# Energy storage and heat into energy

Let me get this out of the way: I am not formally educated in this specific area of physics.

Let’s also get this straight: By spending energy to remove and discard energy (AKA a refrigerator or a house), we are doubly shooting ourselves.

We turn energy into heat all the time, why don’t/can’t we turn heat into energy?

HIVAC systems move hot air out and discard it and move cold air in. Why not have these units move the hot air out and store it in an apparatus designed to suck the molecular kinetic energy out of the air and store it?

Story: I live in South Florida. My AC broke… it’s summer. It’s hot as hell. The thermometer in the house read 90 degrees. My brother has a house in Michigan with same exact structural dimensions. His AC is broken. His thermometer reads 72 degrees. Wouldn’t it be safe to say I have a ton of energy in my air compared to his air. Can’t I take that energy and power my house? Are you guys getting the picture?

I know from deductive reasoning that this is difficult to do, but why? It is so simple in theory…

Drop a rubber ball. Come back in a minute, it’s resting on the ground. Why can’t I take the heat that this process produced and make the ball bounce back up? I’m in theory land here folks, work with me!

If heat is simply kinetic energy at the molecular level, then why can’t we take heat and produce energy at our “macromolecular” level?

I want to take 100 grams of water @ 100Kelvin, convert it into 50grams of water @ 150 Kelvin, and the other 50 grams of water @ 50 Kelvin… because in this example I would have the same amount of, ready for this, “gram/Kelvins” = 10,000 to be exact. So, basically I have created a new unit for energy, or more importantly, work. Does this mean I can convert gram/Kelvins to Joules?

There are ways to convert heat energy into more useful energy, for example, the Peltier effect. The problem is that all methods of energy conversion are less than 100% efficient. (And peltier devices are not particularly efficient at all.) Thus, for any given amount of heat X, you’re going to get less than X worth of electricity. Further, for any amount of heat that you pump from your bedroom, you’re going to be putting more than that amount of heat outside. Such is the price of comfort.

There are ways to recycle waste heat, though. New York City’s central steam system is largely generated by industrial waste heat, for example.

We do. All the time. We use heat to power geothermal, nuclear, coal, gas and oil plants. What you are asking is why can’t we use the ambiant heat in the air to power stuff. Well, the obvious answer is that there isn’t enough of it. To be useful, that heat energy needs to be converted to kinetic energy to turn a generator or something. I suppose theoretically we could make tiny molecular size devices that use atomic motion to power tiny little induction coils and create electrical energy. But I suspect we would need them to cover a very large surface area.

The obvious answer is that in order to turn heat energy into other forms of energy there has to be a temperature difference. The atmosphere provides lots of temperature differences but none of them stay in one place long enough to run any kind of device that converts the heat to mechanical, electrical or what-have-you energy.

Well, if the temperature differences cause the air to move, and it powers a wind generator, isn’t the temperature difference powering the generator?

Sure is. :smack:

I think I see where you are going with this. You want to trap the heat removed from your house or your six pack of pale ale by your refrigeration systems and store it away. Not an unreasonable thing to suggest but there are some problems with that. As Hoodoo Ulove said our old pal the second law of thermodynamics gets in the way. Heat only moves from a hotter place to a cooler place on its own. Refrigeration seems to circumvent that but it really doesn’t and only works by adding extra energy to the system which also has to be discarded. If we try to discard this heat to a closed resovior then the medium we are trying to store heat in becomes hotter than the coils causing heat to flow the other way which will make the system soon fail.

There are ways to do the things that the OP suggests, but none of them are cost effective. Hot air is just not very energy dense, and as stated above, you need a temperature differential to move a heat engine of any type.

So how do we get a temperature differential? What cold thing do you have in your house that you did not just get done spending energy to make cold? In most people’s houses, the answer is water. You could blow air over cold water to heat up the water to concentrate the heat a little, but then you’d have 75 degree water instead of 50-60 degree water. More concentrated than the air, but really not much use. It would cool your house down a little, but the cost of all the water would more than make the little cooling cost ineffective.

What about waste heat from your Ac and refrigerator? This can be made use of. The coils on your AC and fridge are simply there to get rid of heat. They don’t care where the heat is rejected to. You could build a heat exchanger which uses flowing water to cool the coils, which could be used to preheat your hot water tank, thus saving you energy. But unless you always take a shower right when the AC is on, it’s not going to be very practical. Unless you want to build a huge neighborhood insulated preheated storage tank.

As others have mentioned, it is certainly possible to convert a temperature difference into electric power. In fact, that’s how some space probes are powered… the power supply contains hot plutonium, which sets up a temperature difference between two points. A thermopile is then used to convert the temperature gradient into DC electric power.

But it’s not very efficient.

To state the Second Law of Thermodynamics (which pertains to the amount of energy you can extact from a given process) in terms more conceptually visualizable, consider this: heat is a form of energy that, lacks direction; a volume of gas, for instance, that is heated expands out in all directions. (In the atmosphere it goes up because it is less dense and therefore has less pressure than the surrounding atmosphere, but in a no-gravity environment it would merely expand out.) In order to make use of the energy, i.e. do work (movement in opposition of a force or change in velocity of a mass), you have to direct the energy; with a thermodynamic process, however, you only get to use a certain amount of that energy.

For instance, in an internal combustion engine, such as the one that powers your car, work is done upon the pistons (which in turn, connect to a crankshaft, attached to a transmission, and eventually connecting to your wheels) by the combusion of vaporized gasoline combined with air, resulting in a very hot gas which tries to expand. As it expands it cools off; as it cools, it expands progressively less forcefully, until it actually takes more energy to expand against the inertia of the piston, et al, than the heat of the gas can provide. In reality, mechanical losses in the engine and transmission will stop it long before it reaches its thermodynamic limits, and so we design the piston to reciprocate, blowing the non-completely exhausted gaseous residue out through the just-opened exhaust port.

All heat engines have a thermodynamic efficiency which is based strictly upon the ratio between the temperature of the initial fluid and temperature of the exhaust products. (This is assuming that the system is closed and that the fluid doesn’t undergo some kind of major phase isothermic phase transition.) The Otto Cycle engine, for instance, has the displayed efficieny, although the values on the graph actually seem kind of high to me.

In short, the greater the temperature difference, the more of the heat you can put to actual use. Slight temperature differences don’t provide enough of a differential to overcome other system inefficiencies (mechanical friction, thermal losses, et cetera). Burning coal in Hell might actually be a good way to stay cool.

Stranger

Heat IS Energy.

Since it is so readily generated and quantified, it was the first form of energy studied by scientists. The whole field of thermodynamics was founded by studying the equivalence and interconversion of heat energy with mechanical energy via “heat engines” such as a gas-filled piston.

Conside the coal-fired electrical generating plant:

Electromagnetic energy (sunlight) is converted to chemical energy by prehistoric plants. Time passes. The plants are converted to coal.
The coal is burned, converting chemical energy into heat energy.
The heat energy turns water into steam which spins a turbine; heat energy is converted into mechanical energy.
The turnine turns a generator: mechanical energy is converted into electrical energy.

Pretty much every process you can observe involves conversion of energy in one form into another. Heat is just one of the many currencies that energy comes in.

Now think about what the melting point of water is in Kelvins. Now re-read that sentence, make it grammatically correct, have it make some sense, and re-post.

I have nothing to add other then what is stated above, but it seems the logical conclusion to me would be to add solar panels (if you really were that fussed about gathering energy from the hot florida summer), not some magical engine that takes energy from the air in the form of heat.

The Second Law of Thermodynamics is the complete and only answer to this question. The usual statement of the Second Law is that entropy can (and usually does) increase, but it can never, on the whole, decrease. You can actually decrease they entropy in some particular system, but you’ll have to increase the entropy somewhere else to do so.

This still leaves us with defining entropy, though. I will not attempt to do so here, beyond noting that for any given amount of energy, a system with everything at the same temperature will have a higher entropy than a system with parts at different temperatures.

So to use your example of water at 100 Kelvins (yes, this is below the freezing point of water, but this is unimportant: The Second Law works just as well for solids as it does for liquids): 100 grams of water at 100 K will have the same amount of energy as 50 grams at 50 K and 50 grams at 150 K, but it will not have the same entropy. Things with everything at the same temperature have a higher entropy, so the 100 g at 100 K has higher entropy than the 50-50 case. This means that it is possible (and in fact quite easy) to go from 50 grams at 50 K and 50 g at 150 K to 100 g at 100 K. All you have to do is put the two subsystems in contact with each other and wait. But going the other way is impossible, because it would decrease the entropy of the system. Or at least, if you do do it, you’ll have to increase entropy elsewhere by a greater amount, which usually costs you something.