Saturday, June 18, 2011

Thermoelectric Heat Conversion Increased Ten Percent

Converting heat energy directly to electrical power is equivalent to converting solar energy directly to electrical power.  Both are amply available and unsurprisingly we are able to convert some of it.  Again ten percent plus or minus a bit is typical and generally achievable.  Notching either to the twenty percent level in a practical system has stymied researchers for decades in the case of solar.

Perhaps this advance with outright heat conversion will prove to be the exception.

Yet waste heat produced on a commercial heat engine is likely best drained off using a water jacket or its equivalent.  It is possible to recover 75% of that heat as brake horsepower by putting it through a Reverse Rankin Cycle Engine.  Why bother with a twenty percent solution when you have a seventy five percent solution that may possibly lose another twenty percent producing electrical power?

There will still be appropriate applications.  How about on chips to draw of some of the heat when it overheats?

MAY 23, 2011

An alloy of lead telluride (PbTe), for example, which has long been used to generate electricity aboard satellites, has a ZT of around 0.8. Researchers have now made one of the most common thermoelectric materials more efficient with a ZT of 1.8. A ZT of 1.8 at 850K has a energy conversion efficiency of about 20 to 22% instead of 13 % at ZT of 0.8.

To increase ZT, researchers typically try to increase a material's electrical conductivity as much as possible while holding down its thermal conductivity. In 2008, researchers led by Jeffrey Snyder, a materials scientist at the California Institute of Technology in Pasadena, spiked PbTe with thallium, which boosted the ZT to 1.5. The group later determined that the thallium altered the electronic structure of the crystal, improving its electrical conductivity.

But thallium is toxic, so Snyder and his colleagues wanted to determine if they could match the improvement with other additives. Earlier this year, Snyder and his team at Caltech reported in Energy & Environmental Science that substituting sodium for thallium produced a ZT of 1.4. Now, Snyder's team, in combination with researchers from the Chinese Academy of Sciences' Shanghai Institute of Ceramics, report online today in Nature that adding selenium and sodium gives them a maximum ZT of 1.8. The selenium not only further improves the electrical conductivity, it also reduces the thermal conductivity, Snyder explains.

The Caltech Researchers discuss the new bulk thermoelectrics and their applications.

"These new materials are roughly twice as effective as anything seen before, and they work well in a temperature range of around 400 to 900 degrees Kelvin," says Snyder. "Waste heat recovery from a car's engine falls well within that range."

In other words, the heat escaping out your car's tailpipe could be used to help power the vehicle's electrical components—and not just the radio, wipers, and headlights. "You'll see applications wherever there's a solid-state advantage," Snyder predicts. "One example is the charging system. The electricity to keep your car's battery charged is generated by the alternator, a mechanical device driven by a rubber belt powered by the crankshaft. You've got friction, slippage, strain, internal resistance, wear and tear, and weight, in addition to the mechanical energy extracted to make the electricity. Just replacing that one subsystem with a thermoelectric solution could instantly improve a car's fuel efficiency by 10 percent."

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