Thursday, February 12, 2009

Automotive Thermoelectric Generation

This is one of those items that allow significant incremental improvement on the efficiency of an internal combustion engine. I am sure there are still issues that have prevented this from been done a long time ago.

However, our increased knowledge makes this an attractive point of departure for a useful bit of innovation. Energy efficiency is no problem when you are using the waste heat dumped by a traditional gasoline or diesel engine.

Most do not realize that only around twenty five percent of such an engine’s power is delivered as brake horse power. This has notched up over the past few years, but climbing from 25% to 35% is dramatic in terms of fuel mileage, but it is still leaving a huge amount of energy on the table as thermodynamic wastage.

What made the argument for Reverse Rankin Cycle engines so compelling in large power plants, was that this waste heat, usually at 100C could deliver a theoretical 75% brake horsepower while lowering the temperature to ambient.

You still have the problem of stripping the heat which is hardly minor into the working fluid. But why has no one ever tried? If in fact we transition to engine generator combinations, then such integration means a plausible doubling of the output work. This is one of those cases in which the customer is on a budget and this adjustment is likely a doubling of capital costs. For most, it is easier to simply let the end buyer eat the fuel costs. Yet there is a second generation power plant design protocol here that has been outright ignored to our loss.

Using thermocouples to operate a few key engine components is possibly a simple and cheap way to save on brake horsepower if only because it releases the alternator load and perhaps the load generated by peripherals. 600W is a lot of energy to work with. Too bad it is not enough to also take over the full electrical load, but it is effectively free to the car.

Thermoelectrics to replace car alternators and improve MPG

February 9, 2009 Thermoelectrics - the phenomena in which a temperature difference creates an electric potential - have been known about for almost 200 years, but practical applications have not been widespread due to their low energy efficiency. That may all now be about to change as Germany automakers Volkswagen and BMW have developed thermoelectric generators (TEG) that recover waste heat from a combustion engine.

According to a report by Prof. Rowe of the University of Wales in the
International Thermoelectric Society, Volkswagen claims 600W output from the TEG under highway driving condition. The TEG-produced electricity meets around 30% of the car’s electrical requirements, resulting in a reduced mechanical load (alternator) and a reduction in fuel consumption of more than 5%.

BMW and DLR (German Aerospace) have also developed an exhaust powered thermoelectric generator that achieves 200 W maximum and has been used successfully for more than 12,000-km road use.

Thermoelectric refrigeration

Thermoelectric have been used for refrigeration utilizing the Peltier effect originally discovered in 1834. An electrical current at the junction of two different metals results in heat being absorbed by one metal and expelled by the other metal. Thermoelectrics can also be used to generate electricity using the Seebeck effect that dates back to 1770. Thermoelectric power generators convert heat energy to electricity. When a temperature gradient is created across the thermoelectric device, a DC voltage develops across the terminals.

Thermoelectric generators

Typical applications for this technology include providing power for remote telecommunications and navigation beacons. A more familiar application is a thermocouple that is a type of temperature sensor that can generate a current proportional to the amount of heat it is exposed to. Thermocouples were used in remote parts of Russian in the 1920s to power radios from a wood fireplace and they also form the basis of radioisotope thermoelectric generators (RTG) that use heat from a radioactive material to power deep space satellites. The drawback to all thermocouple based electric generation is that they are very inefficient at between 3-7%.

Automotive thermoelectric generators (ATEG) have been developed intermittently since 1988 when Porsche made a exhaust ATEG capable of 20-30 watts out of a 944 exhaust system but they have never made it past the prototype stage of development.

Paul Evan

1 comment:

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