Feasibility of heat conversion to electricity by new spin Seebeck based thermoelectrics

The UK has committed to meet an 80% reduction in greenhouse emissions relative to 1990 by 2050. Currently, it is recognised that this will likely stem from a diverse portfolio of renewable and existing energy sources as well as the development of technologies for energy storage, conversion and usage. As the majority of the UK's total energy consumption can be attributed to heating (48%) and transport (38%) these are clearly significant targets for change.

One possible route for energy storage on a domestic scale is the storage of heat that could later be converted to electricity if required. Energy harvesters designed to recycle or use various forms of energy that would otherwise be wasted (such as kinetic, thermal, acoustic, or solar), could also find applications with regards to reduction of energy demands. A technology that applies to both these applications is the thermoelectric energy generator (TEG).

The TEG is typically based on the Seebeck effect: a physical process that results in the generation of an electric current when a temperature difference exists between two terminals. Advantages of this technology include reliability, flexibility, and relatively small volumes, however due to low efficiencies and high costs it is currently limited to niche markets. One of the bottlenecks for improvement of the TEG efficiency is the co-dependence of the key material properties (i.e., thermal and electric conductivities) according to the Wiedemann-Franz law. Whilst some progress has been made on this by nano-engineering, there is still some way to go before widespread commercialisation becomes viable.

A solution to this bottleneck could be found in a new phenomena that involves the interplay of thermal and electron spin currents: the spin Seebeck effect. It is similar to the Seebeck effect in that a thermal gradient can be used to generate a current, but with two main differences: the material must be magnetic (whether metallic, insulating or semiconducting), and the electric current generated is spin polarised. This is significant as it has led to the observation of spin dependant conductivity, a feature that could allow us to sidestep the limit imposed by the Wiedemann-Franz Law and thus improve the efficiency of TEGs further.

Harnessing the maximum spin polarised current generated by the spin Seebeck effect typically requires the use of expensive platinum contacts. For such technology to become economically viable therefore would require research into cheaper alternatives. It has been shown that small amounts of platinum, bismuth or tantalum in otherwise 'inactive' copper can result in a similar harvested voltage compared to pure platinum contacts. The aim of this research project therefore, is to explore the possibility of alternative metal contacts with respect to spin Seebeck effect based TEGs and to assess the viability of such an application.

The potential technology output of this project to reduce energy use as well as provide a vector for energy storage and conversion ultimately aligns it with the UK's 2050 commitment to reduce carbon emissions. As such, it could impact on several areas of society and industry, the most obvious of which is the potential improvement of thermoelectric generators (TEGs). The beneficiaries of this research are outlined below.

1. Society and the public
An increase in efficiency of thermoelectric generators (TEGs) could have a significant impact on society over the next 10-50 years as they become more economically viable. For example, the largest source of immediate energy savings would be the installation of a TEG module in cars where typically 60% of fuel burnt is lost as waste heat (full installation of a TEG with 20% efficiency would result in a UK annual saving of 4 hundred thousand oil equivalent tonnes). Not only would TEGs be useful as energy harvesters, but they could also serve as vectors for energy storage as they allow for conversion of heat to electricity (and vice versa). Any technology that can utilise or direct heat in this way will contribute to the stability of the UK's energy portfolio and thus contribute to stable or reduced energy costs (in the long-term).

2. Emerging industries and the UK economy
Although thermoelectric cells are not a new concept they have been limited by material costs and relatively low efficiencies. Recent improvements in efficiency have required nano-engineering of the basic material that can be difficult to up-scale. Should a new device be developed that could be used to circumvent the problem of low efficiency and fabrication costs then it is highly likely that it will lead to a new (industrial) market. This could, for example, bring in new jobs for production and distribution of the TEGs, or income from patented technology.

3. The computing industry
For the last 40 years the miniaturisation of transistors in computers has closely followed Moore's law. As transistors continue to be scaled down, however, the increasing power density results in instability of the device and this issue needs to be addressed for continued progress. Spintronics is one research field that aims to move beyond Moore with the manipulation of spin polarised currents. It is not unreasonable, therefore, to argue that the production of a spin polarised current coupled with the control of heat flow in a system could lead to the development of several new spintronic devices. This will have both economic and technological impact on society as it will significantly reduce the heat barrier faced by downscaling components that is common to charge transport based devices. As a result this could lead to new technology and more efficient computing (technology exports and reduced energy costs). For example, magnetic heat switches could be one result of this research area and would lead to intelligent thermal management in electronic devices as they would have the potential to collect waste heat and divert it away from important components. Due to high consumer demand it is realistic to expect commercialisation of this technology within the next 5-20 years.