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A Simple Design For A Cheap Solar Thermal Collector
  • I am proposing a very simple design for a solar thermal collector. The basic idea is shown in the image attached.

    The system shown in the diagram can be used as a
    concentrating solar thermal system.

      It uses a transparent plastic or glass
    cylindrical tube that acts like a cylindrical lens when filled with water. 

    The focus of this
    lens would be a straight line, just like a parabolic trough. Therefore, we can
    a place a metal collector pipe along this focus.

      The heat collected in this pipe can be used
    to boil water and make steam just like the parabolic trough concentrator.

    The sun-tracking of this system is really simple as shown in the second image.

    Advantages of the lens collector over
    the parabolic trough collector:-

    The design is

    It does not require
    very high reflective glass. Simple plastic and water is enough. This will
    reduce the cost of construction since the materials are cheaper.

    In the parabolic
    trough collector, it is required that the entire trough has to track the
    movement of the sun continuously. This complicates the design. But in the
    cylindrical lens collector, only the metal collector at the focus has to be
    adjusted according to the movement of the sun. It is easier.

    The long cylinder
    can be divided into smaller sections. It can be assembled on site and then
    filled with water. This makes the construction process easier for remote
    locations. In parabolic trough collector, the trough has to be pre-assembled
    and then transported in large trucks.

    The cylindrical
    shape provides better  protection against
    strong winds when compared to the parabolic shape. Many parabolic reflectors
    simply break in a strong wind situation.

    Solar Thermal Collector.JPG 53K
    Solar Thermal Collector_Sun-Tracking.JPG 35K
  • 40 Comments sorted by
  • Interesting.
    Some thoughts:
    Water is very heavy. Whatever tube forms the lens body will have to be able to withstand the weight of the water without substantial bending, or the focus will be affected.
    The power of the lens to collect sunlight is determined by its diameter, so a bigger diameter is better.
    The round shape, coupled with the weight of the water will certainly resist shaking from the wind, and since the weight is great, the support structure must be strong to begin with, so damage from high winds is not likely to be an issue, unless we're talking about very strong storms or similar.

    I wonder what the effect might be of introducing various substances into solution in the water. Perhaps something like salt or acid might increase the refractive index and make the water-lens better at focusing light as well as deterring algae or bacterial growth, which will cloud the water and reduce efficiency.
    I'd like to see someone play with: a: solid lens (plastic, glass, etc.), b: water-lens in plastic vs. glass c: water lens made from two concentric cylinders, to reduce weight, etc.
    Perhaps it would be possible to make a Fresnel water lens with a shallow pan, the bottom of which was vacuum-formed polycarbonate.

    A bank of shorter sections of lens with a "serpentine" chain of collector piping below might make a more compact, sturdy structure. The collector could rock back and forth on four or more rocker arms to maintain focal distance.

    Have you done any testing? I'd like to see what a clear polycarbonate tube would do at perhaps four inches diameter (100 mm), and, say, 1 inch/25mm black iron pipe for the collector. I bet this kind of rig would be just the ticket for running a small steam engine or a hot-air engine.

    In short, I think this is an idea worth pursuing.
  • I made a quick model in SW to check the mass of a sample configuration.
    A 100mm (OD) plastic tube with a 2mm wall thickness, having a length of 1 meter, filled with water, would have a mass of only 7.8981 Kg.
    This does not include any structure or even end-caps... it's just the tube and the water.

    This collector body would collect light as an area of 1m x 0.1m, or 0.1 square meters.
    If we take 1KW as the Solar Maximum of power per square meter, then each 1m tube would collect a maximum of 100 Watts.
  • Hi ,
    Thanks for the comments. Good to know someone actually ran a computer simulation of this idea.  If we can get a concentration ratio of 10:1 or higher, we can get really high temperatures.
    I haven't done any actual testing. I have tried to get a working model built by some local water heater companies here in bangalore, but they have not been very enthusiatic since this is experimental. 
  • If the main purpose is to just heat the water... Then simple wound 19mm polytube in a black box with a glass/plexiglass cover, will easily boil the water. To a point where you actually need to have a steam pressure release valve installed.

    Plenty of videos on YouTube using this principle and I can even take a photo of my neighbours setup. It does just as I describe.
  • Concentrated Solar power is required to boil water quickly. Flat plate collectors take several hours to do that. All concentrating techniques use either reflective surfaces or lenses. I have seen several videos on youtube that describe these techniques. The problem with reflective glass is that they are expensive and fragile. You can have a large array of parabolic reflectors but the main limitation there is the cost of the reflective surface that must have at 90-95% reflectivity. Plus they break under strong wind conditions. The SEGS power plant loses 3000 parabolic troughs a year to wind damage. 
    I have also seen people using frensel reflectors, but cost is a hindrance. Some people are experimenting with Frensel lenses.
    Once again, these thin flat plastic surfaces would be very fragile.
    Hence i am suggesting a compromise between all the known techniques. A perfectly round cylindrical plastic surface would be both cheap and sturdy. Cylindrical shapes probably offer the least resistance to wind, thereby making them last really long. 
  • I think you are underestimating how efficient the "Flat Plate" setups as you refer them actually are.

    A decent setup (albiet a bit large) can boil a full winding in about 30-40 minutes in full sun.
    If you need an IHWS (Instant Hot Water Supply/Service) then I would suggest looking at gas heated water coils. Used commonly in caravans and mobile homes, but now becoming the norm in everyday housing.
    With this you can use either LPG/Propane or you can design a system that uses Hydrogen.

    That being said, if you still wish to pursuit this design course, then I suggest looking at a YouTube channel called GreenPowerScience.
    It is hosted by a guy by the name of Dan Rojas and his with Denise.
    Now he explores the very design concept that you are looking at... What he does is take a large diameter PVC pipe (around 150mm diam.) and then coats it with a highly reflective plastic sheet.
    If I remember correctly, he states that it should last you around 10 years before you need to replace it.

    He has a video of how to piece together exactly what you are trying to do;

  • Dan's videos are great, but most of his stuff is limited in power.
    We're talking about a solar collector capable of producing enough steam to drive a steam engine of sufficient capacity to be relevant for farm use, not a toy.
    There's a vast difference between heating domestic hot water with solar energy, versus replacing a fuel-powered boiler by concentrating sunlight.
  • Then I dare say you can't do much better than the tried and true method of Steam Engines that burn a medium for heat.

    Because what you have to take into consideration is the size of the engine and what kind of pressures are needed to get any kind of momentum or torque.
    You would have to have some fairly large collectors to push a tractor... So much so, it would be self defeating due to the weight of the collector itself.
    Then you have to issue of reduced performance under certain conditions; IE Overcast, shadow castings from trees/buildings.

    Perhaps another avenue to explore would be to see if you could use an ICE powered by Hydrogen?
  • It's almost like you're intentionally trying to miss the point of this idea.
    Yes, fuel is convenient, especially in a mobile engine application.
    What we're discussing here however is a robust, inexpensivelow-tech, easy-to-aim, easy-to-track-the-sun version of a solar collector for boiling steam with the concentrated power of the sun, for a stationary power plant.

  • Exactly that. I agree with wolfrick. It's safe to assume that nobody would be mounting one of these on a tractor.


    If you want more power you have to make arrays of small solar thermal collectors. The important requirement is the concentration ratio(10:1 or higher).   No matter what you do, if you want to use solar power you have to cover large areas of land. So the only way to make it economical is to use the cheapest possible material. Out of plastic, reflective surfaces and photovoltaics, Plastic is the cheapest.
  • I apologize, I misread what you meant when you mentioned a Steam Engine.
    I didn't think of an application where the engine would be stationary for generating electricity.

    Then it is more than possible ;p
    Would it also be worth looking at including the Tech behind a Sterling Engine? Use temperature differentials to help drive the pistons and reduce a little of the burden of the steam?
  • Hi, interesting idea, to make a lens from a watertube. I wonder how much energy gets stuck 'in' the tube. Also wonder about how it practically works out, if you get a decent focus to work with. About suntracking, I think soon we will have that nut cracked. I am working on it using software from as a start (open source). We are not there yet, but it's a matter of time!
    Anyway, great you take a whole new approach to sunfocussing. Hope to see a real life image!
  • In order to demonstrate a water tube cylindrical lens i did the following.
    I took a 2.5 litre pepsi bottle. Filled it with water and took it out in the sun and got the focus on the floor. The middle part of the bottle was a perfect cylinder. I got a prettly sharp line focus in the middle part.

    Check out the image attached.
    Water Tube cylinderical lens Demo.jpg 182K
  • Can somebody do a Net-Energy analysis on this? For the same size what would be the net energy gain for Water tube cylindrical lens collector, a parabolic trough collector and frensel reflector type collector. Kind of a comparative study?
  • This may interest you: It's an inflatable solar concentrator.
  • I think the Solar Thermal Concentrator should be build upon raw material find in desert

    THink open, but think also big

    We need a Glass expert,

    A simple design,

    And automated tools for building the glass parts

    In fact, It could be easier to get a walls of glass tubes with steams engine producing electricity : because the raw material is in abundance, and the concentrator should be more difficult ( and expensive ) to build : I SUPPOSE

    Then we directly produce more walls from our energy

    the growth is exponential

    BY THE WAY : In this design, you can use Water for producing electrical energy.... or even Algae producing, hydrogène, oil, methane , and biomass
  • Here are some sites i found that promote renewable energy.
  • Good stuff! Currently not using translucent polymers (mostly acrylic). For algae this is awesome. For solar, you can do with more economical materials?

    Did you have a look at similar existing design via A lot of them are over 20 years and belongs to public domain.
  • I don't think there are any patents on this. I haven't seen any commercial applications that resemble this idea. If there were patents they would have been put to use. This is not an invention anyway. It's just an idea. And its open source :)
  • The cheapest way to do this would be with recycled 2 liter plastic bottles. You can find those for free anywhere in the world. I know you were thinking of a single very large tube, but a hundred or so bottles might work just as well, but cheaper.
  • Yes. Billions of tons of plastic waste is thrown in the dump and into the sea each year. Better use it for something that let animals and fishes choke on it. :-(
  • I've been salvaging plastic bottles for a different hardware project I'm doing. I found five of them in one day, without especially looking for them, just by picking them up when I passed by them. 2l plastic bottles are EVERYWHERE and they're FREE.

    Each 2l plastic bottle will capture sunlight from about a 285 square centimeter area and concentrate it onto a strip about 28cm long and 1cm wide. You'd need to use mirrors to reflect many of these strips of light onto a single tube if you want to boil water.
  • To use mirrors would complicate the design. If you have the means, find a way to recycle all that plastic and use it to make a large transparent cylindrical drum. Then, it will be easier to use.
  • I have an idea that modifies this design.... Instead of trying for steam, if you have smaller versions of this you can try for convection. See my pic for how.... It would still have to rotate the heating pipe to stay in the focal point.

    Due to UV cracking of plastics, I would go with plexi-glass tubing or, even better, lexan.
    convection water lens.jpg 115K
  • Good idea. You can use a low-boiling-point working fluid like ammonia or R134 in place of water. It might improve the conversion efficiency. Keep sharing ideas like this.
  • An engineer/physicist can chip in on this, but my understanding of the Carnot cycle is that the higher the difference in temperature (delta T), the higher a conversion efficiency you get. For converting low temperature differences into useful energy, an open-source Stirling engine would probably get better conversion efficiency than a steam engine.
  • Hi,
    I have posted a follow-up to this idea--Another Design for a Cheap Solar Thermal Collector.

    Check it out.
  • OK, if the soda bottle can focus sunlight by a factor of 10, then line them up in a frame, and run black pipe or something along the focal lines in parallel, then output to a heat engine. You might get by with single vertical axis pivot to track the sun, and a simple adjustment of the pipes relative to bottles to account for sun elevation. Maybe not very efficient, but pretty cheap if you can get lots of bottles for free.

    Concept for "soda bottle array": frame of 1x4 or 2x4 lumber, with crossbars that are notched to hold the bottle neck on one side, and a small shelf screwed into the bottom of the crossbar to hold the bottom of the next bottle. bottles are lined up perhaps three rows of horizontal bottles by 8 ft long in the frame, with three pipes underneath to get the heat. The array is tilted at the best sun angle, and has a vertical pivot such as a vertical pipe to rotate around to follow the sun. The pipes may need to be insulated to reduce losses. The output is manifolded to a heat engine, or hot water storage for home use.
  • Bottles are not very large in size, so the heating effect may not be sufficient. You will need something with a larger surface area.
  • Yeah, when I suggested bottle concentrators, I forgot that the aim was to make steam. You definitely need something bigger for this project, though bottle would be good for concentrating light onto photovoltaic cells
  • There are a variety of needs for energy, from making electricity to simple water heating for washing dishes. A simple, low cost concentrator would be a useful "tool" in the toolbox, in the sense that you have a variety of wrenches for different needs. It could be used for home heating and such, while higher tech concentrators would be used for steam or metal casting. The metric would be lowest cost and complexity to reach a required concentration ratio, with different designs at different values.

    As a side note, even food growing can use low concentration ratios (on the order of 2) to warm a greenhouse and provide extra light to extend the growing season in some climates.
  • Its a good idea.

    I'm concerned about the size of the active components.  Can we do it without large active components?

    Given heat and sand, I can make shiny flat pieces of somewhat reflective stuff.

    Given earth moving and compressing equipment, I can engineer tiers mounting for fixed mounting of reflectors on a south facing hillside, essentially a large fresnel reflector.

    With towers and cables, highly efficient mirrors and shiny tubes, I can concentrate the resulting light onto a steam boiler.

    The big sustainability part is that the active and high tech components are minimized - cable steered reflector/collector that can be rebuilt and replaced as needed, even lowered into a bunker for storage against heavy winds.

    The mirror field itself can be maintained by hand and low technology - the vegetation as relevant cut back, and the mirrors themselves replaced with shinier and/or smoother versions as cheap technology to make them becomes available.  Or with robot tenders and such if people aren't inclined.

    The initial mirror set may not be the most efficient - but when you're covering a hillside, you don't have to be!  There will be plenty of power.

    Naturally, it should be engineered so that the sun at any time of the day/night/year concentrates the beams in human accessible places, that would be... bad.

    The question in the end is that is it more practical to put up more fixed mirrors (even if they're of limited reflectivity) and move the collector around, or is it better to keep the collector in one spot and heliostat the mirrors around...

  • Is it possible, that one may be confusing heat with temperature?

    Yes, a tightly focused beam of sunlight can quickly reach temperatures needed to boil water, but the amount of heat available from that cross-section of sunlight hasn't changed, so I contend that one would still need many such collectors and much much more area to produce enough steam to be able to do meaningful work with it.

    A match may be momentarily hotter than the Mississippi  river, but which contains more heat?  The River by several orders of magnitude.
  • High temperature leads to more power. The laws of thermodynamics state that when converting heat to mechanical work, higher temperature leads to higher conversion efficiency.
  • @DavidIAm
    I prefer collector research over heliostat research on the basis of safety first. Heliostat offtrack fires can be devastating. Collector meltdown only destroys the collector center component(s).

    2 liter bottles are great for short term, pickle or spaghetti sauce jars are glass and hold up under UV and are still obtainable as recycled. just a thought. Not sure about the solar bottle collector viability on the whole....

  • Vote Up0Vote Down
    December 2011
    Greetings All!

    This is my first post on these boards and I'm happy to be able to take part in this conversation. I have a few thoughts that I would like to input here as well. Im not sure if anyone is still working on this concept but I figured I'd throw my input out there anyways

    I love the concept of using a water filled tube for generating steam and thus electricity. It would be marvelous to see such a device in operation. There are a few of considerations to make when upscaling this idea to practically usable size. Materials, weight and climate. It would be important to use a material that would not react significantly to UV radiation from the sun. Typical plastic as found in plastic bottles does photo degrade. Acrylic turns yellow in the sunlight unless it has been treated against UV light, and glass is impractical because of its weight. There most definitely are solutions to this, but it would be fairly costly.

    I did some math of a tube that would equal the large fresnel lens on the measuring 49"x35". In order for the tube to have the same profile to the sun it would have to be 49" long with a diameter of 35". Looking at the weight of the tube and considering that water weighs approx 62lbs per cubic foot, then multiplying that by the 27.3 ft3 of the tube's volume we find the approx weight to be 1692.6 lbs. Perhaps using a lighter medium would help with this. Just for reference, the fresnel lens in this comparison weighs 21lbs in its wooden frame.

    Lastly, climate. This is obvious but bears mentioning. This would not work in climates with moderately cold winters simply because the tubes would freeze. This would not be the case if the liquid in the tubes had a lower freezing temperature, just something to consider.

    Have you thought of creating a long lens shape instead of a tube. It would reduce the weight, the amount of medium the sun passes through, while not loosing any of the surface area. See the attatchment or a basic drawing of what I mean. Also, I would recommend using distilled and/or  de-mineralized water as it will have less particulate matter in it, allowing more sunlight to pass through.

    Anyways, I hope this project is going well. I would love to see an update on the testing anyone has done on this!


    lens comparison.png 12K
  • In terms of using materials that can be manufactured on the Farm, it seems like polished metal is a better bet than UV resistant plastics. 

    Just off the top of my head it seems like the steel tubes used for the tractor could be used as a frame to hold polished aluminum sheets in a parabolic shape to focus reflected sunlight on a heat collection source. Then, water could be run through the steel frame to capture the head that was absorbed by the aluminum instead of being reflected. Aluminum flashing is already pretty shiny, and it comes in long rolls. 
  • Here is a video from some company called Matteran energy. They claim they can generate electricity from warm water. They also say they have a working prototype capable of generating 2.5 KW plus refrigeration. 

  • Vote Up0Vote Down
    January 2012
    "this patented process"

  • wow..5000 views. Keep it up.

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