Design Geothermal Power Generator: Ideas, Physics & Experimentation

[Pythagorean]

I'm looking to make a geothermal power generator out of something like a refrigerator or a small air conditioning unit (or even a car AC unit)

Is there anybody here who as experience with these thing and would like to discuss design ideas and the physics behind them here?

I really have no clue where to start, but I'm willing to work through it and I will be experimenting with it, regardless of success rate.

 

[russ_watters]

I'm not following - what is the heat source, what is the sink, and how exactly is the electricity generated?

 

[Pythagorean]

Up here (in the face of local skepticism) some engineers took a commercial air condition system and made a geothermal power supply out of it for the a pump house resort here. They relied on hot springs for the heat source.

The heat source would be hot springs (probably around 140-180 F, the heat sink would be the environment (it gets down to -40 here easily in the winter).

My problem is building a device that will utilize that temperature gradient to make energy.

I was thinking you could drive a fridge backwards and use it's motor as a generator, but I've never looked inside a fridge (yet). I'm somewhat worried about handling freeon too.

edit: for testing, I imagine I'll be using an electrically driven heat source, like a heater or a boiling pot of water. I of course have no idea if that even sounds reasonable.

 

[russ_watters]

I don't see how it could work. In a heat engine converting heat energy to mechanical energy, you need both a pump and a turbine. In a refrigerator, the pump moves the refrigerant around: what would extract the energy? Or if the pump was meant to work as a turbine (not sure that's possible...), what would cause the refrigerant to move?

 

[Pythagorean]

Yeah, that was my issue. I thought it would be as simple as applying the heat source to one end of the fridge coil and having the other end exposed to the cold atmopshere (to make the stuff move) but I've never taken any engineering classes (only physics) so I don't know how stuff works in the real world intuitively. I'm sure it wouldn't be freon.

The big one based off of the commercial AC unit used a turbine, but I don't see where I'd put a turbine in a fridge.

Perhaps just making a large air conduit with a turbine placed horizontally in it would be sufficient, and use air as the medium. Sound somewhat inefficient, but I'd have to figure out how to calculate the affects of a temperature gradient before I know how useful it would be.

 

[russ_watters]

Well, I suppose you'd put the turbine in place of the expansion valve. An expansion valve is known in thermo as a throttling device, lowering the pressure and temperature of the gas, but without extracting mechanical work. I guess then the idea would be to run the refrigerator with a similar cycle to a steam engine. But the cycle is still sort of backwards - after a turbine, a steam cycle still has waste heat to discharge in order to condense the steam to water. In a refrigeration cycle, the refrigerant leaves the expansion valve as a low temperature liquid and gets heated (absorbs heat from the refrigerator), turning it to a gas. It goes through the compressor as a gas, unlike a steam engine, which has the compressor compressing a liquid.

Here's how a refrigeration cycle works: http://www.geo4va.vt.edu/A3/refrigeration-cycle.gif

Here's a steam cycle: http://images.google.com/imgres?img...cle&um=1&start=1&sa=X&oi=images&ct=image&cd=1

And from the T-S diagram, you can see that the points and processes on the cycle are roughly the same, but reversed. Ie, point 1->2 in the steam cycle is work out via expansion through the turbine, but the same two points in the refrigeration cycle are work in via the pump.

 

[Pythagorean]

Yeah, the more I look at it, the more it seems it would be easier to use the steam diagram and go after the individual components.

It's not your average bathing hot spring. The actual springs are underground. There's big rubber tubs that you fill up.

So.. the heat source has already been tapped. A large rubber hose comes out of the ground, spewing a thick stream of skin-burning temperature water. I'm starting to think I should go with a sort of adaption to the hose so that the water pressure and heat can both drive the system, and also allow exhaust into the river.

On another note, I had thought there was a way to go directly from a temperature gradient to electrical power without using mechanical power. Something to do with two different compositions of wire at different temperatures. Is that not efficient in the real world? Because I've never heard of anything like it besides experimenting in physics lab.

 

[russ_watters]

A stirling engine will sort of do that. It still has a compression stage, but it takes the energy from the cycle for it (of course, ultimately so does any other generator).

 

[Pythagorean]

Isn't stirling a closed system though? With an endless supply of water pressure, I'm assuming it has to be an open system (especially since federal law requires that your hot spring still feed the creeks, so there will have to be an exhaust)

Do you know anything about my last paragraph in my last post?

(At this point, I'm probably going to try a turbine design, just curious)

 

[russ_watters]

Your original idea was for a primary-secondary system, with the geothermal energy being passed through a heat exchanger to put energy into a heat engine. Now you're talking about running the primary geothermal directly through a turbine. That's certainly reasonable if you have a good amount of pressure, but I suspect you don't - I suspect your energy is mostly in the heat (and sensible heat at that - there is no phase change). So you need to transfer the energy to a working fluid that will do a phase change. Refrigerant is good for that, you'd just need to figure out how to make a power cycle with it.

A sterling engine has the benefit of using air as the working fluid and doesn't require a very high delta-T to run it efficiently.

On another note, I had thought there was a way to go directly from a temperature gradient to electrical power without using mechanical power. Something to do with two different compositions of wire at different temperatures. Is that not efficient in the real world? Because I've never heard of anything like it besides experimenting in physics lab.

A thermocouple or peltier device does this. Nuclear-powered satellites actually use peltier devices to convert the heat of nuclear fission directly to electricity. These things actually are more often used for solid-state refrigeration (hobbyists use them for computer cpus).

Unfortunately, they are only something like 5-10% efficient.

 

[Pythagorean]

Ok, I'm starting to get a better idea of this, thank you for your advice; it's done a lot to help my understanding so far. I'm going to have to look out for old fridges at our local transfer station so I can look at the actual system

[1]I'm curious though, what do I want my latent heat of vaporization to be? Do I want it to be at the high-end temp, the low-temp or somewhere in between?

My natural assumption is somewhere in the middle, but closer to the heat end so that it changes phase. In the summer, the relevant temperatures will be higher than those that a fridge acts at.

[2]Is the act of expansion in the expansion valve the direct cause of the temperature lowing in the medium or is there something else causing the temp to lower too in that portion of the cycle?

[3]The augmentation of a turbine to the coolant system sounds tricky. I'm assuming I'd have to keep the system closed, so the turbine would have to be enclosed inside the cycle? Is there not a way to get the the heat energy to drive the pump that's already in the system?

 

[Pythagorean]

Ok, having done a bit more research on the local, commercial example, I've found out that the pump is actually still part of the system and that the plant actually requires electricity to start the process.

 

[dukejaz]

anyone interested in low delta t power should check out >>>yourownpower.com<<<<< . yes, it is a large Carrier vapour compression air conditioner, running in reverse as a organic rankine cycle motor. note to russ watters... the unit has its vane style compressor acting as a turbine when running as a motor. ANY large air conditioning unit will run backwards during stutdown, if the condenser is hot and evaporator cool. as this is damaging to scroll and reciprocating compressors, a backpressure release valve, or other means, is designed to prevent this. the Carrier/utc had only to close this valve; no major overhaul necessary. the working Th is 70C. the heat sink is a cold alaskan creek, at ~ 5C. two such units are now in opperation, producing 2 MW each. there are considerable drilling and then pumping costs but carrier expects a payback on all drilling and infrastructure in under 7 years. the large retrofitted a/c units will retail for ~ $100,000, less than half of a new waste heat to power unit of comparable size. these older rooftop a/c units are rapidly being decomisioned, as li-br absorption, and more importently, silica gell adsorption units become more economic.

it seems clear to me that a solar thermal plus geo-sourced heat sink could provide these delta t's and overall btu's. with carnot at ~ 20% there will be left over heat of course. at chena, this is dedacted to both winter resort heating, (humans can use what carnot cannot!), and to an 'ice palace', year round igloo, that is kept frozen by a li-br absorption chiller, which also uses the geothermal heat as input energy.

 

[russ_watters]

Yes, that's exactly where our discussion was going - thanks, I didn't know Carrier/UTC was doing that. Here's a link to the system itself: http://www.yourownpower.com/Power/#How it Works

There are three main changes made to transform a Carrier chiller into a power plant. The turbine/generator assembly has 13 of 171 parts uniquely manufactured for power production versus its corresponding chiller/compressor assembly. A pump has been added to circulate the liquid refrigerant and maintain operating pressure. Also, the heat exchangers, while standard Carrier manufactured units, are specifically sized for power plant operation at design temperature.

 

[Pythagorean]

Excellent, thank you both for your input.

last summer we salvaged a fridge and harvested the cooling system out of it (complete w/ pump, expansion valve, etc). We'll probably play with it more this summer (we're are scared of getting a blast of freon in the face, needless to say, as none of us have any experience with refrigerator system).

It seems impractical so far, fitting a turbine in the system; it would either have to be a tiny turbine, or we'd have to enlarge the duct system (which, it would seem, would mess up the thermodynamics because of the change in X-section area, but perhaps there's something to make up for this)

I've trying to think of another mechanism that extracts work from flow that would be reasonable (as an alternative to a turbine).

I've also been trying to think of other cooling systems that could work (not looking to power Chena, just for a cabin out in the woods)

 

[3DTOPO]

@Pythagorean

You are missing a fundamental point; most heat engines are reversible processes.

The turbine that you are concerned about is just the compressor inherit in a refrigeration cycle. When energy is applied to the compressor it compresses the working fluid. When a working fluid is expanded through the compressor work is exerted on the shaft of the compressor.

I think you should read the white paper from the Chena folks online at http://www.yourownpower.com. This is clearly explained therein.

I have a 70C hot spring and 6C creek that I am currently working on a system to generate power from. I am looking at a solid state TEC (standard TECs can be used in reverse as a TEG [Thermo Electric Generator]) system and a Organic Rankine Cycle (similar to Chena Hotsprings). I am also talking to the folks at http://www.powerchips.gi - they claim their device can get 70-80% Carnot efficiency from their solid state electron tunneling nano technology (roughly double the efficiency of a turbine system)


Cheers,

-jeshua