I was recently looking at the “System Sizing tool 3/3” in a 2DI3 “Technical evaluation and Methodology” a unicef publication. The publication originated from a 2DI3 SDD industry meeting.
The sizing method prescribed is based on the premise that “the daily potential solar energy supply is calculated as the number of sunshine hours 1000 W/m2 multiplied with the corrected power output.” The compressor of an SDD refrigerator requires about 70 watts to run and start. When the output of the solar array is below 70 watts no useable energy is collected. If for example the refrigerator is connected to a solar array with an actual output (corrected for dust and other losses) of 300 watts the output of the array may not go above 70 watts for a 1.5 hour solar day. From the method in section 3/3 the calculated useable energy collected will be 300 watts x 1.5hr or 450 watt hrs. This result would be correct for a battery based system however zero useable energy would be collected for an SDD system with this sizing method the errors are particularly large when sizing is most critical, during periods of low insolation.
Manufacturers apparently know about the failure in this sizing method because the arrays they are incorporating are about 3x larger than the arrays suggested by this method. If any one knows if there is a sizing method for SDD systems currently being suggested by WHO please let me know.