Recognizing snow on a solar module and melting it, prototyping and profitability

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NTNU Norwegian University of Science and Technology Faculty of Information Technology and Electrical Engineering Department of Electric Power Engineering

Thor Thinius Tuv

Recognizing snow on a solar module and melting it, prototyping and

profitability

Master’s thesis in Energy and Environmental Engineering Supervisor: Steve Völler

June 2021

Master ’s thesis

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Thor Thinius Tuv

Recognizing snow on a solar module and melting it, prototyping and

profitability

Master’s thesis in Energy and Environmental Engineering Supervisor: Steve Völler

June 2021

Norwegian University of Science and Technology

Faculty of Information Technology and Electrical Engineering Department of Electric Power Engineering

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1 Abstract

In this paper the main objective is to understand the behaviour and the consequences of heating solar panels by injecting current. During the winter and spring, solar panels can be covered by snow, unable to generate energy from solar irradiance. The heating method is meant to take care of snow fall on top of the panel, by melting it to ensure continued generation of energy by the panels.

To explore this possibility, a commercial photovoltaic panel is acquired and connected to an external power source. By applying current across this panel, there will be generated heat. This heat is meant to remove the snow on top of the solar panel.

This method is tested, with varying amounts of power, ambient temperatures and other parameters.

By mapping the results there could be constructed a model for when and how the operation should be executed for optimal results.

By testing the process at different parameters, there is discovered that the removal of snow is easier with certain modifications to the panel, and any optimal solution includes melting while the ambient temperature is above zero degree Celsius.

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2 Sammendrag

I denne rapporten er m˚alet ˚a forst˚a hvordan man enklest kan fjerne snø ifra solcellepaneler. Solcellepaneler har en lei tendens til ˚a bli dekket av snø, og ikke motta solstr˚alene som trengs for ˚a produsere energi.

Oppvarmingsmetoden skal fjerne snøfall p˚a toppen av panelet, ved ˚a smelte det for ˚a sikre fortsatt generering av energi fra panelene.

For ˚a utforske denne muligheten anskaffes et kommersielt solcellepanel og kobles til en ekstern strømkilde.

Ved ˚a sende en strøm inn i dette panelet, vil det bli generert varme. Denne varmen skal fjerne snøen p˚a toppen av solcellepanelet.

Denne metoden er testet, med varierende mengder kraft, omgivelsestemperatur og andre parametere.

Ved ˚a kartlegge resultatene kan det konstrueres en modell for n˚ar og hvordan operasjonen skal utføres for optimale resultater.

Ved ˚a teste prosessen ved forskjellige parametere, blir det oppdaget at fjerning av snø er lettere med visse modifikasjoner p˚a panelet, og enhver optimal løsning inkluderer smelting mens omgivelsestemperaturen er over null grader Celsius.

Snøfjerningen kan vise seg ˚a være en mulighet for ˚a optimere profitt p˚a et allerede eksisterende system uten massive modifikasjoner.

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3 Preface

This thesis is the final part of my master’s degree in Electrical energy technology and Energy systems.

The task at hand is:

Recognizing snow on a solar module and melting it, prototyping and profitability.

The thesis is written during the spring semester of 2021. It is written for the Department of Electric Power Engineering, at the Norwegian University of Technology and Science, in Trondheim.

The student is responsible for conducting the experiments, writing the report and making conclusions from the results. Supervision, assistance and guidance has been provided by Steve V¨oller. Both the student and the supervisor are associates of Norwegian University of Science and Technology. The work in this rapport is conducted on campus in the Electro- and Heat-laboratory, as well as just outside the building.

This project includes gathered information, hypothesises, experiment results and conclusions regarding the topic at hand. Ideas for further development have been submitted as well.

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List of Figures

1 How the atom structure is figured at the P- & N-type material[3] . . . . 5

2 How a photovoltaic cell functions.[4] . . . . 6

3 Visual representation of the bypass and blocking diodes in a solar panel array. [5] . . . . . 7

4 The bypass diodes. . . . 8

5 The voltage profile of a PN junction diode.[8] . . . . 11

6 The PN junction diode in forward bias operation. . . . 14

7 A magnifying look at the microscopic level inside of the diode. . . . . 14

8 Illustration of the PN junction diode with a low reverse current applied. . . . 17

9 Illustration of the PN junction diode with a larger reverse current applied. . . . 18

10 The voltage profile of a PN junction diode.[8] . . . . 19

11 Illustration of the avalanche effect. . . . 20

12 Sliding object off an incline.[14] . . . . 22

13 The stumpness of the edge. . . . 26

14 Snow on the panel and the edge holding it back. . . . . 27

15 The net force from the system in Figure 14. . . . 27

16 How a different edge can affect the situation. . . . 28

17 Net force of the system in Figure 16. . . . 28

18 How the edge looks like in the conducted experiments. . . . 29

19 How the edge looks like in the conducted experiments, captured by photo. . . . 29

20 The panel at a vertical orientation. . . . 31

21 The panel at a horizontal orientation. . . . . 32

22 Snow on the panel and the edge holding it back. . . . . 33

23 The panel at a vertical orientation. . . . 34

26 The panel at a portrait orientation. . . . 36

27 How the solar panel looks like when ice is acting as resistance. . . . . 37

28 Where the water turns to ice. . . . 38

29 How the ice is keeping the snow back. . . . 38

30 The construction. . . . 40

31 The construction. . . . 40

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35 How the sensor works. . . . 44

36 How the sensor looks like. . . . 45

37 The snow is melting into water, and a large part of this water refreezes unto the frame. . 53

38 Further melting leads only to further ice. . . . 54

39 How the panel looks like after a long time of running heat generation without being able to remove the snow. . . . 56

40 How the insulation is attached to the panel. . . . 58

41 How the insulation is attached to the panel. . . . 58

42 How the snow looks like below the surface when the process is creating pockets of air. . . 59

43 How the tape is attached to the panel. . . . 60

44 How the tape is attached to the panel. . . . 61

45 Clear spots appearing at the bottom of the frame. . . . . 63

46 How a lot of snow has melted from the bottom of the panel. . . . 64

47 25% of the snow is still remaining after the initial avalanche. . . . . 66

48 15% of the snow is still remaining after the initial avalanche. . . . . 69

49 5% of the snow is still remaining after the initial avalanche and the continuous melting. . 70

50 30-40% of the snow is still remaining after the initial avalanche and the continuous melting. 72 51 How the different scenarios impact slide time. . . . 74

52 How the different scenarios impact the energy spent to slide. . . . . 75

53 The average electricity price by time of day, in March 2021, region Trondheim.[24] . . . . 77

54 A visual representation of the percentage of decrease in spent energy which can be achieved by utilizing time sensitive melting. . . . 78

55 The 2x3 grid at horizontal orientation . . . . 85

56 The 2x3 grid at vertical orientation . . . . 85

57 The 2x3 grid at horizontal orientation with snow. . . . 87

58 The 2x3 grid at vertical orientation with snow . . . . 88

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List of Tables

1 A table describing the snow density over time.[16] . . . . 24

2 In- and output values for scenario 1. . . . 52

3 Input values for scenario 2. . . . . 52

4 Input values for scenario 3. . . . . 55

5 Input parameters for scenario 1. . . . 62

6 Output parameters for scenario 1. . . . . 64

8 Output values for scenario 2. . . . 67

13 Input parameters for scenario 5 . . . . 71

14 Output values for scenario 5 . . . . 72

15 An estimate on how much energy can be saved by increasing the ambient temperature. . . 76

16 Energy spent for each scenario on average, in contrast to the base case. . . . . 82

17 The energy spent, and the cost in NOK, for each scenario. . . . 82

Nomenclature

µ The coefficient of friction between the snow and the solar panel.

θ The incline of the solar panel.

g Gravitational acceleration coefficient.

m Mass of the snow on top of the panel.

N Normal force enacted on the snow from the surface of the solar panel.

VBR Theoretical voltage required to breakdown a PN-junction diode.

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4 Introduction

Solar panels have the ability to provide renewable energy for a long amount of time, which makes them a good source of energy, now and in the future. The solar panel only generates energy when they are exposed to solar rays directly onto the solar cells. When the cells are covered by shade, snow or any other object, potential generated energy is lost.

In this paper there will be focused on the removal of snow, in order to maximize the amount of renewable energy which can be extracted from the solar panels during the course of its lifetime. By removing the snow at strategic times, an extended period of sunlight before the next snowfall could generate energy otherwise squandered.

By coupling a solar panel to an external power source, the possibility of automatically removing the snow on top of the panel is explored. The removal of the snow is achieved by applying an external current into the panel. When this external current is large enough, some of the energy will turn into thermal energy, heat, in the cells of the solar panel.

By heating the panel, the goal is to remove the snow as energy efficient and easy as possible. Several different scenarios and solutions connected to these scenarios using this method has been mapped.

The main motivation throughout the project were to improve the efficiency of the melting process, to reduce the costs. Throughout the project it was enjoyable to attempt finding energy saving solutions. The purpose of this paper is to expand the reader’s knowledge on the possibilities of increasing the profits of a solar energy generation system, by removing snow.

The entire project, spanning from autumn 2020 to summer 2021, could be derived into four parts.

• Design a melting system

• Optimize the melting system

• Automatizing the melting process

• Calculate profitability

It seems natural to focus on the first two parts during the first semester with regards to time. The desired progress would be to finish the first part, while also attaining some insight into the second part, before the end of the first semester. This was achieved during the autumn semester 2020[1]. In the summer semester, there was planned to put a larger emphasis on the second half of the list above. In reality there has been a lot of focus on the optimization of the operation, due to ideas which showed up during the experiments.

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So the focus for this particular rapport is the optimization of this process, the mapping of profitability and optimization margins, and the automatizing of the system.

To explain the melting process, the rapport consists of a theory part, explaining the details of what is happening inside of the diodes, and how the heat is generated. For the experiments in the autumn semester, there was only a solar panel and a power source required for the testing. During the course of the summer semester, where more extensive experiments has been conducted with snow, more equipment has been used. There has been spent some time experimenting with additional external improvements to the system to increase its energy efficiency, which will be discussed later.

By the end of the assessments of the melting process, the goal is to provide a basic model which can be used to find out how much profit can be gained from melting the snow using the chosen method, dependant on a number of different external parameters. There will be a natural focus towards trying to find the most optimized solution for the greatest gain.

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5 Theory

Here follows a complete theoretical backround on solar panels and their properties which can or could have an effect on the process of melting the snow on top of the panel. Section 3.0 to 3.2 are selected paragraphs from the project rapport written during the autumn semester 2020.[1] Most of the theory from that rapport has been cut, but the absolute basics of how a solar panel works and how the diodes can be configured in order to heat up the panel, has been included. This is necessary in terms of context to understand the principles which have been exploited using this method.

Photovoltaic panels have become a reliable and energy efficient way of producing power in areas wherever there is sunlight. There have been a significant increase of installed power in Norway, Europe and the rest of the world. The most important advantages of utilizing solar power is the relative reliability and the renewability of the energy itself.

5.1 Composition of the solar cells

A solar panel is made up of numerous photovoltaic cells and strips of metal in order to lead the induced current through the solar panel.

Every photovoltaic cell in this solar panel is made up of doped Silicon, whereas the backside of the cell is P-type Silicon and the front is N-type Silicon. The difference in charge creates an electric potential in the space in between the front and the back of the cell. This electric potential is inducing a current throughout the metal strips in between the cells. When solar rays hit the cell, a current is induced.

The solar panels are part of an array of panels usually connected in series. All of the panels with access to sunlight are contributing to the total electricity produced by the array. [2]

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5.2 Power generation in the solar panels

In order to generate power from the photovoltaic panels, there has to be produced a current inside of the material. Production of the current comes from electrons knocked off from their respective connections in the N-type material. As one can tell from Figure 1, there is an important characteristic which separates the P & N-type material from each other. The N-type material consists of Silicon doped with Phosphorus. The P-type material consists of Silicon doped with Boron. [2]

Figure 1: How the atom structure is figured at the P- & N-type material[3]

The Silicon in the material has by default 4 electrons in it’s outer shell, which makes it

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is the opposite. Boron has a total of 3 electrons in its outer shell, which results in the P-type to lack an electron for every atom of boron doped into the material. The strength of the electrodes, are dependant on how heavy the material is doped. [2]

The excess of charge carriers inside the diode increases the rate of the ability to conduct current, since the charge carriers are having an easy time of moving about. If there is applied energy to pure Silicon, for example in form of heat or photons, it may cause the already stable and bonded electrons to get knocked out of the electron shell, leaving behind a positive hole. But the pure Silicon does not release enough electrons relative to the energy provided to justify using pure Silicon as a conductor. [2]

That is why it is preferred to have the Silicon doped with Phosphorus, creating a situation where the fifth electron is only held in its place by electromagnetic forces, and can easily be knocked off their spot, to conduct current. When the P-type material and the N-type material connects, it creates an electrical field, making an incentive for electrons to travel towards the positive holes and vice versa. As one can see in Figure 2, the energy from the sunlight is enough to knock out the loose electrons and have them conducting current in intended direction.[2]

Figure 2: How a photovoltaic cell functions.[4]

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5.3 Bypass and blocking diodes

In order to understand one of the methods used to optimize the system later in the rapport, the function and consequences of the bypass diodes in the panel is explored.

Bypass diodes and blocking diodes are central components to PV-panels. In Figure 3, it is shown how they are installed in an array of solar panels.

Figure 3: Visual representation of the bypass and blocking diodes in a solar panel array. [5]

There are installed bypass diodes inside of the coupling box of the panel. How it looks like inside the chosen panel for this project is shown in Figure 4.

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Figure 4: The bypass diodes.

The purpose of the bypass diodes in power generation is to give the current in the circuit an alternative way of traveling. In an array of panels, coupled in a series, the power across the entire array will be limited by any single panel’s ability to conduct current. So if the first panel is partly shaded or covered with snow, the entire array will start generating power at a decreased rate. The bypass diode provides an outlet for the current flow, which avoids the shaded panel, so the rest of the array can continue to operate on a high voltage level. [5]

In heat generation mode, the bypass diodes serves no purpose, since the current is forced through the Silicon diodes inside the panel in forward bias, effectively removing the need for bypass diodes. They are still present, but will have no effect on the forward bias experiment.

When producing energy, the bypass diodes could help in the case when one part of the panel is covered, while the other is not. The halves of the panel can run simultaneously and independently.

Blocking diodes in power generation mode only has a single simple function. It’s purpose is to make sure that the power is conducting the correct way. Blocking diodes are the

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measure taken to avoid accidental reverse bias current when the panel is generating power.

In the heat generation trials the current is forced into the panel in the correct direction, making the installment of a blocking diode unnecessary in this experiment.

Blocking diodes are as one can tell from Figure 3 installed in between panels in an array.

There is no blocking diode in the panel used in the experiments conducted in this rapport.

Since there is only used a single panel at the moment, and not an array, the need for a blocking diode is void.

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5.4 The heating method

When there is snow on the solar panel, the solar rays which is inducing current in a normal operating system, can’t reach through the snow. At this point there is no power produced by the solar rays, and the power output from the panels are zero. Obviously, there is a incentive to produce as much power as possible, so the snow must be removed.

There are several ways of removing the snow.

Manual labour is the easiest, provided the manpower is available at all times. This is more and more difficult the bigger the panel arrays are. An entire park of panels might need a dozen men to clear the snow in a day. Manual removal of the snow also creates the risk of damaging the panels. This is because the surface of them are quite fragile, and scratch marks or dents in the glass might negatively effect the power production.

Panel arrays on roofs creates a whole different set of workplace hazards if the snow is to be cleared manually. The dangers of working on a roof requires sufficient fall-control on site. Therefore a better solution is sought.

Heating the panels becomes a self explanatory alternative for the removal of the snow.

Installing heaters on the back side of the panel, or applying exhaust air from nearby heat sources to the surface of the panel, are different ways of melting snow. These appliances have some advantages and some drawbacks, but in this paper there will be a focus on the method of applying a current from an external power source. Whereas it covers the most significant drawbacks of all the above heating methods. It is easily applied once installed.

By applying energy into the solar panel, one can generate heat in the cells of the panel.

This is because once the applied power is of a certain magnitude, excess energy which it cannot handle is emitted as thermal energy.[6]

External heaters will always come with manufacturing-, installation- and renovation- costs. The method of heating the panels using an external power source is easy because it requires little installation costs, excluding the automatizing system, which will be discussed later.

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5.4.1 PN junction diode

A solar cell diode, is a so called PN-junction diode. A PN junction diode is a diode which is designed to conduct current only one way, and need a certain voltage applied to it to conduct effectively. There is also possible to conduct current in the opposite direction with the right circumstances, this will be explained later. Both opportunities will be inspected throughout as they are both considered for a possible solution in this task. [7]

In Figure 5 the relationship between the flowing current in the forward and reverse bias is illustrated.

Figure 5: The voltage profile of a PN junction diode.[8]

From this figure one can make a number of conclusions;

• V_F is the voltage required for a single diode to conduct anything that is not miniscule current in the intended direction. 0.7V in a Silicon diode.

• Before the voltage VF is applied, there is conducted a very small amount of current

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conducted in the reverse direction.

• When the voltage applied in reverse mode reaches the critical point V_BR the diode reaches its breakdown point.

• When the breakdown point is reached, there is a large amount of current conducted through the reverse direction in the diode.

• |V_F|<<|V_BR|

The PN junction diode is made up of 3 critical areas. The P-doped side of the diode is the anode. It is composed of positively doped atoms. The N-doped side of the diode is the cathode. It is composed of negatively doped item atoms. The depletion region is the area in between the doped areas. The depletion region’s strength determines how easily the charges will be exchanged with each other. [7]

5.4.2 Forward bias

For the provided illustrations in Figure 6, 7, 8 & 9, the representative visualizations are in reality as follows:

• The green large circles represent the P-doped atoms.

• The yellow large circles represent the N-doped atoms.

• The blue small circles with a plus sign represent the positive electron holes.

• The small yellow circles with a minus sign represent the electrons.

• The brown battery shaped figure is the power source.

• The red arrows which goes from the N-doped side to the P-doped side is an representation of the electrical field #»

E which is created by the charge difference, and its direction.

• The blue arrows shows in which direction the 2 different charge carriers flow.

• The black plus and minus signs represent charges. The atoms in the depletion region

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are positive and negative ionized atoms.

• The red circles are the doped atoms in the silicon material.

When the panel is in normal generation mode, the Silicon diode is in forward bias. This means the current is flowing through the diode in the intended direction. For this diode to conduct current, the voltage applied to the diode must approach the forward voltage V_F.

There is already established that this is generally 0.7V for a normal Silicon diode at 25 degree Celsius. This means there must be applied a voltage of 0.7V*36 = 25.2V to induce a current on a panel consisting of 36 cells at room temperature. This is because the cells are coupled together in a series, making the relationship between amount of cells and panel threshold voltage, linear. The value of V_F is 0.7V according to Figure 10. This value is a general value to be used for rough calculations, butV_F is actually not 0.7V in all instances. V_F actually depends on which type of Silicon diode are used in the panel.

The chosen panel’s specifications and it’s threshold voltage is accounted for in the power supply paragraph.

As with any diode, or conductor, it has a limit for how large currents can flow through it before it reaches its capacity. When the applied voltage exceeds it’s limit, the diode will start to heat up, and if the limit is exceeded further the diode might burn up or malfunction. The purpose of this system is to heat up the diodes, in order to melt the snow on top. The voltage applied for snow melting has to be so high that the temperature on the panel will rise sufficiently, but also low enough to not fry the components. This means that for a standardized panel of 36 cells, one must use a voltage which is approaches 25.2V, as of last paragraph. [9]

In the forward bias operation, the charge carriers and depletion region looks like the illustration in Figure 6. The depletion region is narrow in this figure, which represents that some voltage has been applied across the diode. The depletion region being narrow

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Figure 6: The PN junction diode in forward bias operation.

The positive holes will attract towards the cathode and electrons will attract towards the anode due to the flow of current applied. [10]

A magnifying view on the state of the P- and N-type material is shown in Figure 7. In the P-type material, there will exist positive holes in place of electrons in between the atoms.

This positive hole will eventually be filled by a rogue electron roaming in the P-doped material. The doping of the material with for example Boron, will as explained earlier make sure that the net charge in the material allows for a positive hole to be created somewhere else or knock the rogue electron off again. [10]

Figure 7: A magnifying look at the microscopic level inside of the diode.

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As for the N-type material, the situation is similar except opposite. Instead the doping is done using Phosphorus, which results in an extra roaming electron instead of a positive hole. The electron will roam, filling in onto rogue positive holes, or making their way across the depletion region, searching for their electromagnetic counterparts in the anode.

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This difference between electrical charges creates a higher incentive for conducting current in proportion to the amount of voltage applied. This means the concentration of Boron in the anode or the concentration of Phosphorus in the cathode has an impact on how easily the diodes conduct current.

Conducting a higher amount of current across the diode than it can handle with ease, results in a higher amount of electron collisions. These collisions will have a higher amount of energy, dependant on the power applied across the diode. When all this energy is exchanged with each other in such a small and contrived space, the energy the diode is not able to process is emitted out as thermal energy. This means that overloading the diode in a forward bias will effectively heat up the diode. [11]

5.5 Depletion region

In order to easier explain the operation of reverse bias, and the temperature effect on the diode, central aspects of the depletion region is assessed.

The depletion region, in between the anode and the cathode, is an isolated region which counteracts current from traveling through it. The larger the depletion region, the more difficult it is for charge carriers to move across the gap and induce current. The depletion region width decreases when voltage is applied across it in the forward bias, which means that it will be easier for charge carriers to travel across. This situation is shown in Figure 6. This results in a current which flows with more intensity proportional to the voltage applied to the diode. [9]

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carriers are moved away from the depletion region and the width of the region increases.

This is shown in Figure 8 & 9. Without the presence of charge carriers, there barely flows a current through the region. This occurs because it is very difficult for electrons to consistently travel from N-type to P-type electrode. This means the PN-junction is a semiconductor, only conducting current in certain instances. [7]

The depletion region will widen when the temperature of the diode decreases, and shrink when the temperature increases. This makes it consequently, easier for the current to conduct at higher temperatures and harder at lower temperatures.

The correlation is found from the equation giving the real open circuit voltage V_F. The current across the diode is inversely proportional to the temperature.[12]

5.6 Reverse bias

In an array of solar panels, there is a certain direction the current is supposed to go. This direction is defined by the negative and positive terminal on either side of the panel array.

When the current is applied through forward bias there is generated heat in the panels.

When there instead is applied current through the reverse bias, there is also generated heat, but not the same way as in forward bias. When operating a PN-junction diode in reverse bias, the positive side of the battery is coupled with the N-type material and the negative is coupled to the P-type. [7]

Using this direction of current bias for heat generation is experimental, but might have potential. The central points on how it will work and the risks involved will be explained further.

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Figure 8: Illustration of the PN junction diode with a low reverse current applied.

When reverse bias has been applied to the semiconductor, the external power source is coupled with its negative terminal into the P-type material, and the positive terminal is coupled with the N-type material. This negativity onto the P-type material will pull the positive holes away from the junction and towards the negative external source. The same will happen at the N-type material, where the electrons will move away from the junction, and towards the positive external source. This creates a situation where the pools of charge carriers are moved further away from each other, and the consequence is a wider depletion region. The higher the external voltage applied is, the stronger the terminal pull will be, and the depletion width will become even larger. [10]

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Figure 9: Illustration of the PN junction diode with a larger reverse current applied.

5.7 Heating in reverse bias

From the previous paragraph there doesn’t seem like there is any point to applying reverse bias voltage on to a diode. This is because it seems reverse voltage only lowers the chance for charge carriers to pass. This is not the case. There are two central ideas of how the reverse bias operation can heat up the panel.

5.7.1 Reverse bias pre breakdown

In the pink area of Figure 10, the diode operates in reverse bias, but the voltage applied is not enough to break down the diode. Still, it conducts a certain current in this bias.

If this current is large enough to heat up the diodes is yet to be seen. From the graph one can see that the current induced by this method of operation will provide a small amount of current. [7]

At this point, as illustrated in Figure 9, the depletion region tries to stop any charge carrier from crossing the barrier. At either pole there is a large amount of charge carriers amassing, neither interested in being close to the depletion region. But as the energy builds up, there will always be some charge carriers who find themselves between the border of electrode and depletion region. When the concentration of charge carriers are so high in such contrived spaces, some of the charge carriers are bound to break out from the sheer potential of energy, and find themselves closer to the depletion region. [10]

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The moment the charge carrier touches the depletion region it will be swept across to the other side due to the polarity. This exchange of charge carriers eventually becomes a current. The current which flows in the diode in this instance, is the so called drift- current. [11]

5.7.2 Reverse bias post breakdown

In the yellow area of Figure 10, the diode has broken down, and is conducting a large current. This large current would certainly be high enough to heat up the diodes. The breakdown is achieved when the electrons moving across the depletion region as a drift- current, has the energy to knock off other stable electrons inside the depletion region.

[13]

Figure 10: The voltage profile of a PN junction diode.[8]

When the applied reverse voltage reaches the breakdown voltage limit, the electrical field inside of the depletion region is very wide and strong. This electrical field causes a few free electrons to be slung towards the N-type material in a flow of drift current. In the process of moving through the depletion region from the P-type material to the N-type, the electrons will collide with several other ionized atoms on the way, knocking out several other electrons. This is called the avalanche effect, and is illustrated in Figure 11.

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Figure 11: Illustration of the avalanche effect.

This effect further reinforces the huge amount of high energy electrons at the N-type.

The free charge carriers are forced into a tight space when the depletion region widens.

This results in a situation inside the electrodes, where it is quite chaotic, with electrons moving at high speeds with high energy potential. This causes the high speed electrons in the diode to crash into and knock free already established covalent-connections between the silisium-atoms in the N-type as well as in the depletion region. These knocked off electrons, along with the original electron which has knocked off an electron and is bouncing around to its next target will knock off more electrons, yet again, reinforcing the avalanche effect. [13]

At the breakdown voltage, it means that all of the covalent bonds in between the Silicon atoms are breaking, due to the heavy avalanche effect put on the diode. When the breakdown voltage is met, or the voltage is even higher, the diode starts conducting a large amount of current in its opposite direction. Because of the high flowing current and the many collisions happening when the diode is in its breakdown region, the diode will generate a lot of heat as a product of the high energy electrons colliding.[13]

There are central concerns about how this breakdown will affect the diode short and long term. First of all the voltage to achieve this breakdown isV_BR =−50V at 25°C according

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to 10. If this is the amount of voltage required to breakdown several cells in series, the experiment can be conducted, since a 50V power source is not that dangerous. But if there is required 50V for each cell, which seems likely, applying a voltage of 36*50V = 1800V, seems unrealistic, and common sense advocates for caution.

The diode might incinerate or be permanently destroyed. Whether these are legitimate concerns or if the diode can handle it is yet to be researched, and will along with the pre-breakdown reverse bias experiments be conducted in the spring semester if deemed viable.

5.8 Power supply

There is not many complicated demands in order to make the power supply running as intended. The power supply which is being forced through the panel in the reverse direction has to deliver DC current. If the power supply operates with an AC current it has to be converted into DC current before it is fed into the panel.

As already established, the general threshold for conducting current through the diodes in the panel are 0.7V for Silicone diodes at 25°C per cell. In reality, these cells will start conducting later than 0.7V/cell due to temperature being lower than 25°C. Exactly when, is discussed in the Electrical supply subsection of the Method-section.

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5.9 Gravitational force from the snow

Throughout the testing conducted with snow, a lot of knowledge has been gained on the dynamics and the relationships between the initial parameters. In the following sections the different mechanisms which has an effect on the outcome will be accounted for and explored.

When the avalanche effect occurs, there is reached a critical point where the gravitational pull is stronger than the resistance from the panel. The gravitational force downwards, p ulls the snow off the panel if the requirements for this critical point is reached. The principle of the gravitational pull of the snow is illustrated in Figure 12.[14]

Figure 12: Sliding object off an incline.[14]

In theory, the only parameters which effect the snow sliding or not, are

• Mass of the object, m.

• The angle of incline, θ.

• The coefficient of friction, µ.

• Normal force from the surface on the object, N.

These will determine if the snow on top of the panel slides, or not.

The weight of the snow would be calculated by m∗g, and the force towards the edge of the panel, encouraging the avalanche effect, is F_slide = mg∗sin(θ). Once this value is

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higher than the resistance holding the snow up on the panel, the snow slides.[15]

In conclusion: F_total ≤F_slide induces an avalanche.

5.10 Different types of snow

From the last paragraphs, the parametersm,θ,µ,N, are central to the sliding function.

Two of these are related to the object on top of the panel. Different snow types have different properties which directly affect the friction and the weight of the snow. In this section some nuances around which difficulties or opportunities different snow types can offer will be explored.

Since the properties of snow will primarily affect the weight and the friction, this will be the focus of the differences between the snow. This is because the coverage of all the nuances of different snow types is not relevant to this rapport.

5.10.1 Weight of the snow

In terms of weight, one can differentiate between them by their density. Snow with a higher density will naturally have a higher weight in relation to the amount of snow in volume which is situated on top of the panel. The density of the snow on top of the panel is determined by a number of factors. Table 1 is an overview of how the density is affected by time.

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Table 1: A table describing the snow density over time.[16]

There is not really realistic to achieve densities above 550 kgm⁻³, since the snow would for the absolute longest, stay on top of the panels for 3-5 months across winter, to be melted away in the spring. This table is included more to illustrate to point that the longer the snow stays, the easier it is to melt it off. This results in a situation where the longer the heating system is inactive, the easier, and more profitable, it becomes to melt the snow off.

The density is also affected by the ambient temperature. The higher the temperature is during snowfall, the higher the density of the newly fallen snow will be. This is due to the increased water contents at higher temperatures. The ambient temperature will also affect already fallen snow in the direction of higher density. Again, this is due to the temperature increasing the amount of fluid water in the mass of snow relative to frozen ice crystals.[15]

The wind is another factor which impact the density of the snow. If the snow falls during

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times of small magnitude winds, such as below 10 m/s wind speeds, the density of the snow will typically be low. In contrast, higher wind speeds make higher density snow.

This is because a stronger wind would force the individual snow crystals further into the surface snow once it hits the ground, increasing the compactness. [15]

The density of the snow is important because the weight of the snow is the main force behind the sliding effect. The higher the density of the snow is, the higher the weight, which means larger the force.

A higher density snow will also lead to a higher insulation effect due to the compactness of the snow. The pockets of air inside of a compact block of snow, contain air which is warmer than the air outside the block of snow, given there is a source of heat below it.

This is called the igloo-effect.

5.10.2 Friction of snow

To find out how much of an effect the materials used constructing the solar panel has on the sliding, the friction coefficients has been found. Wet snow has been used consistently, because once the melting process has started, the snow will become wet as the surface towards the panel will heat up and melt the innermost layer. The snow in direct contact with the panel will be assumed to always be wet, once the operation has started.

• Wet snow on aluminium at 0 degree Celsius: µ= 0.4. [17]

• Wet snow on glass at 0 degree Celsius: µ≤0.03. [18]

One can tell that the friction coefficient of aluminium is much higher than the friction coefficient of glass, with regards to wet snow. A decrease of the friction force in the system could help the sliding effect initiate easier. A logic area for seeking improvement would be the aluminium frame, which could be replaced with something, which has a lower friction coefficient.

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5.11 Panel edge effect

The edge applies some unknown magnitude of force opposite sliding direction. This will from now on be defined as−→

F⁰. In Figure 14, the edge is modeled as it is post-production, a stump half circle with a gap beneath it. The stump edge is around 1mm above the glass, as in Figure 13.

Figure 13: The stumpness of the edge.

This value, −→

F⁰ is not known. −→

F is the frictional force enacted on the snow from the surfaces parallel to the panel itself. This includes the glass and the aluminium frame which lies in parallel. −→

F is much smaller than −→

F⁰, because glass and aluminium has a low relative friction as expanded upon in the friction section.

In Figure 14, the snow and the panel, along with the gravitational and frictional forces which affect it, is drawn.

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Figure 14: Snow on the panel and the edge holding it back.

There is known from the gravitational pull section, that when N_x = F_slide = mg ∗ sin(θ)< Ftotal =Fx+F_x⁰, the snow will slide downwards. IfNx =Fslide =Ftotal=Fx+F_x⁰, the snow will not slide. θ is the angle of which the panel is tilted up against the surface.

In Figure 15, the forces are added together, and the last arrow, −→

S, is the net force. The snow will move in the direction of the net force.

Figure 15: The net force from the system in Figure 14.

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Figure 16: How a different edge can affect the situation.

The forces are drawn again in this figure. The edge in this figure is modified to be more alike a ramp, to provide incentive for the avalanche effect to occur. −→

F is not necessarily equal to the case in Figure 14, but close. More importantly to the final net force, −→

F⁰ has a different direction.

In Figure 17 the net force is again calculated.

Figure 17: Net force of the system in Figure 16.

In this case the net force has a significantly stronger force pushing the snow downwards the panel. This means that a more optimized edge would help the snow slide faster.

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Unfortunately there was not possible to make an optimized ramp to latch on the edge.

Rather, there was applied tape to create a less optimized ramp. This is shown in Figure 18.

Figure 18: How the edge looks like in the conducted experiments.

Still, the new edge creates an improvement in the slide initiative. This decrease in total friction in the slide direction, means that the snow will slide off the panel at a faster rate, as well as more of it will fall off. Exactly how much will be discussed after the initial results have been presented. In Figure 19, how the tape looks like in reality is shown. As one can tell, it is equal to the representation in Figure 18.

−→

F⁰⁰ represents the friction enacted on the snow from the surfaces parallel to the panel, but touches the duct tape rather than the glass or the aluminium frame. −→

F⁰⁰ is different than −→

F.

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5.12 Panel heating without applied current

The entire principle of the heating mechanism which is being utilized in this project is based on the fact that current throughout the diodes are producing heat. But this is not only limited to applied power. Once the panel is in power generation mode there is also flowing current throughout the diodes, which is also producing heat. This means in practice that once the panel is receiving solar rays from the sun, it is currently self sustainable in terms of heating.

Obviously, this is no use as long as the panel is covered up with snow, since no solar rays will reach the panel. But it means that once the snow is removed, and there is a steady supply of sun exposed to the panel, it will keep its relatively high temperature, becoming resistant to small amounts of snowfall during sunny days.

This is of course a small positive to add to the list of features which can be taken advantage off. But since the profitability of this system is important, there is an incentive to further take as much advantage of this mechanism as possible. During the course of the experiments conducted with snow there was discovered that not all of the snow leaves the panel at the avalanche critical point.

Even though there was current applied to the panel, the scarce amounts of snow left on the panel did not have the weight to start another avalanche effect for a long time. There was observed that it would rather melt off than slide off, due to the lightweight nature of the remaining snow. During the course of this melting, there is produced large amounts of heat, across the entire panel. Even though there is only snow present on the 5-10%

bottom part of the panel, the entire panel is being heated to remove this snow.

This is as one can imagine, not very energy efficient. This is where it is profitable to use the self heating properties of the panel to remove it. If the initial avalanche effect is enough to remove a significant part of the snow, the remaining snow can be removed by the panel itself provided there is sunny weather. Therefore the only power which has to be applied to the panel is the amount required to create the initial avalanche effect.

Provided the melting process has been started on such a day that there is a consistent

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flow of solar rays onto the panels, one can expect a completely clear panel by the end of the day.

In the event that the bottom string has one or more cells completely covered in the leftover snow, that string will not conduct current, and not participate in supplying the panel with self-generated heat. However, the transfer of thermal energy throughout the glass, especially if the operation is executed while the current ambient temperature is above zero degree Celsius, will gradually remove the snow on the bottom string. Once either of the cells are even barely uncovered, this area will gather heat from the solar rays, since the diodes are black, the heat will be absorbed and further help the removal of snow.

Once all of the cells in the bottom string have at least 1% of their diode receiving solar rays, this string will further start producing heat due to the current flow, even below the areas of snow, which are not yet uncovered. This results in the panel becoming basically self sustaining after the initial avalanche.

5.13 Horizontal vs vertical orientation of the solar panels

Below in Figure 20 & 21, is an illustration of the panels. To understand why the orientation of the panel makes any difference to the process of melting the snow the strings have been drawn onto the panel.

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Figure 21: The panel at a horizontal orientation.

Both figures shows the panel when it is clear of snow, and when it has some snow on the bottom. The second instance is representative of the situation after the initial avalanche effect. There was consistently a certain amount of snow left on the bottom of the panel after the initial avalanche. In these figures, it is assumed the amount of snow left on the panel after initial avalanche, is equal across both orientations.

There are two central aspects which differentiate the orientations from each other; the amount of edge effect keeping the snow up, and the post avalanche snow melting.

5.13.1 Less edge effect

Assuming the amount of snow on top of the panel are equal for both instances. It is already concluded that the edge effect of the panel plays a central role in keeping the snow on top of the panel. Once the total weight of the snow is distributed and held back by a lesser edge, the snow will slide easier.

• Edge length horizontal orientation = 100.5 cm.

• Edge length vertical orientation = 52 cm.

As already concluded, R_edge is not known, but it is not necessary. Without it, one can still illustrate the point of minimizing sliding resistance, by minimizing edge area. Again, looking at Figure 22.

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Figure 22: Snow on the panel and the edge holding it back.

The force of resistance is here represented in 2D. But as the length of the edge in focus is shortened, so also is the total resistance.

• Edge resistance horizontal orientation = 100.5cm∗R_edge.

• Edge resistance vertical orientation = 52cm∗R_edge.

Eventually leading to the conclusion that

Rtotal−vertical = _100.5⁵² ∗Rtotal−horizontal = 0.517∗Rtotal−horizontal

With this relationship being the way it is, it is also reasonably to conclude that the orientation with the least resistance is the orientation which encourages the avalanche effect the most. This is, as explained, the vertical orientation.

5.13.2 Post avalanche snow melting

Moving away from the effect of the edge and towards the heat production of the panel, the results are equal as long as the panel is covered in snow.

The panel has a net amount of heat produced through it’s string onto the surface of the snow, and where the heat is applied is universal across the numerous diodes. Therefore the orientation of the panel, whether vertical or horizontal does not have any effect on a

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the case is different once the initial avalanche effect has commenced.

Figure 23: The panel at a vertical orientation.

The central difference between the orientations here, is the difference in what happens after the initial slide.

There are two different values which are key;

• An unknown amount of power generated by a partly clear panel, P_g.

• The power applied on the remaining layer of snow, for melting, P_m.

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To illustrate the point, there is for this specific section assumed the initial avalanche effect removes the same amount of snow both on vertical and horizontal orientation, even if not the case in reality. The amount of snow left on panels used to illustrate the point are fictional, but close to reality.

Even though the total area covered by the snow after the initial avalanche effect might be equal for both panel orientations, the effects after the avalanche is not. This is due to how the strings are configured in the panel. Each string is made up of cells in a series.

The shading of cells inside strings has the following nuances:

• If one or more cell is completely shaded, the entire string conducts no current.

• If one or more cell is partly shaded, the entire string conducts a current inversely proportional to the shaded area of the cell which is most covered.

This means that if just one entire cell in the string is buried beneath snow, this specific string will not be able to self sustain the heat generation since it does not conduct current.

If one or several cells are only partly shaded, the panel will still be able to produce current through the flow of solar rays.[19]

This is important because there is assumed that there is the same amount of snow left on the panel after the avalanche effect, the chance of single cells being covered is larger when the panel is orientated vertically. Looking at the previous examples in Figure 20 & 21.

The same amount of snow, which covers the entire bottom cell row across the bottom of a vertically orientated panel, might only cover 90% of the bottom row on a horizontally orientated panel. In this scenario, a horizontally orientated panel would produce 100%

of capacity on its upper string, while producing 10% of of capacity on its lower string.

The snow on the lower string would slowly but surely melt, and the production would rise with it.[19]

In the event that only one of the strings in a horizontal orientation conduct current, some

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Now considering another scenario, where there is a little less snow left after the avalanche effect. The orientated panels now look like illustrated in Figure 25 & 26. The assumption of same amount of snow still applies.

In this scenario, the horizontal orientated panel has a lesser amount of its lowest row of cells covered by snow. This results in the bottom string operating at 50% of capacity, melting the snow at a faster rate than in the previous scenario, while the upper string produces at 100% capacity. Power faced towards melting the remaining snow in this scenario is 50% of the capacity of 1 string.

Figure 26: The panel at a portrait orientation.

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In the vertical orientated case, both strings has 80% of their bottom cells covered by snow. Both strings will in this case operate at 20% capacity, slowly melting the snow.

Power faced towards melting the remaining snow in this scenario is 20% of the capacity of 2 strings.

This is where the comparisons between the orientations become interesting. Because now there is a question over which panel is clear of snow the quickest, and if the continuous production of the upper string in the horizontal orientated panel makes up for the fact that there is a larger amount of power towards melting the snow in the vertical orientated panel.

This will be covered in the Recap and Reflections-section.

5.14 Panel ice formation

During the course of the experiments conducted at sub-zero temperatures there was a consistent issue of ice forming at the areas of the panel which were not heated. This was an issue at all sub-zero temperatures. The effect is shown both in drawing and in picture in Figure 27, 28 & 29. In Figure 27 the picture is taken far into the experiment at a sub-zero temperature. The snow load was originally large, but has melted away. The snow never started sliding. The added resistance from the ice particles at the bottom of the panel makes sure the snow stays at the panel. [20]

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In Figure 28 the central problem areas are highlighted to show exactly where the problem lies. Even though the diode itself is hot, and there is forming a layer of water beneath the snow to encourage sliding, some of the water pours downward the panel. This water refreezes in areas further down.

Figure 28: Where the water turns to ice.

Figure 29 shows how the bottom ice layer forms on this area of the panel. This layer of ice is cold enough that water particles freeze on top of the ice again, binding the snow crystals above it to the glass and the frame, rather than lowering the resistance for the avalanche effect to happen. This is an issue, solutions must be sought.

Figure 29: How the ice is keeping the snow back.

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6 Prototype

6.1 Components

The system is composed of three components:

• The construction.

• The panel.

• The electrical supply.

6.1.1 Construction

The construction is placed on top of a pallet to provide a stable base and an easy way to dispose of the water which will melt and gather below the panel. Aluminium rails are used to create the structure.

The dimensions of the structure is:

• Vertical rail length L_v = 60cm.

• Horizontal rail length L_h = 70cm.

• Diagonal rail length Ld= 62cm.

Thanks to the number of slots in the rails, the angle tilt of the panel can be easily changed, by using screws and bolts. The final construction is displayed in Figure 30 & 31.

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Figure 30: The construction. Figure 31: The construction.

6.1.2 Panel

There is acquired a 90W panel from Sunwind(Mod nr: GS90W) for the experiments. The panel has the following general data:

• Type of cell: Mono crystalline A-cells.

• Effect: 90W.

• Cells: 18 split cells, total 36 cells.

• Effect tolerance: +/- 5%.

• Cable: 90 cm w/ MC4 cable.

• Current produced: 5.4A.

• Dimensions: 100.5 x 52 x 3.5 cm.

• Weight: 5.77 kg.

The panel originally consists of 18 square cells. But by having a gap in the middle of the square cell where there is no contact, the panel doubles the amount of cells it consists of.

In Figure 32 the picture of a single cell is inserted. The red rectangle indicates where there wouldn’t be a gap if these cells weren’t split. This is the reason for the cells to behave as 36, and not 18. This is very important information as the voltage profile changes when the resistance increases. Resistance increases with the amount of cells, as they are serial coupled together.

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Figure 32: A single cell in the panel, split.

There is used 2 bypass-diodes in this panel, to minimize losses by allowing flowing current if one half of the panel is shaded or in this case buried in snow. How this is installed is captured by photo and shown in Figure 33. This means that if the snow only slides halfway off, one half of the panel will start generating power on its own.

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6.1.3 Electrical supply

In order to inject power into the panel, one would need a stable source of DC current.

This is no issue since the laboratory has easy power access and AC-DC converters which can make sure that the power injected into the panel is of desired magnitude and form.

To be able to connect the power supply to the panel there has to be spliced cables with the MC4 contacts. Once it is possible to connect it to the panel, power can be applied to induce heating.

The data sheet attached to the panels tells a lot about how the panel will behave. The data sheet is photographed in Figure 34.

Figure 34: The I-V curve for the specific solar panel.

From the data sheet the real threshold voltage for current flow is found.

Recognizing snow on a solar module and melting it, prototyping and profitability

Thor Thinius Tuv

Recognizing snow on a solar module and melting it, prototyping and

profitability

Master ’s thesis

Thor Thinius Tuv

Recognizing snow on a solar module and melting it, prototyping and

profitability

1 Abstract

2 Sammendrag

3 Preface

Contents

List of Figures

List of Tables

Nomenclature

4 Introduction

5 Theory

6 Prototype