I read the WHO unicef publication “Solar Direct – Drive Vaccine – Refrigerators & Freezers.” The publication shows the relative size of the power systems for an SDD refrigerator and battery powered refrigerator, Figure 1. This figure is misleading. In the illustration the battery powered refrigerator is powered by four solar modules and four batteries. The SDD refrigerator is powered by only three panels. In actuality a battery powered refrigerator could be a single solar panel and the SDD refrigerator would require an array about four times larger.
An 80 liter refrigerator would typically consume 12 amp hrs/day in an environment with a 24 hr average ampient temperature of 32 deg C. This is typical design temperature for battery powered systems in the tropics. On a 3.5 hr solar day this refrigerator could be powered by a single 60 watt solar module. With a 50 percent safety factor a 90 watt module could be used. This is typically four times smaller than the array used to power the typical SDD refrigerator. To obtain three days of storage a relatively small 36 amp hr battery would be required. If a 25 percent safety factor is desired, the battery size could be increased to 45 amp hrs. This is the size of a small car battery. The required battery size could be further reduced by increasing the size of the solar panel to 125 watts. This would be a relatively low cost measure. High quality lithium ion batteries contain no lead and can provide reliable operation for more than ten years. Currently in California lithium ion batteries in electric vehicles are required to have a performance guarantee of 250,000 kilometers. In a vehicle batteries are subject to much harsher conditions than when powering a vaccine refrigerator. These conditions include higher charge and discharge rates, vibration and high temperatures. Lithium ion batteries which have had problems are in applications where very high energy density is required such as cell phones and hover boards. In a well designed system incorporating lithium batteries the batteries and the control electronics could be pre-wired and built in next to the cooling system of the refrigerator. In the field only the wires to a single solar panel need be connected. These systems would be much easier to transport and install when compared to the multi-panel SDD system. In addition, the refrigerator would be less bulky because the refrigerator itself can be smaller as a consequence of the space taken up and the weight of the phase change storage materials. If the lithium ion battery fails and an original battery cannot be obtained, it could be replaced by a small car type battery. I anticipate that it is inevitable that high quality lithium ion batteries will be approved by WHO.
Almost all the currently installed battery powered solar refrigerators are a combination refrigerator/freezer and require a larger array than a refrigerator only. Perhaps that is why the battery powered system illustrated in figure 1 has the larger solar array.
In the WHO unicef report the relative cost of SDD and battery driven systems was compared. Figure 2 in the publication illustrated a lower yearly cost for the SDD system. With more realistic figures for the size of the solar power system the cost of a battery powered system would be substantially less than an SDD refrigerator.
Before the introduction of SDD refrigerators WHO discouraged manufacturers from building solar powered refrigerators without ice pack freezers. Initially the technology of SDD refrigerators/freezers was not available. With the introduction of SDD refrigerators WHO promoted this refrigeration-only technology. Perhaps readers with field experience could estimate what percent of the installed SDD refrigerators could benefit from an accompanying icepack freezer.
Battery powered refrigerators require a much smaller solar array because they can collect and store energy at low levels of insolation. Typically if the output of the solar array is less than 70 watts the compressor on an SDD refrigerator will not operate. With a battery powered system useable energy will be collected even at very low light levels. Sun Frost has developed a device we call the “PB Charge Controller.” It allows an SDD refrigerator to run at low levels of insolation. Our F-1 freezer contains this technology and was one of the units tested in Columbia by the Solar Electric Light Fund. These test results were described in the WHO unicef report. The F-1 operated successfully for the past three years on a single PV module. This device is available to manufacturers to increase the efficiency and reliability of their SDD refrigerators and freezers.
I would be glad to elaborate on any of the points in this note,
Sun Frost