Waste-heat harvesting in nano-structures

Researchers demonstrate how voltage fluctuations in a micro-fabricated structure can be converted to electricity. This work paves the way for electronic circuits capable of converting excess heat into usable energy. 


Anyone working in an office has witnessed how the heat generated from computers, printers and photocopiers can noticeably increase the ambient temperature. Data storage servers and computer clusters, in particular, generate an enormous amount of heat that ought to be removed by costly air conditioning means. Heat-recycling circuits integrated into everyday electronics could lower our air-conditioning needs and put a fraction of the wasted energy back into use. In this issue of Physical Review Letters, Hartmann and colleagues report a proof-of-principle experiment that brings us a step closer to such a technology [1].


Conversion of heat to electrical power can be achieved through a phenomenon known as the Seebeck effect [2]. The ideal convertor must be a poor heat conductor but have excellent electrical conductivity such that the temperature gradient is maintained despite the current flow. As heat and charge transport are closely linked, no naturally occurring material meets this criterion.  A series of pioneering experiments in the 1990s demonstrated the Seebeck effect in nano-structures [3-4], and researchers began to wonder whether some of the limitations of conventional materials could be overcome through utilization of quantum mechanics at the nano-scale.  In 2011, Sánchez and Büttiker put forward a new design that ingeniously circumvented the conductivity dilemma by decoupling the direction of the heat and the charge currents.

The proposal is a three terminal device consisting of two quantum dots (QDs) and three thermal reservoirs (Fig. 1). The upper QD, known as the conductor, is in contact with hot and cold reservoirs. The lower dot is in contact with a gate reservoir that provides the necessary thermal fluctuations. When the conductor dot is empty, electrons flow from the hot reservoir into the conductor.
The energetic barrier between the conductor and the cold reservoir prevents the tunneling of the electron out of the conductor.
The electron will occupy the conductor until fluctuations in the gate reservoir excite an electron into the lower dot. The resulting Coulomb repulsion forces the electron out of the conductor. The temperature difference between the two reservoirs ensures that on average the flow is from the hot to the cold reservoir.

Schematic diagram of the experiment. The upper QD is known as the conductor dot and is in contact with hot and cold reservoirs. The lower QD is in contact with a fluctuating reservoir, known as the gate. The temperature fluctuations are simulated by applying a voltage with a white noise spectrum.  I and J represent the direction of the current and the heat flow respectively. 

To realize this scheme, Hartmann and colleagues utilized micro-fabricated GaAs/AlGaAs heterostructure as quantum dots.  A high mobility two-dimensional electron gas (2DEG) located about 80 nm below the surface took the role of the electron reservoir. The components were connected via quantum point contacts to allow electron tunneling in either direction.  As achieving thermal fluctuations is difficult experimentally, the fluctuating thermal reservoir was simulated through a gate voltage with a white noise spectrum. Similarly, thermal reservoirs were simulated by applying voltage biases to the 2DEG. The researchers observed output powers (currents) in the pW (nA) region.

For practical applications, to make the device economically viable, the power required to maintain the thermal reservoirs must be lower than the output power of the device.  This demands channeling the heat generated in the electronic circuitry into a small volume, and maintaining a source of cold electrons that are isolated from the ambient temperature. Further experiments, ideally with thermal electron sources, are thus required to assess the efficiency of such a converter.

 This technology may find applications in nano-scale temperature stabilization where a bias is generated in response to temperature fluctuations thus counter-acting undesirable thermal changes. From a theoretical perspective, a working prototype of a quantum thermoelectric device can provide the ideal test-bed of quantum thermodynamic concepts. It would be interesting, for instance, to investigate under what circumstances quantum effects can enhance the classical efficiencies, and whether the efficiencies can be improved via quantum reservoir engineering.

1. F. Hartmann, P. Pfeffer, S. Höfling, M. Kamp, and L. Worschech, Phys. Rev. Lett. 114, 146805 (2015).

2. L. E. Bell, Science 321, (2008).

3. S. F. Godijn, S. Moller, H. Buhmann, L. W. Molenkamp, and S. A. van Langen, Phys. Rev. Lett. 82, 2927 (1999).

4. L. W. Molenkamp, Th. Gravier, H. van Houten, O. J. A. Buijk, M. A. A. Mabesoone, and C. T. Foxon. Phys. Rev. Lett. 68, 3765 (1992).

5. R. Sánchez and M. Büttiker, Phys. Rev. B 83, 085428 (2014).

Quantum Thermoelectricity: Sacrifice Efficiency and Gain Power

Thermoelectricity (TE) is a remarkable quantum mechanical effect that enables the conversion of heat to electrical current and vice versa. In TE devices passing a current through a junction of two distinct electrical conductors results in heating the junction, while reversing the direction of the current removes heat from the junction. This is known as the Peltier effect and is the foundation of TE heaters and fridges. The converse phenomenon utilizes a temperature gradient to generate electricity and is known as the Seebeck effect.

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).