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HeatReCar project demonstrates technical feasibility of thermoelectric generator for waste heat recovery; economic case more difficult

A recently completed European project coordinated by Centro Ricerche Fiat (CRF) demonstrated the technical feasibility of a Bi2Te3-based thermoelectric generator (TEG) for waste heat recovery for application to a diesel light-duty truck (LDT). The project “Reduced energy consumption by massive thermoelectric waste heat recovery in light-duty trucks” (HeatReCar) focused on thermoelectrics to provide electricity, either to on-board components or to the power train of hybrid electric vehicles. Reduced fuel consumption for these purposes translates to emissions reductions.

TE materials have been employed previously in automotive applications but have not achieved reasonable conversion efficiencies. The researchers tackled this issue in two ways. They selected bismuth telluride (Bi2Te3) suitable for lower operating temperatures in a diesel engine. They also optimized the geometry of heat transfer surfaces to maximize the temperature difference available to the TE modules. The technology was implemented in a prototype TE generator (TEG) for a diesel IVECO Daily light-duty truck (LDT) in common use in the EU.

Performance of TE materials was increased by more than 20% through ball milling and subsequent spark plasma sintering (SPS). Sintering is a standard method for consolidating powders without melting them. Compaction is achieved by sintering at high temperatures compared to the melting point of the processed materials. Spark plasma sintering uses an electric current to heat the sample and to activate sintering.

The initial HeatReCar base case was a TEG producing 1 KWe at 130 km/h. At this power level the potential for CO2 emissions reduction ranged from 50 g CO2/km in real traffic at 30 km/h (18.6 mph) average city speed to 10 g CO2/km on the motorway at 130 km/h (81 mph).

Test bench results showed that the TEG was able to deliver 500We at the design point, with inlet gas at 450 ˚C and a mass flow rate of 90 g/s. A thermoelectric by-pass was included also to limit the highest thermoelectric modules temperature at 270 ˚C to prevent Bi2Te3 material damage.

This strategy caused a penalty in the thermoelectric performance, as at medium-high engine load, where the thermoelectric potential is higher, the system was to be switched to by-pass in order to lower the temperature over the thermoelectric modules. With this limitation the thermoelectric was operated at full flow up to a 110 km/h (68 mph) vehicle speed, and run with partial by-pass above this speed. In these conditions the output did not exceed 170 We. After improvement, the by-pass system should allow to reach 250 We at constant speed.

Driving cycle tests results:

  • On the NEDC Cycle the thermoelectric system dropped the fuel consumption by about 2.2% (6.7 g CO2/km reduction) with a peak thermoelectric electric power of 150 We.

  • On the more heavily loaded WLTP Cycle the thermoelectric dropped the fuel consumption by about 3.9% (9.6 g CO2/km reduction) with a maximum thermoelectric electric power of 200 We.

Increases in TEG electric output corresponded to the same decrease in alternator demand.

To enhance marketability, the researchers concluded, further work should be focused on decreasing the cost together with several recommendations regarding engine type, driving conditions and materials properties.

The thermoelectric system was not designed to produce a maximum electric output, but to provide the higher overall energy efficiency benefit at a vehicle level, taking into account constraints on the maximum allowable backpressure over the engine, the heat load increase on the cooling system and the integration aspects with the on board energy management.

Even though efficient, the current prototype is not in a position to result immediately in an economically viable product due to high cost at €8.4 (US$11.5) per We.

The cost breakdown analysis showed that 20% of the cost is due to material cost (Bi2Te3) and up to 73% of this cost is due to thermoelectric module manufacturing.

Given the current design and with the same material (Bi2Te3), the researchers projected that a high volume and highly automated manufacturing process could reduce this to around €3.1 ($4.25) per We with thermoelectric manufacturing cost comparable to material cost—i.e. €300 to 400 (US$411 to 548).

Aiming at a market for cars and light duty trucks, the cost for the automaker should be dropped to €0.5 to €1.5 (US$0.68 to US$2) per We using cheap thermoelectric material and automating the thermoelectric module manufacturing process.

The project concluded that Bi2Te3-based thermoelectrics—even with a highly automated production process—would be too expensive to meet market acceptance, except for uses with a very high number of operating hours such as taxis and large diesels on ships and power plants. The main bottlenecks for cutting production cuts are:

  • the thermoelectric material: Bi2Te3 is too costly for automotive applications, cheaper materials must be found; and

  • the basic design of thermoelectric modules with a very high number of very small legs is too labor (or robot) intensive. Simpler designs must be found.

Economic viability is more likely to be met in the future on gasoline cars (higher exhaust temperature in the city) with short time response thermoelectrics (fast light-up of the thermoelectric), the group concluded. This requires cheap, lightweight thermoelectric material standing temperatures up to 650 ˚C.

The targeted thermoelectric system of the future could be designed for gasoline with a power output of 250 We on the road and 80 We in city traffic, using cheap thermoelectric material (say less than €100 per liter) withstanding temperatures up to 650 ˚C.

Comments

Liviu Giurca

The most efficient and simple method to recover the exhaust heat is that used by the hybrid pneumatic engine with Ericsson cycle. See http://www.scribd.com/doc/169592344/Hybrid-Pneumatic-Engine-With-Exhaust-Heat-Recovery . This concept proposes to use at least one cylinder (of a multicylinder engine) as pneumatic heat engine specially in highway drive. In city drive same engine can recover by its self the braking energy. More info at lgiuca@hybrid-engine-hope.com.

jumpjack

I can't find any official paper from this project. Charts and tables would be much more interesting than single numbers.

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