Charging Curve for Li-Ion Batteries
A typical charging curve for Li-Ion batteries reveals that the stored capacity increases linearly at a constant applied current during charging, and compared to other batteries, it charges very fast. A control unit regulates the applied current to avoid overheating, and at approximately 95% charge, reduce the current to balance out top charge.
A rapid charger for electric vehicles can deliver up to 60 kW, charging a car battery to 95% in less than 30 minutes. A residential charger, typically delivering close to 10 kW, will require six times longer to achieve this charge capacity.
Charging in Parking Garages
Earlier, the owner of parking garages would decide on the number of reserved spaces for electric vehicles, but new regulations dictate that 6% of the available parking spaces in car parks must be reserved electric vehicles - with charging capabilities.
Complying with these new regulations can be demanding, as shown by the following example:
A garage has 500 parking spots, requiring 30 spots reserved for electric vehicles - with charging. Each charging point has the capacity to provide 20 kW, allowing for fully charged cars at the end of a short workday. If all these charging points are utilized at the same time, a likely scenario in a typical office building in Oslo, the total power consumption reaches 600 kW, and this is practically impossible to achieve.
However, there are several solutions to this problem, although not all ideal. The simplest solution is to supply each charging point with less power, but this results in very slow charging times. Alternatively, prioritized charging can be employed, charging a limited number of cars at the same time; first come, first serve. This is unfortunate when a car is needed during the day and it has yet to be charged.
The last solution is to supply extra charging power by on-site Li-Ion batteries. These batteries can charge while the car park is empty and later assist in rapid charging of vehicles. If in the above example, we supplement the system with a 400-500 kWh Li-Ion battery pack, the batteries alone can deliver the first 20 minutes of required electric power to all charging points. After the vehicles has reached 95% capacity, the power required to charge further decreases and can be handled by regular means. Finally, as the required power decreases even further, the batteries can start to recharge.
Rapid Charging of Electric Busses
Several companies have initiated pilot projects with electric busses. Different strategies and battery capacities have been tested. Most common is operating with a 100 kWh capacity and have raping charging capabilities at end stations. On short routes, this setup enables busses to complete a round trip, even if one of the charging points are unavailable.
A significant cost associated with this is infrastructure. In the example given above, the end station would need to provide close to 25 kWh as fast as possible. Where infrastructure already exists, i.e. from tram or subway, this is possible to achieve, but at other locations, significant investments in infrastructure are needed. The solution is again storing electrical power in batteries on site to assist rapid charging, and a relatively small battery pack would be able to provide this power, and at the same time eliminating the need for a high voltage grid connection.
Often, busses make short, natural stops en route at mass transport hubs etc. A significant amount of energy can be transferred at these locations, if equipped with a battery-based charging system. This opens up the possibilities for new, fully electric, bus-routes, and busses with reduced battery capacity to reduce cost. Smaller onboard battery packs weigh less, increasing the possible pay-load.
Marine traffic is seeing the same electrification as land-based traffic. Ferries are an obvious candidate for such a transition towards electricity as the distances traveled are well known and planning of charging points are straight forward. Different strategies have been deployed here as well. Some ferries charge large battery packs overnight, supplementing with as much charging as possible when docked, others scale the battery pack to supply the required electrical power for the whole journey and rely on fully recharging the batteries while docking. Battery/diesel hybrids are also used, but are mostly considered as a step towards fully electrical ferries.
On the Brekstad-Valset crossing in Sør-Trøndelag, Fjord-1 will employ two electrical ferries, able to carry 50 cars, 6 trailers and a total of 200 people, with a battery capacity of 1 MWh. These ferries will charge at each docking.
Even though ferries have natural stops allowing disembarking and embarking of passengers and cargo, docking times are ideally as short as possible for time optimization, and fast charging times are therefore wanted.
Considering the above example: a ferry makes twelve crossings a day (in each direction) and for each crossing the battery consumes 60%. Ideally, it is docked a maximum of 15 minutes each time. The charging terminal will then have a daily consumption of 1 MWh x 0.6 x 12; approximately 7.2 MWh, requiring close to 300 kW on average over 24 hours. The peak consumption on the other hand, when the ferry is charging, is much higher; 600 kWh is needed for 15 minutes, equaling 2.4 MW. In this case, a 600 kWh battery pack would prevent the need to invest in expensive and extensive infrastructure.
Ferries are, of course, not the only maritime vessels suitable for electrification. Successful experiments have been conducted with electric fishing boats and electric fish farming vessels carrying fish from offshore fish farms to onshore facilities.
As a rule of thumb, the following applies:
-Scaling battery packs gets easier as the usage gets more routinely
- If short docking times are desired, battery packs to assist rapid charging are highly recommended
- If increased freedom is wanted, the battery pack could be complemented with a generator to form a hybrid system