I’m doing a science experiment outside of class on solar cells and how the temperature of the solar panel affects the efficiency. Warm temperatures lower output. I want to test for different types of materials to absorb the heat from the panel. Right now, most panels are either cooled with water or just left alone and this basically is a waste of possible energy. What kind of materials can absorb a lot of heat but also dissipate away from the panels?
Also, I need to create a setup to shine the same amount of light at different times since I can’t actually rely on the sun. This would cause too much variation between trials. I have the feeling that a regular light won’t be strong enough to do much. I’m just a little high school student so I don’t have access to much.
I don’t understand the first part of your question. Are you just trying to find a simpler method (i.e. without water) to cool the solar panels? I’ve seen some solar cells bonded to aluminum plates - that looks like a good solution to me. If you’re using some type of material to conduct heat away from the panels to a heat sink or radiator, all that matter is the heat conductivity. Copper is best, but Aluminum is pretty good too. Aluminum is much cheaper and easier to work with.
As for a steady light source, that’s a very difficult problem. It also depends on what you’re trying to do - does your experiment require that the specrum of the light be similar to sunlight? Infrared light contributes a lot to the heating but not much to production of electricity, so if the ratio of infrared to visible light is wrong you might get a different result than you would from real sunlight. Also, is the brightness critical, or does it just need to be stable? Re-creating the full brightness of sunlight is very difficult. At work we’ve used a solar simulator to test sensors and solar cells - ours cost something like $100,000 and is only 60% as bright as the full sun. (But it does produce a very nicely collimated beam.)
If the spectrum and brightness are critical, I’d try to get all the different setups ready side by side and use real sunlight. If not, I’d just use the brightest halogen lamp I could find. Slide projectors might work as well.
Oops, sorry wasn’t being too clear. I’m going to experiment with the solar cells in rolls. Usually they’re attached to a surface with a foam that also serves to absorb heat.
If you want cheap, you could just find a flourescent ballast that will hold say 4 48" tubes. They are easy to wire up and don’t produce as much extra heat as some other more powerful light sources. Just lay the panels a couple inches underneath the lights. To dissipate some of the heat, you could set up a fan to blow air over the panels. Neither this lighting nor the heat “getter-ridder-of” are on the professional level of what researchers would use, but they would give you some results. Come to think of it you were wanting actual materials to remove heat… yeah, copper is best, if you can use the fan to blow air over the copper (sorta like a radiator) that would work better than just having it sit there. Are you to find a new material or cooling method, or just compare several methods/materials? I don’t imagine that a high school science experiment that the students have to design and fund themselves is going to require NASA level sophistication for a good mark:).
Sorry, still a bit unclear to me, probably because I’ve never bought solar cells in rolls. But what exactly is this foam, and what do you mean by “absorb heat”? Most types of foam are good insulators, and it’d be terrible at carrying the heat away from the solar cell.
To carry heat away from the cell, you need a material with high thermal conductivity. You could attach the cells to copper or aluminum, but even a layer of double-sided tape can hinder the heat transfer. You might look into heat-conductive grease or something similar - some computer stores sell decent sized tubes of those. Also, heat is not destroyed, merely moved. If you attach cells to a copper plate, you spread the heat over the entire plate - but you might want a way to cool that plate efficiently. You could attach fins to increase surface area, or provide a fan. (Though I doubt the increase in efficiency will make up for the power used in the fan.)
As for the light source - the more I think about it, the more difficult it seems to rig up an artificial source. Why don’t you set up one cell and see how much variation there really is? If there is a lot, you can keep one cell as a reference and always compare your new experimental setups with that.
Like I think scr4 said, could you simply use more than one “cell?” Mount one on foam, one on aluminum fins, and one sitting in a try of running water. Something like that?
I think you are looking for a shape, not a material so much, to be able to absorb the heat and dissipate it away. As was mentioned, a heat-sink would be a good shape to do so. If you’ve ever seen your CPU in your PC up close, you will know what I mean.
ooooooooooooooo - cell
XXXXXXXXXXXXXXX - heat sink surface
X X X X X X X X
X X X X X X X X
X X X X X X X X - fins
X X X X X X X X
This will have a large surface area to absorb heat by conduction on the cell side, and will have a large surface area to radiate and convect heat on the fin end of the sink.
CPU heat sinks can be costly if you need many of them, especially the nice berylium ones. Also, there is a highly conductive contact paste that is often used between sinks and sources - that can help the heat transfer greatly.
Note that by inserting the fins in water, and by doing measurement of the water temperature rise, you could calculate the heat flux, and get extra credit (probably).
Why not an incandescent lamp with a 150 W bulb in it? Or two lamps?
As someone else has already mentioned, there is a difference between “absorbing” heat and dissipating heat. I put absorbing in quotes, since any energy “absorbed” must be ultimately be transferred to something else. But you know what I mean…
To efficiently “absorb” heat, you basically need a heat sink with a lot of thermal mass. You’ll find these type of heat sinks in systems that are required to absorb a heat from transient power sources. For example, a 1-watt carbon composition resistor (which are just about impossible to find nowadays!) is much better at absorbing large transients than a carbon or metal film resistor, all else being equal.
To efficiently dissipate heat, you need a heat sink with a lot of surface area. You’ll find these type of heat sinks in systems that are required to dissipate heat on a continuous basis. Using the previous example, if a 1-watt carbon composition resistor and a 1-watt carbon film resistor have the same surface area, they will have equal performance when it comes to dissipating heat on a continuous basis. (Note that “equal performance” really means they’ll be at the same temperature for a give power output. Due to differences of materials, they won’t really have equal performance when it comes to long-term reliability…)
So here’s what I’m getting at… your goal is to dissipate heat to the environment on a continuous basis. Therefore, your heat sink should have lots of surface area. Aluminum is usually the material of choice for many practical reasons. You’ll often see the heat sink painted black to better idealize a black-body, but I’m not sure if this will apply to your application. (Any physicists in the audience?) The fins should be vertical. Due to surface irregularities, you should use some kind of heat transfer material between the solar panel and heat sink. Instead of using conductive grease (which is very messy), I would use a silicone-based thermal conductor. It may be a bit more expensive, but much easier to work with. More info here:
http://www.aavidthermalloy.com/products/options/interface.shtml
I’m taking part of your post out of context to try to understand what you are trying to do.
Are you sure you want to absorb the heat or do you want to dissipate it. I would think you need to get rid of it.
Three things come to mind.
A flat plate,probably the most sensible is aluminum if you are considering cost,would allow the heat to dissipate to the air.
A piece of copper plate with a coil of copper tubing soldered on the back connected to a running water source. this allows you to cool below the ambiant air temp but is probably inefficent.
Just use a fan.Also inefficent.
I don’t see why you couldn’t run all your tests at the same time.Just miniatureise your test set up.
The CRC Handbook of Chemistry and Physics, which can be found at any self-respecting library, has heat data for many different materials. Look up thermal conductivity in the index.
I think most of you are writing about present day solar panels. Would the same things you’ve all so kindly presented work with these plastic sheets?
Here’s a rough outline of the experiment:
Project objective: Study the heat transfer characteristics of certain roofing materials as potential backing for flexible plastic solar sheets.
Introduction: Present day solar panels are large and heavy and may not not be that convenient to install on exiting roof tops. These panels are usually made up of arrays crystalline silica wafer cells encapsulated with a top glass layer for ruggedness. These panels usually require additional hardware like metal racks for placement on roofs. Many people are looking at integrating solar cells into roof shingles as a solution for residential and commercial building. However, another possibility solution for commercial buildings is to install solar sheets on flat roof tops. Flexible solar sheets is being researched on by many solar cell manufacturing companies and academic institutions. These solar sheets are usually made up of thin film amorphous silica deposited on a flexible substrate such as stainless steel or plastics (polymer). They are usually light weight and may be installed on new and old roof tops. Although these amorphous silica cells are not as efficient as crystalline cells in converting sunlight into electricity, given the right roof top conditions, they may be laid out in larger surface area and still deliver the necessary amount of power as the traditional solar panels.
Problem Statement: All solar cells, whether they are crystalline or amorphous silica deteriorates in performance under high temperature (temperature coefficient?). The regular solar panels are mounted on racks and hence have sufficient air ciruculation around and underneath these panels to dissipate the heat accumulated on the panels. Not so for the solar sheets if they are laid on roof tops, usually glued to a roofing material like foam (this is the most cost-effective way of installation). A material like foam not only offers a layer for glueing the sheets but also offer additional insulation for the commercial building, and hence savings energy. The performance of the solar sheets will depends on how well the underlayer to which they are glued can dissipate heat. Therefore, the success of deploying this new type of solar technology will be dependent on the appropriate choice of a roofing material.
In this project, we will investigate the following:
- various materials, including foam, which may be viable underlayer for the solar sheets.
- heat transfer characteristics (???) with varying material thickness .
- performances of the solar sheets under certain temperature conditions.
The final deliverables of this project will be: - findings and results
- recommendations for a plausible roofing material for the sheets
Methodolgy:
- use readily available roofing materials
- use readily available temperature measuring devices (meat thermometer?)
Most information provided in this thread are basic physics. You didn’t tell us about constraints specific to your problem so of course it’s not 100% appropriate to your question. Now that you have I’ll babble on a bit more.
So you have a bare roof with a foam coating. Can we assume that the foam is necessary for insulation in cold weather, and removing the foam is not an option?
Then you need to think about how and where you are dissipating the heat to. Or rather, you should experiment with different methods and places to dissipate the heat. What are the common mechanisms for heat transfer? Conduction, radiation, forced convection and natural convection. Forced convection requires a pump or fan, so that may not be practical for your use.
Natural convection is just surrounding air being heated up and rising away from the surface - to maximize that, the best thing is a bare solar cell without a cover. But you probably need some sort of protective transparent cover, right? So if you want as much convective cooling as possible, you want as thin a covering as possible, made of material with high thermal conductivity. Unfortunately we’re a few centuries too early for transparent aluminum, and there aren’t many transparent materials with high thermal conductivity. You can look up some values in the CRC handbook, as suggested.
Radiative cooling depends on the surface properties, not just of the cell but also the covering. The value you want is “emissivity” - that measures how well something radiates heat. But you can’t change the emissivity of the solar cell itself. And if you try to use the surface of the transparent cover as a radiator, you have the same problem as I mentioned above - i.e. heat doesn’t transfer across the cover very well.
So what’s left? Conduction. Basically, you want something between the solar cell and the foam to conduct heat away from the cell. But where does the heat go? Do you absorb it and store it in this layer? Try calculating the amount of heat energy absorbed by the solar cell over the course of one day (maybe you’ve done that already?), assume a reasonable material and thickness for the heat absorption layer, and see how much the energy will raise its temperature. This won’t be exact but it’ll give you some idea of the feasibility of this method.
If the amount is reasonable, your problem is solved. If it heats up too much, it means you need to conduct the heat away from the cells and dump it somewhere else. As already stated by many people, the standard way of dumping heat is to use a “heat sink” - a block of metal cut up to increase surface area, so heat is transferred to the air effectively. If it’s in sunlight, make sure it’s nice and shiny because you don’t want that to absorb more sunlight. I have to say I’m having a hard time coming up with other ways, but that’s your job!
I hope that gives you a better idea of the problems you’re dealing with. Have fun! You’d probably want to pick up a textbook or two on heat transfer. You don’t need a full class on thermodynamics for this stuff - just some understanding on heat conduction and transfer. I recently had a lot of fun solving a similar problem (I’m a grad student) - managed to cool a high-altitude balloon payload in broad daylight to -14 degrees C purely by convection and radiation.
You don’t mention if your solar sheets work as a roofing material all by themselves.If so your problem is simply to radiate the heat into the attic and blow it out the gable end.