Tuesday, July 05, 2016

How to Build a Heat Engine With Guitar Strings and Levers

To most of us, a heat engine is the thing that makes our car run. A refrigerator is the appliance that keeps our milk cold. Scientists, however, tend to think about things on a much more fundamental level.

This week, a new paper by scientists from the Swiss Federal Institute of Technology (ETH) demonstrates how to build a heat engine and refrigerator using a couple of guitar strings and a lever. Their work, published in Physical Review Letters, could pave the way for new ways to produce energy and help us learn more about heat and energy on the microscopic scale.

At the most basic level, a heat engine is something that harnesses heat to do useful work. Common examples include steam engines, steam turbines, gasoline engines, and diesel engines. Essentially, energy from a heat source (like steam from boiling water) powers some mechanical movement (like pushing a piston), which makes a machine do something useful (like turn the wheels of a steam engine).

A refrigerator works in the opposite direction. A refrigerator uses mechanical work, usually the compression of gas in a cylinder, to take energy from the area you want to keep cool (like the box in your kitchen) and deposit it as heat somewhere else (like in your kitchen).

Marc Serra-Garcia, who lead the development of this new type of fundamental heat engine, is a physics PhD student working under Chiara Daraio in the mechanical engineering department at ETH. The group started down this particular path about three years ago. At the time they were working on a way to turn mechanical vibrations into electrical energy. Imagine that you have a temperature sensor on top of the roof of a tall building or a similarly inaccessible place. Instead of running power cables or performing difficult battery changes, wouldn’t you rather have a sensor that could harvest energy from its environment?

While working on this project, Serra-Garcia started thinking about this “energy harvesting” device from another point of view. He realized that naturally occurring vibrations in a string can be interpreted in terms of heat, energy, and random motion.

Particles in a gas or liquid are constantly rattling around, bumping into atoms and molecules and changing directions. Scientists call this Brownian motion, or random motion. A particle’s random motion is associated with its energy, and is also tied to the temperature of the system in which it is moving. Similarly, random vibrations in a string are associated with energy and temperature.

Scientists have been exploring how to capitalize on random motion and turn it into useful work for more than 150 years. In the last few years, research teams have actually built microscopic heat engines that do this. These devices are reimagined versions of the steam engine. They replace steam, which contains a huge number of molecules, with a small number of particles. Most of the designs replace the pistons with computer-controlled lasers, and so far they require more energy to power the lasers than they produce.

The engine created by ETH scientists takes a different, simpler form than the laser-based designs, but it is based on the same principle of using random motion to do useful work. The system is composed of two strings and a lever. As shown in the image, the main string (the top one) is in contact with a cold source, and a second string in contact with a heat source. The main string is tied to the free end of a lever, and its other end is loosely coupled to the second string.

Energy enters the system through the second string, and comes from the random motion of the particles in the heat source. This random motion causes random vibrations in the strings. The pieces of the system interact in such a way that the lever acts like a piston and compresses and expands the main string, which acts like the steam in a steam engine. Put another way, energy from the random motion in the heat source leads to changes in the tension of the main string, and these changes in tension are turned into useable work by the lever.

The engine consists of two vibrating strings (purple) at different temperatures and a lever that moves back and forth (green) doing work (W). The two strings are "loosely coupled" by the blue surface at left.
Image Credit: Marc Serra-Garcia.
The team built their design in the lab using strings 30-cm long, but say that it can be miniaturized. Their experimental results show that this system has all of the characteristics of a heat engine and a refrigerator, and requires less energy to operate than other engines based on random motion.

It’s really hard to simulate what goes on in a steam engine or gas engine, which involve a huge number of molecules all going in their own direction. In contrast, this system involves just a few pieces and it is easy to simulate, understand mathematically, and control. “This simplicity allows us to ‘zoom in’ and visualize thermodynamic processes with full detail in a computer or in the lab,” Serra-Garcia says. In addition, the new design could open the door to new types of energy producing devices. Not bad for a couple of guitar strings and a lever.

Kendra Redmond

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