E.4 Final SPM Product

A total of 15 SPMs were manufactured using commercial mice electronics and rapid prototyping-based plastic encasings. Although the incorporated PV cell is flat and rigid, curved lines and double-bent surfaces dominate the design. Figure E.4.1 shows the final SPM design together with the support structure to give an impression of the design aesthetics.

Figure E.4.1 Rendered pictures of the final SPM

E.4.1 SPM Specifications

The 15 SPM prototypes were manufactured in accordance with the different options discussed in Section E.3. A voltage converter (DC/DC voltage up-conversion) charges the battery (of NiMH type of AAA size and 800 mAh capacity) when the PV voltage is above 0.3 V. The voltage converter applies maximum power point tracking by measuring the IV curve of the incorporated c-Si or mc-Si solar cell every 10 s and adopting switching frequency based on pulse width modulation. The voltage converter is incorporated onto the PCB of the additionally incorporated charge controller, which protects the batteries from both deep discharge and overcharge. Should defined voltage thresholds be exceeded, either the solar cell or the mouse PCB is disconnected from the battery. Battery SOC indication already calls for improvement without user tests: A red LED blinks when battery voltage is below a certain voltage threshold. Nothing, however, indicates an (almost) fully charged battery. Note that we incorporated a smaller battery capacity than we consider optimal for a commercial product, to ease prototype assembly. In addition to the SPM design features discussed in Section E.3, it is possible to recharge the battery by connecting the SPM to a USB port. This allows test users to continue working with the SPM should PV-generated charge be insufficient.

E.4.2 SPM User Tests

Eighteen users tested the SPM at their office desktops in Delft, Twente, and Utrecht, the Netherlands, during October and December 2007. Six participants tested the SPM for over 7 weeks, and the other 12 for only a month. User interviews allowed in-depth understanding of individual user concerns; however, the limited number of test users allowed only qualitative conclusions to be drawn.

The general satisfaction of users with the SPM prototypes greatly diverged. At the start of the test period, most users indicated the mouse to be very big or even bulky. Nevertheless, as the test phase continued, users adjusted to product dimensions so that, once the test period came to an end, some users actually indicated a preference for a larger mouse with dimensions equal to those of the SPM. The influence of different product geometries, however, was apparently underestimated during the design process, as this issue (negatively) concerned almost half of the users at the beginning.

All users, irrespective of their willingness to sunbathe the SPM, perceived the quality of battery SOC indication as insufficient. Suggestions on possible improvements greatly diverged. The majority would be satisfied with a simple battery status indicator, for example a bar graph such as in simple mobile phones. Some desired more sophisticated solutions, especially those users who were sunbathing the SPM on a daily basis. Independently, three of these test users suggested that the SPM energy balance should be accessible via special computer software to adopt optimal sunbathing strategies.

Although sunbathing was accepted by users remarkably well, it was interpreted in a way that did not include direct sunlight exposure. Users working at indoor locations with no access to direct sunlight did not move their mice into direct sunshine, that is, to a windowsill at another room when the current weather condition was sunny. However, users agreed that placing the mouse on the windowsill at their specific workplace could be easily synchronized with daily or weekly work routines. Consequently, roughly two-thirds of all users placed their SPM on the windowsill almost every weekend, roughly one-third did so on a daily basis, and only one user refused in principle to do so. Future research should investigate more quantitatively how many users are willing to sunbathe their PV device. Particularly interesting would be an examination of how far charge-harvesting potentials related to sunbathing are affected when direct sunshine is defined as obligatory. This is important, as direct sunshine implies much higher energy density and thus shorter sunbathing durations. Some of the SPM prototypes were tested at indoor locations, in which solely diffuse solar radiation reached the windowsill. Hence, sunbathing requirements differ widely from those assumed in the preliminary assessments in this study, which suggested sunshine exposure for sunbathing activity.

Sunbathing in the preliminary assessments was assumed during lunchtime, whereas sunbathing of SPM test users occurred before 9:00 a.m., as the mice were most often placed at the windowsill at around 5:00 p.m., when people left work. Note that this would be particularly beneficial for SPM located at east- or west-facing windowsills, respectively, due to direct sunshine exposure for this case. However, time-course data for various use locations and a variety of users would be needed for representative results on whether or not sunbathing will fully provide the SPM charge demand by the incorporated solar cells. This is especially true when considering the great range of irradiance levels that prevail at different indoor locations, when distributing the SPM to the different test users. For the desktop workplaces where the SPMs were used, the average irradiance intensity of the diffuse fraction of daylight was 2.5 W/m² without and 4 W/m² with artificial lighting switched on as the average for all the test sites, with notable deviations, however, of up to −300% to +700% regarding daylight availability. The irradiance intensity of the diffuse solar fraction at windowsills was 13–20 W/m² on average, depending on SPM orientation (horizontal or mounted at 45°), with −1000% to +300% deviation. Illumination levels of artificial lighting were found between 450 and 700 Lux (it can be noted that all sites had fluorescence tubes as artificial-lighting type).