Charging Station for Electric Vehicles Nordic Folkecenter For Renewable Energy

Transcription

1 Charging Station for Electric Vehicles Nordic Folkecenter For Renewable Energy June 2008 Jessica Grove-Smith

2 INTRODUCTION...3 WIND ENERGY...5 SPECIFICATIONS...5 WIND TURBINE...5 INVERTER...6 EXPECTED ENERGY PRODUCTION...7 POWER CURVE...7 WIND DISTRIBUTION...11 ENERGY PREDICTION...13 SOLAR ENERGY...14 SPECIFICATIONS...14 PV PANELS...14 INVERTER...15 EXPECTED ENERGY PRODUCTION...16 THEORETICAL PREDICTION...16 EXPERIMENTAL RESULTS...17 A COMPARISON: THE SUNFLOWER...21 ELECTRIC VEHICLE CHARGING...22 DATA MONITORING AND DISPLAY...23 CONCLUSION

3 Introduction In today s world transportation is a major polluter and energy consumer. People have become used to regularly travelling long distances to work or just for pleasure and modern lifestyles are often arranged around the permanent availability of relatively cheap transport options. The current habits and technologies are not sustainable and solutions need to be found to minimise the environmental impact as much and as soon as possible. One of them is the electric car. The concept of electric vehicles has a very big potential for future applications - especially if charged on renewable electricity - as they can provide pollutionfree 1 transportation for the main part of the population s travel needs. The technologies of electric motors and batteries are already well established; they are now being adapted and developed further in order to produce higher energy efficiencies and larger effective capacities. Some people are worried about the limited range of electric cars, the required charging time or the maximum available speed. Advancement of technology will lessen the magnitude of these problems. At the same time a range of concepts are being suggested and introduced for an efficient and clean transport system based on electricity. This includes, for example, battery leasing (Th!ink s Mobility Pack ), charging stations with battery replacement facilities, vehicle to grid charging etc. The use of electric cars as a replacement for the currently used vehicles based on fossil fuel will only then imply a significant pollution reduction if the energy used to charge the new cars is produced from renewable sources. 2 As with every new technology, a big step in achieving a successful changeover from an old and well-established product is public awareness and genuine acceptance and interest in the new device. For demonstration and awareness raising purposes, a renewably powered electric charging station that incorporates both wind a PV technology, was set up at the Nordic Folkecenter. This charging station is an information point for all visitors an incentive for people to consider electric vehicles as a promising future transport option. The beauty of the installed charging station also lies within the simplicity of the underlying idea: The combination of two complementing renewable energy sources for energy production all year round. This concept can be applied to a wide range of applications of which Folkecenter s charging station for electric cars is only one example. As is the case here, the system can be connected to an electricity grid (national or local). Alternatively, batteries could be used for energy storage, thereby creating a 100% autonomous power production facility. The scale can be as small or even smaller than demonstrated at the centre or be increased to much larger sizes. The dimensions and connection 1 Not taking into account the pollution caused during the entire lifetimes of all components. 2 Do Electrical Cars make sense in Denmark? written by Folkecenter trainee Melissa Valgardson. Published on the centre s website. 3

4 type depend on the specific use - but energy collected from the sun and wind will complement one another in the wider context of every system. This report was written with the aim of giving an introduction to the individual components that make up Folkecenter s charging station and their initial performance. It lists the specifications of the main parts and explains how performance monitoring and calculations were carried out. Where known the part numbers are given in square brackets. Additional information has been collected in a folder for easy access, so that any person wanting to work with the system won t have to spend time on researching instruction manuals. Most documents and files are also saved on the centre s server (Trainees Completed Work\Integrated Systems\Charging Station). The prepared Mathcad files can be used for future data analysis in order to monitor the system s performance and verify the energy predictions made in this report. 4

5 Wind Energy Specifications Wind turbine LAKOTA (Aeromag Corporation) Rotor diameter: 2.09 m Swept area: 3.43 m 2 Rated power output: 900 W (at 12.9 ms -1 ) Peak power output: 1500W (at 17 ms -1 ) The incoming signal from the turbine passes through a commander box where it is rectified before being sent to the inverter. The commander also ensures that the maximum inverter input voltage is not exceeded, by sending any incoming signal above a certain voltage to a dump load (two 1000W, 0.75Ω resistors connected in series [ASE ]). This voltage limit is controlled by the load diversion regulator [LDR 48-30], which can be adjusted with a potentiometer. In the current configuration the potentiometer is set to approx 59V corresponding to the inverter s properties. A brake is installed in the commander before signal rectification (careful, the OFF-positions refers to the brake being turned off, meaning that the turbine is turned on!). The turbine automatically tilts backwards for overspeed protection and should therefore be able to withstand quite high wind speeds without having to be stopped manually. The turbine was previously connected to a 24 V battery bank for which the diversion limit had to be significantly lower. As this is not within the normal range of the potentiometer the addition of two cables in the commander, as shown in the pictures below, was used to halve the voltage. Cables used to halve the diversion voltage LDR potentiometer 5

6 Inverter Windy Boy WB 1100LV (SMA Technologie AG) Nominal output power: 1000 W Maximum output power: 1150 W Maximum input voltage: 60 V Maximum input current: 62 A Maximum efficiency: 92 % Creating a stable electrical signal from the constantly varying power output of a small wind turbine is not an easy task, which is why there are not many suitable inverters on the market at the moment. It is much more common to connect small turbines to a battery bank for energy storage. This is the first time a small turbine has been connected to the grid at the Folkecenter. The Windy Boy installed in the charging station is the smallest in the wind turbine inverter range offered by SMA and so far (for it s operating time of four months) it has worked very satisfactorily without any problems. Windy Boy Efficiency Efficiency 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% AC Power [W] The above graph depicts the inverter s efficiency over a range of output power. The data was recorded during a windy day (maximum one-minute average wind speed of 14.8 m/s) and covered almost the entire power range of the inverter. The overall efficiency for this day was %. 6

7 Expected Energy Production Many factors will influence the annual wind energy contribution of the charging station and before the system has been monitored over a long period of time all production values are based on assumptions and predictions. Theoretically, the energy produced can be calculated by multiplying the wind turbine s power curve with the local wind distribution. Taking into account the inverter s limited efficiency will then allow the determination of the amount of energy fed into the grid and available for car charging. Power Curve The power curve foremost depends on the turbine s fixed properties. The rated power output of the installed turbine is 900 W, which it should reach at around 13 m/s according to the manufacturer s specifications. To ensure production to the turbine s full capacity for a wide range of wind speeds the Windy Boy inverter has an internal voltage versus power curve, which can be adjusted by the user. It is called the Characteristic Curve and the device is delivered with a typical setting for small turbines. It is described by the following parameters: 3 Parameter U.PV Start [V] Definition Defines the voltage at the moment when the Windy Boy is ready to perform grid synchronization. Defines the voltage at the moment when the Windy Boy begins to feed power into the mains grid, and begins to extract power from the turbine. Defines the voltage threshold, after which the curve climbs more steeply. Defines the voltage at the moment when the Windy Boy begins feeding maximum power into the grid. U.DC WindStart [V] U.DC WindMid [V] U.DC WindMax [V] P. WindMid [W] Defines the power threshold, after which the curve climbs more steeply. P. WindMax [W] Reduces the Windy Boy's maximum output power to adapt to wind turbines with lower output power. Different settings were tested under different wind conditions and subsequently analysed with Mathcad (DataAnalysis1). The file is available for review and future use on Folkecentre s server. It requires the input of two text files, one containing the wind speed data (one minute average) and another that contains the data collected from the turbine (recorded every 10 seconds). The Mathcad file sorts the averaged data into wind and voltage bins and then graphically displays the turbine s resulting power curve, efficiency coefficient and characteristic curve. An output file is created in order to facilitate further analysis and comparison of the data. A different Mathcad file (DataComparison1) was created for comparison of the turbine s performance under different conditions. It imports the experimental results from selected 3 As defined in the manual. 7

8 dates and then graphically displays the data. Some results from these two Mathcad files are presented on the following pages to show how the turbine reacted to different Characteristic Curve settings. The approach used to find the optimal setting was to match the curve to the actual output produced by the turbine. Determining the optimal setting was not as simple as it seemed at first because the wind conditions were obviously different for every set of data recorded. As it turned out in the end, the original setting had already been fairly adequate (red coloured data in graphs). The final setting is very similar to the slightly more efficient first correction made to the parameters (dark blue data set recorded on March 21 st ) but with a reduced grid feeding starting voltage U.DC WindStart of 21 V (as for the light blue data set). This reduction allows for power production at lower wind speeds. When looking at the raw data it becomes clear that the turbine could actually produce power at even lower wind speeds. But when the Characteristic Curve was adjusted for this property by lowering U.DC WindStart and U.PV Start, the load became too large at higher wind speeds causing the maximum power production to be strongly limited (green graph). Different settings were tested in order to counteract this effect but none of them were successful so they are not included in this report. Giving the curve a steeper gradient also had an overloading effect (pink data). 800 Power Curve 600 Power [W] original Wind Speed [m/s] 8

9 1 3 Efficiencies Efficiency coeffiecient Cp Wind Speed [m/s] 50 2 Voltage Curves 40 DC Voltage [v] Wind speed [m/s] 9

10 Set Characteristic Curves - Windy Boy 1 Power [P] Voltage [V] The fatter black line in the two graphs displayed on this page represents the final Characteristic Curve setting. 1.5 Measured Characteristic Curves 1 Power [P] Voltage [V] 10

11 Final characteristic curve parameters: Parameter final value U.PV Start [V] 20 U.DC WindStart [V] 21 U.DC WindMid [V] 43 U.DC WindMax [V] 57 P. WindMid [W] 250 P. WindMax [W] 1150 Unfortunately the data collected with the final setting does not cover a large enough wind speed range to produce a complete power curve. For the turbine s annual energy contribution to the system, the dark blue set of data was used, which was collected with a very similar setting. W ind Distribution There are two facilities for recording wind speed at the Folkecenter, one at the front of the main building and one in the Testfield. Both are at a height of approximately 10m. The data from the anemometer at the centre is monitored continuously and can be used to calculate the local wind tendencies and distribution over a chosen period of time. The most suitable set of data for determining the wind distribution over one entire year was recorded during Only 10 days in June and 10 days in December are missing and the entire set is composed of 10-minute average winds speeds. Ideally, the average time should be as low as one minute for a more precise distribution and better correspondence with the determined power curve. Analysis of the available data was carried out using Mathcad. One file compares the wind values at the centre and out in the Testfield (WindComparison1) and a second file is used to calculate the wind distribution (WindDistribution1). It turned out that the relationship between the two sites is fairly linear with the wind values out in the Testfield being only slightly higher, as can be seen in the top graph on the next page. The gradient of the linear fit is and its intercept equals Obviously, the actual difference for every individual value depends on factors such as the momentary wind direction, turbulence and speed but for an annual distribution the linear fit can be used as an average correction. The results are displayed in the graph second graph; they clearly show the significant effect such a small correction has on the annual distribution. For a more accurate distribution a continuous set of wind data should be recorded out in the testfield with one minute averaging over several years. 11

12 Wind comparison Wind speed in the testfield [m/s] Data Linear fit Wind speed at the centre [m/s] 0.15 Wind distribution Wind Speed [m/s] data (Center) Weibull distribution corrected data (Testfield) corrected Weibull distribution 12

13 A summary of the local wind distribution properties: Most Wind Speeds [m/s] Average Median occurring Data Original data Weibull fit Corrected data Data (Testfield) Weibull fit Energy prediction Using the wind distribution based on the 2007 data and the previously determined power curve for the LAKOTA leads to an annual energy production between 1000 kwh and 1200 kwh per year. P w = 1100 kwh/year Instead of multiplying the wind distribution value of every wind bin with the respective amount of power produced by the LAKOTA, a multiplication with the corresponding AC-power fed into the grid by the inverter, gives an indication of the Windy Boy s overall efficiency. This can then be used to calculate how much wind-electricity is being fed into the grid by the charging station, on a yearly basis. Estimated overall inverter efficiency and AC energy production: η WB = 86.5 % P wac = kwh/year Taking into account the DC production range, this final value should fall between 865 kwh/year and 1038 kwh/year. However, these values represent predictions based on sets of experimental data but should be confirmed after long-term monitoring of the stable system. The final version of the characteristic curve was set on May 30 th This date should be used as a starting point for future data analysis the operational time and total energy production of the Windy Boy were reset to zero at 10 a.m. 13

14 Solar Energy Specifications PV panels (SOLEL AS / GAIA Solar) The PV system attached to the charging station for electric vehicles is composed of 25 second-hand monocrystalline silicon solar panels ranging in capacity from 37 W to 41 W with the following known properties: Maximum power W P [W]: Open circuit voltagev oc [V]: Voltage at maximum power V P [V]: Short circuit current I SC [A]: Current at maximum power I p [A]: Minimum number of panels: Area [m 2 ]: The 25 panels cover a total area of m 2. For 17 of these panels, however, the peak power output is not specified. For most of them the measured open circuit voltage at midday is so high that they are likely to be able to produce around 40 W, which sums up to a total system capacity of around 980 W. The system faces south and is mounted at an angle of 30. The panels are all connected in series. In theory, the efficiency of the entire array should be around 10%. However, the actual efficiency is expected to be a little lower due to the age of the cells. The only way to determine this is to compare the production with the momentary incoming solar radiation. This process is subject to fairly high errors, as it was difficult to determine the solar radiation at exactly the right angle with the available equipment. The experimental values listed in the table below result in an average efficiency of 6.28%. This value could be determined more accurately with the recording of more precise values over a longer period of time. Inverter Input Solar radiation Area Efficiency of Date Current [A] Voltage [V] Power [W] [W/m 2 ] [m 2 ] PV array % % % % 14

15 % % % % % % % Average = 6.28% Inverter Sunny Boy SB 1500 (SMA Technologie AG) Nominal output power: 1500 W Input voltage range: V (DC) Maximum input current: 3-8 A Maximum efficiency: 96 % The inverter operates in MPP mode (Maximum Power Point) in order to achieve maximum power production at all times. Some of the internal values can be adjusted by the user, e.g. the start-up and stopping voltages were set to the allowed minimum values, which equal 140 V and 100 V, respectively. As with the Windy Boy, the Sunny Boy s efficiency depends on the momentary power production. However, overall the Sunny Boy is more efficient than Windy Boy, as can be seen in the graph below. This can be easily explained with the fact of a much more constant and stable input signal. Sunny Boy Efficiency Efficiency 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% AC Power [W] 15

16 The data displayed in the graph on the previous page was recorded on March 23 rd, a sunny day on which a production-range from 0 W to 884 W was covered. The maximum efficiency reached on this day was 95.16% and the overall conversion efficiency equalled 93.90%. Expected energy production Theoretical prediction In order to predict how much energy the PV system of the charging station will produce yearly, the annual solar radiation onto the panels has to be known. For the calculations in this report a database provided in the free downloadable software RETScreen International 4 was used. The following table shows the relevant values from the database and the array s resulting monthly power production (based on a PV efficiency of 6.28% and total area of m 2 ). The numbers show very clearly that a tilt of 30 leads to an annual production increase. Daily solar radiation [kwh/m 2 ] Daily solar radiation (south, 30 tilt) [kwh/m 2 ] Monthly power production [kwh] January February March April May June July August September October November December The above numbers yield an expected annual production of: P s = kwh/year As shown in the previous section, the efficiency of the Sunny Boy inverter is above 90% on a long sunny day, which would imply losses of less than 10% before the produced energy is fed into the grid. However, the production on cloudy winter days will be a lot lower, which will dampen the overall efficiency

17 In order to make a reasonable prediction some values were recorded and analysed with Mathcad. Experimental results The following graphs and numbers give an indication of daily energy production and overall daily inverter efficiency for a selection of sunny and cloudier days. Date PV production (DC) Grid feeding (AC) [kwh/day] [kwh/day] Overall efficiency March 19 th % March 20 th % March 21 st % April 22 nd % April 23 rd % April 24 th % April 25 th % April 26 th % Total % 17

18 1000 April 22nd, Power [W] PV production (DC) Grid feeding (AC) 1000 April 24th, Power [W]

19 1000 April 25th, Power [W] April 26th, Power [W]

20 The first graph displays visually how much energy is being lost in the inverter at any given point in time. Overall the graphs demonstrate the energy production on different days and the effect of clouds / shadows. The maximum production seems to lie around 900W. At this time of year the system starts up at around 6:15 to 6:45 a.m. and ceases to produce between 6.30 and 8:30 p.m., depending on the conditions. This corresponds to operating hours per day. An interesting next step would be the comparison to data recorded during the winter months. A Matchcad file called YearlyProdcution was prepared, which calculates the average daily energy production for every month and uses these values to predict the annual energy yield and overall inverter efficiency. However, the measurements so far have only been made on sunny days in March and April and therefore don t take into account the reduced production during winter months. Calculations should be made once the system has been monitored for a period of at least one year. The existing data suggests in overall inverter efficiency of 91.43%. However, the daily efficiency can be as low as 83% on less productive days. Taking into account the winter months and more rainy seasons, the overall annual inverter efficiency is estimated at: η SB = 87% This should be verified after the system has been monitored over a longer period of time. For now, applying this estimated value to the previously calculated annual energy production of the PV system, the amount of energy fed into the grid and available for charging our electric car from the station is predicted to equal: P sac = kwh/year The Sunny Boy installed in the charging station was previously used in a different system. The operating hours and total energy production can therefore not be used as an absolute reference. On March 6 th 2008, when the first set of data was recorded, the total energy processed by the inverter equalled 3552 KWh, which had been collected during operating hours. Taking this as a starting point, the inverter fed kwh of energy into the grid over a period of 7 weeks (April 24 th ). This energy was collected during operating hours. This corresponds to an average power production of W and average operation of 11 hours per day, which would lead to an annual energy production of 1228 kwh. It will be interesting to see how these average values change over the course of the year. Unfortunately it was not possible to reset the Sunny Boy to fix a starting point for the charging station system. On May 30 th at 10 a.m. when the Windy Boy was reset, the Sunny Boy had produced kwh of energy in a time frame of hours. 20

21 A comparison: The Sunflower On May 16 th 2008 a new SMA Sunny Boy was installed by the Sunflower sculpture in the test field, to replace an old broken one. It is of the same type (SB1500) as the one used in the charging station and the PV-array is of very similar size, so a comparison is interesting. The following table lists the average properties of the mono-crystalline silicon Sunflower PV-panels: Maximum power W P : 34 W Open circuit voltage V oc : 10.5 V Voltage at maximum power V P : 8.4 V Short circuit current I SC : 5 A Current at maximum power I p : 4 A Area of one panel: m 2 Number of panels: 36 Total area: m 2 Maximum total capacity: 1242 W The total capacity of the system is higher than that of the PV array connected to the charging station and the cells have a higher theoretical efficiency of 12% (not confirmed experimentally). However, the sculpture faces south-east and is fixed at a much higher angle of around 80, which is less advantageous for a location in Denmark. The annual production depends strongly on the efficiency of the cells and as this is unknown no prediction will be made within this report. After 8 days a first check of the production was made. At 1 o clock in the afternoon on a sunny spring day the system was producing W (234 V * A) and feeding 757 W into the grid. This corresponds to a high inverter efficiency of 95.9 %, as expected in this power range. During the total 127 operational hours of the Sunny Boy, it had already contributed 40 kwh to the electricity grid. This means that the inverter had been running approx 15 hours per day with an average power production of 315W. Based on these values the system would feed 1725 kwh of energy into the grid per year. However, as with the system installed on the charging station, these average values will definitely go down during the less sunny seasons of the year and therefore bring down the total annual energy production. 21

22 Electric Vehicle Charging Currently a variety of electric vehicles are available in different countries spread of the world ranging from very simple one-person cars (e.g. Citycom AG s City EL) to modern sports machines (Venturi s Fetish). A list of currently available cars is saved on the Folkecenter s server in an Excel file (EV list). The next years will surely bring some changes, improvements and the development of entirely new products but the basic technology is most certainly already available today. Currently, the most successful models include Norwegian based Th!nk City (Think Global) and the Indian G-Wiz (RECC), which is used, for example, by people living and working in central London. They are both two-seater city cars with a consumption of 16.5 and 22 kwh per 100 km, respectively. The question most relevant to this report is how far one of these cars would be able to drive powered solely on the renewable energy produced by the wind-solar station? The total annual energy produced by the charging station is predicted as = 1639 kwh/year The table below shows how many kilometres this corresponds to for a selection of currently available electric cars: Number of seats Consumption [kwh/100km] Approx. yearly range [km] City EL ,317 Th!nk City ,933 G-Wiz ,450 Example of a converted Twingo ,927 The Th!nk represents a reasonable average value for a useful city car. The calculated annual range corresponds to a daily distance of 27.2 km per day. This may not cover regular long distance drives but is more than sufficient for carrying out necessary tasks (e.g. shopping, driving to work etc.). 22

23 Data monitoring and display When the wind turbine was first installed the dump load control was still set to 30 V and it was unknown that the two cables described at the start of this report were the cause of the problem. In order to avoid permanent damage to the newly obtained Windy Boy by feeding it with a too high voltage (above 60V), the system was connected to batteries or without a load for a short period of time. The only way of recording data was to attach a voltmeter and ammeter at appropriate places within the turbine s controller and manually taking pictures as quickly as possible. This is method is very time consuming and not very accurate as the analogue meters don t react fast enough to the electronic signal. However, it was good enough to establish that the turbine was working correctly and could produce up to 1600 W at high enough wind speeds. It also gave a first idea of appropriate characteristic curve values. Once the dump load diversion was set correctly at 60 V, data could be recorded directly from the inverter. SMA s USB Service Interface cable allows direct connection from an inverter to a computer two meters away. The downloadable program Sunny Data then allows simple recording, data display and export to Excel for analysis. All data used for this report was recorded in this way. However, this setting has two major disadvantages: Firstly, it requires the lid of the inverter to be removed during data recording, which is a serious safety issue. Secondly, only one inverter can be connected at a time, which makes it impossible to monitor the combined wind-solar system of the charging station. The USB Service Interface in combination with Sunny Data is useful for quickly establishing any SMA inverter s current setting and production or determining a fault but it does not offer a solution for long-term data monitoring. For future use and reference: The data collected with Sunny Data is saved in the local folder C:/Programmer/SMA/Sunny Data/PlantXY/, where XY is a specified plant number. For example, Plant02 contains the data collected from the turbine whereas Plant03 refers to the PV array. SMA provides several options for permanent system monitoring but all of them require the purchase of additional equipment. A list and descriptions of the various elements can be found on their website they range from a small portable display to web-connected data storage devices in combination with a weatherproof display. The easiest and cheapest option, which is adequate for the Folkecenter s needs, is the direct connection of the inverters to a computer via RS485 cables. (SMA advised not to use the option of transferring data via the simpler powerline connection due to excessive noise in the signal.) This method allows for up to 50 inverters to be connected in series with a maximum distance of 1.2 km from the computer where the data is to be recorded. Every inverter has to be equipped with a RS485 piggyback [485-PB-NR] and a conversion interface [I-7520] with an external power supply is needed for the computer connection. For data monitoring and display a program called Sunny Data Control is available on the SMA website, which is slightly more advanced than the previously used Sunny Data. With this setup, a range of useful data can now be collected continuously and simultaneously from both inverters installed in the charging station. The computer screen can be used effectively to show visitors not only the current 23

24 and total energy production of the system but also the saved CO 2 emissions and graphical displays of the turbine s and solar array s daily or monthly power output. At the time of writing this report the new monitoring system had only just been installed. It was working correctly but some time should be invested in figuring out the best settings of the new data analysis and display programme. The location of all recorded data is the local folder C:/Programmer/SMA/Sunny Data Control/Plants/Chargin station for EVs/. Data is recorded once a minute and represents a one-minute average value. This should be more accurate than the previously applied method (momentary data recording every ten seconds) but means that all prepared Mathcad files need to be adapted slightly before they can be used for analysis. According to the manual, Sunny Data Control allows for access and graphical data display using Excel from within the program (macros need to be enabled!). This will be very useful for visitor information display but could not be tested before the completion of this report. If the installation of the monitoring system is successful a future project could be the addition of further useful components. It would make sense, for example, to permanently install a weather station in the charging station in order to record the wind speed and maybe even the solar radiation. This data is essential for the complete analysis of the wind turbine and PV array s performance. It is needed in order to calculate the LAKOTA s power curve to a much more accurate level than achieved in this report and also, in order to determine the PV panel s actual efficiency. Additionally, the wind data could be used over a longer period of time to precisely establish the local wind distribution. Another project suggestion is the addition of Folkecenter s other SMA inverters to the monitoring system. 24

25 Conclusion All the small experiments carried out for this report aimed at optimising the system and confirming the newly acquired Windy Boy s stable performance. Repeated data analysis had the result that I gained a thorough understanding of the system s properties. I hope that with this report I have managed to pass some of this knowledge on to the reader. The poster displayed on the charging station s container contains a set of numbers to give Folkecenter s visitors an idea of the amount of energy that can be produced with such a renewably powered system and how many kilometres this translates into on a yearly basis. The report explains where these numbers came from - it will be interesting to see how accurate these predictions actually are in one year s time. 25