In comparing mature and immature design of sustainable products, appearance will evidently play a role as is their user friendliness, their usability and last but not least the way of PV modules implementation. In practice it turns out there will be a compromise between the appearance, its functionality, usability and utility. Where to draw the precise line will be a matter that can change at each case.

Up to now in PV powered products the main concern has just been the PV performance. However there is also a growing preference for additional comfort, convenience of use in other words improved user friendliness, or good ergonomically designed products and appealing designs are in demand. Depending on the user context as a result, this will have a direct impact on the shape and colour of the PV cells to be used.

In sustainable product design, the actual colour of the PV cells will become a less important factor since a dominant colour will on the long term limit the time a product is in vogue. This is for example demonstrated in the new PV roof projects in which the PV modules on the roof are built without being conspicuous, just resembling normal tiles. The main restriction is that the overall performance of the PV cells is not frustrated by its colour (see section 3.3)

1.1.3 Convenience of use, and appearance

Convenience of use is demonstrated for instance for a product which has to fit nicely into the palm of your hand and therefore should not have sharp edges. In figure 3a [Solar — Home,2004] it can be seen that the rectangular and flat shaped PV module on the PV torch hampers the proper use mainly due to its sharp edges. In addition it is not aesthetically designed. This design ‘craves’ for PV panels with curved surfaces, which follow the cylindrical shape of the battery holder as can be seen in Figure 3b.

1.1.4 PV implementation and appearance

Figure 4a: PV powered lamp

Comparing the PV powered lamp presented in Figure 4a with another lamp presented in Figure 4b namely a PV (streetlight) lamp designed in the framework of a master thesis project [Verkuijl 2000] one can observe the following differences:

In the first case, the PV module is just an add-on unit not well integrated into the overall design. In the second case the curved PV panels will function as Solar Cell on one side and simultaneously as light reflector of the generated light on the other side. A sound integration of the PV panel into the overall design is demonstrated. As a result of this integrated design, synergy between the energy conversion function and the energy use (light) is obtained. The energy storage was done at lamp location, introducing a mobile design concept. In addition the use of curved PV panels enhanced the aesthetics since this street lamp resembles a flying object. The latter design is more mature than the former one.

In both torch and lamp examples above the need of curved PV panels is evident. However one should be cautious to generalize such need to all PV powered products. A design rule could be that curved PV cells are to be used if and only if this feature introduces a new added value.