Myth #3: Energy Efficiency Compromises Temperature Stability and Uniformity

This is my third blog post addressing the claim that “compromises” must be made when applying energy-efficient technology to ultra-low freezers…

It has been suggested that a cooling system has to be anemic to conserve energy and this will degrade the stability and uniformity of temperatures in the box. This is simply not the case for Stirling Ultracold ultra-low freezers, which offer dramatic energy savings without compromising overall temperature stability or uniformity of temperatures mapped at locations within the freezer cabinet.

Dispelling the Temperature Uniformity Myth
An independent biorepository tested our SU780UE upright ULT freezer by monitoring temperatures at eight specified locations within the cabinet during a 3-hour period after door opening. As shown below, the average temperature difference in the SU780UE varied by only +/- 4.2 °C in this spatial uniformity test:

Stirling Ultracold SU780UE temperature chart

Remarkable Temperature Stability
Over time, it should also be noted that the SU780UE temperature varies by only .2 °C in a steady state because the Stirling engine runs continuously, without stop-start operation. This is in contrast to compressor systems that generate a “sawtooth” temperature profile. As shown (below) in independent tests reported by the same biorepository, temperatures moved up and down around the setpoint when compressors turned on and off during normal operation of a leading brand cascade freezer:

leading cascade freezer temperature variation chart

Industry-Standard “Myth Buster” is Ready and Waiting
Thanks to the new EPA test standard for Ultra-Low Temperature Freezers, temperature uniformity performance can now be presented for “apple to apples’ comparison. The ENERGY STAR® “final test method” requires the mapping of temperatures in various conditions, at specified locations, within the interior compartment. This industry standard should lead to more meaningful evaluations for ULT buyers, once other manufacturers present their compliant test data. Unfortunately, we’re still the only ULT freezer manufacturer who has published EPA-compliant test data, as of this posting.

Myth #2: Energy Efficiency Must Compromise Reliability

This is the second of three blog posts confronting the claim that “compromises” must be made when applying energy-efficient technology to ultra-low freezers . . .

It’s not surprising that an ultra-low freezer technology offering an alternative to decades-old, cascade-compressor technology might create some skepticism when it comes to reliability.  And if that alternative consistently consumes less than half the energy of compressor-powered ULTs, as is the case with Stirling freezers, you might expect those defending the status quo to suggest that energy efficiency comes at a price to system reliability.  This is simply not true and I would like to address this myth . . .

The First Step to Recovery is Admitting There’s a Problem

It is surprising that anyone would suggest that the state of cascade-compressor freezer reliability even remotely resembles a healthy picture. When an executive of a prominent manufacturer once publically stated, “All freezers fail” he was referring to the disappointing failure rates of cascade-compressor freezers.  Many of these manufactures have built high failure rates into their selling narrative so that “redundant capacity” has become a prevailing industry standard.  In fact, cascade-compressor systems not only consume enormous amounts of electric power, but that power produces a significant amount of heat that negatively impacts their reliability.

The Stirling Engine: Two Moving Parts in Continuous, Non-Contact Operation

ULT-reliability-smOn the other hand, our Stirling cooling engine generates less than half the heat and has only two moving parts – a piston and a displacer. These two moving components are supported on gas bearings enabling non-contact operation and, unlike compressors; there is no wear during normal operation of the engine. The engine requires neither oil nor any other form of lubrication. The Stirling piston operates at a fixed frequency by an integral permanent magnet linear motor.  Unlike compressors that operate under stop-start conditions that degrade mechanical reliability, we purposefully designed our engine to run continuously with modulation, so that the damaging effects of current surges are avoided during on-off cycling.

Industry’s Best Standard Warranty

Free-piston Stirling engines of this design have been proven in numerous applications, have flown on the Space Shuttle and continue to cool the instruments on the RHESSI satellite.  More than 6,100 Stirling engines have been incorporated into laboratory equipment, including more than 2,450 ULT freezers operating in over 60 research facilities and institutions. The exceptional quality of the Stirling cooling engine provides the foundation of our 7-year standard warranty with all freezers, because we are willing to stand behind the reliability of our product.

The value of having “been around for a long time” in an industry isn’t what it used to be in these times of technological change.  In fact, that could be the real compromise in an industry marked by decades-old technology and lackluster reliability performance.