Hybrid energy storage system optimal sizing for urban electrical bus regarding battery thermal behavior

This paper proposes an algorithm for sizing the hybrid energy storage system of an urban electrical bus regarding battery thermal behavior. The aim of this study is to get the supercapacitors optimal contribution part in the hybrid energy storage system to keep the battery temperature within its allowable limit. A semi-active parallel topology that uses supercapacitors as a main source of energy is considered. According to the bus mechanical parameters and the ARTEMIS driving cycle, the power and energy demand are calculated. Using mathematical models for the battery, supercapacitors and DC-DC converter, several simulations are performed for different hybridization percentages. While observing the evolution of battery temperature, the most favorable hybridization percentage is defined.


INTRODUCTION
The first autonomous electric buses were equipped with batteries as a source of energy [1]. These batteries have some disadvantages, namely a too slow dynamic [2] and overheating problem [3,4]. The combination of batteries and supercapacitors (SC) is the suitable solution for electric vehicle [5]. This combination has complementary qualities and provides an excellent solution that can increase dynamic behavior and cover a wide range of power and energy requirements and it was demonstrated that this combination has lower battery costs [6], a general increase in battery life and higher overall system efficiency [7]. Starting from the observation that battery buses are used almost exclusively in urban areas rather than for long-distance transport. The urban transport has relatively short intervals between recharging possibilities. Externally based energy storage on SC can be a solution since they can charge much faster than conventional batteries [8].
In the literature, most of the reported works focus solely on the electrical behavior of hybrid energy storage system (HESS). While the behavior of the battery temperature in this kind of application has not yet been treated. All HESS sizing methodologies for electric buses do not considerate the battery temperature evolution. And to remedy to this problem, supercapacitors oversizing is done.
Obviously, HESS topologies are very diverse, depending mainly on the type of application [9]. For the studied case, the selected topology will use the SC as the main source [10]. Our study is to redo this dimensioning while considering the temperature of the battery to choose the optimal capacity value of supercapacitors. In this study, an algorithm is proposed to define the minimum SC energy part in the hybrid storage system for electrical bus to maintain the battery in its permissible temperature zone. In this work we neglected the regenerative braking energy. The suggested algorithm is applied to "Irizar ie" bus, with a total mass of 16000 kg following an ARTEMIS driving cycle on a working day of 24 round trips. Total energy required is calculated. Then, capacity part of each element is defined and for each percentage of hybridization the temperature evolution is determined using battery model which consider temperature effect. This paper is organized as follows: After the Introduction is given in the first section, the System Description and Modeling are introduced in section 2. The Research Method is presented in section 3. Then, the Electric Bus Energy Storage Sizing is addressed in section 4. Finally, section Simulation Results develops battery thermal behavior for different percentage of hybridization. The conclusions are given in the last section.

2.
DESCRIPTION AND SYSTEM MODELING 2.1. Hybrid energy storage system topologies Four possible topologies [11,12,13] for the HESS are presented below:

Parallel passive topology
The basic passive parallel hybrid configuration is shown in Figure 1, the SC pack and the batteries are directly connected in parallel to the load. Because of the direct connection, the SC pack basically acts as a low-pass filter. The main advantage is the ease of implementation and no complicated control device required. The disadvantage of this configuration is that the power sharing between the battery and the SC pack is uncontrolled and dictated solely by the parasitic elements. Also the DC bus voltage is not regulated and varies depending on the voltage range of the batteries, which influences the design load.

Parallel active topology
The multi-converter configuration uses two separate bidirectional back-boost converters as shown in Figure 2. The batteries and SC pack voltage can be kept lower than the DC bus voltage, less balancing problems. The voltage of the SC pack can vary in a wide range so that the capacitor is fully used. The advantage of this configuration is that the power of the batteries and SC pack can be individually controlled according to their state of charge and power requirements. The disadvantage of this topology is the increase in the number of components and the cost.

Parallel semi active battery/supercapacitor topology
The Parallel Semi Active battery/supercapacitor configuration is illustrated in Figure 3. In this configuration, the batteries voltage can be kept lower or higher than the SC pack voltage. The SC pack is connected to the DC bus and works directly as a low-pass filter. But the power of the batteries is uncontrollable. The control strategy applied to this topology allows the DC link voltage to vary in a range so that SC pack energy can be used more efficiently.  Figure 4 shows the diagram of the HESS configuration using a bi-directional buck-boost converter for the SC pack interface, the SC pack voltage can be used in a wider range. This configuration has a single controlled power source. However, the bidirectional converter must be oversized to handle the power of the SC pack. In addition, the nominal voltage of the SC pack may be lower. The batteries are connected directly to the DC bus. Therefore, the DC bus voltage is fixed.

Electric bus system description
Our goal is to use SC pack as the main power source for the bus because of their power density and their dynamic, which will be recharged at each bus stop to solve the problem of their low energy density. The battery will intervene in case of the SC discharge. The proper configuration is the Parallel Active Topology. It allows the maximum use of stored energy in SC while keeping the nominal voltage of the load. Due to its cost, the parallel active configuration will be discarded. Due to its advantages [14] the Parallel Semi Active Supercapacitor/Battery Topology will be used in the bus, where the overall system is shown in Figure 5 [15,16].
The supercapacitor (SC1) that can produce and absorb peak power is the main element of the energy storage system of the electric urban bus, which can be charged by the other SC in the bus stop. The battery will be used in extreme conditions when the supercapacitor is almost exhausted. The general structure of the charging station at the bus stop [17] is illustrated in Figure 6. The electric urban bus SC can be charged by (SC2) supercapacitor through a DC/DC converter at each bus stop when passengers get on and off. SC2 can be charged by the power grid via an AC/DC converter between them with a lower power density before the next bus arrives. With this method, the impact of surge on the power distribution network can be avoided.

. Battery model
In this work, we use MATLAB model for lithium-ion battery. Two models will be presented below (with and without temperature effect). For each model the discharge equation will be presented. The lithium-ion battery model without temperature effect is given as [18]: The impact of temperature on the model parameters is represented bellow [19]: with: Q(T a ) = Q| T a + ∆Q ∆T (T a − T ref ) The cell or internal temperature T, at any given time t, is expressed as:

Supercapacitor model
The SC is an emerging technology in the field of energy storage systems. Energy storage is performed by the means of static charge rather than of an electro-chemical process that is inherent to the battery [20]. The supercapacitor model used in this work is a generic MATLAB model parameterized to represent most popular types of SC [21]. The SC output voltage is expressed using a Stern equation as: With : = ∫

DC/DC converter model
DC/DC converters can be represented by two types of models, namely the switching models and the average value models. Switching models are mainly used for design purposes and to study the types of pulse width modulated systems with respect to switching harmonics and losses. These models require a low sampling time to observe all the switching actions, which makes the simulation very long.
On the contrary, medium-valued models take less time because the switches are replaced by controlled voltage/current sources. The switching harmonics are not represented, but all the dynamics of the converter are maintained, which makes these models attractive, because a longer sampling time can be used. Models of DC/DC converters of average value are used in this paper, as shown in Figure 7. The design of the control loops is performed taking into account the dynamics of the model [22].

RESEARCH METHOD
In this article, the choice of the hybrid energy storage system elements for a totally electric bus is carried out according to the diagram represented in Figure 8. The illustrated approach consists in determining all the energies consumed between two consecutive stops on the total path of the bus. These energies are normally determined from the mechanical characteristics of the bus and the bus driving cycle. Then choose a supercapacitor pack to ensure the minimum of calculated energies. Next, determine the battery capacity to ensure the power supply of the bus during a working day, knowing that the supercapacitors pack, which will be recharged at each bus stop, will provide partial or total power between two consecutive stops. Subsequently, use the model developed under MATLAB/Simulink, which uses the parallel semiactive topology Supercapacitor/Battery, with the model of the battery which considers the temperature effect, by applying the chosen value of the supercapacitors and battery. Then observe the evolution of the battery temperature during a day of operation. If the observed temperature exceeds the battery permissible value, the energy value just above the energy used for the supercapacitor must be chosen from the energies already calculated. And repeat the same algorithm until finding the optimum value of HESS elements.

ELECTRIC BUS ENERGY STORAGE SIZING
The objective of this part is to size the energy storage system of a fully electrical bus from well-defined specifications.

Bus mechanical parameters
The main mechanical characteristics of the chosen bus are summarized in Table 1. The vehicle is constructed applying the body of "Irizar ie" bus with a new rear transmission ratio of 8.

Driving cycle
The chosen driving cycle is ARTEMIS Urban [23] illustrated in Figure 9.

Motor torque, angular speed and power calculation
The rotational speed and power demand for the powertrain and the torque demand to overcome friction forces (rolling and air resistance) are depicted in Figure 10. The bus traction force required is given by this equation [24][25][26]: tr = aero + rr + i + gr (10) where:

Bus energy autonomous calculation
We define eight stations in the given driving cycle, the total driving range is 4870 m. The duration and the demand energy between two successive stations are calculated and listed in Table 2. The total energy demand for 1000 seconds ARTEMIS driving cycle is approximately 7.6 kWh. We estimate that the route between two bus terminals is two ARTEMIS cycles followed by a 15 minute break (each trip will last 2900 seconds). For a day operation, we define 24 round trips or 48 ARTEMIS cycles. The total energy required for a day is 364 kWh.

SIMULATION RESULTS
All simulation tests are executed with different operating conditions in MATLAB/Simulink environment. The simulations have been carried out during 69 600 s which represent the bus day operation time. In this study, we proposed eight simulation tests that represent the battery temperature evolution for each HESS combination given by Table 3. The aim is to find the best configuration with minimum SC capacity, to ensure the autonomous and the battery permissible temperature. The chosen battery parameters are summarized in Table 4. For each simulation performed, we present the battery current draw between two bus terminals that lasts about 2000 seconds followed by 900 seconds rest. The evolution of the temperature corresponding to this current draw during a day of operation are presented respectively in Figures 14-20 for different HESS combinations given by Table 3.   The following Table 5 summarizes the results of the simulations performed. We note that from a capacity of 60F we can keep the battery temperature within the allowable limits. For 60F Capacity, the maximum temperature reached is 55°C which is below the battery permissible temperature (60°C). SC energy is about 511 Wh which is 0.16% of the battery energy (330 kWh). With this percentage of hybridization, the battery capacity has been reduced by 32%.

CONCLUSION
Based on the ARTEMIS driving cycle and the mechanical parameters of the bus, we calculated the power required for its training system, then we calculated the energy required for its operation during a day. We have determined the optimal share of the energy to be stored in the supercapacitors, to allow the battery to operate in its zone of admissible temperature. The chosen bus case study with a power of 270 kW, requires a daily energy autonomy of 364 kWh is the equivalent of 871 Ah battery under 557 V. We have found the right combination of supercapacitors and batteries, namely 511 Wh for supercapacitors and 592 Ah for battery, which represents 68% of the energy required by the bus. With this combination we have limited the maximum temperature to 55°C.