Envirocooler: Free Piston Stirling Cycle Chilly Bin

Free Piston Stirling Cycle Chilly Bin

The Stirling thermodynamic cycle is reversible.  When a Stirling cycle engine is driven by an external power source (an electric motor for example), one end becomes hot and the other cold.  Because the Stirling cycle is theoretically the most efficient thermodynamic cycle that’s possible (equivalent to Carnot efficiency), it is often used when very cold temperatures are required (liquifying air for example)- see the Philips and Fabrum cryocoolers in this collection- and for infra-red sensing (high end night vision glasses for example).

This is an “Envirocooler” by Jarden Scientific, made for them by the Twinbird Corporation of Niigata, Japan.

More expensive than traditional vapour cycle refrigerators, their market niche is the medical industry- which is willing to pay a higher price for reliability, lower power use (significant when relying on batteries during transportation) and for not using CFC’s or more recent (but not entirely environmentally friendly) alternatives.   Envirocoolers use compressed Helium as their working fluid.     

It uses a Free Piston system developed by Professor Beale at the University of Ohio in the 1970’s. Free piston Stirling Machines use a concentric piston and displacer (beta layout) constructed to reciprocate appropriately by inertial, spring and gas pressure forces alone.  There is no crank, no rotary motion.  For free piston Stirling engines, power is extracted via a linear alternator.  For free piston cryocoolers power is supplied by a linear electric motor. 

Simple in concept but difficult to design and make in practice, it was not until the 21st century that free piston Stirling machines became reliable enough, and inexpensive enough for uses outside their original military (night vision) and scientific applications.  Stirling engines powered by radioactive isotope heat sources are being actively developed for deep space probes (where sunlight is too weak) and for Moon and Mars colonies.    

A key element of free piston Stirling machine success has been the development of contactless seals – reputed to have cost NASA more than a billion dollars. 

The free piston unit in this unit is smaller but very similar in layout and construction to the Infinia 3.5kw Stirling engine also in this collection. 

This Envirocooler was used for storing and transporting vaccines during the Covid era.  It’s set to 4 degrees Celsius.

Peter Lynn for the Mahan Heritage Centre, July 2025.