Electrical vehicles: state of art and issues for their connection to the network Eduardo Valsera-Naranjo1, Andreas Sumper1,2, Pau Lloret-Gallego1, Roberto Villafáfila-Robles1, Antoni SudriàAndreu1,2 1. Electrical Engineering Department, Universitat Politècnica de Catalunya (UPC) Centre d'Innovació Tecnològica en Convertidors Estàtics i Accionaments (CITCEA), Barcelona, Spain 2. Institut de Recerca en Energia de Catalunya (IREC)
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Abstract— The recent awareness about fossil fuels and the environment has arisen more sustainable alternatives regarding means of transport. The first alternative of green transport has been hybrid vehicles. This kind of vehicles reduces significantly the CO2 emission but not totally. Nowadays, the current trend is the utilization of a unique motor for vehicle. i.e. an electrical motor. It seems that the electric vehicles (EV) will become the cars of the future. Moreover, one kind of such vehicles, the plug-in electric vehicles (PHEV), will not only charge their batteries, but PHEV will also be able to inject power to the network when required. This fact suggests that EV penetration will affect current power system performance. Then, it is necessary to study some scenarios of penetration of such vehicles into the electrical network in order to maintain security and quality of power supply. Electrical vehicles, Plug-in Electrical vehicles, network performance
I.
INTRODUCTION
A combustion engine uses only about a 30 % of fuel of its fuel tank while (the rest is lost in gases and in heat losses). An electrical motor has efficiency greater than an 80 %. This advantage was well known since the 19th century; in 1889 Thomas Edison built the first electrical vehicle powered by nickel alkaline batteries. However, electricity had not become the main energy source to power vehicles due primarily to the batteries. Since the beginning of the electrical vehicle batteries were a limiting factor. Indeed, this limiting factor led to giving up the concept of electrical car to move to the combustion engine car as the main vehicle.
This trend continued until the end of 1990. In this year, the growing awareness for the environment caused an action from the public entities. The CARB (California Air Resources Board) adopted the Zero Emission Vehicle (ZEV) mandate. The ZEV required the automakers California market share include 2% ZEVs in 1998, 5% ZEVs in 2001 and 10% ZEVs in 2003. Unfortunately, the protests of the vehicle manufacturers and the pressure exercised by the oil industry caused the abandon of the ZEV. However, some groups remained and the research in the electrical vehicle field was not completely forgotten. Little independent companies lead the research and propose real alternatives to the combustion vehicle. In addition, important cities of the world have implemented charge points in their electrical local network to promote the use of the electrical vehicle as a real alternative to others ways of transport. There can be distinguished two types of vehicles which may require charging points: pure electrical vehicle (EV) and plug-in hybrid electric vehicle (PHEV) which have the capacity of charging the battery from an external power source. The pure EV have only batteries as power source for powering the electric motor. There are various settings for the layout of the electric motor in the vehicle: a single electric motor (Fig. 1 (a)), an electric motor in each of the guide wheels (Fig. 1 (b)), a motor for each rear wheel ((Fig. 1 (c)) and one motor per wheel (Fig. 1 (d)). The PHEV’s have one or more electric motors and a combustion engine. Before the apparition of PHEV’s, common hybrid vehicle used a combustion engine to charge their batteries to feed the electric motor. Current PHEV’s allow starting from an initial situation of charged batteries; thus it reduces the fuel used by the combustion engine.
Figure 1. Possibilities for the layout of the electric motors in EV’s
Figure 3. Equivalent circuit of an electric battery
Nowadays, there is under discussion the possibility of using the EVs and the PHEVs in combination with renewable energies. Specifically, it is proposed to charge the EV when there is an excess energy from renewable sources. In addition, it is also proposed to go a step further and use the batteries from the EV connected to the grid as a power reserve and inject power to the grid when it is needed. In order to make this possible it is necessary to study the batteries of the electric vehicles, chargers, the location of the charge points and the standardization of the connectors (Fig. 2 shows a proposed standard for a connector). The present paper makes a review of all this items.
As Fig. 2 shows, the battery voltage decreases if it is delivering power and the voltage increases if the battery is charging. B. Capacity The capacity of a battery is the most important parameter and it is usually expressed in Amphour. If a battery is of 10 Amphours, this means that it can provide 1 A for 10 hours. However, the same battery would not deliver 10 A in 1 hour because the capacity of a battery depends directly in the way the energy is extracted; the quickest the energy is extracted the less capacity the battery has. C. Energy stored One of the most important parameters of the batteries in the field of the EVs is the energy stored because this parameter is the responsible of the autonomy of the vehicle. The energy stored in a battery depends on its voltage and on its capacity. The unit for this parameter is Joules, but this is an inconveniently small unit, so Watthour is used instead. D. Specific energy Specific energy is the quantity of energy stored in the battery for every kilogram. The specific energy is typically given in Wh·kg-1.
Figure 2. Connector of an electrical vehicle
II.
BATTERY PARAMETERS
Batteries were the limiting factor that led the electric vehicle to disappear from the transport field; the component that was the only energy storage was the component with the highest cost, weight and volume. In addition, autonomies reached by batteries were significantly lower than autonomies of fuel powered vehicles. Below, a review of batteries and its main parameters are presented in this section. A. Battery voltages The nominal voltage of a battery can be expressed by the electric equivalent circuit of Fig. 3.
E. Specific power Specific power is the amount of power obtained for each kilogram of the battery and is measured in W·kg-1. It is important to differentiate the specific power from the specific energy: a high specific energy means that the battery can store a high energy but this does not imply that the same battery can provide the energy in a fast way which means it has a high specific power. In Fig. 4 a Ragone plot shows the relationship between the specific power and the specific energy of two of the most used types of batteries (adopted from [1]).
A. Definitions The main components of the charging station installation are defined below. 1) Link Station The link station links the General Protection Panel (GPP) with the user installation. Therefore, the Link Station begins at the end of the electrical connection 2) Charging Station All the equipment designed to provide ca to electric vehicles. It contains the plugs to connect the electric vehicle. 3) Control Center Centralized system responsible for manage statistical data and incidents of all the charging stations of the whole installation.
Figure 4. Ragone plot two types of batteries
III.
REVIEW OF BATTERIES
V.
This section presents a review of batteries and the main characteristics of each type. The main types of batteries for commercial use that can be considered to power an EV are (Table I) [2]: Lead – Acid batteries, Ni – Cd batteries, Ni – MH batteries and Li ion batteries. The cheapest type of battery is the Lead-Acid type. However, its low specific energy makes this type of battery inadequate for using them in EVs and PHEVs. The Ni-Cd battery type has a better cycle number than the Lead-Acid type but its specific energy is not high enough to use in EV’s and PHEV’s. The Ni-MH and Li ion battery types have a good specific energy (specially Li ion type) but they have a high cost. TABLE I. Cost Specific Energy (Wh·kg-1) Voltage per cell Charge current Cycle number (charge/discharge) Autodischarge per month (% of total) Minimum time for charge (h) Activity requirement Environmental warning
TYPES OF BATTERIES
Lead - Acid Low 30 -50 2 Low
Ni - Cd Medium 50 -80 1.25 Very Low
Ni - MH High 40 -100 1.25 Moderate
Li ion Very High 160 3.6 High
200 - 500
1000
1000
1200
Low (5%)
ModerateHigh (20%)
High (30%)
Low (10%)
8 - 16
1 – 1.5
2-4
2-4
180 days High
30 days High
90 days Low
None High
As it is mentioned in section 1, one of the biggest challenges that arise in the introduction of electric vehicles in cities is the standardization of the charging stations. The fact that there are still no national laws governing standardization makes difficult the implementation of the electrical vehicles. The group CITCEA-UPC, in collaboration with the Energy Agency of Barcelona (AEB), has developed a project to define standard specifications for electric vehicle charging stations. The specifications are mainly based on European [3] and [4]. One of the significant outcomes of this project was the definition of charging points for underground parking and charging points for surface parking because of the different charging needs of the two locations Another conclusion from the project is the need to create a Europe standard which defines the type of connector to be used by electric vehicles. Until a new standard gets develop, it is proposed to use a SCHUKO (CEE 7/4) connector type for currents up to 16A. In addition, to slow charging, the output values of the charging station are up to 16 A per plug, 230 V ± 10%, and 50 Hz ± 1%. REFERENCES [1]
IV.
CONNECTION ISSUES
In order to achieve a good implementation of the electric vehicle a definition of a standard charging station is needed. The standardization process should guaranty: Easy replication of the charging stations Easy expansion of functions through a modular design Safety for people and for the equipment Interoperability between charge stations and electric vehicles from different manufacturers
IMPLEMENTATION OF CHARGING POINTS FOR ELECTRICAL VEHICLES IN BARCELONA
[2] [3] [4]
J. Larminie, J. Lowry, “Electric Vehicle Technology Explained”. John Wiley & Sons Ltd, 2003. Battery University.com: http://www.batteryuniversity.com/ Conductive charge system for electric vehicles (“Sistema conductivo de carga para vehículos eléctricos”) (UNE-EN 61851) Reglamento electrotécnico de baja tensión (Real Decreto 842/2002, 2 de agosto de 2002)
