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Tangentially Exhaust Heat Recovery is a very interesting concept for ICE vehicles. The side effect of internal combustion engines is the most efficiency you can

Tangentially Exhaust Heat Recovery is a very interesting concept for ICE vehicles. The side effect of internal combustion engines is the most efficiency you can get out of the Otto cycle is like 40% of the heat shoots right out the tailpipe, into the coolant etc. Toyota does this on the Prius where it passes coolant through the exhaust heat to recover it and help get the car up to cycle faster, but there's other cases that have been floated. For example, the reverse of a Peltier cooler is a TEG, which generates electricity from heat directly and use that to charge the battery

semi-extrinsic 2y ago
TEGs are essentially unworkable if you want to produce kilowatt-class energy in a size that can fit a car.
- floatrock 2y ago
  How about in any industrial building that has a smokestack?
  - deleted 2y ago
    [deleted]
- calvinmorrison 2y ago
  You don't want to produce... kilowatt class energy, you would want to offset the losses of running accessories directly off the driveshaft. Spinning an alternator takes energy. Spinning parts also have wear issues like bearings etc. Secondly, in the Prius approach, it's about getting the engine to optimal temperature to reduce emissions and increase efficiency
  - semi-extrinsic 2y ago
    Yeah, but all these things (even the heat transfer from exhaust toncoolant in the Prius) need to transfer many kilowatts of energy to actually work. A kilowatt-class liquid heat exchanger is the size of a large coffee cup. A kilowatt-class TEG is the size of a car engine. See for instance the paper linked below, Fig. 12, their module is 1 m x 0.3 m x 0.35 m. For comparison the Ford 1.0 Ecoboost engine is famously said to fit on an A4 sheet of paper and produces 100 kW. So adding a TEG you would more than double the size of your engine for a 1% efficiency gain. https://pangea.stanford.edu/ERE/db/WGC/papers/WGC/2015/26009.pdf https://pangea.stanford.edu/ERE/db/WGC/papers/WGC/2015/26009...
observationist 2y ago
Unless the temperature differential is huge, hundreds of degrees C, the efficiency of these is tiny - around 5-7%. Incorporating things like stirling generators could theoretically get up to around 35-45% efficiency, but that adds weight and complexity, with the maintenance and production costs offsetting the potential economic gains in reduced fuel usage. Better thermoelectric generator technology is possible - there are lots of exciting possibilities with graphene, and heat pumps, and so forth, but saving $10,000 in fuel costs over 5 years for $15,000 worth of technology in a car doesn't make sense. Auto manufacturers optimize for price and performance and space and maintenance and all sorts of competing factors, so it's not always as easy as engineering a technical solution and slapping it on top of everything else that goes into a product.
- calvinmorrison 2y ago
  We already have heat recovery systems in production models, though not for generating electricity of course. Another driving feature is emissions, not cost. Meeting emissions requirements drives up the cost and complexity of the vehicle.

semi-extrinsic 2y ago

TEGs are essentially unworkable if you want to produce kilowatt-class energy in a size that can fit a car.

floatrock 2y ago
How about in any industrial building that has a smokestack?
- deleted 2y ago
  [deleted]
calvinmorrison 2y ago
You don't want to produce... kilowatt class energy, you would want to offset the losses of running accessories directly off the driveshaft. Spinning an alternator takes energy. Spinning parts also have wear issues like bearings etc. Secondly, in the Prius approach, it's about getting the engine to optimal temperature to reduce emissions and increase efficiency
- semi-extrinsic 2y ago
  Yeah, but all these things (even the heat transfer from exhaust toncoolant in the Prius) need to transfer many kilowatts of energy to actually work. A kilowatt-class liquid heat exchanger is the size of a large coffee cup. A kilowatt-class TEG is the size of a car engine. See for instance the paper linked below, Fig. 12, their module is 1 m x 0.3 m x 0.35 m. For comparison the Ford 1.0 Ecoboost engine is famously said to fit on an A4 sheet of paper and produces 100 kW. So adding a TEG you would more than double the size of your engine for a 1% efficiency gain. https://pangea.stanford.edu/ERE/db/WGC/papers/WGC/2015/26009.pdf https://pangea.stanford.edu/ERE/db/WGC/papers/WGC/2015/26009...

observationist 2y ago

Unless the temperature differential is huge, hundreds of degrees C, the efficiency of these is tiny - around 5-7%. Incorporating things like stirling generators could theoretically get up to around 35-45% efficiency, but that adds weight and complexity, with the maintenance and production costs offsetting the potential economic gains in reduced fuel usage. Better thermoelectric generator technology is possible - there are lots of exciting possibilities with graphene, and heat pumps, and so forth, but saving $10,000 in fuel costs over 5 years for $15,000 worth of technology in a car doesn't make sense. Auto manufacturers optimize for price and performance and space and maintenance and all sorts of competing factors, so it's not always as easy as engineering a technical solution and slapping it on top of everything else that goes into a product.

calvinmorrison 2y ago
We already have heat recovery systems in production models, though not for generating electricity of course. Another driving feature is emissions, not cost. Meeting emissions requirements drives up the cost and complexity of the vehicle.