There is need for a secure backup refrigeration system for the long-term storage of biological material. In addition to freezer component failure natural disasters such as Tropical Storm Allison in 2001 in Houston and Hurricane Katrina in 2005 in New Orleans have caused power outages and flooding, leading to the loss of biological specimens when backup refrigeration systems fail. Freezer component failure includes seal leaks, pump and compressor breakdown, and electrical monitoring malfunction. Power outages often result from high winds or large quantities of rain, snow, and ice that disrupt transmission lines. Floodwater, especially from excessive amounts of rain, fills basements and ground floors where biological storage is often located. A non-electrical backup system that can function in the presence of flooding is needed to preserve specimens when the primary system fails.A liquid nitrogen (LN2) refrigeration system utilizing a thermostatic expansion valve (TEV) was designed, built, and tested for use as a backup refrigeration system. The non-electric TEV is the key component in the system that can provide secure backup refrigeration for long-term storage of biological specimens. The passive LN2 refrigeration system does not depend on electricity so it continues to function where common systems fail. Test results of the LN2 TEV system demonstrated consistent regulation of freezer air temperature within 1°C of the set point. Simulations show that the system's high gain and reserve cooling capacity allows it to hold the temperature steady even during a large increase in ambient temperature.
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