Lightweight Electric/Hybrid Vehicle Design

112 Lightweight Electric/Hybrid Vehicle Design
The technology of solar cells, Fig. 5.5, has been given a recent boost by the Swiss Federal
Institute of Technology who claim to have outperformed nature in the efficiency of conversion of
sunlight to electricity even under diffuse light conditions. The cell has a rough surface of titanium
dioxide semiconductor material and is 8% efficient in full sunlight rising to 12% in diffuse daylight.
For more conventional cells, such as those making a Lucas solar panel, these are available in
modules of five connected in series to give maximum output of 1.3 watts (0.6 A at 2.2 V). Some
ten modules might be used in a solar panel giving 13 watts output in summer conditions. Power vs
voltage and current vs voltage are shown at (a) for so-called ‘standard’ and ‘typical’ operating
conditions. 100 mW/cm 2 solar intensity, 0° C cell temperature at sea level defines the standard
conditions against 80 mW/cm 2 and 25°C which represent ‘typical’ conditions at which power
output per cell drops to 1 W. Temperature coefficients for modules are 0.45% change in power
output per 1° C rise in temperature, relative to 0° C; cell temperatures will be 20° C above ambient
at 100 mW/cm 2 incident light intensity. Variation of solar energy at 52° north latitude, assuming a
clear atmosphere, is shown at (b). On this basis the smallest one person car with a speed of 15 mph
and a weight of 300 lb with driver would require 250 W or 50 ft 2 (4.65 m 2 ) of 5% efficient solar
panel – falling to 12.5 ft 2 (1.18 m 2 ) with the latest technology cells. A 100 Wh sealed nickel–
cadmium battery would be fitted to the vehicle for charging by the solar panel while parked.
The future, of course, lies with the further development of advanced cell systems such as those
by United Solar Systems in the USA. Their approach is to deposit six layers of amorphous silicon
(two identical n-i-p cells) onto rolls of stainless steel sheet. The 4 ft 2 (0.37 m 2 ) panels are currently
6.2% efficient and made up of layers over an aluminium/zinc oxide back reflector. The push to yet
higher efficiencies comes from the layer cake construction of different band-gap energy cells,
each cell absorbing a different part of the solar spectrum. Researchers recently obtained 10%
efficiency in a 12 in 2 (0.09 m 2 ) module.
Rapid thermal processing (RTP) techniques are said to be halving the time normally taken to
produce silicon solar cells, while retaining an 18% energy conversion efficiency from sunlight.
Researchers at Georgia Institute of Technology have demonstrated RTP processing involving a 3
minute thermal diffusion, as against the current commercial process taking 3 hours. An EC study
has also shown that mass production of solar cells could bring substantial benefits and that a £350
million plant investment could produce enough panels to produce 500 MW annually and cut the
generating cost from 64 p/kWh to 13p.
E F
Accel sensor
Fig. 5.6 Overall system configuration.
Remaining capacity
Vehicle
control unit
Battery power limit
Motor Inverter Relay
box
Charger
V
Battery
controller
A
Cell controller Cell controller
modules (8 cells)
T T
battery pack (12 modules)
signal line
power line
Cooling fan
T: temp. sensor
V: voltage sensor
A: current sensor