IEEE Electrification Magazine - December 2013 - 11

require 1 kW of power for 1 h. however, if we wanted to
charge the same battery in 6 min (ten times faster), it would
require 10 kW for 6 min. this example may not mean much
to someone not used to computing power consumption,
but to a grid operator, the peak power required to accommodate recharging large batteries rapidly means knowing
there is enough grid capacity at the location where charging is to occur so as to accommodate the charging without
overheating the nearest transformer. For a light vehicle
with a relatively small battery pack, the problem is minimal. however, for a vehicle with a larger-capacity battery
pack, the problem can be serious and costly for the grid
operator and, therefore, the ratepayer.
as an example, the model s from tesla motors is an
attractive, high-performance all-ev with a high-capacity
battery pack (either 60 kWh or 85
kWh). consider that if you want to
charge the battery to one-half of its
capacity in 30 min, the power required
to charge at that rate (assuming 100%
efficiency) is 60 kW (60 kWh/2*2 = 60
kW). this amount of power needs to
be carefully considered based on the
capacity of the grid's nearest transformer, the time of day, and other local
usage. however, if you want to charge
the battery to 100% of its capacity
overnight, it would be a more moderate charging rate, which would be
handled much more easily by the electric utility. in fact, charging over 8 h
(overnight), when grid usage is lower
and transformers are generally cooler,
reduces the power requirement and
the risk of transformer damage considerably.

charging considerations
there are several considerations for charging plug-in vehicles that impact the power required of the grid. First
among them is the time of day when the battery is
charged. in general, if vehicle batteries are charged
between the hours of 10 p.m. and 6 a.m., when grid usage
is at its minimum and local transformer temperatures are
lower, they have the least negative impact on the grid.
charging during peak grid usage may require upgrades to
the grid to handle peak demand that is higher than is
experienced today. second, chargers should be designed
with a nighttime mode, to draw only enough power to
charge the battery in the 8-h nighttime period, based on
the known state of charge of the battery. charging at
night, when usage is low, improves the economics of the
utility grids, potentially reducing the
cost of electricity. third, with the current technology, optimal battery
usage suggests plug-in hybrid and
range-extended evs are of great interest because of their high energy and
power efficiency. all-electric propulsion is exceptionally more attractive
when employed for short local trips,
while hybrid electric propulsion is
more practical for longer-range trips.
this strategy reduces petroleum
usage dramatically (often 50-70%,
depending on usage patterns) and
optimizes the usage of electric local
travel without prompting high gridupgrade costs. it optimizes petroleum
usage, electric efficiency, battery
capacity, and grid power demand-
the key elements in an electrified transportation system.
some 85% of trips are 20 mi or shorter in the united states
and, thus, would consume only electricity, with petroleum
available for longer trips. With this configuration, the battery size can be constrained to 8 kWh on a traditional
vehicle, providing 20 mi of range, or 6 kWh, providing the
same 20 mi of range, on a vehicle weighing 2,000 lb or less.

Developing the right
mix of elements to
appeal to the senses
and emotions of
customers is as
important as
engineering the right
blend of power
during acceleration.

Plug-In range-extended Alternative
the chevy volt, with a 16-kWh battery pack, has a roughly
38-mi electric range under good conditions before a rangeextender engine starts generating electricity for longerrange travel. the range-extender feature is intended to
counteract the fear of running out of electricity, sometimes
referred to as range anxiety. this strategy accommodates
local electric driving with the ability to travel longer distances by burning petroleum. it is aimed at a customer
seeking all-purpose usage within one vehicle. volt customers
report that they fill their gas tanks about once per month or
even once in several months, so petroleum fuel consumption
is minimal. the battery capacity is noticeably smaller than
that of the model s and the car is less than half the price, but
less often noticed is the demand on the electrical grid. the
volt can be charged at night, demanding less than one
house's daytime peak energy requirement, and is therefore
much more accommodating of the grid. in fact, if these cars
were charged only at night, no upgrades would be required
of the grid in most places for quite some time.

Sensible design Strategies
sensible design strategies using current technologies suggest that vehicles should provide a balance of efficient
energy usage for each type of drive cycle while minimizing the impact on the energy source, whether electricity
from the grid or petroleum from the pump. electricity consumed from the grid is typically 10-20% of the cost of
petroleum and offers considerable improvements in air
quality, so it should be the primary source of energy for
transportation where practical. at the current state of
technology, petroleum is still the primary source of energy
for long-distance travel and will likely remain so for a
decade or more; hevs offer the best powertrain options
for long-distance travel.

IEEE Electrific ation Magazine / d ec em be r 2 0 1 3

Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2013