Page 147 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CH AP TER 6 .1 Battery/fuel-cell EV design packages

The cell type SL09B presently produced in the pilot In the ML1 cell, internal resistance is reduced to in-
production line has an open-circuit voltage of 2.58 V at crease power. The resistance contribution of the cathode is

300 C with a very low temperature coefficient of due to a combination of the ion conduction between the
3 10 4 V/K, a capacity of 30 Ah and an internal re- inner surface of the b-aluminium ceramic with the reaction
sistance that varies between 12 and 25 mW, dependent zone (80%) and electric conduction between the reaction
on temperature, current and rate of discharge. This var- zone and the cathode current collector (20%). The ML1
iation is because, during the charging and discharging has a cloverleaf section shape ceramic to enlarge its surface
process, the electrochemical reaction zone moves from area over the normal circular section, with resultant two-
the inner surface of the b-ceramic electrolyte into the fold reduction in cathode thickness and 20% reduction in
solid electrode. During this process the length of the resistance. Based on this form of cell construction a new,
sodium ion path and the current-density in the reaction Z11, battery has been produced with properties compared
zone increases and so the internal resistance increases. In with the standard design as shown by the table at (c) and
principle this effect is used to enable a stable operation of the battery is under development for series production.
parallel connected strings of cells. But from the vehicle
point of view the available power which is directly re- 6.1.2.5 Solar cells
lated to the internal cell resistance should not depend on
the battery charge status. The redesigned cell type ML1 According to Siemens, solar technology is a probable
is a good compromise between these two requirements. solution for Third World tropical countries. Solar
The battery is operated at an internal temperature range modules are available from the company to supply 12 V,

of 270–350 C. 100 Ah batteries from a 50 W solar module. The com-
The cells are contained in a completely sealed, double pany recently installed a system on the Cape Verde
walled and vacuum-insulated battery box as shown at Islands with a collective power output of 550 kW at each
(b). The gap between the inner and outer box is filled of five island sites. Even in Bavaria, the village of
with a special thermal insulation material which sup- Flanitzhutte, which has an average 1700 hours annual
ports atmospheric pressure and thus enables a rectan- sunshine period, has severed its links with the national
gular box design to be utilized. In a vacuum better than grid with the installation of 840 solar modules, with
1.10 1 mbar this material has a heat conductivity as low a total area of 360 square metres, to provide peak power
as 0.006 W/mK. By this means the battery box outside of 40 kW. Maintenance-free batteries provide a cushion.

temperature is only 5–10 C above the ambient tem- The technology of solar cells, Fig. 6.1-5, has been given
perature, dependent on air convection conditions. a recent boost by the Swiss Federal Institute of Tech-
Cooling systems have been designed, built and tested nology who claim to have outperformed nature in the
using air cooling as well as a liquid cooling. The latter is efficiency of conversion of sunlight to electricity even
a system in which high temperature oil is circulated under diffuse light conditions. The cell has a rough sur-
through heat exchangers in the battery with an oil/water face of titanium dioxide semiconductor material and is
heat exchanger outside the battery. By this means heat 8% efficient in full sunlight rising to 12% in diffuse
from the battery can be used for heating the passenger daylight. For more conventional cells, such as those
room of the vehicle. making a Lucas solar panel, these are available in modules

Fig. 6.1-5 Solar cell technology: (a) cell characteristics; (b) solar energy variation.

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