PART 2: Would the depletion of fossil-fuel reserves and the ban on internal-combustion engines kill the car in its current form?
(See Part 1 here)
T HE automobile symbolises freedom and independence. Life without personal mobility is an unimaginable concept to many people. However, climate change, emissions and declining fossil-fuel reserves are clouding the future of our beloved form of transport. In part one of this series (December 2012), we looked at the options that automakers are considering regarding alternative fuels for the internal-combustion engine (ICE). This month, we remove the ICE from the equation to investigate the alternative forms of energy to power a car.
Batteries
Batteries store chemical-potential energy, which is converted to electricity in order to power the electric motor(s) of an electric vehicle (EV). Pure EVs are nothing new; they were actually preferred to ICE vehicles in the late 1800s and one EV even held the accolade of the first car to break the 100 km/h barrier (a La Jamais Contente driven by Camille Jenatzy in 1899). With the advancements of ICE and the invention of the electric starter motor, electric vehicles soon lost favour owing to their limited range, hefty weight and the charging time of the batteries.
Today, battery technology has improved immensely and has allowed automakers to revisit the EV. Unfortunately, energy storage is still the Achilles’ heel of EVs owing to inferior specific energy (Watt hours per kilogram) of batteries compared with fossil fuels (see table below). Although the latest developments in lithium-ion technology look promising, the higher the specific energy is pushed with new cathode and anode materials, the more unstable the batteries become (rendering them unsuitable to automotive use because of fire risk). That said, electric motors use energy far more efficiently than ICE systems (on average, around 90 per cent versus 35 per cent) while emitting zero local emissions.
Super Capacitors
Capacitors are devices in which two conductive plates are separated by an insulator. Applying a direct-current (DC) voltage across the plates results in a positive and negative charge in respective plates that create an attraction force. This stored potential energy can then be harnessed as an electric current.
The capacitance (capacity) of a capacitor is measured in Farad. The term super capacitor describes a large-capacity unit owing to its increased plate areas and dual-layer construction. Even the most advanced super capacitors have limited energy density (see table on page 107) compared with petrol, which limits their use as an absolute energy-storage device in vehicles. Other factors that count against super capacitors include high self-discharge rates and extreme voltage drops during discharge. Their advantages include quick acceptance and discharge of energy. Therefore, super capacitors can be used as a kinetic energy-recovery system (KERS) and supply an energy-boost function when needed.
Fuel Cells
These release energy in the form of electric current following a chemical reaction between hydrogen and oxygen (water is a by-product of the process). The energy efficiency of a fuel cell can be as high as 80 per cent if the heat generated during the chemical process is captured for use. However, currently the efficiency is considerably lower in automotive use. A fuel-cell system’s energy density is significantly greater than that of lithium-ion batteries (see specific energy table), which improves range and performance over battery-powered EVs. The main advantage over lithium-ion batteries is that no charging is needed when the energy is depleted; a quick stop at a hydrogen filling station to replenish the hydrogen (similar to a normal fuel stop) will do the trick. The problem remains that transportation, storage and infrastructure of hydrogen is problematic, which renders this fuel-cell solution less than ideal.
Solar
Solar panels (photovoltaic type) convert solar energy into electric current. As this energy is free and environmentally friendly, why do carmakers not cover vehicles in solar panels? The reasons are weight penalties, cost and little energy produced per surface area.
Let’s focus on the last point: how much energy is available per square-metre with today’s solar-panel technology? Solar energy varies around the world (higher at the equator than poles) and is very dependent on the time of day, clarity of the sky and other atmospheric conditions. A best value of 1,0 kW/ m2 is assumed on the surface of the Earth perpendicular to the Sun’s rays on a clear day. Current solar-panel efficiencies range between 10 and 15 per cent, with talks of higher efficiencies possible at increased cost. A 15-per-cent efficient, one-square-metre solar panel can extract 150 W (simplified) during the best conditions and only for a short time during the day. Charging the 24 kWh battery pack of a Nissan Leaf (with a roof-mounted panel that measures two square-metres) would take at least 80 hours of perfect sunlight.
Considering only energy density as a factor, it is clear that a large surface area is needed to capture any meaningful energy from the sun. This can be achieved by covering the roofs of buildings and then supplying electric vehicles with this generated electricity.
Future Sources
Nuclear power would have been the perfect solution to power a vehicle were it not for the size and weight of the reactors, and the obvious safety risks. The reason is that the nuclear-energy content of uranium-235 approaches 80 million MJ/kg. This dwarfs the energy content of fossil fuels: 1 gram of uranium-235 contains as much energy as roughly 2 353 litres of petrol.
Summary
It is clear that there is no perfect alternative-energy source to power vehicles in the near future. Converting renewable energy (and even nuclear) into electricity appears to be the most elegant solution. However, storing the energy in an electric-powered vehicle remains the biggest obstacle. Battery technology still carries too much of a weight and cost penalty. Maybe the inventors of Scalextric toys in the 1950s already found a solution to our problem today? Can inductive circuits under motorways recharge our EVs on long-distance journeys and eliminate the need for large batteries? We’ll eventually find out.
Read about fuel here, in part 1 (click).
