Using heat pump to power wind turbine

[harlam2]

Hi everyone,

Just a thought question. I had been reading up on wind turbines and was wondering if anyone had considered attaching a wind turbine to an outdoor heat pump or AC unit? These units produce a lot of "wind" when running and a narrow turbine or blade attached to the front of the heat pump or top of the AC condenser could make use of this. In a way, this would be similar to regenerative braking in EVs. Any thoughts? My main thoughts are space to accommodate the turbine, and if this would put any extra load on the fan of the units. Just wanted to see what others though. Obviously this is just a thought experiment but welcome any feedback. Thanks

 

[sunshine_eggo]

Could be done. Use the HAWT calculator to estimate yield.

Wind Turbine Calculator [HAWT and VAWT]

The wind turbine calculator finds the power output, efficiency, RPM, torque, and revenue of a wind turbine (either HAVT or VAWT).

www.omnicalculator.com

If you get more than 15W or so, that would be a HUGE win.

 

[v_green57]

Might be more efficient to attach some small but powerful rare-earth magnets to the tips of the condenser fan, and mount some coils such that the magnets pass very close to them when the fan is rotating.

This polyphase AC could be rectified to DC and used in the application of your choosing.

But...and this is a big BUT...will the drag introduced by this Rube Goldberg increase the torque needed by the fan motor and effectively cancel out any energy generated? My old college physics education says maybe. Remember the "conservation of energy" law.

 

[sunshine_eggo]

Might be more efficient to attach some small but powerful rare-earth magnets to the tips of the condenser fan, and mount some coils such that the magnets pass very close to them when the fan is rotating.

Nope. That will create additional resistance requiring additional power from the motor. The yield of the fan tip "generator" would be notably less than the additional power consumed by the motor.

Conservation of energy means you NEVER get out what you put into a closed system. It's always less.

 

[harlam2]

Looked at the calculator and there's a lot of info I wouldn't have. It looks though that to produce anything substantial, it would need wind speeds of at least around 20 mph. I'll have to pick up a cheap anemomenter when I'm back from my holiday break to see what kind of wind speeds these heat pumps put out. My unit is a 2 ton, so hopefully can get decent speeds. My thought is there may be slight more strain on the motor on startup, but should balance out after the wind turbine gets going, akin to driving a car at speed on the freeway as opposed to starting it from a stoplight..and most of the time, people tend to keep the heat pump on for hours, most of the day or even overnight for heat and AC.

 

[LakeHouse]

I had been reading up on wind turbines and was wondering if anyone had considered attaching a wind turbine to an outdoor heat pump or AC unit?

I think so far everyone else has been too nice about this question. I'll be a little more blunt: This is a bad idea.
Using a fan to drive a turbine and expecting to get a net gain in power just isn't going to happen. The largest energy savings by far would be to just turn off the fan that's blowing the air, but for obvious reasons that would be bad for the condenser; that fan runs for a reason.

this would be similar to regenerative braking in EVs.

It really isn't similar in an important way. Regenerative braking is useful because it's capturing energy that would otherwise be purposefully bled off as heat (by frictional braking), and instead capturing that energy by charging the battery AND slowing the vehicle. The 'wind energy' from your AC unit is most certainly not wasted; creating that airflow is exactly the point of the fan because moving that air is necessary to cool the fluid in the condenser.
Anything you do to disrupt (or capture) the airflow out of the unit is going to make the fan's job harder to do, making the whole system less efficient.

 

[OzSolar]

Any thoughts? My main thoughts are space to accommodate the turbine, and if this would put any extra load on the fan of the units. Just wanted to see what others though. Obviously this is just a thought experiment but welcome any feedback. Thanks

Conservation of energy means you NEVER get out what you put into a closed system. It's always less.

Fun topic!

So the fundamental formula for energy is F=ma, Force = Mass x Acceleration (Check out Newton's Laws sometime)

My preferred summary for the law of energy conservation is "Energy is neither created or destroyed, it merely changes form and each time it changes forms there is a loss, usually in the form of heat"

So how does this apply to collecting energy from the condenser fan?

It says that the energy collected from the output will be added to the input plus the efficiency losses. AKA "net loser". I'm confident that for every kWh you collected you'd be adding at least 1.5 kWh's to the system.

 

[sunshine_eggo]

Fun topic!

So the fundamental formula for energy is F=ma, Force = Mass x Acceleration (Check out Newton's Laws sometime)

My preferred summary for the law of energy conservation is "Energy is neither created or destroyed, it merely changes form and each time it changes forms there is a loss, usually in the form of heat"

So how does this apply to collecting energy from the condenser fan?

It says that the energy collected from the output will be added to the input plus the efficiency losses. AKA "net loser". I'm confident that for every kWh you collected you'd be adding at least 1.5 kWh's to the system.

If you could harness the pure waste products (heat and forced air) as proposed by the OP, it would be a net energy gain, but a fan-tip "generator" would indeed add load to the fan-driving motor and ultimately consume more than is gained.

 

[harlam2]

Thanks for all the feedback. Just to clarify, I'm simply trying to regain some of the waste mechanical energy ( wind) from the heat pump. I'm not trying to run the heat pump by regaining all of it's energy with the wind turbine, which would be pretty silly.

I think a better analog would be the alternator in an internal combustion engine. The engine needs to run any ways, and the alternator would be recapturing some of the waste mechanical energy through the serpentine belt. I wouldn't expect the alternator to run the engine, but it can run the lights, the radio and other smaller electrical appliance we use in our cars. It has a relatively small load on the engine, so makes sense. In this way, the heat pump would be the engine, the wind turbine would be the alternator, and the "serpentine belt" would simply be the wind energy. I don't think a motor attached to the heat pump would work, but one that is sits say a few inches in front or behind the heat pump blades may not place much load once it gets up to speed. Even if it can recover some waste mechanical energy, that would go toward recharging my battery bank that could power household items I have connected to the inverter.

Like I said, it's a thought experiment. I'll get an an anemomenter next week, and have a box fan laying around so can see if the heat pump even produces enough wind to effectively move the box fan blades. We'll see since it's all theoretical at this point.

 

[v_green57]

"Thanks for all the feedback. Just to clarify, I'm simply trying to regain some of the waste mechanical energy ( wind) from the heat pump."

Right, but then wouldn't you be putting that energy back (as electrical power) into the very system that provided it in the first place?

That's where things go sideways. If you treated the energy from the scheme as a stand-alone "gain", of course it would be such. But you expended a lot more energy in the first place to get it.

I remember being fascinated as a kid with the concept of a "perpetual motion machine". I thought that I could take two identical fans, one of them configured as a generator and both pointing at each other, and using the power generated by one to drive the other. Cool.

Then I learned about mechanical/electrical/other losses. My dream of becoming a zillionaire evaporated.

 

[SparkyJJO]

Thanks for all the feedback. Just to clarify, I'm simply trying to regain some of the waste mechanical energy ( wind) from the heat pump. I'm not trying to run the heat pump by regaining all of it's energy with the wind turbine, which would be pretty silly.

I think a better analog would be the alternator in an internal combustion engine. The engine needs to run any ways, and the alternator would be recapturing some of the waste mechanical energy through the serpentine belt. I wouldn't expect the alternator to run the engine, but it can run the lights, the radio and other smaller electrical appliance we use in our cars. It has a relatively small load on the engine, so makes sense. In this way, the heat pump would be the engine, the wind turbine would be the alternator, and the "serpentine belt" would simply be the wind energy. I don't think a motor attached to the heat pump would work, but one that is sits say a few inches in front or behind the heat pump blades may not place much load once it gets up to speed. Even if it can recover some waste mechanical energy, that would go toward recharging my battery bank that could power household items I have connected to the inverter.

Like I said, it's a thought experiment. I'll get an an anemomenter next week, and have a box fan laying around so can see if the heat pump even produces enough wind to effectively move the box fan blades. We'll see since it's all theoretical at this point.

The alternator isn't capturing any waste energy. It is a direct mechanical load on the engine (through the serp belt), converting mechanical energy into electrical for those lights, ignition, etc.

 

[LakeHouse]

The alternator isn't capturing any waste energy. It is a direct mechanical load on the engine (through the serp belt), converting mechanical energy into electrical for those lights, ignition, etc.

Exactly. I’m not sure how many times we can tell the OP that the air movement isn’t waste energy.
The heat is, and if that could be captured you might have something to gain.
A better analogy for the air movement would be installing a turbine on your bicycle or the roof of your car to capture the ‘wasted wind energy’ of all that air flowing by.

 

[OffGridInTheCity]

Exactly. I’m not sure how many times we can tell the OP that the air movement isn’t waste energy.
The heat is, and if that could be captured you might have something to gain.
A better analogy for the air movement would be installing a turbine on your bicycle or the roof of your car to capture the ‘wasted wind energy’ of all that air flowing by.

Agree. However, just for fun, there's actually a guy trying this 'car wind turbine' idea out to see if he can eek out XXw from the wind turbulence in the front of a vehicle and feed it to reduce the alternator load...

Whether it works at all or not - the effort is just tooooooo much for waaaaay too little. And this overall idea is like using misters on AC unit or a generator on the water flow when flushing the toilet, and in the spirit of @LakeHouse comment - better to make the car more aerodynamic

 

[OzSolar]

If you could harness the pure waste products (heat and forced air) as proposed by the OP, it would be a net energy gain, but a fan-tip "generator" would indeed add load to the fan-driving motor and ultimately consume more than is gained.

What I can remember of my thermodynamics classes from many years ago leads me to believe that overall it will be a net loser.

Here's how I see it: in order to collect energy the turbine will end up so close to the output that it would be restricting the flow of air to the point where it reduces the amount of heat (or cool) it's rejecting thus reducing the complete system efficiency. It may not add watts to the heat pump input but it will likely increase the run time needed to satisfy the home's heating/cooling needs.

And of course, at risk of stating the obvious, if it made sense every condenser fan would come with a wind generator.

 

[sunshine_eggo]

What I can remember of my thermodynamics classes from many years ago leads me to believe that overall it will be a net loser.

Here's how I see it: in order to collect energy the turbine will end up so close to the output that it would be restricting the flow of air to the point where it reduces the amount of heat (or cool) it's rejecting thus reducing the complete system efficiency. It may not add watts to the heat pump input but it will likely increase the run time needed to satisfy the home's heating/cooling needs.

There's certainly a threshold.

Let's consider the far end of things... lighting a single LED @ 2.4V/0.02A - that's 0.05W.

According to the calculator I linked, a 1 ft diameter HAWT can harness 1W in 10mph winds.

So it's within reason that a small HAWT can harness .05W at a distance sufficiently far from the unit to not impact unit efficiency.

And of course, at risk of stating the obvious, if it made sense every condenser fan would come with a wind generator.

Exactly.

 

[RCinFLA]

Any wind turbine coupled to heat pump fan exhaust will restrict air flow through heat pump heat exchanger that will drive up heat exchanger temperature and pressure requiring more compressor work consuming more electrical power by compressor.

Most regular fan motors will rise in rpm's slightly with air flow restrictions and draw less power producing a lot less CFM of air movement amplifying the pressure/temp rise of heat pump heat exchanger. That is why manufacturer specifies a minimum clearance distance to walls and other blockage items to ensure good air flow. Also need to keep the heat exchangers clean to prevent air flow restrictions.

 

[TomC4306]

something to gain.
A better analogy for the air movement would be installing a turbine on your bicycle or the roof of your car to capture the ‘wasted wind energy’ of all that air flowing by.

Right! To power the lights. He isn't spinning the AC fan in order to produce some small DC charging voltage. He's spinning the AC fan to produce heat in the house. Of this process, that wind coming off that AC fan IS waste

 

[TomC4306]

Any wind turbine coupled to heat pump fan exhaust will restrict air flow through heat pump heat exchanger that will drive up heat exchanger temperature and pressure requiring more compressor work consuming more electrical power by compressor.

Most regular fan motors will rise in rpm's slightly with air flow restrictions and draw less power producing a lot less CFM of air movement amplifying the pressure/temp rise of heat pump heat exchanger. That is why manufacturer specifies a minimum clearance distance to walls and other blockage items to ensure good air flow. Also need to keep the heat exchangers clean to prevent air flow restrictions.

Drag could be mitigated with distance between the driven and the driver. Perhaps a ducted fan 2 ft above the condenser fan, with an unsealed pathway.

 

[OzSolar]

This set up would make just as much power (*zero) plus it's a lot more fun to watch.

 

[RCinFLA]

Restricting air flow on outside unit during cooling mode will just drive-up condenser temperature causing compressor head pressure to rise causing compressor to draw more power to pump against a higher refrigerant pressure.

If you want to recover some wasted energy, you can get a heat exchanger that routes air conditioner compressor hot discharge refrigerant gas through heat exchanger to heat water to provide hot water preheating for hot water heater. You can yield 1-7 kbtu's of heat from compressor hot gas discharge depending in incoming water temperature for a 4 ton, 48kbtu air conditioner. That is equivalent to 0.3-2 kW's when A/C unit is running. There are companies that sell these air conditioner heat recovery heat exchangers.

These units work best if you have a separate preheat water tank and you live in area where tap water is less than 65 degrees and you run the preheat tank to get water preheated to 80-100 degrees before tapping preheat tank water output to feed your regular hot water heater.

In Florida, where tap water is typically 75-80 degrees F, they are not worth the trouble.