Energy storage systems boost the construction of electric vehicle fast charging infrastructure

Considering the presence of multiple charging posts, the grid needs to provide a local charging peak power of more than 1MW. The grid may collapse at multiple points, or huge investments will be needed to improve transmission lines and centralized power plants to significantly increase the base load. However, this load is pulsed and must be integrated with intermittent energy generated by renewable energy sources such as solar and wind.

Energy storage systems offer a simple and elegant solution to this problem. In the same way that we use fuels like gasoline and natural gas to store energy and use it again when needed, such as when refueling a car, we can use electronics and chemistry to store electrical energy in batteries. This energy can then be used to increase the charge of electric vehicles, stabilize the grid by smoothing out power peaks, or provide power in the event of a blackout.

The automotive market is already shifting. Nearly 3 million electric cars will be sold in 2020, out of a total of more than 80 million cars sold. While 3 million may seem like a niche market, forecasts show that electric car sales will grow rapidly, reaching 10 million in 2025 and more than 50 million in 2040, out of a total of 100 million cars sold. This means that by 2040, 50% of vehicles sold will be fully electric. For all of these cars, the charging will be done at home using a simple wall charger, overnight slow charging using a multi-kilowatt DC charger in homes with solar power systems and energy storage batteries, and fast charging on the street from a charging station, or super-fast charging at future gas stations.

We see that while the electric vehicle market is rising rapidly, the renewable energy power generation market (which has recently experienced a boom in solar photovoltaic (PV) systems) is still maintaining good growth momentum, which is inseparable from the price drop of about 80% of solar systems in the past 10 years and strong decarbonization initiatives. Today, solar energy accounts for less than 5% of global electricity generation, and is expected to account for more than one-third (33%) of global electricity generation by 2050.

In the context of intermittent electricity loads in the future, charging electric vehicles and intermittent energy sources such as solar and wind energy will face some challenges, such as how to integrate these emerging participants in the energy ecosystem with the grid as the center. Intermittent load demands such as electric vehicles require higher transmission line specifications to meet higher power peak demands.

Solar power generation will change the way centralized power plants operate, ensuring that the power grid is not overloaded; people will demand more convenient power supply methods, and their own electricity in their homes will increasingly be provided by residential solar power generation systems.

In order for all entities to work together smoothly and benefit from renewable energy and zero-emission electric vehicles, energy storage systems must be involved, ensuring that we can store and reuse electricity generated when demand is low (for example, using solar energy generated at noon in the evening) and use excess energy to balance the grid load.

Energy storage systems (ESS) are the equivalent of oil tanks or coal storage in the electrical energy world and can be used in a variety of applications on both the residential and industrial scale. In residential applications, it is easy to connect a PV inverter to a battery to store and use energy at home, or use the energy generated by the sun during the day to charge a car at night. In industrial or utility-scale applications (such as grid-connected services), energy storage systems can be used for different purposes: from regulating PV and wind energy to energy arbitrage, from backup support to black start (eliminating diesel generators), and most importantly from a total cost perspective, it can delay investments. In the latter case, energy storage systems can be used to meet peak load demands at grid nodes, ensuring that there is no need to incur costly upgrades to existing transmission lines. Another relevant application case is off-grid facilities, where energy storage systems make microgrids or islands self-sufficient in terms of electricity.

Figure 1. Integration of renewable energy, energy storage systems and electric vehicle charging infrastructure

Taking all possible applications into account, the energy storage system market will exceed the threshold of 1000 GW of power generation/2000 GWh of production capacity before 2045, which is a rapid growth compared to today's 10 GW of power generation/20 GWh of production capacity.

This article will focus on energy storage systems for electric vehicle charging infrastructure.

Private and public AC charging infrastructure is simple but limited in power. Level 1 AC chargers operate at 120 V and have a maximum output of 2 kW. Level 2 AC chargers operate at up to 240 V and up to 20 kW. In both cases, the onboard charger is required to convert the AC power to DC. Wall-mounted AC chargers are more of a metering and protection device than a charger. Due to cost, size, and weight constraints, automotive onboard chargers are always rated below 20 kW.

On the other hand, DC charging allows electric vehicles to be charged at much higher powers: Level 3 chargers have a maximum rated DC voltage and power rating of 450 V and 150 kW respectively, while the latest superchargers (equivalent to Level 4) can exceed 800 V and 350 kW. For safety reasons, the voltage is capped at 1000 V DC when the output connector is plugged into the vehicle. When using a DC charger, the energy conversion is done in the charging pile, and the DC power output connects the charging pile directly to the car battery. This eliminates the need for an on-board charger, with the added benefit of reduced space and weight. However, during this transition phase, the EV charging infrastructure remains highly fragmented and varies from country to country, with EVs mostly using a small 11kW on-board charger that allows users to charge from an AC outlet when needed.

Increasing charging power requires increasing the operating voltage and ensuring that the current remains within reasonable limits of cable size and cost, which means that the microgrid or subgrid where the charging station is installed must be properly designed and sized.

Let’s imagine a future charging station (in 2030) where the fuel consists of electrons, delivered via pipes called transmission lines, and connected to the medium voltage (MV) grid via transformers. Currently, the fuel is stored in huge tanks underground and brought to the station periodically by tanker trucks. While it may seem like a simple solution to always have new fuel (electrons) delivered via the grid, we can see that this simple approach is not sustainable if we want to enable drivers to fully charge their electric vehicles in less than 15 minutes.

The charging station has five DC chargers, each of which can deliver a maximum peak power of 500 kW. In the worst case, five chargers charge a completely depleted battery at the same time, and the charging station must take this into account. To simplify the calculations, we will now assume that the losses in the power conversion stage and the battery charging path are zero. Later in this article, we will see that even small power losses in the entire power chain can affect the normal design.

Let’s assume that there are five electric vehicles, each with a 75 kWh battery (all-electric vehicles on the market today have batteries ranging from 30 kWh to 120 kWh), that need to be charged from 10% state of charge (SOC) to 80%:

This means that 262.5 kWh of energy needs to be transferred from the grid to the electric vehicle in 15 minutes:

The grid must deliver slightly more than 1 MW of power to these electric vehicles for 15 minutes. The charging process for lithium batteries requires a constant current, constant voltage charging profile, where the power required to charge the battery to 80% is greater than the power required to charge the last 20%. In our example, we assume that charging stops at 80% at maximum power.

The grid (preferably a subgrid) where the charging station is located must intermittently sustain peaks of more than 1MW. Very efficient and highly complex active power factor correction (PFC) stages must be implemented to ensure that the grid remains efficient without affecting the frequency or causing instability. This also means that very expensive transformers must be installed to connect the low-voltage charging station to the medium-voltage grid and to ensure that the transmission lines that carry the power from the power plant to the charging station are sized to handle the peak power requirements. If the charging station is used to charge a mix of cars, trucks and buses, the power required will be even higher.

The simplest and most economical solution is to use electricity produced locally from renewable energy sources such as solar and wind power instead of installing new transmission lines and large transformers. In this way, users can directly connect to charging stations with excess electricity instead of relying entirely on the grid. In fact, 100 kW to 500 kW solar photovoltaic (PV) power plants can be installed near charging stations or subgrids connected to charging stations.

Although photovoltaic power can provide 500 kW of electricity, reducing the power demand on the grid to 500 kW, photovoltaic power is intermittent and not always available. This brings instability to the grid, forcing electric car drivers to charge their cars at the fastest speed only when the sun is shining. This is not what users need and is not sustainable.

Missing from the power electronics puzzle is the energy storage system. Think of the energy storage system as a large battery, just like the underground tanks at gas stations today, that stores the electricity from renewable energy sources and delivers it to the grid, to charging stations, or back to the grid. The first characteristic of energy storage devices is bidirectionality, being on the low-voltage side of the grid. The new installation is designed to have a DC bus voltage of 1500V, connecting renewable energy, electric vehicle charging stations, and the energy storage system battery. The energy storage system must also be properly sized to ensure that the ratio between peak power and energy capacity meets the optimization requirements for the specific installation. This ratio depends largely on the amount of electricity generated locally through solar, wind or other energy sources, the number of charging stations, other loads connected to the subgrid, and the efficiency of the power conversion system.