Master Level Thesis

European Solar Engineering School

No.171, August 2013

Use of Photovoltaic on an E-bike?

A Feasibility Study

Master thesis 18 hp, 2013 Solar Energy Engineering

Student: Thomas Schnabel Supervisor: Jon Persson

Dalarna University

Energy and Environmental

Technology

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Abstract

In recent years the number of bicycles with e-motors has been increased steadily. Within the pedelec – bikes where an e-motor supports the pedaling – a special group of transportation bikes has developed. These bikes have storage boxes in addition to the basic parts of a bike. Due to the space available on top of those boxes it is possible to install a PV system to generate electricity which could be used to recharge the battery of the pedelec. Such a system would lead to grid independent charging of the battery and to the possibility of an increased range of motor support. The feasibility of such a PV system is investigated for a three wheeled pedelec delivered by the company BABBOE NORDIC. The measured data of the electricity generation of this mobile system is compared to the possible electricity generation of a stationary system. To measure the consumption of the pedelec different tracks are covered, and the energy which is necessary to recharge the bike battery is measured using an energy logger. This recharge energy is used as an indirect measure of the electricity consumption. A PV prototype system is installed on the bike. It is a simple PV stand alone system consisting of PV panel, charge controller with MPP tracker and a solar battery. This system has the task to generate as much electricity as possible. The produced PV current and voltage are measured and documented using a data logger. Afterwards the average PV power is calculated. To compare the produced electricity of the on-bike system to that of a stationary system, the irradiance on the latter is measured simultaneously. Due to partial shadings on the on-bike PV panel, which are caused by the driver and some other bike parts, the average power output during riding the bike is very low. It is too low to support the motor directly. In case of a similar installation as the PV prototype system and the intention always to park the bike on a sunny spot an on-bike system could generate electricity to at least partly recharge a bike battery during one day. The stationary PV system using the same PV panel could have produced between 1.25 and 8.1 times as much as the on-bike PV system. Even though the investigation is done for a very specific case it can be concluded that an on-bike PV system, using similar components as in the investigation, is not feasible to recharge the battery of a pedelec in an appropriate manner. The biggest barrier is that partial shadings on the PV panel, which can be hardly avoided during operation and parking, result in a significant reduction of generated electricity. Also the installation of the on-bike PV system would lead to increased weight of the whole bike and the need for space which is reducing the storage capacity. To use solar energy for recharging a bike battery an indirect way is giving better results. In this case a stationary PV stand alone system is used which is located in a sunny spot without shadings and adjusted to use the maximum available solar energy. The battery of the bike is charged using the corresponding charger and an inverter which provides AC power using the captured solar energy.

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Acknowledgment

I would like to thank my supervisor, Jon Persson, for his support and advice throughout this work. Jon’s interest in spreading the thoughts of renewable energy all over the world and in each section of life provided me with this topic and gave me the possibility to work this thesis out. His confidence and positive thinking helped my through “shaded” times during the thesis. I would also thank Kent Börjesson who supported me with his help and all necessary components and measurement devices which enabled me to build and test the PV prototype and measurement systems. He had a key when I was standing in front of looked doors ... and also a new data logger. I am extremely grateful to Bart Omlo and Christian van Dartel from company BABBOE NORDIC for lending me their bikes and for the opportunity to rebuild them and to install the PV prototype system. It was a pleasure to drive around with BABBOE bikes, especially with the one with e-motor! For the knowledge and the background, which is necessary for this thesis, and which I gathered during this one year master program I want to thank Frank Fiedler and all lecturers as well as all students of ESES 2012-2013, which made the studies fun and enjoyable. Last but not least I want to thank Magdalena for going along with me all the time. Thanks for all the support, the discussions, the delicious food and the interest in my “BIKE BABY”.

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Contents

1 Introduction ................................................................................................................................... 1 1.1 Aims 2

2 Methods .......................................................................................................................................... 3 2.1 Analyses of pedelec 4

2.1.1. Bike Battery 4 2.1.2. Sensors on the pedelec 5 2.1.3. Motor controller 5 2.1.4. Motor 5 2.1.5. Available space for on-bike PV prototype system 5

2.2 Analyses of available material for the on-bike PV prototype system 6 2.2.1. PV panel 6 2.2.2. Charge controller 8 2.2.3. Solar battery 9

2.3 Analyses of available material for measurement equipment 9 2.3.1. Datalogger 9 2.3.1. Inverter and 12V power supply 10 2.3.2. Irradiance sensor 10

2.4 Measurement of electricity consumption of the pedelec 11 2.4.1. Energy logger 11 2.4.2. Tests to measure the electricity consumption of the pedelec 12

2.5 On-bike PV prototype system 14 2.5.1. Electrical scheme 15 2.5.2. Mechanical installation 15

2.6 Measurement of generated electricity of on-bike system 17 2.6.1. Electrical scheme 17 2.6.2. Mechanical installation 18 2.6.1. Tests to measure the electricity production of the on-bike system 18

2.7 Measurement of irradiance on stationary system 19 2.7.1. Measurements on stationary system 19

3 Measurements and calculations ................................................................................................. 20 3.1 Measurement of the electricity consumption of the pedelec 20 3.2 Measurement of generated electricity of on-bike system 20 3.3 Measurement of the irradiance on stationary system 21 3.4 Calculation of possible electricity generation of stationary system 22 3.5 Calculation of possible daily electricity generation 22

4 Results ........................................................................................................................................... 24 4.1 Electricity consumption of the pedelec 24 4.2 Generated electricity of on-bike system 24 4.3 Possible electricity generation of stationary system 28 4.4 Comparison between on-bike electricity generation and possible stationary electricity generation 30

5 Discussion .................................................................................................................................... 31 5.1 Electricity consumption of the pedelec 31 5.2 Generated electricity of on-bike system 31 5.3 Possible electricity generation of stationary system 32

6 Conclusion .................................................................................................................................... 33 6.1 Energetic feasibility of an On-bike PV system 33 6.2 On-bike system versus Stationary system 33

References ........................................................................................................................................ 34

ANNEX ........................................................................................................................................... 36

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1 Introduction In the recent years the number of bikes with supporting electric motors has steadily increased. This can be recognized in daily life and is also pointed out in numbers. A press release gives the sales numbers of e-bike in Europe with 900 000 in 2011 and 1 100 000 in 2012 (Zweirad Industrie Verband E. V, 2013). Further it states out that 95% of all e-bikes are pedelec, the rest are electric mopeds or motorcycles. Pedelec stands for Pedal Electric Bike. That means that the electric motor just supports the pedaling and must not work without pedaling of the user. Further regulations like the maximum power of the motor or the maximum allowed speed which is supported by the electric motor are defined in the European Directive 2002/24/EG (Eurlex, 2002). A special kind of pedelec is available from the Dutch company BABBOE which has a sales office in Sweden and Norway. BABBOE bikes are designed to fulfill a transport function. There are boxes mounted on the bikes to make the transportation of children or different kind of goods up to a maximum load of 100kg possible. The design of the bike has to be heavy to support the transportation function. Even with the gearshift it can be exhausting to ride the bike with a heavy load. Therefore an electric drive is an optional feature for the bikes. The drive system consists of electrical motor, battery, controller and sensors which are monitoring the operation mode of the bike. To recharge the battery an external charger is used which is connected to the main grid. Motivated by Jon Persson of Högskolan Dalarna the question appeared within BABBOE NORDIS if it is possible to offer an optional system based on photovoltaic which can be used to recharge the battery during operation. This master thesis proves the feasibility of using a PV system mounted onto the e-bike for recharging the battery. This kind of installation is later referred to as “on-bike”. Therefore the electricity consumed by the bike during operation as well as the electricity that could be generated by an on-bike PV system is evaluated. At this point no commercial products with an on-bike PV system to provide energy for recharging the bike battery are available on the market. A lot of information can be found about prototypes made by more or less professional developers but no data about possible electricity gain is stated. The systems are often developed for very specific uses and therefore the available documentation is insufficient to a more general and scientific feasibility study. Hence this thesis contributes information and empirical data that add to the deeper understanding of the topic. Nowadays the general approach when it comes to the use of solar energy for recharging batteries of electric driven bikes is to transform the DC-voltage from a PV system to 230V or 110V AC and to make use of the external battery charger. For those applications stationary systems are available. In this thesis a comparison between the possible electricity generation of the on-bike PV system and of a stationary system is done. This comparison also shows the influence of partial shadings on the moving bike on the possible electricity generation. Those shadings significantly reduce the performance of the PV system in terms of less electricity output of the PV panel and losses due to necessary adjustments of the used charging controller of the PV system.

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1.1 Aims

The aims of this master thesis are:

to evaluate the energetic feasibility of an on-bike PV system . To what degree can it contribute electricity to power the pedelec and recharge the bike battery during operation in the specific case of using a BABBOE TRANSPORTER.

to evaluate the electricity generation of an on-bike system in comparison to a stationary PV system.

Boundary conditions:

Due to a limited budget and time-frame only materials which are already available at Högskolan can be used to develop the PV prototype and measurement system. The system design and the measurements strongly depend on the availability of the material.

An economical evaluation of an on-bike charging system is not possible due to the fact that the cost for the necessary development and production is not available.

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2 Methods The empirical investigation done in order to collect the data needed to evaluate the questions formulated in the aims can be split into the following subtasks:

Analysis of pedelec and available material for the PV prototype system and the measurement equipment The pedelec is analyzed to find out how to measure the consumption of the motor and to check the possibility to install a PV system including the measurement system. Also the available material for the PV prototype system as well as for the measurement equipment is analyzed.

Measurement of the electricity consumption of the pedelec The measurement of the consumed electricity of the pedelec is important to evaluate if a PV system can fulfill the need. To get realistic data for the consumed electricity test are done under different operation conditions.

Development and installation of a PV prototype system on the bike To get realistic data from a real on-bike system it has to be developed and to be installed on the pedelec. The basic concept of the on-bike system is a PV standalone system which is used to generate as much electricity as possible.

Measurement of the electricity collected by the on-bike PV prototype system After the installation it is possible to measure the delivered electricity with a suitable measurement system which has to be designed and installed as well. Using this system the delivered current and voltage of the PV stand alone system is measured and used to derive the energy. The important task of the measurement system is to deliver data from real test runs during different weather conditions which can be compared to the derived consumption data of the motor.

Comparison of the electricity generation of the on-bike system to the possible electricity which can be generated by an unshaded stationary system with tilted surface

The comparison of the generated electricity by the on-bike PV prototype system possible electricity which could be produced on a stationary system is done using measurement data from the bike and calculated data from the roof using the prevailing irradiance values.

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2.1 Analyses of pedelec

The bike used is a TRANSPORTER (Figure 1) of the company BABBOE which is provided to do the study. The special feature of this bike is that it is designed for transportation and has a big box in the front part of the bike. The top of the cargo box is offering space to put a PV panel.

Figure 1: BABBOE TRANSPORTER

BABBOE offers an electrical drive system for the bikes. It consists of a lithium ion battery, a controller, an electric motor and corresponding switches and sensors.

2.1.1. Bike Battery

An important part of the electric support system of a pedelec is the bike battery. It stores the electrical energy and its capacity defines the energy content which is available to run the motor. The higher the capacity (of a fully charged battery), the more energy can be provided and used to increase the distance which can be covered with motor assistance. For bike batteries different technologies are used. Approximately half of the pedelecs in Europe are equipped with Lithium-Ion batteries and the other half with Nickel-Metal-Hybrid products (ETRA Secretary General, Annick Roetynck, 2010, p. 44). The battery market offers a big variety of voltages and capacities and it is up to the manufacturer of the e-bike to choose a suitable one. The battery in this case is a Lithium-Ion-Polymer battery pack. The voltage is 26.6V and the capacity is 10Ah. An inbuilt electronic unit cares about the charging and discharging procedures. This is very important because lithium ion batteries are very sensitive when it comes to too high charging currents or to too deep discharge. Too high current can lead to overheating and destruction of the battery (Cadex Electronics, 2013). Therefore it is important to use the charger which comes with the system. This external charger, which is connected to the main, is used to recharge the dismounted battery. The time to fully charge an empty battery is given with roughly 4 hours. It is not possible to charge the battery while it is mounted on the bike. This and the fact that the current electrical system on the bike is not designed for battery charging makes it impossible to use solar energy for recharging the bike during operation even if a PV system is installed. That requires a redesign of the electrical system.

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2.1.2. Sensors on the pedelec

Due to the European directive a pedelec has to fulfill different requirements to be free of type approval (Eurlex, 2002). One of those is that the motor must not run when no movement with the pedals – in the correct way – is done. Therefore sensors are built on the bike to monitor if the motor is allowed to operate. Another sensor is monitoring the breaks of the bike. If the bike user operates the break, or if the break is fastened in order to park, the motor is stopped respectively blocked.

2.1.3. Motor controller

The battery is directly connected to the motor controller. This unit gets the energy from the battery and creates the necessary modified signals which are needed to run the motor. Basically it is a control unit for the brushless DC motor which also takes the information of the sensors into account. If the information of the sensors enables the operation of the motor, the motor controller delivers the power to the motor. As mentioned in “2.1.1. Bike battery” the existing control unit is not designed to recharge the battery during operation. To make a recharge of the battery possible the electric system has to be redesigned to fulfill the function of a battery charger as well.

2.1.4. Motor

The electric motor is the part which generates mechanical power using the electrical power which is provided by the motor controller. According the mentioned legislation the maximum continuous rated power of 0.25 kW must not be exceeded (Eurlex, 2002). To integrate an electrical motor in a pedelec different designs are used. This includes motors which are parts of the gearing (Bosch AG, 2013) or which are transmitting the mechanical energy via a belt directly on the wheel (Camcycle, 1997). But the most used types are wheel hub motors nowadays. In this case it is a 250W brushless DC wheel hub motor. The motor has 3 inputs which are used to generate a revolving field which is needed to operate the motor.

2.1.5. Available space for on-bike PV prototype system

Another important point is to have enough space on the bike to fix a suitable size of PV panel. On the bike used this is given by the transport box in the front part of the bike. The available dimensions are 1000mm times 610mm which is suitable for the fixture of the PV prototype system. The depth of the box offers enough space to carry both the PV prototype system and the measurement equipment. Figure 2 shows the mechanical measurements of the transportation box off the BABBOE TRANSPORTER.

Figure 2: mechanical measurements of the transport box on the TRANSPORTER

1000mm

610mm

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The maximum available area is 0.61m². Depending on the efficiency ηPV module [1] of the PV technology the maximum peak power Ppeak [W] of a PV module using a certain module area Amodule [m²] can be calculated using

Table 1: module efficiency values (Häberlein, 2010, p. 15) for different PV technologies and the corresponding maximum peak power values for 0.61m²

technology Max. efficiency

ηPVmodule maximum peak power Ppeak for

Amodule =0.61m²

monocrystalline Si 19.5% 119.0W

polycrystalline Si 16.0% 97.6W

amorphos Si 7.5% 45.8W

CdTe 11.0% 67.1W

Table 1 gives an overview about the maximum possible installed peak power using different PV technologies. It is obvious that even if the whole box is used as PV module area (0.61m²) with a specific designed panel with the highest efficiency (19.5%), the maximum power is just the half of the motor load during operation. The, in comparison to the motor consumption, low power output of the PV system (max. 119W) explains the need to store the produced electricity from the PV in a battery for a later use to run the motor (250W).

2.2 Analyses of available material for the on-bike PV prototype system

The PV prototype system is basically a PV stand alone system and consists of a PV panel, a charge controller and a solar battery. Due to limited resources in terms of time and investment capital a boundary condition is to use existing material which is available at Högskolan. Out of a plenty of different possibilities the following options are chosen.

2.2.1. PV panel

The PV panel is the part of the PV prototype system which generates electrical energy by using the irradiance of the sun. The maximum power (or peak power indicated with Wp) of a PV panel depends basically on the type of panel and on the dimensions and is given for an irradiance of 1000W/m². Some producer dependent parameters affect the efficiency of a new panel. Due to degradation of the active material, soiling and other influences the peak power of PV panels decreases with the years. The power PPV [W] which a PV panel can generate always depends on the peak power Ppeak [W] and on the prevailing irradiance G [W/m²]:

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Example: Ppeak = 55W; G = 200W/m² (barley enough irradiance to effect shadings)

That means, at a prevailing irradiance of 200W/m² a 55Wp module generates in maximum about 11W.

Another important fact is that (in most cases of monocrystalline Si panels) the whole panel has to be illuminated with the same irradiance to deliver the assumed power. If only one cell (part) of the panel has an area of less irradiance – due to partial shading – this has a disproportionate impact on its power production (Deline, 2009). This effect has a big influence on the possible electricity generation on the bike because the panel often is shaded by the driver or the surrounding. The PV panel which is used in the PV prototype system is a 55Wp module which is build of Si monocrystalline cells. It has 2 bypass diodes build in the connection box to improve the performance when it comes to partial shading on a half of the panel. It is chosen because its dimension fits with 985mm times 440mm very good on the available space of the bike. To evaluate the performance of the panel used the IV curve is measured and shown in figure 3.

Figure 3: IV curve of the PV panel used for the on-bike PV prototype system

0.00W

5.00W

10.00W

15.00W

20.00W

25.00W

30.00W

35.00W

40.00W

0.00A

0.50A

1.00A

1.50A

2.00A

2.50A

0.00V 5.00V 10.00V 15.00V 20.00V 25.00V

Current [A]

Power [W]

IMP = 2.06A

ISC = 2.32A

UMP = 16.81V UOC = 21,07V

MPP = 34.61W

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Taking the irradiance of 720W/m² into account the theoretical power is

The measurement of the maximum power point resulted in PMPP720W = 34.6W. That means that the panel delivers, due to degradation and other influences, 12.6% less power than theoretically possible. The peak power of the panel is taken furthermore as 48W.

2.2.2. Charge controller

A charge controller has the function to control the energy flow from the PV panel to the solar battery. It is an important unit in a PV standalone system because it prevents the battery from overloading and respectively too deep discharging. For this reason charge controllers have different operation modes to control the voltage of the battery. If the battery is fully charged the charge controller stops the delivery of energy and, as a matter of fact, doesn’t take any energy of the PV panel. If this case happens no power of the PV panel is withdrawn. To avoid this case a load is introduced. It discharges the battery continuously and keeps the charge controller in a mode to withdraw the available energy from the PV panel. In this case the load is the supply for the measurement system and a 20 Ohms resistor which is optionally used if the PV panel delivers more energy than the demand of the measurement system. Some charge controllers have a special feature to track the maximum power point (MPP) of the PV panel. This means that the charge controller tries to work the PV panel in the optimal operation point. Especially in cases of bad light conditions or unsteady irradiance this feature has big advantages in terms of efficient use of available solar energy. When partial shadings on the PV panel happen the IV curve offers more than one maximum power point. Under partially shaded condition it can happen that the MPP tracker of the charge controller is trapped in a local MPP which is probably not the highest among all existing (Chin, et al., 2011). This leads to a situation where not the highest available power is used. Also the reaction time of the charge controller to adjust to the MPP during dynamic and fast changing shading conditions leads to a reduction of possible energy withdrawal. The effects of the trapping in local MPP and of dynamic behavior can’t be evaluated in this case due to missing measurement equipment and incomplete data of the charge controller. For the PV prototype system the charge controller type NAPS Max power (see figure 4) is used. The reason this type is that it offers a MPP tracking function.

Figure 4: photo of the charge controller used - NAPS Max power

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2.2.3. Solar battery

To store the PV generated electrical energy solar batteries are used. The main task is to preserve electrical energy when it is generated and not directly used by a load and to provide it if a need of the load appears and no solar resource is available. In contrast to the batteries used in e-bikes different properties are necessary. For solar batteries is important that they are easy to handle. Due to the fact that charging and discharging procedures can change very quickly batteries used for solar applications have to deal with this. Another fact is that the weight is not as important as the weight in bike applications where each additional weight causes additional load for the motor. Mostly batteries on the basis of lead acid technology are used especially because their lower costs in comparison to other battery types. The name is familiar due to the batteries used in cars and other similar applications but the internal construction is very different and should not be mixed. Lead acid batteries for solar applications are available in a huge variety of voltages and capacities. Nevertheless the literature clearly indicates that also Li-ion battery technology, as used for e-bikes, is suitable for the application in PV standalone systems (Thiaux, et al., 2009). The used battery is an AGM lead acid battery type Microlyte red top SEC 12MRT20. It has a voltage of 12V and a capacity of 20Ah.

2.3 Analyses of available material for measurement equipment

2.3.1. Datalogger

A data logger is an electronic device which has usually several input channels. During the operation the data logger scans those channels in defined time intervals and reads the voltage levels which are existent at this moment. The values are processed or directly stored in a memory for later use. Corresponding software tools allow to download the data to a computer and to analyze the data afterwards. Depending on what the intended application the data logger is many different products are available on the market. The possible scan interval, voltage range of the channels and many more parameters can vary quite a lot among the devices. In most cases dataloggers are used to fulfill monitoring and measurement tasks. The data logger used is a Campbell type CR10X. The scan interval is set to 0.25s and the average value of the channel voltages is stored each minute. Unfortunately this 1 minute is the shortest available storage time of this data logger. This means that the dynamic behavior of the voltage signals (PV current, PV voltage, irradiance) due to partial shadings or quick changes of the light conditions is lost and can’t be evaluated. In maximum 6 differential inputs can be used. To download the stored data the data logger is connected to the USB of a computer via a special interface device. The needed operating voltage of 12V DC is provided using a power supply. As long the operating voltage is supplied the data logger is working according a program which defines the scan interval and other operation characteristics. As soon as the operation voltage is disconnected the data logger stops working.

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Figure 5 shows the data logger and the necessary interface equipment to connect it with a computer.

Figure 5: photo of the datalogger and the corresponding interface device

2.3.1. Inverter and 12V power supply

As already mentioned the data logger needs a 12V power supply. Due to the fact that the data logger is mounted on the bike and has to be mobile the necessary power supply has to be mobile to. For this reason the power supply takes advantage of the solar battery. An inverter is connected to the solar battery to generate 230V AC. This supply again is used to run a 12V DC power supply which provides the data logger. Both the inverter and the 12V DC power supply are shown in figure 6. The design fulfills basically 2 tasks. Firstly it works as a load for the solar battery to be sure that it never gets fully charged and that the PV panel always works on optimum operation point. Secondly to provide a stable 12V DC supply for the data logger.

Figure 6: photo of the inverter and power supply 12VDC

2.3.2. Irradiance sensor

To measure and evaluate the available irradiance on the bike an irradiance sensor is used. Different technologies are available to measure the irradiance. One of those is to make use of a single PV cell. Irradiance sensors based on this idea using the principle of PV. They amplify the generated voltage of this single cell to a higher voltage potential to ease the measurement of the signal. This signal is directly proportional to the irradiance on the sensor. To facilitate this function an operation voltage has to be provided.

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The available irradiance sensor (see figure 7) is a product of Mencke & Tegtmeyer. The type Si-01TC-T offers the possibility to get irradiance and temperature data of the cell. It is supplied with 5V from the data logger. The output signal of the irradiance channel is proportional to the irradiance and results in 0 to 1V as the irradiance is 0 to 1000W/m².

Figure 7: photo of the irradiance sensor used – Si-01TC-T

2.4 Measurement of electricity consumption of the pedelec

To measure the electricity consumption of the pedelec different methods are possible. The most accurate method for deriving the motor consumption would be to measure the voltage and current directly on the motor. It would be necessary to measure the voltage and the current of the motor continuously to have complete data along the operation. In this case it is not possible due to missing measurement equipment. Also the measurement of the current using a shunt resistor is prevented by too many disturbances. The low voltage level of the current signal (50mV at 5A), is overlaid by noise and gives not a good quality of the collected data. As a matter of fact the overall consumption is measured indirectly. An energy logger is used to measure the energy which is used to recharge the battery using the corresponding battery charger. Before a test run the battery is charged completely and after the test run the necessary energy to recharge it to full capacity is documented.

2.4.1. Energy logger

The tool used is an energy logger 4000 from Voltcraft which can be seen in figure 8. It measures voltage and current which passes through the device and stores the electrical power value which is calculated.

Figure 8: photo of the energy logger used – energy logger 4000 Voltcraft

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2.4.2. Tests to measure the electricity consumption of the pedelec

To evaluate the electricity consumption of the bike as a function of the length and the covered height difference different tracks were defined. Track 1 – Faluvägen 1 (Steep track) The first track is a part of the Faluvägen which leads from Borlänge to Ornäs. The slope in the middle of the track makes some help of the bike rider necessary because the motor power alone doesn’t cover the necessary energy. The track started immediately after the roundabout and ended at the crossing to the Skistadion.

Track specific: Track length: 350m Cover Height difference: 7m (Low: 128m High: 135m) Number of repetitions (#): 4 Track 2 – Faluvägen 2 (Moderate track) The second track is also a part of the Faluvägen. It starts at the crossing to Fågelmyra and goes towards Borlänge. Along this section the bike has to cover a moderate rise and no additional pedaling power is necessary.

Track specific: Track length: 800m Cover Height difference: 10m (Low: 126m High: 136m) Number of repetitions (#): 5

128m

135m 131m

126m

126m

126m

130m

136m

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Track 3 – Ornäs (Flat track) The third track is a part of the Ornäsvägen. It starts at the entry of Ornäs and leads towards the main road and back again. The characteristic of this track is flat. Due to the fact that it is going back again the height difference is just 4m. This is chosen to evaluate the consumption of the pedelec when riding an even road.

Track specific: Track length: 940m Cover Height difference: 4m (Low:121m High:122m / Low:121m High:124m) Number of repetitions (#): 6 Track 4 – Ornäs to Högskolan This track is defined to get a typical run with different slopes and phases where the rider is supporting by pedaling.

122m

124m

123m

121m

118m

120m

122m

120m

127m 126m

136m

131m

136m

138m

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Track specific: Track length: 8500m Covered Height differences: 52m (positive track rises) Number of repetitions (#): 1

2.5 On-bike PV prototype system

The idea is to have a PV system on the bike which can store and use the energy of the sun to recharge the battery of the bike. Even the idea is superior it cannot be realized in this state. The bike is not designed to allow the charging of the battery while it is mounted on the bike. For recharging the battery has to be removed. To enable the system to support charging using solar power while the battery is mounted on the bike the electronic control has to be reengineered. It also has to fulfill the function of a battery charger and has to manage the charging and discharging processes while using the bike. Here the need for specific and detailed development is given. The installed PV prototype system is a simple type of a PV stand alone system. It consists of a PV panel, a charge controller and a lead acid solar battery. The task of the system is to generate and to store as much electricity as possible which is offered by the sun and available on the bike. As a matter of fact the total solar energy is reduced by the efficiency of the PV panel and the efficiency of the function of the charge controller (MPP tracking, battery management, efficiency of charge controller).

135m

128m

118m

126m

136m 142m

140m 140m

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2.5.1. Electrical scheme

Figure 9 shows the scheme of the PV prototype system which is installed on the bike. The PV is connected to the charge controller on the solar side. On the other side of the charge controller the battery and the loads are connected in parallel. The Load resistor as well as the inverter are not connected steadily and can be unconnected if not needed.

Figure 9: Electrical scheme of the on-bike PV prototype system

This system is working like a common PV standalone system. Due to the function of the charge controller the solar battery has to be connected first to the charge controller. After that the PV can be connected to the charge controller as well. Depending on the irradiance on the PV panel the solar battery is charged. To avoid a fully charged solar battery the additional load resistor is used to discharge.

2.5.2. Mechanical installation

To carry the PV prototype system on the bike it has to be fixed first. The PV panel and the irradiance sensor are mounted on a wooden plate with the dimension: 1050mm x 660mm x 10mm. (see figure 10)

Figure 10: photos of the mounting of the PV panel on a wooden plate

PV Charge controller Battery

Shunt

Load resistor

Inverter 12-15V DC 230VAC

Power Supply 12V DC

Data logger

S+

S- B-

B+

-

+ +12V

GND

GND GND

12V 12V

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The BABBOE TRANSPORTER has a lid mounted on the box which can be removed quite easily. Figure 11 shows the bike with and without the lid.

Figure 11: photos of the dismounting of the lid

The wooden plate is fixed to a hinge (see figure 12) to provide the flexibility to open it like a lid.

Figure 12: photos of the mounting of the hinge

The supports (see figure 13 - left) on the edges of the bike are used to fix the panel construction on the bike (see figure 13 - right).

Figure 13: photos of the supports and the finished on-bike PV prototype system on pedelec

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2.6 Measurement of generated electricity of on-bike system

To evaluate the electricity which is generated by the PV prototype system a measurement system is installed on the bike. It measures the current and the voltage which is supplied from the PV panel. The value of the PV current is transformed into a voltage signal using a shunt resistor (50mV at 5A). A voltage divider is installed to minimize the PV voltage by a factor 12 to offer a smaller voltage signal. The measurement signals of PV current and PV voltage are connected to a data logger which reads the values in a time interval of 0.25s and stores the average value of both every minute. To derive the PV power the product of PV current and PV voltage is calculated. Due to the fact that the available data logger just saves the average values the result for the calculated power implies measurement errors which cannot be described completely. During low irradiance and fast changing light conditions the error is higher whereas constant sunshine condition leads to a smaller error. Taking into account that the inaccurate power values happen when the irradiance level is low, the contributed energy during this time is low as well and the resulting failure of the finally calculated energy result is smaller. Assuming that the irradiance during the measurement period is also a parameter which is different from day to day the results of the measurement are capable to evaluate the desired values for the energy delivery of the on-bike PV system.

2.6.1. Electrical scheme

Figure 14 shows the electrical scheme of the measurement system which is installed on the bike. The data logger measures and stores following values:

I_PV : This is the voltage signal caused by the current which flows from the PV panel to the charge controller. Due to the characteristic of the shunt resistor the voltage is 50mV when 5A are passing.

U_PV : Due to the fact that the maximal input voltage of the data logger is 2.5V the voltage of the PV panel has to be increased using a voltage divider. U_PV is 1/12 of the PV panel voltage.

G : G is the irradiance signal of the irradiance sensor. The voltage is between 0 and 1V and has to transformed into an irradiance value afterwards.

T : Due to the fact that the efficiency of PV panels decreases with increase of temperature the voltage signal is logged and stored for later use.

Figure 14: Electric scheme of the measurement system for the generated electricity on the bike

Shunt 5A .. 50mV

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2.6.2. Mechanical installation

The mentioned parts are mounted on a wooden plate which fits into the box of the bike. So it is mobile and can do the measurements while be carried around.

Figure 15: Mechanical installation of measurement system for the generated electricity on the bike

The other shunts which can be seen in the middle of the photo are supposed for load current measurements. Due to issues with different electrical potential they are not measured with the data logger but the voltage signals are used to monitor the battery and load currents with a multimeter.

2.6.1. Tests to measure the electricity production of the on-bike system

To measure the electricity generation of the PV prototype system under several conditions different test runs are made. Those are different to the tests of the motor consumption also because the generation during parking is measured. On-bike energy measurement 1 – Bike ride from Borlänge to Ornäs Performing this test the electricity generation was measured during a bike ride from Borlänge to Ornäs. It was a sunny afternoon with fewer clouds on the sky.

14:52 (figure 16) Start at Högskolan Dalarna. The direction of the ride was almost towards east-north-east so the driver shades the reference cell and the upper part of the PV panel as could be seen at the photo. 15:17 – 15:20 During this time the irradiance was very good but the sun was directly in the back of the driver and the panel was shaded. 15:20 – 15:23 The bike was turned and the sun was shining on both the reference cell and the panel. 15:26 (figure 17) This part of the road was shaded due to forest on the right side of the road. 15:40 End of the ride in Ornäs. During the last part of the panel could get some sunshine.

Data logger

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Figure 17

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On-bike energy measurement 2 – Parking During the test “on-bike energy 2” the bike was parked in a sunny spot with less shading on the panel. On-bike energy measurement 3 – Parking During the test “on-bike energy 3” the bike was parked in a sunny spot with less shading on the panel. It was relocated twice to improve the energy generation. On-bike energy measurement 4 – Parking During the test “on-bike energy 4” the bike was parked in a shaded spot. It was relocated once during the measurement.

2.7 Measurement of irradiance on stationary system

Many companies around the world offer the concept of stationary PV systems to provide energy for charging batteries of electric driven bikes. To compare the electricity which is generated by the PV prototype system on the bike to the electricity which would be possible to generate by a stationary system, the latter was calculated using irradiance data of a tilted and unshaded plane. The irradiance values are determined using the measurement data of a fix installed pyranometer on the roof of Högskolan Dalarna in Borlänge. Again a certain lack of accuracy is given because the position of the bike during the measurement runs is not the same as the stationary measurement. For this reason the results involve some uncertainties but the general comparison can be made.

2.7.1. Measurements on stationary system

To compare the possible electricity generation of a stationary system with those of the PV prototype system, the irradiance on the stationary system is measured at the same time. Stationary irradiance measurement 1 – at the same time as On-bike energy measurement 1 Stationary irradiance measurement 2 – at the same time as On-bike energy measurement 2 Stationary irradiance measurement 3 – at the same time as On-bike energy measurement 3 Stationary irradiance measurement 4 – at the same time as On-bike energy measurement 4

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3 Measurements and calculations

3.1 Measurement of the electricity consumption of the pedelec

To measure the consumption of the pedelec several test runs are made. Before each test run the battery is fully charged and after them the necessary energy is measured. The energy logger is switched between the main and the battery charger and reads the power consumption and stores an average value each minute. This data is transferred to a computer where the corresponding software allows the user to display the data in form of a graph like in figure 18.

Figure 18: available data from the Energy logger 4000

The graph gives information about the apparent power Precharge [W] and the time trecharge [min] which is needed to fully recharge the bike battery. Using these values the recharge energy Erecharge [Wh] is calculated with

3.2 Measurement of generated electricity of on-bike system

Before the data collection procedure works the USB interface and data logger software has to be installed on the computer. Also a program must be written and transferred to the data logger in order to define the input channels and stored values. The procedure for the programming can be found in the annex. To ride the bike and measure the generated electricity of the PV prototype system firstly the systems have to be set up. The PV prototype system as well as the measurement equipment is installed on the bike. To ensure a proper working of the system and to avoid damage of the data logger due to high voltage/current peaks a defined procedure has to be executed to start up the measurement.

1. The solar battery has to be connected first to the Charge controller and afterwards to the PV inputs.

2. The voltage of the solar battery is measured with a multimeter and has to be between 12.0V and 13.0V before starting the measurement. If the voltage is lower the battery is recharged using the PV panel. If the voltage is higher the additional load resistor is connected.

3. When the voltage is in the accepted range the inverter is switched on and the 12V supply is connected to the data logger.

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4. Afterwards the sensor cables to measure I_PV, U_PV and later the G and T wires are connected to the data logger.

5. Now the bike and the systems are ready to data collection. As soon as the data logger is supplied with 12V DC it is working and logging the voltage values according the program in the data logger. It is also possible to monitor the input channels online on a computer or to download the already stored data. The data for the current I_PV and the voltage U_PV which is delivered by the PV prototype system is used to calculate the power of the system P_PV. The values for I_PV and U_PV are calculated based on average values and therefore a lack of accuracy for the calculated power is given. Comparing values derived from data of the data logger with values measured with multimeters a deviation of 10% is found. The average building of the data logger conducts to an underestimation of the generated PV power. As a matter of fact the PV power P_PV is corrected and increased using a factor 1.10 furthermore.

The values for P_PV [W] are average values for one minute and so represent the generated electricity during this minute. The sum of the P_PV values during the minutes of measurement n [min] gives the total transformed PV electrical energy Energy_PV [Wh].

3.3 Measurement of the irradiance on stationary system

The energy data from the on-bike PV system is derived in the way it is described in the former section. To get the irradiance values of the roof the data of the installed measuring system on the roof of Högskolan Dalarna is used. It measures the irradiance on a 40° tilted and south heading plane using a data logger. The data is measured each 10 seconds and automatically saved in an Excel file for further processing. Like the data from the on-bike system also this stationary irradiance data faces the issue of building an average value. To calibrate the irradiance and energy measurement on the bike, it is positioned next to the test rig on the roof of Högskolan Dalarna where the roof irradiance data is measured (figure 19). The comparison of the irradiance values of the 2 systems shows that the measurement system on the bike underestimates the irradiance. As a matter of fact the irradiance value of the bike is corrected and increased using a factor 1.013 furthermore.

Figure 19: Comparison measurement on roof of Högskolan – bike 40°

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3.4 Calculation of possible electricity generation of stationary system

Using the irradiance data G [W/m²] from the stationary system, the generated power P_stationary [W] and the energy Energy_stationary [Wh] can be calculated if the same panel would be installed as on the bike. The value of the peak power Ppeak [W] is assumed to be 48W.

3.5 Calculation of possible daily electricity generation

Before planning and installing the PV prototype system estimations about the possible electrical energy are made. This data is necessary to evaluate the difference between a horizontal PV system on a bike and a stationary system. The daily electrical energy Edaily [Wh] which is transformed by a PV panel is calculated using the average daily insolation Idaily average [kW.h/m².day] and the peak power Ppeak [W]

The data for average solar insolation is available from weather services and literature (Duffie & Beckman, 2006, p. 54). It is always given for a specific location and period of time. The value differs from year to year. Another important parameter is the tilt of the plane where the insolation is measured. For this calculation the data from (Solarelectricityhandbook, 2013) for Uppsala is used. Figure 20 shows the average daily electricity which can be generated using a panel with 48Wp. The premises are that it is has no additional degradation, no shading or other influences that would decrease the efficiency of the PV panel and that all generated electricity is withdrawn.

Figure 20: Average daily electricity using a PV panel with Ppeak = 48W without losses

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Figure 21 includes an efficiency of 85% which is needed to convert the solar energy into suitable AC voltage to run a battery charger and to charge the battery. This is a rough guess and includes the losses of MPP tracking, inverter and battery charger. Detailed efficiency values have to be derived for specific installations. It is important to point out that the generation and the processing of electrical energy using solar energy imply losses so that not all of the available resource can be used.

Figure 21: Average daily electricity using a PV panel with Ppeak = 48W including 15% losses due to conversion

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4 Results In this section the results of the tests and measurements are shown. The results of the electricity consumption of the pedelec and the generated electricity of the on-bike PV system are the basis to evaluate if an energetic feasibility is given to install such a system on a pedelec. To allow the comparison of the generated electricity of the on-bike PV system with the results of the stationary irradiance measurement, the latter is shown together with the possible electricity production on this stationary system. All results of the on-bike system and the stationary system as well are based on the PV panel with a peak power of 48W.

4.1 Electricity consumption of the pedelec

The results of the needed energy to recharge the bike battery after riding cycling different tracks are shown in table 2. The standby consumption of the battery charger is measured with 0.43W without the battery connected to the charger. The resulting standby electricity consumption during charging is already subtracted from the shown figures. Important to mention is that the charger has a higher power consumption while charging due to dissipation loss. That means further that not all of the recharging energy can be associated to the motor consumption. Nevertheless the total recharging energy is needed to fully recharge the battery. The weight of the bike is 150kg. (60kg bike + 90kg additional weight) Table 2: summary of test runs; necessary recharging energy after riding the tracks

Track #

total difference in altitude

[m]

total length

[m]

Recharging power

[W]

Recharging time [min]

Recharging Energy [Wh] Notes

1 4 28 1400 61.95 47 49 slight help

2 5 50 4000 62.62 107 112 no help

3 6 24 5640 63.61 70 74 no help

4 1 52 8500 62.7 104 109 strong help

4.2 Generated electricity of on-bike system

The following section shows the results of the energy measurement of the PV as well as the irradiance values which are measured at the same time on the bike (see figures 22-25). Assumed that the irradiance on the irradiance sensor and on the PV panel is the same, the charts in the following figures 22 – 25 should be congruent. The reason for the larger deviations is that the PV panel due to its larger size was more often influenced by shading than the smaller irradiance sensor. Also the losses due the MPP tracking during rapid changing light conditions can be the reason for the lower power values. The difference during low irradiance can be caused by the difference of the low light performance of the monocrystalline PV module and the PV cell used in the irradiance sensor (Häberlein, 2010, p. 92).

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Figure 22: Generated PV power and prevailing irradiance during-on-bike measurement 1

Figure 23: Generated PV power and prevailing irradiance during-on-bike measurement 2

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Figure 24: Generated PV power and prevailing irradiance during-on-bike measurement 3

Figure 25: Generated PV power and prevailing irradiance during-on-bike measurement 4

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Table 3 presents a summary of the on-bike measurements done and gives the values for the total electricity (Energy_PV) which was generated by the on-bike system. The average PV power (P_PV) during the measurement is shown as well as its relation to the peak power. To use the generated PV electricity directly for recharging the bike battery it has to be controlled and processed with a charging unit within the bike controller. This processing will cause losses in the charging unit so that not all of the energy can be used. Alone the charge controller used has an efficiency of 90% to 94% (Naps, n.d.). Further loss from the bike battery charging unit is hard to guess without any development done. As the standalone consumption of the existing battery charger is known with 0.43W this value is subtracted from the average PV power P_PV. The values for “time for 100% recharge of bike battery” take this standalone consumption, the efficiency of the charge controller used with 92% and the capacity of the solar battery with 266Wh into account. A power dissipation of the charging unit which is higher than 0.43W will lead to an increase of the needed charging time. Table 3: summary of the on-bike energy measurements

On-bike energy

measurement Time Start

Time End duration

total Energy_PV

average P_PV

% of Peakpower

time for 100% recharge of bike battery

1 14:53 15:40 48min 6.20Wh 7.75W 16.1% 39.7h

2 16:05 17:19 74min 20.42Wh 16.55W 34.4% 18.0h

3 8:00 15:15 436min 96.97Wh 13.34W 27.8% 22.5h

4 10:49 16:15 327min 27.11Wh 4.97W 10.3% 64.2h

The results of the generated electricity indicate very clearly that the installed on-bike PV prototype system has a low power output in relation to the peak power of the panel. Even in the test 2 and 3 where the bike was parked in a low shaded spot relocated to avoid complete shading the average PV power was 16.55W and respectively 13.34W. As a result also the electricity generation with this power output is low. Nevertheless having similar weather conditions and low shading an empty battery can be loaded to about 50% during one day. Riding the bike means a continuous changing of position and direction. Because of the position of the PV panel right in front of the driver it gets shaded very often. Also the handlebars of the bike but also houses, trees or other obstacles cause shading on the panel. Already the partly shading of one cell of the module is enough to reduce the extractable energy significantly (Deline, 2009). This explains the low power output figured out in test 1. While doing the measurement the bike was moved from Borlänge to Ornäs. Due to afternoon time the sun was shining in west-south-west and the bike was moved to east-north-east. This caused a lot of shadings from the driver. The resulting 7.75W in average are just 16.1% of the possible peak power of the panel. During a short time the panel was unshaded and delivered 35W that equals 73% of the peak power. If the PV panel of the prototype system is shaded and the light conditions in general are not so good the available energy is very little. The value of test 4 (27.11Wh) proofs that a parking time of nearly 327min under this condition is just enough to deliver a tenth of the capacity of the bike battery which is 266Wh.

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4.3 Possible electricity generation of stationary system

This section shows the results of the irradiance measurement on the roof of Högskolan Dalarna which is a stationary system tilted 40° towards south. It is important to mention that the measurements of the stationary irradiance were done at the same time as the on-bike energy measurements. Those values for the irradiance are used to calculate the possible power of a PV panel with 48Wp as described in “3.4.Calculation of possible electricity generation of stationary system”. That means the following results show the power which would be generated if the same panel is installed on the roof as stationary system as it is installed on the on-bike PV prototype system. An assumption for the calculation of the possible power, and as a result of the possible electricity generation, is that the panel is always working on the maximum power point. The charts (see figures 26-29) for the measured irradiance and the calculated power are congruent because the assumed operation on MPP and the neglect of low light performance of the PV panel (Häberlein, 2010, p. 92).

Figure 26: measured irradiance and possible power generation of the stationary system during on-bike test 1

Figure 27: measured irradiance and possible power generation of the stationary system during on-bike test 2

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Figure 28: measured irradiance and possible power generation of the stationary system during on-bike test 3

Figure 29: measured irradiance and possible power generation of the stationary system during on-bike test 4

Table 4 presents a summary of the possible stationary electricity generation. It gives the average of the possible power of the stationary system (P_stationary) as well as the ratio of this value to the peak power. To recharge the bike battery using a battery charger the stored solar energy has to be transformed into AC voltage. For this reason the value “time for 100% recharge of bike battery” takes a conversion efficiency of 85% into account, it also contains the efficiency of a charge controller (MPP tracker) with 92% (Naps, n.d.). This value is a rough guess and has to be evaluated in case of realization. The necessary energy for total recharge is assumed as 266Wh.

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Table 4: summary of the possible electricity generation of a stationary PV system

Possible stationary electricity generation

Time Start

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% of Peakpower

time for 100% recharge of bike battery

1 14:53 15:40 48min 24.62Wh 30.77W 64.0% 10.17h

2 16:05 17:19 74min 25.10Wh 20.35W 42.3% 15.38h

3 8:00 15:15 436min 240.88Wh 33.15W 69.0% 9.44h

4 10:49 16:15 327min 220.68Wh 40.49W 84.2% 7.73h

4.4 Comparison between on-bike electricity generation and possible stationary electricity generation

Table 5 offers the direct comparison between the on-bike energy measurement and the possible electricity generation of a stationary system. Due to the fact that the values are measured during the same time period a factor can be calculated which shows the relation between the on-bike and the stationary system in terms of electricity generation (P_stationary/P_PV). Table 5: comparison between the electricity generated of the on-bike PV prototype system (P_PV) and the electricity which would be theoretically generated at the same time of a stationary PV system (P_stationary) on the roof of Högskolan Dalarna using the same panel

On-bike energy measurement

average P_PV

Possible stationary electricity generation

average P_stationary P_stationary / P_PV

1 7.75W 1 30.77W 397%

2 16.55W 2 20.35W 123%

3 13.34W 3 33.15W 248%

4 4.97W 4 40.49W 814%

The generated electricity of the stationary system could have had delivered much more energy than the on-bike PV prototype system did in the same time. The primary reason for that is the fact that the stationary system on the roof is not going to be shaded as it is the system on the bike. Test series 1 shows the influence of riding the bike. Whereas the stationary system would have had delivered 30.8W in average the on-bike PV prototype system just provided 7.8W. A stationary system could have had produced 4 times as much electricity as the bike system. The result of positioning the bike in a sunny spot with low shading is shown in test series 2 and equals 16.5W. The corresponding power value on the roof would be 20.4W. In this case a stationary system just would have produced just 23% more than the on-bike system. Especially the comparison of the electricity generation during test series 4 shows a significant difference. Whereas the on-bike system just delivered an average PV power of round 5W the stationary system could have had delivered 40.5W using the same panel. That is 8 times more than the mobile system. The reason is that the bike mainly was left in shadings whereas the stationary system could made use of the good irradiance.

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5 Discussion

5.1 Electricity consumption of the pedelec

Due to limited availability of bike the number of test rides is small. More test runs and recharging procedures have to be done to get more accurate data of the consumption of the pedelec and specifically for the motor. Many influences like distance, covered difference in altitude, weight of the bike and additional load, prevailing wind conditions, effect of pedaling and more could not be evaluated. Nevertheless the available measurements offer a tendency of the level of the electricity consumption of the pedelec. The most realistic test track was number 4. It was a ride over 8.5km, covering 52m difference in altitude and was ridden using human pedaling help. Assuming the constant total weight of the bike plus load of 150kg and that the track displays an average steepness the average electricity consumption for this test is 12.8Wh/km. This value includes the motor consumption and all losses which happen in the battery, in the motor controller and along the power supply from the battery to the motor. Also the power consumption of the battery charger is included in the measured values. The value of 12.8Wh/km is in a range for electricity consumption of e-bikes which was derived in a study done for 12 different products (Timmermans, et al., 2009).

5.2 Generated electricity of on-bike system

To produce a suitable electricity to support the motor a much bigger PV panel must be used which does not fit on the bike. For that reason quite a lot developers tested the concept of using a trailer which is pulled by the bike. This trailers offer more space to install bigger PV panels with higher power output (Knott, et al., 2012) (Solarbikeproject, 2009). Also the positioning of the PV panel overhead the driver was developed and successfully tested doing a long distance ride through the Sahara. In this project just the generated electricity from the PV was used without taking advantages of additional pedaling (Aktfoundation, 2013). Even thou it is not possible to directly power the investigated pedelec with the sun, a certain amount of energy can be used when having an energy storage system. The used concept is a PV standalone system and consists of PV panel, solar battery and charge controller. The solar battery of this system stores the energy during longer time and it can be later used to recharge the bike battery. To make an effective use of this system it is necessary to be ambitious to park the bike always in a sunny spot when it is not used for cycling around. This strategy offers the possibility to partly recharge the bike even if no main grid is available as also a private project states this out in a blog (Bikesastransportation, 2010). The installation of a PV system needs to have enough space where the PV panel and also the battery and charge controller could be installed. This is also a big disadvantage due to the gain of weight and the loss of available space for the primary transport function. As soon as the PV panel is shaded the performance in terms of electricity generation of the stand alone system is dropping and the system can’t deliver a suitable amount of energy, what makes it more or less inappropriate.

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The issue of (partial) shadings is also the reason that the tested system has a very bad performance in terms of electricity delivery when it comes to riding the bike. Especially the used monocrystalline PV panel is very sensitive for this kind of influence. There are other PV panel technologies available on the market, which are less sensitive to partial shadings but those have lower efficiency what means for the same power a panel with larger dimensions has to be used (Wholesale Solar, 2013). Anyway no matter which PV technology is used, there is still an influence of partial shading which is still existent and decreases the possible power generation.

5.3 Possible electricity generation of stationary system

The big influence of shadings on a PV panel is the reason that well designed and well positioned stationary PV systems will always have a better performance in terms of electricity production than mobile systems using same system components. It is not possible to prevent mobile systems as the on-bike system from shadings. The electricity generation of stationary systems depends besides the system components and design, strongly on the location and the choice of tilt and alignment. In “3.5 calculation of the possible average daily energy” the influence of time of the year and the tilt of the PV panel is shown. It is important to point out again that the energy is calculated using the peak power of the available panel (48W). Panels with higher peak power will deliver respectively more electricity. Also the efficiency of the processing of the energy is guessed with 85% which can be different and depends on the system components used. Taking these assumptions into account it is, in average with this 48Wp PV panel, not possible to fully recharge the bike battery once a day. To fulfill this need from March to September a PV panel with 100Wp is needed in an area around Uppsala. Due to save operation and long life of the solar battery the capacity should be at least 120% larger than the capacity of the bike battery. To recharge the bike battery with the corresponding battery charger an inverter has to be used to convert the stored energy into AC power for the charging device. Many simple solar systems which could be used as stationary systems are available on the market. In case of realization of a stationary system a detailed analyzes must be done in order to fulfill the need of the specific application and taking the influences of the location into account.

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6 Conclusion

6.1 Energetic feasibility of an On-bike PV system

The answer to the question if the installed on-bike PV prototype system is feasible to provide enough energy to run the motor of the pedelec, in this specific case, is no. The maximum power output of the PV system with 48W can’t supply the motor which has a power of 250W. Even if the whole possible space on the box is used it can’t deliver enough power. If the electricity consumption of the pedelec is low, due to seldom use, an on-bike system can deliver a feasible amount of energy to recharge the bike battery during parking on sunny spots. That needs a certain effort of the user to take care that the bike is rearranged if shadings happen. In this case it is most important that the electric system of the bike is designed in that way that recharging of the battery is possible. No commercial system could be found doing literature and internet research for this study. To minimize the influence of the partial shading on the PV panel of an on-bike system the position has to be changed. A possibility gives the “sun roof” which is offered by BABBOE as an optional feature. Due to the position overhead the transportation box it is prevented from shadings from the driver and parts of the bike. The possible electricity delivery in this case is hard to guess. For sure it is larger than the on-bike system which is located on the box. Further studies and development have to be done for the “sunroof system” to find a suitable design of PV which fulfills the need for electrical electricity generation, is light build and easy to handle in terms of installation and removal.

6.2 On-bike system versus Stationary system

The comparison of the generated electricity of the on-bike PV prototype system and the possible electricity generation of a stationary system clearly points out the advantage of the latter. The stationary system is also a PV standalone system using a solar battery to store the energy which is generated during sunshine. The advantage is that the tilt and alignment of the PV panel can be adjusted in order to get maximum available and unshaded irradiance. The expression stationary system indicates that it is not carried around on the bike, but it does not necessarily mean that it is fixed on a certain place. It also can be transported easily and put on different places. A possible application of a stationary system is the use during a cycle holiday where no main grid is available. Installed on a sunny spot it can generate enough electricity during the day to recharge the empty bike battery during night. In comparison to the stationary system the on-bike system has a big disadvantage in terms of the amount of electricity which can be generated. The most important advantage is that the on-bike system is mobile and can produce the electricity independent of the location. As soon as the PV panel is illuminated electricity is produced. This factor can be used if the bike is used on places where no grid is available and no base camp is used to take advantage of a stationary system. Anyway, the higher the “time of park” to “time of ride” ratio is the higher is the possibility that even the lower electricity production of the on-bike system can supply a reasonable amount of energy to recharge the bike battery.

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ANNEX

I Technical data of BABBOE TRANSPORTER II Technical data of PV panel III Technical data of charge controller IV Datasheet of solar battery V Datasheet of irradiance sensor VI Technical data of energy logger VII Technical data of data logger

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ANNEX I: Technical data of BABBOE TRANSPORTER

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ANNEX II: Technical data of PV panel

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ANNEX III: Technical data of charge controller

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ANNEX IV: Datasheet of solar battery

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ANNEX V: Datasheet of irradiance sensor

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ANNEX VI: Technical data of energy logger

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ANNEX VII: Technical data of data logger