Application and layout analysis of solar energy and electric vehicle charging systems

Today, with the rapid development of science and technology, various means of transportation and people's daily life require energy, which prompts people to constantly research new energy sources, including fuel cells. As we all know, the Indian government is promoting the electrification of transportation across the country and actively promoting electric vehicles. The electronics industry is optimistic about the development of local electric vehicles. Large OEM manufacturers are developing electric vehicles, and the electric vehicle ecosystem is also under steady construction. Established companies and various startups have begun to make efforts in the development of electric vehicle chargers, charging stations, software and cloud computing services, and initial results have emerged. Effectiveness. However, there are many opportunities for improvement in electronic systems. This article will take India's transportation electrification trend as an example to discuss the application and layout of solar energy and electric vehicle charging systems.

With the help of BIS, ARAI, EESL and other organizations, the Indian government has released technical specifications for charging stations; some of the original specifications such as AC-001 and DC-001 chargers have been developed and deployed at selected locations. stand. The latest guidance requires charging stations to be equipped with a variety of standard chargers, namely CCS AC type 2 and CHADEMO connectors, as well as lower power AC and DC-001 chargers. However, the power supply of these systems is completely dependent on the power grid. In large cities and semi-urban areas, the location of charging stations is a difficult problem, and the question of whether the power grid has reserved load for electric vehicles still exists.

These problems can be solved by solar power generation. Solar power generation and storage can not only supplement the power supply capacity of the grid, but can also be installed across the country to charge electric vehicles even if they are not connected to the grid. Fortunately, India's solar energy promotion is very successful, and its geographical location makes India rich in solar energy. Considering its service life of at least 20-25 years, the one-time installation fee and capital investment of the solar charging station are relatively reasonable and cost-effective, and the return on investment can be obtained within a few years. After being put into use, the power supply of the power station is almost free. The following introduces a feasible solution for charging electric vehicles using solar power generation and storage, which will involve solar power generation and storage methods, distributed battery management, power conversion, communication connections, and the development of a scalable modular solar electric vehicle charging station. Required basic modules.

The figure below is a functional diagram of a typical electric vehicle solar charging station.

The user terminal mainly displays the functions that the end user can see, and is responsible for information exchange and human-computer interaction. The components usually include a TFT touch screen and an NFC card reading device for authentication or payment. In order to achieve more advanced functions, there may be Bluetooth interface. The charging connector supports different charging standards: AC slow charging for small electric vehicles and electric three-wheelers, AC and DC fast charging for electric vehicles. Before charging can begin, users must verify their identity and set charging preferences. More complex functions are performed in the background, and the central controller and other modules are responsible for monitoring background operations.

Current and power management: The charging system has 3 power sources. The first is solar photovoltaic panels. This article does not discuss the calculation of solar panel area. Normally, the power of a charging station must be at least several kilowatts. Under rated illumination conditions, the normal power generation of solar panels is about 150W/square meter. The solar panels send electric energy into the MPPT module, which is an extremely energy-efficient DC-DC converter that runs a maximum power point tracking algorithm internally and has a conversion efficiency of over 98%. These converters are typically multiphase interleaved buck or buck-boost converters, operating at several hundred volts at both the input and output. Electrical isolation may not be a requirement, but most converters are electrically isolated for regulatory and safety reasons. The output is connected to the public DC bus to supply power to downstream loads. Control can be analog, purely digital, or a mixed analog-to-digital approach.

The second power source is the grid. Because this charging station is designed to maximize the use of solar energy, grid charging is the alternative power source. However, in some areas the power grid provides intermittent power supply, and in other areas due to insufficient sunlight, solar panels cannot generate electricity throughout the year, and the grid must supplement solar power generation in certain seasons. This charging system is essentially a solar power storage device that can supplement the power supply capacity of the grid through a two-way grid-connected inverter during peak hours of electricity consumption and function as a power plant. With a reasonable net power settlement policy, solar power plants or captive power plants can legally send power back to the grid. Therefore, this is a "one-stop, two-use" solution.

The third power source is the power storage station. The current trend is to use lithium batteries to build websites because lithium batteries have a long service life, are very suitable for fast charging, and have extremely high depth of discharge and energy density. To save space costs, batteries can be installed underground. These lithium battery packs are connected together in a mixed and series manner, and finally connected to a junction box termination unit with monitoring functions. Each cell has a data port, usually CAN or RS485, which connects the cells together via a daisy chain method and finally to a junction box so that the termination unit can display the health status of each cell, string or the entire battery bank. . Essentially, this is a data concentrator and a switching unit that controls the input and output status of the battery pack circuit. In addition, the junction box communicates with the central controller to determine the charging and discharging operations of the battery.

The figure below shows the power system architecture. This is a scalable modular system, the modules are usually scalable, each module is 3-5kW, equipped with a communication bus, usually CAN or MODBUS/RS485. The central controller can configure modules according to functional requirements at any time, such as charging management, load management, and diagnostic inspection. There is an activation module in the controller to monitor power consumption. The basic parameters include power consumption kWh, power storage kWh and power generation/output power kWh. The controller can also communicate with industry-standard electricity meters to enable billing and rate setting.

Main power management module: DC-DC converter module connected to the DC bus. According to the type of vehicle connected to the charging pile and the voltage and current requirements of the vehicle BMS, the central controller configures the DC-DC converter through the communication bus. This solution is usually used for DC fast charging, and multiple DC-DC converter modules can be connected in series to meet the charging load requirements. The DC-AC inverter is also connected to the same DC bus and is used to charge vehicles that can only accept AC charging or ordinary slow charging. The bidirectional inverter has two purposes: to supply power to the DC bus to meet power demand; to send power back to the grid when the charging station is idle, or to supplement the power supply capacity of the grid during peak hours. Here are the key energy efficiency metrics that any power conversion module needs to achieve today:

1) Extremely high energy efficiency: The actual end-to-end energy efficiency is now higher than 95%

2) Extremely high power density: Because of high property costs, system sizes are becoming smaller and smaller

Taking advantage of the development and progress of silicon technology, the above two requirements can be met. Wide bandgap semiconductor materials, especially silicon carbide devices, have high switching frequencies, higher junction temperatures and energy efficiency. In addition, advances in silicon technology allow passive components such as magnetics and capacitors to become smaller. Better magnetic materials also allow for higher output power in low-power, low-loss designs.

The main central controller is the brain of the charging station and is responsible for all functions such as user/reservation user authentication and human-computer interaction before charging. Its components include high-performance processors, communication connection technology and sensors. The main functions are as follows

1) User ID verification and payment: This is the most common function that users see at charging stations. User verification and payment can be completed through smart cards, OTP, NFC mobile phones and even Bluetooth. These subsystems are controlled by the onboard MPU/MCU.

2) Power management: This is the most important component of the charging station but cannot be seen by users. The system controller continuously monitors the power supply and demand relationship, and then selects the charging mode according to the situation: pure solar charging, hybrid solar and energy storage charging, or grid-assisted solar charging. There may be excess power supply or excessive power demand. The system controller should have certain intelligence to change the power transmission channel by changing the settings of the various power blocks mentioned above according to the relationship between supply and demand.

3) Communication connection: Currently, in order to achieve remote monitoring, charging stations and charging piles need to be connected to the cloud, have regular communication sessions with the CMS (Central Management System), report transactions, parameters, diagnostics and operating data, and receive operating commands from the CMS and settings. Therefore, the solution provides a variety of wireless and wired connection technologies such as 3G/4G, Wi-Fi, Ethernet, and even uses LoRa technology for remote monitoring.

4) Protection, diagnosis and fault reporting: In order to prevent faults, the system protection mechanism responds very quickly and can protect against external events such as surges or lightning strikes; operational problems such as accidental misoperation or intentional incorrect operation/abuse; or short circuit, overheating or overheating. Internal circuit problems such as voltage/overcurrent. To keep operating costs low and minimize downtime, the system is able to automatically report problems that may arise from time to time. The modular construction provides on-site indication of faulty circuits that need to be replaced, so service technicians can prepare parts in advance.

This article provides a brief introduction to how to deploy an electric vehicle solar charging system. Customers can experience viable functional prototypes and various sub-modules at STMicroelectronics' Noida, India development center, which can customize designs according to OEM customer needs. Electric vehicles and electric vehicle charging facilities are the main research directions of the development center. It has conducted a lot of research on how to improve the performance of the above-mentioned functional modules, and can provide customers with all the semiconductor components needed to develop electric vehicle charging stations, as well as a large number of Reference designs to reduce time to market. Although there are fuel cells, the current technology is not enough to ensure the operation of all human beings. This requires us to protect energy. Starting from ourselves and starting from the little things around us, saving energy is what every one of us humans should do. responsibility.