Parasitic Power Devices

Aside from having a very cool name, parasitic power is an innovate way to recapture already spent power. This power can come in the form of wasted heating or cooling of a building for example. Last week the Southern Methodist University activated the first commercial Green Machine from ElectraTherm. The unit recycles residual heat from the building into electricity. So far, the 50kW Green Machine has exceeded expectations. The company also says owners can recoup the units cost after about three years.

Pictured above is a Parasitic Wind Turbine built as an experiment to see if any useable energy could be captured from a standard home air conditioning unit. Although not listed on the website, the 6 phase 90 V 4.0 the motor used here is available from Skycraft or auction websites like eBay. Using the rotor of a standard box fan and placing it at a 45 degree angle yielded the best RPM.

Lastly, these applications of parasitic power remind us of an art project by Michael Rakowitz from 1998. Rakowitz created temporary homeless shelters that were inflated by a building exhaust vents. The air flow from these vents provided both structure and heating for the shelters. They also served to draw attention to the homeless housing situation and encourage cities to think of new ways to address the problem.

25 thoughts on “Parasitic Power Devices

  1. Parasitic is kind of apropos in the case of the air conditioner wind turbine. By disrupting the airflow out the wind turbine probably makes the air conditioner draw more power to do its normal work. I’d like to see some measurements taken of the air conditioner running with and without the turbine, my guess is that much of the ‘reclaimed’ power is actually cancelled out.

  2. bug, i agree.

    The energy used to turn that turbine has to come from somewhere. Not only does the fan in the A/C unit have to draw more current, it holds more heat in the unit causing the compressor to run longer also.

  3. >By disrupting the airflow out the wind turbine probably makes the air conditioner draw more power to do its normal work.

    Indeed. More wind resistance means that the air conditioner does its job less efficiently, or the resistance in the fan goes up – making it consume more electricity.

    Don’t get me wrong, there are certain applications that make make a lot of sense when reclaiming energy, i just don’t think that this is one of them.

    I remember seeing a a project where people reclaimed heat from the water exiting your house (eg, stealing the heat from the water from your shower) something like that makes more sense to me.

  4. Tony, 20% of the air’s energy (contained entirely in kinetic energy)(.35cp x.8 x extra loss) can be recovered, without affecting the fan, however, this is actually only a small fraction of the energy spent across the pressure difference between the two sides of the radiator.

    A more effective way to reduce the electrical consumption of the fan [1/2hp (~400Watts)]
    is to build a larger condenser/evaporator, modify the fan to a lower speed, 5 or more bladed airfoil. (not some sheet metal cr@p) and build a sheet metal exhaust flue to take advantage of the hot air.

  5. >eg, stealing the heat from the water from your shower

    Yeah, that and clothes dryers are relatively unutilized sources of reclaimed energy.. I’ve heard of people going all-out with a big heat exchanger between warm/stale inside air and cool/fresh outside air.

  6. first- kudos to the hackaday team for getting a whole lot more interesting stuff posted every day.

    i’m not sure you’re actually hurting the efficiency on the ac unit- you’d really have to measure. Granted, this fan up on top of the unit probably increases the outgoing pressure of the fan slightly, but that just means it moves a bit less air. Ultimately, the condenser fan motor may be using a bit LESS current since its performing less work (moving less air), and if there was any decreased efficiency, it would be because of less air flow across the condenser coils. I guess we’d really have to see a measurement to be sure., and it would change with outside air temp. Interesting idea though. Of course, if you lived in an apartment, you could always put this on your neighbor’s condensor unit instead!

  7. there is plenty case in nature where parasites help the host in some way other than being slightly annoying i’m sure there is some parasites that don’t do any harm. i think this is an awesome idea… i use the heat from my dryer exhaust (only when i am drying clothes) in the winter to help keep the basement warm. only side effect is a little humidity. there is a dehumidifier running anyways in the basement.

  8. to the people that are saying that its using more power to run the turbine and causing more heat your completely wrong in every way, and its not causing the compressor to run longer . the a/c would never even know its there the only time it would make more heat is if the turbine was blocking airflow by restricting it and since the sides of the turbine and most of the surrounding area are open the air is just diverted somewhere else. so what have we learned.. the turbine isn’t hurting efficiency at all and the a/c isn’t getting any hotter with it on so its a win win. so yes your reclaiming already spent power, i want to see more hacks like this.

  9. Joe – how do you figure the first part of your comment? It sounds like you know what you’re talking about but I don’t follow. By placing a wind turbine in front of the fan, you’re decreasing the pressure difference across the fan, which decreases the fan’s airflow and thus reduces the heat transfered by the heat exchanger. Then the controller has to increase the fan speed to compensate, thus drawing more power and decreasing efficiency. Would you mind going into more detail of where the 20% number comes from? I’m curious. Thx.

  10. seems more than a little futile to run a multi-kw ac and then try and claw back a watt or two by impeding its airflow. perhaps servicing it or just turning it down would be a better solution?

  11. ***(10)*** i use the heat from my dryer exhaust (only when i am drying clothes) in the winter to help keep the basement warm. only side effect is a little humidity. there is a dehumidifier running anyways in the basement.

    Isn’t this defeating the purpose of regaining lost power by running the hunidifier to reduce the humidity? Does that not use more power?

  12. Just spitballing here, but couldn’t the vent fan housing be enlarged some and have the fan increased in size/changed to accomodate the wind turbine? Place the turbine in-line with the current fan axle and have 100% of the fan’s rotation generating energy?

    If the fan is creating windflow that is turning another fan, couldn’t a middle step be cut out?

    And….if that step could be cut out, then we would have electricity turning the fan turning the turbine creating electricity, so we’d be better off redesigning the fan even more so it required less energy to operate in the first place? I’m thinking it would be more efficient to be more efficient in the first place, rather than trying to “win back” energy already expended.

  13. I was kinda mystified by the humidifying the area that requires a dehumidifier as well…
    But what I really want to know is –
    What the heck is a Twin Screw Expander?

  14. to Justin Reed and others who asked why up to approximately 20% of the air’s kinetic energy can be recovered.

    A wind turbine is, in theory, 59.6% efficient, a 3 bladed wind turbine gets about half of that 59.6%. The alternator is 80% efficient, and a box fan is around 10% efficient or less, working backwards.

    if you put a flat disk, the size of the fan, about 1 fan diameter, above the fan outlet, did the back pressure increase? no, not significantly.

    The fan in question is recovering kinetic energy if it is far enough away from the fan, if it is too close, then the air is forced to go through the turbine, and it will recover a lot more.

    a 1 meter diameter fan blowing air at 5 meter’s second requires a [3.9 cubic meter/s x 1.19KG X (5m/s)^2] 233 watt motor.
    If we increase the speed to 10m/s we need 8 times the power, but we only get twice the cooling rate.
    that is why i suggested a larger condenser and a slower fan.

    A good question to ask is: is the air pressure in front of a wind turbine higher than the air pressure a hundred feet up stream?
    you will learn a lot if you try to answer that.

  15. In think calling this device “parasitic” is fitting in a context where the “host” is somebody ELSE’s A/C unit. I would also say that this is probably the most interesting application… in which case it wouldn’t matter whether or not it makes the A/C Unit less efficient, as the user wouldn’t be paying the electricity bills.

  16. and wouldn’t putting this fan near the exhaust accomplish the same thing? if your concern is restricting airflow to your ac, then just put it where the air leaves. hell, put it in front of the vent; that will also help circulate the air, cooling the house faster

  17. Hi everyone, I’m the author of that instructable, I wanted to correct one important thing in the summery, the turbine uses a 2phase 6volt 0.4 amp stepper, the larger one is for an, as yet, unrealized project.

    As to whether it increases the load on the compressor, that is still up for debate. My experiment have been inconclusive due to limitations of the test equipment available.

  18. I think it’s interesting you can take parts from an old box fan or oscillating fan to make a turbine.
    I was wondering, won’t a reverse phase turn a normal electric motor into a generator?

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