solar group project report
TRANSCRIPT
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Prepared for: Dr Senthilarasu Sundaram Prepared by: Hakeem Buge, Chris Aoun & Hugo Tilmouth 8 December 2016
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Solar Powered Electronics Charging Station
CSMM427 SOLAR ENERGY RESEARCH & INNOVATION
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ABSTRACT
In this report, a polycrystalline-module is designed, fabricated and the corresponding analysis has been completed to show its performance. The report includes an in depth analysis of the individual cell testing using an I-V and an external quantum efficiency test (EQE) as well as the overall panel testing with comparisons to industrial modules. Further studies have been completed on the thermal efficiency, Sylgard application, soldering, component efficiencies, a resource analysis using PVSyst, gas strut calculations as well as user testing to provide an all round evaluation of the project.
SPECS is designed for outdoor applications and specifically for UK parks. The module is highly effective at charging a range of devices from mobile phones to tablets and a later iteration of the design can be scaled up for commercial use across the globe.
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TABLE OF CONTENTS
Abstract 3
Table of Contents 4
List Of Figures 6 List Of Tables 7
Aims and objectives 8
Objective 8 Goals 8
Introduction 9
Methods 10
System Design And Cad Drawings 10 Schematic Of Electronics 11 Analytical Calculations 12 Material Purchase List 13 Design Specification 14 Fabrications And Testing Methods 16
Results and Discussions 24
Soldering Review & Cell Breakages 24 Lab Testing Of Cell 24 Theoretical Maximum Calculations 31 Panel Testing 32 Comparison Between Theoretical, Actual & Industrial 36 Cell Thermal Efficiency Study 43 Individual Cell Function Test 44
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Sankey Diagram Of Energy Losses 47 Sylgard Application And Calculation 48 Use Of Acrylic Over Glass. 49 Efficiencies Of Components 51 Economics 53 Potential Sites For Placement Of Specs 56 Pvsyst Analysis 58 Gas Strut Calculations 64 Indoor Testing 65 Outdoor Testing 66 User Testing 67
Evaluation 68
Construction And Project 68 Evaluation Of Testing 69
Conclusion 70
Acknowledgement 71
Bibliography 72
Appendices 75
Sylgard® 184 Silicone Elastomer Properties 75
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LIST OF FIGURES
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LIST OF TABLES
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AIMS AND OBJECTIVES OBJECTIVE
The objective of this project is to build and test the performance of a photovoltaic PV panel. The fabricated panel consists of 36 monocrystalline solar cells. The performance of the fabricated solar panel will be compared to that of a commercial solar panel of similar dimensions. Comparative performance analysis of the IV Curve, Isc, Voc, Fill factor, Pmax and efficiency of the commercial and fabricated panels will be carried out. This report outlines the methodology behind the construction of the panel and outlines the experiments and results from the assessments carried out on an individual solar cell as well as the fabricated panel. The fabrication of the solar panel required the soldering of electrical connections and encapsulation using Sylgard within a glass casing. This document also discusses the design and application of the fabricated panel which has as its objective to be utilised as a source of energy for small electronic devices.
GOALS 1. Optimally design a solar module for an appropriate application
2. Investigate a test cell’s performance to estimate the characteristics of the module
3. Construct the solar module
4. Test the solar module to verify earlier estimated power output and efficiency
5. Suggest reason for underperformance of the solar module
6. Compare module to industrial panels
7. Provide further analysis on various aspects of the design to gain a deeper understanding of performance behaviour
8. To successfully charge multiply electronic devices
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INTRODUCTION
The initial idea of this project stems from the fact that even in the modern day and age, the electronic devices regularly used on a day-to-day basis, such as mobile phones or tablets, consist of batteries that have considerably low discharge times. The SPECS team wanted to create a system that would solve this issue during a time when the electronic device would not be needed, for example, whilst taking a walk in the park or undertaking recreational activities. Therefore, the result is to design a panel that would provide sufficient energy to charge the equivalent of four mobile phones at the same time and be portable to use in outdoor applications. The structure will contain lockers to provide a space for the devices to be safely stored during the period of charging. The idea was further pushed forward after completing some market research because solar charging stations are not regularly used within the UK even though they would have a very useful purpose during summer time.
The report will encompass the methods, results and discussion followed by a reflection section that evaluates the project as a whole as well as ideas on how to commercially implement the system if it is to be taken to large scale applications.
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METHODS
In this section of the report the methods used for fabricating the panel and the locker module are explained and illustrations shown for the key steps. Justification for each stage is also given.
SYSTEM DESIGN AND CAD DRAWINGS
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Four individual lockers for storage of phone while charging, each of these will be lockable for security
36 cell poly crystalline si l icone cel l array, connected in series
Strong metal legs to support the weight of the panel and locker module
20,000 mAh of battery storage comprising of two 10,000 mAh external usb chargers
DC-DC conver ter to step voltage down from panel to 5v
Partition to separate the individual lockers
Ample space to store p h o n e , t a b l e t o r camera while charging Charging cable with
connections suitable for all phones
Hinge to open panel lid
Figures 1, 2 and 3: CAD model of SPECS
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SCHEMATIC OF ELECTRONICS
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Figure 4: Schematic of the electronics in the project
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ANALYTICAL CALCULATIONS
There were some calculations that were required to be completed in order to gain a better understanding of how to match the output and sizing of the panel.
The cell specifications are as follows:
Format : 125 mm x 125 mm +/- 0.5 mm, diagonal:165 mm
Thickness: 200 +/- 20 um
Front (-): Silicon nitride anti-reflecting coating, 1.5 mm wide front silver bus bars.
Back (+): Full aluminium back surface field, 2.5 mm wide (silver/aluminium) soldering pads.
Power: 2.8W
Efficiency: 17.6-17.8%
Vmp: 0.523 V
Imp: 5.215 A
Voc: 0.629 V
Isc: 5.585 A
Calculations based on a panel consisting of 36 cells:
Total cell coverage area of panel: 125mm * 125mm * 36 cells = 0.5625 m2 (0.75m x 0.75m)
Total power: 2.8W * 36 = 101 W
Total Vmp: 0.523 * 36 = 18.8 V
Total Imp: 5.215 A
Total Voc: 0.629 * 36 = 22.6 V
Total Isc: 5.585 A
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MATERIAL PURCHASE LIST
Number Item Cost
1 36 x monocrystalline cells (150mm x 150mm approx.) Supplied by university.
£0.00
2 Tabbing Wire – Supplied by university £0.00
3 Sylgard – Supplied by university £0.00
4 1 x standard wood sheet of top of box (£8) £8.00
5 Planks for frame of the box and compartments (£64.38) £64.38
6 Bottom mdf board (£11.44) £11.44
7 4 x stands (£12) optional £12.00
8 Large sheet of transparent acrylic (£10) £10.00
9 Sheet of glass( £5) £5.00
10 4 x Hinges (£5) £5.00
11 2 x Batteries (£26) £26.00
12 DC-DC Converter (£8) £8.00
13 Phone charging cables (£16) £16.00
14 Other costs (£15) £15.00
Total £180.82
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Table 1: Costings of the project
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DESIGN SPECIFICATION
The solar module in this project is made of of 36 poly-crystaline cells, that are soldered together in a series connection. These panels are first mounted on the MDF base of size 110cm x 110cm. This was chosen for its ability to electrically insulate the cells and its ease of cutting and shape modification. This based then has a 3mm high boarder glued on top to create a space for the encapsulant to be spread without seeping out of the sides. The encapsulent used is Sylgard 184 silicon elastomer, this is a highly transparent and insulating material, and with a long curing time and viscous properties, it is able to fill open spaces automatically. Sylgard also does not deform when exposed to heat while operating in the sun. On top of the encapsulent is a 3mm acrylic sheet to prevent damage to the fragile cells. The series connection was completed using a copper tabbing wire, with each module being soldered individually. The start and end connections were then drilled though the wooden base and secured on the underside to allow connection to the electronic load devices.
Usually, to minimise losses due to resistance, tabbing wires are soldered to top or bottom of each module with enough tabbing wire to connect to the next module. This arrangement can be seen in figure 5.
While starting the soldering process it was found that the panels would break under the weight of the long tabbing wires (type A in figure 5). To avoid putting stress on the very delicate panels, short equal length, tabbing wires protruding from both the top and bottom were soldered onto each cell (type B in figure 5). This method can be seen in figure 5. This allows them to then be soldered to one another once installed onto the full module, and more importantly prevent more cell breakage.
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Figure 5: Two types of tabbing wire methods
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The 36 cells were connected in series in arrangement show below in figure 6. When each cell was connected in series the total circuit was then tested to ensure no cell was disconnected. To connect each line of 6 cells a bus bar was soldered in reversing the direction and forming a zigzag shape for the cell arrangement.
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PositiveNegative
Figure 6: Arrangement of the cells in the array
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FABRICATIONS AND TESTING METHODS
1. After purchasing the wood the first step was to cut it to size using a circular saw. This was done with the necessary safety precautions, using safety glasses and a dust mask.
2. As the wood was only half the width needed for the design, it was decided that two pieces should be sandwiched together. The wood was then measured and drilled in to a precise depth.
3. The wood was then joined using dowel joints. It was also glued and then clamped together to make the joint very strong. Then the gaps between the two pieces was filled with a mixture of wood glue and sawdust. This would then be sanded down at a later date.
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Stage 2Stage 1
Stage 3 Stage 3
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4. The front panel was then jigsawed to create the locker opening as shown below.
5. The frame was then doweled and glued together, before being clamped and left to dry over night. To add additional strength 8 metal angle brackets were installed to ensure a 90 degree joint was maintained at each corner.
6. The locker partitions were then made using a similar technique of dowel joints and wood glue.
7. The box was then assembled and a couple of pieces of wood drilled, screwed and glued in behind the locker partitions so that they were secure, but could be removed at a later date to allow electronics to be installed, and maintenance to be performed.
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Stage 5Stage 4
Stage 6 Stage 7
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8. The solar cells arrived from China, and the soldering could begin. This involved cutting the tabbing wires to the correct length, spreading flux on them, spreading flux on the cells and soldering the tabbing wires to each side.
9. Once soldering was completed each cell was tested using the basic solar simulator in the renewable energy laboratory to ensure that only the most efficient cells were chosen. The results from this testing can found later in the report.
10. The image below shows the cells being numbered as they are individually tested.
11. The top wooden panel was then cut out to allow the solar cells to be installed on the surface. To prevent the Sylgard from spilling out of the top surface, a 5mm border was glued to the surface. To guide the placement of the cells a pencil guide was drawn onto the surface.
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Stage 9Stage 8
Stage 10 Stage 11
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12. The soldering of the cells was completed in situ, on the wooden surface. After soldering each cell in series, the entire chain of cells would best tested to ensure the last solder was completed properly.
13. At the end of each row the cells were connected via a bus bar to form a snake.
14. The next stage was to mix the Sylgard, this was measured and mixed in a 1:10 ratio of activator to carrier fluid. The Sylgard was the left to allow the air bubbles escape the liquid.
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Stage 12Stage 12
Stage 13Stage 14
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15. The Sylgard was then applied to all of the cells using a brush to carefully spread the liquid over the delicate cells.
16. The top surface made from acrylic was then cut down to size using a jigsaw. We then polished the surface to minimise the chance of air bubbles being trapped when placed onto the Sylgard.
17. The acrylic was then placed gently onto the top surface of the Sylgard and pressed to remove the air bubbles.
18. Using the remaining acrylic, the locker doors were made, this was achieved by cutting them down to size, attaching a small door handle and attaching a hinge.
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Stage 16Stage 15
Stage 17 Stage 18
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19. Using reclaimed legs and bolts, a operation height system was constructed.
20. To achieve an antique look, a walnut coloured wood stain was applied to all of the wooden surfaces.
21. To make the top surface more tidy a frame was constructed to be attached on top of the acrylic. To prevent shading a 45 degree was cut off the edge of the frames.
22. The edges were then cut down to the perfect size.
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Stage 20Stage 19
Stage 21 Stage 22
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23. To attach the solar panel to the box a set of three hinges were installed. These were removable to allow testing of the panel at a later date.
24. To prevent the hinge swinging open too much a string was installed.
25. The electronics were then installed inside the box. Leading from the solar panel were a positive and negative connection, this went into a buck converter. This stepped the voltage down to 5v. This was then split into two usb cables. These were then connected to two 10,000 mAh battery packs. These then had two usb connections on each which had phone charging cables connected to each.
26. The phone charging cables had 4 different options for the different phones currently on the market.
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Stage 24Stage 23
Stage 25 Stage 26
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27. It was decided that the best way to support the lid would be to install some gas struts to lift the solar panel lid, and hold it in the 90 degree upwards position. After conducting online research it was found that the geometry of these was more complicated than first thought. A scale model of the gas strut was therefore constructed to optimise the placement. More can be found in a later section of the report.
28. The gas struts were then installed according to the CAD model.
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Stage 28Stage 27
Stage 25 Stage 26
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RESULTS AND DISCUSSIONS
SOLDERING REVIEW & CELL BREAKAGES
At first there were several cells that were used to practice the soldering technique before starting the production of the soldering for the cells that were to be used on the panel. It is important to take good practice soldering because solar cell substrates are extremely delicate. The quality of the soldering process has a direct influence on the series resistance of the photovoltaic module. Using a suitable ribbon thickness and also monitoring how well the solder has been applied is very important in order to maintain constant performance if this kind of project were to be implemented on a larger scale. Although if SPECS is to be pushed forward further, it would be much preferable and efficient to implement the use of automatic machines that can provide a constant soldering process so that series resistance can become negligible.
The manual process of soldering a solar cell requires a certain amount of pressure to be exerted which can result in micro-cracks forming and propagating within the substrate. The formation of micro-cracks occurred several times during the soldering process which were later followed by the complete breakage of the cells. Several others were broken during the management, transportation and testing of the cells. Despite this, there were 36 solar cells that were successfully soldered and ready to be implemented within the panel. However, whilst the cells had been soldered and kept in storage with clear indications that they should not be touched, the cells were moved by some people working in the lab, resulting in the further breakages of 6 more. Fortunately as there were 4 unsoldered cells spare, it only left the panel with 2 cells short and hence only a reduced output of approximately 4W.
LAB TESTING OF CELL I-V Test
As shown in figure 7, one of the solar cells was selected to be tested through a solar simulator which can provide various levels of solar radiation but the test had to be at standard conditions: solar irradiance of 1 sun (1000W/m2), air temperature of 25 degrees celsius and an air mass of 1.5 (AM1.5) spectrum (admin, 2013). This test is to determine the exact open circuit voltage and short circuit current after the soldering as this would have changed from the cell specifications due to soldering or perhaps micro-cracks as previously mentioned. The results of the lab testing are presented in figure 7 below.
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Pow
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Figure 7: I-V curve showing the changes due to temperature effects Source: (Instruments, 2008)
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Figure 7 shows an I-V curve along with the related results that were generated by putting the cell through the solar simulator. The short circuit current and the open circuit voltage are slightly less than the specified rated values of the cell. One factor would be due to the soldering process but also it is important to note that the actual measuring temperature is at 28.7 degrees celsius which is above the standard testing conditions and therefore the extra heating would have a significant affect on these values. Whilst the blue coloured graph represents the I-V curve, the green coloured graph is a representation of the power output for the given voltages and currents.
Despite the fact that the Voc and Isc are the highest values at 0.59V and 5.4A respectively, the overall power at both these values is 0W. Whereas the Vmp and Imp represent the values of voltage and current where the maximum power occurs and this is highlighted on the graph by the point at the peak of the green parabola. The maximum power given by the result of the test is rated at 1.686W which is comparatively lower than the rated. A good indicator to be looking at is the fill factor (FF) which, in this case, has been measured to be at 0.526. A simple way of describing the fill factor is as a measure of the quality of the solar cell where the closer it is to 1, the better the quality. In more technical terms, it is the ratio of the maximum power to the theoretical power and it can be calculated through the equation (Instruments, 2008):
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Therefore for this cell;
In order to analyse the potential causes of this value for the fill factor, the theory of I-V characterisation should be studied.
I-V Characterisation
A good way to model a photovoltaic cell is a current source connected in parallel with a diode. Without any incoming solar radiation (light), the cell simply acts as a diode. As the light intensity increases, current gets generated by the cell and the Isc is dependant upon the value of that light intensity. Isc also occurs at the beginning of a forward-bias sweep (When the positive terminal of the battery is connected to the p-type material and the negative terminal of the battery is connected to the n-type material (Washington (2016)) and
FF =Imp ×VmpIsc ×Voc
FF = 4.519 × 0.3735.417 × 0.592
= 0.526
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for this project, if the soldering was done perfectly, there were no micro cracks and the cell was considered as an ideal cell; this maximum current value would be equivalent to the total current produced in the solar cell by the means of photon excitation.
The crystals within the poly-crystalline structure of the cells that were used in this project, as previously stated, are very sensitive to temperature. It was observed that in the testing condition, the cell temperature was at 28.7 degrees celsius. So how much of an effect does this have on the I-V characterisation for the cell?
The follow figure shows exactly how much affect a temperature differential from the standard conditions can have on the cell performance:
Figure 8 shows that both the Voc and the Isc are affected by a temperature increase but in opposite directions where the Isc increases and the Voc decreases. A more interesting point to consider is the rate at which the variation occurs. The change in Voc is 10 times more than that of the Isc per degree celsius. This would provide a good explanation as to why the test results were slightly different to those of the cell specification. The following table compares the two results:
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Figure 8: I-V curve showing the changes due to temperature effects Source: (Instruments, 2008)
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The results show that in all cases the lab test values are lower than those on the specification but it is clear that the voltage is affected more heavily than the currents. Although the previous table 2 shows that the Isc should increase, the fact that it has decreased practically provides evidence that this could be due to external issues such as the soldering or micro-cracks. Considering the temperature difference of 3.7 degrees Celsius, between the standard conditions and test conditions, using the rate suggested in 8 (0.5%/degree Celsius) the total Voc loss would be:
0.5% * 3.7 degrees celsius = 1.85% loss of Voc, as a result of the elevated temperature of the cell.
In this case the Voc has dropped by 5.9% ; (0.592/0.629)*100
Overall, this suggests that although there is a 1.85% loss from the temperature differential, there is also a 4% loss attributed to those previously suggested external factors.
External Quantum Efficiency (EQE) Test
The second lab test that was performed on the cell was the external quantum efficiency test. A solar cell's quantum efficiency can be defined as the amount of current a cell will produce when irradiated by photons of a current wavelength (PVEd, 2014). However there are two different types of quantum efficiencies: Internal and external (IQE & EQE). The latter can be defined as the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy shining on the cell from the outside (incident photons) (PVEd, 2014). The IQE is fundamentally the same however it takes into account the photons are actually absorbed into the cell as well as those that are shining from the outside. In order to calculate the IQE, the EQE must be calculated first and once this is found, the data can be combined with the transmissivity and reflectivity, however due to the lack of lab instruments to measure these values, a test was only run on the EQE.
Cell Specifications Lab Cell Testing
Open Circuit Voltage (Voc) 0.629 V 0.592 V
Short Circuit Current (Isc) 5.585 A 5.417 A
Maximum Voltage (Vmp) 0.523 V 0.373 V
Maximum Current (Imp) 5.215 A 4.519 A
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Table 2: Comparison between the cell specifications and the results from the lab cell testing
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A formula for the EQE can be expressed as:
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The following figures represent the test result from the EQE readings. There have been two different tests taken because the first one shows a test taken without calibrating the system meaning that the impurities in the air were not taken into account. Therefore, a calibrated cell was used where all the parameters were known and once all these values were run through the machine, the second test can then be made to produce a much smoother curve and also one that does not have any anomalous results as shown in figure 9.
The test was ran for the wavelengths between 400 nm and 1000 nm going up in intervals of 5 nm so that the results are spread out and as accurate as possible. Generally for the modern solar cells, they do not operate outside these wavelengths (ultraviolet and infrared) are either filtered out or absorbed by the cell, which would cause it to heat up, thus decrease its efficiency. In both cases, there positive results where the average efficiency is approximately 90%. The peak efficiency point (93%) occurs at a wavelength of 605 nm according to the second improved test whilst the lowest point (73.5%) occurs at 1000 nm. However, this is a relatively good result and it is close to the ideal shape which would be a square shaped graph. The range that has been used represents the top of that square and it is clear that they are relatively in a straight line as the values are fairly constant throughout the test. The possible reason for the reduction and not achieving 100% is because of the recombination effects where the electrons cannot move into an external circuit.
EQE = electrons/secphotons/sec
= current/(charge of one electron)(total power of photons)/(energy of one photon)
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Figure 9: Testing the cell for its EQE
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Figure 10: EQE Test Results without reference cell
Figure 11: EQE Test Results with reference cell
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THEORETICAL MAXIMUM CALCULATIONS
The following are the analytical calculations from the beginning of the report that represent the ideal case where all the cells are at their specification. However, this is impossible even in industry as when the cells are connected together, there are always losses due to the wiring. This is especially the case because all 36 cells are connected in series and the wiring losses occur most when there is a long and thin wire (which is the case for a solar panel) because the resistance will be increased significantly.
Calculations based on a panel consisting of 36 cells:
Total cell coverage area of panel: 125mm * 125mm * 36 cells = 0.5625 m2 (0.75m x 0.75m)
Total power: 2.8W * 36 = 101 W
Total Vmp: 0.523 * 36 = 18.8 V
Total Imp: 5.215 A
Total Voc: 0.629 * 36 = 22.6 V
Total Isc: 5.585 A
Now that the cell testing has been completed, a new theoretical maximum can be found from the following calculations:
Total power: 1.686W * 36 = 60.7 W
Total Vmp: 0.373 * 36 = 13.4 V
Total Imp: 4.519 A
Total Voc: 0.592 * 36 = 21.3 V
Total Isc: 5.417 A
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PANEL TESTING
The Simulator
The panel was taken for testing under a different simulator which is required to be a larger size due to the larger panel. The simulator that was used to test the cell was the WACOM simulator which can only measure a total area of 210mm x 210mm (WACOM, 2015). Therefore, the Pasan Sunsim Solar module tester was used for the testing of the panel as it can measure modules up to 2m x 2m. This module tester is the first of its kind in the UK having an A+A+A+ certification meaning that it is twice as good as the best class A solar simulators from the IEC ranks (WACOM, 2015). This technology has been designed to not only test silicon-based modules (which were used for this project) but also new technologies such as thin-films.
This solar simulator is an indoor device which provides illumination that is very similar to that of the sun at standard conditions. The advantage of using an indoor facility is that all the variables can be controlled to provide accurate testing of the overall performance of the panel.
Accuracy is very important for module producers as these small details can determine whether the company could make profit or loss especially if this was to be done on a large scale. Although this project only consists of 1 module, it is an advantage to have such precise results so that a conclusion can be drawn on how to make improvements for the future.
Testing
The test was undertaken in a dark room that had to be completely closed off from external light so that there are no extra influences on the panel performance. In order for the program to run, certain parameters had to be pre-entered into the system so that the simulator can make the correct calculations. These parameters included: Total cell area, number of cells, cell open circuit voltage (Voc) and cell short circuit current (Isc).
The first test that was run was under the standard conditions of 1000W/m2, 25 degrees celsius and an air mass 1.5 spectrum. The results for this simulation are as follows:
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Performance measurementPASAN Tester
DUT areaCells in parallel
156.25
Manufacturer Type PolySerial numberSingle cell area 12100.0cm² cm²
unknown
1Cells in series
'''36-SERIES'''111
36
Configuration Module
Operator SAV PasanPASAN Tester version R2.3.4 / PBV100 0.0.0
Measurement 2016/11/30 11.24.15
MC Irradiance Channel 1Serial number 123456Sensitivity 131.684 mV/(kW/m²)Temperature 0.0 %/°C
2016/11/30 2/1
Direct Irradiance Channel 1Compensated Temperature 25.0 °C Fill factor 60.115576 %Compensated Irradiance 1.0 kW/m² Cell efficiency 5.432454 %DUT temperature 24.104035 °C DUT efficiency 2.525418 %Monitor cell temperature 24.104035 °CGavg 1.021 kW/m²GstdDev 0.001 kW/m²Regression linear for Voc 23.997 VLinear regression Isc 2.12 ARegression linear for serial 6.881 ΩRegression linear for Shunt 32.04 ΩMaximum power 30.558 WVoltage at Maximum power 17.115 VCurrent at Maximum power 1.79 A
2016/11/30 2/2
Figure 12: Panel testing results for 1000W/m2 simulation
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Performance measurementPASAN Tester
DUT areaCells in parallel
121.68
Manufacturer Type PolySerial numberSingle cell area 100000.0cm² cm²
unknown
1Cells in series
0.8kw
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Configuration Module
Operator SAV PasanPASAN Tester version R2.3.4 / PBV100 0.0.0
Measurement 2016/11/30 11.26.01
MC Irradiance Channel 1Serial number 123456Sensitivity 131.684 mV/(kW/m²)Temperature 0.0 %/°C
2016/11/30 2/1
Direct Irradiance Channel 1Compensated Temperature 25.0 °C Fill factor 74.541762 %Compensated Irradiance 0.8 kW/m² Cell efficiency 9.275625 %DUT temperature 24.014824 °C DUT efficiency 0.304738 %Monitor cell temperature 24.014824 °CGavg 0.811 kW/m²GstdDev 0.0 kW/m²Regression linear for Voc 18.772 VLinear regression Isc 1.74 ARegression linear for serial 0.68 ΩRegression linear for Shunt 42.36 ΩMaximum power 24.379 WVoltage at Maximum power 17.016 VCurrent at Maximum power 1.43 A
2016/11/30 2/2
Figure 13: Panel testing results for 800 W/m2 simulation
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The results from the first test lower value of the power (at 30.558W) than what was calculated to be the theoretical value (60.7W). This is approximately shown with the fill factor calculation that shows a value of 60%. The conditions this time were at standard temperature and irradiance therefore this was not a factor in the results. The efficiency values have been shown to be very low, much lower than expectations. By looking into this more deeply it seems that the major issue with the power generation is the fact that the current at the maximum power is much lower than expected with a value of only 1.79A. The potential reasons for this could be due to the fact that the soldering was not done to the best standard, or the fact that there had been certain cell breakages throughout the panel during the Sylgard application process but also losses due to the formation of the bubbles from the Sylgard.
The majority of the current losses can be attributed to micro-cracks and even larger cracks within the cells because the cracks result in a much increased rate of recombination which, in turn, creates a higher resistance thus reducing the current. A report by J. I. van Mölken et al (2012), states that the micro-cracks have a direct negative effect on both the Isc and the efficiency of the cell. The micro cracks would have been a result of the manual soldering process which require stress to be applied but also the fact that the bus bars are taken through very fast temperature changes which change the shape of the tabbing wire and therefore form micro-cracks within the substrate.
The efficiency can be also related to the fact that the incorrect pre defined area for the program was used. The area was used (as shown in figure 13) is at 1.21 m2 which is, in fact, the total area of the panel including its border, spacings and edges. Therefore if the simulator is taking into account a larger catchment area then there actually is, the performance related to that catchment area will be minimised in comparison to what the actual performance should be. For example, in this case, the catchment area is 1.2 m2 and it is only producing 30W therefore the efficiency would be calculated to be much lower than if the correct solar area was used which is 0.5625 m2. This perhaps represents the largest loss of efficiency. This is further evident because the collection area for the second test (above) was mistakenly inputted at a value that is an order of 10 higher than it should be. The efficiency in the second test is much less than that of the first test therefore the area definitely plays a big factor in determining the overall efficiency of the module. A study conducted by the National Renewable Energy Laboratory (NREL), states that 'the area definitions used by the PV community can account for large differences (over 100%) in the efficiency between various groups' (Emery, 2010).
Also, the inputted values for the Voc and the Isc were those of the cell specification rather than what was measured for the independent cell. As the specification values are higher than the actual measured, the program would make calculations based on the fact that the module should perform better than it actually did.
Overall, there were certainly issues with cell breakages, Slygard and soldering however, the main source of under-performance is a result of the input parameters into the program.
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COMPARISON BETWEEN THEORETICAL, ACTUAL & INDUSTRIAL
The following tables specifications (electrical and mechanical) of four different industrial panels that all have 36 cells and all of different power ratings.
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Figure 14: Source: (Admin, 2010)
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Tables 3: Compares the values of specs with the values for industrial panels
Figure 15: Source: (Admin, 2010)
The table and graphs below present the comparison of all these industrial panels with the SPECS module.
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Figure 16: Compares the STC Power of SPECS and industrial panels
Figure 17: Compares the STC Power rating per unit area of SPECS and industrial panels
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Figure 19: Compares the Isc of SPECS and industrial panels
Figure 18: Compares the Imp of SPECS and industrial panels
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Figure 21: Compares the Voc of SPECS and industrial panels
Figure 20: Compares the Vmp of SPECS and industrial panels
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Comparison Discussion
Firstly, although all panels have the same number of cells (36), they all have individual characteristics due to other variable factors. The first of which is the power at standard conditions (figure 16). These values have a range from 30W up to 130W which shows the real variability that can occur between the cells and manufacturers. These can perhaps be attributed to the area of each cell therefore a more accurate representation is the STC power per unit area (W/m2) as shown in figure 17. In this figure it can be seen that a lot of the values are relatively close to each other except for the actual SPECS result which lies at 58.2 W/m2. The reasons for this were discussed previously but an important thing to note is that in an ideal (theoretical) scenario, the SPECS would perform very close to the manufacturer's level. Currently, with the issues and breakages, the SPECS module performs an approximate 50% worse than an industrial model of the same size but with improvements it could possibly reach a level that is only 10% less than that of the manufacturers.
The next variables to consider are the current (Isc and Imp) - figures 18 & 19. Both graphs same trend where the better rated modules have better currents. Whilst the SPECS module only produces an Imp value of 1.79A, the next module (KE45) only produces a marginally larger 2.55A. However, the highest performing
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Figure 22: Compares the peak efficiency of SPECS and industrial panels
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module produces a total of 7.47A at the maximum power point so it is definitely a big factor in the overall performance.
The maximum power voltage proved to be very similar throughout all the panels and therefore this value can be regarded as not having an effect on the overall output of the module. The Voc also follows the same trend but this time the Voc of the SPECS module is the highest at 24V whilst the average for the others is approximately 21.7V so the variation is still small. For the industrial panels, it is most likely an advantage to have the lower Voc because when a lot of these panels are connected in series, the total overall voltage will rapidly rise and it may become difficult to match this voltage with that of charge controllers, for example. On the other hand as SPECS is designed for a stand alone purpose, there is no need for controllers and therefore this Voc value is good for the application.
Finally, the last graph (figure 22) shows the variability within the peak efficiencies and this is where the industrial panels differ the most from SPECS. The panels average at 13% efficiency whilst the SPECS only has approximately 3% but as was discussed earlier, despite the soldering, breakages and Sylgard, the main reason for this large difference is the fact that the testing parameters were incorrect. If this was corrected, the SPECS module could see the efficiency rise significantly making it approximately 7% which is still less than that of the industrial panels but it would provide a much better performance. So it is important to consider that the SPECS panel performs better than the test results show but there is a certain room for improvement which will be discussed in the evaluation section of the report.
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CELL THERMAL EFFICIENCY STUDY
In order to determine the thermal efficiency of the cells a data logging micro controller was constructed. This was achieved by using an Arduino micro controller and a temperature probe. The Arduino was then connected to a laptop that plotted its temperature at 1 second intervals. This data was combined with readings from the multimeter to produce the data shown in figure 25. This shows that voltage is inversely proportional to temperature.
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Figure 26: Shows the effect of heat on the cells voltage
Figure 23, 24 and 25: testing apparatus and
testing methods for the thermal efficiency study.
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INDIVIDUAL CELL FUNCTION TEST
Every cell that was used in the creation of the panel was tested individually to see if it functioned according to manufacturers specifications. At the time of testing there were 41 cells available some of which had considerably low voltage readings. All cells were tested in the lab using a basic continuous solar simulator that emitted 1 sun on each cell with the objective of selecting the best 36 cells for the panel construction. 1 sun is typically defined as the nominal full sunlight intensity on a bright clear day on Earth, which measures 1000 W/m2.
The figure below shows the maximum and minimum (after 25 seconds of exposure in the simulator) voltage reading of each tested cell. The yellow line is the open circuit voltage, which is the maximum voltage a cell can produce under no load. All initial voltage readings were below the rated Voc. which is expected due to micro-cracks and other minor miscellaneous imperfections, the stabilised voltage reading after 25 seconds shows a 11.3% drop from the initial Voc reading. This difference was accredited to heat generated efficiency losses caused by solar simulator. This drop in production is to be expected in real life applications on a sunny day. All solar cells functioned, but out of the 41 cells available the 5 cells with the lowest Voc and Stabilised readings were discarded and not used towards the construction of the panel.
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Figure 27: Range of different voltages at max and min, also showing the cut off point and the expected value.
Volta
ge R
eadi
ng
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Cell Number0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42
Max Voltage ReadingMin Voltage Reading
Lowest acceptable value
Expected value
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Cell Number
Max V. Reading Min V. Reading Percentage Drop Between Readings
1 0.592 0.514 13.34
2 0.499 0.462 7.41
3 0.57 0.512 10.18
4 0.601 0.535 10.98
5 0.582 0.521 10.48
6 0.578 0.522 9.69
7 0.604 0.523 13.41
8 0.606 0.518 14.52
9 0.599 0.54 9.85
10 0.608 0.542 10.86
11 0.578 0.543 6.06
12 0.612 0.521 14.87
13 0.589 0.543 7.81
14 0.598 0.567 5.18
15 0.602 0.552 8.31
16 0.587 0.553 5.79
17 0.603 0.557 7.63
18 0.603 0.551 8.62
19 0.608 0.581 4.44
20 0.589 0.539 8.49
21 0.608 0.552 9.21
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Table 4 Part 1: Cell numbers and the voltage readings for each
The table below shows the results from each individual cells testing.
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Cell Number
Max V. Reading Min V. Reading Percentage Drop Between Readings
22 0.589 0.542 7.98
23 0.4930 0.468 5.07
24 0.603 0.547 9.29
25 0.602 0.55 8.64
26 0.578 0.533 7.79
27 0.602 0.54 10.30
28 0.613 0.542 11.58
29 0.594 0.538 9.43
30 0.167 0.11 34.13
31 0.599 0.529 11.69
32 0.621 0.532 14.33
33 0.612 0.513 16.18
34 0.618 0.515 16.67
35 0.612 0.534 12.75
36 0.454 0.41 9.69
37 0.617 0.529 14.26
38 0.625 0.549 12.16
39 0.410 0.384 6.34
40 0.62 0.53 14.52
41 0.617 0.55 10.86
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Table 4 Part 2: Cell numbers and the voltage readings for each
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SANKEY DIAGRAM OF ENERGY LOSSES
The sankey diagram below shows the percentage of losses in each stage of transmission, starting from the solar radiation on the panels through to the received energy in the charging devices.This diagram shows that the majority of the losses in the system are inquired from the solar panel and not within the unit, therefore the majority of efficiency losses in the are beyond control in the assembly stage.
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Figure 28: Sankey diagram of energy losses through the system
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Panel
90%
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SYLGARD APPLICATION AND CALCULATION
The importance of correct module encapsulation can not be overstated, it is needed for the protection of the PV cells from exterior damage and for electrical insulation to minimize losses. 1.1kg of Sylgard was made available for this purpose.
Sylgard 184 silicone elastomer is a transparent encapsulate with good flame resistance and an intrinsic dielectric strength of 19kV/mm. The dielectric strength of an insulating material is the maximum electric field that a pure material can withstand under ideal conditions without experiencing failure of its insulating properties. Practical dielectric strength decreases with increased sample thickness. For this reason, when applying Sylgard the objective was to evenly spread a thin layer that would completely cover the module area.
The plan was to rest the PV modules, against the acrylic cover and insulate the rear, the negative contact area of the module. This was in order to make the best of the amount of Sylgard. The procedure was proposed by supervising PhD student Hassan Baig.
Ideally for a 1m2 module area, the maximum thickness of Sylgard required would be 1.07mm. The calculation is defined below. For a hand made assembly, it would be nearly impossible to efficiently regulate the thickness by eye, so in order to meet the requirements of the thinnest possible layer of Sylgard was poured. Unfortunately the distribution was not shared as evenly as anticipated, because some of the Sylgard seeped into the front of some cells and got trapped between the front of the cells and the acrylic, reducing the amount of encapsulate available for the encapsulation of the rear. This caused visible air bubbles within the structure.
CALCULATION:
We know
Amount of Sylgard required = Sylgard density x module area x required thickness
Therefore:
Required thickness = Amount of Sylgard / (Sylgard density x module area)
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We know the amount of Sylgard available is 1.1kg
Density: 1030
Module area : 1m2
So:
Required thickness = 1100g/ (1030 x 1000 x 1) = 1.067mm
(In this case required thickness is max possible thickness)
USE OF ACRYLIC OVER GLASS.
A rigid outer transparent layer of protection is usually placed at the uppermost layer of the module, allowing light to pass through to the cells all the while ensuring its protection from damage and water. In most assemblies, glass sheets are used to cover the cells. In this assembly an acrylic sheet was used instead.
Acrylic sheets have numerous advantages over glass in this application, some of which include:
• Transparent acrylic glass transmits up to 92% of visible light where as glass transmits 80-90% depending on the manufacturer. (White, 2015)
• Acrylic has a high impact strength which makes it more shatter resistant than glass, improving the safety figures
• Acrylic has a high thermal efficiency, which can be beneficial for the cells, reducing the heat gain on the cells from the sun's radiations
• Acrylic is easier to transport and more easier to assemble.
• Acrylic is significantly cheaper than glass.
Acrylic also has some disadvantages, which include
• It is softer than glass and can be scratched much easier than glass, in this assembly scratching is very plausible because the panel is not elevated beyond the reach of users.
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• Acrylic can warp when overheated, in this assembly the risk of warping has been mitigated, having used wood, a material with low thermal conductivity, in the rear of the panel as an insulator the risk of overheating is significantly reduced.
• An untreated acrylic is susceptible to tarnishing with exposure to sunlight.
Acrylic was chosen over glass because it is fit for function in this assembly, the decision was further encouraged by financial and structural incentives of it being cheaper and more malleable than glass. But for future purposes hardened glass would be worth considering because it us less susceptible to scratches and will also be resistant to shattering.
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EFFICIENCIES OF COMPONENTS
Energy loss is an unavoidable part of energy conversion systems. A 100% efficient system is not achievable, but well-designed power systems can achieve remarkable efficiencies, approaching percentages in the mid to high 90s. This section will analyse the efficiency of each component, with the objective of answering the questions, how much of the generated electricity is utilised, how much is wasted and what could be done to improve efficiencies.
Efficiency of the panel
The calculated device under test efficiency result of the panel is misleading, in the test the area of the panel considered is not the actual panel area but rather the panel frame, which includes the spaces in between each cell and the edge of the frame, giving the area of 1.25m^2 where as the total cell area 0.56m^2. This significant increase in considered area is the reason for a low panel DUT efficiency of 2.5% but under reevaluation the expected efficiency is expected to be closer to 10%.
Efficiency loss due to copper wire
The power losses due to the internal resistance in the 1mm copper wire will be calculated below:
Formula for Power loss calculation: Imp^2*Rwire
The wire resistance R, is calculated using the formula information from an American Wire Gauge (AWG) chart.(School of science, 2011) The chart says that 1mm diameter copper wire is equivalent to 18 AWG, using this information we find the resistance per metre of 18 AWG copper wire to be 20.9 ohms per km (Newton,
1999) We can calculate the resistance of the wire R, by multiplying the length of the wire by the resistance per meter.
The length L: 4.14m (the longer the wire, the greater its resistance)
Rwire = 0.004km x 20.9 = 0.083 ohms
The value of Rwire was confirmed using an online calculator provided by CIRRIC systems which gave Rwire to be 0.087ohms. (Ellsworth, 2015). For the sake of this calculation we will use the calculated Rwire value of 0.083ohms.
The power loss: Imp^2 x Rwire = 4.52^2 x 0.083 = 1.71W
The theoretical power of the module is 60.7W. The power loss from the wire is 1.71W, this equates to a negligible power reduction, bringing the actual power to 97.2% of the original theoretical power.
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Efficiency losses from DC/DC Converter
An ideal DC/DC converter would have 100% efficiency, operate over arbitrary input and output voltage ranges, and supply arbitrary currents to the load. Realistically, DC/DC converters common in battery-driven, portable, and other high-efficiency systems such as the SPECS, can deliver efficiencies greater than 95% while boosting, reducing, or inverting supply voltages. Resistance in the power source is one of the most important factors that can limit efficiency, in this section we analyse the convertors efficiency loss. Lin, W. (2013)
The system will always have substantial loading, either when charging mobile devices directly or charging the batteries for later use, this means the DC/DC converter will always be under substantial load and will work within high efficiencies. For this DC/DC converter the data sheet says it has an efficiency of 96%.
Efficiency losses in battery
Lithium-ion batteries are most commonly valued for their light weight, small size and long life cycles when compared to traditional lead acid batteries. Lithium-ion deep cycle batteries give long operational time if you require a battery that gives you more operational time, and was the best battery fit to purpose for this task.
A lead acid battery’s internal resistance becomes higher the deeper it is discharged. Therefore the charging algorithm is designed to slowly charge the battery at lower voltage levels. Conversely, the constant current algorithm of lithium batteries is preferable due to the high efficiency and low internal resistance. That means you are able to charge at a much higher rate. In turn, reducing downtime and increasing operational time. Rechargeable lithium-ion batteries have an efficiency of 99% and offer a much higher usable capacity at the same Amp hour rating than lead batteries (Hecimovich, 2015).
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ECONOMICS
This section discusses and analyses the financials involved with the project. It will consider the cost of assembly and the financial savings made by the project.
The cost of electricity is used in this economic study is SSE's 11.47 pence per kWh (UKPower, 2016). In this study we assume that mobile devices (phones, tablets & kindles) can be continuously charged for a total of 6 hours a day from the batteries, this is based on the battery specifications. An iphone 6 requires 2 hours to charge from 10% to 100% (Apple, 2015). For this study we assume that the unit charges continoulsy for 5 hours a day everyday with no down time that comes down to 1825 hours of charging per annum for each compartment. The entire unit is able to charge 4 devices simoutaniously which equates to 7300 hours of charge per annum by the station. We also know that most phone chargers consume anywhere between 2-6 watts to charge, iphone chargers consume 5.1 watts while charging (Bonnington, 2013)
Calculating the amount of kWh used per annum by the unit to charge devices will give the amount of energy saved by the device, this value will be converted to monetary terms using the cost of electricity.
7300 hours x 0.0051kW = 37.2kwh
The cost of electricity equates to the multiplication of the unit price of electricity by the amount of electricity being used.
37.2kWh x 11.47
37.2kWh x 11.47p = 426.6p
It will take 43 years to pay back the assembly cost. The assembly cost is different from the total from the material cost list. The assembly cost consists of all purchased materials, as well as the cost of Sylgard and the panels.
Sylgard: £30;
36 solar cells: £56.2
Bringing the total assembly cost to £267.02
The solar cells have a life span of 20 years, meaning the system will never make a payback based on electricity savings. To compensate for this deficit a payment for service system is proposed.
In this proposed system each user will be charged a 10p fee for the service. It is assumed that the payment will be for a single charge to maximum device battery level, assuming it is charging an iphone from 10%-100% which will take two hours. So we assume that the payment of 10p for every two hour of use.
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Dividing the number of hours the system charges per annum for (7300 hours) by the number of hours it takes to charge a phone, (2 hours) gives the number of charges the system will make per annum. This comes down to 3650 charges per annum across all 4 compartments. Multiplying the number of charges by £0.1 (10p) will give the amount of money generated if the system was used contioniously. This comes down to £365 per annum. Realistically the system will not be used continuously, a realistic usage factor must be adopted. Factors such as seasonal variation, area population density will affect the rate of use, to compensate for this the usage factor of 0.2 was adopted.
A similar analysis was made considering a 20p charge
Based on these systems the financial reviews below were made.
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A comparison of the two cash flow models shows that having a 10p charge will have a payback after the 4th year of installation and generate a revenue of £1119.98 at the end of its 20-year life time. Where as a 20p charge will have a payback after its 2nd year of installation and generate a revenue of £2506.98.
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Figur
e 31
: Cas
h Flo
w M
odel
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POTENTIAL SITES FOR PLACEMENT OF SPECS
The SPECS team have identified 4 different sites that would be appropriate for the installation of the unit, around the University of Exeter, Penryn Campus, Cornwall. These were based on a range of factors listed below.
Site Reasoning for choice
1 This site is based in the middle of the student village, with a space used in the summer for socialising and BBQing, the students could charge their phones while relaxing in the sun.
2 This site is adjacent to the university outdoor games area, allowing users of the facility to charge their devices while completing recreational activities.
3 This site located outside the Stannary, the part of the campus with the highest human traffic. This would allow the highest number of people to use SPECS and promote sustainability with the student population.
4 This site is outside the ESI, the leading solar research and innovation centre in the UK. Displaying SPECS here would be a very prestigious opportunity for the team.
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Table 5: Chosen sites and the reasoning behind the site choices
Figure 32: Chosen sites
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Site 1:
Glasney Student Village recreational area
Site 2:
MUGA sports ground
Site 3:
Stannary and Library Entrance
Site 4:
ESI Entrance
Figure 33, 34, 35 and 36: Site photos of the proposed sites at Penryn Campus
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PVSYST ANALYSIS
Using the potential sites that have been allocated on campus, a PVSyst analysis took place to determine the overall incoming solar radiation and the total energy output from the panel.
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Figure 37: Solar radiation monthly data for Penryn Campus, UK
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Figur
e 38
: Sun
Pat
h Di
agra
m
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Figure 39: Simulation report page 1
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Figure 40: Simulation report page 2
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Figure 41: Simulation report page 3
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Figure 42: Simulation report page 4
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GAS STRUT CALCULATIONS
While constructing the solar panel it was decided that to better explain how the panel worked, the inside electronics would be labelled and a poster would be displayed on the bottom of the panel lid. To allow the lid to operate easily and safely a variety of different devices were investigated. Initially a string was installed to hold the lid as a 120 degree angle. This proved to be quite precarious and was discarded. Another idea was for the use of a hanging stick, similar to that of a cars front bonnet was suggested, but was also rejected.
The chosen method was to use two 100Nm gas struts. This value was chosen by measuring the force needed to open the lid with a 50-500Nm gauge. The value recorded was 80Nm per side but 100Nm was chosen to ensure that the lid would be held securely.
Through research it was found the the geometry of the gas strut was very important to gain the greatest leverage and allow the lid to move freely. Therefore to optimise the placement of the strut, a solidworks model was created to model the strut and its movement. An animation of the struts movement can be found by using the link at the side of the document.
After this was tested and analysed, the geometry measurements were taken and used for installing the real gas struts on the device. Due to the careful measurement, planning and testing they worked perfectly in the first attempt of installation.
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goo.gl/nZQ3cA
Figure 43: 3D model of hinge structure
Figure 44, 45 and 46: Animation of the gas strut moving
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INDOOR TESTING
Before testing the rig outside it was important to check that the panel was connected correctly and producing electricity. This was achieved by using a multimeter connected to the panel, and a high powered light. The light proved to be enough to power the panels. The next stage was to connected the dc-dc converter and connect a usb charger for a mobile phone. This also worked perfectly and while testing 4 phones were connected directly to the panel and all were successfully charged at the same time.
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Figure 47, 48 and 49: Images of indoor
testing.
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OUTDOOR TESTING
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https://goo.gl/43uorn
Before testing the unit on the public, it was important it worked reliably. The team tested the unit outside the laboratory for 6 hrs monitoring the status of charging in two test devices throughout. This test proved very successful and the panel managed to continue charging even in the shade. This made the team confident to proceed to user testing. The QR code and link lead to a video showing the panel being testing outside.
Figure 50, 51 and 52: Images of outdoor testing.
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USER TESTING
To test the ease of use of the device we tested the rig on several students from the university. With no instruction they operated and understood the device. This was a great success and proved that the design was user friendly. After each student finished using the device we asked for feedback. The feedback can be seen below:
“The rig is very simple to use and great as it doesn't use energy from the grid” Tom
“I like that rig is easy to understand, it would be good as an educational tool in schools” Bruce
This feedback was useful and positive towards the project.
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Figure 53, 54 and 55: Images of user testing.
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EVALUATION CONSTRUCTION AND PROJECT
Despite this project being a great success and the goals being achieved, there are parts of the project that would be completed in a different manner if completed again. This section of the report will discuss these points.
While the lockers for the phones are adequate for use on a university campus, the lack of locks means that it could not be installed in a public area, due to the ease of theft. It would therefore be recommended for a future project to install locks on each individual lockers.
Similarly with the lockers, in a future model it would be recommended that more lockers be installed, than the four that were chosen for this design, as the panel could power many more devices, and this would allow more users to benefit from the technology.
While completing the Sylgard application to the cells it was found to be difficult due to the large size of the panel and the lack of prior knowledge of the behaviour of the Sylgard polymer. In future it would be recommended that the Sylgard should be applied to the base of the panel, to create a perfectly flat surface to place the cells on, then the cells be installed, then a layer of Sylgard be poured over the cells, and while curing the entire panel placed on a vibrating table for 48 hrs.
The soldering process also turned out to be a difficult part of the project, due to the highly fragile nature of the cells, several cells were broken while starting the soldering process. With a future project, the advice would be to anticipate just how fragile the cells and treat them with more caution from the start.
The soldering process which in this project was efficient and accurate when compared to other manual soldering techniques, in a commercial version of this project the soldering could be improved with the use of an automatic soldering machine.
On a similar topic, when the cells were soldered they were left on the laboratory, and moved by a staff member not familiar with their fragile nature. This unfortunately resulted in several more cells being broken and many others developing micro-fractures.
The panel on this project was very effective despite being angled at 0 degrees at all times. It would be much more efficient for the panel to be angled at a 40 degree angle. It would also be more effective to develop a tracking panel system. This could be the work completed by a future research group.
The method for attaching the cells together was decided due to the fragile nature of the cells, allowing a large amount of space between cells and around the boarder, and with more practise a higher cell density could be
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achieved. This would significantly increase the power generation to size of panel, making it more economically efficient as well as improving the DUT efficiency.
The cells chosen for this project were silicon poly-crystiline cells used had a factory efficiency of 17.7%, although that is high it would be possible to invest in higher efficiency cells to produce more power. This option could be investigated for a future project.
As investigated in the section above, the acrylic used in this project was untreated and therefore would discolour with use over time. It would therefore be suggested in a future project to use either treated acrylic or a sheet of glass. The addition of glass would potentially make the project less safe. An option of tempered or plastic laminated glass should also be explored.
As shown in the section investigating the thermal efficiency of the cells , the efficiency does drop while the panels heat up. The current predict does not include a heat sink to pull the heat away from the panels. It would therefore be suggested for a future project to implement a heat sink, to prevent these efficiency losses.
Due to the nature of this project, time was limited and certain aspects could not be fully investigated. The development of a battery controller is one of these parts. This would improve the delivery of power to the charging devices and improve how the rig handles the incoming energy deciding automatically whether to store or charge a device.
The buck converter used was taken from an car battery supplier, and only starts working when the cell starts producing more than 8v, this could be switched out for an optimised buck converter to start working at a lower voltage and with higher efficiency of conversion.
To allow the rig to me moved more easily, the project recommends that some coasters or wheels be installed on the base of the legs. This would allow the rig to be installed and moved around more easily. This opens up applications for festivals allowing the rigs to be brought onto sites quickly and removed with ease.
EVALUATION OF TESTING Perhaps the largest factor that caused an under achievement in the efficiency performance of the panel was the testing process. Although the ideal conditions were used alongside a well-established solar simulator, the input testing parameters were incorrect. If the testing process were to be re-taken in the future, it would be ensured that the exact parameters for the sizing of the system were entered into the program so that an accurate efficiency can be determined.
As for the cell testing, the temperature conditions were higher than the standard conditions and in the future, the cell testing would take place at exactly 25 degrees celsius so that the cell can be tested at its optimal performance.
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CONCLUSION
Overall, the project can be seen as a very successful because despite all of the difficulties, the panel is fit for the purpose and it can charge multiple electronic devices simultaneously. Those difficulties came from soldering, cell breakages, Sylgard application and bubble formation. The cell performed relatively well under the I-V testing with results similar to those in the specification and it performed with an average of 92% efficiency in the EQE test. The module testing should have been repeated with the correct input parameters however due to time constraints this was not possible however, the important thing that was found was that the panel was generating power and even under the conditions of 800W/m2. Once compared to the industrial panels of the same size, it proved to be under performing however those reasons were attributed mainly to the cell breakages and the fact that the soldering had been completed manually, increasing the wire resistance. The voltage seemed to be very similar to that of the commercial panels and the main reason for the lower power output is a result of the reduced current in comparison.
The poly-crystalline panels have proven to be working however due to their senesitivity within a stand alone structure, there are increased chances of breakages throughout their lifetime. Therefore there is a possibility of also implementing third-generation solar cells into the system as they are rapidly developing to become more space efficient but these technologies will need to be explored further before becoming available commercially.
In conclusion, this panel has proven to provide enough power to successfully charge numerous electronic devices throughout the day and the hope for the future is that the principle of this idea can be expanded, improved and implemented on a larger scale throughout the UK.
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ACKNOWLEDGEMENT
This project was partially financially supported by Exeter University. We thank our colleagues from the ESI who provided insight and expertise that greatly assisted the research, although they may not agree with all of the interpretations/conclusions of this paper.
We would also like to show our gratitude to the Brian, the lab supervisor for sharing his pearls of wisdom with us during the course of this research, and we thank the students that tested and gave comments on the finished product.
Katie Shanks provided guidance on the individual cell testing and we are grateful for her help as well as Dr. Hasan Baig and Prabhu Selvaraj who provided great help with the soldering and Sylgard application.
Finally we are immensely grateful to Dr Senthilarasu Sundaram and Prof. Tapas Kumar Mallick for their organisation of this module and their support and feedback throughout the project, although any errors are our own and should not tarnish the reputations of these esteemed persons.
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APPENDICES
SYLGARD® 184 SILICONE ELASTOMER PROPERTIES
Property Unit Result
Ratio - 1:10
Colour - Colourless
Viscosity (Base) cP 5100
Viscosity (Mixed) cP 3500
Thermal Conductivity btu/hr ft oF 0.15
Cure Time at 25oC hours 48
Refractive Index @ 589 nm 1.4118
Refractive Index @ 632.8 nm 1.4225
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Figure 56: Properties of Sygard 184