Friday, January 6, 2017

New Thermoelectrics with ZT over 7

So long as we are stuck with heat engines and that does look to be for a long time, ZT matters hugely.  So this work is big news and may well pay of with superior hardware.  We often forget just how much human effort went into inventing the engines and refrigerators we now use.  In fact it is a cautionary for all new tech.

  Again this is promising and suggests that superior solutions are not too far along for mass consumption.  Sadly we will barely notice when it lands as what we do use is heavily optimized already and new tech means a new cycle of integration and optimization.

Still good and welcome.

New Thermoelectrics with ZT over 7, could have twice the efficiency of combustion engines, fridges and be lighter, smaller and more reliable

 December 20, 2016

The search for thermoelectrics with higher figures of merit (ZT) will never stop due to the demand of heat harvesting. Single layer transition metal dichalcogenides (TMD), namely MX2 (where M is a transition metal and X is a chalcogen) that have electronic band gaps are among the new materials that have been the focus of such research. Here, we investigate the thermoelectric transport properties of hybrid armchair-edged TMDs nanoribbons, by using the nonequilibrium Green's function technique combined with the first principles and molecular dynamics methods. They found a ZT as high as 7.4 in hybrid MoS2/MoSe2 nanoribbons at 800K, creating a new record for ZT. Moreover, the hybrid interfaces by substituting X atoms are more efficient than those by substituting M atoms to tune the ZT. The origin of such a high ZT of hybrid nanoribbons is the high density of the grain boundaries: the hybrid interfaces decrease thermal conductance drastically without a large penalty to electronic conductance.

In this paper, they investigate thermoelectric properties of hybrid TMD nanoribbons using a ballistic transport approach. The hybrid armchair-edged nanoribbons show high ZT due to the fact that the hybrid interfaces reduce thermal conductance drastically without a large penalty to electronic conductance. The ZT values of pristine armchair TMD nanoribbons are approximately 1.5 ~ 2.1 at room temperature, while those of hybrid nanoribbons are 2 ~ 3 times that of pristine ones, depending on the number of hybrid interfaces. For example, the ZT of the hybrid nanoribbons MoS2/MoSe2 or MoS2/WS2 with three interfaces is around 3.0 while that of pristine MoS2 is only 1.5. With the increase of temperature and interface number, the ZT values are considerably improved. Our calculations indicate that the highest ZT of hybrid nanoribbons can approach 7.4 at a temperature of 800 K. Moreover, the value is expected to be further increased by doping, edge defects, and adsorption. Therefore, the hybrid TMD nanoribbons may have promising applications in thermal energy harvesting.

They investigated the thermoelectric properties of hybrid armchair-edged TMD nanoribbons using the NEGF method combined with first-principles calculations and molecular dynamics simulations. The hybrid armchair nanoribbons show high ZT due to the fact that the hybrid interfaces reduce thermal conductance drastically. In the hybrid structures that have only three interfaces, the highest ZT approaches 6.1 at 800 K, while the ZT can be further optimized to 7.4 by more interfaces, which is a new record of high ZT among all the thermoelectric materials. They find that it is more efficient to improve thermoelectric properties of the hybrid interfaces by substituting X atoms than by substituting M atoms at high temperature, because the latter can pass through more electrons and thus results in a larger ke. This study could be useful to design high efficient thermoelectric devices. In addition, the effect of the hybrid structures’ widths on the ZT is very week. Although thermal conductance increases with the widths, electronic conductance also increases simultaneously, because more transport channels for electrons and phonons are opened together. 

Nextbigfuture notes that mass production of thermoelectrics with figures of merits over 5 transforms the world

Refrigerators convert heat at about a figure of merit of 3.

Thermoelectric materials can be used as refrigerators, called "thermoelectric coolers", or "Peltier coolers" after the Peltier effect that controls their operation. As a refrigeration technology, Peltier cooling is far less common than vapor-compression refrigeration. The main advantages of a Peltier cooler (compared to a vapor-compression refrigerator) are its lack of moving parts or refrigerant, and its small size and flexible shape (form factor).

The main disadvantage of Peltier coolers is low efficiency. It is estimated that materials with ZT over 3 (about 20–30% Carnot efficiency) would be required to replace traditional coolers in most applications. Today, Peltier coolers are only used in niche applications, especially small scale, where efficiency is not important.

Thermoelectric generators serve application niches where efficiency and cost are less important than reliability, light weight, and small size.

Internal combustion engines capture 20–25% of the energy released during fuel combustion. Increasing the conversion rate can increase mileage and provide more electricity for on-board controls and creature comforts (stability controls, telematics, navigation systems, electronic braking, etc.) It may be possible to shift energy draw from the engine (in certain cases) to the electrical load in the car, e.g. electrical power steering or electrical coolant pump operation.

Cogeneration power plants use the heat produced during electricity generation for alternative purposes. Thermoelectrics may find applications in such systems or in solar thermal energy generation

Thermoelectrics with ZT of 7-9 could have twice the efficiency of combustion engines, fridges and be lighter, smaller and more reliable.

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