The main task has been to develop a new cooler, which fulfil the current WHO requirements for vaccine coolers with battery back up, as no standard exist for the battery less type. According to these guidelines the design temperature interval is 0 °C to + 8 °С. The vaccine must also be kept cool for four days without power, and this is the sizing

Fig. 1 Diagram of the first SolarChill prototype

criteria for the ice storage in the cooler. Computer simulation was done based on the most efficient mass-produced cabinets on the market. Those cabinets has 100 mm polyurethane insulation and are of the chest type.

The reason for choosing energy storage in ice was to avoid a lead battery for energy storage. Lead batteries tend to deteriorate, especially in hot climates, or they are misused for other purposes. This makes it necessary to install a new battery after a couple of years, and has in practise been an obstacle for the use of solar powered refrigerators. In addition to that some pollution of lead might be expected from the batteries.

Instead kerosene or gas powered absorption refrigerated coolers are widely used in areas with poor or no grid electricity. Absorption coolers are used for both vaccine storage and for household applications and obviously needs regular supply of fuel. Furthermore, they are difficult to adjust, which does often result in destructive freezing of the medicine.

So far, two generations of prototypes have been build and tested in climate chamber at the DTI and an advanced control were build with the purpose to control the temperature in the cooler and the speed of the DC-compressor in order to exploit maximum power from the solar panels.