Up to 65 percent of heat produced in an average car engine is wasted. No matter how efficient we make our cars and power plants, waste heat will always exist. But rather than let all that heat become waste (when it is released into the atmosphere), University of Cincinnati professor Sarah Watzman is exploring ways to convert it into usable electricity using magnetic field.
“The combustion process in your car burns fuel but also generates significant amounts of heat,” says Watzman, an assistant professor of mechanical engineering in UC's College of Engineering and Applied Science. “If you could recover that, you could make a process that already exists significantly more efficient.” Her lab, set to open next semester, will address this wasteful issue.
Watzman has a lot of irons in the fire, but all her research has a common goal: creating renewable energy from waste products. Watzman originally entered the mechanical engineering field because she saw the need for sustainable energy in an increasingly fossil fuel-reliant world.
“[Entering the engineering field] was a personal interest in wanting to do something that was good for the world,” says Watzman. “Engineering with a cause is really what brought me into this area.”
In an age where renewable energy is on the rise, climate change is a growing concern and fossil fuels are a big part of the problem, how we produce energy will guide our sustainable success in the 21st century. And cutting down waste for energy production is a step in the right direction.
Hot rods
Heat energy flows from hot to cold, and only so much heat can be converted to useful work. That’s why cars emit heat, power plants have massive coolant towers and your dog’s belly is warm. In each case, heat energy is escaping into a cooler environment – usually, into the atmosphere.
A car wastes heat due to the burning of gasoline to power the car’s engine. Gasoline acts within a cycle: in the combustion process of an engine, the gasoline-air mixture pumped from your tank into the engine drives the engine’s pistons down. To keep the cycle going after the piston is driven down, a compressor sends compressed air to the pistons, cooling the pistons and pushing them back up. This is what keeps the car moving down the road, but it’s also what allows heat to be released in the process.
The pistons cannot keep getting hotter and hotter (if you’ve ever had a problem with your car’s radiator, you know this firsthand), so when compressed air cools the pistons, enough heat is released to allow the process to repeat itself.
This cycle of hot and cold ensures that you get from point A to point B, but it also ensures you’ll eventually need to refill your gas tank. In the combustion process, energy released as heat from combustion of gasoline isn’t destroyed – as some of us remember from physics class – it’s just transferred to another form. Unfortunately for us, when that heat energy leaves the car, it has traditionally eliminated the opportunity to use that energy.
Watzman, however, has other plans for it.
Heat and electricity: A love story
Watzman works with thermoelectricity, investigating heat-to-electricity energy conversion. Essentially, if you have a material that has a hot end and a cool end, as in a muffler at the back of a car, the difference in temperature (also known as the temperature gradient) creates a voltage that one can harness as electricity.
Since some materials are more efficient at converting heat to electricity than others, Watzman is determining which material may be most efficient at driving this process. For example, transition metals like iron, cobalt and nickel are extremely popular and abundant, but do not make good thermoelectric materials.
In her new lab at UC, Watzman will be working with topological materials like Weyl semimetals, materials that conduct electricity and heat differently on their surfaces than they would in the bulk of the material.
In her dissertation at The Ohio State University, Watzman showed that properties intrinsic to Weyl semimetals indicated an efficient conversion between heat and electricity, but only in an unconventional geometry. This is good news for eventually trapping the waste heat in cars and other machines.
This conversion, however, was most efficient only when the temperature gradient and voltage of the Weyl materials were perpendicular to one another. Temperature gradients and electron transport induce voltages that naturally run parallel to each other, so Watzman needed something to change the direction of the electrons. Her answer: magnets.
Against the grain
Watzman is exploring the effects of magnets on electrons in thermoelectric conversion.
“If I apply a temperature gradient in one direction and I measure a voltage in a perpendicular direction,” Watzman says, “I’ll need a magnetic field present to do that. The cross product between the perpendicular magnetic field and temperature gradient is actually a force that can accelerate electron movement in the third perpendicular direction.”
On a typical thermoelectric material, a heat source and a heat sink are opposite of each other. Electrons naturally flow from hot to cold, so they go from the heat source to the heat sink, generating a charge disparity that runs parallel to the temperature gradient.
When the temperature gradient and the voltage are parallel to one another, however, one needs two kinds of materials of opposite polarity to convert the heat to electricity for use in a device. For simplicity purposes, Watzman wishes to only use one material. If Watzman can use a magnetic force to change the direction of the electrons, she can get the entire voltage output at the same temperature.
This change of direction would eliminate the need for two materials, one with a positive and one with a negative polarity, in the conversion process. Theoretically, one Weyl semimetal could efficiently complete the conversion on its own.
“I’m trying to focus on new novel materials that are expected to be more efficient but in nontraditional ways,” says Watzman.
Look, mom, no parts!
The problem with cars, as well as any other mechanical devices that rely on energy sources, are their moving parts. Moving parts eventually break and require repairs and maintenance. A thermoelectric material, however, could convert heat energy into electricity on its own, thus eliminating the need for the complications and expenses of maintaining multiple parts.
“These materials are solid state, so there are no moving parts,” says Watzman “Theoretically, if something is solid state like that, it can run forever.”
Watzman and other researchers are already looking at applying this research in reverse: Instead of converting heat energy to electricity, they would convert electricity into a temperature gradient. This technology could be applied to household appliances like heaters and fans. These types of appliances would no longer need motors – the temperature gradient can come straight from a semimetal plugged into the wall.
Converting waste heat into something valuable is only one part of the equation, but it’s a part that could have real, long-lasting impacts that extend far beyond car mufflers. Watzman’s new lab at UC will be another step forward for sustainable energy management in an energy-dependent world, implementing new technology to help meet the complex needs of the 21st century.
By Brandon Pytel