Thermopower! Currently over half of the energy generated in the world is lost as waste heat

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1 SOURCE: CALTECH MATERIALS SCIENCE COVER STORY Thermopower! Are things hotting up in Thermoelectrics? Krys Bangert investigates. E nergy is one of the biggest topics of the moment, and for good reason. Humanity s exploitation of fossil fuels and natural resources has left the planet in a perilous situation; teetering on the brink of global climate change. At the crux of the matter is how we, as a race, use energy. Currently, the bulk of our energy infrastructure is based upon unsustainable green house gas emitting resources, such as fossil fuels. It is widely agreed, that in order to avoid further damaging climate change effects, we need to need to make more efficient use of what non sustainable resources we have left, and further develop alternative energy technologies. Unfortunately, this process takes time. In the intervening period, solutions need to be found to facilitate the transition to a low carbon infrastructure. One of the best ways of achieving this is by increasing the efficiency of existing designs. Currently over half of the energy generated in the world is lost as waste heat 3, due to the parasitic losses inherent in the various systems. If even a fraction of this thermal energy could be recovered, it would greatly increase the efficiency of these various processes. This is where thermoelectrics come in. Harnessing heat Thermoelectrics are solid state devices that can directly convert heat (via a temperature gradient) into electrical energy. They can also work inversely, using electrical energy to move heat (see The thermoelectric effect p2). They are typically constructed in arrays of thermoelectric modules. Each module containing a thermocouple made with two different thermoelectric elements and interconnects (see picture right). The two thermoelectric elements are commonly p-type and n-type semiconductors, with positive and negative free charges respectively. These elements are arranged with the hot ends interconnected and the cold ends separately connected to an external load; this ensures there is an opposing charge buildup at the cold end of the module and a net current flow. The thermocouples are connected electrically in series and thermally in parallel, this allows the electrical voltage to be scaled up (individual thermocouples produce only a few mv). Thermoelectrics have many benefits including, high resistance to extreme environments (temperature, pressure, shock, radiation, EM), low maintenance (no moving parts), high level of redundancy (large array structure), scalability and graceful degradation characteristics 4. These unique properties Currently over half of the energy generated in the world is lost as waste heat have enabled them to be used in many applications as thermoelectric generators (or TEGs, using the Seebeck effect) and heat pumps (using the Peltier effect). One of the first high profile uses of a thermoelectric system came in the early 1960s and 70s when NASA used radioisotope (Pu238) powered TEGs to provide electrical power for their space craft. This power source was chosen over solar panels for the missions because of its more consistent and low maintenance electrical power supply 5. Since that time, the commercial market for thermoelectrics has grown, mainly in the field of remote power generation and solid state cooling applications. With TEG and cooling systems currently estimated to make between and Million USD($) per annum respectively 2. Unfortunately, the commercial uptake of A typical TEG design. 1 E-Futures 26 April 2010

2 SOURCE: CALTECH MATERIALS SCIENCE SOURCE: INTERNATIONAL SYMPOSIUM ON NANO-THERMOELECTRICS - JUNE 2007 the technology has been limited to niche applications, because the thermal efficiency of the units is poor compared to most modern heat engines. Typically only producing 5% of the Carnot cycle efficiency (the maximum efficiency possible by any heat engine). However, in recent years there has been a resurgence of interest in thermoelectrics, driven by the need to find a solution to the current energy problems. This has led to an increase in research funding, and subsequently, breakthroughs have been made in the understanding of thermoelectricity and how to create more efficient thermoelectric materials. Second generation The effectiveness of a thermoelectric device is typically assessed in two ways; by its Carnot efficiency and by a material dependant factor called the thermoelectric figure-of-merit, ZT 4 (see Thermo performance p3). The 1st generation of mass produced, or bulk thermoelectrics, were created over four decades ago. With a typical ZT value of between 0.8 and 1.0 (see pic right). The majority of those designs were based upon alloyed semiconductors such as Bismuth Telluride (Bi 2 Te 3 ). These materials were used because of their high carrier concentrations, which enabled them to achieve conversion efficiencies of 5 to 6%. It was subsequently suggested by some researchers that an alloy limit of thermoelectric performance had been reached 6. This theory was not disproved until the late 1990s, when new ideas about incorporating nanoscale geometry were tested, demonstrating that a ZT above 1.0 was achievable. Since then many different approaches have been tried, with varying degrees of success (see Better by design p5). So far the only 2nd generation bulk thermoelectrics, close to been commercialized, are the nanoscale fine grain BiTe compounds, made by the grind and press technique 2. These thermoelectrics have been demonstrated to have ZT values ranging from 1.3 to 1.7 and have a predicted conversion efficiency of around 11 to 15%. The company, GMZ Energy has announced it is already working on producing commercial quantities of these materials 2. Other research groups have claimed higher values of ZT (4.0 using B 4 C/B 9 C 1 ), but they have yet to demonstrate an economically viable method mass production. However, if this was The figure-of-merit (ZT) timeline. Rapid performance improvements were made in the late 1990s THE THERMOELECTRIC EFFECT The Seebeck effect: In 1821, Thomas Johann Seebeck discovered that a compass reading could be magnetically deflected by a circuit made from two different metals (A, B) with a temperature difference between the junctions (T c,t h ). This effect showed that a thermal energy can produce a voltage, and induce an electric current in certain a pairs of metals or semiconductors. This voltage was found to be proportional to the temperature difference between the two junctions, and was subsequently known as the Seebeck coefficient (or the thermopower). This effect occurs when a temperature difference causes the charge carrying electrons (or holes) to diffuse from the hot side of the material, to the cold. This diffusion causes an opposite charge to be built up in the immobile nuclei, creating a gradient that produces a thermoelectric voltage (ΔV). The Peltier effect: This effect was discovered by Jean- Charles Peltier in He found, that when an electric current flows through a circuit made from two different metals, heat is given off at the upper junction and absorbed at the lower. This occurs because the electrons flow from a high to a low density, causing a cooling effect in a similar fashion to the expansion of an ideal gas. The heat flow between the The basic thermoelectric system. junctions was found to be proportional to the electric current, giving rise to the Peltier coefficient. It was also found, that the direction of the heat transfer could be reversed depending on the polarity of the current. This enables thermoelectric devices to be used as both heaters and coolers. The Thomson effect: In 1851, William Thomson (or as he later became known, Lord Kelvin) unified both the Seebeck and Peltier effects, by explaining their interrelationship using thermodynamics. He demonstrated that heat transfer is proportional to the electric current and temperature difference, in a thermoelectric material. This showed that the Peltier coefficient is equal to the Seebeck coefficient, multiplied by an absolute temperature. The only exception to this rule is when a material superconducts, giving it zero resistance to electrical current. 2 E-Futures 26 April 2010

3 SOURCE: NATURE MATERIALS - FEB 2009 eventually resolved, efficiency ratings of over 20% 7 could be conceivable in the near future, this would have a large impact on the number of applications that thermoelectrics could be used for. A green future? The big question is; can the performance improvements recently made in thermoelectrics, enable them compete with other types of energy harvesting systems, and be financially viable? Over the last few years, many companies and institutions have been conducting studies to find that out. Some of the most recent high profile work has been in the automotive industry, where thermoelectrics are been considered as a way of increasing vehicle fuel economy. Research conducted on petrol powered vehicles has shown that the largest proportion of the energy wasted, is in the exhaust and cooling systems. It is estimated that, light duty vehicles waste the equivalent of 46 billion gallons of petrol annually, through the heat losses in the exhaust system alone. The car manufacturer BMW has designed and built prototype systems incorporating a TEG, into the exhaust and catalytic converters of a vehicle. The electricity generated from the system was used to reduce the alternator load on the engine 3. Their tests have demonstrated that a 4-8% reduction of CO 2 is achievable with current bulk thermoelectrics. They have also estimated that a CO 2 reduction of 6-11% could be possible with the use of second generation materials 8. The U.S. Department of Energy s FreedomCar programme is currently looking at this technology, to see if it can enable them to achieve their target of a 10% increase, in the fuel economy of cars and trucks by Thermoelectrics have also have been looked at to help decrease the engine heating time (via the coolant) and the make air conditioning more efficient using of the Peltier effect 3. It is hoped that if the materials figure-of-merit can be improved even further, enough energy could generated from a vehicle exhaust to replace the alternator 3. The interest in this system is growing rapidly and it may be give added impetus soon, because R-134a, one of the main vehicle air conditioning refrigerants, is due to be banned in European cars by THERMO PERFORMANCE When comparing systems, the thermoelectric figure-of-merit (ZT) is the number that all scientists and engineers look for. It comprises of the three main electric and thermal material transport properties (for the n and p type couple elements), and the absolute operating temperature. It is defined by the equation 1 : Z=α 2 /(ρ.k) Where α, is the Seebeck coefficient, ρ, is the electrical resistivity and k, is the thermal conductivity 4. One of the other big applications currently been looked at in thermoelectrics, is industrial waste heat reclamation. Manufacturing industries on average, loose 33% of their energy as waste heat. The U.S. the manufacturing sector alone losses a staggering 3,000 TWh every year, equating to roughly 1.72 billion barrels of oil 2. Detailed studies carried out in Japan 9, Thailand and the U.S, have looked at industrial waste heat in many applications, such as incinerators, steel mills, diesel engines and electrical transformers. These studies have shown that integrated thermoelectrics can provide a benefit, when used as a part of a co-generation system. They are especially affective when it is difficult to transport the heat to a separate conversion system. The study in Thailand estimated that gas turbine and diesel cycle cogeneration systems produced electricity at 33% and 40% of the fuel input, respectively. Of this Exhaust to energy Materials with a high value of ZT, give high power conversion efficiency. Thus, it is the aim of all thermoelectric designers to create materials with high Seebeck coefficients, high electrical conductivity and low thermal conductivity 7. Unfortunately, this is not a straight forward process, because the properties are highly interdependant 3. amount, the useful waste heat from the exhaust stacks was estimated to be 20% for a gas turbine and 10% for the diesel cycle. This roughly equates to 100MW of It is estimated that, light duty vehicles waste the equivalent of 46 billion gallons of petrol annually net power that could potentially be harnessed. The U.S. study concluded that if thermoelectrics with a ZT value of 1 to 2 could be applied to similar processes, between 0.9 and 2.8TWh of electricity could be produced. Japan is one of the few countries already to have invested in applying this technology, with some of their programs starting as far back as Their most recent 2007 BMW s new TEG system converts waste heat from the exhaust system into electrical energy. Other manufacturers including Honda and VW are also looking into similar systems. 26 April 2010 E-Futures 3

4 SOURCE: NATURE MATERIALS - FEB 2009 study showed that they were making good progress toward their goal of an average 15% system efficiency, thus showing that thermoelectrics can work effectively in an industrial context. Hybrid solar power is another application currently been looked at. These systems would use a combination of solid state photovoltaics (PV) and thermoelectrics to absorb the low and high wavelength solar radiation. This arrangement would enable both systems to operate within their optimum absorption spectrums, and facilitate cooling of the PV junctions, using the thermoelectric modules as the heat sinks 4. Other solar systems, featuring high efficiency thermionic converters (TIC) coupled with thermoelectrics, have also been researched. Efficiencies close to 40% have been claimed to be produced by these systems 10. Thermoelectrics have also been considered for use in combined heat and power (CHP) systems, using a TEG to produce electricity as a by product of the heat production process 10. Similarly, but on a much larger scale, investigations have been carried out to see if the heat from industrial waste hot water, can produce enough electricity to be financially viable. The results from that showed that over a three year period, the system could break even with standard utility rates. Many other smaller niche applications, such as, telecommunications lasers, military smart munitions 11, battery replacements and enhancing the combustion efficiency of wood stoves in developing countries 6, have been looked at. Making the grade So, what does the future hold for thermoelectrics? It is difficult to tell. The science, technology and commercial interest has improved in recent years, but many problems still remain, that could hinder their wide scale application. The biggest of which been, the conversion efficiency is still too low to compete effectively with other energy systems, even using second generation thermoelectrics. Just to put this in context, currently the best performing thermoelectric has a maximum ZT value of 3.5. Even if a thermoelectric with a ZT value of 4 could be created, that worked across the entire temperature range, in both p and n type materials (none can currently), it would be still less efficient than the existing Unless a thermoelectric material is discovered with a ZT value of >4, the impact on the renewable energy sector looks likely to be still limited to niche applications Solid-state vs. Mechanical When compared to large scale mechanical systems, exceptional figure-of-merit (ZT) performance would be needed to make thermoelectrics be competitive. commercially produced mechanical systems (see diagram) 2. However, it is not all bad news, even if thermoelectrics cannot compete in large scale energy production, there is still potential at the milli/microwatt power levels. In these performance ranges thermoelectrics have higher efficiencies than the mechanical systems, allowing them to be more competitive 2. The vehicle exhaust TEG system is a good example of this trend. However, competition from other similar fuel economy systems is likely to still be tough. Honda s Rankine steam engine system generates electricity from the same source, with an overall engine efficiency increase of 3.8% and BMW s own Turbosteamer system has claimed fuel efficiency improvements of up to 15%, by enhancing the cars power train 2. Unless a thermoelectric material is discovered with a ZT value of >4 12, the impact on the renewable energy sector looks likely to be still limited to niche applications, with the prospect of hybrid systems broadening this slightly in the future. However, there are still many grey areas in our understanding of the thermoelectric effect, especially in relating to the fields of nanostructuring and carrier transport 6. If breakthroughs in these areas continue to be made and manufacturability increases, there could still yet be a green future for thermoelectrics. Krys Bangert is a PhD Student studying at the University of Sheffield. 4 E-Futures 26 April 2010

5 SOURCE: ENERGY ENVIRON. SCI 2009, CHEMISTRY OF MATERIALS 2010 BETTER BY DESIGN Theoretically, a material with the best thermoelectric properties would combine the high electrical conductivity of a metal (or a well-ordered crystal), with the low thermal conductivity of glass. Scientists refer to this ideal material as a phonon glass, electron crystal (PGEC) 6. So far no thermoelectrics have been produced that can replicate these ideal properties 7, but with recent advanced in physics, chemistry and materials science, some are now starting to get close. Methods such as increasing the electrical carrier concentration, are currently been looked at by research teams. But due to recent breakthroughs in understanding, the majority of work in this field has been focused upon the idea of reducing a materials thermal conductivity, by disrupting the phonon transport. Phonons are the quantized vibrations in the lattice of a solid material that carry the heat energy. They come in a spectrum of wavelengths, each contributing a different amount, to the overall thermal conductivity of a material. It was found recently, that changes to a materials structure, could scatter these phonon vibrations 6. This has led to the development of many different thermoelectric materials that harness this effect, to increase the thermoelectric figure-of-merit. Two main approaches have been taken by the researchers to achieve this effect, the first, was to create new custom thermoelectric materials with the required properties, and the second, was to use existing thermoelectric materials with a different nanostructure 6. Currently, the most advanced type of nanostructured thermoelectric, are the thin films. They are typically created in many different forms, such as superlattices (2D), nanowires (1D), and quantum dots (0D) 6, using an atomic layer deposition technique. These structures are designed to be small enough to disrupt the mean free path of the phonons, whilst still allowing the electrons and holes to pass easily. This effect lowers the materials thermal conductivity. At present, these designs have some of the highest ZT values in the field thermoelectrics, with the Quantum-dot superlattices and normal superlattices, having ZT values of 3.5 (at 575K) and (at 300/400K) respecively 2. However, there is a downside to these enhanced properties, the thin films are hard to fabricate and the processes are inherently expensive. A lot of work is still needed before these materials can be used at a commercial level, but the data gathered did provide researchers with another potential avenue to explore. It was found in the transport models for the thin films, that an exact nanoscale geometry was not needed to successfully inhibit the phonon thermal conductivity 6. This gave rise to a cheaper, easier to produce, bulk nanostructured thermoelectrics. These materials don t have an exact geometry, but retain the high density interfaces of the thin films. They are often classed as nanocomposites because their structures consist of nanoparticles, embedded in a host heterogeneous structure 13. They are typically made from bulk thermoelectric materials, which have been processed (e.g. ground using a ball milling technique) to break them up into nanocrystalline pieces, which are then hot pressed to made a nanograined material 7. This system has the advantage of been simple and cheap to fabricate and can be practically applied to any material system 6. Other production techniques such as nanoscale precipitate induction and matrix encapsulation 6 have also proved to effective in the creation of nanocomposites. Arguably, the materials that have been shown to be the closest to the PGEC concept, are the custom designed clathrates and skutterudites. These materials have been designed with caged structures formed by the host atoms, which contain much larger guest atoms. These guest atoms rattle against the cage structure and produce low frequencies, which inhibit phonon transport. The clathrates are typically formed from silicides and germanides, and the skutterudites are made from antimony-based compounds 3. The two types of design have produced ZT values in the religion of , but again, the production methods are currently too expensive and time consuming for mass production. b) a) A TEM image of a Si 80 Ge 20 nanocomposite b) Crystal structure of cubic a Ba 8 Ga 16 Si 30 Clathrate 26 April 2010 E-Futures 5

6 REFERENCES 1 Ghamaty, S. & Elsner, N. B. Si/SiGe Quantum Well Thermoelectric Materials and Devices for Waste Heat Recovery From Vehicles and Industrial Plants. International Symposium on Nano-Thermoelectrics (2007). 2 Vining, C. B. An inconvenient truth about thermoelectrics. Nature Materials 8, 83-85, doi: / nmat2361 (2009). 3 Kleinke, H. New bulk Materials for Thermoelectric Power Generation: Clathrates and Complex Antimonides. Chemistry of Materials 22, , doi: /cm901591d (2010). 4 Fleurial, J.-P. Thermoelectric Power Generation Materials: Technology and Application Opportunities. Global Innovations: Materials for Energy 61 (2009). 5 Technology, C. I. o. Caltech Thermoelectrics Website, < (2010). 6 Minnich, A. J., Dresselhaus, M. S., Ren, Z. F. & Chen, G. Bulk nanostructured thermoelectric materials: current research and future prospects. Energy & Environmental Science 2, , doi: / b822664b (2009). 7 Kanatzidis, M. G. Nanostructured Thermoelectrics: The New Paradigm? Chemistry of Materials 22, , doi: /cm902195j (2010). 8 Bell, L. E. Broader Use of Thermoelectric Systems in Vehicles. 1st Thermoelectrics IAV Conference (2008). 9 Kajikawa, T. Approach to the Practical Use of Thermoelectric Power Generation. Journal of Electronic Materials 38, , doi: /s (2009). 10 Riffat, S. B. & Ma, X. L. Thermoelectrics: a review of present and potential applications. Applied Thermal Engineering 23, , doi: /s (03) (2003). 11 Bierschenk, J. & Ieee. in th Ieee International Symposium on the Applications of Ferroelectrics IEEE International Symposium on Applications of Ferroelectrics (Ieee, 2008). 12 Ovsyannikov, S. V. & Shchennikov, V. V. High-Pressure Routes in the Thermoelectricity or How One Can Improve a Performance of Thermoelectrics. Chemistry of Materials 22, , doi: / cm902000x (2010). 13 Dresselhaus, M. S. et al. New Composite Thermoelectric Materials for Energy Harvesting Applications. JOM 61, (2009). 6 E-Futures 26 April 2010