Converting heat into power
Vehicles are not very efficient when it comes to using energy. In an automobile, only 27 percent of the fuel energy is used for propulsion. About 33 percent of the energy is spent cooling the engine, 4 percent is lost as friction, and 36 percent is lost as exhaust. Researchers estimate that recycling only 1 percent of an automobile’s exhaust heat can power all its accessories in a car except the headlights. The challenge lies in converting that waste heat into power.
That’s where Sidney Lin, an associate professor in the Dan F. Smith Department of Chemical Engineering, comes in. In his project for the U.S. Department of Energy’s Oak Ridge National Laboratory, Lin is developing thermoelectric powders to help convert waste heat to electric power. And he’s doing it in a way that is faster and more economical than the previous manufacturing method.
The traditional method takes days and needs a high-temperature environment such as a furnace, which consumes a lot of energy. Lin synthesizes the thermoelectric powders using a process called self-propagating high-temperature synthesis (SHS), which was discovered by Russian scientists in 1967. His process takes minutes instead of days, and the only energy needed to make the powders is that used to ignite the process. When the materials are ignited, the combustion creates a chemical reaction that synthesizes into the final thermoelectric powder. “Other processes need a high-temperature environment for their reaction to occur,” Lin said. “That means huge energy consumption during their synthesis and also means a lot of money. SHS can be done at room temperature.”
The SHS process surpasses the former manufacturing method because it is cheaper, faster and less hazardous. “And the products produced by SHS have unique properties because of the fast heating up and cooling down steps,” Lin added. The process has low materials cost and low energy consumption because a high-temperature reaction environment is not necessary. It has simple reactors and fewer production stages. The process takes only minutes and is less hazardous than the previous method used with no resulting volatile organic compounds and no greenhouse gases.
Once the thermoelectric powders have been made, they will be coated on the surface of a substrate and used as a film or consolidated to a bulk part. The first one produces electricity at a high efficiency, but the total power output is limited. The second one can produce a much higher electric current—more electrical power.
The thermoelectric materials generate electricity directly from the temperature difference between the car’s hot exhaust system and outside cold air; however, they need other devices to integrate the electricity into the car’s electric system, so thermoelectric “modules” will be installed on a car.
Thermopower is the same concept as the Seebeck coefficient. Discovered in 1821 by German physicist J.T. Seebeck, the Seebeck effect explains the production of an electrical current in a closed loop composed of two dissimilar conductors when one of the junctions is heated. It is the result of electrons in the high-temperature zone vibrating and migrating faster than those in a cooler zone and generates net electrical current and potential. The opposite effect, known as the Peltier effect, creates a temperature difference when a current is passed through the closed loop.
In some newly designed cars, the Seebeck effect is used to generate electricity from exhaust waste heat using the temperature difference between the exhaust and surroundings to increase fuel efficiency. The Peltier effect is used to cool down temperatures of car seats in summer.
According to Lin, BMW will launch its new vehicles equipped with thermoelectric power generators in 2014. The auto manufacturer claims that reusing the otherwise wasted exhaust heat to power a thermoelectric generator could reduce fuel consumption by 5 percent.
Part of Lin’s research team includes his graduate and undergraduate students, whom he credits as hard working, creative and responsible. Asami Kikuchi, an exchange student from the Center for Advanced Research of Energy Conversion Materials at Hokkaido University in Japan, worked with Lin from October to February. She synthesized some oxides for thermoelectric materials by using combustion synthesis and took part in mathematical modeling of combustion reactions of silicone nitride.
“From my point of view, my positive learning experience is that I can see and consider details of the process of the reaction,” she said. “In my lab in Japan, what I've focused on were all the effects of some parameters on their properties. Of course I've considered the reasons and relationships between courses and results, but I've never tried to see the details of the reactions.”
Kikuchi said she does all her experiments by herself at her university in Japan, but Lin showed her experimental procedures step by step, so she’s learned correct experimental skills from him and from his other graduate students. She has more opportunities to discuss her research with Lin than with her supervisor in Japan, she noted, “which helps us improve our research much faster.”
Lin’s research is progressing quickly. “Based on our prior experimental results, we proposed a plan to improve the performance of thermoelectric oxides,” he said. “We plan to use the composition-microstructure-process-property concept with the design and synthesize high-efficiency thermoelectric oxide materials to convert waste heat to useful electricity. We have successfully modified the composition of the thermoelectric materials. The new materials have lower thermal conductivity—higher thermal resistance—and high electrical conductivity—lower electrical resistance.”
The work will continue. Oak Ridge National Laboratory and Lamar University are in the process of forming a team to expand the research project to industrial applications, Lin said. “Using the SHS technology we have established, we are studying other materials,” Lin said. His team is working on a new project titled “Combustion Synthesis of Phosphors for White Light-Emitting Diodes (LEDs).” It is funded by the Lamar University Research Office. “We hope the results obtained from this project will bring in external funding,” Lin said.