How photovoltaic energy storage systems can help electric vehicles achieve fast charging

The automotive market is transforming, with sales forecasts revised upward as electric vehicle (EV) adoption rates increase rapidly. While EVs represent only a small portion of the overall market, it is predicted that 10 million EVs will be sold in 2025, and by 2050, more than 50 percent of all vehicles sold will be electric.

Most vehicles will be charged slowly by connecting to wall-mounted charging boxes while parked overnight in driveways. Some vehicles will be charged more quickly at street charging points, and future gas stations will have the potential for ultra-fast charging.

With multiple charging points operating simultaneously, peak demand on the local grid will be so great that local grid collapse could become commonplace without massive investment in transmission lines and power plants to provide the capacity to address this demand.

This paper talks about the current state of electric vehicle charging and considers the electricity demand it will likely generate shortly. It then considers how this demand can be met in a practical, sustainable, and commercially viable manner.

Current State of Electric Vehicle Charging

The current AC charging infrastructure used in public and private facilities varies in the power they provide. Level 1 AC charging ports operate at 120 volts (providing a maximum of 2 kW of power), while level 2 charging ports operate at 240 volts, providing up to 20 kW of power. In both cases, the AC-DC conversion is performed in the onboard charger rather than in the wall-mounted charging box (which primarily performs protection and metering functions).

Due to cost, size, and weight constraints, onboard chargers are typically rated below 20 kW. Alternatively, charging can be performed at higher power levels if DC charging (rather than AC) is used. Level 3 DC charging posts are rated at 450 V (providing up to 150 kW of power), while more recent Superchargers are rated at up to 800 V (providing up to 350 kW of power).

For safety reasons, the maximum voltage is limited to 1000 V when the charging plug is connected to the vehicle. In DC charging, the power conversion takes place in the charging port, which is connected directly to the vehicle's battery, making the vehicle lighter and with more available space as it eliminates the need for an onboard charger.

Future needs

As more electric vehicles hit the road, drivers expect to be able to charge their cars in less time. Consider the following charging scenario, which will likely become a reality in less than ten years. A roadside charging station has five DC charging posts; when five cars stop simultaneously, they charge at each post. If each car is equipped with a 100 kWh battery that is already 25% charged and the driver wants to be fully charged to 75% in 15 minutes, the total amount of power that needs to be delivered from the grid to the charging station is :

5x(75%-25%)x100 kWh/0.25 h = 1 MW

The grid supplying the charging stations needs to be able to manage these intermittent 1 MW peaks. This has several implications for the power delivery infrastructure. Efficient and complex active power factor correction (PFC) segments will be required to ensure that the grid's frequency is not affected and remains stable and efficient. Expensive transformers will also be required to connect the low-voltage charging stations to the high-voltage grid. The cables carrying power from the power plant to the charging stations will need to be properly sized to handle the current level delivered. Peak power requirements will be greater for vehicles with higher-capacity batteries.

Solar to fill the gap

Using electricity generated from local renewable sources such as solar or wind is a simpler and more economical solution, eliminating the need to install new transmission lines and large transformers. By their very nature, these energy sources are also intermittent. Still, if carefully managed, they can be used to meet the intermittent demand on the grid generated by electric vehicle charging.

Over the past decade, the price of solar PV technology has fallen by nearly 80 percent, contributing to the continued growth of renewable energy systems driven by the requirement to reduce carbon emissions. Today, solar power accounts for less than 5% of global electricity generation but is expected to grow to more than a third by 2050.

The growth of solar power will affect how electricity is generated and used - power stations will need to be managed to ensure that the grid is not overpowered. People will increasingly consume electricity from residential solar systems installed in their homes. This will require a careful balance between supply from centralized mains and generation from local renewables, as well as customers' variable needs. For our charging station example, which is connected directly to a sub-grid powered by a solar PV installation with a 500 kW supply capacity, the grid only needs to provide 500 kW.

Energy storage solutions

Using power from PV installations means that the fastest charging rates can only be achieved during the day when the sun is at its brightest, which is an unsustainable proposal.

A more realistic solution can be achieved by using an energy storage system (ESS), which is the equivalent of a natural gas or oil storage tank and can be used for various purposes (domestic and industrial). In-home applications, it is easy to connect a PV inverter to an energy storage battery, which is charged by solar energy during the day and can then be used to charge an electric vehicle at night.

In industrial settings, ESS devices can be used for different purposes - to regulate power from PV and other renewable sources or to provide backup support for black starts, eliminating the need for diesel generators. The use of ESS also makes economic sense, as the market demand for faster charging of electric vehicles is growing, and ESS supports the gradual upgrade or replacement of existing transmission lines over a longer time horizon.

The market for these systems is expected to grow rapidly from 20 GWh today to over 2,000 GWh by 2050. for dry our charging stations, ESS behaves like a large battery, storing and delivering energy from solar installations (or other renewable sources) to charging posts as needed, with any excess energy being delivered to the grid. An appropriately sized ESS should be selected to achieve the best balance between peak power demand and energy storage capacity (the ratio of which depends largely on the amount of locally available generation (solar, wind, or other), the number of charging posts, and other loads connected locally.

As electric vehicle sales increase, drivers expect to be able to charge their vehicles in less time, meaning the demand for fast-charging infrastructure for electric vehicles will grow rapidly. A quick analysis shows that the existing grid is not designed to cope with the resulting intermittent peak demand. Using solar PV installations in combination with energy storage systems may be a realistic and commercially viable alternative to a grid infrastructure that may otherwise need to be overhauled.

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