Archive for the ‘Entropy’ Category

What are we storing when we cool water?

2006-06-08

Suppose we use a heat pump to remove heat from a water tank at night when the ambient temperature is low. Exactly what are we storing? At first glance, the same amount of heat energy enters the heat pump at low temperature and leaves at high temperature. Entropy is heat divided by temperature. More entropy enters at low temperature than leaves at high temperature. The entropy inside the heat pump seems to be increasing.

Now that's not quite right. Energy also enters the heat pump in the form of electricity or work, and if the heat pump doesn't heat up, that additional energy must be leaving as heat. So the entropy inside the heat pump ends up constant.

But the entropy inside the insulated water tank is clearly decreasing. Heat is leaving and none is going in. So we are removing entropy from the water. We are storing an absence of entropy, or making a place where we can dump entropy later. Sometimes this is called "storing negentropy," or even "storing syntropy," but those terms are not widely accepted. So I will just say we are storing "cold" (with the "scare quotes") and link to this page.

Update:  I turned off "allow pings" because every time I link to this, I get a ping by email.  Maybe I won't get them anymore.

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Maybe a little work on Renewable Energy Design…

2006-03-28

Idea for organization of R.E.D. wikia (renewableenergy.wikicities.com now renamed renewableenergy.wikia.com)

  • Problem pages should focus on a particular problem stated at the top and worked out on the page.
  • Need list of problem pages for the project. Use categories for this. Each project has a category. Problem pages belong to project category for each project they are relevant to.

Recent thoughts about the machine itself:

  • Need to keep stresses localized. Huge objects just aren't rigid. The forces which make each part of rotor turn have to arise within that part.
  • The gas is localized at top of the cylinder. As cylinder turns, there shouldn't be much stress on cylinder walls which are surrounded by water. There has to be extra mass above the gas to counteract the extra buoyancy.
    • This ballast needs to stay at top. Friction? How to keep it there?
    • Or maybe ballast is on the axis. Stretching forces appear on connection from axis to outer walls when they are at top. Very little force present between ballast and outer walls when they are not at top.
  • The walls need to be cheap. Gas-tight, but cheap. Maybe some fabric or flexible plastic, with ribs to keep the cylindrical shape. Or the shape follows from the tension (stretching) forces due to gas at top and ballast at center connected by ropes (and fabric for the screw part).
  • Might need buoyancy spheres. (Thought of filling the space with them and letting gas move through the gaps between spheres. Outer walls hold them in place. Probably a dumb idea.) May not have a buoyancy problem if the walls are light and there is a big volume of gas.
  • May need hot water moving downward as hot gas moves upward, and cold water moving upward as cold gas moves downward. Screw would help move water but lack of gas phase makes it more like a turbine than a positive displacement screw. (i.e. blades push on the water, but it can go around them).
  • Alternative is the heat is carried by a refrigerant through heat pipes. (Refrigerant gas carries heat to low pressure area, up or down. Condensed liquid flows down. Heat rises in a gravity feed heat pipe.) Trouble is, the source of heat is at top of ocean and the cold is at bottom. So, need a pump (e.g., a screw since we already have rotation going on) to lift the condensed refrigerant up to place where it can absorb heat and evaporate. Evaporated refrigerant will go wherever the pressure is lower, caused by condensation on a cold surface). But this requires fairly large volume low pressure container deep undersea. The external pressure would tend to crush the heat pipes. So they have to be small bore, but then refrigerant can't move fast enough.

The above was all early last week. More recently, I've been thinking of a cylindrical machine made of flexible gas-tight fabric which rotates about a rigid heavy metal axis. The top end is held above water by an inflated collar, while the bottom end is held at a fixed depth by an anchor cable running from float on surface to lower end of machine to a sufficiently heavy anchor some distance below the machine. The whole machine is sealed except at top. Hot surface water is raised to the entrance by a screw as machine rotates. It flows through the hot helix, maintaining temperature of hot expanding gas, and heating cold compressed gas in counter-current heat exchanger at bottom. At end of heat exchanger, the water is nearly cold, and it feeds into a large storage compartment at very bottom where remaining heat is rejected to cold ocean bottom. This compartment is under pressure because of the continuous addition of hot water at top, with excess pressure over ambient equal to the height of entry port above sea level. Output of cooling compartment is cold water which proceeds upward through the cold helix, removing heat from cold gas as it is compressed. At top, the cold water is used to cool low pressure hot air. The resulting warm water is released into open ocean some distance below the surface.

Air also enters the machine at top, to replace the air which was removed as high pressure cold air into storage compartment at bottom. The rate of rotation is controlled by diverting high pressure cold air into storage at bottom, replacing water which is released into the ocean. The hot air (recycled and newly captured) is first cooled in a liquid-gas counter-current heat exchanger at the top of the machine, rejecting heat into the cold water which came up through the cold helix and which is then released at intermediate depth. The resulting low-pressure cold air proceeds into the cold helix (indeed, there is little difference between the cold helix and the counter-current heat exchanger). As the machine turns, pockets of cold air are pulled downward through the cold helix, as cold water rises on its way out.

Buoyancy of the machine must be adjusted to avoid undue stress. The central axis is made of strong and heavy metal, but it is not strong enough to avoid bending if the helices are unevenly filled with air. Each segment serves as ballast to balance the buoyancy of the air in the hot and cold helices directly above. If there is less air in a helix segment, then some compressed air is admitted to the ballast chamber in the central axis segment, displacing some water at the appropriate position along the axis. If the helix is full of air, then the central axis segment should be full of water. If a given helix segment is full of water, then the corresponding central axis segment will be full of air.