Refrigeration Technology

About this page

This page was written by the New Horizons engineering team, which operated until December 2025. First-person references ("we", "our") are the original authors'. A number of those authors are now working with Phoenix GH, which maintains this material and stands behind it. Questions and comments are welcome — please get in touch.

Last update: July 18, 2025

Overview

New Horizons has designed a number of technologies related to vaccine refrigeration. We share these here in the hope that others will find them useful.

Holdover Calculator

An ice-lined refrigerator (ILR) creates an ice bank when power is available, and relies on the ice bank to cool the vaccine storage compartment when power is unavailable. Holdover is the amount of time that the vaccine storage chamber of a powered vaccine refrigerator or storage device will remain between +2°C and +8°C after the power supply is disconnected.

When users know how much holdover is remaining during a power outage, they are able to make more informed decisions about when to move the vaccines to a safe location. Presently, most fridges do not display this information. Here, we share the approach we have and some sample code derived from our experience implementing a holdover estimator for two different fridge architectures.

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     Holdover calculator

Thermosyphon-based CCE Architecture

New Horizons developed a thermal architecture for vaccine refrigerators that optimizes for thermal stability, energy efficiency, long hold time, and low-cost manufacturing. This architecture has been used in PQS-prequalified vaccine fridge models carrying the MetaFridge brand. It uses a passive thermosyphon to transfer heat from the vaccine compartment to the thermal storage block. There are several design features that ensure vaccines are not frozen, even at low ambient temperatures, without the need for a heater.

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     Thermosyphon-based CCE Architecture