by bob culbertson

caution! this concept uses fire and hot liquids that could hurt you if handled carelessly. i don't like to get hurt and i certainly don't want you to be injured either. i believe it has a valid use but you experiment with it at your own risk. i like cooking in my solar box cooker because it is relatively safe but even then i've burnt my fingers on scalding water and hot cookie pans made in it. let things cool off before messing with them.

if you understand this warning please read on:

without sunlight a box cooker quickly cools to the point where cooking stops. i've decided to try supplying heat from an external source and built a small model to test the concept. i call the apparatus a "heat booster". i've built 2 slightly different models called ttu1 and ttu2 that use basically the same principles but ttu2 is a bit easier to make. referring to the sketches and images while reading this text may make it easier to understand. here is a brief description of what i've done so far:

a heat booster must be safe, cheap to use, easily built using common materials and installed in a box cooker with minor modifications. 70�c was the target temperature since most of my cooking is done below 100�c. to do this i decided to use a "heat-pipe" to transport the heat from a burning wax candle to the inside of a box cooker.

i built a heat-pipe from 2 clean "cream of mushroom" soup cans soldered together with 50 ml of water inside. i cut a round hole in the bottom of an insulated box (to simulate a box cooker) large enough to install one end of the heat pipe (isolated from the box) flush with the bottom of the box. a larger can (chunky pineapple?) covered the exposed end of the heat-pipe and is used as a firebox into which a candle would be placed. the insulated box was then placed on spacers with the firebox can beneath it.

a votive candle was placed in the firebox and lit, the flame heated the water in the lower end of the heat-pipe which boiled and turned to steam; the steam rose and condensed on the upper end of the heat-pipe which is inside the box cooker's cavity. the steam condenses back into water, releasing heat, and falls back to the bottom. this cycle is repeated over and over again, as long as the flame is present. the temperature of the heat-pipe is about 100�c (the boiling point of water) which will not burn the box materials.

ttu2

is very much the same as ttu1 except it doesn't need soldering. a can holding a small amount of olive oil (or any other oil that is stable at 100�c) is heated by a candle. inside this can is a glass jar with a small amount (50 ml) of water in it with a tight metal lid. as the oil heats up it transfers heat to the water in the jar which begins to turn to steam at about 100�c the steam touches the lid and turns back to water which falls back to the bottom of the jar where it is reheated.

an advantage to this method is that soldering isn't required, water doesn't touch the metal cans and rust them and water lost from the jar can easily be replaced.

notes:

the initial intent was to build a candle powered 'slow cooker' but adding backup heat to my solar box cooker seemed like a good idea too. i hope the public disclosure of this concept prevents anyone from patenting it (i would be curious as to why anyone would want to patent it but i've seen stranger things).

some pressure is built up inside the heat-pipe but hasn't caused any problems so far. engineers that i've discussed this with don't seem concerned since the heat source is very small. there is only enough energy available at any one time to turn 2 grams of water into steam.

i tested the prototypes by using a votive candle and the temperature inside the insulated box reached 65�c. the burn rate of the candle was about 8 grams of wax per hour which would mean that 454 grams (1 pound) of wax would burn for about 57 hours. i had previously heated the same box with a 15 watt light bulb and the temperature reached 68�c.

an energy content of 13,360 kj/mole @ 280 g/mole was assumed for the wax. an oil lamp could also be used but candles are easier to use. i think the candle is about 80 watts and at 15 watts into the box would mean a 18% efficiency.

water dripped into the olive oil of ttu2 turns to steam and splatters oil all over everything and makes a mess. don't set a teapot directly on top of it.

the heat-pipe transfers heat best in only one direction (from candle to box) and is in the coolest part of the box (the bottom) so the box can be solar heated without much loss through the heat booster.