A Legacy of Being Cool: The Stirling Engine's Reinvention in Advanced ULT Freezers

The Presbytery of Dumbarton gave him the license to preach in the Church of Scotland in 1816. The man seemed fit for his role as a minister, but more than religious instruction and divinity, he found a greater calling in engineering. In an era when power was defined by steam, a reverend minister invented and secured a patent for a hot-air engine. His name was Robert Stirling, and his invention is what we now call a Stirling.

Stirling’s engine wasn’t the only one that existed in its time. His motivation was to create a safer alternative to the steam engines of his era that were prone to explosions due to high-pressure steam boilers. Little did he know, centuries down the road, that his invention would become the inspiration behind a modern-day cold storage innovation.

But what is the connection?   

From steam engines to cold storage

Stirling’s invention remained a novelty in his time, the initial engine design being far from perfect and limited in scalability by the technology available then. A significant number of advances had to follow in materials science, thermodynamics, and gas sealing techniques before this centuries-old technology could be incorporated into ultra-efficient freezing systems. One major development in this aspect was the birth of the free-piston Stirling engine (FPSE).

“In the free-piston Stirling engine, the piston and displacer don’t touch each other but affect one another using gas pressure,” elaborates Douglas West, product manager at Stirling Ultracold. West compares the inner workings of the FPSE to an air hockey table, where much like a puck glides effortlessly on a cushion of air, the Stirling engine's piston is now supported by gas bearings, creating a near-frictionless environment.

The elimination of traditional mechanical linkages, as observed in the FPSE, paved the way for a far more efficient and refined version of the original Stirling engine. In 2009, David Berchowitz, PhD, the founder of Stirling Ultracold as well as the primary developer and patent holder of the FPSE, further iterated the existing infrastructure of the engine with a linear motor and gas bearings to harness its cooling performance for ultra-low temperature (ULT) storage applications.

Berchowitz’s iterated design is still used in Stirling Ultracold freezers today. The first commercial application of the iterated FPSE was in Stirling’s portable model ULT25U/UE, followed by the undercounter model SU105U/UE, and then by the upright model SU780U/UE, which later evolved to Stirling’s current flagship ULT model, originally introduced in 2017, the SU780XLE. So, what makes Stirling’s ULT freezers stand out from their peers?

         

Chill smarter, not harder

On the underside of any conventional home refrigerator, you can find a compressor. The compressor pressurizes the liquid refrigerant to keep it flowing through the system, enabling your refrigerator to do its job. Compressor-based ULT freezers follow the same principles, but this setup comes with several disadvantages that can impact their performance and reliability. One major concern is the wear and tear on the moving parts, which can lead to failures and costly repairs over time.

Additionally, these systems require regular maintenance to check and replace lubricants and seals, increasing operational costs. Energy consumption is another drawback, as compressors tend to use more electricity, particularly during frequent start/stop cycles to regulate setpoint temperatures. This not only raises utility bills but can also cause temperature fluctuations that can jeopardize the integrity of sensitive samples. Furthermore, the noise generated by compressor operation may be an issue in quiet environments like labs or hospitals. So, what’s the alternative?

“You remove the compressors,” says West, with a laugh. “Long story short, all three Stirling Ultracold ULT models use the FPSE to maximize energy efficiency and minimize mechanical wear. You have a system with only two moving parts without friction. This means no wear and tear, no need for oil or lubrication, no clogs, and less ongoing maintenance.” And that is not the end of the list. Unlike the stop-and-go cycles in compressor-based systems, the Stirling engine’s continuous modulation allows for smoother temperature control, akin to how cruise control maintains a car’s speed, reducing mechanical stress and energy usage. The thermosiphon in the system also ensures interior temperature consistency without the need for high pressure or pumps.

Beyond Stirling

The free-piston design of the SU780XLE provides for many other standout features. In West’s words:

• Compact design: The system features a smaller footprint, allowing for greater sample capacity without sacrificing space

"The mechanicals are at the top of the unit in what our users call ‘green tops,’ leaving more room for storage without increasing the footprint."

     

• Broad temperature range: Operates between -20°C and -86°C, offering flexibility for various applications

“During the COVID-19 pandemic, we had the only freezer that accommodated the differing storage conditions, one at -20°C and the other at -80°C, for the Pfizer and Moderna vaccines.” 

• Energy efficiency: Running more efficiently at lower temperatures reduces heat output, minimizing the need for heavy HVAC systems and promoting energy sustainability

“The introduction of the FPSE reduces energy consumption by as much as 75 percent compared to conventional compressor-based systems.”

• Optimized storage options: Users can group different types of samples at their ideal, safe temperatures, enhancing operational flexibility and saving energy

“The option of freezers with a broad range of stable operating temperatures means users can effectively isolate, group, and store different samples with different cooling needs.” 

• High storage density: Its narrow design allows for more samples to be stored in tight laboratory spaces, enabling increased capacity without compromising accessibility

“We can store more samples in a smaller footprint on a floor space. A narrower design requiring less door swing clearance and a refined venting system that requires less venting from the wall helps accommodate more freezers.”

Navigating challenges and innovations

One may be tempted to think that Stirling Ultracold had the perfect solution, but they would soon find out that perfection is an ongoing journey, not a destination. Early quality issues with their engines presented some speed bumps, as West puts it. “With any technology, there is a journey of continual refinement. Our engines experienced the same, but the gradual introduction of a series of modifications—engine improvements stacks, you might say—have dramatically reduced incident rates and have further enhanced our technology.”

In Stirling’s case, this journey has set the stage for greater enhancements to their present portfolio of ULT technology. In a glimpse of what to expect, West shares, “Our immediate goals are to provide further advances in temperature uniformity throughout the cabinet. We have already seen significant improvements in this statistic from our recent R&D work and this is one of the major features we would like to build upon.”

Expanding on the SU780XLE’s temperature range and further innovations in cabinet and door design to enhance temperature recovery are also in the queue. “Aside from temperature uniformity, these improvements also focus on delivering industry-leading temperature stability across the entire -20°C to -86°C setpoint range,” says West. Energy efficiency also remains a priority with efforts being made to reduce energy consumption through optimized heat exchange and freezer design.

If one thing is clear, it is that Stirling Ultracold is not resting on its laurels. West’s optimism for the future suggests that the company is poised to continue maximizing the advantages provided by the FPSE to redefine the ULT freezer market with cutting-edge tech that can meet the evolving needs of researchers and laboratories.

The path forward

From 1816 to the present day, the Stirling engine has withstood the test of time with a legacy rooted in continuous innovation. By harnessing the free-piston design and improving upon it, Stirling Ultracold has set a new standard in ULT freezers. “We are constantly asking ourselves how to build the best ULT freezer, and that question drives everything we do,” asserts West. Their journey with the SU780XLE upright, ULT25 portable, and SU105 undercounter models focuses on the development of cold storage systems that are not only energy-efficient but versatile enough to cater to the needs of today’s research labs, where precision and reliability are paramount.

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