Materials For Heat Collectors

[Nosetotail]

Hi there. I joined the forum to hopefully get help with building a solar heat collector for my swimming pool. There are several trees primarily to the south of the pool so it only receives about three hours of sun in the early and late months of the year. I'm tossing around a couple of ideas but would like to get away with the most hassle free setup.

 

My first idea was to utilize a large Fresnel lens aimed at a black piece of copper submerged just under the surface of the water. My pool has a significant circular circulation that also stirs the water from the top to the bottom, so I believe surface heating my pool will be amply effective. My theory behind this design is that nearly 100% of the available heat energy from the Fresnel lens will be transferred directly to the pool water without loss from the whole heat transfer setup.

 

This brings me to my first question which I think I already know the answer to. Would a submerged black sheet of copper the same size as the Fresnel lens collect just as much heat energy as the Fresnel lens and spot collector would put out? I'm actually thinking it may even be a little more efficient than trying to collect the same amount of heat in a concentrated spot.

 

This brings me to my second question. Do different materials absorb more heat? I understand completely that metals like copper have excellent heat transfer capabilities and make them the prime candidate for solar heat collectors, however if the material I am using to absorb the heat is submerged in the medium I am trying to heat, I am not worried about how efficiently it transfers heat, as it will all be absorbed by the medium regardless. I'm posing this question due to a claim made by the host of Greenpowerscience in this video:

 

http://www.youtube.com/watch?v=53qw4FdVJGM

 

 

where he states any black object the same size as the Fresnel lens will be just as effective as the lens and collector. I'm actually wondering if perhaps a black piece of copper may not be the best object for strictly absorbing heat from the sun, as it may to some extent reflect some of the heat. I'm thinking that a more porous or looser molecular structure material might actually absorb more of the suns rays and actually produce or collect more overall heat from the sun. Of course that may be exactly what copper is best at.

 

I originally thought it would work out to build a floating platform supporting the Fresnel lens and collector however with the suspicion that the lens won't produce any more heat than a collector of the same size, I may just be building a standard solar heat collector.

[Tormod]

Welcome to our forums - and thanks for posting an interesting topic. I don't know much about heat collectors, but I'm confident that others who hang out here do!

[HydrogenBond]

I think if your copper heat sink is submerged, you will get reflection from the surface water thereby lowering your collection efficiency. Simple things like waves and ripples will refract/reflect your input light away from the target.

 

I think a better way is use your target out of the water, with a heat exchanger that is connected to the pumped pool water. The hot water then needs to be pumped to the bottom of the pool, where it will rise with free convection. The hope is there is enough time for complete water-to-water heat transfer, before the rising free convection gives off heat at the surface.

 

Another trick is fill a larger heat exchanger cavity with containers of low melting point wax. If you generate more heat than you can take away with your water flow, the extra heat will be absorbed into the heat of fusion of the wax. When the sun goes down, you may get a few extra hours of warm water. The wax will re-solidify and give off heat at say 90F, for example. Heat of fusion within a wax can store even more energy than the heat capacity of copper or water.

 

I researched waxes For a DOE energy project, to create a passive heat sink for solar home heating. It could maintain uniform heating, with the sun up or down, since the melting and freezing kept the heat exchange temperature steady at the melting point. Less need for a fancy control system.

[Qfwfq]

he states any black object the same size as the Fresnel lens will be just as effective as the lens and collector. I'm actually wondering if perhaps a black piece of copper may not be the best object for strictly absorbing heat from the sun, as it may to some extent reflect some of the heat. I'm thinking that a more porous or looser molecular structure material might actually absorb more of the suns rays and actually produce or collect more overall heat from the sun.

What you need is a surface of excellent emissivity, which certainly will look matt black. It isn't sufficient for it to look matt black, it might not be good in other parts of the spectrum, but if it is great at all wavelengths, including visible, it will be matt black.

 

The drawback is that when the sun isn't shining on it, the surface will be cooling the pool about as efficiently as it heats it. Actually, without a well designed optical system for the source and target (which implies traccking the sun) there is an imbalance toward cooling. As this isn't so easy, it is simpler to have the absorbing surfaces in when the sun is strong and not at other times.

[Nosetotail]

I think if your copper heat sink is submerged, you will get reflection from the surface water thereby lowering your collection efficiency. Simple things like waves and ripples will refract/reflect your input light away from the target.

 

I think a better way is use your target out of the water, with a heat exchanger that is connected to the pumped pool water. The hot water then needs to be pumped to the bottom of the pool, where it will rise with free convection. The hope is there is enough time for complete water-to-water heat transfer, before the rising free convection gives off heat at the surface.

 

Another trick is fill a larger heat exchanger cavity with containers of low melting point wax. If you generate more heat than you can take away with your water flow, the extra heat will be absorbed into the heat of fusion of the wax. When the sun goes down, you may get a few extra hours of warm water. The wax will re-solidify and give off heat at say 90F, for example. Heat of fusion within a wax can store even more energy than the heat capacity of copper or water.

 

I researched waxes For a DOE energy project, to create a passive heat sink for solar home heating. It could maintain uniform heating, with the sun up or down, since the melting and freezing kept the heat exchange temperature steady at the melting point. Less need for a fancy control system.

 

I knew that reflection may be a problem, however the idea developed when I saw the Fresnel lens. At first I thought I could harness a very large amount of heat from a lens. The thought of placing it over the pool came primarily from the inherent danger of a heat collector that is capable of producing heat to the tune of 2000 degrees in a single spot. There is just too much inherent danger with that, so I settled on a compromise of floating it over a large pool of water to minimize the danger. As I think about it more, I am almost certain that even though it is capable of extreme heat, it isn't magical. It will still only collect the amount of sunlight that is flowing across the surface area.

 

I'm starting to lean more in the direction of building a standard heat collector with copper panels to collect and copper tubing to transfer, which brings me to a whole new set of questions.

 

Copper is expensive, but is simpler to work with than aluminum. I have access to corrugated sheet copper that is about 6 inches wide, but is kind of thin. I couldn't begin to guess what gauge it is. I would say about as thin as a standard paper plate. So I'm wondering if this is going to have diminished collecting capacities due to it's overall lack of volume? Also, Is there a certain kind of paint or glaze that is used for heat collectors? If I build this in a box so to speak and sheet it with plexiglass, will it perform better, or will the reflection make it less efficient? Again with the plexiglass, I thought about using two way reflective film with the mirror facing inward, but am wondering if this will further diminish efficiency?

 

I already considered laying the copper sheet on a thin bed of tar over an insulated board to perhaps increase the heat retaining efficiency of the copper. This would perhaps work in an immediate manner similar to the wax heat battery you mentioned.

[Nosetotail]

What you need is a surface of excellent emissivity, which certainly will look matt black. It isn't sufficient for it to look matt black, it might not be good in other parts of the spectrum, but if it is great at all wavelengths, including visible, it will be matt black.

 

The drawback is that when the sun isn't shining on it, the surface will be cooling the pool about as efficiently as it heats it. Actually, without a well designed optical system for the source and target (which implies traccking the sun) there is an imbalance toward cooling. As this isn't so easy, it is simpler to have the absorbing surfaces in when the sun is strong and not at other times.

 

I understand that the collector can also draw heat out of the pool, so I'll just have my pump running during the optimum times. As far as tracking the sun, I'll set it on an optimum angle toward the south so that it is aimed just a bit lower than the noon day sun. This will grab the most heat across the mid day sun when the sun light is at it's highest intensity. It will continue to collect heat although not as efficiently throughout the day, but I'm mainly concerned with the more intense midday sun.

[Qfwfq]

I'll set it on an optimum angle toward the south so that it is aimed just a bit lower than the noon day sun. This will grab the most heat across the mid day sun when the sun light is at it's highest intensity. It will continue to collect heat although not as efficiently throughout the day, but I'm mainly concerned with the more intense midday sun.

This means that you don't want it too tightly focussed, so increasing the need to "knock it out" when the balance goes the opposite way.