Although TE was discovered over 150 years ago, its uses have remained niche and largely confined to industries where conventional heat engines, fridges and batteries are impractical. The most established TE-based industries are solid-state temperature stabilizers for electronics and power-generators for deep-space probes [1]. In these applications the small size of TE components, their durability and the lack of cooling fluids or moving parts, are indispensable merits that dominate other performance criteria. The inefficiency of TE devices has, however, prohibited their wider usage. In TE coolers, for instance, the power is limited to less than 1000 W, making them impracticable as air-conditioning units for homes and buildings.
The main challenge in improving the TE efficiency is that an ideal TE device must be a poor heat conductor but an excellent electrical conductor [2]. Heat and charge transport are closely linked and no naturally occurring material meets these criteria. This obstacle led to the gradual decline of active research in TE materials in the early 1990s. In recent years however, two factors have revived the interest in TE technologies; these are the global need for alternative sources of clean energy, and the hope that nanotechnology and quantum engineering may lead to superior TE materials. The latter factor has brought the quantum character of TE back into focus. A crucial question facing the researchers at this point is: what fundamental limitations does quantum mechanics impose on the performance of nano-size TE devices that operate in the quantum regime?
In an article published in Physical Review Letters, Robert Whitney, a researcher at University of Grenoble, France, considers TE quantum systems consisting of nano-structures, or molecules that exhibit the Peltier effect, and investigates the maximum operating efficiency for a given output power [3] as dictated by quantum mechanical and thermodynamic considerations. Carnot efficiency is the classical maximum efficiency of a heat engine, but it can only be achieved for a reversible process and at zero output power. As real devices have a finite output power, maximum efficiency at finite power is unavoidably less than the Carnot efficiency: Efficiency must be sacrificed to gain power.
Peltier devices are characterized by the so-called transmission function which determines the tunnelling probability of electrons as a function of their energy. Whitney demonstrates that the transmission function of the junction is the key in maximizing the device efficiency. Specifically, it is shown that maximum efficiency can be achieved if the system allows particles within a given energy window to pass, where the size of the window depends on the output power of the device.
A second question investigated by the author is the quantum upper bound on the output power of TE devices. The bound is demonstrated to be wavelength dependent and more stringent that any bounds imposed by classical thermodynamics. An example of a system in which this bound can be applied is the TE unit in the exhaust of vehicles implemented to convert a fraction of the heat back into electrical power [4]. The temperature in the exhaust of a diesel motor, for instance, can reach in excess of 700 K, in comparison to the surrounding temperature of 300 K. This energy difference can generate a maximum power of 10 nW per transverse mode of the quantum TE convertor as predicted by Whitney. The number of transverse modes, in turn, scales as the Fermi wavelength of the TE material, and can be optimized through material design.
From a theoretical perspective, nano-sized TE devices have renewed the interest in quantum thermodynamics. Quantum TE devices with full external control can provide an ideal test bed for quantum thermodynamic concepts. It would be interesting, in particular, to investigate under what circumstances quantum effects could enhance the classical efficiencies. From a practical perspective, efficiency optimization and nano-fabrication can lead to wider usages of TE technologies as low-wattage power generators, or heat-waste harvesters. TE based wearable devices are an example of a sector with enormous expansion potential. Although rechargeable TE jackets already exist, they weigh over 1 Kg and have a relatively short battery life, and consequently a limited mass-market appeal [5]. Nano-fabrication can lead to improved designs and lightweight, temperature-stabilized TE clothing with dual heating and cooling functionality. Conversely, the temperature difference between the body and the environment can be used to charge portable electronics or wireless health devices [6].
The full potential of TE devices for waste-heat harvesting and clean power generation is yet to be realized. If the legacy of low efficiency and high cost are to be overcome, they may indeed hold the key to a cleaner source of renewable energy.
1. Marlow Industries, www.marlow.com
2. L. E. Bell, Science, 321, 2008
3. R. S. Whitney, Phys. Rev. Let., 112, 130601 (2014)
4. K. Ikoma, M. Munekiyo, K. Furuya, T. Izumi, K. Shinohara, paper presented at the 17th International Conference on Thermoelectrics, Nagoya, Japan,?24 to 28 May 1998.
5. dhama innovations, dhamainnovations.com
6. Paper presented by Perpetua on Integrating Thermoelectric Technology into Apparel during the Wireless Health 2012 Conference, San Diego, USA, 23 to 25 Oct 2012.
7. L. A. Correa, J. P. Palao, D. Alonso and G. Adesso, Scientific Reports 4 (2014).
