Cumulative percent of waste heat and maximum energy generation potential as functions of hot side temperature. Maximum energy generation potential is the amount of waste heat that can be converted to electricity, assuming conversion occurs at the Carnot efficiency limit. Source: DE-FOA-0001607. Click to enlarge.

A majority of the opportunity lies in the transportation or mobile sector, with an estimated work potential of 6.7 quadrillion Btu (Q) from 21.9 Q of waste heat.

Several technologies exist to realize the opportunity of lower-grade waste heat recovery—typically either mechanical, solid state, or hybrid systems.

Examples of mechanical systems include the Organic Rankine cycle, and Kalina cycle. Mechanical systems are often limited by their complexity, large footprint (e.g. size/mass), and parasitic power requirements. These are particularly challenging limitations for waste heat recovery in the transportation or mobile sectors.

Examples of solid-state devices include thermoelectric generators, piezoelectrics, and multiferroics among others. Solid-state devices have advantages in mobile applications due to their small footprint and lack of complexity and parasitic power requirement. Unfortunately, existing solid-state technologies have low efficiency and high cost.

However, according to ARPA-E, there may exist an opportunity to greatly improve most solid-state technologies.

For example, one might seek to improve the performance per unit cost of a thermoelectric generator (TEG). To date, TEG devices remain very inefficient (<6%) and costly. If their figure of merit for TEGs (ZT, a surrogate for device efficiency), were to be improved from current state-of-the-art values of around 0.8 – 1.4 up to 3, device efficiencies could reach approximately 20%. With that level of performance, if device costs could also drop to $1/W, TEGs could significantly penetrate the waste heat recovery market; this would include a significant portion of the work potential. Similar transformative performance and cost goals can be envisioned for the other solid-state waste heat recovery systems. Thus, ARPA-E is seeking input from the broader research and development community regarding lower grade waste heat recovery systems and, in particular, solid-state recovery opportunities.

Since preparing technologies for an eventual transfer from lab to market is a key element of ARPA-E’s mission, concepts should eventually be commercializable with reasonable operational systems costs (e.g. $1/watt).

—DE-FOA-0001607