Naked Science Forum
Non Life Sciences => Technology => Topic started by: jules on 02/11/2008 10:07:25
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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?
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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.
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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.
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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.
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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.
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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.
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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. [:o)]
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There no relation between microwave oven to heat.but its also produce heat internally,that means inside the microwave oven.
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If microwaves are so energy ineficient, why not just cook with electric devices?
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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.
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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.
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I have just put some lunch in the microwave. Cheers, SS, bon apetite!
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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
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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.
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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.
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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.
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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.
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I have no idea offhand the cost of electricity to power microwaves
Higher than the cost of the gas would be.
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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.
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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.
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The rate of heating is directly proportional to volts x amps, so to get the same power from a 12 volt unit, you need 10 times the current that you would use in a 120 volt unit. Higher voltages are preferred because you lose less power in the transmission lines.
If you have solar power, consider using a heat pump and a big hot water storage tank, and also underfloor low temperature (30 - 35 degree) heating rather than traditional 50 degree "radiators" (actually convectors) - "low and slow" is best if your property is adequately insulated.
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If microwaves are so energy ineficient, why not just cook with electric devices?
It's a matter of perceived convenience, not science-based common sense, which dictates whether or not to purchase & or operate a microwave oven in a household. I doubt there are any typical consumers who would consider the energy efficiency of a microwave oven vs something like a toaster oven
What about the matter of keeping the inside of the oven clean from food splatters? Microwave ovens are very good at collecting junk, and then combined with how well they condense moisture onto the inner surfaces of the oven, it's really a wonder that very few cases of food poisoning result! I would like to see a microwave have a removable liner that can go right into a sink or dishwasher and also a blow-down mode for the fan so it runs after the oven turns off
ps - microwave oven is also an 'electric device'
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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.
Yes, you can heat the inside of your chambers, using microwave energy. But is it a worthwhile idea? Possibly not! The common magnetron used has air-cooled fins for a reason, as the magnetron itself is converting a sizeable portion of mains energy into heat. You could do what the microwave oven manufacturers do - route the warmed air from these cooling fins into your heating chambers. But there will still be lost heat due to radiation off of your ducting. You might try insulating all the ducting with aerogel paint. Even better if you place the magnetron itself, inside your chambers, so that all the heat is in the right location. But this will create some overheating issues for the magnetron
So, you will be drastically increasing the cost of heating your chambers without making any improvement on the energy-exchange from mains power to room air. If you can find an application where this increased cost can be offset by a as-yet unknown benefit, such as industrial or laboratory processes, maybe it's worth considering further
Perhaps, if you were to make use of excess microwave energy that is being produced for some other purpose, in order to provide an 'added value' of heated air?
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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.
Any microwave device consuming X power can heat something no faster than a standard radiant electric heater at the same power. Only a heat-pump can be more efficient than this, since it uses heat from a sink in addition to the heat generated by the power source.
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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.
Any microwave device consuming X power can heat something no faster than a standard radiant electric heater at the same power. Only a heat-pump can be more efficient than this, since it uses heat from a sink in addition to the heat generated by the power source.
Being that the region of energy exchange between radiated energy & the material substance of a target food item is many times greater for a microwave oven when compared to resistive heating elements, and that for a microwave oven, such a region could allow a reduced efficiency of exchange and yet a greater total source of conducted heat. And also, that the greater exchange region of a microwave oven also presents a much reduced distance of food material to traverse via secondary heating due to conduction - it may very well be possible that a microwave oven is 'faster'?
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Any microwave device consuming X power can heat something no faster than a standard radiant electric heater at the same power. Only a heat-pump can be more efficient than this, since it uses heat from a sink in addition to the heat generated by the power source.
Being that the region of energy exchange between radiated energy & the material substance of a target food item is many times greater for a microwave oven when compared to resistive heating elements, and that for a microwave oven, such a region could allow a reduced efficiency of exchange and yet a greater total source of conducted heat.
For food, yes, but for house heating, there is 100% efficiency in either case. No energy goes to waste heating something else, as opposed to gas or oil which sends heat up a chimney.
All these fancy technology heaters they push in the stores are still just plain electric heaters which are all 100% efficient. A heat pump might be 130% efficient. All depends on the sort of heat sink available to it, and the temperature differential between the output of the heating unit and that sink.
And also, that the greater exchange region of a microwave oven also presents a much reduced distance of food material to traverse via secondary heating due to conduction - it may very well be possible that a microwave oven is 'faster'?
Yes, a microwave heats food faster since it heats all of it, not just the outside surface. The middle of the food is heated directly in part (although still not as fast as the edges), not requiring all heat to conduct from the surface into the interior. This all seems irrelevant to heating water for a house-heating unit. A resistive heater can be used directly, or indirectly with some sort of forced-air setup. A microwave system would need water and then a heat exchanger to move that heat from the water to the house air. All adds up to more steps and more costs.
I got free electricity one winter and managed to mostly heat my house with several $10 electric appliances put here and there, kind of maxing out my breaker box. Was sad to see that end, I tell ya.
Irrelevant to the original question, yes But I was addressing the speed of heating being questioned, and not the OP. I think that the rate of energy exchange is in itself worth looking into. The dynamics affecting that could be interesting
Even if the originally proposed system was designed to be safe for home use, it still can not avoid the facts that it involves the addition of microwave radiation leakage concerns, high-voltage required for operating the magnetron, additional points of mains power being brought into close vicinity of a water system, and all of that without removing the existing hazards of traditional hydronic heating systems
But, I wasn't interested in considering a practical application of the OP's idea, only in evaluating the idea by itself. Could it be made to work? I think it can. Should it be done? I don't care! I only want to see that it works, and that it works well. If any individual wants to try such a thing, such as the OP, then why not see what they can accomplish? It could be at the very least, very entertaining!
So what I would consider, is to use an existing microwave oven for a base design. It already has a correctly sized tuned cavity. Then I would add testing instrumentation to allow the realtime evaluation of just how well this cavity remains in good working order. The very first item to purchase for any instrumentation should be a well-made microwave leakage detector. But temperature & flow sensors, along with waveguide tuning equipment should also be looked into. I would drill holes in the walls of the cavity and braze into place steel tubing. The diameter of such tubing is a variable to consider from the perspective of waveguide design, so as to avoid generating secondary radiation on the inside of such proposed tubes. But it should be worth noting that the diameter of the holes already present in the walls of the cavity, such as the ones in the door, are the best size already chosen for safety. These holes are engineered to be as large as possible to allow viewing the inside of the cavity while remaining just small enough to block the radiation. So it might be a good idea to choose an ID of steel tubing which is that size or slightly smaller. These tubes then should be coated on their outer surface with a microwave absorbent material that will act as the energy conversion stage. I already know of some very practical research into choice of materials, for melting precious metals inside standard microwave ovens. This prototype design will be much like a water-tube boiler. And as such, the rest of the design should be already established. A set of manifolds will be attached at either side of the cavity, perhaps as a simple box enclosure brazed directly to the outside of the existing cavity. The flow rate of water traveling through this boiler will need to be controlled, maybe using a pump with higher flow rate in combination with restricting orifices & a throttling bypass valve?