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Reduced Energy Consumption by Massive Thermoelectric Waste Heat Recovery in Light Duty Trucks

Final Report Summary - HEATRECAR (Reduced energy consumption by massive thermoelectric waste heat recovery in light-duty trucks)

Executive summary:

The project aims at using thermoelements for converting the waste heat in the exhaust line of internal combustion engines into electric power with a view to reducing fuel consumption and emission of pollutants. The main results of the project are:

- The design, the optimisation and the production of a prototype thermo-electric-generator (thermoelectric): An IVECO diesel Daily light duty trucks was selected as a test vehicle. The HEATRECAR thermoelectric 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. The goal of the optimisation of the thermoelectric prototype was to maximise the temperature difference available to the modules and to minimise the pressure drop on gas flow. Thermal and electric contact resistances and differential expansions were particularly taken care of. Bi2Te3 was selected for diesel engines for performance and temperature range. The electric power output of the thermoelectric being highly variable in voltage and intensity a specific direct current (DC) / DC converter was designed to match board electric net requirements.
- The development of novel thermoelectric materials and modules exhibiting a high figure of merit for other automotive applications such as gasoline: four different thermoelectric material classes suitable for low temperature (Bi2Te3) and high temperature (PbTe, TAGS-85, skutterudites) were synthesised and characterised. thermoelectric material performance was improved (+20 %) through ball milling and subsequent spark plasma sintering (SPS). A novel skutterudite based thermoelectric module suited to high temperature applications was optimised, prepared and tested. It delivered 665mWel at 380-Kelvin temperature difference.
- Rolling bench tests on reference driving cycles
Test bench results: the thermoelectric was able to deliver 500Wel at the design point, with inlet gas at 450 degrees of Celius and a mass flow rate of 90 gr / s. A thermoelectric by-pass was included also to limit the highest thermoelectric modules temperature at 270 degrees of Celsius 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 vehicle speed, and run with partial by-pass above this speed. In these conditions the output did not exceed 170 Wel. After improvement, the by-pass system should allow to reach 250 Wel at constant speed.

- Driving cycle tests results:
(a) 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 Wel.
(b) 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 Wel.

A complete model of the vehicle with thermoelectric was developed on MatLab / Simulink and run according to different reference conditions and driving cycles.

- Targets for future developments
The technical feasibility of a Bi2Te3 based thermoelectric for application to a diesel light duty truck is now demonstrated. Even though efficient, the current prototype is not in a position to result straightforwardly in an economically viable product due to high cost at EUR 8.4 per Wel. Aiming at a market for cars and light duty trucks, the cost for the automaker should be dropped to 0.5 to EUR 1.5 per Wel using cheap thermoelectric material and automating the thermoelectric module manufacturing process.

As concerns thermoelectric materials: Bi2Te3 is a good material for long life cycle applications specifically for low temperature but with some limitation for Diesel applications. TAGs can be considered an alternative for these applications. The best candidate for high temperature application (gasoline or natural gas) can be considered skutterudites or Mg2Si / MnSi: cheap, lightweight, stable up to 650 degrees of Celsius according to the severity of cost criteria.

As concerns module design and production: modules and gas tubes must be optimised altogether. For the modules, research is needed on soldering, and differential expansions. A short time response is essential.

Project context and objectives:

Concept

The concept addressed is to make an efficient use of the energy wasted in the form of heat in thermal engines. This heat is mainly dissipated in the exhaust device and in the cooling circuit. One of the recurrent ideas in this field is the use of thermoelectric devices - that convert heat into electricity statically - which has also failed reaching large-scale deployment because of its lack of performance.

We are currently witnessing three trends that combine in a way that this idea can now be considered with a different perspective: on the one hand, electric power consumption is continuously increasing due to the growing part of electric and electronic devices on board modern vehicles; on the other hand, improvements obtained in thermoelectric specialist laboratories show that it is close to being a viable approach; finally, the environment problems have led to strong commitments concerning the reduction of fossil-energy consumption and the emission of carbon dioxide (CO2).

General objectives

The main objective of the project is to reduce the energy consumption and curb CO2 emissions of vehicles by massively harvesting electrical energy from the heat within the exhaust system and re-use this energy to supply electrical components within the vehicle or to feed the powertrain of hybrid electrical vehicles (HEV). The recovery of the thermal energy is performed by laboratory-available thermoelectric (thermoelectric) materials which are able to work at the adequate high temperatures and exhibit high performance. The main objectives of the project are:

(a) to develop novel thermoelectric materials exhibiting a high figure of merit at high temperature;
(b) to automate the thermoelectric module production process so as to come up with sufficiently low production costs;
(c) to design, optimise and produce a prototype system to be tested on a diesel light duty truck.

Target

The initial HEATRECAR base case was a TEG producing 1 KWel at 130 km / h. At this power level the potential for CO2 emissions reduction rangeed from 50gCO2/km in real traffic at 30km/h average city speed to 10gCO2/km on the motorway at 130km/h. A Light Duty Truck (2.3l diesel engine) was selected to be used as the reference vehicle for the sizing of the system and the final production of the demonstrator. This type of van is representative of a typical light commercial vehicle in extensive use in the European Union (EU).

Project results:

WP1: Detailed specifications, value analysis and market estimate

- Detailed specifications:
Specifications in terms of constraints and of target performance have been established for steady state conditions at 130 kph. Target electrical output was 1kWe. Volume constraints for thermoelectric and DC / DC converter have been defined. The baseline tests on the standard production vehicle have been defined. As the system performance would vary depending on the mission profile and the engine load, several reference cycles have been defined.
An IVECO Daily equipped with a 2.3-l Diesel engine has been identified as a target to be used as the reference vehicle for System sizing and final Demonstrator realisation.
A thermoelectric performance assessment procedure over a test rig defines the experimental and virtual methodologies that allow the quantification of the environmental benefits of the innovative heat recovery system to be developed (measurements procedure, different conditions in NEDC and in real life (such as common Artemis driving cycle (CADC)).

- Value analysis and market estimate

Cost analysis of the HEATRECAR thermoelectric prototype : The cost analysis of the 250 Wel prototype which was built in the framework of HEATRECAR leads to a current specific cost of the electrical watt produced at around EUR 8.4 /Wel. The cost breakdown analysis shows that 20 % of the cost is due to material cost (Bi2Te3) and up to 73 % of this cost is due to thermoelectric module manufacturing.

In the current design and with same material (Bi2Te3), we can expect that a high volume and highly automated manufacturing process could reduce this value to around EUR 3.1 per Wel with thermoelectric manufacturing cost comparable to material cost i.e. EUR 300 to 400 per thermoelectric. Therefore, the objective of the production line developed in WP4 is to reduce the thermoelectric module production cost to a level close to that of material.

Value and market analysis: The value of a thermoelectric system for a buyer depends on the cost of the electric kWh delivered by the alternator to the board net, the number of operation hours per year and the number of years accepted for the pay-back time. Taking into account the standard margin between car maker and customer, the maximum cost accepted by the automaker in terms of EUR / Wel can be calculated for typical application cases as follows:
- Private conventional car or gasoline hybrid taxi : EUR 0.5 per Wel
- light duty truck or freightliner in USA : EUR 0.7 per Wel
- Diesel light duty truck or freightliner : EUR 1.5 per Wel
- Conventional diesel taxi: EUR 3 per Wel
- Large diesel ship: EUR 2 per Wel
- Diesel power plant: EUR 2.5 per Wel

Economic viability

It results from the production cost breakdown that a Bi2Te3 based thermoelectric 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:

(a) the thermoelectric material: Bi2Te3 is too costly for automotive applications, cheaper materials must be found;
(b) the basic design of thermoelectric modules with a very high number of very small legs is too labour (or robot) intensive. Simpler designs must be found.

Market acceptance is not only conditioned by the pay-back time for the user who makes his decision mainly on NEDC figures for opting or not for a thermoelectric. The main driver for the thermoelectric acceptance will be to meet the new regulation on CO2 emission. Diesel engines show low temperature in the urban part of the NEDC cycle and acceleration are short, thus providing a limitation in the thermoelectric effectiveness. On the other hand, the new regulation on CO2 emission (both in the EU and US) will include off-cycle CO2 credits. In addition the tests results show that the thermoelectric performance is higher on the new EU homologation cycle WLTC than on NEDC. Nevertheless given these considerations an 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). This requires cheap, lightweight thermoelectric material standing temperatures up to 650 degrees of Celsius on the long run.

Roadmap for future developments

Considering HEATRECAR project results, the technical feasibility of a thermoelectric for application to a diesel light duty truck is demonstrated and the test results show that in extra-urban driving and on highway, the system can provide a sufficient amount of electric output to replace the alternator. On the other hand in city driving the exhaust gases temperature are too low to provide a significant electric output, nevertheless the thermoelectric by releasing additional heat into the cooling loop is provides a beneficial effect on engine efficiency during the warm-up phase.

Though efficient, the current prototype is not yet in a position to result, straightforwardly , in an economically viable product due to high material and manufacturing costs, but it can be the basis for future developments having such goals.

Evolution since 2007 having an impact on thermoelectric generation:

There has been a strong evolution of the automotive environment since the submission of the proposal as concerns fuel cost, emission limits for automakers, Euro6 limits equal for diesel and gasoline, diesel over-tax, nano-soot particles, trend towards down-sizing, emergence of hybrids that are almost all in favour of gasoline in the years to come. And gasoline is a better opportunity for thermoelectrics as temperatures are higher leading to higher efficiency. Gasoline is also more favorable at low-medium engine load (city driving).

Targets for future developments:

The targeted thermoelectric of the future could be defined as follows: designed for gasoline with a power output of 250 Wel on the road and 80 Wel in city traffic, with cheap thermoelectric material (say less than EUR 100 per litre) withstanding temperatures up to 650 degrees of Celsius. A short time response of the order of that of the catalyst, will be aimed at. If a large market is to be met for cars and light duty trucks, the cost for the automaker should not exceed 0,5 to 1,5 €/Wel according to the application considered.
- As concerns the system aspect anti-overheating concepts and module geometry should be investigated.
- As concerns thermoelectric materials :
(a) Bi2Te3 to be developed aiming at low temperature, long life cycle application (ships and powerplants).
(b) TAGs not to be excluded for long life cycle diesel applications.
(c) Skutterudites: in the medium term a very promising candidate for automotive gasoline applications: efficient and temperature proof up to 650 degrees of Celsius.
(d) Mg2Si / MnSi: alternative candidate for automotive gasoline applications: cheap, lightweight, temperature proof up to 650 degrees of Celsius but scientific and technological development is still necessary to overcome current concerns on soldering, differential expansion coefficients between thermoelectric material and contact layers and connections.
(e) As concerns module design and production : The design of the module is not independent of the design of the gas tubes. They must be optimised altogether. The module must also be designed in such a way that the assembling process will be easy and low cost for meeting the target of EUR 0.5 - 1.5 per Wel.

WP2: Complete vehicle simulation including thermoelectric

The target was the simulation of the complete vehicle to optimise the thermoelectric design and calculate the possible impact on the fuel spending and CO2 emission.
In a first step a conventional model of the vehicle with thermoelectric in was developed on MatLab / Simulink and validated on a roller bench.
In a next step this model was extended by all components needed to evaluate the impact of the thermoelectric integration. For this purpose additional aspects of the vehicle such as the thermal management and the exhaust gas line were introduced into the simulation model. Simulation study shows a very good agreement between the model and the conventional vehicle covering these extensions.
For the simulation of the Impact of the thermoelectric six different cycles were chosen. Two constant speed scenarios at 70 km / h and 130 km / h, the NEDC cycle with hot and cold starting conditions as well as the AR thermoelectric MIS an Torino-Geneva cycle.
The work on this project has increased the knowledge concerning the development and validation of a full scale simulation model. Especially the interaction between important modules could be investigated in detail.

WP3: Thermoelectric material production process development

- Melt synthesis of bulk high ZT materials

Four different material classes suitable for low temperature (Bi2Te3) and high temperature applications (PbTe, TAGS-85, skutterudites) have been synthesised and characterised. Work has been done by Termo-Gen to increase the material production rate with a new higher capacity melting chamber for the induction furnace. A planetary ball mill, Fritsch Pulverisette 7 premium line, has been purchased. An ultrasonic powder treatment unit, Hielscher UDP 1000, to be used for continuous operation has been purchased: It is now possible to produce 250 gr / day of PbTe and TAGS85 in the HF-furnace and 1kg in a Nabertherm furnace.

Concerning Skutterudites, further development is required to produce this material in required quantities and performance. The present TGEN equipment can be used for Skutterudite production using the metallurgical route.

The production capacity of PbTe, TAGS85 and Bi2Te3 is at the level required by the project. CRF has been focusing on the development of filled skutterudites as bulk high ZT materials. This kind of alloys is known to exhibit very good thermoelectric properties, their cost is potentially much lower than the cost of materials based on tellurium, many research groups are working on skutterudites worldwide, but they are not commercially available yet. Two materials (one n-type, one p-type) have been chosen:

(a) CoSb3 n-type material - Yb filled
(b) CoFe3Sb12 p-type material - Mm filled

The synthesis of both materials has been performed starting from the pure elements in precise stoichiometric ratio that have been put into quartz crucibles. Vacuum is made inside the crucibles that afterwards are filled with argon and sealed. The synthesis process consists of two steps: pre-melting in resistance oven and complete melting in induction oven. Thermal annealing has been applied which promotes the formation of the desired phase. X-ray diffraction (XRD) analysis for n-type skutterudites after annealing shows that the CoSb3 phase was successfully obtained.

- Spark plasma sintered nanocomposites

Powder processing is used for thermoelectric materials in order to homogenise the materials, to increase their strength and often to improve the figure of merit (see e.g. Wu et al., J. Electr. Mat. 2012 and references therein).

Sintering is a standard method for consolidating powders without melting them. Hereby, the 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. In contrast to conventional sintering methods such as hot pressing, SPS enables to process powders in short times and achieves higher densities at the same temperatures (Munir et al., J. Am. Ceram. Soc. 94 (1), 2011). Thus, sintering at lower temperatures for shorter times helps to suppress grain growth, to control phase transformations and to minimise material vaporisation.

For thermoelectric materials, especially the suppression of grain growth is relevant. Smaller grain sizes enhance the boundary phonon scattering and thus reduce the lattice thermal conductivity (Zhao et al., J. Electr. Mat. 38, 2009). An increase of the figure of merit can be achieved by decreasing the thermal conductivity without changing Seebeck coefficient and electrical conductivity in an unfavorable way.

Within this project, a variety of material classes has been studied by Fraunhofer IPM under thermoelectrical aspects . The original melt synthesis ingots delivered by Termo-Gen and CRF were pulverised by ball-milling and compacted by spark plasma sintering. Most of the materials show a significant improvement through the optimisation process (PbTe, TAGS-85, n-type Skutterudites). Only p-type Skutterudites don’t exhibit an improvement.

We emphasise that a repeated investigation for the different material classes was not possible because of the limited availability of the materials. Thus, the figures given below are to be understood as a trend rather than final reproducible numbers.

WP4: Thermoelectric module production process development

The challenges for the development of a thermoelectric module for the HEATRECAR thermoelectric were: 10 years expected life, high efficiency, competitive cost level and a high degree of manufacturability. The joining technology for thermal connection between the thermoelectric modules and the heat exchanger is critical for the thermoelectric performance.

The work was divided into 11 tasks:

(1) Specification of thermoelectric-performance and operating point
The operating point was determined by standard driving cycles for a Fiat light duty diesel engine powered truck.
The inlet gas temperature is approximately 450 degrees of Celsius with a varying mass flow. The hot side operating point 300 degrees of Celsius
Power: 500 W
(2) Evaluation of suitable novel element and module concepts

Evaluation of suitable module concepts: A module based on segmented elements has the potential to be very efficient if the temperature profile in the element is close to the design values.

The actual temperature profile in the elements in the HEATRECAR thermoelectric will vary rapidly during the driving cycle so the actual gain using segmented elements will be limited. Segmented elements are also adding to the cost of the thermoelectric in this cost sensitive application. It was decided to use single material elements.

Advanced high temperature modules suitable for use at 600 degrees of Celsius were evaluated with good results. The actual temperature in the thermoelectric is however at the low end for the high temperature materials so the cost of the advanced design could not be justified at this time. It was decided to use medium temperature design modules.

The size of the modules is a trade-off between the cost of manufacturing and assembly and the long-term stability of the thermoelectric. It was decided to use small modules because of the advantages of long term stability and thermal contact to the heat exchanger.

(3) Selection of thermoelectric material and module

Selection of thermoelectric material and module design: The temperature range of the exhaust gases suggested bismuth telluride as the most suitable material for the thermoelectric. The thermoelectric operating point was simulated for different drive cycles and suitable dimensions of the elements and the spacing between the elements were calculated from 3D simulation model calculations.

The exhaust temperatures will normally be approximately 450°C in the inlet to the thermoelectric. The temperature drop from the bulk gas via the heat exchanger and the interfaces is normally approximately 150 degrees of Celsius.

It was decided to use different materials along the temperature gradient of the thermoelectric. Skutterudite should be used in the hotter sections of the thermoelectric and Bismuth telluride in the following sections.

(4) Module design and test

The elements and the module were designed based on the mathematical simulations. The legs are ideally of different cross-section for the p- and the n-leg. It was decided to use the 3 x 3x 3.2 mm elements for both materials.

The modules for the thermoelectric-prototype were based on bismuth telluride with a size of 16 mm x 16 mmx 3.2 mm. The power at a hot side temperature of 270 degrees of Celsius was 1 W per module measured in a module test bench.

Modules based on skutterudites were produced after the thermoelectric-prototype was built and tested.

(5) Optimisation of the module

FEM-analysis was used as a basis for the final design of the module and the complete thermoelectric. The medium temperature bismuth telluride modules has been improved with a higher long-term operating point of up to 330°C, these modules will be tested in February 2013 The skutterudite modules have been optimised by IPM. SPS-compaction is used for the skutterudite material.

Three different contacting methods has been developed; contacting by spark plasma sintering, accelerated hard soldering and mid temperature bonding. A mechanically stable 12 leg module with a maximum power output Pmax = 665 mW at ?T = 380K was produced successfully using the mid temperature bonding method.

(6) Joining of thermoelectric and heat exchanger under high temperature and vibrations Thermal contacts are important issues in thermoelectric devices. In this aim, VALEO realised extensive researches to reduce thermal resistances between thermoelectric modules faces and hot or cold sources. Using firsts thermoelectric modules received from Termogen, VALEO performed tests to quantify the voltage gain with thermal interface materials (TIM) on hot and/or cold face(s) (reference is voltage of thermoelectric module w/o TIM on any face).

(7) Assembly processes

The assembly of the modules has to be both cost efficient and the modules must be of very high quality and similar in thickness and performance.
The automotive application is cost sensitive and the planned production volume is high so the processes must be scalable to take advantage of the larger volumes.
It is necessary to avoid handling of the separate elements and the separate modules since the total economically allowed handling of each element is 1 - 3 s.
21 major module production process steps were analyzed from material production to the final test and verification. The proposed processes are proven and some novel processes proposed for evaluation.

(8) Process equipment design and build

Termo-Gen has worked on the production of telluride materials and we have increased the production volumes to levels suitable for pilot production of the HEATRECAR thermoelectric. An evaluation of freeze granulation has been done, this method is suitable for fine powders reducing the health hazards for the operators and reducing the dust considerably. The material and powder production is done in a LAF bench or in a glove box.
CRF has focused on the development and production of skutterudite for further processing at IPM to test modules.

(9) Cost optimisation of processes, components and material

A cost analysis has been made to investigate the potential for future cost reductions based on material cost and improved production processes.
The challenge is to reduce the manufacturing cost of the modules to be close to the raw material price. This requires a high volume production with a minimum of manual handling. Smaller thermoelectric-modules are generally more expensive per electric W both in production and in assembly compared to larger modules but at the same time the larger modules are more exposed to stresses caused by differential thermal expansion. The 16 mm x 16 mm x 6.8 mm module size in the HEATRECAR project makes it necessary to treat batches of 10 or more modules as the smallest production unit.

(10) Pilot production

Pilot-production of modules has been made. Four series of modules including present state of the art materials, SPS-compacted bismuth telluride and skutterudite materials, modules with improved contact layers and modules based on modified bismuth telluride with improved long term high-temperature stability.

(11) Design of the production line

A module production line has been designed and a three-dimensional (3D) computer-aided design (CAD) model produced. The concept is a highly modular production line where the individual stations can be doubled for increased capacity. The implementation of the automation can be made in several steps focusing on the bottlenecks of the production. The production line is designed to make it possible to handle batches of modules as the smallest production unit.

WP5: Design and manufacturing of heat exchanger

The HEATRECAR thermoelectric demonstrator architecture is described as follows:

- An efficient thermoelectric design

Valeo aimed for HEATRECAR demonstrator to optimise and control thermal interfaces and pressure applied on thermoelectrical modules (consequently on the thermoelectric materials too). Four (4) guidelines have to be underlined to achieve an efficient thermoelectric:
(1) Referring to Valeo formal experience, compacting thermoelectric modules allows a better control of contacts between hot/cold sources and thermoelectric modules. Valeo decided to choose. 16 x 16 mm2 modules (4 times smaller than 32 x 32 mm2 of Honda showed at IAV Berlin, 2010).
(2) A cross flow architecture for an easy fit with fluids network in car has been chosen. thermoelectric modules are tight between hot and cold fluid sources.
(3) This design allows controlling thermoelectric modules tightening by 4 tie rods placed at each corners (see Figure 3 below). Following Termogen studies, a compressive force of 340N/modules has been applied through this system.
(4) Finally, thermal continuity on interfaces is assured and enhanced by graphite films serving as thermal interface material between thermoelectric modules and hot and cold sources.

- Components for each fluids

Stainless steel tubes for hot gas side
For hot side, the objective was to find tubes compatible with selected thermoelectric modules, compatible with diesel engines (ie. anti-fooling internal surfaces), with thin walls to lower weight and resisting to high compressive forces. Pressure drop has also to be limited to 30 mbar.
Answer from Valeo was stainless steel tubes coming from EGR technology. Walls thickness is reduced to 250 µm (limit of laser welding, instead of 500 µm in general), tool press cut turbulators are 100 µm thick with a pitch of 2 mm to minimise soot deposit.

Tubes’ length has been limited to 95 mm (correspond to 4 thermoelectric modules in serial) to keep gas temperature above 350 degrees of Celsius (under 350 degrees of Celsius, thermoelectric module's efficiency is drastically reduced). Also, pressure drop is up to 15 mbar only for gas at 450 degrees of Celsius and 1 bar.

Also, these tubes have been tested against thermal shocks and pressure cycles to secure heat exchanger.
Aluminum extruded tubes for cold water side.
For cold side, needs were tubes resisting to high pressure, with low thickness and minimal space.
Valeo selected aluminum 5 channels extruded tubes, with a cross section of 1.75 x 16 mm (limit of extrusion process).

- Assembly of HEATRECAR demonstrator

Mechanical assembly
Following pictures illustrate the assembly of final HEATRECAR demonstrator, made of 504 thermoelectric modules, 63 hot tubes and 24 cold tubes. Thermoelectric modules are regrouped by 12, hot tubes by 3 (4 thermoelectric modules on each faces).

Electrical architecture

Main goal concerning electrical design was to find the best compromise between load voltage/current values needed by DC / DC converter designers and reliability. Valeo implemented a serial/parallel architecture which limits the power loss of 10 % if 25 % of thermoelectric modules are degraded or out of use.

- Performance of the HEATRECAR thermoelectric

All the tests have been performed in a test center specialising in turbo compressors. Hot gas flow, liquid flow, both fluid temperatures and pressures can be controlled independently. A first step has been to validate measurement methods and thermal aspects by assembling a mock-up with thermoelectric modules replaced with borosilicate plots (allowing measurement of temperatures gradients at its edges). Tests validated that 50 to 60 % of ?T hot gas-cold water is available at thermoelectric borders.

Final demonstrator provided electrical results (and thermal measures consistent with thermal mock-up) illustrated hereafter for gas at 450 degrees of Celsius with a variable mass flow.

At the nominal of 63 gr / s and 450 degrees of Celsius (best compromise between electrical power and pressure drop), output power is up to 450 W and total pressure drop has been reduced to 15 mbar, half of the maximum authorised (30 mbar).

A mass flow rate of 63 gr / s is only 50 % of the maximum flow rate available on an Iveco Daily truck, used for inboard tests. These tests show that the potential 1kW target is reachable with engine at full load.

Also, tests showed that the thermoelectric is extremely sensitive to gas temperature. Indeed, decreasing gas temperature of 100 degrees of Celsius leads to a loss of 40 % of electrical power. In conclusion, Valeo advices for a potential next mock-up to reduce tubes length of 25 % to keep thermoelectric modules in high efficiency conditions and optimise electrical power per module.

WP6: DC / DC converter design

The flexible concept, to use a converter topology (BuckOrBoost) with a wide range of the input voltage, led to a successful energy recovery. The DC / DC converter is able to convert input voltages, supplied by the thermoelectric generator (thermoelectric), from 2.7 V up to 60 V. With a capability of converting 600 W into an automotive board net (nominal voltage: 12 V), the delivered DCDC converter meets the necessities of the realised components of the HEATRECAR project. With an input power higher than 40W the efficiency exceeds 90 % and shows a maximum over 97 % at 250 W throughput power, at 600W it is still higher than 93 %. The DC / DC converter externally can be switched off to interrupt the power supply from the thermoelectric for purposes of board net management. In regular operation its controlling maximum power point tracking algorithm (MPPT) matches the converter to the internal resistance of the thermoelectric, thus at all different exhaust and coolant temperatures the best energy harvesting condition automatically and rapidly can be found.
The experimental prototype is splash water resistant and rather small, and, compared to the other components, reasonably priced.

WP7: System integration, test and validation

The objectives of WP7 were the integration of the thermoelectric in the exhaust gas system of a mobile test rig (IVECO Daily truck) and the design of the mechanical, thermal and control equipment. The thermoelectric was delivered by the team member Valeo. The performance and the boundaries of the integrated system were demonstrated by performing different drive cycles and by testing under stationary operating conditions. This enables the optimisation of the auxiliary and supporting systems and of the thermoelectric itself.

The external cooling system of the thermoelectric was designed to transfer the heat energy from the 'cold-side' of the thermoelectric generator to the coolant system in order to set defined temperature levels, which could not be set by the internal engine cooling system. This results in different temperature gaps of the Peltier elements and in that context to a different performance. By this operational strategy the benefit of an additional coolant system beside the internal one could be determined.

The thermoelectric heat exchanger prototype has been installed into the IVECO test vehicle, alongside the previously reported external cooling system, including the DC / DC converter, delivered by Siemens. Various pressure sensors and temperature sensors in the exhaust gas track as well as mass flow and temperature sensors in the external cooling system of the thermoelectric are placed in the test rig, connected with a measurement system.

The vehicle-integrated thermoelectric heat exchanger has first been put in operation without the DC/DC converter. Temperatures at different locations in the system, especially thermoelectric temperatures, have been observed, while operating the vehicle at different operating points. Second, the same basic test procedures have been applied to the system, after the DC / DC converter has been integrated. Voltage and current of the thermoelectric and the DC / DC converter have been carefully observed within these tests.

Operating the thermoelectric prototype in the moderate NEDC showed huge variations in the output voltage, which even required a modification of the installed DC/DC converter’s firmware. In the more dynamic CADC, overheating of the thermoelectric could be observed. As a workaround, more effective exhaust gas bypassing control could be added to the thermoelectric system. However, specification and dimensioning of the thermoelectric for higher temperatures would allow harvesting higher maximum power from the wasted heat of the exhaust gas.

With the original configuration of the DC / DC converter, very low power output was observed.

As can be seen in the above showed figure No.2 no output current and output power of the DC / DC converter could be measured in the complete urban part of NEDC (0 to 900s). The reason therefore is the characteristic voltage level of the DC/DC converter. The DC/DC converter has been configured with a 'turn-on' voltage level of 12 V. Therefore, if the input voltage level is below 12 V, the DC / DC converter gives out a voltage level of constant 12 V with zero current. This shows, that the thermoelectric delivers a voltage of < 12 V (U_DCDC_input) in many low-power operating points, e.g. the complete urban part of NEDC (Figure 12). With a firmware update of the DC/DC converter the 'turn-on' voltage level has been lowered to 5.4 V, which made a power output in urban driving situations possible (not shown due to lack of space).

As specified by team member Valeo, the thermoelectric temperature should not exceed a temperature of 250 degrees of Celsius, in order to avoid thermo-mechanical damage to the thermoelectric component. Even with the improved cooling system, a critical thermoelectric temperature has been reached in the CADC. Reaching the critical temperature could not be avoided by turning the cooling system to full cooling power and by fully opening the thermoelectric bypass flap. This means that opening the bypass flap does not sufficiently reduce the exhaust gas mass flow through the thermoelectric heat exchanger.

As mentioned above the final system configuration was tested under stationary operating conditions. In the left table the vehicle speed and the inlet temperature of the thermoelectric module was set to three different values. The vehicle speed was varied from 50 km / h to 110 km / h. At green hatched marked combinations the bypass flap was controlled in order to keep the maximum temperature of the Peltier elements at nearly 250 degrees of Celsius, in the other cases the bypass flap was always closed. The measured output power results between 7 Watt and 191 Watt. In the right table the bypass flap was set to fully closed (complete gas mass flow through the thermoelectric module), 50 % closed and fully open (minimum gas mass flow through thermoelectric module). Again the vehicle speed was set to three different values. As can be seen the thermoelectric module generates an electrical power output between zero and 238 Watt. In some cases the test had to be stopped to avoid damage of the Peltier elements by exceeding the temperature limits.

The objective of WP7 was the integration of a thermoelectric, DC / DC converter, separate cooling system and measurement equipment in a vehicle test rig and to show the function of this system by driving different cycles and stationary operating points. With this system a maximum power output of 238 W could be reached at a stationary operating point of 130 km / h.

WP8: Vehicle integration on light duty truck including real world testing

CRF integrated the thermoelectric system and thermal management system into the reference light duty vehicle selected in WP1. The vehicle has been modified to allow system integration. It has been equipped with a dedicated acquisition system for the complete characterisation of the system. This task has been carried out mainly by CRF and BOSCH and supported by Valeo.

Moreover the system control strategies have been defined and implemented on the LCV application and the system architecture has been refined.
All the vehicle aspects and the interactions with the other on-board systems and components has been taken into account.
The installation scheme has been defined taking into account the technological, production and maintenance problems.
The correct operation of the thermoelectric System integrated on the vehicle has been verified by test on a roller bench according to procedures identified in WP1 to assess the target performance.
All the tests performed, except for the FTP cycle, shew that the thermoelectric generator has a good potentiality for energy recovery (up to 4% fuel economy) and in some cases, the HEATRECAR system is able to substitute the alternator:
(a) Comparing the test results with system ON and OFF on NEDC Cycle the system reached a fuel consumption reduction of about 2.2 %.
Moreover, during the first part of the NEDC cycle, the system is able to reduce the fuel consumption even if the exhaust gas temperature is not hot enough to allow the thermoelectric to generate any power. The is why, in any case, some exhaust gas power goes into the cooling loop thereby accelerating engine warm-up and improving engine efficiency.
(b) Comparing the test results with system ON and OFF on WLTP Cycle the system reached a fuel consumption reduction of about 3.9 %.
However, the tests put in evidence some system limitation. In particular during the Artemis test, where the by-pass had been switched ON, a portion of exhaust gas continued to flow through the thermoelectric thereby increasing its temperature.
The constant speed test put in evidence that 270 °C as maximum working temperature is too low considering that during the 110 km/h phase the thermoelectric hot side can reach more than 300 °C.

WP9: Dissemination

- Dissemination activities have been covering two main areas:
(a) Participation in conferences and/or Events to communicate on project results as well as paper preparation for journals and magazine (article to be published at first quarter 2013 in 'Usine Nouvelle').
(b) Website implementation and update to enable global information exchange with the public (open area) and between the partners for project purpose (restricted area).

- Promotion of gender equality within the consortium

The objective was to incite partners to organise girl days with a view to convincing female students aged 16 - 18 that the sector of 'energy technologies' offers interesting job opportunities to them.

A presentation on the environmental aspects of energy technologies was prepared both in French and English. Contacts were made with secondary teaching establishments both in France and Germany to make these presentations. The following events were organised:
(1) A visit of the lab of participant Fraunhofer IPM in Freiburg Germany with presentations of energy related issues, thermo-electricity, and HEATRECAR objectives and results.
(2) A presentation at the Mondial de l'Automobile in Paris by participant Véronique Monnet from Valeo.
(3) A simplified presentation by a 12 year old female pupil at a 'technology course' in Toulouse France.

In addition ROM participated in an event organised by “EllesBougent” of France, an organisation active in 'Gender Issues' in energy and transport science and technology. The extended and simplified presentations are available on the HEATRECAR website. They are free for use by third parties.

Presentation to female students at Fraunhofer IPM on 7 December 2012
Presentation to female students at the Mondial de l'Automobile in Paris in October 2012.

- Health, environment and regulatory aspects

The list of the chemical elements involved in the materials investigated in the project (thermoelectric materials contemplated in WP3) is the following: bismuth; tellurium ; lead ; silver ; antimony; germanium; cobalt; cerium; neodynium; lathanum; ytterbium; misch metals (alloys of rare earth elements such as cerium; Lanthanum and Neodynium).
For each compound contemplated in the project, a systematic analysis has been driven concerning the processes for thermoelectric production regarding potential hazards. A literature review has also been delivered for each above listed chemical element covering the following topics:
(a) Applications;
(b) production, availability and cost;
(c) life cycle, recycling, disposal procedure;
(d) health effects and environmental effects;
(e) regulations.

Combined constraints by health, environment and cost

In the table below the parametrers concerning price and toxicity, not only of the above listed elements but also of the elements contained in candidate silicides are ranked in the order of increasing price per litre. The price per litre, if not only, is a significant proxy for the impact of the cost of material on the overall cost of a thermoelectric in EUR / Wel and its chances for ever becoming of economic interest in the automotive industry. Of course compounds including some of these elements may have prices and toxicity different from those of the elements, so each compound should be evaluated specifically for a final selection. Price considerations:
(a) Below EUR 10 per litre (green), no doubt these elements are acceptable for coming up with a cost effective thermoelectric, namely no more than EUR 1.5 per Wel or EUR 500 per unit. Their cost share is a small part of the overall cost of the thermoelectric.
(b) Up to EUR 100 per litre (yellow) the elements, if used as a base material, may still be candidates for a cost effective thermoelectric, but then, their cost share will be high as compared to the overall cost of the thermoelectric. Their implementation in the modules must be carefully optimised. No problem if used as dopants.
(c) Between 100 and EUR 1 000 per litre (red) they must be included only in small quantities in the thermoelectric compounds as dopants.
(d) Above EUR 1 000 per litre (brown) they may only be included in the form of traces as nano-dopants. They may, even so, substantially increase the cost of the base elements.

Toxicity:

Many of the elements or compounds contemplated as thermoelectric materials are heavy metals or rare-earths. As a result they are toxic or very toxic. Several of them are flammable or highly flammable. So, all of them require severe precautions for their manufacturing and implementation. The higher their toxicity and flammability, the higher their impact on the production cost of the thermoelectric as well as on the cost of its recycling or end of life dumping. The toxicity for the skin or inhaled is all the more of concern as before sintering they are in the form of powders, even in the form of nano-powders difficult to handle and filter. Flammability is perhaps the easiest problem to be overcome economically by the automotive industry.

The combination of price and risk constraints gives an advantage in the automotive industry to such elements as Mg, Si, Zr, Pb, Mn, Sb, or Ni. Among the thermoelectric materials including these elements are the silicides Mg2Si and MnSi which in spite of medium thermo-electric performance are low cost (below EUR 100 per litre) and quite easy to handle. They are valuable candidates for future automotive thermoelectric developments.

Potential impact:

Potential impact of TEGS at European level

As compared to the figures presented in the description of work at the start of the project the cost of road fuels has increased by some 30 %, new EC Directives on average CO2 emission per car only envisioned at the start of the project, are on the verge of being implemented, and diesel engines now considered as carcinogenic because of soot nano-particles are going to be hampered by EC6 diesel emission standards identical to gasoline standards. In addition the EC is now considering imposing an additional tax on diesel precisely because of the carcinogenic risk. The main application may therefore mainly be on gasoline in the future. On the other hand the effective performance level that can be expected from thermo-electric generators now and in the future is now much better appraised than at HEATRECAR project start. These considerations altogether allow us to outline a more realistic perspective of the socio-economic impact and the societal implications of HEATRECAR on the following points:
- EU oil consumption,
- EU CO2 and polluting emissions,
- EU raw material consumption and dependence on imports,
- EU employment,
- EU exports.

For establishing the different estimates of socio-economic impact the following figures are going to be assumed (source ACEA): The overall oil consumption for road transport in the 27 Member States is variable over time according to the inflation of the number of cars, of the occurrence of crises or oil price bumps, but it may be considered that it is normally around 300 million tonnes / yr.
The number of cars running on EU roads is on the order of 250 million.
The average yearly mileage per car is on the order 14000 km.
On the basis of an assumed average speed (including town and country) of 30 km / h, the yearly operating time of a car is around 500 h.

thermoelectric performance

The prototype of the HEATRECAR project based on Bi2Te3 thermoelectric material has been able to deliver up to 170 Watts electric with thermoelectric material hot side at 250 degrees of Celsius. The exhaust gases of the van which was used for the test runs could reach 450 degrees of Celsius but the tests were run at lower gas temperature in order not to destroy the thermo-electric material. It has been calculated that through further optimising the design of the heat exchange surfaces in combination with some improvement of thermoelectric material performance a minimum power level of 250 W is not out of reach. By increasing the volume of the thermoelectric generator more electric power could be produced, mainly on motorway driving. But it turned out that this is of limited interest -except for hybrids- because, according to vehicle type, the baseline power demand, namely that during the NEDC test run varies between 170 and 250 Wel. This baseline power level is that which is going to be the minimum demand all over the life cycle of the vehicle, thereby insuring the higher number of amortisation hours. The figure of 250Wel is based on diesel. It is likely that with gasoline and other thermoelectric materials higher power level at lower cost could be obtained.
All estimates are therefore going to be worked out for 250 Wel.

Preliminary remarks

The following considerations on the socio-economic impact of the emergence of thermoelectric should only be considered as a global approach. As a matter of fact European Directives and standards which will trigger the emergence of thermoelectrics will not affect European made vehicles but also imported vehicles. Therefore the number of yearly registrations should be taken in consideration. But then thermoelectrics mounted on vehicles imported in Europe will possibly be produced elsewhere, thereby not automatically providing European employment.

Conversely, vehicles marketed in the USA with thermoelectrics made in Europe will result in European employment, but are not counted as European registrations. Nevertheless as the number of European registrations is close to the number of vehicles produced in Europe,15 million per year, this number will be kept as a reference figure.

Another point is also that the power level of marketed thermoelectrics may vary from application to application and from country to country as a function of regulations, incentives and the cost of fuel. Nevertheless a power level of 250 Wel will be kept as a reference figure.

Savings in EU oil consumption

It is unlikely that all European cars are going to be equipped with a thermoelectric. But it is quite impossible to assume the share of the European fleet that would effectively be equipped with it. The savings that would result from all vehicles being equipped will therefore be used as a maximum figure, being aware that the real figure would in any case be lower.
The production of 250 Wel over 500-h operating time per year amounts to 125 kWhel.
Assuming an average alternator-belt efficiency at 50 % this car will consume 250 kWh mech / yr on the shaft of the engine. On the basis of an average specific consumption of 350g/kWhmech, the production of electric power on this reference vehicle consumes 87,5kg of fuel per year. For a fleet of 250 million vehicles the consumption devoted to the baseline electric power demand is therefore 0.0875t x 25 million = 2.2 million tons of fuel. This is the potential saving resulting from all conventional cars being equipped with thermoelectrics. It does not include the potential savings if heavy duty trucks were also equipped. Although the number of trucks rolling over Europe is of the order of 1 / 100 the number of cars their electric power demand is higher, the operating time per year is between 2 and 4 times that for cars and the flow and temperature of exhaust gas are much more constant. This last application is potentially of interest to truck operators. But it would be of limited impact on European fuel consumption. The application to hybrid cars is potentially of high interest because the flow and temperature of exhaust gas are always high - when the thermal engine is operating- and because any kwhel coming out of the thermoelectric would be used for propulsion. There would therefore be an interest for designing thermoelectrics of over 250 kWel that would find an application on hybrids. For the time being calculations for determining the European impact of hybrids is risky.

EU emissions of CO2 and other pollutants

The emission of CO2 is proportional to the consumption of fuel in the ration 3.08. The reduction in CO2 emission that could be expected from 250 Wel thermoelectrics is therefore at the highest of 6.8 million tonnes of CO2 / yr. Other pollutants would by reduced proportionally according to Euro6 emission standard limits.

EU raw material consumption and dependence on imports

The perspective of ridding the alternator and replacing it with a thermoelectric should not be regarded as realistic. As a matter of fact, even with a highly efficient thermoelectric, the thermal energy available on exhaust in city traffic is too low - especially with diesel- to supply the entirety of the electric energy demand. A battery capable of storing the electric energy on the road with a view to releasing it in the city would be too big and too costly. It is therefore obvious that no saving in copper imports can be expected.

On the other hand a thermoelectric will consume stainless steel, aluminum for the heat exchangers in addition to the thermo-electric elements. As bismuth telluride cannot be contemplated for the automotive industry because of high cost and toxicity, other materials will have to be used such as magnesium, silicium or manganese. With a European production level at 15 million units /yr the orders of magnitude of the imports would be: for Si, Mg, Mn: 20 000 to 50 000 t / yr; for Fe: 100 000 t / yr; for Ni: 15 000 t / yr; for Cr 30 000 t / yr. These quantities are small as compared to the overall consumption of the industry. All these elements - Si, Mg, Mn, Fe, Ni, Cr - are not scarce for the time being and should not increase European dependence on imports.

Employment

The target cost of the thermoelectric system for the automaker ranges from EUR 300 to 900 according to the application contemplated (cars, heavy duty trucks etc). This cost does not seem to be out of reach, provided further research and development (R&D) will be carried out after HEATRECAR project completion. Assuming an average cost of the man x year at EUR 75000 in the European automotive industry it can be calculated that producing 15 million thermoelectric units would require 60 000 more workers. The actual share of thermoelectric equipped cars will of course be lower but in any case such volume production levels would only be achievable by large companies of the size of most of those active in the automotive sector. Nevertheless there would be room for SMEs at least at the beginning of the implementation of thermoelectrics in the form of options for the 'Green Minded Buyer'.

Exports

Europe is not the only area with regulations or incentives on CO2 emissions. The USA also have stringent regulations favourable to the thermoelectric market. Cars and freightliners almost exclusively run on gasoline there. Gasoline gives higher exhaust temperatures than diesel. in as much as standard driving cycles are much tougher in the USA than in Europe: accelerations are more severe resulting in higher power demand and exhaust temperatures favourable to thermo-electricity. The market for thermoelectrics may therefore be as much in the USA as in Europe, thereby entailing still more employment in Europe.

List of websites: http://www.heatrecar.com