13.2 Automotive thermoelectric generators

Automotive thermoelectric generators are revolutionizing vehicle efficiency. By converting waste heat from exhaust gases and engine coolant into electricity, these devices can boost fuel economy by up to 5%. They're a game-changer for reducing emissions and powering onboard electronics.

Integrating these generators into cars isn't easy, though. Engineers must balance optimal placement, temperature management, and electrical system integration. Despite challenges like added weight and durability concerns, the long-term benefits make automotive thermoelectric generators an exciting frontier in green transportation.

Automotive Thermoelectric Generator Applications

Exhaust Gas and Engine Coolant Heat Recovery

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• Exhaust gas recovery systems capture waste heat from vehicle exhaust
  • Converts thermal energy into electrical power
  • Typically placed near the catalytic converter for optimal temperature differentials
  • Can generate 100-750 watts of power depending on vehicle size and driving conditions
• Engine coolant heat recovery utilizes excess heat from the engine cooling system
  • Placed between the engine block and radiator
  • Generates electricity from temperature difference between hot coolant and ambient air
  • Produces 50-300 watts of power in most passenger vehicles
• Combined systems can recover up to 5% of fuel energy normally lost as waste heat
• Recovered energy used to power vehicle electrical systems or charge batteries
  • Reduces load on alternator
  • Improves overall fuel efficiency by 2-5% in typical driving conditions

Vehicle Fuel Efficiency Improvements

• Thermoelectric generators (TEGs) contribute to increased fuel efficiency
  • Reduce parasitic losses from alternator load
  • Provide supplemental power for hybrid and electric vehicle systems
• Fuel savings vary based on driving conditions and vehicle type
  • Highway driving sees greater benefits due to consistent high exhaust temperatures
  • Stop-and-go traffic reduces TEG effectiveness as exhaust temperatures fluctuate
• Integration with start-stop systems enhances fuel economy in urban environments
  • TEGs provide power to restart engine and run accessories when engine is off
  • Can improve fuel economy by up to 10% in city driving scenarios
• Long-term fuel savings offset initial system costs
  • Payback period typically 3-5 years for passenger vehicles
  • Shorter for commercial vehicles with higher annual mileage

Thermoelectric Module Design Considerations

Optimal Placement and Temperature Management

• Thermoelectric module placement crucial for maximizing power generation
  • Exhaust system modules positioned after catalytic converter
    • Balances high temperatures with material limitations
    • Avoids interference with emissions control systems
  • Engine coolant modules placed between engine block and radiator
    • Captures heat before coolant temperature drops in radiator
• Temperature differentials drive thermoelectric efficiency
  • Larger temperature differences between hot and cold sides increase power output
  • Typical exhaust gas temperatures range from 300°C to 600°C
  • Coolant temperatures usually between 80°C and 110°C
• Heat exchanger design critical for maintaining temperature gradient
  • Fin structures increase surface area for heat transfer
  • Materials with high thermal conductivity (copper, aluminum) improve heat flow
• Thermal management systems prevent overheating of thermoelectric materials
  • Active cooling systems use engine coolant or separate cooling loops
  • Passive heat sinks dissipate excess heat to surrounding air

Power Generation and Efficiency Factors

• Power generation capacity depends on multiple factors
  • Module size and number of thermoelectric couples
  • Quality of thermoelectric materials (figure of merit ZT)
  • Temperature difference across the module
• Typical automotive TEGs produce 100-1000 watts of power
  • Large commercial vehicles can generate up to 5 kilowatts
  • Passenger cars generally in the 200-500 watt range
• Efficiency of thermoelectric conversion typically 3-8%
  • Advanced materials and designs aim to reach 10-15% efficiency
  • Compared to 30-40% efficiency of internal combustion engines
• Power output varies with driving conditions
  • Higher speeds and loads increase exhaust temperatures and power generation
  • Idle and low-speed conditions reduce TEG effectiveness

Vehicle Integration Challenges

Electrical System Integration and Weight Considerations

• Integration with vehicle electrical systems requires careful design
  • DC-DC converters match TEG output to vehicle electrical system voltage
  • Power management systems regulate TEG output and protect vehicle electronics
  • Integration with battery management systems in hybrid and electric vehicles
• Weight considerations impact overall vehicle efficiency
  • TEG systems typically add 10-30 kg to vehicle weight
  • Weight increase partially offset by reduced alternator size
  • Use of lightweight materials (aluminum, composites) minimizes weight penalty
• Packaging constraints in modern vehicles limit TEG size and placement
  • Compete for space with emissions control systems and structural components
  • Modular designs allow for flexible integration in different vehicle platforms
• Electromagnetic compatibility (EMC) issues must be addressed
  • TEGs can produce electromagnetic interference
  • Shielding and filtering required to meet automotive EMC standards

Durability and Environmental Challenges

• Durability in automotive environments crucial for long-term reliability
  • TEGs must withstand vibration, thermal cycling, and shock loads
  • Typical design life of 10-15 years or 150,000-200,000 miles
• Thermal expansion mismatches between materials can cause stress and failure
  • Flexible mounting systems and compliant thermal interfaces reduce stress
  • Advanced bonding techniques improve long-term reliability
• Corrosion resistance necessary for exhaust system applications
  • High-temperature alloys and protective coatings used for TEG components
  • Sealed designs prevent ingress of corrosive exhaust gases
• Environmental considerations include:
  • Resistance to road salt, debris, and water ingress
  • Ability to function in extreme temperatures (-40°C to +85°C ambient)
  • Compliance with end-of-life vehicle recycling regulations