12/17/2020
I have spent way too much time over the past decade or so trying to explain the fundamentals of "heat transfer" "infrared radiation" and "radiant heat" to people.
Any serious conversation usually has to begin with whackamole of the meme (and its logical offspring) that "heat rises" that somehow got drummed into people's heads when they were children, along with the fantasy that every atom looks like a miniature solar system.
This afternoon I took a break from reading about half lives, solubility and fate of the radionuclides released in our 1950s weapons tests to once again ponder how to talk about how we can heat spaces - in particular the interiors of the tiny house villages currently being promoted as housing for people currently living in tents in parks, under freeways, and on the sidewalks of our cities.
The good thing about tiny houses is that they are cheap to assemble, "look sorta like little houses" to the neighbors and offer privacy and some semblance of security and shelter from the challenges posed by the environment.
The bad thing is that they are small.
Initially this seems like a good idea but in fact it is a problem. As a structure enclosing space gets larger, the surface area increases on the square but the volume increases on the cube so it does not not take a lot more material to enclose considerably more space. Similarly as a structure gets smaller its surface area decreases on the square but its volume decreases on the cube. Enclosed volume gets smaller faster. This is a governing factor of life and can be presumed to limit the size of single celled organisms and to actually trigger cell division, because the amount of waste that a cell membrane can pass limits how much metabolism can be contained inside it.
Since the heat lost by a structure heated by the metabolism of its occupant is a function of its surface area - the more surface area it has, the more heat it can lose - it would seem like a good idea to make shelters small. But the problem is that just about any shelter one can imagine except a dry goose-down sleeping bag can radiate the 100 watts a person radiates faster than they can produce it.
This got me pondering the tea candle flower pot heaters that got a lot of press on social media a year or two ago. A tea candle puts out about 80 watts, of which 0.05% is visible.
Most of that chemical energy goes into raising the temperature of the solid wax through 2 phase changes needed to turn it into a gas, and the primary combustion by-products from burning it: C2 soots, CO, CO2, OH and H20, and the air that the flame pulls into a convection loop along with the combustion gasses, all of which DO in fact rise to the ceiling of the room, although the rest of the heat is radiated in all directions.
So in spite of the fact that they burn solid fuel more cleanly than a woodstove, and that tea candles create less metal waste than small bottles of LP gas, and are marginally safer than liquid fuels like kerosene, I dissed them pretty aggressively as a non-solution to the majority of the world's space heating problems.
But there is a core idea in those tea candle heaters that is worth holding onto. They take a small bit of extreme hotness (the flame at over 1200C) and spread some of it over a much larger surface area, from which it radiates at a lower surface temperature.
The same concept applies to things like baseboard heaters, which take something that gets way too hot (a high current resistance wire that would be glowing a dull orange if it were not losing so much heat via the fins) and dispersing the heat by conduction it into a bunch of aluminum fins that then radiate heat toward the panel along the face of the heater, and the rest of the metal casing to be reradiated toward the space, and to raise the temp of the air nearby, which is pulled from near the floor into the case, across the fins and then released to rise along the wall to the ceiling.
Because only the very narrow edges of the fins point toward the enclosure, most of the heat is transferred to the air. Same with a ductless heat pump, another popular system for converting electrical power into heated air.
The problem with heating air is that it rises to the ceiling where a lot of escapes either as black body radiation from the now heated but probably inadequately insulated ceiling to the much colder sky or as direct leaks, pulling in cold air from outside the heated envelope as its replacement.
Which leads back to the tiny house problem: how to deal with the reality that a 1000 sf structure can radiate a lot more than 100 watts even if it is very well insulated, which most will not be. Uninsulated it could easily require 600 watts to heat a smallish one to 45F when the outside world is 35F.
More to follow via edits and comments. This is a basic statement of the problem. Edits and comments will look at options for "solving" various aspects of the problem.
https://en.wikipedia.org/wiki/Rate_of_heat_flow
https://www.thenakedscientists.com/articles/questions/how-many-watts-burning-candle
