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Author Topic: The myth about "free electricity" in winter  (Read 8338 times)
DeathAndTaxes
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Gerald Davis


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October 27, 2011, 12:46:07 PM
 #41

Could someone please explain to me how somthing can be more than 100% efficient without allowing people to use its output to power the device for free?

A heat pump doesn't CREATE heat.  If it did it could never have more than 100% efficiency.  A heat pump MOVES heat.  There is significant heat outside the home.  Even in 0C air there is thermal energy.  The only temp that has no thermal energy is absolute zero.

If a heat pump burned fuel or turned electricity into heat via resistance its max efficiency would be 100%.  You can't get more heat/energy than what existed in the fuel/electricity to begin with.

However as a pump it can pump in huge amounts of outside energy for a small amount of energy and thus achieve >100% efficiency.

The theoretical max efficiency for a heat pump varies by the difference is desired source (outside) & sink (inside) temperatures.  For 10C outside and 35C inside is ~1200%.  The highest efficiency units available for purchase are ~500%.

As energy becomes more scarce/expensive expect heat pumps to replace furnaces.  With technology we potentially could have 600%, 750%, 800% efficiency heat pumps someday.  A furnace simply can never be more than 100% efficient (due to law of conservation of energy).

As for why couldn't you use that energy to power the device.  the thermal energy in your house is not very concentrated.  The less concentrated an energy form is the more difficult it is to convert into another form.  For example a thermalcouple can directly convert heat into electricity but it is usually less than 1% efficient.  For 100 units of heat energy = 1 unit of electrical energy.

BTW:
Air conditioners, refrigerators, freezers are all heat pumps they are simply one way heat pumps.  A residential "heat pump" can pump heat into or out of the house.


uh yeah people have seem to forgotten the law of conservation of energy..

Some people don't understand the law of conservation of energy.

If you have 100 units of energy outside the home and 10 units inside and I move 20 of those units inside via a "pump" (a energy/heat pump) then the end state is 90 units of energy outside and 20 inside.  No energy was created or destroyed.  Energy was simply moved.
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October 27, 2011, 12:54:21 PM
 #42

But if you can use room temperature to make the air hotter, why can't you use that generated hot air to power your air heating device?

Converting heat into electricity is difficult and inefficient.  There are a couple different methods but generally efficiency INCREASES when the difference in temps between input and output is very large.

This is why for example we super heat steam by heating it, putting it under pressure, and heating it more.  The higher the input temp and the lower the output temp the more energy which can be converted into electricity. 

The heat in your house while it may feel warm is very very very low.  It is cooler than the exhaust of a car for example.  With low input temp and only slightly lower output temp you wouldn't get much efficiency (maybe 1% or less).

Concentrating heat would require more energy and thus a heat pump can never be a perpetual motion machine.  You could however use a heat pump as a "battery".

Take a storage area that hold temps well (say crushed granite) and surround it with an insulator.  When you have electricity to store use heat pump to pump heat (form atmosphere) into your "heat battery.  When you need electricity allow that heat to flow out of the "heat battery" into a turbine to produce electricity.

If you can get the round trip efficiency of electricity -> stored heat -> turbine -> electricity close to 100% then you have a battery.  The problems are
1) some heat will be lost.  better cheaper insulation will help
2) turbines are at best 60% efficient that can be compensated by a heat pump which is 500%+ efficient
3) turbines are most efficient when input temp is VERY HIGH so you would need a specialize heat pump designed to concentrate a lot of thermal energy into the granite.
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October 27, 2011, 08:04:00 PM
 #43

I see, people say it's over 100% efficient because the device isn't working alone, the Sun or the Earth's molten core (or some other factor that is already there anyway and doesn't need humans to keep it going) is also participating in the process.


That's like saying a fan pointed at a wind turbine in a windy day is more than 100% efficient, right?

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October 27, 2011, 08:18:37 PM
 #44

I see, people say it's over 100% efficient because the device isn't working alone, the Sun or the Earth's molten core (or some other factor that is already there anyway and doesn't need humans to keep it going) is also participating in the process.

Yes.

Quote
That's like saying a fan pointed at a wind turbine in a windy day is more than 100% efficient, right?

No.  That is a bad example.  Neither the fan nor the turbine are >100%.  Neither are 100% either.
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October 27, 2011, 08:22:48 PM
 #45

I see, people say it's over 100% efficient because the device isn't working alone, the Sun or the Earth's molten core (or some other factor that is already there anyway and doesn't need humans to keep it going) is also participating in the process.


That's like saying a fan pointed at a wind turbine in a windy day is more than 100% efficient, right?
No, not really, no.

By nature, the wind would blow into the turbine whether you had a fan pointed at it or not.
By nature, the heat would stay outside of your house unless you used a heat pump to get it inside your house.
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Gerald Davis


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October 27, 2011, 08:28:00 PM
 #46

I see, people say it's over 100% efficient because the device isn't working alone, the Sun or the Earth's molten core (or some other factor that is already there anyway and doesn't need humans to keep it going) is also participating in the process.

Yes.  I would modified the statement to say.  It is over 100% efficiency because it generates more than 1 unit of heat with 1 unit of electricity.  How it is able to do that is tap into other forms of energy (like the thermal energy in outside air even on a chilly day).

Technically a physicist wouldn't say >100% efficient because that is a vague term.  The technical term is Coeffcient of Performance (COP)
http://en.wikipedia.org/wiki/Coefficient_of_performance

A heatpump has a COP of >1.
At best burning fuel or using using electric resistance heating has a COP of 1.*

In reality most furnances are much less efficient.  The minimum legal standard for NEW natural gas furnaces in the US is only 78% efficient (1 unit of energy via fuel = 0.78 units of heat inside home).  Prior to 1990s many furnaces were <60% efficient.

Quote
That's like saying a fan pointed at a wind turbine in a windy day is more than 100% efficient, right?

No.  That is a bad example.  Neither the fan nor the turbine are >100%.  Neither are 100% either.
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October 28, 2011, 02:09:09 AM
 #47



what about this point.

It does not matter about the efficiency of the miner rig producing heat.  If the bitcoins are sold to pay for the electric, then the electric is free, hence the heat is free. period.

1jimbitm6hAKTjKX4qurCNQubbnk2YsFw
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October 28, 2011, 03:09:15 AM
 #48



what about this point.

It does not matter about the efficiency of the miner rig producing heat.  If the bitcoins are sold to pay for the electric, then the electric is free, hence the heat is free. period.
Just so long as you generate enough bitcoins to cover the cost of the heat.  With the recent difficulty drop, and price hovering around $3.00/coin, I'm finally back in the black.  So free electricity + some more!
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October 28, 2011, 05:29:16 AM
 #49

I always wondered about this. The problem with those heat pumps is that they do not work when the temp is colder than I say around 45 degrees.

The question remains, is the heat thrown from a computer at the same efficiency as electrical heaters?

To also better your argument, the computer uses electric, while most people get much better heat efficiency by using gas or oil for heat.
The question remains? Damnit guys, don't you learn physics at school??? Like, that energy DO NOT disappear? If something use 1kw guess what, it will spit out 1kw of heat, be it a computer or a heater.
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Gerald Davis


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October 28, 2011, 12:29:17 PM
 #50

The question remains? Damnit guys, don't you learn physics at school??? Like, that energy DO NOT disappear? If something use 1kw guess what, it will spit out 1kw of heat, be it a computer or a heater.

Well that statement is equally wrong.  An electric car (or any other non-resistance load that does actual work) that pulls 1kW won't generate 1kW of heat.
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October 28, 2011, 02:11:25 PM
 #51

The question remains? Damnit guys, don't you learn physics at school??? Like, that energy DO NOT disappear? If something use 1kw guess what, it will spit out 1kw of heat, be it a computer or a heater.

Well that statement is equally wrong.  An electric car (or any other non-resistance load) that pulls 1kW won't generate 1kw of heat.
It always ends up as heat, no matter how you complicate the route it always becomes heat in the end. If you keep running circles in a closed (and insulated) room with that electric car the room will warm up, no doubt about it.

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October 28, 2011, 02:34:50 PM
 #52

The question remains? Damnit guys, don't you learn physics at school??? Like, that energy DO NOT disappear? If something use 1kw guess what, it will spit out 1kw of heat, be it a computer or a heater.

Well that statement is equally wrong.  An electric car (or any other non-resistance load) that pulls 1kW won't generate 1kw of heat.
It always ends up as heat, no matter how you complicate the route it always becomes heat in the end. If you keep running circles in a closed (and insulated) room with that electric car the room will warm up, no doubt about it.

1kw of "work" I think is what he meant. If heat is the "work" you are trying to achieve then you will get 100% eventually, as long as no chemical reactions take place to absorb heat. Movement->friction->heat. Even the sound from the fans becomes heat eventually.

It is just a matter of directing the heat to where you want it, and making sure that it is contained as desired.
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October 28, 2011, 04:17:28 PM
 #53

The question remains? Damnit guys, don't you learn physics at school??? Like, that energy DO NOT disappear? If something use 1kw guess what, it will spit out 1kw of heat, be it a computer or a heater.

Well that statement is equally wrong.  An electric car (or any other non-resistance load that does actual work) that pulls 1kW won't generate 1kW of heat.
You're wrong.  Movement has friction (even if it is just friction between air molecules), and 100% of it is turned into heat as soon as movement has ceased.

When you stop your car, tons of heat builds up in your brake rotors, because all of the energy that your car was moving with is turned into heat via friction.
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October 28, 2011, 05:02:41 PM
 #54

what about this point.

It does not matter about the efficiency of the miner rig producing heat.  If the bitcoins are sold to pay for the electric, then the electric is free, hence the heat is free. period.
Just so long as you generate enough bitcoins to cover the cost of the heat.  With the recent difficulty drop, and price hovering around $3.00/coin, I'm finally back in the black.  So free electricity + some more!

I'd take this a step further.  Bitcoin mining is something I am doing anyway.  The heat is a byproduct.  The fact that I can use this byproduct to reduce (or eliminate) my heating costs is where the "free heat" comes into play.  It does not matter that the bitcoin miners cost money to run as long as the heat is a unintended byproduct it is "free".
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October 28, 2011, 06:31:49 PM
 #55

I think the problem some people are having with the 100% efficiency thing is they are comparing the efficiency to the conversion factor.  That is not what the 100% is being compared to.  It's being compared to other heat generating activities.

What is the baseline comparison that is factored at 100%?  Then whatever you are using to provide the heat if requires less energy to produce X amount of heat than the baseline, it's > 100% efficient.  If it uses more energy to produce X amount of heat, it's less than 100% efficient.

If you're searching these lines for a point, you've probably missed it.  There was never anything there in the first place.
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October 28, 2011, 06:46:23 PM
 #56

The baseline for any electricity-based device is what using that amount of electricity would generate in heat, if it were sealed in a closed container.  For example, a 1000w air conditioner "creates" cool air for the indoors, but at the same time, creates the same amount of heat + 1000w outdoors.  The net effect, if you sealed the whole air conditioner unit in a box, is that it would be creating 1000w.  The air conditioner would be 100% efficient at creating heat.  In fact, if sealed in a box, ANY electrical device would be 100% efficient at generating heat equal to its wattage.

Regarding fuel sources, like natural gas and heating oil, efficiency depends on percentage of the fuel burned.  If you burn 70% of the fuel, and the other 30% is wasted through exhaust as unburnt fuel, then the device is 70% efficient.  I don't believe there would be a way to make a natural gas or heating oil device more than 100% efficient.

That's why automobiles are now fitted with EGR valves - it actually recycles some of the exhaust gases in an effort to burn even more of the fuel that may have been left unburnt during the previous burn cycle.  A typical modern internal combustion engine is around 30% efficient at moving the vehicle - the rest of the energy from burning gasoline is wasted as heat, with a tiny percentage wasted as unburnt fuel.
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October 28, 2011, 06:51:15 PM
 #57

Regarding fuel sources, like natural gas and heating oil, efficiency depends on percentage of the fuel burned.  If you burn 70% of the fuel, and the other 30% is wasted through exhaust as unburnt fuel, then the device is 70% efficient.  I don't believe there would be a way to make a natural gas or heating oil device more than 100% efficient.

It isn't just unburnt yet.  Even if you burn natural gas completely some of that heat will escape outside the building (via combusiton vent) and that means efficiency will be <100% although they are getting close.  It is possible to buy 96% efficient furnances today.

As far as >100% efficience.  Through combustion (furnace) you are right it can never be >100% however there are natural gas heat pumps which can achieve a COP of >1.  Yes that means it is alos possible to cool a house with natural gas.

http://www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12680
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October 28, 2011, 06:52:56 PM
 #58

Regarding fuel sources, like natural gas and heating oil, efficiency depends on percentage of the fuel burned.  If you burn 70% of the fuel, and the other 30% is wasted through exhaust as unburnt fuel, then the device is 70% efficient.  I don't believe there would be a way to make a natural gas or heating oil device more than 100% efficient.

It isn't just unburnt yet.  Even if you burn natural gas completely some of that heat will escape outside the building (via combusiton vent) and that means efficiency will be <100% although they are getting close.  It is possible to buy 96% efficient furnances today.

As far as >100% efficience.  Through combustion (furnace) you are right it can never be >100% however there are natural gas heat pumps which can achieve a COP of >1.  Yes that means it is alos possible to cool a house with natural gas.

http://www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12680
Well, yeah, that's true.  Always going to have some heat loss through the exhaust, since you can't vent the burnt natural gas air directly into your home.  Tongue

Whoa, didn't know they even made natural gas heat pumps!  That's pretty neat...
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October 28, 2011, 07:10:24 PM
 #59

Always going to have some heat loss through the exhaust, since you can't vent the burnt natural gas air directly into your home.  Tongue

Well you can have 100% efficiency however you need to just vent it into the living space to ensure no heat escapes.

Like this:
Gas logs are just fancy gas burners (like on a stovetop).


Here is a less fancy version which does the same thing:
http://www.amazon.com/gp/product/B000KKO4WA/ref=pd_lpo_k2_dp_sr_2?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B000J011N6&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0DK64RJY3X5SBS3T7KCG

Quote
Whoa, didn't know they even made natural gas heat pumps!  That's pretty neat...
They never really caught on and since less are made tend to be more expensive.  They also are noisier than electric heat pumps.  The fact that natural gas is so cheap is increasing interest in them again.  Anything that uses a compressor can be made to run on natural gas (or any fuel source).
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October 28, 2011, 07:28:47 PM
 #60

Always going to have some heat loss through the exhaust, since you can't vent the burnt natural gas air directly into your home.  Tongue

Well you can have 100% efficiency however you need to just vent it into the living space to ensure no heat escapes.

Like this:
Gas logs are just fancy gas burners (like on a stovetop).


Here is a less fancy version which does the same thing:
http://www.amazon.com/gp/product/B000KKO4WA/ref=pd_lpo_k2_dp_sr_2?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B000J011N6&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0DK64RJY3X5SBS3T7KCG

Quote
Whoa, didn't know they even made natural gas heat pumps!  That's pretty neat...
They never really caught on and since less are made tend to be more expensive.  They also are noisier than electric heat pumps.  The fact that natural gas is so cheap is increasing interest in them again.  Anything that uses a compressor can be made to run on natural gas (or any fuel source).
You can't vent natural gas fumes directly into a living space... and that's not what that gas log burner does.

EDIT:  That open gas burner still vents into a chimney or pipe.  You'll kill yourself from carbon monoxide (or carbon dioxide?) poisoning if you don't vent it at all.
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