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Do we currently obtain useful energy from Gravity?

  1. Apr 11, 2007 #1
    This idea has really been bugging me lately. It all started with a thought, where does the energy that drives hydroelectric turbines come from? Well you could say the kinetic energy of falling water right? Well then where does this energy come from? You could then say well the potential energy of the water of course! But that merely begs the question, how does this water get this potential energy? Surely there is an answer.

    So I began to think about how water gets transported to such great heights? I thought about it a lot and I think I found the answer (though I still have questions), consider the following scenario.

    Step 1: liquid water molecule on the surface of the earth is heated by the sun and thus becomes a vapor.

    Step 2: This vapor rises due to buoyancy, because its density is less than the surrounding air, the greater gravitational pull on the air causes the water vapor to be displaced upward.

    Step 3: At some height the surrounding air becomes sufficiently cool so as to cause the water molecule to reject the heat it obtained in step 1, thus it condenses and becomes liquid.

    Step 4: This liquid water molecule rains down into a reservoir and its potential energy is converted to kinetic energy.

    Step 5: This liquid water molecule's kinetic energy helps turn a hydroelectric turbine thus producing some amount of electrical energy.

    The only conclusion I can come to based on this scenario is that gravity, with the help of some heat input from the sun (which remember was rejected in the upper atmosphere, heat in was equal to heat out, otherwise it would have condensed earlier), the air as a lifting fluid, the water molecule's two phase changes, and the temperature differential from the lower to upper atmosphere which allowed for heat rejection, has essentially been utilized to perform useful work contrary to the prevailing notion of gravity as a conservative force which can do no net work.

    Am I wrong? If so please show me where I made my mistake and explain to me how the water molecule gains potential energy in this process.

    The reason this has been bugging me so much is that the ramifications of this discovery, if true, are quite frankly in my mind, Earth shattering. If my conclusion is correct it means that gravity can be used to perform work, similar to the way in which wind, geothermal, and solar are used to perform work.

    For example if my conclusion is correct it would be a relatively simple matter (compared to the awe-inspiring technological feats of our day) to construct a machine which could heat some low boiling point fluid into a vapor using geothermal or some other heat source, which would rise through a really tall insulated (to prevent condensation on the way up) pipe (the amount of useful energy obtained in the hydroelectric stage of this process would be proportional to the height, and the temperature decrease with increasing altitude would allow for heat rejection) filled with some dense gas (which does not condense in the temperatures of this process) which would serve to cause vapor to rise due to the density differential, this rising vapor would then be run through a turbine just like in a standard Rankine cycle, after exiting this turbine, the vapor could be channeled into a condenser where heat could be rejected via heat exchangers, the vapor would condense and become liquid with potential energy, this liquid would fall and its fall could be directed into a hydroelectric turbine after which it would go back in the sump to repeat the process. This is but one way to take advantage of gravity if my conclusion is correct there are certain to be a host others.

    Please try to prove me wrong, I am obsessed with this! I cannot stop thinking that this process could help alleviate our world's energy problems. I am a chemical engineering student at the University of Washington and I am seriously considering designing a simple experiment as a project to test my hypotheses.

    One question that this leaves me with however is even if my conclusion is true, which I think it is based on the evidence I have seen, how does this fit in with the first law of thermodynamics, where does this energy come from? I don't know exactly but perhaps the first law is wrong, after all it is only right because it has never been disproven, just as is any scientific law. Maybe a more likely scenario is that our current knowledge of the nature of gravity is insufficient to say where the energy in this process comes from.

    Regardless I am very interested to hear what you all have to say, please be respectful in your responses; oh and don't just say that my hypotheses is incorrect because gravity is conservative or because it violates the laws of thermodynamics (which it may or may not that's debatable) that would be circular reasoning, we are talking about a real world phenomena that is observed everyday (everyone has seen rain) so all responses should, if they believe my hypotheses is wrong, present an alternate scientifically plausible scenario as to how the water molecule obtains potential energy.

    Thank you I look forward to hearing your replies
    Last edited: Apr 11, 2007
  2. jcsd
  3. Apr 11, 2007 #2


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    Think of it this way:
    All energy on this planet comes from the sun in the form of both heat and light and maybe some sound or something.
    All energy leaving the planet leaves in the same way.

    When an area of earth gains energy form the sun, water goes into the air. This area is starting to store potential energy against gravity, and the sun is what provides that potential energy.
    When the sun goes away (due to clouds or something), that area gets cold and the sun can no longer hold water in the air, much like holding a spring at tension. Water condenses and gravity acts on it, all energy stored as potential energy is released as kinetic energy.

    To sum up the path of the energy:
    sun energy -- > water heats up -- > water evaporates --> water is held in the air (it takes energy to stop gravity) --> water falls with kinetic energy.
    It seems like there's a net gain to the system, but there really isn't. Eventually energy leaves the planet in some other form, such as half of the earth not even being exposed to the sun at any given time.
    Last edited: Apr 11, 2007
  4. Apr 11, 2007 #3


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    with the exceptions of nuke and geothermal, virtually every energy source on Earth ultimately gets its energy from the Sun. Sun heats up water in the oceans and lower elevations, evaporates it, and then lifted to a higher elevation where it rains (or snows and melts). now you have water at a higher elevation and more potential energy. Sun also provides for animals and plants millions of years ago, giving them the energy to sythesize complex molecules. these die, get buried and eventually break down to what we call a "fossil fuel".
  5. Apr 11, 2007 #4
    THe main problem that i see is that this device would have to be huge, and something has to heating that water vapor, so in terms of energy efficiency and operating cost, why build a 3000 foot high percipitation chamber when we can just let nature do its work for us and then build dams. also, the added potential energy that the water gets when it goes high up, when it comes down to the earth, getting kinetic energy from that is notimpossible, but it is impractical, because unless your chamber was the size of vermont, the same amount of energy could be attained other ways. also, the energy attained by this machine would be small, which would probably discourage its use

    although, I could imagine a practical application of something similar to this. In existing power plants, such as nuclear or geothermal, the excess steam that runs off from the mahcines that turn the turbines could, instead of being released into the air, channeled into some type of area that will condense it into water, and then have it fall onto some turbines and create some extra energy on the side to increase efficiency, but it would take a heck of a lot of steam and some very efficient generators.
    Last edited: Apr 11, 2007
  6. Apr 11, 2007 #5


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    Well, you are correct about the sun being the source of heat. Where you can see it captured is in what the river would do if the dam wasn't there. If the hydroelectric dam wasn't there, the water would flow faster through the river and lose energy in the form of frictional heat, heat which is intially provided by the sun. We simply capture that energy. In that sense, a hydroelectric plant is actually a solar-powered steam engine.

    Since you say/know that the sun is the source of input energy, I don't understand why you would say that there is a conservation of energy problem here. Gravity does not provide both the input and output work. The sun provides the input (as you said) against gravity and gravity against the turbine provides the output.
    A heat source is still a heat source and is still where the energy actually comes from to drive the cycle.
    What you are saying would work, but it wouldn't be revolutionary and would require enormous and inefficient power plants that still require an input of heat energy, as you indicated above.
    Last edited: Apr 11, 2007
  7. Apr 11, 2007 #6


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    Because of the source of inefficiency you mention, virtually all steam systems are closed cycles. They do recover the bulk of their steam and reuse it.

    After the turbine, the steam is a saturated vapor (all vapor, but at the boiling point) or a mostly vapor mixture. It then goes through a condenser, which turns it into a saturated liquid (all liquid and still at the boiling point) before being pumped back to the boiler. The two sources of inefficiency come in in the condenser if they use evaporative cooling (the water vapor that runs through a nuclear plant cooling tower isn't the same water vapor that goes through the turbine though) and in the energy of the pump.

    Here's the cycle (the cycle may actually include some superheat at state 2 to reduce the amount of liquid water going through the turbine to get to state 3): http://en.wikipedia.org/wiki/Rankine_cycle
  8. Apr 11, 2007 #7
    To ShawnD and rbj and russ_water

    I don't think that your analysis is correct because you are failing to account for the fact that the heat energy from the sun which it took to vaporize the water is returned to the atmosphere when it condenses (which creates currents that help produce wind energy), therefore the heat energy is not being converted to potential energy, in addition to the fact that the amount of heat energy it requires to vaporize the water is the same regardless of how high it goes (though the ambient temperature must be high enough to prevent condensation on it's way up obviously). I think the problem is you guys are thinking the heat is what lifts the vapor, but it is the buoyant force which does the lifting the heat merely causes phase change resulting in a density differential. I'll repeat this part because you guys didn't seem to get it, when the water turns to vapor its density is less than air, therefore the air around it is pulled down by gravity with greater force causing the water vapor to be displaced upwards until such time as it reaches a point in the atmosphere where the ambient temperature is low enough that the heat is rejected and it condenses. Heat in is equal to heat out in this system.

    In response to Ki Man, I'll say this, the number of dams that can be built is limited and also causes other environmental problems, so the advantage of this machine would be that it could be placed virtually anywhere (tall buildings would be a great place, there is an average 5 degree fahrenheit temp difference for every 1000ft of altitude) and generate useful energy from waste industrial heat, solar heat, or it could be dug into the ground and generate energy from geothermal heat (you wouldn't have to dig as far as with conventional geothermal you would only need like a 5 degree fahrenheit difference in temperature for this system to work). As far as the chamber having to be the size of vermont I'm not quite sure I understand your reasoning, my reasoning goes like this, if the water falls from a great height it will have greater kinetic energy when it hits the hydro turbine and so the energy generated by this machine on the hydro side is proportional to the height at which condensation occurs. I'm not so sure that the energy produced by this machine would be small, it would depend on how high you built it and what liquid you used the maximum energy on the hydro side being equal to mgh but obviously the conversion to electricity wouldn't be perfect.

    Anyway thanks for replying
    Last edited: Apr 11, 2007
  9. Apr 11, 2007 #8
    Just to reiterate

    The water vapor does not rise in the air simply because it is hot, it rises because of the buoyancy principal.

    If there were no gravity the water vapor would not rise, why would it?

    The force of gravity pulling down on the denser surrounding air is what causes the water vapor to rise via displacement, if water vapor were not less dense than air it would not rise.

    This is the same principal that cause air bubbles to rise in water, and causes helium and hydrogen to rise in air.

    This is the key to understanding this cycle.
  10. Apr 11, 2007 #9


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    Try not to think of earth as a closed system, because it isn't. Heat from condensing water goes into the atmosphere, then the atmosphere loses it. We know the earth loses heat to outer space because we know winter happens every year. Winter is when loss to outer space is greater than the energy gained from the sun, so the temperature of that hemisphere drops. Winter affects specific hemispheres because the energy loss to outer space is faster than the energy dissipation between the hemispheres.

    While water is a gas, it has potential. It's not until it condenses and starts to fall that it loses both its thermal and gravitational potential energies.

    Water can become less dense than nitrogen and oxygen, but first you need to break the bonds between the water molecules and space everything out; think goatse. It takes a tremendous amount of energy to break those water-water bonds in order to make the water lighter than air, so in a very real sense work is done against the atmosphere, not the other way around. It's like pushing a volleyball to the bottom of a swimming pool then letting it go. It naturally floats up on its own, and with quite a bit of force, but you had to do a hell of a lot of work before that was even possible.
    When you evaporate water into 100L of gas, you have essentially displaced the entire atmosphere of the earth by 100L; all of the existing atmosphere had to move over and make room for this new gas, and that's why vaporization takes a huge amount of energy. It's similar to pushing a volleyball under water and displacing a few litres of water. That water was not displaced until you displaced it, and that buoyant force acting on the volleyball is only there because you put energy into the system. In this case, the volleyball is liquid water turning into a gas, and the guy pushing it down is the sun.
    Last edited: Apr 11, 2007
  11. Apr 11, 2007 #10


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    None of that is correct. The difference in air pressure with altitude changes the amount of energy in the water. This is obvious, since a droplet of water at 50,000 feet in a thundercloud has a lot of potential energy but roughly the same amount of thermodynamic energy as it will have when it hits the ground. Buoyancy is not free energy. This energy transport is very similar to what happens when you expand steam through a turbine.
    Look at a steam table. That buoyant force caused by the state change is the result of the input heat. It isn't free.
    We got it: you got it wrong.

    Do chemical engineers take thermodynamics? Most thermodyamics books actually have a secton where they analyze the earth's energy transport system. If you aren't taking a thermodynamics class, get ahold of a mechanical engineer who is and borrow his book. But if you want, you can construct the cycle yourself. The data is available online. The four points, just like in any steam cyle, are:

    1. Saturated liquid water.
    2. Pressurized liquid water.
    3. High pressure saturated or superheated vapor
    4. Saturated vapor or mixture at a lower pressure.

    In your case, 2->3 comes from the sun instead of a boiler and 3->4 & 4->1 converts the heat energy to gravitational potential energy instead of directly driving a turbine. Energy is then extracted later from the gravitational potential energy. The enthalpy (energy) content of the water is NOT constant in these 3 states! As the air rises, it expands and cools and that lowers its enthalpy before it condenses. Buoyancy is not free energy!

    Remember, as I pointed out to Ki Man, any steam cycle is closed and water is boiled and condensed, yet there is still energy to be extracted. The energy extracted comes primarily from the pressure/volume/temperature difference, not the heat of vaporization of water.
    You certainly could do this, but the amount of energy depends on the amount of water you are transporting. The calculations required there are very simple: get yourself a psychrometric chart and calculate just how much air you need to have a certain mass of water and then at what height to get a certain amount of energy.

    People have proposed the concept of a solar tower to use the stack effect to lift air and drive turbines. If the climate that you locate the solar tower in is humid, condensation would be created, which would also be capable of driving a turbine, but it takes a lot of air transport to get a useful amount of condensation. The business is probably a scam, but the idea is technically sound: http://www.enviromission.com.au/

    Consider that the Hoover Dam is about 700 feet high and consumes 10,000 gallons of water per second to generate 2,000 megawatts.
    Last edited: Apr 11, 2007
  12. Apr 12, 2007 #11
    Maybe I'm just dumb...But I don't see how heat of condensation is dependent on GPE

    Sorry if I'm making you mad, maybe I'm just really dumb.

    To answer your question yes chemical engineers take thermodynamics, in fact I have taken thermodynamics already.

    I agree that the state change is the result of an input in heat energy, and I noted this in my original statement, but isn't this heat energy returned to the atmosphere when the water condenses? You said yourself as the vapor rises into altitudes of decreased pressure it expands and cools, doesn't this mean it's releasing heat? I know I neglected to take the decreasing pressure with increasing altitude into account but I don't think it makes a difference because the decreased pressure just causes the vapor to, as you said, expand and cool, lowering it's enthalpy (by releasing heat to the surroundings) before it eventually condenses (releasing the rest of the heat energy).

    Also, isn't it true that the amount of heat energy it takes to achieve state change is the same whether that water vapor subsequently rises 10 meters or 100 meters before condensing? I believe the latent heat of vaporization for water is 2272 Joules/gram, this is the energy it takes to become a vapor and isn't it also the same amount of energy it releases when it condenses? If the pressure decreases it will just release some of that heat energy before it condenses, expanding and cooling as you said. I might be wrong, but I don't think that the amount of heat energy released by the water when it condenses is reduced by the gravitational potential energy it attains when it changes back into a liquid. To quote wikipedia

    The standard enthalpy change of condensation (or heat of condensation) is numerically exactly equal to the standard enthalpy change of vaporization, but has the opposite sign: enthalpy changes of vaporization are always positive (heat is absorbed by the substance), whereas enthalpy changes of condensation are always negative (heat is released by the substance).

    Isn't this true?

    You said that as the air rises (I'm assuming you meant water vapor) it expands and cools lowering it's enthalpy before it condenses. So yes I neglected to consider the difference in pressure, but the only difference is that some of the heat energy is released before it condenses and releases the rest. Again I might be wrong, but I don't think that the heat of condensation released by a water molecule is dependent on it's gravitational potential energy at the moment of condensation.

    Can you explain how you think the heat energy is converted to GPE? I'm pretty sure I learned in thermodynamics that when water condenses it releases heat into its surroundings, isn't that correct? Are you saying that the amount of heat energy it releases is dependent upon it's gravitational potential energy at the moment of condensation? Where are you measuring the GPE from, sea level? Or perhaps a straight line down from the molecule to the nearest solid surface? What would happen if the GPE at the moment of condensation were greater than the latent heat of vaporization that the water molecule absorbed to become a vapor, would it then heat up when it condensed? (or perhaps you think the highest point the molecule could reach is determined by the latent heat of vaporization? In which case does it release any heat energy when it condenses?)

    You say that the atmospheric energy transport process is very similar to what happens when you expand steam through a turbine but when you expand steam through a turbine isn't it pressure caused by heat in an enclosed space, as oppose to buoyancy caused by gravity, that directs the kinetic energy of the steam through the turbine?

    Anyway maybe I'm stupid but I don't see how the heat energy released by water when it condenses has anything to do with its height above the ground, other than the difference in pressure, which as you and I both pointed out merely causes the vapor to expand and cool (releasing some of its heat to the upper atmosphere) before it condenses some time later (releasing the rest of the energy it got from the sun in the beginning).

    Again maybe I've got it wrong but I don't think the heat of condensation of a water molecule has anything to do with the gravitational potential energy of that water molecule at the moment of condensation. Perhaps you could provide a formula for this? Are you basically saying that the amount of heat released by the water molecule when it condenses is equal to the heat of condensation minus the gravitational potential energy? If so where is the gravitational potential energy measured from?
    Last edited: Apr 12, 2007
  13. Apr 12, 2007 #12


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    You're not (either thing)...
    Yes! Before it condenses!

    I'm a little surprised to hear that you have already taken thermodynamics because the energy transport system is pretty much the same as in any thermodynamic cycle.
    The decreasing enthalpy as it rises is the entire point. That's the output energy! And all of that output happens before the water condenses and is returned to its initial state. Just like in a steam turbine.
    Yes! But please reread what I've said now several times (in bold, no less): the heat of vaporization is not where the energy output comes from! As you say, the heat of vaporization is the same whether vaporizing or condensing.

    Look at the T-S diagram in the wik link I provided. You're harping on the energy of condensation, which is process 3->4 in their diagram, but the useful work out comes from process 2->3, with the adiabatic expansion.

    (note: their description doesn't quite match the diagram - in the diagram, point #2 is saturated, not superheated)
    Yes, and that is all the energy that is available to be harnessed.

    Most of the next few paragraphs just repeats the same misconception over and over....
    No! The heat added by a boiler occurs at constant pressure. The pressure is added before the boiler by the pump (or in the case of the Earth's water cycle by the water falling back to earth). Agan, this is thermo 101 stuff: look at the processes in the steam cycle!
    It doesn't! I've said it about 5 times now and you keep ignoring it.
    Last edited: Apr 12, 2007
  14. Apr 12, 2007 #13


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    Anyway, to examine how much energy could be extracted from this process, the math is pretty easy. First we'll decide how much energy we want.

    The Hoover Dam produces 2,000 MW. By comparison, the largest wind turbines produce about 2MW. So lets shoot for a decent sized wind-farm of 100 turbines or 200MW as our basis of comparison.

    The Hoover dam is about 750 feet high and the world's largest buildings are on the order of 1,500 feet high, so lets shoot for 1500 feet as our target height.

    1 watt is .73 ft-lb/sec. 200 MW is therefore 146 million ft-lb/sec. Divided by our height of 1500 feet, that's 97,000 lb/sec. [note: Looking at what I found earlier about the Hoover Dam I think that number I found online is wrong - looking around more, it looks like that should be 10,000 cubic feet (62.4 lb), not 10,000 gallons (8.3 lb)].

    Now, how much air is that? Pennsylvania has some of the hottest summers in the country: hotter than Vegas or Phoenix from a thermodynamic standpoint, with a dewpoint averaging around 65F, or a specific humidity of about 100 grains. 100 grains is .015 lb of water per pound of air. At a specific volume of 14.2 ft^3/lb, that's about 100 million cubic feet of air per second.

    Note also that this energy would only be available 8 hours a day for 5 months a year.

    Now it wouldn't be a bad idea to combine this with the solar tower idea, which would allow a small reduction in height of the tower while maintaining the same energy output.
  15. Apr 13, 2007 #14
    Sorry if I still don't get it

    So as I understand it, we are in agreement that the heat energy that goes into the water molecule when it vaporizes is equal to the heat energy that comes out of it (a little bit coming out before it condenses as a result of decreasing pressure/temperature with increasing altitude and the rest coming out at the moment of condensation), right?

    Basically as I see it, and as you pointed out, a little bit of the initial heat of vaporization comes out before it condenses because of the decreasing pressure/temperature with increasing altitude and the rest comes out when it condenses, is this correct?

    I've reread what you said several times and have come to the conclusion that you agree with the above statement, but I still don't see where I'm going wrong.

    If what I said above is true, I'm still left with the same question because at the moment the water molecule condenses it has a potential energy, some of which might later be changed into electricity using hydroelectric turbines. What is the source of this potential energy? Note that the amount of potential energy in the water molecule at the moment it becomes a liquid in the atmosphere is a function of how high it rises as a vapor before it condenses.

    Note also that when the water molecule gives off heat to the surrounding cold air it causes the air to become more energetic due to the heat input and causes the air rise and form wind currents which might later be turned into electricity using wind turbines.

    I'm sorry that I don't understand what you said, also sorry that I didn't reply sooner; I am really busy with school right now, I'm taking 15 hard credits this quarter.

    Thank you for your replies I am still struggling to understand this process. You are probably right but I'd like to understand why my hypothesis is wrong.
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