Because of a lack of information on the energy to be harvested with portable solar panels, the PES group decided to do an exploratory research to quantify the difference of harvested energy with portable and stationary solar panels. The results from this research give an indication of the amount of energy that can be harvested with solar panels when integrated in portable products. The experiment was carried out by A. Takahshi and M. Weeda.

For the experiment eight identical RWE-Solutions ASI-OEM amorphous silicon PV-cells are used. The output of these cells has been proven identical under a one-sun halogen lamp. Four cells are placed on a (portable) shoulder bag, see figure 2. A thermo-couple is placed at the cross-point of the four cells. The shoulder bag is furthermore equipped with a Grant Squirrel 800 data-logger, in order to log the output that is generated by the four PV — cells. During use the surface of the bag on which the PV-cells are placed has an inclination of 70 to 90 degrees compared to the horizontal plane. The other four PV-cells are placed on an installation on the roof of a building, see figure 3. The output of these cells is also logged by a Grant Squirrel 800 data-logger. This stationary installation has a panel inclination of 70 degrees and is directed to the south. On the shoulder bag, as well as on the stationary installation, the four PV-cells are connected parallel.

During the experiments data is gathered simultaneously with the portable shoulder bag and with a stationary installation that is placed on a roof within a range of five kilometres of the shoulder bag. Every 30 seconds two parameters are logged: the voltage over a 0,1 Ohm resistor and the temperature at the cross-point of the four cells. The shortcut current is calculated from the voltage that is generated over the 0,1 Ohm resistance. The output power and efficiency of the PV-cell is calculated from the specified operating voltage of the PV-cell, the calculated shortcut current and the surface area of the PV-cell.

The shoulder bag is used on 17 December and 18 December 2002, 24 hours a day.

During the sample the bag is carried around by a student during a usual working day, resulting in measurements mainly inside buildings (93% of the day). Figure 4 shows the power generated on 18 December for both the portable and stationary setup. Figure 5 gives the Power Ratio over the day. This ratio is defined as the "portable power” over the "stationary power,” respectively generated on the shoulder bag and the roof installation. Based on this one day measurement it seems that the portable panels harvest 20 to 60% of the stationary power when outside. When the panels are used inside, for instance in between 9:00 and 11:30, the power ratio is very small, only 3 to 5%. During the sunny period of the day, from 9:00 to 16:50, the overall mean Power Ratio is 12,6%, with a standard deviation of 19%. The measurements should be executed over a longer period of time to give an adequate mean Power Ratio which can be used as an indication for portable products equipped with outside solar-panels.