Abstract:The environmental footprint of ultra-low temperature (ULT) freezers as used in bio-repositories, universities and other research organizations is investigated. These freezers, employing the cascade refrigeration system, use between 10 and 20 times the energy of an average household refrigerator/freezer. In addition, they often require high greenhouse gas potential (GWP) refrigerants. A new technology employing the Stirling cycle machine promises to reduce energy consumption of ULT freezers by 50% or more. The cascade and Stirling systems are compared for equivalent sized freezers in terms of embodied energy and equivalent CO 2 production from cradle to gate and use, including total equivalent warming impact (TEWI) estimations. End-of-life issues are discussed but not quantified. It is shown that Stirling technology is able to significantly reduce the environmental impact of ULT freezers.

In order to improve the specific power of piston-cylinder type machinery, there is a point in capacity or power where an advantage accrues with increasing number of piston-cylinder assemblies. In the case of Stirling machinery where primary energy is transferred across the casing wall of the machine, this consideration is even more important. This is due primarily to the difference in scaling of basic power and the required heat transfer. Heat transfer is found to be progressively limited as the size of the machine increases. Multiple cylinder machines tend to preserve the surface area to volume ratio at more favorable levels. In addition, the spring effect of the working gas in the so-called alpha configuration is often sufficient to provide a high frequency resonance point that improves the specific power. There are a number of possible multiple cylinder configurations. The simplest is an opposed pair of piston-displacer machines (beta configuration). A three-cylinder machine requires stepped pistons to obtain proper volume phase relationships. Four to six cylinder configurations are also possible. A small demonstrator inline four cylinder alpha machine has been built to demonstrate both cooling operation and power generation. Data from this machine verifies theoretical expectations and is used to extrapolate the performance of future machines. Vibration levels are discussed and it is argued that some multiple cylinder machines have no linear component to the casing vibration but may have a nutating couple. Example applications are discussed ranging from general purpose coolers, computer cooling, exhaust heat power extraction and some high power engines.

This paper discusses active cooling devices used to maintain acceptable temperature levels for electronics packages in a high temperature environment. The temperature control is provided by a free-piston Stirling cooler (FPSC). The FPSC is a compact, quiet, low-vibration, environmentally benign cooling device based on the Stirling cycle. In principle, the cold side temperature of the FPSC, that thermally regulates the electronics package, is controllable from a low end, at cryogenic temperatures, upwards to the hot side temperature, which is limited only to the maximum temperature survivable by the materials used to construct the driving motor. General characteristics of the FPSC are discussed from the standpoint of electronics cooling and various options for heat transport are presented. Two prototype FPSCs have been developed for cooling electronics packages in applications where it has not been possible to accomplish even close to the requirements with existing technology. The first prototype is being used to cool an electronics package to below 100°C in a 200°C operating environment. The hot side temperature of the FPSC was designed to operate at 230°C in order to reject heat to the 200°C ambient, while limiting the motor temperature to less than 250°C. A cooling capacity of 120 W was achieved. The second prototype is used to provide conditioned air for a sealed, waterproof, electronics enclosure that is exposed to a 60°C environment. The FPSC operating temperatures for this application are 30°C on the cold side and 80°C on the hot side, with a cooling capacity of 500 W.

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