Truck turns its own heat into power

May 25, 2016

A 195-year-old discovery is behind a new system that will save vehicles hundreds of litres of fuel and reduce their carbon emissions by as much as 2 to 3 tonnes per year.

Working with automotive manufacturer Scania, researchers from Sweden's KTH Royal Institute of Technology have been testing semi trucks equipped with a system that converts exhaust heat into power—through a process called thermoelectric generation (TEG). The voltage produced by the system can power the truck and reduce the strain on the engine, explains researcher Arash Risseh.

The TEG system operates on the principle of the , by which differences in temperature are converted into voltage—a phenomenon discovered in 1821 by German physicist Thomas Johann Seebeck, and often referred to as the "Seebeck effect". 

"Most fuel energy is not used to drive a truck forward," Risseh says. "Some 30 percent of this unused energy is lost as heat from the exhaust pipes."

A truck that generates 440kW would see about 132kW of energy disappear in the form of heat coming out of the exhaust pipes, he says. "That's enough to power a typical passenger vehicle."

Capturing this excess energy takes a load off the truck's generator, and in turn, the engine, Risseh says. That means better fuel efficiency and lower emissions.

The Seebeck effect requires a temperature differential—cool on one end of the circuit and hot on the other, which means a truck must rely on a coolant in order to stimulate the voltage. Cooling the circuit is easier with natural alternatives, such as seawater for a ship's engines. Ships also make good candidates for TEG because their buoyancy offsets the constraints of weight and volume that road vehicles face, he says.

TEG is also regarded as a potential saver in data centres that are located in cold climates. Near the Arctic circle in northern Sweden, a data centre that uses 1 Terawatt hour per year could potentially recover 1 Gigawatt per year—a savings of some EUR 100,000, he says.

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5 / 5 (3) May 25, 2016
A liter of diesel fuel emits about 2.68 kg of CO2 which means a savings of 1,000,000 kg of CO2 emissions corresponds to 373,000 liters of diesel.

A typical truck uses between 30-60 liters of fuel per 100 km, so the supposed savings should be equivalent to nearly a million kilometers of driving per truck, which would be impossible to drive in a year. It would be over 2,000 km per day every single day of the year, which at a top speed of 80 kph would take 27 hours to drive non-stop.

So the figures posted are complete and utter bullshit.

In reality, "hundreds of liters" of fuel savings, let's say 500 liters per year, is on the order of 1% in fuel savings assuming the truck does 100,000 km/year, which isn't a big surprise because the practical conversion efficiencies of TEGs are in the single percentage points themselves.

Every few years someone gets the bright idea to strap a TEG to an engine tailpipe, and every time it turns out not worth the money.
5 / 5 (2) May 25, 2016
BMW had a better idea in the 90's using an organic rankine cycle heat engine to operate on the exhaust heat of an ordinary car. Basically a small turbine that runs by evaporating a fluid on the hot exhaust manifold at 200 C.

The trouble was, how to integrate the extra power back into the drivetrain, and that proved to be the costly bit.

They're still at it:


With a promise of 10% fuel savings at highway speeds. Much better than the thermoelectric generator.
not rated yet May 26, 2016
Probably the number of 1000 tonnes is a mistake in the article since "hundreds of liters".
I have worked with TEG and I can tell you that its much easier to handle than ORC of several reasons. One of those is the problem with the time constant as it was mentioned by Eikka but there are more than that and that is why we don't have any ORC in vehicles yet. I believe that TEG has more potential than ORC, at least in vehicles especially with the new thermoelectric material with high ZT why they has to be tested in a complete system.
not rated yet May 26, 2016
I believe that TEG has more potential than ORC

The major problem with the ORC is the need for complex mechanisms to turn the heat into mechanical power, and it's greatly simplified if you just turn it into electricity in a hybrid car. It works like a combined cycle powerplant.

The major problem with TEGs is the abysmal thermal efficiency of the technology, and the issue of keeping the elements cooled, which requires a very complex and costly heat exchanger built along and within the TEG stack to maintain high dT across the elements to keep what little efficiency they have.

The ORC hasn't got such a problem because it's basically a steam engine - it can use a bog standard condenser much like a standard intercooler already found in trucks and cars.

The major impediment to both is the cost, because for a 1-10% improvement in fuel economy you increase weight, cost and complexity (maintenance) of the vehicle more than the cost of the fuel you save.
not rated yet May 26, 2016
There are no need of complex heat exchanger, and if that would be the case, a large part of the cost is due to the design and thereby a once-cost which can not be compared to the cost of an ORC and its maintenance during the vehicles entire lifetime. Other issues with the ORC are the weight and a complex control system which is extremely difficult to design and implement. Automotive industry prefer the simplest and the most reliable solutions of obvious reasons. Therefore, a simple, functional and implementable TEG is a better solution even it generates lower power than an unimplementable OCR with higher power.

And we have to realize that there is no more room to waste so much energy as we do today! Something has to be done even if it costs and gives back a small amount of energy.

not rated yet May 27, 2016
There are no need of complex heat exchanger

Then how do you get the cold to the other side of the TEG elements that are stuck in and around the exhaust system? The ORC simply boils liquid on the hot exhaust manifold and that's it, whereas the TEG has a complex layering of insulation and wiring and channels around the manifold to stop heat leakage around the TEGs.

and if that would be the case, a large part of the cost is due to the design

You also have to manufacture and assemble it.

and thereby a once-cost which can not be compared to the cost of an ORC and its maintenance during the vehicles entire lifetime.

Depends. At its simplest, the ORC is a self-starting sealed system with a turbine and generator integrated on one shaft that runs whenever there's heat in the tailpipe. There's not much to maintain except re-pressurize it with fluid once in a while like you'd do on the AC. The turbine itself should last the lifetime of the vehicle
not rated yet May 27, 2016
Other issues with the ORC are the weight and a complex control system which is extremely difficult to design and implement.

Again, at its simplest the ORC is like a second alternator in the car. When the exhaust manifold gets hot, the alternator starts cranking out power. In hybrid cars that just goes straight into the battery.

The difficulty with implementing one in a non-hybrid vehicle is what stopped BMW from putting it into production.

And we have to realize that there is no more room to waste

There's much more difficulty in fitting the TEG and the required insulations and heat exchangers around the exhaust manifold than fitting a simple coil boiler for the ORC fluid.

The benefit of the ORC is that the hot and cold sides of the system are physically separated by the piping, whereas the TEG is all compressed into 1/4 inch ceramic plates. You have to bring the cooling medium - air or liquid - right in around the exhaust manifold space to make it work.

not rated yet May 27, 2016
Something has to be done even if it costs and gives back a small amount of energy.

If it costs too much and gives back neglible savings, it will consume more energy indirectly than it saves, because money = economic activity = use of energy and material resources.
not rated yet May 27, 2016

The problem of economic cost is more pronounced in the first world countries, because there's so little primary production still left (~2%), and nearly all economic activity is in the services sector (~80%) which generates money by consuming primary goods and energy.

So when you pay a dollar more to save a dollar's worth of gasoline, you're almost entirely sure to be spending that dollar's worth of gasoline elsewhere in order to make that extra dollar.

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