Electric Vehicles, Renewables and the Grid
Operating the electrical grid is a constant balancing act between supply and demand. In order to keep voltage and frequency within specifications, careful oversight is required. Since demand is highly variable throughout the day and year, a grid operator faces the challenge of matching supply to everchanging demand. While steps can be taken to reduce peak demand, such as variable pricing, the primary means of grid control is on the supply side. Fundamental to the operation of the electrical grid is the ability to increase or decrease the amount of energy coming from generating assets.
Historically, the production of electricity is performed by converting various forms of potential energy to mechanical energy before using a generator to output electricity. From nuclear to coal, this type of generation allows for throttling. In the case of fossil fuel fired power plants, this is done by adjusting the amount of fuel being burned. In the case of nuclear, the reaction rate is altered to change the power output. Some renewable energy sources, such as hydroelectric, can easily be throttled but most cannot.
Since the grid operator does not control when the sun shines, or when the wind blows, it is generally tricky to modulate the output of photovoltaic (solar) panels and wind turbines. Instead, these variations in output must be dealt with in much the same way that fluctuations in demand are: by throttling other power generation assets up or down to compensate. As renewables become more prevalent, it is becoming increasingly difficult to maintain balance on the grid. Furthermore, this phenomenon reduces the efficiency of conventional power plants by preventing them from operating at steady state.
A solution to this problem is to temporarily store energy so that it can be dispatched when required rather than when generated. This practice is known as peak shaving. Storage can take various forms, the most well-known of which is the battery. Grid scale batteries are a relatively new concept, providing an energy reservoir that can be tapped during peak electrical demand and recharged during times of excess generation. However, these massive battery packs are financially and environmentally costly to build.
Enter the electric vehicle (EV)! EVs are effectively rolling battery packs. As electric vehicles achieve mass adoption, a prodigious number of relatively large-scale battery packs could be made available for peak shaving. Specifically, EVs contain high power-density batteries, capable of discharging and charging rapidly, making them ideal for peak shaving. However, until recently, there was no way to get the energy from an EV’s battery pack back into the grid.
Vehicle to grid (V2G) chargers offer a crucial advantage over conventional chargers: they allow the bidirectional flow of electrical energy between an EV and the grid. When charging the EV, they operate as a conventional charger. When providing power to the grid and discharging the vehicle’s battery, a V2G enabled charger acts as a high-power grid-tie inverter; it inverts the battery’s direct current (DC) output into alternating current (AC) that matches the frequency, voltage and phase of the electrical grid. This technology is especially advantageous for fleet vehicles which often adhere to a rigid schedule of use and charging.
A couple of challenges remain. Both the vehicle and charger must be V2G capable in order to function in this capacity. As of my writing, the Nissan Leaf is the only V2G capable passenger car available in the US. However, almost every major car manufacturer has plans to include this capability in upcoming models. Additionally, the discharging of the EV’s battery for the purpose of peak shaving will result in additional load cycles on the battery pack, theoretically reducing its life. Careful software and hardware design can likely mitigate much of this drawback. Furthermore, as both V2G and renewables becomes more commonplace, utilities will likely offer incentives to EV owners willing to allow their cars to be utilized in this capacity.
Starting next year, Block Island will be at the forefront of this new technology. The electric school bus and charger being purchased by the Block Island School, with funds donated by the Solar Initiative, are both V2G capable. This means that the bus will serve as both as a means of transportation as well as a peak shaving battery pack that will help support renewable energy on the grid.