Energy harvest ‘feeds’ road vehicles

New routes to cutting fuel consumption

Recovering energy can offer attractive solutions for providing additional power to motor vehicles at the same time as cutting their fuel consumption and emissions. They rely on a number of systems that recover thermal, kinetic, or other forms of energy (such as solar) that would either be lost or not used in vehicles. 

Thermoelectric generator (TEG) installation in exhaust system of BMW saloon
Thermoelectric generator (TEG) installation in exhaust system of BMW saloon

Use extends increasingly beyond small devices

Energy harvesting is no longer perceived as limited solely to powering small devices such as sensors for Internet of Things (IoT) and wearable or medical devices. New uses are emerging in demanding energy-intensive sectors such as road transport, in particular when associated with innovative or improved storage systems.

In spite of greatly improved fuel consumption, internal combustion engines (ICEs) are still inefficient, wasting 55-65% of the thermal energy of the fuel they burn.

Various forms of energy recovery can improve significantly the overall efficiency of road vehicles, making them less dependent on fossil fuels and cutting emissions of noxious gases.

Urban public transport offers the greatest potential for energy recovery. Not only can it provide additional power but in some cases can even replace the use of fossil fuels entirely.

The same applies to private vehicles, albeit to a lesser extent. 

Multiple forms of recovery

Various sources of energy can be recovered to power road vehicles, or some of their systems. The sources, some of which would otherwise be wasted, include:

● Kinetic energy, recovered through regenerative-charge braking and energy-harvesting shock absorbers. It can be converted into electric power used in full or hybrid electric vehicles (EVs) to charge the batteries and capacitors/ supercapacitors that provide extra power and/or support functions such as start-stop or short electric drive. The kinetic energy from regenerative-charge braking and energy-harvesting systems can also be stored in flywheels for near instantaneous redirection to a powertrain or for powering electric motors.

● Heat recovery from exhaust gases is another interesting solution that can improve overall vehicle fuel economy and cut noxious gas emissions from ICEs. Energy from the hot engine exhaust, which would otherwise be wasted, is converted into electrical energy using thermoelectric generators (TEGs). This additional energy can be used to power the growing number of accessories, such as onboard communication and navigation systems, which rely on power provided by engine-driven alternator/generators and increase fuel consumption. The efficiency of TEGs is currently not very high (around 5%), but improvements are on the way and efficiency is expected to increase to around 15% as materials advance. TEGs can also be used to transfer waste heat to either heat or cool (the latter through heat transfer) the engines and occupants of cars. Racing cars have been using motor generator unit-heat (MGU-H) for some time, recovering energy from exhaust heat to power other systems including the motor generator unit-kinetic (MGU-K), which converts kinetic energy generated under braking into electrical energy (rather than letting it be dissipated/wasted as heat). TEGs rely on the use of semiconductor devices; International Standards for these are being prepared by IEC TC 47: Semiconductor devices.

● Energy harvested from the sun also offers attractive possibilities. Experimental vehicles that draw all their energy from the sun, such as the Nuon Solar, have already proven that this technology is viable. More recently, a leading Japanese car maker has introduced a new model of one of its hybrid EVs with a rooftop photovoltaic (PV) option that provides additional power. According to a report in PV Magazine, the “PV panels also supply power to the traction battery while the vehicle is parked, providing enough of a charge to drive up to a maximum of 6,1 kilometres per day, or an average of 2,9 kilometres (…) In addition, the solar panels generate electricity for the vehicle’s lights, power windows and air conditioning systems”.

Thin-film flexible PV panels present interesting opportunities for the fitting of PV energy systems to road vehicles. IEC TC 82: Solar photovoltaic energy systems, develops “International Standards for systems of photovoltaic conversion of solar energy into electrical energy and for all the elements in the entire photovoltaic energy system”. 

Recovered energy must be stored, somehow…

Energy recovered from heat, kinetic or solar sources needs to / can be stored in chemical, electrostatic, or kinetic form to be delivered either nearly instantaneously or later.

Secondary (rechargeable) batteries are the most mature, widespread and best-known energy storage (ES) system for automotive applications. They were first introduced in lead-acid chemistry in the 1860s. In batteries, electrochemically-active material is used to store electrical energy. In addition to lead-acid, other battery chemistries, such as lithium-ion and nickel-based ones, are finding applications in full or hybrid EVs.

IEC TC 21: Secondary cells and batteries, prepares "product standards for all secondary cells and batteries, irrespective of type or application. The requirements cover all aspects depending on the battery technology such as: safety installation principles, performance, battery system aspects, dimensions, labelling. All electrochemical systems are considered".

Another useful ES system in automotive applications relies on capacitors, which store electrical energy electrostatically on the surface of the material rather than chemically as batteries do. Capacitors can capture energy over a very brief period, such as during braking phases, and release it quickly to boost power or for other uses. In supercapacitors (or double layer capacitors), the electrostatic charge is stored in an electrochemical double layer (see article on capacitors in e-tech June 2014). IEC TC 40: Capacitors and resistors for electronic equipment, prepares International Standards for these.

Kinetic energy that would be lost as heat during braking can also be recovered and stored in mechanical form by accelerating a flywheel via continuously variable transmission (CVT). This energy can be released back to the powertrain by the CVT upon acceleration.

Flywheels are spun at very high speeds, sometimes in excess of 60 000 rpm, and are contained in robust casings in case of failure. This so-called kinetic energy recovery system (KERS) was introduced first in racing cars, but is now being tested for hybrid production cars, notably by a renowned Swedish car manufacturer, which claims that the Flybrid system being tested in one of its models under development delivers up to 30% more power than conventional equivalent models, with a 25% boost in fuel efficiency. Since 2014, some 500 London buses have been equipped with GKN Gyrodrive hybrid power flywheel systems, resulting in fuel savings of over 20% over a two-year period and significantly cutting bus pollution.

Some or all of these energy recovery and storage processes and systems are set to be installed into further full and hybrid EV cars  as well as in vehicles that rely mainly on ICEs for propulsion. They all rely to a significant extent on the International Standards being developed by a number of IEC TCs.