Key towards the design of sustainable photovoltaic powered products
Key towards the design of sustainable photovoltaic powered products
proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. dr. ir. J.T. Fokkema, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op dinsdag, 19 december 2006 om 10.00 uur door
Sioe Yao KAN elektrotechnisch ingenieur
en
Prof. dr. ir. J.C. Brezet Prof. dr. W.C. Sinke
Samenstelling van de promotiecommissie: Rector Magnificus, voorzitter
Prof. dr. ir. J.C. Brezet, Technische Universiteit Delft, promotor Prof. dr. W.C. Sinke, Universiteit Utrecht, promotor
Prof. dr. T.B. Johansson, Universiteit Lund, Zweden Prof. dr. J. Schoonman, Technische Universiteit Delft Prof. dr. W.J. Ockels, Technische Universiteit Delft
Prof. dr. dr. h.c. M. Grätzel, Ecole Polytechnique Fédéral de Lausanne, Zwitserland Dr. ir. S. Silvester, Technische Universiteit Delft
Energy Matching - Key towards the design of sustainable photovoltaic powered products Sioe Yao Kan
Thesis Delft University of Technology, Delft, The Netherlands Design for Sustainability Program publication nr. 14
ISBN-10: 90-5155-030-8 ISBN-13: 978-90-5155-030-6
The research was funded by NWO/SenterNovem Stimuleringsprogramma Energieonder-zoek (Stimulation Program Energy Research)
Coverdesign and layout by Duygu Keskin
Printed by PrintPartners Ipskamp, Rotterdam, The Netherlands Distributed by DfS
DfS@io.tudelft.nl Tel +31 15 278 2738 Fax + 31 15 278 2956
Prologue xiii
Acknowledgment xv
1 Introduction and problem definition 1
1.1 Introduction 1
1.2 PV power supplies versus power supplies based on other energy
conversion methods 2
1.2.1 General considerations 2
1.2.2 Comparison between the power systems 4 1.3 General overview energy demand and trends of mobile products 6
1.3.1 Digital electronics 6
1.3.2 Emerging technologies 6
1.3.3 Status today 7
1.3.4 Other design issues 8
1.4 PV powered mobile/wireless product designs today and
in the coming five years 8
1.4.1 General considerations 8 1.4.2 Renewable energy matching in PV powered products 9
1.4.3 Design integration 10
1.5 Problem definition and research question 10
1.5.1 General 10
1.5.2 Research question and sub-questions 11
1.6 Matching 12
1.6.1 General considerations of matching 12 1.6.2 The energy chain and the energy matching model (EMM) 12 1.6.3 Power matching and energy matching 15 1.6.4 The Figure of Matching (FM) algorithm 16 1.7 Research objective, goals and scope of this dissertation 19
1.7.1 Research Objective and Goals 19 1.7.2 The Scope of this dissertation 19
1.8 Methodology 20
2 Optimizing photovoltaic (PV) energy conversion systems for
mobile/wireless products in outdoor/indoor user contexts 23 2.1 Introduction and general remarks 23 2.2 Characteristics of the user context defined incident light 26 2.2.1 Light Energy, irradiance and illuminance 26 2.2.2 Overview of outdoor light energy sources and spectra 26 2.2.3 Overview of indoor light energy sources and spectra 29 2.2.4 Resume of available incident light energy 33 2.3 Potential photovoltaic power converters performance 34
2.3.1 General PV Review 34
2.3.2 PV output parameters and MPP 34 2.3.3 Overview of photovoltaic cell efficiencies 39 2.3.4 Spectral response of PV cells 44 2.3.5 Résumé of the potential photovoltaic power converter
performances 46
2.4 Optimizing the irradiance matching interface (MI:1) 47 2.4.1 The spectral Figure of Matching (MI:1) 47 2.4.2 Minimise shadows on the PV cells by proper design 55 2.4.3 Increase the incident light 57 2.4.4 User context dependent PV power output and the spectral dependent efficiency 57 2.4.5 Résumé on the irradiance matching interface MI:1 57 2.5 Optimizing the PV power output matching interface (MI:2) 58 2.6 Irradiance and PV type dependent power output 59
2.6.1 General Remarks 59
2.6.2 The PV cell power output of one day 59 2.7 Other relevant design aspects 61
2.7.1 General Considerations 61
2.7.2 Curved PV surfaces 61
2.7.3 Colour and PV cells 63
2.7.4 Matching of the user emotional experience options with
the PV application 63
2.8 Conclusions 65
3 Mobile/wireless electrical energy storage media 67
3.1 Introduction 67
3.2 Energy storage media characteristics and performance 70 3.2.1 General overview electrical energy storage media 70 3.2.2 Battery characteristics and performance 71 3.2.3 Selecting batteries that match the energy chain of
PV powered products 77
3.3 Matching photovoltaic energy converters and energy storage media
(MI:2) 84
3.3.1 General 84
3.3.2 Figure of Matching between PV cell and battery 85 3.3.3 Suboptimal energy matching in the PV -
battery matching interface (MI:2) 88 3.3.4 Improving the matching between photovoltaic cells and
batteries by using capacitors 89 3.4.5 Efficiency of energy transfer from capacitors to Li-Ion batteries 91 3.3.6 Some suggestions for improving the Figure of Matching
between PV and battery 93 3.4 Matching energy storage media and energy use in the functional
application (MI:3) 94
3.4.1 General 94
3.4.2 Figure of Matching between Battery (storage medium)
and energy use in the functional application 94 3.4.3 A suggested solution for improving the Matching between
battery and energy use in the functional application with
the aid of a capacitor 100 3.5 Other advantages of battery - capacitor combinations 101
3.5.1 General 101
3.5.2 Fast recharge options today 102 3.5.3 Fast and large discharge options in battery - capacitor systems 102
3.6 Conclusions 104
4 Optimal matching in the energy chain 107
4.1 Introduction 107
4.2 Summary of energy matching examples as found in the preceding
chapters 109
4.2.1 General considerations 109 4.2.2 Irradiance matching interface (MI:1) 109 4.2.3 Charge energy matching interface (MI:2) 112 4.2.4 The energy use matching interface (MI:3) 113 4.3 Matching parameters outside the energy chain 115
4.3.1 General remarks 115
4.3.2 The construction and embodiment matching 115 4.3.3 Environmental design issues and element matching 116 4.3.4 User context design issues and element matching 117
4.3.5 Standardisation 118
4.4 Overall matching and energy balance in PV powered products 118 4.4.1 General considerations 118 4.4.2 Relating and matching the elements and interfaces in the entire
4.4.3 The PowerQuest tool 122
4.4.4 Design Methodology of PV powered products 126
4.5 Conclusions 126
5 Test cases of mobile/wireless PV powered products 129
5.1 Introduction 129
5.2 Master Graduation project examples at the Delft University of Technology, faculty of Industrial Design Engineering 131
5.2.1 The ‘Backpack’ PV battery charger 131
5.2.2 Solar Rudy and his amazing pupil localizer 135
5.3 Benchmarked existing products 138
5.3.1 The benchmark process 138
5.3.2 The cellular phone powered by a PV battery 139
5.3.3 The universal PV charger ‘Source’ 141
5.4 Test case studies in the framework of the SYN-Energy program 143
5.4.1 General remarks 143
5.4.2 The Solar Mobile Companion 143
5.4.3 The wireless PV mouse 146
5.5 Résumé design steps 148
5.6 Conclusions 150
6 Conclusions and Recommendations 151
6.1 Conclusions 151
6.1.1 General considerations for the conclusions 151
6.1.2 Research question, Energy Matching Model and Figure of Matching algorithm 151
6.1.3 Interface related conclusions 153
6.1.4 Spin-offs to other interfaces outside the energy chain 153
6.2 Design approach and guidelines 153
6.2.1 General approach 154
6.2.2 Energy matching 155
6.2.3 Mechanical physical design aspects 156
6.3 Recommendations for further research 157
6.3.1 Recommendations for the PV and battery suppliers 157
6.3.2 Recommendations for product designers and manufacturers and general research 158
6.3.3 Recommendations for fundamental research 158
Summary 161
Sammenvatting 163
Epilogue 165
List of Figures 179
List of Tables 184
Appendix A: Symbols, Quantities and Units used in this dissertation 185 Appendix B: Abbreviations and Acronyms used in this dissertation 186 Appendix C: Basic Units and fundamentals of light energy and
photovoltaic conversion 188
Appendix D: Measured lamp spectra 195
Appendix E: PV performance measurement set-up 196
Appendix F: Battery fundamentals 198
Appendix G: Electronic circuit components and diagrams 199
Usually one writes a dissertation at the early start of one’s career. The dissertation - they say - will boost your career. Apparently, my karma has been otherwise. This dissertation is written after and despite of an industrial career of over 24 years. So, what have been the drivers. What triggered this project?
In the 2000 I won the first price in an Energy Innovation Competition called ‘INVENTEX’. The main theme of this competition was the big challenge of the coming millennium that would make Energy Conversion and Storage more efficient.
For this competition I submitted two inventions, i.c.;
The ‘Electrical chain motor and power generator’ that won the first price; The ‘Smart photovoltaic (PV) battery’.
In the search for an application of the winning concept, I met Han Brezet who was at that moment involved in the Mitka project, an electrical power assisted bike. Of course at that moment the smart PV battery concept was not mature. But for me, this invention was too dear for just gathering dust. In addition, it turned out that this invention could eventually be extended in a more general context and could also be applied in more configurations than the one originally anticipated. The main focus shifted from one integrated device, the smart PV battery, towards a more general sustainable product design concept. In particu-lar, the original idea of obtaining efficiency improvement by combining energy conversion and energy storage was transferred to the whole energy chain including energy usage. Therefore, gradually the wish grew to do more with this invention. A breakthrough was made possible by an explorative study for NWO applied for by Wim Sinke of ECN to-gether with Han Brezet and Sacha Silvester of the Delft University of Technology as well as Wim Turkenburg of the Utrecht University. And as a spin-off, a PhD project could be start June 1st 2003 marking also the start of the SYN-Energy program. The first move was the production of a paper on SYN-Energy, [Kan and Silvester, 2003]; the next move was writing a dissertation in which also SYN-Energy evolved into energy matching. The realiza-tion of this was however a different story.
So, although, this dissertation is written against all odds, I hope you will enjoy reading it. •
Since this PhD thesis is the result of a fruitful collaboration between and with many people at Delft University of Technology, Utrecht University, University Twente and the Energy research Center of the Netherlands (ECN) I owe a word of thanks to so many people that it is impossible to mention them all by names.
First I want to thank NWO/SenterNovem for funding this PhD research and my promot-ers Han Brezet and Wim Sinke for their support. Han I am grateful for your guidance along a road you have foreseen right from the start. Wim I appreciated your detailed scientific comments and you taught me how to think in small steps. Both promoters have given me positive encouragement and inspiration by their creativity and true involvement.
Also my deep gratitude goes to Sacha Silvester who challenged me by saying ‘I expect that you can make a model of the PV chain, not trying is missing the opportunity’. By read-ing and commentread-ing the various versions of my draft dissertation gradually the emphasis changed from ‘synergy’ to the more precise ‘matching’. The results are indeed an Energy Matching Model and the associated Figure of Matching algorithm.
I acknowledge also all the members of our DfS program in ever changing formation, for the warm welcome in the beginning given by Alexandra, Linda, Sacha and Han. This applies also for DfS today in particular the warm attention of Susan who is a master in organising things and giving the finishing touch to this dissertation.
I want also to thank Herman Broekhuizen and Martin Verwaal for helping me with the experiments on PV cells, batteries, capacitors and benchmark samples.
Thanks to the ECN members especially Jan Kroon who assisted me with the measure-ments of PV cells and offered valuable discussions.
A special acknowledgment should be given here to the people who assisted me in a prac-tical sense to make this dissertation an Industrial Design Engineering worthy publication. Kelsey Snook has corrected the English text, Duygu Keskin took comprehensively care of the layout, enhancing the figures and printing of the book, with Jan Carel Diehl’s continu-ous support.
A special place in this book is for my grandmother, who would always stimulated educa-tion in our family.
1.1 Introduction
The link between light, energy and power has been recognised by mankind since ancient times. Examples can be found in mythology all over the world ranging from the Mayans in the Americas to the Egyptians in Africa [Gallenkamp, 1959; Spence, 1917]. Even today light has a symbolic meaning. For instance the Delft University of Technology in its logo gives tribute to Prometheus who stole the light and fire from the Gods to energize mankind. Could the solution for the energy hungry society of today be found in this light energy? Would the photovoltaic cell that converts light into (electrical) power be the modern equivalent of the ancient ‘fire’ of the Gods?
The global use of energy by mankind has increased dramatically in the last decade. This increase has had an impact on the environment, for example the pollution caused by the combustion of fossil fuel such as natural gas and oil. In addition the rapid diminishing amount of fossil fuels in turn has both political and economical implications. Thus, the increasing energy use triggers an urge for more sustainable solutions in order to support this energy demand. In particular, this calls for solutions that do not rely on depletable resources. Although the main global energy use is in the field of industrial applications it can be noted that there is also a rapid increase of industrial designed electronic products that need energy.
Energy use and in particular energy supply systems play a crucial role in the design of elec-tronic products. Therefore research is ongoing at the faculty of Industrial Design Engineer-ing (IDE) of the Delft University of Technology (DUT) on the optimisation and sustainable design of energy supply systems of electronic product. Of all these electronic products, a growing part consists of so-called ‘portable’, ‘wireless‘ or ‘mobile’ products. Throughout this dissertation the term ‘mobile products’ will be used.
and one could say the drive towards more mobility is not compatible with grid power. At the moment however users are accustomed to this way of recharging even by their own non-standardised battery recharge adapter or grid adapter connected to fixed grid outlets. Would mobile products imply that the energy converter should be connected and integrated directly into the products? Could there be a more general approach which also allows fixed external recharging points? Here for the sake of promoting renewable energy the option of recharging with ‘green grid power’ could also be considered. Green grid power is generated by renewable energy sources such as photovoltaics, wind and bio-mass. Electricity from batteries are characterised by Direct Current or commonly known as ‘DC’ power. Electricity from the grid or sometimes called mains are characterised by Alternating Current or known as ‘AC’ power. From fixed green grid power recharge points it is just a small step to that of battery recharging docking stations connected to a DC net powered by photovoltaic (PV) panels mounted for instance on the façade or sunshade of a house. The main drawback of a DC net is the limited distance of a few meters between the output sockets to the façade. This distance is dictated by the allowed voltage drop induced by the resistance of the cables and the current. An advantage of a DC net is that one can get rid of all those AC/DC converters/adapters currently in use for grid connection. This will eliminate energy losses introduced by AC/DC conversion and unnecessary waste of primary products such as iron and copper. An additional advantage will be the potential innovation cascade of designing with DC electricity such as introduc-ing new lightintroduc-ing concepts, new smaller and more energy efficient power supplies and even new security and reliability concepts [Friedeman, 2002]. As indicated above, optimisation of the energy supply will require an optimisation of the entire energy chain in a product. The main elements of the energy chain are the energy converter, some energy storage media and the energy consumption by the application.
The integration of technologies and devices into products is common practice in Design, and is firmly embedded in the research and educational program of the faculty of Industrial Design Engineering (IDE) at Delft University of Technology (DUT) [Buijs and Valkenburg, 2000]. In particular, the research on renewable energy sources such as photovoltaic cells and efficient energy use is substantial in the Design for Sustainability program (DfS) [Brezet, 1995; Brezet and Silvester, 2004; LCEP course, 2005-2006].This PhD research project is executed within the framework of the SYN-Energy program which explores the feasibil-ity of the transition towards the use of photovoltaic cells in consumer and professional products [Alsema et. al. 2005]. This program is a part of the ‘Energy Research Stimulation Program’ of the Netherlands Organisation for Scientific Research (NWO).
1.2 PV power supplies versus power supplies based on other energy conver-sion methods
1.2.1 General considerations
In general energy and power consumption or use depends on its application. Thus typically in literature on consumer and professional products, applications are divided according to their instantaneous power consumption in:
Low power applications range from 1 mW up to 100 W; High power applications range larger than 100 W. •
Using this division, many products in the extreme of the low power application range such as watches, meters, and calculators are today already powered by photovoltaic cells. As a result of this vast amount of examples, the general public perception of PV powered products is limited to these low power applications [Weitjens, 2003; Keukens, 2005]. Taking the value of 100 W as the upper limit of this low power range, one can divide this range even further as presented in Figure 1-1.
Products with high power applications (>100W) such as electrical drills/screwdrivers, por-table defibrillators and porpor-table ultrasound systems are today either directly powered by the grid or by NiCd rechargeable batteries which are recharged by the grid. From inter-views and brainstorm sessions [Weitjens, 2003; Langeveld et. al., 2004; Keukens, 2005] it became apparent that the general public perception is that these products can not be PV powered. However, by using the nanotechnology batteries as will be presented in section 1.3.3 b), the power range in the coming years will be stretched further up to values in the range of 200 W. Looking at the power range 10 mW up to 200 W, as is needed in mobile products; one could envisage several power source options as replacement for the non-rechargeable batteries.
At the faculty of Industrial Design Engineering of Delft University of Technology, besides photovoltaic cells, human power and fuel cells are also investigated as building blocks for potential energy supply options [LCEP course, 2005-2006]. Before compiling a comparison between the above options, one should start with the common points of departure of these power sources:
All the electrical energy used in a product is in general, a ‘conversion result’ of energy sources around us such as sun-light. Conversion is needed since the available energy generally cannot directly be used as such to power a product.
Quite often, the converted energy cannot be utilized at the very moment of conver-sion to power a product. A logical and a practical solution is just to store this energy and therefore to delay the moment of use until a more convenient point in time. Battery re-chargers with some kind of intelligence i.e. smart power management with the aid of ‘special chips’ have already existed for over 10 years [Swager, 1995]. How-ever, since in these examples no mobile energy converter such as PV cells are used, •
• •
still today there is a need of an additional apparatus such as AC-DC transformer to be carried and taken along for connection to the grid socket. Note that AC-DC trans-formers are not standardised, so each product needs its own transformer.
1.2.2 Comparison between the power systems
Whether based on photovoltaics, human power or fuel cells, each of the three compared power systems consist of (1) an energy source; (2) an energy carrier and conversion part and; (3) an energy storage part. Elaborating the comparison between the power systems, these elements can be described as follows:
(1) The energy source
Photovoltaics (PV): The sun or artificial light source (lamp). Human power (HP): The human body.
Fuel cells (FC): Hydrogen atoms, however since these atoms as such are not readily available to be put into fuel cells some effort (energy) must be put into the gathering, chemical processing, and packaging of hydrogen compounds.
(2) The energy carrier and the energy converter
PV: The energy carrier is light which is converted by a photovoltaic cell directly into electrical power.
HP: The energy carrier is mechanical human muscle force and body heat which can be converted with the aid of a generator or converter into electrical power. Due to the small power yield harnessed with body heat, human power (HP) is generally only associated with mechanical muscle force.
FC: The energy carrier is hydrogen which is converted together with oxygen by the aid of a fuel cell into electrical power. Hydrogen atoms are extracted from the stored ‘fuel’ such as hydrogen gas, methanol or other hydrogen compounds. The oxygen is either extracted from the air or stored separately.
(3) The energy storage media
PV conversion will generally be associated with electrical or electro-chemical energy storage media such as batteries. However in cases in which photovoltaics is used to aid electrolysis, there will also be hydrogen gas storage, and sometimes even separate oxygen gas storage. Combining the two gasses in a FC again yields electricity. HP could be stored as potential energy in a spring or in compressed gas or as kinetic energy in a flywheel. After conversion by a generator, it can be stored as electrical energy in a battery.
by photovoltaic cells could be the proper choice as a power supply. Depending on the ap-plication however, the conversion methods will be complementary to each other. If there is no light available, human power can take over from the photovoltaic cells. An example is a radio which is powered both by human power and photovoltaic cells.
Sunlight is abundant and available for free throughout the world. So an energy converter that can convert sunlight directly into electricity, such as photovoltaic cells, will compare favourably to a human powered one. Therefore, PV cells are one of the most promising and potentially sound sustainable options to meet the above-mentioned mobile renew-able energy need.
In certain applications such as electric cars the potential large energy density of com-pressed hydrogen gas will surpass that of any rechargeable battery making the fuel cells the candidate to power the electric car of the future. Today however, the hazards of compressed hydrogen gas are not yet dealt with nor has any sufficient storage solution been found. This is particularly in regards to security and reliability. The question of how to produce the needed hydrogen gas commercially is still open.
Figure 1-2 PV generated hydrogen fuel-cell car [by courtesy of F. Wouterlood, 2006]
In combining several energy conversion methods, one should keep the total efficiency in mind. This is illustrated by the comic design example from the Engineering School of Rijswijk, as seen in Figure1-2. In this model, light is converted to electricity in the solar panel with an efficiency of about 10% [Green et. al., 2006]. This electricity is used to produce Hydrogen from plain water (H2O) by hydrolyse. The efficiency of this process will be maximum 50%. The hydrogen gas is then used in a fuel cell to generate electricity with an efficiency of maximum 50%. The resulting electricity is used to power an electromotor engine which rotates the tires with an efficiency of less than 70%. The total efficiency will be 0.1x0.5x0.5x0.7x100% = 1.75% maximum. This is far less than the efficiency of petrol in a combustion engine of about 25%* [Flipsen, 2005].
Figure 1-3 Moore’s Law [Moore, 2003]
1.3 General overview energy demand and trends in mobile products
Before going into detail about the parameters that have had an impact on the design of mobile PV powered products one should first consider mobile products in general, their energy demand and the trends found with such devices. Because trends in mobile prod-ucts are closely related with the progress and innovations of electronics in general, these items will also be analysed in this section.
1.3.1 Digital electronics
The digital revolution has become an accomplished fact, demonstrating the triumph of microelectronics. Despite all pessimistic prediction, Moore’s law [Moore, 1965], which states that the number of transistors on an integrated circuit (IC) doubles roughly each year is today still valid. No sign of levelling or saturation has occurred. This can be seen in Figure 1-3, which displays the number of transistors per computer (Intel) processor (IC) throughout the last 35 years.
1.3.2 Emerging technologies
Polymer electronics is an emerging technology which will become a design opportunity to be taken into account within the next 5 years. Of particular interest are the simpli-fied ways of manufacturing these polymer electronic components, namely by using inkjet printing methods. In addition the band-gap of semiconducting polymers may be ‘tuned’ by selective chemistry [Sol, 2005]. This tuning could for example enable the making of photovoltaic cells sensitive in a prescribed spectral range.
conven-tional communication means. As a result, power consumption can be reduced to about 10% of that which today is commonly needed by, for example, cellular phones [Dijkstra and Westerhuijs, 2005].
Ambient intelligent systems i.e. complete systems including (micro)-computers, embedded software and transducers to fulfil a dedicated function in wireless interfaces, will become commonplace. In this case wireless is a must, since these systems are too small for connec-tors. This also means the absence of any connector to the power supply. In addition, even if the connectors could be fitted, the large amount and the vast diversity of locations for these systems will make connection by wiring to the grid expensive and inconvenient or even impossible. Therefore it is often suggested that these systems have to be powered by photovoltaic cells for instance [Sol, 2005].
The use of nanotechnology in electrical storage media is emerging rapidly, as can be seen in examples of novel Super Capacitors [Cap-XX, 2005] and Li-Ion Batteries [Toshiba, 2005; NEC, 2005].
1.3.3 Status today A. General mobility
The exploding market of mobile/wireless products such as cellular phones but also Per-sonal Digital Assistants (PDAs) and Notebooks in combination with Bluetooth systems and Wireless Fidelity (WiFi) sets, demonstrate a well-defined trend in both the consumer and professional market towards increasing mobility and wireless freedom. The main high-lights in the field of Telecommunications are Universal Mobile Telecommunications System (UMTS) and Voice over IP (VoIP) or Web-phone, which have become increasingly attrac-tive for private users as well. As a side effect, mobile and wireless products have almost become a necessity in modern daily life.
The products are no longer designed with their one prime function in mind. Integration with other functions has to be taken into account. Therefore, the replacement of a single function approach by a fusion or integration of several functions into one overall design will become a trend. An example of this is the integration of Global Positioning System (GPS), Wireless Application Protocol (WAP) and camera functions into cellular phones. However the consequence of this trend is an increase in energy demands. On the other hand, there is also a growing demand for the simplest design. For example, the functionality of cellular phones should become more transparent and simple to manage, particularly in light of increasingly large elderly sectors.
The mobile office and higher flexibility in choosing office locations have become a topic. The result is not a fixed working place at the office, but also the possibility to work at home or other locations which reduces traffic jams. This trend has a direct link with the trend of increased mobile and wireless communication.
B. Energy storage
The power sources of mobile products have become an increasing matter of concern and annoyance. In particular it is apparent that:
Energy consumption becomes a limiting factor for the functions offered by mobile products powered by batteries;
As mobile products become increasingly smaller, so will the available space for batter-ies and connecters for electricity grid adapters.
For mobile consumer and professional applications there is a growing demand of (light weight) rechargeable batteries, as a power source. As a consequence, there is also a growing demand by the product functions and user context to recharge these batteries, anywhere at anytime, without limits posed by the presence or absence of electricity grid outlets. Lately this tendency towards grid-independent power has become even more intense due to a series of electrical blackouts and increasing public concern about the reli-ability of electricity networks as a result of privatisation (well known blackouts are e.g.:Cali-fornia, USA, 2001/2004; Haaksbergen, NL, 2005; Twente and Eindhoven, NL, 2006; main part of west Europe, 5 November 2006). An ironic detail is that during such an occasion of an electrical blackout, no mobile communication was possible due to failure of the grid powered repeater/amplifier send masts. Also the docking stations and receivers of digital phones (DECTs) were without power. Li-Ion Batteries that can be quickly re-charged and provide large discharge currents (large power output) have recently been introduced [Toshiba, 2005; NEC, 2005]. These small (55x43x4 mm) and high power (about 35W) batteries are based on nano-technology. Due to their high power capability, these new Li-Ion battery types will open opportunities for various applications. For example they can be used in the energy supply of power tools such as drills, which until now were consid-ered to be limited to energy sources fed either directly by the electricity grid or by NiCd rechargeable batteries which in turn are recharged by the grid.
1.3.4 Other design issues
With the ample availability of powerful and fast electronic (micro)-controller or proces-sor chips, integration of intelligence into the energy consumption chain becomes a must [Scherpen et. al., 1998]. Matching product perception and user contexts becomes an issue. In other words, it would be ergonomically preferable to design a product in such a way that the first impression indicates the way the product is meant to be used. There are already examples of polymer electronic technology in use, as is demonstrated by poly-mer Optical Light Emitting Diode (OLED) displays for example the display of the Philips ‘Sensotec’ shaver [Philips, 2005]. At the moment however, the flexibility of the polymer electronics casing is still dictated by the use of glass substrates which are needed to keep out moisture and gas.
1.4 PV powered mobile/wireless product designs today and in the coming five years
1.4.1 General considerations
1.4.2 Renewable energy matching in PV powered products
In analysing the renewability of energy supplies of products, a Life Cycle Assessment (LCA) is typically done. In the case of PV powered products, as a first order evaluation, only the energy-related aspects will be taken into account. This means that the energy payback time (EPBT) will be estimated. The EPBT is generally defined as the time a PV cell or module has to operate in order to generate the same amount of energy (in equivalent terms) as was needed to manufacture it. The environmental impact of a product is then determined by comparing the EPBT with the time the PV cell will be used for that product. The EPBT depends on the PV cell under investigation, its efficiency and user context.
Figure 1-4 The energy payback time (EPBT) of the main silicon PV technologies calculated for South-Euro-pean (S-Eur.) and Middle-EuroSouth-Euro-pean (M-Eur.) locations [Alsema and Wild-Scholten, 2005b]
With the aid of LCAs the EPBT of the main silicon PV technologies: mono-crystalline (mono), multi-crystalline (multi) and ribbon silicon wafer (ribbon) were analysed [Alsema and Wild-Scholten, 2005b] (see Figure 1-4). In the example of mono-crystalline silicon modules, an EPBT was estimated of 1.5-2.5 years for South-European (S-Eur.) locations (irradiance 1700 kW/m2/year). For the Middle-Europe (M-Eur.) region (irradiance 1000 kW/m2/year), a higher EBPT in the range of 2,6-4,4 years was obtained. However at a low irradiance level, with an indoor user context (about 1 kW/m2/year or even less) a cor-responding longer EPBT could be estimated. In all of these EBPT calculations the energy to laminate and frame the PV cells and the energy input for Balance of System (BOS) components such as cables and inverters, are taken into account.
To be able to claim energy related renewability there must at least be a proper match between the energy and EPBT of the used PV cells and the actual ‘duration of use’ of the product concerned. The duration of use will depend on four factors:
The time a design is in vogue; Introduction of new functionalities; Innovative designs;
User attachment to the product.
Some types of mobile product designs have a short life cycle and as a result they are not likely to survive long enough to realise the PV EPBT. For example some of today’s designs seem to be outdated tomorrow, ready for disposal. Quite often in these product designs, the incorporated PV cells still need to function for years in order to meet their payback target after the product has for example gone out of fashion. Therefore, it is necessary to
find ways to recycle PV cells. A product with a new functionality will be more in demand than the old one. Therefore this old, but still functional item is discarded. One solution for these outdated products would be a second life. In this case the PV cells would meet their payback target. In cases of personal attachment and innovative design, the user tends to keep the products longer. Thus, the energy payback target of the PV cells is more likely to be met. Other component factors such as batteries, touch screens and interconnec-tion fatigue will then set the time limit. There are however some optimistic signs that the antagonism and gap between EPBT, duration of use and fashion seasons could be bridged by future technological improvements and new of innovations in PV cells. An indication could be found in innovations like Dye Sensitised Cells (DSC) made with spray techniques that might have an EPBT of about 1 year in the Netherlands [Broeders and Netten, 2004; Nanu et. al., 2005]. These PV cells are not yet commercially available.
1.4.3 Design integration
The most important criterion for a good design is to assure that the product can fulfil its expected functionality. The functionality is defined as something that a product can perform, independently of an objective. In a certain context there may be an objective (application) for this functionality. Functionality becomes a function in case it can be used in an application [Poelman, 2005].
One of the considerations in designing PV powered products will be the appearance of PV cells. For the long-term this appearance should not be the prominent factor of design. The PV cells simply have to perform well in the intended application but they should neither be allowed to interfere with the design freedom of shape nor with the user friendliness. An example in architecture is the use of PV tiles with the colour of normal tiles. It will not be obvious at first glance that the roof is something special. In other words, it will be a virtually invisible PV application. Of course if the purpose is just to show-off the use of PV on the roof, this example will not be applicable. For the user context of PV on the roof, this user preference has been investigated among the inhabitants of the district Nieuwland in Amersfoort in the Netherlands. In this district a 1 MW PV project was realized. It turns out that the majority (75%) prefer an invisible PV application [Vries and Silvester, 2002]. To avoid the predicate ‘just a gadget’ the added value of the additional use of PV cell must be apparent.
1.5 Problem definition and research question 1.5.1 General
Today quite a lot of ‘photovoltaic (PV) powered’ products are already on the market (see Figure 1-5). In most of these products, the PV cells are combined with some kind of re-chargeable energy storage medium such as a secondary battery.
Therefore, the keyword to obtain a sustainable and well-designed product will be matching.
Figure 1-5 The diversity of PV powered products
1.5.2 Research question and sub-questions
In formulating the research question one keyword is matching. The matching can, how-ever, be achieved in different gradations ranging from barely matching, to a proper match and even an optimized match. There are also several points of views with regard to the expression matching such as matching in appearance, ergonomics etc. To focus in this dis-sertation, the matching concerned will be energy matching.
In addition, as previously stated, the emphasis of this dissertation will be on the electro-technical integration aspects in an optimally combined PV - electrical storage system. The core of such a system will be the energy chain. In particular the focus will be on the opti-mal matching of the elements and interfaces in the energy chain. Taking into account the well defined trend towards increasing mobility and wireless freedom, the Industrial Design Engineering challenge will be mobility. This means the design of PV powered mobile /wire-less products.
As a result the research question of this dissertation is:
What systematic matching can be achieved between the elements and interfaces of the energy chain of photovoltaic powered mobile/wireless products?
Notes:
Energy chain concerns the electrical elements: photovoltaic energy converter, energy storage media and the energy using application of the product;
Current and envisaged future mobile/wireless PV powered products.
To provide a convenient outline of this dissertation, the research question above is divided into specific ‘sub-questions’ (Sub Q.). The first two are in line with the two main elements of the energy chain under investigation. This is namely the photovoltaic converter and the energy storage media:
What photovoltaic conversion systems are optimally matched with applications in an outdoor/indoor user context? Sub Q. 1.
What matching can be achieved between the electrical energy storage media and both the photovoltaic energy conversion systems in an outdoor/indoor user context (1) and the functional application (2)? Sub Q. 2.
In addition, to complete the matching analyses:
What optimisation can be achieved between the elements of the energy chain of a PV powered mobile wireless/products and how does this affects other design engineering aspects? Sub Q. 3.
What insight can be gained by analyses of the energy matching in test cases of PV powered products? Sub Q. 4.
The answers to these sub-questions constitute the contents of the consecutive chapters of this dissertation.
1.6 Matching
1.6.1 General considerations of matching
In finding answers to the research question and sub-questions, the main concern will be ‘matching’. Therefore, for the sake of clarity first this subject matter will be discussed. Throughout this dissertation the search for answers to the research question, namely the analysis of matching, will be guided by an Energy Matching Model (EMM). To probe how well the matching is achieved, a Figure of Matching (FM) algorithm will be used to analyse and quantify the matching.
1.6.2 The energy chain and the Energy Matching Model (EMM)
As mentioned above, in this dissertation optimal matching is sought between the elements of the energy chain of PV powered products. For this purpose a novel ‘Energy Matching Model’ (EMM) and a related ‘Figure of Matching’ (FM) algorithm have been developed. In Figure 1-6 this Energy Matching Model of the energy chain is presented. The main elements of the energy chain are: a) user context defined incident light, b) PV power converter, c) electrical energy storage media, d) energy use in the functional product ap-plication and e) user context defined power/energy use pattern of the product. Ad. a) User context defined incident light
The incident light upon the PV cells depends on the context of when and where the PV powered product is used. The main characteristics of the incident light are its intensity and its spectrum. A product that is used only outdoors requires a different type of used PV cells than one which is used only indoors. In Chapter 2 the matching between various types of PV cells and incident light types for both outdoor and indoor user contexts will be analysed. In this dissertation the incident light will be treated as a given exogenous element so all the light transmission losses due to encapsulation and cover glasses of the PV cells and windows are already taken into account in the incident light [Schmidhuber, 2003; Randall, 2003].
• •
Ad. b) Photovoltaic power converter system
The photovoltaic (PV) power conversion system is comprised of photovoltaic cells, electri-cal interconnection and interfaces, and smart power management circuitry. In conjunction with the energy chain of a PV powered product, the photovoltaic (PV) cells are regarded as light dependent electrical current sources. Whenever there is light available, a current is generated which can either be drained by an electrical energy storage medium or be used directly in an energy consuming product application. In Chapter 2 this PV power converter system and its interfaces will be analysed.
Ad. c) Electrical energy storage media
In an electrical energy storage medium, electrical energy is stored either chemically e.g. in a battery or as potential energy in a capacitor. In Chapter 3 the electrical energy storage media will be analysed.
Ad. d) Energy use in the functional product application
Energy use in functional product application refers to the actual energy consumption needed to fulfil the tasks of the product. For the design of a new product one can analyse datasheets of existing products. In the data sheets and user manuals of these existing products one can usually find some information about power consumption. Information is also sometimes provided in the data sheets, on whether there are any stand-by and sleep modes, including the power demand for those modes. By knowing the duration of use, the energy demand of the newly designed product can be estimated for particular ap-plications. But this is only valid for a limited moment in time and this calculation does not give enough data for the complete design of a new product. Thus the information about energy use in the functional application has to be supplemented by the ‘pattern of use’ of the new product to be designed.
Ad. e) User context defined power/energy use pattern
By knowing the ‘pattern of use’ over a certain period of time by one particular user or user group and combining this information with the power demand from the data sheets of existing products the total energy consumption of the new product over a certain period of time (e.g. for one day of that particular user) can be calculated.
The elements in the energy chain interface with each other through so-called matching interfaces (MI: 1-3).
MI 1: An incident light PV system matching interface which concerns the impact of the outdoor/indoor user context defined incident light on the choice of PV technology to be used in the designed product. For example, as mentioned above, the user context will define the type (spectrum) and magnitude of incident light that will energize the PV powered product.
MI 2: A PV output storage input matching interface which concerns the matching between the PV cells and its output circuitry, in particular to facilitate the proper matching of the PV output with respect to voltage and signal shape, as required for proper charging of the energy storage media.
MI 3: An energy use matching interface which concerns circuitry for the optimal energy transfer from the energy storage media into the application.
To obtain a functional totality in addition to the above interfaces, the overall energy bal-ance also has to be analysed. There are two overall energy balbal-ance tracks that close the feedback-loop through the overall application matching interface (OMI), namely: Track (1): The matching between the outdoor/indoor user contexts defined incident light
and the needed energy by the application in a specific user context.
In the majority of today’s designs of PV powered products, the designer makes up the energy balance by comparing the total incident light energy available in a certain user con-text with the predicted energy use in the functional product application of the product to be designed (track 1) [RetScreen, 2005]. In this comparison the elements and interfaces inside the cadre box of Figure 1-6 are not taken into account. In fact this box is treated as a ‘black box’.
From test cases it became apparent that for a proper design one needs to probe inside this black box [Weitjens, 2003]. Therefore this dissertation will focus on the contents of the cadre box and the energy balance will follow track 2.
Energy balances will have a direct impact on several other interface matching parameters such as the embodiment/dimensional matching and environment matching. Embodiment matching means the matching between mechanical product design parameters such as weight, volume, structure, shape and the available PV mounting area on one side and the other elements of the energy chain on the other. As an example, the energy balance will dictate the necessary size of the PV cells to power the application and the available area to support the PV cells. Environment matching means the matching between the product and the environment in which it has to function. As an example there will be discrepancy between the energy balance at ambient temperature and one at say -24 °C.
Resuming, a PV powered product could be used in a certain user context and energy use pattern, that defines the magnitude and nature of the incident light, the amount of energy use, mechanical dimensions, ergonomics and other environmental parameters such as temperature, humidity etc.
1.6.3 Power matching and energy matching
In this dissertation matching is sought between the elements of the energy chain of PV powered products. The analysis will focus on ‘electrical energy matching’ or more popu-larly known in literature as ‘load matching’ [Jay, 1977]. Load matching is defined as:
The process of adjustment of the load circuit impedance to produce the desired en-ergy transfer from the power source to the load;
The technique of either adjusting the load circuit or inserting a network between two parts of a system to produce the desired power or energy transfer.
In the two definitions mentioned above, the measure of how optimal the matching has been accomplished is the ‘transfer efficiency’ (ηtransfer). This is through an interface be-tween two parts of the system under investigation, and is generally defined as:
ηtransfer = Pload / Ppower source x 100% Eq.1-1
Note: Pload and Ppower source are respectively the power transferred into the load and the power
emanat-ing from the power source.
Since the investigated system is an energy chain in which energy storage media are also in-volved, the emphasis should be focused on the energy transfer efficiency instead of power.
So the energy transfer efficiency (ηE-transfer) can be defined as:
ηE-transfer = ∫ Pload dt / ∫ Ppower source dt x 100% Eq.1-2
active-time1 active-time2
Here the active time is the time the power source or the power load is in use. In most cases active-time 1 would coincide with active-time 2, for a general approach however this option in Eq. 1-2 is still open and will be closed for each specific case.
In the energy chain of PV powered products, the transfer efficiency would be:
What percentage of incident irradiance power is converted into electrical power in the PV cells? And over a certain time sequence, e.g. one day, what percentage of the incident light energy is converted into electrical energy?
What percentage of the converted irradiance energy is emitted from the PV terminals as electrical energy towards the load or in most cases as an energy storage medium? What percentage of the energy emanated at the PV terminals can be transferred, will reach and can be stored in the electrical energy storage medium?
What percentage of the energy available at the energy storage medium terminals will be transferred, reach and be used in the application?
To define the energy transfer efficiency of the entire energy chain one will not only need all the transfer efficiencies between the elements but also a defined user context, a certain pattern of use and well defined environmental conditions. This total energy transfer of the energy chain will not be generally valid but will change in time in accordance to the actual user scenario.
1.6.4 The Figure of Matching (FM) Algorithm
The power and energy transfer efficiency between the elements of an energy chain as pre-sented in section 1.6.3 can not always be measured directly or are typically not available as such for the designer of PV power products. So to analyse, quantify and predict to what extent the matching through the matching interfaces (MI:1 till 3) as presented in Figure 1-6 actually is achieved, a novel generic Figure of Matching (FM) algorithm is developed and introduced [Kan et.al, 2006c]. The term ‘generic’ is stressed since the format of this novel Figure of Matching algorithm is applicable at all the matching interfaces (MI:1 till 3). In this Figure of Matching algorithm, the main criterion for how well two elements in an energy chain are matched will be how efficient the power transfer from one element to the next is conducted. This Figure of Matching algorithm can be seen as a combination, adaptation and extension of two analytical methods namely the Stimulus - Response transfer concept and the correlation concept known from the general theory of information transmission. The rationale for this adaptation is that, in fact, the ‘information transmission or transfer’ in this theory concerns a kind of ‘power transfer’ [Krul, 1976]. Therefore information transfer formulas and nomenclature can be used. In other words, the energy chain of a PV pow-ered product is treated as an ‘energy transmission line’.
In general the Stimulus - Response transfer concept is presented in Figure 1-7. •
• • •
Each Stimulus S applied to an element p will provoke a Response Rp. From the relation between the Stimulus and the Response one can learn about the characteristics of the element under investigation. In comparing the Response of various different Stimuli it is appropriate to first determine the Response of a standardized Stimulus. In such a standard-ized Stimulus - Response verification test, it is common practice that the Response R is measured as a result of a ‘Step Stimulus’ Therefore it is named a ‘Step-Response’ [Blok, 1973], see Figure 1-8.
In the theory of signal transmission the step is presented as: def
Sstep =
0 → t = 0
Eq. 1-3 1 → t > 0
With t the time sequence coordinate of the ongoing process.
In the energy chain the output of one element will be the input of the next element in other words the output of one element will be a stimulus for the next element in the chain as presented Figure 1-9.
Figure 1-8 The presentation of a Step-Response
To analyse the matching between two elements in the energy chain, the correlation be-tween the two is investigated. In particular, the output of the first one is correlated with the step response of the second. With this correlation, according to the theory of transmission lines, the power transfer efficiency between the two is established. Having determined the Step-Response of one element p, RStep-p, the impact of various stimuli of the influencing ele-ments on this element p can be compared with the aid of the Figure of Matching algorithm as presented in Figure 1-10. Now the optimal matched pair can be selected.
Figure 1-9 Energy flow between two elements s and p in the energy chain
With:
Rs = Response coming from Element s = Sp= Stimulus on Element p
RStep-p = Response on the standardised Stimulus i.e. the Step-Response of element p
FM = Figure of Matching
The Figure of Matching between two elements s and p is defined as:
FM = { Ø / ∫ Sp (var) d var } x 100% Eq. 1-4
Here Ø is the correlation between the Stimulus Sp emanated from element s towards the next element in the energy chain, i.e. element p, and the Step-Response Rstep-p of this element p.
Ø = ∫ Sp (var) x Rstep-p (var) d (var) Eq. 1-5 var range
Since the Figure of Matching is a quotient of the correlation Ø and the overall stimulus Sp it can be regarded as a normalised correlation. The Stimulus Sp and the Step-Response
RStep-p are both a function of a given variable ‘var’. The higher this ‘Figure of Matching’ is the
better the matching. This is just the opposite of the convention in the mathematical Figure of Merit in which the best one will have the lowest value [Weisstein, 2006].
A generalisation of the original time frame based concept of step responses is introduced here by replacing the time t with a general variable ‘var’ instead. This Step-Response concept fits into the generic Matching Algorithm described above. A generalised Step-Response could be written as:
def Sstep =
0 → var < var1 Eq. 1-6 1 → var > var1
Here the variable var1 is the starting point of the step.
As mentioned above this generic Figure of Matching algorithm can be applied at all three matching interfaces (MI:1 trough 3) of the energy chain, as presented in Figure 1-6. The Figure of Matching can be made specific for one particular matching interface by selecting the proper power or energy type that has to be transferred between the two elements under investigation and as such should be correlated and matched at that particular inter-face. The related variable ‘var’ can than be selected to fit the Figure of Matching formula. The variable ‘var’ can for example be the wavelength λ, in which case the Figure of Match-ing is used in the matchMatch-ing between the spectrum of the incident light (Sp) coming from a light source (element s) and the spectral response (RStep-p) of the PV cell (element p), as in the matching interface MI:1.
In the Chapters 2 and 3 for each matching interface (MI:1 till 3), the appropriate trans-ferred power type, variable ‘var’ and Step-Responses will be identified and measured to calculate the Figure of Matching which then can be compared with the experimentally found transfer efficiency through the analysed matching interfaces.
The Figure of Matching algorithm is conceived in the direct way as a straight forward solu-tion, giving quantitative results. This solution is chosen in favour of other less direct ways such as first developing theoretical models etc. This direct quantitative result is expected to yield good input for designers of PV powered products since it contributes towards a clear understanding of the design parameters involved.
1.7 Research Objective, Goals and Scope of this dissertation 1.7.1 Research Objective and Goals
The research objective of this PhD project is to contribute to the understanding of how, why and under what circumstances the introduction of the Energy Matching Model (EMM) with the Figure of Matching (FM) algorithm could aid industrial designers to design more properly matched photovoltaic (PV) powered products. These well designed products will enable the diffusion of PV powered products from the niche market towards the mainstream and in addition could even provoke an innovation cascade in which new yet unconceivable products become feasible. The goal of this PhD project is to pave the way towards more sustainably designed PV powered products.
1.7.2 The Scope of this dissertation
These detailed analyses and modelling of elements and parameters will be treated in other research programs and dissertations within the SYN-Energy program [Reich, forthcoming, 2008]. Also further optimisation of the overall efficiency of energy consumption, with the use of energy/power management tools are beyond the scope of this dissertation. A vast amount of literature on these topics can be readily found in open literature [e.g. Pouwelse, 2003, Havinga, 2000]. In addition the emotional experience and user interface aspects will be beyond the scope of this dissertation. In the framework of the SYN-Energy program the research in these fields will be treated in another dissertation [Veefkind, forthcoming, 2007] at Delft University of Technology and research at Twente University. The same applies for the Life Cycle Assessment and Costing aspects and PV Technology modelling which will also be beyond the scope of this dissertation. In the framework of the SYN-Energy program the research in these fields will be conducted at Utrecht University. 1.8 Methodology
In writing this dissertation and in search of answers to the research question presented in section 1.6, a research methodology is used that can be seen as a mixture of the system-atic engineering research methods employed during my 24 years in the industry as well as the design methodology from the faculty of Industrial Design Engineering of the Delft University of Technology [Roozenberg and Eekels, 1998].
Both methodologies have an Orientation and Analysis phase. In these phases both the research question and the related problem definition are crystallised. Important in these phases are literature studies or more general the gathering of data and expert views. Literature studies are needed to establish the baseline what is known at the start of the project and to avoid a ‘re-invention of the wheel’. An extension of these literature studies are the expert views gathered from interviews taken by the author and those taken by Master Project students but also discussions by the author with experts at conferences. In this dissertation the literature will be quoted as [Ref., year]. The expert views will some-times be quoted as [Private Communication] but mostly they will be hidden as the source is from a Master Graduation Thesis. If the expert view is an additional explanation of a presented paper, then only the original paper will be quoted. Experiments will be done to fill the missing data gaps.
For this dissertation the Orientation phase was triggered by the invention of the Smart PV Battery [Kan, 2002a]. In the Analyses phase, this idea is explored further and generalized eventually results in the Research Question (section 1.5.2).
1.9 Outline of this dissertation
The outline of this dissertation is structured by the quest to discover consecutive answers for the research question and sub-questions with the aid of the Energy Matching Model and Figure of Matching algorithm, as stated in section 1.5.2. In the following chapters, therefore the elements and matching interfaces of the energy chain, as presented in Figure 1-6 are to be analysed. The complete structure and outline of this dissertation is visualised in Figure 1-11.
Figure 1-11 The structure and outline of this dissertation
The first part of the research question (Sub Q. 1) will be addressed in Chapter 2. This Chapter 2 deals with photovoltaic performance and methods for optimising the matching with respect to incident light and power output from the PV cells in outdoor/indoor user context. In particular the interaction between the user context defined irradiance and the PV power converter in the irradiance matching interface (MI:1) will be analysed. This will include some examples of the spectral Figure of Matching calculations.
The second part of the research question (Sub Q. 2) will be addressed in Chapter 3. This Chapter 3 deals with energy storage media in conjunction with photovoltaic cells and energy use. In particular the electrical interfaces between the PV power converter and the electrical energy storage Media (IM:2) and between the electrical storage media and the functional application (IM:3) will be analysed. This will include calculations of the Figure of Matching and some suggestions for improvement which are validated by a comparison of the measured transfer efficiencies.
how this effects the other design engineering parameters as stated in the third part of the research question (Sub Q. 3).The found optimisation is presented here as a consequence of combining the results of Chapters 2 and 3 with the overall matching for energy use and energy balance in the design.
Chapter 4 is followed by Chapter 5 which presents an overview of some benchmark experiments, test case studies and product examples compiled from master graduation theses and the SYN-Energy program. In Chapter 5 the fourth part of the research ques-tion (Sub Q. 4) is addressed. This chapter demonstrates and illustrates the feasibility of using the Energy Matching Model and the Figure of Matching algorithm in analysing test cases of PV powered products.
To complete this dissertation it is finished with several conclusions and recommendations for further research (Chapter 6).
For the reader’s convenience, basic information is provided in the Appendices.
systems for mobile/wireless products in outdoor/indoor user
contexts
2.1 Introduction and general remarks
In this chapter the most important element in the energy chain of photovoltaic (PV) pow-ered products, namely the photovoltaic power converter and its interfaces will be anal-ysed. In particular this analysis will include the user context defined incident light flux, the incident light matching interface (MI:1) and the PV power converter. In addition part of the electrical interface that concerns the power transfer from PV cells to the system terminals and the electrical output power from those terminals to the energy storage media will also be treated. In doing so, the first part of the research question will be answered:
What photovoltaic conversion systems are optimally matched with applications in an outdoor/indoor user context?
A photovoltaic conversion system is comprised of photovoltaic (PV) cells, electrical inter-connection and interfaces and smart energy management circuitry. In conjunction with the energy chain of a PV powered product, PV cells are regarded as light dependent electri-cal current sources. Whenever there is light available, a current is generated which can either be drained by an electrical energy storage medium or directly be used in an energy consuming product application. In an ideal case this current would be linearly dependent on the irradiance level while the output voltage remains constant. In practice, however, neither the output voltage nor the directly related conversion efficiency conforms to this ideal behaviour.
With optimally matched primarily energy matching is meant. This includes:
Maximal transfer or conversion efficiency from light power to electrical power by choosing the right type of photovoltaic cell to match the available light type (spectra) and level in accordance to the user context.
Maximal extraction of the electrical power from the PV cells (or modules) to the output terminals.
Optimal match with respect to the user as far as the minimum required availability pat-tern is satisfied. If that required minimum is satisfied then one can speak of an optimal match.
Differentiating what is a real must for the user and what is of secondary importance. For an optimal match weight factors for certain energy use have to be taken into ac-count.
Matching between energy parameters of the used type of PV cells and the charge parameters of the energy storage media. Thus an optimal match with respect to the available energy coming from the PV cells and the capacity of the battery being used (see Chapter 3).
Mobile products are by logical consequence of being carried around applied in an outdoor/indoor user context. This means that to achieve the optimal match, PV per-formances under low irradiance levels, as occur at indoor conditions, will also be important design parameters to be taken into account.
The part that concerns the optimal matching of electrical interface with the electrical energy storage media however will be treated in Chapter 3.
Figure 2-1 presents the Energy Matching Model of the energy chain marking the elements and interfaces analysed in this chapter by including the section numbers.
5.
6.
From literature [e.g. Randall, 2003] it becomes apparent that until now around the world much renewable energy research effort is put into designing and optimisation of PV cells for outdoor operation only. However in matching photovoltaic power conversion system characteristics with the energy use in mobile products different aspects regarding photo-voltaic cells used outdoors have to be taken into account. This is namely:
At this moment the datasheets of photovoltaic cell manufacturers present only PV performances measured under Standard Test Conditions (STC). STC means that the tests are conducted at an irradiance level of 1000 W/m2, which equals that of a clear summer day, with an air mass (AM) 1.5 spectrum and temperature of 25 °C. Since in the Netherlands for instance, there is more often an overcast sky, the irradiance level will in practice be less than 1000 W/m2 and the available light spectrum will not comply to the defined AM 1.5 spectrum. Also indoor light characteristics do not comply with the STC conditions. In addition the PV cells could have been warmed up to a temperature far over 25 °C or cooled down below 25 °C (e.g. mobile consumer products specification: 0 °C up to 40 °C).
The incident angle of irradiance on PV cells mounted on a product will not always be exactly perpendicular.
The size of the PV cells or modules will in most cases not be the common module size used on roof-tops.
The price will depend on application, the more urgent the additional function is need-ed the more money that can be investneed-ed in the PV cells.
Mechanical flexibility is usually not a matter of concern for rooftop application. For integration on products it will determine the feasible design space.
To find answers for the above-mentioned research sub-question the matching through the following matching interfaces will be analysed (see Figure 2-1):
Irradiance interface and its energy transfer efficiency (MI:1);
Electrical PV output and storage input Interface and its energy transfer efficiency (MI:2).
The emphasis is on the energy transfer, therefore mechanical interfaces and user inter-faces are treated on the second place as ‘other relevant design parameters’ (section 2.7). In this dissertation they will be regarded as ‘given exogenous parameters’. Data of these exogenous parameters is obtained from literature, estimations of test cases and other joint measurements within the framework of the SYN-Energy program.
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