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Boys and girls, let's make Solar Power Collector !!

Edited by Prof. WAKAI, Kazunori and
cooperated by Qin, Xiaojin, Doctoral Course Student from China




Caution : Especially, bigger mirror has a big power and it is somewhat dangerous when you misuse it.       Do not set any inflammable things at the focal point. It is absolutely forbidden to put your head or hands at the focul point. If you do it, you will surely be burned your face or hands. It is also forbidden to set your eyes at the focal point. It is really absolutely dangerous. You will become blind even by one glance.       When you try to check the performance of the mirror, please do it with your parents or your teacher for the safety.




CONTENTS

  1. Materials and Budget, needed
  2. Preliminary knowledge
  3. How to obtain draft of mirror？
  4. Let's begin with easier one, namely, Mirror type A
  5. Make the concave (parabolic ) mirror
  6. Make supports of mirror pieces
  7. Make the circular base and draw the lines where support papers stand on it
  8. Attach thick wire which can support the material to be heated at the focul point
  9. Set up the supports and attach the pieces of mirror
  10. Make simple equatorial mounting(unfinished)
  11. Solar Power
  12. How to decide collector size
  13. How to measure the performance of solar power collector
  14. Study what is the attainable temperature
  1. Materials and Budget, needed
    1. Smaller mirror case. Low skilled, rather younger pupils can make. You can make it easier by your mother's help.
           
      Left picture shows the middle process of smaller mirror. It was made from drawing paper. After this, alminum foil was pasted on the paper to increase the reflectivity of the mirror surface.
      Materal and budget (example in Japan)
      Alminum foil(1$), Drawing paper(1$), Cardboard(1$), Binding Material G17(1$). Steel wire of 1mm thick, 50cm long(1$).

    2. Suitable for certainly skilled and elder pupils, you can make it when your father helps you.
           
            This mirror can collect plenty solar energy when it is really well made. You should use stainless steel foil. It is somewhat dangerous although it is really thin. You migth hurt your finger by the edge of stainless steel foil just after you cut it. Please be careful when you treat it. You should use high performance scissors. It should have an ability to cut the foil straight without any wrinkle. You should wear cotten work gloves when you cut it and when you make them mirror.
      Materials and budget (example in Japan)
      Stainless steel foil of 0.1mm thick, 30cm wide and 90cm long.(11$), Cardbord(1$), Binding Material G17(2$), Thin veneer bord of 3mm thick, (3$), Steel wire of 1mm tick(1$), a little expesive?

    3. When you want to make the pedestal       Please imagine the equatorial mounting for the telescope. If you make very simple such system, it is very easy to direct the mirror to the suitable angle. It is simple and it is usable only one season, however it is still convinent to adjust the angle. The material needed is wood chip of 1cm thick, 7-10cm wide and 50cm in total length, plastic water pipe of around 2/3 inch in diameter, 90cm long, two elbows for the pipe and one T-joint for it, as well. Their price is less than 5$ in total.


  2. Preliminary knowledge
          My son did not make complete one from the very beginning. He made smaller one shown above, at the begining. The mirror back was made by a drawing paper. It is assembled by many parts whose shape looks almost triangle. It has truely a complicated shape and it is impossible to draw it by compus and rule. It is drawable by computer and printer. We will prepare soon and you can download the digital file of the shape. After you download it, you can print it by a printer. If you don't have such a system, please send us an airmail in which includes self-addressed envelope with stamp.
          He could succeed to make prototype and proceeded to make bigger one. He cut the veneer bord and made circular plate. Nextly, he attached the small pieces of supporting parts whose side shape is parabollic. His case, number of the pieces were sixteen. On those supporting parts, he attached triangle like stainless steel foil. You can see light bule colored wire. The top of the wire is rounded into ring-shape and its location is set at the focal point. When an egg is set on the ring, it is easily boiled in short time. Please make a hole at any place of the egg, otherwise it explodes and you will be burned. The smell is so bad when it explodes.
          Third figure shows the comparison of two mirrors. Bigger one is set on the simple equator.


    		  
          How do you feel?
    I think some of you are interested in this mirror. Some of them might feel if he/she can get draft of this mirror, he/she doesn't need any explanation and can make it by themself. Such pupil should jump to the chapter which descrives how to get the draft. If you don't have confidence on yourself to make it only by youself, please proceed to following chapter.

  3. How to obtain draft of mirror？
    Please see the figures shown bellow which shows the shape of the parts of mirror.

            


          Left figure shows the parts which will consist the mirror itself. It looks triangle but it has really complicated shape. Longer two sides are not straight. They are drawn by personal computer. Right figure shows the support of mirror part mentioned right before. It is also imposible to draw its shape only by rule and compus. It means that the pupil of elementary school cannot draw their shape.
          If you have brother or sister who is a student of high school or university, he/she can help you to calculate the shape and draw such figures. If you don't have such sister, brother, parents or neighbours, don't worry. If you choose some standard mirror shown in following table, you can get draft for them through internet, easily, as shown several paragraph     bellow.


    Type of Mirror
    (suitable level)
    		Printer Precision(dpi) Condition of Mirror Triangle's Shape : Length of
    300360600720 Diameter Focul Length Number of piece
    Base
    		Height
    A
    about 10yeas old
    		35cm13cm1293.8mm187.4mm
    B
    for old or skilled pupil
    		50cm18cm1888.2mm265.4mm


          If you are not satisfied with above two types, and you want to make different shape mirror, it might be possible. Of course, if you have a skill to calculate the shape of mirror pieces and supports, you can do yourself. However if you want to make different one without such skill, you can ask us via email. We will check if your idea can be realized or not. So, please send us your idea of 1. Diameter, 2. Focul length and 3. Number of pieces of the mirror. Why your idea should be checked is, it is often impossible to make mirror by some condision. For example, it is impossible to make mirror by two pieces. If you want to study what is the limitting condition to make mirror, you should read last part of this page. However, it is not finished. We will soon release that page. So, please wait for more several days.
          Evenif you ask us whether your idea can be realized or not and we reply it is possibe, it does not mean you can get draft. You should do following procedure to obtain the draft.
          There are two ways to get draft.
    1. Send draft via airmail.       You send an airmail in which you should put envelope on which you write your address and you attach stamps of 1.00US$(Japanese stamp price is from 90 to 130 Japanese Yen to send back the airmail which will covey the draft.) Wakai Laboratory address where you send airmail is as follow. I think it takes aroung 10days if your country is not so far from Japan. I am afraid it takes much more days because Wakai Laboratory has a summber vacation in middle August. So, download via internet is reccommended.
      Wakai Laboratory, Department of Mechanical Engineering, Gifu University, Japan, 501-1193
    2. Download the draft file via internet.       Choose which mirror you want to make. Move cursor on either button you chose and click left mouse button. Dropdown menu appears and you choose 'save object file'. The www browser quickly save the draft file whose names are xxxA.EXE for Mirror A and xxxB.EXE for Mirror B. First three figure of the file name means the precisin of printer. After saving. you should move to the folder in which yor saved draft file. When you find it, double click the left button of your mouse adjusting the cursor on the file. Then, there appears new window which shows the part of draft in your computer screen. Top left part of the window is shown bellow.
      　　　　
      Task Bar          Drop down menu


            You should know the precision of printe, first. Click [file] button, then there appears dropdown menu. Click [print view] in the dropdown menu, then two figures appear in the window. If those figrues are completely shown, the file is correctly downloaded. So, you can click [print] button in the dropdown menu and printer starts to draw important figures.
            When you get printed draft, please check the base length and height of the traiangle-shaped piece because the printed size is not always equal to the desired one. If the size of printed draft is different from the size shown in above table, please check your printer precision and download suitable draft file, again. If you cannot get suitable size even at any cost, you can enlarge/reduce the size by copy machine.

  4. Let's begin with easier one, namely, Mirror type A
    If you are to make B type, you can skip to chapter 5

          Prepare the drawing paper from which all pieces of mirror should be taken. If you don't have such a wide paper, you can use several small papers. You should prepare the same number and size of alminum foil which often used for cooking. Your mother might keep such foil in her kitchen room.
    First, paste some glue on whole area of the paper as thin and flat as you can. Cover its surface by alminum foil with bright surface upside. If you see some hump on the glued foil, there are surely air pocket between foil and drawing paper. Such case, you should softly squeeze the surface by thick and soft cloth like towel, and push air out. If the surface of the alminum foil looks mirror, the procedure is complete and you can proceed to next chapter.

  5. Make the concave (parabolic ) mirror
          First, copy the triangle shape from the printed paper to the pastepaper as accurately as you can. Cut it off as accurately as you can, as well, but this time, a little bit smaller than original size is reccommended because you should copy again form cut paper to the stainless steel foil or alminum foil glued paper. The thickness of the pencil make copied shape bigger than original. If the copied size is bigger than ideal (original) one, it makes big problem, namely, you should make 12 pieces or 18 pieces of the mirror parts and attach next by next and the last piece might overlaps on the first piece. For example, evenif each piece is only 0.5mm larger than desired, overlap width is 9mm. If the size is smaller than ideal, the problem is not so severe compared to the bigger case. Only there exist small gap between neighbor pieces.
          Picture bellow shows the back side of stainless steel foil of 0.1mm thick. Eighteen pieces of triangle shaped drawing papers are arranged on it by Dr. course student Mr. Qin. He did it to confirm if he can take all pieces from one foil. This case, Mirror type is B and the foil size is 30cm wide and 90cm long which is generally sold at ordinary DIY shop in Japan. You can understand he can make the mirror by one foil of this size. Nextly, he copied the shape of pieces and cut them. Be careful not cut bigger than ideal size because of what I have mentioned above. Cut side should be very smooth. If it is irregurarly cut or it has some wrinkle, next process become very hard. So, you should couteously cut it. When you cut stainless steel foil, the cut edge is very sharp and might harm your hand or finger. So you should wear gloves. The glove can work not only to protect your hand but also to make clean mirror because the oil of your hand is easily copied to the mirror surface.
  6. Make supports of mirror pieces
          This work is almost the same as forme work. The material for this suppot should be thicker than draft paper and you might spend much power to cut it.
  7. Make the circular base and draw the lines where support papers stand on it
          Prepare the veneer board which size is a little bigger than the diamtere of your mirror. If you want to make A-type mirror, thick pastepaper or boxpaper can be used instead of veneer board. Draw circle whose size is equal to your mirror diameter. If you don't have a big compus, you draw circle with pin, string and pencil. Sting needle or drawing pin at the center of the board. knot the string to the pin. Strech the string and find the point on the string where the distance from the pin is equal to the diameter. Knot the pencil there and draw circle.
    Before cutting, you would better draw the lines which divide the cicle into 12 (case A) or 18(case B). You may have studied how to eaually divide circle into 6.
            




          You can draw small circle instead of big circle because you may not have such a big compus and used substitutes. Dividint process should be very accurate, again. So, you should use compus and rule. Even I say small circle, it should be as big as you can drraw by your compus. Thick circle below shows such one. Draw straight line passing through the center and mark the cross-point on the circle. It is marked as 'A'. Draw circle again with its center at point A and find the cross-points on the circle. They are marked as B and C. Draw the line from point B to point O which is the center of the circle. Enlong the line and find the cross-point on the circle at opposite side. Do the same procedure for point C. Thsose three lines equally divide the circle into six.
    If you are to make A-type mirror, you can easily divide those equally divided angles into twelve, as well. The method is easy and you can do it by the help of the same figure above.
    If you are going to make B-type mirror, it is impossible to sub-divide the angle. So, you need protractor. You might think, if you use protractor, you use it from the first step without compus. However if you do so, the error is accumulated and the last angle might have big error. So, I reccommend to use compus and rule, first, especially when you divied into six equal angles.
    Right figure shows such pattern drawn by Mr. Qin.


  8. Attach thick wire which can support the material to be heated at the focul point
          Drill at the center and make the hole of about 1mm in diameter. Get the vinyl coated wire of around 1mm thick, and bend it and make a shape shown later picture. The one side (top side) of the wire is used to keep materials to be heated and other side (bottom side) is used to fix it at the back of the circular bord. When you fix it, adjust the distance beteween the top side of the wire and the bottom place of the mirror is equal to the focul length.


  9. Set up the supports and attach the pieces of mirror
                    


          Pictures bellow show the scene that Mr. Qin is setting up the supporting papers on the cirlular board. Bond used is G17. I am not sure if G17 is sold in your country. However any bond which can adhere paper and wood is useful.

            
            


          After arranging the support, paint the bond thinly and finely on the edge of the support. Be careful not to use too much bond. Set one piece of the mirror and push it softly, then the bond fixes the piece. Attach small pieces of plastic tape at the edge of the mirror piece of backside as shown in the picture bellow. Paint the bond again on the next support. Contact and attach the next mirror piece to the fixed one using plastic tape. Nextly, set it on the support and push it again. Repeat such procedure one after another. When you cut every piece well, there is no overlap between last and first pieces. If it seems to have some overlap, you might force them not to overlap, but if you do so, the mirror piece angle bends and the reflected light from such mirror piece does not go to focul point. So, don't force them.
          You can see that the finger pattern remains on the mirror surface. If your mirror also have such stain, squeeze the surface by the soft cloth containing soap water. Don't squeeze it circularly. Squeeze toward radially along the edge of mirror pieces. After this treatment, you should squeeze again by the cloth containing clean water. Otherwise, there appears the dirty spot or area of dried soap. If there still remains stain after this, it might be bond. Such case you should use thinner or alchohol.
    Well, Mirror itself is finished.

    Next procedure will be released several days later. Please visit us if you need to make simple polar system or you want to study some informatin about the evaluation method of the finished mirror. See you again !!
  10. Make simple equatorial mounting(unfinished)
          Following picture shows the real equatorial mounting attached to the mirror type telescope for amateur astronomer. You can easily direct the telescope toward the star. If you make the same purpose but simple one, you can chase the Sun.
            


          The telescope, orange color, is Schmidt/Cassegrain type and laterally set thick cylinder is plar axis. It is enlarged in right picture. The polar axis is set on the strong tripods. Silver color circular board has an angle scale and it is set at the latitude of your place. There are two another scale ring. You adjust celestrial declination by one of them and also adjust right ascension by another one of them. Once you adjust the celestrial declination, you should only adjust right ascension and you can chase the star you want to watch. So, if you make the similar but much simpler system, you can chase the Sun only by adjusting some scale. You can make it by thick wood (1cm thick) and screw bolt with butterfly nut. We are now preparing to make such system. Please visit this page again around one week later. Mr. Qin is so busy to earn money, bring his children up, proceed his research project and enjoy making this model.

  11. Solar Power       1kW of the power is illuminated from the sun on the global surface of 1m2 when sun is almost at the top of the sky. It happens during mid summer and around noon. Ordinary comercial solar cell can convert such power with around 10-15％ of conversion efficiency, namely, it can get electricity of 100W to 150% in 1m2.
          How much energy, do you guess, we can collect by the mirror you have made or will make? By the mirror of 35cm in diameter, and suppose the efficiency is 30%, it is around 30W. With 30W of electricity, we can allight one fluoresc ligth. If the diameter is 50cm and with the same efficiency, it is 100W and with this power, we can allight the electric lamp for the room of 20m2. However, we can collect not electric power but heat. The heat for the toaster is from 250W to 500W and it means that the mirror is insufficient to make toast. However, if the bread is black colored, you can make toast by moving it around the focal point, I bet. You might think the mirrors are so powerless but it does not mean the mirrors are bad. We should think that the solar power is not so strong compared to the fossil energy which is accumulated for long long years. We use such energy in very short time and after we spend such energy out, we don't have such a convinent energy at all. After we spend them out , we should survive only by solar energy, wind energy, wave energy, geothermal power, biomass energy and so on but they are so thin.

  12. How to decide collector size       The key point to make the solar collector is the size (diameter of the mirror), focal length and the number of the pieces of mirror. Following is the explanation how you should decide those figures.
    • Diameter
            You have already understood that the bigger the diameter is the more the mirror can collect power of the sun. Namely, 35cm can be 30W, 50cm can be 100W and 50cm can be 200W. 1m mirror can be 400W, as well. However, the bigger the diameter, the harder to make it and more dangeroug it will be to treat.
    • Focul length
            Focul length is the distance between the bottom of the mirror and the place where the light gathers (focal point). It is hard to understand why this length is important. So, now I should tell you from the actual standpoint. When it is too long, the mirror is almost flat and it is hard to make the focus place small (sun light scatters widely) and it is hard to heat something up. If the focal length is too short, this point becomes inside of mirror and the reflected lights from various part of the mirror has too different angle and it is hard to heat something up. For example, the light reflected from the bottom of the mirror heats up the bottom of the pan but the light reflected from the fringe heats up the side of the pan, so if you want to make fried egg by thin pan, it is very inefficient because side area of the pan is too small. When the focal length is one-forth of the diameter, the focul length is equal to the depth of the mirror. So, I reccommend that the focal length should be a little bit longer than one-forth of the diameter, namely, around one-third to half of the diameter.
    • Numbber of pices
            If the number of pieces is small, it is very easy to make mirror. However, if it is so small, the size of the width of the piece become big. Base length decides the degree of concentration of solar power and it should be smaller than what you want to heat up. For instance, if you want to boil egg, the base length should be smaller than 5-7cm. So the number should approximately be equal to 3 times diameter divided by the size that you want to heat up. When the diameter is 50cm and if you want boil eggs, the number of the pieces should be 20. There are another condition for suitable number of pieces. It should be easy to draw the draft and it should also be easy to make. When we draw draft, we should divide the angle. You have seen that division of 360degree into equal 6 angles is very easy. You have also seen that diviion of some angle into equal two angles is very easy. So, the number of pieces should be multiple number of 6, 2, or 4. Such numver around 20 is 18 or 24. If you are good at making such manual training, you can choose 24 pieces. If you are not so skilled person, you should choose 18.

  13. How to measure the performance of solar power collector
    1. Is it possible to evaluate by attainable temperature?       Everyone wants to know the performance when he/she finished it. Everyone also think that he/she should measure temperature and attained temparature is one of the important index of the performance of the mirror. However, to measure temperature is very hard even for professional researchers. If you still want to evaluate by attainable temperatur, we should decide the standard condition otherwise, the temperature depends one each measurement and there are no general understandings. The attained tempereture belongs not only on the performance of the mirror but strongly on the skill of the measuremant. One of the big problem is the standarizatin of the themometer. Ordinary thermometer can measure 100 degree Celsius at highest. However, attainable temperature is much higher than boiling point of water. It might attain 6000 Kelvin if ideal mirror and thermometer is provided. You mirror case, the attainable temperatur is much lower than 1000 degree Celsius but still you need special pyrometer. So, it is impossible to evaluate the performance of mirror by measuring attainable temperature.

    2. What is the best way of general evaluation of mirror ？       There are many methods but I will propose one of the easy method that you can easily try. Use small can which contained vegitable juice or cofee of which diameter is around 5-7cm and the height is 15-20cm. Cut it at the middle place and make half size of can. Use bottom side and attach some tape on the sharp edge to protect your finger to be hurt. The can should big enough to contain one egg if you want to boil egg. Attach it at the top of the wire which you set at the focul point. You should devise the can not to spoil water from the it when the mirror leans to chase the sun. Paint the back of the can black to efficiently absorb the sun power. Poor measured volume of water. Adjust the direction of the mirror to the sun and start the temperature increase and time. The efficiency of the mirrot to collect sun power is shown as follows.
                  4.2 x Watar Volume (cc) x increased water temperature/boiling time (W) This is shown by power (W) and you can compete this power with the same size mirrors. If you want to compare your mirror with another size mirro, folowing efficiency make you satisfied.
                  4.2 x Watar Volume (cc) x increased water temperature/boiling time x 40/(3.14 x Mirror diameter x Mirror diameter) (%)

  14. Study what is the attainable temperature
          I have written that the attainable temperature is 6000 Kelvin which is equal to the surface temperature of the sun. Do you believe it?
          I remember it was around twenty years ago that one of the researcher named Dr. Noguchi of Nagoya city institute of technology made the high precision solar power collector of which diameter was 1m. When Dr. Noguch brought aroudn 5mm of steel bar at the focal point, it quickly melted and the droplet of the iron dropped down one after another. The melting point of the iron is around 1500 degree Celsius and it is sure that the attained temperature exceeded it. To hear it you might belive it is possible to attain 2000 degree C even if 6000 K is impossible.
          Theoretically, it is possible to attain 6000 K with any mirror of any diameter, any focul length. Some clever pupil might think that the longer focul length mirror make bigger image of the sun and the concentration of the image becomes thinner, namely, the temperature of the image becomes lower than that of shorter waver length. Hoever, the angle of the ligth is narrower than the case of shorter focul length when the light pass the image, and the energy in the unit angle is, as a consequence, the same, namely, it is independent on the focul length and the brightness temperature is the same.
          In this way, if there are no loss of the energy, attainable temperature will be 6000K. However, for example, the back side of the thermometer loose energy to the lower temperature than the surface tempereture of the sun and the thermometer cannot attain it. Even if the back side is coated not to radiate energy, like in white, the front surface itself loose energy because the front side faces not only the direction of the sun but also to the sky. The sky temperature is very cold and the big amount of energy is radiated toward the sky and it is a big loss. If we can make the front surface only direct the sun and backside is ideally painted not to radiate energy. the thermometer can attain 6000K.

If you have any questions or comments, please send email to Wakai Labo.            
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