What is a Stirling Engine?

A Stirling engine is a type of motor, similar to a steam engine, that converts heat energy into usable power. It is considered an external combustion engine, as opposed to internal combustion, because the actual energy conversion process takes place through the engine wall and not within it. It is named for Scottish inventor Roger Stirling, who first developed the idea in 1816.

Stirling pursued his idea with the intention of competing with the burgeoning steam engine industry. Though similar in the basic ways they convert heat energy to power, the Stirling engine uses and reuses a set amount of fluid in a permanent gaseous state. This is different from a steam engine, which uses fluid in both gaseous and liquid forms.

Several fundamental components are required for a Stirling engine to work. These include a heat source, heat exchangers, and, ultimately, a heat sink. The heat source is normally a form of combustion, and since — in a Stirling design — this process is sequestered from the conversion into power, a wide range of fuels can be used that would cause an internal combustion engine to fail. Alternative sources, including nuclear, solar, and biofuels can be used to generate the heat that makes Stirling engines work.

With a heat source in place, a Stirling engine can be arranged to convert energy into power in several different ways. These variances center mainly on the placement of the pistons and cylinders. Each different Stirling design is distinguished and referenced by a Greek letter.

For instance, an Alpha design incorporates two pistons in separate cylinders that drive back and forth to generate power. Alternatively, a Beta design houses two pistons in the same cylinder. One piston provides for the power-making of the engine, while the other is limited only to cycling hot gas to the colder end of the cylinder. A Gamma design is similar to a Beta design, but simpler in mechanical terms, with the power-generating piston housed in a separate cylinder than the cycling piston.

Despite the benefits of a Stirling engine, which include relatively high efficiency for a combustion engine, low noise, and wide applicability, the invention was ultimately not able to dethrone steam boilers as an industrial power source in the 1800s. Frequent failures of early designs negatively affected public opinion, and gave the Stirling design a reputation of unreliability through the remainder of the 19th century.

Through the middle and latter decades of the 20th century, there was renewed interest the Stirling engine design. Ultimately, a breakthrough was again thwarted as high production costs kept mass popularity down, however. With the turn of the 21st century, and as a result of rising fuel costs, the use of the Stirling design in combined heat and power units once again brought the external combustion engine back from the dead.

Combined heat and power units (CHPs) are mechanical devices that are intended to be able to provide both the heat and power supply for an individual family dwelling, or single business office. The basic idea is that the excess, or waste, heat produced by a CHP for the generation of the electricity for the building can be used to heat it as well. This constitutes a relatively efficient and cheap way to satisfy the various energy needs of both urban and rural areas, and reduces demand for large, resource-intensive power stations. Given its unique advantages, the Stirling engine is a popular and increasingly common choice as the power source for CHPs.