Regenerative systems using flywheels and hydraulics seem to work a
little better than battery-based systems, salvaging at least 70% of the brak-
ing energy. Figure 20.17 describes a hybrid car with a petrol engine powering
digitally-controlled hydraulics. On a standard driving cycle, this car
uses 30% less fuel than the original petrol car. In urban driving, its energy
consumption is halved, from 131 kWh per 100 km to 62 kWh per 100 km
(20 mpg to 43 mpg). (Credit for this performance improvement must be
shared between regenerative braking and the use of hybrid technology.)
Hydraulics and flywheels are both promising ways to handle regenerative
braking because small systems can handle large powers. A flywheel system
weighing just 24 kg (figure 20.18), designed for energy storage in a
racing car, can store 400 kJ (0.1 kWh) of energy – enough energy to acceler-
ate an ordinary car up to 60 miles per hour (97 km/h); and it can accept or
deliver 60 kW of power. Electric batteries capable of delivering that much
power would weigh about 200 kg. So, unless you’re already carrying that
much battery on board, an electrical regenerative-braking system should
probably use capacitors to store braking energy. Super-capacitors have
similar energy-storage and power-delivery parameters to the flywheel’s.
Hybrid cars such as the Toyota Prius (figure 20.19) have more-efficient
engines and electric regenerative braking, but to be honest, today’s hybrid
vehicles don’t really stand out from the crowd (figure 20.9).
The horizontal bars in figure 20.9 highlight a few cars including two
hybrids. Whereas the average new car in the UK emits 168 g, the hybrid
Prius emits about 100 g of CO2 per km, as do several other non-hybrid
vehicles – the VW Polo blue motion emits 99 g/km, and there’s a Smart
car that emits 88 g/km.
The Lexus RX 400h is the second hybrid, advertised with the slogan
“LOW POLLUTION. ZERO GUILT.” But its CO2 emissions are 192 g/km –
worse than the average UK car! The advertising standards authority ruled
that this advertisement breached the advertising codes on Truthfulness,
Comparisons and Environmental claims. “We considered that ... readers
were likely to understand that the car caused little or no harm to the envir-
onment, which was not the case, and had low emissions in comparison
with all cars, which was also not the case.”
In practice, hybrid technologies seem to give fuel savings of 20 or 30%.
So neither these petrol/electric hybrids, nor the petrol/hydraulic hybrid
featured in figure 20.17 seems to me to have really cracked the transport
challenge. A 30% reduction in fossil-fuel consumption is impressive, but
it’s not enough by this book’s standards. Our opening assumption was
that we want to get off fossil fuels, or at least to reduce fossil fuel use by
90%. Can this goal be achieved without reverting to bicycles?