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Using Stored Solar Energy to Meet the Demands of EV Fast Charging

Level 1 AC chargers operate at 120 V (2 kW maximum) while level 2 chargers operate at 240 V delivering up to 20 kW of power.

-By Jon Harper – Member of Technical Staff, Industrial SiC Discretes & Modules, onsemi

Smart ChargingThe mobility market is experiencing a transformation and as the rate of adoption of electric vehicles (EV) accelerates, sales forecasts are being continuously revised upwards. While representing a small fraction of the overall market, it is forecasted that up to 10 million EVs sold in 2025 and more than 50% of all vehicles sold will be electric by 2050. Most of these vehicles will be charged slowly while parked overnight on a driveway connected to a wall-box. Some will charge more quickly at street charging points, while superfast charging will become possible at the fuel stations of the future. With multiple charging points operating simultaneously, peak demand on the local power grid will be significant and without massive investment in transmission lines and power plants to provide the capacity to deal with these, localized grid collapse could become commonplace. In this article, we look at the current state of EV charging and consider the levels of power demand that it could create in the not-too-distant future. We then consider how that demand could be met in a practical, sustainable, and commercially viable way.

EV charging today

The AC charging infrastructure used in current public and private installations varies in the amount of power it delivers. Level 1 AC chargers operate at 120 V (2 kW maximum) while level 2 chargers operate at 240 V delivering up to 20 kW of power. In both scenarios, the AC to DC power conversion takes place in the charger on-board the vehicle rather than in the wall box (which largely performs protection and metering functions). Due to cost, size, and weight limitations, the on-board charger in a car is typically rated below 20 kW. Alternatively, if DC charging were used (instead of AC) charging could take place at much higher power levels. Level 3 DC chargers are rated up to 450 V (150 kW maximum), and more recent super chargers go up to 800 V (350kW maximum). For safety reasons the upper voltage is limited to 1000 V while the charging plug is connected to the vehicle. In DC charging, the power conversion is performed in the charging pile which connects directly to the car’s battery. This removes the requirement for a vehicle to have an on-board charger making it lighter and increasing the amount of available space.

Future demand

As more EVs appear on the road, drivers will expect to be able to charge their cars in shorter timeframes. Consider the following charging scenario which is likely to become a reality in less than 10 years hence. A roadside charging station has five dc charging piles when five cars stop at the same time to recharge one at each pile. If each car has a 100kWh battery that is already 25% charged and its driver wishes to charge it to 75% full in 15 minutes, then the total amount of power to be delivered from the grid to the charging station is:

5 * (75%-25%) *100 kWh/0.25h = 1MW

The grid supplying the charging station would require the capacity to manage these intermittent 1 MW peaks and this has several ramifications for the power delivery infrastructure. Highly efficient and complicated active power factor correction (PFC) stages would be required to ensure that the frequency of the grid is not affected and that it remains stable and efficient. Costly transformers would also be necessary to link the low voltage charging station to the higher voltage grid and the cables carrying power from the power plant to the charging station would require proper sizing to deal with the level of current being delivered. For vehicles with higher capacity batteries the peak power demand would be even greater.

Solar plugs the gap

A simpler and more economical solution, which avoids the requirement to install new transmission lines and large transformers, is to use the power generated locally from renewable sources like solar or wind. These are by their very nature, also intermittent but if managed carefully could be used to meet the intermittent demands placed on the grid created by EV charging. The price of solar photovoltaic (PV) technology has fallen by almost 80% over the last decade and this is contributing to the continuous growth in renewable energy systems which in turn is being driven by the requirement to reduce carbon emissions. Today solar power represents less than 5% of global electricity being generated but is expected to grow to more than one-third by 2050. The growth of solar power will impact on how electricity is produced and consumed – power stations will require to be managed in ways that ensure that the grid does is not oversupplied and people will increasingly consume electricity produced in residential solar systems installed at their own homes. This will create the requirement for careful balancing the supply of centralized mains power and localized renewable energy production, with variable customer demand. For our example charging station, by connecting it directly to a sub grid powered by a solar photovoltaic (PV) installation with the capacity to supply 500 kW, it would only require 500 kW to be supplied by the grid.

Smoothing out by backing up

However, using power from a PV installation would mean that the fastest charging speeds could only be achieved during daylight hours when the sun is shining at its brightest – an unsustainable proposition. A more realistic solution could be achieved by also using an Energy Storage System (ESS). These are the electrical equivalent of gas or oil storage tanks which can be used in multiple applications (domestic and industrial). In a domestic application, it would be straightforward to connect a PV inverter to a storage battery which is charged by sunlight during the day, and which could then be used to charge an EV overnight. In an industrial environment ESS installation could be used for different purposes – regulating power from PV and other renewable sources or to provide backup support for black start-ups, removing the requirement for diesel generators. The use of ESS also makes economic sense because they allow for existing transmission lines to be upgraded or replaced gradually over longer timeframes as the demand for faster EV charging grows. The market for these systems is projected to grow quickly from 20 GWh today to exceed 2000 GWh by 2050. For our charging station, an ESS would behave like a big battery capable of storing and delivering energy from a solar installation (or other renewable sources) to the charging piles as required, with any excess energy being delivered to the grid. The size of the ESS would be chosen to strike the best balance between peak power demand and energy storage capacity (with the ratio strongly depending on the amount of locally generated power available (solar, wind, or others), the number of charging piles, and other locally connected loads.

As EVs sales increase, drivers will expect it to become possible to charge their cars in shorter timescales and this means the demand for fast EV charging infrastructure will grow rapidly. A quick analysis has shown that the existing grid is not designed to cope with the intermittent peak demands that will result. Using a combination of solar PV installations and energy storage systems may represent a realistic and commercially viable alternative to the wholesale grid infrastructural overhaul that might otherwise be necessary.

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BiS Team

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