[jules]

jules asked the Naked Scientists:

Why can we not install and use microwave technology for domestic central
heating systems, to heat the water  and radiators in the home, cost
effectively?...

What do you think?

[graham.d]

Hi Jules, I am not sure exactly how you are thinking of utilising microwaves to any advantage here. Microwaves, when used for cooking, are basically radio waves around 2.4GHz, which is a good absorption frequency for water molecules. This is why it is good for cooking. It penetrates objects and heats the water within them throughout the object. Like any radio wave it will not pass through metal and will not heat objects that are not electrical conductors. Of course it can cook humans as well as any other meat so these high energy radio wave need to be "contained" in a metal box.

As it uses electrical energy to make microwaves, and not with 100% efficiency, it would not seem practical to use them for heating water (say) as it is much more efficient, and less hazardous, to simply use an immersion heater.

[Pumblechook]

Eaxactly.  Our microwave produces 850 Watts of radio frequency power but it takes 1400 Watts from the mains.  It is more efficient to heat water directly with a heating element.

But it is not true that microwaves will not heat materials which are not electrical conductors.  Materials like PVC will get warm in a microwave due to dielectric heating. Basically the molecules of the materials vibrate (heat up) in the presence of an alternating electric field.   That is why some plastic containers are suitable for microwave use (made of Polyethelene or PTFE - low dieletric loss - don't respond much to an applied field) and some are not. 

[Soul Surfer]

It just depends whether you want to do. Physically warm the room or create the sensation of warmth.
If you actually want to warm things the best way is to use the cheapest source of heat. However if you want to create the sensation of radiated warmth without heating things too much (a bit like an electric bar fire)  you should use microwaves but not the 2.4 GHz of a conventional microwave you should use submillimetere teraherz microwaves.  These waves penetrate your clothes and are used for special kinds of security radar which show you naked!  They also penetrate the skin slightly and stimulate the heat senors in your skin without creating too much overall heat.  These can create the sensation of warmth.  High power versions of this have been proposed as a method of crowd control by creating a non damaging sensation of burning and causing people to turn and run away from the transmitter.

[techmind]

Infra-red radiant heaters are widely used in industrial buildings, warehouses, and some shops to give the feeling of warmth at a much lower cost than actually heating the air. They're also useful in entranceways where the air is continually washing indoors and out. They don't necessarily visibly glow at all.
I believe several Waitrose branches which have IR radiant heaters fixed way up in the ceiling near the entrance and/or tills/reception desk. They look a bit like grey heatsinks.

[daveshorts]

I think the problem with both the microwaves and the terrahertz waves is that pieces of metal can redirect and focus the waves which could increase the intensity to a dangerous level. I think that metal framed glasses may cause issues with the US pain ray that you were talking about.

[AB Hammer]

I' sorry but this gives me the vision of the hamster in the microwave. Beep!! pop! goes the weasel, look you've got a little balloon. [)]

[ironflex]

There no relation between microwave oven to heat.but its also produce heat internally,that means inside the microwave oven.
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ironflex
newbielink:http://"http://www.celebpoker.com/poker-promotions" [nonactive]

[Onlyinterestednotdevoted]

If microwaves are so energy ineficient, why not just cook with electric devices?

[Soul Surfer]

It all a matter of where most of the energy in the radio waves is deposited  this is usualy around one quarter to half a wavelength  inside a poorly conducting material like meat and vegetables ie a few centimeteres from the surface in the case of a 2.45GHz microwave.  This means that what you are cooking is heated FROM THE INSIDE and therefore cooks much quicker.  A conventional cooker heats the surface and this energy has to be conducted inside and this takes time and therefore requires more energy to heat the surroundings.  It is quite possible to cook something in a microwave oven while the oven stays quite cool.

[Soul Surfer]

As for just creating a sense of warmth while using a minimum amount of energy and not heating the surroundings.  Teraherz waves are probably the best because they penetrate the clothing and stimulate the heat sensors in the skin directly.  Microwaves are dangerous because they penetrate much deeper and can overheat and damage living tissue in areas where there are no heat sensors.  There are some terrible stories about early power microwave engineers cooking up their guts with microwaves and not realising it and dying in agony some days later when the dead intestines fell apart inside them givong them preitonoitis.

[lyner]

I have just put some lunch in the microwave. Cheers, SS, bon apetite!

[oldsmobile_man]

I came up with an idea to heat a house using a microwave oven back in 1984. I assembled my invention back in 2004 and it WORKS

[CliffordK]

Oldsmobile Man,
I am curious what you actually did.  What is being heated?  How do electronic gizmos, knives, forks, and etc hold up?

If one could effectively keep the microwaves in a house (foil on the walls), and somehow make a uniform distribution around the house, it would be interesting if one could actually just specifically warm the people in the house with low intensity microwaves. 

Heat the people, not the house.

But, everything would have to be designed to be microwave safe.  For example, one may choose to use button flies rather than zipper flies.  And, it would have to go through years of animal testing before beginning human trials.

[alancalverd]

Heat the people, not the house.

The Inuit solved this problem about 40,000 years ago. Sealskin parkas consume no electricity, and don't melt the roof. Just eat plenty of blubber and sleep with your boots on.

[Trekker]

I have been thinking about this exact thing recently. I have worked on furnaces for over 20 years and am wondering why microwaves can't be used instead of say natural gas? A standard furnace has a heat exchanger that is made up of 2 to 4 chambers. Picture cucumber shapes (standing about 1 1/2 ft. tall and approx. same depth), upright with about 2 inch gaps between them. The inside air in these chambers is heated which heats the metal itself which in turn heats the air surrounding each chamber. The fan blows this air throughout the house via ductwork.

Can these chambers not be heated using microwaves? If needed one could make sealed chambers with water or whatever inside if one needs something else to hold the heat. The microwaves themselves would always be enclosed in each chamber. I have no idea offhand the cost of electricity to power microwaves or if it would be safe using it this way. I also thought that since microwaves will run on 12 volt could one not put up a couple solar panels to power it? If so it could be a 100% free standing, self contained unit. Environmentally friendly as well, no?

There are electric forced air furnaces. But they are like using a conventional oven to melt a slice of cheese compared to doing it in a microwave cost wise.

I would love some opinions.

[alancalverd]

The cost of domestic electricity is the same whatever means you use to heat the house, except by night storage heaters,where you can use offpeak tariffs. All electric heaters are almost equally efficient with the exception of electrically powered heat pumps, which use about 30% of the energy of a direct (resistive or microwave) heater.

[Bored chemist]

I have no idea offhand the cost of electricity to power microwaves

Higher than the cost of the gas would be.

[Trekker]

All electric heaters are almost equally efficient with the exception of electrically powered heat pumps, which use about 30% of the energy of a direct (resistive or microwave) heater.

What about a 12 volt microwave compared to the 110. Would they produce the same amount of heat? If so it could be solar powered with battery storage.

[Trekker]

Higher than the cost of the gas would be.

One of the things here is I am now paying about $70. a month before I use a single wisp of natural gas because of all their fixed monthly charges. We also now have carbon taxes on gas but not electricity which is kind of ridiculous given the carbon footprint to produce electricity when averaged among all forms of generation.


As for microwaves they just seemed to heat things up so much faster which is why I thought it might be feasible. As the heat exchangers could be made of or contain the best heat transferring products.