Fresnel lens solar mass heater?

I have been kicking around the idea of building a sort of kiln like box made from possibly fire brick and clay or some other better type of high temp tolerant material and insulated with a thick layer of dirt. It would be built in a way which would allow air to be drawn through it and heated to a very high temperature with a large Fresnel lens as the top of the box. The air would then be pumped through a large mass inside a tiny house and then on into the house itself.  The air flow could be generated by a small exhaust fan at the end of the pipe inside the home.  does anyone have any thoughts on whether this might work, how efficiently it might work, best possible building materials, ect...   any thoughts are welcome

I have been collecting those lenses from the old big screen TV. Get them now before they dont make them anymore. Your idea soundd great. I am always thinking of things like this and am going to start such experiments as well when i move to my small farm to be in Palatka, Fl. I would love to know of it works. Keep inventing!

I have seen some Fresnel lens stuff on youtube and such, mostly being used to focus solar energy on a fixed point.   The way I see what you described is that it is like a solar cooker, but with a Fresnel Lens instead of glass, and the box, rather than being made of reflective material or black metal for absorbtion/radiation is a mass of clay or brick to hold more of the heat.    I'm sure the Fresnel would focus more energy than simple glazing, but... I'm not sure what sort of efficiency you would get out of such a system.  

I'm assuming that you are planning to place it near or against the building, so you would lose less heat in transit.  It is a good idea to have the mass inside the house, and perhaps not at all in the box.

I would definitely rethink using earth as an insulator.  Earth is an alright insulator, but it's more of a thermal mass than an insulator.  For this purpose, think of an insulator as being something that contains more air.  We think of the insulative properties of Earth but that is because of depth, and the length of time that heat transfers through the depth.  Deep earth is insulating, but for example, fiberglass insulation is a great deal more so per unit of depth, because it contains way more air per volume of mass (which absorbs, conducts and radiates).  Perlite or vermiculite, for instance contain a lot more air than earth (dirt).  And in terms of dirt, sand fractions are better than silt, and silt in turn is better than clay fractions.  Generally the less compacted, and the drier, the less conductive and more insulative a material is, but it's easiest for this purpose to just think of it being those substances which contain more air are more insulative.

If you could get the Fresnel lens to focus on something that heats up a lot, like ceramic tiles (which are dense and are less easily damaged by heat), but transfers that heat to say water in conductive copper pipes for instance, and then ran your fanned air by the pipes and tiles, in an insulated system via a tube to the house then you might have a lot more of an efficient system.

There might be easier ways to get hot air into your house.    If you built your tiny house with good passive solar design, with the none window portions of the South wall being Trombe walls, your heating system would probably be a lot simpler to build AND more efficient.

" I'm sure the Fresnel would focus more energy than simple glazing, but... I'm not sure what sort of efficiency you would get out of such a system.  "


I own just such a Fresnel lens from a scrapped projection TV. (Which we named the giant solar death ray, it will melt anything.  Really, anything...)  And I have been involved in the design and construction of a couple of solar assist houses/structures.

I believe that a box, insulated or not, with a Fresnel lens, will not collect any more heat energy than the same box with plain glass as a covering.  

There are only a fixed number of BTU's coming into a solar collector of a given size, let's call it 1,000 BTU's per hour.  A Fresnel will concentrate those BTU's into a tiny spot, which will get destructively hot and produce temperatures in excess of 1,000 F*.  And it will damage or melt whatever it is that receives that little spot of concentrated heat.  Heat loss out of the collector is directly related to the temperature differential between the inside and the outside of the collector, so the Fresnel will make it much less efficient due to the bigger temperature differential.

1,000 BTU's can be contained in a tiny very hot spot, or a large diffuse warm spot, but it's still just 1,000 BTU's.  The focusing feature provides no additional efficiency.  THat's the difference between heat and temperature.

1,000 BTU's is the exact same amount of heat in both cases, but the Fresnel is 1,000F, and the plain glass and sheet metal collector is 183F.  Far lower temperature, exact same BTU's.

The one advantage I might see to a Fresnel Lense would be in an application where the heat was directed into a very large heatbank with very little exposure to the air of the collector.

Collecting 1,000 btus onto a few square inches of surface [it seems wise to channel the energy a bit more spread out rather than the laser point that melts everything] with several cubic meters of insulated mass below might work out.

Collecting 1,000 btus onto a few square inches of surface [it seems wise to channel the energy a bit more spread out rather than the laser point that melts everything] with several cubic meters of insulated mass below might work out.

 That's more of what I had in mind.  It could focus on a rock, or clay tile, or rammed earth in a slightly more diffused pattern so that it was not melting everything and anything under it's laser gaze.

Stated differently, which is more able to heat a house, a swimming pool full of water at 185F, or a lit match that is 1450F?  The match clearly has a much higher temperature, but far far far less total heat.


For equal amounts of heat (BTU's) then it depends on what you want to do with the heat.  Do you need 1,100F air to heat your house?  Nope.

If you're trying to melt aluminum, then you absolutely need very high temperatures produced by a solar concentrator.

Roberto pokachinni wrote:

Collecting 1,000 btus onto a few square inches of surface [it seems wise to channel the energy a bit more spread out rather than the laser point that melts everything] with several cubic meters of insulated mass below might work out.

 That's more of what I had in mind.  It could focus on a rock, or clay tile, or rammed earth in a slightly more diffused pattern so that it was not melting everything and anything under it's laser gaze.

I believe you would get just as much heat out with a plain glass or plastic glazing than any concentrating lens, even if the focus point is a fuzzy 6" spot instead of an intense 1/2" spot.  The fact that it is concentrated does not make more BTU's.

It would work, but no better than a plain piece of glass.

Water running through pex tubing in the floor, using convection. Much more efficient than relying on air. And just use conventional solar heating systems, no need for the fresnel here.