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Steam Generation with Microwaves

  1. Jan 21, 2010 #1
    Hello reader,

    I was heating some coffee in the microwave oven today when I had an idea come to me.

    Is it Possible to heat water with microwaves, (dielectric heating) to superheat water? Then allow it to form to steam, turn that steam into dry steam. And then use that to turn a generator thus making electricity?

    This idea seems kind of "free energy" as the electricity make would be fed back to the, what ever it is that make microwaves. Which makes me think that it would not work. i.e. electricity made is lower than electricity used.

    Anyway I wasn't even sure where to ask this question.

    Thank you for your time.

    Fitter
     
  2. jcsd
  3. Jan 21, 2010 #2

    mgb_phys

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    yes it just isn't a particularly efficent way to make steam compared to just burning whatever you are burning in the power station to make the electricity.

    You do use microwave heaters sometimes when you don't want to burn other parts - like drying glue in wood/plastics
     
  4. Jan 21, 2010 #3

    Averagesupernova

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    Funny this should come up. Several years ago I run across something on the internet that claimed a sort of perpetual motion thing by fitting a magnetron onto the cylinder head of a small engine. The engine drew in water and the magnetron was fired at the appropriate time to make steam, push the piston down, steam exhausted in the normal manner, cycle starts over again. And can you guess what powered the magnetron? Yep, a generator hooked to the crankshaft. Just another idea that wouldn't work that borders on what the OP has suggested. Thought I'd share.
     
  5. Jan 21, 2010 #4

    sas3

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    Here is the site for that motor/generator that clams to output more power then it uses.

    http://www.keelynet.com/energy/microeng.htm" [Broken]
     
    Last edited by a moderator: May 4, 2017
  6. Jan 21, 2010 #5

    mgb_phys

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    I thought that was www.enron.com ?
     
  7. Jan 22, 2010 #6
    While what you suggest is possible, it is not commercially viable for three major reasons:
    1. Efficiency: generating microwave power is only about 50% efficient, i.e. it takes 2 kilowatts of electrical power to produce 1 kilowatt of microwave power. That's why any scheme to generate household hot water with microwaves is nonsense - an immersion heater is nearly 100% efficient.

    2. Microwaves are not economical when heating large amounts of water to produce steam - one kilowatt will evaporate about 3.5 pounds of water an hour - i.e. create steam; and the installed cost of industrial microwave equipment is about $ 5,000 per kilowatt - do the math - it doesn't work comercially.

    3. Microwaves are not good at heating large bulks of water. The penetration depth into water at room temperature is about 1.3 cm - that means that a distance of 1.3 cm the microwave energy at the surface has diminished by nearly 2/3 - 1000 watts at the surface has only about 350 watts left at one penetration depth.(your coffee doesn't heat evenly - there is more microwave energy at the surface than the interior; actual temperature distribution is very complex and has to include cooling from the surrounding air, the lensing effect of a cylindrical coffee cup, and much more.) So, to heat the tons of water required for electrical generators is not mechanically reasonable.
     
  8. Jan 23, 2010 #7
    Hello Readers and Responders,

    Thank you for taking the time to answer my question. This of course raises a few more questions.

    1. Can the efficiency of generating microwave power be increased? Probably not, but I thought I would ask.

    2. The nuclear powerhouses I have worked in have an estimated cost, of a new plant, in the U.S. ranging from $6 to $10 billion. So in my opinion, the power companies have some money to work with.

    3. Since the penetration depth of microwaves into water is so poor, could the design of a microwave steam boiler be changed to heat a large surface area, i.e. an in closed pool of 4" deep water.

    4. Do microwaves and steam have the same relationship? If not could this be an efficiency type improvement? Reducing the amount of "stuff" one must burn to run the super heater and economizer that power generation requires.

    5. Is there a book or some source of information I could get my hands on? So I can educate myself a bit more about the subject? I find searching the internet to be...unreliable.

    Again thank you for your time.

    Fitter
     
  9. Jan 23, 2010 #8
    Not meaningfully. As pointed out, a resistive heating element is effectively 100% efficient. The only losses are in getting electrical power to the element, and miniscule electromagnetic losses. You could improve the efficiency of a microwave heater enough to approach that of a resistive heater by using waste heat from the magnetron and power supply to pre-heat the water heated by the microwaves, but what's the point? You're adding more and more complex and expensive equipment, not for any advantage, but to reduce the disadvantages of the approach.


    The issue isn't that it's too expensive for the amount of gain, it's more expensive and has negative gain.


    This increases surface area for loss of heat to the environment via conduction and radiation, as well as increasing complexity and material costs.


    Power is power. You can't get more thermal power by first converting electrical power to microwave power. You certainly can't convert that thermal power back into mechanical and then electrical power and get more power out than you put in. You only increase expense and losses.

    Microwave heating could have uses for heating materials to extremely high temperatures or for heating highly corrosive or abrasive materials, environments where resistive elements would be difficult to use or have short lifetimes, or where a hot heating element surface would accumulate deposits of material that would impede with its operation. It could also be useful when localized heating is required, when you're heating individual small objects or thin layers instead of a bulk fluid. There are some uses in chemical industry and manufacturing processes. It's not useful for producing household hot water, and using it for generating steam to produce electrical power is just wasting power.
     
  10. Jan 23, 2010 #9

    RonL

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    This is an area that should offer some of the best options for energy systems and storage in the future (Imho).
    Energy can be stored in many ways and a flywheel has a greater potential than batteries, also thermal can be stored in a mass of steel that can give a release of flash steam that can change to superheated in microseconds.

    My thoughts are, anything that can transfer energy and at the same time remove as many sliding and positive contact parts as possible, will let the efficiency improve in great amounts. Gas flow and electrical transfer, can and will move very close to that impossible 100% mark.

    Ron
     
  11. Jan 23, 2010 #10
    Not sure how flywheels relate to microwaves, but thermal energy storage has some severe problems. The higher the temperature, the greater the losses to the environment through radiation and conduction. Lower the temperature to reduce these, and you decrease the efficiency at which you can recover energy from the stored heat.

    Aside from that...a mass of steel is far more efficiently heated by passing electrical current through it. Microwave heating can not improve over resistive heating in this case. Almost all of the power dissipated in resistive heating is dissipated as heat in the heating element. Magnetrons themselves heat up and need to be cooled, and their cooling systems will consume power, and their power supplies will themselves have conversion losses and need cooling...


    This is an oversimplification. For example, solid state heat pumps using thermoelectric junctions are terribly inefficient compared to heat pumps using compressors full of sliding seals and other moving parts. The effect that makes those heat pumps work makes compressed gas energy storage less efficient...compressing gas leads causes it to heat up. Some of that heat is then lost to the surroundings, reducing pressure and the amount of energy you can recover at a later time.
     
  12. Jan 25, 2010 #11
    this concept is tied to one of the things I'd like to do. Land speed record on steam. Couldn't you use a resistive preheater, then flash the preheated water to dry steam with the microwave? (mythbusters got 350F as a highest temp, if I recall correctly) The advantages of heating the water electrically is: smaller size of combustion chamber, running out of water is not a meltdown, and the glass superheater tube, and heater element could easily be changed, quickly, and insulation would be a breeze. The car could be run for its passes (2 required within an hour) with no re-fueling problems like the brits had. just change the batteries, and fill water tank and the "Onan" (the generator).

    dr
     
  13. Jan 25, 2010 #12

    RonL

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    I do tend to oversimplify my thoughts, but there are so many machine configurations in my mind it is hard to talk about any one thing and not bring in comments that confuse everyone trying to picture what I'm talking about.
    I did buy a CD from microwaveguru, or his recomended source some time ago (well worth the money). One question not answered yet is, can two or more magnatrons be aimed at each other, with only water between them and no damage be incurred, while all energy is absorbed in the water?
    If all parts are prepared correctly, the entire power system of a microwave unit can be enclosed within a shell and volume of non conductive oil on the side of a flywheel, the oil captures otherwise lost heat and can preheat the water (or refridgerant) used in the steam cycle.
    Two flywheels face to face, with just a very small space between them, can be driven in counter rotation, using a very small volume of liquid being flashed to high pressure gas flow.

    The idea beyond here gets into too much detail for a short post.

    To me the best method of heating liquid is using a resistance in wire. (as said, almost 100%)

    Ron
     
  14. Jan 25, 2010 #13

    mgb_phys

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    And what are you using to power the heater and microwave - batteries?

    You can do exactly the same thing with a propane powered water heater

    That's not allowed - it's rather like saying the driver could just get out and take the steering wheel to a second car
     
  15. Jan 25, 2010 #14
    Batteries, and a generator would be the source of power. The sizing between then would be so that a failure of either would be enough to complete the run. A propane hot watter heater will not do that. It would need to be either multiple tube boiler, (with tubes to plug up) or a tank style boiler that would not have the heating surface area. I don't get your last quote. you have one hour to make your return run to be used for minor repairs if needed and re-fueling. Gasious or cryo'd fuels are to unpredictable with the residual heat that will migrate into the car. Thats why the brits couldn't re-fuel, because the residual heat caused the fuel tank to be too hot to fill without causing the CNG to flash to vapor, resulting in more pressure inside the empty tank than the fueling cart could push into the tank. The resistance heater would be an "element in a pipe" and its output would go to the microwave superheater. Ron, I was wondering if you could double the mag's but wondered exactly the same thing. I had thought about some kind of "protection slug" that if water flow stopped, something would absorb the radiation during shut down. Remember, high efficiency is prefered, but not required for this application. Chassis would be a 74 dodge charger in superbird trim. 200 mph is the goal.

    dr

    another "crackpot idea" from dr dodge...lol
     
    Last edited: Jan 25, 2010
  16. Jan 25, 2010 #15

    mgb_phys

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    The electric land speed record, using batteries and an electric motor is 315mph.
    It's unlikely that replacing the electric motor with a microwave, a water supply, a boiler and a steam turbine is lighter and more efficent.
     
  17. Jan 25, 2010 #16
    well, the fact that steam expands at 1600:1 adds aq lot more power potential. Current electric diesel locomotives still do not produce the same power as a steam locomotive that same tonnage. the goal is not to break 315 for electric, its to break 139 mph for steam (and do it for way less than $400,000+)
    dr
     
  18. Jan 25, 2010 #17

    mgb_phys

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    There was an article describing the technical limitations set by the rules but I can't find it on their web page.

    Form what I remember the limit was having to carry enough water for the return trip, they were allowed to refill but not with pre-heated water. So they would have had to cool the boiler, refill and get back upto temperature.
    They claim it is a partial flash boiler - so presumably that means a separate dry steam superheater. So i'm guessing they didn't have enough spare pump power to run with cold water injected directly into a flash boiler.

    To be honest the whole thing is a bit silly, it didn't really improve on the 100 year old steam car result in spite of all the modern technology and didn't really prove anything.
     
    Last edited: Jan 25, 2010
  19. Jan 25, 2010 #18

    mgb_phys

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    They are remarkably similair, I suppose lightweight isn't a big driver for railways.

    The famous Mallard was 100t, 167 with the tender, famously did 125mph on a record breaking run and produced 158Kn (they don't seem to quote power for steam locos)
    the Intercity 125 diesel also did 125mph 9standrd - it did 150mph for a record) weighs 70tons and generates only half as much force (although you do have one at each end).

    The big saving is in man power and maintenance.
     
  20. Jan 26, 2010 #19
    that is very true. I have heard that something like 2/3 of the rail employees, in the age of steam, were to maintain the locomotives. In 10 years those people, and jobs were gone forever. Now, as we restore old loco's there is a "disconnect" between our knowledge and theirs. best practices lost, if you will, only for us to attempt to discover again. I don't think the lack of soot everywhere is a bad thing, though...lol

    dr
     
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