Stirling engines show promise
Although it was originally conceived in 1816, the adoption of Stirling engine technology has been slow. It is powered by cyclic compression and expansion of a working fluid, powered by an external thermal energy source. The working fluid can be air, nitrogen, carbon dioxide, helium, or hydrogen, just to name a few, and the compression and expansion is done by a piston or series of pistons. The attraction is its high theoretical efficiency and the fact that the operating thermal energy can come from a wide variety of sources.
Reusing Heat Energy
To date, most commercial or experimental Stirling engines have been small in scale. Their unique aspect is the use of a regenerator to capture and reuse thermal energy, thus contributing to their high efficiency. The regenerator is an internal heat exchanger and temporary heat storage system placed between the hot and cold spaces such that the working fluid passes through it first in one direction then the other.
Stirling engines have been built to a wide variety of designs, including those with two separate pistons – one for the hot side and one for the cold side – and those with a single piston that operates in a two-ended cylinder with one end hot and the other cold. Modern metallurgy and computer modeling of thermodynamic flows make them more efficient than in the past. Several manufacturers today offer units for European and North American residential and commercial markets.
In European Homes
A current manufacturer of cogeneration units powered by Stirling engine technology is WhisperGen, headquartered in Spain and selling residential-scale units. The WhisperGen engine is a patented four-piston unit with a unique “wobble yoke” that connect the engine to the generator. The WhisperGen residential package is primarily marketed in Europe through a distribution network.
Another current manufacturer of commercially available Stirling engines is Infinia Corporation, headquartered in Ogden, Utah. The Infinia engine design uses a free-piston technology that can accept heat from a variety of sources including natural gas, biogas, propane, or from a solar receiver. One high profile application for Infinia is a 1.5 MWe multiple generator complex powered by solar collectors at Toele Army Base in Utah.
Ideal for Remote Applications
The same technology is ideal for operation at other remote sites where there is no central station power but there is access to natural gas or another fuel. According to Jeffrey Williams, CFO for Infinia, the engines are capable of reliable operation with minimal maintenance or supervision required. This characteristic and their fuel source flexibility also make them good candidates as power sources for villages in Third World countries.
Williams notes that currently the Infinia engine has an electrical output of up to 10 kW. He points out, “It is possible to parallel multiple units to increase power output. Infinia is currently developing a packaged paralleling controller to provide this option.” He stresses that the product has large growth potential. “This small gas fueled generator is a great fit for smaller, remote sites where long term reliability and minimal maintenance is needed.”
Worth Watching Developments
To be sure, the Stirling engine technology today is in no position to overtake turbines or conventional reciprocating engines in size or number of units deployed. But with its unique advantages, this technology is evolving and broader application in the future is likely. It’s one more technology we need to watch.
– This story appeared in the Summer 2013 Gas & Technology supplement. See additional stories below.