Menno Veefkind, Delft University of Technology, Faculty of Industrial Design Engineering, Landbergstraat 15, 2628CE, Delft, phone: +31 15 2783772, fax: +31 15 2781839 Bas Flipsen, Delft University of Technology, Faculty of Industrial Design Engineering Herman Broekhuizen, Delft University of Technology, Faculty of Industrial Design Engineering

In order to increase the diffusion of PV-technology in portable consumer products figures on the amount of energy that can be harvested during the use of such products is needed. This paper presents two experiments that are carried out in order to gain data on the energy that can be harvested by the application of PV-cells on portable products. Although the experiments are not sufficiently extensive for the deduction of clear figures, their results are presented as a first exploration in this field. The discussion is based on a comparison of both experiments and focuses on the equipment that was used and the samples taken.

Introduction

A special group within consumer electronics can be characterised as "personal products," portable products that offer the user a "nomadic lifestyle" due to the absence of data — or power cords. Examples that illustrate the advance of these personal products are the Walkman, the laptop computer and the mobile telephone. When "powered as usual” these products include primary — or rechargeable batteries, which are a source of discomfort due to their weight, necessary replacement or need for recharge. Moreover, from an economic as well as from an environmental point of view, batteries are a costly energy source. The PES (Personal Energy Systems) group does research on the application of alternative energy sources for personal products. At the moment of writing the energy sources under investigation are PV-power, human power and fuel cells. The PES group itself is part of the faculty of Industrial Design Engineering, which mission statement is "Products for people.”

When industrial designers think of alternative energy sources for portable products, they often bring up PV-power as one of the options. The optimism, with which industrial designers approach the use of PV-technology in consumer products, in the first place, often turns into scepticism when the energy that can be harvested under "real" operating conditions is determined. For this reason the expectations for the use of PV-technology as the power source for consumer products seem to have tempered the last years. A framework that provides a rational view on the use of PV-technology in consumer products is needed in order to come out of this impasse. An important part of such a framework is the assessment of product ideas that include a PV energy source in an early stage of product development. A fundamental part of this assessment is the simulation of the product’s energy balance. An earlier study that was presented at the ISES 2003 conference (Veefkind, 2003) shows that the simulation of the energy balance of portable consumer products is difficult, due to the inexistence of data on the amount of energy that can be harvested on portable consumer products.

Method

From a methodological point of view sizing solar power systems for consumer products is not different from sizing any other solar power system. Sizing procedures based on the energy balance of the whole system, such as described in "Solar electricity” (Markvart,
2000) are in theory equally applicable for consumer products. In practice differences occur in the input to the sizing procedure. This is due to the fact that sizing procedures are designed for stationary stand-alone systems to be used outdoors. Typical input parameters that are used in order to determine the available solar irradiation for such systems are meteorological data and the inclination of the solar module. Consumer products will often be used in more complicated environments, for example because they are placed behind a window. Iowa Thin Film (Iowa Thin Film) provides figures for the irradiance in different situations as a percentage of the full sun. Those figures can be used as input for a sizing procedure for stationary consumer products. However, there is no figure available for the irradiance that will be caught by a portable product that is going to be carried around.

If it comes to information on the energy that can be harvested by portable products, three levels can be distinguished: PV-module level, PV-system level and product level (see figure 1). On each level data on the performance of the product exist, as well as data on the conditions that enables this performance. Data gathering on the first two levels takes place under laboratory conditions. Data gathering on product level takes place in the field and includes many variables, amongst others the influence of human behaviour.

This study focuses on the performance of PV-cells on portable products on product level. Over the past few years different experiments in this field have been carried out at the Faculty of Industrial Design Engineering. These experiments have been carried out independently, and therefore differ in terms of the equipment used, the parameters that were under investigation and the conditions under which the experiments have been carried out. None of the experiments is carried out sufficiently extensive to allow the deduction of clear figures for the energy that can be harvested on portable products. In this exploratory study two different experiments are compared. This leads to a discussion concerning the gathering of data on the performance of PV-powered products on product level.