Dear Steve,
After further consideration and some computer simulations we have come to the conclusion that the autonomy tool is not appropriate to use for direct drive systems. Analysis of a direct drive requires hourly data to analyze the performance. The autonomy tool was developed using data for total daily insolation (solar radiation), not hourly data.
Currently almost all, if not all, direct drive refrigerators incorporate a Danfoss compressor. This compressor requires about 60 watts to start and consumes about 60 watts when running. If the output of the solar array is less than 60 watts the compressor will not run. If the output of the array is greater than 60 watts the compressor will still consume about 60 watts. With a battery powered system the energy addition to a battery is directly proportional to the intensity of the incident insolation; the energy accumulated in a battery is proportional to the total daily insolation. This is not true for the direct drive refrigerators incorporating Danfoss compressors. Currently the number of hours a day a direct drive refrigerator will operate depends not only on the total daily insolation, but on the distribution of solar radiation during the day. Two days could have the same total daily insolation, but the number of hours the compressor operates could be different. That is the primary reason daily total insolation cannot be used to calculate the run time of the compressor.
Hourly data is difficult to obtain for developing countries. However, we found one station in Illorin, Nigeria through the WRMC where data is recorded every 3 minutes. The data is recorded at the University of Illorin through their Physics Department. We used data from Illorin to illustrate the importance of knowing the daily temporal variation in insolation. Two days were compared, both received 3.2 wh/m^2/day of insolation. We assumed the solar array connected to the refrigerator was 140 watts and the compressor required 60 watts to run and start, as represented by the horizontal line on the attached graph. As illustrated in the accompanying graphs, the compressor on 8/20/93 ran for 1.45 hours and the compressor on 7/23/93 ran for 2.66 hours, which is 1.8 times longer. When insolation levels are low the difference in run time for different days which have the same level of daily insolation could be larger. This is all when accuracy is most important to insure reliability.
Total daily insolation data could be used for a direct drive system if the speed of the compressor could be varied so that the compressor power requirements range from 0 to 140 watts. At the beginning of the day the motor would spin slowly then midday at full sun it would spin at full speed and consume all 140 watts. With a battery powered system if the batteries are not full almost all the power produced by the array can be captured under both full sun and cloudy conditions and hence hourly data is not needed.
In conclusion to determine the number of hours per day the compressor in a direct drive system will run requires a method of analysis which incorporates solar data showing the temporal variation of incident solar radiation during the day. The autonomy tool uses for its analysis only total daily insolation and cannot be used to determine the performance of a direct drive system.
Attached Graph:
Solar-Insolation-Graphs.pdf