Battery Swapping to Ease G2V and V2G Operations
- Written by Md. Mahmud Hasan and Hussein T. Mouftah
The long idle time of electric vehicles (EVs) during charging and discharging does not comport well with future scenarios in which EVs play a big role. Though fast charging is a proposed solution to this problem, it has some drawbacks such as drawing excessive current and harming battery lifetime. Battery swapping is a promising solution.
Battery driven EVs are expected to be one of the major power consumers in a smart grid, eventually replacing conventional gasoline driven vehicles completely. For short drives, EV batteries can be easily charged at home outlets. But for longer distances, EV batteries require much longer charging time than conventional vehicle refueling, and during charging, the EV has to be idle.
There are two basic modes of charging: AC and DC. Whereas AC is slow—the battery may take several hours to reach full charge—DC is faster. Its charging time can be as low as thirty minutes, drawing around four hundred amperes of current. As current is the rate of change of electric charge with respect to time, faster charging means higher amperage. And so if a significant number of EVs are charging simultaneously in the same distribution network, there can be a sudden surge in electricity demand. That dilemma will limit the number of quick charging outlet in future grids. Another important constraint is that faster charging makes an EV battery less durable—its lifetime will be shorter.
Battery swapping represents an alternative solution that can drastically reduce EV idle time. In this scenario, discharged batteries are immediately replaced by fully charged ones. This makes the idle time negligible. In a battery swapping facility, EV batteries are charged and stored at charging stations. Low-charge EVs come to charging stations to replace their batteries. Charging stations then collect fully and partially discharged batteries. Those batteries are later recharged for future use.
Although one of the earliest battery swapping services, the company Better Place’s, was commercially unsuccessful, there are many successful implementations in operation around the world. (It may be that Better Place’s failure resulted from the relatively small size of the markets—parts of Denmark and Israel—in which battery-swappable EVs were deployed.)
In China, a number of companies are providing battery swapping services, and they are gradually expanding service coverage between major cities. Mainland China (the People’s Republic) has the highest number of battery swapping stations in the world. In Taiwan, battery swapping facilities are available in two major cities, New Taipei and Kaohsiung. Kentfa Corporation, a Taiwanese company has introduced vending machines for electric scooter battery exchange.
The European Union’s parliament recently announced a standardization plan for EV refueling infrastructures that includes battery swapping stations; in Slovakia, Greenway Operator has achieved commercial success in battery swapping business. Last year, Tesla Motors came up with a battery swapping technology that beats conventional vehicle refueling time; it plans to launch battery swapping stations in California.
Battery swapping has many advantages—not all of them obvious at first glance—for both EV owners and the utilities serving them, but also some serious implementation issues.
To start with the benefits for vehicle owners, in the absence of swapping they currently have to put up with long battery charging times and sometimes excessively long queues (lines) at charging stations. Such annoyances send discouraging signals to prospective EV owners.
Utilities, for their part, can expect to see improved profit margins with battery swapping. Utility-owned stations can charge batteries during off-peak hours and store them to help manage peak demand. While acquiring energy from EVs is a commonly proposed peak demand management strategy, EV owners are known to be interested in selling energy from vehicles only when it is quite profitable to them. Swapping reduces the amount of EV-stored energy that utilities would need to purchase at high prices to meet peak-load needs.
Battery swapping can also enhance reliability in peak demand management. EV-stored energy will not always be available at prices mutually agreeable to owners and utilities when power needs to be discharged from cars in real time to meet the grid’s needs. Therefore it will not always be possible to acquire the amount of battery power needed; battery swapping reduces the utility’s dependency on EV owners.
As vehicle-to-grid (V2G) operations are conventionally conceived, EVs are required to be idle to provide ancillary services to the grid. In the case of selling energy, discharging of EV batteries takes a considerable amount of time. In those operations, commonly proposed scenarios include a so-called grid access points where EVs are plugged-in. So EVs have to travel some distance to reach the access points. Once there, the EV cannot sell its entire charge—it always requires a minimal amount of charge to return to quarters. And of course the EV needs to be idle at the access point until a certain amount of battery discharge occurs. The idle time can be more costly to the car owner than the profit anticipated from the sale of power to the grid, and this consideration may reduce the number of interested ancillary service providers.
Battery swapping minimizes idle time and reduces discharging costs, benefiting both vehicle owners and utilities. Mutually beneficial cost reductions also arise from utilities’ recharging batteries at times of low load, and from improved efficiencies.
V2G operations have significant overhead due to communications, security and computational functionalities. They consume power to support processing and memory. EVs must communicate with electric utilities to schedule V2G transactions. Security mechanisms are deployed to maintain confidentiality, integrity and availability of their exchanged messages. A number of computations are required to attain optimality of operations. Battery swapping can reduce V2G overheads significantly.
For all those benefits to be realized, implementation issues must be faced squarely, among them:
- EVs from different manufacturers might have different battery requirements. Battery performance can also vary with manufacturer. These factors can affect implementation of battery swapping technologies. This is why replaceable batteries need to follow a standard like those for EV power plugs. Standardization of EV batteries will bring a higher degree of compatibility.
- Virtually, battery swapping is equivalent to buying a new battery. Pricing of batteries depends on a number of technical and market factors. It is assumed to be dynamic and competitive in a smart grid scenario. For example, an EV battery is not always fully discharged at the time of its replacement by the utility. An EV owner should receive compensation for residual charge. In energy selling, an EV owner should receive compensation for the sold portion of the charge and a minimally charged battery. Another important consideration is battery lifetime. Each EV battery has an estimated lifetime in terms of the number of charging-discharging cycles. Residual lifetimes of the exchanged batteries should be taken into account for pricing. Non-linearity of charging curve, charging instance and residual lifetime are three major factors to be considered in pricing.
- The excessive weight of EV batteries is a discouraging factor in battery swapping. Specialized equipment is needed to handle the heavy batteries at swapping facilities. An advanced electric car requires a fully-charged lithium ion (Li-ion) battery pack of 250-400 kg to travel 100 miles. There exists a trade-off between range and lightness. The improvement of the range-to-weight ratio is an open research problem. Lighter EV batteries are more feasible for swapping. In Taiwan, light-weight batteries are available for electric motor cycles and scooters.
In spite of such issues, battery swapping will ease grid-to-vehicle (G2V) and V2G operations. There are individual implementations and initiatives in progress in some parts of the world. It only requires our combined efforts to move toward large-scale deployments.