Selection of Power Sources for Portable Applications - CiteSeerX

Selection of Power Sources for Portable Applications S.F.J. Flipsen Delft University of Technology, Department of Industrial Design Engineering, Delft, the Netherlands

Abstract

perience from past projects, knowledge developed at school and university, experts, or mother wit. When it comes to power sources no tools are commonly used which support the industrial designer. The goal of the project is to develop a tool for choosing and sizing the right power/energy source combination for a portable electronic application during the concept phase of design. In this paper a literature review on tools and methods is executed, described in detail in section 2. In section 3 a first approach towards a new algorithm for a Power Source Selection Tool is presented. For this tool different transfer functions have to be developed. In section 3.2 one of the transfer functions, the Direct Methanol Fuel Cell hybrid system, is presented. This function is evaluated by designing a fuel-cell powered MP3 player [1, 2], presented in section 3.3. The simple transfer function was found not to be sufficient in the design of a fuel-cell hybrid system, and is updated based on the redesign, section 3.4 and 3.5.

New power sources emerge very quickly. Implementation of hybrid power sources for portable electronics depends on the knowledge of industrial designers. For now this group has little understanding of fuel cells and especially fuel-cell hybrids. This slows down implementation and increases the chance of failure. In this paper a review is given of tools and methods which gives concept designers a first guesstimate of the volumes they have to deal with when designing the power source. Second there will be a first approach to a second-order model and method which should produce an optimized volumetric design of the fuel-cell hybrid, being used in concept phase of design.

1

Introduction

In general 10 to 30% of the total volume in a portable electronic application is used by the power/energy source. This percentage is increasing in newer applications, where electronics size is decreasing and the quest for more power and energy is increasing. This makes the power/energy source an important component influencing a great deal of the applications form. Usually a power source is chosen at the very end of the design process (“of the shelf”) instead at the beginning of the conceptualization phase. Industrial design engineers operate in this phase. Almost 80% of the product is defined in the concept phase and based on first assumptions the industrial design engineer wants to size the application, making important and sometimes irreversible choices. The industrial designer engineer makes use of ex-

2

Selection strategies

To give the designer more insight in the world of (alternative) power sources a review is made on tools and methods in which different power and energy sources are compared. Different search and selection strategies are applicable during the concept phase: 1. Power source selection for a specific application (section 2.1), especially interesting for the industrial designer, designing portable electronics with specific performance characteristics. 2. Application selection (section 2.2), instead of 1

seeking power sources to fit a new application, how can we identify applications for a new power source. 3. Power source design tools (section 2.3), which help the designer in dimensioning the power source. 4. Optimization tools (section 2.4), where all possible configurations of power-sources and energy-containers are evaluated and optimized for single or multi objectives. In the next sections these strategies will be described more in depth.

2.1

Power source selection tools

Three main aspects of power sources for portable electronics are of interest for the designer: (i) size, (ii) weight, and (iii) costs. In a paper written by the author [3] different alternative power sources have been compared to each other based on these aspects. A large database of commercially available power and energy sources (and combination thereof) is used to compare them as pictured in figure 1. Opportunities for short-term but especially long-term developments in portable power sources are in that way overlooked. Emerging power-sources like fuel cells are found to be a very interesting alternative for the lithium-ion battery. Other power sources described and compared are ether-smog, human power, thermo-electric generators, pi¨ezo generators, electro-mechanical devices, photo-voltaic cells, micro-fuel cells and microcombustion engines. The ‘Database and Selection Method for Portable Power Sources’ from Liang Fu et al [4] is based on the ‘free search’ selection strategy for materials and processes from Ashby [5], and is implemented in the Cambridge Engineering Selector (CES) [6]. The CES selection tool is widely used by industrial designers to screen thousands of materials and production processes on their applicability for a specific product, part or function. The search engine of CES is a ‘free search’ selection method, one of three possible selection strategies available [5]:

Figure 1: Log-log plot of the power-specifics for the different power sources [3].

So called ‘design indices’ are used to direct the designers in their search for the optimal choice. 2. Questionnaire based search is based on expertise-capture, guides the uninformed user through a more or less structured set of decisions, using built in expertise to compensate for the lack of it in the user. 3. Inductive reasoning and analogy search is based on a library of previously solved problems or ‘cases’. An analysis of its features, a solution and an assessment of the degree of success of this solution is given, guiding industrial designers in product development or improvement. The power-source selector is mainly based on basic characteristics as mass 𝑚, volume 𝑉 , and cost 𝑐 of existing components. In figure 2 an example is given of the output of the power-source selectionmethod. Besides size and mass, a selection can be made manually based on performance parameters, like average voltage and shelf life, and derived properties like energy per unit of cost and energy density.

1. Free search is based on quantitative analysis of performance and normalized characteristics. 2

Figure 2: An example of the output of the CES based power-source selection method [4]

2.2

Application selection tools

for DMFC power systems is described. A so-called Ragone plot is set up to screen the various field of opportunity for the DMFC and the lithium ion battery, see figure 31 . In the same figure an overlay is made of different existing applications. Especially low power, long-endurance applications seem to be interesting for DMFC systems. Three fields can be distinguished:

The inverse problem of finding an application for a specific power source is also interesting. This problem mostly exists with the power-source manufacturers trying to push their technology on the market.

A. High power, short endurance: for applications demanding high power boosts (>1C), the battery will be the best solution. The graph shows that the fuel cell system is not going to be an improvement, even when the fuel-cell system is optimized B. High power, normal endurance: for applications demanding normal discharge characteristics (0.01