
11.76
0 (Completely Discharged)
Table Relationship between open-circuit battery voltage and state of charge.
The terminal voltage of the battery is measured by voltage sensor and it is given by:
Vbat = V1 + Ibat X R1 (2.1)
where V1 and R1 are governed by a different set of equations depending on which mode of operation the battery is in. Values for the battery current (Ibat) are positive when the battery is in charge mode and negative when the battery is in discharge mode.
Charge Mode [5]
with SOC(t) as the current state of charge(%). SOC (t) will be defined by a set of equations later on in the section.
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Discharge Mode [5]
The most difficult part of the battery model is accurately estimating a value of SOC (t). The estimation of the PSpice model is described by the following equation:
where all the parameters have already been defined.
This is basically the energy balance equation computing the value of the SOC increment as the energy increment in a differential of time taking into account self-discharge and charge discharge efficiency. For this equation, time is assumed to have units of seconds so some terms must be divided by 3600 so that SOC is in Wh. This value can be altered to compensate for time units of minutes (60), hours (1), or any other increment (divisions/hour). For my first simulations, I chose to use units of time in minutes, simplifying the equation to:
This equation can be further simplified by substituting V bat as a function of
It is necessary to use integration to solve for SOC (t):
With t as the number of time units. Therefore, SOC (t) can be found if you know the previous condition. Since SOC (O) = SOC 1 = initial state of charge, SOC(1) can be found, and by looping the result with Simulink, we can estimate the current value for SOC(t) for any 1.
The capacity of the battery is in Ampere-hour (Ah) and it can be converted into Watt-hour by multiplying the voltage of the battery. To find the battery which will be used in the system, the load consumption needs to be figured out. This will help to find the Depth of Discharge (DOD) of a battery which determines the fraction of power can be withdrawn from the battery. The load will draw 30A for 8 hours when the car is fully discharged. Thus, the size of the battery is 30A X 8 hours = 240 Ah. With consideration of
The load draw 3.3kW with 240V and the current will be 13.75A. The normal lead-acid battery voltage is about 12V
All EVs are charged during the peak hour Assumed extra load due to EVs is base the calculation below,
Average daily travel distance of passenger vehicle = 13.9 km
Energy consumption of Leaf = 211 W/km
The number of registered vehicles in Australia = 16 million
Thus, the total number of vehicles in Australia is 16,061,098 which is about 16 million and the number of passenger vehicles are 12,269,305 in 2010.*** Specifically, in NSW, there are 4,681,471 which is about 4.7 million vehicles. If 10% of those vehicles owner switched to EVs, then the number of EVs will be 1.6 million in entire Australia and 470 thousands EVs will be on the street.
6.1.2 Battery Simulation Model
This is the Simulink model of standard 12V lead acid battery.
Figure
Figure
6.2 Solar Panel
The typical commercial solar panel module is consists with screen printed Si solar cell. It has, usually, 36 cells in series and also it is compatible with 12V battery. The EVs consume 3.67 kW daily and it is about 110 kW per month. Thus, the panel need to generate at least 110 kW per month and 150 kW is required value for the stable operation from the above analysis. The
6.3 National Power Grid
6.4 EV Charger
The average travel distance of passenger vehicles is 13.9 km per day in Australia. The Nissan U.S.A announce that the energy consumption of the Leaf will be 34 kW∙h/100miles and it is 0.211 kW∙h/km. To be conservative on this fact, the efficiency of the motor is assumed as 80% and the final value has become 264 W∙h/km. With the average driving distance of the day, it will consume 3.67 kW∙h/day. The charger from the Nissan provide about 3.3 kW to charge 24 kW battery pack inside the Leaf which will takes about 7-8 hours to fully recharge.
6.5 Power Inverter
An inverter is an electrical device that converts direct current (DC) to alternating current (AC); the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits.
Solid-state inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high-voltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries.
There are two main types of inverter. The output of a modified sine wave inverter is similar to a square wave output except that the output goes to zero volts for a time before switching positive or negative. It is simple and low cost (~$0.10USD/Watt) and is compatible with most electronic devices, except for sensitive or specialized equipment, for example certain laser printers. A pure sine wave inverter produces a nearly perfect sine wave output (<3% total harmonic distortion) that is essentially the same as utility-supplied grid power. Thus it is compatible with all AC electronic devices. This is the type used in grid-tie inverters. Its design is more complex, and costs 5 or 10 times more per unit power (~$0.50 to $1.00USD/Watt).[1] The electrical inverter is a high-power electronic oscillator. It is so named because early mechanical AC to DC converters were made to work in reverse, and thus were "inverted", to convert DC to AC.
The inverter used in this system is for invert 12 DC output of battery into 240 AC to supply energy to electric vehicle charger. There are various charger available, typical chargers for the Leaf will draw 3.3kWh. The size of inverter should be 10 to 20% above the load level, so 4 kW will be the size of the inverter.
6.5.1 Power Inverter Type
6.5.2 Inverter Simulation Model
Figure
6.6 Controller & Switch
Simulation Model
On this thesis, MATLAB will be used as simulator. Simulink is inside the MATLAB which offer an environment for multi-domain simulation and model-based design for dynamic and embedded systems. It provides an interactive graphical environment.
7.1 Calculations and Results
Analysis
8.1 Capital Cost Analysis
Discussion
Future Work and Possibilities
Conclusion
Electric vehicle, which has reliable, affordable, and environmental power source, grows more with the support of the public, the federal government, and the states. The EV charging station will play the most important role in mass adoption of EV. The background knowledge of EV and electricity pricing information which found from this report will be used to perform the calculation and cost comparison of charging in various locations on thesis B.
Tasks:
1. Further literature reviews and gather detailed data from automotive industries
2. Capital cost analysis
3. Consumption Estimation and PV system analysis
4. Create database and perform calculations (i.e. losses)
5. Compute and compare the price of charging with different area
6. Research future technologies and recommendation (i.e. Solar-roadway)
7. Write Report
