Introduction
Internal combustion engines (ICE) lose up to 70% of the energy they produce, mainly through heat. As fuel economy becomes more important for the auto industry each year and with the passing of increasingly stringent regulations, car manufacturers have had a watchful eye on an old concept known as the thermo-electric generator (TEG). Thermal energy which is otherwise wasted can potentially be converted to improve the vehicle’s overall efficiency.
Fiat’s HeatReCar project combines the efforts and skillsets of seven partners to develop and integrate a working thermo-electric generator into a vehicle.
The reference vehicle used is an IVECO Daily equipped with a 2.3l Diesel engine. The design reference conditions are:
• Vehicle @130kph
• Exhaust gas temperature: 450°C
• Gas flow : 70g/s (max torque), 140g/s (full load)
TEG Basic Design
The HeatReCar TEG was not designed for max electric power output, rather to achieve a well-packaged, lightweight component providing a maximum overall energy efficiency increase at the vehicle level.
The TEG created utilized cross flow architecture with hot tubes made out of stainless steel and cold tubes of aluminum. This design was used to simplify the connection to the fluid’s network in the car. Material selection is very important and for this project, the group considered the well-established composition of Tellurium, Antimony, Germanium and Silver (TAGS) as well as segmented Bi2Te3-PTe, Skutterudites (developed and manufactured at module level), known for their good electrical conductivity, and Bi2Te3: used for the full scale prototype manufacturing with specific module design. The thermo-electric modules to tubes contact is controlled through ‘tie rods’ at each TEM’s corner. A modular design was chosen for ease-of-fit to a large scale of applications.
The constraints of the design are exhaust gas pressure drop
Thermal Behaviour
The next step of the project was to produce glass ‘dummies’ and a full size mock-up to check sensitivity of thermal gradient on ‘dummies’ opposite faces, tie rods tightening, material type & thickness placed between tubes & ‘dummies,’ measurement of glass ‘dummies’ temperature gradient and to check heat transfer balance of cold / hot fluid.
Experimental tests were performed to evaluate the impact on the thermometric efficiency (η) and the gas ΔP of the following parameters:
• Hot gas temperature - no impact on η, strong impact on Δ p
• Hot gas pressure - no impact on η, strong impact on ΔP (linear dep.)
• Cold liquid temperature - no impact on η
• Hot gas flow - best matching between η and max allowable ΔP
Validation of the Electric Circuit
The second mock up validated the electric circuit. The designs used 4 hot tubes with 8 cold tubes on each face evaluating various in-line vs. parallel circuit connections of TEMs on electrical output.
Final Design
Based on the mock-up testing, a 63 tube TEG was produced (63 hot tubes, 24 cold tubes, 504 TEMs).
TEG Performance on the Test Bench
Hot gaz flow: 90g/s
ΔP hot gaz: 30mbar
T hot gaz: 450°C
Cold liquid flow: 1200l/h
ΔP liquid flow: 0.15 bar
T cold flow 60°C
DC/DC Converter
A highly efficient multiport DC/DC-converter needed to be designed and manufactured to adapt the TEMs voltage level to the vehicle voltage level.
To achieve full energy harvesting the TEG has rather high internal resistance (1 .. 2ohms) requiring an electrical matching to the load, dominated by the automotive battery. The governing algorithm of this matching has a so called maximum power point tracking (MPPT), which allows the DC/DC converter to adapt automatically to the variable exhaust temperature conditions, which cause a wide range of output voltage of the TEG. At any time the DC/DC-converter keeps each module operating at its maximum power point to convert as much thermal energy as possible. This is implemented by parallel DC/DC-converters, each operating at different input voltages and input.
Onboard Overall System Architecture
Heat rejected into NP engine cooling loop has the advantage of having no additional components and a positive effect on engine and cabin thermal transient (fuel economy + thermal comfort). The disadvantage is that a control strategy is required to manage high load conditions on the radiator. An overall energy management review was also conducted.
Emissions and Fuel Economy tests have been performed over different driving cycles and different vehicle speeds in order to evaluate the overall benefit.
•Over the EU Homolog. Cycle (NEDC), the TEG electric output is provided mainly in the extra-urban part of the Cycle.
•Any DC/DC Output current increment causes the same decreasing demand to the alternator
•Over the new homologation Cycle WLTC, the System reached a peak of about 220 W.
•In the last part of the cycle the TEG power output is sufficient to provide the on-board electric need, thus completely replacing the alternator.
The Results are Positive
All the tests performed show that the TEG has a good potentiality and under medium-high engine load, can provide the electric power needed to possibly replace the alternator. A 4% fuel economy improvement over the WLTP cycle has been achieved. The tests gave evidence that 270°C is a maximum working temperature (due to TE material type max allowable temperature). This is too low considering that during the 110 km/h phase the TEM hot side can reach more than 300°C. High temperature material should be used in future applications to fully take advantage of the TEG power generation.
*This article is an adaptation of a presentation by Daniela Magnetto of Centro Recerche Fiat, given at the 3rd International Conference: Thermal Management for EV/HEV; Darmstadt, Germany 24-26 June 2013.
Will Hornick is the Managing Editor of Automotive IQ