Solar Tergo is a charger for small "personal products," such as mobile telephones and portable CD — or MP3 player, for use in combination with a Boblbe-e backpack. It is designed by Bernadette Weitjens in co-operation with Energy research Centre of the Netherlands. When the Solar Tergo is exposed to light it charges a battery pack which is used, at its turn, to charge the product that is to be charged in the first place. A prototype of the Solar Tergo (see figure 6) has been realised, based on a Unisolar US 3 flexible triple junction cell, 8 NiCd AA cells, a SAA1501T Battery charge controller and a set of LEDs for battery level indication. In order to determine the performance of the Solar Tergo it is equipped with a Grant Squirrel 800 data-logger, a Kipp & Zonen SP-lite pyranometer as well as thermo-couple. In February 2004 the first data have been gathered with this "personal laboratory." The Solar Tergo is suitable for outdoors conditions.

During these experiments data is gathered simultaneously with the portable Solar Tergo and with a stationary installation that is placed on a roof within a range of five kilometres of the Solar Tergo. In these experiments the Solar Tegro’s PV-cell has been isolated from the rest of the system. During the experiments the irradiance and temperature on the Solar Tergo are measured every 5 or 10 seconds. Simultaneously the open voltage and the voltage over a 0,1 Ohm resistor that are generated by the Solar Tergo’s PV-cell are measured alternately. Switching between the two modes takes place every 30 seconds, while a measurement is done every 5 or 10 seconds. The stationary installation that is laced on the roof consists of a Kipp & Zonen SP-lite pyranometer, a Unisolar US-3 flexible triple junction cell and a Grant Squirrel 800 data-logger. Here the irradiance and the voltage over a 0,1 Ohm resistor that is generated by the PV-cell are being measured every 10 seconds in a horizontal plane. 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 samples that are presented here are taken on 18 and 31March 2004. Although the Solar Tergo was logging all day long, for both days a shorter sample is isolated and edited. Both samples cover a period of time in which the Solar Tergo is used while biking in an urban environment in the Netherlands. The 18 March sample is taken from 10.11 to 10.41 (winter time) under heavy overcast conditions. The 31 March sample is taken from 15.00 to 16.00 (summer time) under practically cloudless conditions. During this activity the panel inclination is about 90°.

It is clear that reliable figures can not be derived from two samples. Nevertheless, the results of the experiments are presented in order to give an idea of the output that is generated. Average values of each of the quantities that are measured by the Solar Tergo are calculated per minute of time. Table 1 shows the mean value and the standard deviation of those figures. Moreover, mean value and standard deviation of the ratio 0ST/0Roof of irradiance values and the ratio IST/lR00f of the shortcut currents are displayed. These ratio are defined as the "portable irradiance” over the "stationary irradiance,” respectively "portable shortcut current” over the "stationary shortcut current.” The ratio IST/IRoof is in fact the same figure as the "Power Ratio” that is calculated in Experiment one.

Figure 7 and 8 show the irradiance as measured on the roof and on the Solar Tergo during the two samples. The mean value of the ratio 0ST/0Roof is smaller under cloudless conditions than under heavy overcast conditions, while the standard deviation is larger. Logically this difference is the result of variation in shadow and variation in the angle of the PV-cell towards the sun in the horizontal plane, which occurs while biking around. Figure 9 and 10 show the shortcut current that was generated by the cell on the roof as and on the Solar Tergo during the two samples. The identity of shape of the figures 7 and 9, and 8

and 10, in combination with the figures of table 1, suggest that the output of both cells can simply be deducted from the irradiance.

In order to verify whether the relation between the output and the irradiance is equally clear for portable applications as for stationary applications, a scatter diagram is drawn. Figure 11 displays the relation between the irradiance and the shortcut current, for both the Solar Tergo (portable) and the roof installation (stationary). This diagram is based on both the data of 18 March and 31. The diagram suggests that there is not much difference in this relation between the data that is produced by the Solar Tergo and the roof installation. Variation might be a result of factors such as spectral distribution or cell temperature.

Discussion

When comparing the two experiments that are described above, one notes that they differ in the equipment used and in the samples that are studied. The shoulder bag compares the output of PV-cell on a portable device with the output of a stationary PV-cell. The Solar Tergo does the same, but adds information on the conditions under which this output is generated. One can also imagine a piece of equipment that only logs conditions. If the relation between irradiance and shortcut current proves to be equal for portable

applications as for stationary devices, than data on the conditions on product level in combination with data on performance on component level can theoretically serve as input for sizing procedures and energy balance simulations. This is attractive since irradiance data is universally applicable while data gathering on component level takes place under laboratory conditions. In some cases data on component level can be taken from the PV — cells specifications. Therefore it is useful to study the relationship between irradiance and shortcut current for different conditions and activities. The Solar Tergo is equipped to study this relation for the Unisolar cell. The first experiments with the Solar Tergo suggest that, for this cell, the relation between irradiance and shortcut current is similar for portable applications as for stationary applications. Note that an approach as sketched above, that leaves the spectral distribution of light out of consideration, might be valid for outdoors application. For indoors applications spectral distribution is likely to play an important role, particularly in relation with multi-junction cells.

The shoulder bag experiment shows that there is a very large variation in output when a 24-hour sample is taken, for example in terms of the "Power Ratio." This implies that figures based on such a sample provide little support if it comes to taking design decisions, for example based on calculation of probability. For a group of products it will be possible to use more specific scenarios as basis for design decisions, for example because they are usually used in a specific environment and/or under specific conditions. Therefore data that is gathered on product level while controlling some variables, as is done in the Solar Tergo experiment, will prove useful.