Pages

July 25, 2008

Steam power > Combustion > Jet engines > Advanced Thermoelectric > Carnot limit


Thermoelectrics using nanostructures offers the potential of getting very close to the carnot limit of efficiency using very light weight systems for convert heat to electricity. Early versions of thermoelectrics have been sold for many years but are being rapidly improved. About 90 percent of the world’s power (approximately 10 TW) is generated by heat engines that convert heat to mechanical motion, which can then be converted to electricity when necessary. Such heat engines typically operate at 30-40 percent efficiency, such that ~ 15 TW of heat is lost to the environment To be competitive compared to current engines and refrigerators (efficiency 30-40 percent of Carnot limit), one must develop materials with ZT > 3. For the last 50 years, the ZT of materials has increased only marginally, from about 0.6 to 1, resulting in performance less than 10 percent of Carnot limit. There is no fundamental upper limit to ZT.


Some experts doubt that high ZT materials can reach the levels needed to displace the best current systems. The systems shown here represent an estimate of ‘best practice,’ meaning these values are based on the actual performance of up-to-date systems. The comments for each of the ZT levels seems pessimistic. ZT 2.0 is happening now, so each of the level descriptions should be adjusted.
ZT 2 (happening now, commercial by 2011)
ZT 4 (in lab work, commercial 2012-2015)
ZT 20 (ambitious plausible eventually)
ZT infinity (unknown)

These are not ‘best possible’ values as each of these technologies can be expected to improve going forward. The smallest mechanical engine represented is the ‘Solar/Stirling’ machine at 25 kWe. The others are at least 9 times larger and range up to 1600 MWe for the Nuclear/Brayton+Rankine study.


Typical conversion systems become less efficient as they are scaled down to small size. This means there is a crossover point: below some power level thermoelectric technology will tend to be more efficient. Increasing ZT will move the crossover point to higher power levels, increasing the range of applications where thermoelectrics compete. Thus the ZT of 3 to compete with current best car size and refridgerator mechanical systems.


Higher ZT scores are a way to rank materials. The higher the score then the closer the material is getting to Carnots limit (ultimate efficiency for heat engines)

Many people are aware of the importance of getting material that is stronger and lighter like carbon nanotubes. Carbon nanotubes are 100 times stronger than steel by weight. Carbon nanotubes will enable the creation of a space elevator for a new age of cheap access to space. Carbon nanotubes are part of the long progress of human material technology that has defined the times. Stone age, bronze age and iron age.

The dominant Power technology has similarly been important in defining human civilizations. The Age of steam and the age of the internal combustion engine. The overall timeline of heat engine technology. The steam engine has not gone away as 50% of electrical power in the USA is still coal power that is heat water to make steam via modern turbines. Most nuclear power plants use steam turbines.

Technology has been advancing by trying to achieve the theoretical efficiency that is possible with the Rankine Cycle

The heat efficiency of engines are limited by carnot efficiency (theoretical maximum) and endoreversible limits (identical to the Carnot cycle except in that the two processes of heat transfer are not reversible)

Some of the ways of getting to higher thermal efficiency are more expensive and less practical. They involve using gases or liquids that are difficult to handle. The internal combustion engine uses the Otto cycle, diesel uses the diesel cycle, jet engines use the Brayton cycle and supersonic ramjets may use the humprey cycle.

Thermoelectrics offer an alternative conversion of heat to electricity which has simplified handling of liquids or gases and which appear to theoretically and experimentally offer a practical path to higher thermal efficiency. They can also be applied to all aspects of heating and cooling. Power from heat as well as more efficient air conditioners and refridgerators. They also can be systems that are cheaper and lighter than current engines for converting heat. Thermoelectrics have been light enough to already have been chosen for energy conversion in space ships where weight was a critical factor. They can also have fewer moving parts, which means that they can breakdown less frequently and be cheaper to maintain. The actual parts where heat is converted to electricity does not move it is solid state. There are no pistons or gears only a tube that brings hot gas or liquid past the thermoelectrics and tubes for cooling the other side.

Very high efficiency thermoelectrics would be a fundamental upgrade to the power technology of civilization which would effect everything related to heat power and cooling.

The first generation of thermoelectrics are already in beer coolers and car seats with a ZT of 0.7 or less.

The second generation of thermoelectrics will be air conditioners and co-generators in cars with most in power ranges of 100watts to 5kw with ZT 1 to 3. They will be commercial from 2009-2015. (Others had targeted a 2011-2012 start, but progress appears to be going better)

The third generation of thermoelectrics will be replacements for combustion engines or will be significant boosters of efficiency for cars and powerplants and industrial processes. ZT of 4-20. They will be commercial and having wide impact from 2015 onwards. (Others had targeted a 2020 start, but progress appears to be going better.)

Fourth generation thermoelectrics if possible would be able to replace large powerplant thermal converters. ZT 20+.

Second, third and fourth generation thermoelectrics would help to increase the energy of higher temperature RTGs (radioisotope thermoelectric generator) for space vehicles. Most RTGs have been 10% efficient. Advanced thermoelectrics would boost higher temperature RTGs to 40-50% efficiency. RTGs are usually the most desirable power source for unmanned or unmaintained situations needing a few hundred watts or less of power for durations too long for fuel cells, batteries and generators to provide economically, and in places where solar cells are not viable. The advanced thermoelectrics would expand that superior power range to tens or hundreds of kilowatts.


There was the recent announcement from Ohio state University of a material for converting heat to electricity which could enable a 10% increase in car fuel efficiency and that they could soon increase it to achieve 15% improved fuel efficiency. This is 3-4 years from commercialization. Almost all the recent improvements (which have been many significant ones) to thermoelectric materials have come with a decrease in their thermal conductivity. Heremans and his colleagues increased the voltage that the materials create.

There is a lot of existing thermoelectric successes and many promising research projects. Ultrananocrystalline dispersed doped diamond (now available in kg quantities) could achieve a ZT of 4 at 1200C-2000C. This would enable conversion of over 40% of heat to electricity. Getting over 30% heat to electricity would make it practical to replace combustion engines with thermoelectrics.

BMW has found that re-purposing the otherwise wasted exhaust heat to power a thermoelectric generator generating up to 1kW could be used to reduce real-world fuel consumption by as much as 5%. The benefit comes from storing the electricity and using it to pre-heat the engine or power the air-conditioning systems.

Honda’s similar work on the Rankine Cycle [non-thermoelectric form of waste heat capture], which uses exhaust gas to heat water, creating steam that spins a turbine to generate electricity, has found as much as 32kW can be generated by the method, though the weight penalty for the device reduces fuel efficiency benefits to 3.8% at a 100km/h (62mph) cruise in a 2.0L direct-injection petrol four-cylinder.


The Freedom car project which has a portion for researching thermoelectrics is funded for $1.7 billion over 5 years. Cummins (big diesel engine company), GM, Argonne national labs working with the DOE developing and commercializing thermoelectrics.

Thermoelectrics can be used to make zonal air conditioning which would use 700Watts per person instead of 3500-4000Watts for the whole car. 3000 watts for five people will come out 2012 (early target range of 2012-2015). Thermoelectric (TE) HVAC enables the use of distributed cooling/heating units. This approach would cool/heat the specific number of occupants rather than the whole cabin and its components. In addition to decreasing engine load and thus increasing vehicle efficiency, TE HVAC will reduce or eliminate the need for conventional air conditioning working fluids, further reducing greenhouse gas emissions.





Michigan State University has been working with materials with a ZT 1.6-1.8 at 650-700K since late 2007. This is better than what was announced at Ohio university. Michigan State is targeting materials with a ZT 2.6 @ 800K.

Argonne labs working boron doped diamond in Kg quantities. Think they can get ZT over 4 working at 1200-2000C. Temp differentials that high would be like 40+% heat to electricity.

John LaGrandeur, BSST Waste Heat Recovery Program Development of a 100-Watt High-Temperature Thermoelectric Generator John LaGrandeur, BSST LLC

Development of Thermoelectric Technology for Automotive Waste Heat Recovery at General Motors.

GM has 350Watt avg systems on a truck 3% fuel savings can get to 4% with better integration.
The near term target is 10% fuel savings

Efficiency Improvement in an Over-the-Road Diesel-Powered Engine System by the Application of Advanced Thermoelectric Systems Implemented in a Hybrid Configuration Harold Schock, Michigan State University

From Feb 2008. Had ZT 1.6-1.8 at 650-700K. Targeting ZT 2.6 at 800K

Research has been funded for the development of ultrananocrystalline dispersed doped diamond (now
available in kg quantities).


Stable to > 1200°C, mechanically robust
Electrical conductivity increased by heat treatment and boron additions
Seebeck and thermal conductivity measurements in progress
Potential of ZT>4 Bulk TE produced by surface catalyzed reaction with
hydrocarbon molecules binds UNCD particles together covalently in an
sp2 bonded nanocarbon network
Inexpensive, non-toxic, environmentally benign

Future Directions – Long-range program
Processing
– Powder consolidation
– Atmospheres (argon initially, then methane, and hydrogen)
– Temperature (might be as high as 2000°C)
– Doping (p and n type)

FURTHER READING
Thermoelectric news

Advanced aerodynamic solutions are also being developed and should be capable of being economically mass-produced, safe, and amenable to the broad commercial truck market. Factory installed aerodynamic solutions expected to achieve a 20% reduction in trailer aerodynamic drag or 15% improvement in overall fuel economy of the tractor/trailer combination shall be available for purchase by truck fleets within 2 1/2 years from project start date, according to the DOE.

3 comments:

Big Al said...

At 40% conversion efficiency, this could be the "killer app" for solar power.

Cyril R said...

Brian, your own figures prove that thermo-electrics aren't going to make LWRs and other low temp heat sources substantially more efficient even with a ZT of 20.

There is some hope that nano antennas can be used instead, plausibly giving efficiency equal to the carnot limit.

standehaven said...

It is a shame no one considers the use of better technology electric batteries in the quest for energy independence. Thermal efficiencies are fixed and while there are notable advances, all useful systems require some sort of energy storage. That storage is generally chemical, and in the case for high efficiency electric batteries, that is typical. Scavenging waste heat from engines is an excellent way to improve efficiencies... above the Carnot limit! However, complexity must increase involving the use of better electrical storage. Many, many ways of producing electricity are known but few are available to store this energy.