Two solar direct drive ice pack freezers have been successfully tested for seven months in Columbia. The F-1 was manufactured by Sun Frost and a second freezer was produced by Sun Danzer. The freezers were tested by Steve McCarney of SELF and the project was supported by BMGF grant. The tests results were highly successful: To quote Denis Marie the performance of the units was “beyond user expectations”. F-1 Runs at Low Light Levels The Sun Frost F-1 incorporates some unique technology which we call the Power Block. The Power Block allows the compressor to run at low light levels. Typically with a SDD refrigerator or freezer the compressor will not run if the output of the solar array is less than approximately 60 watts. As a consequence, the solar array is typically over sized so that the compressor can run during periods of low insolation. The Power Block is also helpful on sunny days as well as cloudy days because it allows the compressor to run closer to sunrise and sunset, as a result there are more hours per day for ice making. In Columbia the Sun Frost F-1 was powered by a single 180 watt module. A relatively small 135 watt module would be adequate in most locations. Ice Packs are Pre-Conditioned with SDD Freezers Water packs typically freeze during the day, and overnight the temperature of the ice packs will rise. The next morning most of the ice packs will typically contain a small quantity of water while some may be totally frozen. A valuable feature of SDD freezers is that the ice packs containing water will be completely conditioned. The temperature of the ice packs which are totally frozen will be close to 0 deg C and require minimal conditioning. The temperature of the freezer could be adjusted so that all the packs are conditioned the next morning. The partially melted ice packs will typically contain only about 10% water. The health care workers will have to judge how many ice packs to take partially based on the percent of ice in each ice pack. Field Tests are of High Value There are a number of design flaws which may not show up in a lab test but will become evident in a field tests. One example is moisture problems, there have been SDD refrigerators which have successfully passed PQS testing but have serious condensation problems in the field. Another example is one model of an electronic unit for a Danfoss compressor, it would continually try to start at low light levels and burn out. The electronics problem was resolved by Danfoss, yet, it never showed up in lab tests. Another example pointed out on the Tech Net 21 website, the unexplained expansion of containers holding phase change material. An SDD freezer could possibly have problems with ice accumulation or expansion which may not show up in a lab test. Problems such as these did not show up in the field tests. Tests Comparison Refrigerator Versus Freezer With SDD freezers, unlike refrigerators, their performance can be easily evaluated by visually looking at the ice packs in the morning and judging the net ice production. Temperature measured during the day would be of little value to the end user, unlike a refrigerator where too high or too low a temperature could effect the efficacy of the vaccines. With a vaccine storage refrigerator if unusually poor solar weather is encountered the vaccines could be lost if the system was not properly sized. Under very poor solar conditions a likely consequence with a SDD freezer is that there will not be the extra energy available to freeze new ice packs, however, there may be enough output from the array to keep stored ice packs cold. Fortunately even in the unlikely event that the freezer would not run at all and the ice was lost the weather would probably be so bad that the outreach program would be rescheduled. Unlike the operation of an SDD refrigerator vaccine efficacy is not compromised by a weather related change in the freezers performance. High Demand, No PQS There is currently a high demand for SDD ice pack freezers, thousands will be needed in Nigeria. However there are no PQS standards and no PQS approved freezers available. I suggest that SDD freezers that have been field tested by a reputable testing organization have their availability published along with other PQS approved products. They would also need some type of WHO approval so that they can be purchased with support from major donors. If information on SDD freezers is not made readily available SDD freezers with proven performance will not make there way into the cold chain where they are needed. Larry Schlussler, Ph.D. Sun Frost
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