A New Newcomen Solar Steam Engine

Discussion in 'Alternative Energy' started by JAK, Jul 11, 2006.

  1. JAK

    JAK Well-Known Member

    Messages:
    529
    Joined:
    Oct 15, 2005
    Location:
    New Brunswick
    The Newcomen Steam Engine, invented in 1760, is interesting to me because it injects water into the cylinder to create a vacuum rather than having an external condensor. It also works at near atmospheric pressure, and below, which might make it safer and practical for DIY home power. The lower boiler pressure and temperature makes it less efficient, but for a solar steam engine it is impractical to try and achieve too high of a temperature anyhow, due to losses to the environment. It is a tradeoff between collector efficiency and thermodynamic efficiency. The Newcomen Steam Engine would work best if you had really cold spring water or rain water. You could also combine heat and power to produce hot water, or space heat, somewhat at the expense of electrical generation. If you operated a boiler at 2 atmospheres, the steam temperature would be 250F, plus perhaps a little superheat. If you were heating hot water to 140degF with the condensate, your condensor pressure would be about 3psig. The thermodynamic efficiency would work out to about 1-(460+140)/(460+250) = 15.5% and you would get perhaps another 50% for hot water, so perhaps 1kw of electricity for every 3kwh of hot water (10,000 BTU or about 10 gallons). If you operated from 1 atmosphere, the thermodynamic efficiency would be reduced to 1-(460+140)/(460+212) = 10.7% but your solar collection efficiency might improve considerably. Perhaps it might still work with however much sun you get. Assuming a simple solar hot water collector, with a collector efficiency of 30% for losses to the environment, you would need about 200 ft2 of collector area to get 20,000 BTU of heat, which would convert into 10,000 BTU of hot water and 0.5kwh to 1.0kwh depending on how hot you could get the steam to run the engine. A better approach might be to run the condensor as cold as possible to generate power, and then simply use whatever hot water is left at the end of the day after the steam engine stops running. Consider injecting 50degF water to get 78degF condensate. With an atmospheric boiler temperature of 212degF and a condensor temperature of 50F the effiency would be 1-(460+78)/(460+212) = 20%. That is darn good considering the steam engine is cool enough to be made of PVC plastic.

    One disadvantage of mixing water and steam in the engine is you are adding mass when you are trying to create a vacuum. Newcomen's idea was to create a vacuum. James Watts great improvement was to have an external condensor and cool it from the outside. With this improvement you can get theroetical efficiencies as high as 25%, still without having very high boiler temperatures. Only by going to extremely high boiler pressures and temperatures, which push the metallurgical limits of boiler tube, and a much more complicated steam cycle and steam engine, can you get efficiencies approaching 40%. Today these cycles are further improved by combining a gas turbine cycle on top of a steam cycle, for perhaps 60% efficiency. But for a simple DIY solar combine heat and power cycle for 4kwh of electricity and 40 gallons of hot water from 200 square feet of solar collector I think Newcomen's engine should be revisited. Perhaps the same engine could run on wood heat in the winter, and the two could be combines in Spring and Fall with the wood boiler providing the final heating after the solar collectors did there best. By operating at lower temperatures and pressures I think it would also be easier to operate. The solar engine could be fully automated. The solar/wood engine might work by the solar collector doing its preheating on its own, and then the woodstove being fired up in the late afternoon to get whatever additional heat and power is required. The engine and generator could still be fully automated if you were just charging the batteries. In theory you could also hook a well pump and a freezer compressor to the engine directly in addition to a generator, so they don't run off batteries. The freezer could have a small thermal mass of brine so that it only needs to run 4 hours a day, but late afternoon is a good time for it to run anyhow. Washer and dryer are a little more tricky as even the best use about 1500w peak and 1.5kwh over 3 hours for a single load.

    The system would replace about 1000w of solar panels, or about $5000 worth, and might provide more consistent power year round with the combination of solar and wood power. Of course the only practical value might be that your would want to build such a monster for the recreational value.

    Steam Table Online:
    http://efunda.com/materials/water/steamtable_sat.cfm
    (delete cookies to reset the trial)

    Good description of a newcomen engine:
    http://inventors.about.com/gi/dynam...ogy.niagarac.on.ca/people/mcsele/newcomen.htm

    Interesting article about James Watt studying a Newcomen Engine:
    http://www.psigate.ac.uk/newsite/reference/plambeck/chem2/p02032.htm
     
  2. JAK

    JAK Well-Known Member

    Messages:
    529
    Joined:
    Oct 15, 2005
    Location:
    New Brunswick
    Thinking some more, I wonder if you could design a Newcomen engine to operate at a fairly constant speed and power output between a super hot hot water tank and a cold water tank. In this way you could separate your solar or wood heating from you power generation, and simple run the engine to match your AC load. This would be similar to a Stirling Engine, but with water and steam as the working fluids you should be able to get somewhat more power and speed so the engine would not have to be so big. The hot water could be rainwater so that it is already distilled and soft. The cold water could be fresh spring water so that it is colder. Rather than a closed system it could be an open system and the combined condensate and cooling water would be recovered into a cold water tank for daily use. Say the Washer/Dryer needed 1500w and 1.5kwh per load. If super-hot 200degF hot water provided the heat for the condensing dryer instead of electrical heating coils then this might be reduced to as little as 500-1000w peak power and 1000 watt-hours in total over a 3 hour wash/dry cycle. Between 200degF and 50degF the theoretical best thermodynamic efficiency would be 20%. Assuming just 10% efficiency from heat to electricity the amount of heat from the 200degF hot water consumed in an open system would be 34000 BTU. This would normally be about 30 gallons. However, since the system would operate in a partial vaccum the working fluid would be vapourized and take heat from the hot water in the process. If it achieved 100% steam at 200degF it would draw enough energy from the hot water that only about 3 gallons would be needed. In a 300 gallon tank this would drop its temperature by about 10 degrees which would need to be made up during the cycle and by the next day. The cold water required to condense the expanded steam would be considerable. If the condensor temperature was 20deg above the coolant temperature and the expanded steam quality was 80%, you would need about 120 gallons of cold water, which is considerable, but could provide the daily household water use, except perhaps drinking water. Its a practical trade-off between efficiency and cold water use. Also a lower condensor temperature requires greater expansion of the steam, and thus a larger and slower engine. For such a system to be practical you would need very cold spring water to begin with, perhaps 40-50 degF. However, improving efficiency from 10% to 15% would reduce the water use. This could be accomplished by superheating the steam above 200F with a final pass through a solar collector and/or wood stove before entering the steam engine.

    Such an engine is already similar in some ways to the way the condensor dryer works. Perhaps the two could be combined, increasing overall simplicity and efficiency and eliminating the electrical generator and motors. To the logical extreme, you might have a self contained washer/dryer that ran entirely off of a hot water / cold water tank, obtaining its heat and power and condensing coolant from it. Then you would only need to worry about supplying the hot and cold water.
     

  3. JAK

    JAK Well-Known Member

    Messages:
    529
    Joined:
    Oct 15, 2005
    Location:
    New Brunswick
    I was curious about temperatures inside a tumble dryer. According to the link below normal temperature reach 175 degC (350F) and can go higher. I would guess this is lower in a ventless condensing dry, but still this may make the idea of heating the dryer with 200-210degF water impractical, unless a extremely delicate and long cycle was used. 2-3 hours perhaps. The practical trade-off, beside time, is that the motor would have to run longer and consume that much more power.

    Still it might work in theory.

    http://answers.google.com/answers/threadview?id=729373