An Osaka-based venture firm, estir Co, a Panasonic subsidiary, has been conducting experiments resulting in the development of a prototype modern Stirling engine that can harness the waste heat from manufacturing plants to generate electricity.

A Stirling engine is an external combustion engine, converting heat from an outside source into motion by exploiting rapid changes in pressure caused by expanding and contracting air. The basic concept was invented by Scottish clergyman and engineer Robert Stirling about 200 years ago. Air in a cylinder of the engine goes through a cycle of repeated expansion and contraction as it is heated by the air and cooled by the water, driving the engine’s piston up and down.

The main technical problem with Stirling engines, one that has caused the concept to be abandoned as an ICE replacement (except for a couple of attempts by small development companies) since the 1990s, is that they are not highly efficient and work best at temperatures of over 1000°C, and that kind of heat source is not readily available. Manufacturing plants can usually only supply air at less than 500°C and there is no practical solution available today to generate power from it. For example, in generating steam to power turbines the energy efficiency is too low for practical use if the supply temperature is less than 600 C.

Teruyuki Akazawa, the president of estir, decided to try to make the engine work at lower temperatures, such as those produced by Panasonic’s plants, and persuded Panasonic to fund the attempt. Akazawa thought of reducing the achieving this by internal friction of the engine. Using oil as lubricant was not an option, because oil can burn and clog the engine at these temperatures. After several trial-and-error experiments, Akazawa used a mechanism called “Scotch yoke” to give the linear motion of the piston a rotary one. This succeeded in making the engine work without oil lubrication. In this version air discharged from a Panasonic manufacturing plant at about 500 C is delivered to a Stirling engine cooled by water.

The result was a prototype engine that has been running since June 2009, with an output of 500 W. The second version of the engine has been running since December 2009 and provides 10 kW. The company has now conducted a number of demonstration experiments in a Panasonic plant in Yamatokoriyama, Japan, aiming to release the product onto the market in 2011.