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Will this work?
  • Hi,

    I bet a lot of you have heard of "The Solar Updraft Tower". If not, then here are a few links:-

    Now suppose I want to build something like this on a much smaller scale, say, just big enough to provide electricity to a small household, maybe 300-500 Watts. I want to improvise the design since i think that the maintenance of a glass canopy would be too much hassle. Glass is expensive and breaks easily.

    So I have some ideas. Suppose we lay down some metal sheets very close to the ground(maybe half an inch above the ground). We paint the top surface of this metal canopy black, so it absorbs sunlight and gets really hot(I think 60 degree Celsius is easily attainable in tropical regions like southern India). We spread these black painted metal sheets over an area of a few square meters(maybe 100). At the center of this canopy we make a hole and place a 10 feet tall chimney and inside the chimney we place a turbine. So I am hoping that heat absorbed by the metal sheets will be transferred to the thin layer of air underneath the sheets. The hot air will rise through the chimney and rotate the turbine.

    Will this work? I welcome advice/criticism/suggestions/curiosity.
  • 12 Comments sorted by
  • Vote Up0Vote Down
    November 2012
    300 - 500 Watt per hour or instantaneous?
  • The 300-500 watt was just a rough estimation. So basically i want enough power to run a laptop for several hours during the day, a small refrigerator too maybe, with enough left over to charge the batteries to keep a few lights on for 4-5 hours during the night. I may also need to use a small water pump for a few minutes to pump underground water. How much power would be enough to do this?
    Also, is the device technically feasible, even if I lower the requirement to 100 Watts?
  • Vote Up0Vote Down
    November 2012
    You may want to read this article:
  • Thanks. The article was informative, but it deals with thermoelectric effect which is mostly a microscopic phenomenon. My concern is mostly the mechanical aspects of the device. Will the air under the canopy get hot enough to gain enough velocity in order to drive a turbine?
  • Probably. It looks like the design is very easy to scale. If you're not getting enough power just make it bigger. Of course it will be highly dependent on having enough spare land and on appropriate weather. 

    I'm not sure about replacing the plexiglass with black metal, but it seems cheap enough to just give it a try.
  • Thanks Matt Maier. These days I am looking for people in bangalore city who can help me with the technical aspects of design and construction. I hope i am able to build a prototype soon
  • Vote Up0Vote Down
    December 2012
    You could probably get some more ideas and information from here:
  • Hi Rajesh, reading the wikipedia page you supplied tells me that the cost of the collector+tower+generator would be much more than solar PV for the same power, even if you achieve the industrial scale efficiency of the 7km dia, 1km tall version they speak of, which is unlikely (I would guess from my understanding of fluid flow dynamics that the resistances of the scaled down version would reduce the collection rate by 10 or even 100 times). this based on the price we paid last year for PV (~€1/watt) and prices I've been quoted in the past for roofing steel sheet (~€5/m²). The black painted steel sheet would forfeit some of the advantages of the solar tower (able to grow food in the collector and heat storage for night) and at the same time would probably decay faster than the PV too (climate depending, if you're in a really dry place, maybe not!).

    I hope I'm wrong and your idea works wonderfully if you try it. May I ask the reasons for you wanting to do it this way?

    I spent some time looking into sterling engines a few years back and came up with a simillar concept to the one here in OSE of using reflective concentrators for high temperature heat collection with a steam accumulator for storage (8m³ could store enough for 6 months), sterling motor for electric conversion and various other direct heat uses for cooking, heating water and house, and heat powered fridges (amonia constant pressure cycle like gas fridge) for food and air-conditioning, storing day created cool for the night in an ice accumulator, too. All the energy needs of an European home served by its own roof. A 250°C prototype could be made using an LPG bottle for the storage (steam pressure at 250°C is 36bar, same as propane at 50°C, so should be safe enough) and old caravan fridges (we have 4, one from each of the 20 year old caravans we've been living in for the past few years) run happily from 205°C heat. Heat collection and storage can be better than 80% efficient, too, so anything that can be run from heat directly should be (in my opinion).

    One other thought comes to mind - that efficiency number of 0.5% is the same as my estimate of the per land area efficiency of growing trees and using them as fuel. Now a tree is a truly high-tec device that is self assembling, self repairing, self adaptive to its surroundings, mitigates CO2, builds soil, provides food, moderates the environment.....
  • Yes. These are points to think about. But anyway my intention right now is to build a small prototype and see how it performs (not larger that 10-20 m²). I am not really sure how well it will do. I hope it's worth a try.

  • Hi Rajesh

    I'm glad you're going to do it anyway, I hoped you hadn't got disparaged! It will do something, and the materials and
    learning will all be useful to you in your future if you decide its
    not that good. So I've put together some thoughts on how to maximise
    the result. I apologise in advance if the level of my explanations is
    too low or seems condescending, I don't know how much you or anyone
    else knows about these things, or for that matter how much I know (I
    am entirely self taught from Internet and books) - anyone should
    feel free to correct me if my thinking is inaccurate.


    See file Diagram 1: whole system overview

    1: Contrary to my earlier statement
    about viscous resistance, get the main area of the collector as close
    to the ground as possible, down to a total area not less than double
    the area of the chimney pipe. If possible ensure that the ground is
    as flat as possible too, although completely smooth will actual work
    against you as turbulence increases heat transfer. the perimeter is
    going to be so large that the actual velocity of the air at that
    point will be tiny.

    2: Increase the distance between floor
    and collector as you approach the pipe to maintain the 2*pipe area as
    the circumference decreases. Also try to make the transition to the
    pipe as smooth at possible as sharp corners create resistance to

    3: the length of the pipe is a delicate
    balance between losses and gains. Insulating the pipe will keep the
    gasses inside as hot as possible. Draw is affected by 3 things: 1)
    Temperature difference between inside and outside give a density
    difference (boyles law, density or pressure are related to absolute
    temperature ie Kelvin); 2) The hight of the pipe converts the density
    difference into a pressure difference that can be used to drive a
    reaction turbine; 3) The diameter of the pipe affects the resistance
    to flow drastically - about 10 times less resistance for a 50%
    increase in diameter.

    4: Putting the turbine at the top may
    not make any significant difference but it could do on 2 counts: 1)
    theoretically the energy is taken from the heat in the flow so this
    keeps the pipe gasses as hot as possible; 2) The turbine will
    introduce turbulence into the flow which will increase the resistance
    to flow in the pipe, and increase heat loss to the walls of the pipe.
    The turbine should be fairly small diameter, to get a higher
    rotational speed from the air velocity as this helps get better
    results from a small generator. Concentrating the flow into it gives
    it more air velocity and pressure to work with. (see Diagram 2)

    5: The generator mounted here needs to
    be matched to the speed of rotation of the turbine at load and to the
    voltage of the battery or fridge or whatever. It should be a
    non-cogging type (sorry, no DC motors as the magnets grab hold of the
    rotor when it's still and will stop the turbine from starting at all,
    even with a lot of flow).


    See file Diagram 2: Turbine and Generator

    In case you don't know how a reaction
    turbine works, you need 2 sets of blades. The first set is static and
    speeds up the air with a rotational swirl, converting pressure into
    kinetic energy. The second set of blades rotate and are pushed round
    by the swirling air but the shape of them slows down the swirl to
    near zero as they pass. The pressure has been reduced (cooling the
    gas) and the flow volume remains constant up the pipe, as it must.

    I'm imagining this turbine being made
    from a computer cooling fan for the rotor (~80mm diameter) and
    possibly wood and sheet plastic or metal for the stator (cut curved
    slots with a knife and fit the sheet into the curve). The lower
    bearing of the shaft is within the central block of the stator (not
    shown for clarity). Another cooling fan probably cannot be used for
    the stator since the blades must curve the opposite way, and I have
    never seen a cooling fan made to rotate that way. It is possible to
    put multiple pairs of stators and rotors on the shaft, but may bring
    little benefit except at startup.

    An expansion zone after the turbine
    controls the expansion to realise some pressure drop in this zone,
    and any swirl remaining after the rotor will increase this.

    The generator I am imagining is made
    from stock plastic or plywood with some thicker mild steel sheet for
    the top and bottom plates to assist the magnets, which are the 10mm
    diameter 10mm thick ones I have knocking about somewhere. It is a
    miniature version of Hugh Piggotts axial flux 3-phase generator. Cutting circles with a hole-saw gives a good circle with a hole in
    the center. Drill 10mm holes for the magnets in the plywood and push
    them in. Glue the sheet metal to the backs - these are the rotors.
    The stator has the upper bearing in its center. use a larger hole-saw
    to make this circle if available, drill large holes for the coils
    (20mm? number of turns and gauge to be determined when you know how
    fast its all going to spin, but the inner diameter is the same as the
    size of the magnet) glue them in and cover with a thin smooth layer
    of plastic (sticky-backed?) set the upper bearing in the center. Put
    it all together on some threaded bar with a washer either side of the
    top bearing to give some clearance. Fit the stator into a tube for a
    case, sealing the top against weather (obviously). This then gets
    supported far enough away from the top of the pipe to be out of the
    way of the airflow.

    This won't make much power but will be
    enough for a test run and can be scaled up with more/bigger magnets
    and coils if it turns out there's more power available. The voltage
    generated is determined by the equation

    equation 1: V= N×dΦ/dt

    where Φ
    is the total flux change in Webers, t is time in seconds and N is the
    number of turns of wire.


    my 10mm diameter magnets have flux
    density of 0.45 Teslas. 1 Weber = 1 Tesla × 1 m², so total flux =
    0.0000353 Webers.

    If there are 8 magnet pairs and 6 coils
    (as Hugh Piggots design) then each coil will experience 0.000141
    Webers change each 1/8th of a revolution. If the rotor is rotating at
    20 revolutions per second this is .023 V/turn average. With 2 coils
    in series and star wiring of all six this is .069V/turn. to get 20V
    no-load output (remember I×R voltage loss in coils and wiring to
    battery will reduce) to charge a 12V lead-acid or Nickel-Iron battery
    therefore needs 290 turns per coil.

    The steel plates on the outside should
    feed some of the flux from the back of one magnet through to the
    front of its neighbours, but I don't know how much so have left this
    out of the calculations.

    I have another design for a larger
    generator which in theory has many advantages, not least that it
    requires no more magnets than this one. It is a double generator with
    the magnets static, generating a current in small rotating coils that
    is used to energise larger electromagnets which spin round a second,
    static set of coils as per above. some electronics is required to be
    on the rotor but at a minimum this is just 6 diodes to rectify the
    3-phase from the mini generator for the main electromagnets. It could
    have a buck regulator as well to reduce the extra load when enough
    current is already coming to saturate the iron of the main generator.
    It has zero starting torque like the one above because the
    electromagnets have little residual flux until the mini-generator has
    started to turn, but many times the flux density in the main
    generator allowing less turns of fatter wire and reducing losses. An
    alternative is to use the lower starting RPM and high peak voltage
    with a buck regulator matching the output voltage/current to the
    battery/load, reducing wiring losses as well. I originally thought of
    this for use with a wind turbine where the wide possible operating
    range and RPM² output matched the speed³ energy of the wind better
    than a standard generator.

    I hope this all helps.

    Any comments?

    PS. just a thought, but for
    experimentation purposes agricultural silage sheet (100µm thick
    black UV resistant polyethylene sheet available in huge sizes) may be
    a fine heat membrane up to 80°C and could be stretched out into
    shape in one piece. In UK 42×11m can be got for <£100. I use it
    for everything from temporary roofing and vapor barrier to pond

    Diagram 1.jpg 163K
    Diagram 2.jpg 288K
  • Thanks for the suggestions. My first goal is to get a fan to rotate. So I am thinking I will fist build a system without a generator to see whether the turbine spins. Once I achieve that I can start working on the generator and couple it to the turbine later. That would be a good start.
  • I have received suggestions to make the air flow more directional. Is it a good idea to use steel pipes instead of a metal canopy. I lay down a lot of black painted steel pipes on the ground. One end of all the pipes open into the inlet of the chimney. The chimney will create the draft to pull the hot air from inside the pipes. Is that a good idea? Also, I think the pipes will make the device easily portable. Are pipes a lot more expensive than metal sheets?

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