Phase changes and heat pump

CyberNeko · Post by **CyberNeko** » 2006-04-12 08:34pm

For past few days I've been messing around with the idea of using phase change of liquid and vapour in stirling-like engine, when I put it together with a heat pump, I keep seeing positive feed-back, more energy is from the cold to hot end than used. I am not a expert on such physics but I would like some analysis so I understand this.

Post by **Darth Wong** » 2006-04-12 08:36pm

Your terminology "more energy is from the cold to hot end than used" doesn't make much sense, so I'll assume you're just talking about the fact that a heat pump can move more watts of heat than it actually requires in order to operate, which is quite normal and is not a violation of thermodynamics.

CyberNeko · Post by **CyberNeko** » 2006-04-12 08:47pm

Darth Wong wrote:Your terminology "more energy is from the cold to hot end than used" doesn't make much sense, so I'll assume you're just talking about the fact that a heat pump can move more watts of heat than it actually requires in order to operate, which is quite normal and is not a violation of thermodynamics.

yes I understand that much, but I meant that the process gives positive feed-back to a point, as in generating more energy than is used up, I've been told it's a violation of thermodynamics and laws of energy. I think it's the small temperature difference and large volume change of liquid to gas that's making this effect.

Ou des · Post by **Ou des** » 2006-04-12 10:09pm

Do you mean that you're using the heat pump to create the hot and cold heat sources that drive the engine, and then using the engine to power the heat pump?

CyberNeko · Post by **CyberNeko** » 2006-04-13 06:52pm

Ou des wrote:Do you mean that you're using the heat pump to create the hot and cold heat sources that drive the engine, and then using the engine to power the heat pump?

Exactly yes, I'm pretty sure I'm not taking in account of something, but since I can't find anything on direct usage of phase changes, I'm unsure on how to figure out the energy processes.

Braedley · Post by **Braedley** » 2006-04-14 07:38pm

Is the system entirely enlclosed? (Okay, no system can be entirely enclosed, but for this we'll just assume that no significant outside forces, such as huge temperture gradiants outside of the desired case.) Do you have any electrical power into the system (even just a pump to move the water into the cooling jacket of the motor)?

kc8tbe · Post by **kc8tbe** » 2006-04-14 08:21pm

Could you post the calculations that lead to your present conclusion? This way we can all see how you did the experiment and not have to ask you dozens of specific questions.

Master of Cards · Post by **Master of Cards** » 2006-04-15 03:49pm

if it was a closed system that would work with no excess engey

CyberNeko · Post by **CyberNeko** » 2006-04-15 09:54pm

There are no other equpiments in the loop, and I cannot post the calculations since I can't find it right now but to be honest, it's more of a thought experiment. I was just wondering how efficient a carnot cycle-like engine would function through phase change, and the very low temperature difference seems to encourage the heat pump's efficiency.

The calculation assumes that the engine is perfectly contained and that all the parts are working at or near maximumal theorical efficiency, expect for the heat pump, which is assumed to be working at 25% of maximumal.

There are no outside power sources, it's all self-contained and assumed to have already started and is running.

But for this purpose, assume the engine is similar to a stirling engine, with one end cool and other hot at very narrow temperature range, say 99-101 degrees. And each cylinder is compenstated for the volumetrical change of water to steam and back (roughly 1200-1600 change at or near one atmosphere) and there are some gas to buffer the large volumetrical change slightly. This is pretty much what I've been trying to work out.

Braedley · Post by **Braedley** » 2006-04-15 10:14pm

Efficiencies multiply, so you're, at most, only as efficient as your least efficient device. If your heat pump is only 25% efficient, then your entire system can't be more than 25% efficient, in which case, since you have no additional input, your machine will effectively stop very quickly. In fact, it'll will stop before the water runs through the heat pump for the third time. I think you're grossly underestimating the efficiency of the heat pump.

Post by **Darth Wong** » 2006-04-15 10:16pm

Wait a minute, you're asking us to find the error in calculations which you can't post for us to examine?

Winston Blake · Post by **Winston Blake** » 2006-04-17 03:52am

CyberNeko wrote:There are no other equpiments in the loop, and I cannot post the calculations since I can't find it right now but to be honest, it's more of a thought experiment.

When you find and post whatever calculations you have so far, please include a diagram or something. Also, if you don't mind me asking, is English your first language?