How Many Nitinol Engines Would be Needed to Power LA?

Fig. 1: Nitinol transitioning phases. (Source: Wikimedia Commons)

Nitinol is a metal with a memory. When it is heated to a certain temperature, it shapeshifts, changing from its current shape to a previously defined shape. Nitinol has the ability because the arrangement of atoms within Nitinol changes at different temperatures. At high temperatures, Nitinol is rigid because the atoms are well-organized (known as the austenite phase). When it cools down, it becomes more pliable and bendable. However, as long as the chemical bonds are not broken, the nitinol will snap back to its austenite phase shape if it is heated again (see Fig 1).

Nitinol was discovered in 1959 by researchers at the Naval Ordnance Laboratory while they were trying to make better missiles. They found that a 1:1 nickel (Ni) titanium (Ti) alloy created a more durable, robust nose cone. [1] Since then, nitinol has been used in a variety of applications, including in robotics, aerospace, and eyeglasses. Researchers realized that Nitinol could also be used as the workhorse for a heat engine.

A heat engine converts heat into mechanical energy. The efficiency of a heat engine is typically defined as the ratio of energy output to the energy input (for heat engines, the input is a source of thermal energy). The French mechanical engineer Carnot proved that heat engines are fundamentally limited in their efficiency as a consequence of the second law of thermodynamics.

Empirically, the best performing Nitinol heat engine has an efficiency of 11.3%. [2] This inefficiency greatly limits its power output, with the highest power output from any nitinol engine being 4 watts. [3] Could these engines provide the world with significant energy, despite their inefficiencies? As a case study, we will consider Los Angeles County.

In 2010, Los Angeles county consumed 5.6 × 10¹⁰ kWh. [3] If we divide this figure by the number of hours in year (365 days × 24 hours day^-1), we obtain the average power of 6.3 × 10⁹ Watts. This figure indicates that 1.59 × 10⁹ 4-watt nitinol engines (as presented in the aforementioned study) would be needed to power LA, an impractically large number. Furthermore, if we assume that this engine has an efficiency of 11%, then the heat engine would need to be supplied with 5.7 × 10¹⁰ Watts (6.3 × 10⁹ divided by 0.11), and the vast majority of this energy would need to be dumped as waste heat. These limitations prevent nitinol heat engines from being able to make a significant contribution to the world's energy needs.

© Luke Hansen. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] G. B. Kauffman and I. Mayo, "The Story of Nitinol: The Serendipitous Discovery of the Memory Metal and Its Applications," Chem. Educator 2, 1 (1997).

[2] R. A. Abubakar, F. Wang, and N. Wang, "A Review on Nitinol Shape Memory Alloy Heat Engines," Smart Mater. Struct. 30, 013001 (2020).

[3] D. Burillo et al., "Forecasting Peak Electricity Demand For Los Angeles Considering Higher Air Temperatures Due to Climate Change," Appl. Energy 236, 1 (2019).