1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why energy could be "renewable"?

  1. Jun 4, 2014 #1
    we often hear about the terms "renewable" energy, but according to some Physics laws, energy is something that couldn't be created or destroyed. How could energy be "renewable" then? I also know that there is high-grade energy and low-grade energy. When energy is changed from high-grade to low-grade, it's unlikely to reverse the process. When wind energy is collected by the facilities it is then finally converted into heat energy, which is not possible to go back.

    By the way, can the existing non-renewable energy so far last for another 100 years for the world
  2. jcsd
  3. Jun 4, 2014 #2


    Staff: Mentor

    The term "renewable energy" has nothing to do with physics.
  4. Jun 4, 2014 #3


    User Avatar

    Staff: Mentor

    In this context, "renewable" means directly or indirectly extracted from the flood of energy that the sun pours onto the earth every day.
    Hydroelectric: Sun heats ocean water, water evaporates, rises, falls as rain on the land, flows downhill through a hydroelectric plant.
    Wind: Sun heats air causing air currents which drive a windmill.
    Biofuel: Sunlight hits leaves of plants, which use the energy to convert carbon dioxide back into free oxygen and carbon compounds which can be burned for fuel.
    Direct solar: Sunlight heats water or generates electricity in photovoltaic cells.

    There's also tidal and geothermal energy, which are sometimes considered to be "renewable" because the underlying energy source is so enormous compared with our requirements - "inexhaustible" might be a better term.
  5. Jun 5, 2014 #4

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Energy is not lost but it becomes unavailable to perform useful work. You have identified the issue: when low entropy (concentrated) forms of energy are used to do mechanical work - eg. move a truck - the energy is dispersed as low grade heat (high entropy) and is no longer available to do work. The energy problem on earth is really an entropy problem.

  6. Jun 5, 2014 #5
    yea I know that. Is there any scientists ever thinking about turning low grade energy to become high grade again?
    just an imagination
  7. Jun 5, 2014 #6
    Decreasing Entropy

    The Entropy can be decreased, without we doing work to decrease it.

    There many examples.
    One of them is Heat Pipe technology

    We know, that normaly fluids move from high temperature to low temperature zones (high pressure to low pressure). And to move them in the opposite direction, we need to do work.
    But in heat pipes, the fluid moves from colder to hotter zone and we do no work to make that happen.


    There are many examples of selforganising systems, that decrease the entropy instead of increasing it.


  8. Jun 5, 2014 #7


    User Avatar
    Science Advisor

    None of those examples are closed systems. The second law of thermodynamics forbids it. You can decrease the entropy within a system only at the expense of a corresponding increase in the entropy of the outside environment.

    You cannot recycle "low grade" energy into "high grade" energy without an external interaction.
  9. Jun 5, 2014 #8
    In reality, there are no closed systems. Closed system is idealisation.
    And I don't think that if you get decreased entropy here, you will care about it increasing billion miles away.
  10. Jun 5, 2014 #9


    User Avatar
    Science Advisor

    The fact that there are no perfectly closed systems is irrelevant. It does not relieve you of the need to put fuel in the tank to keep the car going.

    The fact that you can sink entropy into a reservoir that is very large or very far away is certainly important. We have a big hot sun that is only 93,000,000 miles away and a big cold reservoir that extends for billions of light years in every direction.

    Your remark that "The Entropy can be decreased, without we doing work to decrease it" sounded wrong. But maybe what you had in mind is that we don't have to do any work -- we can just let some other energy source take care of it.
  11. Jun 5, 2014 #10
    Yes, exactly. We don't have to do any work. And I think that is the important part.
  12. Jun 5, 2014 #11


    User Avatar
    Science Advisor
    Gold Member

    Heat pipes are not heat pumps, they do not create temperatures lower than ambient on the cold side. If they did they would require input work and would operate like an absorption refrigerator.
  13. Jun 5, 2014 #12
    Evaporative coolers cool the air only with water evaporation. And they are much more effective than heatpumps.



    At lower pressure, water evaporates much faster.
    In heatpipes, the wick draws the vapor and transfer it to the hot side and lowers vapor concentration.
    So evaporation can continue.

  14. Jun 5, 2014 #13


    User Avatar
    Science Advisor
    Gold Member

    First of all, heat pipes are not the same as evaporative coolers. Heat pipes are a closed system with recirculating fluid. They do not refrigerate anything, they merely transport heat just like conduction through a solid object, but with higher heat flux. Secondly, evaporative coolers work by mass transfer to the environment and do increase entropy, as evaporated water has higher entropy than liquid water.

    I wasn't debating the efficiency of these processes, only that your assertion that it is possible to decrease entropy without doing work is false.
  15. Jun 6, 2014 #14
    I said We don't have to do any work. Someone(something) does the work, but not us.

    If entropy is decreased here, it is increased somewhere else. We can use that decrease to extract energy, wich is free for us.
    There is no one way process(increasing entropy). That is my point. It can increase, but it can decrease too.
  16. Jun 6, 2014 #15


    User Avatar
    Science Advisor
    Gold Member

    Yes that is true, you could say the same thing about hydroelectric power, "we" don't do the work lifting the water to higher elevations, we just exploit the potential energy arranged by nature. However, heatpipes still do not count, they do no work.
  17. Jun 6, 2014 #16
    We use "Non-renewable" for a source which take long time to renew. It does not mean that it cannot be renew.
    If a source is not taking energy(conserved), then you use it for a limited time.
    Like wind&tidal energy is said to be renewable, but it is not, because it will slow down the earth.
    Solar energy is not renewable because it will be finished someday.
    If you mean "Renewable" to create energy, then it is 100% wrong in any case.
    Energy can change its form. Energy is conserved.

    Which source of energy? Like Coal/petroleum/Minerals
  18. Jun 6, 2014 #17

    Yes. But hydro power is there because there was an evaporation first and then the water falls down.
    Evaporation is increasing the entropy you said. But then, after the rain, we have decreased entropy and we can extract usable energy.

    "Heat pipes do no work. They just transfer heat"

    Following the same logic, we can say - > Heatpumps do no work. They just transfer heat.

    And yes. We can achieve lower than environmental temperature with evaporation.
    Put some alcohol at a cotton piece and put that piece on a room thermometer.
    The temperature of the thermometer will go down bellow the room temperature.


    Last edited: Jun 6, 2014
  19. Jun 6, 2014 #18


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    It depends to some extent on your definition of low and high grade. Normally an energy source is considered high grade if it can easily be turned into another form. So electricity is considered high grade because it's easy to turn into heat. Heat is considered low grade because it's hard to turn into electricity. But everything is relative.

    There is some work being done on how to power things like electrical sensors using low grade waste heat or vibration. The military for example would love to be able to turn vibration (perhaps created by a soldier running) into electricity to power his radio or similar.
  20. Jun 6, 2014 #19

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Yes. It is called a heat pump. Unfortunately, the second law of thermodynamics makes it a futile exercise if you want to do work with that heat.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook