Prepared by DEVELOPMENT OF HYBRID SOLAR SYSTEM

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DEVELOPMENT OF HYBRID SOLAR SYSTEM

Prepared by

Muhammad Irfan shafi (19830526-T151) Md.Maidur Rahman Talukder (840707-3553)

February 2013

Programme

Supervisor: Björn.O.Karlsson Examiner: Taghi Karimipanah

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i Abstract

Technology replaces newer technology with improved efficiency. Solar technology is going to draw out a new life to make a green change in the terms of energy. As a result energy from the sunlight is being changed into electric energy by using solar cell. But still its efficiency could not be able to make a sense as a depending energy technology. In order to look up the solution, solar technology is changing rapidly to get maximum output. To take up this new challenge solar technology is trying to change its building component that are used to make solar cell, for example solar cell material, bypass diode system, blocking diode system etc.

Now-a-days, solar energy system is designed as a hybrid system that can make electricity and hot water at the same time. In the hybrid solar system, photovoltaic and solar thermal systems are integrated at the same system and as a result heat and electricity are produced simultaneously at the same area. Solar cells are attached with both top and the bottom side of the module and the collectors are set up inside the module. By using collector inside the module, re- jected heat from the solar cell is absorbed by the water that flows through the collectors. But a problem arises at the midday or after midday because the reflector of this system cannot reflect sunlight properly on the bottom side of the module. That’s why shading is occurred on the bottom side which reduce the total electrical output of this system.

To work out this shading problem, a bypass diode is connected in parallel with the group of solar cells. Schottky diodes are being used as bypass diodes inside in the most of the solar cells. Schottky diode forward voltage drop is almost 0.45 Volt which is an important cause of reducing the output power as well as the efficiency of this hybrid system. To solve this problem, new lossless diode is attached inside the hybrid solar system instead of schottky diode which can work with a very low forward voltage drop roughly 50mV at 10amp.

To make a comparison between the performance of PVT system with the schottky diode and the new lossless diode, many data has been collected from the outdoor test. After getting the output result, it is clear that the output power and efficiency is going to be changed for using the new lossless diode. For using the lossless diode, the efficiency of the bottom side of the module was increased by 0.31 %.

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ii Acknowledgement

At first, we would like to thank Almighty ALLAH who gave us strength and courage to complete this project.

We would like to express our deepest appreciation to our supervisor Bjorn.O.Karlsson, Pro- fessor, Faculty of Engineering and sustainable development, University of Gävle Sweden for his time and guidance in this project. Also we have the honor to express our heartfelt thanks to Stefan Larsson CTO of Solarus AB. We also thank Björn and Stefan for choosing us for the topic and to provide necessary papers and references and finally for helping us to complete this thesis paper successfully.

Prof. Bjorn.O.karlsson and Stefan Larsson ceaselessly assisted us on this contrives uncondi- tionally and also gave valuable propositions. It was practically impossible for us to consum- mate this project without their interest, untiring efforts and supervision on technical issues.

Solar engineer Joao Gomes should also be mentioned here whom we like to give special thanks to complete our thesis work properly. It’s true that without the help of Joao Gomes it would have really tough for us in preparing this paper. Also some of technical persons helped us precisely on many occasions. They helped and inspired with great passion and of course by providing information. These excellent people are the employees of “Solarus AB”. We are also thankful to them.

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iii Contents

1. Introduction. 1

2. Objective and outline. 3

3. Background. 4

3.1 Solar Energy. 4

3.2 Photovoltaic (PV) System. 5

3.3 Hybrid Solar system. 8

3.4 Shading on solar panel. 8

3.5 Hot-Spot. 10

3.6 Bypass diode. 11

3.7 Schottky diode. 13

3.8 New lossless diode. 14

3.9 Theoretical explanation of Photovoltaic Cell. 14

4. Methodologies. 19

5. Experiment and Experimental Result. 20

5.1 Experimental set up with Hybrid solar collector. 20

5.2 Bypass diode of Solarus hybrid solar system. 23

5.3 Solar lossless bypass diode from Microsemi. 25

5.4 Comparison result between schottky and lossless diode. 29

5.5 Calculation. 33

5.6 Economic outlook. 37

6. Discussion . 38

7. Conclusion. 39

References. 40

List of figures. 42

List of Tables. 44

Appendix. 45

Symbols and Abbreviations. 49

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1 1. Introduction

With the Photovoltaic technology, Shading is a significant problem to reduce the output power. For many reasons when a part of PV module is shaded then it cannot generate the same current like the non-shaded cell. According to the photovoltaic (PV) module construction design, all cells are connected in series, it is important to generate the same current from each cell. Due to shading the non-shading cells force the shaded cell to pass more current then their short circuit current results in negative voltage cause a net voltage loss to the system. The negative power produced by the shaded cells is given by the current times the negative voltage. On the other hand the shaded cells power dissipation results in heat and produce hot spot on the PV module. A shaded cell downs the overall IV curve of group of cells but of course it depends on how many cells are shaded. To shade one cell 75% is far worse than to shade three cells 25% each. So, we should try to spread the shading over the maximum number of cells, if shading cannot be avoided. [1]

In order to reduce shading effect in a single module, bypass diodes is being used in the junction box. To reduce the voltage losses through the module, bypass diode will give another current flow path around the shaded cells. During shading, bypass diodes becomes forward biased and instigate to conduct current through itself. Now the total amount of current higher than the short circuit current of the shaded cell is bypassed through the diode, in this way it will reduce the heat of the shaded cell area. This bypass diode also maintains the total shaded module or group of cells to a small voltage loss approximately 0.7volts, thus restraining the reduction in PV array output. [1]

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PN junction diode was used to give a bypass path but its forward voltage drop was relatively high. That is why as a second technology that is converted to the schottky diode due to low forward voltage. Traditionally they are axial diodes that snap into a connector in a junction box on the back of the panel. The schottky diode forward voltage drop is almost 0.5 V which is still an important cause of power reduction. In order to solve this problem, technology is again changing to reach a point where forward voltage drop for the bypass diode will be very low to increase the overall efficiency. [2]

In this project we are trying to use a new lossless diode technology that is produced by the Microsemi Company. According to their specification its forward voltage drop is almost 0.4mV and has a negligible temperature rise at 10 ampere. Due to low temperature rising system it can work with more reliability and can give a high longevity. With this new technology, photovoltaic efficiency will reach a new door. In order to look through the difference between normal schottky diode and the lossless diode here we use a solar hybrid collector from a Swedish Solar company (Solarus AB).

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3 2. Objective and outline

The main objective of this thesis project is to compare the performance of PVT module with the schottky diode and lossless in terms of output power and the efficiency. Also, this project will present the amount of power that is saved by using the new lossless diode. To write this paper all data is collected by using a solar hybrid collector, IV curve logger, Melacs control device. In order to get a comparisons result between schottky diode and the lossless diode, all data is collected from the outfield at the place of Älvkarleö (small city inside Sweden) during the summer time.

Thesis project is organized as follows:

3: Background of this thesis project and mentioning of the new technology that is changing with time.

4: Methods those are used to reach the goal of our research oriented thesis.

5: Making an experiment and presenting the experimental result that can give an exact solution for this work.

6: Discussion and concluding the best results.

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4 3. Background

3.1 Solar Energy

Because of a many human activities, the deliberation of greenhouse gases is increasing day by day. For using the fossil fuels, depletion of ozone and vanishing of large areas of forest is going to be a big reason to increase the earth temperature; this condition will make an uncer- tain future for the coming generation. To solve this problem solar energy is becoming a good solution as a clean energy, bottomless resource that is easy to install, has an unlimited life as well as may be used to both rural and city environment without difficulty.

Solar energy is the energy that imitative from the sun as the form of solar radiation. Among the total amount of solar radiation, only the half can reach on the earth surface and the rest amount of solar radiation is absorbed by the surrounding atmosphere. Actually solar energy is the key cause for the all of the energy sources in earth like fossil fuel, tidal, wind and the geo- thermal energy. Moreover it produces electricity by using the photovoltaic panel. [3]

Normally the solar energy consigns to utilize solar radiation by humans, and is frequently use interchangeably with solar power.

Fig 2.Incoming solar radiation on Atmosphere and Earth surface [4]

A major amount of sun radiation is reflected by clouds, dust, snows or any reflective surface.

The amount of reflective solar radiation is more than the total present fossil fuel in the earth.

Solar energy production includes quite a lot of power sources, mutually active and passive.

Depends on the selection, it is important to distinguish among the different types of solar energy production systems.

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Two different ways are being used to make solar power, one is the direct transmission from solar energy to current by using the PV panel and the other is solar concentration system to produce steam in order to drive turbine.

Now-a-days solar energy technologies are well developed and improved. Different types of solar technology are being used for the different function. By understanding the difference along with the existing solar energy systems, it can be use more efficiently, cost effectively and environmentally. By evaluating the efficiency, functionality and the economical feature of the accessible systems and product we can get an efficient utilization.

3.2 Photovoltaic (PV) System

Photovoltaic system is the most direct way to make the electricity from solar radiation by using photoelectric effect. This PV technology produces direct current from the semiconductor material when they are light up by Photon and the generated current is the direct propor- tional with the photons that stick on the semiconductor material.

The atoms construct a lattice inside a crystal of pure silicon. Silicon’s atoms have nuclease that contain photon (Positive charge) and electron (Negatively charge) in the shells. The amount of electron in the valance band (outer cell) is not complete that’s why adjacent atoms share electrons and grip each other together in the crystal.

The pure silicon atoms may be doped with a small amount of impurity and this doping material can be containing more electrons in the valance bond compare than the pure silicon. Then the negatively charged electron can move easily around then it is called n-type silicon. This doping material can carry out electricity better than the pure silicon. On the other hand if the pure silicon is doped with a material that has few electrons in the valance bond then there will be a shortage of electron then it’s called P-type silicon. The tiny areas where electrons are well missing are called holes that can as well move around.

Fig 4.Silicon wafer of solar cell [5]

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In photovoltaic cell n-type and p-type silicon will be connected together. Due to attraction of holes, electron will start moving across from n-type to p-type silicon. The junction area will work as a barrier and discontinuing more electrons moving across n-type to p-type that create an electric field across the junction. When the sun light is absorbed by this photovoltaic cell then this energy will drive electrons across the junction. [5]

Fig 5.Solar cell working principle [5]

The efficiency may be 15-20% for this type of cell because silicon wafers cannot absorb all light energy. Now-a-days a new type of complicated cell is being used to make the PV panel that have multi junction and that can be able to absorb different light energy. [5]

Basically most of the photovoltaic devices are integrated by the PN junction in the semiconductor. All photovoltaic cell are constructed by two wafers of doped silicon material one is n- type and another is p-type that can make a junction. These two wafers are connected with electrical connector. Usually photovoltaic cell is incredibly thin that is roughly 100mm di- agonally. Individually each cell can produce around 0.7 volts and the maximum power being generated at a voltage of roughly 0.4 volts. To make a module, some of this cell will be connected in series and make a panel in order to protect from weather. In order to produce 24 volt panel 72 cell may be wired series within it and will give the peak power 28.8 volts. Now these series connected cell are placed on the backing plate and the electrical connection wire are lies above the plate and below the cells. In order to increase the light absorption, a non- reflective layer will be on the top side of the cell. Lastly, a tough glass will be set on top and whole structure will be assembled by an aluminium Frame. [6]

Fig 6. Design of a solar cell [6]

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Fig 7.Complete solar cell [6]

Figure 7 shows silver accomplished strips printed to the upper surface of the cell and these conducting strips are going from the top of one cell to the bottom of the next cell. [6]

In order to use it in practical purpose this photovoltaic panel is enclosed into module including either a number of cell which actually it depends on the application. To Keep up with the demand it is connected in series or parallel, series connection are provided more voltage and the parallel connection always provide the high current, commercially most of the time design is constructed in series in order to decrease the system losses because of two reasons, one is the solar cell protection from ambient and another is, it gives higher voltage then a single cell. Moreover module can give a good structure that may be good for transportation and maintenance. [7]

There are two approaches for utilizing Photovoltaic system: Standalone system and Grid- interactive system.

Standalone system:-To use in the night or cloudy time here a battery is in order to store power.

Grid-interactive system:-Power is used from the utility supplier and the time when the power is surplus then supplies back to the central utility. [8]

3.3 Hybrid Solar System.

The key disadvantage of PV system is its high initial cost and the limited output electricity with compared to input solar light intensity. PV panel can absorb 80% of solar irradiation.

Depending on the PV technology only 5-20% incident energy is converted into electricity and the remaining energy is converted into heat that is the one important reason to decrease the efficiency of photovoltaic system. Temperature of the photovoltaic system is depends on the module design but this adverse effect can be decreased partially by heat extraction with water. [10]

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8 Fig 8. Hybrid solar System [9]

This heat can be used for another purpose like hot water or house heating or other application. After including this heat extraction system it will be construct as a solar hybrid system.

Where PV module and the thermal module may be considered together and it can produce heat and electricity at the same time. This Hybrid system will generate high energy output moreover it will be cost effective then separate PV electricity and thermal system. [11]

3.4 Shading on Solar panel

In order to get good output power by PV panel, position is a very important thing. Solar panel should be tilted in the right position, it should be faced to the sun at day time while the sun is peak in the sky and the important contemplation thing is shading.

Fig 9.Shading on solar panel

Basically two types of shading exist in a system one is permanent shading and the other is partial shading. Permanent means that the surface never receives direct solar radiation. On the other hand partial shading is created by an object that obstructs the sun light intensity. The part of shading may be passed through the array as the sun passes overhead. This partial shading has a big impact on the solar panel. It is clear that it is good to avoid the shading al- together but sometime it is not possible. If only one cell of a solar panel is shaded the efficiency of the photovoltaic cell will decrease [12]. When a small part of the module is shaded then total output of the module is decreased or turns to zero power dramatically. Photovoltaic panel are usually series-connected strings of cell. The most shade cell will limit current.

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Fig 12.Partial-cell shading that can reduce PV module power half [13]

In the photovoltaic cell when one cell is shading then it cannot generate the same current like other cell that are connected in series as a result the un shaded cell will give a force to the shaded cell to pass more current The shaded cell can work with this high current than their short circuit current only in a region of negative voltage that is the main voltage loss of the PV panel. [14]

3.5 Hot-Spot

In the PV panel, hot spot occurs when a cell in a string of series connected cells is negatively biased produce heat. Basically it occurs when a single cell produced less current than the string current that’s why localized heating will happen the current flow through each cell should be the same. [15]

Fig 13. Shaded cell that produce hot spot [16]

This type of unexpected problem is created when the cell is shaded or damaged or just generates lower current then the string current and can cause the micro plasma break down, avalanche break down and structural defects.

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Fig.14. Effect of hot spot on solar panel [16]

In series connection cells work at the same current, during shading cells become reverse biased resulting in power dissipation, which is a cause of heating effects. The reverse biased characteristic is much more extensive and limited by the breakdown voltage. The short circuit current of a cell is less than the string current if the cell is shaded, so that it is operated at the reverse characteristic, causing power to be degenerated [17].

The amount of hot-spot heating of solar cells is almost same to the properties of the semiconductor material. Locally concentrated shunt defects are resulted from non-uniformity. The amount of defects is associated with the slope of the reverse IV-characteristic [17].

The current scatters over the whole cells during low biased voltage and heating occurs, the maximum current density is lower than critical limit, the I-V curve will be steady against thermal effects, for the solar cells the most imperative method in junction breakdown is the avalanche multiplication which is originated from a high electric field in the depletion layer that is produced by the bias voltage. At a definite level of the field strength the generated electron-hole pairs grow enough energy to ionize lattice atoms which again can create charge carrier pairs. Cells do not have a uniform structure, regions with a higher concentration of contamination centers subsist. At high bias voltages these points break down former. If the current density at this point crosses a critical limit the cell is irreversibly dented by thermal breakdown (be exhausted) that forms a shunt path in the cell structure. Now at reverse biased circumstances the current is locally concentrated, focal-point heating is caused and damage to the cell encapsulation is to be expected (hotspot) [18]

The decrease in shunt resistance Rsh has an effect on the I-V slope of a cell in the reverse di- rection that results in high power. [9]

3.6 Bypass diode

This critical effect of hot-spot heating may be solved by using the bypass diode. Inside the PV panel bypass diode is connected in parallel although opposite polarity with the cell. Basi- cally without shading each solar cell is forward biased on the other side bypass diode is reversed biased and work successfully as an open circuit. But for shading if any solar cell is reverse biased creates a short circuit current in the series connected solar cell then the bypass diode is active to pass the current.

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Fig 15.Equivalent circuit of solar cell with bypass diode [19]

According to this equivalent circuit when current (I) is greater than I_Lthen bypass diode is contracting with current I- I_L.The diode prevents the reverse voltage from rising above 0.7V.

Fig 16. I-V curve of solar cell with bypass diode [19]

It is true that one diode for one cell is too much expensive that’s why one diode is attach across a group of cells. The generating capacity of all cells in the group is almost equal to the highest power dissipation in the shaded cell.

In order to prevent the silicon cell damage, basically one bypass diode is used for the 10-15 cells. Generally if a module is constructed with 36 cells then three bypass diode are enough to make sure the module will not be helpless to hot spot damage. [19]

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Fig 17.Bypass mode due to shading [14]

Due to one cell shading, current is pushed in the region of all members of cell’s group. The threshold voltage of the diode is equal to the reverse-biased voltage. That’s why the shading cell is reversed biased to generating capability of other cell as well as the threshold voltage.

Finally it clear that the maximum power degenerate in the shaded cell is equal to the generating capability of all cells in the group. [19]

But still all commercial modules are not designed with bypass diode. For this type of condition it is necessary to ensure that the modules are not short-circuited for long periods as well as the parts of the modules will not be shaded by surrounding constitution or adjacent arrays.

Now a day most of the modules have bypass diodes between strings of cell in order to minimize shading effect and lose the power only in the shading part.

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13 3.7 Schottky Diode

Schottky diode is a special type of semiconductor diode that works with low forward voltage drop. It has some internal resistance to that current flow, that’s the reason why a small voltage drop across the diode terminalswhen current flows through a diode.

Fig 18.Schottky diode schematic symbol

Schottky diode forward voltage drop is around 0.15 volts to 0.45 volts on the other hand it is 0.6-1.7 volts for the normal silicon diode. Due to this low forward voltage drop schottky diode can provide fast switching action and it can increase the system efficiency.

Schottky diode is constructed by metal-semiconductor junction that creates a schottky barrier.

Basically N-type silicon is used as semiconductor that acts as the cathode and the metal part work as an anode of the diode. This schottky barrier can give low forward voltage drop and very fast switching action.

Fig 19. I-V curve for p-n junction diode and schottky diode. [20]

According to the I-V curve comparison figure between PN diode and schottky diode, schottky diode tends to contract at lower forward voltage drop then PN-diode but its rises time is slower than the PN-diode due to the non-ideal factor. On the other hand, schottky diode also provides higher reverse leakage current. The current flow system is different for these two diode, schottky diode is ambitious by thermionic emission that is the overcoming of the po- tential barrier by energetic carriers, where PN-diode current is diffusion driven that is created by carrier concentration differences. [21]

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14 3.8 New Lossless Diode

It is true that the efficiency is a still big question in the photovoltaic technology. To work out this problem photovoltaic technology is altering. To improve this photovoltaic technology lossless bypass diode has an important role as well as the photovoltaic material efficiency.

This lossless diode technology will open a new entrance to increase the photovoltaic efficiency as well as the reliability.

To cope with the shading effect in the photovoltaic technology, in the beginning manufacturer used PN junction diode to give a bypass path. This PN junction diode has a forward voltage drop 0.7 to 1.0 volt and the reverse breakdown voltage was almost 600 volts. This diode could tolerate the heat for the low ampere. But to get more output wafer size increased and the string current increased 5, 6 or more than 8 ampere. That’s why the manufacturer company had changed their technology from PN junction diode to Schottky diode, its forward voltage drop was around 0.5volt that is the half of the PN junction diode and it was good in the heating condition. But unfortunately PN junction diode and the Schottky diode had reverse significant breakdown voltage that is almost 40 to 60 volts. This initiated another problem. On the other hand schottky diodes are absorbent at the high temperature and it can easily be eternally damage by passing energy. Due to fail it may be open then it leave the parallel cell and make a hot spot during the shading period. On the other hand (this term is used for comparisons of two opposite things) due to fail it may be shorted that produce minimum energy.

By-pass diode is going to change the photovoltaic technology. At this moment some re- searchers are going to develop a new type of diode. This new diode is called lossless diode that forward voltage drop is 40 to50 mV that was 0.4 V for the schottky diode. In reverse bias mode this lossless diode has high temperature leakages measured in micro-amperes where the schottky diode was mili-amperes. [2]

3.9 Theoretical explanation of Photovoltaic Cell

The equivalent circuit of the solar cells may be modelled when a current source is parallel with a diode. The solar cells perform like a diode when light is absent to generate any current.

With the increasing of incident light intensity, current is generated by the PV cell. I-V curve of the solar cell given below-

Fig 20.I-V curve of Solar cell and electrical diagram [22]

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According to the Kirchhoff’s current law in an ideal solar cell, the amount of current IL that is generated by photoelectric effect minus the diode current I_Dis equal to the total current I.

I = I L- ID = IL-I0 Where,

I_o=Saturation Current

q =Charge 1.6*10^-19 Coulombs K=1.38*10^-23J/K

T=Temperature (Kelvin) V=Cell voltage

In order to explain detail about the equivalent circuit a new figure shown below where R_S and RSH are going to represent series and shunt resistance. [22]

Fig 21.Equivalent circuit of solar cell [22]

Due to this new model the associate equation will be like- I=I_L-I₀

Where n is the diode ideal factor and the value of n should be between 1 and 2 unit. For the above electrical circuit construction the overall I-V curve of the solar cell drawn in figure 22 below where V_OC is the open circuit voltage and I_SC is the short circuit current and the general performance of solar cell can be determined from this I-V curve:-

Fig 22. I-V curve of solar cell

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16 Short circuit current (I_SC)

When the voltage is zero with the low importance then the short circuit is represent in the term of short circuit condition.

I (when V=0) =ISC

At the maximum current value in the power quadrant and the beginning of the forward-bias sweep is the main cause of I_SC occurs. [22]

I_SC=I_MAX=I_L(Forward bias power quadrant) Open circuit voltage (VOC)

When there is no current passing through the solar cell then it creates the open circuit voltage.

V (When I=0) =VOC

VOC is also represented by the maximum voltage difference across the cell for a forward-bias sweep in the power quadrant. [22]

VOC= V MAX (Forward bias power quadrant)

Generated solar cell power can be calculated by the equation P=VI where power will be zero at I_SC and V_OC points. The maximum value of power will be arising between I_SC and V_OC points. At the maximum power point the maximum current and maximum voltage is represented by IMP and VMP respectively.

Fig 23. Maximum power for PV cell [22]

In order to measure the fundamental quality of solar cell, fill factor is an important term that is symbolized by its abbreviationFF. The fill factor is calculated by the ratio of the maximum power from solar cell to the produce of ISC and VOC. [22]

FF =

=

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Fig 24.Fill factor from the I-V curve [22]

Usually the fill factor vary from 0.5 to 0.82 even though a large fill factor value is desirable as well as the I-V curve should be more square-like.

Efficiency (η)

The efficiency of the solar cell can be measured by ratio between electric output power (Watt), Pout and solar radiation input (Watt) PIN.The Value of POUT is the same of PMAX value because to get maximum efficiency the solar cell can be operated to its maximum power output.

η =

=> ηmax =

The maximum efficiency of the solar panel not only depend on the irradiance of incident light but also like all I-V parameter can also be pretentious by ambient condition like as temperature and the spectrum of the incident light. That’s why it is important to compare the similar light and temperature situation. [22]

Shunt resistance (Rsh) and series resistance (Rs)

The dissipation of power across the Shunt resistance (Rsh) and Series resistance (Rs ) can be an important reason to reduce the efficiency of solar cell. When solar cell is consider as the term of ideal then R_SH should be infinite and R_sshould be zero, at the same time should not provide the alternative path to current flow, ensuing in no future voltage drop before the load.

The relationship between these two parasitic resistances is like- increasing RS and decreasing R_SH will be the causes of decreasing fill factor as well as P_max. [22]

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18 Fig 25. Effect of diverging RS and RSH [22]

Temperature dependence

Semiconductors materials are used to make PV cell that are sensitive to temperature. When the PV cell temperature is increased then V_ocdecrease considerably and I_SC increase slightly as a result decrease the maximum power output P_MAX.

Fig 26. Temperature effect on I-V curve [23]

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19 4. Methodologies

To congregate with a good result this paper was involved by different activities. Since this thesis paper was related with the result of practical aspects that’s why we have done lot of activities in the outdoor. During the time of practical work it was also important to go through the research paper that is directly related without research oriented thesis paper.

Actually this thesis paper was built up by the following approach:- a. Practical approach

 Installing a schottky diode and a new loss less diode inside the module of hybrid solar collector

 Each diode was connected in parallel with the group of solar cells

 Melacs control system was used in order to measure solar radiation, Water input and output temperature and amount of water flow.

 IV loger was used to make an IV curve for the module.

b. Theoretical approach

 Research paper review for the schottky diode and the loss less diode.

 Hybrid solar collector research paper consideration.

 Documentation paper review from Solarus in order to get a clear idea about their hybrid solar system

 Paper review from Micro semi Company.

After using these above approaches it was easy to make a comparison between schottky diode and the lossless diode for the term of output power and efficiency.

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5. Experimental set up and Experimental Result

To establish the comparison between schottky diode and the loss less diode, this experiment was done with a hybrid solar collector that was made by solarus AB, Sweden and the lossless diode from Microsemi Company.

5.1Experimental set up with Hybrid solar collector

In order to produce electricity and heat at the same time, Solarus has developed a solar hybrid collector design. According to their design it can produce electricity and heat by using same surface area. To produce electricity and heat that is really a good solution in order to save the congested roof surface. With comparison to another traditional solar collector system it is more reliable and cost minimizing system due to their two types of sustainable production system. One of the important advantages in this hybrid collector is that, it can operate at its peak electrical output because it can reduce the overheating problem of solar cell. The out coming thermal energy is used for heating, cooling, hot water, water treatment etc. [24]

Fig 27.Solarus Hybrid solar collector [24]

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Technical specification

Structure

Manufacturer :Solarus AB

Brand name :SOLARUS CPC-T

Manufacturer :2368mm*1040mm*235mm

Absorber area :0.68 m2

Aperture area :2.17m2

Gross area :2.40 m2

Weight :30kg

Cells

Number of cells :152

Cell dimension :26.6mm*150mm*200mm

Max power rating(P_max) :Up to 300W Max operating temperature :200C Max power voltage(Vp max) :18.4V Max power current(Ipmax) :14.76V Open circuit voltage(V_oc) :22.8V Short circuit current(Isc) :16.04A Absorber

Material

:Copper tube integrated in aluminium plate

Construction type :Two strips in series

Coating :Nickel selective surface

Peak power :1500 Watt per collector

Capacity :0.364 L/module

Maximum operating pressure :10 bar Stagnation temperature :200C Casing

Material :Anodized aluminium frame

Gross dimension(L*W*H) :1014mm*2368mm*235mm

Sealing material :Silicon

Connection :Copper tube,10mm,2connections

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Fig 28. Double absorber and aluminium reflector [25]

The PV panel output depends upon the angular dependent of the angle of incidence. The sun moves from east to west plane then the angular dependence similar to a flat collector. On the other hand if the sun move in the north-south plane then the angular dependence is quite different that shown in the figure 29. When the angle is greater than the reflector optical axis the efficiency goes down rapidly.

Fig 29.Efficiency at different irradiation angles [25]

The absorber of the Solarus hybrid collector is manufactured in the width of 150mm. The absorber contain the cells on both side and the reflector is used to focus light on to the bottom side solar cell in order to get maximum output by using the same area. This hybrid solar collector system can delivered around 15% electrical energy and the rest of the energy is re- leased as a heat. To use this heat, a tube is used inside the absorber to flow the water that can consume the heat energy in order to get hot water.

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23

12A 100V

100mj

VF at IF=12A 0.58V

Non-repetitive avalanche energy EAS at IAS = 2.0 A, TJ = 25

°C

Primary Specification Maximum average forward rectified current I_F(AV)

Maximum repetitive peak reverse voltage VRRM

Peak forward surge current 10 ms single half sine-wave superimposed on rated load IFSM

200A

5.2 Bypass diode of Solarus hybrid solar system

Fig 30. Solarus CPC-T module with Schottky diode.

The Solarus CPC-T module contains 8 strings that are connected in parallel and each string is constructed with 38 cells. A bypass diode is used for every string that is located in the end section of hybrid collector. This diode is active when the cell cannot produce current or produce less current compare to the other cell due to the shading problem. Basically the bottom side of the Solarus hybrid collector absorber is more shaded due to the gables. The top side of the absorber get the longitudinal light waves that are parallel with the sun radiation and the bottom side of the absorber get transverse light waves that are perpendicular with the sun radiation when the sun moves north-south plane. That’s why it is important to consider the shading problem on the Solarus hybrid solar collector in order to get maximum output.

From the beginning a tradition schottky diode is using inside this solar hybrid collector that forward voltage drop is still a big issue for the term of efficiency. This schottky diode is manufactured by the VISHAY that shown in figure below-

Fig 31.Schottky diode from VISHAY[24]

According to their specification the following are the features for this schottky diode. [23]

 Ideal for automatic placement.

 Very low profile-typical height of 1.1mm

 Trench MOS schottky technology

 Operate with high efficiency

 Low forward voltage drop as well as low power loss

 Halogen-free according to IEC 61249-2-21definition

Table 1: Primary Specification of schottky diode [24]

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24

Table 2. Electrical Characteristics of schottky diode [23]

Fig 32. Maximum forward current duration curve[23]

Fig 33. Forward power loss curve[23]

Breakdown voltage,V_BR I_R=1.0mA I_F= 5 A I_F=12A I_F= 5 A I_F=12A

T_A=125̊ C Test Conditions

T_A=25̊ C

Instantaneous forward voltage,V_F

T_A=25̊ C

T_A=125̊ C

Reverse current,I_R

V_R= 70V T_A=25̊ C T_A=125̊ C V_R= 100V T_A=25̊ C

Electrical characteristics Parameter

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25 Fig 34. Typical Instantaneous forward curves [23]

5.3 Solar lossless bypass diode from Microsemi

Microsemi has developed a new lossless solar bypass diode (LX2400) that forward voltage drop is relatively very low. This diode forward voltage drop is 50mV at 10A that generate 10ºC temperature rise for the coolest operation and it can fully operate from -65ºC to +165ºC.

Moreover in the reverse mode Micro semi diode can block 22V at less than 100 º C leakages current without breakdown. Due to the low forward voltage drop, it produces less heat and can operate with lower temperature that makes the system more reliable and longer life. The reliability and the longer life of this diode can reduce the operational expenses and the warranty cost. For the period of lightning strike, lightning survivability give a bidirectional low impedance path that provide the lowest power dissipation in the LX2400. [24]

Fig 35.Loss less Micro semi Solar bypasses diode [24]

Area of Speciality

 Extremely low operating forward voltage drop, VF=50mV at 10A that enhance the system efficiency and reduce the power dissipation, reverse mode: 100µA at 90ºC.

 Low heat generation, less than 10ºC rise at 10A.

 Life time is around 30years.

 Low operating expense and warranty cost.

 Bidirectional lightning survivability.

 No heat sink required.

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26

Fig 36. LX 2400 on the solar string system [24]

Fig 37. Typical forward voltage and current [24]

Table 3. Function pin description [24]

Name Pin Description

CAT 2 Cathode power pin connected to

positive terminal of string

AN 1 Anode power pin that is connected

negative terminal of string Pin Description

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27

Min Type Max

Diode forward

voltage V_FWD I_FWD=10A 45 65 mV

Start up current I_FWD-SUI

Minimum FWD current needed to

initiate bypass

100 mA

On-to-off

transition time TOFF 0.5 mS

Reverse break

down voltage V_REV Cathode-to-anode; I

REV=10mA 22 V

Reverse leakage

current I_REV Cathode-to-

anode:VREV=20V 250 µA

Off-to-on

transition time TON V_FWD-BD=400mV 5 mS

Electrical Specification

Parameter Symbol Comment

LX2400

Unit

Table 4. Electrical characteristics of lossless diode [24]

Fig 38. Typical forward voltage at 10A VS t [24]

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28

Fig 39. Current VS forward voltage for Schottky diode and the lossless diode [24]

Fig 40. Reverse leakage (µA) VS temperature [24]

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29

5.4 Experiment and Comparisons of the results

The Solarus CPC-T modules contain 8 strings that were connected in parallel and each string is constructed with 38 cells. A bypass diode is used for every string that is located at the end of the hybrid collector. In this experiment, Melacs system was used in order to measure the solar radiation, Output Voltage, output current as well as power. In order to draw out the electric power output behaviour due to using the schottky diode and lossless diode, a logger was connected with the module to get I-V curve simultaneously.

Two modules were selected for the experimentation connected with schottky diode and new lossless diode respectively, experiments took place on the same day, so we had the same light intensity for both of the modules, effects on the performance of the modules are calculated in terms of electrical conversion efficiency, fill factor and thermal power of the same type of the module but different by pass diodes.

First top side of the modules were under examination with schottky and loss less diode at 2011-06-12 and then bottom side of the modules at 2011-05-25.

Top side electrical power output with Schottky

At 12^thof June apparatus was set to record the electrical power output for the top side of the module with schottky by pass diode attached as there was no shading on the top side of the module and the power output pattern was smooth because of low voltage drops as shown in the figure 41.

Fig 41. Electrical Power out Put for the top side of the module at 12^thJune at Älvkarleö.

0 5 10 15 20 25 30 35 40

04:48 07:12 09:36 12:00 14:24 16:48 19:12

Power W

Time HRs

Top Side with schottky diode

Series1

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30

Getting on with the precise calculation many data have been taken. In order to organize this large number of data only the optimum values are considered. For the top side of one module 34.8 W (see appendix) was measured as a maximum power.The power at Y-axis and the time at X-axis shown in the figure 41 and IV curve are shown in figure 42.

Fig 42. Measured I-Vcurves for the cells on the top side of the module during the period 09:00 to 14:00 Hrs , 2011-06-12.

Top side electrical power output with lossless diode

At 12^thof June again the top side of the same module brought under observation with the same procedure with new lossless diode.

The pattren for the electrical power out put was the same as there were no shadow on the top side of the module

Fig 43. Electrical power out for the top side of the module, at 12^st June 2011, Älvkarleö.

On the day of 12^st june,the maximum output power of top side of the module was 37.3 W (see appendix) as shown in the figure 43 and IV curves are shown in the figure 44.

0 5 10 15 20 25 30 35 40

4:48 7:12 9:36 12:00 14:24 16:48 19:12

Power out put with lossless diode

Power

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31

Fig 44, measured I-V curve for the cells on the top side of the module with lossless diode during period 0800 to 1300 hrs at 2011-06-12

Bottom side electrical power output with Schottky diode

Similarly electrical data was gathered by following the same procedure on the bottom side of the module with schott key diode on 25^thof May,

It was observed that there was shadow on the bottom side of the module because of the gables and power out was not good as compared to the top side of the modules because of the voltage drop due to shadow .

Fig 45. Electrical power out put at bottom side, 25^th May 2011, Älvkarleö.

To measure the output voltage, current and power, the logger was connected to the bottom side of the module and was taken a large number of IV-curve data simultaniously in order to draw the output power curve.The maximum output of bottom side of one module was recorded 25.33 W (see appendix) as shown in figure 45 and IV curves are given in figure 46.

0 5 10 15 20 25 30

04:48 07:12 09:36 12:00 14:24 16:48 19:12

Power with schottky diode

Power

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32

Fig 46. I-V curve of solarus module bottom side when schottky diode was connected during period 0800 to 1300 hrs. at 2011-05-25.

Bottom side electrical power output with lossless diode

At the same way 25^th of May was selected to look at the behaviour of bottom side of one module with the new lossless diode. The power out put was on heigher side as compared to the module with schottky by pass diode.

Fig 47. Electrical power out put at bottom side, 25^th May 2011, Älvkarleö.

During this time the maximum output power of bottom side of one module was 37.42 W (see appendix ) as shown in the figure 47 and IV curves are shown in figure 48.

0 5 10 15 20 25 30

04:48 07:12 09:36 12:00 14:24 16:48 19:12

Power W

Time Hrs

Power with Loss less diode

Power W

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33

Fig 48. I-V curve of Solarus module bottom side when lossless diode was connected during 0800 to 1300 Hrs. on 2011-05-25.

5.5 Calculation

After gathering all the data for the top and bottom side of the module with schottky and lossless diode, e.g electrical power out put, voltage, current, flow rate, sun light radiations, temprature. Following three parameters are calculated and the results are compared for the module with schott key diode and the module with loss diode.

Thermal output (PT), fill factor (FF) and electrical conversion efficiency (η) was calculated by using equation 5.1, 5.2 and 5.3 respectively for the same solar CPC-T module with schottky diode and lossless diode.

P_{T =}m^o C_p(T_out– Tin) / Ac 5.1 FF =

5.2 η =

*100 5.3

Where m^ois the mass flow rate and Cp is the heat capacity and Tout and Tin is the inlet and out let temperature of the fluid and V_mpis the voltage at maximum power and same is the I_mpand Voc and Isc open circuit voltage and short circuit current, Pmax is the maximum power output recorded at a particular day and X and Y are the dimensions of the cell side.

Top side with schottky diode

After getting experimental data following calculations made for the module with schottky diode for top side of the module.Following measurements can be seen in appendix.

Date 2011- 06 -12 Time 11: 49 hrs.

Voc=19.989V Isc=2.490A

Maximum electric Power, Pmax= 34.83 W

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34 Vmp=15.633V

I_mp =2.228 A

Resulting FF=

69.98%

Cell side X=15cm=0.15m Cell side Y=204cm=2.04 m A_c = 2.2 m²

Tout =47.1˚C T_in = 46.2 ºC

Water flow = m^o = 5.1 L / min Heat capacity = Cp = 4.18 J / g ˚C

Thermal power = PT = m^o Cp (Tout – Tin) / Ac

= 145 W/m² Solar radiation = 800 W/m²

Electric power resulting efficiency = η =

*100=14.22%

Top side with lossless diode

Similarly calculation is carried out for the top side of the module with lossless diode connected. Following parameter can be seen in appendix

Date 2011- 06 -12 Time 11: 59 hrs.

Voc=19.989 V Isc=2.621 A

Maximum electric Power, Pmax= 37.3 W V_mp=15.7 V

Imp =2.376 A

Resulting FF=

70.21 % Cell side X=15cm=0.15m

Cell side Y=204cm=2.04 m Ac = 2.2 m²

T_out=41.5 ˚C Tin = 40.4 ˚C

Water flow = m^o = 5.1 L / min Heat capacity = C_p= 4.18 J / g ˚C

Thermal power = P_{T =} m^o C_p(T_out– T_in) / A_c = 175 W/ m²

Solar radiation = 840 W/m²

*100=14.51 % As there was no shadow on the top side of the module at 11: 49 Hrs, the maximum power output was recorded for the module with schottky diode, with 14.22 % efficiency and at 11.59 Hrs. The maximum power recorded for the module with lossless diode with 14.51%

efficiency.

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35

Similarly modules with schott key diode and loss less diode were experimented with same light intensity to compare the performance on the bottom side. Measured values can be seen in the appendix

Bottom side with schottky diode

Date 2011- 05 -25 Time 12: 29 hrs.

Voc=19.327 V Isc=2.24 A

Maximum electric Power, Pmax= 25.33 W Vmp=14.235 V

I_mp =1.78 A

Resulting FF=

T_out=43.2 ˚C T_in = 42.2 ˚C

Water flow = m^o = 5.1 L / min Heat capacity = Cp = 4.18 J / g ˚C

Thermal power = P_T=m^o C_p(T_out– Tin) / A_c = 143.89 W/m² Solar radiation = 810 W/m²

Electric power resulting efficiency = η = *100=10.21 %

Lossless diode bottom side

Date 2011- 05 -25 Time 12: 37 hrs.

Voc=19. 238 V Isc=2.26 A

Maximum electric Power, Pmax= 25.78 W Vmp=14.198 V

Imp =1.816 A

Resulting FF=

T_out=44 ˚C Tin = 43 ˚C

Water flow = m^o = 5.1 L / min Heat capacity = C_p= 4.18 J / g ˚C

Thermal power = P_T=m^o C_p(T_out– Tin) / A_c = 159.89 W/m² Solar radiation = 800 W/m²

*100=10.41 %

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36

Bottom side of the module have been measured that was mainly affected by shading because of gables. At 12: 29 Hrs, due to attachment of schottky on the bottom side the efficiency was 10.22% on the other hand, at 12: 37 Hrs, for the lossless diode attachment the efficiency was reached 10.53 % that shows the improvement of the hybrid solar system efficiency.

As shadow affects the bottom side of the module due to gables so by pass diode get affected at bottom side so comparison between schottky diode and lessloss diode can be seen through bottom side results as shown in the Table 5.

Bottom Side Date Time Pmax(W) FF % Efficiency Schottky

diode

25/5/11 12:29 25.33 0.64 10.21 Loss diode 25/5/11 12:37 25.78 0.65 10.41

Table 5. Calculation results for new loss less diode by Micro Semi and schottky diode at bottom sides of modules.

Fig. 49 Comparison of power from bottom side of the module with lossless diode and schottky diode.

As shadow effects the bottom side of the module due to gables during evening and morning time so bypass diode active during this time and above figure 49 gives the clear idea that the power with lossless diode in blue is more the power with schottky diode in red during shading time.

Now it is easy to make a comparison of efficiency between schottky diode and lossless diode inside the solar module from the above table 5. In this experiment, after using the new lossless diode, this increased efficiency amount was 0.31%.

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37

Number of diodes Price per piece

1-200 33 SEK

200-10000 20 SEK

Price of the new lossless diode that will be used inside the solarus module

Number of diode Price per piece

01-99 14.52 SEK

100-249 12.38SEK

250-999 9.71SEK

More than 1000 8.06 SEK

Price of the schottky diode that are using inside the solarus module

5.6 Economic outlook

Economical view is most significant part for all replacement technology. That’s why it should be considered, what will be the economical consequence if the new lossless diode is used instead of normal schottky diode. Actually the price of both schottky and lossless diode are varied depending on their amount of quantity that will be bought.

A shortlist of the price of schottky diode is given below.

Table 6: Price list for the Schottky diode

A shortlist of price of new lossless diode (from Micro semi) given below-

Table 7: Price list for the new lossless diode

Now it is easy to make a price comparison between schottky and lossless diode from the above table-8, Lossless diode is little bit more expensive than the schottky diode. But if the relation is made between cost and longevity, in that case lossless diode is more preferable.

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38 6. Discussion

The main purpose of this study was to compare the results in terms of output power and efficiency using schottky diode and new loss less diode by the company Micro Semi, in thermal solar hybrid collector. The PVT collector used in experiments or data collection was manufactured by Solarus AB Sweden, which integrate heat and electricity by using the same ser- vice area, and can operate at its peak electrical output by reducing overheating problem.

Shading can cause the power loss or hot spot creation. To minimize shading effects by pass diode concept is used to remove power dissipation, traditionally schottky diode is used which has more forward voltage drops decrease the efficiency or electrical power put of the system.

To overcome Micro Semi designed new lossless by pass diode with low forward voltage drop which are 50mV at 10 A generates 10°C temperature rise and can fully operate from - 65°C to + 165°C and reverse Microsemi diode can block 22 V at less than 100 °C leakage current without break down.

So experiments were conducted to investigate the power output and efficiency of the system by using both, new lossless diode by Micro Semi and schottky diode and to compare the results. IV curves were obtained by using logger and solar radiations are measured by using the Melacs control system. The solar radiations are collected from the top side of the solar collector, on 12^th of June with schottky diode and with new lossless diode. Only the peak values are considered between every 500 output value. For the top side 34.8 W and 37.3 W maximum output power was measured for schottky diode and new lossless diode with electrical conversion efficiency 13.39 % and 14.53 % respectively, similarly from the bottom side of the system maximum power at 25^th May was 25.33 W with schottky diode and for lossless diode power output at was 25.78 W with electrical conversion efficiency 10.21% and 10.41% for schottky and lossless diode module respectively.

Shading caused at the bottom of the collector because of gables which decrease the power output. Because the reflectors cannot work properly to reflect light and the current of the module go through by pass diode which cause the forward voltage drop results in reduce power output.

After using the lossless diode as a substitute of schottky diode, the total output power was Increased that’s why we got more efficiency from this one module.

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39 7. Conclusions

In order to have a précised result many data have been collected from the outdoor test even though it was very difficult to make an exact result because of the large range of data. After accomplished all data the result has been reflected that the module with lossless diode has more out power and electrical conversion efficiency as compared to module with schottky diode at the bottom side where shadow is the problem because of the gables. Now, it can be concluded that lossless diode is more effective than the traditional schottky diode.

LX2400 Micro semi new lossless diode operating efficiency and power output is more by using the normal schottky diode.

In terms of cost and life time, lossless diode is more expensive than schottky diode but the lifetime is relatively high then the schottky diode .Considering this all new result solar energy technology can turn to a new way where lossless diode will be used instead of schottky diode inside the solar module.

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40 References

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[2] Eurems; Bypass diode [On line]

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(Accessed 04 May 2011)

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[10] S.A. Kalogirou, Y.Tripanagnostopoulos. “Hybrid PV/T solar systems for domestic hot water and electricity production”; Energy Conversion and Management 47 (2006) 3368–

3382.

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41 (Accessed 24 May 2011)

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