Background:
Using the solar energy (Energy from sun) has been the subject of research for many years. We all believe that resources for fossil based fuels such as oil and coal are limited and soon they will vanish. Many scientists are working to use solar energy for different applications.
Today, solar calculators are very common and samples of solar cars have been made. Solar energy is also being used to warm up homes and charge batteries. But, can we use solar energy to make a furnace that can melt steel?
We have used magnifiers to concentrate the sun light, to burn a paper and light a match. Can we make a very big lens to concentrate more sunlight in one spot? Can we use mirrors to do the same? It seems to be a good project to work on. It is educational and fun.
Introduction: (Initial Observation)
Solar energy is produced in the sun. The sun constantly releases energy at the enormous power of 3.827*10^14 Terawatts.
This radiant energy is transmitted through photons to the earth. About 10^18 KWh of this energy hits the earth every year. This is more energy in one hour than all people in the world use in one year. The photons include small packages of solar energy. This energy can be used in several ways. The first way is to produce heat and transform it to electricity (called solar thermal power stations). The second way is to convert the sunlight into electricity. This technology is called photovoltaics.
Information Gathering:
We know that concaved mirrors (those who show the objects larger) can concentrate light the same way that a magnifier does. But building a large concaved mirror is not an easy task. To find out what others have done in this area we search the Internet and find the following web-sites.
http://www.esru.strath.ac.uk/Courseware/Class-16110/index.htm
http://rhlx01.rz.fht-esslingen.de/projects/alt_energy/sol_thermal/flux.html
The information that we collected shows that many have used an array of flat mirrors to concentrate the sunlight into one spot. We base our project on the same method.
Lenses can focus the sunlight into a small point that is vary hot. The amount of heat depends on the size of the lens and its precision to minimize the area of the focused light.
The larger the lens is the more heat it will concentrate. Making a solar furnace with the Fresnel Lens is easy. All you need is a movable frame to hold it. By moving the frame you can face it to the sunlight for the best results.
This is a sample of solar furnace made with Fresnel Lens. Fresnel lens is made of clear plastic.
Solar Power
Solar power is being used to generate electric power in some places in the world.
How it works
Solar power towers consist of a large field of sun-tracking mirrors, called heliostats, which focus solar energy on a receiver atop a centrally located tower. The enormous amount of energy, coming out of the sun rays, concentrated at one point (the tower in the middle), produces temperatures of approximately 550°C to 1500°C. The gained thermal energy can be used for heating water or molten salt, which saves the energy for later use.
Heated water gets to steam, which is used to move the turbine-generator. This way thermal energy is converted into electricity.
Heat storage and transfer
As already mentioned there are two main fluids which are used for the heat transfer, water and molten salt. Water for example is the oldest and simplest way for heat transfer. But the difference is that the method in which molten salt is used, allows to store the heat for the terms when the sun is behind clouds or even at night. Molten salt – better: the heat of it – can be used until the next dawn when the sun will be back to heat the cooled down salt again.
The molten salt consists of 60% sodium nitrate an 40% potassium nitrate (saltpeter). The salt melts at about 700°C and is liquid at approximately 1000°C, it will be kept in an insulated storage tank until the time, when it will be needed for heating up the water in the steam generator.
This way of energy storage has an efficiency of approximately 99%, i.e. due to the imperfect insulation 1% of the stored energy gets lost .
Scheme
Question/ Purpose:
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
Can we use mirrors to build a solar furnace? How much heat can we collect using a solar furnace? What will be the highest temperature of a solar furnace?
The purpose of this project is building a solar furnace with multiple flat mirrors and find out what factors effect the performance of our solar system.
The main question for this project is:
How does the increase in the number of mirrors affect the temperature in a solar furnace?
This question will be studied in experiment 1 below.
Identify Variables:
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
For our solar furnace we want to use mirrors to collect sunlight from a large area and reflect and focus it in to a small space. Variables that may effect the performance of our solar system are:
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- The number of mirrors
- The size of mirrors
- The distance of mirrors from focal point
For the main question of this project, this is how you may define variables.
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- Dependent variable is the number of mirrors that reflect the sunlight to a certain spot.
- Dependent variable is the temperature at concentration point.
- Controlled variables are the weather temperature and wind.
- Constants are the size and the shape of mirrors.
Hypothesis:
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
Following is a sample hypothesis:
We can use multiple mirrors to concentrate sunlight in to one spot to create high temperature. The energy of the hot spot (furnace) will be the energy of sun on one mirror multiply by the number of mirrors. So the amount of heat must also increase at the same ratio.
Experiment Design:
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Experiment 1: How does the increase in the number of mirrors affect the temperature in a solar furnace?
Introduction:
Direct the reflection of sunlight from different number of mirrors to one spot. Record the maximum temperature of the spot for different number of mirrors.
Procedure:
Use one mirror to reflect the sunlight to a certain spot on the wall. Secure the mirror on that position. Use a thermometer to record the temperature on that spot.
Use a second mirror to reflect light at the same spot. Now your hot spot is getting light from two mirrors. Measure and record the temperature again.
Use a third and fourth mirrors to reflect light at the same spot. Does increasing the number of mirrors create a brighter and hotter spot?
Record your results in a table like this.
| Number of mirrors | Temperature |
| 1 | |
| 2 | |
| 3 | |
| 4 | |
| 5 |
Use the above results table to draw a line graph.
Construct a solar furnace: (Optional)
First we do a test with just one mirror to see if the reflection of the sunlight is as hot as the sunlight, or if it is not, what will be the ratio. Then we put together a mirror array. I suggest for beginners to make a 3×3 or 5×5 mirror array.
5×5 mirror array means 25 mirrors placed in 5 rows and each row has 5 mirrors. The size of mirrors is optional.
These diagrams show a tiny 6 X 6 mirror-array as an example. Also I used the silicone-glue “hinges” so my array could be reprogrammable. If you only want a fixed-focus furnace, you can use screws to adjust the mirrors, then glue them permanently into position. Obviously you can increase the size of the array as to large as you wish. My dream is to build an array the size of a 4ft X 8ft sheet of plywood. With 1 in mirrors, it should create a focused spot which is 4000 times brighter than normal sunlight.
The trick is simple: Make or buy a large number of glass or plastic square mirrors about 2cm or 3cm in size. 12 inch glass mirror tiles can be purchased from the larger hardware stores, and these can be scored and broken with a glass cutting tool. Or, Plexiglas mirror-plastic can be cut up with a utility knife or an electric Jig saw.
Obtain a sheet of plywood and a big bag of screws which are about 1/4 inch longer than the thickness of the wood (if the wood is 1/2 inch thick, buy screws that are 3/4 inch. or slightly longer.) I used 6-32 machine screws, but most any screw will probably work. If your mirrors are larger than 1in, use longer screws. Lay out and drill holes as shown, choosing a drill bit which is only slightly smaller than the screw diameter. The screws should turn easily with a screw driver.
To make “hinges”, cut a number of round wooden toothpicks into 1/8in segments, place a small dollop of silicone sealant at the corner position of a mirror, lay the toothpick in the silicone as a spacer, then lay down the mirror on the correct spot on the wood.
The silicone attaches the mirror to the wood, while the toothpick keeps the mirror from smashing the silicone into a too-thin layer. Carefully perform this operation for all your mirrors. The toothpick tilts the mirror one way, and when you later add the screws, the screws will lift the mirror as desired.
Let the mirrors harden for 24hrs. When the silicone has cured, screw all your screws into their holes just enough to let them touch the mirrors.
Programming the array as a solar furnace
Cut up some papers, and stick a square of paper to each mirror to block the light. LEAVE ONE MIRROR NEAR THE CENTER OF THE ARRAY UNCOVERED. This one mirror will act as a reference for adjusting all the others. (As the position of the sun changes, the position of this spot will also change, so you shouldn’t need to move the whole device to track the sun.) Take your mirror array out in the sun, and position it a few feet from a convenient target. The distance between the mirrors and this target will become the focal length of the Solar Furnace.
First use your screwdriver to adjust the single bare mirror so it is no longer tilted by the silicone glob. Bounce some sunlight from this small mirror towards your target, and move the whole device so the spot of light hits the target. Now remove the paper from one other mirror, and use the screwdriver to adjust this other mirror so the spots of sunlight from the two mirrors combine together. Cover this newly-adjusted mirror, and uncover the next one. Repeat the adjustment process, then cover it and go to the next. Do not of course change the setting of your central “reference” mirror. When each mirror has been adjusted to combine with the “reference” mirror, peel all the paper from the mirrors and see what you’ve accomplished. All the bright spots should now shine on the same place.
Materials and Equipment:
List the materials you use for your experiments in this section of your report. The main items that you need for your experiment are some mirrors and one thermometer that can show high temperature. Additional materials and tools needed to cut, mount and align the mirrors varies depending on your ideas and creativity.
Following is a sample list of materials:
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- Plywood 20″ x 20″ (50cm x 50 cm)
- Flat mirrors 3″ x 3″ (75mm x 75mm) 25 pcs
- Silicon Glue
- 2″ (5cm) bolts/ screws 50 pcs
- Oven thermometer
- Black paint (used to cover the bulb or the sensor of the thermometer)
Safety precautions:
If you decided to use glass mirrors and cut it yourself, make sure that you have leather gloves, protective clothing and eye glasses. Also do it only while supervised by an adult.
Results of Experiment (Observation):
A 6X6 array is pretty safe for experimenting. It won’t set anything on fire, but in the summer sun it will heat a black garbage bag almost to the melting point. Once you have the whole process learned, try making a 16 X 16 array (256 mirrors), or even larger. The above process lets you slowly “coat” any flat wooden surface with solar-furnace arrays. With thoughtful planning you could even cover a non-flat surface with a solar furnace array. If you had enough time, you could build one of ANY size and temperature. ***WARNING*** if you build a big one, keep it covered when not in use. If you leave it around the house, the moving sun might unexpectedly strike it and start a fire!
Calculations:
If you need help on any calculations, contact your project advisor.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Conclusion:
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
Possible Errors:
PROBLEMS WITH A LONG “THROW”
If you adjust your solar furnace for a very long focal length, you’ll find that the hotspot grows larger, fuzzier, and cooler. This occurs because the sun is not a tiny point, it is a disk, and the mirror-facets act as the pinholes of a pinhole camera. They form an image of the sun, rather than an image of the mirror-chip shapes. Each little square of light will develop a blurry edge, and only the center of the square image will have “full sun.” To compensate for this blurring effect, use larger mirrors. The rule: the focal length must be lots shorter than 170 times the width of a mirror-chip. (This 170 comes from 1/tan(1/3), the sun being about 1/3 degree in angular size.)
References:
Visit your local library and find books or magazines related to solar energy.
A creative model of solar furnace (From USNJCS007):
In this model mirrors are mounted on an old satellite dish.
The heat center will be the focal point of the dish.
This model could be improved by having more mirrors and using square mirrors instead of rectangle ones.
