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LD LANGER

Thu Apr 28 2011, 07:06PM

Registered Member #3824 Joined: Sun Apr 10 2011, 08:29PM
Location: Edmonton, Alberta, Canada
Posts: 54

Hello,
So I figured this thread probably goes in this section as induction heaters use medium voltage / HF. I have been for quite some time pondering over the idea of designing an induction heater for large scale space heating applications rather than typical joule heating methods seen right now. Most industrial electric space heaters use just resistive heating elements and a large fan that moves a lot of air over those hot coils. If you were to use some graphite or iron mesh (large surface area) on both sides of two or three pancake coils, it seems to me that this would be a far more efficient method of heating. The mesh would be supported by ceramic insulators and heated up to a very high temperature (approaching probably 8 or 9 hundred degrees C). With large air flow this would be able to heat a shop or large garage very quickly.
As induction heating uses hysteresis losses and eddy currents (basically also joule heating) I cannot figure out the efficiency of induction heating for the method I said previously. I have looked absolutely everywhere and I cannot find articles that cover this area.

Does anyone know methods for calculation of something like this, or if this sounds like a viable idea or not? I know that a lot of heaters are gas powered but in places where only electric service is available this would be an option.

Thanks,

Daniel

James

Thu Apr 28 2011, 08:15PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

All forms of electric space heating are essentially 100% efficient. Where else is the energy to go?

To improve on this, a heat pump is a good option. Since it moves useless heat from outside to the interior, it effectively exceeds 100% efficiency.

LD LANGER

Thu Apr 28 2011, 08:33PM

Registered Member #3824 Joined: Sun Apr 10 2011, 08:29PM
Location: Edmonton, Alberta, Canada
Posts: 54

ahh.. classic overthinking. well that settles that.
Thanks for the input.

Just to be clear: For any given amount of electrical energy, there is a set amount of heat energy that can be produced no matter the method of energy conversion? (I know I'm basically ignoring the 1st law of thermodynamics here)

Xray

Thu Apr 28 2011, 09:16PM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

James wrote ...

All forms of electric space heating are essentially 100% efficient. Where else is the energy to go?

To improve on this, a heat pump is a good option. Since it moves useless heat from outside to the interior, it effectively exceeds 100% efficiency.

You're joking.... right?

James

Thu Apr 28 2011, 09:26PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

No, I'm completely serious. A heat pump sucks heat from outside and pumps it into the interior, so the heat output into the building is greater than the electrical energy consumed. You're not creating something from nothing, the heat comes from outside, but that heat is otherwise wasted so "effectively" you are are over 100% efficiency.

You get roughly 3x the BTUs per Watt from an air source heat pump as you do from straight electric resistance heat, and electric resistance heat is, for all practical purposes, 100% efficient at converting electrical energy into heat. The only waste is whatever light is produced by the element and most of that gets absorbed and turned back into heat.

It's very easy to convert potential energy into heat, the losses are incurred going the other way around.

Mathias

Thu Apr 28 2011, 10:09PM

Registered Member #1381 Joined: Fri Mar 07 2008, 05:24PM
Location: Hungary
Posts: 74

Then again, from an environmentalist point of view. Using electricity for heating is a wasteful thing to do if one would consider how electricity is produced. Basicaly every "conventional" power station boils water (steam) that somehow drives the generators. (So, in practice after the losses ( chemical physical, mechanical , electric) the overall efficiency ends up around 40% at best)

By the way talking about 100% efficiency ~ over unity bring us into pseudoscience territory...

Xray

Thu Apr 28 2011, 10:15PM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

Mathias wrote ...

By the way talking about 100% efficiency ~ over unity bring us into pseudoscience territory...

I agree. The heat pump system is not over 100% efficient. It CANNOT be! In fact, if you bring in air that is cooler than what the heat pump produces, then the heat pump will have to work to heat that air. Bringing in outside air that's warmer than the inside air will INCREASE efficiency, but it can never equal or be greater than 100%. If anyone can find a link that proves me wrong, I'd like to see it.

James

Thu Apr 28 2011, 10:28PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

It's not pseudoscience, it's a well understood process that has been in widespread use for decades. I said effectively, not that it is literally over 100% efficient. There is no mystery here, no free energy, just harvesting of heat from outside the building to relocate it inside. In the process most of the "wasted" energy from the mechanical losses in the heat pump also end up inside the building where they are not really wasted.

The fact is, if you measure the heat output of a heat pump, it will be greater than the electrical energy into the system. This is because the electrical energy is only a portion of the energy going in, the rest is ambient heat from the air outside absorbed by the boiling refrigerant in the evaporator coil. This is the whole point of using a heat pump in the first place instead of just using far simpler resistance heat. You're not getting something for nothing, just using a bit of energy to move other energy to a place you want it.

Yes, using electricity to heat is inherently inefficient, but the inefficiency is in the process of generating electricity from heat, not turning it back into heat. Not everywhere has natural gas though, small coal boilers are less efficient than huge central power plants, and we're not allowed to have nuclear reactors in our basement to heat water that could be circulated to warm our homes.

Xray

Thu Apr 28 2011, 10:30PM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

James wrote ...

No, I'm completely serious. A heat pump sucks heat from outside and pumps it into the interior, so the heat output into the building is greater than the electrical energy consumed. You're not creating something from nothing, the heat comes from outside, but that heat is otherwise wasted so "effectively" you are are over 100% efficiency.

I think that's misleading. We are discussing the efficiency of converting electrical energy into useable heat to heat a room. Even if the heat pump doesn't need to actually "heat" the air because it's bringing in hot air from outside to warm a cold room (that is, only a FAN running) then it's still not anywhere near 100% efficient because electrical energy is used to turn the fan blades. In that situation, the "system" is operating at a very high efficiency, but it would be incorrect to say that it's operating at greater than 100% simply because the heat pump doesn't need to engage.

James wrote ...

You get roughly 3x the BTUs per Watt from an air source heat pump as you do from straight electric resistance heat, and electric resistance heat is, for all practical purposes, 100% efficient at converting electrical energy into heat. The only waste is whatever light is produced by the element and most of that gets absorbed and turned back into heat.

It's very easy to convert potential energy into heat, the losses are incurred going the other way around.

I disagree. Maybe the actually heating element converts nearly 100% of electricity into heat, but it's far from being USEABLE heat. When speaking of efficiency, you need to consider the entire heating system, and not simply the heating element. For example, a worst case scenario would be if an electric heater FAN failed to turn. The elements would get hot, but that heat would not be disbursed into the room. In that case, the "system" would be more like 1% efficient!

Xray

Thu Apr 28 2011, 10:39PM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

James wrote ...

The fact is, if you measure the heat output of a heat pump, it will be greater than the electrical energy into the system.

That's true, but it's not because the heat pump is "effectively more than 100% efficient", it's because the external heat was generated by some outside source.... In this case, the SUN! So, the Sun is what's heating the room, and not the heat pump. True, the heat pump can operate more efficiently when utilizing an outside heat source, but the conversion of electrical energy into another form (in this case we are talking about heat energy) can NEVER be 100%, let along > 100%.

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