The ESS market, considering all its possible applications, will breach the 1000 GW power/2000 GWh capacity threshold before the year 2045, growing fast from today's 10 GW power/20 GWh.
For this article, the focus will be on the ESS installations for the EV charging infrastructure.
The ac charging infrastructure, both for private installations and for public ones, is simple but power limited. Level 1 ac chargers work at 120 V ac, delivering at maximum 2 kW; level 2 is capable of 240 V ac and 20 kW and the power conversion from ac to dc is, for both, demanded to the vehicle on-board charger. The ac wall box is rather a metering and protection device rather than a charger. The vehicle on-board charger, for cars, is always rated lower than 20 kW, for cost, size, and weight limitations.
On the other hand, dc charging enables the possibility to charge the EV at much higher power: level 3 chargers are rated up to 450 V dc and 150 kW, and the newest super chargers (equivalent to a level 4) can go beyond 350 kW and 800 V dc. The upper voltage limit is set to 1000 V dc for safety reasons when the output connector is plugged into the vehicle. While using a dc charger, the power conversion is made in the charging pile, and the dc power output directly connects the charging pile with the car's battery. This removes the necessity of an on-board charger, with all benefits in reduced occupied space and less weight. Nevertheless, in this transition phase, when the EV charging infrastructure is still fragmented and different country to country, region to region, a small 11 kW on-board charger is mostly present in the electric cars, to give users the ability to still charge via an ac outlet if needed.
Increasing the charging power requires an increased operating voltage to make sure the current is kept within reasonable limits for the cable's size and cost, and implies the necessity to properly design and dimension the microgrid or the subgrid where the charging stations are installed.
Let's imagine a charging station of the future (in the year 2030), where the fuel consists of electrons and where the fuel is available from a pipe called transmission line, connected to the medium voltage (MV) grid via a transformer. Today the fuel is stored in big tanks underground and brought to the fuel station regularly by tank trucks. Having the new fuel—the electrons—always available from the grid seems an easy and issue-free solution, but we can see that this simple approach is not sustainable if we want to give drivers the possibility to charge their EVs in less than 15 minutes.
Our charging station has five dc charging piles capable of maximum 500 kW peak power output each. The worst case, for which the charging station must be dimensioned, is represented by five EVs charging fully depleted batteries at the same time. To simplify the calculation, we now consider zero losses in the power conversion stages and in the battery charging path. Later in the article, we will see how a proper design is influenced even by small power losses across the power chain.
Let's consider five EVs, each one with a 75 kWh battery (the cars available in the market today, with a full electric powertrain, have batteries from 30 kWh to 120 kWh) that needs to be charged from 10% state of charge (SOC) to 80%:
This means that an energy of 262.5 kWh must be transferred from the grid to the EVs in 15 minutes:
Slightly more than 1 MW of power must be provided by the grid to the EVs, for 15 minutes. The charging process of lithium batteries will require a constant current, constant voltage charging profile, where the power required to charge up to 80% of the battery is bigger than the last 20%. In our example, we stop the charge at 80% assuming maximum power.
The grid or, better, the subgrid where the charging station is located, must sustain peaks greater than 1 MW intermittently. Very efficient and complex active power factor correction (PFC) stages must be implemented to make sure the grid is kept efficient, without affecting the frequency and without creating instability. This also means that very expensive transformers to link the low voltage charging station to the medium voltage grid must be installed, as well as making sure the transmission lines bringing the power from the power plant to the charging station are properly dimensioned to cope with the peak power required. In case the charging station is charging a mix of cars and trucks or buses, the required power is higher.
The simplest and most economical solution, instead of installing new transmission lines and big transformers, is to use the power locally generated by renewable sources, such as solar and wind. This enables users to have a direct link to the charging station with extra power and not rely only on the grid. Realistically, solar photovoltaic (PV) installations in the range of 100 kW to 500 kW can be done at the charging station or near the subgrid where the charging station is connected.
While the PV source can provide 500 kW, limiting the power requested from the grid down to 500 kW, the PV source is intermittent and not always present. This brings instability in the grid, as well as enabling the EV drivers to charge their cars at the fastest speed only when the sun is shining at its maximum. This is not what the users want, and this is not sustainable.
The missing piece of this power electronics puzzle is the ESS. Acting like the underground tank for the fuel in today's stations, the ESS can be represented as a big battery capable of storing and delivering energy from the renewable sources to the grid or to the charging piles or back into the grid. The first key characteristic of the energy storage unit is being bidirectional and working on the low voltage side of the grid. The new installations will be targeting a dc bus voltage of 1500 V dc linking the renewable sources, the EV charging piles, and the ESS battery. A proper sizing of the ESS also has to be done to make sure the balance between peak power and energy capacity is optimal for the specific installation. This ratio strongly depends on the size of the local power generation, being through solar, wind, or other sources, the number of charging piles, other loads connected to the subgrid, and the efficiency of the power conversion systems.