An analysis in the current Journal of Applied Physics shows how to achieve solid-state conversion of thermal energy to electrical power by exploiting the physics of coupled quantum dots, delivering high power density at an efficiency close to the Carnot limit. The work also provides an excellent example of the methodology of exploratory engineering.
The approach described in the paper would improve on thermophotovoltaic systems by simultaneously achieving well-tuned transfer of energy (in quanta of just the right energy to create charge carriers) and high power density.
The trick involves placing a high-temperature surface extremely close (~5 nm) to a low-temperature surface, and to cover the surfaces with arrays of carefully structured quantum-dot components. A numerical analysis indicates that, with the right choice of materials and structure, most of the heat will be transmitted by transfer of excitation energy from hot-side to cold-side quantum dots, pumping electrons in the cold-side dots to a higher energy, a process which ultimately drives the output current. The scheme uses Förster resonance energy transfer in place of the thermal-radiation or evanescent-wave coupling used in thermophotovoltaics.
Exploratory Engineering
As you can see from the diagram, the system the authors analyzed is relatively complex; it would also be difficult to fabricate. The dimensions of the components shown are in the few-nanometer range; some of the gaps are small enough to permit electron tunneling.
The authors decided to examine this structure in order to make the quantum mechanics of the system more tractable for mathematical analysis:
…the computations increase dramatically in complexity with the participation of more states; hence, a quantum dot implementation restricts the number of states involved and is easier to calculate.
Unfortunately, to minimize the number of available states in the quantum dots, the quantum dots in the implementation under discussion must be very small. We have also relied on tunneling in order to transfer carriers between quantum dots and to contacts.
The fabrication requirements are, in fact, beyond the scope of current technology:
Although impressive advances in quantum dot technology have occured in recent years, the devices under consideration in this work probably cannot yet be fabricated to be consistent with the requirements assumed in the modeling.
Further advances in fabrication technology will occur, so that in the future for such devices we might expect the situation to change. Alternatively, one can envision different implementations which trade off performance to be more closely matched to current fabrication technologies, and which might be available sooner for experimentation.
This trade-off between ease of analysis and ease of fabrication is familiar in exploratory engineering work; it drove several basic choices (rigid materials, vacuum environment, etc.) that determined the class of systems that I chose to analyze in Nanosystems.
A process of exploratory analysis intended explores fundamental physical constraints can and often must consider structures beyond the reach of immediately available fabrication technologies. It is easier to take problems one at a time, first to establish that a realm of technology is possible, and then to explore how best to access it.
See also:
- Exploratory Engineering: Applying the predictive power of science to future technologies
- The Physical Basis of Atomically Precise Manufacturing
