Neill Lane, Stirling Ultracold
A new generation of ultra-low freezers powered by linear free-piston Stirling engines, is now reaching the market. They offer a dramatic reduction in energy use, exceeding 50 percent versus the installed base in some cases, compared with the cascade compressor systems now used in existing ultra-low freezers on the market worldwide. A conventional full-size (25 to 30 cubic feet) -80°C freezer uses about the same amount of energy as the average United States household—up to 22 kWh/day. High energy use results in significant heat rejected to the facility HVAC system, additional requirements for infrastructure and back-up power, and noisy operation. Depending upon the electric power source, the freezer and HVAC system electric use can cause production of up to 100 tons of CO2 over the freezer life.
The industry has recognized the problem of ultra-low freezer energy consumption and the major freezer manufacturers have recently marketed energy-efficient conventional cascade freezers; according to the manufacturers, when operating at -80°C, these freezers use between 0.59 and 0.75 kWh/day/cubic foot of storage space. At this condition the Stirling freezer uses 0.4 kWh/day/cubic foot: between 53 percent and 67 percent of the energy of the best cascade freezers.
The energy performance of Stirling freezers was independently verified at a number of customer beta sites. These include, among others, a bio-repository, a large biotech company, and university and non-university research facilities. The University of California at Davis has published its independent test results.
Lower steady state freezer energy consumption reduces the need for air-conditioning and lowers operating cost. In addition, the beta site data have shown that the Stirling freezer peak current demand is only 37 percent of that measured for a new competing freezer. This dramatically lower peak electrical demand means that electrical infrastructure and back-up power requirements are correspondingly lower. For a new build facility, this combination of lower peak current and lower air-conditioning needs means that the total construction cost is substantially lower if the facility is designed to use Stirling freezers.
These new USA-made freezers are entirely sourced from one manufacturer, which makes both the Stirling cooling engine and the freezer cabinet and controls. This is in contrast to other conventional cascade compressor freezers, whose manufacturers source the compressors from a third party.
The Stirling cooling engine uses environmentally friendly helium as the working fluid and is hermetically sealed at manufacture. Two moving components are supported on gas bearings (similar to the operation of an “air hockey” table). The system uses no compressor, requires no oil, and eliminates all contact between moving parts.
Cooling is distributed to the interior of the cabinet through a sealed gravity-driven thermosiphon with no moving parts; a small amount of refrigerant (less than 20 percent of that used in a cascade system) evaporates at cabinet interior and condenses at the cooling engine. The thermosiphon operates isothermally so there is minimal temperature gradient in the evaporator and interior chamber walls.
The cooling engine modulates continuously to match the heat load on the cabinet, creating straight-line temperature performance, unlike the operation of cascade freezers where temperature is controlled by compressors cycling on and off. Over the life of a conventional ultra-low freezer, the compressors stop and start thousands of times. In contrast, assuming no power outages, the free-piston Stirling engine may start only once in the life of the freezer.
In conventional cascade compressor freezers, it is expected that the compressors fail one or more times in the life of the freezer, necessitating a complicated and costly repair where the refrigerant and oil have to be removed from the freezer. Typically rigid insulation also has to be replaced. In contrast to the oil-lubricated compressor, the Stirling engine uses gas bearings with no-wear operation. Stirling engines of this design are proven in many applications. They have flown on the Space Shuttle and continue to cool the instruments on the Rhessi satellite. Nonetheless the cooling engine has been designed to be easily replaced with no need to remove oil, no cascade experience required, and clip-on flexible insulation to simplify maintenance.
The freezer cabinet employs vacuum-insulated panels, and with the compact cooling system offers the highest vial count per square foot of floor space on any freezer on the market. The freezer has a touch screen controller with a graphical user interface.