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Issues in Electrical Energy Storage for Transport Systems

1.1. Storage requirements for transport systems

For the past century, the difficulty of storing electrical energy in large quantities, within reasonable volume and weight limits, has been a major obstacle in the development of autonomous electric vehicles that are able to travel medium to long distances. This difficulty has been overcome in the case of guided vehicles, trains, trams or underground trains, which capture electrical energy from an overhead line or a third rail during their movement. This solution has also been applied to buses designed to cover only a well-determined route. This result was represented by the trolleybus, which captures electrical energy from an overhead line which is required to be double when there is no possible current return by the rails. With these applications, a stationary electrical energy storage system incorporated into the supply system makes it possible to recover the braking energy of vehicles and to regulate the power demand from the electric power grids prior to the supply with electricity, or to cover particular areas without power supply.

Vehicles that do not complete regular journeys or travel long distances, such as cars, vans, lorries and motor coaches, cannot benefit from the acquisition of energy in motion. In this case, it is therefore necessary to load the electrical energy in sufficient quantities to reach the final destination. An electric car should have 200 to 300 kg of Li-ion batteries on board for approximately 200 km of autonomy. In contrast, a liquid hydrocarbon makes it possible to store approximately 12 kWh of thermal energy in 1 kg; with approximately 50 kg of fuel, tank included, a car with a thermal engine can reach 1,000 km of autonomy.

Other onboard systems produce their electricity on-site: aircraft, vessels and diesel-electric locomotives. The tendency to use the electricity vector more frequently in these systems, for traction and/or auxiliary attachments, generates a growing demand for storing electrical energy to reduce operational risks and also to save the energy generated during the braking phases of engines and actuators.

The hybridization of vehicles and onboard systems using electrical energy and liquid or gaseous fuel of fossil or non-fossil origin is in the course of development, due to the fact that this solution represents an essential intermediate step towards introducing vehicles without fossil fuel consumption. In the case of guided modes of transport, hybridization makes it possible to optimize the energy consumption of trains that complete journeys using electrified and non-electrified lines. Noise pollution may also be reduced by using electricity for shunting locomotives, for example in urban areas.

Space satellites and vehicles are onboard systems that capture electrical energy using solar panels when they are facing the sun, and store the electrical energy to satisfy their energy requirement during movement in shadow.

The significant development of the electricity vector within the framework of transport systems is a consequence of the flexibility of electricity, as well as of its potentially non-polluting nature while being used. However, if electricity is produced from fossil energy, for example in thermal power stations, pollution, including CO2 emissions, is not emitted at the level of the vehicle, but upstream during the production of electricity. To accomplish the objective of reducing polluting emissions, it is necessary to produce electrical energy from non-polluting renewable energy (or potentially nuclear energy, which does not emit CO2, but generates radioactive pollution throughout its lifecycle), but also to reduce the use of energy from non-renewable energy sources and the overall amount of pollutant discharge during the construction and deconstruction phases.

With the purpose of reducing CO2 emissions, as well as the consumption of non-renewable sources (fossil or from nuclear power), and using electricity produced from renewable energy sources, projects have been developed to combine the production of renewable energy and the power supply of trains or electric vehicles. The intermittent nature of these types of energy may require the use of storage systems, knowing that in the case of electric vehicles, the latter already incorporate this storage function (electrochemical batteries).

Storage systems, which in the future will be widely incorporated into electric vehicles, meet the requirements of these applications, but also provide the possibility of contributing assistance among other actors of the electric system. Due to the increased costs of storage systems, this could represent a way to enhance their financial value, including the obligation to control the aging of these systems. Studies have also been conducted to research the possibility of whether the storage capacity of electric vehicles, owing to the flexibility of their charge or discharge, can provide assistance to the electric power grid, or even directly to the buildings connected to the grid; reference is, thus, made to vehicle-to-grid or vehicle-to-home.

1.2. Difficulties of storing electrical energy

A weak point of the electricity vector is that the electrical energy cannot be stored directly and that conversion interfaces are required. It is possible to store electrostatic energy (in capacitors) or magnetic energy (in superconductive coils); however, the storage capacities of these solutions are very limited. To obtain substantial storage capacity, electrical energy must be transformed into another form of energy. Electrochemical storage by means of lead batteries has long been used for onboard applications, as they provide improved mass performance and emergency power supplies. Storage in the form of kinetic energy, by means of flywheels, has been used for several decades for fixed applications, such as emergency power supplies and some onboard applications including satellites.

Electrochemical batteries make it possible to store electrical energy as a direct current voltage source. Inertial energy storage is based on electrical machines that are required to operate at variable speeds, namely variable frequency. With electric power grids supplying electricity in the form of alternating voltage and currents, the implementation of these storage technologies remained complicated until the advent of electronic power, which has been developed since the 1960s and is currently used to transform the form and characteristics of currents and voltages at will. A significant barrier has thus been overcome, allowing for a more extensive use of electrical energy storage today.

Ragone diagrams, which show power and specific energy, are often used in the field of onboard applications to compare technologies and illustrate their energy/power compromise [ROB 15]. Figure 1.1 shows a simplified example comparing several electrochemical technologies and supercapacitors [MUL 13].

Figure 1.1. Example of a Ragone diagram for electrochemical technologies and supercapacitors [MUL 13]

The development of Li-ion technology in the last two decades represents a significant progress for onboard systems, that provides vehicles with a level of autonomy compatible with an increasing number of applications. Figure 1.2 shows the evolution of the energy density of lead, nickel-cadmium, nickel-metal-hydride and lithium-ion batteries over the past 40 years.

Figure 1.2. Evolution of the energy density of lead (Lead), nickel-cadmium (NiCad), nickel-metal-hydride (NiMH) and lithium-ion (Li-ion) batteries [BAS 13]

Lifetime remains a significant technological limitation in terms of lifecycle cost of these types of batteries. This is conditioned by the temperature of the battery, which should not be too high nor too low, the frequency of the charging/discharging cycles and the depth of discharge. Manufacturers estimate between 1,000 and 15,000 lifetime cycles for a maximum depth of discharge to be taken into account, and an operating temperature range. When considering a daily charging/discharging cycle, lifetime is estimated to be between 3 and 15 years. By reducing the depth of discharge, lifetime can be increased significantly. Some electric vehicle manufacturers propose to decrease the risk of premature failure for the operator by introducing rental of the vehicle battery pack.

The use of supercapacitors also contributes to the development of electrification in the case of onboard systems. Their energy capacity is clearly lower than that of batteries; on the contrary, they provide higher dynamics and a number of charging/discharging cycles for their lifecycle, which is 10 to 100 times higher, in the range of 10,000 to 100,000. Combining storage systems with supercapacitors and Li-ion batteries may thus be regarded as an interesting solution to obtain a global dynamic storage system with significant energy capacity, while ensuring satisfactory lifetime for various components. With such systems, supercapacitors generate rapid energy fluctuations, while batteries meet basic energy requirements gradually. For example, this type of solution is considered for trams and electric buses which can only be charged at station stopping times [URI 13].

The hydrogen vector is also considered to meet the requirements of onboard systems, particularly for motor vehicles, because this has a higher energy density than batteries (taking account of the tank and storage means). It makes it possible to generate electricity using a fuel cell, and it can be produced using...