Introduction: Parasitic Wind Turbine

Picture of Parasitic Wind Turbine

This was built as a lark, a whimsical attempt to recapture some of the hundreds of dollars I spend on air conditioning each year. It's a wind turbine powered by the compressor fan on my air conditioning unit.
But, It is a ponderable notion. And you could reverse the rotor, add a tail and yaw bearing and make a regular wind turbine.

Step 1: The Alternator

Picture of The Alternator
I started off by messing around with a stepper motor I had, it was rated 6 volts 0.4 Amps per phase and was a two phase motor, meaning max input would be 4.8 watts. So running it backwards could I get that much out?

I'm not really an electronics guy, so I stuck my meter in AC mode across the terminals, spun it with my fingers, and got a reading of 50 volts! On current it read .2 amps. Umm okay, never mind the meter, would it light LEDs?

So wanting to see if it would light LEDs I wired 5 red LEDs together, four as a full wave bridge rectifier and the fifth as the load. As you can see in the video and pics, it had no problem lighting those LEDs, nor the five super bright white LEDs arranged in the same manner.

Step 2: The Rotor

Picture of The Rotor

In the vein of keeping the design simple I wanted to have a simple rotor. My solution was to pull the blade assembly off of a cheap box fan that had reached the end of it's life. The rotor had been attached to the fan on a 1/2 inch shaft. To accommodate the 5mm shaft on my stepper motor, I press fitted a half inch dowel into the fan and then bored a 7/32nds hole through the dowel, and split the end of the dowel with a razor saw to allow the use of a hose clamp to secure the fan to the motor shaft.

Step 3: Housing and Mount

Picture of Housing and Mount

The housing and mount are made from PVC, the housing is made from two 2" end caps, a 2"ring to align the caps and hold the motor in place and a 1/2" elbow cut down to attach the housing to the mount. The two end caps are held together with aluminum tape.

Step 4: The Load

Picture of The Load

The load is a group of five LEDs per phase arranged as a full wave bridge with the fifth as the load. Instead of poorly drawn schematics, I suggest you read this wikipedia article. Here are some pix.

Step 5: All That's Left

Picture of All That's Left

Is to place the motor mount onto the stand and place it on top of the compressor. adjust for best RPM and you are good to go. You might note that contrary to expectations, ie, that you could just set it on top in line with the fan, the best RPM comes from an offset 45 degree placement.

Addendum, I constructed a diode bridge with diodes salvaged with from a CFL and replaced one of the LED loads and recorded an open circuit voltage of 10 volts. Now here's a question for smarter people. Could I put the two phases in series for an OCV of 20 and charge a 12 volt battery?

Step 6: Acknowledgements

These persons are directly or indirectly responsible for this project.

William Kwamba for his inspiration

Alan Parekh who provided the LEDs.


The Reuk website for the info on steppers.

Step 7: Addendum

Picture of Addendum

Not as neat as I'd like, but I bodgered up the turbine in conventional form for those who might be interested, alas Code Enforcement would be all over me if I actually installed a wind turbine.

PVC fittings
broom stick boom
road spam tail
bamboo stake for yaw bearing.

All held together with screws.


hargee (author)2010-06-11

I'm sure the extra load on the AC is negligible since, with the turbine on top, in open air, it is not a closed system. Unlike the turbo which has a restricting conduit that does not give the exhaust an alternative path to spooling the turbine.

rgerber1 (author)hargee2015-08-03

I agree, I believe the unit is already loaded by " Sucking" air through the coils.. the fan appears high enough to not restrict the air flow out of the collar of the unit ( for lack of a better word, much like the back side of a house fan, at least on my ac unit it has a cone in which the fan sits slightly in front of)

bob8898 (author)2015-06-17

Soooo.... I dont know that much about steppers... Whats single phase vs. dual phase? Also with a stepper that i know has 2 dc power inputs, would those 2 dc inputs (when the stepper is used as a generator) output dc? or AC? if ac... HOW DO I MAKE A FULL WAVE BRIDGE RECTIFIER?

rgerber1 (author)bob88982015-08-03

you can check You tube or perhaps search here, a full wave Bridge will change AC into DC Current. I am sure Wikipedia has something on the subject also.

mgalyean (author)2013-08-16

It would be more efficient to just run the LEDs from a wall power supply because you are robbing Peter to pay Paul. You are making the A/C blower motor work a bit harder by putting that resistance over it. The A/C blower only runs when the A/C condenser coils reach a certain temperature. By putting resistance in the path of the cooling air, you are just delaying the blower motor from shutting off by some amount and make it run longer. You need to make the system do *less* work, not *more*, to get a savings. In dry climates, I've used a water mist on the intake air into the A/C outdoor condenser unit to make the heat exchange more efficient. This allowed the A/C to run more efficiently. At the cost of the water I was spraying anyway. There is no free lunch.

toddk63 (author)mgalyean2015-08-02

My thought exactly on adding resistance to the condenser motor. Spraying water on the condenser coils will certainly improve the system, but at the risk of leaving hard water deposits fouling the coils.

mgalyean (author)toddk632015-08-02

Hard water. Good point. Maybe I should have taken the condensate from the inside evaporator and sprayed that water instead as it should be much more scale free than tap water having lost the scale when it became humidity in the air.

javierkayaker (author)2014-12-17


Radiothermal (author)2014-07-30

I stuck an old 140mm PC fan over my Trane unit and at best generated 4v at 20ma. Cool idea, I'm gonna build by own with a 200mm fan.

paqrat (author)2011-07-24

I must admit to not knowing much of anything about this but wouldn't it be more efficient if the blades of the fan were parallel to the exaust fan of the ac?

I got the idea for using a fan when, a few years ago, I bought a small electric fan and walking to the car I noticed the breeze I was generating by walking slowly was spinning the blades of the fan.

nav_lexy (author)2011-07-18

What about adding two more at 1/3 of the angle? Like a PEACE sign

dangerous dan (author)2011-05-26

I was mislead by the amount of torque you were applying to light the LED's, -and I want to know how hard the wind was blowing for this picture.
I have some steppers laying-around, and your 'ible has got me thinkin'!!!

dangerous dan (author)2011-05-26

All I can say comes from the fertile-mind of Robert Heinlein:

whitten_jaw (author)2011-05-09

To answer your question about phases. Two voltages are only 'summing' if their phases are the same. 180 degrees out of phase would be subtractive, so you would get less power out. 90 Degrees out of phase would give you no effect. You may be better to generate two DC outputs and put those in series (provided that the source resistance is high enough). The other alternative is to look up inductors and capacitors and their corresponding phaser math to offset your motor phases enough to make them additive. I hope that gets you started--doing the vector math isn't hard when you have the circuit in front of you... if my memory serves me right, capacitors are:

Z = -[j]/ [(omega)*(Capacitance) or Z = 1/[(omega)*(Cap)] < -90 degrees
and an inductor is

Z = [j]*(omega)*(inductance) or Z = (omega)*(Ind) < +90 degrees

kenmneth (author)2011-04-15

if you want something to work on then try this take a mirrored satellite dish and put in the center of it a mirrored tube at the focal point must be a mirrored cone shape or small dish your choice sending the light down the mirrored tube into the basement straight into the mirrored box i am using a frig, solar panels are inside of frig i used rollers from a desk to slide solar panels back and forth for cleaning if they need it .small fan takes the hot air out cooling system will cause sweat sending the light into the mirrored box is a GOOD IDEA BUT NOT THE GREAT ONE I AM WORKING NOW.

SNUFFY3724 (author)2011-03-28

OK, I'm just a housewife, never studied thermodynamics or physics, so don't rake me over the coals. In NC, I use my heat pump AC from March (sparingly) to November (again sparingly), with the heaviest usages from last week of April to 3rd week of October. In the worst part of the year, the AC runs CONSTANTLY - not only heating, but pulling out humidity. My condenser unit is under my deck, on purpose, to eliminate freezing over when we get our wonderful ice storms. There is a six foot clearance vertically, and I can tell you when I walk across that part of the deck, especially in a skirt, there is still a strong wind current - certainly enough to power a turbine. I bought a unit a full 1/2 ton larger than my neighbors ( I have 3080 sq/ft heated space, so I got a 4 ton unit), which only cost me about $100 more, and my electric bill is always, year round, cheaper than my neighbors by 10% +/-; my EPP is $199/mo., and the 12th month they always owe me, most years I only pay 10 1/2 months of electricity. I haven't even thought about salvaging the heat loss, because it seems to me it would be complicated and expensive, but maybe that's just me being ignorant. But seriously, if you're wont to create a wind generator, in my climate, this would be an absolutely dependable source of 'wind' to power one, and you wouldn't have to worry about damage due to the HAWT or VAWT being too close to the unit - 6 feet will still 'get her done'! This instructable is the closest thing I've seen to date for someone thinking to salvage this unused energy. Get to work boys, I want to see something real - I'm closing in on retirement, and I still can't get rid of the kids and their electronics.

austinpowerzzzzz (author)2011-02-26

where can i buy the generator thingy? ive been lookign around and cant find it.

dallasmel (author)2010-03-10

All the comment relate to the 2nd law of thermodynamics.  The resultant of any process will always be less that the amount of energy you put into it!

You tap energy off a condensing unit, you'll get less energy out of your process, and increase the amount of energy the condenser uses.  There's just no way around it!  There's no such thing as 'free energy".

unclejoe (author)dallasmel2010-04-05

'honeybees can't fly per law of aerodynamics'
dm, the energy being tapped is the dump/waste heat energy. there is no strain on the condenserunit as it is designed to push air away from said unit to remove the excess temperature, aka cooling effect.
if anything. the small fan blade assists in moving the air by deflecting it.

Callum Snowden (author)unclejoe2011-01-16

I would have thought that the fan on top once it got going if it had no load or very little load, it would have helped to draw the air away from the AC unit's fan because it would be pulling air upwards to keep it spinning?

dallasmel (author)unclejoe2010-04-05

Honeybees can fly per the laws of thermodynamics.  For example, if you quit feeding a honebee, it eventually won't be able to fly (a process), and the second law of thermodynamics state the resultant of a process is alway less than the amount of energy put into it!  Starve yourself for a week and try doing the same amount of work you did before.  Same principle.

Not only does the condenser fan remove heat from the condenser via "forced convection", the system in which it operates is essentially fixed, and fan laws for a fixed system with turbulent flow applies.  The change in air-horsepower as a function of a change in the system resistance of such flow (eg: more restrictive as a result of placing your device in the airstream) changes as a linear funciton if assuming constant mass flowrate.

Strain is a change in length over a unit length, and as a result of compression or tension of a member.  Strain does not apply to a machine, in this case a heat pumping machine.  You restrict airflow in a heat pumping machine, you will encounter an increase in the delta-Temperature (ie: leaving-entering air temperature) at a constant heat transfer rate according to a Sensible heat transfer (ie: another process). 

The First, Second, and Third Laws of Thermodynamics apply to everything.  There's no way you ... or a honeybee can get around it.

unclejoe (author)dallasmel2010-04-05

this device is utilizing the exhaust not the intake and was discussed
in the comments with trebuchet in june 2008. please read.

dallasmel (author)unclejoe2010-04-05

Intake, Exhaust, it doesn't matter ... they are both part of the "system" in which the airflow is operating, and it is related to principles of continuity of mass, momentum, etc.

I typically don't like to use an "appeal to authority" argument ... but since you have .... 

I see this as being a little misunderstanding about the law of thermodynamics, the fan laws, laws involving the continuity of mass, etc.  ... in general, Laws of Nature/Physics ... not Man-made laws.  Even with a minor misunderstanding of the Laws of Nature/Physics, the probably of Rub-Goldberg devices being proposed into society increases expotentially. 

If you truly think you have engineered a devise that does not tap energy off of the condenser, then add several of your devices to the condenser outlet ... and do it again ... and again ... and again ... allowing you to save enough energy to bill back the utility companies.  You'd be creating energy!!!! (a violation of the First Law of Thermodynamics).

It won't and can't happen, and because of the Laws of Thermodynamics.

Mel Presswood, P.E. ... I've been involved with pumping heat, thermodynamics, HVAC, and environmental control systems for over 40-years .. my "Appeal to Authority" argument.

bigjeff5 (author)dallasmel2010-06-17

The point most people are missing (which you are dutifully trying to explain) is that putting a load on the back end of the system is still putting a load on the system.

Basically what adding a turbine to your AC exhaust does is literally make it harder for the AC unit to push the exhaust air out. This will result in one of two things as it relates to the AC unit (and these are the only options): if it's a relatively smart exhaust fan, it will increase it's speed to compensate for the lost air movement. If it's a dumb fan, it will keep cranking at the same speed/power, and the cooling capability of your unit will be decreased. Your AC unit will have to run longer to hit your target temperature, because it cannot cool quite as fast.

If you think it through a little bit, you just have to extrapolate from what you know to understand it won't save you anything. Start with the extreme of covering the AC unit. The heat exhaust is critical to cooling the house, so your AC unit does not cool anything without a way to exhaust the heat. Lifting the cover slightly to allow a small amount of air through will allow the unit to begin cooling, but it won't work well at all. Lift the cover further - say a foot or so - and you've got heavy restriction but it is really starting to cool things down. Lift the cover to 10 feet and it will run at almost peak efficiency, but you also have very little air flow.

Putting a turbine on the back of your AC unit is exactly the same as partially covering it. It adds restriction to the air flow, which means it cannot exhaust the heat as well, which means it loses efficiency.

unclejoe (author)bigjeff52010-06-18

gentlemen, what both of you are explaining is applicable to a 'closed' system, where the air is going thru ducting in a closed cycle similar to central heating systems. it has minimal impact on an 'open' system which is being tapped by this device.

bigjeff5 (author)unclejoe2010-06-19

It applies to both a closed and an open system. Adding anything to the back end of that exhaust fan increases the resistance on the air moving out. Since the point of the air moving out in the first place is to dissipate waste heat, anything that slows that air down is going to slow the heat dissipation as well, which means the exhaust system works less efficiently.

The exact same thing can be seen in reverse in high power carburated engines. Hot rod builders remove the air filter to increase the flow, but they also cut holes in the roof and install flared intake pipes to maximize the air flow.

With no holes, it's still an open system. Air flows freely through the radiator and from under the car, into the intake which provides oxygen to the engine to burn the fuel. Cutting holes in the hood of the car alone increases the air flow, which means the engine has more oxygen for burning fuel. To maximize the air flow, they then add specially engineered flared intake pipes, which provide the best possible air flow into the engine.

They do the exact same thing with the exhaust, putting wide, straight pipes out from the engine to maximize the flow.

Obviously the AC unit isn't that precisely engineered, however any obstruction reduces the ability of the air to dissipate heat, which reduces the efficiency of the AC unit. At the same time, the obstruction makes the air being pushed out more difficult to push, so the fan either pushes out slightly less air or uses more electricity to push the air out a little faster, depending on how smart the fan is.  The only difference between an obstruction inside a sealed tube attached to the exhaust and an obstruction sitting in the open air outside the exhaust is the amount of potential loss it can create.  They both create a loss in efficiency, which cannot be recovered because the turbine is not even remotely close to 100% efficient.  The laws of thermodynamics are pretty clear on this one, even if you don't see the relationship.

The only way I could see this being of any benefit is if you aren't the one paying for the electricity to run the AC. In that case, it's incredibly unethical, but it will result in a net gain for you (but not overall).  And then, if you're going to do it, you might as well seal it off, because you'll collect a whole lot more energy that way.

spiceyweasel (author)bigjeff52010-06-25

Look out everybody! It's the battle of the Physics egos. Kindly stop behaving like a gaggle of self-righteous quacks or we'll have to take your magnadoodles away, you bumblebee haters. Personally, I'd like to know if you could use the motor out of an old fan to make some electricity.

bigjeff5 (author)spiceyweasel2010-07-12

Absolutely, but you'll need high RPMs to get any electricity out of it because of the motor design - the metal of the electromagnet will want to stick to the permanent magnets and you need to overcome that. You could roll your own generator pretty easily that would generate electricity at much lower RPMs (which is what you are more likely to be dealing with). All you need are some properly wound induction coils (how they are wound will depend on your setup) sitting in a circle on a board or other flat plate, and some permanent magnets in a circle on another board. Put a drive shaft between the two circles and suspend the magnets above the induction coils as close as you can without touching, and then just spin the top plate. You'll get AC current flowing through the circuit on the induction coil side. You can hook that up to a wind turbine or any other rotating device and generate electricity. And cheap too.

spiceyweasel (author)bigjeff52010-07-12

Thanks Big Jeff. If you know of an instructable I could use as a reference, it would be greatly appreciated. I really like the concept of producing electricity with these things. I hope I can adapt it up here in big breeze country to my benefit.

bobofett69er (author)dallasmel2011-01-13

So by tapping geothermal power, you're saying we're aging the planet by working it too hard? By using solar power, we're causing the sun to use more power as well so we're helping it to burn out sooner? Heh.

Davidfromcali (author)dallasmel2010-07-16

The laws of thermodynamics do not know if you are simply heating your yard's air with the 'waste' heat or your house. There is no violation of any laws of thermodynamics by using the waste heat. How silly.

kcarring (author)Davidfromcali2011-01-02

@Dave - I agree totally. Dallas you are missing some key elements.. A condenser works BETTER when the heat can be dissipated faster. Think of your fridge, when the condensor gets dusty and dirty, and even the fact that it is crammed into a hole against the wall... these all fight the process. If you capture heat waste and re-use it, you actually IMPROVE the process, if anything. In fact this technology is taking the world by storm, do your homework. Volkswagen are in bed with a West German power company, with over 300,000 installations.

Davidfromcali (author)kcarring2011-01-07

One of the simplest and best examples I've seen was an article about 10 years ago about homes built in Italy in mountain areas with no electricity. It mentioned a small generator used for pumping water into a storage tank, lights, and charging batteries. The generator manufacturer sold a heat exchanger unit that used the exhaust heat (where about 80% of the heat from the engine is usually wasted) to heat an insulated water tank for heat later, washing, etc.
It wouldn't take very sophisticated technology to do this anywhere with even the most primitive set up, although if it is an Italian design it probably looks good, too!

kcarring (author)Davidfromcali2011-01-07

Very Cool! I think you will see a lot of people putting their heat pumps inside of solar collector rooms, or small "sun rooms" if you will... why not? That heat pump is working hard outside to try and do it's job, all winter long. It makes no sense to NOT have the sun help it out. I believe that a lot of people are quick to jump to an incorrect assumption: free energy, overunity - they are not the same thing as COP>1. A co-efficient of performance greater than one, has nothing to do with breaking the laws of physics (dallasmel: There's no such thing as 'free energy".) We are, in fact not even talking about free energy, at all period. If you spun your fan generator by hand, you'd get some juice. By capturing the waste "wind" you are putting that generator COP>1, you don't have to spin it. The air movement upwards, from the AC fan, is only very very slightly affected by this restriction, and that does cost a little bit, HOWEVER - that is offset by the fact that the secondary fan is helping to remove heat FASTER, which in theory will make the lower fan operate less often, in the first place.

dallasmel (author)Davidfromcali2010-07-17

You obviously do not have a working appreciation for the Laws of Thermodynamics to make such a statement. I highly recommend you talk to a High School/College Physis professor for clarification. Even waste heat dosen't get there by itself. It's the inefficient resultant of a "process", and as dictated by the 2nd Law of Thermodynamics. A reviewer had previeously made a suggestion to use your neighbor's condensing unit to power the device (acknowledged unethyical). But ... it wouldn't be you paying for the waste heat to power your device. It would be your neighbor. Wind turbines are in reality powered by the nuclear reaction occurring on our Sun. That reaction heats our planet and causes air to move ... subsequently powering our wind generators ... and generating electricity. All along the way in the process ... energy is being converted from one form to another ... and at each conversion process ... an inefficiency is generated. Try powering a motor with a generator that powers the motor. It dosn't work ... and can't work ... and because of the 2nd Law of Thermodynamics. A parasitic generator is powered by waste heat .... but something has to move that waste heat .... or the parasitic generator cannot work. If you place the parasitic generator in the air path of a condensing unit, the system in which the air-moving device (in the condenser) operates changes (becomes more resistive). Subsequently, the air-moving device will move less air, resulting a higher condensing temperature, less ability to remove heat from the home, and increase electrical usage, an example of the 2nd Law of Thermo.

Davidfromcali (author)dallasmel2010-07-17

I have taken several years of college physics and yes I do understand the basics. You brought up a bunch of scenarios that I did not support or even imply. I was not implying using the parasitic fan or stealing from my neighbor, both ideas that are a bit silly in my book but at least the person is trying to make it more efficient. All I said was that there are other things to do with waste heat than heat the air above the condenser. No laws would be broken building a properly designed heat exchange unit. I was talking about recovering waste heat which is a very common thing to do as any engineer that deals with industrial processes will tell you. Picture the waste heat going out of the condenser into a building for example. There is no net change in the energy consumption, you are simply putting the waste heat to use. If you look at industrial processes you will find that heat exchangers are extremely common in order to retrieve heat that would normally be wasted. They improve the overall efficiency of a large number of industrial processes without violating any laws of thermodynamics. I suggest you look into the engineering of heat exchangers in industrial processes if you would like to gain an understanding of the myriad practical uses of such a device. I have worked with them in industrial conditions and they are not put there by engineers that don't understand thermodynamics, they are used to recover energy that otherwise would go to waste and put it good use, almost always to save money despite their considerable costs. I am trying to bring practical information to this discussion so that all of us can see possibilities through proper use of technology, not win an argument.

dallasmel (author)Davidfromcali2010-07-18

So your taking waste heat and using it in a process, that of recovering a portion of the heat that was formerly being wasted. By definition, it is no longer waste heat if your putting it to work. In addition to having over 41 years of HVAC, Thermo, environmental control systems, etc. under my belt, I have done rocket science, and I am a certified commercial energy plans examiner and inspector. The point I have attempted to make is that you can't get something for nothing. You can't power the wind generator with the the discharge of a condensing unit without imposing additional energy requirement on the prime mover (the air-moving device in the condenser, and compressor since a decrease in the flowrate of air in a more restrictive system will decrease it's ability to pump heat due to higher condensing pressure ... which will also raise the staturated suction pressure and temperature in the evaporator ... also decreasing the ability of the evaporator to absorb heat. I'm quite familiar with energy conservation and recovery principles, and have engineered several such systems. Look up the term "specific speed". It's an engineering principle used to minimize the energy consuption of engineered systems utilizing rotary driven fans, pumps, etc. I believe you have some misunderstanding of the Laws of Thermodynamics, and probably because of the scope that the laws apply. The First Law says that energy can be converted from one form to another (eg: heat, potential, dynamic, etc.). The Second Law says that during any energy conversion process, the resultant of the process will always be less than the amount of energy put into it. Subsequently, waste energy (or heat if you like) is created. If you tap into that waste energy to use it, it's another process ... and if that process affects the prior process that created waste energy ... then you've also affected the prior process. If you use waste energy from the 1st process for a 2nd process, then by definition, it isn't waste heat anymore, and the second process will create it's own waste heat. The argument you've been giving regarding waste heat from a condensing unit, also supported by a photo of a air-powered generator mounted atop a condensing unit will affect the prior process because it affects the system in which the condenser air-moving device operates, which will affect compressor operation, which will affect evaporator operation.

Davidfromcali (author)dallasmel2010-07-18

A heat pump moves heat from one side of a device to the other. In the case of AC we want to use the cold side to cool a space and we need a heat sink, in this case the outside environment, to absorb the heat. If we wanted to heat a space, we would use the hot side to warm a space by pumping heat from somewhere, in this case the outside environment. A defined amount of work is done moving the heat. If I want a cool house and warm water, the water being the heat sink instead of my 100 degree yard, the same amount of work is done. It is simply a different heat sink, water instead of air. Why does using both sides, the cool side and the heat sink, violate laws of thermodynamics? How do the laws change due to my intent in designing the system differently by using water instead of air? Isn't a heat sink a heat sink? BTW - I enjoy an exchange of ideas as much as the next person but please notice that I have not supported the idea of the fan as described above. I have no idea why you feel I am defending it. You're analysis of the problem associated with it is basically correct.

dallasmel (author)Davidfromcali2010-07-19

When I talk of pumping heat, I'm referring to the energy level within the substance I'm pumping. It doesn't matter if it's hot, cold, etc. ... it contains heat as long as it's above absolute zero. My argument has to do with changing the system in which the pumping of heat occurs, and in order to utilize the total heat (evaporator heat + compressor heat) of rejection of the condensing unit. It's not so much as you'd be using the heat rejected to power an air-driven generator as you'd be influencing the mass airflow through the outdoor condensing unit. They heat pumping process is not a batch process (the heat has already been removed from the home?). It's a continuous process as long as the HVAC system is operating. You cannot power something with nothing. The system in which the air-moving device operate will be impared (made more restrictives). Subsequently, the ability of the device to pump heat will be compromised. My analysis is correct ... there's nothing "basically" about it. I enjoy this site as much as you ... and I hope my comments are taken as constructive critizism. I've nothing else to render regarding this subject, and will not be responding to any further comments. I've Clients that expect results ... GO FIGURE ... so it's back to work for me. Regards!

Davidfromcali (author)dallasmel2010-07-19

Again, I am not talking about using the unit as designed and constricting the air flow which obviously would change the dynamics of the system. I am talking about better design so that you use both sides of the heat pump. Even as designed, hanging a pot of water 6 feet above the unit to heat some water instead of air would neither affect the unit or violate any law of thermodynamics while at the same time it would use so-called 'waste heat' for a practical purpose. I saw a generator on a house in Italy that provided electricity from the generator along with hot water and house heat from the exhaust. It did not use a drop more diesel or provide less electricity than the same model generator without the heat exchanger, it simply cost more for the heat exchanger. Use the heat sink. Nothing can be made 100% efficient but nature does not differentiate between heat and 'waste' heat. It has to go somewhere so let's use it.

dave570 (author)2009-07-07

i think u guys are assuming the a/c fan is drawing downward thru the fan thus restricting air flow to the ac fan/ how ever my ac unit draws air UP thru the sides tthru the condencer THEN out thru the my case this is not causing any undo resistance to the ac motor, as all the air movemnet thru th ac unit is complete.

Treknology (author)dave5702010-10-01

Sorry, inlet or outlet, it is still adding to the load on the fan. Block up the exhaust end of your vacuum cleaner and then see how well it sucks. The above arguments about using the waste heat from the condensor for another purpose than further heating summer-air are more realistic, e.g., heating the shower, dishwasher or maybe even the oven...

Grichey5 (author)2010-08-22

Wow! I am not sure if I want to leave a comment or not. I would not wan to insult any engineers. I currently work in Energy management for large industrial and commercial buildings and complexes. I do not believe that this will cause your AC compressor in negligible difference in power usage. Your idea is in fact already being considered on large air cooled chiller systems. I got a chuckle when I saw your little version here. No doubt the energy you recover will be less than the energy put into it. I have worked on many jobs that have us spending millions of dollars pursuing all of that wasted energy that Mel finds so ridiculous. We are doing it more and more and there are huge financial incentives to do it for large commercial buildings. Obviously if it was not worth the effort so much money would not be spent on it. I am sure the purpose of your project was not to bruise the ego of an engineer. I work with engineers all of the time and I find most are now consulting engineers. That is code for not my fault if it don't work. Experimentation is a good thing and SHOULD be encouraged. Is that not the purpose of a site like this?

Davidfromcali (author)2010-07-11

Perhaps a better way to make the whole process more efficient would be to use the heat coming off the condenser for heating your water instead of just heating your yard. That way you would put BOTH sides of the condensor to good use for the same energy expenditure since we use hot water as well as air and refrigerator cooling. It's not free energy, just better use of it. My guess is that if you use your air conditioning much it would eliminate your hot water bill during the hot time of the year.

spa31rky (author)2010-07-07


necromncr (author)2008-06-02

Wait. Isn't the fan pushing the air doing some work? And if you put a fan in the path of the air being pushed, don't you increase the load the pushing fan has to do? In that case, you actually make energy from increase of power consumption of the fan which means larger bill. I might be talking jiberish, my tea didn't kick in yet, please be nice.

_soapy_ (author)necromncr2008-06-02

That's exactly right. See Treb's accurate explanation in the thread. I'll paraphrase it: You don't get anything free, because by putting something in the way of the airflow, the fan has to work harder to do the same cooling it was doing before you put the new fan on top and reduced the airflow.

martyjr9 (author)_soapy_2010-07-04

i think the power loss is negligible as the air is already out of the unit. Now if there was ductwork forcing the air into the fan blades now you are talking less cooling effect on the condenser.

bwcbwc (author)necromncr2008-06-10

Think of a turbo-charger on a car. Yes it pushes back on the engine, but it gives more than it takes because w/o the turbo-charger ALL of the energy in the exhaust is wasted.

doryrhp (author)bwcbwc2008-12-25

Turbo-chargers do not improve the efficiency of an engine, in fact they generally lower the overall efficiency of an engine (more gallons of fuel per horsepower). Turbos only improve the volumetric efficiency of an engine, allowing the engine to burn more fuel-air mixture (since the mixture is pre-compressed) than a naturally aspirated engine. The turbo essentially allows a small displacement engine to produce power similar to a larger displacement engine (which is why they are so popular, large engine power, with a small engine weight and size). The set-up in this Instructable is most likely forcing the motor running the fan to draw more power than what the turban and alternator are producing. One method to improve the efficiency would be to make sure the fan is compressing only cool, fresh air, then forcing the air past the hot compressor motor and condenser before exhausting it through the turbine. With this method the AC unit and turbine would essentially be acting as a jet engine! Of course for the AC-jet engine to work you would most likely need some serious aerodynamic ducting and compressor/condenser streamlining to reduce frictional losses in the unit. You might also need to start out with a larger AC unit since running the compressor and condenser in a hot environment would likely reduce the amount of cooling power available to your house!

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