The limiting factors in automotive electrification today are the speed to charge the battery and the energy conversion efficiency to usable work, such as the EV range or thermal management of the passenger cabin. Extreme temperatures significantly negatively affect the vehicle's performance and battery durability. Energy harvesting can help the battery thermal management systems (BTMS) regulate the battery temperature in extreme ambients to optimize performance and range, increase the charging speed, or control the cabin temperature. For EVs to become truly mainstream, they must provide the performance drivers expect in all conditions, including extreme heat (100°F and above) and cold (20°F and below).
What is Energy Harvesting in EVs?
Energy harvesting, often called energy recovery in automotive applications, captures ambient energy, and converts it to electrical energy. This concept applies to all available ambient energy sources, including solar, wind, vibration, or thermal radiation. It can power ultra-low power MCUs to reduce small-load battery demand or MEMS sensors that monitor vehicle performance items.
For the larger-scale challenge of electric vehicles, the additional recovered energy augments the vehicle's primary energy load during operation, increasing its efficiency, and extending the range. Another benefit occurs during charging, where recovered waste energy from charging can warm the battery or preheat the cabin during extreme cold.
Harvesting three types of energy can supplement the thermal management system in extreme temperatures to protect the battery and enhance its performance: solar energy, thermal energy, and electrodynamic energy.
Solar Energy Harvesting
In the dead of winter in the northern US, temperatures can drop below zero degrees fahrenheit. One of the most significant benefits of internal combustion engines (ICEs) is that the combustion reaction creates an endless heat source to warm the engine and cabin. In EVs, this heat is unavailable, so engineers employed electric resistance heaters to warm the battery, which operates at peak efficiency between 25-35°C, and the cabin. The power for these heaters comes directly from the battery.
Recent developments have focused on automotive heat pumps, which output three units of usable heat for each unit of power consumed through a refrigerant with a boiling point below the ambient temperature. The sun still shines in winter, so adding photovoltaic arrays to the vehicle captures even more ambient solar energy. Researchers have demonstrated solar energy harvesting to improve range by nearly 23 percent. In addition, the approach reduced grid energy draw and charge time by about 10 percent and increased battery life by the same level. In addition, EVs are a natural fit for solar energy harvesting, as the battery provides the storage needed to smooth the power intermittency inherent in solar energy.
Thermal Energy Harvesting
Despite their challenges for EV thermal management, extreme temperatures provide the opportunity for a high-temperature differential to drive rapid heat transfer. In extremely hot weather, a thermoelectric generator converts the temperature differential to electricity, supplementing primary battery power and reducing the load.
This approach is most efficient at high ambient to battery/cabin temperature differentials but is only around 5-10 percent efficient in absolute terms due to the application's low-quality heat (100-150°F). Still, supplemental heat trims the peak power draws when first engaging the thermal management system.
Kinetic Energy Harvesting
While solar and thermal energy harvesting are robust enough to improve efficiency in extreme temperatures, they still depend on the quality of sunlight and ambient temperature conditions (respectively). That reality provides the opportunity for kinetic energy harvesting, which recovers waste energy from actions and characteristics every vehicle takes during operation.
An example of kinetic energy harvesting is regenerative braking, during which a portion of the braking force energy flows back to the battery for supplemental power through a Piezoelectric material. Like temperature differential in thermal harvesting, there is a direct correlation between the driving potential (brake force in this case) and the effectiveness magnitude of energy recovery available to reduce primary battery power draw. The efficiency of this process is much better than thermoelectric generators, however, achieving up to 70 percent of waste energy from braking.
Other applications of kinetic energy harvesting include shock absorbers and vibration sensors, each of which similarly captures higher energy recovery loads with increased mechanical force.
Conclusion
Extreme temperatures can present significant challenges for automotive OEMs, from battery durability to reduced driving range to passenger discomfort. Employing solar, thermal, and kinetic energy harvesting strategies can generate important secondary power sources to offset high loads when the thermal management system first engages.
The sensors described above enable technologies of extreme temperature EVs, as they convert the waste energy sources into usable power at the edges of the operating envelope. Finally, employing solar, thermal, and waste energy recovery dramatically improves the vehicle's sustainability profile.
Original Source: Mouser
About the Author
Adam Kimmel has nearly 20 years as a practicing engineer, R&D manager, and engineering content writer. He creates white papers, website copy, case studies, and blog posts in vertical markets including automotive, industrial/manufacturing, technology, and electronics. Adam has degrees in chemical and mechanical engineering and is the founder and principal at ASK Consulting Solutions, LLC, an engineering and technology content writing firm.
