# Most efficient mechanism for heating air

I was read about small scale ion treatment which uses heats to high temperatures, would this be scalable and if so would there be any more efficient methods.

In summary, if you wanted to heat large volumes of air to 1000 celcius with only electricity what would be the most efficient method? Would it be a laser and if so what frequency or would a superheated coil be best etc. The air is from between 25000 and 35000 ft of altitude, which ever would be more efficient.

• dry air or is there water in it? – docscience Mar 9 '17 at 0:18
• @docscience it will be air from 70,000ft altitude – SRawes Mar 9 '17 at 0:26
• I suspect fairly dry then. Then the next question: do you want to heat the air at the same barometric pressure or are you allowed to compress the volume of air you want to heat? What are the constraints? Without reducing volume, compressing the gas it will be very hard to transfer heat energy to the rarefied gas using electrical energy. – docscience Mar 9 '17 at 2:22
• It can be compressed if that will make it more efficieent. The rest doesn't matter so whatever will make it more efficient @docscience – SRawes Mar 9 '17 at 16:38
• are you talking about this with "ion treatment" ? nasa.gov/centers/glenn/technology/Ion_Propulsion1.html – anna v Mar 26 '17 at 5:55

As the comments have noted, any answer depends on the many missing details about what you are trying to accomplish and any constraints on what you can do or afford. Is a “large volume” 1 cubic cm or 10000 cubic metres? Is it a static volume of air or is it constantly being exchanged for new air? What are the specifications of the available electric power? Must the volume of air be empty, or can structures (e.g. heating elements) pass through it? Is the air enclosed in a container? Is the container insulated? How long do you need the air to be at 1000°C? …

Energy conversion efficiency is not the problem. A simple heating coil connected to a DC battery can have almost 100% efficiency in converting the energy of the battery into heat. The maximum average temperature reached by the air will depend on the balance between the rate at which the electric coil is heating the air and the rate at which the air is cooling at its boundary by conduction, air exchange, or radiation. You want to maximize the surface area over which air is in contact with the heater, e.g. using metallic fins or meshes.

Why do you specify electrical heating? Does the “air” have to be the standard mix of atmospheric gases? The easiest way to get a bunch of very hot gas is to burn a fuel, and this is why hot air balloons up use burners. A tank of propane will produce much more heat than the same weight in batteries. (You may want to look at https://electronics.stackexchange.com/questions/112312.) I assume, however, that your air is not contained in a balloon since 1000°C is above the ignition or melting temperature of the balloon fabrics or films that I can think of.

Using a laser is problematic. See, for example, Optimal laser wavelength for heating air. With even the most efficient laser diode, about half your electrical energy would go into heating up the diode, not producing light that might – if the right wavelength – couple to the air. A laser or microwave beam fired up from the ground might work in some cases, since you could then be inefficient on the ground, but the aimed/focused beam could efficiently couple to a large properly designed air container at altitude.

Without more details or a link, it is not easy to figure out what “small scale ion treatment” you are referring to. Electric arcs have been long used to heat air, e.g. http://www.jhuapl.edu/techdigest/views/pdfs/V01_N1_1961/V1_N1_1961_Olsen.pdf.

If you are trying to design an electric jet engine I suggest looking at aviation stackexchange questions 26910 and 15163. Heating air is a poor way to use electricity for propulsion. Electrically driven propellers or direct ion acceleration are better, e.g. see these articles from The Economist, Popular Mechanics, and MIT.

• mhi-inc.com/PG4/eion-plasma-device-family.html @david bailey also it is constantly moving air and by large volume i mean the same as a jet engine. I am thinking what would be the best way to replace a combustion chamber from turbo fan – SRawes Mar 26 '17 at 19:00
• I'm not sure whether I tagged you properly sorry, @David Bailey – SRawes Mar 26 '17 at 19:07
• @SRawes Since heating the air is for propulsion, I edited my answer to include a few more relevant links. – David Bailey Mar 29 '17 at 5:39