Introduction

Thermoelectric materials directly convert heat to electricity by generating a potential difference in response to a temperature gradient (and vice versa). They hold great promise for applications in power generation and refrigeration technologies. While much of the recent attention given to thermoelectrics has focused on their ability to cool and heat, thermoelectric materials can also be applied to generate power from a wide variety of heat sources. A major challenge in thermoelectric materials development is to manipulate the structure and physical properties in order to improve thermoelectric power, electrical conductivity and simultaneously reduce thermal conductivity. This is needed to improve the so-called figure of merit of the materials

 

where S is the thermoelectric power (Seebeck coefficient), σ is the electrical conductivity, κ is the thermal conductivity, and T is the operating temperature. In general, it is difficult to decrease the thermal conductivity, κ, without simultaneous decrease in σ and novel approaches are needed.

 

The figure of merit ZT for thermoelectric materials from 1950 till today has increased stepwise. In the middle of the 1990s, Slack, Dresselhaus et al. and others developed new approaches toward further increase of ZT. Approaches such as band gap engineering, controlled disorder, nanostructuring and quantum confinement were applied to design new materials and methods of synthesis resulting in a significant increase of ZT (ZT ~ 3). However, these values were obtained using thin-film methods, which are difficult to be incorporate in high energy power generation applications with large thermal gradients. The requirements placed on materials needed for this task are not easily satisfied. Not only must they possess high conversion efficiency, but they must also be composed of nontoxic and abundantly available elemental materials having high chemical stability in air even at temperatures of 700–1200 K. Oxide materials are particularly promising for TE applications because of their oxidation resistance stability even at high temperatures in air, reduced toxicity, easy manufacture and low cost.

 

 

Physics of thermoelectric effect

 

Physiscs of thermoelectric effect

 
The Seebeck effect is the conversion of temperature differences directly into electricity. The Seebeck effect is caused by two things: charge-carrier diffusion and phonon drag. Charge carriers in the materials will diffuse when one end of a conductor is at a different temperature than the other. Hot carriers diffuse from the hot end to the cold end, since there is a lower density of hot carriers at the cold end of the conductor, and vice versa. If the conductor were left to reach thermodynamic equilibrium, this process would result in heat being distributed evenly throughout the conductor The movement of heat (in the form of hot charge carriers) from one end to the other is a heat current and an electric current as charge carriers are moving. In a system where both ends are kept at a constant temperature difference, there is a constant diffusion of carriers.

 

The Peltier effect is the presence of heat at an electrified junction of two different metals. An external electric potential can drive the heat carrying charges, then the heat can be forced to flow from one end to the other. The coefficient of performance and the maximum temperature drop that can be achieved is again related to the efficiency of the thermoelectric materials through the thermoelectric figure of merit ZT.


Oxide thermoelectric materials and modules  
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In high energy power generation applications with large thermal gradients, the requirements placed on materials needed for this task are not easily satisfied. Not only must they possess high conversion efficiency, but they must also be composed of nontoxic and abundantly available elemental materials having high chemical stability in air even at temperatures of 700–1200 K. Oxide materials are particularly promising for TE applications because of their oxidation resistance stability even at high temperatures in air, reduced toxicity, easy manufacture and low cost.

 

Oxide thermoelectric materials have been rapidly developed in the last decade, virtually exclusively in Japan. The discoveries of a large TE response in some transition metal oxides such as manganites, titanates, cobaltites, lanthanide molybdenum oxides and copper aluminate recently gave rise to interest in TE oxides. Among these transition metal oxides, the p-type misfit-layered Ca3Co4O9 exhibits unusual TE properties: coexistence of a large thermopower and a low resistivity. The performance of n-type oxides, however, has remained at a modest level. Recently, the highest performance of n type bulk oxides was achieved for the Al and Ga doped ZnO, ZT = 0.65 at 1247 K. Further improvement of the ZT values in these bulk phases is possible via the control of proper amount nanometer size inclusions in composites, as recently shown theoretically. TE modules using oxide materials have been recently reported. However, their performance is lower than expected, considering the properties of their starting materials. This is thought to be mainly due to the contact resistance at the oxide/metal junctions, thus limiting the magnitude of the output power. Given the high temperatures of applications with TE oxides, conventional materials and methods for constructing electrical contacts cannot be used.

 

ZT line

TE modules using oxide materials have been recently reported. However, their performance is lower than expected, considering the properties of their starting materials. This is thought to be mainly due to the contact resistance at the oxide/metal junctions, thus limiting the magnitude of the output power. Given the high temperatures of applications with TE oxides, conventional materials and methods for constructing electrical contacts cannot be used. If a very wide operational range of temperature (~300-1200 K) is needed, no single thermoelectric material is suitable. It is therefore necessary to combine different materials, each of which has its own optimal temperature range, for achieving optimum performance. This can be done by segmented modules where the p- and n-legs are formed of different materials joined in series. An advanced concept of a segmented p-n thermocouple was proposed for the first time by Jeff Snyder et al. in 1999.

Competing with current electricity costs will mandate the allowable TE system cost. Low cost and high volume production methods of TE materials must therefore be developed in order to achieve this goal. Meanwhile, maintaining a large temperature difference across thin TE devices presents significant engineering challenges. Some of these tasks relate to the design of the module, selection of the material and the assembly principle for avoiding uniaxial stresses in the material. The challenges will also appear both in terms of the conduction and heat transfer rate to and from the module. Obtaining high heat transfer rates will require advances in materials, in thermal contacting and in heat exchange systems with high heat transfer coefficients. The current project will explore them through different contacting technologies and through integration of heat transfer surfaces using e.g. microchannels and/or other layouts, specifically tailored for the individual application, to minimize the temperature drop over the electrically insulating surfaces. Using this approach the heat transfer surfaces, module and elements will be co-designed, leading to unique designs with few tradeoffs and thus enhancing the potential for high outputs. Finally, by carefully designing the load side, power electronics and control strategy, the complete TE system can be optimized to maintain high system efficiencies over a wide range of operating conditions, thus increasing power output over time. 

Steady growth in the demand for energy saving technologies has offered the groups involved in this project a unique opportunity to undertake new research and development in collaborations with a new and dynamic company specializing in thermoelectric system integration (ALPCON A/S) as well as number of end-user companies. The advantage of multi-discipline research groups lies in core of this project and it brings a variety of perspectives to bear on a common problem of high temperature oxide generator for heat recovery. Finally, by sharing knowledge and other resources, the collaborative research groups from three major universities in Denmark, five companies and two of the leading international groups can achieve the ambitious goal of this proposal that individual laboratories or scientists could not hope to accomplish. 

Innovative value, impact and relevance of the project

Waste heat losses arise both from equipment inefficiencies and from thermodynamic limitations on equipment and processes. It is estimated that somewhere between 20 to 50% of industrial energy input is lost as waste heat in the form of hot exhaust gases, cooling water, and heat lost from hot equipment surfaces and heated products. As the industrial sector is continuously investing to improve its energy efficiency, recovering waste heat losses provides an attractive opportunity for an emission free and less costly energy resource. Despite many new materials discoveries in the past decade, the development of large-scale industrial high temperature TE waste-heat recovery technologies remains a challenging issue. There are major challenges in the areas of materials, processes, and system integration. The associated societal impact on reduction of CO2 emissions and thus climate change will be significant, particularly as the project targets sustainably sourced and environmental friendly (e.g., lead-free) compositions. The generic scientific impact of optimized thermal and electronic properties of bulk materials will be significant both in energy application and in other applications of functional materials. These important issues will be addressed in the present project. The technical goal of the project will advance the commercial potential of an oxide-based TE conversion device significantly. The industry partners will be well placed to take the technology to the market, thus demonstrating a clear route to economic impact. The high-temperature modules will both be suited to stationary and automotive applications. Widespread exploitation of waste heat from cars can improve fuel economy further and thus contribute to the reduction of transportation CO2 emissions. This will support the strong Danish and EU commitments for the reduction of future greenhouse gas emissions.