Solar cogeneration and distributed microgeneration: novel opportunities for thermoelectrics?

On many counts, thermoelectricity is still a solution in search of a problem. Beyond its well-known limitations as of their conversion efficiency, thermoelectric devices have not yet found a significant field of application where their unique advantages may prevail over their current restrictions, making thermoelectric generation acceptable to end users while still in their technological infancy. This was instead the case of thermoelectric coolers, which were accepted as the sole viable solution for quiet air conditioning in submarines by the Navy in 1993 [1]. And it was also the case for the use of thermoelectric generators by NASA in deep-space probes [2] – yet a too marginal applicative niche to significantly promote a wide commercial interest for thermoelectric generation. In this talk two instances of applications will be discussed, where thermoelectric generation might meet critical issues that no other energy conversion technology may solve. The first instance is solar energy cogeneration [3]. Over the last years, many ways of pairing photovoltaic and thermoelectric stages have been considered, modeled and, to a lesser extent, experimentally validated. Pairing strategies will be reviewed, showing their points of strength and limitations. A detailed analysis of the energy balance in both photovoltaic-thermoelectric solar generators and in tri-cogeneration systems will be provided. Further to energy profitability [4, 5], special emphasis will be given to criteria of economic convenience, which is required for pairing to be successful. It will be shown how coupling between photovoltaic and thermoelectric stages, while energetically convenient for large-gap photovoltaic materials in non-concentrated solar cells, is instead also economically convenient only when low-concentrated (up to 5-10 suns) solar plants are considered. The second applicative context that will be presented is microgeneration, where thermoelectric generation may complement or compete with conventional batteries as power sources in distributed sensing [6]. Also in this case, both energetic feasibility and economic profitability will be discussed. Results will suggest that microgeneration may be competitive as an alternate powering strategy in the enormously growing fields of the Internet of Thing and of Industry 4.0. In summary, while in general terms thermoelectric generation remains non-competitive with conventional heat conversion approaches, it may be sensible and not too overoptimistic to conclude that thermoelectricity has reached a level of maturity adequate to surface as a key-enabling technology in selected applicative areas. Still, possibly more than higher efficiency, what thermoelectricity keeps missing on the materials side is an extension of device operability to the medium temperature range, where technologically mature materials (and devices) are not yet industrially available.